5. site 983€¦ · water depth (drill pipe measurement from sea level, m): 1985.0 penetration...

Jansen, E., Raymo, M.E., Blum, P., et al., 1996Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 162

5. SITE 9831

Shipboard Scientific Party2

HOLE 983A

Position: 60°24.200'N, 23°38.437'W

Start hole: 1200 hr, 21 July 1995

End hole: 0930 hr, 22 July 1995

Time on hole: 21.5 hr (0.90 days)

Seafloor (drill pipe measurement from rig floor, mbrf): 1994.1

Total depth (drill pipe measurement from rig floor, mbrf): 2248.5

Distance between rig floor and sea level (m): 11.1

Water depth (drill pipe measurement from sea level, m): 1983.0

Penetration (mbsf): 254.4

Coring totals:Type: APCNumber: 27Cored: 254.4 mRecovered: 264.42 m, 103.9%

Formation:Unit I: 0-254.4 mbsf; Holocene to late Pliocene; clay with silt, nannofos-

sils, mixed sediment

HOLE 983B

Position: 60°24.210'N, 23°38.437'W








Penetration (mbsf): 251.7




'Jansen, E., Raymo, M.E., Blum, P., et al., 1996. Proc. ODP, Init. Repts., 162: Col-lege Station, TX (Ocean Drilling Program).

2Shipboard Scientific Party is given in the list preceding the Table of Contents.

HOLE 983C

Position: 60°24.218'N, 23°38.443'W








Penetration (meters below seafloor, mbsf): 260.4




Principal results: Site 983 is located on the Bjorn Drift in approximately1650 m water depth on the eastern flank of the Reykjanes Ridge. A se-quence of sediments ranging in age from late Pliocene to Holocene (2.0 to0 Ma) was recovered at Site 983. Age estimates are consistent betweenbiostratigraphic and magnetic data and indicate that the Site 983 sedimen-tary sequence is continuous, without significant hiatuses. Sedimentationrates were determined using magnetic polarity data combined with thebiostratigraphic datums and range from 9 cm/k.y. in the Pleistocene, up to17 cm/k.y. in the upper Pliocene section. Multisensor track (MST) data al-lowed the construction of a continuous composite section for this site andpreliminary studies done on board indicate strong variance in a number ofparameters on both Milankovitch and sub-Milankovitch time scales,throughout the section.

The sediments at Site 983 are predominantly composed of rapidly ac-cumulated fine-grained terrigenous particles with minor amounts of bio-carbonate and biosilica. Although discrete ash layers are rare, pale to darkbrown glass (tachylyte) commonly occurs as a constituent of the silt andsand size fractions. Authigenic iron sulfides, primarily in the form of dis-seminated pyrite, are also typically present. The dominant lithologies in-clude silty clay, clay, clayey nannofossil mixed sediment, and clay withvariable amounts of nannofossils and silt. Nannofossil oozes with variableamounts of clay and sponge spicules also occur. Lithologic variation ondecimeter to meter scales characterizes the sediment at this site and is dueto changes in the abundance of silt and biogenic materials relative to claycontent. The range of variation in these components is diminished relativeto sediments of comparable age at Sites 980 to 982.

No major lithologic boundaries occur within the 260 m of sediment re-covered at this site and one lithologic unit is recognized. Subtle, but dis-tinct, boundaries that demark three subunits occur at depths of 120 mbsfand 180 mbsf. The shallower boundary is recognized primarily in thespectral reflectance signal. It is characterized by a downcore decrease inthe amplitude of the higher frequency (decimeter- to meter-scale) reflec-

139

SITE 983

tance signal and an absence of the lower frequency reflectance signal(>lO-m scale). The deeper boundary is recognized in visual examinationof split cores and smear slides. It is characterized by a downcore absenceof layers in which biocarbonate is predominant. All dropstones, which arenever common, occur above this deeper horizon. There is no evidence ofsignificant sediment disturbance, winnowing, or erosion at Site 983 al-though bioturbation is ubiquitous throughout the cores.

Calcium carbonate contents fluctuate between 0.7% and 43.3% withan average value of 16.8%, gradually decreasing with increasing sedimentdepth. As at Sites 980, 981, and 982, the carbonate cycles of Hole 983 Aprobably reflect glacial/interglacial fluctuations. Total organic carbon(TOC) contents vary between 0.04% and 0.48%, with an average value of0.18%. Total nitrogen and sulfur contents are generally very low. C/N ra-tios between 1 and 10 are indicative of predominantly marine organic ma-terial.

Calcareous nannofossils are the dominant fossil group at this site andare generally abundant and well preserved. All the standard Quaternarynannofossil zones are recognizable. Planktonic foraminifers are generallycommon to abundant and well preserved throughout the uppermostPliocene to Holocene sequence although rare barren intervals are ob-served. Benthic foraminifers are present at most of the levels examinedand preservation is good throughout. Diatoms at Site 983 were commonto abundant and exhibit moderate to good preservation. Due to the possi-ble influence of the East Greenland Current, warmer water species wererare while many cooler water indicators were more common. Siliceousflagellates (including silicoflagellates, ebridians, and actiniscidians) rangefrom trace to common in abundance with good to moderate preservation.

All archive halves of Holes 983A, 983B, and 983C were measured us-ing the pass-through cryogenic magnetometer. The Brunhes/Matuyamaboundary and the Jaramillo and the Olduvai Subchrons are well defined inall three holes. A prominent normal-polarity subchron observed in thethree holes between the Jaramillo and the Olduvai Subchrons is tentativelyidentified as the Cobb Mountain event. The high magnetic remanence in-tensities and high sedimentation rates of the sediments from this site willenable very interesting shore-based work on magnetic field transitions andpossible secular field changes.

Pore-water profiles from Site 983 are typical of sediments in whichsulfate reduction and methanogenesis are occurring. Sulfate concentra-tions decrease from seawater values at the top of the core to zero at about100 mbsf. Below 120 mbsf, methane begins to increase from 0 ppm,reaching a maximum of about 9000 ppm near the base of the hole. Theboundary between sulfate reduction and methanogenesis is very sharp at120 mbsf, presumably because utilization of methane by sulfate-reducingbacteria prevents significant diffusive penetration of methane into the sul-fate reduction zone above. The sharp sulfate/methane boundary at 120mbsf also corresponds with lithologic subboundary IA/IB and with seis-mic reflector R2. Ethane and propane values occur in detectable amountsbelow 165 mbsf; however, the high C]/C2 ratios also suggest that thesource for methane is most likely in situ bacterial methanogenesis result-ing from decomposition of organic matter in the sediments.

Bulk density, velocity, and shear strength all gradually increase down-core. The physical properties suggest a stratigraphically continuous se-quence and the drilled section has been defined as one geotechnical unit.

The seismic survey, on the other hand, resulted in the designation ofseven seismic units, (GA-I to GA-VII), with the upper five being cored atSite 983. The sedimentary section as a whole is acoustically well-strati-fied, although with variation between units. Lateral changes in sedimentthickness are common and seem to be controlled by basement relief aswell as depositional and erosional processes. The seismic reflectors (Rl toR6), marking changes in seismic character, are also unconformities whentraced laterally away from Site 983. At Site 983, all seismic reflectors ap-pear conformable, further testifying to the completeness of the site.

BACKGROUND AND OBJECTIVES

Site 983 (GARDAR-1) is located on the Gardar Drift in approxi-mately 1985 m water depth on the eastern flank of the ReykjanesRidge (Fig. 1). The primary drilling objective at Site 983 was to re-cover high-sedimentation-rate pelagic sequences with which to studyclimate evolution in the North Atlantic region over the last few mil-lion years. The site is located near the approximate boundary of Gla-

:•

Figure 1. Detailed topographic view of Gardar andBjorn Drift region showing location of Sites 983 and984.

64

15C

20c

25C

30°W

140

SITE 983

cial North Atlantic Intermediate Water (GNAIW) and SouthernSource Water (SSW) during the last glaciation (Oppo and Lehman,1993). Obtaining a long-term history of this GNAIW is one of the pri-mary scientific objectives of this site. In conjunction with Sites 980,981, and 982 to the east, and Site 984 just to the north, this site formsa depth transect of five sites roughly spanning the interval between 1and 2 km water depth.

This same suite of sites also defines a northwest-southeasttransect extending from ~24°W to ~14°W. Intercomparison of car-bonate, faunal, and geochemical records from these sites should de-fine changes in surface-water temperature and hydrography over thePliocene-Pleistocene. In addition, Site 983 should prove very sensi-tive to Nordic Seas overflows across the Greenland-Scotland Ridge.This site lies on the northwest margin of the Iceland Basin directlydownstream of overflows from the Iceland-Faeroe Ridge (see fig. 1,chapter 1, this volume). This water passes as a deep northern bound-ary current through the Charlie-Gibbs Fracture Zone south of Site983 (e.g., McCartney, 1992). Using this site, along with the othersites drilled north and south of the Greenland-Scotland Ridge, wehope to unravel the long-term history of deep- and intermediate-wa-ter formation in the Nordic Seas and subpolar North Atlantic. Com-parison of 613C and trace element gradients between these sitesshould allow us to define glacial-interglacial changes in North Atlan-tic Intermediate Water formation.

As predicted from previously studied piston and gravity cores inthe region, we recovered a sedimentary sequence with unusually highsedimentation rates (between 10 and 20 cm/k.y.). This will provideus with an unprecedented record of both glacial-interglacial and mil-lennial-scale variations in thermohaline circulation, surface-watertemperatures, and ice-rafting history over the late Pliocene and Pleis-tocene. In particular, we plan to investigate rapid century- to millen-nial-scale oscillations, such as those observed in temperature anddustiness in Greenland ice cores (Dansgaard-Oeschger events), aswell as in late Pleistocene marine record (e.g., Bond et al., 1992,1993; Bond and Lotti, 1995; Fronval et al., 1995). In piston cores,such events can be seen as changes in surface foraminiferal fauna(sea-surface temperature), carbonate, color, and deep-ocean chemis-try. The transitions between cold and warm epochs in ice cores areabrupt: ~6°C warmings occur in as little as 10-20 years and four-folddrops in dust content in as little as 20 years. Broecker (1994) reviewspossible causes for these oscillations and the linkages between deep-sea sedimentation and ice-core records. One possibility is that millen-nial-scale climate variations in ice cores are related to the strength ofthe thermohaline "conveyor belt."

Site 983 will be used to address a number of questions relating tothese "sub-Milankovitch" cycles. In particular, we expect to deter-mine whether these rapid climate oscillations characterized the ma-rine record during earlier, warmer climatic regimes, for instance,during the late Pliocene. We will look for variations in color, fora-miniferal faunal composition, stable isotope composition, and lithicconcentration. Many of the continuous records of physical propertiescollected on the ship (e.g., MST and spectral reflectance data) will beused for this purpose, and an early research priority will be to deter-mine how these signals relate to lithologic and textural variations inthe sediment (e.g., Robinson and McCave, 1994).

Superimposed on the "Dansgaard-Oeschger" events of the lastglacial period are "Heinrich" layers, events with longer characteristicrepeat times (-10,000 yr), which may be related to surges of the east-ern Laurentian and other major ice sheets (Heinrich, 1988; Bond etal., 1993; MacAyeal, 1993; Broecker, 1994; Fronval et al., 1995).Site 983 is located just north of the zonal axis of the Heinrich layersdocumented in the last glacial cycle. Determining the geographic dis-tribution of these Heinrich events, their long-term character, and thetiming of their first occurrence is a main objective of our drilling onthe Feni (Sites 980 and 981), Gardar (Site 983), and Bjorn (Site 984)Drifts. Are they restricted to the "100-k.y. world" of the Brunhes

Chron, which was characterized by large marine-based continentalice sheets? Do they occur in intervals characterized by higher fre-quency variations in smaller ice sheets? Are marine-based ice sheetsa prerequisite for climatic instability of this sort? Do Heinrich eventsalways have a characteristic repeat time of 10,000 years?

By studying deep-water variability using carbon isotopes, Cd/Caratios, and other proxies, we will also be able to determine whethermillennial-scale variations in surface temperature are associated withvariations in thermohaline circulation (Boyle and Keigwin, 1987;Rahmstorf, 1994; Weaver and Hughes, 1994; Oppo and Lehman,1995). Combining data from Site 983 with that from shallower anddeeper drift sites (Sites 980, 981, and 984), we will examine if, andhow, changes in the vertical nutrient distribution in the North Atlanticoccur. By studying sedimentation patterns, surface-water properties,and deep-water variability on suborbital time scales and relatingthese observations to ice cores, we hope to better understand the forc-ing and dynamics of decadal to millennial climate variability in theNorth Atlantic-Arctic region. The long-term perspective availablefrom ODP drilling will significantly add to the knowledge fromGreenland ice cores, which cover only the last 200 k.y.

OPERATIONS

Site 983 (GARDAR-1) was a high-priority contingency sitewhich was drilled because the leg was ahead of schedule by nearly 2days. The operational plan called for three piston-cored holes to ap-proximately 300 m below seafloor. Because time was short, we de-cided to drill three shallower holes (<260 m) rather than two deeperholes.

Head winds limited the vessel's speed while en route to Site 983.Before slowing down for pre-site seismic profiling, the vessel aver-aged a mere 8.3 kt. At 0742 hr on 21 July 1995 the ship slowed downto 6.0 kt and the seismic gear was deployed for a limited pre-site sur-vey of the site (see "Seismic Stratigraphy" section, this chapter). By1158 hr on 21 July the seismic profiling gear had been recovered, thevessel had returned to the drilling location based on GPS coordinates,and the positioning beacon had been deployed, initiating Hole 983A.

A standard APC/XCB bottom-hole assembly was used for allholes at Site 983, including a nonmagnetic drill collar. Subsequent (Band C) holes were offset 15 m to the north. The mudline was estab-lished for each hole. The APC firing depth was offset by a few metersfor subsequent holes to establish continuous sediment sections. Posi-tion, depths, and coring totals for each hole are summarized at the topof this chapter. All cores are listed in Table 1.

Coring proceeded without incident at all three holes until scientif-ic target depth had been reached. The following cores were orientedwith the Tensor tool: 983A-3H through 27H; 983B-3H through 27H;and 983C-3H though 27H. The drill pipe was tripped above mudline,clearing the seafloor, and the positioning beacon was released andsubsequently recovered at 2225 hr on 23 July. The vessel was securedfor transit and got underway at 0200 hr 24 July 1995.

COMPOSITE DEPTHS

Based on correlations between magnetic susceptibility, naturalgamma radiation, gamma-ray attenuation porosity (GRAPE), andspectral reflectance data, continuity of the sedimentary sequence wasdocumented for the entire sequence drilled at Site 983, extendingfrom the late Pliocene through the Holocene. A composite sectionwas developed as discussed in the "Composite Depths" section, "Ex-planatory Notes" chapter (this volume). The depth offsets that com-prise the composite depth section for Site 983 are given in Table 2.Continuity of the stratigraphic section was confirmed from the mud-line to approximately 288 mcd (260 mbsf in Hole 983C).

141

SITE 983

MST records that were useful in correlation are displayed on thecomposite depth scale in Figure 2 (see also back pocket). High-am-plitude magnetic susceptibility and GRAPE variations were used todetermine depth offsets for the composite depth section. P-wave ve-locity and natural gamma radiation measurements were also collect-ed over nearly the entire cored sequence of all holes at both sites.These measurements, combined with percentage spectral reflectancemeasurements (see "Lithostratigraphy" section, this chapter), wereuseful in constraining the hole-to-hole correlations.

Overlap between adjacent holes was documented throughout thecomposite depth section. Although the relative agreement of sedi-mentary features in adjacent holes was excellent, stretching and com-pression within the cored sequence occurred over most intervals.Because much of this distortion occurs on a scale of less than 9 m, itwas not possible to align every sedimentary feature using only com-posite depth scale adjustments. Within-core, decimeter to centimeterdepth scale adjustments would be required to align all sedimentaryfeatures simultaneously.

The expansion of the composite depth section relative to the mbsfdepth scale results from physical expansion of the cores after recov-ery as well as stretching of the sequence during the coring process.This expansion is illustrated in Figure 3. Growth of the mcd scale rel-ative to the mbsf scale is about 10% in all holes from Site 983.

Following construction of the composite depth section for Site983, a single spliced record was assembled from the aligned cores asdiscussed in the "Explanatory Notes" chapter (this volume). The tiepoints for the Site 983 splice are given in Table 3. Spliced magnetic

susceptibility, natural gamma radiation, and GRAPE density areshown in Figure 4.

LITHOSTRATIGRAPHY

The sediments at Site 983 are predominantly composed of rapidlyaccumulated fine-grained terrigenous particles, and have an averagecalcium carbonate content of approximately 16%. Biocarbonate and,to a lesser extent, biosilica are present in variable minor amounts.Whereas discrete ash layers are rare, pale to dark brown glass (tachy-lyte) commonly occurs as a constituent of the silt- and sand-sizedfractions. Authigenic iron sulfides, primarily in the form of dissemi-nated pyrite, are also typically present in minor abundances. Thedominant lithologies include silty clay, clay, clayey nannofossilmixed sediment, and clay with variable amounts of nannofossils andsilt. Nannofossil oozes with variable amounts of clay and spongespicules also occur. Lithologic variation on decimeter to meter scalescharacterizes the sediment at this site, and is due to changes in theabundance of silt and biogenic materials relative to clay content. Therange of variation in these components is diminished relative to sed-iments of comparable age at Sites 980, 981, and 982.

The primary lithostratigraphic unit and subunits for the Site 983sedimentary sequence are defined on the basis of data obtained fromseven sources: (1) visual observation of color, (2) smear slide exam-ination, (3) bulk calcium carbonate measurements, (4) spectral re-flectance measurements, (5) magnetic susceptibility measurements,

Table 1. Coring summary for Site 983.

DateCore (July 1995)

162-983A-1H2H3H4H5H6H7H8H9HIOHIIH1211I3HI4H15HI6H17HI8H19H20H21H22H23H24H25H26H27H

162-983B-1H2H3H4H5H6H7H8H9HIOH11HI2H13HL4H15H

212121212121212121212121212122

2222222222222222222222

2222222222222222222222222222

Time(UTC)

163517001740181518451920195020202055213022102240231023450025010501450220031003450420045505350615065007300810

104011101150122012551325135514251500153516051640171517501820

Depth(mbsf)

0.0-7.47.4-16.9

16.9-26.426.4-35.935.9-45.445.4-54.954.9-64.464.4-73.973.9-83.483.4-92.992.9-102.4

102.4-111.9111.9-121.4121.4-130.9130.9-140.4140.4-149.9149.9-159.4159.4-168.9168.9-178.4178.4-187.9187.9-197.4197.4-206.9206.9-216.4216.4-225.9225.9-235.4235.4-244.9244.9-254.4

Coring totals:

0.0-4.74.7-14.2

14.2-23.723.7-33.233.2-42.742.7-52.252.2-61.761.7-71.271.2-80.780.7-90.290.2-99.799.7-109.2

109.2-118.7118.7-128.2128.2-137.7

Lengthcored(m)

7.49.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.5

254.4

4.79.59.59.59.59.59.59.59.59.59.09.59.59.5

Lengthrecovered

(m)

7.429.919.999.97

10.079.989.959.939.969.929.919.989.929.95

10.069.879.819.919.959.909.859.689.809.639.579.629.91

264.4

4.739.97

10.039.849.899.809.949.99

10.089.82

10.029.859.809.93

10.02

Recovery(%)

100.0104.0105.0105.0106.0105.0105.0104.0105.0104.0104.0105.0104.0105.0105.9104.0103.0104.0105.0104.0103.0102.0103.0101.0101.0101.0104.0

103.9

100.0105.0105.6103.0104.0103.0104.0105.0106.1103.0105.0103.0103.0104.0105.5

Core

16H17H18H19H20H21H22H23H24H25 H26H27 H

162-983C1H2H3H4H5H6H7H8H9H10H11H12H13H14H15H16H17H18H19H20H21H22H23H24H25H26H27 H28H

Date(July 1995)

222222222222222222222323

232323232323

232323232323232323232323232323232323232323

Time(UTC)

192019552030210521402210225023202355003501150155

0430050505400615065007300810084509201000103511151150123013101340141514501520155516251655173018101840191019502025

Depth(mbsf)

137.7-147.2147.2-156.7156.7-166.2166.2-175.7175.7-185.2185.2-194.7194.7-204.2204.2-213.7213.7-223.2223.2-232.7232.7-242.2242.2-251.7

Coring totals:

0.0-3.93.9-13.4

13.4-22.922.9-32.432.4-41.941.9-51.451.4-60.960.9-70.470.4-79.979.9-89.489.4-98.998.9-108.4

108.4-117.9117.9-127.4127.4-136.9136.9-146.4146.4-155.9155.9-165.4165.4-174.9174.9-184.4184.4-193.9193.9-203.4203.4-212.9212.9-222.4222.4-231.9231.9-241.4241.4-250.9250.9-260.4

Coring totals:

Lengthcored(m)

9.59.59.59.59.59.59.59.59.59.59.59.5

251.7

3.99.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.5

260.4

Lengthrecovered

(m)

9.839.949.619.939.899.919.889.849.849.779.829.86

261.8

3.899.839.859.999.95

10.0310.029.899.61

10.0210.049.929.92

10.039.929.92

10.029.959.98

10.029.71

10.009.839.809.909.989.789.95

271.8

Recovery(%)

103.0104.0101.0104.0104.0104.0104.0103.0103.0102.0103.0104.0

104.0

99.7103.0103.0105.0105.0105.6105.5104.0101.0105.5105.7104.0104.0105.6104.0104.0105.5105.0105.0105.5102.0105.2103.0103.0104.0105.0103.0105.0

104.4

142

SITE 983

Table 2. Site 983 composite depths.

Core, section

162-983 A-1H-11H-21H-31H-41H-51H-CC2H-12H-22H-32H-42H-52H-62H-72H-CC3H-13H-23H-33H-43H-53H-63H-73H-CC4H-14H-24H-34H-44H-54H-64H-74H-CC5H-15H-25H-35H-45H-55H-65H-75H-CC6H-16H-26H-36H-46H-56H-66H-76H-CC7H-17H-27H-37H-47H-57H-67H-77H-CC8H-18H-28H-38H-48H-58H-68H-78H-CC9H-19H-29H-39H-49H-59H-69H-79H-CC10H-110H-210H-310H-410H-510H-610H-710H-CC11H-111H-211H-311H-411H-511H-611H-711H-CC12H-112H-2

Length(cm)

15015015015012616

1501501501501501507021

15015015015015015070

1501501501501501507918

1501501501501501508324

1501501501501501507919

1501501501501501507124

15015015015015015067

15015015015015015068

1501501501501501506725

1501501501501501506724

150150

Depth(mbsf)

0.001.503.004.506.007.267.408.90

10.4011.9013.4014.9016.4017.1016.9018.4019.9021.4022.9024.4025.9026.6026.4027.9029.4030.9032.4033.9035.4036.1935.9037.4038.9040.4041.9043.4044.9045.7345.4046.9048.4049.9051.4052.9054.4055.1954.9056.4057.9059.4060.9062.4063.9064.6164.4065.9067.4068.9070.4071.9073.4074.0773.9075.4076.9078.4079.9081.4082.9083.5883.4084.9086.4087.9089.4090.9092.4093.0792.9094.4095.9097.4098.90

100.40101.90102.57102.40103.90

Depth(mcd)

().()()1.503.004.506.007.268.59

10.0911.5913.0914.5916.0917.5918.2919.5621.0622.5624.0625.5627.0628.5629.2630.3631.8633.3634.8636.3637.8639.3640.1541.7143.2144.7146.2147.7149.2150.7151.5451.3652.8654.3655.8657.3658.8660.3661.1562.2063.7065.2066.7068.2069.7071.2071.9172.2773.7775.2776.7778.2779.7781.2781.9482.7584.2585.7587.2588.7590.2591.7592.4392.7494.2495.7497.2498.74

100.24101.74102.41103.19104.69106.19107.69109.19110.69112.19112.86113.22114.72

Offset(mcd - mbsf)

0.000.000.000.000.000.00

:

:

.19

.19

.19

.19

.19

.19

.19

.192.662.662.662.662.662.662.66

3.963.963.963.963.963.963.963.965.815.815.815.815.815.815.815.815.965.965.965.965.965.965.965.967.307.307.307.307.307.307.307.307.877.877.877.877.877.877.877.87

8.858.858.858.85

9.349.349.349.349.349.349.349.34

10.2910.2910.2910.2910.2910.2910.2910.2910.8210.82

Core, section

12H-312H-412H-512H-612H-712H-CC13H-113H-213H-313H-413H-513H-613H-713H-CC14H-114H-214H-314H-414H-514H-614H-714H-CC15H-115H-215H-315H-415H-515H-615H-715H-CC16H-116H-216H-316H-416H-516H-616H-716H-CC17H-117H-217H-317H-417H-517H-617H-717H-CC18H-118H-218H-318H-418H-518H-618H-718H-CC19H-119H-219H-319H-419H-519H-619H-719H-CC20H-120H-220H-320H-420H-520H-620H-720H-CC21H-121H-221H-321H-421H-521H-621H-721H-CC22H-122H-222H-322H-422H-522H-622H-722H-CC23H-123H-223H-3

Length(cm)

1501501501507424

1501501501501501506824

15015015015015015069

1501501501501501508026

15015015015015015064

1501501501501501506021

1501501501501501506625

15015015015015015070

1501501501501501506822

15015015015015015059

150150150150150150662

150150150

Depth(mbsf)

105.40106.90108.40109.90111.40112.14111.90113.40114.90116.40117.90119.40120.90121.58121.40122.90124.40125.90127.40128.90130.40131.09130.90132.40133.90135.40136.90138.40139.90140.70140.40141.90143.40144.90146.40147.90149.40150.04149.90151.40152.90154.40155.90157.40158.90159.50159.40160.90162.40163.90165.40166.90168.40169.06168.90170.40171.90173.40174.90176.40177.90178.60178.40179.90181.40182.90184.40185.90187.40188.08187.90189.40190.90192.40193.90195.40196.90197.49197.40198.90200.40201.90203.40204.90206.40207.06206.90208.40209.90

Depth(mcd)

116.22117.72119.22120.72122.22122.96123.51125.01126.51128.01129.51131.01132.51133.19133.71135.21136.71138.21139.71141.21142.71143.40143.71145.21146.71148.21149.71151.21152.71153.51154.30155.80157.30158.80160.30161.80163.30163.94164.62166.12167.62169.12170.62172.12173.62174.22175.08176.58178.08179.58181.08182.58184.08184.74185.55187.05188.55190.05191.55193.05194.55195.25195.83197.33198.83200.33201.83203.33204.83205.51205.69207.19208.69210.19211.69213.19214.69215.28216.44217.94219.44220.94222.44223.94225.44226.10227.12228.62230.12

Offset(mcd - mbsf)

10.8210.8210.8210.8210.8210.8211.6111.6111.6111.6111.6111.6111.6111.6112.3112.3112.3112.3112.3112.3112.3112.3112.8112.8112.8112.8112.8112.8112.8112.8113.9013.9013.9013.9013.9013.9013.9013.9014.7214.7214.7214.7214.7214.7214.7214.7215.6815.6815.6815.6815.6815.6815.6815.6816.6516.6516.6516.6516.6516.6516.6516.6517.4317.4317.4317.4317.4317.4317.4317.4317.7917.7917.7917.7917.7917.7917.7917.7919.0419.0419.0419.0419.0419.0419.0419.0420.2220.2220.22

143

SITE 983

Table 2 (continued).

Core, section

23H-423H-523H-623H-723H-CC24H-124H-224H-324H-424H-524H-624H-724H-CC25H-125H-225H-325H-425H-525H-625H-725H-CC26H-126H-226H-326H-426H-526H-626H-726H-CC27H-127H-227H-327H-427H-527H-627H-727H-CC

162-983B-1H-11H-21H-31H-CC2H-12H-22H-32H-42H-52H-62H-72H-CC3H-13H-23H-33H-43H-53H-63H-73H-CC4H-14H-24H-34H-44H-54H-64H-74H-CC5H-15H-25H-35H-45H-55H-65H-75H-CC6H-16H-26H-36H-46H-56H-66H-76H-CC7H-17H-27H-37H-47H-57H-6

Length(cm)

1501501506020

15015015015015015060

31501501501501501504017

15015015015015015059

31501501501501501506427

15015015023

15015015015015015077

1501501501501501508419

1501501501501501505925

1501501501501501507118

1501501501501501505426

150150150150150150

Depth(mbsf)

211.40212.90214.40215.90216.50216.40217.90219.40220.90222.40223.90225.40226.00225.90227.40228.90230.40231.90233.40234.90235.30235.40236.90238.40239.90241.40242.90244.40244.99244.90246.40247.90249.40250.90252.40253.90254.54

0.001.503.004.504.706.207.709.20

10.7012.2013.7014.4714.2015.7017.2018.7020.2021.7023.2024.0423.7025.2026.7028.2029.7031.2032.7033.2933.2034.7036.2037.7039.2040.7042.2042.9142.7044.2045.7047.2048.7050.2051.7052.2452.2053.7055.2056.7058.2059.70

Depth(mcd)

231.62233.12234.62236.12236.72238.36239.86241.36242.86244.36245.86247.36247.96248.71250.21251.71253.21254.71256.21257.71258.11259.44260.94262.44263.94265.44266.94268.44269.03269.59271.09272.59274.09275.59277.09278.59279.23

0.001.503.004.505.376.878.379.87

11.3712.8714.3715.1416.0917.5919.0920.5922.0923.5925.0925.9326.2227.7229.2230.7232.2233.7235.2235.8136.6338.1339.6341.1342.6344.1345.6346.3447.1048.6050.1051.6053.1054.6056.1056.6457.1858.6860.1861.6863.1864.68

Offset(mcd - mbsf)

20.2220.2220.2220.2220.2221.9621.9621.9621.9621.9621.9621.9621.9622.8122.8122.8122.8122.8122.8122.8122.8124.0424.0424.0424.0424.0424.0424.0424.0424.6924.6924.6924.6924.6924.6924.6924.69

0.000.000.000.000.670.670.670.670.670.670.670.671.891.891.891.891.891.891.891.892.522.522.522.522.522.522.522.523.433.433.433.433.433.433.433.434.404.404.404.404.404.404.404.404.984.984.984.98

4.98

Core, section

7H-77H-CC8H-18H-28H-38H-48H-58H-68H-78H-CC9H-19H-29H-39H-49H-59H-69H-79H-CC10H-110H-210H-310H-410H-510H-610H-710H-CC11H-111H-211H-311H-411H-511H-611H-711H-CC12H-112H-212H-312H-412H-512H-612H-712H-CC13H-113H-213H-313H-413H-513H-613H-713H-CC14H-114H-214H-314H-414H-514H-614H-714H-CC15H-115H-215H-315H-415H-515H-615H-715H-CC16H-116H-216H-316H-416H-516H-616H-716H-CC17H-117H-217H-317H-417H-517H-617H-717H-CC18H-118H-218H-318H-418H-518H-618H-7

Length(cm)

71

15015015015015015071

1501501501501501508325

1501501501501501505923

15015015015015015073

1501501501501501506124

1501501501501501507010

1501501501501501506627

1501501501501501507527

1501501501501501506122

150150150150

1506826

15015015015015015037

Depth(mbsf)

61.2061.9161.7063.2064.7066.2067.7069.2070.7071.4171.2072.7074.2075.7077.2078.7080.2081.0380.7082.2083.7085.2086.7088.2089.7090.2990.2091.7093.2094.7096.2097.7099.2099.9399.70

101.20102.70104.20105.70107.20108.70109.31109.20110.70112.20113.70115.20116.70118.20118.90118.70120.20121.70123.20124.70126.20127.70128.36128.20129.70131.20132.70134.20135.70137.20137.95137.70139.20140.70142.20143.70145.20146.70147.31147.20148.70150.20151.70153.20154.70156.20156.88156.70158.20159.70161.20162.70164.20165.70

Depth(mcd)

66.1866.8967.8369.3370.8372.3373.8375.3376.8377.5478.3879.8881.3882.8884.3885.8887.3888.2188.7690.2691.7693.2694.7696.2697.7698.3598.93

100.43101.93103.43104.93106.43107.93108.66109.47110.97112.47113.97115.47116.97118.47119.08119.69121.19122.69124.19125.69127.19128.69129.39130.36131.86133.36134.86136.36137.86139.36140.02140.65142.15143.65145.15146.65148.15149.65150.40151.75153.25154.75156.25157.75159.25160.75161.36161.54163.04164.54166.04167.54169.04170.54171.22172.32173.82175.32176.82178.32179.82181.32

Offset(mcd - mbsf)

4.984.986.136.136.136.136.136.136.136.137.187.187.187.187.187.187.187.188.068.068.068.068.068.068.06

8.738.738.738.738.738.738.738.739.779.779.779.779.779.779.779.77

10.4910.4910.4910.4910.4910.4910.4910.4911.6611.6611.6611.6611.6611.6611.6611.6612.4512.4512.4512.4512.4512.4512.4512.4514.0514.0514.0514.0514.0514.0514.0514.0514.3414.3414.3414.3414.3414.3414.3414.3415.6215.6215.6215.6215.6215.6215.62

144

SITE 983


Core, section

18H-CC19H-119H-219H-319H-419H-519H-619H-719H-CC20H-120H-220H-320H-420H-520H-620H-720H-CC21H-121H-221H-321H-421H-521H-621H-721H-CC22H-122H-222H-322H-422H-522H-622H-722H-CC23H-123H-223H-323H-423H-523H-623H-723H-CC24H-124H-224H-324H-424H-524H-624H-724H-CC25H-125H-225H-325H-425H-525H-625H-725H-CC26H-126H-226H-326H-426H-526H-626H-726H-CC27H-127H-227H-327H-427H-527H-627H-727H-CC

162-983C-1H-11H-21H-31H-CC2H-12H-22H-32H-42H-52H-62H-72H-CC3H-13H-2

Length(cm)

241501501501501501506924

1501501501501501506524

1501501501501501506328

1501501501501501506424

1501501501501501505826

1501501501501501506123

1501501501501501506215

150" 150

1501501501506319

1501501501501501506818

1501506227

1501501501501501506815

150150

Depth(mbsf)

166.07166.20167.70169.20170.70172.20173.70175.20175.89175.70177.20178.70180.20181.70183.20184.70185.35185.20186.70188.20189.70191.20192.70194.20194.83194.70196.20197.70199.20200.70202.20203.70204.34204.20205.70207.20208.70210.20211.70213.20213.78213.70215.20216.70218.20219.70221.20222.70223.31223.20224.70226.20227.70229.20230.70232.20232.82232.70234.20235.70237.20238.70240.20241.70242.33242.20243.70245.20246.70248.20249.70251.20251.88

0.001.503.003.623.905.406.908.409.90

11.4012.9013.5813.4014.90

Depth(mcd)

181.69182.87184.37185.87187.37188.87190.37191.87192.56193.04194.54196.04197.54199.04200.54202.04202.69203.15204.65206.15207.65209.15210.65212.15212.78213.37214.87216.37217.87219.37220.87222.37223.01223.88225.38226.88228.38229.88231.38232.88233.46234.13235.63237.13238.63240.13241.63243.13243.74244.93246.43247.93249.43250.93252.43253.93254.55255.21256.71258.21259.71261.21262.71264.21264.84265.26266.76268.26269.76271.26272.76274.26274.94

0.021.523.023.645.256.758.259.75

11.2512.7514.2514.9318.6420.14

Offset(mcd - mbsf)

15.6216.6716.6716.6716.6716.6716.6716.6716.6717.3417.3417.3417.3417.3417.3417.3417.3417.9517.9517.9517.9517.9517.9517.9517.9518.6718.6718.6718.6718.6718.6718.6718.6719.6819.6819.6819.6819.6819.6819.6819.6820.4320.4320.4320.4320.4320.4320.4320.4321.7321.7321.7321.7321.7321.7321.7321.7322.5122.5122.5122.5122.5122.5122.5122.5123.0623.0623.0623.0623.0623.0623.0623.06

0.020.020.020.021.351.351.351.351.351.351.351.355.245.24

Core, section

3H-33H-43H-53H-63H-73H-CC4H-14H-24H-34H-44H-54H-64H-74H-CC5H-15H-25H-35H-45H-55H-65H-75H-CC6H-16H-26H-36H-46H-56H-66H-76H-CC7H-17H-27H-37H-47H-57H-67H-77H-CC8H-18H-28H-38H-48H-58H-68H-78H-CC9H-19H-29H-39H-49H-59H-69H-710H-110H-210H-310H-410H-510H-610H-710H-CC11H-111H-211H-311H-411H-511H-611H-711H-CC12H-112H-212H-312H-412H-512H-612H-712H-CC13H-1I3H-213H-313H-413H-513H-613H-713H-CC14H-114H-214H-314H-4

Length(cm)

1501501501506619

1501501501501501508019

1501501501501501506926

1501501501501501508023

15015015015015015070

1501501501501501506425

15015015015015015061

1501501501501501507230

1501501501501501507529

1501501501501501506725

1501501501501501507220

150150150150

Depth(mbsf)

16.4017.9019.4020.9022.4023.0622.9024.4025.9027.4028.9030.4031.9032.7032.4033.9035.4036.9038.4039.9041.4042.0941.9043.4044.9046.4047.9049.4050.9051.7051.4052.9054.4055.9057.4058.9060.4061.1060.9062.4063.9065.4066.9068.4069.9070.5470.4071.9073.4074.9076.4077.9079.4079.9081.4082.9084.4085.9087.4088.9089.6289.4090.9092.4093.9095.4096.9098.4099.1598.90

100.40101.90103.40104.90106.40107.90108.57108.40109.90111.40112.90114.40115.90117.40118.12117.90119.40120.90122.40

Depth(mcd)

21.6423.1424.6426.1427.6428.3028.0629.5631.0632.5634.0635.5637.0637.8638.4639.9641.4642.9644.4645.9647.4648.1549.8051.3052.8054.3055.8057.3058.8059.6060.5362.0363.5365.0366.5368.0369.5370.2371.1972.6974.1975.6977.1978.6980.1980.8381.8283.3284.8286.3287.8289.3290.8291.0392.5394.0395.5397.0398.53

100.03100.75101.72103.22104.72106.22107.72109.22110.72111.47112.23113.73115.23116.73118.23119.73121.23121.90122.24123.74125.24126.74128.24129.74131.24131.96132.33133.83135.33136.83

Offset(mcd - mbsf)

5.245.245.245.245.245.245.165.165.165.165.165.165.165.166.066.066.066.066.066.066.066.067.907.907.907.907.907.907.907.909.139.139.139.139.139.139.139.13

10.2910.2910.2910.2910.2910.2910.2910.2911.4211.4211.4211.4211.4211.4211.4211.1311.1311.1311.1311.1311.1311.1311.1312.3212.3212.3212.3212.3212.3212.3212.3213.3313.3313.3313.3313.3313.3313.3313.3313.8413.8413.8413.8413.8413.8413.8413.8414.4314.4314.4314.43

145

SITE 983


Core, section

14H-514H-614H-714H-CC15H-115H-215H-315H-415H-515H-615H-715H-CC16H-116H-216H-316H-416H-516H-616H-716H-CC17H-117H-217H-317H-417H-517H-617H-717H-CC18H-118H-218H-318H-418H-518H-618H-718H-CC19H-119H-219H-319H-419H-519H-619H-719H-CC20H-120H-220H-320H-420H-520H-620H-720H-CC21H-121H-221H-321H-421H-521H-621H-7

Length(cm)

1501507429

1501501501501501506329

1501501501501501506626

1501501501501501507527

1501501501501501506629

1501501501501501507622

15015015015015015074

15015015015015015057

Depth(mbsf)

123.90125.40126.90127.64127.40128.90130.40131.90133.40134.90136.40137.03136.90138.40139.90141.40142.90144.40145.90146.56146.40147.90149.40150.90152.40153.90155.40156.15155.90157.40158.90160.40161.90163.40164.90165.56165.40166.90168.40169.90171.40172.90174.40175.16174.90176.40177.90179.40180.90182.40183.90184.64184.40185.90187.40188.90190.40191.90193.40

Depth(mcd)

138.33139.83141.33142.07142.72144.22145.72147.22148.72150.22151.72152.35152.63154.13155.63157.13158.63160.13161.63162.29162.91164.41165.91167.41168.91170.41171.91172.66172.65174.15175.65177.15178.65180.15181.65182.31183.42184.92186.42187.92189.42190.92192.42193.18193.73195.23196.73198.23199.73201.23202.73203.47203.85205.35206.85208.35209.85211.35212.85

Offset(mcd - mbsf)

14.4314.4314.4314.4315.3215.3215.3215.3215.3215.3215.3215.3215.7315.7315.7315.7315.7315.7315.7315.7316.5116.5116.5116.5116.5116.5116.5116.5116.7516.7516.7516.7516.7516.7516.7516.7518.0218.0218.0218.0218.0218.0218.0218.0218.8318.8318.8318.8318.8318.8318.8318.8319.4519.4519.4519.4519.4519.4519.45

Core, section

21H-CC22H-122H-222H-322H-422H-522H-622H-722H-CC23H-123H-223H-323H-423H-523H-623H-723H-CC24H-124H-224H-324H-424H-524H-624H-724H-CC25H-125H-225H-325H-425H-525H-625H-725H-CC26H-126H-226H-326H-426H-526H-626H-726H-CC27H-127H-227H-327H-427H-527H-627H-727H-CC28H-128H-228H-328H-428H-528H-628H-728H-CC

Length(cm)

141501501501501501507327

1501501501501501506419

1501501501501501506515

1501501501501501506426

1501501501501501507226

1501501501501501506216

1501501501501501507322

f* •frnm th

Depth(mbsf)

193.97193.90195.40196.90198.40199.90201.40202.90203.63203.40204.90206.40207.90209.40210.90212.40213.04212.90214.40215.90217.40218.90220.40221.90222.55222.40223.90225.40226.90228.40229.90231.40232.04231.90233.40234.90236.40237.90239.40240.90241.62241.40242.90244.40245.90247.40248.90250.40251.02250.90252.40253.90255.40256.90258.40259.90260.63

p tart nf p

Depth(mcd)

213.42214.06215.56217.06218.56220.06221.56223.06223.79224.33225.83227.33228.83230.33231.83233.33233.97235.48236.98238.48239.98241.48242.98244.48245.13246.28247.78249.28250.78252.28253.78255.28255.92256.77258.27259.77261.27262.77264.27265.77266.49267.88269.38270.88272.38273.88275.38276.88277.50278.05279.55281.05282.55284.05285.55287.05287.78

ar li cfr•tir

Offset(mcd - mbsf)

19.4520.1620.1620.1620.1620.1620.1620.1620.1620.9320.9320.9320.9320.9320.9320.9320.9322.5822.5822.5822.5822.5822.5822.5822.5823.8823.8823.8823.8823.8823.8823.8823.8824.8724.8724.8724.8724.8724.8724.8724.8726.4826.4826.4826.4826.4826.4826.4826.4827.1527.1527.1527.1527.1527.1527.1527.15

.n

(6) natural gamma-ray measurements, and (7) X-ray diffraction(XRD) analysis. No major lithologic boundaries occur within the 260m of sediment recovered at this site. Subtle but distinct boundariesthat demark three subunits occur at depths of 120 and 180 mbsf. Theshallower boundary is recognized primarily in the spectral reflec-tance signal. It is characterized by a downcore decrease in the ampli-tude of the higher frequency (decimeter- to meter-scale) reflectancesignal and an absence of the lower frequency reflectance signal (>IO-m scale). The deeper boundary is recognized in visual examination ofsplit cores and smear slides. It is characterized by a downcore ab-sence of layers in which biocarbonate is predominant. All dropstonesoccur above this deeper horizon. Lithostratigraphic Unit I and its sub-units are described in detail in the following section.

Description of Lithostratigraphic UnitUnit I

Intervals:Cores 162-983A-1H through 27H

Cores 162-983B-1H through 27HCores 162-983C-1H through 28H

Age: Holocene to late PlioceneDepth: 0 to 260 mbsf

Unit I consists of three 60- to 120-m-thick subunits. Subunit IAoccupies the uppermost 120 mbsf of Unit I, Subunit IB extends from120 to 180 mbsf, and Subunit IC includes the lowermost sedimentsrecovered, 180-260 mbsf (Table 4; Fig. 5). Unit I sediments are dom-inated by variable amounts of clay, silt, and calcareous nannofossils.Specifically, the unit consists of alternating intervals of two primarysets of lithofacies: (1) dark gray, dark greenish gray, very dark gray,and very dark greenish gray clay, silty clay, clayey silt, nannofossilclay, and clay with nannofossils, and (2) dark gray to very dark graynannpfossil clay mixed sediment, clayey nannofossil mixed sedi-ment, and clayey mixed sediment with nannofossils. A third minor,more nannofossil-rich set of lithofacies occurs in Subunits IA and IB,and is absent in Subunit IC. It consists of light greenish gray, greenishgray, gray, dark gray, and dark greenish gray nannofossil ooze, nan-nofossil ooze with clay, nannofossil ooze with spicules, clayey nan-

146

SITE 983

GRAPE Natural Magnetic GRAPE Natural Magneticdensity gamma ray susceptibility density gamma ray susceptibility(g/cm3) (counts/s) (SI units) (g/cm3) (counts/s) (SI units)1.4 1.8 10 20 30 200 800 1.4 1.8 10 20 30 200 800

10

20

30

40

50 I i , • - l . . I

50

60

70

80

90

100 JL

100

GRAPE Natural Magneticdensity gamma ray susceptibility(g/cm3) (counts/s) (SI units)1.4 1.8 10 20 30 200 800

110

120

130

140

150 V>l

Figure 2. GRAPE density, natural gamma radiation, and magnetic susceptibility data from Site 983 on the mcd (meters composite depth) scale. Lines for Holes983B (dotted) and 983C (dashed) have been horizontally offset from line for Hole 983A (solid) for better display; therefore, values given on horizontal scale arethe true values only for Hole 983A. (See also back pocket.)

147

SITE 983

150

GRAPEdensity(g/cm3)

1.2 1.6 2.0

Natural Magneticgamma ray susceptibility(counts/s) (SI units)10 20 30 200 800

160

170

CDT3

180

190

2001 ^ ) 'I ,1 :?-\

200

GRAPE Natural Magneticdensity gamma ray susceptibility(g/cm3) (counts/s) (SI units)

1.2 1.6 2.0 10 20 30 200 800

210

220

230

240

250

250

GRAPEdensity(g/cm3)

1.2 1.6 2.0


260

270

280 -

290

300 i i i i

Figure 2 (continued).

148

SITE 983

60 120Depth (mbsf)

180

Figure 3. Depth offsets of the Site 983 meters composite depth scale relativeto mbsf depth, illustrating the "growth" of the composite depth scale. Solidcircles = Hole 983A, crosses = Hole 983B, open circles = Hole 983C.

Table 3. Site 983 splice tie points.

Hole, core,section (cm)

162-983-A-1H-5, 53B-2H-2, 62C-2H-5, 104A-2H-7, 8B-3H-3, 137C-3H-6, 146B-4H-3, 29C-4H-5, 128A-4H-7, 53C-5H-4, 134A-5H-7, 5C-6H-6, 124B-7H-4, 136C-7H-7, 17B-8H-3, 131C-8H-6, 47B-9H-5, 20C-9H-6, 125B-10H-3, 115C-10H-7, 44B-11H-4, 25C-11H-6, 68B-12H-3,64C-12H-6, 97B-13H-3, 32C-13H-5, 20A-13H-6, 8B-14H-3,43C-14H-6, 107B-15H-3, 14C-15H-5, 113A-15H-7.56C-16H-2, 146A-16H-7, 32C-17H-5, 130A-17H-7, 38C-18H-6, 83A-18H-6, 134C-19H-6, 128A-19H-6,56B-20H-2, 64C-20H-7, 16A-20H-7, 17C-21H-6, 146A-21H-7, 35C-22H-6,41A-22H-6, 143C-23H-6, 89A-23H-6, 124C-24H-6, 104A-24H-5, 125B-25H-2, 85C-25H-6, 106A-25H-6, 100C-26H-5, 136A-26H-6, 52B-27H-4, 44C-27H-7, 8A-27H-6, 124C-28H-7, 68

Depth(mbsf)

6.536.82

10.9416.4818.5722.3526.9930.1735.9338.2444.9450.6458.0660.5766.0168.8677.3979.1584.8589.3494.9597.58

103.34107.37112.51114.60119.48122.13126.47131.33134.52140.46139.85149.71153-.70159.27164.22168.24174.17176.95177.84184.06187.57193.36197.25201.81206.32211.78215.64221.43223.65225.55230.96234.40239.26243.42247.14250.47253.64260.58

Depth(mcd)

6.537.49

12.2917.6720.4627.5929.5135.3339.8944.3050.7558.5463.0469.7072.1479.1584.5790.5792.91

100.47103.68109.90113.11120.70123.00128.44131.09133.79140.90143.78149.84153.27155.58163.61170.21173.99180.97183.92192.19193.60195.18202.89205.00212.81215.04221.97225.36232.71235.86244.01245.61247.28254.84257.21264.13267.46270.20276.95278.33287.73

tie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie totie to

Hole, core,section (cm)

162-983-B-2H-1, 116C-2H-2, 74A-2H-3, 71B-3H-2, 8C-3H-2, 32B-4H-1, 137C-4H-1, 146A-4H-4, 47C-5H-1, 143A-5H-2, 109C-6H-1,95B-7H-1, 136C-7H-2, 101B-8H-2, 37C-8H-1.95B-9H-1.77C-9H-2, 125B-10H-2, 32C-10H-2, 38B-11H-2, 5C-11H-2, 46B-12H-1,44C-12H-1.88B-13H-1, 101C-13H-1,76A-13H-4, 44B-14H-1.74C-14H-1, 146B-15H-1,25C-15H-1, 106A-15H-5, 13C-16H-1.65A-16H-1, 128C-17H-1,71A-17H-4, 109C-18H-1, 134A-18H-4, 140C-19H-l,50A-19H-5, 64B-20H-l,56C-20H-1, 145A-20H-5, 107C-21H-1, 115A-21H-5, 113C-22H-1,98A-22H-4, 104C-23H-1, 103A-23H-4, 110C-24H-1.38A-24H-4, 116B-25H-1, 68C-25H-1, 100A-25H-5, 14C-26H-1.44A-26H-4, 19B-27H-2, 71C-27H-2, 82A-27H-5, 136C-28H-1.29

Depth(mbsf)

5.866.14

11.1015.7815.2225.0724.3531.3733.8338.4942.8553.5653.9163.5761.8571.9773.1582.5181.7891.7491.36

100.1399.78

110.24109.16116.83119.43119.36128.45128.46137.03137.54141.68147.10155.49157.24165.29165.90175.54176.26176.35185.46185.55195.02194.88202.93204.43212.49213.28222.05223.88223.40232.03232.34240.09244.40243.72252.26251.18

Depth(mcd)

6.537.49

12.2917.6720.4627.5929.5135.3339.8944.3050.7558.5463.0469.7072.0479.1584.5790.5792.91

100.47103.68109.90113.11120.70123.00128.44131.09133.79140.90143.78149.84153.27155.58163.61170.21173.99180.97183.92192.19193.60195.18202.89205.00212.81215.04221.97225.36232.71235.86244.01245.61247.28254.84257.21264.13267.46270.20276.95278.33

nofossil ooze, and clayey nannofossil ooze with spicules. XRD andsmear slide analyses reveal that quartz and feldspar dominate the de-trital silt component found in the silty dark gray clay layers, while ta-chylyte, amphiboles, pyroxenes, micas, and inorganic calcite arelesser constituents.

The mean carbonate content of Unit I is 16.4%, as measured inHole 983A (Fig. 6; see "Organic Geochemistry" section, this chap-ter). Values fluctuate from 1.7% at 208.30 mbsf to 42.4% at 8.03mbsf. The relatively high-amplitude 10-m-scale variations in carbon-ate content are superimposed on a longer trend of diminishing valuesdowncore, which is most noticeable in Subunit IA.

The spectral reflectance signal in Site 983 sediments is remark-ably diminished compared to previous sites, with few peaks above20% (Figs. 5, 6; compare to "Site 980/981" and "Site 982" chapters,this volume). Values for reflectance within the red band (650-700nm) for Subunit IA average 10%, and deviate 4%-5% above and be-low the average reflectance. Toward the base of Subunit IA, the val-ues diminish to an average of 6%. With few exceptions (e.g., 159-162 mbsf and 170-176 mbsf), the reflectance across all spectralbands remains low. Downcore, through Subunits IB and IC, the per-centage reflectance within the red band seldom deviates from 6%.

Dropstones are uncommon at Site 983, averaging fewer than fourper hole. Eleven dropstones between 1 cm and 5 cm in size (Fig. 7)were identified throughout Subunits IA and IB, and none were iden-tified in Subunit IC (Fig. 5). They occur in the more common dark-colored, clay-rich intervals. The distribution, texture, and composi-tion of all dropstones greater than 1 cm in size are summarized in Ta-ble 5.

Scattered thin ash layers and ash pods occur throughout Unit I. Asingle 8-cm-thick (rhyolitic?) ash bed is evident within Subunit IB inall three holes (Fig. 8).

There is no evidence of significant sediment disturbance by natu-ral processes at Site 983. Visible bedding planes and color contactsare horizontal and parallel, with no erosional relief, although most aremottled by bioturbation (Fig. 9). Bioturbation is ubiquitous through-out the cores, and is characterized by discrete burrows, blurred tran-sitions, and fine-scale mottling.

Interpretation

Sediments accumulating at Site 983 include terrigenous and bio-genic components. Both calcium carbonate and siliceous microfos-sils are well preserved throughout the recovered sequence (see"Biostratigraphy" section, this chapter) and there is no sedimentolog-ical evidence for erosion or winnowing. Consequently, changes inrelative proportions of these two sediment types must reflect variabledeposition rates rather than selective sediment removal. The mainte-nance of higher-than-typical sedimentation rates (100-150 m/m.y.),dominated by fine-grained elastics, throughout the late Pliocene andPleistocene may be related to local surface water and atmospheric ef-fects such as dust plumes or sea-ice melting, but is more likely due todeep-sea sediment focusing at this site (McCave and Tucholke,1986). The low sand content (3%-5%) and paucity of dropstones, onthe other hand, indicate the occurrence but relatively limited influ-ence of ice-rafting as a depositional mechanism during this same in-terval.

The biogenic component of sediments at Site 983, as observed insmear slides, consists primarily of calcareous nannofossils. Bulk car-bonate values peak at -40%, lower than peak values for the same timeinterval at Sites 980, 981, and 982. This may reflect diminished pro-ductivity and/or enhanced dilution by the high terrigenous sedimen-tation rates at this site. Diminished terrigenous sediment input wouldexplain both the general increasing trend in carbonate content upsec-tion and somewhat decreased overall sedimentation rate within Sub-unit IA (see "Sedimentation Rates" section, this chapter). Varyingdilution might also account for the slight covariance of calcareous

149

SITE 983

0

GRAPEdensity(g/cm3)

1.0 1.3

Naturalgamma ray(counts/s)

10 14 18

Magneticsusceptibility

(SI units)300 600

10

20

CDT3

30

40

50

50

GRAPEdensity(g/cm3)

1.2 1.4

Natural Magneticgamma ray susceptibility(counts/s) (SI units)14 20 300 600

60

70

80

100

90 - <_--_-

100

GRAPEdensity(g/cm3)

1.2 1.4 1.6

Natural Magneticgamma ray susceptibility(counts/s) (SI units)14 20 300 600

110

120

130

140

150

Figure 4. Spliced records of GRAPE density, natural gamma radiation, and magnetic susceptibility data from Site 983. Tie points for forming the splice aregiven in Table 3. Holes are 983A (solid line), 983B (dotted line), and 983C (dashed line).

150

SITE 983

150

GRAPE Natural Magnetic GRAPEdensity gamma ray susceptibility density(g/cm3) (counts/s) (SI units) (g/cm3)

1.2 1.4 1.6 12 20 300 600 1.2 1.4 1.6200

160

170

Q.CD

"O

3'woCL

oo

180

190

200 r I

210

220

230

240

250

Natural Magnetic GRAPEgamma ray susceptibility density(counts/s) (SI units) (g/cm3)12 16 20 300 600 1.2 1.4 1.6

250


260

270

280

290

300

Figure 4 (continued).

151

SITE 983

Table 4. Summary table of sediment characteristics found in Iithostratigraphic Unit I at Site 983.

Calcium carbonate (%)

Unit SubunitDepth(mbsf)

Thickness(m) Dominant lithologies/criteria Avg. Min. Max.

IA

0-260.2 260.2 Pleist.-late Plio. Cyclic (0.5 to 10 m) lithologic changes

0-120.0 120.0 Pleistocene Clay, silty clay, clayey silt, clayey nannofossil mixed sediment nannofossil ooze,

16.4 1.7 42.4

nannofossil clay/cyclic lithologic changes, terrigeneous and biogenic components oftennearly equal, variable color

IB 120.0-180.0 60.0 Pleistocene Clay, silty clay, clayey silt, clayey nannofossil mixed sediment, nannofossil ooze,nannofossil clay/cyclic lithologic changes, terrigeneous and biogenic components oftennearly equal, invariant color

IC 180.0-260.2 80.2 Pleist.-late Plio. Clay, silty clay, clayey silt/cyclic lithologic changes, terrigeneous components dominant,invariant color

and siliceous microfossil abundance, although this might also reflectenvironmental influences on productivity. It is interesting to note thecarbonate minimum from 130 to 160 mbsf (1.1-1.4 Ma) at Site 983is approximately equivalent in time to a minimum carbonate intervalobserved in sediments recovered from the Nordic Seas during Legs104 and 151 (Baumann et al., unpubl. data).

The carbonate content of Site 983 sediments appears to influencespectral reflectance even at low concentrations, although values ob-served at this site fall on the portion of the curvilinear reflectance/car-bonate relationship where spectral reflectance seems somewhat lesssensitive to carbonate content (fig. 9, "Site 984" chapter, this vol-ume). Noncarbonate sediment components appear to influence themagnetic susceptibility and natural gamma radiation records at Site983, as the two vary independently of reflectance, and appear tomove in and out of phase with each other. Both signals fluctuate atdepth scales that approximate orbital periodicities, varying from ec-centricity to obliquity band (see "Sedimentation Rates" section, thischapter) as well as suborbital time scales. The spectral reflectancevariations are greatest in Subunit IA, where relatively large changesoccur with a period of approximately 40 m.

BIOSTRATIGRAPHY

A continuous sequence of sediments ranging in age from latePliocene to Holocene (2.0 to 0 Ma) was recovered at Site 983, locatedon the Gardar Drift on the eastern flank of the Reykjanes Ridge. Cal-careous nannofossils are the dominant fossil group at this site. How-ever, all fossil groups exhibit variable abundance and preservation,possibly correlated to glacial/interglacial events. Biostratigraphic zo-nations for all microfossil groups are summarized in Figure 10.

Age information, with the exception of the occurrence of the dia-tom N. fossilis in sediment younger than expected, indicates that theSite 983 sedimentary sequence is continuous, without significant hi-atuses. Age boundaries at this site are based on nannofossil data (Fig.10). Sedimentation rates were determined using magnetic polaritydata combined with the biostratigraphic datums presented in Table 6.For a detailed discussion of sedimentation rates at Site 983, see the"Sedimentation Rates" section (this chapter).

Calcareous Nannofossils

Calcareous nannofossils are generally abundant and well pre-served at Site 983. However, a number of samples from variousstratigraphic levels yielded rare and poorly preserved nannofossils.Commonly, Cretaceous nannofossils are present in these intervals.Most of these Cretaceous nannofossils are likely to be ice-rafted fromthe North Sea area, which is the highest latitude area in the North At-lantic that contains abundant Cretaceous nannofossils (see detaileddiscussion by Donnally, 1989). Nannofossil assemblages recovered

from Site 983 are similar to those from previous sites, with relativelylow species diversities and a general dominance of Gephyrocapsa.Other than the Cretaceous nannofossils in a number of samples asmentioned above, reworking of older fossils into younger sediment isminimal or undetectable. All the standard Pleistocene nannofossilzones are recognizable (Fig. 10). Nannofossil zonal boundaries, aswell as some nontraditional markers, are summarized in Table 6.

Planktonic Foraminifers

Planktonic foraminifers are generally common to abundant andwell preserved throughout the latest Pliocene to Holocene sequencein Hole 983A. A single barren interval is found in Sample 162-983A-16H-CC. Planktonic foraminifers are frequently dominated by Neo-globoquadrina pachyderma (sinistrally coiling variety) but also in-clude Globigerina bulloides, Globorotalia inflata, Globorotaliascitula, Globigerinita glutinata, Globigerina quinqueloba, and N.pachyderma (dextrally coiling variety). Assemblages dominated byN. pachyderma (sinistrally coiling) are inferred to reflect glacial con-ditions, while assemblages with abundant Gg. bulloides, Gr. inflata,and N. pachyderma (dextrally coiling variety) are inferred to reflectinterglacial conditions. Assemblage composition is dependent uponwhether the core-catcher sample chanced to fall within an interglacialor glacial interval. Subpolar paleoenvironmental conditions are indi-cated throughout the sequence. The start of the acme zone of N.pachyderma (sinistrally coiling), between Samples 162-983A-25H-2,20 cm, and 5H-4,20 cm, falls just below the base of the Pleistocene(Fig. 10).

Benthic Foraminifers

Benthic foraminifers are present at most of the levels examined,although the lowermost samples from Site 983 exhibit more impov-erished benthic faunas than the overlying late Pleistocene sequence.A single barren interval is recorded in Sample 162-983A-16H-CC.Diversity is highly variable, but generally low below Sample 162-983A-18H-CC. Preservation is good throughout, although it is nota-ble that the aragonitic species Hoeglundina elegans only occurs be-low Sample 162-983 A-24H-CC.

The most common taxa include Cassidulina teretis, Cibicidoideswuellerstorfi, Epistominella exigua, Melonis spp., Pullenia spp., Pyr-go spp., and Stainforthia spp. The relative abundances of these taxaare variable, presumably in response to inferred glacial/interglacialcycles. The last occurrence of Stilostomella lepidula in Sample 162-983A-15H-CC represents the only significant biostratigraphic event.Falling at an interpolated age of 1.22 Ma, this LO appears to occurearlier here than at Sites 980, 981, and 982. There is no published agefor this datum. (For further discussion, see "Biostratigraphy" section,"Sites 980/981" chapter, this volume.)

SITE 983

Reflectance (%),450-500 nm

0 10 20 0

Carbonate

20 40

Calcareousnannofossils (%)

0 20 40 60

240-

Figure 5. Core recovery, lithostratigraphy, age, spectral reflectance (redband), magnetic susceptibility, and natural gamma radiation of sedimentsrecovered in Holes 983A, 983B, and 983C. Locations of dropstones (opendiamonds) are shown in the column adjacent to the lithostratigraphy. Percent-age reflectance, magnetic susceptibility, and natural gamma radiation recordsare from Hole 983A. (Key to symbols used in the "Generalized Lithology"column can be found in fig. 4, "Explanatory Notes" chapter, this volume.)

e 100

200

Figure 6. Spectral reflectance (blue band), bulk carbonate content, and calcar-eous nannofossil content of sediments recovered from Holes 983A, 983B, and983C. Carbonate content and spectral reflectance records are from Hole983A.

Diatoms

Diatoms were common to abundant throughout the Site 983 se-quence and exhibited moderate to good preservation. This allowedthe determination of several diatoms' first and last occurrences. Dueto possible cold-water influences, species such as Nitzschia reinholdiiand Pseudoeunotia doliolus, which are important late Pleistocenemarkers in the North Atlantic, were rare. Occurrences of many cool-er-water indicators, such as Neodenticula seminae and Rhizosoleniacurvirostris, were more common. The first and last occurrences ofRhizosolenia curvirostris, the last occurrence of Nitzschia reinholdii,and the first occurrence of Pseudoeunotia doliolus were reliable da-tums for the Site 983 sedimentary sequence. The presence of Nitzs-chia fossilis in sediments younger than its expected last occurrencesuggests that sediment reworking may have occurred.

Siliceous Flagellates

Siliceous flagellates (including silicoflagellates, ebridians, and ac-tiniscidians) are present between Samples 162-983A-3H-CC and27H-CC, with abundances ranging from trace to common. Preserva-tion of silicoflagellates is usually moderate, while preservation of ac-tiniscidians and ebridians is good.

Silicoflagellates

Below a barren interval, Samples 162-983A-3H-CC to 26H-CCcan be placed in the Distephanus speculum Zone, which is Holoceneto latest Pliocene in age. The Mesocena quadrangula Subzone occursfrom Sample 162-983A-9H-CC to 11H-CC, with an upper age of 0.83Ma. The lowermost Sample 162-983A-27H-CC belongs to the upperPliocene part of the Distephanus aculeatus Zone.

153

SITE 983

2 5 -

3 0 -

3 5 -

Table 5. Summary table of dropstones greater than 1 cm in size found atSite 983.

Figure 7. Photograph of a 5-cm angular crystalline dropstone found in Section162-983B-5H-3, 22-37 cm.

Actiniscidians and Ebridians

Below a barren interval, Samples 162-983A-3H-CC to 26H-CCare assigned to the Actiniscus pentasterias Zone. The lowermostSample 162-983A-27H-CC is attributed to the Ammodochium seroti-num Zone of late Pliocene age, with an upper age of 2.0 Ma.

PALEOMAGNETISM

At Site 983, all archive halves of Holes 983A, 983B, and 983Cwere measured using the pass-through cryogenic magnetometer witha 5-cm measurement interval. Demagnetization was carried out at

Core, section,interval top (cm)

162-983 A-3H-4, 414H-6, 68

162-983B-5H-3, 2611H-4, 3418H-4, 5119H-7, 30

162-983C-9H-3, 199H-4, 14317H-3, 5119H-7, 1320H-1, 110

Depth(mbsf)

21.835.7

39.295.1

161.7175.5

73.676.3

149.5174.5176.0

Size(cm)

1.01.2

5.04.01.51.0

1.02.01.52.57.5

Composition

Black, basaltBlack, soft, mudstone

BasaltFelsic igneousIgneousIgneous

BasaltBasaltSandstoneBlackDiorite

Shape

SubroundedSubangular

SubroundedRoundedSubangularAngular

SubroundedSubroundedSubangularSubangularSubangular

peak alternating fields of 25 mT for all core sections. Additional de-magnetization at 30 mT was carried out on some core sections, astime allowed (Table 7). At the 25-mT demagnetization level, magne-tization intensities fluctuate around 150 mA/m. Large variations inmagnetic intensity are observed close to polarity reversals, where in-tensities often decrease to about 10 mA/m.

Discrete-sample stepwise AF demagnetization indicates that thesteeply inclined drill-string (viscous) magnetization was removed bydemagnetization fields exceeding 10 mT (Fig. 11). In some cores, theupper 25%-50% of Section 1 appears to have a high coercivity (drill-string) remagnetization which was not removed at peak alternatingfields of 25 or 30 mT.

The Brunhes/Matuyama boundary, the Jaramillo Subchron, andthe top of the Olduvai Subchron are well defined in all three holes(Fig. 12). Subbottom depths of polarity chron boundaries are given inTable 8. The base of the Olduvai Subchron is observed only in Hole983C. A prominent normal-polarity subchron observed in all threeholes between the Jaramillo and Olduvai Subchrons is tentativelyidentified as the Cobb Mountain Subchron.

A thin interval within the Brunhes Chron of low negative inclina-tions, close to 20 mbsf (Fig. 12), occurs in all three holes and mayrepresent a magnetic excursion within the Brunhes Chron. Thin inter-vals of low positive inclinations within the Matuyama Chron, someof which are present in all three holes (for example, close to 125 mbsfand near 190 mbsf), may represent magnetic events within theMatuyama Chron or intervals in which the steep downward drill-string remagnetization has not been completely removed.

The high fidelity of the magnetic polarity stratigraphy and thehigh sedimentation rates (-11-13 crn/k.y.) bode well for the possibil-ity of high-resolution polarity transition records at this site. Transi-tional directions are recorded over 50-150 cm of core at mostpolarity zone boundaries; however, some may be artifacts of theNRM acquisition process.

Trial normalizations of NRM intensity records (after demagneti-zation at 25 mT) by MST volume susceptibility produce coherent"paleointensity" records which do not appear to co-vary with suscep-tibility and correlate among the three holes. The potential of theabove will be explored by high-resolution shore-based studies.

SEDIMENTATION RATES

A sedimentary section 260 m thick was recovered at Site 983, ex-tending to the late Pliocene. Sedimentation rate reconstructions werebased on magnetic polarity events from all three holes and calcareousnannofossil datums from Hole 983A (see "Paleomagnetism" and"Biostratigraphy" sections, this chapter). The Site 983 compositedepth section (see "Composite Depths" section, this chapter) wasused to relate events recorded in multiple holes to a common depth

154

SITE 983

cm

8 0 -

8 5 -

9 0 -

cm90—1

100-

110-

120-

95-1

Figure 8. Photograph of an 8-cm colorless volcanic ash layer found in Section162-983B-17H-6, 77-95 cm.

130 - 1

Figure 9. Photograph showing a typical bioturbated color contact in Site 983sediments (Section 162-983C-11H-5, 90-130 cm).

155

SITE 983

100-

2 0 0 -

Nannopl.zones

OkadaandBukry(1980)

Planktonicforam. zones

DiatomzonesBaldauf(1984)

Silico-flagellate

zonesLocker and

Martini (1989)

Ebridian/actiniscid.

zonesLocker and

Martini (1989)

Table 6. Depth range of biostratigraphic datums, Site 983.

Figure 10. Biostratigraphic summary, Site 983. Hatched intervals indicateabsence of fossils. The location of the Pliocene/Pleistocene boundary is basedon nannofossil data.

scale. To facilitate comparison between sites, sedimentation rateswere estimated from age vs. depth plots by drawing straight-line seg-ments (uniform sedimentation rate) between selected datums.

Sedimentation rates were calculated for both the meters belowseafloor (mbsf) depth scale and the meters composite depth (mcd)depth scale. Figure 13 presents sedimentation rates as a function ofage and composite depth. Sedimentation rates are calculated based onan integrated magnetobiostratigraphy (Table 9; Fig. 14), and confirmthe expected high sedimentation rates for this site. Magnetic polarityage control points included are the Brunhes/Matuyama Chron bound-ary and the Jaramillo and Olduvai Subchrons. Calcareous nannofossildatums are the FO of Emilianii huxleyi at 0.26 Ma, the LO of Pseu-doemilianii lacunosa at 0.46 Ma, and the LO of Gephyrocapsa spp.A/B at 1.23 Ma. These datums are considered synchronous from trop-ical through temperate zones (see "Biostratigraphy" section, "Ex-planatory Notes" chapter, this volume).

ORGANIC GEOCHEMISTRY

The shipboard organic geochemistry program at Site 983 consist-ed of analyses of volatile hydrocarbons, determinations of inorganiccarbon, total nitrogen, total carbon, and total sulfur, and pyrolysismeasurements (for methods, see "Explanatory Notes" chapter, thisvolume).

Volatile Hydrocarbon

As part of the shipboard safety and pollution monitoring program,concentrations of methane (Q), ethane (C2), and propane (C3) gaseswere measured every core using standard ODP headspace-samplingtechnique. One sample was taken from each core immediately afterits arriving on deck and was measured using a Hewlett Packard 5890Series II gas chromatograph. The results are presented in Table 10

DatumAge(Ma)

Core, section,interval (cm)

Depth Depth(mbsf) (mcd)

FO E. huxleyi (N)

LO R. curvirostris (D)

LO P. lacunosa (N)

LO N. reinholdii (D)

FO Gephyrocapsa spp. C/D (N)

LO M. quadrangula (S)

LO H. sellii (N)

LO Gephyrocapsa spp. A/B (N)

FO R. curvirostris (D)

LO C. macintyrei (N)

FO Gephyrocapsa A/B (N)

S, acme N. pachyderma s. (F)

FO P. doliolus (D)

LO A. serotinum (E)

0.26

0.30

0.46

0.65

0.78

0.83

1.22

1.23

1.58

1.59

1.70

1.80

1.89

2.00

162-983A-3H-CC, 26-294H-1, 20-21

3H-CC, 26-294H-CC, 13-18

5H-6, 22-235H-CC, 19-24

6H-CC, 14-197H-CC, 21-24

8H-3, 10-118H-4, 10-11

8H-CC, 21-269H-CC, 23-28

14H-CC, 21-2615H-CC, 24-26

16H-2, 20-2116H-3, 20-21

19H-CC, 23-2520H-CC, 20-22

22H-CC, 24-2623H-1, 20-21

23H-7, 20-2123H-CC, 18-20

25H-2, 20-2225H-4, 20-22

26H-CC, 0-327H-CC, 24-27

26H-CC, 0-327H-CC, 24-27

26.8626.60

26.8636.32

43.6245.92

55.3364.82

67.5069.00

74.2883.81

131.30140.94

142.10143.60

178.83188.28

207.30207.10

216.10216.68

227.60230.60

244.99254.78

244.99254.78

29.5230.56

29.5240.28

49.4351.73

61.2972.12

75.3776.87

82.1592.66

143.61153.75

156.00157.50

195.48205.71

226.34227.32

236.32236.90

250.41253.41

269.03279.47

269.03279.47

Notes: FO = first occurrence; LO = last occurrence; S = start. In parentheses: N = calcar-eous nannofossil, D = diatom, S = silicoflagellate, F = planktonic foraminifer, and E= ebridian.

Table 7. Summary of pass-through cryogenic magnetometer measure-ments at Site 983.

Measurement Core sections

162-983A-25 mT 1H-1 through 27H-630 mT 14H-4, 15H-4, 17H-7

162-982B-25 mT 1H-1 through 27H-730 mT 20H-1, 20H-2, 21H-6, 22H-1, 23H-6

162-982C-25 mT 1H-2 through 28H-730 mT 13H-4, 15H-6, 16H-1, 28H-3, 28H-4

and displayed in Figure 15. Twenty-seven sediment samples werecollected between 4.5 and 250.9 mbsf in Hole 983A. Methane con-centration in sediments are very low above 120 mbsf (from Sample162-983A-1H-4, 0-5 cm, to 13H-5, 0-5 cm). Below 120 mbsf, CH4

concentration shows a distinct downhole increase (Fig. 15). At 241.4mbsf (Sample 162-983A-26H-5, 0-5 cm), methane concentrationreaches a maximum of 8522 ppm (Fig. 15; Table 10). Ethane and pro-pane occur in detectable amounts below 165.4 mbsf (Fig. 15; Table10).

In general, the Q/C2 (methane/ethane) ratios consistently de-crease with burial depth due to the increasing influence of early di-agenetic generation of hydrocarbons. At Site 983, however, no

156

SITE 983

983A-15H-6, 103 cmW, up

S -

E, down

Jo= 2.5e+2 mA/m

0 50Demag. level (mT)

N

Figure 11. Stepwise AF demagnetization of a sample from Hole 983A. Top:orthogonal projection of end points of the magnetization vector. Open andsolid symbols represent projection on the vertical and horizontal planes,respectively. Middle: change in magnetization intensity during AF demagne-tization. Bottom: equal area projection of magnetization vector duringdemagnetization.

significant amounts of higher hydrocarbons (such as ethane or pro-pane) were detected, the high CyC2 ratios suggest a biogenic originof the methane. The methane at Site 983 was most likely derivedfrom in situ bacterial methanogenesis resulting from decompositionof organic matter in the sediments. The pore-water sulfate concentra-tion is depleted at a depth of about 120 mbsf, where methane starts toincrease rapidly (see "Inorganic Geochemistry" section, this chap-ter).

Carbon, Nitrogen, and Sulfur Concentration

Determinations of inorganic carbon, carbonate, total carbon, totalnitrogen, and total sulfur in Hole 983A are summarized in Table 11.Calcium carbonate content in the sediment sequence of Hole 983Afluctuated between 0.7% and 43.3%, with an average value of 16.8%(Fig. 15; Table 11), and shows a gradual decrease with increasingdepth. The concentration of carbonate in sediment is primarily con-trolled by the flux of biogenic carbonate and supply of terrigenousmaterial. As at Sites 980, 981, and 982, the carbonate cycles of Hole983A certainly reflect glacial/interglacial fluctuations.

Total organic carbon (TOC) contents vary between 0.04% and0.48%, with an average value of 0.18% (Fig. 15; Table 11). A maxi-mum TOC value of 0.48% occurs in the upper part of the sedimentaryrecord of Hole 983A (Sample 162-983A-5H-2, 43-44 cm; 37.84mbsf) Total nitrogen contents are generally very low (Fig. 15; Table11; 0.04%-0.08%). Total sulfur values vary between 0% and 0.59%,with an average value of 0.15% (Fig. 15; Table 11).

Composition of Organic Matter

Due to the organic carbon-poor nature of the sediments, pyrolysisanalyses were not made. C/N ratios varied between 0.9 and 9.9 (Fig.

15; Table 11), indicating a predominance of marine organic material(Fig. 16). Further qualitative and quantitative organic geochemicaldata are required before a detailed paleoceanographic interpretationof the organic carbon data can be made.

INORGANIC GEOCHEMISTRY

Pore-water profiles from Site 983 are typical of sediments inwhich sulfate reduction and methanogenesis are occurring. Sulfateconcentrations decrease from seawater values at the top of the core tozero at about 100 mbsf (Fig. 17A; Table 12). Below 120 mbsf, meth-ane begins to increase from 0 ppm, reaching a maximum of 9000 ppmnear the base of the core (Fig. 17B). Methanogenesis is an importantprocess in sediments where sulfate reduction has depleted availablesulfate. In this process, organic matter is degraded and carbon diox-ide, methane, ammonium, and phosphate are produced with the fol-lowing Redfield stoichiometry:

(CH2O)106(NH3)i6(H3Pθ4) = 53CO2 + 53CH4 + 16NH3 + H3PO4 (1)

The boundary between sulfate reduction and methanogenesis isvery sharp at Site 983 at 120 mbsf, presumably because utilization ofmethane by sulfate-reducing bacteria prevents significant diffusivepenetration of methane into the sulfate reduction zone above(Gieskes, 1983). The sharp sulfate/methane boundary at 120 mbsfalso corresponds with a lithologic subboundary (see "Lithostratigra-phy" section, this chapter) and with a seismic reflector (see "SeismicStratigraphy" section, this chapter).

Ammonia progressively increases with depth due to productionby both sulfate reduction and methanogenesis (Fig. 17C; Table 12;Equation 1). Alkalinity increases with depth from 4.5 mM to a max-imum of about 13 mM between 80 and 140 mbsf, reflecting bicarbon-ate production from sulfate reduction (Fig. 17D; Table 12). Below-130 mbsf, in the zone of methanogenesis, alkalinity decreases, indi-cating either diminished production or a removal mechanism not re-lated to calcite precipitation, as calcium values increase during thisinterval (Fig. 17H). Salinity decreases sharply from 34 to 32 in thezone of sulfate reduction, reflecting the removal of dissolved sulfatefrom pore waters (Fig. 17E; Table 12). Phosphate concentrationsgenerally increase in the top of the core, reaching a maximum atabout 80 mbsf, and then decrease toward the bottom of the core, in-dicating removal of phosphate at depth by mineral precipitation (e.g.,francolite).

Magnesium concentrations are near seawater values at the top anddecrease linearly downcore (Fig. 17G; Table 12), reflecting (1) up-take by clay mineral formation during in situ alteration of volcanicmaterial in the sediments and (2) chemical interaction with basalticbasement below. Calcium decreases in the upper 80 mbsf from 9.7mM at the top to a minimum of 3 mM at 80 mbsf, and then increasestoward the bottom of the section (Fig. 17H; Table 12). The Ca:Mg re-lationship is typical of interstitial waters in which both sulfate reduc-tion and alteration of volcanic material are important processes (Fig.171). In the upper 80 m, calcium and magnesium show a positive re-lationship because calcium is decreasing due to precipitation of cal-cite in the zone of sulfate reduction. Below this level, calcium andmagnesium display the typical negative relationship expected fromalteration of basaltic material in the oceanic basement and/or dis-persed in sediments.

Chloride concentrations increase with depth from near seawatervalues (-560 mM) to a maximum of 576 mM at 250 mbsf (Fig. 17J;Table 12). Similarly, sodium values also generally increase down-core from near seawater values at the surface (481 mM) to a maxi-mum of 517 mM at the base of the core (Fig. 17K; Table 12). TheNa:Cl relationship is linear with a slope of approximately 2:1 (Fig.17L). The increase in chloride and part of the increase in sodium mayresult from hydration of clay minerals during in situ alteration of vol-canic material (Shipboard Scientific Party, 1995). This process re-

157

SITE 983

Hole 983CΦ Inclination (°)

δ -90 0

Figure 12. Inclination of the magnetization vector vs.depth (mbsf) for Site 983, after AF demagnetization atpeak fields of 25 mT.

240

Hole 983B

Φ Inclination (°)

δ -go 0 90

Hole 983Aα> Inclination (°)

δ -90 0 90

Table 8. Preliminary positions of polarity chron boundaries at Site 983.


162-983 A-9H-6, 4512H-3.9013H-6, 015H-3.5024H-7, 0

162-983B-10H-2, 5012H-6,4014H-2, 3015H-5, 12525H-2, 60

162-983C-9H-6, 12012H-4, 5014H-l,015H-4, 7525H-1, 3028H-3, 75

Depth(mbsf)

81.85106.30119.40134.40225.40

82.70107.60120.50135.35225.30

79.10103.90117.90132.65222.70254.65

Interpreted boundary

Brunhes/MatuyamaJaramillo (top)Jaramillo (bottom)Cobb Mountain (?)Olduvai (top)

Brunhes/MatuyamaJaramillo (top)Jaramillo (bottom)Cobb Mountain (?)Olduvai (top)

Brunhes/MatuyamaJaramillo (top)Jaramillo (bottom)Cobb Mountain (?)Olduvai (top)Olduvai (bottom)

Age(Ma)

0.780.991.071.211.77

0.780.991.071.211.77

0.780.991.071.211.771.95

Comments

Section break

Section break

Core break

moves water, enriching the remaining pore waters in dissolvedconstituents. Hydration of clays should enrich both chloride and so-dium equally, but the observed 2:1 relationship requires an additionalsource of dissolved sodium to the pore waters.

Potassium concentrations decrease in the upper 100 m, reflectinguptake into clay minerals, and then values remain relatively constantto the base of the hole (Fig. 17M; Table 12). Lithium concentrationsaverage about 25 µM in the upper 150 m of Site 983 and then increase

toward the base of the hole (Fig. 17N; Table 12). This increase prob-ably reflects a diffusion gradient, with oceanic crust below providinga source of lithium to pore waters.

Dissolved silica increases from surface values of 430 µM to amaximum of 945 µM at 165 mbsf, and then values decrease towardthe base of the core (Fig. 170; Table 12). The increasing silica valuesreflect dissolution of siliceous microfossils and/or alteration of vol-canic material. The profile of strontium is similar to that of calcium,showing a modest decrease in the upper 80 mbsf and then increasingsteadily toward the base of the core (Fig. 17P; Table 12). The corre-lation of strontium with calcite is what would be expected from bio-genic calcite dissolution and subsequent recrystallization.

PHYSICAL PROPERTIES

Shipboard measurements of sediment physical properties at Site983 included nondestructive near-continuous measurements of bulkdensity, bulk magnetic susceptibility, compressional wave (P-wave)velocity, and natural gamma radiation on whole-round core sectionsof the three holes using the multisensor track. Velocities were alsomeasured on split-core sections from Hole 983A, either parallel orperpendicular to the length of the core sections. Measurements weremade at an average of six per core. The velocities were measuredalong core sections above 69.6 mbsf (Table 13). Below this depth thesediment became too consolidated to insert the DSV transducers, andsonic measurements were performed perpendicular to the core axisusing the Hamilton Frame velocimeter. Undrained shear strengthmeasurements were made using a motorized vane (Table 14). At ap-proximately 70 mbsf (corresponding to a shear strength of approxi-

158

SITE 983

mately 50 kPa), the vane started to produce cracks in the sediments.A pocket penetrometer was therefore used as an additional tool belowthis level.

Index properties were measured and calculated according toMethod C at a rate of one per section (Table 15). The gravimetric wetbulk density data are generally higher than the GRAPE values (Fig.18). The difference is approximately 0.1 g/cm3 in the upper part of thehole but increases to approximately 0.2 g/cm3 below 210 mbsf. Po-tential causes for this difference are discussed in "ExplanatoryNotes" (this volume).

Geotechnical Stratigraphy

One geotechnical unit (Gl) has been defined at Site 983, and thephysical properties show a gradual downhole effect of increasedoverburden. The unit consists of two subunits G1A and GIB, prima-

200

I1'"5E 1 2 0 - - .

80

1 1

—

1 1

"1

i••u

1

1 , ' - . . .

i i

100 200Depth (mcd)

300

Figure 13. Site 983 sedimentation rates vs. age (A) and vs. composite depth(B). Solid lines indicate rates in mbsf/m.y.; dashed lines indicate rates inmcd/m.y.

rily defined by the downhole change in velocity. Subunit G1A occu-pies the uppermost 60 m in Hole 983A, and is characterized by adownhole increase in wet bulk density (from 1.3 to approximately 1.5g/cm3), decreasing porosity (from 80% to 70%), fairly low (around1500 m/s) velocities, and low (about 18 kPa) shear strength values(Fig. 18).

In geotechnical Subunit GIB, the wet bulk density values arequite constant down to 95 mbsf, and increase between 95 and -155mbsf (Fig. 18; Table 13). The rest of the hole is characterized by val-ues around 1.6 g/cm3. The porosity drops from values of 71%-74%to approximately 65%-68% at the bottom of Hole 983A. This trendis inverse to the wet bulk density record, with two intervals of con-stant values separated by an interval characterized by slightly de-creasing values (Fig. 18; Table 13).

There is a good agreement between the two velocity records fromthe PWL and the split-core measurements, both of which show agradual increase in velocity downhole due to the effect of increasedoverburden (Fig. 18): The velocity increases downhole from approx-imately 1500 m/s near the upper boundary of Subunit GIB to 1530-1550 m/s at the bottom of Hole 983A.

Age (Ma)1

100 -

Q.CD

a

^ \ ^ FAD E. hu×leyi

^~^~m LAD P. lacunoεa

B/M \ .

tJ >»b J \ %

LO Gephyrocapsa spp. A/B N.

I I I 1 I

_

_

t 0 \ .

i i

200 -

300

Figure 14. Site 983 age vs. depth (mcd) curve based on integrated magneto-stratigraphic and biostratigraphic datums. Solid circles = nannofossils; opencircles = diatoms; open squares = foraminifers; open triangles = siliceousflagellates; solid triangles = magnetostratigraphic datums. B/M = Brunhes/Matuyama Chron boundary; tJ and bJ = Jaramillo Subchron (top and bot-tom); tO and bO = Olduvai Subchron (top and bottom).

Table 9. Age control points, Site 983.

Event

Core top

FO E. huxleyi (N)

LO P. lαcunosα (N)

Brunhes/Matuyama

Jaramillo top

Jaramillo bottom

LO Gephyrocapsa spp. A/B (N)

Olduvai top

Olduvai bottom

Age(Ma)

0.00

0.26

0.46

0.78

0.99

1.07

1.23

1.77

1.95

983A(mbsf)

0.00

26.73

44.77

81.85

106.30

119.40

142.85

225.40

983A(mcd)

0.00

30.69

50.58

90.70

117.12

131.01

156.75

247.36

983B(mbsf)

0.00

82.70

107.60

120.50

225.30

983B(mcd)

0.00

90.76

117.37

132.16

247.03

983C(mbsf)

0.00

79.10

103.69

117.90

222.70

254.65

983C(mcd)

0.02

90.52

117.23

132.33

246.58

281.80

Avg.depth(mbsf)

0.00

26.73

44.77

81.22

105.86

119.27

142.85

224.47

254.65

Avg.depth(mcd)

0.00

30.69

50.58

90.66

117.24

131.83

156.75

246.99

281.80

Rate(mbsf/m.y.)

102.80

90.20

113.91

117.37

167.54

147.40

151.14

167.69

Rate(mcd/m.y.)

118.04

99.45

125.25

126.57

182.42

155.73

167.11

193.39

Note: Ages are from Berggren et al. (1995). N = calcareous nannofossil.

159

SITE 983

Methane (ppm).0 4000 8000

Ethane (ppm)Propane (ppm) C/C2 ratio CaCO3 (%)

0 4000 8000 0 20 40 0

TOC (%)

0.2 0.4

TN (%)TS (%)

0.4 0.8

C/N ratio4 I

300

Figure 15. Methane, ethane, and propane concentrations, methane/ethane (C]/C2) ratio, calcium carbonate (CaCO3), total organic carbon (TOC), total nitrogen(TN), and total sulfur (TS) contents, and TOC/TN (C/N) ratio in Hole 983A.

Table 10. Results of headspace gas analysis of Hole 983A.


162-983A-1H-4, 0-52H-5, 0-53H-5, 0-54H-5, 0-55H-5, 0-56H-5, 0-57H-5, 0-58H-5, 0-59H-5, 0-510H-5, 0-511H-5, 0-512H-5, 0-513H-5, 0-514H-5, 0-515H-5, 0-516H-4, 0-517H-5, 0-518H-5, 0-519H-5, 0-520H-5, 0-521H-5, 0-522H-5, 0-523H-5, 0-524H-5, 0-525H-5, 0-526H-5, 0-527H-5, 0-5

Depth(mbsf)

4.5313.4322.9332.4341.9351.4360.9370.4379.9389.4398.93108.43117.93127.43136.93144.93155.93165.43174.93184.43193.93203.43212.93222.43231.93241.43250.93

c,(ppm)

2345

56665328381351201131593827439258955463666171718024752985227398

c2(ppm)

1111122222

c3(ppm)

1347

77798

C,/C2

3827439258955463666135864012376542613699

C,/C3

3827146414747801332102411461076947925

Notes: C] = methane, C2 = ethane, C3 = propane, CilC2 = methane/ethane ratio, C1/C3 =methane/propane ratio.

The undrained shear strength record shows a downhole trendcaused by a gradually increased downhole overburden (Fig. 18; Ta-ble 14). The values near the upper boundary of Subunit GIB are closeto 30 kPa, and increase downhole to approximately 100 kPa (vane)and 177 kPa (pocket penetrometer) at the bottom of the hole.

The magnetic susceptibility record shows a large number of peaks(Fig. 18) throughout Hole 983A, suggesting that during the last 2m.y. (see "Biostratigraphy" and "Paleomagnetism" sections, thischapter) accumulation of terrigenous material at the site has followeda cyclic pattern. Increases in magnetic susceptibility appear to corre-late with peaks in shear strength below -170 mbsf, suggesting thatshear strength increases in zones that contain relatively more conti-nental-derived material with high susceptibilities.

The natural gamma radiation record from Hole 983 A also revealsa cyclic pattern consisting of high and low counts (Fig. 18). Thepeaks most likely reflect zones enriched in clay minerals. The fre-quency pattern with which the peaks appear is similar to that of themagnetic susceptibility, but the cycles are not in phase. Thus, fluctu-

ations in clay/carbonate accumulation are not necessarily driven bythe same factors that control the frequencies of terrigenous materialdeposition at the site.

Correlation with Lithostratigraphyand Seismic Stratigraphy

Hole 983A penetrates three seismic reflectors at 60, 121, and 163mbsf, respectively (see "Seismic Stratigraphy" section, this chapter),and recovered one lithostratigraphic unit which has been divided intothree subunits (see "Lithostratigraphy" section, this chapter). Reflec-tors Rl and R2 correspond to lithostratigraphic boundaries, but Re-flector R3 does not seem to correspond to changes in lithology.Reflector Rl is most likely associated with the change in velocity thatoccurs at approximately 60 mbsf (Fig. 18), which may produce achange in acoustic impedance. Reflectors R2 and R3 are relativelystrong, and may be associated with velocity peaks that occur at 121mbsf and close to 160 mbsf, respectively (Fig. 18).

Physical properties in Hole 983A suggest that the entire drilledsection is continuous, without significant stratigraphic breaks. This isin agreement with the biostratigraphic studies (see "Biostratigraphy"section, this chapter). The conformable nature of the sediments is alsoreflected by the internal seismic reflection pattern of the sedimentarybasin at Site 983 (see "Seismic Stratigraphy" section, this chapter).The general picture of the sediments as viewed from their physicalproperties is in good agreement with their lithology, which is charac-terized by rapidly accumulated fine-grained terrigenous and biogenicsediments (see "Lithostratigraphy" section, this chapter).

SEISMIC STRATIGRAPHY

The proposed site GARDAR-1 (Site 983) was selected on the ba-sis of the multichannel seismic EW-9302 Line lb, shot in 1993 (Oppoet al., unpubl. data). To obtain a crossing line and to survey GGC-11,a nearby giant gravity core site off the existing seismic line, a surveywas designed to connect the two sites and then duplicate EW-9302Line lb. The survey (Fig. 19) was carried out using the 80-in.3 watergun as source (see "Explanatory Notes" chapter, this volume), as wellas the 3.5-kHz and 12-kHz PDR systems. In order to tie the drill siteto results from pre-cruise gravity cores, PDR data were considered tobe of particular importance. The pre-cruise seismic data showed thatthe area is characterized by a high basement relief, and it was there-fore important to locate the site between basement highs. Further-more, the survey showed that the sedimentary section in this area isdisrupted by small faults associated with the basement highs.

160

Table 11. Summary of organic geochemical analyses of Hole 983A samples.


162-983 A-1H-1, 107-1081H-2, 40-411H-3, 134-1351H-4, 33-342H-1, 62-632H-4, 63-642H-6, 59-603H-1, 124-1253H-3, 72-733H-5, 125-1264H-1, 81-824H-4, 92-934H-6, 14-154H-7, 37-385H-2, 43-445H-5, 80-816H-1, 70-716H-3, 27-286H-5, 129-1307H-1,68-697H-3, 59-607H-6, 79-808H-1,58-598H-3, 70-718H-5, 47-489H-1,73-749H-2, 89-909H-6, 7-681 OH-1,80-8110H-4, 75-7610H-5, 83-8411H-3, 139-14011H-4, 117-11811H-6, 50-5112H-1, 135-13612H-4, 101-10212H-5, 134-13512H-7, 57-5813H-2, 99-10013H-5, 99-10014H-2, 102-10315H-1, 100-10115H-6, 101-10216H-3, 133-13416H-4, 138-13916H-5, 98-9917H-2, 129-13017H-4, 128-12917H-5, 122-12318H-1, 135-13618H-4, 128-12919H-2, 137-13819H-6, 135-13620H-2, 115-11620H-4, 127-12820H-4, 135-13621H-1, 81-8221H-3, 130-13122H-1, 137-13822H-5, 130-13123H-1, 139-14023H-4, 139-14023H-6, 134-13524H-2, 78-7924H-3, 97-9824H-5, 117-11825H-2, 119-12025H-5, 121-12225H-6, 112-11326H-1, 131-13226H-3, 130-13126H-6, 122-12327H-1, 130-13127H-2, 130-13127H-4, 116-11727H-5, 123-124

Depth(mbsf)

1.081.914.354.848.03

12.5415.5018.1520.6324.1627.2231.8334.0535.7837.8442.7146.1148.6852.7055.5958.5063.2064.9968.1170.8874.6476.3082.0884.2188.6690.2497.3098.58

100.91103.76107.92109.75111.98114.40118.90123.93131.91139.42144.74146.29147.39152.70155.69157.13160.76165.19171.78177.76181.06184.18184.26188.72192.21198.78204.71208.30212.80215.75218.69220.38223.58228.60233.12234.53236.72239.71244.13246.21247.71250.57252.14

IC(%)

3.682.862.181.655.094.231.441.492.612.683.582.021.251.293.544.331.565.025.204.631.122.471.631.761.062.790.622.191.111.304.020.804.223.312.462.030.812.110.733.570.551.100.520.872.061.930.760.433.060.912.664.783.333.380.960.931.281.911.732.660.201.440.620.440.341.160.092.270.431.560.561.701.651.680.951.49

CaCO3

(%)

30.723.818.213.742.435.212.012.421.722.329.816.810.410.729.536.113.041.843.338.69.3

20.613.614.78.8

23.25.2

18.29.2

10.833.57.4

35.227.620.516.96.7

17.66.1

29.74.69.24.37.2

17.216.16.33.6

25.57.6

22.239.827.728.2

8.07.7

10.715.914.422.2

1.712.05.23.72.89.70.7

18.93.6

13.04.7

14.213.714.07.9

12.4

TC(%)

4.10

1.795.244.39

1.53

3.62

1.48

4.02

1.64

4.79

1.74

2.95

1.35

1.09

2.61

0.91

0.711.21

1.07

0.94

1.07

4.87

3.60

1.40

1.89

0.52

0.61

0.30

1.66

1.83

TOC(%)

0.42

0.140.150.16

0.04

0.04

0.23

0.48

0.08

0.16

0.11

0.16

0.24

0.20

0.15

0.18

0.160.11

0.20

0.18

0.16

0.09

0.22

0.12

0.16

0.32

0.17

0.21

0.10

0.18

TN(%)

0.08

0.050.040.04

0.04

0.05

0.05

0.05

0.04

0.05

0.05

0.05

0.04

0.05

0.04

0.04

0.040.04

0.06

0.06

0.05

0.04

0.05

0.05

0.05

0.06

0.05

0.06

0.04

0.05

TS(%)

0.10

0.100.000.15

0.10

0.05

0.26

0.13

0.05

0.06

0.29

0.34

0.00

0.00

0.31

0.22

0.110.31

0.07

0.00

0.36

0.18

0.27

0.12

0.06

0.59

0.00

0.06

0.13

0.21

C/N

5.3

2.53.53.7

1.1

0.9

4.3

9.9

2.0

3.0

2.4

3.2

5.4

4.1

3.4

4.1

3.82.5

3.6

3.2

3.1

2.3

4.5

2.3

2.9

5.0

3.2

3.7

2.3

3.5

Notes: IC = inorganic carbon, CaCO3 = calcium carbonate, TC = total carbon, TOC = total organic carbon, TN = total nitrogen, TS = total sulfur, and C/N = total organic carbon/totalnitrogen ratios.

SITE 983

161

SITE 983

The site survey revealed that the originally proposed site was nearoptimal, both with respect to distance from any obvious faults, andthe thickness and continuous character of the sedimentary section.The position chosen for the site was therefore within a 100-m radiusof the original, and corresponds to shotpoint 669 of Line S3 (Fig. 20).

Description of Seismic Stratigraphic Units

Seven seismic units (GA-I to GA-VII; Fig. 20B) were definedfrom the site survey data, but only the upper four were penetratedduring drilling at Site 983. The sedimentary section as a whole isacoustically well stratified, although variable between units. Lateralchanges in sediment thickness are common and seem to be controlledby basement relief as well as by depositional and erosional processes.

0.08

0.04

oon

-<—

-* mmt/ /

-Marine1 ' 1 /

i 1

-V

0.0 0.4 0.8Total organic carbon (%)

Figure 16. Total organic carbon vs. total nitrogen in Hole 983A. Lines showC/N ratios of 5, 10, and 20.

Basement shows high relief, resulting from the formation of half gra-bens, each bounded by steep normal faults on its eastern side. Thesediments fill the basins in between the basement highs, and closelyspaced faulting of the sedimentary section is observed primarily overthe highs. As at Site 982 (see "Site 982" chapter, this volume), thefaults extend to the seafloor, where they produce small depressions.Faulting can readily be observed in the high-resolution 3.5-kHz PDRrecords (Fig. 21), and the displacement rarely exceeds 7-8 m.

The seismic stratigraphy is based on the identification of six sig-nificant seismic reflectors (Rl through R6) between the seafloor andacoustic basement. In addition to marking changes in seismic charac-ter, these reflectors also become unconformities when traced laterallyaway from Site 983. Erosional truncation of underlying strata, as wellas onlapping reflectors are observed, with the latter being more prom-inent towards the margins of each individual basin. With the excep-tion of Reflector Rl, the erosion related to each of theseunconformities does not appear to be significant as judged from theseismic records alone. At Site 983, all seismic reflectors are conform-able, suggesting that no major hiatuses would be found at the site. _

Unit GA-I extends from the seafloor at 2.755 seconds two-waytraveltime (s TWT) to approximately 2.835 s TWT (Fig. 22). Thisunit is penetrated by the 3.5-kHz PDR (Fig. 21), and displays a dis-tinct layering of evenly spaced reflectors. The distance between indi-vidual reflectors in the PDR records is about 10-12 ms TWT at Site983. Using an interval velocity of 1.55 km/s, as measured on splitcores (see "Physical Properties" section, this chapter) and adjustedfor core expansion (see "Explanatory Notes" chapter, this volume),the spacing of reflectors is 8-9 m and the total thickness of Unit GA-I is about 60 m. In the lower resolution seismic section, this unit ap-pears as an almost acoustically transparent layer. Farther east alongLine S1, Reflector Rl seems to be a major unconformity in the upper

A SO42-(mM) B CH4

+(ppmx103) C NH4+(µM) D Alkalinity (mM)

0 15 30 0 4 8 0 1500 3000 4 8 12E Salinity F PO4

3"(µM)32 33 34 0 30 60

'150

300

G Mg2+(mM) H Ca2+(mM)15 30 45 2 6 10

'150

300

I Ca(mM) J cr(mM) K Na+(mM)6 10 555 565 575 470 490 510

- 150 -

300

L Cl- (mM)560 570 580

- ji•

/

'80-250 mbsf

.. upper 50 m

520

K+(mM) N ü+(µM) O Si (µM) P Sr2* (µM)10 11 12 20 40 ' 60 400 700 100070 100 130

-150

300

Figure 17. Vertical profiles of interstitial waters and comparisons of interstitial components: sulfate (A), headspace methane (B), ammonium (C), alkalinity (D),salinity (E), phosphate (F), magnesium (G), and calcium (H), calcium vs. magnesium (I), chloride (J), sodium (K), sodium vs. chloride (L), potassium (M),lithium (N), silica (O), and strontium (P) in Hole 983A.

162

SITE 983

Table 12. Composition of interstitial waters in Hole 983A.


Depth(mbsf)

Na(mM)

K(mM)

Li(µM)

Mg(mM)

Ca(mM)

Sr(µM)

Cl(mM)

S04

(mM)NH4

(µM)Si

(µM)P04

(µM) PHAlkalinity

(mM) Salinity

162-983A-1H-3, 145-1502H-4, 145-1503H-4, 145-1506H-4, 145-1509H-4, 145-15012H-4, 145-15015H-4, 145-15018H-4, 145-15021H-4, 145-15024H-4, 145-15027H-4, 145-150

4.4513.3522.8551.3579.85

108.35136.85165.35193.85222.35250.85

485482482

493498501510513517516

11.812.112.011.311.010.410.810.510.510.710.7

19.018.418.319.919.120.722.730.436.944.451.4

51.951.149.043.537.734.830.626.924.321.118.6

9.78.06.43.62.93.13.63.94.95.56.0

9493

817481869399

103113

561561560564566570569573575575576

27.623.519.310.24.11.00.20.30.00.10.2

217430717

11441728186421112511247824202747

429637646709745780

945906792

37.128.628.941.053.443.436.817.811.16.05.4

7.617.547.627.637.887.757.837.837.727.808.01

4.5756.1647.483

10.85113.05713.29113.18610.8399.2478.0676.267

34.034.034.033.533.032.032.032.032.032.032.0

Table 13. Compressional-wave velocity measurements from Hole 983A. Table 14. Undrained shear strength measurements from Hole 983A.


Depth Velocity Temperature(mbsf) (m/s) (°C) Direction


Depth Undrained shear Penetrometer(mbsf) strength (kPa) Spring no. (kPa)

162-983A-1H-1,361H-1, 1081H-2,411H-2, 1211H-3, 66lH-4,871H-5, 532H-1,632H-2, 89

0.361.081.912.713.665.376.538.039.79

151215111507150315011496149914991492

20.719.518.')19.920.320.220.220.019.4

zz/.zzzI7.z

162-983 A-1H-1,421H-2,411H-2, 1201H-3, 651H-4, 871H-5, 512H-1.622H-2, 882H-3, 17

0.421.912.703.655.376.518.029.78

10.57

8.88.7

24.87.4

12.930.612.220.76.3

111111111

Note: For explanation of measurement directions, see "Explanatory Notes" chapter (thisvolume).

Only part of this table is reproduced here. The entire tableappears on the CD-ROM (back pocket).

Only part of this table is reproduced here. The entire tableappears on the CD-ROM (back pocket).

Table 15. Index properties of samples from Hole 983A.


162-983 A-1H-1, 35-371H-1, 106-1081H-2, 40-421H-2, 119-1211H-3, 64-661H-3, 134-1361H-4, 32-341H-4, 86-881H-5, 51-53

Depth(mbsf)

0.351.061.902.693.644.344.825.366.51

Water content

(wet %)

61.163.457.256.056.561.060.665.255.5

(dry %)

157.4173.5133.4127.2130.0156.3153.5187.3124.8

Bulk density

Method C(g/cm3)

1.341.301.411.411.401.341.331.321.42

Grain density

Method C(g/cm3)

2.632.442.862.692.712.612.492.932.74

Dry density

Method C(g/cm3)

0.520.480.610.620.610.520.530.460.63

Porosity

Method C(%)

80.180.578.876.977.579.978.984.377.0

Void ratio

Method C

4.034.123.723.343.443.983.735.353.34

Only part of this table is reproduced here. The entire table appears on the CD-ROM (back pocket).

part of the sedimentary section, and most of the sequence drilled atSite 983 pinches out against Reflector Rl within a distance of 12 kmeast of the site.

Unit GA-II, bounded by Reflectors Rl and R2, thins both west-ward and eastward of Site 983. Its thickness is approximately thesame as that of Unit GA-I, extending from 60 to 125 mbsf (Figs. 20,22) and appears slightly more acoustically stratified than the upperunit.

The next seismic unit, GA-III, differs from the two upper units inhaving distinct, closely spaced, high-amplitude reflectors, both inter-nally and as its upper and lower boundary Reflectors R2 and R3, re-spectively. Both reflectors are unconformities, with onlappingoverlying reflectors, but none of them appear to be erosional in char-acter. Toward the east, both Reflectors R2 and R3 terminate againstRl. The seismic velocities measured for this site show no distinctsteps, but rather a small but steady downhole increase. Using an in-terval velocity of 1.6 km/s, Unit GA-III extends from 125 mbsf to ap-proximately 170 mbsf.

Drilling at Site 983 terminated approximately 85 m into seismicUnit GA-IV (Fig. 22). Using an interval velocity of 1.65 km/s in theupper part, and slightly higher, 1.8 km/s, below, this unit extendsfrom 170 to about 420 mbsf. The lower boundary is uncertain, how-ever, since velocity measurements do not exist for most of the unit.Most of this seismic unit also pinches out against Reflector Rl in theeastern part of the survey area (Fig. 20). Unit GA-IV shows generallylow-amplitude stratification, although some variation is observed.Reflections are slightly stronger in the upper part of the unit than inthe lower. At mid-depths, 10-20 m below the bottom of Hole 983A,a wide reflector, consisting of a band of individual reflections, isseen. The base of this unit, Reflector R4, shows the same unconform-able but nonerosive character as Reflectors R2 and R3, with onlap-ping strata towards the margins of the sub-basin in which Site 983 islocated. A similar reflection configuration is even better developed inthe larger sub-basin farther east along Line S2 (Fig. 20).

Unit GA-V is defined between Reflectors R4 and R5, from 3.270s and 3.485 s TWT below seafloor. It is characterized by high-ampli-

163

SITE 983

GRAPEdensity(g/cm3)

.1.2 1.6

Porosity

50 70 90

Velocity(m/s)

1450 1650

Undrainedshear

strength(KPa)

0 100 200

Magneticsusceptibility

(SI units)0 1000

Naturalgammaradiation

(counts/s)10 20

Geotechnicalunit

Figure 18. Plots of GRAPE wet bulk density (thin line) andgravimetric bulk density (line with solid circles), porosity,P-wave velocity (thin line) and split-core velocity (linewith solid circles), undrained shear strength (vane = opencircles; pocket penetrometer = solid circles), magnetic sus-ceptibility, natural gamma radiation and geotechnicalstratigraphy (with seismic Reflectors R2 and R3 marked bydashed lines) for Hole 983A.

240 -

60°24'N

Figure 19. Map showing the site survey at Site 983.GGC-10 and GGC-11 are giant gravity coring stationsfrom pre-cruise site survey.

(10:00^^

S2 ^

'. * '

^ Site 983

i i

i

09:0

0

+

1

-H — ^ —S1

+

I

I

GGC-11

+

I

8:00

7/2

U

23°44'W

tude stratification, and the lower strata onlap the basal Reflector R5.Again, as for Unit GA-IV, this is best developed in the eastern sub-basin (Fig. 20), but is also clearly observed near Site 983.

The disposition of the two lowermost units, GA-VI and GA-VII,is controlled by the underlying basement topography to a higher de-gree than that of the overlying units. Unit GA-VI has a band of rela-tively strong reflections in its upper part, with decreasing amplitudesdownsection. Unit GA-VII, on the other hand, is nearly transparent,with the exception of a strong, flat reflector approximately 0.1 s TWTabove basement.

Relationship to Core Data

The sediments cored at Site 983 are homogeneous in characterand comprise only one lithostratigraphic unit. Subdivision into thethree lithostratigraphic Subunits IA, IB, and IC is based on subtlechanges in the carbonate vs. terrigenous content and in the spectralreflectivity trend (see "Lithostratigraphy" section, this chapter). Thecyclic character of the upper sedimentary section, as observed in the3.5-kHz PDR records (Fig. 21), is probably caused by these changes,and could therefore be related to lithologic differences between gla-cial and interglacial periods. The boundary between lithostratigraph-ic Subunits IA and IB corresponds to seismic Reflector R2, while the

23°36' 23°28' 23°20'

lower lithostratigraphic subunit boundary is defined approximately10 m below Reflector R3 (Fig. 22).

The shipboard measurements of sediment physical propertiesshow steady, normal trends of increasing compaction with depth, andonly one geotechnical unit, consisting of two subunits, G1A andGIB, is defined (see "Physical Properties" section, this chapter). TheP-wave velocity curve shows distinct highs of approximately 50 m/sabove the average trend both at 120-125 mbsf and around 160 mbsf.These depths correspond to Reflector R2 and near Reflector R3, re-spectively. At 160 mbsf, there is also a general increase in the veloc-ity gradient vs. depth. The geochemical characteristics of thesediments also change at 120 mbsf, in association with a pronouncedincrease in methane concentration (see "Inorganic Geochemistry"and "Organic Geochemistry" sections, this chapter).

Overall, the correlation between seismic unit boundaries and sed-imentary changes is good. Reflector Rl is defined because it changesinto a main unconformity both westward and eastward of the site.Based on seismic data alone, no significant change in the cored sedi-ments was to be expected at this level. Reflector R2, however, corre-sponds to noticeable changes in lithology, as well as in physical andgeochemical properties.

The seismic data show that the Gardar Drift represents a dynamicdepositional environment. About 1 km of sediment fills the basins

164

SITE 983

2.5 •

3.0-

3.5-

4 . 0 -

W Site 983

Acoustic basement

Line 983-S3

Figure 20. A. Seismic Line 983-S1. B. Interpretation of Line 983-S3, with seismic units and reflectors shown. The figure shows the entire line. See Figure 19 forlocation.

formed between basement highs. The thickness of individual strati-graphic units shows large lateral variability, and onlap relationshipsmay indicate near-bottom sediment transport and/or syndepositionaltectonic movements. Only the upper seismic unit boundary, ReflectorR l , is clearly erosional in character when followed laterally awayfrom the site. At Site 983, all seismic reflectors are conformable, andthe seismic stratigraphy supports the results from other shipboard in-vestigations, namely that the drilled sequence represents a continuoussedimentary section.

REFERENCES

Baldauf, J.G., 1984. Cenozoic diatom biostratigraphy and paleoceanographyof the Rockall Plateau region, North Atlantic, Deep Sea Drilling ProjectLeg 81. In Roberts, D.G., Schnitker, D., et al., Init. Repts. DSDP, 81:Washington (U.S. Govt. Printing Office), 439-478.

Berggren, W.A., Hilgen, F.J., Langereis, C.G., Kent, D.V., Obradovich, J.D.,Raffi, I., Raymo, M.E., and Shackleton, N.J., 1995. Late Neogene chro-nology: new perspectives in high-resolution stratigraphy. Geol. Soc. Am.Bull., 107:1272-1287.

Bond, G., Broecker, W., Johnsen, S., McManus, J., Labeyrie, L., Jouzel, J.,and Bonani, G., 1993. Correlations between climate records from theNorth Atlantic sediments and Greenland ice. Nature, 365:143-147.

Bond, G., Heinrich, H., Broecker, W., Labeyrie, L., McManus, J., Andrews,J., Huon, S., Janitschik, R., Clasen, S., Simet, C , Tedesco, K., Klas, M.,Bonani, G., and Ivy, S., 1992. Evidence for massive discharge of ice-bergs into the North Atlantic ocean during the last glacial period. Nature,360:245-249.

Bond, G.C., and Lotti, R., 1995. Iceberg discharges into the North Atlanticon millennial time scales during the last glaciation. Science, 276:1005-1010.

Boyle, E.A., and Keigwin, L.D., 1987. North Atlantic thermohaline circula-tion during the past 20,000 years linked to high-latitude surface tempera-ture. Nature, 330:35-40.

165

SITE 983

Site 983 W

5 2.8α>

Figure 21. Sample of 3.5-kHz PDR record along Line SI.Note the evenly spaced reflectors.

2.9

3.0 -÷

Figure 22. Relationships between seismic stratigraphy,lithostratigraphy, and geotechnical stratigraphy. Thedepth scale in mbsf is linear within the depths drilled,but note that the figure is not to scale below the drilleddepth. AB = acoustic basement.

2.835-

2.915-

2.970-

Φ

f 3.075-

i

| 3.270-

3.485-

3.735-

3.875-

Ref

lect

orR3

R2

R3

R4

R5

R6

AB

Seismicunit

GA-I

GA-II

GA-III

GA-IV

GA-V

GA-VI

GA-VII

Acousticcharacter

Lowamplitude,

cyclicstratification

Low amplitudestratification

belowunconformity

Highamplitude

stratification

Varying/lowamplitude

stratification

Mediumamplitude

stratification

Lesscontinuous

stratification

Nearly

transparent

Intervalvelocity

1.55 km/s

1.57 km/s

1.60 km/s

1.65 km/s

?

?

9

?

Thickness(two-way time)

0.08 s

0.08 s

0.055 s

0.105 s

0.195 s

0.215 s

0.250 S

0.140

Thickness(meter)

62

63

44

86

?

9

?

7

Simplifiedlithology

Nannofossilclay/clay withnannofossils

Clay/siltyclay with

nannofossils

Clay/silty clay

Lith.unit

IA

IB

IC

Geotechn.unit

G1A

G1B

Age

Ple

isto

ce

ne

Plio

.

-o

-100

-200

-255

•<>1000)

Nondrilled section

Broecker, W.S., 1994. Massive iceberg discharges as triggers for global cli-mate change. Nature, 372:421-424.

Donnally, D.M., 1989. Calcareous nannofossils of the Norwegian-GreenlandSea: ODP Leg 104. In Eldholm, O., Thiede, J., Taylor, E., et al., Proc.ODP, Sci. Results, 104: College Station, TX (Ocean Drilling Program),459-486.

Fronval, T., Jansen, E., Bloemendal, J., and Johnsen, S., 1995. Oceanic evi-dence for coherent fluctuations in Fennoscandian and Laurentide icesheets on millennium timescales. Nature, 374:443-446.

Gieskes, J.M., 1983. The chemistry of interstitial waters of deep-sea sedi-ments: interpretation of deep-sea drilling data. In Riley, J.P., and Chester,R. (Eds.), Chemical Oceanography (Vol. 8): London (Academic), 222-269.

Heinrich, H., 1988. Origin and consequences of cyclic ice rafting in thenortheast Atlantic Ocean during the past 130,000 years. Quat. Res.,29:142-152.

Locker, S., and Martini, E., 1989. Cenozoic silicoflagellates, ebridians, andactiniscidians from the V0ring Plateau (ODP Leg 104). In Eldholm, O.,Thiede, J., Taylor, E., et al., Proc. ODP, Sci. Results, 104: College Sta-tion, TX (Ocean Drilling Program), 543-585.

MacAyeal, D., 1993. Binge/purge oscillations of the Laurentide ice sheet as acause of the North Atlantic's Heinrich events. Paleoceanography,8:775-795.

McCartney, M.S., 1992. Recirculating components to the deep boundary cur-rent of the northern North Atlantic. Prog. Oceanogr., 29:283-383.

McCave, I.N., and Tucholke, B.E., 1986. Deep current-controlled sedimenta-tion in the western North Atlantic. In Vogt, P.R., and Tucholke, B.E.(Eds.), The Western North Atlantic Region. Geol. Soc. Am., Geol. ofNorth Am. Ser., M:451-468.

Okada, H., and Bukry, D., 1980. Supplementary modification and introduc-tion of code numbers to the low-latitude coccolith biostratigraphic zona-tion (Bukry, 1973; 1975). Mar. Micropaleontol, 5:321-325.

166

SITE 983

Oppo, D.W., and Lehman, S.J., 1993. Mid-depth circulation of the subpolarNorth Atlantic during the last glacial maximum. Science, 259:1148—1152.

, 1995. Suborbital timescale variability of North Atlantic deepwater during the past 200,000 years. Paleoceanography, 10:901-910.

Rahmstorf, S., 1994. Rapid climate transitions in a coupled ocean-atmo-sphere model. Nature, 372:82-85.

Robinson, S., and McCave, I., 1994. Orbital forcing of bottom-currentenhanced sedimentation on Feni Drift, NE Atlantic, during the mid-Pleis-tocene. Paleoceanography, 9:943-971.

Shipboard Scientific Party, 1995. Site 918. In Larsen, H.C., Saunders, A.D.,Clift, P.D., et al., Proc. ODP, Init. Repts., 152: College Station, TX(Ocean Drilling Program), 177-256.

Weaver, A.J., and Hughes, T.M.C., 1994. Rapid interglacial climate fluctua-tions driven by North Atlantic ocean circulation. Nature, 367:447-450.

Weaver, P.P.E., and Clement, B.M., 1986. Synchroneity of Pliocene plank-tonic foraminiferal datums in the North Atlantic. Mar. MicropaleontoL,10:295-307.

Ms 162IR-105

Note: For all sites drilled, core description forms ("barrel sheets") and core photographs canbe found in Section 3, beginning on page 391. Forms containing smear slide data can be foundin Section 4, beginning on page 1147. On the CD-ROM enclosed in the back pocket of this vol-ume are all tables from this chapter (including an extended coring summary table) and ship-board measurements (files containing GRAPE density, P-wave velocity, natural gammaradiation, magnetic susceptibility, index properties, and spectral reflectance data).

167

5. site 983€¦ · water depth (drill pipe measurement from sea level, m): 1985.0 penetration...

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