marine micropaleontology, 4(1979): 45--60 45...

Marine Micropaleontology, 4(1979): 45--60 45 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

P A L E O S A L I N I T Y , P A L E O T E M P E R A T U R E , A N D ISOTOPIC F R A C T I O N A T I O N R E C O R D S OF N E O G E N E F O R A M I N I F E R A F R O M DSDP SITE 173 AND THE C E N T E R V I L L E BEACH SECTION, C A L I F O R N I A

THOMAS W. KAMMER 1

Department of Geology, Indiana University, Bloomington, Ind. 47401 (U.S.A.)

(Approved July 18, 1978)

Abs t rac t

Kammer, T.W., 1979. Paleosalinity, paleotemperature, and isotopic fractionation records of Neogene foraminifera from DSDP site 173 and the Centerville Beach Section, California. Mar. Micropaleontol., 4: 45--60.

The oxygen isotope record of the planktonic foraminifera Globigerina buUoides and Neogloboquadrina pachyderma from Pliocene and early Pleistocene sediments at both DSDP site 173 and the Centerville Beach section in California suggests a large influx of isotopically light water in this area during late Pliocene and early Pleistocene time. Salinity may have been reduced by as much as 2 to 4°/00. Surface sea water paleotemperatures for the lower Pliocene range from 9.5°C to 15.5°C. The oxygen isotope record of the benthonic genus Uvigerina shows little variation indicating environmental stability at depth. At DSDP site 173 the small variation in Uvigerina is due to variation in the oxygen isotopic composition of the oceans as glaciers waxed and waned. At the Centerville Beach section the oxygen isotopic composition of Uvigerina reflects the gradual shoaling of the Humboldt Basin. Car- bon and oxygen isotope ratios in G. bulloides and N. pachyderma are inversely correlated at the 95% confidence level. This may indicate that the oxygen and carbon isotopic composition of foraminifera are influenced by the same factors. On the other hand, the inverse correlation may be due to metabolic fractionation. No correlation was found between oxygen and carbon isotopic composition in Uvigerina.

I n t r o d u c t i o n

The purpose o f this s tudy of the oxygen and ca rbon isotopic compos i t i on of foraminiferal tests f rom Deep Sea Drilling (DSDP) site 173 and the Centerville Beach section, California is th reefo ld : first, to de te rmine the climatic h is tory during the Pl iocene and early Pleis tocene of this region; second, to determine the oxygen isotopic variat ion in sea water due to glacial mel t ing and f reshwater runoff; and third, to establish be t te r corre la t ion be tween the deep-sea record and the mar ine record exposed on land.

1 Present address: Shell Oil Co., P.O. Box 60775, New Orleans, La. 70160 (U.S.A.)

This s tudy is the first of its kind in the nor theas t Pacific Ocean. The two sites were chosen for s tudy because o f their close proxi- mi ty to one ano the r (Fig. 1) and because o f previous biostrat igraphic (Ingle, 1973b , 1976) and pa leomagnet ic (Heinrichs, 1973; D o d d et al., 1977) studies of these sites.

The geological significance of stable isotopes was first realized by Urey (1947) . The t h e r m o d y n a m i c proper t ies o f oxygen isotopes are such tha t the oxygen isotopic composi t ion of calcium ca rbona te is direct ly related to the t empera tu re dur ing prec ip i ta t ion and the isotopic compos i t i on o f the available ions. A few years later this relat ionship was used to de te rmine the thermal h is tory o f Cre-

4 6

128 ° 126 ° 1 2 4 ° 122 °

4 o

~2 o

40 °

Fig. 1. Index mal~ showing the loca t ion o f DSDP site 173 and the Centerville Beach sect ion.

taceous belemnites (Urey et al., 1951). Many early studies of oxygen isotopic variations in fossils inadequately evaluated the contribu- tion of changes in the 180 content of sea water {Emiliani, 1955; Shackleton, 1967), thus causing an inaccurate estimate of paleotemperatures. Dansgaard and Tauber (1969) have suggested that as much as 70% of the variation in the 180 content of foraminifera may reflect changes in glacial ice volumes on the continents. Emiliani and Shackleton (1974), however, propose that the variation due to glacial effect is only approximately 25%. 16 O, the lighter isotope, is concentrated in rain and snow because of fractionation during evaporation. In the interpretation of oxygen isotope values the effects of both temperature and the 150 content of the original sea water must be considered, especially when there are major isotopic fluctuations resulting from the growth or melting of ice caps.

The oxygen isotopic composition of foraminifera from deep-sea cores have been extensively used for studying the earth's climatic record (Emiliani, 1954, 1955; Douglas and Savin, 1972, 1973, 1975; Saito and Van Donk, 1974; Savin et al., 1975; Savin, 1977; Shackleton and Kennett, 1975a, b; and many others). For the most part, investigators have tended either to make a detailed study of the Quaternary record (e.g. Emiliani, 1955; Broecker and Van Donk, 1970; Shackleton and Opdyke, 1973), or they have studied the record of the entire Cenozoic (Savin et al., 1975; Savin, 1977; Shackleton and Kennett, 1975a, b). The records for the individual Cenozoic epochs, other than the Pleistocene, have been studied only briefly. With the exception of Devereux and others study (1970) most oxygen isotopic studies have only considered a few points in the Pliocene.

The significance and causal mechanism of changes in stable carbon isotopes in organic~ ally precipitated calcium carbonate is not well understood. The bicarbonate ion in sea water serves as the source for carbon isotopes in calcium carbonate (Deuser and Degens, 1967; Degens, 1969; and Emrich et al., 1970). The bicarbonate ion is in equilibrium with aqueous carbon dioxide, which in turn is in equilibrium with atmospheric carbon dioxide. Thus, the isotopic composition of the bicarbonate ion can be ultimately influenced by the isotopic composition of the atmospheric carbon dioxide. Whether or not the carbon isotopic variation of calcium carbonate actually reflects changes in atmospheric carbon dioxide is unknown. The fractionation of carbon isotopes during the secretion of calcium carbonate is poorly understood. There is some evidence that the origin of the carbon in the test is a mixture of metabolic and inorganic carbon dioxide (H. Craig, in Revelle and Fairbridge, 1957, p. 275; Vinot-Bertouille and Duplessy, 1973).

Carbon isotope ratios have been found to reflect depth in the water column at the time of secretion of calcite by foraminifera (Deu- ser and Hunt, 1969; Saito and Van Donk,

47

1974; Weiner, 1975). Primary product ion by phytoplankton causes a depletion of '2C in the bicarbonate ion in surface waters. Hence, planktonic foraminifera have a higher 13C content than benthonic foraminifera.

Geographic and stratigraphic setting

DSDP site 173

Site 173 (Fig. 1) is located at the base of the continental slope 100 km southwest of Cape Mendocino. The site was drilled on June 10--12, 1971 at a depth of 2927 m (Kulm et al., 1973). The upper 138 m of section con- sists of Pliocene and Pleistocene terrigenous muds and some fine sands. The fauna contains abundant foraminifera, diatoms, and silico- flagellates. The planktonic foraminifera are especially well preserved. Ingle (1973a, b) as a member of the shipboard scientific party, extensively studied the planktonic foraminifera and subdivided the section into the planktonic zones of Blow (1969). Other organisms used for biostratigraphic zonation of site 173 include diatoms, coccoliths, and radiolarians (Kulm et al., 1973). Because of its rich fauna, site 173 has been proposed as a standard section for the Neogene in the northeast Pacific region (Ingle, 1973b). Because the biostratigraphic zonation of site 173 is well unders tood it was chosen for comparison with the on-land section at Cen- terville Beach.

Centerville Beach section

The Centerville Beach section is exposed in sea cliffs south of Centerville Beach and along the southern margin of the northwest trend- ing Humbold t or Eel River Basin surrounding Eureka, California (Fig. 1). Seismic studies by Silver (1969) and Hoskins and Griffiths (1971) indicate that the major part of the basin lies on the submerged continental margin off the coast of California and Oregon. At Centerville Beach 1900 m of late Neogene marine terrigenous sediments are in fault con-

tact with the metasediments of the Upper Jurassic--Lower Cretaceous Yager Format ion (Ogle, 1953). These Neogene sediments make up the Pullen, Eel River, and Rio Dell For- mations of Ogle (1953) and are part of the larger Wildcat Group, which also includes the overlying marine and nonmarine Scotia Bluffs Sandstone, and Carlotta Formation. Mud- stones, siltstones, and fine to coarse sand- stones make up the basinal turbidites, slope deposits, and continental shelf deposits of these marine units (Piper et al., 1976). The sequence of sediments records a gradual shoaling of the area through geologic time.

The Centerville Beach section has been assigned various ages. Gabb (1866) was the first to assign an age to the rocks of Hum- boldt County when he described six molluscan species of Pliocene age. Lawson (1894) assigned the name Wildcat "series" (more properly called the Wildcat Group) to the rocks of the Humbold t Basin and assigned them a Pliocene age on the basis of molluscs and echinoids. Subsequent workers (Martin, 1916; Smith, 1919; Grant and Gale, 1931; and Faustman, 1964) have also assigned a Pliocene age to these sediments based on molluscan assemblages.

Age assignments based on foraminifera have not always agreed with those based on molluscs. Although Cushman et al. (1930) placed the Wildcat Group in the Pliocene, Stewart and Stewart (1949) later recognized the possibility of the younger beds (e.g. Car- lotta Formation) being Pleistocene in age. On the basis of benthonic foraminiferal assemblages Ogle (1953) placed the Miocene-- Pliocene boundary in the basal Pullen and the Pliocene--Pleistocene boundary in the Carlot- ta Formation. Haller (1967) in a more com- plete s tudy of the foraminifera of the Hum- boldt Basin placed the Pliocene--Pleistocene boundary in the upper Rio Dell Formation.

Recent advances in planktonic foraminiferal biostratigraphy and paleomagnetic and radiometric time scales (Berggren, 1969; Blow, 1969; Cox, 1969; Opdyke, 1972; Berggren and Van Couvering, 1974) allow a

48

more precise subdivision of Neogene rocks. Analysis of planktonic foraminifera from Centerville Beach (Ingle and Takayanagi, 1968; Ingle, 1969; Orr and Zaitzeff, 1971) indicates that the Wildcat Group was deposited between early Pliocene and early Pleis- tocene time. Biostratigraphic comparison of the Centerville Beach section and DSDP site 173 by Ingle (1973a, b, 1976) suggests that the Pliocene--Pleistocene boundary is in the middle Rio Dell Formation. Dodd et al. (1977) in a paleomagnetic s tudy of the Cen- terville Beach section placed the beginning of the Olduvai event, which is also defined as the beginning of the Pleistocene (Berggren and Van Couvering, 1974), in the middle Rio Dell Formation.

Samples

DSDP site 173

Twenty-three core samples, regularly spaced approximately every 3.5 m, from cores

U N22

Rio Dell Fm. M

N2t L

Eel River Fro. N20

NI9 Pullen Fro.

N18

CENTERVlLLE BEACH SECTION

PLEISTOCENE

PLIOCENE

MIOCENE

Core No.

6

. . . . . . -1

N 2 t

10 N20

11 N20/191

12 . . . .

',2, N~9

- N f 8 |

DSDP SITE t73

Fig. 2. Age comparison, based on the planktonic z o n e s of Blow (1969), between the Centerville Beach section and the cores studied from DSDP site 173. Age assignments from Ingle (1976)and Ingle (1973a), respectively.

6-14 of site 173 were obtained from the DSDP curator at Scripps Institution of Oceanography. Cores 6 through 14 were chosen for s tudy because they are approximately stratigraphically equivalenf to the section at Centerville Beach (Fig. 2). Samples were chosen on the basis of preservation and abundance data presented by Ingle (1973a) for site 173. The average sediment sample size was 10 cm 3 . The rate of sedimentation at site 173 was approximately 3 cm per 1000 years (Heinrichs, 1973), indicating that samples were spaced at approximately 130,000- year intervals. All samples from site 173 are fossiliferous and most of the foraminifera appear to be bet ter preserved than those from the Centerville Beach section.

Centerville Beach section

Thirty-four 750 g sediment samples were collected, approximately every 60 m stratigraphically, from the Pullen, Eel River, and Rio Dell Formations in June 1976 (Fig. 3). On the basis of the paleomagnetic work of Dodd et al. (1977) the rate of sedimentation during Rio Dell time was found to be approximately 70 cm per 1000 years. Thus, the spacing between samples from the Rio Dell Formation represents approximately 85,000 years. Sedimentation rates for the Eel River and Pullen were probably significantly less (Piper et al., 1976). The samples collected from the upper Rio Dell were more widely spaced to compensate for increased rates of sedimentation (Ingle, 1976; Piper et al., 1976). Some samples were more widely spaced than desired due to lack of outcrop.

Not all the samples contained specimens which could be used for isotope analysis. Those samples collected from the Pullen Formation (CB-1 through CB-5) were either barren or yielded too few foraminifers for analysis. A few samples had undergone extensive diagenetic alteration (heavy iron staining is visible on the foraminifers) and were omit ted from the study.

49

Centerville Beach

Location Map CB-~,

LEGEND

Hookston Fro.

Rio Dell Fro. (upper, midd le , and

lower member )

Eel River Fro.

CB-

Lu CB-

Pul len Fm,

False Cape shear zone

Formot ion comtoct

2 0

Bedd ing a t t i t ude

Sample site

0 2 0 0 0 Feet i I t I I

0 L I i I I

CB

C B - 1 5 ~ _ 34 - - - - - -

• Trd I

C B - I O ~• - - ~

• 34 D ~ Ter

C B - 5 / ! ~ - - - ~

C B - I/~• Tp

10001 Meters / - . . . . . \

/

Fig. 3. Geologic map of the Centerville Beach, Cali- fornia area showing the location of samples collected for this study. Geology from Ogle (1953).

Experimental methods

Sample preparation

The sediment samples from DSDP site 173 were boiled in water with Calgon to aid in deflocculation of the clay particles. The Centerville Beach samples were first crushed and then boiled in water. All samples were washed and sieved through a 100-mesh screen. The washed residue was picked under a mi- croscope with a fine-haired brush. For each sample, specimens of Globigerina bulloides, Neogloboquadrina pachyderma, and Uvigerina were separated onto cardboard slides. Speci- mens which displayed heavy secondary cal- citic crusts and/or chamber infilling were avoided.

The experimental procedure for preparation of CO2 from the carbonate samples was modified from Shackleton (1965) and Shack- leton and Opdyke (1973). The forams were placed in quartz thimbles and roasted in a vacuum at 425°C for 15 min, the temperature and roasting time suggested by N.J. Shackleton (pers. comm.). During roasting, the foraminifers darkened due to charring of the organics.

CO2 samples were prepared, purified, and collected in a standard vacuum line. The vacuum was maintained by a mechanical pump backing an all-glass mercury diffusion pump. The apparatus was evacuated to better than [}.01 Torr for an hour or more to remove any trapped gas molecules from the phosphoric acid. The sample, in its quartz thimble, was dropped from the bucket of the side arm of the reaction vessel into 3 ml of 95% phosphoric acid where the reaction took place at 50°C. At this temperature the reaction is rapid and is completed within 5 rain. The reaction products were instantaneously pumped off cryogenically to prevent any exchange of oxygen isotopes between CO2 and H20. The sample CO2 was passed through a pentane slush trap at -130°C to remove water and other impurities before being frozen in a liquid nitrogen trap at

50

- 196°C. The volume and pressure of the CO: was measured in order to calculate the moles of carbon dioxide for use by the mass spectrometer operator. The sample COs was pack- aged finally in a glass tube and sealed with a torch (Des Marais and Hayes, 1976).

Measurement o f stable isotope ratios

Measurements of ~80/160 and 13C/I~C ratios were made on a Varian GD150 mass spectrometer equipped with a Nuclide dual electrometer and radiometer. The isotope ratios from a known carbonate standard (Harding Iceland Spar, 5 ~ OpD B = - 18 .16%0, 513CpDB _ 4 .91%0) were compared with those from the sample CO2. Corrections were made for the peak overlap for masses 45 and 46, for background ion currents, for ~70 con- tributions to the ion current at mass 45, and for cross contamination due to valve leakage at the dual inlet (Craig, 1957). The difference between sample and standard is expressed by the following equation:

5 = -Rsample 1-I × 1000

_ R standard

where R = the 180/160 or la C/~: C ratio; and 6 is expressed in parts per thousand (% 0). The standard deviation of the mean for oxygen and carbon isotope measurements of standards was 0.4°/00 and 0.1°/00, respectively. Such a large variance in the oxygen isotopic composi t ion of standards would be un- acceptable if the isotopic variation of the experimental samples was small. The standard deviation of the mean for planktonic oxygen values from the Centerville Beach section and DSDP site 173 was 1.9°/0o and 1.1°/00, respectively. These large standard deviations help to reduce the significance of the large variance of the oxygen isotope measurements.

In order to compare the oxygen isotope values of different labs, isotope workers reference their results to a standard (Craig, 1957). All oxygen and carbon isotope values

from this s tudy are referenced to the PDB standard. The preparation of carbonate standards requires that the calcium carbonate be dissolved in 100% phosphoric acid and the products allowed to equilibrate for 12 to 24 h. If samples are prepared differently from the above method they can not be directly compared with the PDB standard. To correct for the lack of oxygen isotopic exchange between CO: and H20 in the Shackleton method an in-house standard was prepared by both the McCrea method (1950) and the Shackteton method. Oxygen isotopic values were found to be 1 .22%o lighter when prepared by the Shackleton method. A correction constant of 1.22°/00 was added to the oxygen data from all the samples. No such correction was required for the carbon data because no exchange of carbon isotopes can take place during the preparation of CO2.

Choice of taxa for analysis

Planktonic foraminifera

Previous studies of the Centerville Beach section (Hailer, 1967) and cores from DSDP site 173 (Ingle, 1973a) indicate that the genera Globigerina and Neogloboquadrina are the most common planktonic foraminiferal genera. Haller reproted (1967, fig. 7) N. pachyderma and G. bulloides as the most common planktonic foraminifera from the Center- ville Beach section. On the average, planktonic foraminifera are not abundant in the Centerville Beach section (Haller, 1967, fig. 9). Ingle's work (1973a) on the foraminifera from DSDP site 173 indicates a high diversity and abundance of planktonic foraminifera. Species equitability is low, however, for in most samples G. bulloides and N. pa- chyderrna comprise 80% or more of the planktonic foraminifera.

To minimize differences in isotopic fractionation resulting from analysis of different species, only one species was analyzed where possible. Because planktonic species are used

51

to determine surface, or near-surface, water temperatures, a species should be used which lives high in the water column. Studies on living foraminifera indicate that G. bulloides inhabits shallower depths than does N. pachyderma (Bradshaw, 1959; Casey, 1963; B~ and Hamlin, 1967; and Berger, 1969). Bradshaw (1959) found G. bulloides concentrated in the upper 100 m of the water column, whereas N. pachyderma was most ly restricted to depths greater than 100 m. Van Donk and Mathieu (1969) found that N. pa- cyderma secreted half of its calcite below 300 m. Oxygen isotopic studies by Kennet t et al. (1977) indicate the reverse situation; oxygen isotopic values for N. pachyderma and G. bulloides indicated that N. pachyderma secreted calcite in shallower water.

Based on the conflicting literature it is hard to make a choice as to which taxon is best for isotope analysis. It was decided that G. buUoides was preferred to N. pachyderma for this study. Nearly all the samples from DSDP site 173 yielded sufficient quantities of G. bulloides for isotopic analysis; however, a few required the combined analysis of G. bulloides and N. pachyderma so that enough calcite was available for analysis (see Fig. 4). In most instances the Centerville Beach samples did not yield enough specimens of either species for a single species analysis. Because of this a mixed assemblage of G. bulloides and N. pachyderma was analyzed from the Centerville Beach section.

Benthonic foraminifera

A benthonic genus was also analyzed in order to moni tor changes in the isotopic composit ion of the ocean. Shackleton (1974) found that Uvigerina deposits its test at or near isotopic equilibrium in the temperature range from 0.8°C to 7°C. Thus, values for the isotopic composi t ion of tests can be explained in terms of changes in the mean oxygen isotopic composi t ion of the ocean if abyssal temperatures are assumed to remain constant. Oceanographic studies of m o d e m water mass-

es have shown that deep water greater than 1000 m is constant in temperature and free from thermal fluctuations (Barbee. 1965). Study of late Pleistocene benthonic foraminiferal assemblages from the North Atlantic (Streeter, 1973) suggests that bo t tom waters may have varied by as much as 2°C in the last 150,000 years. Studies of this nature are lacking for the Pacific Ocean, bu t it is probably safe to assume no large variation in abyssal temperatures during the late Neogene.

The genus Uvigerina is common at Center- ville Beach and DSDP site 173. Hailer (1967, fig. 7) lists U. peregrina as the dominant species of Uvigerina at Centerville Beach. Ingle (1973a, table 8, p. 536) lists seven species of Uvigerina from site 173. Due to the complex taxonomy no a t tempt was made to identify specimens of Uvigerina from site 173 to the specific level. The first 15 to 2b specimens encountered were used for isotopic analysis.

Adequacy of the data

Whenever a s tudy of oxygen isotopes is a t tempted, certain a priori assumptions must be justified. First, did the organism deposit its calcite in isotopic equilibrium with sea water? Second, has there been any diagenetic alteration of the calcite (or aragonite)? Sever- al studies of oceanic foraminifera have indicated that foraminiferal tests are deposited at or near isotopic equilibrium (Shackleton, 1974; Savin et al., 1975). However, Shackle- ton et al. (1973) found some planktonic species deviated from isotopic equilibrium by as much as - 0 . 5 0 % 0 . Providing that the foraminifera were deposited on the sea floor above the carbonate compensation depth, foraminifera in deep-sea clays should have experienced a minimum of diagenesis. Dia- genetic alteration could be more of a prob- lem with the Centerville Beach samples, bu t for the most part the mudstones should have low permiability to groundwater flow.

The standard deviation of the mean for the oxygen isotopic analysis was found to be

52

0.4%0 (see above section of Measurement of Stable Isotope Ratios). Due to the large isotopic variation found in this study, such a large standard error does not greatly limit the interpretation of the data.

Measurement of carbon isotope ratios is easier because the natural abundance of ~3C is one part in 90 as compared with 180 which is one part in 500. The standard deviation of the mean for carbon isotope measurements was 0.1°/o0.

Interpretation of paleotemperatures and

paleosalinities

If the isotopic composi t ion of sea water is known, oxygen isotope values can be used

for paleotemperature determinat ions (Savin et al., 1975; Shackleton and Kennett, 1975a). However, in working with fossils of post- Miocene age, particularly Quaternary fossils, there is the strong possibility that the oxygen isotope record of these fossils reflects glacial events rather than actual changes in ocean surface temperatures. Early studies (Emiliani, 1955) of Pleistocene paleotemperatures did not believe that the changes in the oxygen isotopic composit ion of the sea water due to waxing and waning of glaciers was a signifi- cant factor in determining paleotemperature (Shackleton, 1967). Later workers have made estimates of 70% (Dansgaard and Tauber, 1969) and 25% (Emiliani and Shackleton, 1974) for the amount of variation in isotopic

Worm Cool % Sinistral Coiled 618 OpoB% o

0 I00 -3 -2 -i 0 i 2 1 ' ' ' i I F- - I I .... q 7 7

~13CpoB%O 5 -I -2 -3 -4

. -J ' E, ~,, 4 9 . 5*

~ "~ J 7 > 48- 50

~ ~ . * ' / ~ ~ ~ . . . ~ , 8 -4 . 50-- 52

t ~ i 8 6, 5 0 - 5 2 "~- .... . . ~ ~ EISTOCENE " " ~ . ~ , ~ .--.="~ d - -~D~EN~-- qE 9-~, 49-5~

- ' - - 1 9-5 48 -5o

i} ec,,......... / ~ I / t0- I, 4 8 - 5t

10--4 4 9 - 51

- 1 0 ~ 6 , 5 0 - 5 2

i t 5 0 - - 52

~', S. 5 0 - 52 ! *~, ~ 12 4 9 - 5i

! 12-5, 4 9 - 5 ~

12-5, 55 - 55

"ll" ~ ,~ '~ ~ 13" ; , 4 7 - 4 9

~ \ ' ~ 1 ~ ' ~ 13-3, 5 0 . 52

13-5, 4 8 - 5 0

- " ~ ~4-,, 49- 5~ B ,/ ' ~ , ~ ,4-2, ~oo-,oz

PL~OCENE

Fig. 4. DSDP site 173: sample numbers indicate core number, section number, and the number of centimeters below top of section. A, coiling ratios of N. pachyderma (Ingle, 1973a); B, oxygen isotopic composit ion of G: bulloides; C, oxygen isotopic composition of Uvigerina; D, carbon isotopic composition of G. bulloides; E, carbon isotopic composition of Uvigerina. (* indicates mixed assemblage of G. bulloides and N. pachyderma, )

53

-5 I

6 1 8 0 p D B % o

- 4 - 3 -2 - I 0 I 2 I I I I I I I

~I3CpDB%o

3 -I -2 -3 - 4 I I I I I

, ( / %.

\ \

\ /

I I

I I

I or_ ,

o

~'~/ / / ' #

A' BI

\ \ \ \ 4 k ~

~'C D ~ j

CB-54 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18

17 16 15 14 ~3 i2 II I0 9 8

7 6

I I I I I I I I I I I I I

Fig. 5. Centerv i l l e B e a c h s e c t i o n : A , o x y g e n i s o t o p i c c o m p o s i t i o n o f N. pachyderma--G, bulloides m i x e d assem- b lages ; B, o x y g e n i s o t o p i c c o m p o s i t i o n o f Uvigerina; C, c a r b o n i s o t o p i c c o m p o s i t i o n o f N. pachyderma--G, bulloides m i x e d a s s e m b l a g e s ; D, c a r b o n i s o t o p i c c o m p o s i t i o n o f Uvigerina.

values due to changing ice volumes on the earth's surface.

Salinity changes, reflecting mixing of fresh and marine waters, can greatly influence the oxygen isotope record. Brackish and freshwater environments are characterized by lighter oxygen isotope values (Epstein and Mayeda, 1953). Dodd and Stanton (1975) successfully used oxygen isotope analysis to differentiate open marine environments from brackish and freshwater environments in

the Pliocene of California. Study of the oxygen isotopic composit ion of foraminifera and water samples from the Arctic Ocean by Van Donk and Mathieu (1969) indicated a deple- t ion in 1sO of about 40/00, corresponding to a salinity of approximately 300/00, due to melting of glacial ice. The estimated isotopic composi t ion of freshwater runoff was - 22°/00. The is O profile was found to closely follow the salinity profile with 8 is O changing by about 0.8°/00 per 1°/00 salinity change. At

54

depths greater than 350 m, normal marine salinity was reached. In a s tudy of three Gulf of Mexico deep-sea cores, Kennet t and Shackle- ton (1975) found an oxygen isotopic anomaly as large as - 1 . 5 % 0 hundreds of miles from the mouth of the Mississippi River. This anomaly is representative of a reduction in salinity of approximately 3% 0 and was interpreted by Kennet t and Shackleton as representing the rapid melting of the Laurentide ice sheet during late Quaternary time. The estimated isotopic composi t ion of the glacial meltwater was - 300/00.

The present s tudy also documents the oc- currence of isotopically light water and reduced salinity conditions offshore (Figs. 4 and 5). At both of the s tudy sites the oxygen isotopic record for the late Pliocene and early Pleistocene shows large negative anomalies that are interpreted as low salinity conditions. Such anomalies could represent changes of as much as 10--15°C in surface water temperatures, if interpreted in light of the paleotemperature equation; however, such large changes over so short a time period does not seem geologically feasible. Instead the anomalies more likely represent an influx of isotopically light water from the ancestral Eel and Columbia Rivers brought south along the coast by the southward flowing California Current. At DSDP site 173, anomalies as large as 2--3%0 are recorded (8-6, 50-52; 10-4, 49-51) and may represent reduced salinity conditions of approximately 2°/o0. At Center- ville Beach, anomalies as large as 3--4.5°/00 are recorded (CB-19 through CB-26) and may correspond to reduced salinities of as much as 3 - -4%o. The greater reduction in salinity at Centerville Beach is to be expected because of its proximity to the shoreline. Absolute changes in salinity can only be estimated because there is n o accurate way to estimate the isotopic composit ion, or the volume, of the freshwater influx.

The Centerville Beach section was deposited a few tens of kilometers from the mouth of the Eel River and D S D P site 173 was ap- p r o x i m a t e l y 120 km from the Eel River. The

benthonic record at DSDP site 173 shows fluctuations in the 5 ' sO values but no major anomalies like those at the surface. The bottom waters at DSDP site 173 have a subarctic source (Sverdrup et al., 1942) and are thus not influenced by local effects. The benthonic record for the Centerville Beach section shows no major anomalies either, but it does show a gradual decrease in ' sO content probably due to warmer bo t tom water temperatures as the section gradually shoaled {Ingle, 1976; Piper et al., 1976). Major anomalies in the planktonic but not the benthonic record probably indicate that isotopically light fresh water, which is less dense than sea water, may have moved across the ocean surface for a long distance before being thoroughly mixed with seawater. The surface record at DSDP site 173 shows strong fluctuations between isotopically light conditions and normal marine conditions, bu t at Centervilte Beach there was a long period of isotopically light conditions unpunctuated by normal marine conditions (CB-19 through CB-26). This should be the expected pattern because the Centerville Beach section is much closer to the source of isotopically light water than is DSDP site 173.

Both sites show stability of the record for the lower Pliocene. Based on an assigned temperature of 3°C [the temperature at 1.500 m depth (Barbee, 1965)] for Uvigerina, the isotopic composit ion of the sea water can be calculated by the following equation which ap- proximates conditions in the 0--10°C range (Shackleton, 1974):

T~C = 3°C ~ 16 .9 - 4.0 (tic - 8w)

where tic = the isotopic composit ion of Uvi. ger ina; and 8 w = the isotopic composit ion of the sea water. Knowing the value of 5 w, the surface water temperature can be calculated by either the above equation or by the following equation, both of which approximate conditions in the 10--25 ° range (Shackleton, 1974):

T = 1 6 . 9 - 4 . 3 8 ( ~ c - 5 w ) + 0 . 1 0 ( t i c - 8w) 2

55

TABLE I

Calculated surface water paleotemperatures for the lower Pliocene at DSDP site 173 and the Centerville Beach section

DSDP site 173 Centerville Beach section

Sample No. °C Sample No. °C

12-5, 53-55 9.6 CB-16 10.8 13-1, 47-49 12.4 CB-13 10.8 13-3, 50-52 15.4 CB-10 13.9 13-5, 48-50 10.5 CB-9 15.6 14-1, 49-51 12.7 CB-8 15.0 14-2, 100-102 13.4

where 6 c = the isotopic composi t ion of Globi- gerina or Neogloboquadrina. Calculated paleotemperatures for the lower part of the record for each section show the expected temperatures for the temperate North Pacific (Table I).

The upper parts of both sections show a large influx of light water which masks the temperature effect. This influx of light water in the late Pliocene and early Pleistocene might represent a period of heavier precipitation in the continental Pacific northwest than today, or a change in drainage pattern for this region. According to Faustman (1964), molluscan faunas record cool temperatures for this period.

Studies of coiling ratios in Neogloboqua- drina pachyderma (Ericson, 1959; Bandy, 1960) have shown this foraminiferal species to be sensitive to changes in sea water surface temperatures. Predominantly left-hand coiled (sinistral) assemblages of N.Pachyderma are in- dicative of cold water conditions; right-hand coiled (dextral) assemblages indicate warm conditions. Coiling ratios of N. pachyderma from DSDP site 173 reveals no positive correlation with the planktonic oxygen isotope record (Fig. 4). If there was no influence from isotopically light water, then light oxygen values would be associated with dextral coiling populations and vice versa. In fact,

two apparent intervals of obvious negative correlations occur at the level of 10-1 and 7-2. This indicates that great caution msut be used in the interpretation of paleotemperatures that are based on the ~180 record at DSDP site 173. The negative anomaly at level 8-6, 50-52 does correspond well with the warm period indicated by foraminiferal coiling ratios.

Another indicator of a strong influence of fresh water is the mean value for the carbon data (Figs. 4 and 5). Typical values for the carbon isotope ratios in Uvigerina for open ocean conditions are about 0°/00 to 40/00 during the Pliocene (Shackleton and Ken- nett, 1975b). The mean carbon isotope ratio for Uvigerina from DSDP site 173 is -2.850/00 and that from the Centerville Beach section i n - 2.36%0. This is significantly lighter than the carbon isotope ratio for any open ocean benthonic foraminifera (Savin et al., 1975) and thus indicates a strong terrestrial component in the carbon isotope values or possibly diagenetic alteration. The oxygen isotope data, however, does not support the latter possibility.

Relationship of carbon and oxygen isotopes in foraminifera

When the oxygen and carbon isotopic records for the planktonic foraminifera were compared, an inverse trend was observed between carbon and oxygen (Figs. 4 and 5). As the isotopic values for oxygen became lighter, carbon isotopic values became heavier, and vice versa. To test for the significance of this trend, the number of times that carbon and oxygen values varied together was calculated and compared to the expected value (50%) by use of the X 2 test (Table II). The null hypothesis would be a situation where oxygen and carbon isotope values would vary randomly so that only 50% of the time there would be an inverse correlation. It was found that at the 95% confidence level the null hypothesis can be rejected. Thus it can be reasonably assumed that in G. bulloides and N. pachy-

56

TABLE II

x 2 values and confidence levels for the inverse association between oxygen and carbon isotope values in foraminifera from the two study sites

6 18 0--8 23 C Observed

inverse correlation

G. bulloides 14 DSDP 173

AT. pachyderma-- 8 5 1 3.60 > 90% G. bulloides Centerville Beach

N. pachyderma-- 22 14.5 1 3.88 >95% G. bulloides total

Uvigerina 7 9.5 1 1.32 > 70% DSDP 173

Uvigerina 6 7 1 0.29 > 30% Centerville Beach

Uvigerina total 13 16.5 1 0.74 > 50%

Expected d.f. ×~ Confidence level inverse of rejecting correlation null hypothesis

9.5 1 4.26 >95%

derma carbon and oxygen isotope values are inversely correlated. This was not the case for Uvigerina (Table II). The binomial test was also applied to the data and an inverse correlation was also indicated for the planktonic foraminifera at the 95% confidence level.

Previous workers have also noted an inverse correlation between oxygen and carbon isotope ratios in foraminifera. In a detailed study of DSDP site 284, west of New Zealand, Shackleton and Kennett (1975b) found a good inverse correlation in Uvigerina for parts of the Miocene and Pleistocene but not the Pliocene. Because the two isotope patterns were found to be related, and oxygen isotopes are in part temperature controlled, they proposed that carbon isotope values are also temperature controlled. This inverse relationship is the reverse of what would be predicted on the basis of thermodynamics (Urey, 1947). Savin et al. (1975) have also noted the same relationship in both planktonics and ben- thonics. They merely point out the relationship and offer no explanation.

The oxygen isotope data from this study is best explained by large changes in the

120 content of the sea water and not nec- essarily by temperature changes. Hence, if carbon isotope values are temperature controlled, as proposed by Shackleton and Ken- nett (1975b), then the inverse relationship between G lsO and 513C noted in this study would not be present. Because this relationship is found, the hypothesis of temperature control seems dubious. It is possible that the environmental conditions of the study area are unique and different from open ocean conditions and thus are responsible for this unique correlation. Perhaps the correlation is the result of fractionation during metab- olism and the secretion of calcite by the foraminifera. As Savin et al. (1975, p. 1509) have stated, too little is presently known about these processes to speculate further.

Carbon i s o t o p e variat ion

Traditionally the results of carbon isotope analysis in foraminifera have been largely ignored because the factors controlling their distribution are poorly understood. Based on the measurements of inorganic carbon isotope ratios in ocean water, Deuser and

57

Hunt (1969) found that surface waters are characterized by heavier carbon isotope values because of the selective removal of 12 C by phytoplankton during photosynthesis. Saito and Van Donk (1974) and Weiner (1975) have used this relationship to s tudy depth stratification in foraminifera. In this s tudy planktonic carbon isotope values were found to be heavier than benthonic values (Figs. 4 and 5). This is in agreement with the work of Deuser and Hunt (1969).

The pattern of change in carbon isotope ratios is independent of depth in the water column (Figs. 4 and 5). Though the patterns appear to be cyclic there is no known mechanism to explain the changes. The carbon isotope composit ion of dissolved bicarbonate in sea water is controlled by factors near the surface such as primary production rates and the equilibrium exchange between atmospheric carbon dioxide, aqueous carbon dioxide, and the bicarbonate ion (Deuser and Degens, 1967; Emrich et al., 1970). It has already been shown that carbon isotope variation is related to oxygen isotope variation thus ad- ding to the complexi ty of the controlling factors. Too little is presently known to explain the controlling mechanism behind carbon isotope variation.

S u m m a r y and c o n c l u s i o n s

(1) During the late Pliocene and early Pleistocene there was a large influx of isotopically light water at DSDP site 173 and the Centerville Beach section. Salinity may have been reduced by as much as 2--40/00.

(2) During the early Pliocene surface sea water temperatures ranged from 9.5 to 15.5°C.

(3) Benthonic oxygen isotope data from the Centerville Beach section records the gradual shoaling of the Humbold t Basin.

(4) Oxygen and carbon isotope variations are inversely correlated at the 95% confidence level in Globigerina bulloides and Neoglobo- quadrina pachyderma. This inverse correlation is probably no t related to temperature changes. Presently, there is no known mech-

anism to explain this correlation. (5) Planktonic carbon isotope values are

heavier than benthonic values; this is in agreement with the work of Deuser and Hunt (1969).

A c k n o w l e d g e m e n t s

I wish to thank J. Rober t Dodd of Indiana University for help in planning this project, assistance in the field, and guidance through- out the project. John Hayes and Dwight Matthews of Indiana University offered much assistance and advice with high vacuum techniques and the GD150 mass spectrometer, for which I am grateful. I would also like to thank Sam Savin of Case Western Re- serve University for providing an independent analysis of the isotopic standards used in this study. John Hayes, Rober t Shaver of Indiana University, Sam Savin, and James Ingle, Jr. of Stanford University critically read a preliminary draft of the manuscript and offered numerous suggestions.

The Deep Sea Drilling Project Samples were supplied through the assistance of the Nation- al Science Foundat ion, wi thout which this s tudy would not have been possible. Field work was supported by a Penrose Bequest Research Grant from the Geological Society of America and by a Grant-In-Aid of Research from Sigma Xi, the Research Society of North America. I am most grateful to both of these societies. Lab work was partly supported by NSF Grant GA30878 to J. Rober t Dodd for study of the biostratigraphy and paleoecology of the Humbold t Basin.

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