magnetic signals in plio-pleistocene sediments of the ...from wefer g, mulitza s, ratmeyer v (eds),...

From WEFER G, MULITZA S, RATMEYER V (eds), 2004, The South Atlantic in the Late Quaternary: Reconstruction of MaterialBudgets and Current Systems, Springer-Verlag Berlin Heidelberg, pp 261-277

Magnetic Signals in Plio-Pleistocene Sediments of the South Atlantic:Chronostratigraphic Usability and Paleoceanographic Implications

Frank Schmieder

Universität Bremen, Fachbereich Geowissenschaften, Postfach 33 04 40,D-28334 Bremen, Germany

e-mail: [email protected]

Abstract: The origin of the magnetic signals used to build age models for marine sediments recov-ered in the framework of the long-term Quaternary South Atlantic research project SFB 261 is two-fold. Conventional magnetostratigraphy makes use of well-dated polarity reversals of the Earth'smagnetic field recorded in the natural remanent magnetization (NRM) of the sediments. In addition,magnetic cyclostratigraphy has been successfully established as a very efficient dating tool for marinesediment sequences during recent years. In the oligotrophic South Atlantic, confirmation of orbitalforcing of magnetic susceptibility records made it possible to establish high-resolution age models,by tuning the respective components to astronomical variations. A set of twelve individually tunedand well-correlated Pleistocene magnetic susceptibility records were stacked within the stratigraphicnetwork SUSAS and can now be used as a correlation reference for other cores recovered in thisregion. The suitability of this target curve for age control is tested against paleomagnetic ages. Pat-tern correlation is possible for nine of ten selected cores recovered in the oligotrophic South Atlanticbetween 15°S and 35°S, but seems to be only partly successful for sediments from the Congo Basin.In the Pleistocene sequences, the magnetic age models provide further evidence for the simulta-neous deposition of previously reported unusual diatom ooze layers between 23°S and 33°S at ap-proximately 540 – 530 ka, at the end of the Mid-Pleistocene climate transition (MPT). The age modelsalso indicate enhanced carbonate dissolution during the MPT interim state (920 – 640 ka). The con-cept of tuning magnetic susceptibility records to orbital variations is extended to the late Plioceneand reveals characteristics obviously related to the rearrangement of ocean circulation as NorthernHemisphere glaciation intensified. Enhanced carbonate preservation since approximately 3.0 Ma andthe establishment of obliquity-driven dissolution cycles since about 2.5 Ma document increasinginflux of North Atlantic Deep Water (NADW) into the subtropical South Atlantic. In a deep core fromthe Rio Grande Rise area, an abrupt change from red deep sea clay to carbonaceous sediments isrecorded at 2.73 Ma, exactly the time proposed for the major intensification of Northern Hemisphereglaciation.

Introduction

A crucial requirement for understanding the dy-namics of geological and paleoenvironmental pro-cesses documented in sediments is age control.Knowledge of the timing of the reversals of theEarth's magnetic field and the fact that these re-versals are frequently recorded in the natural re-manent magnetization (NRM) of marine sedimentsenables dating via a comparison of normally andreverse magnetized intervals to a geomagneticpolarity timescale (GPTS). Magnetostratigraphyprovides quite reliable age control, although its value

is limited by the number of reversals documentedin the studied material. For the Pleistocene, thefrequently used GPTS of Cande and Kent (1992,1995; Fig. 1) contains three such reversals. Addi-tionally, the authors included two short geomagneticevents (therein called "cryptochrons") in their tem-plate, the Emperor (504 to 493 ka) and the CobbMountain event (1212 to 1201 ka).

With regard to high-resolution dating, astronomi-cal tuning has proven to be one of the most pow-erful tools during the last two decades. It has been

262 Schmieder

Fig. 1. Geomagnetic polarity timescale (GPTS) for the last4000 ka (left) after Cande and Kent (1992, 1995) and thecommonly used indications of reversals (right).

successfully used for late Pliocene and Pleistocenesections, but it is by no means restricted to theseepochs (e.g., Schwarzacher 1993a, b; Shackletonet al. 1995; Lourens et al. 1996; D’Argenio et al.1998). Oxygen isotopes are by far the most fre-quently used and best studied proxy regardingcyclostratigraphy. However, because of much lesslaboratory effort and proven suitability, other cli-matically induced signals have also become popu-lar and are now accepted as useful candidates (e.g.Langereis and Dekkers 1999). In this context, physi-cal properties are of special interest because theycan be measured relatively rapidly and with highstratigraphic resolution. Their usefulness has beenproven by numerous chronostratigraphies (e.g.Shackleton et al. 1995; Shackleton and Crowhurst1997; Bickert et al. 1997; Heslop et al. 2000).

In many marine environments magnetic suscep-tibility can be used to refine the age models for sedi-mentary depositions due to certain, regionally vary-ing, orbital linkages. As a function of concentrationand composition of the magnetic mineral fraction,

the magnetic susceptibility of marine sediments isin the first instance a measure of the content of(titano-) magnetite (Thompson and Oldfield 1986).Most magnetite reaches the sediment as part of theterrigenous input (an important exception in someregions is bacterial magnetite, e.g. Petersen etal.1986), and in the majority of cases the magneticsusceptibility record reflects the variable ratio ofbiogenic to lithogenic components (Robinson 1990).Variations in the magnetic susceptibility records ofmarine carbonate sediments can be influenced bycarbonate dissolution, varying terrigenous input,dilution of a constant terrigenous input by varyingcarbonate production, or a combination of theseoften climatically controlled processes. Conse-quently, magnetic susceptibility can serve as aproxy for the carbonate record which mirrors paleo-climatic history. Therefore, magnetic susceptibilityrecords can be tuned to orbital variations and canadditionally aid in paleoceanographic reconstruc-tions.

Magnetic susceptibility records from twelveGeoB cores recovered in the framework of SFB 261in the South Atlantic were used by von Dobeneckand Schmieder (1999) to construct the SubtropicalSouth Atlantic Susceptibility (SUSAS) stack bytuning characteristic Pleistocene Milankovitch-stylepatterns to orbital obliquity and precession. TheSUSAS cores are located between the Rio GrandeRise in the west and the Walvis Ridge in the eastand form an ocean-wide transect between 22°S and34°S. In the South Atlantic the ice age rhythmcauses cyclic variations of the sediment CaCO3content with increased carbonate dissolution dur-ing glacial periods (Volat et al. 1980; Tappa andThunell 1984; Balsam and McCoy 1987) becauseglacial reduction of North Atlantic Deep Water(NADW) gives way for a vertical expansion ofmore corrosive Lower Circumpolar Deep Water(LCDW) (e.g. Bickert and Wefer 1996). The re-sulting dissolution cycles are mirrored in magneticsusceptibility with higher values during glacial pe-riods.

Oxygen isotope records exist for four SUSAScores, but their stratigraphic interpretability is lim-ited by very low sediment accumulation rates, gen-erally as low as 0.5 - 1.5 cm/kyr. Nevertheless,astronomical tuning of the magnetic susceptibility

4000

3500

3000

2500

2000

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0

Age

[ka]

C1n

C1r

C2n

C2r

C2An

C2Ar

C1n-1

C1r.2r-1n

C2r.2r-1

C2r.1n

Brunhes

Matuyama

Gauss

Gilbert

C1r.1n

Olduvai

Réunion

Kaena

Mammoth

Jaramillo

Cobb Mountain

C2An.1r

C2An.2r

Emperor / Big Lost

Plio

cene

Ple

isto

cene

Magnetic Signals in Plio-Pleistocene Sediments of the South Atlantic 263

Fig. 2. Locations of GeoB cores investigated (red dots). SeaWIFS image (SeaWiFS Project, NASA Goddard SpaceFlight Center) depicts primary productivity as visualization of chlorophyll a content in the surface water, valuesincrease from dark blue to red (data collected over the time period September 1997 through August 1998). All coresare located in the oligotrophic South Atlantic. Yellow dots mark locations where an unusual diatom ooze layer ofEthmodiscus rex has been found in the sediments. GeoB 3801-6 and 3813-3 are SUSAS cores containing this layer.

records could be established based on magnetostra-tigraphic tie points. The applied phase lag was de-termined by cross spectral analysis of δ18O andmagnetic susceptibility. It yielded for the 100 and41 kyr cycles a coherence of 0.99 and 0.96 andreasonable phase angles (von Dobeneck andSchmieder 1999). In the obliquity band, magneticsusceptibility lags the orbital forcing by 4.5 kyr.

The individually tuned and well-correlatedSUSAS records form a master-curve, and can beused as a target record for other cores in this re-gion. The remarkably good similarity of magneticsusceptibility profiles in the whole oligotrophic SouthAtlantic has been used to construct preliminary agemodels for sediments recovered between 20°S and38°S during RV Meteor cruises M 41/3 (Frederichsat al. 1999) and M 46/4 (Donner and Schmieder2001) by comparing typical patterns to the SUSASstack. The post-cruise spot sensor magnetic sus-ceptibility data measured in the laboratory for theseand other cores allow for a more detailed signaldefinition and hence improved age modeling. Bysubsequently tracing the magnetic signal back to its

paleoceanographic origin it can be interpreted interms of modifications of deep water chemistry.

Material and MethodsFrom several hundreds of cores recovered between1988 and 2000 in the South Atlantic within theframework of Sonderforschungsbereich (SFB) 261at the University of Bremen, ten cores were cho-sen to test the suitability of the SUSAS stack as atarget curve to construct high-resolution age mod-els. A main criterion for the selection was the re-cording of at least one paleomagnetic reversal toprovide an independent age control. As gravitycoring on board RV Meteor is limited to 15 - 20 m,only cores from regions with relatively low sedimentaccumulation rates of less than 2.5 cm/kyr reachthe youngest well-defined polarity reversal, theBrunhes/ Matuyama boundary at 780 ka (Fig. 1).Therefore, regions with enhanced productivity areexcluded from this study. The chosen cores wereall recovered from the oligotrophic South Atlantic(Fig. 2) in water depths between 3789 and 5073 m

.01 .02 .03 .05 .1 .2 .3 .5 1 2 3 5 10 15 20 30 50

Chlorophyll a concentration [mg/m3]

264 Schmieder

Table 1: Water depth, position, core length, and sampling interval of the GeoB gravity cores studied in this paper.MTS: Magnetostratigraphy; Demag.: Demagnetization. Demagnetization was performed at A: 10, 20, 30, 40, 50, (70,100) mT, B: 5, 10, 20, 30, 40, 50, 70, 100 mT, C: 5, 10, 15, 20, (25), 30, (35), 40, (45), 50, 65, 80, (95, 100, 110, 120) mT.

METEOR GeoB Position Water Core Sampling Demag. MTS Cruise

Expedition core Latitude Longitude Depth [m] Length [m]

interval [cm]

scheme published in

report

M 6/6 1036-1 21°10.4'S 03°19.7'E 5073 10.76 10 (5) A Thießen ‘93 Wefer et al. (1988) M 15/2 1307-1 33°36.1'S 27°39.9'W 4017 6.77 10 B this paper Pätzold et al. (1993)

M 16/1 1413-4 15°40.8'S 9°27.3'W 3789 10.10 10 A Thießen ‘93 Wefer et al. (1991) M 34/2 3724-4 26°08.2’S 08°55.6’E 4759 11.41 10 C this paper Schulz et al. (1996)

M 41/3 5112-4 23°49.5'S 16°15.5'W 3842 5.59 10 C this paper Pätzold et al. (1999)

5142-1 19°05.4'S 17°08.7'W 3946 5.64 5 C this paper M 46/4 6425-2 33°49.5'S 23°35.2'W 4352 10.73 5 C this paper Wefer et al. (2001)

6426-1 33°30.0'S 24°01.5'W 4385 13.41 5 C this paper 6428-1 32°30.6'S 24°14.9'W 4015 7.26 5 C this paper 6429-2 31°57.0'S 24°14.9'W 4335 7.44 5 C this paper

during several RV Meteor expeditions. The car-bonaceous sediments are mainly composed ofnannofossil ooze. Table 1 summarizes the position,water depth, core length, and the sampling intervalfor paleomagnetic analyses of the cores, and liststhe respective cruise reports. The sediments wererecovered using a gravity corer with an internaldiameter of 12 cm. Subsampling for paleomagneticmeasurements was carried out on board, generallyat a 10 or 5 cm sampling interval (Table 1), usingcubic plastic boxes with a volume of 6.2 cm³.

The natural remanent magnetization (NRM)was measured using two different three axis SQUIDmagnetometers. A stepwise alternating field (AF)demagnetization was performed for all samples toisolate the characteristic remanent magnetization(ChRM). The different demagnetization schemesused are listed in Table 1. For the samples ofGeoB 1036-1, 1307-1, and 1413-4 (Thießen 1993)a Cryogenic Consultants GM 400 and a singleaxis demagnetizer 2G Enterprises 2G 600 wereused. In 1998, the laboratory of Marine Geophys-ics in the Department of Geosciences at BremenUniversity was equipped with a new 2G Enter-prises magnetometer (Model 755R) with an inte-grated demagnetizing unit. This fully automatedmagnetometer was used for all other samples.Directions of the ChRM were calculated by usingthe principal component analysis (Kirschvink 1980)and at least three successive demagnetization steps.

Magnetic susceptibility was measured post-cruise on archive halves using a Bartington In-struments spot sensor (F) at a 1 cm spacing. Be-fore measuring, the archive halves, which arestored at 4°C, were warmed up to room tempera-ture to avoid temperature effects. Each measure-ment was corrected with a separate backgroundreading.

Results and Discussion

MagnetostratigraphyIn most cases the ChRM could be determined un-ambiguously. Stable ChRM directions were isolatedfrom the linear sections of the demagnetizationcurves. Figure 3 summarizes ChRM inclinationsand declinations of all cores. Depths of all identi-fied reversals and the corresponding ages referringto Cande and Kent (1992, 1995) are listed inTable 2. Superimposed trends on some declinationvalues, e.g. between 10 and 13.5 m in GeoB 6426-1,point to rotation during the core recovery. As theydo not complicate the identification of geomagneticpolarity, these intervals are plotted unchanged.

By containing the Gauss/Gilbert boundary(3580 ka), core GeoB 1307-1 contains the oldestsediments of this study. In GeoB 6429-2, theMatuyama/Gauss boundary (2581 ka) is recorded,all the other cores end within the Matuyama Chron.


-90

0 90

Incl. [°]

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GeoB 5142-1

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th [m

]

Decl. [°]

0 180

GeoB 5112-4

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0 90

Incl. [°]

131211109876543210

Decl. [°]

0 180

GeoB 6426-1-9

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Incl. [°]

11109876543210

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GeoB 6425-2

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GeoB 1307-1-9

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11109876543210

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th [m

]

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0 180

GeoB 1036-1

-90

0 90

Incl. [°]

1110

9876543210

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0 180

GeoB 3724-4

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0 90

Incl. [°]

10

9

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6

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GeoB 1413-4

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]

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0 180

GeoB 6428-1

Fig. 3. Magnetostratigraphic results for GeoB 1036-1,1307-1, 1413-4, 3724-4, 5112-4, 5142-1, 6425-2, 6426-1,6428-1, and 6429-2. Data of GeoB 1036-1 and 1413-4are from Thießen (1993). Inclination (Incl.) and decli-nation (Decl.) of the characteristic remanent magneti-zation (ChRM) were defined after stepwise demagneti-zation of single samples. Initial age models were builtby relating identified normally (black) and reversed(white) magnetized sections to the GPTS of Cande andKent (1992, 1995) shown in Fig. 1. Negative depth val-ues for GeoB 5112-4 result from usage of well-pre-served sediments recovered in the bomb of the grav-ity corer.

266 Schmieder

No short polarity events could be identified unam-biguously within the Brunhes Chron. Only one ofall analyzed samples in the Brunhes Chron recordeda positive inclination, the one from 1.35 m depth inGeoB 6425-2 (+10.4°; α95 = 0.4°). The subsequentrefinement of the age model (see below) results inan age of 135 ka for this sample. The nearest po-tential event, the Blake event, has been dated byLangereis et al. (1997) at 110 - 120 ka. In theseoligotrophic environments, a combination of verylow sediment accumulation rates and bioturbationis likely to hinder the recording of very short geo-magnetic reversals or excursions.

However, under certain conditions a duration ofabout 10 kyr seems to be sufficient for a record-ing, since within the Matuyama Chron the shortCobb Mountain event is clearly documented in twoof the six cores which reach the necessary age. InGeoB 1036-1, three samples with inverse inclina-tion and/or declination are found between 5.23 mand 5.38 m, and in GeoB 6426-1 between 12.13 mand 12.28 m (Fig. 3). The well-defined inversedeclination at 6.25 m (α95 = 3.2°) in GeoB 6428-1does not correspond to the Cobb Mountain event.Relating the normal interval at the bottom of thiscore to the Olduvai event results in an age of1602 ka for the depth of this sample. Subsequentrefinement of the age model (see below) producesan age of 1582 ka in marine isotope stage (MIS)

54. It can be assumed that this feature correspondsto a short event first found by Clement and Kent(1987) in North Atlantic sediments from ODP Site609. By retuning the ODP 609 data to the recordof ODP Site 677 (Shackleton et al. 1990) C. Lange-reis and D. Heslop dated this "Stage 54 Event" to1580 ka (pers. comm.).

In Figure 4 age/depth relations based on identi-fied geomagnetic reversals (open symbols) areshown for all cores. When more than one tie pointis available these results indicate more or less con-stant sediment accumulation rates during the Pleis-tocene. Slightly lower rates are calculated betweenthe top of the Jaramillo event at 990 ka and theBrunhes/Matuyama boundary at 780 ka in GeoB1036-1, 6428-1, and 6429-2 (Fig. 4 b). This resultis in agreement with the finding that the time inter-val between 920 and 640 ka is characterized bymarkedly lowered carbonate accumulation in thesubtropical South Atlantic as a result of enhancedinfluence of corrosive bottom waters of southernorigin (Schmieder et al. 2000).

The paleomagnetic analyses indicate muchlower sediment accumulation rates in the lowest~ 2 m of GeoB 1307-1 and 6429-2. At a first glance,this change could be assumed to result from sedi-ment compaction during the core recovery. Butremarkably, the change occurs in both cores ex-actly at the same time, between top and bottom of

Table 2: Depths of identified geomagnetic reversals and ages corresponding to the geomagnetic polarity timescaleof Cande and Kent (1995).

CK ’95 GeoB

1036

GeoB

1307-1

GeoB

1413-4

GeoB

3724-4

GeoB

5112-4

GeoB

5142-1

GeoB

6425-2

GeoB

6426-1

GeoB

6428-1

GeoB

6429-2

Age [ka] Depth [m]

780 3.38 1.2 8.645 6.2 5.3 5.375 8.275 8.075 2.575 1.875

990 3.98 1.8 - 8.2 - - 10.225 10.025 3.175 2.125

1070 4.525 1.9 - 8.7 - - - 10.875 3.575 2.375

1201 5.225 - - - - - - 12.125 - -

1211 5.375 - - - - - - 12.275 - -

1770 9.625 3.9 - - - - - - 7.175 4.975

1950 - 4.2 - - - - - - - 5.425

2581 - 4.8 - - - - - - - 6.575

3050 - 5.3 - - - - - - - -

3110 - 5.5 - - - - - - - -

3220 - 5.6 - - - - - - - -

3330 - 5.775 - - - - - - - -

3580 - 6.15 - - - - - - - -


0 500 1000 1500 2000 2500 3000 3500Age [ka]

11

10

9

8

7

6

5

4

3

2

1

0

Dep

th [m

]

GeoB 1036-1GeoB 1307-1GeoB 3724-4GeoB 6428-1GeoB 6429-2

1 cm/kyr

0.5 cm/kyr

0.25 cm/kyr

0.1 cm/kyr

b

0 200 400 600 800 1000 1200 1400Age [ka]

131211109876543210

Dep

th [m

]

GeoB 1413-4GeoB 5112-4GeoB 5142-1GeoB 6425-2GeoB 6426-1

1 cm/kyr

0.5 cm/kyr

1.5 cm/kyr

a

the Olduvai event (1950 - 1770 ka). Additionally, apotential compaction of the lower part of the coreis not observed for any of the other cores whichdo not reach the top of the Olduvai, except for thelowermost ~ 40 cm of GeoB 3724-4, where therefined age model described below (small symbolsin Fig. 6) could be interpreted in terms of compac-tion (or strongly lowered sediment accumulationrates). Furthermore, seven late Pliocene magneto-stratigraphic tie points in GeoB 1307-1 denote nofurther sediment accumulation rate reduction priorto the Olduvai which would be expected in case ofcompaction as a result of an increasing effect. In-stead, nearly constant sediment accumulation ratesof about 0.15 cm/kyr are observed during the wholelate Pliocene sequence, except for the 70 kyr longKaena subchron wherein they even increase to0.29 cm/kyr.

For these reasons, we rule out sediment com-paction and conclude that indeed sediment accu-mulation rates increased significantly in the west-ern subtropical South Atlantic during the Olduvai,thus sometime between 1950 and 1770 ka. Aver-age values calculated for the Pleistocene are in bothcores approximately 40% higher than in the Plio-cene sections. In GeoB 1307-1 they shift from 0.15

to 0.21 cm/kyr and in 6429-2 from 0.18 to 0.26 cm/kyr. The subsequent refinement of the age models(small symbols in Fig. 4) results in the identifica-tion of hiatuses in the Pleistocene sections of thesetwo cores. Taking these gaps into account wouldeven slightly enhance the average sediment accu-mulation rates calculated for the Pleistocene.

Magnetic Susceptibility:Tuning and Interpretation

Within the SUSAS stack (Pleistocene)

Based on the magnetostratigraphic tie points astraightforward top to bottom correlation even ofsmall-scale characteristics in magnetic susceptibil-ity to the SUSAS stack was feasible for the sedi-ments of GeoB 1413-4 (Fig. 5 a), 5142-1 (b), 5112-4(c), 6425-2 (d), and 6426-1 (e). The correlationindicates that MIS 1 to 3 are missing at the top ofGeoB 5142-1 (Fig. 5 b). MIS 1 to 4 are missing atthe top of GeoB 5112-4 but in accordance withFrederichs et al. (1999) the well-preserved sedi-ments recovered in the bomb of the gravity corerhave been added to complete the record as far as

Fig. 4. Age/depth relations for GeoB cores resulting from the magnetostratigraphy shown in Fig. 3 (open symbols)and refined age models after correlation to SUSAS stack and obliquity tuning shown in Figs. 5, 6, and 8 (small sym-bols). Dotted lines depict different sediment accumulation rates. Note different age and depth scales on both plots.

268 Schmieder

possible (Fig. 5 b). This approach is justified by avery good correlation of the resulting magneticsusceptibility pattern to the SUSAS stack andneighboring cores. Mean magnetic susceptibilityvalues of the cores, calculated only for the Brunheschron to enable a comparison, increase from northto south (Table 3). A lower contribution of biogeniccomponents in the deeper southern cores, e.g

caused by enhanced influence of corrosive south-ern source deep waters, is unlikely, since sedimentaccumulation rates are similar or even higher in thisarea (see also Fig. 5). Consequently, it can be as-sumed that the trend to higher magnetic suscepti-bilities in the south indicates enhanced terrigenousinput.

Fig. 5. Combined age models for GeoB a) 1413-4 , b) 5142-1, c) 5112-4, d) 6425-2, and e) 6426-1 based on paleomagnetictie points and correlation to f) SUSAS stack. Average sediment accumulation rates (Ø SR) range between 0.7 and1.1 cm/kyr. Identified geomagnetic reversals are marked by dots. As a consequence of the lock-in process they aregenerally located deeper in the cores than the corresponding reference ages of Cande and Kent (1992, 1995) whichare marked by vertical dotted lines. Labels at the SUSAS stack indicate even numbered cold oxygen isotope stages.

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600Age [ka]

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Magnetic susceptibility [10

-6 SI]

-2

-1

0

1

2

100

150

200

250


-6 SI]

100

150

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250

20

40

60

80

100

GeoB 6426-1(Ø SR = 1.01 cm/kyr)

Brunhes Matuyama

SUSASstack

JaramilloCobb

Mountain

2 4 6

810 12

14

16 18

2022

24

2628

30

32

34

38 40

42

4446

48

50

36

GeoB 6425-2(Ø SR = 1.03 cm/kyr)

GeoB 5112-4(Ø SR = 0.73 cm/kyr)

GeoB 5142-1(Ø SR = 0.73 cm/kyr)

GeoB 1413-4(Ø SR = 1.11 cm/kyr)

d

e

f

c

b

a


Apart from a good resemblance of characteris-tic Milankovitch style patterns with the main orbitalperiods of 41 kyr in the older part of the stack andapproximately 100 kyr during the late Quaternary,high-frequency features are as well recorded as thelonger-term changes. A good example is the 'MPTinterim state' (Schmieder et al. 2000), between 920and 640 ka (Fig. 5). The evolution of climate vari-ability within the Mid-Pleistocene climate transition(MPT) causes much higher magnetic susceptibili-ties during this time period. These higher values arepresumed to result from strongly reduced carbon-ate accumulation caused by enhanced influence ofsouthern source deep waters. The discovery ofconsiderably raised magnetic susceptibilities dur-ing the MPT interim state in the more northerncores GeoB 1413-4, 5112-4, and 5142-1 (Fig. 5) isfurther clear evidence for an ocean-wide changeof sedimentation conditions in the middle Pleisto-cene.

Within the generally carbonaceous sequences,an unusual diatom layer was found in cores GeoB5112-4, 6425-2, and 6426-1 (Pätzold et al. 1999,Wefer et al. 2001). All three layers are predomi-nantly composed of the giant diatom Ethmodiscusrex (O. Romero pers. comm.) and the preliminaryage estimations made on board by comparing themagnetic susceptibility records to the SUSAS stack(Frederichs et al. 1999; Donner and Schmieder2001) suggest that they have been deposited at thesame time as comparable laminated diatom oozelayers in the SUSAS cores GeoB 3801-6 and 3813-3(Fig. 2). Together with unusual lithological featuresin other cores, the latter have been interpreted bySchmieder et al. (2000) as a documentation of theterminal event of the MPT related to the rearrange-ment of ocean circulation after a period of reducedNADW production and the onset of late Quater-

nary 100 kyr climatic cycles. This event has beendated at 540 - 530 ka and the diatom layers wereassumed to have been deposited during a time in-terval of 10 kyr. A prominent Ba/Al peak is asso-ciated with the diatom layer in GeoB 3813-3 indi-cating that the layers recorded a period of increasedorganic carbon flux to the sea floor (Gingele andSchmieder 2001).

In the case of GeoB 5112-4 and 6425-2, the topand bottom of the diatom ooze layer can be clearlydetected by strongly enhanced P-wave velocities(Frederichs et al. 1999, Dillon et al. 2001) measuredusing an automated full waveform logging system(Breitzke and Spieß 1993) and are visible by eye.Therefore, the relevant section could easily be re-moved from the record before it was correlated tothe stack. Unlike the dating procedure used within theSUSAS chronology for GeoB 3801-6 and 3813-3,no time interval has been attributed to this layer.Assuming a very rapid deposition of the diatoms(< 1 kyr) the correlation of the resulting magneticsusceptibility pattern to the SUSAS stack resultsin an age for the event of 524 ka in GeoB 5112-4and of 528 ka in GeoB 6425-2. In core GeoB 6426-1 the diatom zone is not as clearly detectable as inthe other two cores and could not be removed. Theage model relates the sequence containing frag-ments of Ethmodiscus rex to the time interval 548- 530 ka. The detection of this diatom layer in sedi-ments from more northern and southern positionsin the South Atlantic (Fig. 2) and the evidence fora simultaneous deposition strengthen the hypoth-esis that a major paleoceanographic event tookplace at the end of the MPT interim state.

In contrast to the cores shown in Fig. 5, GeoB3724-4 from the Congo Basin displays strongerdeviations from the SUSAS pattern (Fig. 6). Theshift to higher magnetic susceptibilities during the

Table 3: Mean magnetic susceptibilities, standard deviation and avearge sediment accumulation rates during theBrunhes chron for four investigated GeoB cores which form a transect from north to south.

Core Latitude Mean susceptibility

[10-6 SI] (780 – 0 ka)

Standard deviation [%] Sediment

accumulation rate

[cm ka-1] (780 – 0 ka)

GeoB 1413-4 15°40.8'S 33.1 48.3 1.11

GeoB 5142-1 19°05.4'S 124.9 43.2 0.73

GeoB 5112-4 23°49.5'S 128.7 34.9 0.73

GeoB 6425-2 33°49.5'S 347.7 34.5 1.06

270 Schmieder

MPT interim state is still visible, but a correlationof small scale patterns, especially during the earlyPleistocene, is only feasible in some intervals. Nocorrelation has been performed after MIS 13,where two minima with extremely low magneticsusceptibility values probably indicate significantdisturbances.

Beyond the SUSAS stack (Pliocene)According to the magnetic reversal stratigraphy(Figs. 3 and 4), sediment cores GeoB 1036-1, 1307-1,6428-1, and 6429-2 reach farther back in time thanthe 1534 ka old SUSAS stack. This results mainlyfrom very low average sediment accumulationrates, ranging from 0.20 to 0.54 cm/kyr. The re-duced temporal resolution makes it more difficultto relate magnetic susceptibility variations unam-biguously to specific SUSAS features or orbitalcycles, as demonstrated for the cores shown inFigure 5. To get a better impression of pattern vari-ability, a simultaneous correlation strategy wasapplied. This parallel core processing makes useof all available magnetostratigraphic tie points, in-cluding those of the oldest, low-resolution cores aswell as the better preserved magnetic susceptibil-

ity patterns in the cores with higher sediment ac-cumulation rates. Subsequent to paleomagneticdating the logs were correlated to the SUSAS stackas far as possible (Fig. 7). Generally, this was moredifficult in the youngest part of the records. InGeoB 1036-1, no unambiguous correlation waspossible between MIS 2 and 7. The correlationresults in a hiatus in GeoB 1307-1 between MIS 9and 21 and in GeoB 6429-2 between MIS 19 and23. These hiatuses result in higher average sedi-ment accumulation rates for the Pleistocene sec-tions than those calculated above, based on themagnetostratigraphic tie points alone. AlthoughGeoB 6428-1 and 6429-2 are located near the re-gion where several cores indicate a synchronousdeposition of diatom ooze at approximately 530 kathis layer could not be detected in these two cores.Probably, sediment accumulation rates were toolow at these locations to enable preservation of thediatoms.

In the older Early Pleistocene and late Pliocenesections of these cores, variations with a typical41 kyr periodicity are better preserved because ofhigher sediment accumulation rates. Explicit evi-dence for obliquity forcing of magnetic suscepti-bility comes from an additional tie point beyond the

Fig. 6. Combined age model for a) GeoB 3724-4 based on paleomagnetic tie points (dots) and correlation to b)SUSAS stack. This correlation is not as good as those shown in Fig. 5 and not at all feasible for the youngest partof the record plotted with a dotted line. Reference ages of Cande and Kent (1992, 1995) are marked by verticaldotted lines. Labels at the SUSAS stack indicate even cold oxygen isotope stages.

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600Age [ka]

50

100

150

200

250


-6 SI]

-2

-1

0

1

2

GeoB 3724-4(Ø SR = 1.01 cm/kyr)

Brunhes Matuyama

SUSASstack

JaramilloCobb

Mountain

2 4 6

810 12

14

16 18

2022

24

2628

30

32

34

38 40

42

44

4648

50

36

a

b


SUSAS stack provided by detection of the top ofthe Olduvai subchron at 1770 ka in GeoB 6428-1and 1036-1. Six nearly equidistant magnetic sus-ceptibility cycles recorded in both cores betweenthis reversal and the last applied SUSAS tie pointat 1508 ka result in a dominant period of approxi-mately 44 kyr. Spectral analysis of the magnetostra-tigraphically dated magnetic susceptibility record ofGeoB 6428-1 also verifies strong obliquity forcing(Fig. 8). These cycles were used to refine the pa-leomagnetic age models by tuning to the respec-tive orbital variations (Fig. 7 c). In accordance tothe procedure utilized during the construction of theSUSAS stack, obliquity variations calculated byBerger and Loutre (1991) shifted by -4.5 kyr wereused as target curve. During the tuning procedure,low obliquity values, indicating colder conditions,were related to magnetic susceptibility maxima.This strategy was substantiated during the construc-tion of the SUSAS stack and is also supported byseveral magnetostratigraphic tie points in thePleistocene and Pliocene (Fig. 7 a and b). After-wards, the tuned sequences of GeoB 1036-1 and6428-1 helped to adjust the record of GeoB 6429-2(Fig. 7 b). In the older, late Pliocene section of thiscore, tuning of the filtered record to the shiftedobliquity target curve was performed as far aspossible. Below the last tie point at 2673.5 ka, therecord was extrapolated with the last calculatedsediment accumulation rate. Below the SUSASstack, the obliquity tuning results only in small de-viations from the magnetostratigraphic age model(Fig. 4 b). The average difference of allocated agesfor all measured data points of GeoB 6429-2 is12.2 ± 12.4 kyr. Extremely low sediment accumu-lation rates in core GeoB 1307-1 (Fig. 7 a) re-stricted identification and correlation of particularmagnetic susceptibility patterns. The SUSAS stackhas been used as target record as far as possible,in the older part obvious similarities to GeoB 6429-2were used to adjust the record.

In the SUSAS cores, magnetic susceptibility isa reliable proxy for carbonate content (Schmiederet al. 2000). The remarkable similarity of thesemagnetic records to Pleistocene climate variationsresults from glacial/ interglacial carbonate dissolu-tion cycles induced by the interplay of NADW andmore corrosive bottom waters of southern origin.

The SUSAS pattern is dominated by typical 100 kyrcyclicity in the late Quaternary and distinct 41 kyrcycles in the older part back to 1500 ka (vonDobeneck and Schmieder 1999). When applyingthis interpretation as a proxy for carbonate pres-ervation, magnetic susceptibility in the older por-tions of the presented cores appears to be closelylinked to the late Pliocene reorganization of oceancirculation.

This reorganization is coupled to the intensi-fication of Northern Hemisphere glaciation whichtook place between 3100 and 2500 ka with a ma-jor step at about 2750 - 2730 ka (Maslin et al. 1996;Maslin et al. 1998; Haug et al. 1999). Accordingto Turnau and Ledbetter (1989), NADW was muchreduced or even absent in the Rio Grande Riseregion before the onset of Northern Hemisphereglaciation at about 3100 ka (ages adjusted to thenew timescales of Shackleton et al. 1990 and Hilgen1991). A first incursion of NADW to this regionnear 30°S occurred at approximately 3000 ka. Theinterval from 3000 to 2700 ka was dominated byclimatic cooling and increasing influx of deep wa-ters originating in the North Atlantic. During thisinterval, NADW expanded and deepened in theSouth Atlantic (Turnau and Ledbetter 1989).

Very low carbonate contents in the oldest sedi-ments of cores GeoB 1307-1 and 6429-2 (Pätzoldet al. 1993, Wefer et al. 2001) are indicated by highmagnetic susceptibilities (Fig. 7 b). During thesetimes the absence of NADW in the subtropicalSouth Atlantic distinctly hampered carbonate pre-servation at greater water depths. Extremely highmagnetic susceptibility values around 800 · 10-6 SIare found in the time range between 4000 and3100 ka in GeoB 1307-1, two peak values of 2800and 3500 · 10-6 SI are dated to 3720 and 3400 karespectively. In striking contrast to the otherwisecarbonaceous sequences, the lowermost 0.5 m ofGeoB 6429-2 consists of dark colored red deep seaclay (Eberwein et al. 2001; see Fig. 7b). The com-bined magneto- and cyclostratigraphic age modelrelates the depth of this remarkable change in sedi-ment composition to an age of 2730 ka, exactly thetime proposed by Haug et al. (1999) for the abruptmajor intensification of Northern Hemisphere gla-ciation. The southward expansion and deepeningof NADW (Turnau and Ledbetter 1989) appar-

272 Schmieder

Fig. 7. Combined age models for a) GeoB 1307-1, b) 6429-2, d) 1036-1, and e) 6428-1 based on paleomagnetic tiepoints and correlation to f) SUSAS stack and tuning of extracted components to c) lagged obliquity variationscalculated by Berger and Loutre (1991). Susceptibility data of GeoB 1036-1 are from Thießen (1993). The cores arecharacterized by very low average sediment accumulation rates (Ø SR). Identified geomagnetic reversals are markedby stars for positions kept in the final age model and by dots for those adjusted during the correlation and tuningprocedure. As a consequence of the lock-in process the latter are generally located deeper in the cores than thecorresponding reference ages of Cande and Kent (1992, 1995) which are marked by vertical dotted lines. Labels atthe SUSAS stack indicate even numbered cold oxygen isotope stages. NHG: Northern Hemisphere glaciation.

24.5

24

23.5

23

22.5

0 200 400 600 800 1000 1200 1400 1600 1800 2000Age [ka]

100

200

300

300

400

500

600

700


-6 SI]

0

50

100

150

200


-6 SI]

-2

-1

0

1

2

300350400450500550

GeoB 6428-1(Ø SR = 0.41 cm/kyr)

Hiatus

Brunhes Matuyama Olduvai

SUSASstack

JaramilloCobb

Mountain

2 4 6

810 12

14

16 18

2022

24

2628

30

32

343840

42

4446

48

50

36

GeoB 1036-1(Ø SR = 0.54 cm/kyr)

GeoB 6429-2(Ø SR = 0.25 cm/kyr)

GeoB 1307-1(Ø SR = 0.20 cm/kyr)

Obliquity [°] (+ 4.5 ka)

d

c

b

a

e

f

Hiatus

52

54

5658

60

6264

52

52

64

64


ently reached the location of GeoB 6429-2 in awater depth of 4335 m not before 2730 ka. After-wards, it resulted in enhanced carbonate preser-vation at this location. The red clay is not found inGeoB 1307-1, probably because in this slightly shal-lower water depth of 4017 m at the eastern flank ofthe Rio Grande Rise, the carbonaceous shells weresomewhat better preserved. Nevertheless, thelowermost part of this core is also characterizedby a very low carbonate content (Donner 1993).

After the onset of the intensification of NorthernHemisphere glaciation at approximately 3100 ka,

increasing carbonate content in both cores is indi-cated by decreasing mean magnetic susceptibilityvalues. This trend continues until approximately2100 ka and is superimposed on developing obliq-uity-driven cycles. The 41 kyr cyclicity is estab-lished in GeoB 6429-2 between 2800 and 2500 kaon a nearly constant background level of617 (± 128) · 10-6 SI (arithmetic mean ± standarddeviation). At that time, a deepwater stratificationsimilar to that of today was established in the RioGrande Rise area as a result of southward expand-ing NADW (Turnau and Ledbetter 1989). The

Fig. 7. continued (note 200 kyr overlap with left side).

24.5

24

23.5

23

22.5

300

400

500

600

700

Mag

netic

sus

cept

ibilit

y [1

0-6 S

I]

300350400450500550

1000

2000

5000

500

200


-6 SI]

Matuyama Gauss GilbertKaena Mammoth

2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000Age [ka]

GeoB 1307-1(Ø SR = 0.20 cm/kyr)

Obliquity [°] (+ 4.5 ka)

Olduvai

Log-scale→

GeoB 1307-1

2730 ka: Major Step in Intensification of Northern Hemisphere glaciation (Haug et al., 1999)

Red Clay

~ 2500 ka: Establishment of "Modern" Deepwater Stratificationin the South Atlantic (Turnau and Ledbetter, 1989)

2800 ka: Shift from Precession to Obliquity Forcingin African Climate (de Menocal, 1995)

CarbonaceousSediments IncreasingObliquityForcing

Intensification of NHG

b

a

c

GeoB 6429-2(Ø SR = 0.25 cm/kyr)

274 Schmieder

Northern Hemisphere glaciation intensified be-tween 3100 and 2500 ka (e.g., Shackleton et al.1984; Tiedemann et al. 1994) with a major step at2750 - 2730 ka (Maslin et al. 1996; Maslin et al.1998; Haug et al. 1999). African climate variabil-ity proxies document enhanced obliquity forcingsince approximately 2500 ka (e.g., Bloemendal andde Menocal 1989; de Menocal 1995). After a long-term minimum between 4500 and 3100 ka, obliq-uity forcing of solar insolation intensified consider-ably between 3100 and 2500 ka (Fig. 7 b). Thisincrease in obliquity amplitudes is asserted by sev-eral authors as an important additional factor(Maslin et al. 1995; Maslin et al. 1998; Haug andTiedemann 1998) within the scope of explanationsgiven for the cause of the initiation of NorthernHemisphere glaciation (for a summary see Maslinet al. 1998), like the uplift of the Tibetan-Himalayanplateau (Raymo et al. 1988) or the gradual closure

of the Isthmus of Panama (Haug and Tiedemann1998). The latter led to a changed ocean circula-tion which supplied more moisture to, and hencemore precipitation in the Northern Hemisphere.More obtuse angles of the Earth rotational axisoccurred periodically and prevented the increasedsnow cover of the winters from melting duringsummers. Within this time period the magnetic sus-ceptibility records of GeoB 1307-1 and 6429-2 in-dicate significant changes in deep water chemis-try in the subtropical South Atlantic. Well-preservedobliquity cycles since 2500 ka can be assumed toresult from periodically enhanced influence ofNADW in this region.

The record of GeoB 6429-2 implies a furthermajor change at about 2150 ka when the backgroundlevel of magnetic susceptibility drops to the muchlower early Pleistocene level of 429 (± 112) · 10-6 SI(between 2150 and 1000 ka). Like a reduced in-fluence of NADW during the MPT interim statecaused much higher magnetic susceptibilities(Schmieder et al. 2000), the late Pliocene declineis interpreted as a further abrupt intensification ofNADW influx to the subtropical South Atlantic.Most likely this major increase of NADW influxand the consequential better carbonate preserva-tion accounts for the above discussed 40% in-crease of sediment accumulation rates near thePliocene/Pleistocene boundary.

Conclusions• The chronostratigraphic validity of the SUSAS

stack (von Dobeneck and Schmieder 1999) asa target record has been demonstrated for tenpreviously magnetostratigraphically dated GeoBcores recovered in the oligotrophic South Atlanticbetween 15.5 and 33.5°S.

• Obliquity forcing of magnetic susceptibility isdocumented since 2500 ka in the late Plioceneand has been used to refine magnetostrati-graphic age models in sections older than theSUSAS stack.

• Further evidence for a major paleoceanographicevent at the end of the Mid-Pleistocene Transi-tion at 540 – 530 ka (Schmieder et al. 2000)comes from the concordant dating of unusualdiatom layers consisting of the giant diatom

124kyr

41kyr

30

40

50

60

Est

imat

ed S

pect

rum

[dB

]

Estim

ated Spectrum

0.00 0.01 0.02 0.03 0.04 0.05 0.06Frequency [1/kyr]

95kyr

23kyr

19kyr

GeoB 6428-1Magnetostratigraphicage model

510kyr

Fig. 8. Spectral analysis of magnetostratigraphicallydated magnetic susceptibility of GeoB 6428-1 on a lin-ear (solid black) and logarithmic (line) scale. Main or-bital periods are marked by vertical dotted lines. Notethe strong obliquity forcing (41 kyr) of the (untuned)record. The spectral peak near 500 kyr results fromstrongly enhanced susceptibilities during the MPT in-terim state. The cross is related to the logarithmic scaleand depicts 6 dB bandwidth and 90% confidence inter-val. Spectral analysis were done using the program SPEC-TRUM (Schulz and Stattegger 1997).


Ethmodiscus rex in three more cores (GeoB5112-4, 6425-2, 6426-1).

• Enhanced magnetic susceptibilities and hencelowered carbonate accumulation rates during theMPT interim state are also documented at morenorthern locations and strengthen the hypothesisof an oceanwide enhanced influence of corrosivebottom waters of southern origin during that in-terval.

• Paleomagnetic age models indicate that sedimentaccumulation rates in the Rio Grande Rise areaduring the early Pleistocene are higher by ap-proximately 40% than during the late Pliocene.

• In addition to the paleoceanographic history re-corded in the Pleistocene SUSAS sequences thelate Pliocene and early Pleistocene rearrange-ment of ocean circulation is mirrored in oldermagnetic susceptibility records. In the Rio GrandeRise area, obliquity forcing of sedimentationevolved simultaneously with the intensificationof Northern Hemisphere glaciation between3100 and 2100 ka. In the deep core GeoB 6429-2,the major intensification of the glaciation is docu-mented in an abrupt change from dark coloredred deep sea clay to carbonaceous sediments at2730 ka. Inflow of NADW to the Rio GrandeRise region increased steadily during the latePliocene with major steps at approximately2700 – 2600 and at 2150 ka.

AcknowledgementsLiane Brück provided technical assistance duringNRM measurements of GeoB 1307-1, 5112-4, and5142-1. Thomas Frederichs, Christian Hilgenfeldtand Karl Fabian automated the cryogenic magneto-meter and the magnetic susceptibility measurementsystem in Bremen and developed various softwareto analyze the data. Thanks to Ian Snowball andCor Langereis for thoughtful reviews and helpfulcomments on the manuscript and to David Heslopfor improving the English. Captains and crews ofRV Meteor are acknowledged for their efficientsupport during the cruises M 6/6, M 15/2, M 16/1,M 34/2, M 41/3, and M 46/4. This study was fundedby the Deutsche Forschungsgemeinschaft (Sonder-forschungsbereich 261, contribution no. XXX).Data are available under www.pangaea.de.

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