Sedimentary record of the mid-Pleistocene climate transition in the southeastern South Atlantic (ODP Site 1090)

Palaeogeography, Palaeoclimatology, Palaeoecology 182 (2002) 241^258 www.elsevier.com/locate/palaeo

Sedimentary record of the mid-Pleistocene climate transition in the southeastern South Atlantic (ODP Site 1090) Bernhard Diekmann  , Gerhard Kuhn Alfred Wegener Institute for Polar and Marine Research, P.O. Box 12 01 61, 27515 Bremerhaven, Germany Received 14 August 2000; accepted 5 December 2001

Abstract One important goal of Leg 177 of the Ocean Drilling Program (ODP) was to explore the nature of the midPleistocene climate transition (MPT) on the southern hemisphere. A suitable MPT record was encountered at Site 1090 in the southeastern South Atlantic, where a 44-m-thick sequence of Quaternary diatom-bearing foraminiferal muds and oozes was recovered on the Agulhas Ridge. Environmental responses to the MPT comprised changes in terrestrial climate, biological productivity, and regional ocean circulation, as inferred from compositional sediment data and clay mineralogy. A shift towards more arid conditions occurred between 900 and 800 ka in southern Africa. Changes in palaeoceanography already started earlier. Since 1150 ka, northward displacements of the Polar Front appeared during glacial periods and shifted the area of dominant diatom deposition towards Site 1090. Likewise, glacial^interglacial contrasts in regional conveyor circulation strengthened after 1200 ka and became most severe after 650 ka. However, while changes in regional conveyor circulation likely responded in tune with global ice-volume changes and show the onset of 100-kyr cycles after 1200 ka, an unusual 130-kyr pattern characterises the pattern of frontal movements between 1200 ka and 650 ka, probably in response to imperfect adaptation of regional climate to the global 100-kyr climate cycles. < 2002 Elsevier Science B.V. All rights reserved. Keywords: South Atlantic; Pleistocene; Palaeoclimate; Antarctic Circumpolar Current; biological productivity; clay minerals

1. Introduction The climate system of the Quaternary is most signi¢cantly controlled by cyclic variations in the Earth’s orbital parameters, dictating quantitative insolation with mediate e¡ects on global ice vol-

* Corresponding author. Present address: Alfred Wegener Institute for Polar and Marine Research, Research Unit Potsdam, P.O. Box 60 01 49, 14401 Potsdam, Germany. Tel.: +49-331-288-2170; Fax: +49-331-288-2137. E-mail address: [email protected] (B. Diekmann).

ume and environment (Hays et al., 1976; Imbrie et al., 1992). During the mid-Pleistocene climate transition (MPT) climate-induced responses of environmental change switched from a dominant 41-kyr cyclicity of obliquity towards a strong 100-kyr cyclicity of eccentricity, although orbital forcing of insolation did not change fundamentally (Pisias and Moore, 1981; Ruddiman et al., 1989; Imbrie et al., 1993; Berger et al., 1994). During the MPT, glacial^interglacial contrasts became more severe and the 100-kyr climate cycles developed their typical asymmetric pattern of the late Quaternary, with long phases of climate cool-

0031-0182 / 02 / $ ^ see front matter < 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 0 1 ) 0 0 4 9 8 - 9

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Fig. 1. Location of ODP Site 1090 and sediment core PS2489-2 in the southeastern South Atlantic. The schematic illustration of the modern surface circulation shows the £ow paths of the Benguela Current and the Agulhas Current, as well as the frontal system of the Antarctic Circumpolar Current (ACC), comprising the sub-Antarctic Zone (SAZ), the Polar Front Zone (PFZ), and the Antarctic Zone (AZ) (Peterson and Stramma, 1991). Distal eddies and ¢laments of the Agulhas Current retro£ection may reach as far as the core location (Lutjeharms, 1996). Sea £oor below 2000 m is gradually shaded in steps of 1000 m.

ing and progressive ice build-up that are terminated by stages of relatively rapid climate warming and ice decay. The timing of the MPT is documented by benthic foraminiferal N18 O records in marine sediments of the world oceans, which document a general increase in global ice volume and the onset of weak 100-kyr cycles between 1250 ka and 900 ka and the establishment of strong 100-kyr cycles since 650 ka (Ruddiman et al., 1989; Imbrie et al., 1993; Berger et al., 1994; Chen et al., 1995; Mudelsee and Schulz, 1997). The MPT was likely caused by readjustments of the climate system in response to long-term climate deterioration of the late Cainozoic. It resulted in the expansion of northern-hemisphere ice sheets in the Svalbard^Barents Sea that commenced around 1.6 Ma and was followed by a major pulse between 1.2 and 0.9 Ma (Berger and Jansen, 1994; ElverhLi et al., 1998; Solheim et al., 1998). Since then, di¡erent kinds of nonlinear subglacial ice dynamics (Berger and Jansen,

1994; Mudelsee and Schulz, 1997; Clark et al., 1999) and their positive feedback impacts on ocean conveyor circulation (Berger and Jansen, 1994; Denton, 2000) probably exerted a strong in£uence on climate variability and gave rise to the prominent 100-kyr glacial^interglacial oscillations. In contrast, other hypotheses invoke changes in atmospheric conditions, ocean and air temperatures, and sea-ice coverage to have led to and triggered 100-kyr ice-volume £uctuations (Petit et al., 1999; Shackleton, 2000; Gildor and Tziperman, 2000). Thus, sea-ice coverage in high latitudes in£uences air temperatures and atmospheric moisture supply as an important prerequisite for ice build-up (Gildor and Tziperman, 2000). On the other hand, increased heat £ow from the tropics to higher latitudes around 1500 ka ago might have strengthened semiprecessional cycles in the northern hemisphere and induced sustained 100-kyr glacial^interglacial cycles (Rutherford and D’Hondt, 2000).

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One important goal of Leg 177 of the Ocean Drilling Program (ODP) was to explore the nature of the MPT on the southern hemisphere (Gersonde et al., 1999). At Site 1090, a 44-m-thick sequence of Quaternary diatom-bearing foraminiferal muds and oozes was recovered on the Agulhas Ridge in the southeastern South Atlantic (Fig. 1). They were deposited at average sedimentation rates between 1.2 and 3.1 cm kyr31 , providing the favoured temporal resolution for a MPT investigation (Shipboard Scienti¢c Party, 1999). The site is situated within the sub-Antarctic Zone of the Antarctic Circumpolar Current (ACC), bounded to the north by the Subtropical Gyre and the Benguela Current (Peterson and Stramma, 1991; Orsi et al., 1995). Moreover, it is in£uenced by the distal eddies and ¢laments of the Agulhas Current retro£ection introduced from the Indian Ocean around the Cape region of South Africa (Lutjeharms, 1996). The water depth (3702 m) places Site 1090 near the boundary between Circumpolar Deep Water (CPDW), which makes up the principal water mass of the ACC, and the North Atlantic Deep Water (NADW) tongue, which is injected into the ACC at intermediate water depths (2000^3800 m) (Reid, 1989). The regional current system thus is situated in the southern-hemispheric ‘junction box’ of global water masses (Keir, 1988). Moreover, it forms an integral part of global conveyor circulation, which drives interhemispheric heat exchange between the world oceans and which is assumed to in£uence global climate (Broecker and Denton, 1989; Berger and Wefer, 1996). Roughly, two opposite modes of operation can be de¢ned. The modern interglacial warm-route conveyor mode implies a far southward injection of relatively warm and saline NADW into the ACC, compensated to a large extent by the northward out£ow of warm surface and intermediate waters, which enter the South Atlantic via the Agulhas Current (Gordon et al., 1992). During glacial stages, the cold-route conveyor mode is realised, which is characterised by prevailing cold southern-source water masses with a diminished NADW in£ux (e.g. Charles and Fairbanks, 1992) in combination with an only sporadic in£uence of Agulhas Current leakage

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(e.g. Flores et al., 1999; Goldstein et al., 1999; Kuhn and Diekmann, 2002). Here we present a reconstruction of sub-Antarctic environmental change during the Quaternary on the basis of compositional data, which document southern-hemispheric responses to the MPT between 1200 and 650 ka. Discussed parameters comprise abundances and mass-accumulation rates (MAR) of biogenic opal, carbonate and Corg concentrations as indicators of changes in biological productivity and deep-water ventilation. Mineralogical parameters of terrigenous clay are used as tracers of water-mass con¢gurations and as indicators of climatic changes in the continental source areas.

2. Material and analytical procedures 2.1. Selected sediment cores, stratigraphy, and sample base Three holes (B, D, E) were drilled at Site 1090 (42‡54.8PS, 8‡54.0PE, 3702 m water depth) that yielded a spliced Quaternary section down to 44 metres composite depth (mcd) (Shipboard Scienti¢c Party, 1999). Because the uppermost part of the section seems to be disturbed, we connected the 1090 record with nearby sediment core PS2489-2 (42‡52.4PS, 8‡58.4PE, 3794 m water depth), taken during a pre-site survey with RV Polarstern (Gersonde, 1995). Both records show a good overlap in their benthic foraminiferal N18 O records for the interval between 560 and 340 ka (Becquey and Gersonde, 2002). For this study, we spliced the 1090 record at 12.40 mcd with the PS2489-3 record, corresponding to an age of 408 ka. The applied age models for sediment core PS2489-2 (Becquey and Gersonde, 2002) and Site 1090 (Venz and Hodell, 2002) are based on the correlation of their benthic foraminiferal N18 O records with the benthic foraminiferal N18 O record of ODP Site 607 in the North Atlantic (Raymo et al., 1990), using the the age scale of Mix et al. (1995). In the investigated Pleistocene section of Site 1090, a small hiatus is present in marine isotope stage 48 at 40.37 mcd, spanning a 18-kyr

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time interval between 1476 ka and 1458 ka (Venz and Hodell, 2002). Samples were taken at 10-cm intervals, representing ^ with a few exceptions ^ time steps between 5 and 15 kyr for the interval between 1800 ka and 1200 ka, and time steps between 1.5 and 6.0 kyr for the younger interval 6 1200 ka. These sample spacings are su⁄cient to resolve Milankovitch cycles in the 41-kyr31 and 100-kyr31 frequency domains with con¢dence. We present the original data at age scale together with the benthic foraminiferal N18 O curve (Becquey and Gersonde, 2002), as a reference for glacial^interglacial £uctuations in global ice volume. Illustrated time series of MARs and the data sets for time-series analyses were resampled by linear integration at equal time increments of 5 kyr, representing the average temporal resolution of the samples. Time-series analyses included Gaussian ¢ltering and spectral analyses after the Blackman^Tuckey method, applying the AnalySeries software (Paillard et al., 1996). Sample speci¢cations and data lists can be extracted from the PANGAEA data information system (www. pangaea.de) 2.2. Measured sediment parameters Bulk sediment composition was analysed on freeze-dried and ground sub-samples. A Leco CS 125 device was used to determine organic carbon with a relative precision of S 3%. Bulk carbon was measured with a Leco CNS 200 device, yielding a relative precision of S 1%. The percentage of carbonate was calculated from the di¡erence between percentage bulk carbon and percentage organic carbon, multiplied by 8.33. The carbonate fraction consists of biogenic remains of calcareous foraminifers and nannofossils (Shipboard Scienti¢c Party, 1999). For the determination of opal contents, we applied the Automated Leaching Method with a relative analytical precision of 4^10% (Mu«ller and Schneider, 1993). Opaline sediment constituents mainly comprise diatoms with small amounts of radiolarians and accessory volcanogenic glass shards (Shipboard Scienti¢c Party, 1999). Therefore, we consider opal proportions to represent the biosiliceous sediment frac-

tion. The proportion of non-opaline and non-calcareous constituents is regarded as the lithogenic siliciclastic sediment fraction. MARs (in g cm32 kyr31 ) of biogenic carbonate (MARCarbo ), biogenic opal (MAROpal ), lithogenic matter (MARLitho ), and Corg (MARC org ) were calculated by multiplying linear sedimentation rates with values of dry-bulk density and the proportions of the respective sediment fractions. Drybulk densities were inferred from calculated grain densities and continuously measured wet-bulk densities by Q-ray attenuation with a multisensor track (Shipboard Scienti¢c Party, 1999). The calculation of grain densities is based on the good correlation between carbonate concentrations and grain densities in shipboard samples, which were analysed with laboratory methods in the course of physical-property investigations (Shipboard Scienti¢c Party, 1999). Also the inferred dry-bulk densities of the sample set used for this study agree well with shipboard measurements of dry-bulk density. We refer to MARs in a broader sense, taking into account the combined e¡ects of sediment £uxes and the rates of deposition, preservation and burial. Mineralogical analyses were carried out by means of X-ray di¡raction measurements on glycolated preferentially oriented clay mounts, following techniques explained in detail elsewhere (Ehrmann et al., 1992; Petschick et al., 1996). Mineral proportions were calculated semiquantitatively from weighted peak areas recorded in the W /10X-ray di¡ractograms (Biscaye, 1965). The 5-A W A peak intensity ratio of illite is a measure of illite chemistry, with low values 6 0.75 representative of Fe^Mg-rich illites (biotitic illite) and those values s 0.40 indicative of the presence of Al-rich illites (muscovite, sericite) (Esquevin, 1969). Relative analytical precision for major clay components is 6^9%, and 8^14% for minor clay components (Ehrmann et al., 1992).

3. Results 3.1. Bulk sediment parameters Bulk sediment composition shows cyclic varia-

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Fig. 2. Time series of the benthic foraminiferal N18 O record (Becquey and Gersonde, 2002) and bulk sediment parameters. Grey stripes with numbers to the right indicate glacial marine isotope stages (MIS).

tions related to the alternation of glacial and interglacial stages, well displayed by the benthic foraminiferal N18 O record. The relative abundances of biogenic and lithogenic sediment constituents reveal a strong dependence on carbonate concentrations. Interglacial intervals show maximum carbonate proportions mirrored by low proportions of lithogenic matter, while a reverse pattern characterises glacial intervals (Fig. 2). Since 1150 ka, glacial intervals beside high proportions of lithogenic matter show increased concentrations of Corg and opal peaks after 920 ka. Moreover, a signi¢cant change in average sedimentation rates with low values of 1.2 cm kyr31 prior to 1150 ka and elevated values of 3.1 cm

kyr31 after 1150 ka characterises the mid-Pleistocene. The increase in sedimentation rates implies increased MARs of all sediment constituents, particularly of carbonate (Fig. 3). MARs of the various sediment components covary with high-amplitude £uctuations in linear sedimentation rates, and MARC org values are strongly increased in glacial intervals of the last 1150 kyr (Fig. 3). Between 1150 ka and 340 ka, glacial stages also show increased MAROpal values. High Corg concentrations and MARC org values in glacial intervals are also displayed by low carbonate/Corg , opal/Corg and litho/Corg ratios (Fig. 4). High opal/litho ratios without distinct glacial^interglacial contrasts occur between 1200 and 400 ka and document an episode of relatively increased opal

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Fig. 3. Time series of benthic foraminiferal N18 O record (Becquey and Gersonde, 2002). Sedimentation rates and MARs of major sediment constituents are integrated over 5-kyr intervals. Grey stripes with numbers to the right indicate glacial marine isotope stages (MIS).

deposition or preservation in respect to lithogenic matter (Fig. 4).

illite chemistry, indicates a relative increase of Fe^Mg-illite after 900 ka.

3.2. Clay mineralogy

3.3. Time-series analyses

The clay-mineral spectrum is dominated by smectite (30^55%) and illite (35^50%) with minor kaolinite (5^15%) and chlorite (5^15%), and exhibits high Qz/Fsp ratios (2.0^4.0) (Fig. 5). Although the relative abundances of individual clay minerals exhibit low variability through the section, Qz/Fsp ratios and Kao/Chl ratios display marked glacial^interglacial £uctuations, with lower values in glacial intervals. Moreover, the W /10-A W peak intensity ratio, as a measure of 5-A

Time-series analyses were conducted on ¢ve sediment parameters exhibiting pronounced glacial^interglacial cyclicity, in order to deduce their frequency domains through time (Figs. 6 and 7). The data sets comprise the benthic foraminiferal N18 O record, carbonate and Corg concentrations as well as Qz/Fsp ratios and Kao/Chl ratios. The power spectra and Gaussian ¢ltering of the ¢ve parameters each yield a low-amplitude 41-kyr periodicity throughout the Quaternary. Stronger 41-

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Fig. 4. Time series of benthic foraminiferal N18 O record (Becquey and Gersonde, 2002) and ratios of major sediment constituents. Grey stripes with numbers to the right indicate glacial marine isotope stages (MIS).

kyr power is only evident for the carbonate and Corg parameters between 1000 and 800 ka. Between 1200 and 650 ka the carbonate and Corg parameters show a strong and unusual 130-kyr periodicity, while low-frequency variations of the benthic foraminiferal N18 O record as well as Qz/Fsp and Kao/Chl ratios are centred around a 100-kyr periodicity with moderate power. For the last 650 kyr, all parameters exhibit strong power in the 100-kyr band. Thus the clay-mineral ratios and the stable isotope parameters show consistent changes in their spectral characteristics through time, while the carbonate and Corg parameters are out of tune, particularly in the mid-Pleistocene interval between 1200 and 650 ka.

4. Discussion The sedimentary parameters and their temporal variability at Site 1090 point to di¡erent kinds of environmental change across the MPT. Since 1150 ka, northward movements of the ACC frontal system, which a¡ected biological productivity, sea-ice distribution, and deep-water ventilation, took place during glacial periods. Between 1200 ka and 650 ka, glacial^interglacial contrasts in regional conveyor circulation strengthened roughly in accordance with global ice-volume £uctuations. A one-time shift towards drier climate conditions occurred in southern Africa between 900 ka and 800 ka.

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Fig. 5. Time series of benthic foraminiferal N18 O record (Becquey and Gersonde, 2002) and mineralogical parameters of the lithogenic clay fraction. Grey stripes with numbers to the right indicate glacial marine isotope stages (MIS). Other abbreviations: Kao = kaolinite, Chl = chlorite, Qz = quartz, Fsp = feldspar, Sm = smectite, Ill = illite.

4.1. Depositional pattern of biogenic matter and shifts of the ACC frontal system The proportions and MARs of biogenic components in the Pleistocene section of the Site 1090 sedimentary record vary well within the characteristic glacial^interglacial value ranges of late Quaternary sediments in the sub-Antarctic Southern Ocean, which are mostly composed of calcareous oozes and muds with minor opal concentrations (Charles et al., 1991; Howard and Prell, 1994; Bareille et al., 1998; Frank et al., 2000). Biogenic sedimentation at Site 1090 was dominated by 41-kyr carbonate cycles during the early Pleistocene with high carbonate proportions in

interglacial stages and reduced carbonate concentrations and increased proportions of lithogenic matter in glacial sediments (Fig. 2). Along with a general increase in sedimentation rates (Fig. 3), a modi¢cation in the depositional mode of biogenic matter started with the glacial period of MIS 34 around 1150 ka, when carbonate cycles ¢rst switched to a 130-kyr periodicity and after 650 ka to a 100-kyr periodicity (Figs. 6 and 7). Since then, reduced carbonate concentrations and higher proportions of lithogenic matter in glacial intervals went along with increased opal proportions and 6^10-fold increases in Corg concentrations and MARC org values, particularly between 900 and 340 ka, while the interglacial values re-

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Fig. 6. Evolutionary power spectra of selected sediment parameters, estimated from Blackman^Tuckey spectral analyses on 400kyr intervals at steps of 100 kyr. Each 400-kyr time series included 81 data points (5-kyr increments) and was measured with a Bartlett window at a number of 24 lags (30% of series) with 80% con¢dence. Abbreviations: Kao = kaolinite, Chl = chlorite, Qz = quartz, Fsp = feldspar.

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Fig. 7. Cyclic variability and spectral amplitudes of selected sediment parameters, deduced from Gaussian ¢ltering at frequencies of 0.0100 S 0.0025 (100-kyr periodicity, thick black curves) and 0.0244 S 0.0025 kyr31 (41-kyr periodicity, thin grey curves). Abbreviations: Kao = kaolinite, Chl = chlorite, Qz = quartz, Fsp = feldspar.

sembled those of the whole preceding early Pleistocene (Figs. 2 and 3). The pattern of biogenic deposition since 1150 ka suggests repeated northward displacements of the ACC frontal system and associated shifts of the zone of prevalent diatom ooze accumulation during glacial periods, as also proposed by other authors who investigated late Quaternary burial rates of biogenic sediment constituents in the Southern Ocean (Charles et al., 1991; Kumar et al., 1995; Bareille et al., 1998; Frank et al., 2000). In this context, the position of the Polar Front plays an important role, because it forms an ecological and physical water-mass boundary. Today, it separates warmer and saltier surface waters to the north dominated by calcareous plankton from cold and silicate/nutrient-rich waters to the south, where diatoms represent important phytoplankton producers (Burckle and Cirilli, 1987; Nelson et al., 1995; Smetacek, 1999). Farther south, the extension of seasonal Antarctic sea ice limits primary biological productivity through the

availability of light (Burckle and Cirilli, 1987; Gersonde and Zielinski, 2000). The sea £oor between the Polar Front and the seasonal sea-ice limit actually is occupied by a high-accumulation belt of diatomaceous oozes enriched in organic carbon (Burckle and Cirilli, 1987; Nelson et al., 1995; Kumar et al., 1995). Since primary biological productivity is not unusually high in the modern Southern Ocean, the manifestation of this circum-Antarctic opal belt does not solely re£ect the quantity of primary biological productivity, but also documents the high preservation e⁄ciency of biogenic opal in that zone (Nelson et al., 1995; Schlu«ter et al., 1998). The postulated northward shifts of dominant biosiliceous deposition in response to frontal movements is mostly consistent with micropalaeontological reconstructions of palaeo-summer sea-surface temperatures from planktonic foraminifers (Becquey and Gersonde, 2002). Accordingly, the early Pleistocene was characterised by generally low temperatures with less developed

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glacial^interglacial contrasts, which increased after 900 ka. The temperature record moreover suggests extremely cold phases during the glacial stages between 870 Ka and 430 Ka (MIS 24 to MIS 12), which correspond to maximum opal and Corg concentrations revealed by our data and thus document strong advances of the Polar Front close to Site 1090. The reconstruction of frontal movements, however, may su¡er from the assumption that environmental conditions separated by the fronts remain constant through time. Furthermore, the frontal positions are also controlled by other factors like topography and the con¢gurations of wind belts (Moore et al., 2000). Beside frontal con¢gurations, atmospheric circulation represents another important boundary condition for biological productivity, as it determines the intensity of down- and upwelling processes along open-ocean fronts that control the availability of nutrients and other dissolved trace elements (Yoder et al., 1994; Archer and Johnson, 2000). Clues for a wind-driven invigoration of ACC circulation during the middle Pleistocene actually arise from erosional features on Meteor Rise, which is situated south of Site 1090 (Westall and Fenner, 1991). It can only be speculated whether the latter process changed the mode of biological productivity during glacial periods. However, we may assume that increased sedimentation rates at Site 1090 after 1150 ka indeed re£ect ACC invigoration, which caused stronger lateral sediment transport and led to sediment winnowing on topographic highs and augmented sediment focusing in the deeper ocean. Such a topographically in£uenced pattern of sediment redistribution is not unlikely, because it is well documented for late Quaternary sediment sequences of the southeastern South Atlantic by 230 Thex data (Frank et al., 2000). Another aspect of enhanced atmospheric circulation implicates a stronger supply of particulate iron that contributes to the fertilisation of surface waters (Martin, 1990; Kumar et al., 1995; Moore et al., 2000). From our data, however, we are not able to infer contributions of aeolian dust, because the terrigenous sediment record mainly monitors detrital particle supply through water-mass advection, as discussed further below.

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Under the assumption that changes in biological productivity were mostly triggered by frontal movements, the unusual spectral pattern of Corg variations with marked 130-kyr cyclicity between 1200 and 650 ka re£ects an imperfect adaptation of the ACC environment to the upcoming 100-kyr climate cycles, which are already indicated by the Site 1090 benthic foraminiferal N18 O record (Figs. 6 and 7). Thus, frontal movements appeared out of tune with changes in global (mainly northernhemispheric) ice volume and may point to individual regional climate dynamics. The close relationship between the postulated frontal movements and variations in sea-ice coverage and sea-surface temperatures at Site 1090 (Becquey and Gersonde, 2002) suggests a link between sub-Antarctic air temperatures and ACC surface circulation rather than an interhemispheric teleconnection. For the late Quaternary, a leading role of the southern hemisphere in terms of climate change has been revealed by Vostok ice-core data and Southern Ocean marine proxy records, which indicate that variations in regional air and sea-surface temperatures generally precede £uctuations in global ice volume (Charles et al., 1996; Blunier et al., 1998; Kim et al., 1998; Brathauer and Abelmann, 1999). From the temporal resolution of our data, we are not able to decipher such phase relationships for the mid-Pleistocene and can only highlight the divergent spectral features of the Site 1090 proxy records. 4.2. Carbonate cycles and deep-water ventilation Glacial^interglacial carbonate cycles at Site 1090 may have been caused by mutual dilution e¡ects of biogenic and lithogenic sediment components, by changes in the mode of biological productivity, and by dissolution e¡ects both in the water column and within the sediment. Dilution of biogenic carbonate by lithogenic mud during glacial periods is very likely, because terrestrial erosion and detrital sediment supply by glacial ice and wind were enhanced during cold climate stages. Moreover, cold climates in combination with low stands of sea level facilitated glacigenic and £uvial sediment supply beyond the shelf edges and sediment gravity transport to-

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wards the deep sea (see discussion in Diekmann et al., 1999). Another minor dilution e¡ect arises from the higher supply of opal and organic carbon during glacial periods after 1150 ka (Figs. 2, 3) as pointed out in Section 4.1. Since lateral sediment focusing and winnowing is evident, it is di⁄cult to decipher mutual dilution e¡ects from sediment composition and MARs with con¢dence. Likewise, quantitative changes in carbonate export production from the surface waters can hardly be inferred, because they are overprinted by lateral sediment transport. However, clues for augmented carbonate export production during interglacial periods arise from an extensive study of late Quaternary carbonate cycles throughout the Southern Ocean (Howard and Prell, 1994). A water depth of s 3500 m places Site 1090 near the regional lysocline, where the settling of calcareous particles is a¡ected by dissolution processes (Howard and Prell, 1994). From a traditional view, variations of carbonate preservation in the deep South Atlantic and Southern Ocean re£ect the distribution of corrosive CPDW with respect to the less corrosive NADW that controls the position of the lysocline (Volat et al., 1980; Howard and Prell, 1994; Schmieder et al., 2000). Thus, enhanced carbonate dissolution takes place, while CPDW expands in response to the shallowing and northward retreat of NADW during glacial periods. Indeed, carbonate £uctuations at Site 1090 roughly match the composite pattern of carbonate preservation in the South Atlantic (Schmieder et al., 2000), although in detail some discrepancies exist. For instance, the Site 1090 carbonate record does not document less developed carbonate preservation due to a general weakening of NADW in£uence (stronger dissolution) during both glacial and interglacial stages between 900 and 650 ka, as suggested by the South Atlantic carbonate pattern. Rather, the Site 1090 record illustrates improved carbonate preservation between 1100 and 400 ka, particularly during interglacial stages and less pronounced during glacial stages (Fig. 2). This contradiction might be explained, when considering generally increased sedimentation rates during this interval that might have promoted survivabil-

ity of calcareous particles at Site 1090. In turn, an extreme dissolution event around 900 ka (MIS 24) and changes in the periodicity of carbonate cycles during the mid-Pleistocene are nearly conform with the South Atlantic carbonate record. Though marked by 125-kyr cycles at Site 1090, the general change in the periodicity of carbonate cycles around 1200 ka away from dominant 40-kyr cycles is comparable with the onset of nearly reversed 100-kyr carbonate-preservation cycles in the equatorial Paci¢c after 1200 ka (Weber and Pisias, 1999), with carbonate maxima during glacial-to-interglacial transitions and carbonate minima during interglacial-to-glacial transitions (Berger, 1970; Farell and Prell, 1991; Weber and Pisias, 1999). The reversed spatial and temporal carbonate patterns between the Paci¢c and the South Atlantic/Southern Ocean support causal linkages to changes in thermohaline deep-water circulation and their related impacts on interbasinal fractionation processes in carbonate chemistry (Berger, 1970; Farell and Prell, 1991). However, ACC deep-water chemistry depends not simply on the mixing ratio of NADW and CPDW, but also on other oceanographic processes that a¡ect the deep-water CO2 budget. For Site 1090, this holds particularly true for the time after 1150 ka. According to the polar alkalinity model, increases in alkalinity and CO2 concentrations make CPDW more corrosive during glacial periods (Broecker and Peng, 1989). Variations in deep-water chemistry thus can be achieved by both modi¢cations of biological export production (lower carbonate/Corg rain rates) that changes the nutrient inventory and the CO2 gradient between surface and deep water masses as well as the associated reduction in the admixture of NADW. Although proxy records on the variability of palaeo-nutrient distributions and biological productivity show inconsistent results, concerning the role biological productivity plays in the ACC region (see review in Anderson et al., 1998), the Site 1090 data document increased export production of organic carbon during glacial periods after 1150 ka that might have a¡ected deep-water chemistry. A new physical model proposes that reduced

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deep-water ventilation during glacial times due to stronger density gradients between NADW and underlying water masses may have changed CPDW alkalinity independently of biological processes (Toggweiler, 1999). An alternative view relates reduced deep-water ventilation during glacial times to an extended year-round Antarctic sea-ice coverage and increased vertical strati¢cation of surface waters (Stephens and Keeling, 2000). In fact, the Site 1090 compositional data are in support of reduced deep-water ventilation due to seaice extension. Evidence for sluggish abyssal circulation arises from the high organic carbon concentrations and the concomitant decreases in carbonate/Corg , opal/Corg , and litho/Corg ratios in glacial intervals (Fig. 4). These features strongly suggest improved preservation of organic matter due to suboxic conditions in abyssal waters and sediment pore £uids, as also revealed by similar compositional variations of glacial^interglacial sediments in the Indian Ocean ACC sector (Bareille et al., 1998). Moreover, the postulated northward movements of the Polar Front during glacial periods after 1150 ka, as indicated by the opal data (Fig. 2), were actually associated with very cold seasurface temperatures and a wider extension of Antarctic sea ice (Becquey and Gersonde, 2002). These ¢ndings underline the potential in£uence of variable sea-ice coverage on deep-water ventilation and carbonate preservation beside the e¡ect of variable NADW injection on the expansion of corrosive CPDW. In summary, the palaeoclimatic signal of the carbonate parameter is di⁄cult to assess, because it depends on a variety of environmental boundary conditions. 4.3. Terrigenous clay minerals and regional climate and ocean circulation The Site 1090 clay-mineral assemblage with high portions of relatively Fe^Mg-rich illite (Fig. 5) is representative for the Agulhas Ridge and the Cape Basin (Petschick et al., 1996; Diekmann et al., 1996; Kuhn and Diekmann, 2002). Most illite in marine sediments around southern Africa originates from highly illite-bearing soils and dusts of southern Africa supplied to the ocean by the southeastern trade winds and to a lesser extent

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through £uvial input (Chester et al., 1972; Kolla et al., 1976; Bremner and Willis, 1993; Gingele, 1996; Petschick et al., 1996). Long-distance transport of this detritus to the study area, however, is mainly achieved by ocean currents (Petschick et al., 1996). Since Site 1090 lies in the reaches of the southern-hemispheric westerlies, a direct in£uence of the southern trade winds is not realised (Lindau, 2001). The atmospheric boundary conditions, however, do not virtually rule out that vagabonding dust particles from southern Africa occasionally reach Site 1090. This holds particularly true when considering that under distinct meteorological conditions dusts from Morocco are dispersed as far as southern England in the northern hemisphere (Pitty, 1968), although ^ in analogy to the study area ^ the con¢guration of the trade-wind belt and west-wind system normally argues against such a dust trajectory. The presence of iron-bearing illite, whether delivered by winds or ocean currents, actually is indicative of arid sources with prevailing physical weathering. Since the clay-mineral distribution lacks profound long-term modi¢cations through the studied Pleistocene sequence, the terrestrial sources of clay minerals apparently did not change fundamentally through time. So the change in illite chemistry towards more iron-rich varieties between 900 and 800 ka (Fig. 5) can be explained by the climatic e¡ect of long-term aridi¢cation on the terrestrial weathering regime, permitting a better persistence of unstable ironbearing clay minerals susceptible to chemical attack. This ¢nding agrees with geomorphological and sedimentological land records that point to the desiccation of southern Africa through the Quaternary (Lancaster, 1984). In eastern Africa, £oral populations indicate an increased appearance of arid-adapted species already since 1000 ka (De Menocal, 1995). Other clay-mineral parameters underline the importance of regional ocean circulation in the dispersal of ¢ne-grained terrigenous sediment, as also observed at nearby Site 1089 in the southern Cape Basin (Kuhn and Diekmann, 2002). Fluctuations of Kao/Chl ratios and Qz/Fsp ratios with high values during interglacial periods (Fig. 5) sensitively monitor the operation of the warm-

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route conveyor mode that facilitates the southward propagation of kaolinite- and quartz-rich suspensions, entrained in NADW and in the eddies of the Agulhas Retro£ection (Diekmann et al., 1996; Kuhn and Diekmann, 2002). The suspensions originate from the £uvial discharge of distant tropical and subtropical African rivers, diluting the dominant and relatively proximal input of illite-rich material from southern Africa (Kolla et al., 1976; Petschick et al., 1996; Goldstein et al., 1999). During glacial periods, represented by low Kao/Chl ratios and Qz/Fsp ratios, the ACC region is mainly bathed by CPDW, which bears chlorite-rich and quartz-poor particulates, derived from Patagonia and the Antarctic Peninsula (Diekmann et al., 1996, 2000). At the same time, increased glacigenic sediment delivery added by minor aeolian contributions from the latter sources result in higher £uxes of these detrital components within the CPDW (Diekmann et al., 2000). As pointed out in Section 1, the southeastern South Atlantic represents the ‘junction box’ of major global water masses. Therefore, variations of the regional current pattern likely re£ect changes in global thermohaline conveyor circulation. Although we are not able to deduce the exact phase relationships, similar temporal variations and long-term modulations in the spectral characteristics of both the benthic foraminiferal N18 O record and the water-mass-relevant clay mineral ratios (Figs. 5^7) suggest an intimate relationship between changes in global ice volume and conveyor circulation. Particularly the onset of 100-kyr cycles and strengthening of glacial^interglacial contrasts across the MPT is nicely displayed by both parameters and opposes trends in ACC frontal movements, which are driven by regional climate change, as pointed out previously. In comparison, the clay-mineral results are in so far consistent with ¢ndings from other watermass proxies like the benthic foraminiferal N13 C record at Site 1090 (Venz and Hodell, 2002) that they indicate a wider propagation of CPDW and a diminished NADW injection during glacial periods throughout the Pleistocene. On a longer time scale, however, Pleistocene benthic forami-

niferal N13 C records consistently point to more suppressed glacial NADW £uxes since 1600^ 1500 ka in the North Atlantic (Raymo et al., 1990), in the equatorial Atlantic (Bickert et al., 1997), and in the Atlantic ACC sector (Hodell and Venz, 1992; Venz and Hodell, 2002). In contrast, the clay-mineral data at Site 1090 indicate reorganisations in thermohaline conveyor circulation to have started not before 1200 ka. However, just as the reliability of the clay-mineral parameter has its limitations, because of variable detrital in£uxes in the source areas (Diekmann et al., 1999), the benthic foraminiferal N13 C parameter may be a¡ected by changes in the rain rates of both marine and terrestrial organic carbon (Mackensen and Bickert, 1999; Lea, 1995; Raymo et al., 1997). Thus, both proxies may be biased to some degree. Nevertheless, the 1600-ka and 1200-ka datums, identi¢ed as turning points in conveyor circulation by the N13 C parameter (1600 ka) and the clay-mineral parameter (1200 ka), both correspond to stepwise increases in Arctic ice volume (ElverhLi et al., 1998; Solheim et al., 1998). The 1200-ka datum indeed is consistent with the onset of the last fundamental reorganisation in the glacial^interglacial modes of NADW production in the North Atlantic and Nordic Seas, which since then have been dominated by 100-kyr cycles that became most severe during the late Pleistocene (Henrich and Baumann, 1994; Henrich et al., in press). This timing and cyclicity agrees with our reconstruction of regional conveyor circulation in the southeastern South Atlantic and suggests a causal linkage to northern-hemispheric ice-sheet dynamics via the positive feedback loop of global thermohaline circulation.

5. Conclusions Sedimentary parameters and their temporal variability at ODP Site 1090 document environmental responses of the southeastern South Atlantic to the MPT that implied the following aspects. b Since 1150 ka, northward displacements of the Polar Front and the circum-Antarctic opal

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belt appeared during glacial periods and shifted the area of dominant diatom deposition towards Site 1090. These shifts were most pronounced during the glacial stages between 900 and 340 ka. At the same time, extended sea-ice coverage likely reduced deep-water ventilation and thus favoured the preservation of organic matter during glacial stages. An unusual 130-kyr cyclicity in biogenic deposition between 1200 ka and 650 ka appeared out of tune with £uctuations in global (mostly northern-hemispheric) ice volume and points to individual regional climate dynamics, at least during the mid-Pleistocene. b Glacial^interglacial contrasts in regional conveyor circulation strengthened after 1200 ka with a northward expansion of CPDW during glacial periods and the realisation of the warm-route conveyor mode during interglacial periods, implying frequent leakage of warm surface waters to the South Atlantic through the Agulhas Retro£ection that compensates the southward propagation of NADW. The changed pattern of glacial^interglacial conveyor circulation after 1200 ka appeared in tune with ice-volume £uctuations and was dominated by 100-kyr cycles that particularly became severe after 650 ka. b In the terrestrial realm, the MPT implicated a one-time shift towards more arid conditions in southern Africa, centred around 900^800 ka.

Acknowledgements This study was funded by the Deutsche Forschungsgemeinschaft through Grants Di-655/2 and Ku 683/6. We thank the organisers of the Ocean Drilling Program for inviting us to participate in Leg 177 and we appreciate the kind assistance of the Joides Resolution ship crew and scientists. For sample preparation and technical support we are indebted to Rita Froehlking, Silvia Janisch, Claudia Leng, Norbert Lensch, Walter Luttmer, Maren Thomas, Jutta Vernaleken, and numerous student trainees. The paper benefited from fruitful discussions with Sabine Becquey and Claus-Dieter Hillenbrand. Finally, we acknowledge the reviews and constructive recommendations of Steve Hovan and one anonymous colleague.

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