Past hydrographic and climatic changes in the Subantarctic Zone of the South Atlantic – The Pleistocene record from ODP Site 1090

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Abstract

Pleistocene summer sea-surface temperatures (SSST) have been reconstructed on a composite core section recovered in the Subantarctic Zone of the Southern Ocean from planktonic foraminifers applying the Modern Analog Technique. The composite consists of Core PS2489-2 and the sections recovered at ODP Site 1090, and documents the last 1.83 Ma. Three distinct climatic periods can be identified that mirror the Pleistocene development of the Southern Ocean hydrography. Cold climatic conditions prevailed at 43°S during glacial as well as during interglacial periods during the early Pleistocene (1.83–0.87 Ma), indicating a northward shift of isotherms that characterize the present-day Polar Front Zone by about 7° of latitude. Evidence shows a strong linkage between Southern Ocean and low latitude climate during that interval time. Between the Mid-Pleistocene Revolution (ca. 0.9 Ma) and the Mid-Brunhes Event (ca. 0.4 Ma), we observe higher amplitude fluctuations in the SSST between glacial and interglacial periods, corresponding to the temperature range between the present Polar Front and Subantarctic Front. These climatic variations have been related to changes in the northern hemisphere ice sheets. The past 0.4 Ma are characterized by strong SSST variations, of up to 8°C, between glacials and interglacials. Only during the climatic optima (stages 11.3, 9.3, 7.5, 7.1, 5.5, and the early Holocene), SSST exceeded present SSST at the core locality (10.2°C). Although the carbonate dissolution record exhibits high variability during the Pleistocene, it can be shown that SSST estimates were not significantly biased. The Mid-Brunhes dissolution cycle as well as the Mid-Pleistocene enhanced carbonate preservation appear to belong to a global long-term variability in carbonate preservation.

Introduction

One of the goals of Ocean Drilling Program (ODP) Leg 177 was to document the Quaternary paleoclimatic history of the Southern Ocean based on the reconstruction of the paleolatitudinal position of the Antarctic Circumpolar Current (ACC) and related oceanic frontal systems (Gersonde et al., 1999).

The Quaternary is characterized by two climatic transitions, the Mid-Pleistocene Revolution (MPR) and the Mid-Brunhes Event (MBE). The MPR has been documented to represent a complex global change leading to Late Pleistocene ice ages with increased mean ice volume and the shift of the 41-kyr dominated cycle to the 100-kyr dominated cycle in ice volume variations (Ruddiman et al., 1986b, Raymo et al., 1990, Berger et al., 1993a, Berger and Jansen, 1994). The major change of the climate system occurs around 920–900 ka and involves the buildup of unusually large amounts of glacial ice as recorded by increasing δ18O values (Berger and Jansen, 1994). Changes in the solid boundary conditions of the ocean–sea-ice system and particularly in mountain elevation are implicated (Ruddiman et al., 1986b; see for review Imbrie et al., 1993, Berger and Jansen, 1994). The second aspect of the MPR is the transition from the 41-kyr (obliquity) to 100-kyr (eccentricity) dominated oscillations. This transition has been reported to have either a rather progressive character, taking place approximately in the period between 1.2 and 0.6 Ma, as proposed by Ruddiman et al., 1989a, Ruddiman et al., 1989b, Berger et al., 1993a, Berger et al., 1993b, Imbrie et al., 1993 and Berger and Jansen (1994), or to occur abruptly (Pisias and Moore, 1981). Recent studies (Mudelsee and Schulz, 1997, Mudelsee and Stattegger, 1997) indicate that the full establishment of the 100-kyr dominated periodicity controlling climatic variability in the middle and late Pleistocene was rather more abrupt than progressive, and occurred around 640 ka. As a consequence the MPR-related change in 100-kyr amplitude significantly lags behind the change in the δ18O mean values (at about 920 kyr) by ∼280 kyr (Mudelsee and Schulz, 1997, Mudelsee and Stattegger, 1997). These authors speculate that the climate system ‘needs’ this delay to adapt itself after the initial disturbance (increased ice volume) occurring at 920 kyr, and is the result of the coincidental combination of insolation, ice mass and bedrock depression. Different theories have been proposed to explain the change in climate cyclicity at the MPR. According to the Milankovitch theory, orbital forcing is the primary cause of the cyclicity in ice volume (Ruddiman et al., 1986b). However, because there is only negligible 100-kyr power in the signal of insolation directly received on earth (Berger, 1984), insolation cannot explain the 100-kyr response that dominates δ18O signals and other records (Hays et al., 1976) and therefore cannot explain the MPR shift (Ruddiman et al., 1986b), unless resonant responses are invoked to amplify the 100-kyr solar insolation forcing such as ice–albedo feedbacks or ice–bedrock dynamics. It has been proposed that the inclination theory linked to the flux of meteoric dust would explain the 100-kyr cyclicity (Muller and MacDonald, 1997, Patterson and Farley, 1998, Patterson et al., 1999), however, by itself such flux does not necessarily imply a climatologic effect (see Berger, 1999). Despite the great interest of the climatic and frequency change during the MPR, the debate on the driving mechanism is still ongoing.

The second distinct Pleistocene climatic change, the MBE, occurred around 400 and 300 ka (Jansen et al., 1986). The MBE has been interpreted to represent a global event resulting from a decoupling of climate conditions in different latitudes. While equatorial and southern hemisphere regions experienced an intensification of interglacial climate conditions, conditions in the northern hemisphere became more glacial (Jansen et al., 1986). The cause of the event is unknown but has been attributed to both external forcing, such as variations in insolation (Jansen et al., 1986), and internal forcing (Pisias and Rea, 1988) of the climate system.

A better knowledge of the past hydrographic changes in the Southern Ocean surface frontal system is crucial for understanding the evolution of Pleistocene climate. In fact, the ACC plays an important role in global ocean circulation by serving as a pathway for inter-ocean exchange of water, thus it is important to establish its meridional extent during the past. The ACC is divided into three zones, defined by oceanographic frontal systems, the Polar Front (PF), the Subantarctic Front (SAF) and the Subtropical Front (STF) (Fig. 1). The STF, the most northerly frontal system, exhibits the strongest horizontal thermal and saline gradients, both at the sea surface and at depth. By definition, the STF is the hydrographic boundary between subtropical and subpolar waters (Stramma and Peterson, 1990). Mean temperatures drop from 17.9 to 10.6°C and salinity drops from 35.5 to 34.3‰ (Lutjeharms and Valentine, 1984, Lutjeharms, 1985, Stramma and Peterson, 1990). North of the Agulhas Ridge (between 10°E and 20°E), temperature gradients between 17 and 13°C have been observed at the STF (Gersonde, 1995). On average, in the eastern portion of the Atlantic Sector the STF is located at around 41°S (Lutjeharms and Valentine, 1984, Lutjeharms, 1985, Stramma and Peterson, 1990). The characteristic temperature decrease at the sea surface for the SAF is about 4°C, from 9.0 to 5.1°C (Lutjeharms and Valentine, 1984) while the salinity drops from 33.89 to 34.11‰ (Allanson et al., 1981). The geographic position of the SAF generally lies at about 45°S (Lutjeharms, 1985, Peterson and Stramma, 1991). The PF occurs at the northernmost limit of the 2°C isotherm, which forms part of the subsurface temperature minimum. In the African sector, the average surface temperature range of the PF is 2.5–4.1°C (Lutjeharms and Valentine, 1984). The mean latitude based on the surface as well as subsurface expressions of the front is at about 50°S (Lutjeharms, 1985).

The signatures of the oceanographic fronts and their associated sea-surface temperatures (SST) are well delineated by planktonic foraminiferal faunas in surface sediments (Howard and Prell, 1984, Labracherie et al., 1989, Thiede et al., 1997, Weaver et al., 1997, Niebler and Gersonde, 1998) as well as by radiolarian and diatom assemblages (Pichon et al., 1992, Zielinski and Gersonde, 1997, Abelmann et al., 1999). Previous attempts of SST reconstructions were mainly focused on the past 300 ka (Prell et al., 1979, Morley, 1989, Howard and Prell, 1992, Labeyrie et al., 1996, Brathauer and Abelmann, 1999) and recently over the past 550 ka (Becquey and Gersonde, 2002a). Since the latitudinal fluctuations of the isotherms play a crucial role in the understanding of the development of the Southern Ocean surface water hydrography, the surface water biological productivity, the formation of intermediate and deep water masses and the stability of the Antarctic ice sheets, there is an urgent need to estimate the SSTs on longer time records.

The aim of this study is to reconstruct the summer SSTs (SSST) from planktonic foraminiferal assemblages using the Modern Analog Technique (MAT), to compare these estimates with carbonate dissolution as well as with ice-rafted debris (IRD) input, and to insert these results in a global context. We choose to reconstruct SSST because the microplankton flux, including foraminifers, is strongly focused on austral summer in southern high latitude areas, as documented by sediment trap experiments (Abelmann and Gersonde, 1991, Wefer et al., 1988, Wefer and Fischer, 1991, Donner and Wefer, 1994, Fischer et al., 2002). To reconstruct the quaternary SSST in the southern Atlantic Ocean, we used a composite sediment sequence consisting of Core PS2489-2 and sediment recovered at Site 1090 documenting the past 1.83 Myr.

Section snippets

Core material

ODP Leg 177 Site 1090 (42°54.8′S, 8°53.9′E, 3700 m) was drilled in the central portion of the Subantarctic Zone on the southern Agulhas Fracture Zone Ridge (Fig. 1), where present SSSTs are 10.2°C, at 10 m water depth (Levitus and Boyer, 1994). The upper 44 meters composite depth (mcd) at Site 1090 correspond to the Pleistocene and the sediments consist of alternating pale gray foraminifer nannofossil ooze and darker greenish gray mud- and diatom-bearing nannofossil ooze (Shipboard Scientific

Planktonic foraminifers

The distribution pattern of foraminiferal assemblages preserved in the studied 1.83-Ma record shows three distinct intervals (Fig. 2). Between 1.83 and 0.87 Ma, both glacial and interglacial assemblages are dominated by the cold-water species Neogloboquadrina pachyderma sin. This is followed between 0.87 and 0.43 Ma by assemblages that display stronger glacial/interglacial variability of the relative abundance of N. pachyderma sin. Although glacial planktonic assemblages are still dominated by

Validity of the foraminifera-derived SSST estimates

Considering that carbonate dissolution may have biased the foraminifera-derived SSST estimates we compare the SSST with the IRD and the planktonic oxygen isotope record (Venz and Hodell, 2002). Both records should provide additional information on the surface hydrology at the site location and represent useful tools for the validation of our SSST estimates. Although the occurrence of IRD cannot be explained solely in terms of the pattern surface water temperature because it is also controlled

Acknowledgments

We thank Bernhard Diekmann and Martina Kunz-Pirrung for fruitful discussions, and J.D. Ortiz and an anonymous reviewer for constructive comments for the final version of this paper. Isotope measurements were accomplished in the Alfred-Wegener Institute isotope laboratory under the helpful guidance of A. Mackensen. We are grateful to Ute Bock and Ruth Cordelair for technical support. This research was supported by the Deutsche Forschungsgesellschaft (DFG Grant Ge516/6). Abundance data of the

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