Mid-Brunhes century-scale diatom sea surface temperature and sea ice records from the Atlantic sector of the Southern Ocean (ODP Leg 177, sites 1093, 1094 and core PS2089-2)

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Abstract

Diatom assemblages from ODP Leg 177 sites 1093, 1094 and core PS2089-2, from the present Antarctic sea ice free zone and close to the Polar Front, were analyzed in order to reconstruct the climate development around the Mid-Brunhes Event 400 000 yr ago, as reflected by summer sea surface temperature (SSST) and sea ice distribution. Dense sample spacing allows a mean temporal resolution during Marine Isotope Stage (MIS) 11 (423–362 ka) of 300–400 yr. SSST values were estimated from diatom assemblages using a transfer function technique. The distribution pattern of sea ice diatoms indicates that the present-day ice free Antarctic Zone was seasonally covered by sea ice during the cold MIS 12 and MIS 10. These glacial periods are characterized by sea ice fluctuations with a periodicity of 3 and 1.85 kyr, suggesting the occurrence of Dansgaard–Oeschger-style millennial-scale oscillations in the Atlantic sector of the Southern Ocean during the glacial stages MIS 12 and MIS 10. Termination V (MIS 12/11) is characterized by a distinct temperature increase of 4–6°C, intersected especially at the southern site 1094 and core PS2089-2 by two distinct cooling events reminiscent of the Younger Dryas, which are associated with a northward shift of the winter sea ice edge in the Antarctic Zone. The SSST record is characterized by distinct temperature intervals bounded by stepwise, rapid changes. Maximum temperatures were reached during Termination V and the early MIS 11, exceeding modern values by 2°C over a period of 8 kyr. This pattern indicates a very early response of the Southern Ocean to global climate on Milankovitch-driven climate variability. The SSST optimum is marked by millennial-scale temperature oscillations with an amplitude of ca. 1°C and periodicities of ca. 1.85 and 1.47 kyr, probably reflecting changes in the ocean circulation system. The SSSTs during the MIS 11 temperature optimum do not exceed values obtained from other interglacial optima such as the early periods of MIS 5 or MIS 1 from the Antarctic Zone. However, the total duration of the warmest period was distinctly longer than observed from other interglacials. The comparison of the South Atlantic climate record with a high-resolution record from ODP Leg 162, site 980 from the North Atlantic shows a strong conformity in the climate development during the studied time interval.

Introduction

The Mid-Brunhes Event (MBE) is a phase of global climatic change around 400 ka (ka=1000 years), which was forced by the orbital eccentricity cycle, according to Jansen et al. (1986) and Pisias and Rea (1988). Terrestrial and marine geological records indicate a trend towards more ‘glacial’ conditions in the northern hemisphere and more ‘interglacial’ conditions and higher sea surface temperature values in the southern hemisphere (Jansen et al., 1986). This is also documented from a subantarctic temperature record, which at Termination V shows a change from a period with glacial temperatures that display the coldest Pleistocene summer sea surface temperatures (SSSTs) to a time period characterized by glacial and interglacial temperatures that exceed those of the earlier Pleistocene period and are marked by strong glacial/interglacial variability at a 100-kyr-dominated cyclicity (Becquey and Gersonde, in press). However, there is still ongoing debate whether the rather negligible eccentricity power in the insolation signal can explain the 100-kyr response that dominates the middle and late Pleistocene ice volume record (Berger, 1999).

The MBE includes MIS 11 (423–362 ka, Imbrie et al., 1984), which represents one of the longest and warmest Pleistocene interglacials, as reported from marine and terrestrial records (Rousseau et al., 1992, Oppo et al., 1990, Howard and Prell, 1992, Howard, 1997). The largest sea level rise during the last 500 ka occurred from a MIS 12 low stand of 139±11 m below present sea level (Rohling et al., 1998) to the maximum MIS 11 high stand of up to 20 m above present sea level (see also Kaufman and Brigham-Grette, 1993, Bowen, 1999, Hearty et al., 1999, Kindler and Hearty, 2000). Based on a combined micropaleontological and geochemical study, Scherer et al. (1998) suggested that the West Antarctic Ice Sheet (WAIS) collapsed during the late Pleistocene, probably during MIS 11. Studies from Bermuda and the Bahamas also support the hypothesis of a partial collapse of the WAIS during the Mid-Brunhes (Hearty et al., 1999). However, the drawdown of the WAIS would only explain a sea level rise of 5–6 m. To produce a 20-m sea level rise during MIS 11, which according to Hodell et al. (2000) represents a value at the upper limit, also a deglaciation of the Greenland Ice Sheet (GIS) must be considered, and to, a smaller extent, also the ice volume of the East Antarctic Ice Sheet (EAIS) should be affected. Studies from the North Atlantic ODP site 982 indicate a major deglaciation of Greenland during MIS 11 (Stanton-Frazee et al., 1999).

Speculations that, in contrast to other interglacials, an unusual MIS 11 warming can be explained by higher atmospheric CO2, related to massive reef build-up (Droxler et al., 1997) or changes in the alkalinity of surface water masses (Broecker and Peng, 1989), are not confirmed by atmospheric CO2 reconstructions of Berger et al. (1996) or CO2 measurements from the Vostok ice core (Petit et al., 1999).

The reason why MIS 11 lasted longer than the following interglacials is up to now not sufficiently explained. The orbital forcing during MIS 11 was relatively weak due to low eccentricity (Imbrie et al., 1993). Nevertheless, an about 60-kyr-lasting interglacial (Imbrie et al., 1984) has developed, at a time when insolation fluctuation was minimal (Berger, 1978). This misfit is known as the ‘Stage 11 Problem’ (Imbrie and Imbrie, 1980, Imbrie et al., 1993, Berger, 1999). Muller and MacDonald (1997) proposed to explain the 100-kyr cyclicity mode of Pleistocene climate by the ‘orbital-inclination-dust model’, which is independent of insolation-related forcing and thus would resolve the ‘Stage 11 Problem’. However, this model is discussed controversial (Paul and Berger, 1999, Berger, 1999, Ridgwell et al., 1999).

Considering that MIS 11 represents a potential analogue for a future warmer climate because the changes in orbital parameters during MIS 11 were similar to those in the Holocene with respect to Milankovitch forcing (Berger and Loutre, 1991, Howard, 1997, Hodell et al., 2000) there is further strong need to document the establishment and the structure of MIS 11 in more details from different latitudes on land and marine records. To date, such information on century-scaled time scales is only available from the North Atlantic ODP Leg 162, site 980 (Oppo et al., 1998, McManus et al., 1999) and from the South Atlantic ODP Leg 177, site 1089 (Hodell et al., in press). Here we present an analogue with century-scale time resolution from the ODP Leg 177 records (sites 1094 and 1093) and core PS2089-2, recovered from the present northern sea ice free Antarctic Zone and close to the Polar Front in the Atlantic sector of the Southern Ocean (Fig. 1).

By means of a high-resolution diatom record from the Eastern South Atlantic the following questions will have to be answered:

(1) Is there evidence for an abrupt transition between the ‘cold’ and ‘warm’ period during the MBE?

(2) Was MIS 11 warmer than the Holocene although insolation conditions were similar?

(3) How stable were the climate conditions during the MIS 11 climate optimum?

(4) Is there any evidence in sediments of the Southern Ocean that the WAIS disappeared during the MBE?

Section snippets

Study area, preparation and counting

Site 1094 (53°10.8′S, 5°7.8′E) and piston core PS2089-2 (53°11.3′S, 5°19′E) are located south of the Polar Front (PF) in the southern part of the ice free Antarctic Zone (Shipboard Scientific Party, 1999a, Bathmann et al., 1992) (Fig. 1). Site 1094 is drilled in 2807 m of water depth in a small sedimentary basin northeast of Bouvet Island and core PS2089-2 was collected in the vicinity of site 1094 in a water depth of 2611 m. Site 1093 (49°58.58′S, 05°51.92′E, 3624 m water depth) is located

Stratigraphy

The basic middle and late Pleistocene stratigraphy, and proposed correlation between sites 1094 and 1093 was developed from established lithologic variations as reflected in multiple shipboard analyses (Shipboard Scientific Party, 1999a,b). These reveal in great detail the glacial and interglacial cycles of the late Pleistocene. Terminations are characterized by abrupt decreases of magnetic susceptibility and natural gamma radiation (NGR) and by increases in high gamma ray attenuation (GRA)

The winter sea ice record

The sea ice diatoms indicate that the present-day ice free Antarctic Zone and the area close to the PF was covered by winter sea ice during the cold MIS 12 (Fig. 7). Starting at around 432 ka the winter sea ice edge retreated southwards, and between 430 and 427 ka the core locations were no longer influenced by sea ice. In the upper part of Termination V, between ca. 426.5 and 425 ka, the sea ice indicators increase, indicating a northwards displacement of the winter sea ice edge to the

Environmental conditions during the middle Pleistocene glacials MIS 12 and MIS 10

MIS 12 is one of the most extreme glacials during the last 500 ka (Shackleton, 1987, Howard, 1997). The global ice volume during MIS 12 exceeded the ice volume during the Last Glacial Maximum by about 15% (Shackleton, 1987, Rohling et al., 1998). According to a sea surface temperature record presented by Becquey and Gersonde (2002) from the subantarctic site 1090, MIS 12 represents the latest glacial from a period starting at MIS 22 (870 ka) that is characterized by coldest Pleistocene

Summary and conclusions

Based on the diatom assemblage obtained at the Southern Ocean sites 1094, 1093 and from core PS2089-2 submillennial-scale SSSTs during the Mid-Brunhes period have been reconstructed using the IKM transfer function technique. The sea ice field was estimated based on the occurrence of diatom sea ice indicators. The final results and their interpretation can be summarized as follows:

(1) During the glacial MIS 12 and 10 the present-day ice free Antarctic Zone in the Atlantic sector of the Southern

Acknowledgments

We are grateful to H.-W. Hubberten, G. Kuhn and U. Zielinski, who provided unpublished data from core PS2089-2. The authors acknowledge the constructive reviews of J. McManus, R. Scherer and an anonymous reviewer. U. Bock and R. Cordelair technically supported this study. This research used samples provided by the Ocean Drilling Program (ODP). The ODP is sponsored by the U.S. National Science Foundation and participating countries under management of Joint Oceanographic Institutions, Inc.

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