Elsevier

Marine Micropaleontology

Volume 160, September 2020, 101894
Marine Micropaleontology

Research paper
Antarctic sea-ice and palaeoproductivity variation over the last 156,000 years in the Indian sector of Southern Ocean

https://doi.org/10.1016/j.marmicro.2020.101894Get rights and content

Highlights

  • New quantitative records for the Antarctic Indian Sector over the last 156,000 years.

  • Winter sea-ice limit north of 50°S, APF at ~46°S and SACCF at ~55°S during glacial stages.

  • Ocean thermodynamics governed sea-ice dynamic.

  • Winds and ocean currents drove zonal sea-ice amplitude changes.

  • Contrasted paleoproductivity patterns across the Antarctic Polar Front.

Abstract

Antarctic sea ice plays a vital role in global climate via its impact on ocean circulation, biological productivity and CO2 partitioning between the ocean and the atmosphere. However, very little is known about its past history, especially in the southwestern Indian sector of the Southern Ocean (SO). We here provide new quantitative records of winter sea-ice concentration (WSIC) and duration (WSID), sea-surface temperatures (SST) and productivity in sediment core SK 200/33 (55°S – 45°E) from the Permanently Open Ocean Zone over the last 156,000 years. The new records, combined with regional records, indicate that the hydrological structures migrated northward by a few degrees of latitude during all glacial periods with the Southern Antarctic Circumpolar Current Front reaching the core site, the Antarctic Polar Front located at ~46°S and the winter sea ice (WSI) probably extending to ~49°S. In contrast, hydrological fronts and WSI edge migrated poleward by a couple of degrees of latitude during the early Holocene and last interglacial. Comparison to SST and WSI records from different sectors of the SO suggests higher amplitude variations in WSI in the Atlantic sector as compared to the Indian and western Pacific sectors over the last glacial-interglacial cycle, which we attribute to the presence of the Weddell Gyre transporting far to the north the sea ice produced in the Weddell Sea. The new records also suggest a drop in productivity in the POOZ during glacial periods, probably related to greater WSI extent and reduced growing season.

Introduction

Antarctic sea ice is the most seasonal feature on Earth. Its annual cycle modulates the radiative balance of the Southern Ocean (SO) (Comiso, 2010), the heat-gas exchange between the ocean and the atmosphere (Delille et al., 2014; Ferrari et al., 2014), the global oceanic circulation (Abernathey et al., 2016) along with the SO productivity (Arrigo and Thomas, 2004). Of special interest is that all these processes affect the partitioning of carbon between the ocean and the atmosphere (Kohfeld and Chase, 2017). More precisely, Antarctic sea-ice expansion during glacial periods is thought to be the main process lowering atmospheric CO2 concentration via its direct insulation effect (Ferrari et al., 2014), its impact on the carbon storage in abyssal SO waters (Bouttes et al., 2011) and on the regional and distal productivity (Moore et al., 2000; Matsumoto, 2007; Jaccard et al., 2013). Hence, it is necessary to document the past changes in Antarctic sea-ice extent in all SO basins to better understand its interactions with climate drivers on a multi-millennial timescale.

Most reconstructions of past Antarctic sea-ice dynamic are either qualitative or restricted to the last 30,000–40,000 years (Burckle and Mortlock, 1998; Crosta et al., 1998a, Crosta et al., 1998b; Gersonde and Zielinski, 2000; Gersonde et al., 2005; Collins et al., 2012; Ferry et al., 2015; Xiao et al., 2016). Only few quantitative studies cover the last glacial-interglacial cycle (Kunz-Pirrung et al., 2002; Crosta et al., 2004; Esper and Gersonde, 2014a; Nair et al., 2019). These studies suggested that the winter sea-ice (WSI) limit was located 5–10 degrees of latitude northward during the last glacial stage relative to its modern position. Although still under debate, this northward shift probably resulted from a congruent shift in the Southern Hemisphere Westerlies Winds (SWW) and Antarctic Circumpolar Current (ACC) along with a drop in atmospheric and oceanic temperatures (Martinson, 2012; Kohfeld et al., 2013; Bostock et al., 2015; Nair et al., 2019). However, there are currently not enough studies to document potential regional differences and therefore robustly assess drivers of past sea-ice dynamic and its feedbacks on the climate system.

Here we used the narrow ecological preferences of diatoms to infer past oceanographic conditions such as sea-surface temperature (SST) and winter sea-ice concentration (WSIC) and duration (WSID), and compared these with palaeoproductivity records in the under-studied Indian sector of the SO over the past 156,000 years. Quantitative estimates of both SST and WSIC-WSID are provided through the Modern Analogue Technique (MAT) and are compared to similar records from the Atlantic and southwestern (SW) Pacific to evaluate whether sea-ice dynamic was comparable in each SO basin over the past ~156,000 years.

Section snippets

Study area and core details

Sediment core SK 200/33 was obtained on-board ORV Sagar Kanya in 2004 from 55°01′S-45°09′E, at a water depth of 4204 m. The core site is located in the SW Indian sector of the SO within the Permanently Open Ocean Zone (POOZ), sea-ice free region, where mean summer SST is ~2 °C which was derived from World Ocean Atlas 2013 (Locarnini et al., 2013) (Fig. 1). The present core site is bound by the Antarctic Polar Front (APF) to the north and Southern ACC Front (SACCF) and WSI limit to the south

Down-core variations in diatom assemblages

The sea-ice diatom group represents the second most abundant group in the core SK 200/33 with the relative abundances ranging from 0 to 8% over the last 156 ka (Fig. 3a). The highest occurrences (> 3%) of this group were observed during the glacial stages (MIS 2–4 and 6) and the lowest abundances (0–2%) were found during the interglacial periods (MIS 1 and 5). Similarly, the water stratification group was more present during the glacial stages, when abundances were 0–5% than in the interglacial

Latitudinal shifts in Antarctic winter sea-ice extent and hydrological fronts in the Indian sector

At present time, core SK 200/33 is located in the POOZ, north of the SACCF and WSI edge but 2–3° of latitude to the south of the southern APF (Fig. 1). The most recent diatom assemblages, dated from the early-mid Holocene, preserved in the core are strongly dominated by the POOZ diatom group (Fig. 3c), with few percent of SAZ diatoms (Fig. 3d), and the absence of sea-ice diatoms (Fig. 3a). These assemblages resulted in SST estimates of ~4 °C (Fig. 3g), which are ~2 °C higher than the modern

Conclusion

The diatom assemblages preserved in sediment core SK 200/33 allowed to estimate SST and WSI conditions in the POOZ of the SW Indian sector of the SO over the past 156 ka. The new records suggest SST of ~1–2 °C and WSID of ~2 months/year during each glacial period, with very little difference in between the mean values during each glacial period. The combination of the new data with published regional SST and WSI data suggests that all hydrological features migrated northward by a few degrees of

Author statement

Pooja Ghadi: Prepared diatom slides, performed diatom counts, graphs and schematic preparation, conceptualization, writing-original draft, reviewing and editing.

Abhilash Nair: Conceptualization, plotting graphs, writing-original draft, reviewing and editing, providing supporting data.

Xavier Crosta: Estimated the sea-surface temperature and sea ice extent records, revising and drafting the manuscript.

Rahul Mohan: Project administration, core collection, obtaining the radiocarbon dates for core

Declaration of Competing Interest

The authors declare no competing interests.

Acknowledgement

The authors wish to thank the Director, ESSO-NCPOR, Ministry of Earth Sciences (MoES), India, and Council of Scientific and Industrial Research (CSIR) for providing the funds for the research program at NCPOR. Pooja Ghadi thanks CSIR , New Delhi, India for the award of Junior Research Fellowship [Ref No. 18/12/2016(ii)EU-v]. We would also like to express our sincere gratitude toward the members of the pilot expedition to the Southern Ocean and Polar Micropaleontology and Past Climate Section,

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