Paleoceanographic dynamics of the Southern Indian Ocean reconstructed from geochemical and sedimentological proxies across the last glacial cycle

Amsler, Helen Eri (2020). Paleoceanographic dynamics of the Southern Indian Ocean reconstructed from geochemical and sedimentological proxies across the last glacial cycle (Unpublished). (Dissertation, University of Bern, Institute of Geological Sciences & Oeschger Centre for Climate Change Research)

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The Southern Ocean is believed to play an important role in Earth’s climate system as a region where a large part of the exchange between the deep ocean and the atmosphere occurs. Several processes have been identified to impact Southern Ocean dynamics and to contribute to the glacial-interglacial redistribution of CO2 between the ocean and atmosphere and therefore substantially influencing climate. The aim of this study is to further investigate the interplay of these processes during the last glacial cycle, focusing on the Indian sector of the Southern Ocean.

Our geochemical and sedimentological reconstructions based on five marine sediment cores retrieved from the Southern Indian Ocean, increase the spatial and temporal resolution of available paleoceanographic records, and thereby advance our understanding towards a more complete representation of Southern Ocean dynamics.

The studied cores span a meridional transect across the Subantarctic Zone (SAZ) to the Antarctic Zone (AZ) of the Southern Ocean. The results show consistently lower bottom water oxygenation during glacial compared to interglacial periods. We propose these oxygenation changes to be primarily controlled by ocean circulation dynamics and by a general reorganization of deep-water masses. The impact of local organic carbon respiration is only observed in the SAZ, as biogenic opal export increased as a result of iron fertilization.

The input of lithogenic material increased during glacial periods in both the SAZ and AZ, similar to existing records from the Southeast Atlantic and the Eastern Indian Ocean. This may have fuelled export production in the SAZ to some extent, but co-limitation by macronutrients inhibited a continued increase throughout MIS 2. We ascribe the lithogenic material to originate from multiple sources including southern America, southern Africa and more local volcanogenic sources, and possibly from Antarctica for the southernmost core.

Furthermore, changes in near-bottom water flow, reconstructed based on sortable silt records, imply a highly dynamic Antarctic Circumpolar Current (ACC) through time. The flow path is strongly directed by bathymetry. However, the ACC can overcome these structural constraints as fronts migrate, possibly impacting upwelling patterns.

The results of this dissertation highlight the importance of the Southern Indian Ocean as an active player in the evolution of the climate system. The paleoceanographic records generated as part of this thesis may contribute to build a more complete picture of the driving mechanisms in the Southern Ocean dynamics and improve predictions of future climate scenarios and adaptation studies.

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