Applications of the Sortable Silt-Proxy on formerly glaciated continental shelves – a case study from subantarctic South Georgia

Johann.Klages [ at ]


Sedimentological grain size studies on marine sediments provide a unique insight into geological and paleoclimatically-driven processes. In pelagic and hemipelagic settings, one widely applied method is the measurement and calculation of the sortable silt mean grain size within the terrigenous sediment fraction, which has been proven to be a powerful proxy for the reconstruction of paleo-bottom current flow speeds. It has been often used in the Southern Ocean, recently in combination with XRF-scanning-derived natural logarithmic ratios of Zircon to Rubidium (or ln(Zr/Rb)), which exhibit a significant cor-relation to sortable silt mean sizes. The method, however, remains largely untested in coastal or shal-low-water environments, particularly in high latitudes with past glaciation or ice transported debris histories. In this thesis, a sediment core from a formerly glaciated subantarctic island shelf was used to test the applicability and limitations of the sortable silt size proxy. The selected gravity core PS119_5-1 from the South Georgian shelf comprises the last glacial to modern time interval, including the deglacial ice retreat from the shelf area and Holocene open water conditions. This position makes it a good archive to study potential flow speed changes in the Southern Antarctic Circumpolar Front (SACCF), as the latter’s position and strength likely influences the shelf circulation. Based on a set of 134 samples, sortable silt size values were calculated and compared to ln(Zr/Rb) ratios, grainsize distributions, and physical properties. The results confirm the established lithostratigraphic core subdivision into three facies and show that sortable silt shows reliable flow speed results in a formerly glaciated shelf envi-ronment. Apart from the lowermost, glacier-proximal diamicton facies, the proxy is applicable to the entire sedimentary core sequence for current speed reconstructions with calculated scalar flow speeds adequately reflecting Holocene current speeds. The latter show significant variations that correlate with climatic changes in the South Georgia glacier dynamics and climate. Comparison of the recon-structed shelf flow speed record with previously published data implies a potential connection to the Atlantic overturning circulation, SACCF changes and its underlying forcings through a combination of atmospheric circulation, tidal, solar and ENSO forcing.

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Thesis (Bachelor)
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Published online
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Ivanova, A. (2022): Applications of the Sortable Silt-Proxy on formerly glaciated continental shelves – a case study from subantarctic South Georgia , Bachelor thesis, University of Bremen.

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