Late Quaternary variability in sea ice cover, primary productivity and terrigenous input along the Transpolar Drift System, Arctic Ocean: A biomarker approach

Tanja.hoerner [ at ]


Arctic sea ice and river run-off contribute to the Transpolar Drift System of the Arctic Ocean and affect substantially Arctic and global climate. Given the recent environmental changes in the Arctic with drastic sea ice reductions and strongly increasing river run-off, paleoenvironmental investigations are essential to assume prospective impacts resulting from the current global warming. The ideal region to observe paleo-sea ice cover and river run-off within the Arctic are the Siberian marginal seas where enormous amounts of freshwater are drained onto the shelves and sea ice is produced. Therefore, the overall aim of this thesis is to examine how sea ice cover, river run-off and biological production (influenced by both parameters) varied on the Siberian shelves and if they might have affected the Transpolar Drift System during the Deglacial to Holocene. To achieve these objectives molecular biomarkers (sea ice biomarker IP25 ; the phytoplankton biomarker-IP25 (PIP25 index); the HBI-diene (highly-branched isoprenoids); brassicasterol and dinosterol as indicators for marine organic matter; campesterol and β -sitosterol as indicators for terrigenous organic matter) , were analyzed on five well-dated, continuous sediment cores from the Laptev Sea (Core PS51/154-11, Core PS51/159-10) and Kara Sea (Core BP99-04/7, Core BP00-07/7, Core BP00-36/4) shelves. These biomarker records revealed high-resolution sea ice, river run-off and biological production variability over the last ~17 and ~12 ka, respectively. During the Deglacial, when the sea level was low, a significantly different environment compared to the modern prevailed on the Laptev Sea shelf. The upper continental slope was mostly covered with permanent sea ice (probably landfast ice) between 17.2 and 16 ka. In addition, river run-off and biological production were extremely restricted. In the course of the Holocene transgression the proportion of the organic carbon sources changed on the Laptev Sea shelf reflected by a long-term decrease of terrigenous (riverine) organic matter and a synchronous increase of marine organic matter from ~16 ka until 7.5 ka. This evolution demonstrates the gradual establishment of fully marine conditions on the shelves, caused by the post-glacial transgression. In addition, strong environmental alterations during colder and warmer phases could be observed on the shelf during the post-glacial period. Cold phases with increased sea ice cover as well as decreased biological production and diminished river run-off occurred between 17.2 and 16 ka (Deglacial), 15.2 and 11 ka (including the Younger Dryas (12.9 - 11.6 ka) with maximum sea ice cover) as well as during the last 7 ka (late Holocene). Reduced sea ice cover as well as enhanced river run-off and more biological production could be observed between 15.2 and 12.9 ka (including the Bølling/Allerød warm period) and between 10 and 8 ka (the Holocene Thermal Maximum with minimum sea ice cover) indicative for warmer phases. Between 15.2 and 12.9 ka, the intensive warmer conditions are accompanied by prominent peaks of the DIP25 index (ratio of the HBI-diene and IP25 ). The late Holocene shows a distinct cooling trend but is further characterized by a conspicuous short-term variability of the sea ice indicators. In the central Kara Sea (Core BP00-36/4) maximum sea ice cover could be recorded between 12.4 and 11.8 ka coinciding with the Younger Dryas cold reversal. Afterwards, a climatic amelioration is documented by minimum sea ice conditions between 10 and 8 ka (Holocene Thermal Maximum). The sea ice records of Core BP00-07/7 from the southern Kara Sea reflect predominantly changes in the fast ice - polynya constellation. During the late Holocene expanding fast ice could be observed documenting the cooling trend. Additionally, conspicuous (multi-) centennial sea ice variations are notable during the last 6.5 ka, consistent with global paleo-climate reconstructions. A cyclic variability of the sea ice indicators (~400-, ~950-year and 1500-year cycles) was discovered, but a relation to Arctic Oscillation changes, proposed by various studies, could not be constantly identified in these records. The southernmost core from the Yenisei Estuary (Core BP99-04/7) recorded consistently seasonal sea ice cover since ~9.3 ka, apart from five short phases of permanent sea ice cover at ~7.3, 4.8, 4.4, 3.3 and 2 ka, documenting most likely fast ice expansion to the core site. The strong influence of river run-off as well as estuary processes might prevent the detection of (short-term) climatic signals at this study site. These studies revealed substantial environmental variations on the shelf, especially in the course of the Holocene transgression. Moreover, the environment seems to be more sensible when the shelves were flooded, since a conspicuous short-term sea ice variability appeared in both study areas during the last ~7 ka. These considerable alterations on the shelf as well as the short-term variability probably impacted the Transpolar Drift and further Arctic and global climate.

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Thesis (PhD)
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Hörner, T. (2016): Late Quaternary variability in sea ice cover, primary productivity and terrigenous input along the Transpolar Drift System, Arctic Ocean: A biomarker approach PhD thesis, University of Bremen.


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