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Given the pivotal role of Arctic sea ice in the global climate system, its excessive loss over the last decades is highly concerning. According to common climate models, an ice-free Arctic Ocean during summer may be reached by the middle of the century. While these profound changes are clearly visibly in the Arctic realm, the far-reaching consequences remain a matter of speculation. As the reliability of model simulations depend on their data base, the acquisition of comprehensive knowledge about past variation of Arctic sea ice cover is absolutely crucial. An ideal study area in this regard is the Fram Strait reflecting changes in the highly dynamic marginal ice zone of the Arctic Ocean. Therefore, the primary objective of this thesis was the reconstruction of sea ice variability in the Fram Strait throughout glacial-interglacial transitions of the Late Quaternary. For this purpose, analyses of molecular biomarkers were carried out on eight sediment cores along the western and northern Svalbard Shelf (PS93/006-1, PS92/039-2), the East Greenland Shelf (PS93/016-6, PS93/031-4) as well as the central Arctic Ocean (PS87/023-1, PS87/030-1, PS87/070-1, PS87/079-1). These records provided valid insights into the Arctic paleoenvironment since Marine Isotope Stage 11. In order to reconstruct past sea ice cover, the sea ice proxy IP25 was applied in combination with the related phytoplankton-IP25 sea ice index PIP25. Further specific biomarkers were used to reflect the sea surface temperatures (glycerol dialkyl glycerol tetraethers), the marine production (brassicasterol, dinosterol, tri-unsaturated highly-branched isoprenoids) and the input of terrigenous material (campesterol, β-sitosterol). The first study focused on the variation of sea ice at the eastern Yermak Plateau north of Svalbard (Core PS92/039-2) since MIS 6 (~160 ka). Here, the activity of the Svalbard Barents Sea Ice Sheet had a major impact on the sea ice cover. During intervals of ice sheet advance onto the

The Yermak Plateau is located north of Svalbard at the entrance to the Arctic Ocean, i.e. in an area highly sensitive to climate change. A multi proxy approach was carried out on Core PS92/039-2 to study glacial-interglacial environmental changes at the northern Barents Sea margin during the last 160 ka. The main emphasis was on the reconstruction of sea ice cover, based on the sea ice proxy IP25 and the related phytoplankton - sea ice index PIP25. Sea ice was present most of the time but showed significant temporal variability decisively affected by movements of the Svalbard Barents Sea Ice Sheet. For the first time, we prove the occurrence of seasonal sea ice at the eastern Yermak Plateau during glacial intervals, probably steered by a major northward advance of the ice sheet and the formation of a coastal polynya in front of it. Maximum accumulation of terrigenous organic carbon, IP25 and the phytoplankton biomarkers (brassicasterol, dinosterol, HBI III) can be correlated to distinct deglaciation events. More severe, but variable sea ice cover prevailed at the Yermak Plateau during interglacials. The general proximity to the sea ice margin is further indicated by biomarker (GDGT) - based sea surface temperatures below 2.5 °C.

Over the last about four decades, coinciding with global warming and atmospheric CO2 increase, the extent and thickness of Arctic sea ice has decreased dramatically, a decrease much more rapid than predicted by climate models. The driving forces of this change are still not fully understood. In this context, detailed paleoclimatic records going back beyond the timescale of direct observations, i.e., high-resolution Holocene records but also records representing more distant warm periods, may help to to distinguish and quantify more precisely the natural and anthropogenic greenhouse gas forcing of global climate change and related sea ice decrease. Here, we concentrate on sea ice biomarker records representing the penultimate glacial/last interglacial (MIS 6/MIS 5e) and the Holocene time intervals. Our proxy records are compared with climate model simulations using a coupled atmosphere-ocean general circulation model (AOGCM). Based on our data, polynya-type sea-ice conditions probably occurred off the major ice sheets along the northern Barents and East Siberian continental margins during late MIS 6. Furthermore, we demonstrate that even during MIS 5e, i.e., a time interval when the high latitudes have been significantly warmer than today, sea ice existed in the central Arctic Ocean during summer, whereas sea ice was significantly reduced along the Barents Sea continental margin influenced by Atlantic Water inflow. Assuming a closed Bering Strait (no Pacific Water inflow) during early MIS 5, model simulations point to a significantly reduced sea ice cover in the central Arctic Ocean, a scenario that is however not supported by the proxy record and thus seems to be less realistic. Our Holocene biomarker proxy records from the Chukchi Sea indicate that main factors controlling the millennial Holocene variability in sea ice are probably changes in surface water and heat flow from the Pacific into the Arctic Ocean as well as the long-term decrease in summer insolation. H

The objective of this thesis was the reconstruction of sea ice variability in the Fram Strait throughout glacial-interglacial transitions of the Late Quaternary. For this purpose, analyses of molecular biomarkers were carried out on eight sediment cores along the western and northern Svalbard Shelf (PS93/006-1, PS92/039-2), the East Greenland Shelf (PS93/016-6, PS93/031-4) as well as the central Arctic Ocean (PS87/023-1, PS87/030-1, PS87/070-1, PS87/079-1). These records provided valid insights into the Arctic paleoenvironment since Marine Isotope Stage 11. In order to reconstruct past sea ice cover, the sea ice proxy IP25 was applied in combination with the related phytoplankton-IP25 sea ice index PIP25. Further specific biomarkers were used to reflect the sea surface temperatures (glycerol dialkyl glycerol tetraethers), the marine production (brassicasterol, dinosterol, tri-unsaturated highly-branched isoprenoids) and the input of terrigenous material (campesterol, I -sitosterol).

The objective of this thesis was the reconstruction of sea ice variability in the Fram Strait throughout glacial-interglacial transitions of the Late Quaternary. For this purpose, analyses of molecular biomarkers were carried out on eight sediment cores along the western and northern Svalbard Shelf (PS93/006-1, PS92/039-2), the East Greenland Shelf (PS93/016-6, PS93/031-4) as well as the central Arctic Ocean (PS87/023-1, PS87/030-1, PS87/070-1, PS87/079-1). These records provided valid insights into the Arctic paleoenvironment since Marine Isotope Stage 11. In order to reconstruct past sea ice cover, the sea ice proxy IP25 was applied in combination with the related phytoplankton-IP25 sea ice index PIP25. Further specific biomarkers were used to reflect the sea surface temperatures (glycerol dialkyl glycerol tetraethers), the marine production (brassicasterol, dinosterol, tri-unsaturated highly-branched isoprenoids) and the input of terrigenous material (campesterol, I -sitosterol).

The objective of this thesis was the reconstruction of sea ice variability in the Fram Strait throughout glacial-interglacial transitions of the Late Quaternary. For this purpose, analyses of molecular biomarkers were carried out on eight sediment cores along the western and northern Svalbard Shelf (PS93/006-1, PS92/039-2), the East Greenland Shelf (PS93/016-6, PS93/031-4) as well as the central Arctic Ocean (PS87/023-1, PS87/030-1, PS87/070-1, PS87/079-1). These records provided valid insights into the Arctic paleoenvironment since Marine Isotope Stage 11. In order to reconstruct past sea ice cover, the sea ice proxy IP25 was applied in combination with the related phytoplankton-IP25 sea ice index PIP25. Further specific biomarkers were used to reflect the sea surface temperatures (glycerol dialkyl glycerol tetraethers), the marine production (brassicasterol, dinosterol, tri-unsaturated highly-branched isoprenoids) and the input of terrigenous material (campesterol, I -sitosterol).