Description: The recent dramatic decline of Arctic sea over the last decades and its controlling processes are still poorly understood. In order to distinguish between natural and anthropogenic processes controlling these changes in sea ice, we have to look back to the past beyond the times of direct measurements. For this purpose, we carried out a multi-proxy approach combining organic-geochemical data (bulk parameters: C/N, TOC, δ13Corg; biomarkers: IP25, sterols, GDGTs) with sedimentological data (core lithology, physical properties, IRD counting, XRF scanning) determined in sediments of Yermak Plateau Core PS92/039-2. This core is situated close to the modern summer ice edge and thus very sensitive for environmental changes. Based on magnetostratigraphy and correlations with dated sediment cores, this core represents the time span from MIS 6 to 1 (ca. 180,000 years) and allows the reconstruction of sea ice variability and related changes in oceanic circulation patterns and the Svalbard Barents Ice Sheet (SBIS) fluctuations during glacial/interglacial changes. As sea ice and phytoplankton biomarkers occur throughout the entire sedimentary section but show some strong variability, a more seasonal sea ice cover was probably predominant during the entire time interval, superimposed by a distinct short-term variability in extent. Significant fluctuations in most of our proxy records indicate highly variable sea ice conditions over the Yermak Plateau during MIS 6. Based on our biomarker data, the SBIS could not have reached the Yermak Plateau during MIS 6. During MIS 4 and 2, coevally elevated concentrations of the sea ice proxy IP25 and the biomarkers for phytoplankton productivity and terrigenous input point to a stationary ice margin above the core position at that time. Strengthened Atlantic Water inflow possibly coupled with katabatic winds from the protruding SBIS may have created this stable ice edge situation and the related sedimentary regime.

Description: One of the main characteristics of the Arctic Ocean is its seasonal to perennial sea-ice cover. Variations of sea-ice conditions affect the Earth’s albedo, primary production, rate of deep-water etc.. During the last decades, a drastic decrease in sea ice has been recorded, and the causes of which, i.e. natural vs. anthropogenic forcings, and their relevance within the global climate system, are subject of intense scientific and societal debate. In this context, records of past sea-ice conditions going beyond instrumental records are of major significance. These records may help to better understand the processes controlling natural sea-ice variability and to improve models for forecasts of future climatic conditions. During RV Polarstern Cruise PS92 in summer 2015, a 860 cm long sediment core (PS92/039-2) was re- covered from the eastern flank of Yermak Plateau north of the Svalbard archipelago (Peeken, 2015). Based on a preliminary age model, this sediment core probably represents the time interval from MIS 6 to MIS 1. This core, located close to the modern summer ice edge, has been selected for reconstruction of past Arctic sea-ice variability based on specific biomarkers. In this context, we have determined the ice-algae-derived sea-ice proxy IP25 (Belt et al., 2007), in combination with other biomarkers indicative for open-water conditions (cf., Müller et al., 2009, 2011). Furthermore, organic carbon fluxes were differentiated using specific biomarkers indicative for marine primary production (brassicasterol, dinosterol) and terrigenous input (campesterol, β-sitosterol). In this poster, preliminary results of our organic-geochemical and sedimentological investigations are presented. Distinct fluctuations of these biomarkers indicate several major, partly abrupt changes in sea-ice cover in the Yermak Plateau area during the late Quaternary. These changes are probably linked to changes in the inflow of Atlantic Water along the western coastline of Svalbard into the Arctic Ocean. Furthermore, the repetitive advance and retreat of the Svalbard Barents Sea Ice Sheet might have influenced the terrigenous input and the environmental setting north of Svalbard, as reflected in the sediment composition of Core PS92/039-2.

Description: IODP site U1417 in the Gulf of Alaska provides a continuous sedimentary record of environmental changes (e.g., in sea surface temperature (SST), marine productivity, ice-rafting) in the subpolar NE Pacific through the Plio-Pleistocene time interval. Here, we present a multi-proxy data set, which allows us to discriminate between different fertilization mechanisms that promoted primary productivity events in the study area between 1.5 Ma and 0.5 Ma. Based on biomarker, micropaleontological, XRF, and sedimentological data, we find that diatom growth benefited from iron-fertilization from aeolian dust, iceberg, and volcanic ash input. Glacial-interglacial SST fluctuations were superimposed by a slight cooling trend with a first pronounced temperature drop during MIS 38 and significantly lowered SSTs persisting through MIS 30 and MIS 28. While the diatom productivity pulses were mainly independent from SST changes, they coincide with terrigenous organic matter input and their occurrence during both glacial and interglacial periods suggest that iron supply from glacigenic dust was mainly controlled by local ice-sheet dynamics. This data set highlights the complexity of fertilization mechanisms in areas affected by evolving ice-sheets and their potential control on the biological carbon pump in subpolar ocean environments.

Description: Past ice sheet conditions in the southern Weddell Sea remain poorly known. Previous studies have led to contradicting scenarios of maximum ice extent during the Last Glacial Maximum (LGM). Scenario A is mainly based on terrestrial data indicating limited ice sheet thickening in the hinterland and suggests a LGM grounding-line position on the inner shelf. Scenario B is based on marine geological/-physical data and concludes that the grounding line was located on the outer shelf (~650 km further offshore than in scenario A). In addition, studies suggest a complex history of ice retreat and drainage pattern since the LGM that needs further constraint. We investigated hydroacoustic data acquired during 17 expeditions. A key finding is a previously unknown stacked grounding zone wedge (GZW) located in Filchner Trough on the outer shelf showing that a palaeo-ice stream stabilized at this position at least twice. Radiocarbon dates from sediment cores indicate that (i) the GZW was formed in the early Holocene and (ii) grounded ice did not extend seaward at the LGM. Hence, the grounding line in Filchner Trough experienced dynamic changes in the Holocene and ice sheet retreat after the LGM was not linear. Ice-flow switches in the hinterland possibly explain this behaviour. Further interesting findings are made in Brunt Basin suggesting the existence of cold-based ice or impacts of large icebergs. In addition, new data will be acquired in the area with RV Polarstern in Jan-Mar 2018.

Description: IODP Expedition 341 succeeded in recovering a continuous sedimentary record of Miocene to Late Pleistocene climate history at drill Site U1417 in the Gulf of Alaska, NE Pacific. Site U1417 sediments provide an excellent opportunity to reconstruct North Pacific sea surface conditions during late Neogene large-scale (global) climate transitions. The Mid Pleistocene Transition (MPT) - one of the most prominent intervals of global Quaternary climate change - is clearly identifiable in Site U1417 sediments (Jaeger et al., 2014). To fully exploit the environmental information archived in U1417 sediments, a sampling strategy has been pursued that permits direct correlation of different (independent) proxy data obtained from biomarker, micropalaeontological, sedimentological and geochemical (XRF) analyses. Mid Pleistocene SSTs in the Gulf of Alaska are in good agreement with SST reconstructions for the North Atlantic and the NW Pacific. A general cooling at about 1 Ma supports earlier hypotheses of an overall Northern Hemisphere ocean cooling as a prerequisite for the increase in continental ice volume. While phytoplankton productivity seems rather independent from SST at Site U1417, it is strongly related to elevated TAR values depicting enhanced input of terrestrial leaf-wax lipids (Meyers, 1997). The transport of these lipids is supposed to be effected by strong winds carrying dust from Alaskan loess deposits to the open ocean as well as by icebergs released from Alaskan tidewater glaciers. The latter is supported by the occasional coincidence of high IRD contents and TAR values. The close relationship between the TAR record, Ba/Al values and the abundance of diatoms, however, strengthens that together with the leaf-wax lipids also iron-bearing dust was exported leading to high productivity events at Site U1417 throughout the Mid Pleistocene. The distinct "on-off" pattern in diatom productivity evolved with the onset of the MPT, which suggests that the expansion of the Northwest Cordilleran Ice Sheet lead to an effective production of glacigenic iron-rich dust that was exported i) by strong northwesterly winds and ii) by icebergs. The observation that productivity peaks in the Gulf of Alaska are not confined to glacial or interglacial periods points to a rather local feedback between the export of iron-bearing dust and an immediately responding ocean surface. The identification of these hitherto unconsidered fertilization mechanisms that potentially fostered ocean productivity and hence the sequestration of atmospheric carbon into the deep ocean are further detailed by Müller et al. (2018).

Description: The Antarctic Ice Sheet extent in the Weddell Sea Embayment (WSE) during the Last Glacial Maximum (LGM; ca. 19-25 calibrated kiloyears before present, cal. ka BP) and its subsequent retreat from the shelf are poorly constrained, with two conflicting scenarios being discussed. Today, the modern Brunt Ice Shelf, the last remaining ice shelf in the northeastern WSE, is only pinned at a single location and recent crevasse development may lead to its rapid disintegration in the near future. We investigated the seafloor morphology on the northeastern WSE shelf and discuss its implications, in combination with marine geological records, for reconstructions of the past behaviour of this sector of the East Antarctic Ice Sheet (EAIS), including ice-seafloor interactions. Our data show that an ice stream flowed through Stancomb-Wills Trough and acted as the main conduit for EAIS drainage during the LGM. Post-LGM ice-stream retreat occurred stepwise, with at least three documented grounding line still stands, and the trough had become free of grounded ice by ~10.5 cal. ka BP. In contrast, slow-flowing ice once covered the shelf in Brunt Basin and extended westwards toward McDonald Bank. During a later time period, only floating ice was present within Brunt Basin, but large ‘ice slabs’ enclosed within the ice shelf occasionally ran aground at the eastern side of McDonald Bank, forming ten unusual ramp-shaped seabed features. These ramps are the result of temporary ice-shelf grounding events buttressing the ice further upstream. To the west of this area, Halley Trough very likely was free of grounded ice during the LGM, representing a potential refuge for benthic shelf fauna at this time.