Late Quaternary glacial/interglacial variability in Arctic sea ice and related organic carbon flux: A 180 ka record from Yermak Plateau

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

Evidence for a palaeo-subglacial lake on the Antarctic continental shelf

Subglacial lakes are widespread beneath the Antarctic Ice Sheet but their control on ice-sheet dynamics and their ability to harbour life remain poorly characterized. Here we present evidence for a palaeo-subglacial lake on the Antarctic continental shelf. A distinct sediment facies recovered from a bedrock basin in Pine Island Bay indicates deposition within a low-energy lake environment. Diffusive-advection modelling demonstrates that low chloride concentrations in the pore water of the corresponding sediments can only be explained by initial deposition of this facies in a freshwater setting. These observations indicate that an active subglacial meltwater network, similar to that observed beneath the extant ice sheet, was also active during the last glacial period. It also provides a new framework for refining the exploration of these unique environments

First evidence for a late LGM subglacial lake in Pine Island Bay, Antarctica

Subglacial lakes are widespread beneath the Antarctic Ice Sheet and as a source for subglacial meltwater they are assumed to modulate ice stream velocity. Further, the evacuation of subglacial meltwater at the ice sheet margin influences ocean circulation and geochemical cycles. However, despite their importance„ subglacial lakes are one of the least explored environments on our planet. As a consequence, their importance for ice sheet dynamics and their ability to harbour life remain poorly characterised. We present the first direct evidence for a palaeo-subglacial lake on the Antarctic continental shelf, document- ing that subglacial meltwater was stored during the last glacial period and evacuated during the subsequent deglaciation. A distinct sediment facies observed in a core recovered from a small bedrock basin in Pine Island Bay, Amundsen Sea, is indicative of deposition within a low-energy subglacial lake setting. Diffusive modelling demonstrates that low chloride concentrations in the pore water of this characteristic sediment facies can only be explained by original deposition in a freshwater setting. We also show that the location of the subglacial lake within a basin on the inner shelf is consistent with the predicted distribution of subglacial lakes based on bathymetric data. This finding will enable future modelling studies to investigate how the geometry and capacity of subglacial lake systems can influence ice dynamics when the substrate and profile of the ice sheet is known – especially in the highly sensitive area known as the "weak underbelly" of the WAIS. With the exception of a direct lake water access at Subglacial Lake Vostok, and some centimetres of sediment retrieval from Subglacial Lake Whillans, the subglacial hydrological system in Antarctica has hitherto mostly been explored using remote sensing and numerical models that suggest the number of potential lake sites to more than 12.000. Our study not only provides first empirical evidence for a palaeo-subglacial lake but also delivers a framework for investigating and refining exploration of these unique subglacial lake environments and their sediments beneath thick contemporary ice sheets. Our approach, however, is easier and cheaper to conduct by using ship borne coring equipment on the seasonal ice-free continental shelf

Stratigraphic Occurrences of Sub-Polar Planktic Foraminifera in Pleistocene Sediments on the Lomonosov Ridge, Arctic Ocean

Turborotalita quinqueloba is a species of planktic foraminifera commonly found in the sub-polar North Atlantic along the pathway of Atlantic waters in the Nordic seas and sometimes even in the Arctic Ocean, although its occurrence there remains poorly understood. Existing data show that T. quinqueloba is scarce in Holocene sediments from the central Arctic but abundance levels increase in sediments from the last interglacial period [Marine isotope stage (MIS) 5, 71–120 ka] in cores off the northern coast of Greenland and the southern Mendeleev Ridge. Turborotalita also occurs in earlier Pleistocene interglacials in these regions, with a unique and widespread occurrence of the less known Turborotalita egelida morphotype, proposed as a biostratigraphic marker for MIS 11 (474–374 ka). Here we present results from six new sediment cores, extending from the central to western Lomonosov Ridge, that show a consistent Pleistocene stratigraphy over 575 km. Preliminary semi-quantitative assessments of planktic foraminifer abundance and assemblage composition in two of these records (LOMROG12-7PC and AO16-5PC) reveal two distinct stratigraphic horizons containing Turborotalita in MIS 5. Earlier occurrences in Pleistocene interglacials are recognized, but contain significantly fewer specimens and do not appear to be stratigraphically coeval in the studied sequences. In all instances, the Turborotalita specimens resemble the typical T. quinqueloba morphotype but are smaller (63–125 μm), smooth-walled and lack the final thickened calcite layer common to adults of the species. These results extend the geographical range for T. quinqueloba in MIS 5 sediments of the Arctic Ocean and provide compelling evidence for recurrent invasions during Pleistocene interglacials

Enviromagnetic and sedimentologic characteristics of sediments from inner Pine Island Bay (Amundsen Sea): implications for the paleoenvironmental and deglaciation history

The Amundsen Sea sector of the West Antacric Ice Sheet (WAIS) has shown significant changes over the last few decades and may rise global sea level by 1.5 m, if complete disintegration takes place in the future. Pine Island and Thwaites Glacier, which drain into the Pine Island Bay show accelerated ice flow, thinning and grounding line retreat. Herein the results of a combined ap- proach of magnetic and sedimentologcial methods on a sediment core from inner Pine Island Bay are presented. They aim to contribute to the understanding of past WAIS behaviour in the Pine Island Bay and help to improve future projections of ice retreat. A minimum deglaciation age of 9.4 ka BP was obtained by radiocarbon dating. Unfortunately tuning of RPI failed because the sediments was inadequate for this technique. Nevertheless a possible subglacial lake deposit is described which may hold significant informations about pre- Holocene sediment deposition in inner Pine Island Bay. The sequence also indicates a possible ice readvane in Pine Island Bay associated with the Antarctic Cold Reversal. The most prominent feature however, is a change in the sediment source with the onset of the Holocene documented by different clay and magnetic mineral assemblages. The high resolution of the magnetic measurements allowed to identify this change as rather sharp than indicated by the clay minerals. The pre-Holocene sediments at the core site are characterized by high percentages of hematite/goethite and coarse magnetite, whereas Holocene sediments are dominated by fine grained magnetite. Sedimentological, the early Holocene sequence in the core is characterized by meltwater flow de- posits with a gradual transition to hemipelagic sedimentation in the mid-late Holocene. A decrease in magnetic particle size shows phases of possible grounding line retreat during the Holocene

The Arctic Ocean Palaeomagnetic Record : A Stratigraphic Approach

The Arctic Ocean is an essential component of the global climate system, yet understanding of its geological archives is hampered by difficulties in age modelling. In addition to the scarcity of dateable microfossils, the palaeomagnetic record of Arctic sediments is unusual. Palaeomagnetic inclination sequences from Arctic sediments display numerous changes from steep positive to steep negative values that do not match the established geomagnetic polarity timescale. Independent age constraints suggest that most changes in the upper few meters below the sea-floor took place within the Brunhes normal chron. It has been suggested that zones of reversed inclination contain reversed titanomaghemite, formed by sea-floor oxidation of titanomagnetite. Until now, self-reversed components of Arctic records have not been studied in the context of regional stratigraphic frameworks, which could elucidate the relative timing of significant diagenetic changes, or their synchronicity between different records. This thesis examines marine sediments from three different areas of the Arctic. All records were evaluated within their regional stratigraphic frameworks and cross-correlated with existing records. A combined approach that used magnetic and sedimentological data was employed to identify the effect of depositional and diagenetic processes on the palaeo- and rock magnetic properties. Chemical, palaeo- and rock magnetic investigations on sediments from the Arlis Plateau and the Lomonosov Ridge revealed a complex magnetic mineralogy and constrained reversed inclinations to a medium to high coercivity magnetic phase. The important role of manganese in the Arctic Ocean, its involvement in iron (oxyhydr)oxide enrichment during interglacial periods and its role in diagenesis led to the hypotheses that an unidentified magnetic ferromanganese phase is involved in the anomalous palaeomagnetic record. Elevated pore water manganese concentration a few meters below the sea-floor in central Arctic sediments is evidence of ongoing diagenesis that involves manganese-oxides. A relationship with iron-oxides is likely and can lead to alteration of existing magnetic minerals and the precipitation of new magnetic phases. This approach revealed 1) an inconsistent alignment of zones of negative inclination when different records were correlated using lithological parameters, 2) no correlation of inclination changes with stratigraphic boundaries and 3) a link between diagenesis and the palaeomagnetic record

The Arctic Ocean Palaeomagnetic Record : A Stratigraphic Approach

The Arctic Ocean is an essential component of the global climate system, yet understanding of its geological archives is hampered by difficulties in age modelling. In addition to the scarcity of dateable microfossils, the palaeomagnetic record of Arctic sediments is unusual. Palaeomagnetic inclination sequences from Arctic sediments display numerous changes from steep positive to steep negative values that do not match the established geomagnetic polarity timescale. Independent age constraints suggest that most changes in the upper few meters below the sea-floor took place within the Brunhes normal chron. It has been suggested that zones of reversed inclination contain reversed titanomaghemite, formed by sea-floor oxidation of titanomagnetite. Until now, self-reversed components of Arctic records have not been studied in the context of regional stratigraphic frameworks, which could elucidate the relative timing of significant diagenetic changes, or their synchronicity between different records. This thesis examines marine sediments from three different areas of the Arctic. All records were evaluated within their regional stratigraphic frameworks and cross-correlated with existing records. A combined approach that used magnetic and sedimentological data was employed to identify the effect of depositional and diagenetic processes on the palaeo- and rock magnetic properties. Chemical, palaeo- and rock magnetic investigations on sediments from the Arlis Plateau and the Lomonosov Ridge revealed a complex magnetic mineralogy and constrained reversed inclinations to a medium to high coercivity magnetic phase. The important role of manganese in the Arctic Ocean, its involvement in iron (oxyhydr)oxide enrichment during interglacial periods and its role in diagenesis led to the hypotheses that an unidentified magnetic ferromanganese phase is involved in the anomalous palaeomagnetic record. Elevated pore water manganese concentration a few meters below the sea-floor in central Arctic sediments is evidence of ongoing diagenesis that involves manganese-oxides. A relationship with iron-oxides is likely and can lead to alteration of existing magnetic minerals and the precipitation of new magnetic phases. This approach revealed 1) an inconsistent alignment of zones of negative inclination when different records were correlated using lithological parameters, 2) no correlation of inclination changes with stratigraphic boundaries and 3) a link between diagenesis and the palaeomagnetic record

Natural remanent magnetization data and derived parameters of sediment core PS92/039-2

Natural remanent magnetization data and derived parameters

Natural remanent magnetization data and derived parameters of sediment core PS92/045-2

Natural remanent magnetization data and derived parameters

Rock magnetic parameters of sediment core PS92/045-2

Rock magnetic parameters for the sediment core from anhysteretic remanent magnetization, isothermal remanent magnetization and hysteresis measurements