Beschreibung: In order to understand the growth and retreat of glaciers in response to the glacial-interglacial changes, subglacial marine sedimentary sequences have been studied extensively in the continental shelf areas of the Ross Sea. The purpose is to comprehend the glaciomarine sedimentation change on the continental slope of eastern Pennell-Iselin Bank in the Ross Sea, using three gravity cores (C1, C2, C3) and three box cores (BC1, BC2, BC3) collected from sites (RS14-1, 2, 3), respectively, across the continental slope to the eastern side of the Pennell-Iselin Bank during XXIX° (2014) PNRA expedition (Rosslope Ⅱ project). Several sedimentological (grain size, magnetic susceptibility), elemental (XRF), geochemical (biogenic opal, total organic carbon, total nitrogen, C/N ratios, CaCO3), and isotopic (δ13C and δ15N of organic matter) parameters were measured along sediment cores with AMS 14C dating of bulk sediments. Core-sediments consist mostly of hemipelagic sandy clay or silty clay with scattered IRDs (Ice-Rafted Debris). A comparison of sediment properties between box cores and the top of gravity cores reveals that the loss of sediment during sampling is trivial. Sediment colors of gravity cores alternate between brown and gray downward. Based on the variation patterns of sediment properties, sediment lithology was divided into different units (A and B), and subunits (B1 and B2). AMS 14C dates and sediment properties assign Unit A, Unit B1, and Unit B2 to interglacial, deglacial, and glacial conditions, respectively. Unit A represents the Holocene and interglacial sediments deposited mainly by the suspension settling of biogenic particles with IRDs in the open marine condition. Unit B1 reflects the deglacial sediments with an increase in IRDs showing the transition of sediment properties from Unit B2 to Unit A by the retreat of subglacial ices. Unit B2 is characterized by different sediment properties, mainly supplied by the continuously lateral melt-water plume or distal part of debris flow originating from the front of grounding floes in the subglacial continental shelf under the ice shelf during the glacial period. Thus, Unit B contains mostly reworked and eroded sediments from the continental shelf with scattered IRDs. The influence of subglacial continental shelf sedimentation in terms of melt-water transport and/or distal stage of debris flow was limited as far as to the middle slope areas (Site 2) during the deglacial and glacial periods. The deeper Site 1 remains in seasonally open marine conditions during the glacial period, due to the peaks of biogenic opal and TOC contents. Keywords: sediment property, subglacial activity, continental slope, Ross Sea

Beschreibung: The majority of glaciers draining the Antarctic Peninsula Ice Sheet are thinning and retreating rapidly1. It is widely understood that these changes are driven by both a warming ocean and atmosphere. However, there are other mechanisms, including pinning points created by bathymetric highs and a reverse bed gradient, that are thought to have an important control on ice stream behaviour (Weertman, 1974; Jamieson et al., 2012). Our understanding of the interplay between these mechanisms and time-scales over which they are important is currently limited in time to the advent of satellite monitoring. By reconstructing the cause and style of ice stream retreat following the Last Glacial Maximum (LGM; 25-19 ka BP), it is possible to gain a greater insight into the mechanisms which drive glacier retreat (Ó Cofaigh et al., 2014). Sedimentary sequences deposited during the LGM and the subsequent deglaciation on polar continental shelves, provide an important archive of past changes (Ó Cofaigh et al., 2014). Previous studies have typically identified three sediment facies assemblages; sub-glacial, transitional and open marine (Ó Cofaigh et al., 2014; Domack et al., 1988; Smith et al., 2011). Transitional sediment facies are deposited at the grounding line and are often targeted for radiocarbon dating, as they represent the onset of glaciomarine sedimentation following the retreat of grounded ice (Domack et al., 1988; Smith et al., 2014; Heroy et al., 1996). Despite the development of depositional models to help explain the processes occurring at grounding lines (Powell et al., 1995 and 1996), there is still significant uncertainty about the temporal and spatial variations in grounding line sedimentation along and across a palaeo-ice stream trough. Here we use a multi-proxy approach (water content, shear strength, magnetic susceptibility, density, contents of biogenic opal, Total Organic Carbon and CaCO3, grain size distribution and X-radiographs) on marine sediment cores recovered from the Anvers-Hugo Palaeo-Ice Stream Trough (AHT), western Antarctic Peninsula shelf, to identify variability in transitional sediment facies deposited along and across the trough. We discuss possible controls on the variability in transitional sediment facies and how this is related to the rate and style of ice stream retreat. Our data reveal systematic variability in the types and volume of transitional sediments deposited during the last deglaciation of AHT. A detailed analysis of the transitional sediment facies shows that this variability reflects different phases of ice stream behaviour. Large volumes of ice proximal sediment facies recovered seawards of grounding zone wedges are indicative of episodes of grounding line still-stands. Re-advances of the grounding line, concurrent with a shallowing of the reverse bed gradient and a narrowing of the trough, appear to have occurred during the final stages of deglaciation. This is indicated by interlaminated ice-proximal and ice-distal sediment facies within inner shelf cores. Transitional sediment variability additionally captures the evolution of the ice stream during deglaciation, including the formation of a small ice shelf on the inner shelf. Keywords: Antarctic Peninsula, Last Glacial Maximum, ice stream, sediment cores References Cook, A. J., Holland, P. R., Meredith, M. P., Murray, T., Luckman, A. & Vaughan, D. G, 2016. Ocean forcing of glacier retreat in the western Antarctic Peninsula. Science, 353, 283-286. Weertman, J, 1974. Stability of the Junction of an Ice Sheet and an Ice Shelf. Journal of Glaciology, 13, 3-11. Jamieson, S. S. R., Vieli, A., Livingstone, S. J., Cofaigh, C. O., Stokes, C., Hillenbrand, C.-D. & Dowdeswell, J. A, 2012. Ice-stream stability on a reverse bed slope. Nature Geoscience, 5, 799-802. Ó Cofaigh, C., Davies, B. J., Livingstone, S. J., Smith, J. A., Johnson, J. S., Hocking, E. P., Hodgson, D. A., Anderson, J. B., Bentley, M. J., Canals, M., Domack, E., Dowdeswell, J. A., Evans, J., Glasser, N. F., Hillenbrand, C.-D., Larter, R. D., Roberts, S. J. & Simms, A. R, 2014. Reconstruction of ice-sheet changes in the Antarctic Peninsula since the Last Glacial Maximum. Quaternary Science Reviews, 100, 87-110. Domack, E. W. & Harris, P. T, 1998. A new depositional model for ice shelves, based upon sediment cores from the Ross Sea and the Mac. Robertson shelf, Antarctica. Annals of Glaciology, 27, 281-284. Smith, J. A., Hillenbrand, C.-D., Kuhn, G., Larter, R. D., Graham, A. G. C., Ehrmann, W., Moreton, S. G. & Forwick, M, 2011. Deglacial history of the West Antarctic Ice Sheet in the western Amundsen Sea Embayment. Quaternary Science Reviews, 30, 488-505. Smith, J. A., Hillenbrand, C.-D., Kuhn, G., Klages, J. P., Graham, A. G. C., Larter, R. D., Ehrmann, W., Moreton, S. G., Wiers, S. & Frederichs, T, 2014. New constraints on the timing of West Antarctic Ice Sheet retreat in the eastern Amundsen Sea since the Last Glacial Maximum. Global and Planetary Change, 122, 224-237. Heroy, D. C. & Anderson, J. B, 1996. Radiocarbon constraints on Antarctic Peninsula Ice Sheet retreat following the Last Glacial Maximum (LGM). Quaternary Science Reviews, 26, 3286-3297. Powell, R. D., Dawber, M., McInnes, J. N. & Pyne, A. R, 1996. Observations of the Grounding-line Area at a Floating Glacier Terminus. Annals of Glaciology, 22, 217-223. 1Powell, R. D. & Domack, E, 1995. Modern Glacimarine Environments. In: Glacial Environments, Volume 1 (ed. J Menzies). Butterworth-Heinemann, 445-486.

Beschreibung: Modern global change affects not only the polar north but also, and to increasing extent, the southern high latitudes, especially the Antarctic regions covered by the West Antarctic Ice Sheet (WAIS). Consequently, knowledge of the mechanisms controlling past WAIS dynamics and WAIS behaviour at the last deglaciation is critical to predict its development in a future warming world. Geological and paleobiological information from major drainage areas of the WAIS, like the Amundsen Sea Embayment, shed light on the history of the WAIS glaciers. Sediment records obtained from a deep inner shelf basin north of the Getz Ice Shelf document a deglacial warming in three phases. Above a glacial diamicton and a sediment package barren of microfossils that document sediment deposition by grounded ice and below an ice shelf or perennial sea ice cover (possibly fast ice), respectively, a sediment section with diatom assemblages dominated by sea ice taxa indicates ice shelf retreat and seasonal ice-free conditions. This conclusion is supported by diatom-based summer temperature reconstructions. The early retreat was followed by a phase, when exceptional diatom ooze was deposited between 12,000 and 13,000 cal. years B.P. Microscopical inspection of this ooze revealed excellent preservation of diatom frustules of the species Corethron pennatum together with vegetative Chaetoceros, thus an assemblage usually not preserved in the sedimentary record. Sediments succeeding this section contain diatom assemblages indicating rather constant Holocene cold water conditions with seasonal sea ice. The deposition of the diatom ooze can be related to changes in hydrographic conditions including strong advection of nutrients. However, sediment focussing in the partly steep inner shelf basins cannot be excluded as a factor enhancing the thickness of the ooze deposits. It is not only the presence of the diatom ooze but also the exceptional preservation and the species composition of the diatom assemblage, which point to specific scenarios involving e.g. changes in the food web that can be related to warmer surface water temperatures. Such warming of shelf waters may be related with an overshooting Atlantic Meridional Overturning Circulation (AMOC) and strong injection of warmer North Atlantic Deep Water into the Southern Ocean water masses at Termination I. Such finding may highlight the effects of AMOC changes on Antarctic ice shelf extent and coastal ecosystems. Keywords: WAIS, Amundsen Sea Embayment, diatoms, deglacial warming

Beschreibung: We will present new multibeam bathymetry data that make the Anvers-Hugo Trough west of the Antarctic Peninsula one of the most completely surveyed palaeo-ice stream pathways in Antarctica. We interpret landforms revealed by these data as indicating that subglacial water availability played an important role in facilitating ice stream flow in the trough during late Quaternary glacial periods. Specifically, we observe a set of northward-shoaling valleys that are eroded into the upstream edge of a sedimentary basin, extend northwards from a zone containing landforms typical of erosion by subglacial water flow, and coincide spatially with the onset of mega-scale glacial lineations. Water was likely supplied to the ice stream bed episodically as a result of outbursts from a subglacial lake previously hypothesized to have been located in the Palmer Deep basin on the inner continental shelf. In a palaeo-ice stream confluence area, close juxtaposition of mega-scale glacial lineations with landforms that are characteristic of slow, dry-based ice flow, suggests that water availability was also an important control on the lateral extent of these palaeo-ice streams. These interpretations are consistent with the hypothesis that subglacial lakes or areas of elevated geothermal heat flux play a critical role in the onset of many large ice streams. The interpretations also have implications for the dynamic behaviour of the Anvers-Hugo Trough palaeo-ice stream and, potentially, of several other Antarctic palaeo-ice streams. Keywords: multibeam bathymetry, ice stream, subglacial water, landform

Beschreibung: Reconstruction of the glacial dynamics of the Antarctic ice sheets during the past by studying records from their margin is essential to evaluate their stability and to anticipate their contribution to future sea level rise. Recently, the first direct evidence for a paleo-subglacial lake on the Antarctic continental shelf was reported from a small bedrock basin in Pine Island Bay, West Antarctica (Kuhn et al., 2017). The evidence is based on a distinct sediment facies and geochemical pore water signatures, i.e. low chloride concentrations, in a marine sediment core (PS69/288). These data indicate that the sediment in the lower part of the core was deposited under a low-energy subglacial lake setting. They also show that the location of the subglacial lake is consistent with the predicted distribution of subglacial lakes based on bathymetric data. Here we report further evidence for a paleo-subglacial lake based on changes in Be-10 concentrations in the sediments. A significant down-core decrease in the Be-10 concentration indicates very limited input of meteoric Be-10 to the sediments in the lower part of the core, suggesting a depositional environment that was isolated from the open ocean. This is consistent with the proposed subglacial lake setting. In detail, the Be-10 concentration shows a further drop within a sand, silt and mud interval from ca. 580 to 470 cm core depth that was interpreted to have been deposited during the transition from the subglacial lake to a sub-ice shelf cavern by grounding line retreat in that area at about 11 kyrs B.P. (Hillenbrand et al., 2013, Kuhn et al., 2017). The lowered Be-10 concentration at the base of this interval probably results from the dominant supply of sediment that had been deeply buried under the West Antarctic Ice Sheet (WAIS) for a very long time. Above a minor up-core increase from 464 to 324 cm, the Be-10 concentration decreases again at about 260 cm. This decrease may correspond to three possible factors: 1.) increased supply of sediments from below the ice sheet (possible meltwater plumes), 2.) an episode of permanent sea-ice cover, or 3.) a re-advance of the ice shelf. Above 260 cm the Be-10 concentration increases significantly toward the top of the core, indicating that an open marine setting had established at the core site. This data provides new insight into a more dynamic behaviour of the WAIS in Pine Island Bay during the Holocene. Overall, the Be-10 concentration of the sediments is a powerful tool to study paleo-subglacial lakes in Antarctica and processes of ice sheet to ice shelf transition during the subsequent deglaciation. Keywords: Subglacial Lake, Be-10, Ice sheet retreat, West Antarctica References Hillenbrand, C.-D., Kuhn, G., Smith, J.A., Gohl, K., Graham, A.G., Larter, R.D., Klages, J.P., Downey, R., Moreton, S.G., Forwick, M., Vaughan, D.G., 2013. Grounding-line retreat of the West Antarctic Ice Sheet from inner Pine Island Bay. Geology 41, 35–38. doi:10.1130/G33469.1. Past Antarctic Ice Sheet Dynamics (PAIS) Conference September 10-15th 2017, Trieste - Italy Kuhn, G., Hillenbrand, C.-D, Kasten, S., Smith, J.A., Nitsche, F.O., Frederichs, T., Wiers, S., Ehrmann, W., Klages, J.P., Mogollón, J.M. (in press). Evidence for a palaeo-subglacial lake on the Antarctic continental shelf. Nature Communications.

Beschreibung: Subglaziale Seen sind unter dem Antarktischen Eisschild weit verbreitet. Sie bilden eine Quelle von subglazialem Schmelzwasser und modulieren weitgehend die Fließgeschwindigkeiten des überlagernden Eisstromes. Der Ausbruch von subglazialem Schmelzwasser am Rande des Eisschildes kann die globale ozeanographische Zirkulation, den Meeresspiegelanstieg und geochemische Zyklen beeinflussen. Trotz ihrer Bedeutung sind subglaziale Seen eine der am wenigsten erforschten Umgebungen auf unserem Planeten. Ihre Auswirkungen auf die Dynamik der Eisbedeckung und ihr Potential primitives aber autarkes Leben zu beherbergen, sind weitgehend schlecht charakterisiert. Wir präsentieren den ersten direkten Beweis für einen paläo-subglazialen See auf dem antarktischen Kontinentalschelf. Sedimentablagerungen beweisen, dass subglaziales Schmelzwasser während oder kurz nach der letzten Eiszeit unter dem Antarktischen Eisschild vorhanden war und erst bei der Enteisung entleert wurde. Im Pine Island Bay (Amundsen Sea Embayment, ASE) haben wir zum ersten Mal in der Antarktis mehr als drei Meter dieser typischen Sedimentfazies, die auf wenig dynamische Ablagerungsbedingungen in einem subglazialen See schließen lassen, beprobt. Niedrige Chloridkonzentrationen im Porenwasser des Sedimentkernes und diffus-advektive Modellierung der zeitlichen Entwicklung dieser Chloridkonzentrationen im Porenwasser zeigen eindeutig die Genese der Sedimente in einem subglazialen, schmelzwassergefüllten See. Basierend auf bathymetrischen Daten können wir zeigen, dass die Lage des subglazialen Sees mit einer prognostizierten Verteilung von subglazialen Seen in diesem Gebiet übereinstimmt. Dieser Befund ermöglicht Modellierungsstudien der Eisdynamik bei bekannter Geometrie des Eisuntergrundes, des Volumens der subglazialen Seen, der Eigenschaften des unterlagernden Substrates und der Form des Eisstroms. Dies ist besonders wichtig für den Pine Island Gletscher, der als "Weak Underbelly" des Westantarktischen Eisschildes (WAIS) gilt. Bisher wurde nur die Eis/Wasser Grenzfläche im subglazialen Wostoksee und wenige Zentimeter diamiktischen Sedimentes vom Whillans-See beprobt. Ansonsten wurde das antarktische subglaziale hydrologische System durch Fernerkundung und numerische Modelle rekonstruiert und die Anzahl der potenziellen subglazialen Seen auf mehr als 12000 geschätzt. Unsere Studie liefert nicht nur den ersten Nachweis für eiszeitliche subglaziale Seen und Beprobung dieser Sedimente in der Antarktis, sondern auch einen Rahmen für weitere Untersuchungen dieser einzigartigen subglazialen See-Umgebung in einer kostengünstigen Variante durch Schiffsexpeditionen auf dem saisonal eisfreien inneren Kontinentalschelf.