Description: Throughout the transition from the last Glacial to the current Interglacial, rising atmospheric CO2 levels were accompanied by declining atmospheric Δ14C values. A likely mechanism, influencing both components is the deglacial release of CO2, stored for millennia in the deep Ocean, to the atmosphere. Due to its long residence time within the oceans interior this CO2 rich water mass was considerably depleted in radiocarbon. Although a large number of studies address this topic, the extent, location and pathways of the glacial carbon pool are still subjects of an ongoing debate. As deep water masses are upwelled and new intermediate waters are formed around Antarctica, the Southern Ocean is a potential area for the deglacial release of stored CO2. Here we present radiocarbon and carbonate ion data from a transect of sediment cores off New Zealand that covers the major water masses in this area, from the AAIW down to the AABW. During the Glacial, our data locate a significantly 14C depleted pool in a water depth between 2000 and 4500 m. The combination of Δ14C and [CO32-] records provides new insights into the process of oceanic-atmospheric CO2 exchange in the Southern Ocean. In addition, our results yield new implications for contradicting Δ14C records from the Southern Ocean and lower latitudes.

Description: The last deglaciation was characterized by rising concentration in atmospheric CO2 (CO2atm) and a decrease in its radiocarbon content (Δ14Catm). Mobilization of 14C-depleted terrestrial organic carbon, which was previously frozen in extensive boreal permafrost soils, might have contributed to both changes, and was potentially caused by coastal erosion during deglacial sea-level rise and warming. Since parts of this potentially mobilized organic carbon was reburied in marine sediments, records of accumulation of terrigenous biomarkers and their compound-specific radiocarbon ages can provide insights into the timing and controls on permafrost decomposition. We present data from three marine sediment cores, two cores off the Amur River draining into the Sea of Okhotsk, and one core from the Northeastern Bering Sea adjacent to the Bering shelf (one the largest shelf areas flooded during the deglaciation) receiving input from the Yukon River. During the Last Glacial Maximum all catchments were completely covered with permafrost. Today, the Amur drainage basin is free of permafrost while the Yukon catchment is covered by discontinuous permafrost. All sites show three distinct deglacial maxima (at 16.5, 14.5, 11.5 ka BP) in accumulation of old terrigenous biomarkers (5-20 kyr old at the time of deposition). The peaks occurred during meltwater pulses suggesting that sea-level rise remobilized old terrestrial carbon from permafrost on the flooded shelfs. In the Bering Sea fossil, mature organic matter, mobilized by erosion of organic rich rocks during the retreat of Brooks Range glaciers and the Laurentide ice sheet additionally contributed to the first peak via increased fluvial runoff. Deglacial changes in abundance ratios of long-chain n-alkanes record gradual changes in vegetation type and wetland extent in the Amur-river catchment. Since wetland expansion is closely linked to permafrost thaw this implies that permafrost decomposition in the Amur drainage basin was a gradual process. By contrast sea-level rise caused abrupt decomposition events across the Okhotsk and Bering Shelfs. We extrapolate our localized findings to an overall potential carbon release during deglaciation of 285 PgC from coastal erosion in the Arctic Ocean and the related permafrost decomposition. By analysing some idealized scenarios using the global carbon cycle model BICYCLE we estimate the impact of such a release on the atmosphere. We find that it might have accounted for a deglacial rise in CO2atm of up to 15 ppm, and to a decline in ∆14Catm of 15‰. These results, if restricted to the three peak events connected to rapid sea-level rise, as supported by our data, might have contributed particularly to abrupt changes in CO2atm and ∆14Catm, corresponding to 15-20% of both, the observed rise in CO2atm of ~90 ppm, and the residual in ∆14Catm that is unexplained by changes in the 14C production rate.

Description: Understanding the preservation and deposition history of organic molecules is crucial for the understanding of paleoenvironmental information contained in their abundance ratios such as Uk’37 and TEX86 used as proxies for sea surface temperature (SST). Based on their relatively high refractivity, alkenones and glycerol dialkyl glycerol tetraethers (GDGTs) can survive postdepositional processes like lateral transport, potentially causing inferred SSTs to be misleading. Likewise, selective preservation of alkenones and GDGTs may cause biases of the SST proxies themselves and can lead to decoupling of both proxy records. Here we report compound-specific radiocarbon data of marine biomarkers including alkenones, GDGTs, and low molecular weight (LMW) n-fatty acids from Black Sea sediments deposited under different redox regimes to evaluate the potentially differential preservation of both biomarker classes and its effect on the SST indices Uk’37 and TEX86. The decadal Δ14C values of alkenones, GDGTs, and LMW n-fatty acids indicate similar preservation under oxic, suboxic, and anoxic redox regimes and no contribution of pre-aged compounds, e.g., by lateral supply. Moreover, similar 14C concentrations of crenarchaeol, alkenones, and LMW n-fatty acids imply that the thaumarchaeotal GDGTs preserved in these sediments are produced in the euphotic zone rather than in subsurface/thermocline waters. However, we observe biomarker-based SSTs that strongly deviate (ΔSST up to 8.4°C) from in situ measured mean annual SSTs in the Black Sea. This is not due to redox-dependent differential biomarker preservation as implied by their Δ14C values and spatial SST pattern. Since contributions from different sources can largely be excluded, the deviation of the Uk’37 and TEX86 proxy-derived SSTs from in situ SSTs requires further study of phylogenetic and other yet unknown environmental controls on alkenone and GDGT lipid distributions in the Black Sea.

Description: The past ice sheet conditions in the southern Weddell Sea Embayment (WSE) are only poorly known. Studies from this area have led to two contradicting scenarios of maximum ice extent during the Last Glacial Maximum (LGM). The first scenario is mainly based on terrestrial data indicating only very limited ice sheet thickening in the hinterland and suggests a grounding-line position on the inner shelf. The alternative scenario is based on marine geological and geophysical data and concludes that the LGM grounding line was located on the outer shelf, about 650 km further offshore than in the other scenario. Three hypotheses have been brought forward to explain these two apparently contradictory scenarios. A) An ice plain was present on the shelf that enabled a large ice extent while maintaining little ice thickness in the hinterland. B) The maximum grounded ice advance lasted for a short period only and was probably caused by a short-termed touch down of an ice shelf on the outer shelf, which did not cause sufficient ice sheet thickening in the hinterland to be traced today. C) Due to an ice flow switch, Filchner Trough was fed by an area further to the west where ice had thickened at the LGM. Besides the poorly constrained LGM ice extent, studies suggest a complex development of its retreat speed and drainage pattern in succession of the LGM that needs to be further constraint. For example, radar data from ice rises in the southwestern hinterland of the WSE suggest that ice flow switches occurred as late as the Mid-Holocene and cosmogenic exposure ages indicate an early Holocene ice sheet thickness in the Ellsworth Mountains comparable to that of the LGM. We investigated multibeam bathymetry data (ATLAS Hydrosweep DS3), acoustic sub-bottom profiles (ATLAS Parasound P-70) and marine sediment cores collected from Filchner Trough during RV “Polarstern” expedition PS96 in Dec 2015-Feb 2016. Our key finding is a previously unknown stacked grounding zone wedge (GZW) located on the outer shelf. This GZW shows that the Filchner palaeo-ice stream stabilized at this position at least two times. Two sediment cores were recovered seaward of the GZW and on top of the lower part of the GZW, respectively. Radiocarbon dates from these cores indicate that (i) the GZW was formed in the Early Holocene and (ii) grounded ice did not extend seaward of the GZW at the LGM. Hence, our data provide evidence that the grounding line in Filchner Trough experienced dynamic changes in the Holocene and that no linear ice sheet retreat occurred within this trough after the LGM.

Description: The history of glaciations on Southern Hemisphere sub-polar islands is unclear. Debate surrounds the extent and timing of the last glacial advance and termination on sub-Antarctic South Georgia in particular. Using sea-floor geophysical data and marine sediment cores, we resolve the record of past glaciation offshore of South Georgia giving insight into glacier response to climate variability through the transition from the Last Glacial Maximum to Holocene. We show a widespread, coherent sea-bed imprint of shelf-wide ice-sheet advance and retreat in the form of glacially-carved cross-shelf troughs, suites of end and recessional moraines, as well as populations of streamlined bedforms. Glacial troughs began to infill with sediments after c. 18 ka B.P. consistent with interpretations of an extensive last glacial advance and early onset of a progressive, and potentially rapid, deglaciation to coastal limits. A fjord-mouth moraine formed during renewed glacier resurgence between c. 15,170 and 13,340 yrs ago. From the geometry of moraines in adjacent fjords, we infer that many of South Georgia’s glaciers advanced during this period of cooler, wetter climate, known as the Antarctic Cold Reversal, extending the geographic footprint of the cryospheric response to an Antarctic climate pattern into the Atlantic sector of the Southern Ocean. We conclude that the last glaciation of South Georgia was extensive, and the sensitivity of its glaciers to climate variability during the last termination more significant than implied by previous studies. Keywords: Sub-Antarctic; ice-cap reconstruction; multibeam bathymetry; sediment cores

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: Radiocarbon (14C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric 14C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international 14C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable 14C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the 14C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine 14C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.

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.

Description: Past ice dynamics are so far only poorly resolved in the southern Weddell Sea. This is highlighted by previous studies that led to two contradicting scenarios for the grounding line location during the Last Glacial Maximum (LGM), which differed by up to ~650 km. Another study suggested that the maximum ice extent locally was not reached during the LGM but in the early Holocene, indicating that there was also a highly dynamic ice sheet system during deglaciation. There is ambiguity about the history of ice advance and retreat in the region offshore Brunt Ice Shelf based on current data. Only one radiocarbon dated marine geological core is available, contains age reversals, and can be interpreted as indicating ice free conditions during the LGM or having been overrun by grounded ice between 30.2-20.3 cal ka BP. Today, the Brunt Ice Shelf itself is a focus of interest due to the critical crack/fracture development since 2016. This endangers Halley research station, which is situated on the ice shelf, and has resulted in the third consecutive year of austral winter closure. Geophysical ice shelf investigations revealed that, unlike usual ice shelves, the Brunt Ice Shelf consists of numerous blocks of meteoric/glacial ice that are “glued” together by freezing sea ice and snow drift. It is hypothesized that the Brunt Ice Shelf sustains its stability due to buttressing at the McDonald Ice Rumples, which form the only remaining ice shelf pinning point. Improved understanding of the past development of the ice shelf system may also aid understanding the processes active today. We investigated hydroacoustic data that were acquired offshore Brunt Ice Shelf over the last decades with RV Polarstern and RRS James Clark Ross for geomorphological indications of past ice sheet dynamics. The identified landforms show that major ice discharge during the LGM was not via Brunt Basin just in front of the modern-day Brunt Ice Shelf, but via an ice stream that occupied Stancomb-Wills Trough, which is located northeast of Brunt Ice Shelf and extends about 200 km upstream of the modern-day grounding line. We identified at least three still stand phases during retreat in this trough. Marine geological data revealed a minimum age for grounding line retreat before 8.5 cal ka BP. In contrast, we found no indications of fast flowing ice in Brunt Basin. Instead, we infer slow flowing, cold-based ice and found uniquely formed ramp-shaped bedforms. We suggest that these ramps were formed due to the unusual structure of the ice shelf, which led to temporary grounding of ice shelf keels that acted as buttressing points for a more extensive ice shelf in the past. We will present the new ice sheet reconstruction and will discuss the formation process of the ramps.