Description: The mobilization of glacial permafrost carbon during the last glacial–interglacial transition has been suggested by indirect evidence to be an additional and significant source of greenhouse gases to the atmosphere, especially at times of rapid sea-level rise. Here we present the first direct evidence for the release of ancient carbon from degrading permafrost in East Asia during the last 17 kyrs, using biomarkers and radiocarbon dating of terrigenous material found in two sediment cores from the Okhotsk Sea. Upscaling our results to the whole Arctic shelf area, we show by carbon cycle simulations that deglacial permafrost-carbon release through sea-level rise likely contributed significantly to the changes in atmospheric CO2 around 14.6 and 11.5 kyrs BP.

Description: Changes in the Holocene interaction of the (i) cold/fresh East Greenland Current (EGC) and (ii) warm/saline Irminger Current (IC) in northern Denmark Strait have been reconstructed from benthic and planktic foraminifera assemblages, ice-rafted debris, grain-size analyses and quantitative X-ray diffraction. During the time from c. 10 600 to 8000 cal a BP, palaeoceanographic reconstructions reveal waning deglacial influence from the receding Greenland Ice Sheet and presence of a strong EGC caused low surface water productivity. From c. 8000 cal a BP, a predominant influence of Atlantic-sourced IC waters on subsurface water conditions became established in northern Denmark Strait, which accompanied low surface water productivity. Relatively warm surface and subsurface water conditions, i.e. reduced EGC and strong IC influence, are found from c. 6500 to 4500 cal a BP, representing Holocene optimum-like conditions. A mid- to late Holocene EGC strengthening caused increased stratification and formation of a distinct halocline. However, we recognize millennial-scale periods of reduced stratification by an enhanced influence of Atlantic-sourced IC Water on surface water conditions: (i) at c. 2500–1400 cal a BP the time of the Roman Warm Period and (ii) at c. 300 cal a BP the later part of the Little Ice Age. These periods of oceanic warming probably relate to changes in the Subpolar Gyre dynamics that led to enhanced entrainment of Atlantic-sourced IC Water into northern Denmark Strait.

Description: There is a converging body of evidence supporting a measurable slowdown of the Atlantic Meridional Overturning Circulation (AMOC) as climate warms and Northern Hemisphere ice sheets inexorably shrink. Within this context, we assess the variability of the AMOC during the Holocene based on a marine sediment core retrieved from the deep northwest Atlantic, which sensitively recorded large‐scale deglacial transitions in deep water circulation. While there is a diffuse notion of Holocene variability in Labrador and Nordic Seas overturning, we report a largely invariable deep water circulation for the last ~11,000 years, even during the meltwater pulse associated with the 8.2‐ka event. Sensitivity tests along with high‐resolution 231Pa/230Th data constrain the duration and the magnitude of possible Holocene AMOC variations. The generally constant baseline during the Holocene suggests attenuated natural variability of the large‐scale AMOC on submillennial timescales and calls for compensating effects involving the upstream components of North Atlantic Deep Water.

Description: During the last deglaciation (18–8 kyr BP), shelf flooding and warming presumably led to a large-scale decomposition of permafrost soils in the mid-to-high latitudes of the Northern Hemisphere. Microbial degradation of old organic matter released from the decomposing permafrost potentially contributed to the deglacial rise in atmospheric CO2 and also to the declining atmospheric radiocarbon contents (Δ14C). The significance of permafrost for the atmospheric carbon pool is not well understood as the timing of the carbon activation is poorly constrained by proxy data. Here, we trace the mobilization of organic matter from permafrost in the Pacific sector of Beringia over the last 22 kyr using mass-accumulation rates and radiocarbon signatures of terrigenous biomarkers in four sediment cores from the Bering Sea and the Northwest Pacific. We find that pronounced reworking and thus the vulnerability of old organic carbon to remineralization commenced during the early deglaciation (~16.8 kyr BP) when meltwater runoff in the Yukon River intensified riverbank erosion of permafrost soils and fluvial discharge. Regional deglaciation in Alaska additionally mobilized significant fractions of fossil, petrogenic organic matter at this time. Permafrost decomposition across Beringia's Pacific sector occurred in two major pulses that match the Bølling-Allerød and Preboreal warm spells and rapidly initiated within centuries. The carbon mobilization likely resulted from massive shelf flooding during meltwater pulses 1A (~14.6 kyr BP) and 1B (~11.5 kyr BP) followed by permafrost thaw in the hinterland. Our findings emphasize that coastal erosion was a major control to rapidly mobilize permafrost carbon along Beringia's Pacific coast at ~14.6 and ~11.5 kyr BP implying that shelf flooding in Beringia may partly explain the centennial-scale rises in atmospheric CO2 at these times. Around 16.5 kyr BP, the mobilization of old terrigenous organic matter caused by meltwater-floods may have additionally contributed to increasing CO2 levels.

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.