Description: Fjords are recognized as hotspots of organic carbon (OC) burial in the coastal ocean. In fjords with glaciated catchments, glacier discharge carries large amounts of suspended matter. This sedimentary load includes OC from bedrock and terrigenous sources (modern vegetation, peat, soil deposits), which is either buried in the fjord or remineralized during export, acting as a potential source of CO2 to the atmosphere. In sub‐Antarctic South Georgia, fjord‐terminating glaciers have been retreating during the past decades, likely as a response to changing climate conditions. We determine sources of OC in surface sediments of Cumberland Bay, South Georgia, using lipid biomarkers and the bulk 14C isotopic composition, and quantify OC burial at present and for the time period of documented glacier retreat (between 1958 and 2017). Petrogenic OC is the dominant type of OC in proximity to the present‐day calving fronts (60.4 ± 1.4% to 73.8 ± 2.6%) and decreases to 14.0 ± 2.7% outside the fjord, indicating that petrogenic OC is effectively buried in the fjord. Beside of marine OC, terrigenous OC comprises 2.7 ± 0.5% to 7.9 ± 5.9% and is mostly derived from modern plants and Holocene peat and soil deposits that are eroded along the flanks of the fjord, rather than released by the retreating fjord glaciers. We estimate that the retreat of tidewater glaciers between 1958 and 2017 led to an increase in petrogenic carbon accumulation of 22% in Cumberland West Bay and 6.5% in Cumberland East Bay, suggesting that successive glacier retreat does not only release petrogenic OC into the fjord, but also increases the capacity of OC burial.

Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

Description: Fjords are recognized as hotspots of organic carbon (OC) burial in the coastal ocean. In fjords with glaciated catchments, glacier discharge carries large amounts of suspended matter. This sedimentary load includes OC from bedrock and terrigenous sources (modern vegetation, peat, soil deposits), which is either buried in the fjord or remineralized during export, acting as a potential source of CO2 to the atmosphere. In sub-Antarctic South Georgia, fjord-terminating glaciers have been retreating during the past decades, likely as a response to changing climate conditions. We determine sources of OC in surface sediments of Cumberland Bay, South Georgia, using lipid biomarkers and the bulk 14C isotopic composition, and quantify OC burial at present and for the time period of documented glacier retreat (between 1958 and 2017). Petrogenic OC is the dominant type of OC in proximity to the present-day calving fronts (60.4 ± 1.4% to 73.8 ± 2.6%) and decreases to 14.0 ± 2.7% outside the fjord, indicating that petrogenic OC is effectively buried in the fjord. Beside of marine OC, terrigenous OC comprises 2.7 ± 0.5% to 7.9 ± 5.9% and is mostly derived from modern plants and Holocene peat and soil deposits that are eroded along the flanks of the fjord, rather than released by the retreating fjord glaciers. We estimate that the retreat of tidewater glaciers between 1958 and 2017 led to an increase in petrogenic carbon accumulation of 22% in Cumberland West Bay and 6.5% in Cumberland East Bay, suggesting that successive glacier retreat does not only release petrogenic OC into the fjord, but also increases the capacity of OC burial.

Description: South Georgia, one of the largest sub-Antarctic islands, is located within the Southern Ocean and is influenced by the moisture-supplying Southern Westerlies and the Antarctic Circumpolar Current, which are highly susceptible to Southern Hemisphere climate variability. Its unique location causes South Georgia's remaining ice masses to react sensitively to climate change, resulting in highly dynamic ice cap waxing and waning, as well as in geomorphological and sedimentological changes on the island and its continental shelf. Sediments around the island have been archiving this ice cap behaviour since at least the Last Glacial Maximum (LGM) and are therefore an important target to investigate past ice cap evolution and climate. Despite several interdisciplinary studies on land and in coastal areas, much of the glacial history is still poorly constrained due to a lack of offshore data. This study presents the, thus far, most distal marine sediment succession from outer Drygalski Trough on the mid-continental shelf of South Georgia. Composite multi-proxy-analyses, together with radiocarbon dating, sub-bottom profiler and high-resolution bathymetric data provide first insights into the evolution of a large glacial trough south of South Georgia since the LGM. Several moraines close to the shelf edge indicate shelf-wide glaciation during the local LGM, which, based on extrapolation of the oldest reliable radiocarbon date, occurred before 30 ka BP at the earliest. Basal stratified diamicton at the core site was interpreted as waterlain till deposited in a subglacial cavity with restricted seawater access and suggests grounding zone-proximal sedimentation in the early phases of deglaciation. The ice margin remained stable until ∼17.5 cal ka BP, when ice quickly retreated from the mid-continental shelf and sedimentation at the core site was dominated by hemipelagic suspension settling with some iceberg melting. Further retreat was interrupted by a local ice readvance and associated increased hinterland erosion during the Antarctic Cold Reversal (14.5–12.8 ka), as indicated by peak sedimentation rates (〉190 cm ka−1) between 13.4 and 12.4 cal ka BP. In contrast, the continuous deposition of ice-distal to open-marine mud at rates of only ∼30 cm ka−1 throughout the Holocene indicate that any other LGM-subsequent glacier readvances reached significantly lesser extents and were probably confined to the fjord. Our data provide new evidence in support of a shelf-wide glaciation during the LGM and suggest rapid, but stepwise ice retreat during deglaciation.