Land-ocean transport in the Laptev Sea during the last deglaciation: Sediment chronology, organic matter characteristics and freshwater dynamics

The Arctic continues to warm at an accelerated pace, about four times faster than the global average, and Arctic coastlines continue to erode, and permafrost containing considerable amount of ancient organic carbon continue to thaw. The insights gained from past deglaciation scenarios offer a valuable framework for anticipating the Arctic’s response to ongoing, and future climate change. As such, one of the main objectives of this dissertation is to provide additional perspectives on ancient carbon release resulting from permafrost degradation during the last deglaciation. This will help to advance our understanding of permafrost thaw dynamics and the release of aged carbon in the context of anticipated climate warming and rising sea levels. Furthermore, the last deglaciation period witnessed the disintegration of ice sheets in the Northern Hemisphere, and consequently large volumes of freshwater were released into the Arctic Ocean, with major impact on ocean circulation patterns and overall climate. Understanding the sources and timings of freshwater runoff are crucial for reconstructing abrupt climate shifts and for improving cryosphere-ocean response for the current global warming. Moreover, in order to undertake correlation between different environmental archives, and determine the exact timing of past climatic events, it is imperative to establish precise chronological frameworks. This dissertation integrates three interconnected studies from the Laptev Sea in the Siberian Arctic, offering a comprehensive perspective on Arctic deglaciation, encompassing improved age-depth model establishment, permafrost carbon release, and freshwater influxes.

The reliability of paleoclimate reconstructions, especially in polar regions, is often constrained by uncertainties in marine reservoir ages. In the Laptev Sea, precise chronological frameworks have historically been limited. In the first study, we used a pioneer approach to synchronize authigenic 10Be/9Be of sediment core PS2458-4 from the Laptev Sea with 10Be-records from absolutely dated ice cores. This method successfully provided a refined local marine radiocarbon reservoir correction (ΔR) of +345 ± 60 14C years, equating to a marine reservoir age of 848 ± 90 14C years. This ΔR value was used to improve the age-depth model of core PS2458-4. The approach offered a reliable temporal framework, potentially enhancing our capacity to precisely align Laptev Sea paleoclimatic records with other globally distributed archives, and improving our understanding of regional climate interactions during the last deglaciation.

The second study investigated the sources and mechanisms of ancient carbon release from thawing Arctic permafrost. Utilizing the refined age-depth model for core PS2458-4 from the Laptev Sea, we analyzed terrigenous biomarkers (lignin phenols, HMW n-alkanoic acids, and brGDGTs), and compound-specific radiocarbon analyses of HMW n-alkanoic acids to reconstruct the dynamics of ancient organic carbon mobilization during the last deglaciation. The results demonstrated that the highest accumulation rates of strongly pre-aged terrestrial biomarkers occurred during the warm Bølling-Allerød and Pre-Boreal periods, coinciding with rapid sea level rises. These observations indicated that ancient permafrost carbon release primarily resulted from coastal erosion driven by rapid sea level rise. Additionally, during the cold Younger Dryas period relatively lower accumulation rates of terrigenous biomarkers with younger radiocarbon ages were noted, suggesting input predominantly from surface runoff rather than from inland and coastal erosion. These findings significantly enhance our understanding of the interactions between climate-driven sea level changes and permafrost dynamics. They emphasized the role of coastal erosion as a potent mechanism for ancient carbon mobilization, and highlighted its implications for future carbon feedback mechanisms under continuing Arctic warming.

In the third study, we aimed to provide answers with regards to the source of the freshwater signal that was recorded in the Laptev Sea at the onset of the Younger Dryas. We used the authigenic Pb isotopic ratios as a sensitive tracer for freshwater, and the Pb isotopic results revealed no significant abrupt shifts indicative of large-scale freshwater runoff from the Eastern Siberian hinterland during the Younger Dryas period. Instead, the Pb isotopic composition showed subtle and gradual variability over several millennia. These results favored the scenario that the freshwater signal observed in the Laptev Sea during the Younger Dryas probably originated from the decaying Laurentide Ice Sheet from the North America continent. The freshwater was probably then advected to the Laptev Sea via Arctic Ocean circulation patterns. This finding highlighted the complex interplay between freshwater dynamics, ocean currents, and regional climate during major climatic transitions in the Arctic.

In summary, this dissertation provided an integrated view of Arctic deglacial history through the integration of precise chronological reconstruction, permafrost carbon feedbacks, and freshwater dynamics. The multidisciplinary approach provided a critical context for the forecasting of future climate trends within Arctic regions experiencing amplified warming and extensive permafrost degradation.