Permafrost carbon mobilization into the Laptev Sea since the last deglaciation

Warming in the Arctic has been observed to exceed the global average by more than four times, which can accelerate circumarctic permafrost thaw, leading to the release of carbon into the atmospheric carbon pool. Once thawed, circumarctic permafrost is transported away from the land and eventually deposited in the marginal seas of the Arctic Ocean. During these processes, carbon can be released through microbial decomposition. Several questions related to this process remain insufficiently understood: Which environmental conditions favor permafrost thawing, and through what mechanisms? How vulnerable is organic matter released from thawing permafrost to decomposition? What is the key step in carbon release during permafrost mobilization? To address these questions, this thesis investigates the dynamics of mobilized terrestrial permafrost deposited on the Laptev Sea shelf, where terrestrial sources comprise an approximately equal mix of Pleistocene and Holocene permafrost deposits.
The first manuscript (Chapter 3) evaluated the organic matter characteristics of terrestrial Pleistocene and Holocene permafrost by analyzing their bulk organic composition, isotopic signatures, and thermal reactivity properties. Terrestrial Pleistocene permafrost was found to exhibit lower reactivity and higher degradation status compared to Holocene permafrost. Surface and downcore sediments (PS51/154 and PS51/159) from the Laptev Sea shelf were subsequently analyzed for the same parameters, and lower reactivity of the Laptev Sea surface sediment was found in the region with higher input from mobilized terrestrial Pleistocene permafrost. A decrease in organic matter reactivity was observed with the increase in transport distance from the shoreline, indicating that degradation during transport is a major control for the organic matter reactivity of Laptev Sea surface sediments. A drastic decrease in organic matter reactivity near the coast was observed, indicating the nearshore region as a hotspot of rapid organic matter degradation.
The second manuscript (Chapter 4) aimed to identify potential environmental causes of rapid terrestrial permafrost thawing by examining sediment core records from the western Laptev Sea (PS51/154 and PS51/159) spanning the last deglaciation. Three periods of high mass accumulation rates of terrestrial biomarkers were identified and linked to distinct environmental conditions. Comparisons with published records from other Arctic marginal seas revealed that enhanced coastal erosion driven by accelerated sea-level rise during meltwater pulse 1A (mwp-1A) was a widespread phenomenon across different marginal seas in the Arctic. Additional periods in rapid terrestrial biomarker mobilization were identified asynchronously between regions and attributed to various region-specific factors, such as freshwater flooding events, enhanced inland warming, and prolonged sea ice-free seasons.
The third manuscript (Chapter 5) further traced the sources of mobilized terrestrial organic matter during these periods of rapid terrestrial biomarker mobilization. Age-depth models for the two cores were first refined using an updated estimation of the local marine reservoir age, which was determined by aligning authigenic 10Be/9Be ratio variations in marine sediment and 10Be fluxes in ice cores. Pre-depositional ages of terrigenous biomarkers, including C28:0 fatty acids, C23 + C25 n-alkanes, and C29 + C31 n-alkanes, were analyzed to assess terrestrial source changes across different periods. The pre-depositional ages of C28:0 fatty acids indicated a higher contribution of aged material during periods of rapid terrestrial biomarker mobilization, whereas inputs from younger permafrost increased during intervals of enhanced river discharge. In contrast, the pre-depositional ages of C23 + C25 and C29 + C31 n-alkanes suggested a consistently predominant input from aged terrestrial materials.
Overall, this thesis provides a multifaceted investigation of terrestrial organic matter transport into the Laptev Sea. It includes differences in reactivity between young and aged terrestrial permafrost, variations in organic matter reactivity during transport and post-burial processes, environmental factors fostering rapid terrestrial permafrost mobilization, and the sources of mobilized materials. These findings contribute to a better understanding of permafrost-derived carbon dynamics in the Arctic and their implications for global carbon cycling.