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Theses

Paleoenvironmental reconstructions and greenhouse gas characterization in permafrost aquatic systems of Central Yakutia (Siberia)

Abstract : In Central Yakutia (Eastern Siberia, Russia), persistent freezing temperatures, flat topography, and a lack of glaciation during the Last Glacial Maximum (~20,000 years ago) have created optimal conditions for the proliferation of deep, ice-rich permafrost. Past and present climate change and other disturbances can have significant effects on permafrost landscapes and the global carbon budget. In areas of ice rich permafrost, degradation can result in the formation of thermokarst (thaw) lakes, which are hotspots for biological activity and greenhouse gas (GHG) emissions. The three axes of this thesis combine provide insights into 1) past lake dynamics, 2) spatial and temporal heterogeneity in GHG concentrations and diffusive fluxes from thermokarst lakes, and 3) large scale analysis of recent (since 1960s) lake dynamics in response to local climate trends and human activities.To understand lake development, sediment and organic carbon accumulation, and changes in primary productivity, within the context of Late Pleistocene and Holocene climate change, a multi-proxy paleolimnological analysis of a sediment core sequence from Lake Malaya Chabyda in Central Yakutia (Eastern Siberia, Russia) was conducted. Age-depth modeling with ¹⁴C indicates that the maximum age of the sediment core is ~14 cal kBP. Three distinct sedimentary units were identified within the sediment core. Sedimentological and biogeochemical properties in the deepest section of the core suggests a lake environment mostly influenced by terrestrial vegetation, where organic carbon accumulation might have been relatively low (average ~ 100 g OC m-2 a-1), although much higher than the global modern average. The middle section of the core was characterized by higher primary productivity in the lake, much higher sedimentation, and a strong increase in organic carbon (OC) delivery (average ~300 g OC m-2 a-1). Conditions in the upper section of the core (< 376 cm; < 9.0 cal kBP) suggest high primary productivity in the lake and high OC accumulation rates (average ~ 200 g OC m-2 a-1), with stable environmental conditions. Changes in the character and quantity of OC accumulation can have important implications for future GHG release. These results help to understand the developmental history of lakes in permafrost landscapes within the context of past climate change.To analyze spatial and temporal heterogeneity in GHG concentrations and diffusive flux, dissolved CO₂ and CH₄ concentrations in thermokarst lakes in a study site in Central Yakutia were measured over four seasons. Lakes formed over the Holocene (alas lakes) are compared to lakes that developed over recent decades. The results show striking differences in dissolved greenhouse gases (up to two orders of magnitude) between lake types and seasons. Shallow lakes located in hydrologically closed alas depressions acted as CO₂ sinks and strong sources of diffusive CH₄ during some seasons. Recent thermokarst lakes were moderate to extremely high sources of diffusive CO₂ and CH₄, with considerable accumulation of greenhouse gas under the ice cover (winter) or in the deepest water layers (summer). The diffusive fluxes measured from thermokarst lakes of this typical taiga alas landscape of Central Yakutia are among the highest presented across Arctic and subarctic regions. Lastly, a remote sensing analysis was conducted to investigate recent (since 1960s) lake development and quantify differences in developmental history between lake types. This study indicated that certain lake types (unconnected alas lakes) are more sensitive to annual changes in precipitation. This information will be used to upscale in situ greenhouse gas measurements to a larger landscape unit.This thesis combines paleolimnological analysis, in situ observations, and remote sensing analysis to provide a comprehensive understanding of lake development in permafrost landscapes and the contribution of these lakes to the global carbon budget.
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Submitted on : Wednesday, July 6, 2022 - 4:25:00 PM
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  • HAL Id : tel-03715697, version 1

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Lara Hughes-Allen. Paleoenvironmental reconstructions and greenhouse gas characterization in permafrost aquatic systems of Central Yakutia (Siberia). Global Changes. Université Paris-Saclay, 2022. English. ⟨NNT : 2022UPASJ011⟩. ⟨tel-03715697⟩

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