Geochemical, sedimentological and microbial diversity in contrasting thermokarst lake sediments of Far Eastern Siberia

Abstract Thermokarst lakes are important conduits for organic carbon (OC) sequestration, soil organic matter (SOM) processing and atmospheric greenhouse gas (GHG) release in the Arctic. They can be classified as either floating-ice lakes, which sustain a zone of unfrozen sediment (talik) at the lakebed year-round, or as bedfast-ice lakes, which freeze all the way to the lakebed in winter. This difference in winter ice regime, as well as their eroding shorelines which mainly determine the quantity and quality of organic matter (OM) input into the lakes, is thought to have a strong influence on the lake’s sediment characteristics and general lake biogeochemistry. We used an array of physical, geochemical, and microbiological tools to identify the differences in the environmental conditions, sedimentary characteristics, carbon stocks and microbial community compositions in the sediments of a bedfast-ice and a floating-ice lake in Far East Siberia with different eroding shorelines. Our data show strong differences across most of the measured parameters between the two lakes. For example, the floating-ice lake contains considerably lower amounts of SOM and dissolved organic carbon (DOC), both of which also appear to be more degraded in comparison to the bedfast-ice lake, based on their stable carbon isotope composition (δ13C). We also document clear differences in the microbial community composition, for both archaea and bacteria. We identified two main drivers of the sedimentary, microbial and biogeochemical diversity in thermokarst lakes. First, the lake depth, which defines the ice regime (bedfast-ice and floating-ice) and thereby also determines the absence or presence of taliks. Second, the input of eroded shoreline soils and OM into the lakes from different allochthonous sources. With ongoing climate warming, it is likely that an increasing number of lakes will shift from a bedfast- to a floating-ice state, and that increasing levels of shoreline erosion will supply the lakes with sediments. Yet, still little is known about the physical, biogeochemical and microbial differences in the sediments of these lake types and how different eroding shorelines impact these lake systems.