Geochemical, sedimentological and microbial diversity in two thermokarst lakes of far Eastern Siberia

Abstract

Thermokarst lakes are important conduits for organic carbon sequestration, soil organic matter (soil-OM) decomposition and release of atmospheric greenhouse gases 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. Another key characteristic of thermokarst lakes are their eroding shorelines, depending on the surrounding landscape, they can play a major role in supplying the lakebeds with sediment and OM. These differences in winter ice regime and eroding shorelines are key factors which determine the quantity and quality of OM in thermokarst lake sediments. 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 sediment organic matter and dissolved organic carbon, 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 the lake water depth (bedfast-ice vs. floating-ice) and shoreline erosion to be the two most likely main drivers of the sedimentary, microbial and biogeochemical diversity in thermokarst lakes. 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.

Arctic Thermokarst Lakes – Carbon Comes and Goes

“…The LOI data represent averages of the top 10 cm of the lake sediment cores (n = 2) from nearby (within 1.5 m) duplicate cores (Meisel et al, , 2023. 4.1 Overview of sediment core field data.…”

“…A total of 11 sediment cores were taken in separate field campaigns in November of 2015 (Alaska, 7 cores) and November of 2016 (Siberia, 4 cores) for this incubation study (Supplementary Table 4.1). Both field campaigns as well as the biogeochemical and sedimentary characteristics of the collected sediment cores were described in further detail by Meisel et al ( , 2023. The sediment cores were collected from the frozen lake surfaces during November 2015 and 2016 with a hand-held UWITEC gravity coring system which deployed 9 cm outer diameter PVC tubes at the lakebed.…”

“…The sediments collected in the PVC tubes were sufficiently stiff and compact to maintain their sedimentary structure. Freezing of the sediment cores was prevented in the field to avoid overpressurization and cracking of the PVC core tubes (Meisel et al, , 2023.…”