Thermokarst Lagoons: A Core-Based Assessment of Depositional Characteristics and an Estimate of Carbon Pools on the Bykovsky Peninsula

Abstract

Permafrost region subsurface organic carbon (OC) pools are a major component of the terrestrial carbon cycle and vulnerable to a warming climate. Thermokarst lagoons are an important transition stage with complex depositional histories during which permafrost and lacustrine carbon pools are transformed along eroding Arctic coasts. The effects of temperature and salinity changes during thermokarst lake to lagoon transitions on thaw history and lagoon deposits are understudied. We analyzed two 30-m-long sediment cores from two thermokarst lagoons on the Bykovsky Peninsula, Northeast Siberia, using sedimentological, geochronological, hydrochemical, and biogeochemical techniques. Using remote sensing we distinguished between a semi-closed and a nearly closed lagoon. We (1) characterized the depositional history, (2) studied the impact of marine inundation on ice-bearing permafrost and taliks, and (3) quantified the OC pools for different stages of thermokarst lagoons. Fluvial and former Yedoma deposits were found at depth between 30 and 8.5 m, while lake and lagoon deposits formed the upper layers. The electrical conductivity of the pore water indicated hypersaline conditions for the semi-closed lagoon (max: 108 mS/cm), while fresh to brackish conditions were observed beneath a 5 m-thick surface saline layer at the nearly closed lagoon. The deposits had a mean OC content of 15 ± 2 kg/m3, with higher values in the semi-closed lagoon. Based on the cores we estimated a total OC pool of 5.7 Mt-C for the first 30 m of sediment below five mapped lagoons on the Bykovsky Peninsula. Our results suggest that paleo river branches shaped the middle Pleistocene landscape followed by late Pleistocene Yedoma permafrost accumulation and early Holocene lake development. Afterward, lake drainage, marine flooding, and bedfast ice formation caused the saline enrichment of pore water, which led to cryotic talik development. We find that the OC-pool of Arctic lagoons may comprise a substantial inventory of partially thawed and partially refrozen OC, which is available for microbial degradation processes at the Arctic terrestrial-marine interface. Climate change in the Arctic leading to sea level rise, permafrost thaw, coastal erosion, and sea ice loss may increase the rate of thermokarst lagoon formation and thus increase the importance of lagoons as biogeochemical processors of former permafrost OC.

Sediment and carbon accumulation in a glacial lake in Chukotka (Arctic Siberia) during the Late Pleistocene and Holocene: combining hydroacoustic profiling and down-core analyses

“…The small carbon pool is especially evident when compared with carbon budgets and sediment volumes reported for Pleistocene-Holocene age yedoma and thermokarst and drained thermokarst lake basin records from Alaska, as well as Holocene TOC pool and sediment volumes of Alberta glacial lakes, Canada (Fig. 9b) (Campbell et al, 2000;Jongejans et al, 2018). Pleistocene-Holocene sediments obtained from thermokarst lagoon and permafrost sediments from the Bykovsky Peninsula, northern Yakutia, also show significantly higher estimated TOC pool (Bykovsky lagoons 5.72 Mt) and TOC density (mean 15.29 kg m −3 ) relative to their small spatial extents (Jenrich et al, 2021;Schirrmeister et al, 2011).…”

“…Epilimnetic water samples were collected in 2016 (Huang et al, 2020) and in 2018 (this study) from multiple surface locations and were immediately measured for pH, conductivity and oxygen content (WTW Multilab 540, Germany). Water subsamples were then filtered (0.45 µm) and transported for analyses of dissolved organic carbon (DOC), anions and cations (Table S1).…”

“…In order to estimate sediment (Sed pool , Mt) and carbon pools (TOC pool , Mt) at Lake Rauchuagytgyn, we used an established bootstrapping approach modified from Strauss et al (2013), Jongejans et al (2018 and Windirsch et al (2020) that excluded ice wedges not present within the lake sediments and was enhanced to include sediment volumes estimated from GIS-based methods (Eqs. 3 and 4).…”

“…Such studies have shown that lake systems may operate in transitional modes between carbon sink and source stages depending on the prevailing climatic background conditions (Anthony et al, 2014). Recent syntheses have characterised the sedimentological characteristics and carbon budgets of Bykovsky Peninsula lagoons of northern Yakutia, eastern Siberia, and carbon inventories within alas and yedoma deposits from central Yakutia and are interpreted in terms of palaeoenvironmental variability since the Pleistocene (Jenrich et al, 2021;Windirsch et al, 2020). Such works have also been extended to drained thermokarst lake basins (DTLBs) and Pleistocene yedoma deposits in north-western Alaska (Jongejans et al, 2018).…”

“…Recent syntheses have characterised the sedimentological characteristics and carbon budgets of Bykovsky Peninsula lagoons of northern Yakutia, eastern Siberia, and carbon inventories within alas and yedoma deposits from central Yakutia and are interpreted in terms of palaeoenvironmental variability since the Pleistocene (Jenrich et al, 2021;Windirsch et al, 2020). Such works have also been extended to drained thermokarst lake basins (DTLBs) and Pleistocene yedoma deposits in north-western Alaska (Jongejans et al, 2018). The reconstruction of accumulation rates in these syntheses has, however, been avoided due to significant reworking of carbon material within per-mafrost landscapes (Strunk et al, 2020;Windirsch et al, 2020).…”