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

Maren Jenrich,Jens Strauss,Ingmar Nitze,Boris K Biskaborn,Lutz Schirrmeister,Loeka L Jongejans,Michael Angelopoulos,Andrew Murray,Pier Paul Overduin,Susanne Liebner,Mikhail Grigoriev,Guido Grosse

doi:10.3389/feart.2021.637899

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.

Highlights

Over the last 2 decades, the Arctic has been warming more than twice as fast as the global average (Johannessen et al, 2004; Berner et al, 2005; Notz and Stroeve 2016)
The results suggest that Holocene permafrost degradation subsided the Yedoma surface by up to 25 m for Uomullyakh Lagoon (UoL) and 27 m for Polar Fox Lagoon (PFL), potentially causing mass flux of sediment and its organic carbon
This study shows that thermokarst lagoons are highly dynamic landforms at the boundary between terrestrial permafrost and marine systems

Summary

Introduction

Over the last 2 decades, the Arctic has been warming more than twice as fast as the global average (Johannessen et al, 2004; Berner et al, 2005; Notz and Stroeve 2016) This amplified warming has led to rapid surface warming in Siberia with modeled temperature rise of up to 4°C over the last 5 decades (Romanovsky et al, 2010; Biskaborn et al, 2019; Lenssen et al, 2019; GISTEMP Team, 2020). Arctic coastal systems are especially affected by rapid permafrost thaw and mobilization of organic matter by erosion and marine inundation (Fritz et al, 2017). As lagoons are complex and diverse systems, there are many different approaches to classify them, for example based on geomorphological and geological origin, their morphological properties such as size and water depth (Kosyan, 2016), the degree of isolation from the sea (Kjerfve, 1994), their physicochemical state (for example, salinity, ionic composition, temperature, turbidity, nutrients) (Tagliapietra et al, 2009), or the degree of influence by living organisms such as corals and humans (Brovko 1990)

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