Evidence of multiple thermokarst lake generations from an 11 800‐year‐old permafrost core on the northern Seward Peninsula, Alaska

Abstract

Permafrost degradation influences the morphology, biogeochemical cycling and hydrology of Arctic landscapes over a range of time scales. To reconstruct temporal patterns of early to late Holocene permafrost and thermokarst dynamics, site-specific palaeo-records are needed. Here we present a multi-proxy study of a 350-cm-long permafrost core from a drained lake basin on the northern Seward Peninsula, Alaska, revealing Lateglacial to Holocene thermokarst lake dynamics in a central location of Beringia. Use of radiocarbon dating, micropalaeontology (ostracods and testaceans), sedimentology (grain-size analyses, magnetic susceptibility, tephra analyses), geochemistry (total nitrogen and carbon, total organic carbon, d13 C org ) and stable water isotopes (d 18 O, dD, d excess) of ground ice allowed the reconstruction of several distinct thermokarst lake phases. These include a pre-lacustrine environment at the base of the core characterized by the Devil Mountain Maar tephra (22 800AE280 cal. a BP, Unit A), which has vertically subsided in places due to subsequent development of a deep thermokarst lake that initiated around 11 800 cal. a BP (Unit B). At about 9000 cal. a BP this lake transitioned from a stable depositional environment to a very dynamic lake system (Unit C) characterized by fluctuating lake levels, potentially intermediate wetland development, and expansion and erosion of shore deposits. Complete drainage of this lake occurred at 1060 cal. a BP, including post-drainage sediment freezing from the top down to 154 cm and gradual accumulation of terrestrial peat (Unit D), as well as uniform upward talik refreezing. This core-based reconstruction of multiple thermokarst lake generations since 11 800 cal. a BP improves our understanding of the temporal scales of thermokarst lake development from initiation to drainage, demonstrates complex landscape evolution in the ice-rich permafrost regions of Central Beringia during the Lateglacial and Holocene, and enhances our understanding of biogeochemical cycles in thermokarst-affected regions of the Arctic.

Paleolimnology of thermokarst lakes: a window into permafrost landscape evolution

“…These results suggest that the evolution of aquatic ecosystems in the SSP is variably influenced by catchment-mediated processes, in addition to the direct effects of climate, as documented on a different, much longer time frame in northern Seward Peninsula (Lenz et al 2016). In the study by Medeiros et al (2014), an increase in supply of dissolved inorganic carbon and nitrogen related to shoreline erosion appeared to be associated with enhanced productivity in an otherwise nutrient-limited thermokarst lake.…”

“…Although some studies (summarized by Jorgenson and Shur 2007) have proposed that thermokarst lake stages from inception to termination may be cyclical, field observations focusing specifically on ground-ice content and aggradation prior to lake inception, rates of changes and associated processes during the Holocene, and diatom-based paleoecological reconstructions do not support such a recurrent succession (Jorgenson and Shur 2007;Ellis et al 2008;Grosse et al 2013;Lenz et al 2016). Instead, these findings indicate that (1) geomorphological and limnological processes occurring in thermokarst terrain do not allow the surface to return to original conditions (i.e.…”

“…Even though the expansion of thermokarst lakes has likely been occurring for centuries to millennia since deglaciation (Lenz et al 2016), increases in the extent of permafrost degradation have been observed in the northern Seward (Jones et al 2011a) and the interior of Alaska (Jorgenson et al 2006). Likewise, several studies have also noted a recent expansion of tall woody shrubs in response to earlier snowmelt, a deeper and drier active layer, and longer growing seasons linked to permafrost degradation throughout the Seward Peninsula (Sturm et al 2001;Lloyd et al 2003;Tape et al 2012).…”

“…As demonstrated in this review paper, thermokarst lakes represent more than just "by-products" of permafrost degradation; they are unique "sediment sinks" that can collect useful environmental archives over their life span (e.g., Dallimore et al 2000;Pienitz et al 2008;Edwards et al 2016;Lenz et al 2016). For example, using a set of thermokarst lakes spanning an ecoclimatic gradient at a given site, it is possible to gain knowledge about past environmental changes related to local geomorphological and hydrological processes, in addition to regional climate (Dallimore et al 2000;Wolfe et al 2011b).…”

“…This includes documenting terrestrial vegetation change, transport of dissolved organic matter related to the thawing of peat-rich permafrost, and lake expansion and subsequent drainage caused by increased summer rainfall (e.g., Bouchard et al 2011Bouchard et al , 2013aBouchard et al , 2014MacDonald et al 2012;Coleman et al 2015). Yet, some thermokarst lakes have persisted for several thousand years, spanning the Holocene and beyond, and their sediment records have yielded temporal information about the succession of cool and warm climate episodes, influence of thermokarst activity on sediment input to lakes, and carbon exchange with the atmosphere (e.g., Dallimore et al 2000;Biskaborn et al 2012Biskaborn et al , 2013aLenz et al 2013Lenz et al , 2016Walter Anthony et al 2014;Edwards et al 2016). Thus, sediments that accumulate in thermokarst lakes provide promising archives to examine a multitude of environmental changes, including temporal insights into permafrost landscape evolution.…”