Holocene Thermokarst Lake Dynamics in Northern Interior Alaska: The Interplay of Climate, Fire, and Subsurface Hydrology

Lesleigh Anderson,Bruce P Finney,Mary Edwards,Catherine Langdon,Mark D Shapley

doi:10.3389/feart.2019.00053

Abstract

The current state of permafrost in Alaska, and meaningful expectations for its future evolution are better informed by long-term perspectives of previous permafrost degradation. Thermokarst processes in permafrost landscapes often leads to widespread lake formation. The spatial and temporal evolution of thermokarst lake landscapes reflects the combined effects of climate, ground conditions, vegetation, and fire. To further explore the Holocene history of Yukon Flats thermokarst lakes, we conducted a detailed study of Greenpepper lake, located on the southern loess uplands, including bathymetry and sediment core analyses across a water depth transect. The sediment results, dated by radiocarbon and 210Pb, indicate that Greenpepper basin development initiated ~8000 cal yr BP through inferred thermokarst processes. Thermokarst expansion to the modern lake’s near-shore shallow water depths continued to ~5000 and 4000 cal yr BP. Subsequently stable sediment chronologies and properties indicate the lake established levels ~4 m higher than present. At that time, water overflowed into an over-deepened gully system no longer occupied by perennial streams and connected to nearby partially drained lakes. By ~1000 cal yr BP, Greenpepper lake levels had lowered below the outflow elevation, which is interpreted to reflect a response to drier conditions and possibly to downgradient catastrophic lake drainage that increased subsurface water loss, although this mechanism is untested. Timing of Greenpepper thermokarst initiation and lake expansion was later than previously documented elsewhere in the southern loess upland and post-dates the establishment of spruce forest. After the lake stabilized by ~4 ka, the late Holocene millennial trend of increased aridity corresponds with Yukon Flats regional fire and paleoclimate reconstructions and suggest linkages with large scale Holocene patterns of North Pacific atmospheric circulation.

Highlights

Lakes of thermokarst origin are hallmark surface features of arctic and sub-arctic permafrost landscapes
Water column measurements indicate that Greenpepper Lake is dimictic with summer thermal stratification indicated by declines in temperature, dissolved oxygen (DO), specific conductance (Spc) and pH between 4 and 6 m water depth (Figure 2A)
The lake is supersaturated in carbon dioxide and methane and has high concentrations of dissolved organic (DOC) and inorganic carbon (DIC); a DOC age of ∼125 years was determined by the National Ocean Sciences Accelerator Mass Spectrometer at the Woods Hole Oceanographic Institution (Wickland, 2018 personal communication)

Summary

Introduction

Lakes of thermokarst origin are hallmark surface features of arctic and sub-arctic permafrost landscapes. Ground-collapse following initial permafrost warming is referred to as thermokarst, whereas water-filled depressions that further deepen and expand by talik formation and thermal/mechanical erosion are thermokarst lakes, or thaw lakes (Grosse et al, 2013). Deep (>10 m) extant thaw lakes in Siberia have been described in relation to carbon fluxes (Zimov et al, 1997; Walter et al, 2006) and two studies in the Old Crow Basin, Yukon, Canada document the evolution of thaw lakes through the Holocene (Lauriol et al, 2002; Burn and Smith, 2006) It has only been with the advent of high-resolution digital elevation detection that additional thermokarst features have been ‘uncovered’ in the densely forested regions of interior Alaska. Holocene forest establishment in interior Alaska enhanced fire (Edwards et al, 2016), which can act to trigger thermokarst initiation (see Brown et al, 2015 for contemporary evidence)

Methods

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Discussion

Conclusion