A 6000-year record of permafrost melt events from the Alaskan North Slope

Background/Question/Methods

Permafrost melting may be accelerated by rising temperatures in the Arctic, which can dramatically alter carbon cycling in a region that stores approximately twice as much carbon as the global atmospheric pool.  Thermokarst, areas of rapid permafrost subsidence, expose this previously unavailable soil carbon to microbial decomposition.  Furthermore, sediments and cations are rapidly released during thermokarst events, which can have profound effects on ecosystem productivity and nutrient cycling. We explore long-term trends in thermokarst activity on the basis of lithologic and geochemical analyses of lake sediments from the Alaskan North Slope. We compare sediment records from Lake NE14, which has an active shoreline thaw slump, to sediments from Perched Lake, which has no discernible thermokarst feature in its watershed.  The slump at Lake NE14 released fine clay particles and cations to the lake during its formation. Using X-ray fluorescence technology, we identify changes in the elemental composition of lake sediments that have similar characteristics to this modern influx of sediments from permafrost soils.  This study offers a first glimpse into long-term permafrost dynamics in the Alaskan Arctic. 

Results/Conclusions

The sediments from Lake NE14 are characterized by the presence of several distinct and thick clay intervals throughout the core that are similar to clays deposited during the recent thermokarst event. Distinct peaks in Ca/K and Ca/Sr, which correspond to these fine-grained sediments, are interpreted as intervals of increased erosion of permafrost soils relative to active layer and organic soil horizons. Using these metrics, we identify at least nine potential periods of enhanced thermokarst activity over the past six millennia at Lake NE14, suggesting that the modern thermokarst feature in the watershed is not unprecedented. Sediments from Perched Lake do not exhibit these sedimentological or geochemical signatures, confirming long-term stability in our “control” watershed.  Furthermore, peak amplitudes in the NE14 geochemical record are high 6000-1500 cal BP and lower thereafter, suggesting a decrease in intensity of these thermokarst events after ~1500 cal BP that may be related to Neoglacial cooling in the region.  These results are a substantial first step in assessing the controls of long-term permafrost dynamics, with important implications for ecosystem productivity, nutrient cycling, and long-term carbon storage.