ARCTIC STREAM GEOCHEMISTRY AS A MONITOR OF RECENT PERMAFROST DEGRADATION

Arctic permafrost has strong regional effects on hydrology, mineral weathering, nutrient supply, and organic carbon storage; therefore, understanding the response of permafrost to climatic warming is critical to predicting regional effects of global climate change. Recent research suggests that arctic permafrost degradation may be occurring in response to warming, but the evidence is not yet conclusive. Thaw depth records may be confounded by interannual changes in soil conditions, and active-layer thickness increases may occur in areas where thaw depth is not easily measured, e.g. beneath lakes and streams. As an alternative to measuring thaw depth, we employed geochemical tracers of permafrost degradation in an arctic watershed.

Permafrost developed on young (<100 ka) glacial till on the Alaskan North Slope has substantially greater carbonate content than the overlying seasonally thawed soil. Thawing and subsequent exposure of carbonate-rich permafrost to weathering would release carbonate weathering products into stream waters on young till surfaces, providing geochemical tracers of permafrost degradation such as increases in Ca/Na and Ca/Ba and decreases in ⁸⁷Sr/⁸⁶Sr. We analyzed element concentrations and Sr isotope ratios in summer stream water samples from the Arctic Long Term Ecological Research site in northern Alaska, and observed such geochemical changes through time.

From 1994 to 2003, July-August stream water Ca/Ba and Ca/Na showed significant increasing trends (n=85; R²=0.3, p<0.001; R²=0.1, p=0.02). ⁸⁷Sr/⁸⁶Sr was strongly correlated to stream discharge (n=114, R²=0.3, p<0.001), likely as a result of a high ⁸⁷Sr/⁸⁶Sr signal acquired by overland flow during storm events. Excluding samples taken during periods of elevated discharge, July-August ⁸⁷Sr/⁸⁶Sr also showed a significant trend toward increasing contribution of carbonates over the past decade (n=51, R²=0.3, p<0.001). These results suggest that active-layer thickness has been increasing over the past decade in at least some environments, concurrent with a regional warming trend. Stream geochemistry may be a useful monitor of changes in active-layer thickness and permafrost stability in other areas where active-layer mineralogy differs from that of the permafrost.