Stream geochemistry as an indicator of increasing permafrost thaw depth in an arctic watershed

Abstract

The presence of permafrost has a strong influence on arctic hydrology, ecology, and engineering. Therefore, understanding the response of permafrost to arctic warming is critical to predicting the regional effects of global climate change. Recent research suggests that thaw depth may be increasing in response to warming, but physical thaw depth surveys in the Alaskan arctic are often not sensitive enough to detect incremental increases and cannot measure increases in the permafrost thaw bulbs beneath lakes and streams. Here we assess the use of geochemical tracers in stream water to identify changes in thaw depth in an arctic watershed. Based on marked differences in geochemistry with depth in soils and permafrost on the Alaskan North Slope, we used 87Sr/ 86Sr and elemental ratios in an arctic stream as tracers of increases in the maximum depth of soil water flow and therefore the integrated thaw depth in the watershed. From 1994 to 2004, stream water 87Sr/ 86Sr, Ca/Na, and Ca/Ba at base flow showed significant trends with time, consistent with increasing depth of soil water flowpaths. Although long time series will be necessary to identify long-term trends, stream geochemistry may be useful as a qualitative indicator of changes in thaw depth in other areas where permafrost and active layer soil geochemistry differs.