Does stream chemistry reflect thaw depth on a seasonal scale across Alaskan Arctic permafrost catchments?

The Arctic is rapidly changing due to increasing temperatures and hydrologic intensification. The Arctic is also data-limited, necessitating the development of new tools to document and quantify ecosystem responses to these changes. Some of the hardest changes to observe are in the subsurface, including thaw depth conditions in continuous permafrost regions. Thaw depth is dynamic across the thaw season as well as on a longer, interannual scale as the Arctic warms and permafrost degrades. Measuring thaw depth often requires intensive sampling or remote sensing capabilities that have spatiotemporal limitations; therefore, little is known about complex subsurface dynamics and how they will affect Arctic ecosystems in the future. Often, surface waters are our best proxy for subsurface dynamics, as streams integrate signals from the landscape as water travels through the hillslope subsurface. In permafrost systems, water and solute flowpaths are governed by thaw depth dynamics, and flowpaths through variable soil chemical conditions govern downgradient stream water chemistry. Hence, thaw depth may be reflected in stream chemistry. Prior work looking at long-term stream chemistry data suggests there are multiple soil-derived solutes that are tracers of thaw in Arctic catchments, and some increase with thaw over decadal timescales. Building on this work, we want to know if this stream chemical tracer approach works at different spatiotemporal scales, such as within a thaw season as thaw depth increases, and across catchments with varying characteristics (e.g., slope, vegetation). We hypothesize that with more frequent stream chemistry observations across a thaw season, we will also see these soil chemical tracers signal seasonal thaw, and that the signal’s strength will vary depending on catchment characteristics.  

To determine whether we can use chemical tracers as proxies of thaw depth across a diverse set of catchments on Alaska’s North Slope, we sampled the stream outlets of three catchments underlain by continuous permafrost across three thaw seasons (2021-2023). We measured continuous discharge and analyzed nine different ions including Ca, Fe, Na, and S to identify seasonal patterns in stream chemistry, as these element concentrations change with soil depth in this region. As discharge could impact instream solute concentrations, we also analyzed concentration-discharge relationships to determine whether discharge was significantly influencing concentration and reducing the tracer’s utility in signalling thaw depth.  

We found that the efficacy of the stream tracer approach to detect thaw on a seasonal scale is seemingly dependent on catchment characteristics, as we suspected. Our two low-gradient tundra systems did not show consistent patterns in the tracers; however, we saw patterns in multiple tracers in our high-gradient alpine catchment. In our low-gradient catchments, solute concentration was often impacted by discharge, making it difficult to assess the impact of thaw depth on chemistry. Overall, understanding thaw depth dynamics will become increasingly important with climate change, necessitating the development of tools to document and predict thaw depth at a range of scales. Here, we find that stream tracers of thaw at the catchment scale show promise, but it is much more nuanced and complex than preliminary studies indicated.