Abstract
Permafrost in the Arctic is decreasing in extent and the depth of the seasonally thawed layer, the active layer, is increasing. Increased exposure to water is increasing fluxes of organic and inorganic solutes with potential impacts for the global carbon cycle and downstream ecosystems. Understanding the relationship between solute release and active layer depth will be critical for modelling environmental impact, especially in inaccessible regions where there is a lack of data. In this study, we focus on the potential for the isotopes of lithium (Li) and uranium (U) to track active layer extent in two permafrost-dominated catchments in Svalbard: one glaciated and one unglaciated. These isotope systems can be measured to a much higher precision than concentration measurements and act as sensitive tracers of environmental change. The extent of Li isotope fractionation provides information on the balance between dissolution of primary phases and formation of secondary phases, such as clay minerals and oxides. The U activity ratio provides information on water-rock interaction times and physical properties. We observe contrasting behaviour between the two catchments. The highest U activity ratios and Li isotope values (those most distinct from bedrock) are observed in summer in the unglaciated catchment, when the active layer depth is expected to be at its maximum extent, whereas negligible seasonal variation and the lowest values are observed in the glaciated catchment. We therefore propose that the extent of solute acquisition is directly linked to the active layer depth, which is restricted in the glaciated catchment due to a layer of `dead ice' underneath the glacial outwash plain, and could therefore provide a valuable tool to assess changes in active layer depth at catchment scales.
Highlights
The Arctic is currently experiencing a marked warming trend in comparison to other regions of the world, with average temperatures more than 1.5◦C warmer than the 1971–2000 average (Overland et al, 2013)
There are a number of scenarios which can lead to high U activity ratios in solution: (1) increased physical erosion which decreases the average particle size, increasing the likelihood 234U will be ejected, (2) rapid chemical weathering of lattice-damaged minerals, releasing 234U, and (3) increased water residence time, which increases the time in which 234U can build up in the surrounding solution
As for U activity ratios, we suggest that the positive relationship between ionic strength and δ7Li (Figure 3B) arises from a residence time control based on reactive transport models that predict increasing δ7Li with increasing subsurface residence time (Wanner et al, 2014)
Summary
The Arctic is currently experiencing a marked warming trend in comparison to other regions of the world, with average temperatures more than 1.5◦C warmer than the 1971–2000 average (Overland et al, 2013). In the Mackenzie and Yukon rivers, an increase in sulfate flux has been linked to the exposure of previously frozen and unweathered material to water and the atmosphere, leading to an increase in the chemical weathering of sulfide minerals (Tank et al, 2016; Toohey et al, 2016). Trace metal concentrations (Barker et al, 2014) and several metal isotope systems (Sr, U, Ca, Mg, Si, Keller et al, 2010; Bagard et al, 2011, 2013; Pokrovsky et al, 2013; Mavromatis et al, 2016; Lehn et al, 2017) have been used as tracers of chemical weathering during the seasonal cycle of active-layer thaw and re-freeze. In this study we explore the utility of two different isotope systems (lithium and uranium), which are widely used in chemical weathering studies (e.g., Bourdon et al, 2003; Tomascak et al, 2016), to track the extent of chemical weathering in permafrost catchments as a proxy for the depth of the active layer
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