Li and U Isotopes as a Potential Tool for Monitoring Active Layer Deepening in Permafrost Dominated Catchments

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 modeling 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 behavior 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.

Experimental constraints on Li isotope fractionation during clay formation

“…Long-term reproducibility for seawater (OSIL IAPSO batch P157) is 30.8±1.1 (2SD, n=45) in agreement with the compiled values of 31.1 (Carignan et al, 2004) and 30.8 (Rosner et al, 2007). We obtained a value of 4.9±1.9 (2SD, n = 10) for USGS shale rock standard SGR-1b (Hindshaw et al, 2018), in agreement with previously published values (Phan et al, 2016;Pogge von Strandmann et al, 2017b;Bohlin et al, 2018). The long-term external reproducibility of seawater is applied to the samples measured in this study (1.1 ), as this value is greater than the 2SD of individual sample measurements.…”

Experimental determination of Li isotope behaviour during basalt weathering

“…8.9 ± 0.4‰ (Magna et al, 2006)); SGR-1b: d 7 Li = 4.00 ± 0.24‰ (e.g. 4.7 ± 0.7‰ (Phan et al, 2016); 3.6 ± 0.4‰ (Pogge von Strandmann et al, 2017b); 4.9 ± 1.9‰ (Hindshaw et al, 2018)). Hence, our standard results show that our Li isotope analyses are accurate, and have a long-term external precision (for the twelve months the new laboratory has been operational) of better than ±0.4‰ (2sd).…”

The Response of Magnesium, Silicon, and Calcium Isotopes to Rapidly Uplifting and Weathering Terrains: South Island, New Zealand

“…There is no apparent influence by vegetation, as both glacial and vegetated terrains cover the entire range of observed δ 26 Mg. Uranium activity ratios are controlled by the ratio of physical erosion to mineral dissolution (Henderson, 2002;Chabaux et al, 2003;Robinson et al, 2004;Pogge von Strandmann et al, 2006, 2011aAndersen et al, 2009;Hindshaw et al, 2018). Relatively high physical erosion rates increase mineral surface area, promoting both recoil of 234 U and the leaching of 234 U from recoil-damaged lattice sites, and driving riverine ( 234 U/ 238 U) to high values (Figure 3E).…”