Spatial Drivers in the Origin and Composition of Dissolved Organic Matter in Snow: Implications for Proglacial Stream Biogeochemistry

Abstract:

The Coast Mountains of southeast Alaska are currently experiencing high rates of glacier volume loss. Continued glacier wastage therefore has the potential to decrease the proportion of streamflow derived from glacial runoff, which could alter the nature of dissolved organic matter (DOM) delivered to proglacial streams. We collected snow from ten locations along a transect that extended from the coast 47 km across the Juneau Icefield, southeast Alaska and analyzed the snow for δ¹⁸O and DOM for ¹³C, ¹⁴C and fluorescence characteristics. Our goal was to assess the origin and quality of DOM in snow to better understand how continued glacial recession in the region may influence the transfer of organic matter to proglacial aquatic ecosystems. The δ¹⁸O of snow decreased with distance from the coast (r²=84, p<0.01) indicative of the natural fractionation or fallout of heavy δ¹⁸O that occurs along elevation or spatial gradients. This depletion in the isotopic signature of snow across the Icefield transect was reflected in the origin and quality of DOM. Concentrations of dissolved organic carbon (DOC) varied from 0.13 to 0.29 mg C L^-1 and progressively decreased (r²=43, p<0.05) as δ¹⁸O became more depleted. The Δ¹⁴C-DOC varied from -742 to -420‰ and showed progressive depletion with decreasing δ¹⁸O (r²=56, p<0.01). Older DOC corresponded to a decrease in the percent contribution of humic-like fluorescence (r²=74, p<0.01) suggesting an overall decrease in modern continental DOM across the transect. A three-source isotope mixing model showed that DOM in snow originates mainly from anthropogenic aerosols from fossil fuel combustion (45-74%) and marine sources (17-34%). These results suggest that anthropogenic aerosols are a quantitatively important source of relic DOM to the glacier ecosystem. Given relic DOM exported from glaciers is highly bioavailable, anthropogenic aerosols could profoundly influence the transfer of DOM from glaciers to proglacial aquatic ecosystems.