Spatial characterization of chemical weathering in a proglacial river system, southcentral Alaska

Abstract

Major constituent concentrations (Na+, K+, Ca2+, Mg2+, HCO3−, SO42−, Cl−, NO3−) were determined for a 121 km reach of the proglacial Matanuska River, associated tributaries, and five additional glacial rivers on the Kenai Peninsula of southcentral Alaska during peak flow conditions in July 2019. Mass balance and mixing models were utilized to gain insight to chemical weathering processes and carbon feedback implications along the sampling transect. Solute compositions of the Matanuska River varied spatially, shifting from carbonate dominance near the terminus of the Matanuska Glacier to a carbonate-silicate weathering signature in the lower reaches of the river. Geochemical modeling suggests that carbonate dissolution closest to the Matanuska Glacier terminus is driven by sulfuric acid produced from the oxidation of sulfide minerals in the subglacial and proglacial system that acts as a source of CO2. This contrasts with the lower reaches of the Matanuska River, its tributaries, and other Kenai glacial rivers, where mineral dissolution is dominated by carbonic acid and weathering acts as a sink of atmospheric CO2. The spatial variations observed in stream and river compositions are predominantly attributed to the local lithological variations across the catchment, with subglacial weathering signatures observed in the meltwater solute compositions closest to the Matanuska Glacier terminus. This study highlights the importance of spatial sampling in proglacial systems, as proglacial water chemical signatures and carbon feedback implications can shift significantly away from the glacial terminus.