CHARACTERIZATION OF CHEMICAL WEATHERING IN PROGLACIAL MATANUSKA ENVIRONMENT, SOUTHCENTRAL ALASKA

Changes in chemical weathering rates and CO₂ sequestration by retreating glaciers have been thoroughly studied on glacial-interglacial timescales, but the impact of unprecedented modern rates of climate change on glacial chemical weathering signatures are not fully understood. To quantify geochemical weathering reactions in an alpine glacial system experiencing accelerated retreat, a sampling campaign was conducted during peak ablation in July 2019 in the proglacial zone of the polythermal Matanuska Glacier. Proglacial meltwater samples were analyzed for major cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) and anions (SO₄^2-, Cl^-, NO₃^-).

Results show that meltwater solute compositions are dominated by Ca⁺² and SO₄^2- ions derived from silicate, carbonate, sulfate, sulfide minerals present in the system. Dissolved inorganic carbon (DIC, mainly HCO₃^-) and cations are sourced from carbonic acid (H₂CO₃) or sulfuric acid (H₂SO₄) reactions, both acids act as proton donors. However, H₂CO₃ reactions sequester atmospheric CO₂ whereas H₂SO₄ release CO₂. In this study, the contribution of DIC from silicate weathering via H₂CO₃ (14% on average) is minor, reflecting the dominance of carbonate weathering in the proglacial zone. Ratios of the sum of cationic charge (TZ⁺) over HCO₃^- and SO₄^2- indicate the extent of weathering from H₂CO₃ and H₂SO₄. Analyses suggests that H₂SO₄ weathering processes dominate the subglacial and immediate proglacial signatures. Hence, chemical weathering close to the glacial terminus may act as a source of atmospheric CO₂. However, this signature may transition to reflect other weathering regimes with increasing distance from the glacial terminus. Local lithology and rates of glacial physical erosion that expose fresh mineral surfaces of trace sulfide and carbonate minerals, and associated kinetics strongly influence weathering reactions and carbon feedbacks in the proglacial environment. Additional investigation will be conducted to estimate the proportion of DIC generated during carbonate weathering via H₂CO₃ versus H₂SO₄. The goal of this research is to investigate the impact of accelerated climate change on glacial weathering signatures: identifying where the proglacial chemical environment behaves as a carbon sink or source depending on distance from terminus, and proton donors and minerology present within the catchment.