T180 Weathering and Soils: Advances in Understanding Rates, Mechanisms, Controlling Factors and Feedbacks

Tuesday, 12 October 2021
1:30 PM - 5:30 PM
D139/D140

CHARACTERIZATION OF CHEMICAL WEATHERING IN PROGLACIAL MATANUSKA ENVIRONMENT, SOUTHCENTRAL ALASKA

Paper number: T180-11
Program: Technical Sessions
Time: 3:55 PM – 4:10 PM

Authors

Abstract

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 (SO42-, Cl-, NO3-).

Results show that meltwater solute compositions are dominated by Ca+2 and SO42- 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 SO42- 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.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-366841

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