SUBGLACIAL CHEMICAL WEATHERING OF TWO ALASKAN GLACIERS: INSIGHTS FROM DIURNAL VARIATIONS IN WATER CHEMISTRY

The water chemistry of glacial streams provides information about subglacial chemical weathering reactions. Temperate glaciers are prolific agents of physical erosion that produce abundant fresh mineral surface area available for chemical weathering. Subglacial weathering can act as either a net source or sink of atmospheric CO₂ depending on whether the dominant reaction is silicate weathering (a CO₂ sink) or coupled carbonate dissolution-sulfide oxidation (a CO₂ source). Therefore, glacial erosion and weathering have been invoked as mechanisms that alter atmospheric pCO₂ and Earth’s climate on Quaternary glacial-interglacial and longer timescales. To better define subglacial weathering’s impact on Earth’s climate, there is need for additional data constraining the relationship between weathering, erosion, and meltwater discharge. On shorter timescales, variations in water chemistry provide insights into subglacial hydrology, as waters with different residence times will have different chemical composition.

Here, we present major ion chemistry data from proglacial stream water samples from near the terminus of Bench Glacier (Chugach Range) and Kasidaya Glacier (Coast Range), Alaska. We collected water samples multiple times a day to capture diurnal variations in water chemistry and meltwater discharge. We measured major cations (Na, K, Ca, Mg) and major anions (SO₄, HCO₃, Cl, NO₃), and used a weathering mass balance model to determine the dominant chemical weathering reactions for each sample. We combine our chemical measurements with proglacial stream discharge records to examine how water chemistry evolves alongside discharge. Our preliminary results spanning three diurnal cycles at Bench Glacier suggest that coupled sulfide oxidation-carbonate dissolution (a CO₂-releasing reaction) is the dominant chemical weathering reaction over silicate weathering, despite silicate-rich bedrock lithology. We find that the relative dominance of chemical weathering reactions exhibits substantial diurnal variation that is not entirely explained by variations in discharge. Our work has implications for understanding subglacial hydrologic flow paths and water residence times, as well as understanding the effect of glaciation on long-term atmospheric CO₂ and climate.