Mitchell, A. C. and Brown, G. H. (2008)

Modeling geochemical and biogeochemical reactions in subglacial environments

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Arctic, Antarctic, and Alpine Research
This study examines current subglacial biogeochemical weathering models using PHREEQCi, a computer-based speciation mass-balance (SMB) model, parameterized using hydrochemical and mineralogical field data from Haut Glacier d’Arolla, Switzerland. The aim is to investigate the utility of SMB models for quantifying subglacial biogeochemical mechanisms, and the mathematical and thermodynamic robustness of current subglacial weathering models in a temporally variable, spatially heterogeneous hydrological and geochemical subglacial environment. The chemical evolution of meltwaters between their source in the supraglacial environment and their output from the hydroglacial system as bulk meltwaters are modeled, and compared with in situ meltwaters collected from the base of boreholes drilled to the glacier bed. SMB modeling produced a broad range of weathering outcomes, but delivered no unique weathering scenario which could account for the observed changes in water chemistry between input and output waters over the ablation season. Organic carbon oxidation and sulfide oxidation by dissolved O2, coupled to carbonate dissolution and incongruent silicate dissolution, could account for seasonal changes in meltwater chemistry, supporting current subglacial biogeochemical weathering models. Atmospheric CO2 was not required under any weathering scenario, and organic carbon and atmospheric CO2 dissolution was only possible in one weathering scenario, at a mass ratio of 10:1, further suggesting that CO2-driven dissolution is unimportant in subglacial environments. This investigation indicates the utility of SMB models applied to subglacial hydrological systems for determining geochemical and biogeochemical processes and interpreting water quality.