Modelling geochemical and biogeochemical reactions in subglacial environments

The determination of geochemical and biogeochemical processes in subglacial environments is critical to our understanding of processes of terrestrial chemical denudation and global biogeochemical cycles in glaciersed environments. This study examines current subglacial biogeochemical weathering models using PHREEQCi, a computer-based speciation mass-balance (SMB) model, parameterised using hydrochemical and mineralogical field data from a small Alpine glacier (Haut Glacier d'Arolla, Switzerland). The aim is to explore the utility of SMB models for identifying and quantifying subglacial biogeochemical mechanisms, and the potential of such approaches in polar and sub-Antarctic hydrological systems. The chemical evolution of meltwaters along a subglacial flow path is modelled, and the mathematical and thermodynamic robustness of current subglacial weathering models in a temporally-variable, spatially- heterogeneous hydrological and geochemical subglacial environment is tested. SMB modelling produced a broad range of weathering outcomes, but delivered no unique weathering scenario which could account for the observed changes in water chemistry. Organic carbon oxidation and sulphide 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 relatively unimportant in subglacial environments. The suggestion of current subglacial biogeochemical weathering models that Fe3+(aq) can act as a sulphide oxidising agent under anoxic conditions was also tested. Under suboxic conditions (pE ~ 0) conditions, abiotic speciation could not generate aqueous Fe3+ (aq), suggesting that abiotic processes cannot account for the generation of Fe3+. This suggests that microbial oxidation of Fe2+ to Fe3+ is required if Fe3+ is indeed an important oxidising agent in subglacial environments.