Understanding of Silicate Weathering in Subglacial Environment by Inverse Modeling of West Greenland Glacial Meltwater

The acceleration of chemical weathering due to physical processes in glaciers has been studied in various systems. In our study we consider the potential role of microbes in addition to physical weathering to enhance weathering in runoff from the Greenland Ice sheet (GrIS). Sub- and supra- glacial weathering products in bulk meltwater are used to determine reaction processes and assess solute provenance. Most current models estimate that 80-100% of the dominant ion Ca2+ is derived from weathering of trace carbonates. The potential for significant silicate-derived Ca2+ is not generally considered. We hypothesize that seasonal changes in subglacial water routing and water residence times have a large impact on the weathering of lithologies with differing geochemical reactivity. This study deconvolutes the solute chemistry in runoff from the GrIS using PHREEQCi, a computer-based speciation mass-balance model. The model utilizes a mass balance approach and allows multiple alternative weathering scenarios under different hydrological conditions to be tested simultaneously. It is parameterized using seasonal chemical and mineralogical field data from Thule (76°N, 68°W), West Greenland. Hypothetical geochemical weathering scenarios suggest Ca-feldspar dissolution is an important solute source and silicate weathering is likely to be dominated by Ca-feldspar weathering in GrIS, due to their relatively high dissolution rates. The proportion of silicate dissolution decreases with increasing discharge on the seasonal timescale, and this reflects the seasonal expansion of the channelized system. At places where the development of channelized system is limited and most waters are routed through a distributed system, silicate minerals, which are more abundant but less reactive than carbonates, have sufficient time to dissolve and may have a greater contribution, approximately equal amounts of Ca2+ as carbonates, to the major solutes