UNDERSTANDING CHEMICAL WEATHERING IN PERIGLACIAL/SUBGLACIAL ENVIRONMENTS BY INVERSE MODELING OF WEST GREENLAND GLACIAL MELTWATER USING PHREEQCI

Biogeochemical processes in periglacial and subglacial environments play an important role in regulation of atmospheric CO₂ contents, generation of soils, the evolution of landscape, and are critical to our understanding of terrestrial chemical denudation and global biogeochemical cycles, particularly as changes are occurring in glaciated systems. Two study sites located adjacent to the western Greenland Ice Sheet in this study are at Thule (76°N, 68°W) dominated by supraglacial melting with considerable contribution of solutes from periglacial environment, and at Kangerlussuaq (67°N, 50°W) with a well-developed subglacial drainage system as major source of solutes. PHREEQCi, a computer-based speciation mass-balance model, solves a series of chemical reactions simultaneously based on the input parameters of supraglacial meltwater as inflow and bulk meltwater as outflow, as well as being constrained by local mineralogy. PHREEQCi is an informative model for generating a range of weathering scenarios that could account for the observed changes in water chemistry between the supraglacial and bulk meltwater, and it makes the comparison between two sites with different climatic, glacio-hydrological and lithological settings straightforward. Model results indicate that the main weathering regime at Thule is sulfide oxidation coupled with silicate weathering, whereas at Kangerlussuaq carbonation dominants with a few exceptions and sulfide oxidation coupled with silicate weathering dominants over the late season. The feasibility of anoxic weathering, which is hypothesized to be present in subglacial environments, was also tested at the Kangerlussuaq site. For this scenario, the model demonstrates that methanogenesis can account for the bulk meltwater evolution by reducing organic carbon to methane. This study provides a suite of possible geochemical weathering reactions for interpreting water quality in the periglacial/subglacial environments. Inverse modeling typically does not provide a unique solution and additional constraints such as primary and secondary minerals are helpful to select the most likely weathering scenario.