CHEMICAL WEATHERING IN PERMAFROST ASSESSED USING MAGNESIUM ISOTOPES

While it is recognized that chemical weathering occurs in permafrost soils, there are few studies that document in-situ weathering of the frozen sediment. This study analyzes the chemical composition, including magnesium isotopes, that have accumulated in the ice phase of a 30-meter permafrost core collected in the Antarctic Dry Valleys. This study reveals that Mg is sourced from two end members with distinct isotopic signatures: (1) marine aerosols that are captured and deposited by snowfall, and (2) chemical weathering of dolerite. Weathering products accumulate in the ice and have Mg isotopic values between the compositions of the two end members. The conditions under which chemical weathering occur is characterized by measured temperatures, soluble salt concentrations, pH, rock-ice ratios, Mg isotopes, and modeled unfrozen water content using the chemical thermodynamic models, PHREEQC and FREZCHEM. The data reveal that Mg isotopic ratios and core composition change abruptly at 7 meters. The Mg isotopic values in the upper 7 meters follow a mixing curve of the two Mg end members. The upper 7 meters are characterized by seasonal temperatures rising above -20°C, with higher soluble salt concentrations, pH values, rock-ice ratios, and percentage of unfrozen water content compared to below 7 meters. Below 7 meters, Mg isotopic values fall within the range of the marine aerosol end member with little deviation. The results show chemical weathering occurring in the upper 7 meters and provide evidence of the degree and potentially the rate of chemical weathering in permafrost. Primary controls on chemical weathering are the rock-ice ratios and the amount of unfrozen water, which is dependent on both temperature and the soluble salt concentrations.