Silicate Weathering in Antarctic Ice-rich Permafrost: Insights Using Magnesium Isotopes

This study reports that substantial chemical weathering occurs at subzero temperatures in ice-and-salt-rich permafrost in the McMurdo Dry Valleys, Antarctica. Chemical weathering is documented in a 30.0-m core collected in Beacon Valley by measuring the ionic composition, pH, and Mg isotopes of water extracted from thawed ice-rich sediment. Evidence of rock weathering is revealed by coinciding increases in the Mg isotopic composition and pH values. The primary factor that controls weathering is the salt content that leads to unfrozen brine; this is most apparent in the upper 7.0 m where salt content is high, temperatures rise above −21 °C and modeled unfrozen water reaches up to 4.0% of ice-content. In the upper 7.0 m, up to 60% of soluble Mg in the thawed permafrost ice is sourced from Ferrar Dolerite (δ26Mg = −0.22 ± 0.07‰) weathering, resulting in δ26Mg values ranging from of −0.82 ± 0.05‰ to −0.64 ± 0.05‰. Below 7.0 m, temperatures remain below −21 °C, unfrozen water is less than 2.0% of ice-content, and on average, 5% of soluble Mg is sourced from dolerite weathering with δ26Mg values ranging from −1.05 ± 0.05‰ to −0.76 ± 0.05‰. Regions of the core that are modeled to have no unfrozen water show little or no evidence of chemical weathering and relatively constant δ26Mg values close to Taylor Glacier and Beacon Valley snowfall values (–0.93 ± 0.06‰). This study demonstrates that significant chemical weathering occurs at subzero temperatures in permafrost where liquid brines form.