Two chloride sources in soils of the McMurdo Dry Valleys, Antarctica

Abstract

Sulfate, nitrate, and chloride are major water‐soluble anions in soils of the McMurdo Dry Valleys (MDV), Antarctica. We measured the chloride stable isotope composition of 40 soil samples from 16 different soil profiles. The soil profiles come from three different microclimate zones in the MDV: a coastal thaw zone, which fosters a seasonally saturated active layer; a stable upland zone, in which air and soil temperatures are too cold to foster active layers and/or significant snowmelt; and an intermediate, inland mixed zone. Results show that the total water‐soluble chloride content is low in both the coastal and the upland zones, but highly variable in the inland mixed zone (average 770 to 11520 ppm). Individual soil δ37Cl values range from −2.10‰ to +0.56‰, with profile average δ37Cl values broadly decreasing from the coast toward upland valleys. Vertically, the δ37Cl increases with depth in soil profiles developed on basal tills but decreases on sublimation tills. We propose that the spatial [Cl−] and δ37Cl patterns are the result of continuous influx and leaching of two major chloride sources, an airborne sea‐salt chloride (SSC) with a δ37Cl value ≥ ∼0.0‰ and a secondary atmospheric chloride (SAC) with a δ37Cl value ≤ −2.10‰. Our model suggests that sublimation tills have their Cl− input from two boundaries: a till‐atmosphere interface at the top and a buried‐ice‐till interface at the bottom, differing distinctively from basal tills where there is no salt contribution from the bottom. Our model also suggests a higher influx of SSC relative to SAC in the MDV in the past. Overall, characterizing the sources, budgets, and spatial/vertical patterns of chloride constitutes a first step toward unraveling climate, tectonic, and landscape changes archived in MDV soil profiles.