Field Investigations of Chemical Partitioning and Aqueous Chemistry of Freezing Closed-Basin Lakes in Mongolia as Analogs of Subsurface Brines on Icy Bodies

Several icy bodies (e.g., Europa and Ceres) likely possess near-surface brine reservoirs, which are considered to be sources of salts on their surfaces. Previous studies have proposed processes whereby an overlying ice layer captures salinity from a freezing, subsurface reservoir. However, few field investigations have been conducted to investigate chemical partitioning and aqueous chemistry in freezing brine reservoirs in terrestrial analogs. Here we report results of winter field surveys to ice-covered, closed-basin saline lakes in the Valley of the Gobi Lakes, Mongolia. We found that the surface ice exhibited complex features, including both pressure ridges formed by compressive forces and wet cracks formed by tensile forces. The latter provides salinity to the ice layer and surface. In addition to wet cracks, saline lake bottom-water, along with suspended matter, was continuously captured within the pores of the ice layer during downward freezing. Using a combined mass balance and low-temperature aqueous chemistry model, we reproduced the ice salinity due to entrapment of Na⁺, Cl⁻, and SO₄²⁻, suggesting that chemical partitioning of these species in freezing brine reservoirs on icy bodies can be understood based on our current knowledge. In the lake bottom-water, the metastable phase of carbonates (i.e., amorphous Mg-carbonate and monohydrocalcite) play key roles in controlling the concentrations of precipitable Mg²⁺ and Ca²⁺ and alkalinity, highlighting the importance of metastable phase of carbonate in the aqueous chemistry of subsurface brine reservoirs on icy bodies. Metastable phases might be indicators of freshly erupted materials from brine reservoirs.