Hydrogeochemistry on lake systems in cold and semi-arid climate of the Valley of the Gobi Lakes, Mongolia: Implications for hydrology of paleolakes on Mars

Abstract:

According to climatological and geochemical studies, the climate of early Mars would have been semi-arid when the surface temperatures were above freezing. In such semi-arid climates, closed-basin lakes would have been created on early Mars; however, the hydrogeochemical cycles of the lake systems are poorly constrained. Mars rovers can acquire detailed geochemical data via in-situ measurements; nevertheless, the data are spatially limited near the landing sites. In this regard, field investigations of a terrestrial analog of Martian paleolakes would provide insights into the hydrogeochemical cycles.

Here we report results of our field surveys to terrestrial analogs of closed-basin lake systems that developed in cold and semi-arid climates; the Boon Tsagaan, Orog, and Olgoy lakes of the Valley of the Gobi Lakes of Mongolia. Our results show that groundwater plays a central role not only in hydrology, but also in geochemical cycles in the lake systems. We find that groundwater predominantly flows into the lakes through local seepage and regional flows in semi-arid climates. Through the interactions with calcite-containing soils, local groundwater seepage provides Ca and HCO₃ to the lakes. In the wetland located in between the lakes, high-salinity shallow pools would provide Cl and Na to the groundwater through infiltration. If similar processes occurred on early Mars, local seepage of groundwater would have provided magnesium and alkalinity to the early Jezero lakes, possibly leading to authigenic precipitation of lacustrine carbonates as suggested by Horgan et al. (2020). On early Mars, infiltration of surface brine may have transported salts and oxidants on the surface to lakes via regional groundwater flows, as proposed by Melwani Daswani and Kite (2017) and Mitra and Catalano (2019). We suggest that inflows of multiple types of groundwater in semi-arid climates could have caused redox disequilibria in closed-basin lakes on early Mars, which could have affected the past habitability.