In Situ Formation of Monohydrocalcite in Alkaline Saline Lakes of the Valley of Gobi Lakes: Prediction for Mg, Ca, and Total Dissolved Carbonate Concentrations in Enceladus’ Ocean and Alkaline-Carbonate Ocean Worlds

Abstract

The nature of mineral precipitations in terrestrial alkaline soda lakes provides insights into the water chemistry of subsurface oceans on icy bodies in the outer solar system. Saturation analyses of terrestrial alkaline lakes have shown that the solution chemistries of lake waters are generally controlled by the presence of monohydrocalcite (MHC) and amorphous Mg-carbonate (AMC). However, direct observations of the formation of these metastable carbonates in natural alkaline lakes have been limited. This study provides evidence of in situ MHC formation in alkaline lakes, based on the water chemistry and mineralogy of suspended matter in Olgoy, Boon Tsagaan, and Orog Lakes (Valley of Gobi Lakes, Mongolia). The solution chemistries were close to saturation with respect to MHC and AMC, consistent with other alkaline lakes worldwide. Suspended matter was separated by the ultracentrifugation of lake water following freeze-drying. Our results show that MHC is the common mineral phase in the suspended matter. These observations confirm that MHC is the direct authigenic product of evaporation in alkaline lakes. The carbonate fraction in suspended matter from Olgoy Lake has a Mg/Ca ratio of 0.4, suggesting the formation of AMC in association with MHC. Based on the dissolution equilibria of AMC and MHC, we predict the Mg2+, Ca2+, and total dissolved carbonate concentrations in Enceladus’ ocean to be ~1 mmol/kg, ~10 μmol/kg, and 0.06–0.2 mol/kg, respectively, in the presence of AMC and MHC. We propose that the measurements of Mg contents in plumes will be key to constraining the total dissolved carbonate concentrations and chemical affinities of subsurface oceans on Enceladus and other alkaline-carbonate ocean worlds.

Hydrogeochemical Study on Closed-Basin Lakes in Cold and Semi-Arid Climates of the Valley of the Gobi Lakes, Mongolia: Implications for Hydrology and Water Chemistry of Paleolakes on Mars

“…Precipitation of MHC is the precursor of authigenic deposition of calcite in alkaline lacustrine environments [35]. Furthermore, our companion paper found MHC present in the suspended materials in the lake waters of the Böön Tsagaan lake [36]. Since MHC and AMC are considered to be the initial phases of precipitation of Mg− and Ca−bearing carbonate minerals [35,36], formations of magnesite and calcite are suppressed in our calculations.…”

“…The saturation indexes of MHC of the Böön Tsagaan, Orog, and Olgoy lakes are commonly slightly supersaturated with respect to MHC from 0.3 to 0.6, suggestive of precipitation of MHC in the lakes. The precipitations of MHC were confirmed through collection and analysis of suspended materials in these lakes by our companion paper [36]. The saturation indexes of AMC of the lakes during a rainy season in 2018 range from -0.1 to −0.6, although those during a dry season in 2017 are slightly low-down to −1.0-compared with those in the wet season (the seasonal difference in the saturation indexes cannot be explained by the difference in surface temperatures).…”

“…Furthermore, our companion paper found MHC present in the suspended materials in the lake waters of the Böön Tsagaan lake [36]. Since MHC and AMC are considered to be the initial phases of precipitation of Mg− and Ca−bearing carbonate minerals [35,36], formations of magnesite and calcite are suppressed in our calculations. On the other hand, we considered Na + , K + , Cl − , and SO 4 2− were conservative (i.e., no removals from lake water via precipitation of minerals) upon evaporative concentration, which meant that no precipitations of chloride and sulfate minerals were taken into account.…”

“…Local seepages of the groundwater would have then supplied these dissolved species to the lake. The supplied Mg 2+ and HCO 3 − would have been consumed within the early Jezero lakes through evaporation and consequent precipitation of authigenic carbonate minerals, such as AMC, in a similar manner to the terrestrial saline lakes found in the Valley of the Gobi Lakes [35,36]. In fact, recent remote−sensing observations found possible evidence for lacustrine carbonates (hydrated magnesite and/or hydromagnesite) near a large inlet river and delta deposits of Jezero Crater [43].…”

“…Upon evaporation, we consider the precipitations of both MHC and AMC at the fixed ambient CO 2 pressure of 4 × 10 −4 atm. This is the case because the water chemistry (Mg 2+ and Ca 2+ concentrations) of the lakes is thought to be governed by the formation of and dissolution equilibria with MHC and AMC [35,36], and because calcite is found in the sediments of the Böön Tsagaan lake (Figure 4 and Section 3.2). Precipitation of MHC is the precursor of authigenic deposition of calcite in alkaline lacustrine environments [35].…”