^1,2Shuya Tan et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [10.1016/j.gca.2026.06.034]
¹Earth and Space Exploration Center, Ritsumeikan University, Kusatsu, Japan
²Institute for Extra-cutting-edge Science and Technology Avantgarde Research of Life (X-star), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan
Copyright Elsevier

The geochemical effects of freezing are becoming important in the investigation of closed aqueous environments, such as inland water on Earth and early Mars, and liquid water on planetesimals. Carbonate-bearing alkaline saline lakes in Mongolia are frozen in the cold season, with chemical species being partitioned among surface ice, lake water, and sediments. Freezing of the lakes leads to the accumulation of dissolved carbonate species, thereby decreasing the pH. The lakes are enriched not only in heavy metals, such as As, Mo, and U, but also in phosphorus. However, little is known about how metals and phosphorus are affected by chemical changes during freezing. Moreover, the mechanisms of major chemical changes are poorly understood and reproduced. Here we performed field surveys to investigate the behavior of these elements during lake freezing. Heavy metals and phosphorus accumulate in lake water during freezing, similar to major elements such as Cl−, with Mo and U concentrations reaching ∼1 mg/L. On the other hand, As and P accumulations are limited. Concentrations of heavy metals and phosphorus in ice increase with depth in the ice. We interpret the observed behavior using a geochemical model that accounts for their adsorption reactions coupled with water removal by freezing and carbonate precipitation. The model successfully reproduces the major chemical changes, including the decrease in pH, achieving quantitative accuracy by accounting for the combined effects of freezing and the revised solubility of carbonate minerals. The pH decrease promotes the adsorptions of As and P on sedimentary ferrihydrite particles, suppressing their accumulation in lake water. However, the decrease in pH is insufficient to promote adsorptions of Mo and U, resulting in their accumulations as major dissolved species. Adsorptions of heavy metals and phosphorus by iron oxides may be an important factor in their behaviors at low temperatures near the freezing point of water. Based on our model and observations, we discuss phosphate/carbonate precipitation in freezing porewater of icy planetesimals and phosphate availability in lake water on early Mars.