Deep-time alkaline lake enigma: Rare or undiscovered?

Abstract

Alkaline lakes are significant for exploring the evolution of life, reconstructions of environments, and exploration and exploitation of energy resources; however, despite modern alkaline lakes being widely distributed on Earth, deep-time (pre-Neogene) examples are scarce, in general, compared with other types of saline lake deposits (e.g., chloride and sulfate salts). This raises the question as to whether this scarcity is due to their extreme formation conditions or because they have yet to be discovered, owing to the difficulty in identification of these deep-time lakes. This paper addresses this question by analyzing the distribution and formation mechanisms of modern alkaline lakes in order to interpret the past based on the present. These results indicate that alkaline lakes can be identified from the presence of alkali minerals, such as trona and nahcolite, and heavy N isotopes (>10‰) caused by NH₃ volatilization. Modern alkaline lakes can be classified into two main types, including those in tectonically active and inland arid zones. Their formation mechanisms are controlled by multiple factors, such as a (semi-)closed basin geography, (semi-)arid climate that favors high degrees of evaporation, and the presence of strongly alkaline fluids that are rich in HCO₃⁻ and CO₃²⁻. The formation mechanisms of discovered deep-time alkaline lakes are similar to those of modern examples. The δ¹³C, and δ¹⁸O of carbonates, Mg/Ca, chemical index of alteration, and chemical index of weathering indicate that deep-time alkaline lakes were also generally deposited in (semi-)closed basins with a (semi-)arid climate, which were conducive to strong evaporation and concentration of saline waters. Furthermore, chemical weathering and hydrolysis of silicate minerals, volcanic–hydrothermal activity and circulation of basinal brines, and microbially mediated processes also produced alkaline fluids rich in HCO₃⁻ and CO₃²⁻ necessary for the formation of ancient alkaline lakes. Alkaline lakes should be widespread throughout geological history, but may be poorly preserved in old stratigraphic sequences or difficultly in identification because of their limited outcrop and depth. Future studies of evaporite mineralogy and the N isotope geochemistry of ancient saline lacustrine basins are necessary to further our understanding of their identification and formation mechanisms. Such studies should also investigate the favorable development zones, characteristics of organic matter accumulation, astrobiology, etc.