Magnesium isotope fractionation during alkaline brine evaporation and implications for Precambrian seawater chemistry

Abstract

The chemical evolution of the ocean is one major component of the puzzle of how climate and life have co-evolved over the Earth's history. A “soda ocean” with high alkalinity, high pH, and low calcium concentration in Precambrian has been proposed to explain the emergence and evolution of early life. However, this hypothesis has not been widely accepted due to the lack of reliable tracers for the chemical composition of Precambrian seawater. Evaporite is formed during seawater/brine evaporation and thus has been widely used to reconstruct the ancient seawater/brine chemical composition. Here, evaporation experiments were conducted using Qinghai Lake (QHL) water, a modern soda lake, to provide an analogy of the Precambrian “soda ocean” evaporation and investigate the mineralogical and Mg isotopic signatures of alkaline brine-derived evaporites. Our results show that the evaporation path of QHL water overall covers the stages of hydrous Mg carbonates (hydromagnesite), halite, and bloedite [Na2Mg(SO4)2·4H2O] precipitation. The precipitation of hydrous Mg carbonates and bloedite is distinct from the modern seawater evaporation and is accompanied by the removal of up to 85% Mg from brine. The brine gradually becomes enriched in heavy Mg isotopes during evaporation due to the preferential incorporation of light Mg isotopes into precipitates. The fractionation of Mg isotopes is dominantly controlled by the bond structure during hydrous Mg carbonates and bloedite precipitation, and the latter is also slightly influenced by the kinetics in the highly concentrated brine. The Mg isotope fractionation during bloedite dissolution is limited due to the rapid congruent dissolution. The significant Mg isotopic fractionation observed within the hydrous Mg carbonates precipitation during the earliest QHL water evaporation indicates the potential Rayleigh distillation of Mg by alkalinity in the “soda ocean”, which is absent in the Phanerozoic oceans. Therefore, the large Mg isotope fractionation during alkaline brine evaporation can be applied to test the existence of Precambrian “soda oceans”.