Seawater Mg/Ca controls polymorph mineralogy of microbial CaCO3:: A potential proxy for calcite-aragonite seas in Precambrian time

A previously published hydrothermal brine-river water mixing model driven by ocean crust production suggests that the molar Mg/Ca ratio of seawater (mMg/Ca-sw) has varied significantly (similar to 1.0-5.2) over Precambrian time, resulting in six intervals of aragonite-favouring seas (mMg/Ca-sw > 2) and five intervals of calcite-favouring seas (mMg/Ca-sw < 2) since the Late Archaean. To evaluate the viability of microbial carbonates as mineralogical proxy for Precambrian calcite-aragonite seas, calcifying microbial marine biofilms were cultured in experimental seawaters formulated over the range of Mg/Ca ratios believed to have characterized Precambrian seawater. Biofilms cultured in experimental aragonite seawater (mMg/Ca-sw = 5.2) precipitated primarily aragonite with lesser amounts of high-Mg calcite (mMg/Ca-calcite = 0.16), while biofilms cultured in experimental calcite seawater (mMg/Ca-sw = 1.5) precipitated exclusively lower magnesian calcite (mMg/Ca-calcite = 0.06). Furthermore, Mg/Ca-calcite varied proportionally with Mg/Ca-sw. This nearly abiotic mineralogical response of the biofilm CaCO3 to altered Mg/Ca-sw is consistent with the assertion that biofilm calcification proceeds more through the elevation of CO32- via metabolic removal of CO2 and/or H+, than through the elevation of Ca2+, which would alter the Mg/Ca ratio of the biofilm's calcifying fluid causing its pattern of CaCO3 polymorph precipitation (aragonite vs. calcite; Mg-incorporation in calcite) to deviate from that of abiotic calcification. If previous assertions are correct that the physicochemical properties of Precambrian seawater were such that Mg/Ca-sw was the primary variable influencing CaCO3 polymorph mineralogy, then the observed response of the biofilms' CaCO3 polymorph mineralogy to variations in Mg/Ca-sw, combined with the ubiquity of such microbial carbonates in Precambrian strata, suggests that the original polymorph mineralogy and Mg/Ca-calcite of well-preserved microbial carbonates may be an archive of calcite-aragonite seas throughout Precambrian time. These results invite a systematic evaluation of microbial carbonate primary mineralogy to empirically constrain Precambrian seawater Mg/Ca.