title: Seawater Mg/Ca controls polymorph mineralogy of microbial CaCO3: a potential proxy for calcite-aragonite seas in Precambrian time.
abstract: 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 (approximately 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 , 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.
authors.affiliation: Department of Geology and Geophysics, The Woods Hole Oceanographic Institution, MS #23, Woods Hole, Massachusetts 02543, USA. jries@whoi.edu
authors.forename: J B
authors.initials: JB
authors.lastname: Ries
authors.forename: M A
authors.initials: MA
authors.lastname: Anderson
authors.forename: R T
authors.initials: RT
authors.lastname: Hill
mesh_terms: D001419:Bacteria / Q000145:classification / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D018441:Biofilms / Q000254:growth & development; D002118:Calcium / Q000032:analysis*; D002119:Calcium Carbonate / Q000032:analysis / Q000737:chemistry* / Q000378:metabolism; D011232:Chemical Precipitation; D017753:Ecosystem; D005580:Fossils; D019015:Geologic Sediments / Q000382:microbiology; D055593:Geological Phenomena; D005844:Geology; D008274:Magnesium / Q000032:analysis*; D010163:Paleontology; D012623:Seawater / Q000737:chemistry* / Q000382:microbiology; D013995:Time
chemical_list: D002119:Calcium Carbonate; D008274:Magnesium; D002118:Calcium