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Seawater Mg/Ca controls polymorph mineralogy of microbial CaCO 3 : A potential proxy for calcite‐aragonite seas in Precambrian time
Author(s) -
RIES J. B.,
ANDERSON M. A.,
HILL R. T.
Publication year - 2008
Publication title -
geobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.859
H-Index - 72
eISSN - 1472-4669
pISSN - 1472-4677
DOI - 10.1111/j.1472-4669.2007.00134.x
Subject(s) - calcite , aragonite , seawater , geology , precambrian , artificial seawater , mineralogy , strontium , geochemistry , chemistry , oceanography , organic chemistry
A previously published hydrothermal brine‐river water mixing model driven by ocean crust production suggests that the molar Mg/Ca ratio of seawater ( m Mg/Ca sw ) has varied significantly (~1.0–5.2) over Precambrian time, resulting in six intervals of aragonite‐favouring seas ( m Mg/Ca sw > 2) and five intervals of calcite‐favouring seas ( m Mg/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 ( m Mg/Ca sw = 5.2) precipitated primarily aragonite with lesser amounts of high‐Mg calcite ( m Mg/Ca calcite = 0.16), while biofilms cultured in experimental calcite seawater ( m Mg/Ca sw = 1.5) precipitated exclusively lower magnesian calcite ( m Mg/Ca calcite = 0.06). Furthermore, Mg/Ca calcite varied proportionally with Mg/Ca sw . This nearly abiotic mineralogical response of the biofilm CaCO 3 to altered Mg/Ca sw is consistent with the assertion that biofilm calcification proceeds more through the elevation of , via metabolic removal of CO 2 and/or H + , than through the elevation of Ca 2+ , which would alter the Mg/Ca ratio of the biofilm's calcifying fluid causing its pattern of CaCO 3 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 CaCO 3 polymorph mineralogy, then the observed response of the biofilms’ CaCO 3 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.