| Autor: |
Ries, J. B., Anderson, M. A., Hill, R. T. |
| Předmět: |
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| Zdroj: |
Geobiology; Mar2008, Vol. 6 Issue 2, p106-119, 14p, 2 Diagrams, 1 Chart, 2 Graphs |
| Abstrakt: |
A previously published hydrothermal brine-river water mixing model driven by ocean crust production suggests that the molar Mg/Ca ratio of seawater ( mMg/Casw) has varied significantly (~1.0–5.2) over Precambrian time, resulting in six intervals of aragonite-favouring seas ( mMg/Casw > 2) and five intervals of calcite-favouring seas ( mMg/Casw < 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/Casw = 5.2) precipitated primarily aragonite with lesser amounts of high-Mg calcite ( mMg/Cacalcite = 0.16), while biofilms cultured in experimental calcite seawater ( mMg/Casw = 1.5) precipitated exclusively lower magnesian calcite ( mMg/Cacalcite = 0.06). Furthermore, Mg/Cacalcite varied proportionally with Mg/Casw. This nearly abiotic mineralogical response of the biofilm CaCO3 to altered Mg/Casw 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/Casw was the primary variable influencing CaCO3 polymorph mineralogy, then the observed response of the biofilms’ CaCO3 polymorph mineralogy to variations in Mg/Casw, combined with the ubiquity of such microbial carbonates in Precambrian strata, suggests that the original polymorph mineralogy and Mg/Cacalcite 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. [ABSTRACT FROM AUTHOR] |
| Databáze: |
Complementary Index |
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