Mineralogic control on the calcium and magnesium stable isotopic compositions of modern microbial carbonates

Modern microbialites grow in a variety of environments including the hypersaline, turbid, low alkalinity, high magnesium (Mg) and calcium (Ca) concentrations (and Mg/Ca ∼ 4.5 mol/mol), shallow (less than 2 m deep) Storr’s Lake on San Salvador Island, The Bahamas. Rather than growing via the trapping and binding of sediments, these largely micritic microbialites form via microbial processes creating crusts and mounds with laminated to clotted structures comprised of both high-Mg calcite and aragonite. The primary objective of this study was to determine whether the mineralogy of representative microbialite mounds dictates their pre-burial metal isotopic compositions. The Mg and Ca isotopic compositions (δ26Mg, δ44/40Ca) of two mounds collected from 0.6 to 1.1 m water depth range considerably, from −3.04 to −2.33 ‰ (relative to DSM3) and 0.38 to 0.94 ‰ (relative to SRM 915a), respectively. Along with Sr/Ca and Mg/Ca molar ratios, δ26Mg and δ44/40Ca are unambiguously related to mineralogy. This stands in contrast to the carbon and oxygen isotopic compositions of the microbial carbonates, which do not correlate solely with mineralogy. A simple bimineralic mixture of aragonite and high-Mg calcite can explain the observations; such a mixture could be a consequence of both phases forming independently or as one phase recrystallizes from another prior to burial. To evaluate the latter hypothesis, we used a time-dependent advection-recrystallization model. We found that recrystallization of high-Mg calcite to aragonite would require an unreasonable Sr partition coefficient, while recrystallization of aragonite to high-Mg calcite could explain the observed geochemical variations in the two mounds. No microbial isotopic effect is apparent for Mg or Ca, however a difference is seen in the carbon isotopic composition of inorganic carbon where or when aragonite and high-Mg calcite form suggesting they form in two different (micro)environments. Consequently, investigations of ancient microbialites (and carbonates in general) need to consider whether mixtures of primary (original) carbonate minerals and/or recrystallization and transformation pre-burial could impact δ26Mg, δ44/40Ca, δ13C and their interpretation using a combination of isotopic, elemental, petrographic, and modeling methods. This is especially important for Mg, whose elemental and isotopic composition can be dominated by even small amounts of calcite or dolomite.