Geochemistry of low-temperature dolomite formation in coastal and continental saline to sabkha-like environments

Abstract

Early diagenetic dolomite forms at low temperatures (<50°C) in settings where relative contributions of microbial influences and pore-fluid chemistry are difficult to separate. Here, dolomite and associated aragonite, calcite and Mg-calcite from modern and ancient coastal and continental settings in Spain, Australia and Brazil were analysed using a multi-proxy geochemical approach. Scanning electron microscopy (SEM) images reveal filamentous structures, extracellular polymeric substances (EPS) and nanoglobules intergrown with the carbonate, consistent with microbial mediation nucleation. Stable carbon and oxygen isotope compositions (δ¹³C = −10.0–3.4‰; δ¹⁸O = −1.6–7.3‰) reflect variations in palaeoproductivity, temperature and hydrological regimes, and ⁸⁷Sr/⁸⁶Sr ratios (0.708213–0.709243) indicate fluid sources. Sr (up to 5828 ppm), Fe (up to 5187 ppm) and Mn (up to 1035 ppm) reach concentrations above those expected for equilibrium precipitation. Sr enrichment does not correlate with Mg/Ca, which we attribute to kinetically controlled uptake during rapid, microbially mediated growth rather than to equilibrium partitioning into the Ca lattice site. Mn and Fe concentrations vary independently, suggesting that they are not controlled by the same redox conditions. Instead, their distribution mainly reflects environmental conditions rather than direct microbial control. In addition, δ⁵⁶Fe values vary between sites (−0.98–0.09‰), providing additional insights into redox conditions during carbonate formation. Together, the data indicate that early diagenetic dolomite formation is governed by site-specific combinations of sulphate reduction, aerobic heterotrophy, EPS-mediated divalent cation binding and fluid pathways, rather than a single mechanism. This study provides a geochemical framework for low-temperature, microbially mediated dolomite formation and identifies which proxies are, and are not, diagnostic of microbial influence in ancient sedimentary records.