Dolomite formation during penecontemporaneous subaerial diagenesis: Evidence from modern dolomite crusts forming in lagoon Brejo do Espinho, Brazil

Early lithification of carbonate mud during the subaerial exposure stage, under semiarid conditions, has been proposed to facilitate dolomite formation. However, how the biogeochemical processes during subaerial diagenesis promote dolomite formation remains unclear. Here, we employ a multiproxy approach to investigate the process of dolomite formation by analyzing modern dolomite crusts forming in lagoon Brejo do Espinho (LBE). Petrological analysis reveals that the crusts comprise coexisting high-Mg calcite (HMC) and dolomite. Low Fe and Mn concentrations indicate the formation of dolomite under oxic conditions, while a higher Sr concentration in well-lithified crust suggests primary bacterial-induced dolomite precipitation. The Mg isotopic composition of the crusts exhibits a lighter value compared to that of modern sabkha dolomite, suggesting different dolomitization processes and Mg sources. More negative δ¹³C values of the crusts than unlithified carbonate mud in LBE, indicating the incorporation of ¹³C depleted organic carbon. The biogeochemical processes related to decaying organic matter during subaerial diagenesis generate partially oxidized organic matter that promotes Mg²⁺ dehydration and enhances the dissolution of primary HMC, ultimately triggering the transition of HMC to dolomite or/and direct dolomite precipitation. The ancient "dolomite factory" operated through cyclic deposition of carbonate sediments and penecontemporaneous subaerial diagenesis.