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

The 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 analysing modern dolomite crusts forming in lagoon Brejo do Espinho. Petrological analysis reveals that the crusts consist of coexisting high-Mg calcite and dolomite. Low Fe and Mn concentrations indicate the formation of dolomite under oxic conditions, whereas 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 than that of modern sabkha dolomite, suggesting different dolomitization processes and Mg sources. The more negative δ 13 C values of the crusts than those of the unlithified carbonate mud in lagoon Brejo do Espinho indicate the incorporation of 13 C-depleted organic carbon. The biogeochemical processes related to decaying organic matter during subaerial diagenesis generate partially oxidized organic matter that promotes Mg 2+ dehydration and enhances the dissolution of primary high-Mg calcite, ultimately triggering the transition of high-Mg calcite to dolomite and/or the direct precipitation of dolomite. The ancient ‘dolomite factory’ operated through the cyclic deposition of carbonate sediments and penecontemporaneous subaerial diagenesis. Thematic collection: This article is part of the Towards unravelling the ‘Dolomite Problem’: new approaches and novel perspectives collection available at: https://www.lyellcollection.org/topic/collections/towards-unravelling-the-dolomite-problem