The “dolomite problem”, which has confused scientists for nearly two centuries, is an important fundamental geological problem. The mineralogical characteristics of carbonate minerals show that the dolomite structure consists of an ordered arrangement of alternating layers of Ca²⁺ and Mg²⁺ cations interspersed with ${\mathrm{C}\mathrm{O}}_3^{2-}$ anion layers normal to the c-axis. The dolomite structure violates the c glide plane in the calcite structure, which means that dolomite has R$\overline{3}$ space group symmetry. The ordered dolomite has superlattice XRD reflections [e.g., (101), (015) and (021)], which distinguish it from calcite and high-Mg calcite. The calculation of thermodynamic parameters shows that modern seawater has a thermodynamic tendency of dolomite precipitation and the dolomitization reaction can be carried out in standard state. However, the latest thermodynamic study shows that modern seawater is not conducive to dolomitization, and that seawater is favorable for dolomitization in only a few regions, such as Abu Dhabi, the Mediterranean and the hypersaline lagoons in Brazil. The kinetic factors of dolomite formation mainly consist of the hydration of Mg²⁺, the presence of sulfate and the activity of carbonate. Current studies have shown that the presence of microorganisms, exopolymeric substances (EPS), organic molecules, carboxyl and hydroxyl functional groups associated with microorganisms and organic molecules, clay minerals with negative charges and dissolved silica facilitate magnesium ions to overcome hydration and thus promote Mg²⁺ incorporation into growing Ca-Mg carbonates. Similarly, the metabolic activity of microorganisms is conducive to the increase in alkalinity. However, the inhibitory effect of sulfate on dolomite formation seems to be overestimated, and sulfate may even be a catalyst for dolomite formation. Combining the carbonate crystallization mechanism with thermodynamic and kinetic factors suggests that the early stage of dolomite precipitation or the dolomitization reaction may be controlled by kinetics and dominated by unstable intermediate phases, while metastable intermediate phases later transform to ordered dolomite via an Ostwald’s step rule.

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