The ‘Sulfate enigma’ in Dolomite Genesis: Review and Perspectives

Key words: 
• Dolomite problem /
• Kinetic barrier /
• Sulfate /
• Inhibiting mechanism /
• Facilitating mechanism /
• Microbial dolomite

Abstract: The dolomite problem remains one of the most contentious and prominent issues in the field of sedimentology. Previous studies have demonstrated that the direct precipitation of ordered dolomite under low-temperature, inorganic conditions is challenging both in laboratory settings and modern natural sedimentary environments. The formation of dolomite is a kinetically controlled process and several crucial factors have been identified, including the hydration of Mg2+, the presence of sulfate inhibitor, nucleation sites, and the ordering of cations. The role of sulfate in the formation of dolomite has garnered considerable attention, yet it remains a subject of substantial debate. The hypothesis that sulfate inhibits the formation of dolomite has long been widely accepted by geologists, serving as an explanatory framework for elucidating the evolution of dolomite abundance and seawater properties throughout geological history. However, with the advancement of research, particularly in ongoing investigations into the formation mechanism of microbial dolomite, a contentious debate has arisen regarding the role of sulfate, as some scholars argued that the presence of sulfate does not inhibit low-temperature dolomite precipitation. The controversy regarding the controversial role of sulfate arises from: (1) The large variation in SO42- concentration in modern natural sedimentary environments where dolomite is precipitated; (2) The distinct roles of SO42- vary under inorganic and organic，high-temperature and low-temperature conditions; (3) The limitations of applying conceptual patterns derived from laboratory contexts with singular factors to complex sedimentary environments. The ‘Sulfate enigma’ remains an unresolved issue in understanding the dolomite genesis. Here, we present a systematic review of the diverse perspectives regarding the role of sulfate in dolomite formation over the past 60 years. Firstly, we elaborate on the two prevailing perspectives regarding how SO42- inhabits dolomite formation. Subsequently, we assess the efficacy of SO42- as an inhibitor under various conditions, and reinterpret its role within the microbial dolomite model. Finally, we summarize the role of sulfate in the laboratory synthesis of dolomite analogs while addressing existing challenges and limitations. This study aims to enhance the understanding of the ‘dolomite problem’ and provide theoretical support for further elucidating the relationship between dolomite abundance in geological history and the evolution of paleoceanic properties.