Formation of molar tooth structures in low sulfate Precambrian oceans

Abstract

Molar tooth structures (MTS) comprise calcite microspar-filled voids in fine-grained shallow-water carbonate, and were commonly formed in the Mesoproterozoic and early Neoproterozoic. However, the origin of MTS and links between the temporal distribution of MTS and contemporaneous seawater chemistry remains poorly understood. Here we report elemental and isotopic data for MTS and host rocks from the Mesoproterozoic Gaoyuzhuang Formation (∼1,600–1,550 Ma), North China Craton. The results reveal similar C, S and Sr isotope signatures between MTS and host rocks, which are close to the isotopic compositions of contemporaneous global seawater, suggesting an early diagenetic, seawater-buffered origin for MTS. A small sulfur isotopic fractionation between seawater sulfate and pyrite (Δ34SCAS-Py) of 4.1 ± 1.5‰ in host rocks is consistent with previously reported data, providing support for sub-millimolar sulfate concentrations in Mesoproterozoic seawater. Our observations suggest that the widespread occurrence of MTS through the Mesoproterozoic to early Neoproterozoic was broadly linked to sulfate scarcity in the ocean. We further propose that in Proterozoic oceans with sub-millimolar seawater sulfate concentrations, where aerobic and anaerobic methane oxidation was likely inhibited, methane produced via methanogenesis may have been more prone to accumulate in sediments, creating voids during escape. The absence of MTS across periods of higher sulfate concentrations during the Palaeoproterozoic and after the mid-Neoproterozoic, suggests that elevated sulfate concentrations promoted consumption of methane via anaerobic methane oxidation, thus preventing methane accumulation and the formation of sediment voids. Rapid lithification of the substrate as a result of elevated carbonate saturation may have also hindered the formation of MTS during these intervals. The link between MTS and changes in both oceanic sulfate levels and benthic methane fluxes gives a new perspective on temporal fluctuations in Earth’s redox state through time.