Mesoproterozoic Molar Tooth Structure Related to Increased Marine Oxygenation

Abstract

AbstractMarine carbonate fabrics are intrinsically related to ocean chemistry, physical processes and biological activity. Molar tooth structure (MTS), a globally distributed structure in Proterozoic carbonate sediments, has been widely studied for more than a century; yet its connections with physical and biological processes remain unclear. Using multiple techniques, we studied ∼1.57 Ga MTS and identified a connection between its occurrence and increased marine oxygenation. In our samples, the matrix surrounding MTS is typically dominated by carbonate mud with early diagenetic dolomite crystals. High I/(Ca + Mg) ratios (up to 4.1 μmol/mol) and negative Ce anomalies (∼0.8) detected in the matrix are consistent with the oxidative removal of inhibitors such as Fe2+ and Mn2+ in the water‐column that permitted carbonate “whiting” mud precipitation stimulated by cyanobacterial photosynthesis. This cohesive but not rigid seafloor carbonate mud was a prerequisite for synsedimentary MTS crack formation. Systematically higher carbon isotope (δ13C) values in MTS microspars, relative to host sediment, support origination of the cracks by methane degassing in the organic‐rich carbonate mud. Low, but non‐zero, I/(Ca + Mg) values of the MTS microspar suggest that the precipitation of the microspar that filled the MTS cracks was triggered by oxidative removal of residual Fe2+ and Mn2+ in porewater through mixing with overlying oxygenated seawater. We therefore propose that MTS formed under moderately oxygenated conditions and that its sporadic occurrence prior to ∼1.2 Ga reflects episodes of pulsed marine oxygenation in an overall anoxic setting.