Abstract
Molar tooth structures are ptygmatically folded and microspar-filled structures common in early- and mid-Proterozoic (∼2,500–750 million years ago, Ma) subtidal successions, but extremely rare in rocks <750 Ma. Here, on the basis of Mg and S isotopes, we show that molar tooth structures may have formed within sediments where microbial sulphate reduction and methanogenesis converged. The convergence was driven by the abundant production of methyl sulphides (dimethyl sulphide and methanethiol) in euxinic or H2S-rich seawaters that were widespread in Proterozoic continental margins. In this convergence zone, methyl sulphides served as a non-competitive substrate supporting methane generation and methanethiol inhibited anaerobic oxidation of methane, resulting in the buildup of CH4, formation of degassing cracks in sediments and an increase in the benthic methane flux from sediments. Precipitation of crack-filling microspar was driven by methanogenesis-related alkalinity accumulation. Deep ocean ventilation and oxygenation around 750 Ma brought molar tooth structures to an end.
Highlights
Molar tooth structures are ptygmatically folded and microspar-filled structures common in early- and mid-Proterozoic (B2,500–750 million years ago, Ma) subtidal successions, but extremely rare in rocks o750 Ma
S isotopic data indicate that molar tooth (MT) microspar was precipitated within microbial sulphate reduction (MSR) zone and Mg isotopic data suggest that microspar precipitation predated the dolomitization of host rock
We propose that MT microspar was precipitated in the sediment column where MSR and methanogenesis occur simultaneously underneath sulphidic seawaters and where the production of CH4 from methyl sulphides and the inhibition of CH4 oxidation by methanethiol allowed CH4 to build up in the sediments
Summary
Molar tooth structures are ptygmatically folded and microspar-filled structures common in early- and mid-Proterozoic (B2,500–750 million years ago, Ma) subtidal successions, but extremely rare in rocks o750 Ma. We propose that MT microspar was precipitated in the sediment column where MSR and methanogenesis occur simultaneously underneath sulphidic seawaters and where the production of CH4 from methyl sulphides and the inhibition of CH4 oxidation by methanethiol allowed CH4 to build up in the sediments. Isotopic compositions of the MTCs. Sulphur isotopic values of carbonate-associated sulphate (CAS) extracted from MT microspars (d34SMT: 31.9–42.8%) are higher than those of CAS from calcareous host rock (d34SHR: 19.1–27.6%; Fig. 1a, Supplementary Fig. 4 and Supplementary Table 2).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
