CO2-clathrate destabilization: a new model of formation for molar tooth structures

Abstract

Abstract Molar tooth structures (MT) are unusual and enigmatic, temporally restricted, calcite bleb- and sheet-like structures hosted almost exclusively in Mesoproterozoic and Neoproterozoic continental ramp and shelf carbonates. Characteristically, MT are sharply bounded, interconnected, often ptygmatically folded or broken structures filled with uniform fine blocky calcite, but with a distinct lack of detrital infill. Collectively, their sedimentological characteristics indicate that the calcite-filled “channels” were not open to the sediment–water interface, and that they were an early, possibly pre-diagenetic, feature. Several models have been proposed for their formation, involving various physical, chemical and biological processes. Two models are presently favoured; gas expansion [J. Sediment. Res. 68 (1998) 104] and seismic shaking [Geol. Soc. Am. Bull. 110 (1998) 1028], but the temporal restriction of these sedimentological features to the pre-Phanerozoic suggests that unique atmospheric or biospheric conditions at this time in the Earth’s evolution may be responsible for MT generation and/or preservation. This paper proposes a new hypothesis for the origin of deeper water MT that invokes the decomposition of CO2-clathrates in the ramp carbonate sediment column. We suggest that the higher CO2 partial pressures thought to be prevalent in the Proterozoic atmosphere allow for the stabilization of CO2-clathrates in the sediment column not unlike the methane clathrates observed in the seafloor environment today. It is further postulated that the dramatic increase in molar volume associated with clathrate breakdown opens cracks and voids in the host sediment, and the precipitation of calcite associated with the breakdown of the CO2-clathrates results in rapid (but not instantaneous) infill, and therefore subsequent preservation, of MT. In this model the decomposition of the CO2-clathrates may have been triggered by, or in turn have triggered, seismic shock waves. The collapse of the CO2-clathrates could have expelled CO2 gas forming pressurized voids in the sediment, which minimized clastic infill. The released CO2 could have combined with seawater to precipitate CaCO3. Thus, this model complements both the gas-bubble expansion and seismic shaking models, and can explain some, but not all, of the many diagnostic characteristics and occurrences of these intensely studied and oft-debated structures, especially those formed in deeper cooler waters. 13 C and 18 O depletions in MT relative to the immediate host rock are consistent with MT formation at lower temperature than the host rock deposition. The host rocks have previously been interpreted to form above storm wave base, outside the stability field of clathrate. However the exact water depth at which the MT were formed is ambiguous. The cooler temperatures of MT formation indicated by the stable isotope data are consistent with deeper water and higher pressure, or lower temperature, conditions which may bring the sediments to within the CO2-clathrate stability field.