197 / 2025-04-14 19:29:46

‘Molar-tooth’ calcite microspar: highly saturated oceans after supercontinent amalgamation

seawater,Molar-tooth calcite,supercontinent

Theme 4: Microbial and Biogeochemical Evolution through Time > Session 4c: Reconstructing Biogeochemical Cycles in Deep Time: Assumptions, Bias, Constraints, and Directions

Early diagenetic, calcite microspar cement (CMC), which commonly fills vertically crumpled cavities referred to as molar-tooth structure (MTS), can be found in many Proterozoic sedimentary basins. Since the first report of such cracks “resembling the markings in the molar tooth of an elephant” from the Mesoproterozoic Belt-Purcell Supergoup of North America, various workers have interpreted MTS-CMC as a sign of exceptionally early lithification by calcium carbonate. In many cases, CMC clearly lithified earlier than the surrounding, often microbially-textured rock matrix, causing an intricate series of brittle fractures along their lengths due to compaction. Cemented crack fill could also be resuspended during storms, forming carbonate lag breccia. As ptygmatic cracks are not unique to the Precambrian, while CMC can fill other kinds of cavities in the same formations, it is the early lithification and characteristic fill that sets MTS-CMC apart from other types of early sedimentary cavities.

Several compilations have been made over the years, concluding that MTS formed throughout the interval from c. 2.6 to 0.5 Ga, with a possible acme during the Tonian Period. Based on a critical evaluation of all reported examples, we show here that MTS-CMC formed during two specific time intervals: c. 1600 – 1400 Ma and c. 1100 – 720 Ma only, with one Neoarchean exception. All other reported Paleoproterozoic, Ediacaran and Phanerozoic examples require further substantiation. Strontium isotope studies confirm that MTS-CMC precipitated from marine porewaters during times of low seawater 87Sr/86Sr, consistent with zircon evidence that these were anorogenic intervals after supercontinent amalgamation.

MTS-CMC appears to have precipitated beneath the seafloor, under fully marine, or sometimes partially restricted environments during times of exceptional tectonic quiescence and low continental freeboard. We propose that limited accommodation space on marine shelves may have contributed to high saturation states at these times. Here we outline the evidence for MTS-CMC as an age-diagnostic facies associated with extreme supersaturation during supercontinent tenure. We speculate further that MTS formed beneath oxygenated seafloor above the storm wave base, with more reducing conditions at depth leading to precipitation directly on the seafloor.