Stromatolite Declines and Molar-Tooth Carbonates:Contributions to the Understanding of the Precambrian Carbonate World

Stromatolite and molar-tooth structures are two kinds of peculiar Precambrian sedimentary phenomena,attracting great attention from geologists.Despite the well-known fact that stromatolites are induced by a variety of abiotic and microbial processes,they remain a Precambrian enigma because no significant microbial evidence has ever been found.Molar-tooth(MT) is an enigmatic sedimentary structure which is mainly distributed in the subtidal carbonate rocks of the Precambrian.The particular configuration of MT cracks filled with the equally-sized microspar calcite results in continuous debating for their genesis.The latest studies reveal that the number of stromatolite morphotype taxa changes over earth history that include a peak of abundance in Proterozoic and several dramatic declines from then on.It appears that MT stucture was developed in episode of relatively high abundance after the declining of the stomatolite and occurres in the deeper water than stromatolite in the Precambrian.Both stromatolite declines and MT episodes may be greatly associated with ocean chemistry changes,instead of metazoan interference.The understanding of this may contribute to the invetihation of the insight into the complicated and multivariant Precambrian carbonate world.