Potential photosynthetic impact on phosphate stromatolite formation after the Marinoan glaciation: Paleoceanographic implications

Abstract This study investigated the origin and the depositional age of phosphate stromatolites contained in the Neoproterozoic Salitre Formation, Brazil. The stromatolites exhibited columnar shapes and were intercalated with laminated dolostones. Their depositional environment was interpreted to have been an evaporitic ramp where erosion and reworking by waves prevailed. Laser ablation-inductively coupled plasma-mass spectrometry measurements of the stromatolites yielded a U Pb age of 616 ± 32 Ma, suggesting that they were deposited after the Marinoan glaciation. Several types of filamentous structures were recognized in the stromatolites; many of them were “pseudofossil” ambient inclusion trails, but some were possibly microfossils. To consider the general presence of photosynthetic microorganisms on the surface of shallow-water stromatolites, the involvement of microbial photosynthesis in phosphate stromatolite formation was expected. Numerical calculations determined that photosynthesis could have induced phosphate mineral precipitation at the time of deposition if the phosphorus concentration was above ca. 5 μM. To achieve such high concentration at the shallow ocean, globally elevated phosphorus concentration was considered to be the ultimate factor in addition to the local process(es) including upwelling and/or evaporation. Therefore, the phosphate stromatolites in the Salitre Formation are evidence of the development of a phosphorus-rich ocean after the Marinoan glaciation.