Post-depositional modification of carbonate ooids by sulfate-reducing bacteria: Evidence from the Lower–Middle Jurassic, Tethyan Himalayas of southern Tibet

Abstract The role of microbial vs. abiotic processes in ooid formation has been a controversial topic for over a century. Recent sedimentological, geochemical and biomarker data indicate that microbes make an important contribution to the construction, destruction and modification of ooids. Here, we present a detailed petrographic and chemical investigation of well-preserved red oolites from the Lower–Middle Jurassic Nieniexiongla Formation in the Tethyan Himalaya of southern Tibet. Petrographic studies and electron probe microanalysis indicate that the red coloration is due to the presence of hematite minerals. The hematites show euhedral and amorphous spheroidal morphology and residual elemental sulfur was detected within minerals. The δ34S values in the leachate (δ34SNaCl) and in carbonate associated sulfate (δ34SCAS) of red oolites are lower than gray oolites and micritic limestones. Moreover, δ34SNaCl and δ34SCAS values are lowest in samples with the highest content of hematitic ooids. Together, these observations suggest that the hematites are transformed from pyrite. The Fe2+ (and possibly SO42−) provided by pore-water combined with degradation of syn-depositional organic matter within ooids produced anoxic microenvironments that facilitated local pyrite precipitation. Some spheroidal pyrites were subsequently transformed to euhedral crystals through continuous growth of the constituent microcrystals. This study demonstrates that organomineralization mediated by sulfate-reducing bacteria can produce pyrite grains within carbonate ooids. The results emphasize the important role that microbial communities play in the deposition and modifications of ooids.