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Whether ooids are formed through biotic or abiotic progress seems to become increasingly controversial in recent decades. New studies indicate that the microbial effect could make important contributions to the construction, destruction and modification of ooids. Previous studies mainly focused on the organomineralization of carbonate minerals in ooids. This paper investigates the formation process of iron minerals in red oolitic grainstones of the Lower Jurassic Nieniexiongla Formation in the Tethyan Himalaya of southern Tibet. Petrographic studies and electron probe microanalysis revealed that the colorful minerals present in red ooids are hematites. Sulfur was detected in the hematites, which indicates that they were transformed from precursor pyrites. These hematites retain euhedral or framboid shapes of the pyrites. Hematite pseudomorphs distribute randomly in ooids and could not form a whole concentric layer of the ooids. Euhedral crystals usually have larger diameters than amorphous spheres and are scattered mostly in nuclei. Based on the distribution and morphology of hematites, we speculate that precursor pyrites were formed through metabolism of sulfate-reducing bacteria. Moreover, the timing of the pyrite formation was after the deposition but before the lithification. The decomposition of the microorganisms within the ooids by bacterial sulfate-reducing provides the HS-, the degradation of organic matters would produce spaces for the intrusion of pore water with sufficient Fe2+ to facilitate the pyrite deposition. In nuclei, the framboid aggregates of pyrites were transferred to euhedral crystals through continuous growth of the constituent microcrystals. Our study demonstrated organomineralization of sulfate-reducing bacteria mediated pyrites in carbonate ooids.

The mid-Cretaceous greenhouse climate was induced by volcanic outgassing and the release of biogenic or thermogenic methane into the ocean-atmosphere system. During this period, major episodes of oceanic anoxic conditions enabled the large scale deposition of marine black shales rich in organic carbon, serving as a source for methane production. Studies on the anaerobic oxidation of methane, the key biogeochemical process at sites where methane is transported toward the seafloor, and its associated authigenic carbonates from mid-Cretaceous strata can contribute to elucidate such extreme climatic conditions. A total of nine layers of authigenic carbonates were recognized in the sequence of mid-Cretaceous inner shelf sedimentary rocks of the Qiangdong section, Gamba area, southern Tibet. In this study, we decipher the mode and causes of carbonate authigenesis by combining field observation, thin section petrography, mineralogy, and carbon and oxygen stable isotope geochemistry. The lowest obtained δ13Ccarb value of −30.5‰ is not low enough to exclude oil compounds as a carbon source, but since other evidence of oil seepage is lacking, methane is the most likely carbon source that was admixed to dissolved inorganic carbon to form authigenic carbonate. Typical seep carbonate phases including 13C depleted matrix micrite, clotted micrite, and banded and botryoidal cement as well as three types of authigenic pyrite comprising framboids, zoned aggregates with radial overgrowths surrounding a framboidal core, and euhedral pyrite crystals indicate the occurrence of methane seepage in the mid-Cretaceous depositional environments. In situ brecciation and subvertical tubular carbonates – the former reflecting rupture caused by local fluid overpressure and the latter interpreted as part of a plumping system – agree with widespread and vigorous seepage. Expanded shallow and deep-water anoxia in combination with low seawater sulfate concentration prior to the oceanic anoxic event 2 (OAE 2) event apparently caused an increased flux of organic matter to the methanogenic zone, stimulating methanogenesis and enabling a higher flux of methane toward the seafloor. In the water column, aerobic oxidation of methane would have enhanced oxygen depletion, thereby contributing to black shale deposition and providing a positive feedback to organic matter preservation and burial. Based on correlation with coeval authigenic carbonate deposits from the Tethyan continental margin, we conclude that organic matter accumulation and sea-level change were the main factors controlling carbonate authigenesis in the mid-Cretaceous.

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.