FORMATION OF MICROBIALITES THROUGH TIME: HOLOCENE CORAL REEFS COMPARED TO EARLY TRIASSIC BIOHERMS

In Holocene post-glacial coral reefs, the occurrence of microbialites has been interpreted to reflect an ecosystem response to environmental change. The microbialites are well preserved and volumetrically abundant, offering an excellent opportunity to study formation mechanisms. Our evidence shows that sulfate-reducing bacteria played an intrinsic role in the precipitation of microbial carbonate in coral reefs off Tahiti, Vanuatu, Belize and the Maldives. With more nutrients and organic matter distributed in the reef ecosystem during the last sea-level rise, anoxic microenvironments preferentially developed. Such conditions favored heterotrophic, particularly, sulfate-reducing bacteria. It is suggested that matrix-solute interaction related to the activity of sulfate reducers induced carbonate precipitation in extracellular polymeric substances.

In contrast, after the End-Permian mass extinction, the Paleozoic skeletal carbonate factory was replaced by microbial carbonate production. With the lack of metazoan reef builders, microorganisms formed microbialite bioherms, which covered wide areas on the Early Triassic ocean margins. Well preserved microbialites from Iran and Turkey (both Neotethys) were studied for their lipid biomarker inventories. Molecular fossils of cyanobacteria, archaea, anoxygenic phototrophs, and sulfate-reducing bacteria indicate the presence of layered microbial mats on the seafloor. In association with metazoans other than corals (bivalves, gastropods, sponges, ostracods) and foraminifera bioherms developed on the ocean margins. In contrast to reef-microbialite formation in the Holocene, the biomarker evidence suggests that photosynthetic CO₂ removal by cyanobacteria possibly contributed more significantly to carbonate precipitation.