EVIDENCE FOR BIOLOGICALLY INFLUENCED PRECIPITATION IN OOIDS FROM SHARK BAY, WESTERN AUSTRALIA

Ooids are typically small (<1 mm in diameter) rounded carbonate grains that form in warm agitated water. Most ooids form through a cycle of precipitation of a mineral, often calcium carbonate, followed by abrasion. However, there remains an open debate over the involvement of microbes in ooid formation. We collected ooid-containing subtidal sand near Carbla beach in Shark Bay, Western Australia to constrain the microbial contribution to ooid formation at this locality. Shark Bay ooids abundant in the >180 µm size fraction show distinct organic-rich nuclei that contain iron sulfide. The nuclei are surrounded by 5-10 µm-thick inner cortices composed of high-magnesium calcite and multiple aragonitic outer layers (25-50 µm). The presence of a single layer of high-magnesium calcite in close association with iron sulfide and organic-rich nuclei suggests that microbial sulfate reduction facilitated precipitation of both iron sulfide minerals and that the degrading organic compounds helped nucleate and stabilize high-magnesium calcite. The aragonitic outer layers likely precipitated in equilibrium with the surrounding water. This spatial distribution of iron sulfide minerals with high-magnesium calcite in ooids from Shark Bay is a textural biosignature of the microbial influence on the nucleation and initial formation of ooid cortices.