Possible microbial origin of phosphorites on Error Seamount, northwestern Arabian Sea

Abstract

Petrographical, mineralogical and microprobe analysis of phosphorites from Error Seamount, in the northwestern Arabian Sea, demonstrates that microbial processes played an important role in the early diagenetic formation of the phosphorites during subaerial exposure of the seamount. The phosphorites on Error Seamount occur as laminated crusts and massive slabs. Low-magnesium calcite and carbonate fluorapatite are the major minerals of the phosphorites, with goethite being important only in the massive slab phosphorites. Elemental sulphur, pyrrhotite, gypsum and chlorite are the mineral phases present in the acid-insoluble residues of the prosphorites. Elemental sulphur, which is exclusively formEd. by microbial processes, occurs as submicrometre-sized granules on gypsum surfaces. The laminated crust phosphorites consist of organic-rich and organic-poor laminae interlayered with ghost pellets, index fossils of Oligocene to lower Miocene age, peloids and coated grains