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 phosphorites. 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. Massive slab phosphorites are fine-grained, and consists of goethitic and non-goethitic zones. Non-goethitic zones comprise mainly microsphorite with Oligocene to lower Miocene fossils and microfossils. Goethitic zones contain abundant Quaternary skeletons. Microprobe analyses of the Error Seamount phosphorites indicate that the geochemistry of these phosphorites is similar to that of the Tertiary seamount phosphorites in the Atlantic and Pacific Ocean and that they are enriched in Ca and depleted in P compared to other seamount and island phosphorites in the Indian Ocean. Microbial activity is evident in both types of phosphorites and occurs in the form of microborings in peloids and on skeletal fragments, encrustation of phosphate crystals on hollow, globular microstructures and phosphate sheaths enclosing organic filaments. The presence of calcite rim cements, microspar-filled patches and recrystallised grains indicates exposure of the seamount under subaerial conditions, during which phosphatisation took place.