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Phosphatic Rocks and Manganese Crusts from Seamounts in the EEZ of Kiribati and Tuvalu, Central Pacific Ocean

1992; Springer Nature; Linguagem: Inglês

10.1007/978-1-4612-2896-7_15

1068-6517

V. Purnachandra Rao, William C. Burnett,

Paleontology and Stratigraphy of Fossils

Phosphorite-manganese crusts were dredged from the flanks of seamounts of the Kiribati and Tuvalu EEZ (Exclusive Economic Zone) from water depths between 1450 and 2355 m. Samples from Kiribati consist of phosphatized algal laminations, coated grains, filamentous and unicellular cyanobacteria. Manganese oxide occurs in the phosphate matrix in the form of dendritic growths, as a coating on intraclasts, and as infillings in foraminifers and algal tissues. Weathered clasts derived from mafic igneous rocks are the dominant constituents in the samples from Tuvalu seamounts and phosphate occurs mainly as a cementing material for these clasts. Calcite and carbonate fluorapatite are the dominant minerals present in the matrix material of both areas. It appears that the Kiribati seamounts were at very shallow depths during the phosphatization process, and microbial activity may have taken an active role during the diagenetic transformation of algal sediments into phosphorites. The Tuvalu samples, however, showed no evidence of shallow water conditions although some microbial influence during phosphogenesis was still apparent. Thin manganese crusts in Kiribati and thick crusts in the Tuvalu samples suggest that depth and/or length of exposure determined the relative crust thicknesses observed.