Phosphatised limestones and associated sediments from the Western continental shelf of India

Abstract

The phosphatised limestones and associated sediments of the outer continental shelf of the west coast of India between Goa and Ratnagiri at depths of between 70 and 120 m have been studied. The highest P₂O₅ content (9%, ranging from 2 to 9% depending on location) occur in the algal nodules. Polished and thin sections show that the algal nodules grew with the occasional microbial mat. These mats apparently formed by trapping and binding of sediment particles, which were subsequently phosphatised. Phosphate also occurs as pellets and rounded structures. High-magnesium calcite and carbonate fluorapatite are the major minerals. Aragonite, low-magnesium calcite and quartz are also present in some samples. Microprobe geochemistry of the phosphatised material of the algal nodules reveals the presence of land-derived siliciclastic detritus and the possibility of more than one source for Ca and P. Phosphate also occurred as cavity fillings in coral and bryozoan limestones, but not in oolitic limestones, regardless of their occurrence at the same depths and in the vicinity of the other limestones. The associated sediments are mostly sandy and show smaller concentrations of P₂O₅ (average 0.3%) and organic carbon (average 0.93%) as compared to the regions of upwelling where phosphorites are forming; the correlation exhibited between the P₂O₅ and the organic carbon is negative. The environmental conditions for the formation of phosphatised limestones and possible sources of phosphate are discussed. Sheath organic matter associated with filaments appears to be the major source for phosphorus; the phosphatisation is a microbial process which takes place at shallow depths. The impact of present-day upwelling is insignificant in the phosphatisation of the limestones.