Abstract: A sediment core collected from a bathymetric high off Goa on the western continental margin of India has yielded phosphatic nodules at various subsurface depths (at 110, 150, 305, 355, 435, 500, 505, and 525 cm). The nodules are hosted by sediments of Pleistocene age. They are <1-5 cm in size, have carbonate flourapatitc (CFA) as a single autliigenic mineral phase, are free of detrital inclusions, and have very high P 2 0 5 contents (<30%). In addition, soft and hard phosphatic nodules have also been recovered from the eastern continental margin of India. Eight nodules from various subsurface depths of the sediment core from the western continental margin and two nodules from the eastern continental margin were analyzed for uranium and rare-earth elements (REEs) by inductively coupled plasma-mass spectrometry (ICP-MS). Total REE contents are very low (8–21 ppm) in western continental margin nodules and only slightly higher in eastern continental margin nodules (maximum is 42 ppm). REE abundances relative to shale (ΣREE in sample/ΣREE in shale) are less than 0.22 and are comparable to phosphatic nodules from the Namibian continental margin, and are slightly lower than the nodules from the Peruvian continental margin. Light REEs are depleted, with La n /Yb n r(La sample /La shale )/(Yb sample /Yb shale )] values lower than unity, indicating a minimal contribution from terrestrial sources. The cerium anomaly (Cc/Ce*), which is a measure of Ce fractionation relative to the neighboring REEs, ranges between 0.64 and 1.38 with most close to 1, indicating very little Ce fractionation. Uranium concentrations are very high in nodules from both margins. Low REE contents and LREE depletion collectively indicate either a seawater or porewater source for these elements. Since Ce is mostly stable in its trivalent stale and U precipitates under reducing conditions, the absence of Ce fractionation in association with U enrichment indicates that the nodules may have formed by autliigenic precipitation in reducing porewaters. High productivity in association with upwelling might have driven the accumulation of organic matter, which ill turn would help enrich phosphate in porewaters, eventually leading to the formation of high-grade phosphorites. Ce anomaly values in nodules do not correlate with paleoredox conditions in the water column, thus the REEs for these nodules are interpreted to mainly reflect porewater conditions.

Abstract All available evidence which relates to the question of Recent formation of indurated phosphate deposits on the sea floor off the coasts of Peru and Chile is reviewed. Uranium-series results, oceanographic parameters, and sedimentary associations are all shown to be consistent with a Recent age for those nodules with uranium isotopic compositions in equilibrium with seawater. New data are presented concerning uranium distribution at different levels within two individual phosphate nodules. Uranium isotopic compositions are constant throughout the thickness of both samples analyzed. Furthermore, variations in the absolute concentration of uranium at different levels within the nodules display a sympathetic association to the relative amounts of apatite present in the same layers. This argues against secondary addition of uranium and supports our hypothesis of Holocene formation of phosphate nodules which contain no significant 230 Th and have 234 U/ 238 U activity ratios close to that of modern seawater.