TRACE AND RARE-EARTH ELEMENTAL VARIATION IN ARABIAN SEA SEDIMENTS THROUGH A TRANSECT ACROSS THE OXYGEN MINIMUM ZONE

We have determined the calcium carbonate (CaCO3), organic carbon (C-or g), trace element, and rare earth element (REE) composition of surface sediments collected from a transect on the central western continenta l shelf and slope of India in the Eastern Arabian Sea. The transect sa mples across the oxygen minimum zone (OMZ) allows us to compare the re lative abundances of trace elements and REEs in the sediments beneath and beyond the OMZ. Shale-normalized REE patterns, La-n/Yb-n ratios, a nd Eu/Eu anamolies indicate that the sediments in the study area are either derived from the adjoining Archaean land masses or from distal Indus source. Sediment deposited in the OMZ have high U Values from 3. 6 to 8.1 ppm, with their U-excess (of that can be supplied by continen tal particles) values ranging between 82-91% of the total U, indicatin g that the U may be precipitated as U+4 in the reducing conditions of OMZ. Sediments deposited beneath the intense OMZ(<0.2 mL/L) and away f rom the OMZ(1-2 mL/L) show slight negative Ce anomalies, with no signi ficant differences between these two sets of sediments. The Ce/Ce-shal e values are poorly related to U and C-org which are indicators of su boxic bottom waters. Normative calculations suggest that two sources, namely, terrestrial and seawater( terrestrial much greater than seawat er) contribute to the total Ce anomaly of the sediments. The Ce anomal y values of the calculated seawater derived component are similar to t he anomalies reported for other coastal waters and the oxygenated surf ace waters of the Arabian Sea and do not show any correspondence to th e lowered redox state of the overlying water, probably due to the redi rection of dissolved Ce into the oxic deeper water. This data on recen tly deposited sediments from a known oxygen-poor environment suggest t hat the Ce/Ce-sh in high sedimentation areas of continental margin en vironments may not be a good paleoredox proxy. U/Th ratios above 1.25 noticed in OMZ are in accordance with the earlier usage of this ratio for identifying reducing environment of ancient sediments. Copyright ( C) 1997 Elsevier Science Ltd.