Trace and rare earth elemental variation in Arabian Sea sediments through a transect across the oxygen minimum zone

Details Trace and rare earth elemental variation in Arabian Sea sediments through a transect across the oxygen minimum zone Trace and rare earth elemental variation in Arabian Sea sediments through a transect across the oxygen minimum zone

Jun 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.2375n&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 61, Issue 12, pp.2375-2388

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7

Scientific paper

We have determined the calcium carbonate (CaCO 3 ), organic carbon (Co org ), trace element, and rare earth element (REE) composition of surface sediments collected from a transect on the central western continental shelf and slope of India in the Eastern Arabian Sea. The transect samples across the oxygen minimum zone (OMZ) allows us to compare the relative abundances of trace elements and REEs in the sediments beneath and beyond the OMZ. Shale-normalized REE patterns, La n /Yb n ratios, and 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 continental particles) values ranging between 82-91% of the total U, indicating 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 from the OMZ (1-2 mL/L) show slight negative Ce anomalies, with no significant differences between these two sets of sediments. The Ce/Ce shale * values are poorly related to U and C org which are indicators of suboxic bottom waters. Normative calculations suggest that two sources, namely, terrestrial and seawater (terrestrial >> seawater) contribute to the total Cc anomaly of the sediments. The Cc anomaly values of the calculated seawater derived component are similar to the anomalies reported for other coastal waters and the oxygenated surface waters of the Arabian Sea and do not show any correspondence to the lowered redox state of the overlying water, probably due to the redirection of dissolved Ce into the oxic deeper water. This data on recently deposited sediments from a known oxygen-poor environment suggest that the Ce/Ce sh * in high sedimentation areas of continental margin environments 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.

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