Cerium Anomalies in the Arabian Sea Sediments Beneath and Beyond Oxygen Minimum Zone (OMZ)

B. Nagender Nath Geological Oceanography Division, National Institute Oceanography,

Dona Paula, Goa-403 004, India

nagender@bcgoa.ernet.in

M. Bau GeoForschungsZentrum Potsdam, Projektbereich 4.3, Telegrafenberg A 50, D-14473 Potsdam, Germany

B. R. Rao Geological Oceanography Division, National Institute of Oceanography, Dona Paula, Goa-403 004, India

Ch. M. Rao Geological Oceanography Division, National Institute of Oceanography, Dona Paula, Goa-403 004, India

Rare-earth elements (REE) alongwith calcium carbonate, C-org and few trace elements have been determined in the surficial sediments of the Arabian Sea in a transect off central western coast of India cutting across the oxygen minimum zone (OMZ). The Ce anomalies (with respect to it's neighbouring elements La and Pr and denoted as Ce/Ce*) recorded in the
sediments impinged by OMZ vary only marginally from the sediments deposited away from the OMZ, with both the types of sediments displaying negative Ce anomalies. Though, Ce is redox-sensitive in nature, do not follow the trend of other redox proxies such as uranium and C-org. Both U and C-org show significant enrichments in the sediments under OMZ. Normative calculations suggest that two phases, viz., terrestrial and seawater sources (terrestrial >> seawater) contribute to the total Ce anomaly of the sediments. The seawater derived component show Ce/Ce* values typical for the coastal waters and the oxygenated surface waters, and do not show any correspondence to the lowered redox state of the overyling water, probably due to the redirection of dissolved Ce into the oxic deeper waters to eventually precipitate as Ce(IV). This data on the recently deposited sediments from a known oxygen-poor environments suggest that the Ce/Ce* in high sedimentation areas of
continental margin environments cannot be used for paleoredox reconstructions.