Evaluating the Sedimentary Re/Mo Ratio as a Paleoindicator of Oxic, Suboxic and Anoxic Conditions
J. Crusius, S. E. Calvert, T. F. Pedersen, and D. Sage
Past changes in the intensity and areal extent of reducing sediments may be linked to the glacial atmospheric pCO2 decrease either: (1) in a direct response to changes in organic carbon fluxes, or (2) through changes in water-column O2 and NO3- consumption (and inventories) that respond to changes in circulation or carbon fluxes.
The trace elements Re and Mo behave conservatively in seawater yet are strongly enriched in reducing sediments (due to the high ratio of [metal]sw/[metal]crust). We present Re and Mo data from cores spanning a range of redox conditions. In the oxic sites, Re and Mo accumulate at close to crustal concentrations. Re shows clear enrichment in suboxic (low O2, low HS-) sediments of the Sea of Japan, in sediments underlying the O2 minimum of the Arabian Sea, as well as in sediments underlying the anoxic (sulfate-reducing) waters of the Black Sea and Saanich Inlet. Re enrichment occurs in these cores just below the depths of Fe and U reduction. Yet, only in the sediments underlying the anoxic (sulfate reducing) waters of the Black Sea add Saanich Inlet is substantial Mo accumulating. Absence of Re and Mo enrichment in sediment trap data suggests that both metals are enriched at or below the sediment-water interface.
Our data thus suggest that Re is enriched in suboxic sediments, while Mo enrichment requires more strongly reducing sediments (strong sulfate reduction). The Re/Mo ratio, together with Re and Mo concentrations in sediments, can be used to hindcast a range of reducing conditions of deposition, even in environments (i.e. continental margins) where a substantial lithogenic flux can obscure smaller enrichments of other metals.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California