Oxyanions in metalliferous sediments: Tracers for paleoseawater metal concentrations?

The concentration of carbon, Fe, Mn, S, P, V, Cr, As, Mo, Cd, U, and Re were determined in sediment core Y-71-7-53P near an active hydrothermal location on the southern East Pacific Rise crest (10 degreesS). The goal was to determine the usefulness of hydrothermal iron oxides as carriers of elemental tracers of seawater concentrations. The sediments are 70% carbonate and about 1.3% detrital material, with the remainder having a hydrothermal origin. Rhenium concentrations are at detrital levels suggesting little anaerobic diagenesis beyond manganese remobilization; however, U, Cd, and Mo concentrations all indicate alteration after deposition. Oxyanion/Fe ratios of P, V, and As in surface sediments are lower than the ratios in plume particles and covary with Fe content downcore. The probable explanation for these variations is variable efficiency of incorporation of these metals during iron oxide formation or early diagenetic processes after deposition. The P/Fe ratio in the core sediment is about half that in the plume particles. Among the oxyanion ratios, only the V/As ratio in the sediments is relatively constant at the present plume particle ratio during the past 60-70 kyr. Changes in sediments older than this could reflect variability in the incorporation efficiency or changes in V/As ratio in seawater. This study demonstrates the difficulties of using hydrothermal sediment element enrichments as tracers of past seawater chemistry changes. In this core it appears that all oxyanion ratios change for reasons other than seawater metal concentration changes, with the possible exception of the V/As ratio. Copyright (C) 2000 Elsevier Science Ltd.