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 wer e determined in sediment core Y-71-7-53P near an active hydrothermal locati on on the southern East Pacific Rise crest (10 degreesS). The goal was to d etermine the usefulness of hydrothermal iron oxides as carriers of elementa l tracers of seawater concentrations. The sediments are 70% carbonate and a bout 1.3% detrital material, with the remainder having a hydrothermal origi n. Rhenium concentrations are at detrital levels suggesting little anaerobi c diagenesis beyond manganese remobilization; however, U, Cd, and Mo concen trations all indicate alteration after deposition. Oxyanion/Fe ratios of P, V, and As in surface sediments are lower than the ratios in plume particle s and covary with Fe content downcore. The probable explanation for these v ariations is variable efficiency of incorporation of these metals during ir on oxide formation or early diagenetic processes after deposition. The P/Fe ratio in the core sediment is about half that in the plume particles. Amon g the oxyanion ratios, only the V/As ratio in the sediments is relatively c onstant at the present plume particle ratio during the past 60-70 kyr. Chan ges in sediments older than this could reflect variability in the incorpora tion efficiency or changes in V/As ratio in seawater. This study demonstrat es the difficulties of using hydrothermal sediment element enrichments as t racers 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.