VANADIUM IN FORAMINIFERAL CALCITE: EVALUATION OF A METHOD TO DETERMINE PALEO-SEAWATER VANADIUM CONCENTRATIONS

Abstract

We assess the potential of using foraminiferal calcite as a paleoceanographic indicator of seawater V concentrations. Laboratory culture experiments show that living benthic and planktonic foraminifera incorporate V into their test in direct proportion to seawater concentrations. Distribution coefficients relative to the culture solution are D = 2.1 × 10−3 and 2.8 × 10−3 for G. calida and A. lobifera, respectively. We use a cleaning procedure that effectively removes most V-rich contaminant phases from foraminiferal calcite preserved in the fossil record including organic matter and MnFe oxyhydroxides. MnCO3 overgrowths cannot be eliminated. Since V is conservative in the ocean, foraminiferal calcite recently accreted and found in surface sediment samples should have the same V content if this tracer accurately reflects seawater concentrations. V /Ca values for the same species of foraminifera are constant for core-top samples collected above the foraminiferal lysocline in different ocean basins. The mean distribution coefficients relative to seawater are D = 5.8 (± 1.0) × 10−3, 10.3 (±0.7) × 10−3 and 32 (+2.5) X 10−3 for G. sacculifer, G. tumida, and C. wuellerstorfi, respectively. These differences suggest that V incorporation is species dependent. Core-top analyses along two submarine rises in the Equatorial Atlantic and Pacific oceans indicate significant dissolution effects. With increasing depth of deposition, and thus more extensive partial dissolution of the test, VCa decreases by a factor of three in G. tumida, increases by up to a factor of four in G. sacculifer, and increases by a factor of two in C. wuellerstorfi. No exchange between foraminiferal V and detrital V in sediments is observed over an interval of 200 kyr.