Vanadium in foraminiferal calcite as a tracer for changes in the areal extent of reducing sediments

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Abstract
We have used the vanadium concentration in cleaned foraminiferal calcite as a tracer of seawater V changes in the past. Since the benthic flux of vanadium is sensitive to the redox potential of sediments, changes in the vanadium concentration of seawater should be a reflection of changes that control the redox state of sediments. V/Ca in G. sacculifer from an eastern equatorial Atlantic core (ENO66‐17GGC) is 21.6 (±2.8) nmol V/mol Ca. This value does not change over 35 kyr, indicating that there was no measurable change in seawater vanadium levels over this period. Potential artifacts from partial dissolution are not significant based on low, constant values for foraminiferal fragmentation (6–7%) in the top 50 cm of the core. A minor correction to account for vanadium associated with Mn carbonate overgrowths, estimated from two Caribbean cores where this mixed phase dominates the deeper V/Ca values, has been applied. Changes in the areal extent of anoxic and suboxic sediments are thus constrained by this constant value and the standard deviation of the measurement, ±12%. Based on a mass balance for vanadium where suboxic sediments are a source and anoxic sediments are a sink to the ocean, suboxic sediments are predicted to have changed by no more than 0.5–1.5 times the current value, assuming no change in the areal extent of anoxic sediments. This corresponds to 1.3–3.5% of total ocean sediments. Given a constant area of suboxic sediments, the areal extent of anoxic sediments did not increase by more than fivefold, or 1.5% of the ocean floor over the past 35 kyr. The significant reductions in deep water oxygen levels and consequent changes in sediment redox conditions required by polar nutrient depletion scenarios are not reflected in the foraminiferal vanadium data over the past 35 kyr. This suggests that models which call on less or no changes in deep water oxygen are more likely alternatives.