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Laboratory Determination of Molybdenum Accumulation Rates as a Measure of Hypoxic Conditions
Citation:
BOOTHMAN, W. S. AND L. L. COIRO. Laboratory Determination of Molybdenum Accumulation Rates as a Measure of Hypoxic Conditions. Estuaries and Coasts. Estuarine Research Federation, Port Republic, MD, 32(4):642-653, (2009).
Impact/Purpose:
This manuscript describes the results of a laboratory study determining rates of accumulation of molybdenum (Mo) in sediments as a function of concentrations of dissolved oxygen (DO) in overlying water. The research shows that concentrations of Mo are elevated in sediments underlying seawater with low DO concentrations (<3 mg/L), with accumulation of Mo occurring entirely within the top 1 cm of sediments, and the accumulation rate of Mo is independent of DO concentration as long as [DO] is below the threshold of 2.8 mg O2/L. Consequently, Mo accumulation depends on the length of time hypoxic conditions are maintained. If these findings are validated by determinations in known field conditions, determination of Mo accumulation rates in sediment samples could provide estimates of the frequency of hypoxia, integrated over multi-year or even decadal time scales, in both recent and historical samples. Such a capability could be quite valuable to monitoring programs and ecological assessments.
Description:
Redox sensitive metals, such as molybdenum (Mo), are enriched in reducing sediments due to authigenic fixation in anoxic interstitial waters of sediments. This study tested whether the process of fixation and accumulation of Mo in sediments could provide a geochemical indicator of hypoxic conditions that integrates over long periods of time and large spatial extents. Sediments maintained under controlled dissolved oxygen (DO) conditions for up to 12 weeks showed elevated concentrations of Mo in the top 1 cm when exposed to low DO concentrations. Mo accumulation was linearly related to time of exposure in the four lowest treatments (0.7 – 2.8 mg O2/L), while treatments with higher DO concentrations showed lesser or no accumulation. Rates of Mo accumulation were independent of DO concentration in the 0.7 to 2.8 mg/L treatments and were not statistically significantly different from each other. The accumulation of Mo occurred at DO concentrations substantially higher than those found to limit Mo accumulation in field settings (<0.32 mg/L). Accumulation rates in the lowest treatments were similar to those found in sediments of deep basins with restricted water circulation and consequent low dissolved oxygen concentrations. The degree of Mo accumulation was found to depend on the length of time hypoxic conditions were maintained rather than on DO concentrations themselves. We suggest, therefore, Mo accumulation rates in sediments could provide estimates of the frequency of hypoxia integrated over multi-year or even decadal time scales.