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Field Validation of Molybdenum Accumulation as an Indicator of Hypoxic Water Conditions
Citation:
BOOTHMAN, W. S. Field Validation of Molybdenum Accumulation as an Indicator of Hypoxic Water Conditions. Presented at SETAC North American 29th Annual Meeting, Tampa, FL, November 16 - 20, 2008.
Impact/Purpose:
The research described is intended to demonstrate that Mo accumulation rates determined as a function of DO exposure in laboratory experiments are also obtained from field samples where detailed DO exposure histories are available. The rates can be used to determine recent historical exposure of sediments to hypoxic water column conditions.
Description:
Laboratory experiments have shown that the accumulation rate of authigenic molybdenum (Mo) in marine sediments may serve as a quantitative surrogate for direct measurement of hypoxic conditions in overlying waters: Mo accumulation in the top 1 cm of sediment is linearly related to the amount of time of exposure to dissolved oxygen (DO) concentrations below ~3 mg/L, and accumulation rates are independent of DO concentrations when [DO] < 2.8 mg/L (a value very close to the U.S. EPA saltwater criterion for survival). To validate the laboratory results, accumulation rates of Mo in field sediments were related to the frequency of hypoxia in bottom waters. Sediment cores were collected from 6 sites in Narragansett Bay (RI, USA) encompassing a range of hypoxic exposures. The frequency of bottom-water hypoxia was determined from RI Dept. of Environmental Management monitoring data for the years 2003-2007. 210Pb dating of selected core sections at each site established sedimentation rates. Authigenic Mo concentrations were determined in surficial sediments by total digestion and ICP-MS analysis and corrected for lithogenic contributions by multiplying measured Al concentrations by a mean crustal Mo:Al ratio. The Mo concentrations and sedimentation rates were used to calculate Mo accumulation rates, and the rates examined as a function of the frequency of hypoxia. Rates were also compared with those from U.S. EPA laboratory experiments and literature references to oceanic basins that experience various degrees of hypoxia. Validation of the laboratory results will demonstrate the utility of measurements of Mo in sediments as a useful, long-term DO monitoring tool.