T6 AM Environmental Hypoxia
Tuesday, 15 November 2005: 8:00 AM - 11:40 AM in 324-326

(BOO-1117-816920) Laboratory Determination of Molybdenum Accumulation Rates as a Measure of Hypoxic Conditions.

Boothman, W¹, Coiro, L¹, ¹ U.S. Environmental Protection Agency, Narragansett, RI, USA

ABSTRACT- Previous studies indicate that molybdenum (Mo) in marine sediments may serve as a useful surrogate for direct measurement of hypoxic and anoxic conditions in overlying waters. Analyses of field samples show a relationship between accumulation of Mo and the period of time that dissolved oxygen (DO) concentrations in near-bottom waters are below 2.3 mg/L; lab experiments show significant Mo accumulation at 0.6 mg DO/L, but none at 2.9 mg DO/L. Field samples also indicate that accumulated Mo is stable in sediments for at least 3 months when DO levels rise, whereas the lab experiments show possible loss of Mo from reduced sediments when exposed to DO-saturated seawater. To refine the quantitative relationship between Mo accumulation and DO concentrations, homogenized marine sediments were exposed for up to 12 weeks to seawater maintained at 5 concentrations of DO between 0.7 and 3.5 mg/L, as well as a control (saturated), with DO concentrations and temperature monitored daily throughout the experiment. Sediments initially were either anoxic throughout or modified to present a 2-cm deep oxic layer. Sediments were sampled at 2 week intervals, at which times depth profiles of redox potential were measured in one oxic and anoxic core at each DO level. Profiles show a rapid convergence of redox conditions, regardless of starting conditions, but depth of the redox potential discontinuity remained sensitive to the concentration of DO in overlying water. Mo accumulation rates determined for each level of DO are compared to determine the critical concentration of DO in seawater below which Mo accumulation is constant and concentrations are a function of time below the critical concentration. The results, combined with sedimentation rates in field locations, may be used to determine the fraction of time waters overlying the sediments may have suffered from hypoxia.

Key words: molybdenum, sediments, hypoxia, redox