Osmium and the History of Anoxia in Chesapeake Bay

George R. Helz Water Resources Research Center and Department of Chemistry and Biochemistry,

University of Maryland, College Park MD 20742 USA


Jordan M. Adelson Department of Chemistry and Biochemistry, University of Maryland,

College Park MD 20742. USA

Richard J. Walker Department of Geology, University of Maryland, College Park MD 20742, USA

Like many other populous coastal regions, Chesapeake Bay is experiencing seasonal anoxia, with devastating consequences for benthos. Anoxia is controlled both by human activities, which affect Corg production by supplying N and P, and by
hydrologic process, which affect density stratification and hence deep-water reaeration. To understand which has the greatest influence on anoxia, long term records would be useful. The span of monitoring data in coastal environments is usually
inadequate, so geochemical indicators of the history of anoxia are needed. Among the promising elements whose sediment concentrations might record overlying anoxia are Mo, Re and U. Osmium also is enriched in the euxinic sediments of the Black Sea, suggesting that it too could be a candidate. The advantage of Os is that isotopic variations offer additional information on provenance.

Total Os concentrations in a reconnaissance suite of samples from Chesapeake Bay and its tributaries range for the most part from 0.21 to 0.04 ppb. 187Os/188Os values for these samples fall in the range, 0.64-0.85, substantially lower than modern seawater and presumed to be the crustal signature in this region. Miocene marine deposits along the Bay's shoreline also fall in this range when corrected for production of 187Os since deposition. (The Miocene deposits are sulfidic and contain 17.6 ppb Re.) Four samples have been found that contain much higher Os concentrations (0.7-1.6 ppb) and low 187Os/188Os ratios
(0.14-0.27). Lower Susquehanna river suspended and bed sediments are in this latter group, suggesting anthropogenic
contamination of the lower Susquehanna. Curiously, samples from Baltimore Harbor and the lower James River, areas of high anthropogenic impact, fall with in the "normal crustal" suite. On the other hand, relatively low, non-radiogenic ratios are observed at a deep water site far from obvious anthropogenic sources. In one case, a low 187Os/188Os ratio was observed below the oak-ragweed pollen transition which marks the introduction of European style agriculture. Thus there must be a non-
anthropogenic source of low 187Os/188Os material. Possibly, meteoritic Os from the Exmore crater, 150 km to the south, is still being cycled in the Bay owing to erosion of Eocene and Miocene shoreline deposits.

The absence of a modern seawater 187Os/188Os signature indicates that total Os in Chesapeake Bay sediments will not provide direct information about euxinic conditions in the water column. In contrast, previous evidence (C.V. Miller, Ph.D. Dissertation, 1992) suggests that Mo is useful in this regard; investigations of Re are underway. The value of Os isotopes lies in their provenance information which can be useful in combination with Mo and Re data. In Chesapeake Bay, some of the structure in Mo profiles in sediment cores correlates with shifts in 187Os/188Os, implying that Mo concentrations are being affected both by changes in water column conditions and by changes in provenance.