Osmium Concentration and 187Os/186Os in Surficial Sediments of Long Island Sound

G. Williams Dept. of Geology and Geophysics, Yale University, Box 208109, New Haven, CT 06520, USA


F. Marcantonio Dept. of Geology and Geophysics, Yale University, Box 208109, New Haven, CT 06520, USA

K. K. Turekian Dept. of Geology and Geophysics, Yale University, Box 208109, New Haven, CT 06520, USA

The supply of Os from continents to the oceans must occur either by stream transport through estuaries or via the
atmosphere. In order to understand how osmium moves through estuaries, we have addressed the problem in Long Island Sound (LIS). We determined osmium concentration and 187Os/186Os for bulk samples and acidic hydrogen peroxide leaches of surficial sediments along the axis of Long Island Sound using methods developed at Yale (Esser and Turekian, 1993; Pegram et al., 1992). Acidic hydrogen peroxide leaches reduce Fe-Mn oxides and release the associated hydrogenous component of osmium. Sediment 187Os/186Os (187Os/188Os) increases from west (bulk 5.2 (0.62), leach 7.1(0.85)) to east (bulk ~9.0(1.08), leach 10.3(1.24)) and bulk osmium concentrations decrease from west (127 ppt) to east (20 ppt). Following the study of New Haven harbor (Esser and Turekian, 1993), we ascribe the variation to the supply of the low 187Os/186Os osmium from New York City (NYC) via the East River. Bulk 187Os/186Os is strongly inversely correlated (R2 = 0.74) with the organic carbon concentration. The fine-grained fraction rich in organic matter is the carrier of the low 187Os/186Os signature and the coarser grained, low organic carbon end member represents the surrounding glacial sediments with an 187Os/186Os of about 10 (1.2). Hydrogen peroxide leach values are consistently more radiogenic than associated bulk values. If the leach fraction represents more labile osmium, the difference between bulk and leach values indicates that there has been a decrease in the low 187Os/186Os component supplied to the Sound in recent times. The leach 187Os/186Os is also strongly inversely correlated (R2 = 0.87) with bulk organic carbon concentration. The observations require that LIS sediments record a mixture of a radiogenic continental sediment (187Os/186Os = 10-11(1.20-1.32), [Os] = 50 ppt (Esser and Turekian, 1993b), a hydrogenous component and a fine, organic rich relatively unradiogenic phase. Although the 187Os/186Os ratios and osmium concentrations close to the source of pollution are well demarked in western LIS and in New Haven harbor, the distribution of the pollutant signal throughout the rest of LIS is homogenized. This distribution is compatible with the strong horizontal turbulent transfer coefficients measured in the Sound (Torgersen et al., 1996) and the 100 day residence time of water in LIS (Turekian et al., 1996). The behavior of pollutant Os in LIS is similar to that of natural 210Pb LIS (Benninger et al., 1979) and 234Th LIS (Cochran and Aller, 1979); the standing crop of 210Pb or 234Th, or the osmium concentration, in sediments is determined by the redistribution of a fine-grained sedimentary component rather than the sites of supply or production.


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