Rates of silicate dissolution in deep-sea sediment:: In situ measurement using 234U/238U of pore fluids

Bulk dissolution rates for sediment from ODP Site 984A in the North Atlantic are determined using the U-234/U-238 activity ratios of pore water, bulk sediment, and leachates. Site 984A is one of only several sites where closely spaced pore water samples were obtained from the upper 60 meters of the core; the sedimentation rate is high (11-15 cm/ka), hence the sediments in the upper 60 meters are less than 500 ka old. The sediment is clayey silt and composed mostly of detritus derived from Iceland with a significant component of biogenic carbonate (up to 30%). The pore water U-234/U-238 activity ratios are higher than seawater values, in the range of 1.2 to 1.6, while the bulk sediment U-234/U-238 activity ratios are close to 1.0. The U-234/U-238 of the pore water reflects a balance between the mineral dissolution rate and the supply rate of excess U-234 to the pore fluid by a-recoil injection of Th-234. The fraction of U-238 decays that result in a-recoil injection of U-234 to pore fluid is estimated to be 0.10 to 0.20 based on the U-234/U-238 of insoluble residue fractions. The calculated bulk dissolution rates, in units of g/g/yr are in the range of 4 X 10(-7) to 2 X 10(-6) yr(-1). There is significant down-hole variability in pore water U-234/U-238 activity ratios (and hence dissolution rates) on a scale of ca. 10 m. The inferred bulk dissolution rate constants are 100 to 10(4) times slower than laboratory-determined rates, 100 times faster than rates inferred for older sediments based on Sr isotopes, and similar to weathering rates determined for terrestrial soils of similar age. The results of this study suggest that U isotopes can be used to measure in situ dissolution rates in fine-grained clastic materials. The rate estimates for sediments from ODP Site 984 confirm the strong dependence of reactivity on the age of the solid material: the bulk dissolution rate (R-d) of soils and deep-sea sediments can be approximately described by the expression R-d approximate to 0.1 Age(-1) for ages spanning 1000 to 5 X 10(8) yr. The age of the material, which encompasses the grain size, surface area, and other chemical factors that contribute to the rate of dissolution, appears to be a much stronger determinant of dissolution rate than any single physical or chemical property of the system. Copyright (C) 2004 Elsevier Ltd.