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

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doi: 10.1016/j.gca.2004.04.024
Author(s): Maher, Katharine; DePaolo, Donald J.; Lin, Jo Chiu-Fang
Author Affiliation(s): Primary:
University of California, Department of Earth and Planetary Science, Berkeley, CA, United States
Other:
Lawrence Berkeley National Laboratory, United States
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, 68(22), p.4629-4648. Publisher: Pergamon, Oxford, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. 119 refs.; illus., incl. 6 tables, sketch map
Summary: Bulk dissolution rates for sediment from ODP Site 984A in the North Atlantic are determined using the 234U/238U 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 234U/238U activity ratios are higher than seawater values, in the range of 1.2 to 1.6, while the bulk sediment 234U/238U activity ratios are close to 1.0. The 234U/238U of the pore water reflects a balance between the mineral dissolution rate and the supply rate of excess 234U to the pore fluid by α-recoil injection of 234Th. The fraction of 238U decays that result in α-recoil injection of 234U to pore fluid is estimated to be 0.10 to 0.20 based on the 234U/238U of insoluble residue fractions. The calculated bulk dissolution rates, in units of g/g/yr are in the range of 4×107 to 2×106 yr1. There is significant down-hole variability in pore water 234U/238U activity ratios (and hence dissolution rates) on a scale of ca. 10 m. The inferred bulk dissolution rate constants are 100 to 104 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 (Rd) of soils and deep-sea sediments can be approximately described by the expression Rd ∼ 0.1 Age1 for ages spanning 1000 to 5×108 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. Abstract Copyright (2004) Elsevier, B.V.
Year of Publication: 2004
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 06 Petrology, Sedimentary; Actinides; Atlantic Ocean; Bjorn Drift; Carbonates; Chemical composition; Clastic sediments; Clay; Deep-sea sedimentation; Europe; Fluid phase; Geochemistry; ICP mass spectra; Iceland; Iceland-Faeroe Ridge; In situ; Isotope ratios; Isotopes; Leg 162; Liquid phase; Marine sedimentation; Marine sediments; Mass spectra; Mathematical methods; Measurement; Metals; Mineral composition; Models; North Atlantic; ODP Site 984; Ocean Drilling Program; Pore water; Radioactive isotopes; Recrystallization; Reykjanes Ridge; Rockall Plateau; Rockall Trough; Sedimentation; Sedimentation rates; Sediments; Silicates; Silt; Solid phase; Solution; Spectra; Thermal ionization mass spectra; U-238/U-234; Uranium; Weathering; Western Europe; Wyville-Thomson Ridge
Coordinates: N612532 N612532 W0240457 W0240457
Record ID: 2005026036
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands