Osmium and helium isotope data are reported on aliquots of the same sediment samples from the first 200 ka of an Indian Ocean core retrieved from near the 90° East ridge (V34-53) at 6°S. Initial data on Holocene biogenic samples from near the coretop reveal Os concentrations of 30-50 ppt and lower-than-seawater Os isotopic compositions (187Os/188Os~0.84) for these predominantly biogenic, low-terrigenous sediments. Given sediment accumulation rates of about 1 g.cm-2.ka-1 (Peterson and Prell, 1985), and assuming Os is derived solely from meteoritic (187Os/188Os~0.12) and hydrogenous (187Os/188Os~1.02) components, the maximum contribution of Os from the meteoritic component to the bulk sediment is 6 pg.cm-2.ka-1. Assuming an average Os concentration in meteoritic dust of about 0.5 ppm, a dust flux of about 60x106 kg.a-1 over the earth's surface is derived from these data. This value is equal, within error, to values obtained by Esser and Turekian (1988), based on Os data from pelagic clays and Mn nodules, and Love and Brownlee (1993), based on direct examination of high velocity impact craters on an earth-orbiting satellite.
The best estimate for the extraterrestrial helium flux during this time period, based on Pacific deep-sea sediments, is (1.0 to 1.5)x10-12 cm3STP.cm-2.ka-1 (Marcantonio et al., 1995; Farley, 1995). Extension of this flux estimate to the Indian Ocean seafloor will be reported. To constrain the cosmic dust flux based on a constant 3He flux to the seafloor, we assume that, on average, meteoritic particles contains 2x10-5 cm3STP 3He per gram. This yields a global cosmic dust flux of approximately 0.35x104 kg.a-1, or less than 1% of that inferred from the Os and impact data. The Os and He methods of calculation of meteoritic dust flux suggest that while all of the Os is retained in the dust particles upon atmospheric impact, most of the He (>99%) is lost, presumably due to melting of particles larger than about 30-40 mm in diameter. The amount of meteoritic fine material which dissolves in the ocean and does not reach to the seafloor can be estimated by comparing the fluxes of 3He and Os to the sediments with the fluxes obtained by directly measuring the amounts of 3He and Os in wet- and dry-deposition collectors. The difference in the fluxes will shed light on the role of dissolved meteoritic debris in the seawater Os isotope record.
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Farley, Nature 376, 153-156 (1995).
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