Cosmic Dust: Terrestrial Accretion Rate and Solubility in Seawater

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We have measured Os-isotope ratios and Os concentrations of bulk pelagic sediments and the hydrogenous (seawater-derived) fraction from DSDP site 596 and other Pacific DSDP/ODP sediments, spanning the past 80 million years. These new data exhibit a general increase in the lS7Os/IStOs of seawater since the K-T boundary, confirming the general shape of the previously reported Cenozoic seawater Os curve (Pegram et a/., 1992; Ravizza et al., 1993). In addition these data, in conjunction with Os isotope mass balance calculations, constrain past variations in the terrestrial accretion rate of cosmic dust during the Cenozoic. The model calculations indicate a nearly constant flux which is similar to Os-based estimates from analyses of recent pelagic clays. This stands in marked contrast to the pronounced minimum in the Os isotope ratio of seawater and large cosmic flux inferred from K-T boundary samples. Both the K-T noble metal spike and the constancy of Cenozoic cosmic dust flux are consistent with Ir data from pelagic clays (Kyte and Wasson, 1986). However, the average cosmic dust flux based on Os isotope data (35000• 12000 tons/a) is much lower than the It-based estimates (96000_+32000 tons/a), suggesting that underest imation of terrestrial Ir accumulation in pelagic clays biases Ir-based cosmic dust flux estimates toward high values. Fig. 1 shows compiled and standardized literature estimates of cosmic dust flux from both the deep sea (filled symbols) and the Earth's surface (open symbols), which can be used to assess the extent of cosmic dust dissolution in seawater or in the sedimentary pile. Various techniques such as mechanical separation of cosmic spherules from sediments or ice, direct space observations and particle collection, trace elements and isotope tracers have been used in more than 50 studies (numbers on the x-axis refer to references below the diagram) to estimate the cosmic flux. Mechanical separations often yield lower bounds (upright triangles) because isolation of all cosmic par t ic les from sediments is impossible. In contrast, some chemical tracers such as Ni give upper bounds (inverted triangles) because the terrestrial contribution is underestimated. The most reasonable flux estimates converge at about 20000 to 60000 tons of cosmic matter accreted each year to the surface as well as the deep sea. Thus, the compiled data do not yield clear evidence supporting significant dissolution.