Osmium isotopic composition and platinum group element concentrations in sediments from ODP Hole 145-886C

Paired analyses of Os isotope composition and concentration of bulk sediment and leachable Os in a metalliferous pelagic clay sequence from the North Pacific, ODP Site 886C, are used to reconstruct the marine Os isotope record and the particulate meteoritic Os flux between 65.5 and 78 Ma. Measured 187Os/188Os of bulk sediments ranges from approximately 0.64 to 0.32 and those of leach analyses are very similar to bulk analyses. Hydrogenous Os dominates the sedimentary Os inventory throughout most of the studied interval. As a result the measured 187Os/188Os of leachable Os approximates that of contemporaneous seawater. The ODP 886C record shows rising 187Os/188Os in the deepest portion of the core, with a local maximum of 0.66 close to 74 Ma. The 67-72 Ma portion of the record is characterized by nearly constant 187Os/188Os ratios close to 0.6. The structure of the marine Os isotope record from ODP 886C differs markedly from the seawater 87Sr/86Sr curve, which rises monotonically throughout the time interval studied here. Calculated particulate meteoritic Os fluxes are between 0.5 and 2 pg/cm**2/kyr throughout most of the studied interval. Two discrete intervals of the core (one of which is within Cretaceous Tertiary, boundary KTB interval) are characterized by higher fluxes of meteoritic Os. Excluding these two intervals, the average background flux of particulate meteoritic Os is roughly half of that estimated from analyses of Cenozoic marine sediments. These are the first Os isotope data to provide evidence of resolvable temporal variations in the background flux of particulate meteoritic material to the Earth.

Nd, Sm and carbon isotopic data for samples from ODP Legs 115, 145, 171, and 207

Late Cretaceous fish debris from Demerara Rise exhibits a dramatic positive excursion of 8 e-Nd units during ocean anoxic event 2 (OAE2) that is superimposed on extremely low e-Nd(t) values (-14 to -16.5) observed throughout the rest of the studied interval. The OAE2 e-Nd excursion is the largest yet documented in marine sediments, and the majority of the shift is estimated to have occurred over <20 k.y. Low background e-Nd values on Demerara Rise are explained as the Nd isotopic signature of the South American craton, whereas eruptions of the Caribbean large igneous province or enhanced mixing of intermediate waters in the North Atlantic could have caused the excursion.

Strontium isotope ratios of fish teeth from ODP sites in the central North Pacific

Strontium isotopic compositions of ichthyoliths (microscopic fish remains) in deep-sea clays recovered from the North Pacific Ocean (ODP holes 885A, 886B, and 886C) are used to provide stratigraphic age control within these otherwise undatable sediments. Age control within the deep-sea clays is crucial for determining changes in sedimentation rates, and for calculating fluxes of chemical and mineral components to the sediments. The Sr isotopic ages are in excellent agreement with independent age datums from above (diatom ooze), below (basalt basement) and within (Cretaceous-Tertiary boundary) the clay deposit. The 87Sr/86Sr ratios of fish teeth from the top of the pelagic clay unit (0.7089891), indicate an Late Miocene age (5.8 Ma), as do radiolarian and diatom biostratigraphic ages in the overlying diatom ooze. The 87Sr/86Sr ratio (0.707887) is consistent with a Cretaceous-Tertiary boundary age, as identified by anomalously high iridium, shocked quartz, and sperules in Hole 886C. The 87Sr/86Sr ratios of pretreated fish teeth from the base of the clay unit are similar to Late Cretaceous seawater (0.707779-0.7075191), consistent with radiometric ages from the underlying basalt of 81 Ma. Calculation of sedimentation rates based on Sr isotopic ages from Hole 886C indicate an average sedimentation rate of 17.7 m/Myr in Unit II (diatom ooze), 0.55 m/Myr in Unit IIIa (pelagic clay), and 0.68 m/Myr in Unit IIIb (distal hydrothermal precipitates). The Sr isotopic ages indicate a period of greatly reduced sedimentation (or possible hiatus) between about 35 and 65 Ma (Eocene-Paleocene), with a linear sedimentation rate of only 0.04 m/Myr The calculated sedimentation rates are generally inversely proportional to cobalt accumulation rates and ichthyolith abundances. However, discrepancies between Sr isotope ages and cobalt accumulation ages of l0-15 Myr are evident, particularly in the middle of the clay unit IIIa (Oligocene-Paleocene).