Variations of the Os Isotopic Composition of Seawater Acrossthe Cretaceous-Tertiary Boundary

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9417948 Ravizza Recent work has shown that the 187Os/186Os of seawater, as inferred from analyses of marine sediments, has risen dramatically during the course of the Cenozoic Era. Based on conservative estimates, the inventory of Os associated with the Cretaceous-Tertiary (K-T) boundary horizon exceeds several hundred thousand years of riverine Os flux. The isotopic composition of Os contained in K-T boundary clays is distinct from typical continental crustal matrial, but is similar to meteoritic and mantle-derived material. Even if only a small fraction of this isotopically distinct K-T Os dissolved in seawater, the effect on the Os isotopic composition of the K-T oceans would have dramatic, shifting seawater 187Os/186Os ratios abruptly to very low, nearly meteoritic, values. The resulting excursion in the Os isotopic composition of seawater, and subsequent recovery to more radiogenic values, is of interest for two main reasons. First, the duration of the excursion provides an empirical measure of the recovery rate of the marine Os system following a strong external perturbation. The longer the duration of the excursion the more likely it is that the seawater Os isotope record can be used to identify other impact events in the geologic record. Alternatively, a very rapid recovery would indicate that the marine Os system has the potential to resolve high frequency tectonic pulses which the marine Sr system cannot. Second, the anticipated rapid, monotonic recovery of the seawater 187Os/186Os ratio to higher values can potentially be employed as a stratigraphic tool in a manner similar to the Neogene seawater Sr isotope record. Here we present preliminary data which show that a minimum in the Os isotopic composition of seawater did occur at the K-T boundary, and we propose to examine the shape of the seawater Os curve across this boundary in greater detail. We will focus almost exclusively on samples from DSDP and ODP cores, however some marine K-T boundary clays from terrestria l sections will also be analyzed. Our specific goals are: (1) To better constrain the 187Os/186Os ratios of Late Maastrichtian seawater, of the K-T impactor, and of seawater immediately following the K-T event; (2) To determine whether the initial increase in the 187Os/186Os ratio of seawater following the K-T minimum was a protracted recovery spanning several million years, or was relatively rapid; (3) To assess the length scale of Os redistribution about a K-T Os peak; and (4) To couple measurements of Os concentration and isotopic compositions with determinations of other PGE concentrations in both K-T boundary clays and in early Cenozoic sediments. Three different types of sedimentary sections well be investigated in order to achieve these goals: Metalliferous sediments, rich in seawater-derived OS, which accumulated on early Cenozoic oceanic crust; Pelagic carbonates from a K-T section which, due to a brief hiatus, contain no Ir anomaly; and Pelagic carbonates from a K-T section which, due to a brief hiatus, contain no Ir anomaly; and Pelagic carbonates with a complete K-T section , including a well delineated Ir anomaly. Analyses of sediments from the former two type of K-T sediment records will enable us to define the seawater Os curve in samples free from the potential influence of diagenetic redistribution of the K-T Os peak. In the latter setting the issue of diagenetic redistribution of the K-T Os will be addressed directly by comparison to data from the other two settings.