187Re is a radioactive isotope that decays with a half- life of about 4.6x1010 years to 187Os. The 187Os of a system depends on the age of that system, the initial 187Os/186Os ratio and the 187Re/186Os (where 186Os is used as the non-radiogenic osmium tracer). The continental crust has 187Re/186Os about 100 times higher than the mantle so that the 187Os/186Os of the average continental crust with a mean age of about 20 billion years is about 10.5 whereas osmium immediately derived from the mantle has a value of about 1. The seawater 187Os/186Os of 8.5 implies that 80% of the present-day osmium supplied to the sea comes from the continents and 20% from oceanic mantle sources. This ratio of sources is expected to vary over time based on the marine 87Sr/86Sr record. The hydrogenous fraction of oxidized deep-sea sediments and organic-rich reduced deposits from stratigraphically dated systems can provide records of past seawater 187Os/186Os. The "giant piston core" from the central North pacific (LL GPC-3) and material from DSDP and ODP cores are ready sources for the study. The procedures involving leaching of the hydrogenous fraction of oxidized cores using dilute hydrogen peroxide have been developed. Preliminary results shows a first order correlation with the marine 97Sr/86Sr record indicating that the relative roles of continental and oceanic mantle source affect both elements. Imposed on this effect is the inverse relationship between 87Sr/86Sr and 187Os/186Os in the latest Pleistocene indicating different sources for the two elements. We will explore the full 187Os/186Os seawater record over the past 100 million years, compare it with the known 87Sr/86Sr record and thereby to add to our understanding of the relationship of two different isotopic signatures to the geologic record of alteration and weathering.
