THE MARINE OS-187 OS-186 RECORD OF THE PAST 80-MILLION YEARS/

We report new Os-187/Os-186 data and Re and Os concentrations in metal liferous sediments from the Pacific to construct a composite Os isotop e seawater evolution curve over the past 80 m.y. Analyses of four samp les of upper Cretaceous age yield Os-187/Os-186 values of between 3 an d 6.5 and Re-187/Os-186 values below 55. Mass balance calculations ind icate that the pronounced minimum of about 2 in the Os isotope ratio o f seawater at the K-T boundary probably reflects the enormous input of cosmogenic material into the oceans by the K-T impactor(s). Following a rapid recovery to Os-187/Os-186 of 3.5 at 53 Ma, data for the early and middle part of the Cenozoic show an increase in Os-187/Os-186 to about 6 at 15 Ma. Variations in the isotopic composition of leachable Os from slowly accumulating metalliferous sediments show large fluctua tions over short time spans. In contrast, analyses of rapidly accumula ting metalliferous carbonates do not exhibit the large oscillations ob served in the pelagic clay leach data. These results together with sed iment leaching experiments indicate that dissolution of non-hydrogenou s Os can occur during the hydrogen peroxide leach and demonstrate that Os data from pelagic clay leachates do not always reflect the Os isot opic composition of seawater. New data for the late Cenozoic further s ubstantiate the rapid increase in the Os-187/Os-186 of seawater during the past 15 Ma. We interpret the correlation between the marine Sr an d Os isotope records during this time period as evidence that weatheri ng within the drainage basin of the Ganges-Brahmaputra river system is responsible for driving seawater Sr and Os toward more radiogenic iso topic compositions. The positive correlation between Sr-87/Sr-86 and U concentration, the covariation of U and Re concentrations, and the hi gh dissolved Re, U and Sr concentrations found in the Ganges-Brahmaput ra river waters supports this interpretation. Accelerating uplift of m any orogens worldwide over the past 15 Ma, especially during the last 5 Ma, could have contributed to the rapid increase in Os-187/Os-186 fr om 6 to 8.5 over the past 15 Ma. Prior to 15 Ma the marine Sr and Os r ecord are not tightly coupled. The heterogeneous distribution of diffe rent lithologies within eroding terrains may play an important role in decoupling the supplies of radiogenic Os and Sr to the oceans and acc ount for the periods of decoupling of the marine Sr and Os isotope rec ords.