MIOCENE SEAWATER OS-187 OS-188 RATIOS INFERRED FROM METALLIFEROUS CARBONATES/

Seawater Os-187/Os-188 ratios for the Middle Miocene were reconstructe d by measuring the Os-187/Os-188 ratios of metalliferous carbonates fr om the Pacific (DSDP 598) and Atlantic (DSDP 521) oceans. Atlantic and Pacific Os-187/Os-188 measurements are nearly indistinguishable and a re consistent with previously published Os isotope records from Pacifi c cores. The Atlantic data reported here provide the first direct evid ence that the long-term sedimentary Os-187/Os-188 record reflects whol e-ocean changes in the Os isotopic composition of seawater. The Pacifi c and the Atlantic Os measurements confirm a long-term 0.01/Myr increa se in marine Os-187/Os-188 ratios that began no later than 16 Ma. The beginning of the Os isotopic increase coincided with a decrease in the rate of increase of marine Sr-87/Sr-86 ratios at 16 Ma. A large incre ase of 1 parts per thousand in benthic foraminiferal delta(18)O values , interpreted to reflect global cooling and ice sheet growth, began ap proximately 1 million years later at 14.8 Ma, and the long-term shift toward lower bulk carbonate delta(13)C values began more than 2 Myr la ter around 13.6 Ma. The post-16 Ma increase in marine Os-187/Os-188 ra tios was most likely forced by weathering of radiogenic materials, eit her old sediments or sialic crust with a sedimentary protolith. We con sider two possible Miocene-specific geologic events that can account f or both this increase in marine Os-187/Os-188 ratios and also nearly c onstant 87Sr/86Sr ratios: (1) the first glacial erosion of sediment-co vered cratons in the Northern Hemisphere; (2) the exhumation of the Au stralian passive margin-New Guinea are system. The latter event offers a mechanism, via enhanced availability of soluble Ca and Mg silicates in the are terrane, for the maintenance of assumed low CO2 levels aft er 15 Ma. The temporal resolution (three samples/Myr) of the Os-187/Os -188 record from Site 598, for which a stable isotope stratigraphy was also constructed, is significantly higher than that of previously pub lished records. These high resolution data suggest oscillations with a mplitudes of 0.01 to 0.02 and periods of around 1 Myr. Although variat ions in the Os-187/Os-188 record of this magnitude can be easily resol ved analytically, this higher frequency signal must be verified at oth er sites before it can be safely interpreted as global in extent. Howe ver, the short-term Os-187/Os-188 variations may correlate inversely w ith short-term benthic foraminiferal delta(18)O and bulk carbonate del ta(13)C variations that reflect glacioeustatic events, (C) 1998 Elsevi er Science B.V. All rights reserved.