Miocene seawater 187Os=188Os ratios inferred from metalliferous carbonates

Seawater 187Os=188Os ratios for the Middle Miocene were reconstructed by measuring the 187Os=188Os ratios of metalliferous carbonates from the Pacific (DSDP 598) and Atlantic (DSDP 521) oceans. Atlantic and Pacific 187Os=188Os measurements are nearly indistinguishable and are consistent with previously published Os isotope records from Pacific cores. The Atlantic data reported here provide the first direct evidence that the long-term sedimentary 187Os=188Os record reflects whole-ocean changes in the Os isotopic composition of seawater. The Pacific and the Atlantic Os measurements confirm a long-term 0.01=Myr increase in marine 187Os=188Os 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 87Sr=86Sr ratios at 16 Ma. A large increase of 1 ‰ in benthic foraminiferal 18O values, interpreted to reflect global cooling and ice sheet growth, began approximately 1 million years later at 14.8 Ma, and the long-term shift toward lower bulk carbonate 13C values began more than 2 Myr later around 13.6 Ma. The post-16 Ma increase in marine 187Os=188Os ratios was most likely forced by weathering of radiogenic materials, either old sediments or sialic crust with a sedimentary protolith. We consider two possible Miocene-specific geologic events that can account for both this increase in marine 187Os=188Os ratios and also nearly constant 87Sr=86Sr ratios: (1) the first glacial erosion of sediment-covered cratons in the Northern Hemisphere; (2) the exhumation of the Australian passive margin–New Guinea arc system. The latter event offers a mechanism, vi