The variation of sea water 187Os/186Os during the Cenozoic has been determined by analyzing the hydrogenous Os released by acidic hydrogen peroxide leaching of layers in an abyssal North Pacific pelagic clay core. Overall, sea water 187Os/186Os has increased from 3.2 at 58 Ma to the present-day value of ∼ 8.6. This pattern is similar to the well-known 87Sr/86Sr evolution of sea water. The increase in the proportion of continental Os (187Os/186Os⩾ 10) could be due to enhanced weathering rates of black shales associated with the uplift of the Himalayas. Other elements associated with black shale weathering, such as U and P, should show effects in the sedimentary record.
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Pegram et al. (1992) studied this question.
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