Seawater 186 Os/188 Os Variations During the Cenozoic

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Providing better constraints on the geologic history of chemical weathering is critical to understanding why the earth has remained habitable for hundreds of millions of years. Chemical weathering plays a fundamental role in controlling the composition of the earth's atmosphere. On million year time scales changes in the rate of silicate weathering are thought to have influenced atmospheric CO2, with lower weathering rates favoring higher CO2 and thus warmer global climate. Changes in the rates burial and erosion of sedimentary organic matter also play a role in regulating the evolution of earth's atmosphere, influencing not only atmospheric CO2, but also atmospheric O2 concentration. The job of reconstructing the history of chemical weathering is as difficult as it is important. Rivers, and the oceans into which they flow, are influenced by chemical weathering but they are ephemeral. Consequently we rely upon subtle changes in the chemistry of ancient marine sediments to make inferences about the changing history of riverine input the oceans. Heavy isotope ratios are particularly valuable in this regard because ancient marine sediments are known to accurately record important attributes of ancient seawater. In this project we will explore for the first time the 186Os/188Os record of seawater and compare it with the already established seawater 187Os /188Os record. Variations in 187Os abundance arise from 187Re decay (~ 45 billion year half-life), while 186Os abundance variations arise from the decay of 190Pt (~ 450 billion year half-life). The chemical behavior of Re, Os and Pt differ, with Re and Os being concentrated on sedimentary organic matter while Pt is not. This contrasting behavior provides a means of using the changing 187Os/188Os and 186Os/188Os ratios of ancient seawater to infer changes in the relative importance of silicate and organic matter weathering through time, and thereby gain a better understanding of the role these fundamental geologic processes may have played in the evolution of the earth's atmosphere.