Seawater 186 Os/188 Os Variations During the Cenozoic

新生代海水 186 Os/188 Os 变化

• 批准号: 9819296
• 负责人: Gregory Ravizza
• 金额: $ 20.41万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1999
• 资助国家: 美国
• 起止时间: 1999-03-01 至 2001-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9819296&HistoricalAwards=false
• 关键词: Seawater; 186; Os; 188; Variations

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.

为化学风化的地质历史提供更好的限制对于理解为什么地球在数亿年的时间里一直适合居住至关重要。 化学风化作用在控制地球大气成分方面起着重要作用。 在百万年的时间尺度上，硅酸盐风化速率的变化被认为影响了大气中的CO2，较低的风化速率有利于较高的CO2，从而使全球气候变暖。 沉积有机物埋藏和侵蚀速率的变化也在调节地球大气的演变中发挥作用，不仅影响大气CO2，而且影响大气O2浓度。 重建化学风化历史的工作既困难又重要。 河流及其流入的海洋受到化学风化的影响，但它们是短暂的。 因此，我们依靠古代海洋沉积物化学成分的微妙变化来推断河流输入海洋的变化历史。重同位素比值在这方面特别有价值，因为已知古海洋沉积物准确记录了古海水的重要属性。 在这个项目中，我们将首次探索海水的186 Os/188 Os记录，并将其与已经建立的海水187 Os/188 Os记录进行比较。 187 Os丰度的变化来自187 Re的衰变（半衰期约为450亿年），而186 Os丰度的变化来自190 Pt的衰变（半衰期约为4500亿年）。 Re、Os、Pt的化学行为不同，Re、Os富集在沉积有机质中，Pt不富集。 这种对比行为提供了一种方法，可以利用古代海水中187 Os/188 Os和186 Os/188 Os比值的变化来推断硅酸盐和有机物风化作用的相对重要性随时间的变化，从而更好地了解这些基本地质过程在地球大气演化中可能发挥的作用。