Seawater 187Os/186Os Variation Over Time

187Re是一种放射性同位素，衰变的半衰期约为4.6x1010年至187Os。一个系统的187Os取决于该系统的年龄、初始的187Os/186Os比值和187Re/186Os(其中186Os被用作非放射性成因的Os示踪剂)。陆壳的187Re/186Os比地幔高约100倍，因此平均年龄约200亿年的平均陆壳的187Os/186Os约为10.5，而直接来自地幔的Os的值约为1。海水的187Os/186Os为8.5今天供应给海洋的Os的80%来自大陆，20%来自大洋地幔来源。根据海洋87Sr/86Sr的记录，这一来源的比率预计会随着时间的推移而变化。来自地层测年系统的氧化深海沉积物和富含有机质的还原沉积物的含氢分数可以提供过去海水187Os/186Os的记录。来自北太平洋中部的“巨型活塞芯”(11GPC-3)以及来自dsdp和dpp岩芯的材料都是这项研究的现成资料。开发了用稀过氧化氢浸取氧化岩心中含氢部分的方法。初步结果表明，与海洋~(97)Sr/~(86)Sr记录具有一级相关性，表明大陆和海洋地幔源区的相对作用对这两种元素都有影响。受此影响的是最新更新世87Sr/86Sr和187Os/186Os的反相关关系，表明这两个元素的来源不同。我们将探索过去1亿年的全部187Os/186Os海水记录，并将其与已知的87Sr/86Sr记录进行比较，从而加深我们对两种不同同位素特征与蚀变和风化地质记录的关系的理解。

187Re is a radioactive isotope that decays with a half- life of about 4.6x1010 years to 187Os. The 187Os of a system depends on the age of that system, the initial 187Os/186Os ratio and the 187Re/186Os (where 186Os is used as the non-radiogenic osmium tracer). The continental crust has 187Re/186Os about 100 times higher than the mantle so that the 187Os/186Os of the average continental crust with a mean age of about 20 billion years is about 10.5 whereas osmium immediately derived from the mantle has a value of about 1. The seawater 187Os/186Os of 8.5 implies that 80% of the present-day osmium supplied to the sea comes from the continents and 20% from oceanic mantle sources. This ratio of sources is expected to vary over time based on the marine 87Sr/86Sr record. The hydrogenous fraction of oxidized deep-sea sediments and organic-rich reduced deposits from stratigraphically dated systems can provide records of past seawater 187Os/186Os. The "giant piston core" from the central North pacific (LL GPC-3) and material from DSDP and ODP cores are ready sources for the study. The procedures involving leaching of the hydrogenous fraction of oxidized cores using dilute hydrogen peroxide have been developed. Preliminary results shows a first order correlation with the marine 97Sr/86Sr record indicating that the relative roles of continental and oceanic mantle source affect both elements. Imposed on this effect is the inverse relationship between 87Sr/86Sr and 187Os/186Os in the latest Pleistocene indicating different sources for the two elements. We will explore the full 187Os/186Os seawater record over the past 100 million years, compare it with the known 87Sr/86Sr record and thereby to add to our understanding of the relationship of two different isotopic signatures to the geologic record of alteration and weathering.