Variations of the Os Isotopic Composition of Seawater Acrossthe Cretaceous-Tertiary Boundary

小行星9417948 最近的研究表明，海水中的187 Os/186 Os，根据海洋沉积物的分析推断，在新生代期间急剧上升。 根据保守的估计，与古近纪-第三纪（K-T）边界层相关的Os的库存超过了数十万年的河流Os通量。 K-T边界粘土中的Os同位素组成不同于典型的陆壳物质，而与陨石和幔源物质相似。 即使只有一小部分这种同位素不同的K-T Os溶解在海水中，对K-T海洋的Os同位素组成的影响也会显着，使海水187 Os/186 Os比值突然降低到非常低的，几乎是陨石的值。 由此产生的海水中的Os同位素组成的偏移，并随后恢复到更多的放射性值，是有意义的两个主要原因。 首先，漂移的持续时间提供了海洋Os系统在强外部扰动后的恢复率的经验测量。 偏移持续的时间越长，海水Os同位素记录就越有可能用于确定地质记录中的其他撞击事件。 另外，一个非常快速的恢复将表明，海洋Os系统有可能解决高频构造脉冲的海洋Sr系统不能。 第二，海水187 Os/186 Os比值的预期快速、单调恢复到较高值，可能以类似于新近纪海水Sr同位素记录的方式用作地层工具。 在这里，我们提出的初步数据表明，在海水中的Os同位素组成的最低发生在K-T边界，我们建议检查的形状的海水Os曲线在这个边界上更详细。 我们将主要关注DSDP和ODP岩心样品，但也将分析陆地剖面的一些海洋K-T边界粘土。 我们的具体目标是：（2）确定K-T极小期后海水中187 Os/186 Os比值的初始升高是否是一个持续数百万年的恢复过程，还是一个相对较快的恢复过程;（3）估算K-T Os峰附近Os再分布的长度尺度;（4）将K-T界线粘土和新生代沉积物中Os含量和同位素组成的测定与其它PGE含量的测定相结合。 为实现这些目标，应调查三种不同类型的沉积剖面：富含海水成因的OS的含金属沉积物，它们堆积在早新生代洋壳上; K-T剖面的远洋碳酸盐，由于短暂的间断，不含Ir异常;和远洋碳酸盐岩，具有完整的K-T剖面，包括良好描绘的Ir异常。 通过对前两类K-T沉积记录的分析，可以确定不受K-T Os峰成岩再分布影响的海水Os曲线。 在后一种情况下，通过与其他两种情况下的数据进行比较，将直接讨论K-T Os的成岩再分配问题。

9417948 Ravizza Recent work has shown that the 187Os/186Os of seawater, as inferred from analyses of marine sediments, has risen dramatically during the course of the Cenozoic Era. Based on conservative estimates, the inventory of Os associated with the Cretaceous-Tertiary (K-T) boundary horizon exceeds several hundred thousand years of riverine Os flux. The isotopic composition of Os contained in K-T boundary clays is distinct from typical continental crustal matrial, but is similar to meteoritic and mantle-derived material. Even if only a small fraction of this isotopically distinct K-T Os dissolved in seawater, the effect on the Os isotopic composition of the K-T oceans would have dramatic, shifting seawater 187Os/186Os ratios abruptly to very low, nearly meteoritic, values. The resulting excursion in the Os isotopic composition of seawater, and subsequent recovery to more radiogenic values, is of interest for two main reasons. First, the duration of the excursion provides an empirical measure of the recovery rate of the marine Os system following a strong external perturbation. The longer the duration of the excursion the more likely it is that the seawater Os isotope record can be used to identify other impact events in the geologic record. Alternatively, a very rapid recovery would indicate that the marine Os system has the potential to resolve high frequency tectonic pulses which the marine Sr system cannot. Second, the anticipated rapid, monotonic recovery of the seawater 187Os/186Os ratio to higher values can potentially be employed as a stratigraphic tool in a manner similar to the Neogene seawater Sr isotope record. Here we present preliminary data which show that a minimum in the Os isotopic composition of seawater did occur at the K-T boundary, and we propose to examine the shape of the seawater Os curve across this boundary in greater detail. We will focus almost exclusively on samples from DSDP and ODP cores, however some marine K-T boundary clays from terrestria l sections will also be analyzed. Our specific goals are: (1) To better constrain the 187Os/186Os ratios of Late Maastrichtian seawater, of the K-T impactor, and of seawater immediately following the K-T event; (2) To determine whether the initial increase in the 187Os/186Os ratio of seawater following the K-T minimum was a protracted recovery spanning several million years, or was relatively rapid; (3) To assess the length scale of Os redistribution about a K-T Os peak; and (4) To couple measurements of Os concentration and isotopic compositions with determinations of other PGE concentrations in both K-T boundary clays and in early Cenozoic sediments. Three different types of sedimentary sections well be investigated in order to achieve these goals: Metalliferous sediments, rich in seawater-derived OS, which accumulated on early Cenozoic oceanic crust; Pelagic carbonates from a K-T section which, due to a brief hiatus, contain no Ir anomaly; and Pelagic carbonates from a K-T section which, due to a brief hiatus, contain no Ir anomaly; and Pelagic carbonates with a complete K-T section , including a well delineated Ir anomaly. Analyses of sediments from the former two type of K-T sediment records will enable us to define the seawater Os curve in samples free from the potential influence of diagenetic redistribution of the K-T Os peak. In the latter setting the issue of diagenetic redistribution of the K-T Os will be addressed directly by comparison to data from the other two settings.