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

9417948拉维扎最近的工作表明，根据对海洋沉积物的分析推断，海水的187Os/186Os在新生代期间急剧上升。根据保守估计，与白垩纪-第三纪(K-T)界线有关的Os存量超过数十万年的河流Os通量。K-T界面粘土中Os的同位素组成与典型的陆壳物质不同，但与陨石和地幔物质相似。即使只有一小部分同位素不同的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急剧地使海水的187Os/186Os比值突然变化到非常低的、几乎是陨石的值。由此导致的海水Os同位素组成的漂移，以及随后恢复到更具放射性的值，是有意义的，主要有两个原因。首先，漂移的持续时间为海洋OS系统在强烈外部扰动后的恢复率提供了一个经验衡量标准。漂移的持续时间越长，海水Os同位素记录就越有可能被用来识别地质记录中的其他撞击事件。或者，非常迅速的恢复将表明，海洋Os系统具有分辨海洋锶系统所不能处理的高频构造脉冲的潜力。其次，预期海水187Os/186Os比值快速、单调恢复到更高的值，有可能以类似于新近纪海水锶同位素记录的方式用作地层学工具。这里我们提供的初步数据表明，海水的Os同位素组成的最小值确实出现在K-T边界上，我们建议更详细地研究跨越这一边界的海水Os曲线的形状。我们将几乎只关注钻探钻探和钻探钻探岩心的样品，但也将分析L地体剖面中的一些海相K-T界线粘土。我们的具体目标是：(1)更好地约束K-T事件后马斯特里赫特晚期海水、K-T撞击器和海水的187Os/186Os比值；(2)确定K-T极小值后海水187Os/186Os比值的最初增加是跨越数百万年的长期恢复，还是相对快速的恢复；(3)评估K-T Os峰附近Os重新分配的长度尺度；以及(4)将K-T界线粘土和早期新生代沉积物中Os浓度和同位素组成的测定与其他PGE浓度的测定相结合。为了实现这些目标，将研究三种不同类型的沉积剖面：富含海水来源的有机化合物的金属沉积，它堆积在早新生代洋壳上；来自K-T剖面的远洋碳酸盐，由于短暂的间断，不含Ir异常；来自K-T剖面的远洋碳酸盐，由于短暂的间断，不含Ir异常；以及远洋碳酸盐，具有完整的K-T剖面，包括很好地圈定的Ir异常。对前两种类型K-T沉积物记录的分析将使我们能够确定样品中的海水Os曲线，而不受K-T Os峰成岩重新分布的潜在影响。在后一种环境中，K-TOs的成岩再分配问题将通过与其他两种环境中的数据进行比较来直接解决。

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.