An isochron method for burial dating with cosmogenic nuclides: Application to river incision in southern Africa

河流阶地沉积物的年代可以提供有关河流切割、构造抬升速率以及河流如何响应气候变化的重要信息。然而，在没有可测年的火山岩的情况下，阶地砾石的年龄通常很难确定。一种用于测定阶地砾石年代的方法是宇宙成因核素埋藏测年，即用加速器质谱仪测量矿物石英中的稀有核素铝-26和铍-10。这两种核素是由宇宙射线产生的，宇宙射线起源于太空，在大气层中传播，但在穿过岩石时被阻挡。如果石英颗粒首先暴露在地表附近的宇宙射线中，但随后被埋在至少10米深的沉积物中，那么铝26和铍10的逐渐放射性衰变提供了一种测定过去300-500万年沉积时间的方法。然而，这项技术需要很深的埋藏深度，以便通过深入穿透宇宙射线来最大限度地减少埋葬后的产量。这个项目介绍并测试了一种新的方法，通过从较浅的深度分析几个单独的鹅卵石来绕过埋葬后的生产问题。每一块鹅卵石都会有完全相同的墓葬后产量。这允许使用等时线方法，其中铝-26与铍-10浓度的曲线图产生一条斜率取决于年龄的线条。这项研究将通过对地层序列中的样品进行分析，在已知年代的地点测试等时线埋藏测年方法。然后，这种测年方法将被应用于南部非洲桑迪斯河上的一个阶地序列，以确定过去300万年的隆升速率。南部非洲的隆升速率仍然受到很少的制约，可以提供有关侵蚀和隆升之间的相互作用以及地幔在支持非洲地形方面的作用的重要信息。过去500万年来一种新的砾石测年方法的发展，将为确定侵蚀和抬升速率提供机会，并可用于测定含有石器或化石的砾石沉积的年代。在非洲南部的实地调查结果将有助于揭示气候何时变得更加干燥，以及河流如何通过改变它们的负荷或切割率来做出反应。这些结果还将首次提供南非南部海岸河流切割和构造抬升的过时记录，那里的抬升速度和模式已经争论了50多年。这项工作还将提供包含重要石器组合的阶地砾石的日期，并将为早期石器时代的技术发展提供一个时间表。预计本研究开发的宇宙成因核素埋藏测年等时线方法将在过去500万年来研究地球和人类历史的许多领域得到广泛应用。

The age of river terrace sediments can provide important information about river incision, tectonic uplift rates, and how rivers respond to climate change. However, the age of terrace gravels is usually difficult to determine in the absence of datable volcanic rocks. One method that has been used to date terrace gravels is cosmogenic nuclide burial dating, in which the rare nuclides aluminum-26 and beryllium-10 are measured in the mineral quartz by accelerator mass spectrometry. These two nuclides are produced by cosmic rays that originate in space and travel through the atmosphere, but are blocked as they travel through rock. If quartz grains are first exposed to cosmic rays near the ground surface, but are then buried within a deposit that is at least 10 meters deep, then the gradual radioactive decay of aluminum-26 and beryllium-10 offers a means to date deposition over the past 3-5 million years. HOwever, this technique requires great burial depths in order to minimize post-burial production by deeply penetrating cosmic rays. This project introduces and tests a new method that circumvents the problem of post-burial production by analyzing several individual pebbles from a shallower depth. Each pebble will have exactly the same post-burial production. This allows the use of an isochron method, in which a plot of aluminum-26 versus beryllium-10 concentrations yields a line whose slope depends on age. This study will test the isochron burial dating method at sites of known age, and by analyzing samples in stratigraphic sequence. The dating method will then be applied to a terrace sequence on the Sundays River in southern Africa, to determine uplift rates over the past 3 million years. Uplift rates in southern Africa remain very poorly constrained, and can provide important information on the interactions between erosion and uplift, as well as the role of Earth's mantle in supporting African topography. The development of a new dating method for gravels over the past 5 million years will open opportunities for determining erosion and uplift rates, and can be used for dating gravel deposits that contain stone tools or fossils. Results of the field investigations in souther Africa will help show when the climate became drier, and how rivers responded by changing their load or their incision rate. The results will also provide, for the first time, a dated record of river incision and tectonic uplift on the southern coast of South Africa, where rates and patterns of uplift have been debated for over 50 years. This work will also provide dates for terrace gravels that contain important assemblages of stone tools, and will provide a timescale for technological developments in the Earlier Stone Age. It is anticipated that the isochron method of cosmogenic nuclide burial dating developed in this study will receive wide applications in many fields studying earth and human history over the past 5 million years.