Improving the Accuracy of U-Th Coral Dating: A Test of Diagenetic Pathways, Models, and Effects

这项资助将试图回答关于铀和钍同位素的表观流动性的三个关键问题，这些问题严重影响了对化石珊瑚进行铀-钍测年的准确性：1)α衰变能量动员的铀衰变产生的同位素在多大程度上是由阿尔法衰变能量动员起来的？2)在化石珊瑚中经常观察到的过剩的铀衰变产物是否被吸附到晶体表面？3)珊瑚和水之间的铀交换是否会在不改变珊瑚的同位素组成的情况下改变珊瑚的同位素组成？S铀浓度？学术价值：鉴于目前人们对海平面上升对人类福祉影响的担忧，了解未来海平面变化的潜在幅度和速度至关重要。过去海平面变化的详细历史将提供这方面的重要信息。重建海平面历史的主要方法之一是对曾经生长在海面附近的珊瑚化石进行测年。这种方法的优势包括广泛分布的化石珊瑚，改进的质谱学可以精确地追溯到60万年前，以及可以用来确定亚米级海平面突然变化的详细的地层对比。不幸的是，这些潜在的优势在很大程度上没有实现，因为用于测年的铀和钍同位素的流动性得到了广泛的认可，但人们对其知之甚少。对化石珊瑚系统性变化趋势的观察，以及关于这些趋势可以通过系统地添加反冲动员的铀衰变产物来解释的想法，重新唤起了人们对珊瑚成岩作用的兴趣。已经提出了几个模型来修正这种女儿增加的U-Th珊瑚年龄，并被证明改善了来自同一化石礁的珊瑚的年龄一致性。然而，这种修正年龄的准确性仍然存在争议，部分原因是这些模型所假设的物理机制尚未得到直接测试。迫切需要更好地了解这些机制，以便我们能够更好地了解过去和未来的海平面变化。这笔赠款将进行一系列实验，旨在测试这些关键假设。广泛的影响：收集的数据将为科学家研究广泛的领域提供基本信息：碳酸盐化学、矿物-流体相互作用、古海洋学、地质年代学和古气候。结果和方法将通过在同行评议的期刊上发表文章和在国际会议上作介绍的方式向科学界公布。结果和影响将通过采访和讲座向公众传达。该团队将利用与这项研究相关的小规模项目，向本科生介绍实验室技术以及U-Th测年的理论和实践。实习将通过世界卫生组织暑期学生奖学金计划进行。此外，这项提议将支持两名从事合作的初级科学家的职业生涯，以此来扩大他们综合科学见解的影响。

This grant will attempt to answer three key questions regarding the apparent mobility of uranium and thorium isotopes that severely compromises the accuracy of uranium-thorium dating performed on fossil corals:1) To what extent are the isotopes produced by uranium decay mobilized by the energy of alpha decay?2) Are excess uranium decay products, which are frequently observed in fossil corals, adsorbed onto the crystal surfaces?3) Can exchange of uranium between corals and water alter the isotopic composition of the coral with no change to the coral?s uranium concentration?Intellectual merit: In light of current concerns over the impacts of rising sea levels on human welfare, understanding potential magnitudes and rates of future sea level change is crucial. A well-dated and detailed history of past sea level change would provide important information in this regard. One of the premier methods for reconstructing the history of sea level is the dating of fossil corals that once grew near the sea surface. The advantages of this approach include the wide spread distribution of fossil corals, improvements in mass spectrometry allowing precise dating as far back as 600,000 years, and detailed stratigraphic correlations that can be used to determine abrupt, sub-meter scale sea-level changes. Unfortunately, these potential advantages are largely unrealized due to the widely recognized but poorly understood mobility of the uranium and thorium isotopes used for dating. Observations of systematic alteration trends in fossil corals and the idea that these trends might be explained by the systematic addition of recoil-mobilized uranium decay products have reinvigorated interest in coral diagenesis. Several models have been proposed to correct U-Th coral ages for such daughter addition, and have been shown to improve the age agreement of corals from the same fossil reef. However, the accuracy of such corrected ages remains controversial, in part because the physical mechanisms assumed by the models have not been tested directly. There is an urgent need to better understand these mechanisms, so that we might improve our understanding of past and future sea level change. This grant will undertake a series of experiments designed to test these critical assumptions.Broader Impact: The data that collected will provide fundamental information for scientists studying a broad variety of fields: carbonate chemistry, mineral-fluid interactions, paleoceanography, geochronology, and paleoclimate. Results and methods will be made available to the scientific community through publications in peer-reviewed journals and presentations at international conferences. Results and implications will be communicated to the public through interviews and talks. The team will use small-scale projects related to this research to introduce undergraduate students to laboratory techniques and the theory and practice of U-Th dating. Internships will take place through the WHOI Summer Student Fellowship Program. In addition, this proposal will support the careers of two junior scientists engaging in collaboration as a way of amplifying the impact of their combined scientific insights.