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

这笔拨款将试图回答有关铀和钍同位素的明显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-钍定年的准确性：1)铀衰变产生的同位素在多大程度上被α衰变的能量调动起来？2)在珊瑚化石中经常观察到的过量铀衰变产物是否吸附在晶体表面？3)珊瑚与水之间的铀交换是否会改变珊瑚的同位素组成，而珊瑚本身却没有变化？铀浓度？知识价值：鉴于目前人们对海平面上升对人类福祉的影响的担忧，了解未来海平面变化的潜在幅度和速度至关重要。关于过去海平面变化的准确而详细的历史将在这方面提供重要的信息。重建海平面历史的主要方法之一是测定曾经生长在海面附近的珊瑚化石的年代。这种方法的优点包括化石珊瑚的广泛分布，质谱法的改进可以精确地追溯到60万年前，以及详细的地层相关性，可以用来确定亚米尺度的海平面突变。不幸的是，这些潜在的优势在很大程度上没有实现，因为人们广泛认识到用于测年的铀和钍同位素的流动性，但对其了解甚少。对珊瑚化石的系统蚀变趋势的观察，以及这些趋势可能由反冲动员铀衰变产物的系统添加来解释的想法，重新激发了对珊瑚成岩作用的兴趣。已经提出了几个模型来校正U-Th珊瑚的年龄，并且已经证明可以提高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，部分原因是模型所假设的物理机制尚未得到直接测试。我们迫切需要更好地了解这些机制，这样我们才能更好地了解过去和未来的海平面变化。这笔拨款将用于进行一系列旨在检验这些关键假设的实验。更广泛的影响：收集到的数据将为研究广泛领域的科学家提供基础信息：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和在国际会议上发表的报告提供给科学界。结果和影响将通过访谈和讲座向公众传达。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WHOI暑期学生奖学金计划进行。此外，该提案将支持两位从事合作的初级科学家的职业生涯，作为扩大他们结合的科学见解的影响的一种方式。

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.