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

提高 U-Th 珊瑚测年的准确性：成岩途径、模型和效应的测试

• 批准号: 0819714
• 负责人: Laura Robinson
• 金额: $ 42万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-03-15 至 2012-02-29
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0819714&HistoricalAwards=false
• 关键词: Improving; Accuracy; Th; Coral; Dating

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.

这项资助将试图回答关于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在化石珊瑚上进行铀-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衰变的能量所动员？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上？3)珊瑚和水之间的铀交换能否改变珊瑚的同位素组成而不改变珊瑚？铀浓度？智力优点：鉴于目前人们对海平面上升对人类福祉的影响感到关切，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资料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。这种方法的优点包括化石珊瑚的广泛分布，质谱法的改进可以精确测年到60万年前，以及详细的地层相关性，可以用来确定突然的，亚米尺度的海平面变化。不幸的是，这些潜在的优势在很大程度上没有实现，这是因为铀和钍同位素的流动性被广泛认可，但人们对其了解甚少。对化石珊瑚的系统蚀变趋势的观察以及这些趋势可能被反冲动员的铀衰变产物的系统添加所解释的想法重新激发了人们对珊瑚成岩作用的兴趣。已经提出了几个模型来纠正U-Th珊瑚年龄这样的女儿除了，并已被证明可以提高年龄的一致性珊瑚从同一个化石礁。然而，这种校正年龄的准确性仍然存在争议，部分原因是模型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。更广泛的影响：收集的数据将为研究碳酸盐化学、矿物-流体相互作用、古海洋学、地质年代学和古气候等广泛领域的科学家提供基础信息。研究结果和方法将通过在同行评审的期刊上发表文章和在国际会议上发表演讲的方式提供给科学界。调查结果和影响将通过访谈和讲座向公众传达。该团队将使用与本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，该提案还将支持两名年轻科学家的职业生涯，他们将合作作为扩大其综合科学见解影响的一种方式。