Collaborative Research: Constraining rates of C-O bond reordering in biogenic calcite: Implications for clumped isotope thermometry

温度是气候的一个中心方面，然而重建地质时间尺度上地球表面的温度历史仍然是一个具有挑战性的目标。利用贝壳等化石碳酸盐的氧同位素组成已经取得了很大进展，但这些组成既取决于碳酸盐矿物生长过程中的温度，也取决于矿物生长所在水的氧同位素组成。因此，氧同位素古温度计需要估计古代水域的氧同位素组成，如果这些估计不正确，重建的温度将是错误的。碳酸盐“块状同位素”测温法是一种新的测温方法，由于它不需要对过去的水同位素组成进行假设，而且能够重建过去的温度和过去的水同位素组成，因此在地科学界引起了广泛的兴趣。温度信息不是包含在矿物的整体同位素组成中，而是包含在重同位素碳-13和氧-18相互成键的优先“聚集”中。然而，虽然这一特征为解决古气候、地质生物学、构造学和岩石学中的长期存在的问题提供了巨大的希望，但同样的特征也导致了关于原始同位素信号保存的一个令人不快的事实：在埋藏过程中，碳-13-氧-18键的丰度比碳或氧同位素组成的变化要容易得多。碳-13-氧-18键的丰度可以通过简单的矿物埋藏加热来改变，这会导致碳和氧原子通过一种称为固态扩散的过程在矿物晶格中迁移。本研究以腕足类贝壳为研究对象，通过实验室实验和自然实验相结合的方法，研究了C-O键重排的动力学过程。实验室实验将使用从实验岩石学借用的方法来确定阿雷尼乌斯参数，从而预测固态碳-氧键重排的温度相关速率。自然实验将有助于评估实验室实验结果，并将重点放在北美3亿年前的腕足类化石上。腕足动物是此类研究的理想材料，因为它们被广泛用于古气候研究，它们的初始温度和形成时间大致已知，它们对重结晶具有抵抗力，而且可以比较对比的埋藏史。实验室和自然实验的一个主要目标是确定可以保存原始块状同位素成分的温度-时间域。换言之，研究人员试图回答这样的问题：“由于固体状态的重新排序，化石壳在什么温度下开始失去其原始的块状同位素成分。”拟议的工作将导致对至少两名研究生和两名本科生进行科学培训。德克萨斯农工大学(TAMU)和约翰霍普金斯大学(JHU)的研究生将分别访问佩雷斯-韦尔塔在阿拉巴马大学的实验室进行电子背向散射衍射分析，学生们将访问合作机构(TAMU或JHU)，学习块状同位素、电感耦合等离子体质谱、阴极发光和其他研究中使用的技术。学生们将在国际会议上展示研究结果，并准备发表研究结果。这项工作是定量的，将提供与地球科学的学术和应用方面有关的培训。

Temperature is a central aspect of climate, yet reconstructing the temperature history at Earth's surface over geological timescales has remained a challenging goal. Much progress has been made using the oxygen isotopic compositions of fossil carbonates such as shells, but these compositions depend both on the temperature during growth of the carbonate mineral, and on the oxygen isotopic composition of the water in which the mineral grew. Thus the oxygen isotope paleothermometer requires estimates of the oxygen isotopic composition of ancient waters, and the reconstructed temperatures will be in error if these estimates are incorrect. Carbonate "clumped isotope" thermometry is a new method that has generated wide interest within the geoscience community because it does not require assumptions about past water isotopic compositions, and moreover the method is capable of reconstructing both past temperatures and past water isotopic compositions. The temperature information is contained not in the overall isotopic composition of the mineral, but in the preferential "clumping" of the heavy isotopes carbon-13 and oxygen-18 into bonds with each other. However, while this feature lends the method great promise for solving long-standing questions in paleoclimate, geobiology, tectonics, and petrology, the same feature also leads to an inconvenient truth about preservation of the original isotopic signal: It is far easier, chemically and kinetically, for the abundances of carbon-13 ¬ oxygen-18 bonds to be altered during burial than it is for the bulk carbon- or oxygen-isotopic composition to be altered. The abundances of carbon-13 ¬ oxygen-18 bonds can be altered by simple burial heating of the mineral that causes carbon and oxygen atoms migrate through the mineral lattice through a process called solid-state diffusion. This research investigates the kinetics of such C-O bond reordering using a combination laboratory and natural experiments focusing on brachiopod shells. The laboratory experiments will use methods borrowed from experimental petrology to determine Arrhenius parameters allowing prediction of the temperature-dependent rates of solid state C-O bond reordering. The natural experiments will help to evaluate the laboratory experimental results, and will focus on 300 million-year-old brachiopod fossils from North America. Brachiopods are an ideal material for such a study because they are widely used in paleoclimate studies, they have approximately-known initial temperatures and times of formation, they are resistant to recrystallization, and because contrasting burial histories can be compared. A major goal of the laboratory and natural experiments is to define the temperature-time domain in which original clumped isotope compositions can be preserved. Stated differently, investigators seek to answer questions such as "at what burial temperature does a fossil shell begin to loose its original clumped isotope composition due to solid state reordering." The proposed work will result in the scientific training of at least two graduate students and two undergraduate students. The Texas A&M University (TAMU) and Johns Hopkins University (JHU) graduate students will each visit Perez-Huerta's lab at the University of Alabama to conduct electron backscatter diffraction analysis, and the students will visit the collaborating institution (TAMU or JHU) to learn clumped isotope, inductively coupled plasma mass spectrometry, cathodoluminsecence, and other techniques utilized in the study. The students will present findings at international meetings and prepare results for publication. The work is quantitative in nature and will provide training relevant both to academic and applied aspects of geoscience.