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

温度是气候的一个核心方面，但在地质时间尺度上重建地球表面的温度历史仍然是一个具有挑战性的目标。利用化石碳酸盐（如贝壳）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时的温度，也取决于矿物生长的水的氧同位素组成。因此，氧同位素古温度表需要估计古代水体的氧同位素组成，如果这些估计不正确，重建的温度将是错误的。碳酸盐“团块同位素”测温法是一种新方法，由于它不需要假设过去的水同位素组成，而且该方法能够重建过去的温度和过去的水同位素组成，因此在地球科学界引起了广泛的兴趣。温度信息并不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相互形成键的优先“团块”中。然而，尽管这一特征为解决古气候、地质生物学、构造学和岩石学中长期存在的问题带来了巨大的希望，但同样的特征也导致了一个关于原始同位素信号保存的难以忽视的事实：从化学和动力学角度来看，在埋藏过程中改变碳-13 -氧-18键的丰度要比改变整体碳或氧同位素组成容易得多。碳-13 -氧-18键的丰度可以通过简单的矿物埋藏加热来改变，这种加热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。本研究以腕足类动物壳为研究对象，采用实验室和自然实验相结合的方法研究了这种C-O键重排序的动力学。实验室实验将采用借鉴实验岩石学的方法来确定Arrhenius参数，从而预测固态C-O键重排序的温度依赖性速率。自然实验将有助于评估实验室实验结果，并将重点放在3亿年前的北美腕足动物化石上。腕足类动物是进行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，它们具有抗再结晶性，并且可以比较不同的埋藏历史。实验室和自然实验的一个主要目标是确定可以保存原始团块同位素组成的温度-时间域。换句话说，研究人员试图回答这样的问题：“在什么温度下，化石壳由于固态重排序而开始松散其原始的块状同位素组成。”建议的工作将导致至少两名研究生和两名本科生的科学培训。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Perez-Huerta在阿拉巴马大学的实验室进行电子后向散射衍射分析，学生将访问合作机构（TAMU或JHU）学习团块同位素，电感耦合等离子体质谱，阴极发光，以及研究中使用的其他技术。学生们将在国际会议上展示他们的发现，并准备结果发表。这项工作本质上是定量的，并将提供与地球科学的学术和应用方面相关的培训。

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.