ABSTRACT

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.

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