Constraints on Phanerozoic paleotemperature and seawater oxygen isotope evolution from the carbonate clumped isotope compositions of Late Paleozoic marine fossils (Invited)

A long-standing geoscience controversy has been the interpretation of the observed several per mil increase in the oxygen isotope compositions of marine calcites over the Phanerozoic Eon. Explanations for this trend have included decreasing seawater paleotemperatures, increasing seawater oxygen isotope values, and post-depositional calcite alteration. Carbonate clumped isotope paleothermometry is a useful geochemical tool to test these hypotheses because of its lack of dependence on the bulk isotopic composition of the water from which carbonate precipitated. This technique is increasingly applied to ancient marine invertebrate shells, which can be screened for diagenesis using chemical and microstructural approaches. After several years of clumped isotope analysis of these marine carbonates in a handful of laboratories, a long-term temperature and isotopic trend is emerging, with the results pointing to relatively invariant seawater δ18O and generally decreasing seawater temperatures through the Phanerozoic. Uncertainties remain, however, including the effects of reordering of primary clumped isotope compositions via solid-state diffusion of C and O through the mineral lattice at elevated burial temperatures over hundred million year timescales. To develop a quantitative understanding of such reordering, we present data from laboratory heating experiments of late Paleozoic brachiopod calcite. When combined with kinetic models of the reordering reaction, the results of these experiments suggest that burial temperatures less than ~120 °C allow for preservation of primary brachiopod clumped isotope compositions over geological timescales. Analyses of well-preserved Carboniferous and Permian brachiopods reinforce these results by showing that shells with apparent clumped isotope temperatures of ~150 °C are associated with deep sedimentary burial (>5 km), whereas those with putatively primary paleotemperatures in the 10-30 °C range experienced no more than ~1.5 km of burial. Consideration of the thermal history of samples is essential for identifying the possibility of clumped isotope alteration via C-O bond reordering. Our experimental results further suggest that even nascent reordering (<5-10%) is enough to measurably bias reconstructed paleotemperatures (and calculated seawater oxygen isotope compositions) to higher values. We also investigate the use of electron backscatter diffraction (EBSD), which resolves crystallographic orientation at high spatial resolution, as an alternative method for identifying sub-microstructural diagenesis in calcite. Preliminary results of EBSD on the experimental and natural Paleozoic brachiopod shells will be presented and discussed with respect to their clumped isotope compositions and the Phanerozoic record.