Measuring the Temperature of the Dinosaurs - Clumped Isotopes

 Paleothermometry, the act of determining the temperature of formation of geologic materials including fossils, has been a central thrust of geological sciences since shortly before Harold Urey won the Nobel Prize in Chemistry.  The oldest “paleothermoter” is the oxygen isotope composition of carbonate minerals such as CaCO3, a major component of the sedimentary record.  This thermometer allows paleotemperature measurements by proxy measurement of the 18O/16O ratio of the mineral and some knowledge or assumption about the same ratio of the water.  The latter requirement has always been confounding – was the ocean chemistry the same as now hundreds of millions years ago?  Do large organisms control this ratio in internal fluids or are they subject to environmental fluctuation?  Poor assumptions about this part of the paleotemperature equation can lead to inaccuracies and biases in reconstructing temperature.

Recently, a new paleothermometer has been developed – the ordering of bonds between 13C and 18O, both rare isotopes of common elements, compared to stochastic distribution.  These isotope “clumps” tend to form more readily than predicted except at very high (~1000°C) temperatures, and magnitude of “clumping” is related thermodynamically to temperature.  This paleothermometer does not need an assumption of the isotopic composition of the water in which the mineral was formed, and, coupled with standard 18O/16O ratio measurements, can yield temperature and isotopic composition of water from a mineral-water system.

However, the measurement of the rare isotope clumps is very difficult.  Five years after the inception of this measurement, all measurements had been made at one laboratory – Caltech.  In the last 3 years, publications have come from Yale, Harvard, the Swiss Federal Institute of Technology (ETH), Johns Hopkins University, University of Chicago, and the National Technical University of Japan.  All but one of these measurements has still come from the same type of mass spectrometer. 

The research group of Brad Rosenheim (Stable Isotope Laboratory, Tulane University (SILT U)) has recently had a manuscript accepted for publication introducing concepts of making these difficult measurements on a completely different mass spectrometer.  In doing so, the group has improved the theory behind the measurement in order to explain differences between their measurements and those of other laboratories.  The results will allow many other laboratories to join the clumped isotope wave and will allow better measurements to be made in existing laboratories. 

Scientifically, the measurement of clumped isotopes has allowed improved understanding of the body temperatures of large extinct reptiles (dinosaurs), the environmental temperatures of areas where modern humans evolved, ancient ocean isotope composition, and the exhumation rates of buried limestones.  *Although Rosenheim and his collaborators at Tulane (Jianwu Tang, laboratory manager and Alvaro Fernandez, Ph.D. student) have no intentions of taking dinosaur temperatures, they have two important calibration papers in the pipeline.  By putting those through the review process and ultimately publishing them, the group will raise Tulane’s profile in geological and especially geochemical circles. 

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