'Clumped isotope' thermometry in bioapatite

Abstract

The stable isotope compositions of biologically precipitated apatite in bone, teeth, and scales are widely used to reconstruct past climate and to obtain information on the diet, behavior, and physiology of extinct organisms. Apatite is an attractive target for paleoclimate studies because biogenic and inorganic apatites are frequently preserved in the geologic record, including in locations where carbonates are not found. Here we report the application of a new type of geochemical measurement to bioapatite, a 'clumped isotope' paleothermometer, based on the thermodynamically driven preference for ^(13)C and ^(18)O to bond with each other within carbonate ions in the bioapatite crystal lattice. This effect is dependent on temperature but, unlike conventional stable isotope paleothermometers, is independent from the isotopic composition of water from which the mineral formed. We show that the abundance anomaly, relative to a stochastic distribution, of ^(13)C-^(18)O bonds in the carbonate component of tooth bioapatite from modern specimens decreases with increasing body temperature of the animal, following a relationship between isotope 'clumping' and temperature that is statistically indistinguishable from inorganic calcite. This result is in agreement with a theoretical model of isotopic ordering in carbonate ion groups in apatite and calcite. This thermometer constrains body temperatures of bioapatiteproducing organisms with an accuracy of 1-2°C. Analyses of fossilized tooth enamel of both Pleistocene and Miocene age yielded temperatures within error of those derived from similar modern taxa. In addition to the study of paleoclimate, the clumped isotope analysis of bioapatite also represents a new approach in the study of the thermophysiology of extinct species, allowing the first direct measurement of their body temperatures. Our initial application of this thermometer has been to measure the body temperatures of Mesozoic dinosaurs to shed light on the 'cold-blooded' versus 'warm-blooded' dinosaur debate.

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