Calibration of the carbonate ‘clumped isotope’ paleothermometer for otoliths

Paleothermometry is an essential tool for understanding past changes in climate. The ‘carbonate clumped isotope thermometer’ is a temperature proxy related to ordering of ¹³C and ¹⁸O in the carbonate lattice (based on measurements of ¹³C¹⁸O¹⁶O in CO₂ produced by acid digestion of carbonate). This thermometer has been previously calibrated for inorganic calcite and aragonitic corals Ghosh P., Adkins J., Affek H., Balta B., Guo W. F., Schauble E. A., Schrag D., and Eiler J. M. (2006) C-13-O-18 bonds in carbonate minerals: a new kind of paleothermometer. Geochim. Cosmochim. Acta70 (6), 1439-1456]. Here we determine the relationship between growth temperatures of aragonitic fish otoliths and abundances of ¹³C¹⁸O¹⁶O produced by acid digestion of those otoliths. Our calibration is based on analyses of otoliths from six species from four genera of modern fish sampled from a latitudinal transect of the Atlantic Ocean between 54° S and 65° N, plus one species from the tropical western Pacific. The temperatures at which fish otoliths precipitated were estimated by the mean temperature in the waters in which they lived, averaged over their estimated lifetimes. Estimated growth temperatures of our samples vary between 2 and 25 °C. Our results show that the abundance of ¹³C¹⁸O¹⁶O in CO₂ produced by acid digestion of fish otolith aragonite is a function of growth temperature, following the relationship: , where Δ₄₇ is the enrichment, in per mil, of ¹³C¹⁸O¹⁶O in CO₂ relative to the amount expected for a stochastic (random) distribution of isotopes among all CO₂ isotopologues, and T is the temperature in Kelvin. This relationship closely approaches that previously documented for inorganic calcite and aragonitic coral (Ghosh et al., 2006).