Oxygen-18 thermometer in carbonate

    One of the first quantitative geochemical paleothermometers is based on the oxygen-18 composition (δ18O) of oxygen-bearing minerals such as carbonates. In a landmark study, Urey (1947) calculated the temperature sensitivity of 18O/16O equilibrium fractionation between calcium carbonate and water, and described how the isotopic composition of marine carbonates could be used to reconstruct past temperatures of the ocean.

      The first experimental calibrations of this technique focussed on the fractionation between biogenic carbonates (Abalone: Haliotididae) and seawater (Epstein et al., 1951, 1953). Numerous calibrations based on marine biocarbonates (corals, foraminifera, molluscs, fish otoliths, bivalves, coccoliths) were subsequently published (e.g. Devereux, 1967; Shackleton, 1974; Wellington et al., 1996; Bemis et al., 1998; Goodwin et al., 2001; Juillet-Leclerc and Schmidt, 2001; Candelier et al., 2013; Marchitto et al., 2014). These studies highlight substantial differences between the expected values and those observed, possibly due to kinetic isotope fractionation effects and/or apparent isotopic disequilibria associated with the biology of the calcifying organisms (the so called "vital effect" in biocarbonates).

      A well-known feature/limitation of oxygen-18 thermometry is its sensitivity to the isotopic composition of the parent waters. In many cases, it is challenging to precisely constrain past or present seawater δ18O values (δ18Osw). For instance, the conspicuous δ18O record of glacial/interglacial cycles observed in benthic carbonates (e.g. Zachos et al., 2001a; Lisiecki and Raymo, 2005) reflects both temperature variations and the changing oxygen-18 composition of the ocean due to variations in the size of the continental ice caps, deconvolving these two signals is far from straightforward (Shackleton, 1967).