An introduction to the techniques, limitations and landmarks of carbonate oxygen isotope palaeothermometry

The fractionation of oxygen isotopes is temperature dependent. The ratio of the two commonest isotopes ( 16 O and 18 O) in carbonate fossils can, in principle, be used to reconstruct the temperature of ancient oceans. Fossil foraminifera are commonly analysed in the Cenozoic and late Mesozoic and, when appropriately identified and separated, seasurface, deeper-water and bottom-water temperatures can be inferred. More ancient carbonate precipitating macrofossils (e.g. molluscs, brachiopods) have also been used, as well as inorganically precipitated carbonate cements. Drawbacks to the oxygen isotope method of palaeotemperature determination are the uncertainties in the isotopic composition of the water of ancient oceans, the occurrence of non-equilibrium fractionation in organically precipitated calcites (especially in macrofossils) and diagenetic alteration to the δ 18 O values of carbonate fossils. Notwithstanding these limitations, trends in palaeotemperature and/or the δ 18 O of seawater, such as the Palaeozoic 18 O enrichment, the long-term Cretaceous and Tertiary climatic cooling, the middle Miocene 18 O enrichment, as well as the Pleistocene succession of glaciations, are discernible from appropriate studies of fossil carbonates.