A calibration equation between Δ48 values of carbonate and temperature.

RATIONALE: Information on the temperature of formation or alteration of carbonate minerals can be obtained by measuring the abundance of the isotopologues 47 and 48 (Δ47 and Δ48 values) of CO2 released during acid dissolution. The combination of these two proxies can potentially provide a greater insight into the temperature of formation, particularly if the carbonate minerals form by non-equilibrium processes. METHODS: We have precipitated calcium carbonates at seven temperatures between 5 and 65°C and measured their δ48 values using a Thermo-253 plus isotope ratio mass spectrometer. The values were transformed to Δ48 values in the conventional manner and then converted to the carbon dioxide equilibrium scale. RESULTS: Using the Δ48 values, we have established an empirical calibration between temperature and Δ48 values: [Formula: see text] CONCLUSIONS: The calibration line produced allows the determination of the temperature of natural carbonates using the Δ48 values and agrees with the measurements of the Δ47 and Δ48 values of some carbonates assumed to have formed under equilibrium conditions.

Prehistoric cooking versus accurate palaeotemperature records in shell midden constituents.

The reconstruction of pre-depositional cooking treatments used by prehistoric coastal populations for processing aquatic faunal resources is often difficult in archaeological shell midden assemblages. Besides limiting our knowledge of various social, cultural, economic and technological aspects of shell midden formation, unknown pre-depositional cooking techniques can also introduce large errors in palaeoclimate reconstructions as they can considerably alter the geochemical proxy signatures in calcareous skeletal structures such as bivalve shells or fish otoliths. Based on experimental and archaeological data, we show that carbonate clumped-isotope thermometry can be used to detect and reconstruct prehistoric processing methods in skeletal aragonite from archaeological shell midden assemblages. Given the temperature-dependent re-equilibration of clumped isotopes in aragonitic carbonates, this allows specific processing, cooking or trash dispersal strategies such as boiling, roasting, or burning to be differentiated. Besides permitting the detailed reconstruction of cultural or technological aspects of shell midden formation, this also allows erroneous palaeoclimate reconstructions to be avoided as all aragonitic shells subjected to pre-historic cooking methods show a clear alteration of their initial oxygen isotopic composition.