Correcting 18O/16O vital effects in biogenic carbonate using 17O/16O and clumped isotope analysis.

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Paleo temperatures (T) are routinely derived from biogenic carbonates using their oxygen isotopic composition (δ18O). Such T estimates require the – generally unknown - isotopic composition of the water. Novel clumped isotopes (Δ47) allow calculating paleo T only from carbonate. Both methods, however, require carbonate-water equilibrium. Disequilibrium is frequently induced by so called “vital effects” which have been attributed to several processes including: (i) diffusion; (ii) hydration (CO2 + H2O) and hydroxylation (CO2 + OH-) of CO2; and (iii) pH effects or disequilibrium at the liquid/solid interface. Isotopic shifts associated with each of these processes can alter paleo T estimates. Especially coral derived δ18O-T can be offset by as much as 20°C from known growth-T. Apparent offsets by up to 9°C are now also observed for the novel Δ47 proxy. Thus both methods fail at constraining accurate paleo T from such samples. Here I propose to perform δ17O analysis to correct for vital effects. If a sample forms in equilibrium with the respective water, all three isotope ratios (δ17O, δ18O and Δ47) must indicate identical T. If the thermometers yield different T, biogenic carbonate must have formed out of equilibrium due to vital effects. Each underlying physical process falls on a unique and well resolvable slope in δ18O vs. δ17O space. Hence, a sample suite with variable vital effect will fall on a process-specific slope in triple oxygen isotope space allowing to distinguish between diffusion, hydration, hydroxylation and a pH effect. The linear correlation can be extrapolated to the equilibrium point where both the δ17O and δ18O give the same paleo T, which corresponds to the real growth T of the carbonate. Additional clumped isotope analysis are required if the water isotopic composition is not known. This approach may also be promising to correct for kinetic isotope effects observed in inorganic carbonates such as pedogenic carbonates, speleothems or hydrothermal carbonates.