Abstract
The carbonate clumped isotope thermometer is a promising tool for determining past ocean temperatures. It is based on the temperature dependence of rare isotopes ‘clumping’ into the same carbonate ion group in the carbonate mineral lattice. The extent of this clumping effect is independent of the isotope composition of the water from which carbonate precipitates, providing unique advantages over many other paleotemperature proxies. Existing calibrations of this thermometer in cold-water and warm-water corals suggest clumped isotope ‘vital effects’ are negligible in cold-water corals but may be significant in warm-water corals. Here, we test the calibration of the carbonate clumped isotope thermometer in cold-water corals with a recently collected and well characterised sample set spanning a range of coral genera (Balanophyllia, Caryophyllia, Dasmosmilia, Desmophyllum, Enallopsammia and Javania). The clumped isotope compositions (Δ47) of these corals exhibit systematic dependences on their growth temperatures, confirming the basis of the carbonate clumped isotope thermometer. However, some cold-water coral genera show Δ47 values that are higher than the expected equilibrium values by up to 0.05‰ (equivalent to underestimating temperature by ∼9°C) similar to previous findings for some warm-water corals. This finding suggests that the vital effects affecting corals Δ47 are common to both warm- and cold-water corals. By comparison with models of the coral calcification process we suggest that the clumped isotope offsets in these genera are related to the kinetic isotope effects associated with CO2 hydration/hydroxylation reactions in the corals’ calcifying fluid. Our findings complicate the use of the carbonate clumped isotope thermometer in corals, but suggest that species- or genus-specific calibrations could be useful for the future application of this paleotemperature proxy.
Highlights
The carbonate clumped isotope thermometer is a promising tool for determining past ocean temperatures
Coral d18OVPDB ranged from À0.85‰ to 4.2‰, and d13CVPDB ranged from À9.07‰ to 1.89‰
We found excellent agreement between laboratories once the data were normalised with reference to the in-house carbonate standards, suggesting that this normalisation may be a useful way to remove inter-laboratory biases which were not removed by converting clumped isotope data into the absolute reference frame
Summary
The carbonate clumped isotope thermometer is a promising tool for determining past ocean temperatures. The stable isotope compositions (d18O and d13C) of cold-water coral skeletons are known to be depleted relative to the expected carbonate-seawater equilibrium values by up to 6–12‰ (Adkins et al, 2003; Lopez Correa et al, 2010; Lutringer et al, 2005; Mortensen and Rapp, 1998; Smith et al, 2000; Spiro et al, 2000; Rollion-Bard et al, 2003, 2010; Marali et al, 2013) The magnitude of these stable isotope vital effects varies even within a coral skeleton and is thought to be related to kinetic and/or pH-driven processes during coral calcification An example of such an isotope equilibrium is: Ca13C16O3 þ Ca12C18O16O2 () Ca13C18O16O2 þ Ca12C16O3
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