Clumped isotopes are heavy isotopes that are bonded to other heavy isotopes. The carbonate clumped-isotope thermometer, or "13C–18O order/disorder carbonate thermometer", is a new approach for paleoclimate reconstruction, basing on the temperature dependence of the clumping of 13C and 18O into bonds within the carbonate mineral lattice. This approach has the advantage that the 18O ratio in water is not necessary (different from the δ18O approach), but for precise paleotemperature estimation, it also needs very large and uncontaminated samples, long analytical runs, and extensive replication. Commonly used sample sources for paleoclimatological work include corals, otoliths, bivalves, and foraminifera. Results are usually expressed as Δ47 (said as "cap 47"), which is the deviation of the ratio of isotopologues of CO2 with a molecular weight of 47 to those with a weight of 44 from the ratio expected if they were randomly distributed.
Contents
Mechanisms
When a heavier isotope substitutes for a lighter isotope (e.g., 18O for 16O), the chemical bond's vibration will be slower, lowering its zero-point energy. In another words, thermodynamic stability is related to the isotopic composition of the molecule.
12C16O32− (~98.2%), 13C16O32− (~1.1%), 12C18O16O22− (~0.6%) and 12C17O16O22− (~0.11%) are the most abundant isotopologues (~99%) for the carbonate ions, controlling the bulk δ13C, δ17O and δ18O values in natural carbonate minerals. Each of these isopotologes has different thermodynamic stability. For a carbonate crystal at thermodynamic equilibrium, the relative abundances of the carbonate ion isotopologues is controlled by reactions such as [3]:
13C16O32− + 12C18O16O22− = 12C16O32− + 13C18O16O22− (Reaction 1)The equilibrium constants for this reactions are temperature-dependent, with a trend that heavy isotopes tend to "clump" with each other (increasing the proportions of multiply substituted isotopologues) as temperature decreases [6][7]. Reaction 1 will be driven to the right with decreasing temperature, to the left with increasing temperature. Therefore, the equilibrium constant for this reaction can be used as an paleotemperature indicator, as long as the temperature dependence of this reaction and the relative abundances of the carbonate ion isotopologues are known[3].
Differences from the conventional δ18O analysis
In conventional δ18O analysis, both the δ18O values in carbonates and water are needed to estimate paleoclimate. However, for many times and places, the δ18O in water can only be inferred, and also the 16O/18O ratio between carbonate and water may vary with the change in temperature. Therefore, the accuracy of the thermometer may be compromised.
Whereas for the carbonate clumped-isotope thermometer, the equilibrium is independent of the isotope compositions of waters from which carbonates grew. Therefore, the only information needed is the abundance of bonds between rare, heavy isotopes within the carbonate mineral.
Methods
1. Extract CO2 from carbonates by reaction with anhydrous phosphoric acid. (there are no direct way to measure the abundances of CO32−s in Reaction 1 with high enough precision)
2. Purify the CO2 that has been extracted. This step removes contaminant gases like hydrocarbons and halocarbons which can be removed by gas chromatography.
3. Mass spectrometric analyses of purified CO2, to obtain δ13C, δ18O, and Δ47 (Abundances of mass-47 CO2) value. (precision need to be as high as ~10−5, for that isotope signals of interest are often less than 10−3)
Limitations
The temperature dependent relationship is subtle (-0.0005% per oC) (Quade 2007).
13C18O16O22− is a rare isotopologue (~60 ppm [3]).
Therefore, this approach requires long analyses and very large and uncontaminated samples.