Clumped Isotopes Kinetic Effects: Insight from Synthetic Carbonate and its Implication for Speleothems
Carbonate clumped isotopes is a new paleothermometer based on the relative abundance of 13C-18O bonds in CaCO3 (∆47). Being an internal property of the carbonate lattice, it provides a temperature estimate that is independent of the isotopic composition of the water in which the carbonate was formed. As such it is most relevant on land where the complexity of water δ18O, associated with the hydrological cycle, makes temperature reconstruction difficult. A large variety of marine biogenic carbonates adheres to a common ∆47-T calibration relationship, developed through calcite synthesized at known temperatures. This relationship is therefore assumed to reflect equilibrium conditions. The application of clumped isotopes on land, however, has been elusive due to non-equilibrium, kinetic isotope effects, that are observed primarily in speleothems. These effects are associated with the degassing of CO2, and reflect the long time required for the solution to regain isotopic equilibrium. We observe that the time required for regaining equilibrium through isotope exchange between DIC and water is similar for ∆47 and δ18O; therefore ∆47 non-equilibrium also implies δ18O non-equilibrium in speleothems. We hypothesize that these kinetic isotope effects are related to speleothems formation occurring from a thin film of solution, where fast degassing lead to an isotopic offset in DIC, that is recorded in CaCO3 which is forming soon thereafter. To gain further insight we synthesized CaCO3 from a stagnant solution, leading to crystals forming at the water-air interface, the site of CO2 degassing, thus mimicking carbonate formation in thin films. The temperature dependence of ∆47 in these samples was significantly less steep than that observed in marine biogenic carbonates, consistent with a kinetic effect that offsets ∆47 to lower values, especially at low temperatures. The observed ∆47-T relationship, although not a direct calibration, is consistent with modern speleothems from two caves, Soreq (Israel) and Bunker (Germany). These observations support a mechanism associated with the competition between the rate of CO2 degassing and the rate of DIC-water exchange. It further suggests a stronger kinetic effect at lower temperature, consistent with slower DIC-water exchange at low temperatures. Preliminary results of ∆47 in Porites corals show a deviation of the opposite sign from the common biogenic ∆47-T relationship and may reflect a similar, though reversed, process.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 4924 PALEOCEANOGRAPHY / Geochemical tracers;
- 4958 PALEOCEANOGRAPHY / Speleothems