Quantifying Kinetic Isotope Effect in Speleothems Through Clumped and Oxygen Isotopes in Laboratory Precipitation Experiments
In many caves, speleothems δ18O deviate from the values expected for cave temperatures and drip water composition. These expected values are based on synthetic CaCO3 precipitation experiments1 that attempted to reflect isotopic equilibrium, although variations among precipitation experiments lead to uncertainties about non-equilibrium effects. Clumped isotopes ∆47 - T calibration2 is also based on synthetic CaCO3, but it is consistent with a wide variety of biogenic carbonates, suggesting isotopic equilibrium. ∆47 in speleothems may therefore help clarify observations in δ18O. Speleothems clumped isotopes are a notable exception from the calibration conformity, with an offset to lower ∆47 values, yielding erroneously high temperatures. This offset, due to kinetic isotope effects in CO2 degassing, is consistent with the δ18O values above the nominal 18O equilibrium, typically observed in speleothems. Speleothems are especially sensitive to these effects due to their formation in thin films of solution, where degassing isotope effects directly affect the mineral growing in close proximity, without the buffering provided by large volumes of solution. Isotopic dis-equilibrium is then due to the long time required for complete isotope exchange between DIC and water. We mimic this mechanism in precipitation experiments where CaCO3 is formed at the water-air interface and therefore strongly reflects the isotope effect of CO2 degassing. This results in offsets from the nominal equilibrium in both ∆47 and δ18O. For ∆47, a larger kinetic offset is observed at low temperatures; whereas δ18O shows the opposite trend with larger offsets from equilibrium at high temperatures. These relationships are consistent with observations in modern speleothems, providing a preliminary process based calibration for the kinetic effects in speleothems. Testing this calibration in stalagmites from Soreq cave (Israel) yields clumped isotope temperatures that are consistent with other temperature estimates in the Eastern Mediterranean. The drip water δ18O values derived from these temperatures and the kinetic calibration provide a basis for understanding the measured isotopic values of fluid inclusions. Preliminary data suggests that although fluid inclusion δ18O in Soreq stalagmites are enriched (by ~1.5%) relative to these calculated drip water, the glacial-interglacial difference is consistent between the two approaches. 1 Kim and O'Neil (1997), GCA. 2 Ghosh et al (2006), GCA.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 4958 PALEOCEANOGRAPHY / Speleothems