Development of the Mass-47 Clumped Isotope Paleothermometer: Methods, Theory, and Application to Climate and Diagenetic Reconstructions.

Abstract

The mass-47 CO2 clumped isotope paleothermometer is one of the most exciting developments in the field of paleothermometry of the last 50 years. This technique improves upon previous approaches in that it constrains the formation temperature of carbonate minerals in a single measurement without assumptions about the composition of the precipitating fluid. This allows the clumped isotope thermometer to be applied to situations where traditional techniques cannot discern the relative influences of temperature versus fluid composition, and allows more accurate reconstructions of Earth climate. Several challenges remain before the thermometer can be accurately applied to Earth materials. Two separate calibrations must be employed to convert measured clumped isotope abundance into temperature estimates: a calibration of the effects of phosphoric acid digestion on CO2 isotopologues; and a calibration of the corrected mass-47 CO2 abundance to formation temperatures. Investigation of these calibrations reveals that they are likely mineralogy independent, suggesting that the cation and crystallographic structure plays a negligible role in clumped isotope equilibria. A second challenge relates to the sample size required for clumped isotope studies: at ~ 20 mg per sample, sample homogeneity cannot be assumed. Full investigation of mixing effects using a computer model and laboratory tests shows that sample heterogeneity can have a measureable effect on clumped isotope temperature estimates, but that these effects can be mitigated by the use of subsampling and mathematical mixing calculations. Further application of the clumped isotope thermometer requires detailed understanding of different proxy materials and the effects of geologic processes on these materials. Calcite formed by meteoric diagenesis was investigated in Pleistocene rocks and sediments. It was determined that meteoric phreatic cements accurately record mean annual temperature, while carbonate from vadose environments record kinetic departures from isotopic equilibrium. To demonstrate the utility of clumped isotopes a suite of Cenozoic New Zealand bivalves were examined. The clumped isotope thermometer reveals that New Zealand climate warmed from the Eocene to Miocene, which is the opposite trend inferred using traditional approaches. The ability to constrain temperatures independent of water isotopic composition is a major breakthrough, and will continue to refine our understanding of Earth history.