CARBONATE CLUMPED ISOTOPE THERMOMETRY OF PALEOSOL CARBONATES (Invited Presentation)

The carbonate clumped isotope geothermometer is a potentially powerful tool for reconstructing past terrestrial environments because of its ability to constrain temperature during carbonate mineralization, and oxygen isotopic composition of aqueous fluids during mineralization. Our understanding of clumped isotopes in soil and paleosol carbonates has advanced significantly in the past few years, and this talk will summarize some of the key findings. The most prominent of these is that soil carbonates in temperate latitudes appear to form primarily during the warm season. Thus in most cases the clumped isotope compositions of paleosol carbonates will be a proxy for warm season soil temperature. This finding leads to the following questions for future research: How universal is this ‘warm season bias’, and how do we make the best use of ‘warm season soil temperature’ as a new kind of paleoclimate information?

Preservation of original clumped isotope signals is also a critical aspect about which we are learning more. Unlike oxygen or carbon isotopes, clumped isotopes can be altered by solution-precipitation processes under closed-system (or low water-rock ratio) conditions. Solid-state diffusion will also act to ‘reorder’ ¹³C-¹⁸O bond abundances at temperatures far lower than would be necessary to change carbon and oxygen isotope ratios. Empirical field-based studies, as well as experimental studies in the lab, suggest that such reordering will become significant at temperatures around 100 °C and higher. Thus clumped isotope paleothermometry will be most successful when studying suites of samples that did not experience deep burial. For deeply buried (and hence altered) materials, the clumped isotope geothermometer will help to reconstruct the thermal history of the sedimentary archive, which in turn will be useful for gauging preservation of other isotopic and chemical proxies of past environmental conditions.