Collaborative Research: Calibrating the Isotopologue-47 Geothermometer in Soils

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Use of isotopologue-47 paleothermometer over the past several years is an exciting new development that holds enormous potential for reconstruction of paleotemperatures in the deeper geologic record. Paleosol carbonates are a particularly attractive target for such analysis because they are so abundant in the geologic record, and because temperature in soils displays a highly predictable relationship to air temperature (unlike in many other systems). Before this potential can be realized, the relationship between temperatures estimated from isotopologue-47 analysis (expressed as Ä47) of modern soil carbonate, and that of local air and soil temperature must be rigorously established. We present unpublished results from a number of modern soils that show that temperature estimates from Ä47 values systematically exceed those of modeled summer maximum temperatures by 6±2°C. Part of the excess?up to 2-4°C?may be explained asymmetric summer heating of soil. The remaining excess may be the result of contamination, the wrong assumed soil temperatures, or perhaps non-equilibrium kinetic effects. Whatever the cause, the more key question is whether the offset between actual and estimated values is systematic. This must be tested with a much larger sample set and with direct measurements of soil temperature. Here we propose to monitor temperature and other parameters in modern soils from a range of settings for comparison to Ä47 estimates of temperatures. Our study sites include Arizona, Nevada, Hawaii, and NW Argentina, taking in most of the natural temperature range for soil carbonate formation. Broader impacts of this on the larger scientific community will be substantial, given the intense interest in using this new geothermometer on a variety of research questions. Exciting potential applications of the new paleothermometer include reconstruction of paleotemperature (±2°C) using such long paleosol time series (17 Myr) as the Siwaliks in Pakistan, and of paleoelevation, through the strong sensitivity of the temperature to elevation in the world?s major orogens. Field and laboratory measurements and data analysis are ideal training opportunities for undergraduates in the sciences, who will be recruited from several institutions to participate in field and labwork.