Early Paleogene clumped-isotope soil temperatures from southwestern North America

The upper North Horn formation in the Axhandle Canyon of central Utah contains a paleosol record spanning the Paleocene-Eocene boundary, including the Paleocene-Eocene Thermal Maximum (PETM). We have previously analyzed a suite of pedogenic carbonate nodules from this section utilizing clumped isotope thermometry. The vulnerability of this climate proxy to diagenetic resetting remains poorly understood. We have determined the likelihood of diagenetic resetting through two primary approaches. First, we have compared the temperature measurements with other measured isotopic parameters, such as δ13C and δ18O of the bulk carbonate, to identify trends suggested to be characteristic of resetting. Second, we have conducted petrographic analysis of thin sections taken from each nodule, including point counts of primary constituents and cathodoluminescence (CL) microscopy. Point counting identified the primary component of all nodules to be micritic carbonate, with variable, low-to-moderate proportions of both siliciclastic grains and sparry calcite crystals. CL microscopy was utilized to qualitatively assign nodules to several categories, based on the degree to which the observed mineral luminescence is likely to reflect major burial alteration and resetting of the clumped thermometer to higher temperatures. These analyses suggest that the nodules with the warmest measured formation temperatures have experienced some degree of burial alteration, but that the majority of measured nodules preserved the isotopic composition of their original formation. The results of these analyses suggest that warm-season temperatures in mid-latitude continental interiors were considerably warmer than is suggested by previous proxy and modeling studies.