A NEW PALEOTHERMOMETER FOR FOREST PALEOSOLS AND IMPLICATIONS FOR CENOZOIC CONTINENTAL PALEOCLIMATE

Paleosols provide an extensive continental paleoclimate archive that reaches further back into geologic time than many other archives, such as ice-cores and marine sediments. While quantitative paleosol proxies have been developed for certain climate processes, an effective, widely applicable, paleothermometer has remained elusive. Part of this difficulty can be attributed to the fact that various soil orders behave differently due to their respective physical and chemical properties. Thus, when trying to tease out one particular climate process, it is practical to focus on an individual order or a sub-set of the twelve soil orders that exhibit similar process behavior. We aimed to create a paleosol thermometer for soils that typically form under forest vegetation, including Inceptisols, Alfisols, and Ultisols. This study compiled previously published major element data from 158 modern soils, covering nine different soil orders. As with previous studies, Vertisols were found to be an outlier relative to other soil orders, and to behave in chemically distinct ways relative to other clay-rich soils. Using a weathering index based on the loss of major cations (Na, K, Mg, Ca), we found a significant (r² = 0.56) relationship with temperature for forest soils. This was used to define a new paleothermometer that has a standard error of 2.2°C. In order to compare our paleothermometer against other temperature proxies, we evaluated previously published chemical data from 161 paleosols in Oregon. Not only did our proxy compare well with paleobotanical data, but it also captures many of the same events and trends found in the Cenozoic deep-marine oxygen isotope record. These include the Eocene-Oligocene transition, Late-Oligocene warming, and the Mid-Miocene Climatic Optimum. This suggests that our new paleothermometer is effective and that detailed, long-term terrestrial temperature records can be reconstructed from paleosols that record both local and global events.