MULTI–PROXY PALEOSOL EVIDENCE FOR TRIASSIC CLIMATES IN EASTERN UTAH

Prior studies suggest that alluvial lowlands of Pangea in the southwestern U.S.A. during the middle to late Triassic experienced an arid to semiarid climate with significant seasonality. However, our research finds evidence for a pluvial episode from paleosols in the Anisian to Norian section of eastern Utah. Multi–proxy paleosol evidence is used to quantify rainfall amounts, ambient temperatures, and atmospheric CO2 concentrations. Rainfall estimates are derived from geochemical molecular weathering ratios. Temperature and atmospheric CO2 levels are derived from stable oxygen and carbon isotopes of pedogenic carbonate, respectively. Eight pedotypes were identified, and include poorly drained aquic Entisols, well drained non–aquic Entisols, calcic Aridisols, calcic Inceptisols, cambic and calcic Vertisols, dystric Inceptisols, and argillic Alfisols. The Anisian Moenkopi Formation was dominated by Castle Valley paleosols with mean annual rainfall between 300 and 400 mm (semiarid) and mean annual temperature between 13 and 23°C (mesic to hyperthermic). The lower Chinle Formation (Carnian) contains a succession of Vertisols that changes up–section to argillic Alfisols and dystric Inceptisols. In accordance with geochemical and isotopic signatures from these pedotypes, rainfall amounts initially increased to between 700 and 900 mm, and then to between 1,300 and 1,400 mm. Temperatures estimated from a Vertisol in the basal lower Chinle are approximately 18°C (thermic). The upper Chinle Formation (Carnian–Norian) reveals a return to semiarid to subhumid conditions and the formation of calcic Inceptisols and non–aquic Entisols. Rainfall amounts decreased to between 400 and 600 mm with temperatures eventually increasing to 29°C (hyperthermic). Using the paleosol isotopic barometer, atmospheric CO2 estimates generally correlate with other proxy for the Triassic. The well developed Alfisols and noncalcareous Inceptisols identified during the Carnian of the lower Chinle correlate with a previously identified global pluvial episode based on sedimentological and marine isotopic evidence, possibly in response to rifting of Pangea. It should not be assumed that the middle to late Triassic in continental alluvial lowlands supported a uniform, semiarid to arid climate, with strong seasonality.