RECONSTRUCTION OF LATE MIOCENE - EARLY PLIOCENE CLIMATE IN SONOITA CREEK BASIN, ARIZONA

The Basin and Range topography characterizing southeast Arizona is commonly referred to as the Sky Island topography because of the high elevation/low relief mountains separated by broad, low relief basins. The cessation of Basin and Range tectonics in southeast Arizona over the past 10 million years has created a natural laboratory to explore how climate has affected the landscape evolution. Previous work by Menges and McFadden (1981) provide a detailed stratigraphy of an exposed section of upper sedimentary basin fill containing 33 calcareous paleosol horizons in Sonoita Creek Basin, AZ. Magnetostratigraphy constrained the age of the deposit to late Miocene – early Pliocene (~6-3Ma), and a preliminary study conducted on 5 paleosol horizons confirmed that this deposit is suitable for carbon and oxygen isotope measurements. We collected bulk sample from each paleosol horizon, separated the carbonate nodules, and measured ¹³C and ¹⁸O isotope ratios to reconstruct a paleoclimate record. Here we present new paleoclimate results from 33 carbonate paleosol horizons spanning the upper basin fill deposit. The carbon and oxygen ratios are reported relative to the Vienna PeeDee Belemnite (VPBD) global standard for carbonates. The ẟ¹⁸O values reflect the composition of meteoric water in the soil at the time of formation, and the ẟ¹³C values indicate the ratio of C3:C4 plant biomass of the sediment. Based on isotopic values from the preliminary dataset also taken from this deposit, we hypothesize that the climate during this time period was cooler and wetter than the modern-day climate. The purpose of this study is to (1) reconstruct a paleoclimate record for Sonoita, Arizona during late stage basin aggradation and (2) assess climate variability across southeast Arizona by comparing the carbon and oxygen isotope ratios to a previous paleoclimate study conducted in the San Pedro Valley (Wang, 1993). Further, our dataset provides a high-resolution climate record for late stage basin aggradation in Sonoita Creek, which will enable us to explore the influence of climate on the landscape evolution of southeast Arizona.