RECONSTRUCTING LATE MIOCENE-EARLY PLIOCENE PALEOCLIMATE OF SONOITA CREEK BASIN, AZ

The relative importance of climate vs tectonics driving landscape evolution is complex and nontrivial. The tectonically quiescent Sky Islands (high elevation/low relief mountains) of SE Arizona enable a unique opportunity to explore the impact of climate on landscape evolution over the past 10Ma. Here we present preliminary results from a paleoclimate study using ¹³C and ¹⁸O isotopes preserved in Miocene-Pliocene (6-3Ma) carbonate horizons in Sonoita Creek basin. We sampled five paleosol horizons containing carbonate nodules to analyze ¹³C and ¹⁸O isotopic ratios in a bulk sample isotope ratio mass spectrometer. The isotopic values are reported in context of the Vienna PeeDee Belemnite (VPDB) global standard, and range from -3.2 to -0.9‰ for ¹³C and -5.4 to -3.5‰ for ¹⁸O. These isotopic values are consistent with the Wang (1993) paleoclimate study in the nearby St. David Formation, indicating that calcrete formation occurred from the per descensum model of pedogeic caliche formation in which carbonate nodules develop in the zone of carbonate accumulation above the water table. We therefore attribute the ẟ¹⁸O isotope to reflect soil the oxygen isotopic composition of meteoric water and the ẟ¹³C to reflect the proportion of C3:C4 biomass present during formation. This proof of concept study enables a novel, high resolution landscape evolution analysis using a suite of paleoclimate, paleomagnetic, and cosmogenic ¹⁰Be paleoerosion rate data. In addition to these preliminary results, we present our study design of sampling every calcareous paleosol of the Sonoita Creek basin fill (~34 horizons) to reconstruct the paleoclimate record during sediment deposition. We will also provide an independent time constraint with a magnetostratigraphic study to correlate polarity reversals in the basin fill with the Geomagnetic Polarity Time Scale. Finally, we present our methodology for collecting suites of sediment samples constrained within single paleosol horizons to determine ¹⁰Be cosmogenic paleoerosion rates. Our goal with this project is to relate paleoerosion rates to climate to understand the erosional history of the basin and range topography of SE Arizona.