Carbon isotopes on pedogenic carbonates of the Fray Bentos Formation (late Oligocene-early Miocene): a geochemical approach for calculating atmospheric paleoCO2

Abstract

In the context of modern climate change, constraining past atmospheric greenhouse gas concentrations and their paleoenvironmental settings is essential for improving predictions of future climate responses to elevated CO₂ levels. This contribution presents the first atmospheric paleo-CO₂ estimates derived from calcretes of the Fray Bentos Formation (late Oligocene–early Miocene) based
on the application of the pedogenic CO₂ paleobarometry model. An 11-m-thick profile at Picada de Berget (Canelones Department, Uruguay) was analyzed and subdivided into ten units according to sedimentary discontinuities and field and laboratory observations.  The succession consists of loess deposits and paleosols that, under semi-arid conditions, developed into calcretes dominated by
powdery and laminar pedogenic varieties. Stable-isotope analyses from seven calcrete horizons yield δ¹³C values between −10.2‰ and −8.3‰, δ¹³C of organic matter of approximately −25‰, and δ¹⁸O values ranging from −4.7‰ to −4.1‰. Using these data, we obtained reasonable paleo-CO² estimates for the late Oligocene–early Miocene, ranging from ~280 to 550 ppmv. These results fall within the range reported by independent proxies for this interval and support the potential of Fray Bentos calcretes as reliable archives for reconstructing atmospheric CO² concentration during the Paleogene–Neogene transition.