Using Terrestrial Organic Matter to Reconstruct the Stable Carbon Isotope Composition of the Atmosphere

Abstract Carbon fluxes to Earth's atmosphere are reflected in the carbon isotope composition of atmospheric CO 2 (δ 13 C atm ). δ 13 C atm is used to track past changes to the global carbon cycle and is a variable in many paleoclimate proxies. However, existing δ 13 C atm proxy records derived from marine carbonates are relatively low‐resolution prior to the middle Eocene (45 Ma) and do not extend beyond the Cretaceous (145 Ma). Here, we use carbon isotopes of terrestrial organic matter (δ 13 C org ) from the early Eocene and late Paleocene to reconstruct δ 13 C atm . We present the first δ 13 C org data from the Hoback Formation (late Paleocene) and Pass Peak Formation (early Eocene) in the Hoback Basin, WY. We use these along with published late Paleocene/early Eocene δ 13 C org data to reconstruct δ 13 C atm . These terrestrial δ 13 C atm reconstructions generally align with established foraminifera‐based Cenozoic δ 13 C atm reconstructions, particularly for sites where mean annual precipitation (MAP) was 1,000–2,500 mm yr −1 . We thus propose that terrestrial organic carbon isotopes can supplement and expand the paleo‐δ 13 C atm record beyond the Cenozoic.