Atmospheric pCO2 Reconstructed across the Early Eocene Hyperthermals

Abstract:

Negative carbon isotope excursions (CIEs) are commonly associated with extreme global warming. The Early Eocene is punctuated by five such CIEs, the Paleocene-Eocene thermal maximum (PETM, ca. 55.8 Ma), H1 (ca. 53.6 Ma), H2 (ca. 53.5 Ma), I1 (ca. 53.3 Ma), and I2 (ca. 53.2 Ma), each characterized by global warming. The negative CIEs are recognized in both marine and terrestrial substrates, but the terrestrial substrates exhibit a larger absolute magnitude CIE than the marine substrates. Here we reconcile the difference in CIE magnitude between the terrestrial and marine substrates for each of these events by accounting for the additional carbon isotope fractionation by C₃ land plants in response to increased atmospheric pCO₂. Our analysis yields background and peak pCO₂ values for each of the events. Assuming a common mechanism for each event, we calculate that background pCO₂ was not static across the Early Eocene, with the highest background pCO₂ immediately prior to I2, the last of the five CIEs. Background pCO₂ is dependent on the source used in our analysis with values ranging from 300 to 720 ppmv provided an injection of ¹³C-depleted carbon with δ¹³C value of -60‰ (e.g. biogenic methane). The peak pCO₂ during each event scales according to the magnitude of CIE, and is therefore greatest during the PETM and smallest during H2. Both background and peak pCO₂ are higher if we assume a mechanism of permafrost thawing (δ¹³C = -25‰). Our reconstruction of pCO₂ across these events is consistent with trends in the δ¹⁸O value of deep-sea benthic foraminifera, suggesting a strong link between pCO₂ and temperature during the Early Eocene.