Reconciliation of marine and terrestrial carbon isotope excursions based on changing atmospheric CO₂ levels.

Negative carbon isotope excursions measured in marine and terrestrial substrates indicate large-scale changes in the global carbon cycle, yet terrestrial substrates characteristically record a larger-amplitude carbon isotope excursion than marine substrates for a single event. Here we reconcile this difference by accounting for the fundamental increase in carbon isotope fractionation by land plants in response to increasing atmospheric CO₂ concentration (pCO₂). We show that for any change in pCO₂ concentration (ΔpCO₂), terrestrial and marine records can be used together to reconstruct background and maximum pCO₂ levels across the carbon isotope excursion. When applied to the carbon isotope excursion at the Palaeocene-Eocene boundary, we calculate pCO₂=674-1,034 p.p.m.v. during the Late Palaeocene and 1,384-3,342 p.p.m.v. during the height of the carbon isotope excursion across all sources postulated for the carbon release. This analysis demonstrates the need to account for changing pCO₂ concentration when analysing large-scale changes in the carbon isotope composition of terrestrial substrates.