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 (pCO(2)). We show that for any change in pCO2 concentration (Delta pCO(2)), terrestrial and marine records can be used together to reconstruct background and maximum pCO(2) levels across the carbon isotope excursion. When applied to the carbon isotope excursion at the Palaeocene-Eocene boundary, we calculate pCO(2)=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 pCO(2) concentration when analysing large-scale changes in the carbon isotope composition of terrestrial substrates.