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 CO 2 concentration (pCO 2 ). We show that for any change in

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... pCO 2 concentration (DpCO 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 ¼ 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.