The changing stable carbon isotope composition of atmospheric CO₂ reflects discrete carbon flux events in Earth's history. One such event is the Cretaceous-Tertiary boundary, which is recognized by an approximately -1 0/00 to -2 0/00 excursion in marine carbonate carbon isotopic value. This excursion is used to infer changes in atmospheric CO₂ (and resulting climate change) associated with the terminal Cretaceous bolide impact. Because the excursion is modulated through the globally well-mixed atmosphere, this event offers a robust chemostratigraphic marker.

We couple a model of isotopic fractionation during C3 land plant carbon assimilation with an extensive database (506 measurements on 207 species) to develop and test a relationship for reading the carbon isotopic composition of the paleo-atmosphere from land plant tissue. Applying this relationship to the land-plant organic carbon record as preserved in terrestrial sediments of the Hell Creek and Tullock formations of McCone and Garfield counties, Montana, we are able to reconstruct the isotopic history of the latest Cretaceous and Danian atmospheric CO₂. In stratigraphic sections where the K-T boundary is recognized by the presence of iridium and shocked minerals, we observe a -1.5 0/00 to -2 0/00 carbon isotopic excursion in land plant organic carbon immediately above the K-T boundary. This excursion is correlated with the negative excursion in marine carbonates recognized at the same chronostratigraphic horizon.

We use this excursion to correlate precisely between sections containing shocked-mineral impact beds and those in which this sedimentological feature was not preserved. This technique offers good results in several sections where the K-T boundary could not previously be placed. Our results show that the Z-coal, the sedimentological boundary between the Hell Creek and Tullock formations, was not deposited synchronously across the eastern Montana outcrop belt and thus does not represent an impact-caused climate-facies change. Variation in carbon isotopic values stratigraphically below the K-T boundary suggest that the impact was preceded by an interval of decreasing then increasing global biomass, which may be reflective of global-scale climatic instability in the latest Maastrichtian. Our revised K-T chrono-stratigraphy will allow precise calibration of floral and faunal change associated with the terminal Cretaceous event, thus constraining hypotheses of timing and mechanism.

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