High-resolution organic carbon isotope record across the Cretaceous–Tertiary boundary in south-central Alberta: implications for the post-impact recovery rate of terrestrial ecosystems and use of δ13C as a boundary marker

Abstract

A high-resolution study identified a δ13C excursion of –1.8‰ to –2.3‰ in terrestrial organic matter across the Cretaceous–Tertiary (K–T) boundary at two localities in the Scollard Formation of south-central Alberta, one of the northernmost occurrences of the K–T boundary in terrestrial settings. δ13C values are at their lowest within 6 cm above the K–T boundary claystone and return to pre-boundary levels within 10 cm above the boundary claystone. Statistical analyses reveal that the K–T isotopic shift in Alberta is related to the nature of floral changes that occurred across the K–T boundary. A radiometrically dated bentonite resting on the boundary-hosting Nevis coal at one of the localities permits us to estimate that the terrestrial carbon cycle recovered ~100 000 years after the K–T boundary event, a value that supports an existing hypothesis that terrestrial ecosystems recovered more rapidly than marine ecosystems. The organic carbon isotope record of the entire Scollard Formation demonstrates that the δ13C excursion across the K–T boundary did not reach anomalously low values by late Maastrichtian standards in Alberta. Furthermore, the occurrence of the K–T carbon isotope shift within a restricted stratigraphic interval (<10 cm) greatly limits the probability of its preservation in the context of terrestrial sedimentary environments. These observations suggest that, on their own, δ13C profiles may be unreliable for locating the K–T boundary (and possibly other geologically instantaneous events) and that they should be used in combination with other approaches (e.g., palynology) to identify the K–T boundary in sections lacking the boundary claystone and iridium anomaly.