DIRECT EVIDENCE OF CANOPY DENSITY LOSS ASSOCIATED TO THE CRETACEOUS-PALEOGENE BOUNDARY CHICXULUB IMPACT IN SOUTHWESTERN NORTH DAKOTA

The Cretaceous–Paleogene (K–Pg) mass extinction event is associated to profound changes in terrestrial plant ecosystems. These changes are especially well documented in North America, specifically from the Hell Creek and Fort Union formations in the Marmarth area (North Dakota) where a drastic reduction in plant species diversity is recorded in leaf fossil assemblages, directly coinciding with the mineralogical (boundary clay, shocked quartz) and geochemical (iridium anomaly) evidences of the Chicxulub asteroid impact. This reduction in diversity is also mimicked in palynological assemblages by the loss of several pollen taxa and the high abundance of fern spores (the fern spike event) immediately following the K–Pg boundary.

In addition to pollen and spores, dispersed leaf cuticles are an abundant component of palynological preparations. A taxonomically independent relationship has been evidenced between the size and shape of leaves epidermal cells and vegetal cover density, allowing for the use of reconstructed Leaf Area Index (rLAI) from cuticles as a new proxy to track changes in plant cover architecture across the K–Pg boundary. We have conducted a preliminary test on the John’s Nose section, north of Marmarth, which has been sampled at a ~1 cm resolution for palynological analysis across the boundary clay. Results from the cuticle analysis shows a significant decrease in rLAI directly coincident with the markers of the Chicxulub impact and the palynologically defined K–Pg boundary. These results provide the first direct evidence of deforestation as a result of the impact blast and/or associated wildfires. The temporary loss of plant cover represent a key element in the discussion of extinction selectivity and recovery dynamics following the K–Pg mass extinction event. Ultimately, the use of rLAI on dispersed cuticles could serve as a new independent marker to identify the K–Pg boundary at localities where the direct evidences of the Chicxulub impact are not preserved.