292-2 Global Terrestrial Primary Productivity Collapse at the K–Pg Boundary Supported by New Evidence for a Fungal Spore Spike from West Bijou (Colorado, USA)

Abstract:

The Cretaceous–Paleogene (K–Pg) mass extinction (66.0 million years ago) is marked by a catastrophic disruption to Earth’s biosphere, coincident with the Chicxulub asteroid impact. Geophysical modelling has suggested that impact-generated dust and combustion-derived soot severely reduced incoming solar radiation, suppressing photosynthesis on a planetary scale. In addition to the immediate mass devastation to terrestrial plant ecosystems caused by the blast of the impact, wildfire ignited by the release of thermal energy, and acid rains from sulfate aerosols, the post K–Pg “impact winter” is hypothesized to have led to a collapse of primary productivity lasting several years. This hypothesis is supported by a unique occurrence of a fungal spike at the Moody Creek Mine K–Pg section in New Zealand.

We report the second discovery of a fungal spike at the Bowring Pit K–Pg section in West Bijou Creek, east of Denver, USA. From relative abundance counts of pollen and spores in 40 samples spanning ±30 cm across the K–Pg boundary, a spike of 31.9% fungal spores is observed within the topmost 3 cm of the boundary clay layer. The West Bijou spike is remarkable as it shows, for the first time, a germinating succession unfolding in two distinctive phases. This particular succession represents a completed asexual reproductive cycle of hyphal growth to mass sporulation, which has a duration of days to weeks in modern Ascomycetes. The fungal spike is immediately followed by a 20–40% Cyathidites spp. fern spore spike, indicating the recovery of devastated landscapes by pioneer plant recolonizers as photosynthesis resumed after post-impact darkness. This new evidence of a fungal spike from Colorado underscores devastation of forests at the K–Pg boundary which provided a dramatic increase in available substrates for saprophytic organisms and that this response occurred at a global scale. The collapse of primary productivity during the years of post-impact darkness would have greatly affected higher trophic levels relying on disappearing resources, while food webs based on detritus and decomposing communities would have been more resilient to the K–Pg mass extinction event.

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