Last month myself and collaborators presented new work at the Fall Meeting of the American Geophysical Union (AGU). The meeting was held in New Orleans, but many presenters, myself included, participated virtually in what turned out to be an extremely well organized virtual system (in my opinion). Our new work is an early result from a much larger project in which we are reconstructing, in as fine systematic, paleontological and ecological detail as possible, the paleocommunities that were present “immediately” prior to and in the aftermath of the end Cretaceous asteroid impact, specifically in the region covered by the Hell Creek Formation in the United States. Our early effort was to subject our reconstruction of the uppermost Hell Creek community, at the end of the Cretaceous, to the darkness that is believed to have followed the impact. Geophysical models of the past 10 years or so have all pointed to the likelihood that soot suspended in the upper atmosphere would have reduced light levels at the surface below that required for photosynthesis, and up to a period of perhaps two years. This shutdown of photosynthesis and the energy input to the ecosystem would have sent a shockwave of effects cascading through the system. In the link below you will be able to access the presentation and therefore all the details of how we went about our investigation, but I also list our summary points.
- Recent geophysical modeling has proposed that the bolide impact at the end of the Cretaceous Period 66 mya would have caused global atmospheric soot-driven darkness. Solar radiation may have been reduced below levels required for photosynthesis for up to 2 years.
- Here we reconstructed an ecological model of the upper Hell Creek Fm. representing a community that would have been present at the time of impact. Simulations subjected the model community to intervals of darkness up to 700 days, during which phtosynthesis and hence primary production were prohibited.
- The community exhibited a nonlinear response to increasing levels of perturbation (intervals of darkness). At a level of 100-150 days, the community was resilient to perturbation, returning to the initial state after the cessation of darkness. At 200 days and greater, however, a tipping point is reached and the community underwent regime shifts, converging to alternative states in which some species went extinct and patterns of dominance shifted. As perturbations approach 700 days duration, extinction levels increased dramatically and the community, up to at least 40 years after bolide impact, would have resembled conditions during the intervals of darkness.
- The model predicts a linear response to extinction with increasing durations of darkness. Primary producer extinction is greater than consumer, but recovery would have been impossible for most consumers, whereas it is generally accepted that many primary producer species could have recovered because of resistant characteristics (e.g. root systems) and seed banks.
- Important validation of the model is given by the close correspondence between the geophysical and ecological models, and the actual fossil record. The range of extinction predicted by the model when darkness lasted 650 or 700 days is 65-81%, which is consistent with our estimated level of extinction, i.e., 73.3%, as measured from the Hell Creek and overlying Tullock fossil assemblages.