Mass extinctions and their causes are topics of intense interest. Nearly forty years after the 1980 paper on the end-Cretaceous impact, significant new research on it and on the resulting extinction continue to appear. Other mass extinctions, such as the ones at the end of the Ordovician and Permian, likewise are fertile areas for investigation. But what has become of equal interest to paleontologists is the aftermath of these cataclysmic episodes: how does life recover after extinctions and how does the rebuilding biosphere differ from what went before? Four very different recent papers show the paths that life took after the Cretaceous-Paleocene (K-Pg) extinction event. Together, they are a warning to us.
A consortium of thirty-eight scientists (Lowery et al. 2018. Nature) examined the recovery at “Ground Zero,” within the Chicxulub crater itself. They drilled some 130 m into the peak ring of the crater, formed at the time of impact, and examined the change in the rock types and fossils over time. In particular, they looked at small single-celled species that live in the water column (plankton), including foraminifera and the tiny photosynthetic nanoplankton. Directly above fractured granites and melted rock, they encountered suevite, a distinctive mix of angular fragments (breccia) and melted rock. Above the suevite is a dark fine-grained limestone that is remarkable for two reasons: first, the fossils are almost all reworked remains of species that went extinct at the time of the impact; and second, the sediments are thinly layered (laminated), with none of the burrows that indicate that the sea floor was inhabited. Small and isolated burrows appear at the very top of the dark limestone, showing that life has returned, perhaps in just years after the impact. The dark limestone gives way to a mottled white limestone; the mottling shows that there is abundant life in the seafloor. The appearance of the white limestone also marks the appearance of an abundant and diverse group of foraminifera, including both planktic and benthic (bottom dwelling) forms. The researchers dubbed the dark limestone the “transitional unit,” and estimated that it represents the ca. 30,000 years it took for a high-productivity ecosystem to be reestablished in the area. This is considerably faster than other sites in the Gulf of Mexico, which may have taken as long 300,000 years to recover.
Another study suggests that ecosystem recovery may have been ephemeral. Again looking at nanoplankton, Sarah Alvarez and her co-authors (2019, Nature) looked at the history of biological communities over 13-million years post-impact, using data from a core far away from the impact site, in the Pacific Ocean. They found that the composition of the communities was very volatile for the first 1.8 million years following the mass extinction and then became much more stable. Alvarez and her colleagues suggest the onset of stability reflects the restoration of the “biological pump” that moves organic carbon from the shallow ocean to the deep sea.
Productivity is only one measure of recovery. Another is biodiversity, the number and kinds of species present. According to Alvarez and her colleagues, nanoplankton diversity appears to have taken about 8–10 million years to recover to pre-extinction levels, considerably longer than the renewal of ecosystems. Similarly, Lowery and Frass (2019 Nature Ecology & Evolution; also see the discussion by Weil and Kirchner) examined the post-impact record of planktonic foraminifera, which suffered greatly in the extinction, declining from seventy to three species. The surviving species were not complex; they lacked spines and did not possess symbiotic algae, common features before the extinction. Remarkably, although diversity took about 20 million years to recover, complexity returned much more rapidly, reaching pre-extinction levels in about 5 million years. Still long, but much quicker than diversity.
How about on land? A remarkable section in Colorado records the first one million years of the Paleocene (Lyson et al 2019, Science). The sequence contains remarkably complete remains of plants, reptiles (crocodiles and turtles), and mammals. They found that the first 100,000 years post-extinction was a time of ecosystem instability, with full recovery taking about 300,000 years. Marked increases in both mammal size and diversity also occurred in this interval. Interestingly, a major increase in body size coincides with the first appearance of legumes (beans, peas, lentils).
What is the lesson of all this? Humans certainly played a key, perhaps even the major role, in the extinction of large mammals at the end of the last ice age. We are certainly responsible for ongoing loss of biodiversity called the “Sixth Extinction”. The aftermath of the K-Pg extinction teaches us that recovery of ecosystems takes tens to hundreds of thousand years and can be highly volatile. Diversity may take tens-of-millions years to reach pre-extinction levels. If left alone, life will locate a path to recovery. However, at best, this process takes far longer than the span of modern human existence. Best to stop the loss now!