The Last Mass Extinction Event Occurred 66 Million Years Ago – Here’s How Ocean Ecosystems Recovered

A light microscope image of a plankton community in the mass extinction recovery phase, dominated by small cells of one species. Superimposed on this, is a high-resolution scanning electron microscope (SEM) image of the same species. These cells are around seven microns in diameter (7/1000ths of a millimeter). Credit: P. Bown

An international team of scientists, led by the University of Bristol, have produced an unprecedented record of the biotic recovery of ocean ecosystems that followed after the last mass extinction, 66 million years ago.

In an article published in the journal Nature, the team, which includes researchers from Southampton, University College London, Frankfurt, and California, present a 13 million-year record of fossil plankton dynamics in the aftermath of near annihilation, providing a remarkable glimpse into how the marine ecosystem ‘reboots’.

The Cretaceous/Paleogene (K/Pg) mass extinction occurred when an asteroid impact caused global environmental devastation and is well known for the extinction of dinosaurs, ammonites, and many other groups.

However, in addition to the loss of these larger animals, there were equally devastating extinctions in the ocean plankton, which eliminated food production at the base of the marine ecosystem and crippled important ocean functions, such as carbon delivery to the deep sea, which is a critical control on atmospheric carbon dioxide.

Lead author, Sarah Alvarez, who carried out the research at Bristol’s School of Geographical Sciences and is now based at the University of Gibraltar, said: “We wanted to find out how long the ocean ecosystems took to recover and how this happened.

“We looked at the best fossil record of ocean plankton we could find – calcareous nannofossils (they are still around today) – and collected 13 million years of information from a sample every 13,000 years.

“We measured abundance, diversity, and cell size from over 700,000 fossils, probably the largest fossil dataset ever produced from one site.”

The team found that these plant-like (photosynthetic) plankton bounced back almost immediately after the mass extinction but that the early communities were highly unstable and cell sizes were unusually small.

Major ecosystem disruption persisted for two million years after the mass extinction, but the gradual appearance of new species and larger cells helped re-establish a stable and resilient ecosystem.

At the same time, carbon delivery to the sea floor returned to pre-extinction levels, marking the restoration of this critical ocean function. This occurred long before species and ecological diversity fully recovered, as key forms filled essential functional roles. It took eight million more years for species numbers to fully recover to previous levels.

The marine ecosystem is dependent on the plankton at its base, just as much today as in the past.

This study, say the authors, highlights the risks posed by diversity loss which may result in highly unstable communities, loss of important ecosystem functions, and long timescales of recovery.

Co-author, Dr. Samantha Gibbs, from the University of Southampton, added: “Losing species today runs the risk of eliminating key players in the ecosystems.

“What we’ve demonstrated from the fossil record is that function is achieved if you have the right players fulfilling key roles.

“Today, by reducing biodiversity, we are running the risk of losing our critical ecosystem players, many of whose importance we don’t yet fully appreciate.”

Reference: ‘Diversity decoupled from ecosystem function and resilience during mass extinction recovery’ by Sarah A. Alvarez, Samantha J. Gibbs, Paul R. Bown, Hojung Kim, Rosie M. Sheward and Andy Ridgwell, 25 September 2019, Nature.
DOI: 10.1038/s41586-019-1590-8