How ancient ocean life in the Gulf of Mexico escaped mass extinction

A bout of global warming 56 million years ago acidified oceans and wiped out marine life. But in the Gulf of Mexico, the effect was much milder. Research on the unique geology of this North American basin has revealed how life was able to escape local extinction.

The Palaeocene-Eocene Thermal Maximum (PETM) was a brief period of global warming, which saw a 5°C to 8°C spike in temperature and caused large changes in ocean chemistry. This caused a severe deep-sea benthic foraminifera extinction, known as the Benthic Extinction Event (BEE).

“This event, known as the Paleocene-Eocene Thermal Maximum, or PETM, is very important to understand because it’s pointing towards a very powerful, albeit brief, injection of carbon into the atmosphere that’s akin to what’s happening now,” says lead author Dr Bob Cunningham, from the University of Texas, US. 

New research, published in Marine and Petroleum Geology, explores the biological responses during this geological time period, including the mud, sand and limestone deposits found across the Gulf of Mexico. Sifting through rock chips brought up during oil and gas drilling across 25 sites, the researchers found an abundance of fossilised radiolarians – a type of plankton – that appeared to survive, and even thrive, during a time when other ocean organisms were being wiped out.

“In a lot of places, the ocean was absolutely uninhabitable for anything,” says co-author Marcie Purkey Phillips, from the Institute for Geophysics at the University of Texas. “But we just don’t seem to see as severe an effect in the Gulf of Mexico as has been seen elsewhere.”

The team also found calcareous nannofossils of algae, and foraminifera (single-cell organisms) – together with the radiolarians, the depositional environment, water temperature, salinity, river input and turbidity could be interpreted. The researchers concluded that a steady supply of river sediments and circulating ocean waters helped radiolarians and other microorganisms survive this hostile time.

“The Gulf of Mexico is a tremendous natural archive of geologic history that’s also very closely surveyed,” says co-author Dr John Snedden from the Bureau of Economic Geology at the University of Texas. “We’ve used this very robust database to examine one of the highest thermal events in the geologic record, and I think it’s given us a very nuanced view of a very important time in Earth’s history.”