The great die off 250 million years ago may have been due to a change in the pH level of the oceans.
- As the ocean's become more acidic again today, fears of another impending extinction crisis are escalating.
- Marine creatures that make shells are hardest hit by ocean acidification.
Earth experienced its most dramatic extinction crisis of all time 250 million years ago when about 90 percent of ocean-dwelling species and 70 percent of land-dwellers disappeared. Exactly what caused the massive die-off, however, has long been a matter of debate.
Now, a new study offers new clues.
Ocean acidification caused by rising levels of carbon dioxide in the air, the study suggests, may have played a major role in the ancient extinction event, which was particularly hard on marine creatures that make calcium carbonate shells -- a process that's much harder for them to do in acidic conditions.
The findings have potentially major implications for what we can expect to happen on Earth in the near future. As atmospheric CO2 levels rise in the coming decades and the oceans absorb the gas, which ends up making the water more acidic, we might be headed for another round of serious extinction events.
"While it's hard to actually quantify the magnitude of the role played by ocean acidification compared to many other things going on, our results clearly show that ocean acidification played a role and maybe an important role in this extinction event," said Alvaro Montenegro, a physical oceanographer at St. Francis Xavier University in Nova Scotia.
"It is quite evident that we have pushed and continue to push the oceans toward more acidic conditions, and we already can foresee impacts on certain types of ocean organisms become significant quite soon," he added, with clams, mussels and other shellfish at biggest risk. "Our results are showing that organisms don't necessarily adapt quickly or quick enough to changes in ocean pH."
The history of life on Earth has been punctuated by a number of major extinction events, including the one that knocked off most of the dinosaurs about 65 million years ago. By far the most extreme sequence of events, though, struck just over 250 million years ago.
Now called the Permian-Triassic Boundary (PTB) extinction, it was unique in a few ways. Besides killing so many species both on land and in the water, for example, it was the only known extinction that wiped out a lot of insects, too. Unlike other extinction episodes, the cause of the PTB event is also surrounded by mystery. There is, for instance, not convincing evidence for a major meteor impact like the one that killed the dinosaurs.
Scientists do know that gigantic volcanic eruptions were spitting large amounts of methane and CO2 into the air at the time. Temperatures were very warm. And the geological record shows that parts of the seafloor were very low in oxygen. But it is still not clear what caused what.
To begin to untangle some of the mystery, Montenegro and colleagues created a computer model that simulated conditions before and during the extinction. The model included a historically accurate arrangement of continents. It also, for the first time, included a seafloor that was full of ridges and other sculpted features to create a realistic picture of water circulation patterns.
After setting temperatures and CO2 concentrations to the high levels that are thought to have existed at the time, the researchers ran experiments to see what would happen in the oceans.
Results, published in the journal Paleoceanography, showed that a warm climate couldn't explain documented changes in the oceans during the PTB extinction. Instead, high CO2 levels alone could have accounted for the serious drop in ocean pH.
The model offered little insight into the question of why there was so little oxygen at the ocean's bottom during the extinction, which suggests that there is plenty of work to be done before we can truly understand what happened during the biggest extinction event in Earth's history, said Paul Wignall, a geologist at the University of Leeds in the United Kingdom.
Still, both ocean acidification and lack of oxygen in the oceans are impending crises for modern oceans, Wignall said. The fossil record shows that shelled marine creatures suffered the most during the PTB extinction, as acidic conditions make it harder for these kinds of animals to secrete and maintain material for their shells.
Cold waters exacerbate the problem, which means that the biggest threats today face high-latitude shell-making creatures. By between 2030 and 2050, Wignall said, predictions suggest that the first victims of a drop in ocean pH are likely to be pteropods. These tiny snails live in the surface waters of high-latitude regions and form the base of the food chain for many fish and birds.
Today's oceans are growing more acidic because of carbon dioxide belched into the atmosphere by fossil fuel and other sources. Effects are likely to spread southward into warmer waters, where threatened creatures include mussels, oysters and corals.