HARVARD GAZETTE ARCHIVES

Mass Extinction is Explained

By William J. Cromie

Gazette Staff

The worst killing period in the history of Earth occurred 250 million years ago. About 90 percent of all animal and plant species in the oceans went extinct. Most of the advanced life forms at the time -- reptiles and amphibians, along with many varieties of insects and vegetation -- disappeared forever from the land. The carnage was far worse than the catastrophe that eliminated dinosaurs 65 million years ago.

A new study, led by a Harvard biologist, lays the blame for this mother of all extinctions on a combination of overturning oceans, release of deadly gases, and global climate change.

"The deep ocean can sow the seeds of biological disaster," says Andrew Knoll, professor of biology. "Rapid overturn of carbon dioxide-charged, oxygen-depleted waters . . . cause[d] widespread physiological shock, climatic shock, or both, resulting in mass mortality and extinction."

Knoll and his colleagues pieced the story together from fossils and records of chemical changes preserved in 250-million-year-old layers of rocks. Their reconstruction involves a time when Earth's surface was a much different place. All the land clustered together to form a single supercontinent called Pangaea. North America was attached to Eurasia and South America in the northern hemisphere. Africa, India, Australia and Antarctica joined South America to form the southern half of Pangaea. A single ocean, as wide as the present-day Atlantic and Pacific combined, covered the rest of the planet.

Sluggish vertical circulation depleted oxygen in this superocean and in shallow basins and bays on the big continent. Land and ocean plants drew carbon dioxide from the air, died, and the gas accumulated on the ocean bottom, together with the corpses of dead sea animals. The decay of their remains in the anoxic conditions led to buildup of hydrogen sulfide, a highly toxic gas with the pungent odor of rotten eggs.

Steady withdrawal of carbon dioxide from the air, which keeps heat from being lost into space, triggered colder weather. Glaciers and ice caps formed and grew. Surface waters became colder and heavier.

Eventually, these waters became dense enough to sink, turning over the ocean and bringing up a deadly combination of carbon dioxide and hydrogen sulfide. Excessive carbonation in the blood of animals impairs their breathing and causes a loss of consciousness and death. As a dramatic modern example, an eruption of carbon dioxide from Africa's volcanic Lake Nyos in 1986 killed 1,746 people and thousands of animals in villages near the lake.

No humans or dinosaurs lived 250 million years ago, but the suffocating ocean and air took a terrible toll on sea creatures, especially corals, sea lilies, clamlike brachiopods, and other animals attached to the sea floor. The more active ancestors of modern fish, squids, snails, and crabs fared better, but an estimated 90 percent of the world's sea species succumbed.

The amount of carbon dioxide in the air could have increased five or six times, according to the researchers. That's probably not enough to directly kill off most land animals. But that much of the greenhouse gas probably led to global warming. The rise in temperature and destruction of their habitats could be responsible for the dying off of most varieties of reptiles and amphibians. These "cold-blooded" animals cannot adjust their metabolism to the same range of temperatures as warm-blooded birds and mammals. Such a lack of adjustment can be deadly.

Even the resilient insects lost many species. By one estimate, 72 percent of all animal genera (groupings of related species) became extinct compared to 47 percent of genera wiped out by the natural holocaust that eliminated dinosaurs.

Severe warming would also kill plants, particularly those adapted to colder, higher latitudes. And it would melt glaciers and ice caps, causing more annihilation through flooding of coastal areas.

The scientists think the carbonation occurred in two gigantic burps over a period of 2 million to 3 million years. Eventually, the world settled down and temperatures stopped rising. The oceans stabilized. Then the cycle of gas buildup in the depths -- cooling, overturning, death, and heating -- started over again.

The extinctions changed the fundamental character of life on Earth. Sluggish animals who lived sedentary lives on the sea floors lost their dominance to mobile predators with higher metabolisms. In the ocean, ancient relatives of fish, squid, and crabs expanded in number and variety. On land, the predecessors of mammals came into their own. These include cynodonts, aggressive four-legged animals, some of which had heads and teeth like dinosaurs and bodies like wolves.

When a comet or asteroid, or some another calamity, killed off the domineering dinosaurs some 185 million years later, the way was cleared for an explosive expansion of mammals, including rats and cats, deer and dogs, apes, and humans.

Participating in the research with Knoll were Richard Bambach, Virginia Polytechnic Institute and State University; Donald Canfield, Max Planck Institute of Marine Microbiology in Germany; and John Grotzinger of M.I.T.