Deadly Ocean Burp
by Jack Penland

Could a dangerous gas buildup at the bottom of the ocean bubble up and wipe out most life on Earth? According to some researchers, it already has done so; several times. As this ScienCentral story explains, scientists are worried that global warming is making conditions ripe for another deadly ocean burp.

Attack from Below

Scientists mostly agree that it was an object from space that crashed into Earth 65 million years ago, killing off much of the planet's life, including most of the dinosaurs. But geologists have found evidence of four other major mass extinctions and more than a dozen other smaller similar extinctions over the past 500 million years. In addition they've not found any corresponding evidence of a collision from an object from space.

Peter Ward, Biology and Earth and Space Sciences Professor at the University of Washington says, "Try as people would, they could not find evidence for impact other than at, really, the age of dinosaurs." Writing in Scientific American, Ward says instead that, "A new type of evidence reveals that the earth itself can, and probably did, exterminate its own inhabitants."

The killer is a type of bacteria that needs lots of sunlight but very little oxygen to thrive. It gives off the gas hydrogen sulfide, which even in small concentrations is lethal. People generally call the gas "sewer gas."


This photo from space shows a small temporary bacteria bloom off the coast of Africa.
image: NASA
Because there was no evidence linking space objects to the remaining mass extinctions scientists began looking back to earth. Ward says, "while geologists discovered, better and better dating methods," other scientists discovered, "a fantastic new method called biomarker methodology."

Ward explains, biomarkers are, "small bits of fossil material. But, it's not a shell; it's organic remains, microscopic ... It could be as small as an organic chain of carbon that came out of the side of a bacterium, but it is particular to that particular bacteria species."

Scientists then began searching the layers of rocks that delineate mass extinctions. Ward says, "at four of the five mass extinction boundaries, the biggest ones, the microbes are microbes we find associated with lots of very toxic sulphurs." He says the evidence is quite strong, emphasizing, "We've got a mass extinction, we've got a lot of dead bodies and we have this fossil evidence, molecular fossil evidence that a very bad bug was there."

But what caused these bacteria to grow? Ward says long ago volcanic activity spewed carbon dioxide into the air and warmed the oceans. He says, "You simply need to warm the oceans, they lose their oxygen and bad bacteria take over." He adds, "There are hydrogen sulfide producing bacteria growing at several places on our Earth's oceans' bottoms today." In fact, NASA has caught from space, evidence of such growth off the coast of Africa.


image: The NewsMarket
Today, instead of volcanoes, it's human activity that is creating greenhouse gasses like carbon dioxide, causing the earth to warm. Ward worries, "The last time we had one of these extinctions, the amount of carbon dioxide in the atmosphere was a thousand parts per million. Today, we're at 385 parts per million and rising." He says some estimates put us reaching a thousand parts per million in 200 more years.

Ward says scientists now need to better understand this chain reaction, explaining, "We absolutely have to know how long it takes between a lot of carbon dioxide in the atmosphere and enough hydrogen sulfide to kill things." He says while industrial accidents have sadly provided us a good understanding of how lethal hydrogen sulfide is on humans, we know little about how it impacts other living things, such as plants. Since that's an important part of the food chain, that will be his next research goal in this area.

This research was published in the October 2006 issue of Scientific American and funded by the NASA Astrobiology Institute, the National Science Foundation, and the National Research Foundation of South Africa.