Reno, Nev. - If the initial rise in the Earth's atmospheric oxygen occurred between 2400 and 1800 million years ago, as most researchers agree, but oxygen producing bacteria existed more than 300 million years before that, Penn State geologists wonder what caused the delay?
"Oddly enough, the rise of oxygen seems to be linked to what may have been Earth's first glaciation," says Dr. Lee R. Kump, professor of geosciences. "After the glaciation that occurred 2.4 billion years ago, the amount of oxygen in the Earth's atmospheric may have been about the same as it is today. Prior to that glaciation, the amount of oxygen was essentially zero, far below the amount necessary to support oxygen breathing life."
Kump and James F. Kasting, professor of geosciences and meteorology, together with their Australian Colleague Mark Barley, have developed a conceptual model that suggests vulcanism caused a rapid change in oxygen content and the glaciation, but this was a different type of vulcanism than had occurred up until then.
"Previous to 2.4 billion years ago, volcanoes spewed hydrogen, carbon monoxide and methane into the atmosphere because their magma source from the near upper mantle, was very reduced," Kump told attendees today (Nov. 15) at the annual meeting of the Geological Society of America in Reno, Nev.
Cyanobacteria produce oxygen from photosynthesis, but none of that oxygen remained in the atmosphere because the hydrogen, carbon monoxide and methane rapidly reduced it. These reducing gases produced a strong greenhouse effect keeping the Earth warm.
The action of water, which contains oxygen, on the iron in basalts emerging from mid-ocean ridges, set up the potential for a more oxygenated atmosphere. The iron in basalt rusted in contact with the water. The hydrogen produced escaped to the atmosphere but the rust - iron oxide -- deposited on the ocean floors. This oxygen rich layer eventually was subducted and accumulated at the core-mantle boundary, far from the area generating volcanic magmas.
"The likelihood that these deep mantles would rise as plumes of oxygenated magmas increased as more and more iron oxide rich magma was buried," says Kump. "What we do not know is why these deep plume volcanos appeared on three or four continents at the same time."
The rising plumes began to spew carbon dioxide and water, rather than methane and hydrogen, and this allowed the oxygen levels to rise.
"The weaker greenhouse caused by lower methane and carbon monoxide levels allowed glaciation to occur," says Kump.
The researchers suspect that glaciation came on rapidly and existed for only a short time. Once carbon dioxide built up in the atmosphere, its greenhouse warming potential would melt the glaciers.
"Geological observation shows that the same sequence of events occurs around the world at this time," says Kump. "There is evidence of reduced iron deposits, then glacial deposits and then oxidized sandstones indicating an oxygen rich atmosphere in Africa, Canada and Australia."
Kump, Kasting and Barley believe that their conceptual model of rapid oxygenation of the atmosphere by deep magma plume volcanos is self consistent and ties together a series of occurrences on different continents.
EDITORS: Dr. Kump may be reached at 814-863-1274 or lkump@psu.edu.
Dr. Kasting may be reached at 814-865-3207 or kasting@essc.psu.edu.