Denver, Colo. -- Iron-rich rock formations dating to 2.3 billion years ago suggest that the Earth's land masses were covered with living things at least a billion years earlier than previously thought, according to a Penn State geologist.
"Until now, the earliest accepted date for land-based life was 1.2 billion years ago, but now we can push that back at least another billion years," says Dr. Hiroshi Ohmoto, professor of geosciences and director of the Penn State Astrobiology Research Center. "Of course, terrestrial life back then was more in the nature of bacterial mats than oak trees and mammals."
Ohmoto, in collaboration with Nick Beukes of Rand Afrikaans University, Johannesburg, South Africa, investigated laterites, iron-rich deposits that form when organic acids -- those acids created when living things decay -- leach iron from upper layers of rock and then deposit them as oxides in layers below. The normal make-up of a laterite is three bands -- an iron-deficient layer covered by an iron-rich layer that is covered by an iron-deficient layer. Modern laterites form in the tropics where large amounts of organic material rapidly decay.
"In order for laterites to form, there must be organic material and atmospheric oxygen," Ohmoto told attendees at the annual meeting of the Geological Society of America today (Oct.26) in Denver. "Since we have now traced these laterites to 2.3 billion years ago, there must have been atmospheric oxygen and terrestrial life at that time." Ohmoto originally looked at formations in Waterval Onder, South Africa, an area near Pretoria. However, in this area, the iron-rich layer and upper iron-poor layer have eroded away. To get the big picture, the researchers looked at core samples drilled by miners who typically drill through these layers looking for gold and uranium ores buried much farther down.
"We looked at two cores in South Africa and one in Botswana that showed the complete series of rocks," says Ohmoto. "It appears that this laterite formation covers a rather large area."
There are several places further west where the formation is actually exposed, but these locations had previously been dated as much younger rock. The researchers have now identified these laterite formations as part of the same, much older formation, because it sits directly on the Hekpoort basalt, an extensive basalt formation that was extruded 2.3 to 2.4 billion years ago.
"Because we can trace the basalt all the way across, even to a depth of 2,600 feet, we know that the laterite deposits directly above are only slightly younger than the basalts," says Ohmoto. v Geologists currently are involved in a debate as to when significant amounts of oxygen appeared in the Earth's atmosphere. These laterites suggest that oxygen was plentiful 2.3 billion years ago, both for the generation of land-based biota and to convert iron to iron oxides.
EDITORS: Dr. Ohmoto is at (814) 863-8761 or ohmoto@geosc.psu.edu by email.