University Park, Pa. -- Remnants of organic matter in ancient soil more than 2.6 billion years old may be the earliest known evidence for terrestrial life, according to a team of Penn State astrobiologists.
"Our work shows that the organic matter in this soil very probably represents remnants of microbial mats that developed on the soil surface between 2.6 and 2.7 billion years ago," says Dr. Hiroshi Ohmoto, professor of geochemistry and director of The Penn State Astrobiology Center. "This places the development of terrestrial biomass more than 1.4 billion years earlier than previously reported." Evidence that microorganisms flourished in the oceans since at least 3.8 billion years ago exists, but when these microorganisms colonized on land is not clear. The oldest undisputed remnants of terrestrial biomass have been 1.2 billion-year-old microfossils found in Arizona.
Examining samples taken from Mpumalanga Province, South Africa, using a variety of geochemical methods, the researchers report in this week's issue of Nature, that a paleosol dating to between 2.6 and 2.7 billion years ago contains organic carbon that was neither created by high temperature fluids nor is the remnant of later petroleum migration, but is in-situ biological in origin.
A paleosol is a layer of ancient soil, in this case buried and preserved where it formed. Because the 55-foot thick layer of soil found at Schagen is located between a layer of 2.7 billion-year-old serpentine and a 2.6 billion-year-old quartzite bed, the researchers can date the soil to between 2.6 and 2.7 billion years ago. Showing that the carbon in the soil is biological in origin and that it accumulated during soil formation is much more difficult.
The researchers, who include Ohmoto; Yumiko Watanabe, Ph.D. candidate at Penn State and at Tohoku University, Sendai, Japan; and Jacques E.J. Martini, Geological Survey of South Africa, evaluated three possibilities for the formation of
Contact: A'ndrea Elyse Messer