Ancient South African Soils Point To Early Terrestrial Life

"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 reduced carbon in the soil.

The first of these was that the carbon was graphite crystals created when the underlying serpentine formed under high temperatures. The graphite then was concentrated during the soil formation. "The crystallinity and hydrogen/carbon rations of the organic matter suggest it is not of igneous or hydrothermal origin," says Ohmoto, a faculty member in Penn State's College of Earth and Mineral Sciences.

The second possible origin of reduced carbon is liquid hydrocarbons introduced after the soil formation ended. Materials introduced after formation should show up along fractures in the rocks. "The organic matter is almost always concentrated in clay-rich parts of the rocks paralleling the ancient surface," says Ohmoto. "Organic matter and clays are so intimately mixed together that the size and morphology of individual ‘grains' of organic mater can only be recognized under electron microscopes."

The Penn State researchers conclude that the reduced carbon was not produced by high heat and then incorporated into the soil as it formed nor was it deposited after the soil formed by migrating petroleum. The third possibility then is that the organic carbon represents remnants of biomats developed on the soil surface. The researchers found that the organic-rich clays in the upper portion of the paleosol appeared as seams between fine-grained and coarse-grained layers of quartz. "These features suggest that the organic matter in the uppermost soil zone is an indigenous remnant of microbial mats that developed on the surface of clay-rich soil during the rainy season," says Ohmoto. "The mats were blanketed by aerosol deposits laid down during the dry season."

In the lower portion of the paleosol, things are less clear because the effects of seeping water and the dissolution and precipitation of materials suggest some decomposition. While identifying the organism in the microbial mats is difficult, the researchers are certain that they were not photosynthetic sulfur bacteria as there is no sulfur present. Photosynthetic blue-green algae, however, are a likely possibility for the mat formation because the ancient remnants have nearly identical carbon isotope ratios as modern blue-green algal mats in fresh water.

The researchers are also certain that the mats formed on land, not in the oceans, because the carbon isotope values for the carbon in the paleosol are distinctly different from the organic carbon found in marine sedimentary rock.

"Although terrestrial bacterial communities were predicted by previous researchers, this is, to our knowledge, the first study presenting several lines of evidence for an extensive development of microbial mats on soil surfaces in the Archaean," says Ohmoto. "Our finding may then imply that an ozone shield developed before 2.6 billion years ago.

"The ozone shield would have protected land-based biological forms from the effects of cosmic radiation. Development of the ozone shield requires an oxygen-rich atmosphere. Our finding of ancient biomats on land is an important addition to a growing line of evidence suggesting that the rise of atmopsheric oxygen took place more than 2.6 billion years ago."

Penn State is a member of and receives research funding for this and other efforts, through the NASA Astrobiology Institute, a research consortium involving academic, non-profit and NASA centers. NASA's Ames Research Center is the agency's lead center for astrobiology, the study of the origin, evolution, dissemination and future of life in the universe.