Geochemical evidence for terrestrial ecosystems 2.6 billion years ago

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Abstract

Microorganisms have flourished in the oceans since at least 3.8 billion years (3.8 Gyr) ago1,2, but it is not at present clear when they first colonized the land. Organic matter in some Au/U-rich conglomerates and ancient soils of 2.3–2.7 Gyr age has been suggested as remnants of terrestrial organisms3,4,5. Some 2.7-Gyr-old stromatolites have also been suggested as structures created by terrestrial organisms6,7. However, it has been disputed whether this organic matter is indigenous or exogenic, and whether these stromatolites formed in marine or fresh water. Consequently, the oldest undisputed remnants of terrestrial organisms are currently the 1.2-Gyr-old microfossils from Arizona, USA8. Unusually carbonaceous ancient soils—palaeosols—have been found in the Mpumalanga Province (Eastern Transvaal) of South Africa9. Here we report the occurrences, elemental ratios (C, H, N, P) and isotopic compositions of this organic matter and its host rocks. These data show that the organic matter very probably represents remnants of microbial mats that developed on the soil surface between 2.6 and 2.7 Gyr ago. This places the development of terrestrial biomass more than 1.4 billion years earlier than previously reported.

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Figure 1: A simplified map of Mpumalanga Province, South Africa.
Figure 2: Mineralogical and chemical characteristics of the 17-m palaeosol section at Schagen (see Methods for details).
Figure 3: Concentration relationships between organic carbon and phosphorus, and organic carbon and nitrogen.
Figure 4: Association of organic matter with clays in the palaeosol.

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Acknowledgements

We thank T. Nagase for TEM analyses, L. R. Kump, J. F. Kasting, H. Naraoka, K. Yamaguchi, S. Ono, Y. Huang, K. M. Towe and J. A. Lamberski for comments on an earlier manuscript, and R. Capo, B. Stewart, D. J. Des Marais, R. Buick and J. M. Hayes for discussions. This work was supported by grants to H.O. from the Japanese Ministry of Science and Education, NSF and NASA.

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Correspondence to Yumiko Watanabe.

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