24 settembre 2012
Chi furono i primi a colonizzare la Terra?
Nella foto: una carota vecchia di 2,5 miliardi anni raccoltsa nel Monte McRae Shale in Australia; in origine era la sabbia sottile di un sedimento marino. Il campione presenta alte concentrazioni di solfuro e molibdeno: probabilmente la maggior parte del solfato veniva liberato dall'attività microbica sulle rocce.
La ricerca è stata finanziata dalla National Science Foundation e dal Virtual Planet Laboratory del Dipartimento di Astronomia dell'Università di Washington.
Contributions to late Archaean sulphur cycling by life on land.
Eva E. Stüeken, David C. Catling, Roger Buick
Nature Geoscience, 2012. Published online 23 September 2012.
Evidence in palaeosols suggests that life on land dates back to at least 2.76 Gyr ago. However, the biogeochemical effects of Archaean terrestrial life are thought to have been limited, owing to the lack of a protective ozone shield from ultraviolet radiation for terrestrial organisms before the rise of atmospheric oxygen levels several hundred million years later. Records of chromium delivery from the continents suggest that microbial mineral oxidation began at least 2.48 Gyr ago but do not indicate when the terrestrial biosphere began to dominate important biogeochemical cycles. Here we combine marine sulphur abundance data with a mass balance model of the sulphur cycle to estimate the effects of the Archaean and early Proterozoic terrestrial biosphere on sulphur cycling. We find that terrestrial oxidation of pyrite by microbes using oxygen has contributed a substantial fraction of the total sulphur weathering flux since at least 2.5 Gyr ago, with probable evidence of such activity 2.7–2.8 Gyr ago. The late Archaean onset of terrestrial sulphur cycling is supported by marine molybdenum abundance data and coincides with a shift to more sulphidic ocean conditions. We infer that significant microbial land colonization began by 2.7–2.8 Gyr ago. Our identification of pyrite oxidation at this time provides further support for the appearance of molecular oxygen several hundred million years before the Great Oxidation Event.
Large bacterial population colonized land 2.75 billion years ago (By Vince Stricherz, University of Washington, September 23, 2012)