Earth's Early Atmosphere and Biosphere

J.F. Lindsay and M.D. Brasier

How ancient is oxygenic photosynthesis? This is a question with considerable relevance to the origins of life and the evolution of the atmosphere. If earth's atmosphere had not become significantly enriched in oxygen, the biosphere would have evolved in an entirely different direction.

 

The evolution of earth's early biosphere and atmosphere may be examined through analysis of the stable isotopes of carbon. Over the past several years, in a cooperative program with the Earth Sciences Department at Oxford University, we have assembled a large set of carbonate carbon isotopic data from dolostones preserved in the Hamersley and related basins, which lie along or either side of the Capricorn Orogen in Western Australia. Fom this work, we have a carbon isotope curve for carbonates from 2.6 to 1.9 Ga, a period in earth history when the earth's atmosphere was oxygenating rapidly. The curve is complex but suggests that active plate tectonics around 2.2 Ga played a major role in driving the isotope balance of both the atmosphere and biosphere.

 

Prior to 2.6 Ga relatively little is known of oxygenic photosynthesis. Given the paucity of platform carbonates prior to this time and the fact that nonphotosynthetic means of isotopic fractionation are possible, an isotopic approach by itself seems unlikely to be fruitful. The best opportunity appears to come from the identification of microfossil assemblages. Relatively few Archaean microfossil assemblages have been described to date, of which those from c. 3.46 Ga chert in the Apex Basalt, Warrawoona Group in Western Australia hold a key position because of their supposedly excellent state of preservation and general acceptance by the scientific community. Recently, we have re-examined both the geological setting and biogenicity of these very early microfossil-like structures, which have been taken to support an early beginning for oxygen-producing photosynthesis. Recent field mapping and sampling reveals that the 'fossiliferous' cherts occur in a hydrothermal complex.

 

Micro-Raman and thin section petrography of the 'microfossil' filaments suggest that their septate appearance is largely created by microcrystalline quartz grains (c. 1 µm) interspersed with darker amorphous graphite that makes up the bulk of the filament and suggests that these structures may be pseudofossils. However, high-resolution stable isotope analyses from freshly cut rock slices reveal delta13Cpdb values of -30 to -26permil for reduced carbon, comparable with results from earlier studies and consistent with biogenic fractionation. The evidence is consistent with a hydrothermal setting for this deposit, but raises questions about the presence of cyanobacteria. The isotope values imply a significant biological contribution to the carbon cycle, perhaps from hyperthermophile methanogenic bacteria, which have been claimed on the basis of bacterial rRNA sequencing to be an early branch of the tree of life, and are arguably of much greater antiquity than cyanobacteria. If so, then oxygenic photosynthesis was a relative late development in the evolving biosphere and the earliest life perhaps evolved in high temperature hydrothermal settings independent of sunlight. Thus, life may have evolved independently many times throughout the universe, on any planetary body where surface conditions were suitable for the development of a sustained hydrothermal system, independent of the distance of the planet from its star, the magnitude of the star or even whether the planet is in orbit around a star. As long as the planet has sufficient endogenic energy resources at least primitive life could be sustained.