The origin of carbonaceous matter in pre-3.0Ga greenstone terrains: A review and new evidence from the 3.42Ga Buck Reef Chert
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The geological record of carbonaceous matter from at least 3.5Ga to the end of the Precambrian is fundamentally continuous in terms of carbonaceous matter structure, composition, environments of deposition/preservation, and abundance in host rocks. No abiotic processes are currently known to be capable of producing continuity in all four of these properties. Although this broad view of the geological record does not prove that life had arisen by 3.5Ga, the end of the early Archean, it suggests a working hypothesis: most if not all carbonaceous matter present in rocks older than 3.0Ga was produced by living organisms. This hypothesis must be tested by studies of specific early geological units designed to explore the form, distribution, and origin of enclosed carbonaceous matter. The carbonaceous, environmentally diverse 3416Ma Buck Reef Chert (BRC) of the Barberton greenstone belt, South Africa, provides an opportunity for such a study. Upward facies progressions in the BRC reflect deposition in environments ranging from shallow marine evaporitic brine ponds to a storm- and wave-active shelf to a deep, low-energy basinal setting below storm wave base. Abundances and ratios of Al2O3, Zr, TiO2, and Cr track inputs of various types of volcaniclastic and terrigenous clastic materials. In particular, Zr/Al2O3 and Zr serve as proxies for concentration of windblown dust and, indirectly, as proxies for sedimentation rate. Cu, Zn, Ni, and FeO were concentrated in the most slowly deposited transitional and basinal sediments, inconsistent with a hydrothermal setting but consistent with a normal marine setting. The distribution of microfacies defined by associations and layering of clastic, ferruginous, and carbonaceous grains correlates with facies transitions. Fine carbonaceous laminations, which occur only in shallow platform settings, represent photosynthetic microbial mats. These were ripped up and the debris widely redistributed in shallow and deep water by waves and storms. The isotopic composition of carbonaceous matter ranges from - 35 to - 30 in shallow-water settings and to - 20 in deep-water units. The heavier 13C in deep-water carbonaceous matter is thought to reflect microbial processing, possibly by fermentation and methanogenesis, of organic matter originally produced in shallow water. Hydrothermal origins for BRC carbonaceous matter are clearly excluded by the inferred depositional setting of the rocks as a whole, an inference supported by field, petrographic, and geochemical analysis. We suggest that the biological model proposed here for BRC carbonaceous matter is the best currently available. The hypothesis that "at least some carbonaceous matter present in rocks older than 3.0Ga was produced by living organisms" should be regarded as likely until extraordinary contradictory evidence is presented. 2006 Elsevier B.V. All rights reserved.
