Stromatolites Belingwe belt

Figure 4 Stromatolitic limestone, Cheshire Formation, Belingwe belt, Zimbabwe (2.6-2.7 Ga old) (a) outcrop surface—structures occur on a variety of scales, from microscopic to metre relief (b) detail of one outcrop (from Nisbet, 1987) and (c) shallow-water shale associated with Cheshire stromatolites.

To summarize, the isotopic and textural evidence collectively implies the activity in the Belingwe belt of a variety of prokaryotic processes (1) sulphate reduction and possibly photosynthetic sulphide oxidation (2) operation of rubisco both in cyanobacterial stromatolites (as expected) but also possibly in non-photosynthetic sulphur-bacterial mats (3) oxygenic photosynthesis (in stromatolites) (4) methanogenesis and methane oxidation. Most probably, other sulphur-based metabolic reactions (e.g. dissim-ilatory sulphate reduction) were also taking place. This complexity is consistent with the relative timing of the metabolic phylogeny deduced from rRNA studies (Woese 1987 Pace 1997). 

Bickle M. J., Martin A., and Nisbet E. G. (1975) Basaltic and peridotitic komatiites, stromatolites, and a basal unconformity in the Belingwe Greenstone belt, Rhodesia. Earth Planet. Sci. Lett. 27, 155-162. 

Martin A., Nisbet E. G., and Bickle M. J. (1980) Archean stromatolites of the Belingwe greenstone belt, Zimbabwe (Rhodesia). Precamb. Res. 13, 337-362. 

Nisbet, E. G., Martin, A., Bickle, M. J. Orpen, J. L. 1993. The Ngezi Group komatiites, basalts and stromatolites on continental crust. In Bickle, M. j. Nisbet, E. G. (eds) The Geology of the Belingwe Greenstone Belt, Zimbabwe. Geological Society of Zimbabwe Special Publications, 2, 69-86. 

Abell, P. I., McClory, J., Martin, A. Nisbet, E. G. 1985. Archaean stromatolites from the Ngesi Group, Belingwe Greenstone Belt, Zimbabwe. Preservation and stable isotopes - preliminary results. Precambrian Research, 27, 357-383. 

Martin, A., Nisbet, E G. Bickle, M. J. 1980. Archaean stromatolites of the Belingwe Greenstone Belt. Precambrian Research, 13, 337-362. 

More controversial is whether there is evidence for former life in greenstone sediments. At present the much publicized carbon isotope evidence for life in the 3.8 Ga Isua succession (Rosing, 1999) is under severe scrutiny. This is discussed in some detail in Chapter 6 (Section 6.3.2.1). In contrast stromatolite fossils have been documented in greenstone belt sediments from about 3.5 Ga onwards, and sulfur isotope evidence from the Belingwe Greenstone Belt demonstrates that there were complex algal communities by 2.7 Ga (Grassineau et al., 2001 - see Chapter 6, Section 6.3). 

Late Archaean stromatolites Younger stromatolites have been described from 3.0 Ga rocks at Steep Rock in Canada (Wilks St Nisbet, 1985), from the 2.7 Ga Cheshire formation of the Belingwe Greenstone belt (Martin et al., 1980) and from 2.52 Ga shelf carbonate sediments in the Campbellrand subgroup, South Africa. In this latter locality there are also well-documented calcified microfossils of cyanobacteria (Kazmierczak St Altermann, 2002). It is also possible that at this locality the mineralization of the dead bacteria was the result of the action of het-erotrophic bacteria. 

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