Cheshire Formation

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.

Carbon isotope compositions of organic carbon in the stromatolites of the Macgregor Member of the Cheshire Formation are variable, but... 

To summarize, the most probable explanation of the C isotope spread in the shallow-water facies of the Manjeri Formation (Spring Valley and Shavi sediments and Rupemba (Spring Valley) stromatolites), and in the Cheshire Formation stromatohtes and black shales is thus that carbon isotope fractionation records a diverse ecology that existed based primarily on oxygenic photosynthesis, but with associated organisms... 

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. 

The carbonate horizons of the Cheshire and Manjeri Formations contain extensive well-preserved stromatolites. These were formed in shallow or intertidal waters, as demonstrated by interbedded ripple-marked and mud-cracked silts. Texturally, the limestones show many indications of organic activity and gas release structures (Martin et al. 1980). The simplest uni-formitarian explanation is that the structures were built by cyanobacteria. Isotopically, carbon from kerogen in the stromatolites typically has 6 C = -25 to -30%o. This implies but does not prove fractionation by rubisco of carbon captured from the atmosphere-ocean system. Carbonate in Cheshire limestones is typically close to 0%o, suggesting that carbon in the atmosphere-ocean system was modulated by rubisco and dominated by oxygenic photosynthesis on a planetary scale (Fig. 1) at least by this date. It should be noted that the c. 3 Ga Steep Rock stromatolites are similar (work by Abell, Grassineau and Nisbet). 

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