DEVELOPMENT OF MICROBIAL MATS DURING SOIL FORMATION 2.6-2.3 GA AGO IN EASTERN TRANSVAAL, SOUTH AFRICA

In Eastern Transvaal, South Africa, many paleosols have been recognized on the Archean basement complex of 2.7 – 3.6 Ga. At Schagen, the paleosol is between 2.7 Ga and 2.6 Ga in age because it is overlain by a 2.56 Ga marine-quartzite unit. At Kalkkloof, located ca. 50 km S of Schagen, the paleosol is older than 2.3 Ga because it is overlain by a 2.3 Ga marine shale unit. Both paleosols are developed on serpentinized ultramafic rocks and are rich in organic carbon (up to 1.2 wt %).

 The textures and the elemental ratios of C, H, N, and P of the organic-rich rocks, as well as the absence of S but the abundance of ferric-bearing minerals in the soil sections, all suggest that the organic matter in these paleosols is a remnant of microbial mats composed mostly of oxygenic cyanobacteria. The microbial mats probably formed mostly in shallow (< 1 m?) ponds developed over the soil during rainy seasons; some mats may have developed on and inside the soil during dry seasons. The d¹³C values of organic matter from these two localities fall within the range for modern cyanoacteria-dominated microbial mats on land, but there are noticeable differences at the two localities: –18 to –14 ‰ at Schagen vs. -25 to –22 ‰ at Kalkkloof. Carbon isotopic fractionation during CO2-fixation by mat-organisms is most strongly affected by the availability of CO2. Therefore, the differences in the d¹³C and the lithology of the two paleosol sections (calcrete and dolocrete vs. silcrete) can be explained by the difference in climatic conditions: hotter and drier conditions at Schagen 2.6 Ga ago compared to those at Kalkkloof ~2.4 Ga ago. The various types of (bio)geochemical data obtained in this study are also consistent with the model of development of an oxygenated atmosphere prior to 2.5 Ga.