HR-TEM CHARACTERIZATION OF CA-CARBONATE NANOPHASES IN A 3.334 GA-OLD MICROBIAL MAT FROM THE BARBERTON GREENSTONE BELT, SOUTH AFRICA

Microorganisms indirectly precipitate many minerals, including a variety of carbonates. Carbonate mineralisation within microbial mats is one of the key signatures of photosynthetic microbial mats and is related to a number of microbial metabolisms, principally heterotrophic sulphur reducing bacterial degradation of the carbonaceous remains of photosynthetic organisms. Microbial mats have been described in Early Archaean sediments from the Barberton greenstone belt in South Africa and the Pilbara of Australia. It has been hypothesised that the microbial mats were photosynthetic on the basis of their morphology, shallow-water environment of formation, and fractionated carbon isotope signature. Unfortunately, the C isotope signature for photosynthesis overlaps with that produced by the abiogenic Fischer Tropsch synthesis. With respect to stromatolites, a priori, the identification of a structure as stromatolitic implies that it was formed by photosynthetic microorganisms. However, the biogenicity of the Early Archaean stromatolites is still controversial.

We have identified two carbonate phases in a silicified 3.33 Ga-old microbial mat from Barberton. The 5-10 µm thick filamentous mat formed in a littoral environment. It shows a distinct lamination. Much of the lower part of the mat has been calcified, the alveolar degraded organic matter acting as a template for the growth of nanocrystals of carbonate. HR-TEM analysis of the nanoparticles indicates that two Ca-carbonate phases are present, calcite and aragonite with crystalline and non-crystalline diffraction. Synchrotron mapping documents additional metals associated with the carbonate phase, Fe and Cr, while Mg has been detected by EDX. This is the oldest known occurrence of aragonite. Both aragonite and calcite can be microbially precipitated in a calcifying mat but is also possible that some conversion of aragonite to calcite has taken place. NanoSIMS elemental mapping demonstrates that the mat has been thoroughly permeated by silica of hydrothermal origin that killed and “fixed” the living mat. It is the silica matrix that preserved the aragonite from conversion to calcite. Thus, the unequivocal evidence for calcification in a filamentous microbial mat formed in a littoral environment is also unequivocal evidence of photosynthesis.