EXPERIMENTAL AND FIELD INVESTIGATIONS IN SEARCH OF BIOSIGNATURES IN EARLY PROTEROZOIC MANGANESE DEPOSITS

Early Proterozoic sedimentary manganese accumulations represent a key period in Earth history when life was likely having a considerable impact on the evolution of the planet. Biological processes may have been in part responsible for the origin of sedimentary manganese deposits, although evidence of microorganisms (i.e. microfossils or chemical biosignatures) have yet to be identified within these sequences. The purpose of this study was to identify potential biosignatures in early Proterozoic manganese deposits, then to investigate biosignatures (if present) in samples collected from the ~2.0 Ga Kalahari Manganese Field of South Africa. The experimental degradation of microorganisms (Bacillus subtilis and Escherichia coli) was simulated in the laboratory by exposing the organisms to various concentrations of Mn(II). A short-lived biomarker (DNA) and a long-lived biomarker (phospholipid fatty acids and their derivative n-alkanes) were monitored over time using real-time PCR and GCMS techniques.

Preliminary analysis shows that both Bacillus subtilis and Escherichia coli DNA is preserved less over time in samples which contain up to 1000 ppm Mn(II). After two days of exposure to Mn(II) Bacillus subtilis showed near complete cell death in 10, 100, and 1000 ppm Mn(II), but DNA from ~10⁶ microorganisms could still be recovered. Total lipids extracted from Bacillus subtilis samples showed a complete conversion to hydrocarbons (n-alkanes) after 72 days exposure to 1000 ppm Mn(II). Samples collected from Mamatwan Mine of the Kalahari Manganese Field have been microdrilled along a length of core and carbon isotopes have been analyzed. A range of carbonate carbon isotopic values from -6‰ δ¹³C to -15‰ δ¹³C have been documented. Further analysis using scanning electron microscopy coupled with energy-dispersive x-ray analysis has confirmed the presence of barite in the Kalahari samples, as well as nanometer-size tube structures of unidentified composition.