GEOCHEMISTRY AND MINERALOGY OF IRON MICROSTRUCTURES IN A NEOARCHEAN STROMATOLITE, CAMPBELL GROUP, SOUTH AFRICA

Archean stromatolites have been well-studied as early evidence for biologic activity in the rock record, but preservational biases and analytical uncertainties have complicated our understanding of the biosphere’s origins, early evolution, and environmental impact. In particular, the microfossil record is largely skewed towards silicate substrates due to structural degradation that occurs during carbonate crystallization, leaving gaps in the fossil record and hindering attempts to discover biosignatures in carbonates that may be relevant to planetary exploration efforts. Preserved microfossils have generally been subjected to intense metamorphism and other forms of alteration that make them difficult to identify and distinguish from inorganic microstructures based on morphology alone.

A study was conducted on a unique cluster of semi-spherical microstructures embedded in a Neoarchean conical stromatolite from the Campbell Group, South Africa (2.6 Ga). The structures (approximately 5-10 micrometers in diameter) have the morphological attributes, dense distribution, and size uniformity expected for a community of fossilized coccoidal cyanobacteria, but petrographic data and scanning electron microscopy (SEM) imaging revealed that they are mineral precipitates. Energy dispersive spectroscopy (EDS) showed significant concentrations of iron and sulfur, suggesting the structures are an iron sulfide or iron oxide mineral. Interestingly, larger spheroids are encircled by dense organic carbon rings that were detectable using Raman spectroscopy, lending support to the hypothesis of a biogenic origin. Carbon isotope values gathered from secondary ion mass spectrometry (SIMS) will be used to verify this and corroborate preliminary laser ablation isotope ratio mass spectrometry (LA-IRMS) data that detected regions of low δ¹³C values (around -27‰) in the stromatolite’s carbonate lamina.

Precipitation of ferrous iron minerals has been shown to often be coupled to metabolic processes such as dissimilatory iron reduction (DIR) and bacterial sulfate reduction, particularly when sulfide is available. It is plausible that the observed microstructures were formed as a product of such microbial processes occurring when this anoxic Archean stromatolite formed.