Limited taxonomic classification is possible for Archaean microbial mats and this is a fundamental limitation in constraining early ecosystems. Applying Fourier Transform Infrared spectroscopy (FTIR), a powerful tool for identifying vibrational motions attributable to specific functional groups, we characterised fossilised biopolymers in 3.5-3.3 Ga microbial mats from the Barberton greenstone belt (South Africa). Microbial mats from four Palaeoarchaean horizons exhibit significant differences in taxonomically informative aliphatic contents, despite high aromaticity. This reflects precursor biological heterogeneity since all horizons show equally exceptional preservation and underwent similar grades of metamorphism. Low methylene to end-methyl (CH₂/CH₃) absorbance ratios in mats from the 3.472 Ga Middle Marker horizon signify short, highly branched n-alkanes interpreted as isoprenoid chains forming archaeal membranes. Mats from the 3.45 Ga Hooggenoeg Chert H5c, 3.334 Ga Footbridge Chert, and 3.33 Ga Josefsdal Chert exhibit higher CH₂/CH₃ratios suggesting longer, unbranched fatty acids from bacterial lipid precursors. Absorbance ratios of end-methyl to methylene (CH₃/CH₂) in Hooggenoeg, Josefsdal and Footbridge mats yield a range of values (0.20-0.80) suggesting mixed bacterial and archaeal architect communities based on comparison with modern examples. Higher (0.78-1.25) CH₃/CH₂ ratios in the Middle Marker mats identify Archaea. This exceptional preservation reflects early, rapid silicification preventing the alteration of biogeochemical signals inherited from biomass. Since silicification commenced during the lifetime of the microbial mat, FTIR signals estimate the affinities of the architect community and may be used in the reconstruction of Archaean ecosystems. Together, these results show that Bacteria and Archaea flourished together in Earth’s earliest ecosystems.

These files are raw data obtained using transmission Fourier Transform Infra-red spectroscopy (FTIR). All data were acquired using a a Bruker VERTEX 70 instrument equipped with a GLOBAR (silicon carbide) source and a deuterated triglycine sulphate (DTGS) detector with a KBr window. All measurements were obtaines at the Centro Interdipartimentale Grandi Strumenti (Università degli studi di Modena e Reggio Emilia, Modena, Italy). Each reported FTIR file gives the average of 16 spectra accumulated within the studied area within a spectral range of 7500-370 cm^–1 with a 4 cm^–1 spectral resolution.

All files are raw numerical data obtained using transmission Fourier Transform Infra-red spectroscopy (FTIR). No treatment has been applied to these data and there are no missing values. Please see Hickman-Lewis et al. "Diverse communities of Bacteria and Archaea flourished in Palaeoarchaean (3.5-3.3 Ga) microbial mats" for the full interpretation of these data and explanations of the geological and petrographic contexts of the materials studied. The present dataset provides all data used in the aforementioned study.

Addtional information is available in Hickman-Lewis_et_al._Palaeontology_FTIR_dataset_README.txt