Microbial diversity and palaeoenvironmental reconstruction of the 1.4 Ga Roper Seaway, McArthur Basin, northern Australia

The 1.4 Ga Roper Group of the McArthur Basin, northern Australia is one of the most extensive Proterozoic hydrocarbon-bearing basins preserved in the geological record (e.g. Jackson et al. 1984;). It is interpreted to have been deposited in a large redox stratified epeiric sea known as the Roper Seaway (Abbott & Sweet, 2000, Cox et. al. 2016). Black shales from the Velkerri Formation, a deep water lowstand systems tract, thermally well preserved in drillcore Altree 2 were analysed to assess microbial diversity and palaeoenvironment, and compared with previously published inorganic geochemistry and microfossils (Cox et al., 2016; Javaux et al., 2001; Figure 1). Indigenous biomarkers describe a microbial environment dominated by bacteria. All extracts are characterised by a large unresolved complex mixture (UCM) and high ratios of monomethyl alkanes relative to n-alkanes - characteristic features of indigenous Proterozoic bitumen (Pawlowska et al. 2013). Regular hopanes in the C27 to C35 range and their diagenetically rearranged isomers were detected in all samples from 410.55 to 793.37 m. C29 to C36 3 beta- and 2 alpha-hopanes were detected in lower concentrations than regular hopanes in the same samples. The combination of these biomarkers describes a water column dominated by bacteria with heterotrophic reworking of the organic matter. Steranes, biomarkers for single-cell and multicellular eukaryotes, were below detection limits in all extracts analysed, although eukaryotic microfossils have been identified in the same drillcore analysed in this study (Javaux et al., 2001). These results suggest that eukaryotes, while present in the Roper Seaway, were ecologically restricted and contributed little to the net biomass. The dibenzothiophene/phenanthrene (DBT/P) ratio increases from the upper Velkerri to the middle and lower Velkerri Formations broadly correlating with increasing sulphide concentrations in the deep water column (Figure 1). The elevated sulphide would have been subsequently incorporated into the organic matter (Hughes et al. 1995), broadly consistent with the onset of euxinia (based on elevated Mo, V, U enrichments) at similar depths in the drillcore (Cox et al., 2016; Figure 1). The 2,3,4- and 2,3,6-trimethyl arylisoprenoids were in low concentrations or absent through the Velkerri Formation. The absence was not due to thermal destruction since the maturity level at the base of the analysed interval is pre-oil window, hence the water column at the time of high total organic carbon (TOC) deposition was not euxinic in the photic zone, but likely only euxinic at depth. The biomarker composition and water column chemistry in the Roper Group described in this study is in contrast to the 1.64 Ga Barney Creek Formation of the McArthur Basin where pervasive euxinia extended into the photic zone (Brocks et al., 2005). This contrast demonstrates that microbial environments and water column geochemistry were heterogeneous during the Mesoproterozoic.