Abstract
The ca. 1.38 billion years (Ga) old Roper Group of the McArthur Basin, northern Australia, is one of the most extensive Proterozoic hydrocarbon‐bearing units. Organic‐rich black siltstones from the Velkerri Formation were deposited in a deep‐water sequence and were analysed to determine their organic geochemical (biomarker) signatures, which were used to interpret the microbial diversity and palaeoenvironment of the Roper Seaway. The indigenous hydrocarbon biomarker assemblages describe a water column dominated by bacteria with large‐scale heterotrophic reworking of the organic matter in the water column or bottom sediment. Possible evidence for microbial reworking includes a large unresolved complex mixture (UCM), high ratios of mid‐chained and terminally branched monomethyl alkanes relative to n‐alkanes—features characteristic of indigenous Proterozoic bitumen. Steranes, biomarkers for single‐celled and multicellular eukaryotes, were below detection limits in all extracts analysed, despite eukaryotic microfossils having been previously identified in the Roper Group, albeit largely in organically lean shallower water facies. These data suggest that eukaryotes, while present in the Roper Seaway, were ecologically restricted and contributed little to export production. The 2,3,4‐ and 2,3,6‐trimethyl aryl isoprenoids (TMAI) were absent or in very low concentration in the Velkerri Formation. The low abundance is primary and not caused by thermal destruction. The combination of increased dibenzothiophene in the Amungee Member of the Velkerri Formation and trace metal redox geochemistry suggests that degradation of carotenoids occurred during intermittent oxygen exposure at the sediment–water interface and/or the water column was rarely euxinic in the photic zone and likely only transiently euxinic at depth. A comparison of this work with recently published biomarker and trace elemental studies from other mid‐Proterozoic basins demonstrates that microbial environments, water column geochemistry and basin redox were heterogeneous.
Highlights
The relationship between ocean chemistry, microbial metabolisms, and the evolution and proliferation of eukaryotes are themes of long-standing interest in Proterozoic geobiology (Anbar & Knoll, 2002; Johnston et al, 2012; Nursall, 1959)
With the recent publication of high-resolution inorganic geochemistry of the Velkerri Formation in the Altree 2 drillcore (Cox et al, 2016), this study provides a complementary contaminant-free analysis of hydrocarbon biomarkers preserved in the ca. 1.38 Ga Roper Seaway sediments in a palaeoenvironmental context
Indigenous hydrocarbon biomarkers extracted from siltstones in the Velkerri Formation of the McArthur Basin from drillcore Altree 2 have been used to determine the microbial ecology of the ca. 1.38 Ga Roper Seaway
Summary
The relationship between ocean chemistry, microbial metabolisms, and the evolution and proliferation of eukaryotes are themes of long-standing interest in Proterozoic geobiology (Anbar & Knoll, 2002; Johnston et al, 2012; Nursall, 1959). | 362 deep-water conditions are considered hostile for the expansion of eukaryotes which are largely aerobic and poisoned by sulphide (Anbar & Knoll, 2002) Such conditions were presumably widespread in the mid-Proterozoic based on biomarker evidence for photic zone euxinia in the 1.64 Ga Barney Creek Formation (Brocks et al, 2005), and the 1.1 Ga Atar Group, Mauritania (Blumenberg, Thiel, Riegel, Kah, & Reitner, 2012; Gueneli, Brocks, & Legendre, 2012), in addition to iron speciation and trace element geochemistry (Canfield, 2004; Poulton et al, 2004) suggested euxinia was a defining feature of the Mesoproterozoic and led to the restriction of eukaryotes to local oases with stable oxygenated conditions
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