Microbial assemblage and palaeoenvironmental reconstruction of the 1.38 Ga Velkerri Formation, McArthur Basin, northern Australia

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.

Weathering in a world without terrestrial life recorded in the Mesoproterozoic Velkerri Formation

“…The Velkerri Formation itself has experienced limited deformation and is recognized as a series of parallel reflectors across the MacArthur basin in seismic profiles 59 . It is shallowly buried and unmetamorphosed, and of a low thermal maturity, entering or within the oil generation window including live oils shows 42,47 . This study utilized 26 samples from the Urupunga-4, McManus1, Lady Penrhyn 2 and Walton 2 cores spanning ~200 km across the MacArthur basin in Northern Australia (Fig.…”

“…Here we directly determine the composition and origin of individual clay minerals in the Mesoproterozoic Velkerri Formation (see Methods). The Velkerri Formation was selected for this study because it is a well preserved, (pre-soil) clay-mineral rich, black shale that is commonly used as an informal reference section for Mesoproterozoic environmental conditions 30,38–42 . Although there is no single Formation capable of recording a comprehensive global view of weathering processes, the Velkerri Formation captures a broad regional pattern of weathering and is a reasonable basis for exploration of weathering processes in this time period.…”

Discovery of the oldest known biomarkers provides evidence for phototrophic bacteria in the 1.73 Ga Wollogorang Formation, Australia

“…This article is protected by copyright. All rights reserved mid-Proterozoic marine ecosystems were dominated by bacteria (Blumenberg, Thiel, Riegel, Kah, & Reitner, 2012;Brocks et al, 2005;Gueneli et al, 2018;Jarrett et al, 2019;G. Luo, Hallmann, Xie, Ruan, & Summons, 2015;Q.…”

“…Worldwide, there are currently only 6 geological sequences  1.0 Ga that yield demonstrably indigenous biomarkers: the 1.0 Ga Nonesuch Formation (Gueneli, 2016), 1.1 Ga El Mreïti Group (Blumenberg et al, 2012;Gueneli et al, 2018), 1.38 Ga Xiamaling Formation (G. Luo et al, 2015, 1.38 Ga Roper Group (Jarrett et al, 2019;Nguyen et al, 2019), 1.4 Ga Hongshuizhuang Formation (Q. Luo et al, 2016), and the aforementioned 1.64 Ga BCF (Brocks et al, 2005).…”

Fossilised Biomolecules and Biomarkers in Carbonate Concretions from Konservat-Lagerstätten

“…As such, biomarkers allow for the detailed investigation of conditions and processes which allow for exceptional fossil preservation [9,55], as well as provide important information regarding microbial systems and preservation conditions [10,35,60]. Similar to biomarkers, DNA can provide a breadth of information on fossilised species and preservation conditions; however, it is limited in its usefulness as it can only survive for up to one million years in sediments [100], whereas biomarkers have been shown to survive for over one billion years in ideal conditions [101].…”