The aerobic diagenesis of Mesoproterozoic organic matter

Origin and Geochemical Implications of Hopanoids in Saline Lacustrine Crude Oils from Huanghekou East Sag and Laizhouwan Northeastern Sag, Bohai Bay Basin

A suite of low-mature crude oils (five high-sulfur oils and six low-sulfur oils) from the Huanghekou and the Laizhouwan Sags, Bohai Bay Basin, are analyzed to investigate the fate of the hopanoids. Abundant hopanes, such as secohopanes, 25-norhopanes, benzohopanes, aromatized secohopanes, and sulfide hopanes, are identified, and their carbon isotope compositions are determined. Varying ¹³C isotope values of C₃₁ hopane (-38.7-34.0‰) and C_29-30 hopanes (-38.5-31.5‰) suggest different bacterial sources of these compounds. The presence of 25-norhopanes with enriched heavy carbon isotopes in severely biodegraded oils suggests that they are microbially mediated products. The detection of the isotopically depleted C₂₉ and C₃₀ D-ring-8,14-secohopanes (-45.6-41.2‰) indicates that secohopanes are from methane-oxidizing bacteria (methanotrophs). The presence of isorenieratane, lower aryl isoprenoid ratios, and a good correlation between the sulfur content and the gammacerane index indicate the presence of green sulfur bacteria (Chlorobiaceae) under photic zone euxinic conditions. Water column stratification results in good preservation of the organic matter, and it is in favor of diversity of aquatic microorganisms. The ratios of C₃₅/C₃₄ sulfide hopane, C₃₅ sulfide hopane-2/C₃₅ sulfide hopane-1, and C₃₅/C₃₄ benzohopane are influenced by the reducing environments in this region. In addition, the D-ring monoaromatized 8,14-secohopanoid/(D-ring monoaromatized 8,14-secohopanoid + benzohopanes) and C₃₁-C₃₅ secomoretanes/secohopanes are affected by the maturity. We hypothesize that the reducing environments and thermal effects are important markers for the hopanoid transformation, including the incorporation of inorganic sulfur in substituting functional groups, cyclizing, aromatizing, and opening ring C of the hopanoids.

Geochemical Signatures and Controlling Factors of Rearranged Hopanes in Source Rocks and Oils from Representative Basins of China

The origin and geochemical significance of the rearranged hopanes in hydrocarbon source rocks or crude oil have attracted extensive attention. Despite numerous studies, there is not yet a proper conclusion. Therefore, this paper discusses the formation conditions of such compounds and points out their geochemical significance in more detail using a remarkably broad range of source rocks and crude oils from four basins in China. Varying content of rearranged hopanes was found in a total of 19 source rocks and oils from the Ordos, Sichuan, and Tarim basins and the North China Block. Gas chromatography-mass spectrometry (GC-MS) in combination with X-ray diffraction (XRD) and conventional geochemical parameters was used for Pearson correlation analysis to reveal the enrichment mechanisms of rearranged hopanes in the studied rock and oil samples. The GC-MS and XRD results showed that the studied source rocks with high rearranged hopane contents are closely associated with the high abundance of quartz rather than that of clay. Furthermore, the present study reveals that anoxic lacustrine conditions are the primary controlling factors of relatively high abundance of rearranged hopanes in the studied rocks and oils, whereas thermal maturity and terrigenous organic matter input are the secondary factors.

On the co-evolution of surface oxygen levels and animals

Few topics in geobiology have been as extensively debated as the role of Earth's oxygenation in controlling when and why animals emerged and diversified. All currently described animals require oxygen for at least a portion of their life cycle. Therefore, the transition to an oxygenated planet was a prerequisite for the emergence of animals. Yet, our understanding of Earth's oxygenation and the environmental requirements of animal habitability and ecological success is currently limited; estimates for the timing of the appearance of environments sufficiently oxygenated to support ecologically stable populations of animals span a wide range, from billions of years to only a few million years before animals appear in the fossil record. In this light, the extent to which oxygen played an important role in controlling when animals appeared remains a topic of debate. When animals originated and when they diversified are separate questions, meaning either one or both of these phenomena could have been decoupled from oxygenation. Here, we present views from across this interpretive spectrum-in a point-counterpoint format-regarding crucial aspects of the potential links between animals and surface oxygen levels. We highlight areas where the standard discourse on this topic requires a change of course and note that several traditional arguments in this "life versus environment" debate are poorly founded. We also identify a clear need for basic research across a range of fields to disentangle the relationships between oxygen availability and emergence and diversification of animal life.

Free and kerogen-bound biomarkers from late Tonian sedimentary rocks record abundant eukaryotes in mid-Neoproterozoic marine communities

Lipid biomarker assemblages preserved within the bitumen and kerogen phases of sedimentary rocks from the ca. 780-729 Ma Chuar and Visingsö Groups facilitate paleoenvironmental reconstructions and reveal fundamental aspects of emerging mid-Neoproterozoic marine communities. The Chuar and Visingsö Groups were deposited offshore of two distinct paleocontinents (Laurentia and Baltica, respectively) during the Tonian Period, and the rock samples used had not undergone excessive metamorphism. The major polycyclic alkane biomarkers detected in the rock bitumens and kerogen hydropyrolysates consist of tricyclic terpanes, hopanes, methylhopanes, and steranes. Major features of the biomarker assemblages include detectable and significant contribution from eukaryotes, encompassing the first robust occurrences of kerogen-bound regular steranes from Tonian rocks, including 21-norcholestane, 27-norcholestane, cholestane, ergostane, and cryostane, along with a novel unidentified C₃₀ sterane series from our least thermally mature Chuar Group samples. Appreciable values for the sterane/hopane (S/H) ratio are found for both the free and kerogen-bound biomarker pools for both the Chuar Group rocks (S/H between 0.09 and 1.26) and the Visingsö Group samples (S/H between 0.03 and 0.37). The more organic-rich rock samples generally yield higher S/H ratios than for organic-lean substrates, which suggests a marine nutrient control on eukaryotic abundance relative to bacteria. A C₂₇ sterane (cholestane) predominance among total C₂₆ -C₃₀ steranes is a common feature found for all samples investigated, with lower amounts of C₂₈ steranes (ergostane and crysotane) also present. No traces of known ancient C₃₀ sterane compounds; including 24-isopropylcholestanes, 24-n-propylcholestanes, or 26-methylstigmastanes, are detectable in any of these pre-Sturtian rocks. These biomarker characteristics support the view that the Tonian Period was a key interval in the history of life on our planet since it marked the transition from a bacterially dominated marine biosphere to an ocean system which became progressively enriched with eukaryotes. The eukaryotic source organisms likely encompassed photosynthetic primary producers, marking a rise in red algae, and consumers in a revamped trophic structure predating the Sturtian glaciation.

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

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.