Biomarker stratigraphy in the Athel Trough of the South Oman Salt Basin at the Ediacaran‐Cambrian Boundary

Abstract

Over recent decades, the South Oman Salt Basin (SOSB) has attracted attention owing to the presence of commercial oil deposits derived from Neoproterozoic-Cambrian sedimentary sequences. The very unusual biomarker characteristics of the early SOSB oil discoveries enabled their attribution to units within the Huqf Supergroup

Redox-Controlled Ammonium Storage and Overturn in Ediacaran Oceans

“…Together with previously published accounts of fossil pigments (French et al, 2015;Roussel et al, 2020), the distribution and stable nitrogen isotope composition of vanadyl porphyrins and porphyrin-derived maleimides in sediments of the Ediacaran SOSB is indicative of shallow euxinia. The δ 15 N values of surface water (Chl-a) derived maleimides likely indicates the presence of both cyanobacteria as well as eukaryotic algae, as confirmed by steroid abundances in SOSB rocks (Grosjean et al, 2009), yet ranges into values that may point towards the use of ammonium as a nitrogen substrate.…”

“…Considering the possibility of a preservational bias, a larger proportion of Chl-a produced in surface waters may indeed be oxidatively degraded than BChl produced at the chemocline, thereby artificially shifting the balance of preserved porphyrins towards anoxygenic phototrophs. However, episodic euxinia with a shallow chemocline was corroborated for deep-water facies of the SOSB by the presence of the molecular markers okenane and chlorobactane (French et al, 2015;Roussel et al, 2020). In the deep-water depositional environments of the SOSB, the observed carotenoid pigments cannot derive from benthic mats but must represent planktonic purple sulfur bacteria and the green strains of green sulfur bacteria, respectively-microbes requiring access to reduced sulfur and with high light requirements.…”

“…Nevertheless if such biases were large, they should have similarly affected the Kupferschiefer samples that are not shifted towards an anoxygenic phototrophic source of MMM and EEM despite only mild euxinia (Grice et al, 1997). Most recently, a study of fossil aromatic carotenoids in terminal Ediacaran SOSB sediments (Roussel et al, 2020) reported an elevated proportion of structures that were likely sourced by cyanobacteria rather than by anoxygenic phototrophs (Cui et al, 2020). However, the taphonomy of carotenoids is complicated by sulfide as a preferential preservative (e.g., Koopmans et al, 1996) and although we cannot exclude principally-cyanobacterial productivity in the SOSB, it is unlikely that 15 N-depleted MMM, EEM and BChl-derived MPM were all largely produced by cyanobacteria.…”

Synergy of Analytical Approaches Enables a Robust Assessment of the Brazil Mystery Oil Spill

“…Additional insights into the geologic origin of the Brazil mystery oil were uncovered from the analysis of carotenoidderived HCs via GC-QQQ-MS analysis. 33 While not previously employed for oil spill forensics, this class of biomarkers captures the upper ocean conditions of primary productivity (e.g., light availability, taxa, and oxygenation) and depositional environment, which in turn helps characterize the source organic matter for petroleum generation. 65 Sample 12 contained an abundance of β-carotane, isorenieratene, and paleorenieratiane and lesser amounts of β-renieratane and βpaleoreneiratane (Figure 6).…”

“…The solvent extract for Sample 12 was additionally processed with activated copper to remove elemental sulfur and fractionated into nonpolar and polar fractions with silica gel chromatography. 33 The nonpolar fraction was analyzed on an Agilent gas chromatograph (GC, 7890B) coupled to an Agilent triple quadrupole MS system (QQQ, 7010A) operated in multiple reaction monitoring mode to analyze C 26 −C 30 steranes, C 21 −C 26 tricyclic terpanes, and C 27 −C 35 triterpanes, together with saturated and aromatic carotenoids. The isolated saturated HC fraction was also analyzed by GC-MS in full-scan mode for biomarkers.…”