An understanding of the temporal evolution of a petroleum system is fundamental to interpreting where hydrocarbons may be trapped in the subsurface. However, traditional exploration methods provide few absolute constraints on the timing of petroleum generation. Here we show that 187Re/187Os geochronology may be applied to natural crude oil seepage to determine when petroleum generation occurred in offshore sedimentary basins. Using asphaltites collected from the South Australian coastline, our determined Re-Os age (68 ± 15 million years ago) is consistent with their derivation from a Late Cretaceous source rock in the nearby Bight Basin, an interpretation similarly favoured by source-specific biomarker constraints. Furthermore, the calculated initial 187Os/188Os composition of the asphaltites, a value inherited from the source rock at the time of oil generation, suggests that the source rock represents the later stage of Oceanic Anoxic Event 2. Our results demonstrate a new approach to identifying the origin of crude oils encountered in coastal environments by providing direct constraints on the timing of petroleum generation and potential source rock intervals in poorly characterised offshore sedimentary basins prior to exploratory drilling.

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Abstract. The termination of Cryogenian glaciations would have undoubtedly impacted the chemistry of Neoproterozoic oceans, with possible consequences for life; but the extent and duration of this impact are poorly constrained. In this study, we use the lithium (Li) isotope composition of Ediacaran cap dolostones from South Australia (Nuccaleena Formation) and China (Doushantuo Fm) to investigate changes in ocean chemistry that followed the Marinoan deglaciation. The effect of diagenesis was evaluated and while the Nuccaleena Fm is likely to have preserved the primary composition of cap dolostone deposition, the offset in Li isotope ratios observed for the Doushantuo Fm could possibly reflect partial overprinting by diagenetic fluids. The Li isotope composition of Ediacaran seawater was estimated and we suggest it was similar to that of late Cenozoic oceans for most of the cap dolostone deposition. Using a box model for the oceanic Li cycle, we show that at the onset of deglaciation, the supply of riverine Li to the oceans was up to 50 times the modern flux. The modelled riverine Li isotope composition suggests that continents resembled modern high-latitude regions during this time. This episode was short-lived (up to 1 Myr) and the subsequent supply of riverine Li was similar to modern conditions, both in flux and isotope composition, for the whole duration of cap dolostone deposition. These results suggest that Ediacaran oceans and continents rapidly recovered from the Marinoan glaciation to reach environmental conditions similar to the late Cenozoic. From the standpoint of the Li oceanic budget, the Ediacaran oceans in which complex lifeforms emerged may have not been that different from our modern oceans.

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