Australasian asphaltite strandings: Their origin reviewed in light of the effects of weathering and biodegradation on their biomarker and isotopic profiles |
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| Institution: | 1. Chemical and Biological Engineering Department, University of Ottawa, 161 Louis Pasteur, Ottawa, Ontario K1N 6N5, Canada;2. CanmetENERGY, 1 Haanel Drive, Ottawa, Ontario K1A 1M1, Canada;1. School of Geographical Sciences, Northeast Normal University, Changchun 130024, China;2. College of Earth Sciences, Jilin University, Changchun 130061, China;3. Key Laboratory of Oil Shale and Coexistent Energy Minerals of Jilin Province, Changchun 130061, China;4. CNOOC Research Institute, China National Offshore Oil Corporation, Beijing 100000, China;5. Department of Earth and Planetary Sciences, MQ Marine Research Centre, Macquarie University, Sydney, NSW 2109, Australia;1. Faculty of Earth Sciences, University of Silesia, Będzińska 60, 41-200 Sosnowiec, Poland;2. Université de Lorraine, UMR 7359 GeoRessources, Ecole Nationale Supérieure de Géologie, 2 rue Marcel Roubault, TSA 70605, 54518 Vandoeuvre-lès-Nancy, France |
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| Abstract: | Asphaltites, long known to strand along the coastline of southern Australia and as distantly as New Zealand and Macquarie Island, are widely regarded as artefacts of submarine oil seepage. Their remarkably uniform composition suggests a common source: marine shale containing sulphur-rich Type II kerogen, probably deposited during an Early Cretaceous oceanic anoxic event (OAE). Suitable hydrocarbon kitchens may exist in the offshore Bight and Otway basins. The physical character of the asphaltites, including laminations and flow structures, and their degree of alteration, which is not the result of biodegradation or extensive water washing, suggest an origin from subsurface tar mats subsequently exposed by the incision of submarine canyons, with the possible formation of asphaltic volcanoes. API gravities of 4–18° impart quasi-neutral buoyancy, implying many asphaltites were submerged drifters prior to stranding, their degree of weathering reflecting, at least in part, the residence time in the marine environment. For any individual asphaltite specimen, this will depend on the proximity of the seafloor seep to the stranding site, an important consideration when attempting to locate their point of origin.This study investigates the hydrocarbon biomarker signatures and n-alkane δ13C profiles of asphaltite specimens from stranding sites on the Eyre Peninsula (n = 2), Kangaroo Island (n = 4) and the Limestone Coast (n = 3), South Australia, and the south island of New Zealand (n = 2). Sub-samples of the interior and weathered surface of each specimen were analysed. No distinction could be made between strandings based on their source-dependent molecular and isotopic signatures, confirming their common origin. Comparison of the interior and exterior sub-samples revealed subtle although consistent differences. Given their degree of degradation and isotopic variance, these Australasian asphaltites seem to be products of low intensity seeps in the Ceduna Sub-basin of the Bight Basin and/or Morum Sub-basin of the Otway Basin. |
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| Keywords: | Australasian asphaltites Weathering Oil seeps Tar mats Biomarkers Carbon isotopes Otway Basin Bight Basin |
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