Australasian asphaltite strandings: Their origin reviewed in light of the effects of weathering and biodegradation on their biomarker and isotopic profiles

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 sulfur-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 2 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 δ 13 C 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.

Remotely constraining the temporal evolution of offshore oil systems

“…Australia’s southern margin collects bitumens emitted from a variety of different petroleum systems 2,3,17,18 . One of the rarest, but most distinctive, types encountered is known as asphaltite 2,17 . These bitumens are easily identified by their striking appearance defined by deep shrinkage cracks, conchoidal fracture and jet-black colour (Fig.…”

“…1). A combination of highly reproducible source-specific biomarkers and other hydrocarbons constrains the origin of these asphaltites to a single oil family expelled early in the oil generation window from a Mesozoic marine shale deposited under anoxic/sulfidic conditions 2,17 . Their biomarker composition is unique, and yet to be linked to any produced oil worldwide 19 .…”

“…This suggests that the asphaltites are not the result of anthropogenic spillage, but rather the product of seafloor seepage from an unidentified petroleum system. Despite their prior extended immersion in seawater, recently stranded asphaltite specimens also preserve high abundances of slightly water-soluble aromatic hydrocarbons such as methylnaphthalenes and phenanthrene, indicating that the parent petroleum system is proximal to their region of stranding 2,17 . Presently, the origin of these asphaltite strandings is attributed to a source rock deposited during the Late Cretaceous global oceanic anoxic event (OAE2) at the Cenomanian-Turonian boundary (93.9 Ma) within the nearby Bight Basin 17,20 .…”

“…Despite their prior extended immersion in seawater, recently stranded asphaltite specimens also preserve high abundances of slightly water-soluble aromatic hydrocarbons such as methylnaphthalenes and phenanthrene, indicating that the parent petroleum system is proximal to their region of stranding 2,17 . Presently, the origin of these asphaltite strandings is attributed to a source rock deposited during the Late Cretaceous global oceanic anoxic event (OAE2) at the Cenomanian-Turonian boundary (93.9 Ma) within the nearby Bight Basin 17,20 . This predominantly offshore Jurassic-Cretaceous rift basin formed during the separation of Australia and Antarctica 21,22 and remains relatively unexplored.…”

Oil Seeps and Coastal Bitumen

“…Reports of such bitumen stranding date back to the mid-1800s, although tanker cleaning operations in the Java and Banda seas may augment the current influx of tar balls to the beaches of southern Australia. A study of six beaches in 1991-1992 revealed a maximum waxy bitumen loading of 2 kg per 100 m. Asphaltite, a heavier variety of local coastal bitumen, is less common, arguably less traveled and considered by Hall et al (2014) to be a manifestation of low-intensity seepage from tar mats exposed by submarine canyons on Australia's southern continental slope. It is carried ashore by a combination of seasonal upwelling currents and winter storms.…”