Fluxes and fate of petroleum hydrocarbons in the Timor Sea ecosystem with special reference to active natural hydrocarbon seepage

A sediment and sediment trap study of natural oil seeps in the Timor Sea off the north coast of Australia has been conducted. Sediment traps were set at 40 m below sea level on top of a benthic carbonate dome, formed near the Cornea oil and gas field, which was visibly leaking oil and gas around its edges in similar to 90 m of water. Other traps were set at the continental shelf edge at 100 m and 400 m below sea level on the 500 m bathymetric contour to gauge the long range dispersion of the Cornea seep. Sediment cores from the trap sites and other relevant sites were also collected. Sea surface microlayer samples of the oil slicks emerging with gas bubbles were also sampled. These were used to characterize the oil as it moved from surface slicks, through settling particles, to surface sediment. Downward fluxes of total hydrocarbons (THC) ranged from similar to 500 mu g m(-2) d(-1) at the shallow Cornea seep site to similar to 100 mu g m(-2) d(-1) in 100 m depth traps at the offshore stations. Fluxes of oil polynuclear aromatic hydrocarbons (PAH) ranged from similar to 6000 ng m(-2) d(-1) at Cornea to similar to 1000 ng m(-2) d(-1) at off shore stations. Principal Hierarchical Cluster Analysis and Principal Component Analysis (HCA-PCA) of the petroleum biomarkers confirmed that the surface slick samples and sediment traps from Cornea and some of the offshore traps had similar hydrocarbon compositions. However, some of the offshore traps also contained hydrocarbons from an apparently different source. This second source appeared to be "background oil", probably due to a mixture of natural seeps, oil industry activities, or release from shipping traffic, and was similar in composition to that seen in previous sediment trap studies on the Northwest shelf The Cornea seep area was deemed a significant point source of oil and gas to the coastal environment that was dispersed by tidal movements across the continental shelf and contributed to settling particles up to 300 km away on the continental slope of the Timor Sea. An extension of a diagenesis model showed that 44 to 77% of the (THC) and 27 to 83% of the (PAH) were degraded in the water column. Of the remaining oil reaching the sediment surface 99 to similar to 100% of THC and 98 to similar to 100% of PAH were degraded in the near surface sediments. Microbial biomarkers indicated the presence of sulphate reducing bacteria in both sediment traps and sediments and only trace amounts of the markers associated with Archaea which together accomplish the anoxic oxidation of methane (AOM). Rather than this well known deep sea microbial consortia, aerobic hydrocarbon oxidizing bacteria constituted a small proportion of the bacterial community in the surface sediment layer associated with the largest seep plume observed at the Cornea seep area. These hydrocarbon degraders were not visible in the offshore sediments against the background of other microbes. The rapid water column and surface sediment degradation rates indicated that the Timor Sea is well adapted to degrade natural oil seeps. This rapid degradation rate makes detection difficult for specific oil biomarkers due to low hydrocarbon concentrations in sediments. Such studies are more successfully accomplished with sediment trap deployments. Crown Copyright (c) 2009 Published by Elsevier B.V. All rights reserved.