Seabed environmental survey of Angola Blocks 18WAD and 31: analysis of seabed samples

This report details the analysis and interpretation of seabed sample data carried out on behalf of BP Angola for an environmental survey of Angola Blocks 18 and 31 (bathyal SE Atlantic). Seabed sampling was undertaken with the NOC Megacorer to produce material for the assessment of hydrocarbons, heavy metals, particle size, total organic carbon and nitrogen and macrobenthos. Seabed sampling spanned water depths of 1300-2050m over the Angolan Margin and included sites on the open continental slope and on and around seabed pockmarks and salt diapirs. Analysis of the resultant data indicates that many physical / chemical parameters vary systematically with water depth and with seabed type (background, diapir and pockmark habitats). Contrary to expectation, sediments coarsen and organic matter (total organic carbon and total nitrogen) increases with depth, this, however, may be a local rather then regional trend. A number of the sampled diapir and pockmark sites are notable for possible indicators of fluid flow, elevated total hydrocarbon levels, unusual hydrocarbon compositions and variant macrobenthos species composition, all potentially indicative of fluid escape from the seabed (? seep sites). The macrobenthos are highly diverse and likely include a majority of species new to science. There is a strong trend of increasing biodiversity, particularly species richness, with depth and some indication that biodiversity is enhanced at diapir sites and somewhat reduced at pockmark sites. Controls on the species composition of the macrobenthos are undoubtedly complex and include both a suite of depth varying factors together with local habitat variation, which likely includes the influence of fluid flow. Should you wish to consult or cite this report please contact the author directly (Brian Bett, bjb@noc.soton.ac.uk, +44 (0)23 80596355).