Asphalt mounds and associated biota on the Angolan margin

Abstract

Keywords: 6145 0 S 11100 0 E Deep-sea benthic biology Megafauna Hydrocarbon seep Chemosynthetic environment Remotely operated vehicle Seafloor geology Habitat heterogeneity a b s t r a c t Release of hydrocarbons from sediments is important in increasing habitat heterogeneity on deep ocean margins. Heterogeneity arises from variation in abiotic and biotic conditions, including changes in substratum, geochemistry, fluid flow, biological communities and ecological interactions. The seepage of heavy hydrocarbons to the seafloor is less well studied than most other cold seep systems and may lead to the formation of asphalt mounds. These have been described from several regions, particularly the Gulf of Mexico. Here, we describe the structure, potential formation and biology of a large asphalt mound province in Block 31SE Angola. A total of 2254 distinct mound features was identified by side-scan sonar, covering a total area of 3.7 km 2 of seafloor. The asphalt mounds took a number of forms from small (o0.5 m diameter; 13% observations) mounds to large extensive (o50 m diameter) structures. Some of the observed mounds were associated with authigenic carbonate and active seepage (living chemosynthetic fauna present in addition to the asphalt). The asphalt mounds are seabed accumulations of heavy hydrocarbons formed from subsurface migration and fractionation of reservoir hydrocarbons primarily through a network of faults. In Angola these processes are controlled by subsurface movement of salt structures. The asphalt mounds were typically densely covered with epifauna (74.5% of mounds imaged had visible epifauna) although individual mounds varied considerably in epifaunal coverage. Of the 49 non-chemosynthetic megafaunal taxa observed, 19 taxa were only found on hard substrata (including asphalt mounds), 2 fish species inhabited the asphalt mounds preferentially and 27 taxa were apparently normal soft-sediment fauna. Antipatharians (3.672.3% s.e.) and poriferans (2.671.9% s.e.) accounted for the highest mean percentage of the observed cover, with actinarians (0.970.4% s.e.) and alcyonaceans (0.470.2% s.e.) covering smaller proportions of the area. Asphalt mounds represent a common and important habitat on several margin systems globally and should be recognised in future environmental assessment and management of these areas.

Bacterial diversity in deep-sea sediments under influence of asphalt seep at the São Paulo Plateau

“…The discovery of asphalt seeps in North São Paulo Plateau was an important milestone in studies of hydrocarbon seep environments and their associated chemosynthetic communities. This asphalt seep is similar to asphalt systems found in Campeche Knolls of southern Gulf of Mexico [28] and Angola Margin [29]. However, Fujikura et al (2017) analyzed the oil from the asphalt seep and their results indicated that this system was not capable of sustaining chemosynthetic communities.…”

Characterization of Methane-Seep Communities in a Deep-Sea Area Designated for Oil and Natural Gas Exploitation Off Trinidad and Tobago

“…Methane seeps frequently occur in areas of economic interest because of their direct association with oil and gas-rich fluids (Sibuet and Olu, 1998;Bergquist et al, 2003;Cordes et al, 2003Cordes et al, , 2016Bernardino et al, 2012;Jones et al, 2014). Although there has been wide-ranging deep-sea exploration off Trinidad and Tobago, the majority of this work has been undertaken by oil and natural gas companies and the data have not been made publically available.…”

Direct monitoring of active geohazards: emerging geophysical tools for deep‐water assessments

“…Natural seepage of fluids at the sea‐floor ranges from kilometre‐scale eruptive mud volcanoes with associated caldera collapses (Gray, Dingler and Wood ) to decametre‐scale extrusion of asphalt at the seafloor formation, pockmarks, and authigenic bacteria‐mediated carbonate crusts (Hovland, Gardner and Judd ; Jones et al . ). Fluid expulsion can create corrosive pore fluids and local modification of seafloor geotechnical properties (Thomas et al .…”