Asphalt mounds and associated biota on the Angolan margin

Daniel O.B Jones,Anne Walls,Michael Clare,Mike S Fiske,Richard J Weiland,Robert O’Brien,Daniel F Touzel

doi:10.1016/j.dsr.2014.08.010

Abstract

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 3.7km2 of seafloor. The asphalt mounds took a number of forms from small (<0.5m diameter; 13% observations) mounds to large extensive (<50m 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 (3.6±2.3% s.e.) and poriferans (2.6±1.9% s.e.) accounted for the highest mean percentage of the observed cover, with actinarians (0.9±0.4% s.e.) and alcyonaceans (0.4±0.2% 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.

Highlights

Release of hydrocarbons from sediments is important in increasing habitat heterogeneity on deep ocean margins
In at least one location a fault in the shallow section appears to correspond with locations of surface expressions of asphalts, asphalt mounds are more commonly observed in areas with no obvious shallow faulting in the sub-bottom profile data (Fig. 3)
Seepage of heavy hydrocarbons may lead to the formation of asphalt mounds in many ocean margins around the world

Summary

Introduction

Release of hydrocarbons from sediments is important in increasing habitat heterogeneity on deep ocean margins. The seepage of heavy hydrocarbons can lead to the formation of asphalt mounds, large structures of solidified asphalt on the seafloor distinct from irregular mats and pools of viscous tar described from coastal (Hornafius et al, 1999) and continental slope (MacDonald et al, 2003) oil seeps (MacDonald et al, 2004). Asphalt mounds appear to be an important habitat in the deep sea, providing a source of hard substratum that attracts an apparently large variety of epifauna It appears that areas of asphalt extrusion are often associated with chemosynthetic communities living on asphalt mounds or authigenic carbonates. Asphalt mounds are likely to be considerably more widespread than the current limited evidence suggest, in the vicinity of shallow salt structures, and may be important to consider in regional assessments of deep-water biodiversity

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