On the fate of plumes from deep-sea oil well blowouts, and their effects on sediment communities

Abstract

Little is known about the fate of subsurface hydrocarbon plumes from prolonged oil well blowouts, and their effects on deep-sea sediment communities. As deepwater drilling expands in the Faroe-Shetland Channel (FSC), oil well blowouts are a possibility, and the complex ocean circulation of this region presents challenges to understanding possible subsurface oil pathways. Here, questions on the fate and effects of subsurface oil plumes were addressed with experiments and modelling. Experiments were performed on uncontaminated deep-sea sediments from a site remote from the FSC. A three dimensional ocean general circulation model (GCM) was used with a Lagrangian particle tracking algorithm to study patterns of subsurface oil distribution from a release in the FSC for the period 1994 – 2009.Deep-sea sediment microcosms were incubated in the presence of water accommodated fraction of crude oil (WAF) at low to high (5 % to 50 % by water volume) treatment levels, with sediment community oxygen consumption (SCOC) measured and compared to controls. Sediment macrofauna and bacteria were studied using measures of biomass. The microbial community was further studied using relative abundance and stable carbon isotope ratios (?13C values) of phospholipid fatty acids (PLFAs).Oxygen demand increased significantly at higher treatment levels (25% to 50% WAF). Treatment effects on macrofauna biomass were non-significant. However, there was significant WAF-driven reduction in bacteria biomass, and clear changes in microbial community composition. There were non-significant shifts in the ?13C values of PLFAs.Release depth of modelled subsurface plumes affected both the direction and extent of their transport and there was both seasonal and interannual variability. Plumes that were advected into overflow water were carried the furthest (>2500 km in June 2008); westwards through the Faroe Bank Channel towards the Iceland basin, reaching as far as southern Greenland, the Labrador Sea and the Northwest Atlantic. An index of temperature-controlled oxygen consumption rate (simulating oil respiration) indicated that large areas of the subsurface ocean (>50,000 km2 in 300 m depth water after one month) would be impacted by increased oxygen consumption in the event of a prolonged oil well blowout and the seafloor footprint beneath the plumes could be up to 100,000 km2. Quantifying impacts of any potential future spill in the FSC will benefit from these investigations on the distribution and effects of subsurface oil in this region