Deflection of natural oil droplets through the water column in deep-water environments: The case of the Lower Congo Basin

Abstract

A B S T R A C TNumerous recurrent seep sites were identified in the deep-water environment of the Lower Congo Basin from the analysis of an extensive dataset of satellite-based synthetic-aperture radar images. The integration of current data was used to link natural oil slicks with active seep-related seafloor features. Acoustic Doppler current profiler measurements across the water column provided an efficient means to evaluate the horizontal deflection of oil droplets rising through the water column. Eulerian propagation model based on a range of potential ascension velocities helped to approximate the path for rising oil plume through the water column using two complementary methods. The first method consisted in simulating the reversed trajectory of oil droplets between sea-surface oil slick locations observed during current measurements and seep-related seafloor features while considering a range of ascension velocities. The second method compared the spatial spreading of natural oil slicks from 21 years of satellite monitoring observations for water depths ranging from 1200 to 2700 m against the modeled deflections during the current measurement period. The mapped oil slick origins are restricted to a 2.5 km radius circle from associated seep-related seafloor features. The two methods converge towards a range of ascension velocities for oil droplets through the water column, estimated between 3 and 8 cm s −1 . The low deflection values validate that the sub-vertical projection of the average surface area of oil slicks at the sea surface can be used to identify the origin of expelled hydrocarbon from the seafloor, which expresses as specific seafloor disturbances (i.e. pockmarks or mounds) known to expel fluids. standing the hydrocarbon plumbing system (Serié et al., 2016). Pockmarks, which are local depressions on the seafloor associated with contemporary focused fluid flow (e.g. King and MacLean, 1970;Hovland and Judd, 1988), are widely recognized as the seafloor outlet of expelled fluids. Even if hydrocarbon migration across the water

Middle to late Pleistocene palaeoceanography inferred from ridge-furrow structures on the continental slope offshore Angola

“…Recent measurements and numerical model observations show that in general the bottom-current regime in the regional study area are shaped by longshore currents that periodically shift between southward and northward flow directions across biweekly, seasonal, and interannual timeframes (Arhan et al, 2003;Guiavarc'h, 2008Guiavarc'h, , 2009Jatiault et al, 2018;Stramma et al, 2003;Vangrieshiem et al, 2005). The biweekly shifting is influenced by the propagating deep Yanai waves formed by the remote equatorial winds (Guiavarc'h, 2008(Guiavarc'h, , 2009Vangrieshiem et al, 2005).…”

“…The SW that is carried by the AC is characterised as a fast, narrow, stable, warm, and saline water mass that occupies a water depth range of 0-300 m (Table .3) (Jatiault et al, 2017(Jatiault et al, , 2018Hardman-Mountford et al, 2003). While the SW carried by the BCC is characterised as cold with a lower salinity but with a similar velocity to the AC (Table .3) (Jatiault et al, 2017(Jatiault et al, , 2018Hopkins et al, 2013). The SW carried by the BCC is commonly located on the surface or beneath water carried by the AC (Jatiault et al, 2017(Jatiault et al, , 2018.…”

“…While the SW carried by the BCC is characterised as cold with a lower salinity but with a similar velocity to the AC (Table .3) (Jatiault et al, 2017(Jatiault et al, , 2018Hopkins et al, 2013). The SW carried by the BCC is commonly located on the surface or beneath water carried by the AC (Jatiault et al, 2017(Jatiault et al, , 2018. The Congo River discharge has a similar velocity magnitude, but warmer and less saline water than the SW, while it disperses in a basinward direction in water depths 0-20 m (Jatiult et al, 2017(Jatiult et al, , 2018.…”

“…2, Table .3) (Arhan et al, 2003;Stramma and Schott, 1999;Jatiault et al, 2017). The AAIW and uCPW are circulated southward by the South Intermediate Counter Current (SICC) (Stramma and England, 1999;Stramma and Schott, 1999), while the NADW is circulated southward by the Deep Western Boundary Current (DWBC) (Jatiault et al, 2018;Garzoli et al, 2015). Those circulating water masses are characterised as sluggish and cold with a maximum velocity magnitude <20 cm/s (Table .3) (Jatiault et al 2017(Jatiault et al , 2018.…”

“…The seasonal and interannual variations are influenced by variations in solar radiation, Atlantic El Niño conditions and anthropogenic aerosol activity (Svendsen et al, 2014;Tokinaga and Xie 2011;Xie and Carton, 2004). Despite shifting directions, the bottom-current regime in the region is generally recognised to flow along the margin towards the south/southeast and is guided by the seafloor topography (Arhan et al, 2003;Garzoli et al, 2015;Jatiault et al, 2018). The domination of the southeastward flow is from the AAIW and NADW components of the thermohaline circulation driven by the SICC (Fig.…”