Amount and Fate of Gas and Oil Discharged at 3400 m Water Depth From a Natural Seep Site in the Southern Gulf of Mexico

Miriam Römer,Gerhard Bohrmann,Susan Mau,Christian dos Santos Ferreira,Heiko Sahling,Markus Loher,Thomas Pape,Ian R. MacDonald,Chieh-Wei Hsu

doi:10.3389/fmars.2019.00700

Miriam Römer, Gerhard Bohrmann + Show 7 more

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https://doi.org/10.3389/fmars.2019.00700

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Abstract

This multi-disciplinary study of the hydrocarbon seepage system at Tsanyao Yang Knoll in the southern Gulf of Mexico illustrates the amount and fate of hydrocarbons (mainly oil and methane) emanating from the seafloor structure and rising through a 3400 m water column. Tsanyao Yang Knoll forms part of the Campeche Knolls and was found to be one of the most active seepage structures at such an exceptional depth. Combining ship-based and AUV-based hydroacoustic mapping with direct seafloor observations and investigations, which used a TV-sled and a ROV with gas and water sampling devices provided an integrated view for the various transport pathways of hydrocarbons from the seafloor to the sea surface. In total, 32 acoustic ‘flares’, indicative of gas bubble emission sites, were detected emanating from depressions on top of the knoll. Most of the emission sites were concentrated in two depressions that comprised a main seep field. An estimated volume of 1359–4900 litres of hydrocarbons per hour are released in the form of gas bubbles, which dissolve almost entirely during their rise in the water column. However, echograms showed gas anomalies to about 500 meters below sea surface and some bubbles were seen to burst at the sea surface. Ex situ concentrations of dissolved methane were highly elevated (~30 000 nmol/L) directly above the seafloor emission site, but decreased to background concentrations (3–5 nmol/L) within the lowermost 100 m. Smaller quantities of oil also escaped from the seafloor, rose to the sea surface and generated natural oil slicks visible from the ship and in satellite images. This study shows that hydrocarbon seepage at ~3400 m water depth can be followed to the sea surface. However, most of the methane dissolves in deeper waters, whereas oil reaches the sea surface.

Highlights

Release of light hydrocarbons and oil from the seafloor into the water column are common processes in various geological settings along the world’s ocean margins
Tsanyao Yang Knoll (TYK) is located in the northernmost part of the Campeche Knoll area and emerges from a relatively flat seabed at 3650 to 3700 m below sea level (Figure 2A)
In the entire Campeche Knolls area, most of the seafloor elevations are situated along ridges that typically trend from SW to NE (Figure 2A)

Summary

Introduction

Release of light hydrocarbons and oil from the seafloor into the water column are common processes in various geological settings along the world’s ocean margins. Numerous seep sites have been discovered and investigated, so far the variations in seepage activities, individual fluxes of light hydrocarbons and oil as well as their specific fate is still poorly explored. This holds true for deep-water seep sites. Major factors controlling the fate of bubbles in the water column are water depth of the emission site, initial bubble size, presence of surfactants, as well as temperature, salinity and dissolved methane concentrations of the ambient water (Leifer and Patro, 2002) and gas composition (McGinnis et al, 2011). The presence or absence of oil can significantly increase the bubble lifetime, because oil coating limits gas exchange (Leifer and MacDonald, 2003; Solomon et al, 2009)

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