Geochemistry, Geophysics, Geosystems
  • Open Access

Natural hydrocarbon seepage on the continental slope to the east of Mississippi Canyon in the northern Gulf of Mexico

Authors


Corresponding author: A. R. Talukder, CSIRO Earth Science and Resource Engineering (CESRE), 26 Dick Perry Avenue, Kensington, Western Australia 6009, Australia. (asrar.talukder@csiro.au)

Abstract

[1] From 5 June to 15 September 2010, a multidisciplinary marine survey was undertaken onboard the M/V Ryan Chouest in the region of the BP Deepwater Horizon incident site in the Gulf of Mexico. The primary objective of the survey was the continuous monitoring of hydrocarbon abundance from sea surface down to a maximum depth of 120 m. Compound abundances were inferred using a hydrocarbon sensor array with associated vertical cast system. In order to better understand the potential inputs from natural seepage in the vicinity of the spill, a Simrad EK60 high-resolution split beam echo sounder, operated at 38 kHz, was included in the survey between 7 July and 15 September 2010. During this period, three fields of natural seeps characterized by hydroacoustic flares were studied in detail. These seep fields are at water depths of approximately 430 m, 880 m, and 1370 m. They are associated with extensive cold seep systems. In particular, the area around Seep Field 1 (the vicinity of Deepwater Horizon) seems to present a vast area of active natural seepages in the Gulf of Mexico. The repeat surveys at two of the fields suggested that the cold seep systems here were active, with expulsions of hydrocarbons into the water column, at least during the periods of our acoustic surveys. Multiple lines of evidence gathered during the survey indicated that the observed hydroacoustic flares at the three fields identified consisted of oily bubble streams of gases of thermogenic origin. However, direct observation and sampling are required to reveal the precise nature of the flares. In the deep water Gulf of Mexico, the formation of a hydrate rim around bubbles seems to be a very important mechanism for the long transport of methane and oil in the water column.

1 Introduction

[2] Submarine natural seepage is a major source of geological emissions of methane and crude oil entering the marine environment [Kvenvolden and Cooper, 2003; Kvenvolden and Rogers, 2005]. However, it is difficult to estimate total submarine hydrocarbon emissions because natural seep systems are highly transient in space and time [Greinert et al., 2006; Leifer et al., 2004; Tryon et al., 1999]. Repeat offshore surveys reveal that vigorous fluxes of free gas bubbling can be intermittent over periods as short as 2 or 3 h [Heeschen et al., 2003]. Even repeat surveys over the same location give only snapshots of the seafloor at discrete times, and a full appreciation of the temporal variability can only be achieved with long-term continuous monitoring [Talukder, 2012]. A second source of uncertainty relates to the fate of the released hydrocarbons in the water column. At sites of natural seepage, dissolved methane is anaerobically oxidized and precipitated as authigenic carbonate [Boetius et al., 2000; Ritger et al., 1987]. Methane released as free gas escapes anaerobic oxidation of methane and enters the water column as a bubble stream capable of transporting methane to the surface [Talukder, 2012 and references therein]. Recent literature provides contradictory information about the proportion of methane from seepage at water depths greater than 200 m that reaches the atmosphere via the sea surface [Hu et al., 2012; Schmale et al., 2005; Schubert et al., 2006; Solomon et al., 2009]. On the other hand, oil slicks on the sea surface originating from natural seepage show clearly that liquid hydrocarbon components released from natural seepage even at water depths greater than 1000 m reach the atmosphere via the sea surface [Garcia-Pineda et al., 2010; Leifer and MacDonald, 2003; MacDonald et al., 1993]. MacDonald et al. [2002] show that the change in relative abundance of n-alkanes of varying carbon chain lengths in floating oil slicks is very rapid: the modal peak in normal alkanes shifted from nC17 to nC23 within 12 min of oil surfacing. One kilometer away from the surfacing point, the slick contained barely detectable traces of oil. Nevertheless, the slick was still visible in the remote-sensing images over distances of 10 km [MacDonald et al., 2002]. Therefore, any volumetric estimate based on slick dimensions using remote-sensing imagery would be an overestimation [Kvenvolden and Cooper, 2003]. Real data from oil and gas seepages from water depths more than 1000 m are limited to studies of gas exchange occurring between bubbles and the surrounding water during their long transport through the water column, as well as bubble transport mechanisms from deep water and the formation of a gas hydrate skin and/or thin oil layer around the bubble [e.g., Greinert et al., 2006; Leifer and MacDonald, 2003].

[3] After the BP Deepwater Horizon incident, a marine survey was conducted by Commonwealth Scientific and Industrial Research Organisation (CSIRO) scientists over a period of 4 months (from 5 June to 15 September 2010) onboard the M/V Ryan Chouest. Initially, the main focus of this multidisciplinary survey was the continuous monitoring of hydrocarbon concentrations at the sea surface using a novel hydrocarbon sensor array. The continuous hydrocarbon monitoring was accompanied by collection of water/slick samples of observed sea surface anomalies for geochemical analysis. It has been well established that the Gulf of Mexico exhibits high levels of active, natural oil and gas seepage [e.g., Anderson et al., 1983; Brooks et al., 1984; Kennicutt II et al., 1988]. Therefore, from 7 July onward, a Simrad EK60 high-resolution split beam echo sounder, operating at 38 kHz, was added to the onboard instrumentation to establish the presence or absence of any natural seepage in the water column and to better understand the natural seepage activity around the incident site. During the survey, many interpreted instances of natural seepage with hydroacoustic flares were observed from water depths as shallow as 20 m down to depths of 1400 m. Three seep fields were studied in detail on the continental slope east of Mississippi Canyon at water depths between approximately 430 m (MC110), 880 m (MC118), and 1370 m (MC294) (Figure 1). The aim of this work was

  1. [4] First, to determine the nature (gas and/or oil) and origin (source: biogenic versus thermogenic) of the observed seepages, temporal variability of their activity, and the evolution of the released hydrocarbons during their long transport through the water column from the seabed to the sea surface.

  2. [5] Second, to constrain the mechanisms for the long transport of the released oil and gas through water column occurring at the seep fields in the deep water of the Gulf of Mexico.

Figure 1.

Bathymetric map of the Gulf of Mexico showing the locations of three seep fields to the east of Mississippi Canyon. The 3-D seismic attribute map is obtained from BOEMRE webpage at http://www.boemre.gov/offshore/mapping/SeismicWaterBottomAnomalies.htm. SAR data have been collected by Fugro NPA during the period of 29 August to 21 October 2010.

2 Geological Context, Natural Seeps, and Gas Hydrates in the Gulf of Mexico

[6] The study area is located in the eastern part of the Mississippi Canyon on the continental slope between water depths of 400 m and 2000 m (Figure 1). The seabed morphology is characterized by a relatively smooth upper slope and very complex lower slope with submarine canyons, slide scarps, ridges, and salt domes. The Deepwater Horizon incident site is located at a water depth of 1500 m in the intraslope/intrasalt basin, surrounded by the Biloxi Dome to the southwest, Gloria Dome to the southeast, Mitchell Dome to the northeast, and Whiting Dome to the north (Figure 1). High sediment loading and salt remobilization are the key processes driving both the morpho– dynamics and the cold seep systems of the margin [Chen et al., 2007; McGee and Macelloni, 2006; Mello and Karner, 1996; Roberts et al., 2010]. Due to the very high sediment loading supplied by the Mississippi River, the thick layer of synrift salt covered by post-rift shale and limestone started to deform from the Cretaceous onward [Lawton et al., 2001; Weimer et al., 1998]. The mobilization of salt-induced faulting and gravity slope failure resulted in complex seabed morphology: intraslope/intrasalt basins, submarine canyons (e.g., Green Canyon, Mississippi Canyon), slide scarps, and ridge features along the margins [Cooper and Hart, 2003; McAdoo et al., 2000; Seni, 1992]. High sedimentation rates also created overpressured hydrocarbon reservoirs which apparently leak, bleeding off fluids upward and providing the temporary accumulations that supply natural seepage [Mello and Karner, 1996; Roberts et al., 2010; Sager et al., 2003; Seldon and Flemings, 2005].

[7] In the Gulf of Mexico, natural seepage occurs often at the edges of salt diapirs where overpressure and buoyancy forces drive entrapped fluids and gases vertically to the seabed through faults and fractures [e.g., Roberts et al., 2010 and references therein]. Cold seep features on the seabed include mud volcanoes, pockmarks, brine pools, authigenic carbonate and barite deposits, widespread occurrences of gas hydrates, chemosynthetic communities, and hydrocarbon (hydroacoustic) flares [Chen et al., 2007; De Beukelaer et al., 2003; MacDonald et al., 2003; MacDonald and Peccini, 2009; Prior et al., 1989; Roberts et al., 2010]. Widespread occurrences of oil slicks on the sea surface have also been reported as a consequence of natural seepage [De Beukelaer et al., 2003; Garcia-Pineda et al., 2010; MacDonald et al., 1993; MacDonald et al., 2002]. Hydrocarbons released as a result of natural seepage can include biogenic gas, thermogenic gas, or oil [De Beukelaer et al., 2003; Kennicutt II et al., 1988; Lorenson et al., 2008; Sassen et al., 2003]. Both bacterial (structure I) and thermogenic (structure II) gas hydrates occur in the Gulf of Mexico [Boswell et al., 2012; Brooks et al., 1984; Milkov and Sassen, 2003]. According to thermodynamic modeling, the minimum calculated water depths at which structure I and structure II hydrates would be stable are 615 m and 300–400 m, respectively [Cooper and Hart, 2003; Milkov et al., 2000]. However, the minimum depth at which gas hydrates have been sampled to date is 420 m [Milkov and Sassen, 2000]. Although the cold seep systems in the northern Gulf of Mexico are some of the best studied in the world, most of the work, especially that related to the evolution of released hydrocarbons in the water column, has been carried out in the area of Green Canyon, to the west of the Mississippi Canyon, at water depths of 550–650 m [De Beukelaer et al., 2003; Feng and Roberts, 2011; Leifer and Judd, 2002; Leifer and MacDonald, 2003; Leifer et al., 2004; MacDonald et al., 2000; MacDonald et al., 2002; Sassen et al., 2003; Solomon et al., 2009]. To the east of the Mississippi Canyon, studies have mainly focused around block MC118, in the region of our Seep Field 2 [Hu et al., 2012; Ingram et al., 2010; Sassen et al., 2006; Sleeper et al., 2006]. Little has been published about the area near the Deepwater Horizon incident site, where our Seep Field 1 is located. However, a detailed bathymetric map of numerous salt diapirs adjacent to Biloxi Dome (Figure 1) suggests that cold seep systems with active expulsion of oil and gas into the water column could be extensive.

3 Methods Employed in the Discrimination of Potential Seep Fields

[8] During the survey, once an acoustic flare had been identified on the echo sounder profile, the survey track was redesigned to form a survey box with sufficient overlap of track lines to ensure full coverage of the seafloor in and around the seep fields (Figure 1). For example, Seep Field 1 lies at a water depth of approximately 1370 m. The EK60 echo sounder operating at 38 kHz has a beam angle of 7°. At a depth of 1400 m, the beam footprint is approximately 172 m wide (tan 3.5° × water depth × 2). As such, survey lines spaced approximately 152 m apart were planned to allow enough overlap to fully survey the seabed. Often flares were tilted in the direction of currents, making it difficult to capture the whole flare in a 2-D echo sounder profile. Therefore, the survey track was also redesigned as a clover leaf over the seep field in order to determine the maximum height of the rising flares (Figure 1). In total, 11,045 km (5967 nm) of echo sounder profiles were completed during the survey. Acoustic data interpretation was based on several criteria: (i) overall geometry of the hydroacoustic flare (ratio between height and width) and the consistency of the flare in the acoustic profiles with different directions and times, (ii) mean volume backscatter (MVB) values, and (iii) where possible, calculated bubble-rising velocity [Greinert et al., 2006].

[9] In addition to the acoustic survey, profiles of inferred dissolved hydrocarbon concentrations in surface waters (1–2 m) were logged every 2 s using a CSIRO-developed hydrocarbon sensor array (Appendix A). The results presented in this paper were obtained using one of the sensors in the array: a Chelsea Technologies Aquatracka. The Aquatracka is a commercially available oceanographic fluorometer tuned for the detection of polyaromatic hydrocarbons (PAHs). Surface observations and photography were also undertaken, and samples of water or surface slick (where present) were collected in areas of observed surface anomalies and/or enhanced sensor response. Water and slick samples were analyzed onboard the vessel for organic compounds by gas chromatography-mass spectrometry (GC-MS), after first extracting the samples with dichloromethane.

[10] In order to understand the extent of the cold seep systems associated with the observed seep fields, we have correlated our data set with the 3-D seismic amplitude anomaly maps of the water bottom horizon published by geoscientists at the Bureau of Ocean Energy and Management (BOEM) for the northern Gulf of Mexico. This is available through their webpage (http://www.boem.gov/Oil-and-Gas-Energy-Program/Mapping-and-Data/Map-Gallery/Seismic-Water-Bottom-Anomalies-Map-Gallery.aspx). Over 400 of the amplitude anomalies have been confirmed as natural seeps by direct observations (by submersible, autonomous underwater vehicle, remotely operated vehicle, and underwater camera) and seabed sediment sampling (see URL link mentioned above). We have also correlated our data set with the Synthetic Aperture Radar (SAR) data set collected by Fugro NPA during the period of 29 August to 21 October 2010 to map the oil slicks associated with the interpreted seep fields. The data were acquired by ERS-1 and ERS-2 satellite SAR with coverage ~10,500 km2 each scene. The SAR data were processed and geo-referenced at Fugro NPA using proprietary software, and interpretation of the SAR images was carried out at full resolution (approximately 20 m) [Lawrence, 2011].

4 Description and Characterization of the Observed Seeps

4.1 Field 1 (MC294)

[11] The first interpreted seep field is located approximately 12 km southwest of the incident site in offshore block MC294 at a water depth of 1370 m (Figure 1). This field is characterized by a hydroacoustic flare rising from the seabed up to a water depth of 730 m (Figures 2a and 2b). Thus, the whole flare lies in the gas hydrate stability field. This flare was imaged twice during the survey between 26 and 27 July 2010: it was imaged the first time in a profile crossing from a northwest to southeast direction (Figure 2a) and the second time, approximately 1 h later, from a southeast to northwest direction (Figure 2b). The MVB of the flare near the seabed was −35 dB, which gradually decreased toward the top of the flare (at 710 m water depth) to a value of −52 dB. Structurally the flare is associated with the Biloxi Dome (Figure 2c). The dome is (approximately 12 km × 6 km) elongated in a northwest-southeast direction and has a complex morphology with fault traces, peripheral synclines found along the southwest and northwest sides, and scarps created by mass wasting. The flare is located in the syncline at the southwest edge of the dome (Figure 2c).

Figure 2.

(a and b) Images of hydroacoustic plumes obtained by Simrad EK60 split beam echo sound systems operated at 38 kHz. (c) Detailed bathymetric map of Field 1 (Biloxi Dome).

4.2 Field 2 (MC118)

[12] This field, located approximately 18 km northwest of the incident site in offshore block MC118 at a water depth of 880 m (Figure 1), was visited three times: on 28 July, 4 August, and 7 September 2010 (Figure 3). On the first visit, the hydroacoustic flare was imaged as a midwater contact from 776 m (107 m above seabed) rising up to a water depth of 400 m (Figure 3a). Though it is a midwater contact, the narrow subvertical geometry, as well as observations of the flare at different times and from different directions during subsequent surveys, suggest that it's a seep flare. On the second visit, in order to image the whole flare, the survey track was designed as a clover leaf over the seep field, and the flare was traversed five times, each time approaching the emanation point from a different direction. The root of the seep on the seabed was imaged on the first attempt, but the maximum penetration into the water column appeared to be truncated (Figure 3b). The MVB value near the seabed was −38 dB. On the third visit, the clover leaf survey pattern detected the flare five times but again failed to image the whole flare (Figure 3c).

Figure 3.

(a–c) Images of hydroacoustic flares obtained by Simrad EK60 split beam echo sound systems operated at 38 kHz on 28 July, 4 August, and 7 September 2010, respectively. (d) Detailed bathymetric map of Field 2.

4.3 Field 3 (MC110)

[13] This field is located 58 km west-northwest of the incident site in offshore block MC110 at a water depth of approximately 430 m (Figure 1). The hydroacoustic flare rises up to a water depth of 160 m (Figures 4a–4c). It appears to originate from a seabed scarp located on the upper slope (Figure 4d). This flare was imaged on three occasions: on 28 July, 4 August, and 6 September 2010. On the first visit, a complex flare was imaged on a profile tracking from east to west, and the maximum MVB value of −24 dB was observed near the seabed (Figure 4a). The MVB values decreased gradually to −64 dB at the top of the flare at a water depth of 170 m. On the second visit, the flare was again imaged, this time on a profile tracking from west to east (Figure 4b). The maximum MVB value near the seabed was −18 dB. This time, the flare was inclined upslope (to the west) to a water depth of 250 m. The flare then appeared to be pushed back to normal, or downslope (to the east), indicating the possible layering of two currents of opposing direction (Figure 4b). On the third visit, a clover leaf survey pattern was performed at very slow velocity. The vessel was also allowed to drift over the field. The echo sounder image obtained during the drifting allowed the calculation of the bubble-rising velocity near the seabed (Figure 4c) by the method described in Greinert et al. [2006]. The calculated rising velocity near the seabed was 0.15–0.20 m/s. The maximum MVB value observed near the seabed was −30 dB.

Figure 4.

(a–c) Images of hydroacoustic flares obtained by Simrad EK60 split beam echo sound systems operated at 38 kHz on 28 July, 4 August, and 6 September 2010, respectively. (d) Detailed bathymetric map of Field 3.

5 Correlation of Observed Seeps with Hydrocarbon Sensor Data, Geochemical Data, and Surface Observations

5.1 Field 1 (MC294)

[14] On 26 and 27 July 2010 (12 days after the MC252 well was sealed), the overall Chelsea Aquatracka fluorometry measurement values were low in the survey area, with the responses observed being equivalent to those obtained with calibration solutions of polyaromatic hydrocarbons in a concentration range of 0.01–0.4 ppb (Figure 5). The maximum sensor responses, equivalent to 0.4 ppb of dissolved polyaromatic hydrocarbons, were observed in a well-defined area 1–3 km southwest of the acoustic flare. Five water samples were taken over the period, from water depths of approximately 1 m (Figure 5). Four of these samples were taken to the southwest and one to the northwest of the seep location. All samples were taken in response to elevated sensor responses, with GC-MS analyses of the samples indicating that they all contained trace levels of hydrocarbons. However, GC-MS analysis of one water sample taken from 1 m water depth to the southwest of the seep location on 27 July (latitude 28.661393°, longitude −88.475237°) revealed aliphatic hydrocarbon compound concentrations of 4 ppb, which include a homologous series of normal alkanes from nC17 to nC31. In addition, this sample contains very low levels of xylenes, toluene, and trimethyl-benzenes (Figure 6a) indicative of a potential thermogenic origin with a proximal source (as the low molecular weight aromatic compounds would be rapidly lost through evaporation). The isoprenoid pristane/phytane source ratio of 0.5 is within the range for oils sourced in the area. No surface sheens or oil were observed in this location. The echo sounder data suggest that the possible seep flare identified may curve toward the sea surface area of enhanced sensor response, although further definition of the fluorescent feature would be required to provide additional evidence that the two features are related.

Figure 5.

Sensor response plotted with position at Field 1, where the amplitude of the sensor response is expressed as equivalent total polyaromatic hydrocarbon (PAH) concentration. Equivalent PAH concentration was obtained by laboratory calibration of the sensor against solutions of MC252 source oil/water extracts, where the total PAH of the water extract had been quantified by GC-MS for 16 PAHs. Low sensor responses were recorded overall in this area with slightly elevated responses to the south/southwest and northwest of the seep field.

Figure 6.

Geochemistry of solvent-extracted seawater samples collected in the vicinity of natural seepage at Seep Fields 1, 2, and 3. Large ion chromatograms show n-alkane distribution using m/z = 57 ion, and smaller inset ion chromatograms show distribution of ethylbenzenes and xylenes based on m/z = 106 ion. Pie charts detail total quantified hydrocarbon abundance, plus relative proportion of aliphatic and aromatic components. (a) Sample collected at 1 m water depth to the SW of Seep Field 1 on 27 July 2010 (latitude 28.661393°, longitude −88.475237°) showing predominance of aliphatic over aromatic components. m/z = 57 ion chromatogram to demonstrate homologous series of n-alkanes from nC17 to nC31. m/z = 106 shows very low levels of xylenes. (b) Geochemistry of water sample taken at 1 m water depth within the area of elevated sensor response at Seep Field 2 on 4 August (latitude 28.87643°, longitude −88.50789°) showing only trace amounts of measured hydrocarbons, with values less than 1 ppb. (c) Sample collected from surface waters on 4 August (latitude 28.853280°, longitude −88.825563°) showing a high relative proportion of aliphatics, with m/z = 57 ion chromatogram illustrating a complex distribution of n-alkanes, high relative abundances of nC15 and nC17, and a pronounced, later eluting, unresolved complex mixture (UCM). (d) Homologous series of compounds visible in m/z = 57 ion chromatogram, suggesting a possible thermogenic origin from this surface sample collected on 5 August (latitude 28.859788°, longitude −88.736393). Low levels of xylenes in m/z = 106 ion chromatogram (low signal to noise). (e) m/z = 57 ion chromatogram shows this surface sample (collected at the same location and on the same date as Figure 6d) exhibits a less pronounced series of n-alkanes compared to Figure 6d; however, m/z = 106 shows presence of xylenes, suggesting the source of hydrocarbons is within close proximity.

5.2 Field 2 (MC118)

[15] The survey grid was approximately 9.5 km × 6 km trending north-northwest to south-southeast in water depths ranging from 700 m to 1200 m (Figure 1). Overall Chelsea Aquatracka measurements indicated baseline levels of inferred hydrocarbons. A relative rescaling (using corresponding highest and lowest sensor voltage values within the area surveyed) of the track traveled shows a greater variation within the data, especially in the northeast-southwest trending tracks, possibly due to a shift in currents. The water depth at this field is deep (700 m to 1200 m), therefore, any expression of hydrocarbons on the sea surface may be offset, by some distance, relative to the possible seep origin.

[16] Between 2 and 4 August 2010, the surface waters adjacent to, and over, Seep Field 2 were surveyed using a rectangular survey box trending north-northwest to south-southeast (9.5 km long by approximately 6 km wide) (Figure 7). Over the period of the grid survey, the overall Chelsea Aquatracka measurements were low: 0.1–0.7 ppb PAH equivalent. The time taken to perform the north-northwest to south-southeast survey box was greater than 24 h. There was a distinct elevated response between 2.3 and 4 km to the north-northwest of the observed acoustic flare, equivalent to between 0.5 and 0.6 ppb PAH (Figure 7). However, GC-MS analysis of the water sample taken at 1 m water depth within the area of elevated response (latitude 28.87643°, longitude −88.50789°) shows only trace amounts of total measured hydrocarbons, with values of less than 1 ppb (Figure 6b). Once west-east trending survey tracks were commenced, the Chelsea Aquatracka showed an overall increase in response to this level, which could be due to changing current directions or other, presently unknown, factors. Therefore, the elevated response in the north-northwest trending track cannot be singularly assigned to the acoustic flare. It is possible that the bulk of the leaking hydrocarbons from MC118 are gaseous. The only observed surface features in the area were convergence lines. No surface slicks were observed in the survey box, although a satellite “hit” indicating an oil slick occurred after the survey had ended in this area (Figure 1).

Figure 7.

Sensor response plotted with position at Field 2, where the amplitude of the sensor response is expressed as equivalent detected total polyaromatic hydrocarbon (PAH) concentration. Increased responses observed on the north-south survey lines cannot be assigned singularly to the acoustic flare due to increases that were also observed when the vessel began west-east survey lines.

5.3 Field 3 (MC110)

[17] The majority of time spent in the vicinity of Field 3 was between 4 and 6 August 2010. The predominant current direction in this area at the time of the survey was to the east-northeast up to approximately 2 knots (based on vessel drift speed measured by GPS). The area is close to the outfall of the south pass of the Mississippi River, and there is significant observed mixing between distinct water masses in the area. The survey grid was approximately 4 × 5 nautical miles trending north-south in water depths ranging from 250 to 550 m (Figure 8). The Chelsea Aquatracka measurements show low levels of inferred polyaromatic hydrocarbons (0.02 to 1.00 ppb). A relative rescaling of the Chelsea data (using corresponding highest and lowest sensor voltage values within the area surveyed) shows a broad increase in responses in the southern half of the survey box (Figure 8). There are distinct localized areas of enhanced response in the northern part of the grid and also on the survey tracks entering the grid from the east. Differences between the north-south track lines and the east-west track lines could possibly be due to the variable high current regime observed over the 3 day period in the area. The most significant Aquatracka measurements (inferred concentrations from 0.50 to 1.00 ppb) at this site were a well-defined zone of enhanced responses to the east-northeast of the seep sites (Figure 8).

Figure 8.

Sensor response plotted with position at Field 3, where the amplitude of the sensor response is expressed as equivalent detected total polyaromatic hydrocarbon (PAH) concentration. Areas of increased response were recorded to the south, north, and on west-bound survey lines entering the survey box from the east, in the vicinity of two seep fields. Samples containing homologous series of compounds and volatile xylenes (sample location SS-D3: also see Figures 6d and 6e) and aliphatics (sample location SS-D2: also see Figure 6c) were collected in this area.

[18] Eight valid samples were taken in the survey box and along the vessel track, with six of the samples collected from surface waters. These surface samples were taken in response to observed anomalies, such as particles on the water surface, land-derived material, foam, rainbow sheen, or these features in combination. The concentration of measured hydrocarbons in surface water samples ranged from approximately 1.9 to 33.5 ppb, with the majority of samples containing low levels of aliphatic, aromatic, and benzene, toluene, ethylbenzene, and xylene compounds. Comparison of the geochemistry of the samples shows that the samples exhibit distinct differences in the compounds that are present and the abundance of these compounds, for example, the relative abundance of aliphatic and aromatic components (Figures 6c–6e, pie charts), and the abundance and distribution of the n-alkanes (Figures 6c–6e, m/z = 57 ion traces). The samples with distinct homologous series of hydrocarbons were collected on the survey tracks entering the grid from the east.

[19] Geochemical results from the sample shown in Figure 6c show a high relative proportion of aliphatics. The n-alkane (m/z 57 ion) profile shows a complex distribution and a pronounced, later eluting, unresolved complex mixture (UCM), which together with the visual observations of the sample suggest that the material may not be locally derived and has been subjected to biodegradation.

[20] The two samples shown in Figures 6d and 6e, respectively, collected to the east of the survey grid have similar geochemical signatures to each other; however, the compounds differ in abundance (probably due to the level of success of sampling the surface interface). These samples were taken after the observation of rainbow sheen on the surface. Figure 6d shows that this sample contains a homologous series of compounds suggesting a possible thermogenic origin. The sample illustrated in Figure 6e does not have a pronounced homologous series of n-alkanes but does contain xylenes: compounds which would be quickly removed by water washing or evaporation. This suggests that the source of this material may be close by. The location of these three samples (Figure 6c-6e) correlates well with enhanced sensor responses to the east of Field 3 survey areas (location SS-D2 and SS-D3 in Figure 8).

6 Discussion

[21] The footprints of the echo sounder systems on the seabed were approximately 170 m, 110 m, and 60 m for Fields 1, 2, and 3, respectively. The maximum resolution of 60 m obtained at Field 3 is still too low to distinguish discrete streams of bubbles and/or oil droplets on the seabed [De Beukelaer et al., 2003; MacDonald et al., 2002]. During our return visits, the locations of the hydroacoustic flares were usually slightly different. The difference in distance was in the range of the footprints on the seabed, which suggested they were probably in the same seep field. Nevertheless, it is possible that every seep field comprises multiple bubble streams, spaced over a distance less than the echo sounder's footprint on the seabed.

[22] The location of the seep fields indicates that they are structurally controlled. Field 1 is located at the edge of Biloxi Dome (Figure 2c). Seeps developing at the edge of salt domes are very common in the Gulf of Mexico [Roberts et al., 2010]. Interpreted 3-D seismic amplitude anomaly maps of the sea bottom horizon from our study area show that the edge of Biloxi Dome is characterized by larger patches of high-amplitude positive anomalies and smaller patches of negative anomalies (Figure 2c). Ground truth surveys have shown that high-amplitude positive anomalies generally correspond to carbonate deposits, gas hydrates, and/or chemosynthetic communities, while negative anomalies correspond to areas of gas escape through the seafloor, characterized by gassy soft sediments [Roberts et al., 2010]. Thus, the seismic amplitude anomaly maps suggest that Biloxi Dome has a prolonged history of natural seepage.

[23] Since 2004, offshore block MC118 has been the site for a gas hydrates seafloor observatory in the Gulf of Mexico [McGee and Macelloni, 2006]. As a result, very high resolution acoustic data sets are now published on the cold seeps and gas hydrate field in MC118 where our Seep Field 2 is located [Sassen et al., 2006; Sleeper et al., 2006]. It presents an excellent opportunity to characterize the cold seep systems associated with the observed hydroacoustic flares at Seep Field 2. Likewise, our data sets would also enable the construction of a time series of seep fluxes at the hydrate observatory. The cold seep systems at MC118 are characterized by deposits of authigenic carbonates and gas hydrates, chemosynthetic communities, pockmarks, and complex vent morphologies around 100 m in diameter [Sassen et al., 2006; Sleeper et al., 2006]. During their 2005 marine survey, C&C Technologies, Inc. discovered three active vents along with several inactive vents [Sassen et al., 2006]. The hydroacoustic flares of Field 2, found during our survey, seem to originate from the southwestern vent discovered by C&C Technologies (Figure 3d). Gas hydrates sampled from this vent were oil stained and consisted of structure II hydrates [Sassen et al., 2006]. Sub-bottom profiles across this vent show that it is associated with an acoustic wipe-out zone [Sleeper et al., 2006]. In the Gulf of Mexico, this type of acoustic blanking is interpreted as an accumulation of free gas [MacDonald et al., 2003]. This blanking can also be partially caused by the carbonate and/or gas hydrate deposits on the seabed, inhibiting seismic penetration. Oily bubble streams and associated oil slicks at the sea surface directly above the field have also been observed [Sassen et al., 2006]. Data obtained during successive surveys in the framework of the hydrate observatory show that Field 2 has developed as a result of deep thermogenic gas seepage and venting through faults and fractures induced by salt diapirs lying 300 m beneath the seafloor [Ingram et al., 2010; McGee and Macelloni, 2006; Sassen et al., 2006].

[24] In the absence of a detailed bathymetric map of Seep Field 3, the correlation with the 3-D seismic amplitude anomaly maps shows small patches of positive seismic anomalies approximately 300 m to the north of the seep field (Figure 4d) indicating that the cold seep system in Field 1 (Figure 2c) is more extensive than in Field 3. Our calculated bubble-rising velocities at Field 3 are in the range of 0.15–0.20 m/s. These rising velocities are in the typical range for rising dirty bubbles between 3 and 10 mm in diameter [Greinert et al., 2006; McGinnis et al., 2006]. Leifer and MacDonald [2003] measured bubble-rising velocities of three bubble streams in offshore blocks GC185 and GC294 in the Gulf of Mexico at water depths of 525–550 m, comparable to the water depth of Field 3. They found that rising velocities of 0.15–0.20 m/s correspond to oily gas bubbles of 4–8 mm in diameter.

[25] Flux rates at natural seep fields are highly variable over time [Greinert et al., 2006]. Vigorous fluxes of hydrocarbon can diminish over time scales of hours to minutes [Greinert, 2008; Heeschen et al., 2003; Schneider von Deimling et al., 2010]. The values of MVB can be used as a qualitative indicator of relative seep intensity [Greinert and Nuetzel, 2004]. The MVB value would depend on initial bubble size, density, presence or absence of surfactant, and rising velocity [McGinnis et al., 2006]. Bubbles are found to be bigger, denser, and faster in the center of the bubble streams compared to at their edges [MacDonald et al., 2003]. Thus, the MVB values corresponding to the maximum flux can only be obtained if we can image through the center of the flare. The overall Gaussian shape of the flare at Fields 1 and 3 (Figures 2 and 5) coupled with the consistent shapes during the surveys at different times and from different directions suggest that it is probably the cores of the flares at Fields 1 and 3 that were imaged. At Field 1, the second visit was only 1 h after the first visit. MVB values indicated that the seep intensity remained the same over this time period. At Field 3, MVB values were stronger during our second visit (7 days after the first visit), which suggests that there was increased seep flux at this time.

[26] Hydroacoustic flares can be composed of gas bubbles with or without oil and/or hydrate coating, oil droplets, hydrate particles, and/or lower density water bodies [Greinert et al., 2006]. Only sampling can reveal the exact nature of the flares at Fields 1, 2, and 3. Seismic amplitude anomaly maps and published literature show that all three fields are associated with cold seep features, which suggest that the hydroacoustic flares have developed as a result of active expulsion of gases and, potentially, oils, creating bubble streams in the water column. In the Gulf of Mexico, streams of both oily and non-oily gas bubbles have been found [De Beukelaer et al., 2003]. At each of the fields reported herein, although overall sensor readings were low, a relative scaling of these responses revealed areas of elevated sensor (fluorescence) values. At Field 1, water samples taken from the area of elevated sensor responses contain trace levels of hydrocarbons potentially indicating the presence of oil. One sample taken on 27 July to the southwest of the seep location reveals the presence of a homologous series of normal alkanes from nC17 to nC31 and very low levels of xylenes, toluene, and trimethyl-benzenes (Figure 6a). The presence of these aromatics of lower molecular weight characterized by short residence periods in the water column suggests that the source is likely to be local, possibly the seeps at Field 1. At Field 3, the predominant current direction during the second visit on 4 August, which was toward the east-northeast, suggests that the elevated sensor responses observed to the east-northeast of the seep location could be associated with the seeps of the field. Water samples collected from this area of enhanced responses contained n-alkanes and xylenes suggesting a possible thermogenic origin (Figures 6c–6e). The presence of volatile compounds such as xylene further indicates that it could be associated with the seep.

[27] Satellite remote-sensing technologies have been widely used in the Gulf of Mexico to detect oil slicks that form over natural seepages [Garcia-Pineda et al., 2010; MacDonald et al., 2002]. SAR (Synthetic Aperture Radar) imagery collected by Fugro NPA during the period of 29 August to 21 October 2010 shows oil slicks (Figures 1-3, and 5), which seem to be surfacing from Seep Fields 1 and 2. This indicates that the flares at these two fields could consist of oily bubbles. At Field 1, slicks present the distinctive shape of four parallel curvilinear lines, which could be the result of multiple discrete seeping points or pulses from the same seep repeating over short periods of time [Garcia-Pineda et al., 2010; MacDonald et al., 2002]. It should be noted that we detected the seep at Field 1 on 26–27 July, approximately 1 month before the SAR survey periods. SAR data correlate positively with the observations from successive sampling surveys in the framework of the hydrate observatory at Field 2 (MC118), which suggest that the hydrocarbon flare consists of an oily bubble stream of thermogenic origin.

[28] The observed oil slicks associated with Seep Fields 1 and 2, as well as the results from our chemical sampling, show that at least part of the released oil from the seeps is reaching the sea surface. MacDonald et al. [2002] suggest that the oil slicks that consistently form near seep fields are the result of bubble-mediated transport of oil to the surface. At the water depths of our seep fields in particular, the rise speed of oil droplets without associated gas bubbles would be too slow to reach the sea surface in the vicinity of the seeps and create oil slicks on the surface. One important limitation of our work is that we did not measure if there was any dissolved methane in sea surface waters. The echo sounder images from different directions and at different times show consistently that at Fields 1 and 2, hydroacoustic flares rise from the seabed at approximately 660 m and 400 m, respectively, and seem to disappear beneath the top edge of the hydrate stability field. The repeat and cloverleaf pattern of the surveys over the seep fields (Figures 2-4) suggest that the disappearance of the flare in the echo sound profiles is not the result of flares being in outer beam sections; rather it suggests the formation of a hydrate rim as the potential mechanism for the long transport of the bubbles. When bubbles are released deep inside the hydrate stability field, formation of gas hydrate rims occurs around them upon their emission into water [Riedel et al., 2002]. By substantially slowing the rate of gas transfer into surrounding waters, the hydrate rims extend the lifetime of bubbles during their long ascent through the water column [McGinnis et al., 2006; Rehder et al., 2002]. Near the top of the hydrate stability field, at the point where dissociation of hydrates occurs, the bubbles—which already have low gas content—shrink quickly [Rehder et al., 2002]. The fate of gas bubbles in the water column depends on initial bubble size, presence or absence of hydrate/oil coating, internal gas circulation, and rising velocity [McGinnis et al., 2006; Rehder et al., 2002] With a hydrate coating, an initial diameter of 6 mm would be sufficient to allow a bubble to rise up to 800 m [McGinnis et al., 2006]. Thus, with a hydrate coating, an initial bubble diameter of less than 6 mm would be enough for the observed hydroacoustic flares in Fields 1 and 2. The oily bubble streams would release the oil droplets upon their dissolution near the top of the hydrate stability field [Leifer and Judd, 2002]. The trajectory of these oils to the sea surface would depend on the prevailing current profiles in the upper layer of the water column. During transport to the surface from depths of 700 to 400 m (at Fields 1 and 2, respectively), oil droplets would undergo further dissolution and dispersion into the water column, particularly of the lighter volatile soluble components. The low concentration of hydrocarbons surfacing after this transportation process would explain the absence or very low sensor responses at Fields 1 and 2.

[29] Like Biloxi Dome, the other salt domes around the Deepwater Horizon area such as Gloria Dome, Dauphin Dome, Mobile Dome, and Mitchell Dome show similar morphological characteristics on the seabed (Figure 1). The edges of these other domes are also characterized by extensive seismic amplitude anomalies. Dauphin Dome in particular shows larger patches of negative anomalies indicating a high potential of active gas escape from soft gassy sediments. SAR images suggest that many of these domes are associated with oil seepage. Therefore, the area around Seep Field 1 seems to present a vast area of active natural seepage in the Gulf of Mexico about which very little has been published. Unlike other deep water (>1000 m) natural seep areas, for example the Black Sea [Greinert et al., 2006; Sahling et al., 2009], Barents Sea [Foucher et al., 2010], at the Hikurangi margin, offshore New Zealand [Greinert et al., 2009], or at the Makran margin, offshore Pakistan [Romer et al., 2012], where there exists only methane seepage, at Seep Field 1, there are both oil and gas seepage within the gas hydrate stability field. This may present an excellent opportunity to study the combination of both oil coating and hydrate rim formation during the long transport of gas bubbles through the water column. Finally, any realistic estimate of the methane or hydrocarbon budget from the seabed to the water column for the Gulf of Mexico requires more detailed study and further seep surveys in and around Seep Field 1.

7 Conclusion

  1. [30] In this paper, we report the natural seepage activities around the Deepwater Horizon incident site in the Gulf of Mexico from 7 July to 15 September 2010. Three seep fields characterized by hydroacoustic flares were studied in detail on the continental slope east of Mississippi Canyon at water depths of approximately 430 m, 880 m, and 1370 m. We interpret these hydroacoustic flares as hydrocarbon bubbles and droplets actively expelled from cold seep systems into the water column.

  2. [31] At Fields 2 and 3, the hydroacoustic flares were observed during all three surveys from 26 July to 9 September 2010, suggesting that the cold seep systems at these two fields were active with vigorous fluxes of hydrocarbons, at least during these periods.

  3. [32] Cold seep systems at all three fields are structurally controlled. At Fields 1 and 2, they are controlled by salt tectonics.

  4. [33] Results from the sensor surveys and water sampling carried out at the seep fields, combined with SAR imagery as well as published literature in the framework of the hydrate observatory at Field 2, suggest that the observed hydroacoustic flares at all three fields could consist of oily bubble streams of gases of thermogenic origin. Nonetheless, only direct observation and sampling can reveal the precise nature and origin of the flares.

  5. [34] At Fields 1 and 2, the disappearance of the seep flares near the top edge of hydrate stability field suggests that the formation of a hydrate rim around a bubble is potentially the main mechanism for the long transport of the gas bubble. The oily bubble streams, upon dissolution of the gas bubbles above the hydrate stability fields, would release the oil, which would undergo further dissolution and dispersion into the water column during their rise to the sea surface. The low concentration of hydrocarbons resulting from this transportation process would explain the absence of, or very low, sensor responses at Fields 1 and 2.

Appendix A: Description of Sensor Calibration

[35] The sensor results used in this paper were obtained using UV Aquatracka (Chelsea Technologies Group, UK), a portable oceanographic fluorometer for the detection of polyaromatic hydrocarbons in water [Lawford et al., 1990]

[36] The output of the Chelsea Aquatracka has been expressed in this paper in units of equivalent polyaromatic hydrocarbon concentration in parts per billion (ppb). This calibration was derived by measuring the sensor response to polyaromatic compounds in the target oil (in this case, MC252 source oil) that would dissolve into solution to form an “aqueous extract.” This extract was intended to represent the true soluble component of the oil likely to be detected in the field by the sensor.

[37] The method for preparing the extract was adapted from Lang et al. [2009]. Milli-Q water (Millipore) and MC252 oil contained in a glass bottle were slowly stirred over a 48 h period to produce an aqueous extract of the oil containing its soluble components. Details of the extract preparation procedure have been described elsewhere [Qi et al., 2010].

[38] Total polyaromatic hydrocarbon concentration of the oil/water extract was determined by GC-MS for a total of 16 polyaromatic compounds. Note that not all PAH homologs could be detected.

[39] The calibration of sensor output against total PAH was obtained by immersing the sensor in a fixed volume of Milli-Q water in a stirred stainless steel vessel. Standard additions of oil/water extract were made to the vessel as the sensor output was recorded. The 16 EPA (United States Environmental Protection Agency) priority PAHs targeted in the GC-MS analysis are listed as follows: naphthalene, acenaphthene, phenanthrene, fluoranthene, benz(a)anthracene, benzo(b)fluoranthene, benzo(a)pyrene, dibenz(a,h)anthracene, acenaphthylene, fluorene, anthracene, pyrene, chrysene, benzo(k)fluoranthene, indeno(1,2,3,c,d)pyrene, and benzo(g,h,i)perylene.

Acknowledgments

[40] The survey work was carried out on behalf of the Unified Area Command for the Deepwater Horizon Incident Response in the Gulf of Mexico, and funded by BP America. We thank the Editor and reviewers for their constructive criticism. We thank C&C Technologies, Inc. for their collaboration during shipboard scientific investigations. We thank Captain William Smith and his crew for their efficient help during the survey onboard the M/V Ryan Chouest. Finally, we thank Cedric Griffiths for his careful and detailed linguistic revision of the manuscript. Analysis and preparation of this paper (CSIRO manuscript ref. no. EP122851) were funded by BP Gulf Coast Restoration Organization.

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