Heterogeneous hydrocarbon seepage at Mictlan asphalt knoll of the southern Gulf of Mexico
Introduction
In addition to gas and oil venting at the seafloor, submarine asphalt discharge became important as a new type of natural hydrocarbon seepage. Since asphalt volcanism was first described at Chapopote Knoll in the southern Gulf of Mexico (GoM) in 2004 during the R/V SONNE cruise SO174 (Bohrmann and Schenck, 2004), there has been an increasing interest in the investigation of submarine asphalt deposits. Consequently, the presence of solidified petroleum deposits at Chapopote Knoll was interpreted by MacDonald et al. (2004) as a result of asphalt volcanism as the deposits resembled massive lava-like flow structures covering a crater-like central depression of this knoll. Subsequent investigations with a remotely-operated vehicle (ROV) including the sampling of hydrocarbon materials from Chapopote Knoll led to the interpretation that seafloor asphalt deposits originated from seepage of very heavy oil with a density slightly greater than water (Bohrmann, 2016; Brüning et al., 2010; Schubotz et al., 2011). Recent visual seafloor observations conducted with an optical mosaicking technique documented several asphalt flow units, illustrating distinct eruption phases of intensified asphalt flows at Chapopote Knoll (Marcon et al., 2018).
Besides Chapopote Knoll, asphalt deposits were confirmed at ten further locations within the Campeche-Sigsbee salt province (Fig. 1a) over the past two decades, based on visual seafloor observations (Bohrmann and Schenck, 2004; Bohrmann et al., 2008; Sahling et al., 2016, 2017). Similar asphalt deposits at the seabed were discovered at two commercial hydrocarbon appraisal areas in the northern GoM: Puma (Weiland et al., 2008) and Shenzi (Williamson et al., 2008). Worldwide, asphalt deposits have been reported from an increasing number of locations: seven extinct asphalt volcanoes in the Santa Barbara Basin (Valentine et al., 2010), more than 2000 asphalt mounds at the Angolan Margin in the southern Congo fan (Jones et al., 2014), and on the North São Paulo Plateau offshore Brazil (Fujikura et al., 2017). Whereas asphalt deposits in the GoM and Angolan Margin are associated with salt diapirism, those in the Santa Barbara Basin as well as on the North São Paulo Plateau are related to compressional tectonics. Such dynamic geological processes allow the migration of asphalt, oil and gas through the sediments to the seafloor and into the water column (Ding et al., 2008; Keller et al., 2007; Loher et al., 2018; Vernon and Slater, 1963; Williamson et al., 2008).
Asphalt deposits provide essential habitats for dense and diverse communities of chemosynthetic fauna, particularly tube worms and mussels (Jones et al., 2014; Sahling et al., 2016; Weiland et al., 2008). Relatively fresh asphalt structures were settled by chemosynthetic communities including bacterial mats and vestimentiferan tube worms, whereas older flow deposits appeared largely inert and devoid of corals and anemones at the deep sea sites (Sahling et al., 2016). Gas hydrates outcropping at some knolls in the GoM were covered by a 5–10 cm thick reaction zone composed of authigenic carbonates (Smrzka et al., 2019), sedimentary detritus, and microbial mats, and were densely colonized by chemosynthetic fauna as well. Investigations on the symbionts of chemosynthetic fauna from Chapopote Knoll expanded the still poorly understood range of substrates known to support chemosynthetic symbioses (Rubin-Blum et al., 2017).
The Campeche-Sigsbee salt province located in the southern GoM (Fig. 1a inset) comprises a hummocky seafloor morphology associated with salt tectonism (Sánchez-Rivera et al., 2011). Several sequences of salt deposition during the Jurassic followed by regional deformation events characterized its tectonic evolution (Angeles-Aquino et al., 1994; Cruz-Mercado et al., 2011). The results from deep seismic reflection together with Deep Sea Drilling Project (DSDP) drilling suggested that units overlying the salt deposits in this area consist of late Mesozoic to early Tertiary pelagic sediments overlain by mid-Tertiary through Pleistocene deposits (Ewing et al., 1969; Ladd et al., 1976). The area is characterized by the wide distribution of active hydrocarbon seepage, which originated from abundant source rock deposition during the Late Jurassic (Magoon et al., 2001). The southern part of Campeche-Sigsbee Knoll is associated with the Pimienta-Tamabra supercharged petroleum system, which has estimated remaining reserves of 66.3 billion barrels of oil and 103.7 trillion cubic feet of natural gas (Magoon et al., 2001). Evidence of the widespread natural hydrocarbon seepage comes from the documentation and study of oil slicks and by monitoring their occurrence over time at the sea surface through high-resolution satellite imagery (MacDonald et al., 2015; Suresh, 2015; Williams et al., 2006).
Furthermore, over 200 hydroacoustic anomalies interpreted as gas flares from gas bubble emissions into the water column have been detected in February and March 2015(Sahling et al., 2017). The spatial distribution of fluid release at the seafloor is suggested to be controlled by the shallow sediment deformation styles associated with salt tectonics (Ding et al., 2010; Hsu et al., 2019).
This study sets out to investigate the diversified hydrocarbon seepage at a recently discovered asphalt volcano Mictlan Knoll in the Campeche-Sigsbee salt province. It describes the extent and seafloor manifestations of asphalt deposits and oil seepage using hydroacoustic datasets and optical seafloor surveys. The current active hydrocarbon seepage sites including fresh asphalt deposits, oil seepage and gas seepage are shown and discussed. Their distribution and the different phases of hydrocarbon seepage give new insights into the mechanism of the subsurface hydrocarbon migration at Mictlan Knoll that link to the near-surface release processes and allow a better understanding of asphalt volcanism in the southern GoM.
Section snippets
Hydroacoustic data
Hydroacoustic data presented in this study were collected during R/V METEOR cruise M114 in spring 2015 (Sahling et al., 2017). Bathymetry, backscatter and water column data were collected with the ship-based multibeam echosounder (MBES) Kongsberg EM122 (12 kHz) system as well as with a Kongsberg MBES EM2040 (300 kHz) system mounted on the autonomous underwater vehicle (AUV) MARUM-SEAL 5000. Bathymetry data were processed with the open-source package MB-System (Caress and Cheyes, 2017). The AUV
Seafloor bathymetry and seafloor backscatter
Mictlan Knoll forms an isolated sub-circular feature located in the central region of the Campeche-Sigsbee salt province (Fig. 1a), about 20 km northeast of Chapopote Knoll and 6 km north of Knoll H2154 (Fig. 1b). Mictlan Knoll is about 7 km in diameter at the base and rises ~300 m above the surrounding seafloor located at about 3300 m water depth (Fig. 1b). Our high-resolution AUV bathymetry map covers an area of 9.3 km2 (Fig. 2a) and illustrates a large crater-like depression at the center of
Seafloor morphology
The seafloor morphology of Mictlan Knoll is a sub-circular knoll characterized by a distinct crater-like depression at the top that is directly connected to the southern flank of the knoll. This is the most significant contrast to the crater-shaped top of Chapopote Knoll, which is fully enclosed within an elevated rim feature. The crater-like depression at Mictlan Knoll (~1.5 km in diameter, up to 60 m deeper than the rim) is larger and deeper than the depression at Chapopote Knoll (~500 m in
Conclusions
The occurrence of various types of near-seafloor hydrocarbon migration and storage at Mictlan Knoll, includes gas venting, gas hydrate deposits, oil seepage, and fresh as well as aged asphalt deposits. In this study, we combined visual seafloor observations with AUV-based high-resolution bathymetry and seafloor backscatter to document seafloor manifestations of hydrocarbon seepage at Mictlan Knoll and quantify a selected gas seepage system.
Mictlan Knoll hosts the largest area of extensive
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We sincerely appreciate the excellent cooperation with the captain and the crew of R/V METEOR during cruise M114 as well as the professional assistance of the scientific and technical operating teams of the ROV ‘MARUM-Quest 4000 m’ and AUV ‘MARUM-Seal 5000’. We are very grateful to the Mexican authorities for granting permission to collect the multidisciplinary data from Mexican national waters (permission of DGOPA: 02540/14 from November 5, 2014). Our sincere thanks go to Marta Torres (Oregon
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