Asphalt Volcanism and Chemosynthetic Life in the Campeche Knolls, Gulf of Mexico

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Science  14 May 2004:
Vol. 304, Issue 5673, pp. 999-1002
DOI: 10.1126/science.1097154


In the Campeche Knolls, in the southern Gulf of Mexico, lava-like flows of solidified asphalt cover more than 1 square kilometer of the rim of a dissected salt dome at a depth of 3000 meters below sea level. Chemosynthetic tubeworms and bivalves colonize the sea floor near the asphalt, which chilled and contracted after discharge. The site also includes oil seeps, gas hydrate deposits, locally anoxic sediments, and slabs of authigenic carbonate. Asphalt volcanism creates a habitat for chemosynthetic life that may be widespread at great depth in the Gulf of Mexico.

Salt tectonism in the Gulf of Mexico hydrocarbon province controls the development of reservoirs and faults that allow oil and gas to escape at the sea floor (1). More than 30 years ago, investigators studying the Gulf's abyssal petroleum system (2) photographed an asphalt deposit (3) among salt domes in the southern Gulf of Mexico. During exploration of the Campeche Knolls, about 200 km south of the photographed site (Fig. 1, A and C), we have now found numerous, deeply dissected salt domes with extensive slumps and mass wasting at depths of 3000 m or greater. Massive, lava-like flow fields of solidified asphalt, evidently discharged at temperatures higher than the ambient bottom water (4°C), have been colonized by an abundant chemosynthetic fauna.

Fig. 1.

(A and B) Maps of (A) the swath-mapped region of the Campeche Knolls and (B) the Chapopote site were compiled onboard the RV Sonne. Contour lines are in meters below sea level. Yellow dots mark locations where floating oil was seen in satellite images throughout the knolls. Gray dots mark bottom navigation fixes during the photo-sled survey of Chapopote. Red dots show locations of asphalt pieces or asphalt flows. Yellow diamonds are grab-sample locations. (C) The regional setting of the swath map (rectangle) and the location of a 1971 photograph (3) of an asphalt pillow (arrow).

The Campeche Knolls are salt diapirs rising from an evaporite deposit that underlies the entire slope region (4) and hosts the Campeche offshore oil fields (5). Numerous reservoir and seal facies have also been attributed to breccia associated with the Chicxulub impact, which occurred ∼200 km to the east (6). Guided by data from satellite imagery (7) that showed evidence for persistent oil seeps in this region (8), we mapped the bathymetry of a 57-by-87-km area with the German ship RV Sonne (9).

Resulting swath data show that the northern Campeche Knolls are distinct, elongated hills that average 5 by 10 km in size, with reliefs of 450 to 800 m and slopes of 10 to 20% (Fig. 1A). The crests and flanks on 9 of the 22 knolls mapped contain linear and crescent-shaped faults and slump scarps. In many cases, the slumps are associated with downslope sediment lobes that extend as far as 4 km out over the adjacent sea floor. The locations of persistent oil seeps detected by satellite correspond to the dissected salt structures, which indicate that considerable sea-floor instability is associated with hydrocarbon discharge.

Visual surveys of one dissected knoll (21°54′N by 93°26′W), which we named Chapopote (10), revealed extensive surface deposits of solidified asphalt, emanating from points along the southern rim of a broad, craterlike graben near the crest of the structure (Fig. 1B). One subcircular flow measured at least 15 m across and comprised numerous concentric lobes stacked higher toward the center; the entire flow was fractured by ramifying radial joints (Fig. 2, A and B). Other flows were linear, bifurcated in places, up to 20 m wide or greater, and extended far down the slope. The morphologies of these deposits were often blocky (Fig. 2B) or ropy (Fig. 2C), similar to a'a or pa'hoehoe basaltic lava flows. The video and navigation data indicate that asphalt flows cover almost 1 km2 of the upper structure.

Fig. 2.

Photographs of asphalt flows and associated organisms at Chapopote were taken with a remote photo-sled. Scale bars are ∼20 cm as determined by parallel lasers projected on sloping sea floor. (A) Asphalt flows typically had shrink fractures normal to the flow direction. (B) Blocky a'a-like morphology was found in the center of flows measuring ∼20 m across. (C) Pa'hoehoe-like folds in the freshest materials were lined with white mats or films. (D) Clusters of living tubeworms were observed growing through fissures in asphalt. (E) Some vestimentiferan clusters appeared to have become embedded in tar. (F) At the edges of the flow field, small clusters of vestimentiferans grew under eroded asphalt deposits with living Calyptogena, bivalve shells, and galatheid crabs.

The biological community at Chapopote was extensive and diverse. Concave joints in the ropy asphalt were coated with white microbial films (Fig. 2C). Vestimentiferan tubeworms (cf. Lamellibrachia sp.) were common, but were always observed in close proximity to asphalt flows, which they colonized by extending the posterior ends of their tubes into sediments beneath the flow edges (Fig. 2A) or into fissures (Fig. 2D). Some tubeworm aggregations were completely embedded in solidified tar, indicating that they were later overcome by flows (Fig. 2E). Large bivalve shells, including the chemosynthetic family Vesicomyidae (cf. Calyptogena sp.), were widespread on the sea floor surrounding the asphalt flows and among asphalt pillows and cobbles (Fig. 2F). Shells and living specimens of chemosynthetic mussels (cf. Bathymodiolus sp. and Solemya sp.) were recovered by grab sample along with highly oiled sediments. Heterotrophic fauna included galatheid crabs (Munidopsis sp.) and shrimp resembling Alvinocaris sp., as well as nonendemic deep-sea fish and invertebrates (Benthodytes sp., Psychropotes sp., and Pterasterias sp.). Crinoids and soft corals were attached to asphalt pillows found farthest downslope from the rim.

A video-guided grab recovered ∼75 kg of asphalt, tubeworm tubes, and additional associated sediments from the crest of the knoll (Fig. 1B). There was scant hydrocarbon gas and no oil in these sediments (11) (Table 1). The asphalt pieces included small fragments and large, irregular blocks weighing more than 10 kg. This material, which was brittle and had no residual stickiness, shows columnar jointing and chilled margins that indicate molten flow followed by rapid cooling (fig. S1). A medical computerized tomography scan of one of the large blocks revealed a relatively low-density mass with an outer, “weathering” rind, an interior with regular folding, and numerous occluded pebbles, the density of which resembled carbonate (fig. S2). Sediments surrounding the asphalt were composed of a thin layer of brown organic material overlying clayey, nannofossil ooze. No H2S was detected (the detection limit was 2 μM), and the presence of NO 3 in a gradient from 14 to 4 μM over sediment depths from ∼1 to 10 cm below the interface indicated that the surface sediments were oxidized.

Table 1.

Hydrocarbon gas composition of sediment and gas hydrate collected in video-guided grabs from Chapopote. Gas hydrate concentrations (Conc.) are reported as parts per million by volume (ppmv) of hydrate gas. Sediment gas concentrations are reported as ppmv of interstitial water. Stable carbon isotopes (δ13C) are reported as parts per thousand relative to Pee Dee Belemnite standard.

Sample CO2 Methane Ethane Propane i-Butane n-Butane
Conc. δ13C Conc. δ13C Conc. δ13C Conc. δ13C Conc. δ13C Conc.
Gas hydrate 3,000 -19.9 962,000 -50.1 29,000 -33.2 33,000 -27.1 8,000 -27.6 1,700
Oily sediment 22,200 -7.5 47,400 -55.1 4,830 -34.1 9,217 -29 6,660 -39.9 295
Asphalt sediment 1,330 17.4 1.5 0.8 0.04 0.2

A second grab targeted one of the few bacterial mats observed at Chapopote. About 20% of this sample volume consisted of viscous, liquid petroleum dispersed in veins and pockets; asphalt was entirely absent. A surface crust comprised slabs of authigenic carbonate with layers of oil pooled beneath. Sediments were entirely anoxic with H2S concentrations of 8 to 13 mM. Gas hydrate formed thin layers in the surface sediments, and numerous pieces floated in the surface water as the grab was recovered on board the ship. A negative chloride anomaly (482 mM) in the upper 4 cm was consistent with gas hydrate layers. An alkalinity profile showed extremely high values from 29 to 35 mM, which indicate the oxidation of hydrocarbons by reduction of seawater sulfate.

Molecular and isotopic compositions of the gas hydrate and sediment headspace from the second grab sample indicate moderately mature, thermogenic gas (Table 1). Aliphatic and aromatic biological markers indicate an Upper Jurassic–sourced, carbonate-rich oil of at least moderate maturity, which is typical of deep-water hydrocarbon seeps in the Gulf of Mexico (12). Oily sediment extracts and asphalt pieces were composed of a degraded, unresolved complex mixture of hydrocarbons with a peak at n-C30 and a few resolved C29 to C32 hopanes. Concentration of carbon dioxide in the oily sediment is high compared to values from deep-water sediments of the Gulf of Mexico (13). The high concentration of carbon dioxide with a heavy carbon isotopic composition may represent carbon dioxide migrating from a deep source with the hydrocarbons or the dissolution of sediment carbonates under acid conditions.

The size, extent, and morphology of the asphalt flows observed at Chapopote entirely distinguish asphalt volcanism from irregular mats and pools of viscous tar described from coastal (14) and continental slope (15) oil seeps. Furthermore, the chemosynthetic biota at Campeche Knolls exploit a variety of biogeochemical niches within the site, including an unexplained association with asphalt. Localized seepage of oil and gas produces gas hydrate, oil-saturated sediments, and oil traces that float to the ocean surface. High concentrations of H2S within the upper sediment column at these localities result from the anaerobic oxidation of hydrocarbon (16, 17), generating authigenic carbonates and a more typical substratum for Lamellibrachia (18). In contrast, sediments associated with asphalt flows may remain little altered by anaerobic oxidation of hydrocarbons; additional biogeochemical processes must occur within or beneath the asphalt flows to support the prevalent tubeworm aggregations.

The collective data indicate that Chapopote has been subjected to repeated, extensive eruptions of molten asphalt under conditions that are probably incompatible with gas hydrate stability (19). The mechanical energy of these eruptions coupled with the violent destabilization of gas hydrate deposits contribute to the faulting, slope failures, and mass wasting mapped at Chapopote and other salt domes in the Campeche Knolls. Additional sampling and measurement will be required to clarify the characteristics of asphalt discharge and the biogeochemical processes that allow chemosynthetic organisms to thrive in association with asphalt deposits. Pequegnat's 1971 photograph (3) of an asphalt pillow shows lava-like morphology as well as a galatheid crab, a crinoid, and, although it was not noted by the author, a solitary vestimentiferan (fig. S3). Asphalt volcanism and associated deep-sea life may therefore be a widespread process in the Gulf of Mexico abyss. Satellite surveillance could be an effective tool for finding more of these features.

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Figs. S1 to S3

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