KeywordsCold Seep Salt Diapir Heavy Petroleum Salt Tectonism Asphalt Deposit
Asphalt volcanism is a type of hydrocarbon seepage associated with submarine mounds found in the Gulf of Mexico and the Santa Barbara Basin. The term asphalt volcanism was introduced because of the lavalike appearance of the asphalt flows and several other indications, which resemble magmatic eruptions.
Asphalt Seepage Versus Asphalt Volcanism
Based on the collected data and observations, MacDonald et al. (2004) postulated repeated, extensive eruptions of molten asphalt under conditions which could destabilize gas hydrates on the seafloor. A violent destabilization of hydrates could contribute to slope failures and mass wasting mapped on Chapopote Knoll, as well as documented on other Campeche Knolls. Based on the idea that the asphalt on Chapopote was molten during extrusion, Hovland et al. (2005) argued for a model that relies on supercritical water being transported vertically upward through a suspected internal conduit within the salt diapir, from near the base of the sedimentary column at perhaps 13 km depth. Organic material including bitumen should have been transported upward together with “hydrothermal-like” components as a hot substance. At the summit of the Chapopote structure, a hot slurry flowed out onto the seafloor where bitumen and asphalt devolatilized rapidly, eventually building up the asphalt volcano’s structure.
The Chapopote Knoll was surveyed in greater detail by Brüning et al. (2010) using the MARUM ROV QUEST. The results support the concept that the asphalt deposits on Chapopote originate from seepage of heavy oil with a density slightly greater than water, which leads to remaining petroleum and oil residues on the seafloor. During extrusion of the heavy petroleum, the viscosity increased due to the loss of volatiles, and the heavy petroleum forms the lavalike flow structures along the distance where continuous solidification occurs. The investigations of Brüning et al. (2010) documented that the asphalt is subject to sequential alteration. While fresh asphalt was gooey, older asphalt appeared fragmented and brittle. Highly altered asphalt was often colonized by further chemosynthetic fauna like mytilid clams and others. The change in the consistency of the asphalts goes along with a change in the geochemical composition and microbial signatures (Schubotz et al., 2011). Besides the unusual asphalt formation, the putative “volcanic structure” is representing a very interesting seepage area which extended our knowledge about the broad spectrum of seafloor venting phenomena.
Beside the Gulf of Mexico, asphalt volcanoes are known from the Santa Barbara Basin, California, in much shallower water depths close to the coast. Seven of those morphological structures were described as extinct asphalt volcanoes by Valentine et al. (2010). Radiocarbon dating of carbonate layers intercalated with the asphalt deposits indicated formation of two of the volcanoes between 44 and 31 kyr ago. Based on quantitative assumptions and the geochemistry of samples taken from the volcanoes, the authors estimated the amount of oil and accompanied methane gas, which are emitted at the sites where the residues of the hydrocarbon seepage (i.e., the asphalt) currently are deposited. Since the amount of greenhouse gas (in this case methane) emissions is not known during former times, the study is of great value to reveal estimates of former seepage rates.
- MacDonald, I. R., Bohrmann, G., Escobar, E., Abegg, F., Blanchon, P., Blinova, V. N., Brueckmann, W., Drews, M., Eisenhauer, A., Han, X., Heeschen, K. U., Meier, F., Mortera, C., Naehr, T., Orcutt, B., Bernard, B., Brooks, J., and de Farágo, M., 2004. Asphalt volcanism and chemosynthetic life, Campeche Knolls, Gulf of Mexico. Science, 304(5673), 999–1002, doi:10.1126/science.1097154.CrossRefGoogle Scholar
- Valentine, D. L., Reddy, C. M., Farwell, C., Hill, T. M., Pizzarro, O., Yoerger, D. R., Camilli, R., Nelson, R. K., Peacock, E. E., Bagby, S. C., Clarke, B. A., Roman, C. N., and Soloway, M., 2010. Asphalt volcanoes as a potential source of methane to late Pleistocene coastal waters. Nature Geosciences, 3, 345–348, doi:10.1038/NGEO848.CrossRefGoogle Scholar