Hydrocarbon-derived carbonates along the upper-lower continental slope, Gulf of Mexico: A mineralogical and stable isotopic study

• Autores: Ahmed S. Mansour
• Localización: Carbonates and Evaporites, ISSN 0891-2556, Vol. 29, Nº. 1, 2014 (Ejemplar dedicado a: Dr. Gerald Friedman Memorial), págs. 89-105
• Idioma: inglés
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• Resumen

  • The authigenic carbonates from three hydrocarbon seep sites (AC 645, MC 118 and GC 234), located at different depths in the Gulf of Mexico Continental Slope, exhibit similar megascopic and petrographic characteristics. The carbonates occur in the form of nodules, irregularly shaped fragments and slabs and are composed either of homogeneous mudstones microfacies or cemented carbonate grains. A considerable variation in the mineralogical (aragonite and/or HMC) and isotopic compositions of carbonates was noted, indicating varied geochemical setting and carbon sources. This variation is noted also within the individual sites suggesting that the formation of carbonates occurred during activity of variable hydrocarbon seepage in different times. The [delta]18O values of carbonates of the deepest AC 645 site point toward their precipitation from fluid in equilibrium with the ambient bottom seawater. The slight increase of [delta]18O values of the more shallower MC 118 and GC 234 carbonates may be attributed to the involvement of 18O-enriched water from partial decomposition of gas hydrate, the prevalence of HMC relative to the aragonite or their precipitation from cold water than the ambient bottom seawater. The moderate negative [delta]13C values (av. -28.3 %o) and highly occurrence of biodegraded crude oil within AC 645-I aragonitic carbonate samples suggest that the carbon is mainly derived from microbial degradation of crude oil. Alternatively, the [delta]13C values of MC 118 (av. -27.86 %o) and GC 234 (av. -20.3 %o) calcitic-carbonates are consistent with those values of high-molecular-weight hydrocarbon gases derived from the decomposition of structure-II gas hydrate available in these sites, suggesting that these gases could represent the primary source of carbon, whereas the AC 645-II calcitic-carbonates of mussel bed have more negative [delta]13C values (av. -47.2 %o), indicating that the carbon is mainly derived from the anaerobic oxidation of biogenic methane. From the observed distribution of the carbonate mineralogy in each seep site and compared with other data available in the literature, it can suggest that the type seep hydrocarbons (crude oil, thermogenic gases, biogenic methane, gas hydrate) play a certain role not only in the carbon isotope composition of the authigenic carbonates but also in their mineralogy. The oxidation of crude oil results mostly in the precipitation of aragonite, whereas the oxidation of hydrocarbon gases results in the precipitation either of HMC at low to moderate rate of gases venting or aragonite at high rate of gases venting.