A COMPARATIVE STUDY OF HYDROCARBON-DERIVED AUTHIGENIC CARBONATES ALONG THE NORTHERN GULF OF MEXICO CONTINENTAL SLOPE

Comparative study of authigenic carbonates collected from the seafloor of three hydrocarbon seep sites located at different depths (Alaminos Canyon (AC) 645, Mississippi Canyon (MC) 118 and Green Canyon (GC) 234) at the Gulf of Mexico continental slope reveals that the carbonates are, more or less, similar to each other and occur in the form of nodules, irregularly-shaped fragments and slabs. They are composed of homogeneous microcrystalline carbonate of mudstone microfacies or cemented mussels shell fragments, peloids and small intraclasts. The mineralogical compositions vary from mainly aragonite and/or HMC in AC 645 to HMC in MC 118 and GC 234. No obvious relationship can be noticed between the mineralogical and oxygen isotopic composition, water temperature or water depth in the study sites. The mineralogical variation is possibly related to the variation in the porewater composition and degree of sulfate reduction process during carbonate precipitation.

δ¹⁸O values point toward precipitation from fluid in equilibrium with the bottom seawater at temperature slightly lower than ambient bottom water temperature as in AC 645 carbonates or may reflect precipitation from colder water than present bottom seawater or the contribution of slightly ¹⁸O-enriched pore fluid liberated from localized decomposition of gas hydrate as in MC 118 and GC 234 carbonates. In addition, the enrichment of ¹⁸O in the MC 118 carbonates may be attributed to the presence of some dolomite mixed with the HMC. Rather, the observed slightly variation in the δ¹⁸O values of AC 645 group-I carbonate samples is related to the local variation in the mineral composition.

Based on the type of hydrocarbon which dominated in each site and the moderate negative δ¹³C values, the carbonate carbon has been contributed from degradation of crude oil and/or thermogenic hydrocarbon vent gases for AC 645-group I carbonates and from degradation of crude oil and/or high molecular weight hydrocarbon gases derived from the decomposition of structure-II gas hydrate for MC 118 and GC 234 carbonates. Alternatively, the AC 645- group II carbonates (mussels bed) have more negative δ¹³C values (-48‰), suggesting their formation from anaerobic oxidation of microbially produced biogenic methane.