@article {Feng2014,
	title = {{Time integrated variation of sources of fluids and seepage dynamics archived in authigenic carbonates from Gulf of Mexico Gas Hydrate Seafloor Observatory}},
	journal = {Chemical Geology},
	volume = {385},
	year = {2014},
	pages = {129{\textendash}139},
	abstract = {Authigenic carbonate rocks recovered from the Gulf of Mexico Gas Hydrate Seafloor Observatory in Mississippi Canyon block 118 (MC118) at approximately 900 m water depth were studied using mineralogical, bulk geochemical, and lipid biomarker analyses. Carbonate rocks occurred as fractured blocks and nodular masses incorporated in carbonate breccias. The carbonates were comprised mainly of high-Mg-calcite and aragonite. The stable carbon isotope composition ($δ$13C) of authigenic carbonate varied from - 29.8{\textperthousand} to - 18.1{\textperthousand} vs. V-PDB, suggesting a complex mixture of various carbon sources, including dissolved marine inorganic carbon (DIC), oil, as well as methane. Oxygen isotopes ($δ$18O) varied from + 3.4{\textperthousand} to + 5.8{\textperthousand}. The observed 18O-enrichment in relation to calculated equilibrium values in the carbonates probably reflects decomposition of gas hydrates. The most abundant lipid biomarkers in the carbonates were isoprenoidal glycerol dibiphytanyl glycerol tetraethers (GDGTs), predominated by GDGT-2 and GDGT-3, which are typically indicators of anaerobic methane oxidizing archaea (ANMEs). Mono- and bicyclic biphytanes (derived after ether cleavage of GDGT-2 and GDGT-3) showed strong 13C-depletion, which is characteristic for ANMEs. Interestingly, large differences between the $δ$13C values of the archaeal diether archaeol and acyclic biphytane on the one hand and monocyclic biphytane on the other hand suggest the presence of archaea other than ANMEs. Archaeol and GDGT-0 (containing two acyclic biphytane moieties) are commonly assigned to various methanogenic archaea. Where methane seepage activity is intermediate or low within acoustic wipeout zones at the MC118 gas hydrate site nowadays, microbial communities must have coped with changing conditions as well as longer-term fluctuations in oil and gas seepage or the temporary cessation of hydrocarbon flux in the past. The change from methane seepage to oil seepage or vice versa in addition to flux variability apparently favors the establishment of complex prokaryotic communities dominated by archaea. In addition to anaerobic oxidation of methane, local production of methane is apparently prominent at the study site based on the occurrence of biomarkers of methanogens in the authigenic carbonate. This finding adds to the ongoing multidisciplinary effort to better constrain the environment at the MC118 observatory site and to determine the locally dominant biogeochemical processes.},
	keywords = {HOV Alvin (Human Occupied Vehicle), ROV Jason (Remotely Operated Vehicle)},
	doi = {10.1016/j.chemgeo.2014.07.020},
	author = {Feng, D and Birgel, D and Peckmann, J and Roberts, H H and Joye, S B and Sassen, R and Liu, X-L and Hinrichs, K -U and Chen, D}
}
@article {Mansour2011,
	title = {{Mineralogical and stable isotopic characterization of authigenic carbonate from a hydrocarbon seep site, Gulf of Mexico slope: Possible relation to crude oil degradation}},
	journal = {Marine Geology},
	volume = {281},
	number = {1-4},
	year = {2011},
	pages = {59{\textendash}69},
	abstract = {One of the characteristic features on the cold deep seafloor on Atwater Valley (AT) Block 425 (1930~m water depth) in the Gulf of Mexico is the occurrence of seepage of crude oil and hydrocarbon gases associated with thermogenic gas hydrate and authigenic carbonate deposits. Mineralogical, petrographic and isotopic analyses revealed that the precipitation of these carbonates was related to the microbial oxidation of hydrocarbons. Carbonate deposits are composed of heavily oil-stained, small nodules to large cobbles of dense limestones of fine-grained textures. Other carbonate fragments are composed of small intraclasts, pellets and some bivalve shell fragments cemented together by aragonite cement. XRD analysis and petrographic investigations revealed that the authigenic carbonate is predominantly composed of aragonite that occurs as microcrystalline matrix and cement. Microcrystalline, sparitic and botryoidal acicular aragonite crystals are observed as void-filling cement. The $δ$18O values of the authigenic carbonate (3.80 to 4.16{\textperthousand} V-PDB) suggest that the precipitating fluid has equilibrated with the cold bottom seawater. The calculated $δ$18O values (0.18 to 0.54{\textperthousand} SMOW) of the precipitating fluid show subtle deviation from the values reported for the present bottom seawater. This is attributed possibly to the precipitation from pore water with temperature ranged from 3.3 to 1.7~{\textdegree}C, which is less than the measured bottom seawater (4~{\textdegree}C). However, the mixing of the marine pore water with some deep hydrocarbon-rich brines or hydrate-water would provide a source of 18O-enrichment fluid for the precipitation of the authigenic carbonates. Based on the predominance of aragonite with botryoidal and clotted fabric, the occurrence of pyrite and from the oxygen isotopic data, it is plausible that the authigenic carbonates may have formed near the sediment/water interface in an oxygen-depleted environment with low temperature. The narrow range of the $δ$13C values of the authigenic carbonate (-~23.88 to -~28.62{\textperthousand} V-PDB) suggests that a single source of carbon is predominant. Based on the moderately low $δ$13C values and the occurrence of spots of crude oil coatings of the carbonate samples, in addition to the application of a carbon isotope mass balance model, the carbon involving in the formation of the carbonates of the AT 425 seep site could be mainly derived from microbial degradation of crude oil. Microbial oxidation of non-methane thermogenic hydrocarbon gases such as ethane, propane and isobutane would play some role as a source of carbonate-carbon.},
	keywords = {HOV Alvin (Human Occupied Vehicle)},
	issn = {0025-3227},
	doi = {10.1016/j.margeo.2011.02.004},
	url = {http://dx.doi.org/10.1016/j.margeo.2011.02.004},
	author = {Mansour, A S and Sassen, R}
}
@article {Roberts1990,
	title = {{Seafloor responses to hydrocarbon seeps, Louisiana continental slope}},
	journal = {Geo-Marine Letters},
	volume = {10},
	number = {4},
	year = {1990},
	pages = {232{\textendash}243},
	keywords = {HOV Alvin (Human Occupied Vehicle)},
	issn = {0276-0460},
	doi = {10.1007/BF02431070},
	url = {http://dx.doi.org/10.1007/BF02431070},
	author = {Roberts, H H and Aharon, P and Carney, R and Larkin, J and Sassen, R}
}
