@article {Hu2014,
	title = {{New insights into cerium anomalies and mechanisms of trace metal enrichment in authigenic carbonate from hydrocarbon seeps}},
	journal = {Chemical Geology},
	volume = {381},
	year = {2014},
	pages = {55{\textendash}66},
	abstract = {Authigenic carbonates that form at marine hydrocarbon seeps provide a unique geological archive of past local environmental conditions and pore fluid geochemistry. Recent work on such carbonates revealed variable cerium (Ce) anomalies and anomalous enrichments of certain trace metals. However, the mechanisms accounting for such anomalies remain poorly constrained. Here, we characterize the rare earth element (REE) patterns of carbonate phases and the trace metal patterns of bulk carbonate rocks sampled at three hydrocarbon seeps located at Congo Fan pockmarks (CF) and the Gulf of Mexico sites AC645 and GB425. The analyzed CF, GB425, and AC645 carbonates yielded different REE patterns, displaying positive, no, as well as negative Ce anomalies. The covariation of molybdenum (Mo) with uranium (U), including authigenic Mo (Moauth) and U (Uauth) enrichments as well as (Mo/U)auth ratios proved useful to obtain new insight into the applicability of Ce anomalies to constrain past redox conditions. Trace element patterns suggest that (1) CF carbonates formed in a restricted sulfidic environment, while (2) the AC645 site experienced intermittent oxygenation causing negative Ce anomalies, and (3) environmental conditions were variable at the GB425 mud volcano site. Interestingly, GB425 carbonates show significant Mo, arsenic (As), and antimony (Sb) enrichments with the enrichment factor of As (AsEF) correlating well with the authigenic Fe fraction. These results suggest that iron oxyhydroxides played an important role in the adsorption of Mo, As, and Sb in the water column and their transfer to the sediment. The combination of trace metal and REE geochemistry of authigenic carbonates used here is a promising tool to better assess past variability of redox conditions and biogeochemical processes at marine hydrocarbon seeps.},
	keywords = {HOV Alvin (Human Occupied Vehicle)},
	issn = {0009-2541},
	doi = {10.1016/j.chemgeo.2014.05.014},
	url = {http://www.sciencedirect.com/science/article/pii/S0009254114002538},
	author = {Hu, Y and Feng, D and Peckmann, J and Roberts, H H and Chen, D}
}
@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 {Feng2013,
	title = {{A comparative study of authigenic carbonates from mussel and tubeworm environments: Implications for discriminating the effects of tubeworms}},
	journal = {Deep-Sea Research. Part I: Oceanographic Research Papers},
	volume = {75},
	year = {2013},
	pages = {110{\textendash}118},
	abstract = {The Gulf of Mexico hydrocarbon seeps are often populated by dense mussel beds and tubeworm aggregations, as well as exposed authigenic carbonate outcrops. Previous studies suggest the activity of mussels and tubeworms could influence the sediment geochemistry of their habitats, resulting in variations in the stable carbon isotopes of the associated carbonates. However, this conclusion was based on the analyses of samples from a single site. To better understand whether there are consistent differences in the geochemical environments of mussels and tubeworms, mineralogical and stable isotopic compositions of authigenic carbonates from mussel and tubeworm environments from four seep sites were analyzed. The studied sites span a depth range of 1200~m to 2800~m on the northern Gulf of Mexico continental slope. We found that carbonate samples from tubeworm environments were more prone to contain aragonite whereas carbonates from mussel environments were more likely to have calcite. This finding supports the hypothesis that vestimentiferans release sulfate across their roots into the pore waters of the surrounding sediments, a process that could generate a locally sulfate-enriched environment that favors the precipitation of aragonite instead of calcite. Moreover, the $δ$13C values of tubeworm carbonates are generally lighter than that of mussel carbonates from the same site, which is consistent with the fact that tubeworms are fueling extra subsurface methane oxidation through the release of sulfate into the sediment. Such a process, consequently, enriches the subsurface dissolved inorganic carbon pool with light carbon derived from the seeping hydrocarbons. Taken together, our data suggest that tubeworms could produce a carbon isotope shift that is sufficient to influence the sediment geochemistry of their immediate area, and that this impact is reflected in the associated authigenic carbonates.},
	keywords = {HOV Alvin (Human Occupied Vehicle), ROV Jason (Remotely Operated Vehicle)},
	issn = {0967-0637},
	doi = {10.1016/j.dsr.2013.02.002},
	url = {http://www.sciencedirect.com/science/article/pii/S0967063713000435},
	author = {Feng, D and Cordes, E E and Roberts, H H and Fisher, C R}
}
@article {White2012,
	title = {{Impact of the Deepwater Horizon oil spill on a deep-water coral community in the Gulf of Mexico}},
	journal = {Proceedings of the National Academy of Sciences of the United States of America},
	volume = {109},
	number = {50},
	year = {2012},
	month = {dec},
	pages = {20303{\textendash}20308},
	abstract = {To assess the potential impact of the Deepwater Horizon oil spill on offshore ecosystems, 11 sites hosting deep-water coral communities were examined 3 to 4 mo after the well was capped. Healthy coral communities were observed at all sites {\textgreater}20 km from the Macondo well, including seven sites previously visited in September 2009, where the corals and communities appeared unchanged. However, at one site 11 km southwest of the Macondo well, coral colonies presented widespread signs of stress, including varying degrees of tissue loss, sclerite enlargement, excess mucous production, bleached commensal ophiuroids, and covering by brown flocculent material (floc). On the basis of these criteria the level of impact to individual colonies was ranked from 0 (least impact) to 4 (greatest impact). Of the 43 corals imaged at that site, 46{\%} exhibited evidence of impact on more than half of the colony, whereas nearly a quarter of all of the corals showed impact to {\textgreater}90{\%} of the colony. Additionally, 53{\%} of these corals{\textquoteright} ophiuroid associates displayed abnormal color and/or attachment posture. Analysis of hopanoid petroleum biomarkers isolated from the floc provides strong evidence that this material contained oil from the Macondo well. The presence of recently damaged and deceased corals beneath the path of a previously documented plume emanating from the Macondo well provides compelling evidence that the oil impacted deep-water ecosystems. Our findings underscore the unprecedented nature of the spill in terms of its magnitude, release at depth, and impact to deep-water ecosystems.},
	keywords = {HOV Alvin (Human Occupied Vehicle), ROV Jason (Remotely Operated Vehicle)},
	issn = {0027-8424},
	doi = {10.1073/pnas.1118029109},
	author = {White, H K and Hsing, P-Y and Cho, W and Shank, T M and Cordes, E E and Quattrini, A M and Nelson, R K and Camilli, R and Demopoulos, A W J and German, C R and Brooks, J M and Roberts, H H and Shedd, W and Reddy, C M and Fisher, C R}
}
@article {White2012a,
	title = {{Reply to Boehm and Carragher: Multiple lines of evidence link deep-water coral damage to Deepwater Horizon oil spill}},
	journal = {Proceedings of the National Academy of Sciences of the United States of America},
	volume = {109},
	number = {40},
	year = {2012},
	month = {oct},
	pages = {E2648{\textendash}E2648},
	abstract = {Our original study (1) used visual inspection as well as biological and geochemical analyses of corals and the surrounding sediment to provide complementary and compelling evidence linking the Deepwater Horizon (DWH) oil spill to the presence of damaged deep-water corals and brittle stars 11 km from the site of the leaking oil.},
	issn = {0027-8424},
	doi = {10.1073/pnas.1210413109},
	author = {White, H K and Hsing, P-Y and Cho, W and Shank, T M and Cordes, E E and Quattrini, A M and Nelson, R K and Camilli, R and Demopoulos, A W J and German, C R and Brooks, J M and Roberts, H H and Shedd, W and Reddy, C M and Fisher, C R}
}
@article {Birgel2011,
	title = {{Changing redox conditions at cold seeps as revealed by authigenic carbonates from Alaminos Canyon, northern Gulf of Mexico}},
	journal = {Chemical Geology},
	volume = {285},
	number = {1-4},
	year = {2011},
	pages = {82{\textendash}96},
	abstract = {Anaerobic oxidation of methane in anoxic sediments at cold seeps often leads to formation of authigenic carbonates close to the seafloor along continental margins. Recent work, however, indicated that the redox conditions in sediments may vary to some degree during seepage activity. In order to shed new light on the extent of this variability, authigenic carbonates from Alaminos Canyon lease block 645 of the northern Gulf of Mexico have been characterized by means of inorganic and organic geochemistry. The carbonates were collected from seep deposits representing various seafloor morphologies, including extensive pavements, mounds, fractured carbonate slabs surrounded by dense bivalve shells, and vestimentiferan tubeworm colonies. The deposits almost entirely consist of aragonite. The $δ$18O values of aragonite vary from +~2.6 to +~5.8{\textperthousand} V-PDB, suggesting precipitation in slight disequilibrium with the surrounding pore fluids. The $δ$13C values of aragonite between -~33.9 and -~20.4{\textperthousand} V-PDB agree with variable amounts of carbonate derived from oxidation of thermogenic methane and crude oil. Methane was primarily oxidized in an anaerobic process as revealed by the presence of 13C-depleted molecular fossils of methane-oxidizing archaea ($δ$13C values as negative as -~118{\textperthousand}) and sulfate-reducing bacteria ($δ$13C values as negative as -~97{\textperthousand}), the syntrophic partners in the anaerobic oxidation of methane. The observed inventories of molecular fossils in the authigenic carbonates mirror those of known consortia of anaerobic methane oxidizing archaea (ANME) and sulfate-reducing bacteria, namely the ANME-2/Desulfosarcina/Desulfococcus (DSS) and ANME-3/Desulfobulbus (DBB) consortia. In contrast, the same carbonates exhibit shale-normalized rare earth elements patterns that all display real negative Ce anomalies (Ce/Ce*~ In contrast, the same carbonates exhibit shale-normalized rare earth elements patterns that all display real negative Ce anomalies (Ce/Ce*~13C values of the biomarkers of aerobic methanotrophs are as negative as -~58{\textperthousand} and are consequently less 13C-depleted than the molecular fossils of the prokaryotes performing anaerobic oxidation of methane, a pattern in accord with culture experiments. Overall, our results suggest that redox conditions at cold seeps are variable. This variability probably reflects changes in seepage flux. The combination of an inorganic and an organic geochemical approach used here is promising to better assess the variability and diversity of past fluid and gas expulsion at seeps.},
	keywords = {ROV Jason (Remotely Operated Vehicle)},
	issn = {0009-2541},
	doi = {10.1016/j.chemgeo.2011.03.004},
	url = {http://dx.doi.org/10.1016/j.chemgeo.2011.03.004},
	author = {Birgel, D and Feng, D and Roberts, H H and Peckmann, J}
}
@article {Roberts2010a,
	title = {{Cold-seep carbonates of the middle and lower continental slope, northern Gulf of Mexico}},
	journal = {Deep-Sea Research. Part II: Topical Studies in Oceanography},
	volume = {57},
	number = {21-23},
	year = {2010},
	pages = {2040{\textendash}2054},
	abstract = {Authigenic carbonates from cold seeps on the middle and lower continental slope of the northern Gulf of Mexico (GOM) exhibit a wide range of mineralogical and stable isotopic compositions. These carbonates consist of concretions and nodules in surface sediments, hardgrounds of crusts and isolated slabs, and mounded buildups of blocks and slabs of up to over 10 meters in relief above the surrounding seafloor. Mineralogically, the carbonates are dominated by high-Mg calcite (HMC) and aragonite. However, low levels ({\textless}5 wt{\%}) of dolomite are present in most samples. Petrographically, Mg-calcite peloidal matrix and acicular to botryoidal aragonitic void-filling cements are the most frequent associations. The carbon isotopic compositions of the carbonates range from -60.8 to 14.0{\textperthousand} PDB, indicating complex carbon sources that include 13C-depleted biogenic and thermogenic methane, biodegraded crude oil, seawater CO2, and 13C-enriched residual CO2 from methanogenesis. A similarly large variability in $δ$18O values (2.5 to 6.7{\textperthousand} PDB) demonstrates the geochemical complexity of the slope, with some samples pointing toward an 18O-enriched oxygen source that is possibly related to advection of 18O-enriched formation water and/or to the decomposition of gas hydrate. A considerable range of mineralogical and isotopic variations in cold-seep carbonate composition was noted even within individual study sites. However, common trends occur across multiple geographic areas. This situation suggests that local controls on fluid and gas flux, types of seep hydrocarbons, the presence or absence of gas hydrate in the near-surface sediment, and chemosynthetic communities, as well as the temporal evolution of the local hydrocarbon reservoir, all may play a part in determining carbonate mineralogy and isotope geochemistry. The carbon isotope data clearly indicate that between-site variation is greater than within-site variation. Seep carbonates formed on the middle and lower continental slope of the GOM do not appear to be substantially different from those found on the upper slope ({\textless}1000-m water depth). The highly variable fluids and gases that leave their geochemical imprints on seep carbonate of the middle and lower continental slope are similar to their outer shelf and upper slope counterparts.},
	keywords = {HOV Alvin (Human Occupied Vehicle), ROV Jason (Remotely Operated Vehicle)},
	issn = {0967-0645},
	doi = {10.1016/j.dsr2.2010.09.003},
	url = {http://dx.doi.org/10.1016/j.dsr2.2010.09.003},
	author = {Roberts, H H and Feng, D and Joye, S B}
}
@article {Roberts2010,
	title = {{Dive site geology: DSV ALVIN (2006) and ROV JASON II (2007) dives to the middle-lower continental slope, northern Gulf of Mexico}},
	journal = {Deep-Sea Research. Part II: Topical Studies in Oceanography},
	volume = {57},
	number = {21-23},
	year = {2010},
	pages = {1837{\textendash}1858},
	abstract = {Use of DSV ALVIN (2006) and ROV JASON II (2007) provided access to never observed or sampled sites of fluid-gas expulsion from the little-studied middle and lower continental slope of the northern Gulf of Mexico (below water depths of 1000 m). Dives were focused on 15 locations selected by 3-D seismic surface attributes and shallow subsurface geologic analyses. The linkage between highly positive seafloor reflectivity and hard bottoms proved to be an efficient indicator of potential sites of interest. Through observation and sampling of reflective sites, starting in the mid-1980s, it has become apparent that most hard bottoms on the northern Gulf{\textquoteright}s continental slope are created by the precipitation of authigenic carbonates at hydrocarbon seep sites. Access to the Bureau of Ocean Energy Management, Regulation and Enforcement{\textquoteright}s extraordinary archive of slope-wide 3-D seismic data made efficient site selection possible. From thousands of sites that display the characteristics of fluid-gas expulsion, 15 were observed and sampled during the 2006 and 2007 cruises. Water depths in which these 15 sites were located ranged from \~{}2750 to \~{}970 m. All sites exhibited evidence of hydrocarbon seepage or more rapid venting. Chemosynthetic organisms, authigenic carbonates, barite, gas hydrates, highly anoxic surface sediments, brine pools, and hydrocarbon-laced brine flows were identified and sampled. High-resolution acoustic Autonomous Underwater Vehicle (AUV) data, including multibeam bathymetry, side-scan sonar swaths, and chirp sonar subbottom profiles, were collected at four locations (AC601, WR269, GC852, and AT340). Data sets from the 2006 and 2007 dives resulted in a greatly improved understanding of both cross-slope and along-slope variability in the characteristics of fluid-gas expulsion sites and associated habitats. Our studies confirmed the importance of fluid-gas expulsion processes for sustaining chemosynthetic communities and impacting seabed geology on the middle and lower continental slope of the northern Gulf of Mexico.},
	keywords = {HOV Alvin (Human Occupied Vehicle), ROV Jason (Remotely Operated Vehicle)},
	issn = {0967-0645},
	doi = {10.1016/j.dsr2.2010.09.001},
	url = {http://dx.doi.org/10.1016/j.dsr2.2010.09.001},
	author = {Roberts, H H and Shedd, W and Hunt, J}
}
@article {Feng2010,
	title = {{Initial results of comparing cold-seep carbonates from mussel- and tubeworm-associated environments at Atwater Valley 340, northern Gulf of Mexico}},
	journal = {Deep-Sea Research. Part II: Topical Studies in Oceanography},
	volume = {57},
	number = {21-23},
	year = {2010},
	pages = {2030{\textendash}2039},
	abstract = {Chemosymbiotic macrofauna (such as mussels and tubeworms) and authigenic carbonates are typical of many hydrocarbon seeps. To address whether mussels and tubeworms could impact the sediment geochemistry of their habitat where authigenic carbonates are precipitated, a comparative study of petrographic and geochemical features of the authigenic carbonates from mussel- and tubeworm-associated environments at hydrocarbon seeps in Atwater Valley lease area block 340 (AT340) of the Gulf of Mexico was undertaken. Both mussel- and tubeworm-associated carbonates are dominated by high-magnesium calcite (HMC) and aragonite, and two tubeworm-associated carbonate samples have minor amounts of dolomite. The $δ$13C values of all carbonates are low, ranging from -60.8{\textperthousand} to -35.5{\textperthousand} PDB. Although there is much overlap, surprisingly the $δ$13C values of mussel-associated carbonates are generally higher than those of tubeworm-associated carbonates (-51.8{\textperthousand} vs. -54.8{\textperthousand} for an average of over 60 subsamples). It is suggested that (1) carbon isotopic vital effect of seep mussels and tubeworms, (2) fluid physical pumping of mussels, and (3) release of sulfate by tubeworm roots may be responsible for the relatively lower $δ$13C values of tubeworm-associated carbonates. It has been suggested that the heterogeneities in mineralogy and stable carbon isotope geochemistry of the seep carbonates may be attributed to the activity of macrofauna (mussels and tubeworms) and associated microbes. Our observations also suggest that at AT340 the geochemical evolution of seep macrofauna is from a mussel-dominated environment to a mixed mussel-tubeworm environment, and finally to a mostly tubeworm-dominated environment. This evolution is controlled mainly by the habitat, e.g., hydrocarbon seep flux.},
	keywords = {HOV Alvin (Human Occupied Vehicle), ROV Jason (Remotely Operated Vehicle)},
	issn = {0967-0645},
	doi = {10.1016/j.dsr2.2010.05.004},
	url = {http://dx.doi.org/10.1016/j.dsr2.2010.05.004},
	author = {Feng, D and Roberts, H H}
}
@book {Roberts2007,
	title = {{Expedition to the Deep Slope 2007, June 4 - July 6, 2007}},
	volume = {2008},
	year = {2007},
	publisher = {U.S. Dept. of Commerce, National Oceanic and Atmospheric Administration, Office of Ocean Exploration},
	organization = {U.S. Dept. of Commerce, National Oceanic and Atmospheric Administration, Office of Ocean Exploration},
	address = {Washington, D.C.},
	keywords = {ROV Jason (Remotely Operated Vehicle), ROV Medea (Remotely Operated Vehicle)},
	url = {http://oceanexplorer.noaa.gov/explorations/07mexico/},
	author = {Roberts, H H and Potter, J M}
}
@booklet {Roberts2006,
	title = {{NOAA Ocean Explorer: Expedition to the deep slope}},
	volume = {2007},
	year = {2006},
	publisher = {U.S. Dept. of Commerce, National Oceanic and Atmospheric Administration, Office of Ocean Exploration},
	address = {Silver Spring, Md.},
	keywords = {HOV Alvin (Human Occupied Vehicle)},
	author = {Roberts, H H and Potter, J M and Wolff, G A}
}
@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}
}
