B53C-01 INVITED
Different Methane Sources Feed Cold Seeps Along the Northeast Pacific Margin
Analyses of carbonate samples recovered from Heceta Bank on the Cascadia margin reveal contrasting carbon source regions consistent with a pattern of mixed sources. This pattern is typical of the northeast Pacific continental shelf and slope from Vancouver to California, where the methane seepage regime is defined by juxtaposition of a young prism in which methane is generated by bacterial activity - against older sequences that serve as a source of thermogenic hydrocarbons that vent onto the shelf. These two sources play a significant role in the carbon cycling and methane inventories of the margin. Biogenic methane is known to vent from several topographic highs along the margin, of which Hydrate Ridge is the most extensively studied. The methane discharge at these discrete ridges in the accretionary prism generates large structures and extensive chemoherms that have been well documented in the literature. Here we present evidence for an additional location of methane discharge at the upper limit of gas hydrate stability, which in this region occurs at ~ 500 meters water depth. The high backscatter observed along the 490 to 610 meter corridor of the upper continental slope suggests that this region is likely to have experienced methane discharge events, followed by carbonate deposition, at the depth where the seafloor intersects the gas hydrate stability curve. Seismic data confirm the correlation between seafloor indicators of venting and subsurface structures. Water column profiles are consistent with a prevalent methane source at this benthic corridor. These observations are significant not only for methane inventories, but because the upper limit of gas hydrate stability defines one of the most climate-sensitive boundaries in this carbon reservoir.
B53C-02
Origin of sedimentary organic matter at the Northern Cascadia Margin
Gas hydrate in marine sediments may have important roles on global carbon cycle and climatic change. We examined origins of sedimentary organic matter and bacterial activity in deep and hydrate-bearing sediment cored in Site U1327 and U1328 at northern Cascadia Margin by IODP Exp311, using σ13C of total organic carbon (TOC), σ15N of total nitrogen (TN), σ34S of total sulfur (TS), and σ13C of biomarkers in hydrocarbon fraction. In both sites, TOC/TN ratios and σ13C of TOC values ranged from 5.5 to 18.0 and 25.7 to 21.5 ‰, respectively, suggesting that sedimentary organic matter is a mixture of terrestrial and marine sources. Long chain (n)-alkanes (C27, C29, and C30), known as biomarkers of terrestrial higher plant were most abundant components (up to ~50 μg/gCorg) through down to 300 mbsf, and their σ13C values (-34.3 to 28.7 ‰) reveal their C3 plant origin. In addition, very long-chain alkene (C37) occurred in some sediments, which suggests the blooming by coccolithophore in the past. σ34S of TS values at both sites show large variation between 30 to +20 ‰. Most of σ34S of TS values were less than present σ34S value of seawater sulfate (+20.3 ‰). This is attributable to isotope fractionation during microbial sulfate reduction. Crocetenes including one double bond occurred in deep sediments with higher σ13C values (23 ‰) than the reported σ13C values (\< ~ -100 ‰, Elvert et al, 2000), providing possibility of heterotrophic archaea using marine organic matter as a carbon source. Pentamethylicosane (PMI) was detected in relatively high concentrations at 249 mbsf at Site U1328 and its σ13C value was 46.4 ‰. This PMI could be chemoautotrophic archaea in origin such as methanogen. Diploptene was also detected in most sediments with the σ13C value of 37 to 35 ‰, probably being characteristic of chemoautotrophic bacteria.
B53C-03
Carbonate-cemented hardgrounds: a subtle indicator for seep activity offshore Humboldt Bay
Active hydrocarbon seeps are common in the accretionary prism of the Cascadia subduction zone. In Humboldt County, California, the prism is exposed at the surface as a series of fault-propagated anticlines trending NW-SE. Offshore of the town of Samoa, a northwest-plunging anticline is breached at approximately 40 meters water depth, allowing hydrocarbons to seep out to the seafloor (40.8° N, 124.25° W). The assumed microbial activity at the seep leads to the production of interstitial carbonate cements forming hardgrounds. Cementation is pervasive and blocks eroded from the seep area of the seabed are transported onshore during storm events. Blocks collected from the beach range from 340 centimeters across. The sediments of the blocks are palimpsest transgressive deposits composed mostly of immature fine sand, but ranging from very fine to rounded gravels 4 cm diameter. Cementation is not dependent on grain size as all of the sediment sizes are cemented. In rare void spaces, a concentric banding of cements is obvious. The interstitial cements preserve original sedimentary structures including graded beds and high-angle cross-beds. Centimeter-scale subspherical concretions occur on the undersides of some blocks. There is no disruption of bedding in contrast to other seeps where the expulsion of gas can create pockmarks, brecciation, and other disturbances. Unlike the better studied seeps farther south in the Eel River basin, the Samoa seeps do not seem to host a rich chemosynthetic fauna. Whole and (mostly) fragmented shells preserved by the cemented sands represent a typical benthic inner shelf community including Dendraster, Macoma, and Olivella. Burrows preserved in the sands are largely horizontal and 12 mm diameter. Seep carbonate-cemented hardgrounds are less well studied then the more obvious meter-scale chemoherm' deposits. However, they may be more prevalent in the rock record and provide a new proxy for locating ancient seeps and hydrocarbon conduits.
B53C-04
Discovery and Description of Extinct Asphalt Volcanoes Along the Southern California Margin
Asphalt volcanism is increasingly being recognized as an important process at cold seeps, linking ancient subsurface carbon reservoirs with more rapid biogeochemical processes at the surface. Here we describe two extinct asphalt volcanoes discovered off the coast of Santa Barbara, CA, using the DSV Alvin during the July 2007 SEEPS (Studies on the Ecology and Evolution of Petroleum Seeps) cruise. These structures are located approximately 10 kilometers offshore and 2 kilometers apart from each other, at a water depth of 150 to 200 meters. The volcanoes occur as asphalt mounds closely associated with sediment-laden depressions, suggesting extrusion of liquid petroleum coupled with localized subsidence or gas blowout. The volcanoes range from 10 to 30 meters in height off the sea floor and may extend below the present level of sediment cover. No active seepage was observed during approximately 10 hours of visual and video surveys from the DSV Alvin, but the volcanoes appear to serve as an oasis for benthic life when compared to the surrounding sediment. Four asphalt samples were collected throughout each site during these surveys and all show remarkable similarity in their structure and chemical composition. Organic carbon comprises 50 percent of the mass for each sample, with sulfur, hydrogen and nitrogen comprising another 10 percent in aggregate. Inclusions of fine-grained sediment and microfossils comprise much of the residual mass and are being used in an attempt to determine the timing of the eruptive events. Each sample was analyzed for the stable isotope composition of carbon, nitrogen and sulfur, and results are consistent with a petroleum source from the Miocene-age Monterey Formation. Analysis of biomarkers using comprehensive two-dimensional gas chromatography yields a suite of hopanes and steranes also consistent with petroleum from the Monterey Formation, but with anomalously high concentrations of bisnorhopane. To our knowledge, this is the first report of extinct asphalt volcanoes along the Pacific margin of North America, and provides an important window into the activity of cold seeps by capturing important structures that are unlikely to survive into the geologic record.
B53C-05
Methane Consumption in Marine Waters Impacted by Gas Seepage
Microbially mediated methane oxidation in the coastal marine water column is an important sink term in the global methane budget, but remains a relatively uncharacterized process. While oxidation in the water column prevents up to two-thirds of methane released from the sea-floor from transiting to the atmosphere, only a limited number of oxidation rates have been measured, and identities of the microorganisms responsible for methane consumption in this environment remain unknown. To date, there have been no comprehensive studies on the relationship between methane oxidation rates and the microbial population responsible for methane oxidation. As a result of the perennially elevated methane concentrations in the water column at cold seeps, these environments are ideal for investigating the composition and efficacy of pelagic marine methane oxidizing communities. A suite of filter samples were collected, and corresponding methane concentrations and oxidation rates measured, in a series of vertical hydrocasts at seep and background sites in the Santa Barbara and Santa Monica Basins during the SEEPS'07 cruise. Here we present data from samples collected in a grid through the methane plume emanating from the Coal Oil Point (COP) seep field in the Santa Barbara Basin, and from a series of vertical casts above a mud volcano in the Santa Monica Basin. Methane oxidation rate measurements were made using a 3H-CH4 tracer. Preliminary calculations of fractional turnover rates in the water column down- current from the COP seep field indicate a maximum fractional turnover rate of 0.04 day-1 at 50-70 m depth, approximately 25 km from the seep field, along the path of plume advection. In the Santa Monica Basin, fractional turnover rates are highest at the bottom (0.03 day-1 at 800 m), decrease to a minimum (0.002 day-1) at 600 m, and increase from 0.003 day-1 at 300 m to 0.009 day-1 at 25 m. Current molecular work will be presented, and is focused on identifying methanotrophic bacteria based on both functional (pmoA) and phylogenetic (16S rDNA) genes, and on comparing community composition across gradients in methane concentration and oxidation rate, and between seepage sites.
B53C-06
Sulfur and Trace Metal Chemistry of a Methane Charged Brine Pool and Adjacent Porewaters in the Northern Gulf of Mexico
A systematic study of a methane brine pool on the Louisiana continental slope reveals the extent to which steep chemical gradients associated with an anoxic hypersaline basin control the establishment and distribution of chemosynthetic organisms. The seep site, located in the Green Canyon lease block (GC233), provides habitat for a bivalve-dominated community of chemosynthetic mussels ( Bathymodiolus childressi). The pool is a brine-filled pockmark centered over a salt diapir buried within 500 m of the seafloor along which methane and vent fluids migrate to the surface. The depression slopes along its southern margin where brine overflows onto the seafloor. This study sought to establish the chemistry of the brine in an effort to better understand fluid transport to the seafloor and the extent to which brine influences chemosynthetic activity at a methane seep site. Ten sediment push cores were collected during submersible operations within the brine spillway and in upslope background sediments distal to the pool. Initial chemical analyses indicate the brine (128 ppt) is anoxic, chloride-rich (1994.8 mM) and sulfur-poor ([SO42-] = 0.4 mM, [HS-] = 8.2 uM). Steep porewater Cl- and Sr2+ concentration gradients observed in sediments downslope of the brine pool clearly indicate mixing between brine and seawater end members. Porewater sulfur profiles from sediments within the brine outflow indicate complete sulfate consumption within 30 cm below seafloor and sulfide production as great as 5 mM. The paired isotopic composition of dissolved sulfate and sulfide (Δ34SSO4-HS = 40‰) is consistent with bacterial sulfate reduction, potentially driven by the anaerobic oxidation of methane or non-methane hydrocarbons. The brine was nearly devoid of dissolved Mo (22 nM) and enriched in Mn (6.3 uM), relative to measured seawater casts ([Mo] = 112 nM; [Mn] below detection). Dissolved Mo enrichments, up to 392.8 nM, in surficial sediments decrease with depth may indicate brine mixing coupled with accumulation of Mo in sulfidic sediments and subsequent removal during authigenic precipitation. The chemical cycling of sulfur and redox sensitive elements (Mo, Mn, and Fe) will better inform our understanding of modern seep formation and provide a baseline for comparison to ancient seep environments.
B53C-07
Methane concentrations and biogeochemical gradients within acoustic wipe-out zones at a Gulf of Mexico cold seep
The spatial distribution of methane concentrations and biogeochemical gradients were assessed within surficial sediments overlying acoustic wipe-out zones at a cold seep, Mississippi Canyon 118, Gulf of Mexico. We hypothesized that the wipe-outs were caused from saturated methane entrained within upward fluxing hydrocarbon-rich fluids and, since these fluids have been shown to stimulate sulfate reduction, anaerobic oxidation of methane, and methanogenesis, we also hypothesized that we would observe steep biogeochemical gradients, indicating high microbial activity, in sediments overlying these features. We tested these hypotheses by collecting thirty sediment gravity cores both within and outside the wipe-out zones and measuring resultant pore-fluids for dissolved methane and sulfate concentration gradients, methane and dissolved inorganic carbon isotope gradients, and organic matter chemical composition. Outside the wipe-outs, fifteen cores resulted in methane concentrations below 10 uM, limited down-core sulfate or methane concentration gradients, and down- core d13C-CH4 values averaging -52+/-2 ppt. While these background cores exhibited low microbial activity, the cores collected within the wipe-outs resulted in moderate to high activity. Moderate activity was exhibited in eight cores where methane concentrations reached 20 uM and had gentle sloping sulfate and methane concentration gradients. The d13C-CH4 values showed little change with depth and averaged -67+/-4 ppt while the d13C-DIC values decreased from -7ppt at the SWI to -32ppt at the bottom of the cores at a rate of 0.22ppt/cm. In stark contrast, high activity was seen in four cores collected within the wipe-outs. Methane concentrations reached above 4 mM, sulfate was depleted by ~50 cmbsf, and down-core profiles of d13C-CH4 and d13C-DIC were indicative of distinct depth zones of sulfate reduction coupled to anaerobic methane oxidation and methanogenesis. Bulk organic matter analysis suggested that the high activity cores were being supported by a source that is enriched in carbon (C:N=15) and depleted in d15N and d13C compared to other activity groups, possibly due to petroleum influx or chemosynthetic carbon. In the high activity cores, the isotopic values of the DIC were similar to the authigenic carbonates whereas in the moderate activity cores they were not, suggesting not only spatial but temporal variability in microbial processes. While the wipe-outs were correlated with saturated methane and were indicators of active microbial processes, the cause of the wipe-outs was not solely methane saturated pore-fluids. This novel data set was then compiled into a seep characterization model to determine the relative upward fluid flux.
B53C-08
Modern Gas Vents in the St. Lawrence Estuary (Eastern Canada); Linking Palaeozoic Rocks, Quaternary Sediments and the Marine Environment.
Since 2003, the seafloor and the subsurface of the St. Lawrence Estuary (eastern Canada) were surveyed with various geophysical tools such as high and very-high resolution seismic reflection systems, sidescan sonar and multibeam echo sounder; moreover several cores were collected. The most striking features on the multibeam bathymetric imagery are numerous pockmarks (n>750) ranging in diameter from tens to hundreds of meters. The use of multi-resolution geophysical systems allowed the documenting of their fractal nature as micro and macro seeps are observed. Pockmarks are isolated, associated with submarine landslides, linearly distributed or included in cluster features. Sidescan sonar coverage confirms that some vents are active whereas backscatter images show several highly- reflective pockmarks. Carbonate crusts were sampled in some of these highly-reflective features whereas in other pockmarks similar crusts, located at various depths in the sedimentary column, indicate possible intermittent chemosynthetic microbial activity through recent times. Seismic sections permitted to image gas chimneys beneath the pockmarks that are rooted into the bedrock and gas-related amplitude anomalies within the Quaternary succession. In addition several seismic anomalies are found over either bedrock highs or slightly- dipping features imaged near the top of the bedrock. The pockmarks distribution correlates with both the bedrock geology and the thickness of the Quaternary deposits. Pockmarks are preferentially found above the Paleozoic autochtonous domain (St. Lawrence platform) and located where the Quaternary sediment cover is the thinnest (<200 m) although seismic anomalies attributed to the presence of gas, are observed over the entire basin including where the Quaternary succession is the thickest (>420 m). The results of the study suggest that gas release in the marine environment is likely of thermogenic origin and controlled by one or several of the following parameters: 1) the occurrence of a leaking seal layer in some parts of the basin, 2) the lithostatic pressure induced by the Quaternary succession or 3) the physical properties of the underlying breached Paleozoic reservoirs.