Seep Field Research Articles

Cold-seep habitat mapping: High-resolution spatial characterization of the Blake Ridge Diapir seep field

Relationships among seep community biomass, diversity, and physiographic controls such as underlying geology are not well understood. Previous efforts to constrain these relationships at the Blake Ridge Diapir were limited to observations from piloted deep-submergence vehicles. In August 2012, the autonomous underwater vehicle (AUV) Sentry collected geophysical and photographic data over a 0.131km2 area at the Blake Ridge Diapir seeps. A nested survey approach was used that began with a regional or reconnaissance-style survey using sub-bottom mapping systems to locate and identify seeps and underlying conduits. This survey was followed by AUV-mounted sidescan sonar and multibeam echosounder systems mapping on a mesoscale to characterize the seabed physiography. At the most detailed survey level, digital photographic imaging was used to resolve sub-meter characteristics of the biology. Four pockmarks (25–70m diameter) were documented, each supporting chemosynthetic communities. Concentric zonation of mussels and clams suggests the influence of chemical gradients on megafaunal distribution. Data collection and analytical techniques used here yield high-resolution habitat maps that can serve as baselines to constrain temporal evolution of seafloor seeps, and to inform ecological niche modeling and resource management.

A serpentinite-hosted ecosystem in the Southern Mariana Forearc

Several varieties of seafloor hydrothermal vents with widely varying fluid compositions and temperatures and vent communities occur in different tectonic settings. The discovery of the Lost City hydrothermal field in the Mid-Atlantic Ridge has stimulated interest in the role of serpentinization of peridotite in generating H(2)- and CH(4)-rich fluids and associated carbonate chimneys, as well as in the biological communities supported in highly reduced, alkaline environments. Abundant vesicomyid clam communities associated with a serpentinite-hosted hydrothermal vent system in the southern Mariana forearc were discovered during a DSV Shinkai 6500 dive in September 2010. We named this system the "Shinkai Seep Field (SSF)." The SSF appears to be a serpentinite-hosted ecosystem within a forearc (convergent margin) setting that is supported by fault-controlled fluid pathways connected to the decollement of the subducting slab. The discovery of the SSF supports the prediction that serpentinite-hosted vents may be widespread on the ocean floor. The discovery further indicates that these serpentinite-hosted low-temperature fluid vents can sustain high-biomass communities and has implications for the chemical budget of the oceans and the distribution of abyssal chemosynthetic life.

Quantification of CH4 loss and transport in dissolved plumes of the Santa Barbara Channel, California

Methane (CH4) emitted into the coastal ocean faces two primary fates: escape to the atmosphere or prolonged dissolution that allows sufficient time for oxidation by methanotrophic bacteria. The partitioning of CH4 between these fates is modulated by physical, chemical and biological factors, including the distribution of CH4 in the water, temperature, wind speed, water movement, and the biological CH4 oxidation rate. Because of the underlying complexity, studies rarely quantify all of these factors in unison, thereby leaving gaps in our understanding of CH4 biogeochemistry in the coastal ocean. In this study we estimated the partitioning of CH4 between transport, microbial oxidative loss, and sea–air transfer in a defined plume of dissolved CH4 originating from one of the world's largest seep fields, near Coal Oil Point (COP) in the Santa Barbara Channel, California. Depth distributions of CH4 concentration, biologically mediated oxidation rate, and current velocity were quantified at 12 stations in a 198km2 area down-current from COP on July 4–5, 2007. Six stations were sampled again on July 7, 2007 to evaluate temporal plume variability. The observed CH4 distribution revealed two distinct CH4 plumes: a shallow plume centered at ∼40m and a deeper plume centered at ∼200m. The shallow plume originates at COP; the source of the deeper CH4 plume is not known. Cross-sections of both plumes were used to calculate transport and loss terms for dissolved CH4. The results indicate that the majority of the dissolved CH4 is advected and diffuses horizontally by turbulence whereas microbial oxidation, sea–air gas transfer, and vertical turbulent diffusion are less significant. Based on rates estimated in the study area, a model was developed to simulate the fate of the dissolved CH4. The model results suggest that 60% of the CH4 of the sampled plumes will ultimately be microbially oxidized and 40% will be transferred to the atmosphere by sea–air gas exchange under the sampling conditions. These results illustrate the significance of microbial CH4 oxidation in coastal oceans.

Fluid origin, gas fluxes and plumbing system in the sediment-hosted Salton Sea Geothermal System (California, USA)

The Salton Sea Geothermal System (California) is an easily accessible setting for investigating the interactions of biotic and abiogenic geochemical processes in sediment-hosted hydrothermal systems. We present new temperature data and the molecular and isotopic composition of fluids seeping at the Davis-Schrimpf seep field during 2003–2008. Additionally, we show the first flux data for CO 2 and CH 4 released throughout the field from focused vents and diffuse soil degassing. The emitted gases are dominated by CO 2 (~ 98%) and CH 4 (~ 1.5%). By combining δ 13C CO2 (as low as − 5.4‰) and δ 13C CH4 (− 32‰ to − 17.6‰) with 3He/ 4He (R/Ra > 6) and δD CH4 values (− 216‰ to − 150‰), we suggest, in contrast to previous studies, that CO 2 may have a significant Sub-Continental Mantle source, with minimal crustal contamination, and CH 4 seems to be a mixture of high temperature pyrolitic (thermogenic) and abiogenic gas. Water seeps show that δD and δ 18O increase proportionally with salinity (Total Dissolved Solids in g/L) ranging from 1–3 g/L (gryphons) to 145 g/L (hypersaline pools). In agreement with elemental analyses, the isotopic composition of the waters indicate a meteoric origin, modified by surface evaporation, with little or no evidence of deep fossil or magmatic components. Very high Cl/Br (> 3,000) measured at many seeping waters suggests that increased salinities result from dissolution of halite crusts near the seep sites. Gas flux measurements from 91 vents (pools and gryphons) give a conservative estimate of ~ 2,100 kg of CO 2 and 11.5 kg of CH 4 emitted per day. In addition soil degassing measured at 81 stations (20x20 m grid over 51,000 m 2) revealed that 7,310 kg/d CO 2 and 33 kg/d CH 4 are pervasively released to the atmosphere. These results emphasise that diffuse gas emission from soil can be dominant (~ 75%) even in hydrothermal systems with large and vigorous gas venting. Sediment-hosted hydrothermal systems may represent an intermediate class of geologic methane sources for the atmosphere, with emission factors lower than those of sedimentary seepage in petroleum basins but higher than those of traditional geothermal-volcanic systems; on a global scale they may significantly contribute to the atmospheric methane budget.

Detection of marine methane emissions with AVIRIS band ratios

[1] The relative source contributions of methane (CH4) have high uncertainty, creating a need for local-scale characterization in concert with global satellite measurements. However, efforts towards methane plume imaging have yet to provide convincing results for concentrated sources. Although atmospheric CH4 mapping did not motivate the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) design, recent studies suggest its potential for studying concentrated CH4 sources such as the Coal Oil Point (COP) seep field (∼0.015 Tg CH4 yr−1) offshore Santa Barbara, California. In this study, we developed a band ratio approach on high glint COP AVIRIS data and demonstrate the first successful local-scale remote sensing mapping of natural atmospheric CH4 plumes. Plume origins closely matched surface and sonar-derived seepage distributions, with plume characteristics consistent with wind advection. Imaging spectrometer data may also be useful for high spatial-resolution characterization of concentrated, globally-significant CH4 emissions from offshore platforms and cattle feedlots.

Active multibeam sonar-derived bubble plume fluxes and dynamics.

By their nature, seeps are spatially and temporally variable and episodic; thus, effective emissions’ quantification presents significant challenges because local measurements likely are unrepresentative. A multibeam sonar system was developed to observe ebullition spatially and used to study water column bubble plume processes, as well as subsurface migration processes for seepage from the Coal Oil Point seep field and the East Siberian Arctic Sea is presented. The system is designed for deep-sea application related to hydrates as part of a benthic observatory.

Characteristics and scaling of bubble plumes from marine hydrocarbon seepage in the Coal Oil Point seep field

The fate of marine seep gases in the ocean and atmosphere is intimately connected with bubble and bubble‐plume processes, which are strongly size‐ and depth‐dependent. Size‐dependent flux distributions, Φ, and vertical velocity functions, VZ(r), were measured with a video bubble measurement system in the Santa Barbara Channel, California. Several distinct plume types were identified for which size distributions were measured, major, minor, intermediate, and obstructed. A further vent class is elastic (which was observed but not quantified). In addition, any plume class could be oily. Minor plumes generally produced a lower emission flux, Q, and showed narrow, peaked Φ that were well described as Gaussian. The radius of the dominant peak, RP, of minor plumes varied as Q0.40, with correlation coefficient, R2 = 0.84, in agreement with theoretical relationship of RP∼Q0.4 for Q above a critical flow rate. Oil contamination was found to affect RP and was not used in the fit. A probability distribution, Ψ(RP), for all Gaussian bubble plumes was itself well described by a combination of Gaussian functions, which were different for different seep areas. Major plumes showed a broad distribution including very small and very large bubbles and were well described by a power law with exponent a, which varied with Q according to a = 0.43 + 0.55 log(Q) with R2 = 0.77. One obstructed vent was analyzed and shared characteristics with the minor bubble plumes. Mixed bubble plume size distributions showed characteristics of both major and minor plume classes, i.e., were described by a combination of Gaussian functions and power laws, and were steeper (higher a) than major plumes for the same Q. Oily plumes produced complex, confused bubble size distributions. Upwelling velocities, VUP(r), were derived from VZ(r) and increased as VUP∼Q0.66 (R2 = 0.64); however, consideration of the more intense plumes (Q > 2 cm s−1) showed VUP∼Q0.35 in agreement with other published field measurements. Thus, the weaker bubble plumes were observed during the acceleration phase.

Methane oxidation in permeable sediments at hydrocarbon seeps in the Santa Barbara Channel, California

Abstract. A shallow-water area in the Santa Barbara Channel, California, known collectively as the Coal Oil Point seep field, is one of the largest natural submarine hydrocarbon emission areas in the world. Both gas and oil are seeping constantly through a predominantly sandy seabed into the ocean. This study focused on the methanotrophic activity within the surface sediments (0–15 cm) of the permeable seabed in the so-called Brian Seep area at a water depth of ∼10 m. Detailed investigations of the sediment biogeochemistry of active gas vents indicated that it is driven by fast advective transport of water through the sands, resulting in a deep penetration of oxidants (oxygen, sulfate). Maxima of microbial methane consumption were found at the sediment-water interface and in deeper layers of the sediment, representing either aerobic or anaerobic oxidation of methane, respectively. Methane consumption was relatively low (0.6–8.7 mmol m−2 d-1) in comparison to gas hydrate-bearing fine-grained sediments on the continental shelf. The low rates and the observation of free gas migrating through permeable coastal sediments indicate that a substantial proportion of methane can escape the microbial methane filter in coastal sediments.

Long-term monitoring of a marine geologic hydrocarbon source by a coastal air pollution station in Southern California

Hourly total hydrocarbon (THC) data, spanning 1990–2008 from a California air pollution station located near the Coal Oil Point (COP) seep field, were analyzed and clearly showed geologic CH 4 emissions as the dominant local source. Annual COP emissions are conservatively estimated as 0.015 Tg CH 4 year −1 and represent a natural and concentrated geologic methane source (24 m 3 m −2 day −1 gas flux at some active seeps, Clark et al., 2010). For a sense of the scale and potential importance to the regional Southern California methane budget, COP emits an amount equivalent to 8% of the estimated Los Angeles County anthropogenic emissions. Station THC measurements near COP showed a strong wind dependency with elevated levels closely correlated with a sonar-derived spatial distribution of seep field emissions. THC varied seasonally, with a maximum in January and minimum in July and a peak-to-peak amplitude of 0.24 ppm. The seasonal signal was more readily apparent midday ( R 2 = 0.69 harmonic fit), compared to nighttime and morning ( R 2 < 0.45). The bimodal diel THC pattern consisted of seasonally-modulated peaks in the morning and evening. THC temporal and spatial trends were consistent with both transport and source emission variations. Long-term, annual seep field emissions consistently decreased on a field-wide basis until the late 1990s, before increasing consistently, most likely as a function of underlying geologic processes. This study demonstrates the value of municipal air quality monitoring stations for insight into local greenhouse gas sources and highlights the non-negligible and variable contribution from marine geologic seepage.

Identification of Novel Methane-, Ethane-, and Propane-Oxidizing Bacteria at Marine Hydrocarbon Seeps by Stable Isotope Probing

Marine hydrocarbon seeps supply oil and gas to microorganisms in sediments and overlying water. We used stable isotope probing (SIP) to identify aerobic bacteria oxidizing gaseous hydrocarbons in surface sediment from the Coal Oil Point seep field located offshore of Santa Barbara, California. After incubating sediment with (13)C-labeled methane, ethane, or propane, we confirmed the incorporation of (13)C into fatty acids and DNA. Terminal restriction fragment length polymorphism (T-RFLP) analysis and sequencing of the 16S rRNA and particulate methane monooxygenase (pmoA) genes in (13)C-DNA revealed groups of microbes not previously thought to contribute to methane, ethane, or propane oxidation. First, (13)C methane was primarily assimilated by Gammaproteobacteria species from the family Methylococcaceae, Gammaproteobacteria related to Methylophaga, and Betaproteobacteria from the family Methylophilaceae. Species of the latter two genera have not been previously shown to oxidize methane and may have been cross-feeding on methanol, but species of both genera were heavily labeled after just 3 days. pmoA sequences were affiliated with species of Methylococcaceae, but most were not closely related to cultured methanotrophs. Second, (13)C ethane was consumed by members of a novel group of Methylococcaceae. Growth with ethane as the major carbon source has not previously been observed in members of the Methylococcaceae; a highly divergent pmoA-like gene detected in the (13)C-labeled DNA may encode an ethane monooxygenase. Third, (13)C propane was consumed by members of a group of unclassified Gammaproteobacteria species not previously linked to propane oxidation. This study identifies several bacterial lineages as participants in the oxidation of gaseous hydrocarbons in marine seeps and supports the idea of an alternate function for some pmoA-like genes.

Acoustic imaging of natural gas seepage in the North Sea: Sensing bubbles controlled by variable currents

Natural seepage from the seafloor is a worldwide phenomenon but quantitative measurements of gas release are rare, and the entire range of the dynamics of gas release in space, time, and strength remains unclear so far. To mitigate this, the hydroacoustic device GasQuant (180 kHz, multibeam) was developed to monitor the tempo‐spatial variability of gas bubble releases from the seafloor. GasQuant was deployed in 2005 on the seafloor of the seep field Tommeliten (North Sea) for 36 h. This in situ approach provides much better spatial and temporal resolution of seeps than using conventional ship‐born echo sounders. A total of 52 gas vents have been detected. Detailed time series analysis revealed a wide range of gas release patterns ranging from very short periodic up to 50 min long‐lasting events. The bulk gas seepage in the studied area is active for more than 70% of observation time. The venting clearly exhibits tidal control showing a peak in the second quarter of the tidal pressure cycle, where pressure drops fastest. The hydroacoustic results are compared with video observations and bubble flux estimates from remotely operated vehicle dives described in the literature. An advanced approach for identifying and visualizing rising bubbles in the sea by hydroacoustics is presented in which water current data were considered. Realizing that bubbles are moved by currents helps to improve the detection of gas bubbles in the data, better discriminate bubbles against fish echoes, and to enhance the S/N ratio in the per se noisy acoustic data.

Geologic control of natural marine hydrocarbon seep emissions, Coal Oil Point seep field, California

High-resolution sonar surveys, and a detailed subsurface model constructed from 3D seismic and well data allowed investigation of the relationship between the subsurface geology and gas-phase (methane) seepage for the Coal Oil Point (COP) seep field, one of the world's largest and best-studied marine oil and gas seep fields, located over a producing hydrocarbon reservoir near Santa Barbara, California. In general, the relationship between terrestrial gas seepage, migration pathways, and hydrocar- bon reservoirs has been difficult to assess, in part because the detection and mapping of gas seepage is problematic. For marine seepage, sonar surveys are an effective tool for mapping seep gas bubbles, and thus spatial distributions. Seepage in the COP seep field occurs in an east-west- trending zone about 3-4 km offshore, and in another zone about 1-2 km from shore. The farthest offshore seeps are mostly located near the crest of a major fold, and also along the trend of major faults. Significantly, because faults observed to cut the fold do not account for all the observed seepage, seepage must occur through fracture and joint systems that are difficult to detect, including intersecting faults and fault damage zones. Inshore seeps are concen- trated within the hanging wall of a major reverse fault. The subsurface model lacks the resolution to identify specific structural sources in that area. Although to first order the spatial distribution of seeps generally is related to the major structures, other factors must also control their distribution. The region is known to be critically stressed, which would enhance hydraulic conductivity of favorably oriented faults, joints, and bedding planes. We propose that this process explains much of the remaining spatial distribution.

Formation of seep bubble plumes in the Coal Oil Point seep field

The fate of marine seep gases (transport to the atmosphere or dissolution, and either bacterial oxidation or diffusion to the atmosphere) is intimately connected with bubble and bubble-plume processes, which are strongly size-dependent. Based on measurements with a video bubble measurement system in the Coal Oil Point seep field in the Santa Barbara Channel, California, which recorded the bubble-emission size distribution (Φ) for a range of seep vents, three distinct plume types were identified, termed minor, major, and mixed. Minor plumes generally emitted bubbles with a lower emission flux, Q, and had narrow, peaked Φ that were well described by a Gaussian function. Major plumes showed broad Φ spanning very small to very large bubbles, and were well described by a power law function. Mixed plumes showed characteristics of both major and minor plume classes, i.e., they were described by a combination of Gaussian and power law functions, albeit poorly. To understand the underlying formation mechanism, laboratory bubble plumes were created from fixed capillary tubes, and by percolating air through sediment beds of four different grain sizes for a range of Q. Capillary tubes produced a Φ that was Gaussian for low Q. The peak radius of the Gaussian function describing Φ increased with capillary diameter. At high Q, they produced a broad distribution, which was primarily described by a power law. Sediment-bed bubble plumes were mixed plumes for low Q, and major plumes for high Q. For low-Q sediment-bed Φ, the peak radius decreased with increasing grain size. For high Q, sediment-bed Φ exhibited a decreased sensitivity to grain size, and Φ tended toward a power law, similar to that for major seep plumes.

Compositional variability and air-sea flux of ethane and propane in the plume of a large, marine seep field near Coal Oil Point, CA

Large quantities of methane (C1), ethane (C2), and propane (C3) emanate from shallow marine seeps near Coal Oil Point (COP), California. Concentrations of these gases were analyzed in the surface water down-current of the seep field over a 15-month period. The variable proportions of C1, C2, and C3 analyzed in gas bubbles emitted from 16 distinct seeps in the COP field encompass much of the variability found in the surface waters down-current. However, waters with disproportionate levels of C1 suggest the presence of additional C1 sources. Based on three spatial surveys, covering areas up to 280 km2, C2 and C3 air-sea fluxes were estimated to be in the order of 3.7 and 1.4 μmol day−1 m−2, respectively. Only 0.6% of C2 and 0.5% of C3 in the dissolved plume originating from the COP seep field are transferred to the atmosphere in the study area, with the fate of the remainder uncertain.

Gas flux and carbonate occurrence at a shallow seep of thermogenic natural gas

The Coal Oil Point seep field located offshore Santa Barbara, CA, consists of dozens of named seeps, including a peripheral ∼200 m2 area known as Brian Seep, located in 10 m water depth. A single comprehensive survey of gas flux at Brian Seep yielded a methane release rate of ∼450 moles of CH4 per day, originating from 68 persistent gas vents and 23 intermittent vents, with gas flux among persistent vents displaying a log normal frequency distribution. A subsequent series of 33 repeat surveys conducted over a period of 6 months tracked eight persistent vents, and revealed substantial temporal variability in gas venting, with flux from each individual vent varying by more than a factor of 4. During wintertime surveys sediment was largely absent from the site, and carbonate concretions were exposed at the seafloor. The presence of the carbonates was unexpected, as the thermogenic seep gas contains 6.7% CO2, which should act to dissolve carbonates. The average δ13C of the carbonates was −29.2 ± 2.8‰ VPDB, compared to a range of −1.0 to +7.8‰ for CO2 in the seep gas, indicating that CO2 from the seep gas is quantitatively not as important as 13C-depleted bicarbonate derived from methane oxidation. Methane, with a δ13C of approximately −43‰, is oxidized and the resulting inorganic carbon precipitates as high-magnesium calcite and other carbonate minerals. This finding is supported by 13C-depleted biomarkers typically associated with anaerobic methanotrophic archaea and their bacterial syntrophic partners in the carbonates (lipid biomarker δ13C ranged from −84 to −25‰). The inconsistency in δ13C between the carbonates and the seeping CO2 was resolved by discovering pockets of gas trapped near the base of the sediment column with δ13C-CO2 values ranging from −26.9 to −11.6‰. A mechanism of carbonate formation is proposed in which carbonates form near the sediment–bedrock interface during times of sufficient sediment coverage, in which anaerobic oxidation of methane is favored. Precipitation occurs at a sufficient distance from active venting for the molecular and isotopic composition of seep gas to be masked by the generation of carbonate alkalinity from anaerobic methane oxidation.

Mapping methane emissions from a marine geological seep source using imaging spectrometry

Methane (CH 4) is an extremely important greenhouse gas that has increased significantly in pre and post-industrial times. Due to CH 4's strong absorptions in the shortwave infrared (SWIR), the potential exists to use imaging spectrometers, such as the Airborne Visible Infrared Imaging Spectrometer (AVIRIS), to map CH 4 emissions. Here, we present research evaluating the ability of AVIRIS to map CH 4 emitted by one of the largest marine geologic CH 4 sources in the world, the Coal Oil Point seep field in the Santa Barbara Channel, California. To develop algorithms for detecting CH 4 and to establish detection limits, initial analysis focused on simulated radiance spectra, calculated using Modtran 5.2 radiative transfer code that was parameterized to match scene conditions for a 6 August 2007 AVIRIS flight over the area. Model simulations included a range of surface albedos, variable column water vapor, and CH 4 concentrations ranging from 1.7 ppm (background) up to the equivalent of 2500 ppm in the lower 20 m of the boundary layer. A multistep CH 4 detection algorithm was developed using Modtran simulations. First, surface albedo was estimated from the radiance at 2139 nm. Next, albedo-specific radiance for background CH 4 was used to calculate spectral residuals for CH 4 above background. A CH 4 index, C, calculated as the average residual between 2248 and 2298 nm, showed high sensitivity to CH 4, with minimal sensitivity to water vapor or surface albedo. Detection limits in simulations depended on CH 4 concentrations and albedo, ranging from 990 ppm for a 0.5% albedo surface, to as low as 18 ppm for albedos higher than 22%. Application of this approach to the AVIRIS data demonstrated considerable potential for mapping CH 4. Due to specular reflectance off of the ocean surface, albedo in the scene varied significantly, from less than 0.5% to over 30%. Strong CH 4 anomalies were observed in the data acquired over the seep field, which produced large C values with spectral residuals consistent with CH 4 and estimated radiance spectra that matched measurements. All strong anomalies were located in close vicinity to and downwind from known CH 4 sources. However, contrary to simulated data, C was overly sensitive to albedo, restricting high confidence anomalies only to the brightest surfaces, and showing high frequency spatial variation throughout the AVIRIS image. CH 4 concentration was overestimated by C, potentially due to a spectral trend in sea surface reflectance and/or the impact of diffuse light on dark surfaces (< 1%) leading to the over-expression of CH 4 absorptions.

Variability of gas composition and flux intensity in natural marine hydrocarbon seeps

The relationship between surface bubble composition and gas flux to the atmosphere was examined at five large seeps from the Coal Oil Point seep field (Santa Barbara Channel, CA, USA). The field research was conducted using a flux buoy designed to simultaneously measure the surface bubbling gas flux and the buoy’s position with differential GPS, and to collect gas samples. Results show that the flux from the five seeps surveyed a total of 11 times ranged from 800–5,500 m3 day−1. The spatial distribution of flux from the five seeps was well described by two lognormal distributions fitted to two flux ranges. The seafloor and sea surface composition of bubbles differed, with the seafloor bubbles containing significantly more CO2 (3–25%) and less air (N2 and O2). At the sea surface, the mole fraction of N2 correlated directly with O2 (R 2 = 0.95) and inversely with CH4 (R 2 = 0.97); the CO2 content was reduced to the detection limit (<0.1%). These data demonstrate that the bubble composition is modified by gas exchange during ascent: dissolved air enters, and CO2 and hydrocarbon gases leave the bubbles. The mean surface composition at the five seeps varied with water depth and gas flux, with more CH4 and higher CH4/N2 ratios found in shallower seeps with higher flux. It is suggested that the CH4/N2 ratio is a good proxy for total or integrated gas loss from the rising bubbles, although additional study is needed before this ratio can be used quantitatively.

Dynamics of hydrothermal seeps from the Salton Sea geothermal system (California, USA) constrained by temperature monitoring and time series analysis

Water‐, mud‐, gas‐, and petroleum‐bearing seeps are part of the Salton Sea geothermal system (SSGS) in southern California. Carbon dioxide is the main component behind the seeps in the Davis‐Schrimpf seep field (∼20,000 m2). In order to understand the mechanisms driving the system, we have investigated the seep dynamics of the field by monitoring the temperature of two pools and two gryphons for 2180 h (90.8 days) in the period from December 2006 to March 2007, with a total of 32,700 measurements per station. The time series have been analyzed by statistical methods using cross correlation, autocorrelation and spectral analysis, and autoregressive modeling. The water‐rich pools never exceed 34.0°C and are characterized by low‐amplitude temperature variations controlled by the diurnal cycles in air temperature. The long‐term validity of these results is evident from a second period of temperature monitoring of one of the pools from December 2007 to April 2008 (120 days). In contrast to the pools, the mud‐rich gryphons have a strikingly different behavior. The gryphons are hotter (maximum 69.7°C) and have large amplitude variations (standard deviation of 6.4) that overprint any signal from external diurnal forcing. Autoregressive modeling shows the presence of distinct hot and cold pulses in the gryphon temperature time series, with amplitudes up to 3°C. These pulses likely reflect a combination of hydrothermal flux variations from the SSGS and the local temporal changes in bubbling activity within the gryphons.

Methanotrophic bacteria occupy benthic microbial mats in shallow marine hydrocarbon seeps, Coal Oil Point, California

Microbial mats composed of giant sulfur bacteria are observed throughout the benthos along continental margins. These communities serve to oxidize dissolved sulfides to sulfate, and are typically associated with the recent exposure of sulfide‐rich sediments. Such mats are also ubiquitous in areas of hydrocarbon seepage, where they are thought to consume sulfide generated in underlying sediment. Despite the high abundance of dissolved methane in hydrocarbon seeps, few studies have considered the importance of methanotrophy in mat communities. To assess the importance of methanotrophs in microbial mats from hydrocarbon seeps, an approach involving lipid biomarkers, stable isotopes and enrichment culturing was applied. Microbial mat samples were collected from benthic surfaces at two hydrocarbon seeps located in the Coal Oil Point seep field, offshore from Goleta, California. Both samples display a high abundance of 16:1 fatty acids, including two isomers specific to type I methanotrophic bacteria, 16:1(ω8) and 16:1(ω6). Depleted values of δ13C found in 16:1 fatty acids suggests methane assimilation into biomass, whereas three separate investigations of sulfide‐oxidizing bacteria yield fractionation factors too small to account for these values. On the basis of these observations and experiments, an isotope mass balance was applied to fatty acids present in the microbial mat samples which indicates methanotrophs contribute up to 46% of total fatty acids. These results implicate methanotrophy as an important function for microbial mats in seep areas, despite the visual appearance of these mats as being composed of giant sulfur bacteria.

Gaseous emission rates from natural petroleum seeps in the Upper Ojai Valley, California

The atmospheric fluxes of methane (CH4), carbon dioxide (CO2), and reactive organic gases (ROGs) were determined for natural terrestrial petroleum seeps in the Upper Ojai Valley, California, by measuring the emission rates from five vents and scaling these measurements with the known distribution of seeps within the valley. The Upper Ojai Valley seeps emit about 55 m3/day (1942 ft3/day) of gas, of which about 15 m3/day (529 ft3/day; 3.6 Mg/yr) is CH4, about 40 m3/day (1412 ft3/day; 27 Mg/yr) is CO2, and less than 0.05 m3/day (1.765 ft3/day; 0.04 Mg/yr) are ROGs. CH4 and ROG fluxes in the Upper Ojai Valley are, respectively, three and five orders of magnitude less than at the well-characterized Coal Oil Point field, a large offshore seep field located approximately 70 km (43 mi) to the west of the valley. The CO2 flux from these two fields is about the same. The compositions and 13C values of seep and reservoir gases were also quantified and indicate extensive biodegradation of gaseous hydrocarbons and the input of isotopically enriched CO2 during ascent from the reservoir. Unlike the nearby CH4-dominated marine seeps, the largest percentage of gas emitted by seeps in the Upper Ojai Valley is CO2.