Natural Seeps Research Articles

SATELLITE MONITORING OF THE BLACK SEA ECOLOGICAL RISK AREAS

Results of multiyear monitoring of zones of persistent ecological risk in the Black Sea are presented. These ones are most affected by oil pollution, raised concentration of suspended particulate matter and harmful algae bloom. Studies are based on satellite remote sensing data obtained over the Black Sea from 2000 to 2017 from radar sensors ERS-2 SAR, Envisat ASAR, Sentinel -1A, -B, Radarsat 2, TerraSAR-X; visible and infrared data from Envisat MERIS, Landsat-5 TM, Landsat-7 ETM+, Landsat-8 OLI, MSI Sentinel-2A and Terra/Aqua Modis. An analysis of radar data indicated areas most affected by ship spills of bilge waters. Greatest polluted area were found along the main ship routes, near biggest ports and at sites of anchor positions.It is necessary to separate anthropogenic oil pollution and oil-containing slicks caused by natural hydrocarbon seeps including mud volcanoes, natural gas and oil outflows from the sea bottom, and gas hydrates. The geographical distribution of these slicks is defined by their permanent locations that well correlate with locations of natural hydrocarbon seeps from the sea bottom in this region.Areas characterized by increased concentration of suspended particulate matter were revealed at maps compiled from Envisat MERIS, MSI Sentinel-2A data and Landsat colour composites. Among the most notable ones are river plume zones, first of all, those of the Danube River and mountain rivers of Georgia. Results of satellite data processing were used for detecting impacts of various natural factors, such as precipitation, rivers flows, wind-driven water circulation and vortex activity, on suspended matter proliferation. Mapping of zones of maximum propagation of suspended solids in different seasons was performed for the examined areas. Eutrophication has recently become a very important problem worldwide. It is true also for the Black Sea, especially for its western part.

Airborne DOAS retrievals of methane, carbon dioxide, and water vapor concentrations at high spatial resolution: application to AVIRIS-NG

Abstract. At local scales, emissions of methane and carbon dioxide are highly uncertain. Localized sources of both trace gases can create strong local gradients in its columnar abundance, which can be discerned using absorption spectroscopy at high spatial resolution. In a previous study, more than 250 methane plumes were observed in the San Juan Basin near Four Corners during April 2015 using the next-generation Airborne Visible/Infrared Imaging Spectrometer (AVIRIS-NG) and a linearized matched filter. For the first time, we apply the iterative maximum a posteriori differential optical absorption spectroscopy (IMAP-DOAS) method to AVIRIS-NG data and generate gas concentration maps for methane, carbon dioxide, and water vapor plumes. This demonstrates a comprehensive greenhouse gas monitoring capability that targets methane and carbon dioxide, the two dominant anthropogenic climate-forcing agents. Water vapor results indicate the ability of these retrievals to distinguish between methane and water vapor despite spectral interference in the shortwave infrared. We focus on selected cases from anthropogenic and natural sources, including emissions from mine ventilation shafts, a gas processing plant, tank, pipeline leak, and natural seep. In addition, carbon dioxide emissions were mapped from the flue-gas stacks of two coal-fired power plants and a water vapor plume was observed from the combined sources of cooling towers and cooling ponds. Observed plumes were consistent with known and suspected emission sources verified by the true color AVIRIS-NG scenes and higher-resolution Google Earth imagery. Real-time detection and geolocation of methane plumes by AVIRIS-NG provided unambiguous identification of individual emission source locations and communication to a ground team for rapid follow-up. This permitted verification of a number of methane emission sources using a thermal camera, including a tank and buried natural gas pipeline.

Enhanced subsea leakage detection with broadband active acoustic sensor system

For field re-activation, improved/enhanced oil recovery through injection of gas in reservoirs, and geological storage of CO2 (carbon dioxide), robust barriers must be provided and confirmed to avoid leakage. Natural seeps from the sea floor represents an additional challenge, as many detection systems will misinterpret them as an infrastructure leakage. Active acoustics (backscattering) has been successfully used for detection of even moderate underwater hydrocarbon gas releases at long ranges, using, e.g., scientific echosounders. However, CO2 leakage behavior is particularly complex and operations in cold and deep waters make such leakages difficult to detect with active acoustics, due to its behavior and low acoustic impedance in liquid phase. Methodology for detecting CO2 even in liquid phase is a prerequisite for many deep water operations, and at present no adequate technology exists for this purpose. In order to design an active acoustic system that can detect subsea leakage of hydrocarbons and CO2 in liquid phase, the behavior and backscattering by droplets as a function of environmental parameters and acoustic frequency is simulated. Theoretical backscattering is further compared with in situ acoustic and optical measurements of CO2, CH4 (methane), and air, released and measured at depths from 1300m to surface, using a custom built gas release and measurement frame.

Acoustic investigations of natural seeps at GC600

We examine the acoustic signature of natural seeps in the Green Canyon block 600 of the Gulf of Mexico. The survey site was 2200'1600 m2 at a depth of 1200 m. Near-bottom multibeam backscatter, side-scan sonar mosaic, and chirp sub-bottom profiles were collected using an AUV cruising at 40 m above the seafloor. The water-column profiles collected by the Kongsberg EM2040 at 200 kHz central frequency were utilized to detect and localize seeps. Geological information from the chirp sonar and side scan images will also augment these data to further our understanding of the geologic structure of natural seeps. Identified seeps from the backscatter intensity were visited with a Comanche ROV for a visual inspection. An oily plume was selected for further investigations and vertical upwelling will be monitored with an Acoustic Scintillation Flow Meter (ASFM), for the first time. Concurrent measurements include nearby Acoustic Doppler profilers, a vertical array of conductivity/temperature instruments and two video cameras (VTLC) for identifying bubble/droplet sizes and distribution and will provide invaluable reference data on temporal variability.

Hyperspectral and Radar Airborne Imagery over Controlled Release of Oil at Sea.

Remote sensing techniques are commonly used by Oil and Gas companies to monitor hydrocarbon on the ocean surface. The interest lies not only in exploration but also in the monitoring of the maritime environment. Occurrence of natural seeps on the sea surface is a key indicator of the presence of mature source rock in the subsurface. These natural seeps, as well as the oil slicks, are commonly detected using radar sensors but the addition of optical imagery can deliver extra information such as thickness and composition of the detected oil, which is critical for both exploration purposes and efficient cleanup operations. Today, state-of-the-art approaches combine multiple data collected by optical and radar sensors embedded on-board different airborne and spaceborne platforms, to ensure wide spatial coverage and high frequency revisit time. Multi-wavelength imaging system may create a breakthrough in remote sensing applications, but it requires adapted processing techniques that need to be developed. To explore performances offered by multi-wavelength radar and optical sensors for oil slick monitoring, remote sensing data have been collected by SETHI (Système Expérimental de Télédection Hyperfréquence Imageur), the airborne system developed by ONERA (the French Aerospace Lab), during an oil spill cleanup exercise carried out in 2015 in the North Sea, Europe. The uniqueness of this dataset lies in its high spatial resolution, low noise level and quasi-simultaneous acquisitions of different part of the EM spectrum. Specific processing techniques have been developed to extract meaningful information associated with oil-covered sea surface. Analysis of this unique and rich dataset demonstrates that remote sensing imagery, collected in both optical and microwave domains, allows estimating slick surface properties such as the age of the emulsion released at sea, the spatial abundance of oil and the relative concentration of hydrocarbons remaining on the sea surface.

Fault Permeability and CO2 Storage

Faults comprise zones of crushed, sheared and fractured rock that have the potential to influence the migration of stored CO2. Fault-zone permeabilities of 10-9 to 10-19 m2 are controlled by many interdependent factors including; fault-zone architecture and rock types, mechanical strength and permeability of host rock, orientation and magnitude of in situ stresses, fracture aperture size and connectivity, fluid properties and burial history. Mitigating the risk of CO2 migration via faults to the atmosphere or into economically valuable resources requires an understanding of the conditions under which they promote fluid flow from the reservoir. In situ flow data from natural seeps indicate that faults can promote the upward flow of CO2, with flux rates being greatest where the highest densities of fractures occur. Flow simulation modelling suggests that low-permeability fault rock may compartmentalise reservoirs giving rise to increased pressures and promoting upward flow of CO2. Migration rates along faults of up to 1000 m/yr are possible and could produce leakage rates of up to 15000 t/yr at natural seeps. These rates are likely to be site specific and positively related to reservoir pressures. Present understanding of fault hydraulic properties is generally not sufficiently complete to predict when and where faults will influence CO2 migration. To improve understanding of fault hydraulic properties, studies of outcrop, analogue and numerical models are required. In situ flow measurements are critical for testing site-specific and generic fault fluid-flow models that are important in establishing guidelines for the inclusion of faults in risk assessment and determining what mitigation measures are most appropriate.

Natural CH4 Gas Seeps in the French Alps: Characteristics, Typology and Contribution to CH4 Natural Emissions to the Atmosphere

Two CH4-rich (> 80% vol.) gas seeps situated in the French Alps have been characterized in order to get a detailed overview of the gas phases and their spatial variability. Albeit set in similar geological settings, the Le Gua and Rochasson gas seeps present some differences in C2H6 or CO2 contents. Gas fluxes emitted into the atmosphere suggest that few hundreds of kg of CH4 are emitted each day from the seep area of Le Gua together with tens of kg of CO2. Emissions at Rochasson are one order of magnitude lower and only consist in CH4 venting. Site monitoring techniques and strategies are discussed.

Evaluating the Performance of Soil Flux Surveys and Inversion Methods for Quantification of CO2 Leakage

Understanding the near surface migration patterns and rates of efflux of CO2 is important for developing effective monitoring and verification programs for the geological storage of CO2. Soil flux surveys are a well-established technique for characterising surface CO2 emission sources from controlled release sites, CO2 storage sites or natural CO2 seeps. The performance of four interpolation methods; arithmetic mean (AM), two minimum variance unbiased estimators (MVUE), and a newly developed geostatistical cubic surface were evaluated using 21 soil flux surveys conducted over two controlled release experiments in 2012 and 2013, at the Ginninderra controlled release facility, Australia. Data was binned to approximate a regular sampling grid for improved performance of the whole-of-field AM and MVUE averaging techniques. The AM and MVUE methods were highly sensitive to deviations in the statistical distribution of the data, and performed inconsistently across the two experiments. These two methods proved ill-suited for application to CO2 leak quantification due to their inflexible sampling and distribution requirements. The cubic technique provided the best net emission estimates across both experiments, and when applied at different bin sizes, estimating the true release rate to within 20% for the 2012 experiment and 45% below the release rate for the 2013 experiment. The cubic method is well-suited for CO2 leak quantification because it is not limited by assumptions of the data's spatial or statistical distribution. Net H2O emissions of 29 kg/d were observed coincident with the high CO2 flux zones in the field. The interpolation methods were applied with similar results on soil flux surveys taken from a natural seepage site in Qinghai, China. Gravity currents appear to describe the observed soil flux and soil gas behavior at Ginninderra, i.e. the observed lateral migration of CO2 in the subsurface. Subsurface migration was also strongly influenced by the relative depth of the groundwater. Thus the low water table and greater vadose zone in the 2013 experiment is suspected to facilitate greater lateral CO2 migration and explain the poor closure of the CO2 balance.

Fungal diversity in deep-sea sediments associated with asphalt seeps at the Sao Paulo Plateau

We investigated the fungal diversity in a total of 20 deep-sea sediment samples (of which 14 samples were associated with natural asphalt seeps and 6 samples were not associated) collected from two different sites at the Sao Paulo Plateau off Brazil by Ion Torrent PGM targeting ITS region of ribosomal RNA. Our results suggest that diverse fungi (113 operational taxonomic units (OTUs) based on clustering at 97% sequence similarity assigned into 9 classes and 31 genus) are present in deep-sea sediment samples collected at the Sao Paulo Plateau, dominated by Ascomycota (74.3%), followed by Basidiomycota (11.5%), unidentified fungi (7.1%), and sequences with no affiliation to any organisms in the public database (7.1%). However, it was revealed that only three species, namely Penicillium sp., Cadophora malorum and Rhodosporidium diobovatum, were dominant, with the majority of OTUs remaining a minor community. Unexpectedly, there was no significant difference in major fungal community structure between the asphalt seep and non-asphalt seep sites, despite the presence of mass hydrocarbon deposits and the high amount of macro organisms surrounding the asphalt seeps. However, there were some differences in the minor fungal communities, with possible asphalt degrading fungi present specifically in the asphalt seep sites. In contrast, some differences were found between the two different sampling sites. Classification of OTUs revealed that only 47 (41.6%) fungal OTUs exhibited >97% sequence similarity, in comparison with pre-existing ITS sequences in public databases, indicating that a majority of deep-sea inhabiting fungal taxa still remain undescribed. Although our knowledge on fungi and their role in deep-sea environments is still limited and scarce, this study increases our understanding of fungal diversity and community structure in deep-sea environments.

Quantification of methane fluxes from hydrocarbon seeps to the ocean and atmosphere: Development of an in situ and online gas flux measuring system

Natural hydrocarbon seeps in the marine environment are important contributors to greenhouse gases in the atmosphere. Such gases include methane, which plays a significant role in global carbon cycling and climate change. To accurately quantify the methane flux from hydrocarbon seeps on the seafloor, a specialized in situ and online gas flux measuring (GFM) device was designed to obtain high-resolution time course gas fluxes using the process of equal volume exchange. The device consists of a 1.0-m diameter, 0.9-m tall, inverted conical tent and a GFM instrument that contains a solenoid valve, level transducer, and gas collection chamber. Rising gas bubbles from seeps were measured by laboratory-calibrated GFM instruments attached to the top of the tent. According to the experimental data, the optimal anti-shake time interval was 5 s. The measurement range of the device was 0–15 L min−1, and the relative error was ± 1.0%. The device was initially deployed at an active seep site in the Lingtou Promontory seep field in South China Sea. The amount of gas released from a single gas vent was 30.5 m3 during the measurement period, and the gas flow rate ranged from 22 to 72 L h−1, depending on tidal period, and was strongly negatively correlated with water depth. The measurement results strongly suggest that oceanic tides and swells had a significant forcing effect on gas flux. Low flow rates were associated with high tides and vice versa. The changes in gas volume escaping from the seafloor seeps could be attributed to the hydrostatic pressure induced by water depth. Our findings suggest that in the marine environment, especially in the shallow shelf area, sea level variation may play an important role in controlling methane release into the ocean. Such releases probably also affect atmospheric methane levels.

Pontellid copepods, Labidocera spp., affected by ocean acidification: A field study at natural CO2 seeps.

CO2 seeps in coral reefs were used as natural laboratories to study the impacts of ocean acidification on the pontellid copepod, Labidocera spp. Pontellid abundances were reduced by ∼70% under high-CO2 conditions. Biological parameters and substratum preferences of the copepods were explored to determine the underlying causes of such reduced abundances. Stage- and sex-specific copepod lengths, feeding ability, and egg development were unaffected by ocean acidification, thus changes in these physiological parameters were not the driving factor for reduced abundances under high-CO2 exposure. Labidocera spp. are demersal copepods, hence they live amongst reef substrata during the day and emerge into the water column at night. Deployments of emergence traps showed that their preferred reef substrata at control sites were coral rubble, macro algae, and turf algae. However, under high-CO2 conditions they no longer had an association with any specific substrata. Results from this study indicate that even though the biology of a copepod might be unaffected by high-CO2, Labidocera spp. are highly vulnerable to ocean acidification.

Experimental observations of acoustic backscattering from spherical and wobbly bubbles

Methane bubbles released from the seafloor transport gas through the water column to the atmosphere. Direct or optical methods via underwater vehicles are often used for quantifying methane gas flux in the water column, however these methods are time consuming and expensive. Acoustic measurements, using split-beam and multibeam echo sounders that are readily available on most sea vessels, provide a more efficient method for determining methane gas flux. These acoustic methods typically convert acoustic backscatter measurements of bubbles to bubble size using analytical models of bubble target strength. These models assume that bubbles have uniform shape; however it has been shown that bubbles with a radius greater than 1 mm, which have large Eötvös and/or large Reynolds number, are non-spherical. To investigate the error associated with assuming large bubbles are spherical, a 6 m deep tank experiment was conducted to compare calibrated target strength measurements of both small spherical and large wobbly bubbles to existing acoustic scattering models. Bubble sizes observed in this experiment ranged from a fraction of 1 mm to 6 mm in radius. This experiment used a broad range of frequencies (10-300 kHz) to cover typical echo sounder frequencies utilized in field measurements of natural methane seeps.

Fingerprinting of Weathered Oil Residues from the Deepwater Horizon Oil Spill: The Importance of Multiple Lines of Investigation

ABSTRACT 2017-189 The crude oil released from the Macondo Well, also known for its location in Mississippi Canyon area as the MC252 well during the Deepwater Horizon Oil Spill, entered an environment already containing a complex mixture of hydrocarbons from both natural and anthropogenic sources, many of which have closely related chemical profiles. To understand the impact of the released oil in offshore areas, a method was needed to distinguish MC252-related hydrocarbons from other sources. A multiple lines of evidence approach was developed to identify weathered MC252 oil in offshore sediments in the Gulf of Mexico. Chemical data for alkanes, PAH, petroleum biomarkers and metals were combined with spatial, temporal, and observational information to examine the fingerprints for more than 4,000 sediment samples collected over the span of five years. The unique conditions of the Gulf of Mexico (GOM), with many natural petroleum seeps and tepid seas, provided an ideal environment to support microbial degradation of petroleum. As a result of these conditions, the initial fingerprint of the MC252 was rapidly and extensively altered in the environment including depletion of petroleum biomarkers, usually assumed to be recalcitrant and often used in ratios to identify petroleum residues. Revised biomarker match criteria were defined to account for biodegradation within this fraction. Applying this methodology to the offshore sediment data from the GOM provided a comprehensive understanding of the distribution of the MC252 oil in offshore sediment and an understanding of the various transport pathways which conveyed the oil to the sediments.

Refugio Incident SCAT Operations for Santa Barbara and Ventura Counties Case Study

The Refugio Incident occurred May 19, 2015 along the Gaviota coastline in Santa Barbara County, California. The spill impacted many miles of shoreline and a variety of habitat types. Habitats impacted included exposed wave-cut platforms, man-made structures, cobble, fine-grained sand and boulder beaches. Because of the large scale response, Shoreline Clean-up Assessment Technique (SCAT) surveys were utilized to provide a more systematic assessment of impacted shorelines following a standardized approach. SCAT packets were created to document the progression of clean-up and to direct the Operations Section on where, what, and how to clean the shoreline. One difficulty with this response was that the spill occurred along a dynamic shoreline that constantly changed; and the response was in an area with known significant natural oil seepage. Photographic monitoring points were established throughout the impact area which showed dramatic changes along the coastline throughout the response. A few months into the response, sand and kelp accumulated along the shoreline covering up the incident oil making it difficult to complete the clean-up. SCAT maps were created to make sure that impacted areas that had been previously documented as needing to be cleaned were eventually addressed once the sand eroded from these areas. There was about a two month long time frame where the beach appeared to be clean because of the natural buildup of sand and kelp that covered some of the contamination. The SCAT maps helped to direct the Operations Section by visually identifying specific areas within each segment that still needed cleaning. During a large scale oil spill response when many miles of coastline are impacted in a dynamic environment, it is important to document in a systematic way all field observations so all areas impacted are eventually addressed. This provides the response and the Unified Command with direction and guidance to remediate the spill while taking into account a Net Environmental Benefit Analysis to ensure the established clean-up endpoints are met.

The Refugio Oil Spill Response: Case Study and Lessons’ Learned/Best Practices for the Future

ABSTRACT The Refugio Oil Spill occurred on May 19, 2015, due to the failure of an underground pipeline, owned and operated by a subsidiary of Plains All-American Pipeline near Highway 101 in Santa Barbara County. The Responsible Party initially estimated the amount of crude oil released at about 104,000 gallons, with 21,000 gallons reaching the ocean. A Unified Command (UC) was established consisting of Incident Commanders from the U.S. Coast Guard (USCG), California Department of Fish and Wildlife (CDFW) Office of Spill Prevention and Response (OSPR), Santa Barbara County, and Plains Pipeline with additional participation by the U.S. Environmental Protection Agency and California State Parks. Within hours, the CDFW closed fisheries and the following day Governor Brown declared a state of emergency for Santa Barbara County. The released oil caused heavy oiling of both on and offshore areas at Refugio State Beach and impacted other areas of Santa Barbara and Ventura. A number of factors created unique challenges for the management of this response. In addition to direct natural resource impacts, the closure of beaches and fisheries occurred days before the Memorial Day weekend resulting in losses for local businesses and lost opportunities for the public. The Santa Barbara community, with its history with oil spills and environmental activism, was extremely concerned and interested in involvement, including the use of volunteers on beaches. Also this area of the coast has significant tribal and archeologic resources that required sensitive handling and coordination. Finally, this area of California’s coast is a known natural seep area which created the need to distinguish spilled from ‘naturally occurring’ oil. Most emergency responses, including oil spills, follow a similar pattern of command establishment, response and cleanup phases, followed by non-response phase monitoring, cleanup and restoration. This paper will analyze the Refugio oil spill response in three primary focus areas: 1) identify the ways in which this spill response was unique and required innovative and novel solutions; 2) identify the ways in which this response benefited from the ‘lessons’ learned from both the Deepwater Horizon and Cosco Busan oil spills; and 3) provide a summary of OSPR’s response evaluation report for Refugio, with specific focus on how the lessons learned and best practices will inform future planning efforts within California.

Hydrocarbon Degradation in Caspian Sea Sediment Cores Subjected to Simulated Petroleum Seepage in a Newly Designed Sediment-Oil-Flow-Through System.

The microbial community response to petroleum seepage was investigated in a whole round sediment core (16 cm length) collected nearby natural hydrocarbon seepage structures in the Caspian Sea, using a newly developed Sediment-Oil-Flow-Through (SOFT) system. Distinct redox zones established and migrated vertically in the core during the 190 days-long simulated petroleum seepage. Methanogenic petroleum degradation was indicated by an increase in methane concentration from 8 μM in an untreated core compared to 2300 μM in the lower sulfate-free zone of the SOFT core at the end of the experiment, accompanied by a respective decrease in the δ13C signal of methane from -33.7 to -49.5‰. The involvement of methanogens in petroleum degradation was further confirmed by methane production in enrichment cultures from SOFT sediment after the addition of hexadecane, methylnapthalene, toluene, and ethylbenzene. Petroleum degradation coupled to sulfate reduction was indicated by the increase of integrated sulfate reduction rates from 2.8 SO42-m-2 day-1 in untreated cores to 5.7 mmol SO42-m-2 day-1 in the SOFT core at the end of the experiment, accompanied by a respective accumulation of sulfide from 30 to 447 μM. Volatile hydrocarbons (C2–C6 n-alkanes) passed through the methanogenic zone mostly unchanged and were depleted within the sulfate-reducing zone. The amount of heavier n-alkanes (C10–C38) decreased step-wise toward the top of the sediment core and a preferential degradation of shorter (<C14) and longer chain n-alkanes (>C30) was seen during the seepage. This study illustrates, to the best of our knowledge, for the first time the development of methanogenic petroleum degradation and the succession of benthic microbial processes during petroleum passage in a whole round sediment core.

Epsilonproteobacteria dominate bacterial diversity at a natural tar seep

The bacterial diversity of a naturally seeping bitumen source was investigated by 16S rRNA gene cloning and sequencing. Epsilonproteobacteria were shown to dominate the bacterial diversity in the underground water and within the bitumen, representing ca. 75% of the total bacterial diversity. These Epsilonproteobacteria were dominated by Sulfurimonas OTUs, while Sulfurovum and Arcobacter OTUs completed the remaining diversity. Epsilonproteobacteria are sulfur-oxidizer, nitrate-reducing chemo-lithoautotrophic bacteria, unable to use most organics for growth but capable of CO2 fixation. Thus, reduced sulfur species, but not the complex organic matter of the tar, are utilized for growth by bacterial communities at the Puy-de-la-Poix. The large prevalence of populations of Epsilonproteobacteria is a clear indication that crude oil offers a competitive ecological niche for these organisms.

Monitoring of natural oil seepage in the Lower Congo Basin using SAR observations

Synthetic Aperture Radar (SAR) is a spaceborne tool allowing near real-time imagery over large ground footprints of areas hundreds of kilometres wide. SAR also allows persistent observations of the process of oil discharges, benefiting from (1) day and night observations, (2) independence of cloud cover and (3) high revisiting frequencies. The interpretation of 82 SAR scenes over the Lower Congo Basin for the 1994–2012 period allowed the recognition of 1400 natural seepage slicks associated with 102 individual seep sites. We acquired an additional SAR data set with a short revisit time over a selected prolific area. The data set consisted of 22 SAR scenes acquired over a 10-day period, meaning a maximum revisiting period of 12h between subsequent SAR acquisitions. The short-term approach shows that seepage slicks were detected with wind speeds between 1.5 and 6.5m/s (2.91 to 12.63knots). Both long-term and short-term monitoring evidences that seepage events are intermittent, with the occurrence rate of oil release ranging from 5 to 80%. Short-term monitoring shows that the seepage pace is independent between seep sites, suggesting contrasted controlling factors. It also shows that the residence time of seepage slicks is systematically shorter than 12h, which is far below reported values. Understanding the seepage slick residence time is essential to provide an accurate estimation of the fluid flow and to compute the volumetric outflow. The integration of in-situ mooring points measuring the current velocity 10m below the sea surface, in addition to local wind field and slick length, allowed us to more accurately estimate the residence time of oil slicks at the sea surface before vanishing. The results show that the sea surface residence time of oil slicks is limited to a few hours, with a median value of 3h15min. These new residence time estimations allowed us to propose a quantification of the regional oil output in the Lower Congo Basin, estimated at 4380m3/year. This area may therefore be considered as the world's third biggest oil-supplying province from natural leakages.

Time series video analysis of bubble release processes at natural hydrocarbon seeps in the Northern Gulf of Mexico

This research quantifies the rate and volume of oil and gas released from two natural seep sites in the Gulf of Mexico: lease blocks GC600 (1200 m depth) and MC118 (850 m depth). Our objectives were to determine variability in release rates and bubble size at five individual vents and to investigate the effects of tidal fluctuations on bubble release. Observations with autonomous video cameras captured the formation of individual bubbles as they were released through partially exposed deposits of gas hydrate. Image processing techniques determined bubble type (oily, gaseous, and mixed: oily and gaseous), size distribution, release rate, and temporal variations (observation intervals ranged from 3 h to 26 d). A semi-automatic bubble counting algorithm was developed to analyze bubble count and release rates from video data. This method is suitable for discrete vents with small bubble streams commonly seen at seeps and is adaptable to multiple in situ set-ups. Two vents at GC600 (Birthday Candles 1 and Birthday Candles 2) were analyzed. They released oily bubbles with an average diameter of 5.0 mm at a rate of 4.7 bubbles s−1, and 1.3 bubbles s−1, respectively. Approximately 1 km away, within the GC600 seep site, two more vents (Mega Plume 1 and Mega Plume 2) were analyzed. These vents released a mixture of oily and gaseous bubbles with an average diameter of 3.9 mm at a rate of 49 bubbles s−1, and 81 bubbles s−1, respectively. The fifth vent at MC118 (Rudyville) released gaseous bubbles with an average diameter of 3.0 mm at a rate of 127 bubbles s−1. Pressure records at Mega Plume and Rudyville showed a diurnal tidal cycle (24.5 h). Rudyville was the only vent that demonstrated any positive correlation (ρ = 0.60) to the 24.5 h diurnal tidal cycle. However, these observations were not conclusive regarding tidal effects on bubble release.

Microbial response to oil enrichment in Gulf of Mexico sediment measured using a novel long-term benthic lander system

Weathered crude oil sank to the seafloor following the Deepwater Horizon disaster in 2010, removing this oil from further physical and photo-chemical degradation processes and leaving benthic processes as the mechanisms for altering and remediating this hydrocarbon source. To quantify potential microbial oil degradation rates at the seafloor, and associated changes in sediment microbial community structure and pore fluid composition, we used a benthic lander system to deploy novel sediment flow-through chambers at a natural hydrocarbon seep in the Gulf of Mexico (at a depth of 1226 m in lease block GC600) roughly 265 km southwest of the Deepwater Horizon wellhead (at 1500 m depth). Sediment amended with 20% unweathered crude oil had elevated rates of sulfate reduction over the course of the 5-month-long experiment as compared to an unamended control, yielding potential rates of sulfate reduction (600–800 mmol m–2 d–1) among the highest measured in hydrocarbon-influenced seafloor sediment. Oil amendment also stimulated methane production towards the end of the experiment, and led to slightly higher cell densities without significant changes in microbial community structure, based on 16S rRNA gene sequence libraries and fatty acid profiles. Assuming a link between sulfate reduction and hydrocarbon degradation, these results suggest that electron acceptor availability may become limiting in heavily oiled deep-sea environments, resulting in minimal degradation of deposited oil. This study provides unique data on seafloor sediment responses to oil deposition, and reveals the value of using observatories to fill the gap in understanding deep-sea microbial processes, especially for ephemeral and stochastic events such as oil spills.