Coal Oil Point Research Articles

Strong Release of Methane on Mars in Northern Summer 2003

Living systems produce more than 90% of Earth's atmospheric methane; the balance is of geochemical origin. On Mars, methane could be a signature of either origin. Using high-dispersion infrared spectrometers at three ground-based telescopes, we measured methane and water vapor simultaneously on Mars over several longitude intervals in northern early and late summer in 2003 and near the vernal equinox in 2006. When present, methane occurred in extended plumes, and the maxima of latitudinal profiles imply that the methane was released from discrete regions. In northern midsummer, the principal plume contained approximately 19,000 metric tons of methane, and the estimated source strength (>/=0.6 kilogram per second) was comparable to that of the massive hydrocarbon seep at Coal Oil Point in Santa Barbara, California.

Monitoring Shorebird Habitat using Photogrammetry: The case of Western Snowy Plover at Coal Oil Point Reserve, Santa Barbara, California

The goal of this study was to determine how digital photogrammetry can be used to describe and monitor shorebird habitat. Methods previously used for monitoring beach erosion were applied to shorebird habitat, concentrating on the requirements of Western Snowy Plovers. How the physical habitat of Western Snowy Plovers has changed over time was investigated at Sand Beach, which is part of Coal Oil Point Reserve in Santa Barbara, California, from 1928 to 2004, using width, area, and elevation data gathered from a series of aerial photographs. Single georeferenced photographs were used to measure width at regular intervals and area for each year a photo was available. Georeferenced aerial photographs can be used to obtain accurate measurements of beach area and width. Orthorectification is not necessary if photos can be georeferenced with minimal error. Sands Beach was found to be increasing in area. This was due mainly to the retreat of the vegetation line as the mouth of Devereux Slough shifted in 1992 rather than an accumulation of sand on the beach.

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.

Dissolved methane distributions and air‐sea flux in the plume of a massive seep field, Coal Oil Point, California

Large quantities of natural gas are emitted from the seafloor into the stratified coastal ocean near Coal Oil Point, Santa Barbara Channel, California. Methane was quantified in the down current surface water at 79 stations in a 280 km2 study area. The methane plume spread over an area of ∼70 km2 and emitted on the order of 5 × 104 mol d−1 to the atmosphere. A monthly time series at 14 stations showed variable methane concentrations which were correlated with changing sub‐mesoscale surface currents. Air‐sea fluxes estimated from the time series indicate that the air‐sea flux derived for the 280 km2 area is representative of the daily mean flux from this area. Only 1% of the dissolved methane originating from Coal Oil Point enters the atmosphere within the study area. Most of it appears to be transported below the surface and oxidized by microbial activity.

Beach tar accumulation, transport mechanisms, and sources of variability at Coal Oil Point, California

A new field method for tar quantification was used at Coal Oil Point (COP), California to study the mechanisms transporting oil/tar from the nearby COP natural marine hydrocarbon seep field. This method segregates tar pieces into six size classes and assigns them an average mass based on laboratory or direct field measurements. Tar accumulation on the 19,927 m 2 survey area was well resolved spatially by recording tar mass along twelve transects segmented into 4-m 2 blocks and then integrating over the survey area. A seasonal trend was apparent in total tar in which summer accumulations were an order of magnitude higher than winter accumulations. Based on multiple regression analyses between environmental data and tar accumulation, 34% of tar variability is explained by a combination of onshore advection via wind and low swell height inhibiting slick dispersion.

Carbon and hydrogen isotope fractionation associated with the aerobic microbial oxidation of methane, ethane, propane and butane

Carbon isotope fractionation factors associated with the aerobic consumption of methane (C1), ethane (C2), propane (C3), and n-butane (C4) were determined from incubations of marine sediment collected from the Coal Oil Point hydrocarbon seep field, located offshore Santa Barbara, CA. Hydrogen isotope fractionation factors for C1, C2 and C3 were determined concurrently. Fresh sediment samples from two seep areas were each slurried with sea water and treated with C1, C2, C3 or C4, or with mixtures of all four gases. Triplicate samples were incubated aerobically at 15 °C, and the stable isotope composition and headspace levels of C1–C4 were monitored over the course of the experiment. Oxidation was observed for all C1–C4 gases, with an apparent preference for C3 and C4 over C1 and C2 in the mixed-gas treatments. Fractionation factors were calculated using a Rayleigh model by comparing the δ 13C and δD of the residual C1–C4 gases to their headspace levels. Carbon isotope fractionation factors (reported in ε or ( α-1) × 1000 notation) were consistent between seep areas and were −26.5‰ ± 3.9 for C1, −8.0‰ ± 1.7 for C2, −4.8‰ ± 0.9 for C3 and −2.9‰ ± 0.9 for C4. Fractionation factors determined from mixed gas incubations were similar to those determined from individual gas incubations, though greater variability was observed during C1 consumption. In the case of C1 and C3 consumption, carbon isotope fractionation appears to decrease as substrate becomes limiting. Hydrogen isotope fractionation factors determined from the two seep areas differed for C1 oxidation but were similar for C2 and C3. Hydrogen isotope fractionation factors ranged from −319.9‰ to −156.4‰ for C1 incubations, and averaged −61.9‰ ± 8.3 for C2 incubations and −15.1‰ ± 1.9 for C3 incubations. The fractionation factors presented here may be applied to estimate the extent of C1–C4 oxidation in natural gas samples, and should prove useful in further studying the microbial oxidation of these compounds in the natural environment.

Tracking an oil slick from multiple natural sources, Coal Oil Point, California

Oil slicks on the ocean surface emitted from natural marine hydrocarbon seeps offshore from Coal Oil Point in the Santa Barbara Channel, California were tracked and sampled over a 2-h period. The objectives were to characterize the seep oil and to track its composition over time using a new sampling device, a catamaran drum sampler (CATDRUMS). The sampler was designed and developed at UCSB. Chromatograms showed that oil originating from an informally named, very active seep area, Shane Seep, primarily evolved during the first hour due to mixing with oil originating from a convergence zone slick surrounding Shane Seep.

Submarine landslides in the Santa Barbara Channel as potential tsunami sources

Abstract. Recent investigations using the Monterey Bay Aquarium Research Institutes (MBARI) Remotely Operated Vehicles (ROVs) "Ventana" and "Tiburon" and interpretation of MBARI's EM 300 30 kHz multibeam bathymetric data show that the northern flank of the Santa Barbara Basin has experienced massive slope failures. Of particular concern is the large (130 km2) Goleta landslide complex located off Coal Oil Point near the town of Goleta, that measures 14.6-km long extending from a depth of 90 m to nearly 574 m deep and is 10.5 km wide. We estimate that approximately 1.75 km3 has been displaced by this slide during the Holocene. This feature is a complex compound submarine landslide that contains both surfical slump blocks and mud flows in three distinct segments. Each segment is composed of a distinct head scarp, down-dropped head block and a slide debris lobe. The debris lobes exhibit hummocky topography in the central areas that appear to result from compression during down slope movement. The toes of the western and eastern lobes are well defined in the multibeam image, whereas the toe of the central lobe is less distinct. Continuous seismic reflection profiles show that many buried slide debris lobes exist and comparison of the deformed reflectors with ODP Drill Site 149, Hole 893 suggest that at least 200 000 years of failure have occurred in the area (Fisher et al., 2005a). Based on our interpretation of the multibeam bathymetry and seismic reflection profiles we modeled the potential tsunami that may have been produced from one of the three surfical lobes of the Goleta slide. This model shows that a 10 m high wave could have run ashore along the cliffs of the Goleta shoreline. Several other smaller (2 km2 and 4 km2) slides are located on the northern flank of the Santa Barbara Basin, both to the west and east of Goleta slide and on the Conception fan along the western flank of the basin. One slide, named the Gaviota slide, is 3.8 km2, 2.6 km long and 1.7 km wide. A distinct narrow scar extends from near the eastern head wall of this slide for over 2km eastward toward the Goleta slide and may represent either an incipient failure or a remnant of a previous failure. Push cores collected within the main head scar of this slide consisted of hydrogen sulfide bearing mud, possibly suggesting active fluid seepage and a vibra-core penetrated ~50 cm of recent sediment overlying colluvium or landslide debris confirming the age of ~300 years as proposed by Lee et al. (2004). However, no seeps or indications of recent movement were observed during our ROV investigation within this narrow head scar indicating that seafloor in the scar is draped with mud.

Restoration of Breeding by Snowy Plovers Following Protection from Disturbance

Promoting recreation and preserving wildlife are often dual missions for land managers, yet recreation may impact wildlife. Because individual disturbances are seemingly inconsequential, it is difficult to convince the public that there is a conservation value to restricting recreation to reduce disturbance. We studied threatened western snowy plovers (Charadrius alexandrinus nivosus) at a public beach (Sands Beach, Coal Oil Point Reserve) in Santa Barbara, California (USA) before and during a period when a barrier directed foot traffic away from a section of upper beach where snowy plovers roost. The barrier reduced disturbance rates by more than half. Snowy plovers increased in abundance (throughout the season) and their distribution contracted to within the protected area. Snowy plovers that were outside the protected area in the morning moved inside as people began using the beach. Experiments with quail eggs indicated an 8% daily risk of nest trampling outside the protected area. Before protection, plovers did not breed at Coal Oil Point. During protection, snowy plovers bred in increasing numbers each year and had high success at fledging young. These results demonstrate how recreational disturbance can degrade habitat for shorebirds and that protecting quality habitat may have large benefits for wildlife and small impacts to recreation.

Time Evolution Of Beach Tar, Oil Slicks, And Seeps In The Coal Oil Point Seep Field, Santa Barbara Channel, California

ABSTRACT Linking beach tar with sources in a complex natural marine seepage area presents numerous challenges. Efforts at Coal Oil Point (COP), CA included beach tar distribution surveys, oil slick tracking, sampling, and chemical analysis, underwater scuba surveys, aerial surveys, and numerical modeling. Despite a wind from the east and current to the west, a slick was tracked initially north from its source, presumably due to spreading, then it drifted east, ending in a kelp bed off COP. Sample chromatograms showed mixing with another oil slick by the appearance of a heavier series of n-alkane peaks where the trajectory changed direction. Trajectory simulations suggested that interface currents were poorly described by parameterizations of wind and surface currents, and/or the existence of small-scale circulations not resolved by CODAR or the drift buoy. Detailed tar accumulation surveys covered 175-m (4400 m2) of COP beach where tar accumulation generally is greatest. Maximum total beach tar observed was 1.5 kg, with significant variability. Modeling suggested a similar source location for the three analyzed surveys. Analysis also suggested kelp canopies can play a significant roll in the arrival time and location of beach tar by blocking onshore transport. However, wind, current, and kelp conditions were such that much of the variability in tar accumulation for these surveys probably was from source emission variability.

USE OF NATURAL OIL SEEPS FOR EVALUATION OF DISPERSANT APPLICATION AND MONITORING TECHNIQUES

ABSTRACT The natural oil seeps off Coal Oil Point (Santa Barbara), California, release an estimated 100–150 bbl of oil per day to the marine environment. This project proposed to conduct a series of dispersant trials using these seeps to intercalibrate NOAA's Scientific Monitoring of Advanced Response Technologies (SMART) UV/Fluorescence-based protocols with finite measurements of dissolved aromatics and dispersed oil droplets in the water column and to evaluate a unique oil-boomldispersant-application technology (NeatSweep). Following an elaborate and lengthy permitting process including cooperation from multiple regulatory agencies and organizations, laboratory tests indicated that although fresh produced oil from nearby Platform Holly could be treated (>70% effectiveness), dispersing the weathered 11° API gravity seep oil was totally ineffective (0%). Limited field tests then verified the laboratory findings that the seep oil could not even be dispersed with Corexit 9500, a commonly used dispersant for heavily weathered and viscous oils. Lacking reasonable alternatives (including the use of intentional spills), the project was halted before full-scale field implementation. This paper documents the development of the research plan, the steps required to obtain the necessary permits, and the results from the limited laboratory and field tests that were completed. The planning and permitting efforts for this project are provided so that others with similar needs or goals might benefit. A brief discussion is provided on the limitations of using natural seep oils for spill response research and on the difficulties with spill-of-opportunity research during actual spill events. The importance of controlled experimental discharges of oil is discussed along with the pros and cons of such deliberate spills.

Evaluation of the relationships between biochemical endpoints of PAH exposure and physiological endpoints of reproduction in male California Halibut (Paralichthys californicus) exposed to sediments from a natural oil seep

Coal Oil Point (COP) is a natural oil seep off the coast of Santa Barbara, California. Although most studies examining the fate and effects of petroleum have focused upon urbanized or anthropogenic sources of inputs, few have examined the effects of polyaromatic hydrocarbons (PAHs) derived from natural seeps. In order to evaluate the effects of PAHs derived from COP on marine fish populations, hatchery-reared California Halibut (Platichthys californicus) were exposed for 30 days to seven dilutions of sediments collected from COP. Hepatic cytochrome P450 1A (CYP1A), biliary fluorescent aromatic compounds (FACs), gonadal somatic indices, and plasma steroid concentrations. Sixteen USEPA priority PAHs were targeted for analysis in each sediment dilution. In general, biochemical responses were somewhat recalcitrant to dose-response relationships and were less sensitive than the literature values established for the same indicators following exposure to urbanized PAHs. Trends toward reductions in plasma 17beta-estradiol concentrations were observed, but reductions in gonadal somatic indices were not observed. FAC values for naphthalene, benzo(a)pyrene, phenanthrene-related compounds reached maximums at 33-100% COP sediment. The resulting insensitivity may be unique for exposure to "natural" petroleum due to a higher concentration of lower molecular weight PAHs or uncharacterized inhibitors.

The spatial scales, distribution, and intensity of natural marine hydrocarbon seeps near Coal Oil Point, California

Natural hydrocarbon seepage from marine environments is an important source of methane and other gases to the atmosphere. Quantifying this flux is necessary for constraining global budgets and understanding local air pollution sources. A field of strong hydrocarbon seepage offshore of Coal Oil Point near Santa Barbara, California produces extensive areas of bursting bubbles at the sea surface. An instrumented buoy was deployed in the Coal Oil Point field to measure directly the atmospheric gas flux from three seeps of varying size and intensity. Spatial scales of continuity of the seeps, quantified by semivariograms, are small, ranging from <1 to about 9 m such that flux values on larger scales are uncorrelated. These de-correlation scales are comparable to the horizontal extents of individual bubble plumes estimated visually at the sea surface. Semivariograms for each seep are used in an ordinary Kriging procedure to interpolate flux measurements onto a regular grid and produce objective maps of the spatial distribution of flux. Spatial integrals yield total flux estimates from the three seeps of ∼7400 m 3 day −1 which amount to 4–13% of the total flux from the Coal Oil Point field based on estimates from previous studies.

Turbine tent measurements of marine hydrocarbon seeps on subhourly timescales

Three turbine seep‐tents simultaneously measured marine seep gas fluxes with high time resolution (0.2 s) at multiple locations. Tents were inverted polyvinyl cones, 2‐m diameter, 1‐m tall, and weighted on their lower skirt edges. Rising gas bubbles induce vertical fluid motions, which were measured by laboratory‐calibrated turbines in chimneys on top of the tents. Initial deployment was at an active seep area in the Coal Oil Point seep field, in the Santa Barbara Channel, California. The three tents simultaneously collected data for continuous time periods of 2 hours in both the morning and afternoon. Seabed temperature and pressure were acquired every 3 s over the same time periods as the flux measurements from a conductivity temperature depth, CTD, mounted on one tent. Results strongly suggest that oceanic swell had a significant forcing effect on the flux at a subhourly timescale. There was an inverse relationship between effect of swell height on the flux and flux. Swells from 1 to 4 m height and periodicities of 7 and 12 s caused variations of ∼1% to 4% from the average flux. Proposed mechanisms to explain the observations are diffusion with surrounding sediments, termed gas charging, swell induced changes in fracture size, termed fracture forcing, and swell induced vent activation/deactivation, termed pore activation. On the basis of the seep frequency response, we propose pore activation was dominant.

Using the HyMap airborne hyperspectral sensor for offshore seepage mapping

The HyMap airborne spectrometer is an airborne remote sensing instrument collecting data in 126 spectral channels from the visible (VIS) to the shortwave infrared (SWIR) wavelength regions. Applications are wide spanned ranging from mineral exploration to environmental monitoring. Offshore seepage mapping has always been a potential application, but not many open file reports can be found in freely accessible literature though oil&gas exploration companies are known to use the technology.As part of a 2003 airborne survey campaign in the USA, HyVista Corporation collected airborne HyMap data over one of the most active natural oil seeps in the world offshore Santa Barbara, California. These seeps are just off Coal Oil Point and spread over an area of 10’s of sqkm. UCSB has been studying this seepage for many years and slick distribution and flow rates are monitored regularly. A production platform called ‘Holly’ is also located in the seep area and can be clearly identified on the imagery.Airborne hyperspectral seep mapping offers a new possibility of characterising seeps in an exploration area of interest. It offers the advantage of allowing spectral discrimination of seep components which may not be separable with conventional techniques like radar. HyMap’s capability to sense in the SWIR offers the additional possibility of properly separating an effect called ’sunglint’ from any data over open water, since water will absorb radiation almost 100 % at these wavelengths. Due to the same effect it is also possible to discriminate seepage with a strong surface expression from others, which is not possible with sensors having no SWIR spectral region.The Santa Barbara oils seeps were surveyed with 4 parallel HyMap lines covering the major seepage and migration path of the slicks at the time. Different spectral components in the open water as well as the seep area are identified and mapped. Seamless data products are produced using advanced mapping techniques and utilising the hyperspectral nature of the data. Results of these different processing steps will be presented.

Bacterial diversity in marine hydrocarbon seep sediments.

Marine seeps introduce significant amounts of hydrocarbons into oceans and create unusual habitats for microfauna and -flora. In the vicinity of chronic seeps, microbes likely exert control on carbon quality entering the marine food chain and, in turn, hydrocarbons could influence microbial community composition and diversity. To determine the effects of seep oil on marine sediment bacterial communities, we collected sediment piston cores within an active marine hydrocarbon seep zone in the Coal Oil Point Seep Field, at a depth of 22 m in the Santa Barbara Channel, California. Cores were taken adjacent to an active seep vent in a hydrocarbon volcano, on the edge of the volcano, and at the periphery of the area of active seepage. Bacterial community profiles were determined by terminal restriction fragment length polymorphisms (TRFLPs) of 16S ribosomal genes that were polymerase chain reaction (PCR)-amplified with eubacterial primers. Sediment carbon content and C/N ratio increased with oil content. Terminal restriction fragment length polymorphisms suggested that bacterial communities varied both with depth into sediments and with oil concentration. Whereas the apparent abundance of several peaks correlated positively with hydrocarbon content, overall bacterial diversity and richness decreased with increasing sediment hydrocarbon content. Sequence analysis of a clone library generated from sediments collected at the periphery of the seep suggested that oil-sensitive species belong to the gamma Proteobacteria and Holophaga groups. These sequences were closely related to sequences previously recovered from uncontaminated marine sediments. Our results suggest that seep hydrocarbons exert a strong selective pressure on bacterial communities in marine sediments. This selective pressure could, in turn, control the effects of oil on other biota in the vicinity of marine hydrocarbon seeps.

Compositional changes in natural gas bubble plumes: observations from the Coal Oil Point marine hydrocarbon seep field

Detailed measurements of bubble composition, dissolved gas concentrations, and plume dynamics were conducted during a 9-month period at a very intense, shallow (22-m water depth) marine hydrocarbon seep in the Santa Barbara Channel, California. Methane, car- bon dioxide, and heavier hydrocarbons were lost from rising seep bubbles, while nitrogen and oxygen were gained. Within the rising seawater bubble plume, dis- solved methane concentrations were more than 4 orders of magnitude greater than atmospheric equilibrium concentrations. Strong upwelling flows were observed and bubble-rise times were � 40 s, demonstrating the rapid exchange of gases within the bubble plume.

Biochemical effects of petroleum exposure in hornyhead turbot ( Pleuronichthys verticalis) exposed to a gradient of sediments collected from a natural petroleum seep in CA, USA

Concentrations of serum/plasma estradiol, biliary fluorescent aromatic compounds (FACs), levels of hepatic CYP1A expression, and DNA damage were measured in sexually mature hornyhead turbot ( Pleuronichthys verticalis) exposed in the laboratory for 7 days to a gradient of sediments collected from a natural petroleum seep in the Santa Barbara Channel. Coal oil point (COP) sediments were homogenized and divided into four treatments containing 0 (sediment from the Orange County Sanitation District's reference location), 33, 66, and 100% (COP) sediments. Sediment concentrations of 20 PAHs ranged from below the detection limit for the 0% COP sediment treatments to 105 μg/g in the 100% treatments with lower molecular weight compounds predominating. Concentrations of biliary FACs were not linear with COP treatment but levels of hepatic DNA damage increased linearly with increasing concentrations of high molecular weight PAHs. Hepatic CYP1A expression was elevated only in the 100% treatments. A reduction of plasma estradiol in male and female fish was observed in all COP exposures. These results demonstrate that acute sediment-only exposure of flatfish to naturally-derived PAHs elicits alterations in biochemical endpoints indicative of PAH bioavailability and adverse effects with different sensitivities.

Temporal variation in natural methane seep rate due to tides, Coal Oil Point area, California

Two large steel tents (each 30 m by 30 m), open at the bottom to the seafloor, capture ∼16,800 m3 d−1 (594 MCF) of primarily methane from a large natural hydrocarbon seep, occurring a kilometer offshore in 67 m of water. The gas is piped to shore where it is metered and processed. The seep flow rate was monitored hourly for 9 months. Our results show that the tidal forcing causes the flow rate to vary by 4–7% around the mean. These results are the first quantitative documentation of the effect of tides on natural gas seepage in relatively deep water. Time series analyses of the 9 month record clearly show four principal tidal components with periods of 12.0, 12.4, 23.9, and 25.8 hours. High tide correlates with reduced flow, and low tide correlates with increased flow. The correlation indicates that each meter increase of sea height results in a decrease of 10–15 m3 hr−1 or 1.5–2.2% of the hourly flow rate. The observed changes are best accounted for by a pore activation model, whereby gas is released from small pores at low pressures but is inhibited at higher pressure. Pressure‐dependent gas solubility changes are a less likely cause of flow variation. Our study implies that sea level differences, on a tidal timescale, can significantly change the gas seepage rate from sediments. Lower sea level in the last hundred thousand years would presumably allow higher gas loss from the sediment, assuming sufficient gas present, because of reduced hydrostatic pressure at the sediment‐sea interface. The magnitude of this long‐term change cannot be extrapolated from our tidal data.