Anthropogenic oil in the ocean is of great concern due to its potential immediate and long-term impacts on the ecosystem, economy, and society, leading to intense societal efforts to mitigate and reduce inputs. Sources of oil in the ocean (in the order of importance) are natural marine seepage, run-off from anthropogenic sources, and oil spills, yet uncertainty and variability in these budgets are large, particularly for natural seepage, which exhibits large spatial and temporal heterogeneity on local to regional scales. When source inputs are comparable, discriminating impacts is complicated, because petroleum is both a bioavailable, chemosynthetic energy source to the marine ecosystem and a potential toxic stressor depending on concentration, composition, and period of time. This synthesis review investigates the phenomena underlying this complexity and identifies knowledge gaps. Its focus is on the Coal Oil Point (COP) seep field, arguably the best-studied example, of strong natural marine hydrocarbon seepage, located in the nearshore, shallow waters of the Northern Santa Barbara Channel, Southern California, where coastal processes complicate oceanography and meteorology. Many of our understandings of seep processes globally are based on insights learned from studies of the oil and gas emissions from the COP seep field. As one of the largest seep fields in the world, its impacts spread far as oil drifts on the sea surface and subsurface, yet much remains unknown of its impacts.
Understanding seepage requires knowledge of the geological and oceanographic controls on its emissions (where and how strong) and then oceanographic and meteorological factors that control its fate (transport and the chemical and biological evolution of seep hydrocarbons). California marine seepage over evolutionary timescales has impacted marine ecosystems both as a chemical stressor and as a chemosynthetic energy source, leading to adaptation at the species and community levels.
In this review, we summarize the state of knowledge of emissions and impacts from one of the most prolific and arguably best-studied natural marine hydrocarbon seep fields, the Coal Oil Point (COP) seep field in the northern Santa Barbara Channel, offshore Southern California, and place it in the general global context of seepage understanding.
The COP seep field lies in nearshore waters (10-70 m), and thus, its emissions and their fate are governed by transport and weathering in the complex Pacific coastal environment. This synthesis review places context on the role of seep petroleum hydrocarbons in the California coastal marine environment and highlights how current understanding of the COP seep field provides insights into seep processes elsewhere.
Hydrocarbon seepage is the process by which hydrocarbon gases and liquids (fluids) associated with petroleum accumulation in a reservoir layer escape from the lithosphere to the hydrosphere and/or atmosphere. The reservoir layer must be fractured or porous to allow migration within. Critical to seepage is a capping layer that allowed hydrocarbon accumulation over geologic timescales and fractures and/or faults that provide migration pathways within the reservoir and through the capping layer (if not eroded allowing outcropping) to the seabed or atmosphere [
Natural marine hydrocarbon seepage occurs from intertidal depths (e.g., San Simón Bay, Galícia, Spain) to more than 4,000 m in the Aleutian Trench and in every sea and ocean [
Natural hydrocarbon seepage is widespread onshore and offshore and has played a notable role in human societies as far back as the Paleolithic when it was used as an adhesive in tool construction [
Bitumen supported extensive ancient trade, with modern chemical fingerprinting showing trade routes [
Seepage impacts a range of important geophysical and biological processes as well as economic activities. These include the climate, marine carbon cycling, oil spill science, oil spill response, and exploration, discussed below.
The important greenhouse gas CH4 has anthropogenic (360-430 Tg yr-1;
Fossil fuel industrial (FFI) emissions comprise both anthropogenic and geologic, i.e., ancient CH4 that is isotopically light. The IPCC estimates that geologic CH4 contributes ~20-33% of the natural budget or 54 Tg yr-1, i.e., 33-75 Tg yr-1 [
Estimates of global seep emissions are based on a very limited number of measurements. Fluxes for individual marine seep vents have been reported for the Gulf of Mexico [
The COP seep field is the first seep field for which gas emissions were estimated, at 105 m3 dy-1 CH4 in 1995 based on sonar data [
Extending seep surveys to annualized emissions is problematic as a survey provides snapshots of geological systems, which exhibit high temporal variability on timescales from the subhourly [
The two primary marine petroleum sources [
Contribution of average annual oil releases (in ktonnes) for 1990-1999 for (a) the globe and (b) North America. From NRC [
Over recent decades, oil transport safety has increased while the amount spilled into the ocean has decreased. As a result, natural seepage plays an increasingly important role in the marine petroleum budget, both globally and regionally. Natural oil seepage is estimated as the largest marine source, estimated by NRC [
There are two published quantitative oil emission estimates for a seep field or region as opposed to a single vent. One is for the COP seep field [
A second value was published more recently for the Gulf of Mexico and found
Accidental tanker spills, which in the 1970s released oil quantities comparable to seepage (320,000 tons yr-1), contributed just 3,900 tons yr-1 for the period 2010-2016 [
Seep petroleum hydrocarbons (including oil and natural gas) provide bioavailable chemosynthetic energy [
Impacts from cold seep ecosystems extend beyond the seepage area by mobile upper trophic-level species [
In cold seep ecosystems, the spatial distribution of bacterial mats and microbial symbiont-bearing species, such as tube worms, varies with chemical indicators (CH4, sulfur, reducing activity, oxygen content, etc.), including transport by currents [
Both the presence of the chemosynthetic source and its chemical stresses are important to the community structure. Thus, within habitat patches along geochemical gradients, species self-sort, creating complex and heterogeneous community assemblages, while dispersal efficiency and species competition drive community succession [
Associated, higher trophic level species are found clustered around cold seeps but with ranges that extend beyond the seep area’s geochemical footprint [
The importance of seep-related substrate, typically authigenic carbonates, is significant to the diversity and richness of seep ecosystems [
In shallower (<400 m) water, “normal” organisms that rely on energy derived from the photic zone tend to outcompete seep-specialist organisms [
Oil spill response depends strongly on spill models that include characterizations of a wide range of oil spill processes that vary on day to week timescales. These processes include wind and wave advection, spreading and surface diffusion, compression from waves and currents (into wind rows or narrow slicks), sedimentation and dissolution into the water column, emulsification, dissolution and evaporation, and photochemical and biological degradation [
Natural marine oil seepage and slicks (e.g., Figure
Thick oil slick from the Coal Oil Point (COP) seep field. This slick stretched for kilometers. Similar COP slicks have been used for spill science, including dispersant tests [
The modern oil industry largely began in Pennsylvania in 1859 with a well drilled into an oil spring (e.g., a terrestrial hydrocarbon seep) that had been used previously by the Seneca Indians and early settlers [
Marine hydrocarbon seepage occurs where hydrocarbons escape from a reservoir formation that is capped by a relatively impermeable rock layer, which allows hydrocarbon accumulation, along migration pathways to the surface through the capping layer and overlying sediments [
Seepage occurs through the capping layer and overlying sediments, migrating to the seabed along faults and fractures in these layers [
Marine seepage occurs on all continental shelves, spanning geological settings from passive margins (e.g., riverine deltas, like the Mississippi or Nile Rivers, which deposit organic materials in thick sediment layers) to convergent and divergent plate boundaries where sediments are buried in subducting sediments, to transform plate boundaries (e.g., the Pacific California coast). In the latter, compression from tectonic stresses pressurizes sediments [
Many authors have noted a relationship between seepage and geologic structures [
Sediment overburden can be a major factor in the seepage spatial distribution, particularly in accretionary settings like the Gulf of Mexico where sediment layers are kilometers thick [
Migration pathways through sediment can be highly variable, unlike pathways through fractures in rock layers. Where sediments are coarse-grained, emissions show similarity to percolation beds as used in chemical engineering with seepage appearing to shift rapidly and randomly between semistationary vents [
Seepage is driven by a pressure difference between the reservoir and the seabed (above hydrostatic) along multiple migration pathways with higher seepage associated with higher permeability pathways. One approach to describing this complex system is a seep electrical (SE) model (Figure
Seep electrical model for a simple seep area. Table defines the physical seep flow process and corresponding electrical component. Adapted from Leifer and Wilson [
Seepage is pressure-sensitive with emissions finely balanced between near-seabed reservoir recharge from below and discharge to the seabed. This drives a highly nonlinear response of seepage to changes in seabed hydrostatic pressure, with the sensitivity inversely related to flux [
For example, consider decreasing hydrostatic pressure, which leads to increased emissions. The increased emissions discharge the near-seabed reservoir until it is depleted. Meanwhile, deeper recharge eventually achieves a new equilibrium. Higher emissions also increase resistance through the active migration pathways in a negative feedback that limits the increase in emissions, analogous to resistive heating. Because the driving force remains, this emission self-limits, maintaining the hydrostatic overpressure thereby allowing activation of inactive migration pathways with higher
There is asymmetry in the seep response to changing hydrostatic pressure. Specifically, decreasing pressure leads to a greater increase in emissions than increasing pressure decreases emissions. The underlying mechanism is unknown but could have an elastic component—higher flow and pressure open bottlenecks, decreasing migration pathway resistance to flow, further increasing the flow. To summarize, waves pump emissions [
Geological structures affect the details of hydrocarbon seepage to the seabed; thus, seepage from different seep vents can be hydraulically related [
Generally, seepage is not zero sum (i.e., just shifting between vents). This is illustrated in Figure
Seep electrical model schematic illustrating effect of blockage in one of multiple migration pathways from a shallow reservoir, which is fed from a deeper source. Arrow thickness indicates the flow rate.
In the above discussion, emissions from the shallow reservoir change from changes in migration resistance (or permeability) between the shallow reservoir and the seabed or from seabed (hydrostatic) pressure changes. These changes in emissions and permeability drive the interplay between vents. In contrast, the effect of a change in migration flow or resistance from the deep source that feeds the shallow reservoir is different. Here, an increase in flow to the shallow reservoir also increases seabed emissions. However, higher flow in these pathways increases the resistance, i.e., seabed emissions increase less than the increase in flow from below. This increased pressure in the shallow reservoir partitions the increased flow among all migration pathways to the seabed, favoring larger vents, and/or by pathway activation until inflow and outflow from the shallow reservoir balance. This mechanism was proposed to explain long-term seep emission trends in the COP seep field reported by Bradley et al. [
For seepage through unconsolidated sediment, emissions are far more variable and sporadic, often transient, with high variability in the migration pathways. This is akin to a chemical engineering trickle bed. In this case, the absence of a shallow reservoir where pressure can build up (or draw down) prevents communication between vents, i.e., emissions remain uncorrelated. This uncorrelated behavior was observed by Greinert [
Hydrocarbon seepage escapes the seabed as individual bubbles, bubble streams, bubble plumes, or intense megaseep (order 106 L dy-1 or greater) bubble plumes (Figure
Example images of (a) mobile sediment seepage including many minor bubble plumes, (b) a megaseep where emissions arise from numerous vents that combine into a single bubble plume, IV Super Seep, (c) obstructed seep that leads to bubble aggregation into very large bubbles from Shane Seep, and (d) oil seepage from Jackpot Seep in 10 m water depth. From Leifer [
The fate of seep bubbles and their impact strongly depend on bubble size and seabed depth. Bigger and shallower bubbles [
For bubble flows below a critical flow rate, bubble size depends on orifice size while for flows above the critical flow rate, bubble size depends on both orifice size and flow rate [
Any plume type can contain oily bubbles; however, identifying oily bubbles by appearance can be difficult (except for very oily bubbles). Thus, the presence of oil is inferred from slowed bubble rise [
As bubbles rise, their contents—CH4, higher alkanes, and other trace gases [
Synergies allow bubbles in a plume to rise higher, retaining more of their initial gases to shallower depths or the sea surface. The most important synergy is the upwelling flow, where a fluid flow is driven by the bubbles’ buoyancy [
Seep emissions vary on second [
The first reports of tidal forcing of seep emissions were in the COP seep field for seep oil emissions [
Seasonal trends have been identified in the North Sea passive acoustic data (Figure
Long-term acoustic spectral observations for the North Sea 22/4b seep site and daily average (right axis). Fine striations (subdaily) are tidal emission variations. The acoustic spectrum relates to the bubble size spectrum. The September data gap reflects retrieval and deployment of a second recorder. From Wiggins et al. [
Interannual data only are available from two COP seep field datasets: repeat (i.e., discontinuous) sonar surveys [
Temporal and spatial changes are related at large (interannual and kilometer—e.g., Bradley et al. [
Where sediments are unconsolidated, emissions are sporadic and highly variable, often transient, with rapidly varying vent location [
As with many other geologic systems, large and abrupt emissions (eruptions) can occur. Eruptions of, for example, mud volcanoes (terrestrial and marine seeps that emit both CH4 and mud fluids) are well known globally and may be triggered by seismic activity [
Given the absence of long-term, seep-monitoring studies, there are few observations of large seep eruptions. Leifer et al. [
Seep migration often occurs through faults and fractures that penetrate the overlying capping layer, which provide migration pathways. As a result, there is a connection between seismic activity and emissions [
Seismicity need not be local to affect geofluid flows. For example, geyser systems have been affected by earthquakes thousands of kilometers distant [
Seismicity also affects overlying marine sediments (that may be gas-charged) by liquefaction, subsidence, and dislocation, as well as sediment failure and translation including submarine landslides [
Marine oil seepage escapes the seabed as oily bubbles or oil droplets, rises, and surfaces within a footprint whose size relates to varying water column currents [
Seep oil rises slowly as droplets, or far more rapidly as oily bubbles, which have a far greater buoyancy [
For shallow oil seepage, water column dissolution and current drift are minimal and the oil arrives at the sea surface in a narrow surfacing footprint and forms a slick with similar composition to when it was released. These slicks drift and weather, with the latter transforming the petroleum’s chemical and physical properties [
Schematic of important early oil slick processes on timescales of up to a few days. Oil image derived from the Airborne Visual Infrared Imaging Spectrometer of the Deepwater Horizon oil spill. Wind and current directions are approximate based on the spatial pattern of the oil slicks. Adapted from Leifer et al. [
Wind and currents drive large-scale oil slick movement. Although winds are generally referenced to 10 m height and currents are measured at a depth of tens of centimeters to meters, the actual interfacial drift velocity lies between the two. Generally, oil is presumed to drift at the vector sum of the currents and wind speed after applying a windage factor of ~2-3% [
Although several slick processes suggest that oil slicks should disperse [
Evaporation causes the slick to lose its lighter components, with faster loss of lighter components [
For surface slicks and for shallow oil seepage, dissolution negligibly affects oil mass, generally much less than 1%, although this varies with oil type and age, i.e., the volatile fraction. In general, dissolution preferentially involves the lightest components, such as the gasoline fraction, which also are the most toxic [
Dissolution competes with evaporation, which occurs faster and dominates in the loss of soluble compounds for surface slicks and shallow pipeline leaks and seepage. For deep sea emissions, dissolution losses can remove most of the volatiles before the oil surfaces and forms a slick—with decreased evaporation [
Emulsification is the process of water incorporation into oil, creating an oil-water mixture. Oil slick emulsions can be 85% or more water, greatly expanding slick volume [
Solar photooxidation breaks molecular bonds in oil components, converting larger and less volatile molecules into smaller and more volatile molecules and fragments that may react with other components. This increases evaporation [
Turbulence can break up surface oil slicks into a dispersion of fine oil droplets that remain “suspended” subsurface [
Submerged droplets drift with currents until resurfacing once winds subside and turbulence decreases; larger droplets first. For example
Additionally, if the oil is negatively buoyant, or becomes negatively buoyant from evaporation or dissolution, it will sink. For very weak currents (less than ~10 cm s-1), the oil will sink to the seabed, but stronger currents and turbulence can maintain sunken oil in suspension [
Biodegradation by natural microbial populations is an important mechanism that removes petroleum and other hydrocarbons from the environment on days to week and longer timescales depending on the petroleum compounds and environmental factors [
Schematic of nearshore transport. Shape function of velocity moments of the equation for time-averaged sediment transport.
Once oil reaches the shoaling zone, the rising seabed causes waves to become nonlinear (i.e., not deep water waves) with an onshore transport component [
The undertow transports sediments offshore, across the surf zone, towards the wave breaking point, and beyond, where the seabed flow reverses to weakly onshore [
As the beach slope changes due to sediment deposition and erosion, the breaking point shifts. Thus, for high wave-energy conditions, sediment and bars move offshore, whereas for low wave-energy conditions, they move onshore [
Rip currents are high-speed (>0.5 m s-1) offshore flows that can be persistent or transient and are driven by uneven alongshore flow. Uneven alongshore flow arises from variations in wave breaking height, seabed bathymetry, wave focusing, and wave shadowing along the coast [
Where waves impact the beach at an angle, the onshore flow has a longshore component, typically 0.3-1 m s-1 [
Although more sophisticated treatments of the longshore current exists, beach parameters are continuously changing, highly complicated, and generally unknown, due in part to sand removal during winter storms and summer buildup. Additionally, adaptation of these sediment transport models to tar is not trivial; tar does not behave as sand. Specifically, direct observations (Leifer, unpublished observation, 2005) suggest that longshore transport occurs beyond the swash zone, though details such as the water depth of transport and where in the swash zone this transport occurs remain unknown. Also, tar is a minor fraction of the beach (sand is the beach) that is nonuniformly distributed on the beach. Specifically, tar strands mostly at the high tide mark [
In the Gulf of Mexico, burial and subsequent reintroduction of beach tar into the surf zone by beach erosion have been documented for stranded Deepwater Horizon tar [
Stranded tar weathers more rapidly than floating oil (same solar insolation) due to warmer beach sand temperatures. Weathering increases the tar’s density, as does incorporation of sand into its matrix, rendering it less buoyant. Note that tar buoyancy can increase by incorporation of vegetative material. Upon retrieval from the beach by the flood tide, weathered beach tar is more likely to roll along the seabed or drift and bob above in a sufficiently energetic wave field, than to drift at the sea surface.
The COP seep field has played an important role in the overall understanding of marine hydrocarbon seepage, due to its significant size and accessibility—an hour by boat from Santa Barbara, CA. COP seep field emissions are the most intense in North America. For comparison, COP emissions are 6-25% of the estimated Gulf of Mexico oil emissions, arising from just ~13 km2 [
Seepage occurs along much of the southern and central California coast; thus, as the influence of COP seep field emissions diminishes with distance, the impact from local seepage becomes increasingly important. Still, even in the Santa Monica Bay, 130 km from the COP seep field, the COP seep field’s influence is notable. For example, Hartman and Hammond [
The ecosystem impacts of seep oil decrease with distance due to dispersion and weathering (which reduce its toxicity). In the case of the COP seep field, it has influenced the local and regional ecosystems for at least 500,000 years [
Seepage has influenced regional indigenous people’s culture and lives for thousands of years in the Santa Barbara area and elsewhere in California. For example, the Chumash Indians of the Santa Barbara region used tar for waterproofing canoes [
Geologic basins with gas and oil fields are found along the entire coastal length of California, in the Transverse Mountain Range, and in the San Joaquin Valley (Figure
California Geologic Basins with oil potential and oil and gas fields. Adapted from WESTCARB [
The presence of a trap is critical. Much of the onshore Santa Barbara Basin has no trap potential and thus no potential for accumulation and associated seepage [
There is active hydrocarbon formation in these basins. In the COP area, active formation occurs in the Monterey Formation deeper than ~3 to 4 km [
California offshore marine seepage is widespread and has been documented in the Ventura-Santa Barbara Basin, with seepage near Point Conception, COP, and Rincon Point, and in the Los Angeles Basin, with seepage in the Santa Monica Bay and offshore Long Beach (Figure
Southern California offshore seepage and major faults. Arrows show fault direction. From Wilkinson [
The COP seep field is arguably the best-studied offshore seepage and among the largest in the world [
Important seepage also occurs to the west of the COP seep field, in waters offshore Point Conception, where asphalt mounds to 18 m tall cover an estimated 5.4 km2 of seafloor. These seep fields also emit oil and gas [
Seepage is widespread in the Los Angeles Basin, including the oil and gas seepage of the La Brea Tar Pits [
The COP seep field is by far the largest and most intense marine California seep field with respect to gas emissions and its oil emissions are the largest or second largest in California with the other largest oil seepage from seep fields near Point Conception [
The geologic setting is critical—the strongest marine seepage in the Santa Barbara-Ventura Basin occurs where the Monterey Formation crests—around Point Conception and offshore Goleta (i.e., the COP seep field, where portions of the formation outcrops). The Monterey Formation also outcrops offshore Summerland, but folds isolate the shallow reservoir that drives Summerland offshore seepage from the deeper Monterey Formation (Figure
Generalized northern Santa Barbara Channel geological cross section from Point Conception to Ventura. Adapted from CDOG [
Leifer et al. [
Coal Oil Point (COP) seep field sonar map (red: strong emission, blue: weak) and underlying geologic structure: (a) oblique view from above looking northwest; (b) overhead view of Monterey and Rincon Formations and faults labeled. The Monterey Formation outcropping is a sea surface projection. Projection is uniform with orientation axis with size scale dots on the axis every 200 m; contours are 100 m. Figure by Mark Kamerling, Venoco Inc.
The strongest seepage occurs close to above the anticline crest, slightly offset to the south due to the dip of the S. Ellwood Fault—more properly the S. Ellwood Fault System (SEFS) with several parallel faults mapped. A thick damage zone is associated with the SEFS. Additional seepage is found inshore of the Red Mountain Fault. In contrast, minimal seepage is found from where the Monterey Formation outcrops at the seabed. In part, this is from near-surface biodegradation that forms tar seals that block hydrocarbon migration [
South of Point Conception, seepage is noted over two trends, the Conception seep field and the Cojo seep field, which follow the major offshore anticline and associated faults that allow hydrocarbons to accumulate (Figure
Geology and gas and oil seepage off Point Conception. Legend on the figure. Adapted from Fischer ([
More recently, north-south trending seepage in the offshore extension of the Santa Monica Basin (Figure
Point Conception seepage includes gas, oil, and tar, with observations of tar whips escaping from tar mounds (see Figure
Fischer [
The Monterey Formation outcrops in Summerland, ~40 km east of the COP seep field, which was the site of the first offshore US oil well [
Seepage has been documented for a number of locations in the Santa Monica Bay extending along the Newport-Inglewood Fault into deeper water and across to offshore of Point Dumé (Figure
The Monterey Formation, which is the source of seepage on the northern rim of the Santa Barbara Chanel where it rises near the coast, also rises on the southern rim of the Santa Barbara Channel around the Channel Islands. Fischer [
Visualization of major faults and location of seeps around the Channel Islands. Based on data in Fischer [
Terrestrial seepage in coastal hills and mountains potentially can enter streams and rivers and ultimately the ocean. Distance to flowing water is important; thus, seepage near or in streambeds and riverbeds is far more likely to contribute to riverine oil fluxes. Oil readily forms oil mineral aggregates in energetic, shallow rivers and can be buried in river/streambed sediments or permanently attaches to debris and vegetation; thus, distance to the coast also is important. For example, seepage in the Amazon River mouth [
Where geologic basins extend from onshore to offshore, there is increased potential for riverine seep oil inputs to the ocean. For example, Leifer and Wilson [
Photo of oil accumulation in the Santa Paula River from riverbank seepage. Tar is visible in fractures in the exposed rock matrix. Inset shows an enlarged area where oil enters the water and flows downstream. Photo: Ken Wilson and Ira Leifer.
For oil seepage near a river, downslope flow is very slow and decreases as the oil weathers, leading to the tar flow freezing in structures resembling lava flows, preventing them from reaching flowing water [
Riverbed catchment structures (natural from fallen vegetation, and rocks, or engineered) play a dualistic role—preventing downstream oil transport while enabling accumulation of floating oil in pools (Figure
Despite the well-documented widespread occurrence of terrestrial seepage in several coastal petroleum basins [
Natural hydrocarbon seepage varies on timescales from second to decadal both for continuous emissions and for episodic emissions. Additionally, continuous emission systems can episodically erupt. Finally, temporal and spatial emission variability often is interconnected with emissions shifting between connected subsurface migration pathways.
Hydrostatic pressure variation affects emissions on timescales as short as those of ocean swell—first documented in the COP seep field [
Isolation from surge is important in this type of measurement as Leifer and Boles [
Seasonal variations in emissions have been identified in decadal-long COP seep field air quality data [
The reaction of seepage to very small pressure fluctuations at 1,250 m suggests extreme pressure sensitivity with emissions finely balanced between near-seabed reservoir recharge from below and discharge to the seabed. This drives a highly nonlinear response of seepage to changes in seabed hydrostatic pressure, with sensitivity inversely related to flux [
Interplay between gas vents (Figure
Whereas hydrostatic pressure affects the driving pressure at the seabed, geological processes can affect the driving pressure on the reservoir side by altering the reservoir pressure through increases or decreases in recharge and by alterations in the migration pathway resistance(s), including opening or activation of new migration pathways and narrowing through deposition or destruction of existing migration pathways. Geological-driven changes can be slow and evolutionary, or they can be abrupt, producing large transient (eruptive) emissions.
Few quantitative observations are available for eruptive emissions for the COP seep field or elsewhere. As a result, the eruptive contribution to overall annualized emissions remains uncharacterized.
The only detailed, quantitative observation of a seep eruption is from a turbine seep-tent network deployed in 20 m water at Shane Seep in the COP seep field, which recorded seep fluxes at 1 Hz [
(a) Emission time series of a seep eruption at Shane Seep (20 m) in the Coal Oil Point seep field. (b) Detailed time series of the eruption. Adapted from Leifer and Boles [
Based on these observations, Leifer et al. [
Eruptive events can affect migration path permeability by freeing (eroding) and transporting sediment, tar, and other unconsolidated blocking materials on timescales that are much longer than the eruptive event (Figure
Seasonal variations in COP seep field emissions were identified in long-term atmospheric total hydrocarbon (
Normalized probability histogram (
Seasonal storms likely play an important role in seep emission seasonality. Potential storm forcing mechanisms could relate to hydrostatic pressure, seabed scouring, and aquifer recharge. Aquifer recharge causes increased reservoir pressure from seasonal rains. For both offshore British Columbia [
Interannual to decadal datasets are only available for the COP seep field. Data include repeat (i.e., noncontinuous) sonar surveys [
Decadal trends are evident in areal extent changes from 1946 to 2005 (Figure
Sonar return map for 2005 from Leifer et al. [
New data outlines were drawn for the 1940s data, ignoring oil reports, because oil droplets can drift significant distances before surfacing. Specifically, the map shows oil seepage (but not gas seepage) between the Ellwood and COP seep trends where the geologic model [
The surveys mapped a significant decrease in the overall seep field area over the 60-year period (Figure
One possible example is for the field’s largest seep, the Seep Tent Seep. The Seep Tent Seep was tented in 1982, with two tents collecting oil and gas that were piped to shore [
Overall, long-term, continuous WCS data (Figure
The eastern Ellwood trend features the persistent La Goleta Seep area, whose extent remained similar from 1946 to 1972 (with an eastward offset in 1972 relative to the 2005 survey, possibly from current differences). The La Goleta Seep clearly decreased by 1995 but remained unchanged between 1995 and 2005. Similarly, the Patch Seep area appears largely unchanged over the decades.
The La Goleta Seep lies along a projection of the eastern coast of Campus Point, suggesting that a fault connects the two. Additional support for an unmapped fault is the trend of oil seepage noted in 1946 along this trend, a trend that matches a linear trend of oil seepage in the 1953 survey. Leifer et al. [
Both the Seep Tent Seep and La Goleta Seep overlie shallow crests on the Monterey Formation anticline, delineated to the north by the multiple parallel faults of the S. Ellwood Fault System and thus are preferential seepage locations. The Seep Tent Seep also is located adjacent to a major crossing fault, the Wolf Fault (Figure
For the western area of the S. Ellwood trend, a significant decrease in seepage extent was mapped between the 1970s and 1990s. Quigley et al. [
The inshore COP trend, which includes the modern Trilogy and Horseshoe Seeps, appears to occupy a similar area extent over the period, with apparent spatial reduction due in part to improved resolution and better mapping accuracy in 2005 than the 1970s. In 2005, more extensive seepage was found than in 1995 and also more focused along the faults controlling seepage from the Trilogy Seep area. The major controlling fault trends towards one side of the Coal Oil Point fault and was active in 1946.
In 1946, two seepage trends were identified in Goleta Bay [
Oil seepage was reported along linear north-south trends over the dropped, compressional block between the inshore and offshore seep trends south of both COP and Campus Point in both 1946 and 1955. This seepage was not mapped in recent sonar surveys yet is suggested in the 1994 aerial imagery (Figure
Shane Seep was mapped in 1946 and is suggested in 1953. Currently (Ira Leifer, unpublished observations, 2018), as in the earlier studies, Shane Seep produces oil; however, it was absent in the 1972 map, which included very shallow seepage offshore Isla Vista (even among the kelp beds). During the 2000s, Shane Seep has been a persistent feature and the subject of numerous studies [
Overall, the changes in area extent are consistent with the ~18-year WCS data (Figure
Frequent field studies in the 2000s observed a significant change first documented in 2005, the appearance of the Trilogy Seeps. Prior work in the 1990s identified the largest inshore seepage as from the Coal Oil Point Seep, which became inactive around 2000 (Thor Egleton, Personal Communication, 2005). The Trilogy Seeps are aligned with the fault that defines the Coal Oil Point. Their appearance was first noted after two exceptional rainstorms in January 2005, during an airborne survey of massive oil slicks off Coal Oil Point (Figure
(a) Extensive oil slicks offshore Coal Oil Point following strong storms in Jan. 2005 and (b) comparison image for Jun. 2003. From Del Sontro [
Winds and currents drive oil slick transport, with currents comprising bulk currents, tidal currents, wind-driven surface currents, current shears, convergence flows, and Langmuir circulations. As the oil is transported, it rapidly emulsifies (if not a sheen) and loses its more volatile components to evaporation. Evaporation increases rapidly with temperature and thus is slower at night and during winter. For reference, the volatile component of oil from the COP seep field and for typical temperatures is reported at 30% [
In the following sections, we describe oil transport at the sea surface as fresh oil slicks, (Section
The meteorology of the Santa Barbara Channel results from the coupled interactions between the lower troposphere, ocean, and coastal mountain ranges [
Prevailing winds are from the northwest with weaker and far less frequent winds from the east-southeast. Winter prevailing winds are primarily from the west with very infrequent and weak easterly winds. Prevailing summer winds are from the west-northwest with significant winds from the east-southeast (Figure
Wind roses for the NOAA East (46053 - labeled 54) Channel buoy for (a) full year, (b) winter (January), and (c) summer (June) and West (46053 - labeled 53) Channel buoy for (d) full year, (e) winter (January), and (f) summer (June). Adapted from Beckenbach [
The strongest winds tend to occur in early afternoon, particularly in spring, and the weakest winds in the early morning (Figure
An alternate wind pattern of strong offshore flow occurs in fall and winter, the Santa Ana winds. The Santa Ana winds are strong, mountain lee side, surface-following winds. Santa Ana winds are driven by synoptic-scale pressure and/or temperature gradients between the coast and the interior desert [
Another key meteorological feature of the coastal Santa Barbara Channel is the cool, dense marine atmospheric boundary layer, which is particularly important in the summer when its thickness varies from 300 to 350 m for late morning to late afternoon, thinning significantly at night [
The Santa Barbara Channel (Figure
(a) Santa Barbara Channel bathymetry and generalized currents. Adapted from Divins and Metzger [
These two currents define a channel-scale counterclockwise eddy at shelf depth or less [
Near-surface water currents (Figure
Mean wind stress patterns, currents, and CODAR flow patterns for (a) upwelling 1 and (b) relaxation, (c) upwelling 2, and (d) convergent flow patterns. See text for details. Current data from Oct. 1992 to Jan. 1996 also shown. Data key on figure. After Beckenbach [
Upwelling generally occurs late winter through early summer when winds are stronger (Figure
Swell generally arrives from the west, driving a strong, eastward longshore current along the beaches west of COP, which is significantly weaker for the sheltered beaches east of COP (Figure
Generalized nearshore currents (not surface) for the COP seep field area showing generalized offshore currents for the COP seep field. COP seep field sonar map for 2005 added for spatial reference (see Figure
COP seep oil primarily rises buoyantly on bubbles or as droplets to the sea surface where it forms surface slicks. The nonvolatile oil slick components then either wash onto nearby or more distant beaches, sink and deposit into seabed sediments, form tar balls, or disperse into the water column and ultimately drift out out of the channel. Partitioning between these fates is largely unknown, although the lack of significant permanent stranded tar on Santa Barbara Channel area beaches [
Although poorly documented, sinking appears to play a critical role in the fate of COP seep field oil slicks. The initiation of oil sinking has been observed during slick tracking studies, e.g., Del Sontro [
Weathering and subsequent oil sinking also were documented during an oil booming experiment in the COP seep field (Figure
Image of submerged (dispersed) oil in the Coal Oil Point seep field during a bubble oil boom test, described in McClimans et al. [
(a, b) Photos of a seep oil assessment experiment for a single, isolated seep plume near Horseshoe Seep. (c) Zoomed image of collected surface oil. From Lorenson et al. [
In addition, during several slick-tracking experiments (Leifer, unpublished observations), it was observed that about an hour after the morning fog (also termed "marine layer") clears, oil slicks weather to where the slightest disturbance causes dispersion, i.e., the oil does not resurface. In contrast, entrained oil from a similar disturbance an hour prior would have resurfaced. These observations are consistent with photooxidation fragmenting larger hydrocarbon molecules into smaller ones that evaporate (Section
Submerged oil can sink to the seabed and become buried under sediments. Farwell et al. [
Comparison between the sediment plume volume and seep field emissions implies that a very small fraction of overall field emissions are buried in downcurrent sediments. When oil sinks or disperses naturally, it is slightly negatively buoyant—likely comparable to marine snow and smaller MOS [
A more plausible hypothesis is that under typical (nonstorm) Santa Barbara Channel conditions, the majority of dispersed COP oil remains in the wave-mixed layer, forming oil mineral aggregates with sediment (OMA) or with algae and microbes (MOS) that slowly sink while drifting west with the Davidson Current into the open Pacific Ocean, and/or recirculated by the Santa Barbara gyre into the California Current and then drifting south and east.
Under the very infrequent stormy conditions in Santa Barbara, the wave-mixed layer extends to the shallow seabed, mixing oil directly to the seabed, as well as forming fast-settling (large) OMAs with storm-mixed sediments. Thus, storms could provide an efficient deposition mechanism. The storm deposition hypothesis could explain in part the several orders of magnitude mismatch between seep field emissions and the sediment plume’s mass based on the infrequency of storms—one to three per year - but OMAs would have to permanently deposit into sediments - equally likely storm-disturbed sediments could resuspend sediment oil into the water column.
The fate of COP seep field oil depends on currents (Davidson Current, longshore current, nearshore countercirculation eddies, and wind-driven surface currents) and winds.
Data from West Campus Station, ~1 km inshore of COP (see Figure
Thus, the typical pattern is the formation of a series of quasiparallel, along-coast oil slicks oriented west-northwest arising from different active portions of the seep field once winds calm down in early evening (~18:00 PST). Absent wind forcing, oil slicks cannot approach land [
Drifter data for Santa Barbara Channel, west of Coal Oil Point (COP). Dashed green line shows where offshore currents shift to an inshore countercurrent. Box shows CODAR coverage. Adapted from Ohlmann [
Winds compress the oil slicks into streamers due to higher wind stress at the slick’s leading edge (where the sea surface is mobile) than over the slick where oil suppresses capillary waves [
If conditions are such that oil weathering is slow, oil washes ashore onto area beaches in pulses corresponding to different streamers associated with different seep sources in the seep field. Offshore, the Davidson Current transports oil slicks westward, which shifts in the shoaling zone to an eastward transport by the longshore current. Complementing the longshore transport is onshore transport by the sea breeze and the counterclockwise inshore eddy, which advects oil slicks into the surf zone. In the surf zone, wave breaking fragments the oil slicks. Del Sontro [
There are several atypical wind patterns that affect the oil’s fate, including calm winds all day, strong prevailing winds into the night, and storm winds. For calm winds, there are necessarily no onshore wind components and the oil does not approach the shoreline, drifting westwards along the coast. If foggy and cold, oil can drift tens of kilometers on the sea surface, e.g., Estes et al. ([
If strong prevailing winds continue through late evening, COP seep oil can be driven far east. For example, floating, oil slick streamers tens of meters to hundred meters long are observed offshore Santa Barbara early in the morning (Leifer, personal observation, 2016). Oil that is driven this far east contributes to beach tar accumulations between Santa Barbara and Goleta, where COP seep field tar balls are found [
Both field observations and quantitative assessment of beach tar (Section
Tar is common on southern and central California beaches due to chronic oil emissions from natural oil seeps in its petroliferous regions [
Surface oil slick and tar ball transport depend on the combined effect of surface currents and winds, typically calm overnight with currents transporting the oil along the coastline—to the west-northwest and towards shore by the sea breeze and prevailing westerlies. The nearshore current flow field is complex, particularly due to a clockwise recirculation eddy inshore of a rough line between COP and the El Capitan Beach (Figure
Location of transects 1-12 for COP and coverage by 2 m quadrats as used in beach tar surveys. Inset: example images of tar ball size classes 1-5.
In the only published quantitative longitudinal beach tar study, Del Sontro [
A total of 57 irregularly timed surveys were conducted in 2005, reported in Del Sontro et al. [
Summer beach tar accumulation is higher than winter accumulation (Figure
Beach study area tar (a) mass accumulation (M), (b) precipitation at West Campus Station, (c) swell height, and (d) daily and 5-day averaged wind speed. Adapted from Del Sontro [
Important differences between winter and summer include temperature (cooler winter implies slower weathering), winds (stronger winter winds imply reduced transport time to beaches but also more days with wave breaking and sinking), and low clouds associated with the atmospheric marine layer (less marine layer (as occurs in winter) implies more photooxidation and sinking). Whereas cooler winter temperatures increase the likelihood of oil reaching the beach than in summer, reduced marine fog (mostly summer) counters this as does wave breaking and sinking from higher winds.
For long-range transport, losses from photolysis and wave breaking during the multiple days of transport and interaction with currents likely are dominant. For short-range transport (i.e., from the COP seep field to nearby beaches), other slick processes are more important and the dependency on winds is complex [
Specifically, coastal marine winds exhibit clear diurnal cycles including sea and land breezes as well as prevailing winds and nighttime decoupling of surface winds from upper level winds. Additionally, the strengths of these winds and the timing of shifts exhibit seasonal cycles. Del Sontro [
Based on field observations, it is likely that the timing of the disappearance of the marine layer and the onset of morning winds within a few hours after the marine layer breaks down are important to beach tar. As noted in Section
Winter (May-Aug) and summer (Jan-Apr) afternoon (1200 - 1800 LT) wind speed (
The 40 kg of tar for the study area on 27 Feb. 2005 was exceptional compared to the annual average of 3.49 kg for the study area, moreso for winter when tar coverage is less. Beach tar coverage was near 75% from the swash zone to the bluffs and extended with similar coverage at least several kilometers to the west based on a beach survey. If we assume that the tar distribution falls off as a Gaussian with a length scale on the order of ~5 km (based on the beach survey), there could have been several cubic meters of oil on the beach. Based on Hornafius et al. [
A more extensive beach tar dataset spanning 2005-2011 than just the 2005 data that was available to Del Sontro et al. [
(a) Total beach tar accumulation for the Coal Oil Point study area. (b) Study area tar accumulation versus Julian day with 28-day smoothing with 75% overlap (7/28) and 7-day smoothing with 50% overlap (3.5/7), respectively. Low data density in late December-January reflects poor student availability. (c) Beach tar accumulation versus wind speed for individual and monthly averaged surveys.
If oil emission rates relate to swell, then low-swell periods correspond to periods of low emissions during which migration pathways likely become clogged, i.e., self-seal, as discussed in Hovland [
Although individual surveys showed no clear wind speed-tar accumulation relationship, monthly averaged data show a clearly increasing trend to ~3 m s-1, above which tar mass appears to be either unrelated to wind speed or slightly negative (Figure
Due to longshore current and the beach configuration around COP, Devereaux Beach to the east of COP is far broader than Sands Beach to the west (Figure
(a) Daily mean tar mass, 2007-2009, for Coal Oil Point (COP), versus distance from the bluffs and transect. Tar mass was 4 m smoothed along the distance axis. Dotted line shows midbeach point. (b) Aggregated tar mass, integrated along the distance axis for the full beach and upper and lower halves.
Under high swell, sand accumulates on the beach, whereas under low swell, the beach loses sand. Thus, under high swell, which affects the upper beach, waves increase tar accumulation on Sands Beach, with even higher tar accumulation on upper Devereaux beach due to deceleration of the longshore current as it rounds COP (Figure
Wave-breaking turbulence in the surf zone strongly modifies the tar ball size distribution depending on the oil’s physical characteristics. Greater wave height leads to more energetic wave breaking on the beach, which fragments tar balls into smaller tar balls. Del Sontro et al. [
Tar mass accumulation ratio (
Tar is unevenly distributed across the beach, tending to be stranded in tar aggregates. A total of 1,119 beach tar aggregations were identified for the 12 transects for 2007-2009. The tar aggregate probability distribution (Figure
Tar aggregate mass (
Three different probability regimes were identified in the beach
(a) Occurrence of tar aggregate in three aggregate mass (
The aggregate mass probability distribution (Figure
Buried tar is not found on beaches around COP [
Beach-stranded tar weathers faster than floating oil due to warmer sand temperature for the same solar insolation. Weathering increases its density, as does incorporation of sand into its matrix, rendering it less buoyant. Note that buoyancy can be increased by incorporation of vegetative material such as kelp, often found in berms on beaches during storms.
Upon retrieval by the next flood tide, the weathered tar likely rolls along the seabed or drifts and bobs above the seabed in a sufficiently energetic wave field, rather than floating at the sea surface. In the surf zone, the near-seabed current is offshore and will transport tar towards the shoaling zone, beyond which near-seabed currents are onshore. Here, the longshore current transports submerged oil towards the east.
For typical Santa Barbara summer waves (~0.5 m height, ~7-second period) impinging at 15° to a 3° sloped beach, the longshore current is 1.2 m s-1 or 50 km per tidal cycle based on a simple empirical model of Harrison [
More sophisticated treatments of longshore current have been developed; however, their application requires numerous beach and oceanography and meteorology parameters that are largely unknown. Beach parameters are highly dependent on oceanography and orientation and are continuously changing, due in part to sand removal during winter storms and buildup in the summer. Additionally, these models are for sand transport, yet tar is not sand, and thus its longshore transport is likely different from that of sand. For example, transport occurs beyond the swash zone although where and at what height above the seabed (and hence velocity relative to sand transport velocities) remain unknown.
The ultimate fate of beach tar is unclear—while on the beach, tar clearly weathers—losing volatiles (becoming less pungent) and less deformable (or more brittle) and incorporating sand into its matrix. This ensures that after removal from the beach with the next high tide, the tar balls travel as submerged oil, likely along the seabed and/or becoming buried in nearshore sediments. Nearshore offshore sediment burial is unlikely to be permanent—seasonal winter storms tend to transport sand from area beaches to deeper waters.
Rip currents are observed offshore of COP (Figure
The utilization by chemosynthetic microbial communities of seep CH4 and higher hydrocarbons in waters from the deep sea to shallow coastal is well documented [
Compared to the numerous deep sea ecosystem studies, few studies have investigated shallow photic-zone CH4 seep ecosystems (beyond the microbial community structure), even less involving oil and gas seepage. Most of these studies have been for the COP seep field, where seep emissions have occurred over geologic timescales [
COP oil and gas seep hydrocarbons provide bioavailable chemosynthetic energy that enters the shallow water food chain through microbial oxidation [
Thick
Studies for the COP seep field have investigated the spatial distribution of community structure in relation to chemical gradients and identified significant complexity. However, the reality is far more complicated as seepage is highly variable on a range of time and spatial scales (Section
In general, most studies of the interaction of marine oil and the shallow marine ecosystem (outside microbial levels) have been from oil spills, which generally introduce petroleum hydrocarbons abruptly raising local concentrations to well above ambient petroleum (polycyclic aromatic hydrocarbons (PAHs)) levels, which includes pollution from industrial and transportation sources as well as natural PAHs [
Beyond the immediate lethal and sublethal effects of petroleum hydrocarbon toxicity from a spill or natural seep eruption, there are rapid shifts in species composition towards species that are more tolerant of petroleum toxicity and/or with less sensitive feeding strategies and a decrease in species abundance and diversity [
For ecosystem settings where petroleum hydrocarbon seepage has occurred on geological timescales, such as the COP seep field, ecosystem adaptation occurs at all levels. This leads to higher species diversity and abundance [
In 2003, a seepage workshop was held to identify major knowledge gaps in our understanding of seep processes and key research needs [
Since, techniques to quantitatively map seep emissions have been applied to seep fields across the world’s oceans. In several areas, particularly the COP seep field, studies have related geologic controls to seep flux (prior studies had related geology with seep presence/absence). Additionally, new measurements and numerical bubble model studies have improved understanding of the fate of seep methane and other trace gases across the water column in the COP seep field and elsewhere. Furthermore, significant insights have been made in understanding how seepage enters the food chain at the microbial level, although our understanding of the role of seep hydrocarbons, particularly oil, to higher tropic levels remains poor. Equally important, little is known for seepage as a chemosynthetic energy source to the food chain in the photic zone.
Quantitative seep emission studies have documented high variability, implying significant uncertainty in extrapolating from short campaign measurements to annualized emissions. Underlying this uncertainty is a lack of understanding of the factors driving variability including eruptive emissions. It also should be noted that although quantitative emissions are available for a dozen or so seep areas around the globe, most areas globally where seepage is feasible remain uninvestigated and/or uncharacterized.
Although the bubble processes controlling the fate of nonoily gas seepage are reasonably well characterized, the effect of oil coatings on bubble processes remains largely unknown. Once the oil surfaces, its drift and evolution are well studied based on decades of research into surface oil spills; however, investigation of subsurface oil transport after it sinks remains a poorly understood area.
Bubble size distributions and gas emissions have been assessed for individual seep vents, for major areas of active seepage, and for the entire COP seep field [
Recommendation: field studies should assess seabed sediments to understand the controlling factors of seep bubble size distribution and how it changes over the lifetime of a seep vent, facilitating extension of results from one seep field to another.
To date, seep emission measurements have been of noneruptive emissions; thus, our knowledge of eruptive emissions, e.g., Leifer and Boles [
Recommendation: field studies should include benthic observatories and be integrated with studies to understand how the fate of eruptive emissions differs from steady-state emissions.
Although a range of measurement tools for gas seepage quantification are available and have been used in the COP seep field and elsewhere, seep oil quantification is largely absent from the literature. Tools are needed, with remote sensing showing promise [
Recommendation: remote sensing approaches available for oil spill remote sensing should be adapted to assess quantitatively seep oil emissions. Further field data on oily bubbles are needed that characterize the factors controlling oiliness of different size bubbles in the plume. New, validated, underwater remote sensing methods are needed, such as high fidelity multibeam sonar, for characterizing and quantifying oil and oily bubble emissions.
Although studies long have investigated geological controls on the spatial occurrence of seepage [
Recommendation: whether through repeat multibeam sonar surveys or rotating multibeam sonar benthic observatories, quantitative 4D data are needed to relate temporal emission trends to shallow and deep geological structures. The COP seep field can continue to play a key role in developing insights due to its diverse types of seepage and sediment from bare rock to sandy, nonoily to oily, single bubble to megaplumes and its unique multidecadal data. Such data should be used to test hypothesized processes by numerical models, such as a numerical seep electrical model.
Although understanding of the primary mechanisms driving the water column fate of seep field gas emissions is well studied and understood for shallow seepage, significant uncertainty remains with respect to the importance of hydrates—particularly Type 2 hydrates [
Recommendation: laboratory studies of oily bubbles for a range of oiliness and oil types and bubble sizes are critically needed to provide parameterizations for numerical bubble models. Additional field data are needed to validate parameterizations and numerical bubble models.
Subsurface oil transport from both COP seep field slicks and after remobilization from area beaches is critical to understanding the fate and impacts of oil from the COP seep field (and other coastal oil seeps). Although validation efforts of oil slick trajectory and weathering models demonstrate significant uncertainties in the underlying processes, at least there are parameterizations for most processes (except perhaps photooxidation, which at least for COP seep field oil slicks plays an important role). In contrast, there are few field studies that shed light on how oil and tar drift and evolve subsurface, particularly in the complex nearshore environment. In part, this is from a paucity of measurement tools. In contrast, quantitative and semiquantitative oil thickness remote sensing approaches are becoming available and can provide important oil slick process validation data.
Recommendation: natural oil seepage provides an invaluable natural laboratory with no release permissions required. Natural seep oil slicks should be studied to improve understanding of oil slick processes at the sea surface and floating oil subsurface. Field studies should leverage and develop the latest remote sensing approaches both airborne and vessel borne, including multibeam sonar. Furthermore, field and lab photooxidation studies are needed.
There are large gaps in our knowledge on how seep hydrocarbons move through the food chain above the microbial level [
Recent studies have investigated the flow of chemosynthetic energy to higher trophic levels, including top predators, although most aspects remain uncharacterized. Leifer et al. [
Recommendation: since the COP seep ecosystem studies in the 1970s [
Most California seep research has focused on the COP seep field for reasons including proximity to Santa Barbara, protection against waves and wind (compared to seepage near Point Conception), size, and shallowness and thus diver accessibility (i.e., at a low cost compared to ROV or submersible depth seepage); however, marine seepage is widespread in coastal southern and central California waters [
In the case of riverine seepage, almost nothing is known [
Marine hydrocarbon seepage extends along much of the southern and central California coasts and likely further north, albeit largely uncharacterized and/or undiscovered. Thus, “pristine” reference sites free from seepage along the southern California coast do not exist. Ecosystem impacts of the COP seep field decrease with downstream distance while impacts from other seep fields, such as the Point Conception seep field, increase. Little is known about how impacts transition between seep fields. Schmale et al. [
The COP seep field is far from industrial and urban areas and extremely strong; thus, its waters are dominated by its seep PAHs. However, other seep areas, particularly near Los Angeles, are much smaller, and in waters with significant PAH inputs from pollution as well as other nearby seep sources, and natural PAHs from microbial processes. Ecosystem impacts from seepage in these waters could be amplified due to synergies between these environmental stressors or diminished due to evolutionary adaptations. For example, few dolphins were found killed from two major oil spills in the Persian Gulf in 1986 and 1991, whereas repeat dolphin die-offs in other years likely associate with red tide events [
Recommendation: studies that extend to beyond the COP seep field are needed to characterize other California seep fields including river seepage. Additionally, comparisons with seep ecosystems close to major urban pollution centers, like Los Angeles, are needed.
Views and conclusions in the document are those of the author and should not be interpreted as necessarily representing the official policies either expressed or implied, of the supporting institutions.
The author declares that they have no conflicts of interest.
Tar data analysis was supported by the US Geological Survey through a Cooperative Agreement #M07AP12407 NSL 06-03b between the Bureau of Ocean Energy Management Regulation and Enforcement (BOEMRE) and the University of California, Santa Barbara (UCSB). Structural geology analysis was supported by the California State Lands Commission, US Minerals Management Service, US Geological Survey, University of California Energy Institute, California Sea Grant, and Institute of Crustal Studies. I also thank Venoco Inc., Carpinteria, CA, for providing subsurface data and Marc Kamerling, Venoco Inc., for the subsurface geological model. Support for writing was provided by Plains All American Pipeline, LLC.