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Oil in the Sea III: Inputs, Fates, and Effects
regulators and resource managers, by helping to establish linkages between petroleum inputs and potential effects, leading to prioritized actions needed to reduce the most significant risks.
Natural seeps are the highest contributors of petroleum hydrocarbons to the marine environment (Figures 2-2A and 2-2B). Coastal seeps occur mainly in southeast Alaska where they are a minor source of petroleum introduced to the region during the reporting period (1990-1999).2 Offshore seeps are only known to occur in the northern and southern Gulf of Mexico, where they represent 95 percent of the total oil inputs to the offshore region, and southern California, where they represent 98 percent of oil inputs to the offshore zone. Considering the size of the inputs into these regions, and that the releases are composed primarily of unweathered crude oil, one would expect seeps to have a significant impact to marine resources in these areas. Yet, studies have shown that benthic communities have acclimated and even evolved to utilize some of the hydrocarbons (Spies et al., 1980; Spies and Desmaris, 1983; Montagna et al., 1986, 1989). The uncertainty in these estimates is large, complicating attempts to understand the potential risks to marine ecosystems. It is also uncertain whether the rates of release from seeps will decrease as the reservoirs are depleted over the next coming decades.
Crude oil released as seeps constitutes the single largest source of PAH input to the sea; however, these seeps most often occur in specific oil-production areas, and release rates are relatively low and chronic. As the relatively unweathered crude oil enters the ocean from the seabed, the soluble compounds dissolve and the volatile compounds volatilize, but much of the oil rises to the surface, forming slicks. For most crude oils, about one-third is lost by evaporation-volatilization in the first 24 hours. Intermediate-weight compounds can degrade by photooxidation and microbial processes over periods of weeks, and the residues form tarballs. Where seeps occur close to shore, such as in the Santa Barbara Channel off the coast of southern California, tar forms persistent deposits on the shoreline. Seeps are a significant contributor to the coastal PAH budget, adding 2.5 thousand metric tonnes, or one half of the estimated annual PAH loading (Table 2-4). Unlike much of the coastal waters, waters in areas dominated by seeps are likely enriched in dissolved PAH, and net volatilization of PAH occurs.
Petroleum seeps occur in many parts of the ocean and have served as natural experiments for understanding the relationship between chemical persistence and biological response among organisms comprising the seep community, including adaptive responses that have occurred over generations of exposure. As petroleum enters the ocean from the seabed, it is relatively unweathered and provides an energy source to microbial populations (Bauer et al., 1988; Spies et al., 1980; Spies and Desmaris, 1983; Montagna et al, 1986, 1989). This enrichment of the benthic environment by microbial turnover of organic material alters the benthic community by depleting local oxygen concentrations in addition to altering the hydrocarbon concentrations for exposure.
The most detailed investigations of petroleum seepages have been carried out in the Santa Barbara Channel off the coast of southern California. In heavy seepage areas, the benthic community has low diversity of a few species or invertebrates and is dominated by mats of sulfur-oxidizing bacteria (Beggiatoa) and a few species of invertebrates (Spies et al., 1980; Montagana et al., 1987, 1989, 1995). Pore-water concentrations of aromatic hydrocarbons within a few centimeters of an active seep were about 1 ppm. Within several meters of the very active seeps and where a small amount of seepage is still found, a diverse benthic community exists, similar in composition to benthic communities of the inner continental shelf in southern California with a few differences in species abundance (Spies, 1987). Natural biogeochemical tracers (13C, 14C, 35S) indicate that both the petroleum carbon, particularly the lighter fractions, and the sulfur from sulfide are incorporated into benthic meiofauna and macrofauna (Spies and DesMarais, 1983; Bauer et al., 1990). Therefore, even though the input from seeps is very large, ecological impacts appear to be limited in area, suggesting that the slow rate of release allows biota to acclimate to PAH and other toxic compounds in the releases.
Extraction of Petroleum
Historically, extraction of petroleum hydrocarbons has represented a significant source of spills and other releases of petroleum to the marine environment. The second largest marine spill event in the world was the IXTOC I blowout that released 476,000 tonnes of crude oil into the Gulf of Mexico in 1979. In the past decade, however, improved production technology and safety training of personnel have reduced significantly both blowouts and daily operational spills. Today, roughly 150 tonnes of petroleum hydrocarbons per year are discharged as accidental spills from platforms in North American waters (Table 2-2). Another source of petroleum hydrocarbon pollution in the extraction process is from produced waters. Presently, this is the largest source, approaching 2,700 tonnes per year into North American waters and 36,000 tonnes worldwide (Table 2-2). Although these amounts may seem high at first glance, these contributions represent less than 2 percent of the amount entering the marine environment from natural sources (Figures 2-5A and 5B).
As discussed in Chapter 3 and Appendix I, erosion of organic rich source rocks can yield petroleum-bearing sediment in locally significant amounts. However, this material is largely bound within sediment particles, thus suggesting it is largely not biologically available.