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Oil in the Sea: Weathering
Travis Hudson, American Geological Institute, 4220 King Street, Alexandria, VA 22302

The National Academy of Science’s National Research Council (NRC) has investigated the inputs, fates, and effects of petroleum (hydrocarbon gases, liquids, and in some cases solids) released to the marine environment (see The National Academy of Science’s Oil in the Sea Studies). The NRC’s most recent assessment (Oil in the Sea III published in 2003) included an investigation of how oil changes in the marine environment, changes called weathering. The processes responsible for weathering include evaporation, emulsification, dissolution, and oxidation.

Evaporation is a major process affecting oil in the sea. Because crude oil is a complex mixture of different hydrocarbon compounds, the amount that evaporates varies significantly depending on its composition. Oils lacking more volatile components (heavy oils) may lose only about 10% of their volume whereas oils with abundant volatile components (light oils) may lose up to 75% of their volume to evaporation. Evaporation is a key process in changing oil to a more viscous or tar-like substance in the marine environment.

Emulsification is the physical mixing of sea water and oil. The resulting gooey mass (nicknamed “mousse” by some) can contain 60 to 85% water. This significantly increases the volume of the oil-bearing material. Emulsification can also lead to increased oil viscosities and formation of heavier semi-solid masses. Stable oil and sea water emulsions tend to have a reddish color.

Some oil components will dissolve in sea water. Overall the amount of oil that dissolves is a small proportion but it tends to be the more soluble and smaller hydrocarbon compounds that can be more toxic to marine organisms.

The combining of oxygen with hydrocarbons, oxidation, affects all oils. This is the same general process as respiration in organisms and combustion in vehicle engines. If oxidation reactions go entirely to completion they convert hydrocarbons to carbon dioxide and water. But oxidation of oil in marine environments is incomplete, affects some oil components more easily than others, and involves photooxidation (sunlight-assisted reactions) and microbial oxidation. Photooxidation affects small amounts of oil but some of its products can be more toxic and water-soluble than the original oil. Microbial oxidation is respiration that ultimately produces carbon dioxide and water. It is a key way that oil is removed from the sea but the amount of oil microorganisms can oxidize varies depending on the type of oil and physical conditions.

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Santa Barbara County, California/USGS

An oil-sea water emulsion formed from a natural oil seep offshore Santa Barbara, California.

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Santa Barbara County, California/USGS

Evaporation, emulsification, and oxidation convert oil to a viscous tarry mass.

Additional Resources:
The above images are from the Natural Seep Inventory Final Report, conducted by Santa Barbara County and the U.S. Geological Survey.

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EarthNote No. 3 , © 2010-2014 American Geosciences Institute,
P. Patrick Leahy, Director, 4220 King Street, Alexandria VA 22302