This process forms an outer protective coating of heavy compounds that results in the increased persistence of the oil as a whole. Tarballs, which are often found on shorelines and have a solid outer crust surrounding a softer, less weathered interior, are a typical example of this process. However sea water has a density of approximately 1.025 and very few crudes are dense enough or weather sufficiently, so that their residues will sink in the marine environment. Sinking usually occurs due to the adhesion of particles of sediment or organic matter to the oil. Shallow waters are often laden with suspended solids providing favourable conditions for sedimentation.
Oil stranded on sandy shorelines often becomes mixed with sand and other sediments.
If this mixture is subsequently washed off the beach back into the sea it may then sink. In addition, if the oil catches fire after it has been spilled, the residues that sometimes form can be sufficiently dense to sink. Biodegradation Sea water contains a range of micro-organisms or microbes that can partially or completely degrade oil to water soluble compounds and eventually to carbon dioxide and water. Many types of microbe exist and each tends to degrade a particular group of compounds in crude oil. However, some compounds in oil are very resistant to attack and may not degrade.