THE WIDESPREAD OCCURRENCE OF HIGHLY BRANCHED ACYCLIC C-20, C-25 AND C-30 HYDROCARBONS IN RECENT SEDIMENTS AND BIOTA - A REVIEW

The occurrence of widely distributed acyclic isoprenoid hydrocarbons (C20, C25 and C30) with highly branched structures, in sediments and biota is reviewed. The compounds occur as alkanes and alkenes with from one to at least four double bonds in young aquatic (both marine and lacustrine) sediments from many parts of the globe (e.g. Peru, Antarctica, Gulf of Suez, North Sea, Atlantic) sometimes in high concentrations (e.g. 40-mu-g g-1 sediment) relative to other hydrocarbons. However, the compounds rapidly disappear in older sediments, possibly due to biodegradation and reaction with sedimentary sulphur. These and other factors are discussed. The sources of this fascinating group of hydrocarbons remain largely unknown, though evidence points to algae (possibly diatoms) as one possibility. The recent identification of related sulphur-containing compounds in a crude oil promises to extend the number of reports of these compounds still further, and to increase their importance as environmental biological markers.