LIPID BIOGEOCHEMISTRY IN THE LAURENTIAN TROUGH .2. CHANGES IN COMPOSITION OF FATTY-ACIDS, STEROLS AND ALIPHATIC-HYDROCARBONS DURING EARLY DIAGENESIS

Interfacial sediments of the 300-m deep Laurentian Trough contained 3. 2-11, 1.2-6.2, 0.4-2.4, 0.2-0.5 mg g(-1) OC of fatty acids, unresolved hydrocarbons (UCM), sterols, and resolved aliphatic hydrocarbons, res pectively, reflecting a 46-93% loss relative to settling particles int ercepted at 150m depth. A further 22-66% lipid loss was observed betwe en 0-3 and 35 cm depth in the sediments. Lipid accumulation efficienci es in the top 3 cm sediment inventory averaged 21 +/- 29% of water col umn fluxes, and indicated a clear reactivity trend: pheopigments > ste rols > fatty acids approximate to total lipids > resolved hydrocarbons > UCM. However, within each lipid class, marine-derived components we re as highly reactive as pheopigments, whereas terrestrial compounds b ehaved akin to the UCM. Extremely labile marine lipids that were enric hed in settling particles, such as pristane, 24-methylcholesta-5, 22E- dien-3 beta-ol, and heneicosahexaene, were quickly lost near the sedim ent-water interface. Only moderately reactive components (16:0, 16:1 f atty acids, squalene) and the more stable 22:0 and 24:0 acids, fichtel ite and UCM persisted deep enough to preserve the geographical trends observed for rapidly settling particles, i.e. predominance of terrestr ial and petrogenic compounds at a landward site and prevalence of mari ne lipids at a station located 120 km further seaward. The preferentia l decay of marine components continued with depth in the cores, result ing in similar residual lipid patterns at 35 cm depth. However, each s ite could still be discriminated clearly by its fatty acid and hydroca rbon composition. In situ sedimentary processes, such as the vertical distribution of bacteria and the production of coprostanol, were also recorded in the lipid composition of these deep coastal sediments. (C) 1997 Elsevier Science Ltd.