Lipid biogeochemistry in the Laurentian Trough—II. Changes in composition of fatty acids, sterols and aliphatic hydrocarbons during early diagenesis

Abstract

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, respectively, reflecting a 46–93% loss relative to settling particles intercepted at 150 m depth. A further 22–66% lipid loss was observed between 0–3 and 35 cm depth in the sediments. Lipid accumulation efficiencies in the top 3 cm sediment inventory averaged 21 ± 29% of water column fluxes, and indicated a clear reactivity trend: pheopigments > sterols > fatty acids ≈ total lipids > resolved hydrocarbons > UCM. However, within each lipid class, marine-derived components were as highly reactive as pheopigments, whereas terrestrial compounds behaved akin to the UCM. Extremely labile marine lipids that were enriched in settling particles, such as pristane, 24-methylcholesta-5, 22E-dien-3β-ol, and heneicosahexaene, were quickly lost near the sediment-water interface. Only moderately reactive components (16:0, 16:1 fatty acids, squalene) and the more stable 22:0 and 24:0 acids, fichtelite and UCM persisted deep enough to preserve the geographical trends observed for rapidly settling particles, i.e. predominance of terrestrial and petrogenic compounds at a landward site and prevalence of marine lipids at a station located 120 km further seaward. The preferential decay of marine components continued with depth in the cores, resulting in similar residual lipid patterns at 35 cm depth. However, each site could still be discriminated clearly by its fatty acid and hydrocarbon 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.