Novel mono-, di- and tri-unsaturated very long chain (C37–C43) n-alkenes in alkenone-free lacustrine sediments (Lake Masoko, Tanzania)
Résumé
Different positional isomers of C37–C43 mono-, di- and (to a lesser extent) tri-unsaturated n-alkenes maximising at C39 and C40 and showing no odd or even carbon chain predominance are present among the major hydrocarbons of two recently deposited (>1850–1900 AD) sedimentary horizons from Lake Masoko, Tanzania. Structural similarities (i.e. double bond positions) between mono- and dienes indicate biosynthetic relationships. These very long chain alkenes are accompanied by numerous isomers of C19–C35 mono-unsaturated alkenes having either a terminal or a central unsaturation and showing a strong predominance of odd numbered chains. The wide variety of C19–C43 n-alkenes and their distinct distributions suggest different biogenic sources (and/or biosynthetic pathways) for the n-alkenes in the lake. Till now, the presence of very long chain (>C35) alkenes in aquatic environments has been attributed to the haptophyte microalga Emiliania huxleyi known to produce structurally related alkenones. However, these ketones are absent from Lake Masoko sediments, suggesting an alternative source for the very long chain alkenes. The compounds, as well as the shorter chain n-alkenes, appear relatively well preserved in the early lipid sedimentary record of Lake Masoko, due to the anoxic conditions prevailing at the sediment–water interface