Identification of double‐bond positions in isomeric alkenones from a lacustrine haptophyte

<jats:sec><jats:title>Rationale</jats:title><jats:p>Measurements of alkenone unsaturation ratios are widely used for paleotemperature reconstructions in ocean and lake environments. Previously, we reported the discovery of a series of tri‐unsaturated alkenone positional isomers (Δ<jats:sup>14, 21, 28</jats:sup>) from oligosaline and freshwater lakes in Greenland and Alaska. In this work we provide a detailed analysis of the structures and isotopic compositions (<jats:italic>δ</jats:italic><jats:sup>13</jats:sup>C and <jats:italic>δ</jats:italic><jats:sup>2</jats:sup>H) of the alkenones produced by the “Greenland haptophyte”.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>Alkenones were extracted from sediments of Lake BrayaSø, Greenland. Alkenone double‐bond positions were determined by GC/EI‐MS analysis of alkenone dimethyl disulfide and cyclobutylimine derivatives. Alkenones were purified by semi‐preparative HPLC using a silver(I) thiolate stationary phase. Carbon and hydrogen isotope analysis was performed by gas chromatography/isotope ratio mass spectrometry (GC/IRMS).</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>A series of novel tri‐unsaturated alkenone positional isomers were identified among four alkenone homologues (i.e. C<jats:sub>37 Me</jats:sub>, C<jats:sub>38 Me</jats:sub>, C<jats:sub>38 Et</jats:sub>, and C<jats:sub>39 Et</jats:sub>) with double‐bond positions at Δ<jats:sup>14, 21, 28</jats:sup>. The hydrogen isotope compositions (<jats:italic>δ</jats:italic><jats:sup>2</jats:sup>H, VSMOW) of the tri‐unsaturated positional isomers from C<jats:sub>37 Me</jats:sub> and C<jats:sub>38</jats:sub> <jats:sub>Et</jats:sub> were slightly depleted (~ −11 ‰) relative to the common tri‐unsaturated alkenone. The carbon isotope composition (<jats:italic>δ</jats:italic><jats:sup>13</jats:sup>C, VPDB) of the tri‐unsaturated positional isomers from the C<jats:sub>37 Me</jats:sub>, C<jats:sub>38 Me</jats:sub>, C<jats:sub>38 Et</jats:sub>, and C<jats:sub>39 Et</jats:sub> alkenones were significantly enriched (~ +4 ‰) relative to the common alkenones (di‐, tri‐, and tetra‐unsaturated).</jats:p></jats:sec><jats:sec><jats:title>Conclusions</jats:title><jats:p>The novel tri‐unsaturated alkenone positional isomers produced by the Greenland haptophyte possess Δ<jats:sup>14, 21, 28</jats:sup> double‐bond positions, instead of the common Δ<jats:sup>7, 14, 21</jats:sup> double‐bond positions. The hydrogen isotope values suggest the novel tri‐unsaturated positional isomers could be biosynthetic precursors to the tetra‐unsaturated alkenones (Δ<jats:sup>7, 14, 21, 28</jats:sup>). However, the significantly higher carbon isotope values of the tri‐unsaturated positional isomers relative to the common di‐, tri‐ and tetra‐unsaturated alkenones suggest these positional isomers may have different/additional biosynthetic precursors. Copyright © 2015 John Wiley & Sons, Ltd.</jats:p></jats:sec>