oa Identification of methyl-branched alkenones with up to five double bonds in saline lakes

We demonstrate new C38 and C39 found in Lake Van and Lake Fryxell are methyl-branched alkenones (brC38Me and brC39Et). Previous assignments of these compounds as straight chain C38Me and C39Et were incorrect. Double bond positions, determined based on mass spectra of corresponding DMDS adducts, are Δ 4, Δ 7, Δ 14, Δ 21 and Δ 28 for penta-unsaturated branched alkenones. Based on published DNA data and changes of branched alkenone percentages of Lake Van, we hypothesize that branched alkenones function as storage of energy and are produced by a yet unknown species of Isochrysidales. UKbr38Me based on brC38Me alkenones has a similar trend with UK37 in the past 270 ka of Lake Van, suggesting the high temperature sensitivity of branched alkenones, same as other straight chain alkenones. Such proxies based on branched alkenones might be more reliable than UK37 and UK'37 to provide temperature record in sites with mixed alkenone productions.