Alkenones are long-chain unsaturated methyl and ethyl n-ketones produced by a few phytoplankton species of the class Prymnesiophyceae. Alkenones have been observed containing between 35 and 41 carbon atoms and with between one and four double bonds. Uniquely for biolipids, alkenones have a spacing of five methylene groups between double bonds, which are of the less common E configuration. The biological function of alkenones remains under debate although it is likely that they are storage lipids. Alkenones were first described in ocean sediments recovered from Walvis Ridge and then shortly afterwards in cultures of the marine coccolithophore Emiliania huxleyi. The earliest known occurrence of alkenones is during the Aptian 120 million years ago. They are used in organic geochemistry as a proxy for past sea surface temperature.
Alkenone-producing species respond to changes in their environment — including to changes in water temperature — by altering the relative proportions of the different alkenones they produce. At higher temperatures more saturated alkenones are produced proportionally. This means that the relative degree of unsaturation of alkenones can be used to estimate the temperature of the water in which the alkenone-producing organisms grew. The relative degree of unsaturation is typically described as an Unsaturation Index of di- versus tri- unsaturated C37 alkenones according to:
- UK′37 = C37:2/(C37:2 + C37:3) 
The UK′37 can then be used to estimate sea surface temperature according to an empirical relationship determined from core-top calibrations. The most commonly used calibration is that of Müller et al., 1998:
- UK′37 = 0.033T [°C] + 0.044 
The Müller et al. (1998) calibration is not suitable for all environments and, in particular, different calibrations are required for high latitudes and lacustrine settings.
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