A group of long-chain unsaturated methyl and ethyl n-ketones with 35–41 carbon atoms and 1–4 double bonds that are produced by a few species of haptophyte phytoplankton (e.g. coccolithophorids). These organisms require sunlight and occur mainly in the photic zone. Alkenones are found in marine sediment cores of Eocene to modern age, in some lacustrine environments, and in exposed outcrops of uplifted marine sediment. Species producing alkenones alter the relative proportions of their alkenones by increasing the proportion of saturated alkenones as the water temperature rises. Thus, the degree of unsaturation allows the past water temperature to be estimated. See unsaturation index.