Alkenone

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 as first described (U^K₃₇) included the tetra unsaturated C₃₇ alkenone:

U^K₃₇ = (C_37:2 - C_37:4)/(C_37:2 + C_37:3 + C_37:4)

However, a simplified Unsaturation Index (U^K′₃₇), generally more useful in marine settings, is based on di- versus tri- unsaturated C₃₇ alkenones and defined as:

U^K′₃₇ = C_37:2/(C_37:2 + C_37:3)

The U^K′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:

U^K′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.