An index of di- versus tri-unsaturated C₃₇ alkenones that is used to estimate past water temperature. The index is calculated as: U^K’₃₇ = C_37.2/(C_37.2 + C_37.3). Temperature for most marine environments is then given by: U^K’₃₇ = 0.033 T (°C) + 0.044. A different calibration is used for samples from high latitudes and lacustrine environments.