Alkenone unsaturation indices (UK’ 37 and UK 37) in marine sediments have been extensively used for reconstructing paleo-sea surface temperature (SST). The long-chain alkenones are produced by haptophytes, especially Emilinia huxleyi and Gephyrocapsa oceanica, in present oceans, and the unsaturation indices-T calibrations are based on the physiological features of these present alkenone producers. However, these species occurred since the Pleistocene, and thus, it needs to confirm applicability of the unsaturation indices-T calibrations in the sediment older than the Pleistocene. Long time-scale variations in compositions of the alkenones as well as related compounds such as alkenoates and their unsaturation ratios provide considerable insights for evaluating the reliable application of the T calibrations to older sediments.
Our recent researches have reported a distinctive compound, C38 di-unsaturated (C38: 2) alkenedione, from the Miocene to Pliocene sediments recovered from the NE Atlantic (Furota et al., 2016). The alkenedione shares the most characteristics in molecular structure with the alkenones, but it has not been identified from the present alkenone producers. Moreover, the alkenedione/alkenone ratios tend to increase in the older sediments, and the comparison with the coccolithophorid calcareous nannofossil assemblage implied that the alkenedione was produced only by the ancient alkenone producers like genus Reticulofenestra. Thus, the alkenedione can be a key compound to study the physiological features of the ancient alkenone producers.