Alkenone

Alkenones are highly resistant organic compounds (ketones) produced by phytoplankton of the class Prymnesiophyceae. The exact function of the alkenones remains under debate.

Coccolithophoroids, for instance Emiliania huxleyi, respond to changes in water temperature by altering the production of long-chain unsaturated alkenones in the structure of their cell. At higher temperatures, more of the di-unsaturated molecules are produced than tri-unsaturated [Prahl and Wakeham]. The molecules are resistant to diagenesis, and can be recovered from sediments up to 110 million years old.

The ambient water temperature in which the organisms dwelt can be estimated from ratio of their unsaturated alkenones (C₃₇-C₃₉) that are preserved in marine sediments. The modified Unsaturation Index of “di” versus “tri” unsaturated C₃₇ alkenone is calculated according to the following relationship from [Prahl and Wakeham], which is modified after the original Unsaturation Index from [Brassell et al] that included the tetra-unsaturated alkenone:

U^K′₃₇ = C_37:2/(C_37:2 + C_37:3)

The Unsaturation Index can then be used to estimate the water temperature according to the following experimental relationship [Prahl and Wakeham]:

T [°C] = (U^K′₃₇ - 0.039)/0.034

References

• Bradley, S R. (1999) Paleoclimatology: Reconstructing Climates of the Quaternary. Second edition. Academic Press
• Brassell, S.C., Eglinton, G., Marlowe, I.T., Pflaumann,U. and Sarnthein, M. (1986) Nature, 320, 129-33.
• Prahl, F.G. & Wakeham, S.G. (1987) Calibration of unsaturation patterns in long-chain ketone compositions for palaeotemperature assessment, Nature 330, 367-369