Biotic and abiotic degradation of alkenones and implications for U37K′ paleoproxy applications: A review

Lipid biomarkers in sediments are widely used to infer environmental conditions occurring in the geological past, but such reconstructions require careful consideration of the biotic and abiotic processes that degrade and alter lipid biomarker compositions before and after deposition. In this study, we use alkenones produced by haptophyte microalgae to explore the range of effects of these degradative processes. Alkenones are now perhaps the best studied of all biomarkers, with several hundred references on their occurrence in organisms, seawater and sediments. Much information has been obtained on their degradation from laboratory incubation studies and inferences from changes in their distribution in aquatic environments. Although alkenones are often considered as more stable than many other lipid classes, it is now clear that their distributions can be affected by processes such as prolonged oxygen exposure, aerobic bacterial degradation and thiyl radical-induced stereomutation which, in some cases, can lead to changes in the proportions of the alkenones used in the U37K′ temperature proxy. The same set of chemical and biological processes act on all lipids in aquatic environments and, in cases where there is a marked difference in reactivity, this may lead to significant changes in the biomarker distributions and relative proportions of different lipid classes.