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Alkenones are long-chain unsaturated methyl and ethyl ''n''-
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''''K''37) included the tetra unsaturated C37 alkenone:
''U''''K''37 = (C37:2 - C37:4)/(C37:2 + C37:3 + C37:4)
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 = C37:2/(C37:2 + C37:3)
The ''U''''K''′37 can then be used to estimate
ketone
In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...
s produced by a few phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
species of the class Prymnesiophyceae
Prymnesiophyceae is a haptophyte class. Although it was originally described by Casper in 1972, it did not receive a Latin diagnosis (a requirement for valid publication under the International Code of Botanical Nomenclature
The ''Internat ...
. Alkenones typically contain between 35 and 41 carbon atoms and with between two and four double bond
In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...
s. Uniquely for biolipids, alkenones have a spacing of five methylene groups between double bonds, which are of the less common E configuration. The biological function of alkenones remains under debate although it is likely that they are storage lipids. Alkenones were first described in ocean sediments recovered from Walvis Ridge and then shortly afterwards in cultures of the marine coccolithophore
Coccolithophores, or coccolithophorids, are single-celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingdom ...
'' Gephyrocapsa huxleyi''. The earliest known occurrence of alkenones is during the Aptian
The Aptian is an age (geology), age in the geologic timescale or a stage (stratigraphy), stage in the stratigraphic column. It is a subdivision of the Early Cretaceous, Early or Lower Cretaceous epoch (geology), Epoch or series (stratigraphy), S ...
120 million years ago. They are used in organic geochemistry as a proxy for past sea surface temperature
Sea surface temperature (or ocean surface temperature) is the ocean temperature, temperature of ocean water close to the surface. The exact meaning of ''surface'' varies in the literature and in practice. It is usually between and below the sea ...
.
:
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''''K''37) included the tetra unsaturated C37 alkenone:
''U''''K''37 = (C37:2 - C37:4)/(C37:2 + C37:3 + C37:4)
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 = C37:2/(C37:2 + C37:3)
The ''U''''K''′37 can then be used to estimate sea surface temperature
Sea surface temperature (or ocean surface temperature) is the ocean temperature, temperature of ocean water close to the surface. The exact meaning of ''surface'' varies in the literature and in practice. It is usually between and below the sea ...
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 = 0.033''T'' �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.
References
Further reading
* Bradley, S R. (1999) ''Paleoclimatology: Reconstructing Climates of the Quaternary.'' Second edition. Academic PressExternal links
*{{Commons category-inline Paleoclimatology