Difference between revisions of "Alkenone"
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| − | The C<sub>37</sub> alkenone unsaturation index (<math> U_{37}^{k'} </math>) is a firmly established tool for past [https://en.wikipedia.org/wiki/Sea_surface_temperature sea surface temperatures] reconstruction and is based on the | + | The C<sub>37</sub> alkenone unsaturation index (<math> U_{37}^{k'} </math>) is a firmly established tool for past [https://en.wikipedia.org/wiki/Sea_surface_temperature sea surface temperatures] reconstruction and is based on the relative abundance of di- (C<sub>37:2</sub>) and tri- (C<sub>37:3</sub>) unsaturated ketones with 37 carbon atoms. The index varies between 0 and 1, thus it may saturate in the temperature extremes as it becomes more challenging to determine since C<sub>37:3</sub> and C<sub>37:2</sub> alkenones approach their detection limits. Since alkenones come exclusively from a few [https://en.wikipedia.org/wiki/Species species] of [https://en.wikipedia.org/wiki/Haptophyte haptophyte] [https://en.wikipedia.org/wiki/Algae algae] which require [https://en.wikipedia.org/wiki/Sunlight sunlight], alkenone thermometry offers the advantage of direct estimate of near-surface ocean temperatures. The ubiquitous presence of alkenone-synthesizing organisms (most commonly the coccolithophorids [https://en.wikipedia.org/wiki/Emiliania_huxleyi ''Emiliania huxleyi''] and [https://en.wikipedia.org/wiki/Gephyrocapsa_oceanica ''Gephyrocapsa oceanic''] through the world's ocean and the rapidity and high-precision of the alkenone analyses had made <math> U_{37}^{k'} </math> a valuable proxy for paleoceanographic reconstructions. |
However, the <math> U_{37}^{k'} </math> proxy is subject to non-temperature effects, such as lateral transport through [https://en.wikipedia.org/wiki/Ocean_current oceanic currents] ([http://science.sciencemag.org/content/298/5596/1224.abstract Ohkouchi et al. (2002)]) and preferential post-depositional [https://en.wikipedia.org/wiki/Redox oxidation] of C<sub>37:3</sub> compared to C<sub>37:2</sub> ([http://onlinelibrary.wiley.com/doi/10.1029/97PA02893/full Hoeffs et al. (1998)]; [http://science.sciencemag.org/content/298/5596/1224.abstract Ohkouchi et al. (2002)]). Furthermore, changes in the seasonality of the proxy (i.e., which part of the seasonal cycle the proxy is recording) may bias the inferred temperatures toward winter or summer conditions ([http://www.sciencedirect.com/science/article/pii/S0146638006001884 Herfort et al. (2006)]). Finally in oceanic regions where the [https://en.wikipedia.org/wiki/Photic_zone photic zone] extends below the surface mixed layer, the sedimentary signal may not strictly represent sea surface temperatures but rather a composite temperature of the mixed layer and the thermocline ([http://www.sciencedirect.com/science/article/pii/S0016703798000970 Müller et al. (1998)]; [http://ceoas.oregonstate.edu/people/files/mix/Prahl_etal_2006_uk37_Chile.pdf Prahl et al. (2006)]). | However, the <math> U_{37}^{k'} </math> proxy is subject to non-temperature effects, such as lateral transport through [https://en.wikipedia.org/wiki/Ocean_current oceanic currents] ([http://science.sciencemag.org/content/298/5596/1224.abstract Ohkouchi et al. (2002)]) and preferential post-depositional [https://en.wikipedia.org/wiki/Redox oxidation] of C<sub>37:3</sub> compared to C<sub>37:2</sub> ([http://onlinelibrary.wiley.com/doi/10.1029/97PA02893/full Hoeffs et al. (1998)]; [http://science.sciencemag.org/content/298/5596/1224.abstract Ohkouchi et al. (2002)]). Furthermore, changes in the seasonality of the proxy (i.e., which part of the seasonal cycle the proxy is recording) may bias the inferred temperatures toward winter or summer conditions ([http://www.sciencedirect.com/science/article/pii/S0146638006001884 Herfort et al. (2006)]). Finally in oceanic regions where the [https://en.wikipedia.org/wiki/Photic_zone photic zone] extends below the surface mixed layer, the sedimentary signal may not strictly represent sea surface temperatures but rather a composite temperature of the mixed layer and the thermocline ([http://www.sciencedirect.com/science/article/pii/S0016703798000970 Müller et al. (1998)]; [http://ceoas.oregonstate.edu/people/files/mix/Prahl_etal_2006_uk37_Chile.pdf Prahl et al. (2006)]). | ||
Latest revision as of 16:09, 1 February 2016
The C37 alkenone unsaturation index (
) is a firmly established tool for past sea surface temperatures reconstruction and is based on the relative abundance of di- (C37:2) and tri- (C37:3) unsaturated ketones with 37 carbon atoms. The index varies between 0 and 1, thus it may saturate in the temperature extremes as it becomes more challenging to determine since C37:3 and C37:2 alkenones approach their detection limits. Since alkenones come exclusively from a few species of haptophyte algae which require sunlight, alkenone thermometry offers the advantage of direct estimate of near-surface ocean temperatures. The ubiquitous presence of alkenone-synthesizing organisms (most commonly the coccolithophorids Emiliania huxleyi and Gephyrocapsa oceanic through the world's ocean and the rapidity and high-precision of the alkenone analyses had made a valuable proxy for paleoceanographic reconstructions.
However, the proxy is subject to non-temperature effects, such as lateral transport through oceanic currents (Ohkouchi et al. (2002)) and preferential post-depositional oxidation of C37:3 compared to C37:2 (Hoeffs et al. (1998); Ohkouchi et al. (2002)). Furthermore, changes in the seasonality of the proxy (i.e., which part of the seasonal cycle the proxy is recording) may bias the inferred temperatures toward winter or summer conditions (Herfort et al. (2006)). Finally in oceanic regions where the photic zone extends below the surface mixed layer, the sedimentary signal may not strictly represent sea surface temperatures but rather a composite temperature of the mixed layer and the thermocline (Müller et al. (1998); Prahl et al. (2006)).
