2.1.2 Alkenone Unsaturation Indices U
K
37 and U
K
0
37
The index U
K
37 (U = unsaturation, K = ketone, 37 = chain length of ketone) was first
proposed by Brassell et al. (1986) to quantify the unsaturation of alkenones. Alkenones are derived from marine phytoplankton of the class Prymnesiophyceae; in
cold, polar waters, they are limited to the coccolithophorid Emiliania huxleyi
(Volkman et al. 1980; Bendle et al. 2005). This alga is widespread in the euphotic
zone of all modern oceans, from tropical to polar waters (Okada and Honjo 1973;
Okada and McIntyre 1979), and is tolerant of different nutrient and light conditions
(Prahl et al. 2003). E. huxleyi responds to changes in water temperature by altering
the molecular composition of their cell membranes; as the water temperature
decreases, they increase the production of alkenones (Bradley 2014).
The standard U
K
37 index measures the relative abundance of the di-, tri- and tetraunsaturated C 37 alkenones (C 37:2 , C 37:3 , C 37:4 ) in sediments and is defined as follows:
U
K
37 ¼
C 37:2 À C 37:4
C 37:2 þ C 37:3 þ C 37:4
C 37:4 alkenone is rarely detected in open sea sediments from low- and mid-latitudes. Thus, the equation for those areas can be written as (Prahl and Wakeham 1987):
U
K
0
37 ¼
C 37:2
C 37:2 þ C 37:3
C 37:4 alkenone becomes more important with decreasing temperature (<10 °C), so
U
K
37 is more reliable in high latitude areas, such as the northern North Atlantic and the
Southern Oceans (Rosell-Melé et al. 1995; Sikes et al. 1997; Andresen et al. 2013).
Fig. 1 Schematic illustration of the sedimentary contents of IP 25 and the phytoplankton-derived
biomarkers and resulting PIP 25 indices for each setting (modified after Belt and Müller 2013)
130
M. Łącka et al.
K
37 and U
K
0
37
The index U
K
37 (U = unsaturation, K = ketone, 37 = chain length of ketone) was first
proposed by Brassell et al. (1986) to quantify the unsaturation of alkenones. Alkenones are derived from marine phytoplankton of the class Prymnesiophyceae; in
cold, polar waters, they are limited to the coccolithophorid Emiliania huxleyi
(Volkman et al. 1980; Bendle et al. 2005). This alga is widespread in the euphotic
zone of all modern oceans, from tropical to polar waters (Okada and Honjo 1973;
Okada and McIntyre 1979), and is tolerant of different nutrient and light conditions
(Prahl et al. 2003). E. huxleyi responds to changes in water temperature by altering
the molecular composition of their cell membranes; as the water temperature
decreases, they increase the production of alkenones (Bradley 2014).
The standard U
K
37 index measures the relative abundance of the di-, tri- and tetraunsaturated C 37 alkenones (C 37:2 , C 37:3 , C 37:4 ) in sediments and is defined as follows:
U
K
37 ¼
C 37:2 À C 37:4
C 37:2 þ C 37:3 þ C 37:4
C 37:4 alkenone is rarely detected in open sea sediments from low- and mid-latitudes. Thus, the equation for those areas can be written as (Prahl and Wakeham 1987):
U
K
0
37 ¼
C 37:2
C 37:2 þ C 37:3
C 37:4 alkenone becomes more important with decreasing temperature (<10 °C), so
U
K
37 is more reliable in high latitude areas, such as the northern North Atlantic and the
Southern Oceans (Rosell-Melé et al. 1995; Sikes et al. 1997; Andresen et al. 2013).
Fig. 1 Schematic illustration of the sedimentary contents of IP 25 and the phytoplankton-derived
biomarkers and resulting PIP 25 indices for each setting (modified after Belt and Müller 2013)
130
M. Łącka et al.