3:45 PM - 4:00 PM
[MIS14-27] Variety of Cretaceous alkenones as product of haptophytes: perspective for understanding high latitude paleotemperature during Cretaceous
Keywords:alkenones, paleotemperature, Cretaceous
Alkenones are strait-chain alkylketones with 37-42 carbon atoms synthesized by haptophyte algae. C37 alkenones with two or three unsaturation sites (C37:2Me and C37:3Me) are often employed for paleothermometry. For this proxy, a parameter, UK’37, is calculated from relative abundances of these two molecules obtained with gas-chromatography (GC).
The alkenone paleo-thermometer is a method frequently used in discussions of oceanic environmental changes in the late Quaternary, but it has also been reported from the pre-Neogene. Alkenones are often reported as molecules with carbon numbers ranging from 36 to 42, and their structural isomers are also known. In the alkenone distribution of the Cenomanian stage of the Cretaceous, recently reported by the authors, the most abundantly contained molecule was the C40:2Et molecule, a di-unsaturated compound with 40 carbon atoms. In this study, we first identify and report the position of the unsaturation in this C40:2Et molecule using the derivatization method with DMDS. Previous study have shown that the tri-unsaturated C40:3Et molecule and an isomer of C40:2Et elute at the same GC retention time, resulting in overlapping peaks of two different alkenones. By separating the peaks in GC-MS SIM mode and determining their relative abundance variations, we found a high positive correlation of these compounds. If UK’40 = [C40:2Et] / ([C40:3Et] + [C40:2Et]) indicates temperature variations, then the relative abundance of this C40:2Et isomer should also vary in relation to temperature changes. In this report, we discuss the structure of this isomer and its potential as a new paleotemperature proxy.
The alkenone paleo-thermometer is a method frequently used in discussions of oceanic environmental changes in the late Quaternary, but it has also been reported from the pre-Neogene. Alkenones are often reported as molecules with carbon numbers ranging from 36 to 42, and their structural isomers are also known. In the alkenone distribution of the Cenomanian stage of the Cretaceous, recently reported by the authors, the most abundantly contained molecule was the C40:2Et molecule, a di-unsaturated compound with 40 carbon atoms. In this study, we first identify and report the position of the unsaturation in this C40:2Et molecule using the derivatization method with DMDS. Previous study have shown that the tri-unsaturated C40:3Et molecule and an isomer of C40:2Et elute at the same GC retention time, resulting in overlapping peaks of two different alkenones. By separating the peaks in GC-MS SIM mode and determining their relative abundance variations, we found a high positive correlation of these compounds. If UK’40 = [C40:2Et] / ([C40:3Et] + [C40:2Et]) indicates temperature variations, then the relative abundance of this C40:2Et isomer should also vary in relation to temperature changes. In this report, we discuss the structure of this isomer and its potential as a new paleotemperature proxy.