Salinity-Related Occurrence of Unusual Alkenoate and Its Involvement in the Past Temperature Estimation

Alkenone temperature estimation technique uses the physiological response of the unsaturation degree of C37 alkenones to growth temperature (Prahl and Wakeham 1987, Prahl et al., 1988). On the other hand, some recent culture observations have been revealed the extra-temperature limited fluctuations of the alkenone composition (e.g., Yamamoto et al., 2001). The novel alkenoate ethyltetratriacontadienoate (C34:2EE) was identified in late Quaternary sediments of the Japan Sea. This novel alkenoate accompanied hexatriacontadienone-2-one (C36:2EK), which first reported from Black Sea sediments (Xu et al., 2001). The concentrations of C34:2EE has good correlation with the concentrations of C36:2EK, suggesting both compounds must have synthesized under the specific ecological or physiological mechanism. The relative abundance of C34:2EE in total alkenones and alkenoates was significantly high in the last glacial maximum when the d18O in planktonic foraminifera is anomalously light (Oba et al., 1991, 1995), strongly suggesting that C34:2EE is diagnostic of low salinity environments. Samples with extraordinarily high C34:2EE content correspond to anomalously high alkenone temperatures during LGM that are about 3 degree Celsius higher than present SST. This could be explained by the low salinity that may have an ecological or physiological influence on the unsaturation and carbon length of alkenones and alkenoates.