Exploring the theoretical upper temperature limit of alkenone unsaturation indices: Implications for paleotemperature reconstructions

TitleExploring the theoretical upper temperature limit of alkenone unsaturation indices: Implications for paleotemperature reconstructions
Publication TypeJournal Article
Year of Publication2023
AuthorsLiao S, Novak J, Huang Y
JournalOrganic Geochemistry
KeywordsCulture experiments, RCC3483, RCC6484, Temperature calibrations

The structural specificity, exceptional diagenetic stability, and linear response of unsaturation to temperature have made alkenones an indispensable tool for reconstructing past sea surface temperatures, with the well known U37K′ proxy widely applied in the past 40 years. However, U37K′ approaches unity at around 28 °C in cultures of Emiliania huxleyi (E. huxleyi), the most widely distributed alkenone producer in global oceans. Recent studies using surface sediments suggest U38MeK′ has ∼1.5 °C higher upper temperature limits than U37K′. However, E. huxleyi generally does not grow above 28 °C. Gephyrocapsa oceanica (G. oceanica), on the other hand, is the dominant alkenone producer when sea surface temperature is above 22 °C and thrives in ocean regions such as the Pacific Warm Pool. So far there have been no culture data for G. oceanica above 29 °C to evaluate the temperature response of alkenone distributions and the (theoretical) upper temperature limit of alkenone unsaturation indices. Here we performed the first culture experiments on two strains of G. oceanica isolated from warm ocean sites at up to 32 °C: RCC6484 from the Pacific Warm Pool and RCC3483 from the South China Sea. We show both strains display higher growth rates at higher temperature, with the highest growth rates at 32 °C (the highest temperature we tested), although alkenone production per cell decreases as temperature increases. Importantly, U37K′ and U38MeK′ values of RCC6484 and RCC3483 continue to respond to temperature changes beyond 28 °C, although temperature sensitivity decreases significantly above 28 °C. Above 30 °C, U37K′ and U38MeK′ responses to temperature further diminish, limiting their potential for paleo-SST applications using the currently available analytical technology. The ratio of C38:3 over C38:2 methyl ketone is on average 11 times higher than the ratio of C37:3 over C37:2 from 24 to 32 °C, suggesting a greater potential of using U38MeK′ for paleotemperature reconstruction in high temperature settings.