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

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 U-37(K ') proxy widely applied in the past 40 years. However, U-37(K ') approaches unity at around 28 degrees C in cultures of Emiliania huxleyi (E. huxleyi), the most widely distributed alkenone producer in global oceans. Recent studies using surface sediments suggest U-38Me(K ') has similar to 1.5 degrees C higher upper temperature limits than U-37(K '). However, E. huxleyi generally does not grow above 28 degrees C. Gephyrocapsa oceanica (G. oceanica), on the other hand, is the dominant alkenone producer when sea surface temperature is above 22 degrees 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 degrees 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 degrees 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 degrees C (the highest temperature we tested), although alkenone production per cell decreases as temperature increases. Importantly, 37 and U-38Me(K ') values of RCC6484 and RCC3483 continue to respond to temperature changes beyond 28 degrees C, although temperature sensitivity decreases significantly above 28 degrees C. Above 30 degrees C, U-37(K ') and U-38Me(K ') responses to temperature further diminish, limiting their potential for paleo-SST applications using the currently available analytical technology. The ratio of C-38:3 over C-38:2 methyl ketone is on average 11 times higher than the ratio of C-37:3 over C-37:2 from 24 to 32 degrees C, suggesting a greater potential of using U-38Me(K ') for paleotemperature reconstruction in high temperature settings.