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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° C in cultures of Emiliania huxleyi (E. huxleyi), the most widely distributed alkenone producer in global oceans. Recent studies using surface sediments suggest U 38 M e K′ has∼ 1.5° C higher upper temperature limits than U 37 K′. 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 …