Physiological impacts on alkenone paleothermometry

[1] We conducted isothermal (15degreesC) batch culture experiments with the coccolithophorid Emiliania huxleyi (strain NEPCC 55a) to evaluate the extent to which nutrient and light stress contribute to variability in the alkenone unsaturation index U-37(K'). Alkenone content and composition were constant throughout exponential growth in both experiments when nutrients (nitrate and orthophosphate) were replete. Stationary phase (nutrient-starved) cells continued to produce alkenones, amassing concentrations (SigmaAlk) greater than or equal to 3 times higher than those dividing exponentially (1.5-2 pg cel(-1)), and the U-37(K') of excess'' alkenone dropped by 0.11 units. In contrast, 5 days of continuous darkness resulted in a 75% decrease in cellular SigmaAlk and a significant U-37(K') increase (+0.11 units). Given an established 0.034 unit/degreesC response for exponentially growing cells of this strain, the observed range of U-37(K') variability at 15degreesC corresponds to an uncertainty of +/-3.2degreesC in predicted growth temperature. This level of variability matches that of the global U-37(K') annual mean sea surface temperature calibration for surface marine sediments, begging the question: What is the physiological condition of alkenone-producing cells exported to marine sediments? Comparison of our laboratory results for a strain of E. huxleyi isolated from the subarctic Pacific Ocean with depth profiles for alkenones in surface waters from two contrasting sites in the northeast Pacific Ocean suggests that the answer to this question depends on the ocean regime considered, a possibility with significant bearing on how stratigraphic U-37(K') records in marine sediments are to be interpreted paleoceanographically.