Alkenone unsaturation index UK37 in surface sediments

C37-C39 long-chain unsaturated ketones (alkenones) and alkenes were identified in samples of particulate organic matter obtained from surface waters spanning the temperature range -0.7–12.2°C in the Southern Ocean south of Australia. The carbon number distribution indicates that these compounds are derived from the prymnesiophyte alga Emiliania huxleyi which is the predominant coccolithophorid in these waters. Values of Uk'37 were calculated according to the definition UK'37 = [37:2]/([37:2] + [37:3]), where [37:x] is the concentration of the C37 alkenone with “x” double bonds, and plotted against sea surface temperature (SST). Our data form a contiguous set with earlier compilations of data from warmer waters. Over much of the temperature range the relationship between Uk'37 and temperature is remarkably linear, and is best fitted by the simple equation Uk'37 = 0.0414T - 0.156 (for temperatures 4-25°C). This calibration of Uk'37 with temperature for field samples is significantly different from that established from analyses of prymnesiophyte algae in culture, and it diverges especially at the colder temperatures. Data for temperatures below about 4-6°C show increased scatter and the calibration at these low temperatures may be better fitted by a nonlinear exponential function, but this requires further testing. Certainly, the predictive power of the linear calibration is severely constrained at temperatures less than 5.0°C, but at temperatures above this our data establish the usefulness of Uk'37 as an SST indicator for waters north of the polar front in the Southern Ocean and for several regions of the world ocean.