Evaluation of long-chain alkenones as paleo-temperature indicators in the Mediterranean Sea

Long-chain alkenones were analyzed in samples collected from February to July 1989 by a time-series sediment trap located at 200 m depth in the northwestern Mediterranean Sea. The alkenone temperature signal was calibrated using water column particles collected at the sediment trap mooring site. The calibration (U37K′ = 0.041T−0.21, r2 = 0.97, 13°C < T < 19°C), differed from the culture calibration of Prahl et al. (1988, Geochimica et Cosmochimica Acta, 52, 2303–2310) and the calibration for the eastern North Atlantic waters between 16°C and 25°C (Cenre and Eglinton, 1993, Deep-Sea Research 1, 40, 1935–1961). The slope of the calibration was, however, similar to the slope of the calibration established using all Pacific Ocean samples (Sikes and Volkman, 1993, Geochimica et Cosmochimica Acta, 57, 1883–1889). The correlation of U38MeK and water temperature was also significant (U38MeK =0.042T−0.25, r2=0.87). Sediment trap results revealed that the production of alkenones during high flux periods predominantly occurred in subsurface waters. This was further supported by the depth distribution of alkenones observed during the monthly cruises: C37 alkenone concentrations in the upper 100 m of the water column indicated a subsurface maximum at approximately 50 m depth in May, June and July, whereas in March and November alkenones were more abundant in the upper 50 m. These results emphasize that accurate reconstruction of SST in core-tops must take into account the seasonality and depth variations of alkenone production. An intercalibration exercise, conducted on Mediterranean and Norwegian Sea water samples, demonstrated that high resolution chromatographic methods are essential to obtain reliable values of U37K′ and avoid the overestimation of production temperatures, particularly for colder waters where alkyl alkenoates are abundant.