ALKENONES IN GEPHYROCAPSA-OCEANICA - IMPLICATIONS FOR STUDIES OF PALEOCLIMATE

Emiliania huxleyi is widely regarded as the most likely source of C-37 -C-39 alkenones in present-day seawater and Recent sediments, but othe r sources are required to account for the presence of alkenones in sed iments that predate the first occurrence of E. huxleyi about 265,000 y ears ago. Analysis of the lipids of a laboratory culture of the closel y related marine coccolithophorid Gephyrocapsa oceanica (strain JB02) isolated from a massive bloom in Jervis Bay, eastern Australia showed that this species also synthesizes C-37-C-39 alkenones and esters of d i- and tri-unsaturated C-36 fatty acids. This confirms earlier predict ions based on the fossil record that species of Gephyrocapsa should co ntain these distinctive biomarkers. In this strain of G. oceanica the sum of the C-38 ethyl ketone concentrations is similar to, or greater than, that of the C-37 methyl ketones over the temperature range 11-29 degrees C, whereas the reverse is true for Emiliania huxleyi. It shou ld thus be possible to determine whether there is a contribution from Gephyrocapsa to the alkenones in seawater and sediments. The concentra tions per cell of the major alkenones showed different responses to in creasing growth temperature. The 37:3 and 38:3 methyl alkenones and 38 :3 ethyl alkenone showed an approximately linear decrease in cellular concentration over the entire temperature range, whereas the 37:2 and 38:2 methyl alkenones and 38:2 ethyl alkenone concentrations showed al most no change from 11 to 20 degrees C followed by a dramatic increase above 20 degrees C. As a result, the ratio of di- to tri-unsaturated methyl alkenones as measured by U-37(k') changes greatly with growth t emperature, but the relationship is different from that found for E. h uxleyi in culture. The temperature response can be approximated by the linear relationship U-37(k') = 0.049T - 0.520 (r(2) = 0.89), although a better fit can be obtained using polynomial expressions. These data might account for some of the apparent anomalies in predictions of se a surface temperature (SST) derived from Emiliania-based alkenone-SST calibrations in those sediments which contain contributions from Gephy rocapsa.