C₃₇-Alkenones as Paleotemperature Tool: Fundamentals Based on Sediment Traps and Surface Sediments from the South Atlantic Ocean

Abstract

The alkenone paleotemperature method has gained wide acceptance, but questions remain concerning the water depth and seasonality of alkenone production or the temperature calibration of the U^K’ ₃₇ unsaturation index. In this paper, we summarize alkenone results from the South Atlantic Ocean which were obtained within the scope of the collaborative research project SFB 261 at Bremen University. We present sediment trap time-series from the eastern Equatorial Atlantic, the Northern and Southern Benguela, the Polar Frontal Zone and the Antarctic Zone, and compare the U^K’ ₃₇ records to concurrent temperature variations in the surface waters (Reynolds and Smith 1994). To convert U^K’ ₃₇ into temperature, we used the Emiliania huxleyi calibration of Prahl et al. (1988). In addition, we recapitulate surface sediment results and provide an update of the global core-top calibration. Our sediment trap results confirm earlier conclusions deduced from surface sediments that U^K’ ₃₇ principally reflects mixed-layer temperatures in the eastern South Atlantic. A shallow alkenone source is indicated, for example, by coinciding SST and U^K’ ₃₇ records, comparable temperature amplitudes and identical flux-weighted SST and U^K’ ₃₇ values within ±1°C. The sediment traps further reveal that seasonal variations in alkenone production have little effect on the overall U^K’ ₃₇ signal exported out of the euphotic zone. Canonical spring-autumn blooms as observed in the Northern Benguela and episodic flux events prevailing in filamentous upwelling regions produce average U^K’ ₃₇ signals not significantly different from the annual mean SST. Additional interannual variations weaken seasonal effects. In the Polar Frontal Region, where the dominant alkenone flux occurred in late winter and spring, the flux-weighted U^K’ ₃₇ signal was lower by about 1°C compared to the mean SST in the collection period. Only at site BO1 south of the Polar Front, did the U^K’ ₃₇ time series fail to reproduce the annual SST cycle. Relatively low alkenone temperatures (−0.4° to 0°C) obtained for the productive summer season at this site may be attributed to the calibration, although other factors cannot be ruled out. Altogether, our sediment trap and sediment results suggest that U^K’ ₃₇ reflects the mean annual temperature of the mixed layer in most regions of the South Atlantic. An exception is the western Argentine Basin, where the sedimentary U^K’ ₃₇ ratios appear to be biased by offshore and northward redistribution processes. An update of the global core-top calibration (n=518) using annual mean SST data of World Ocean Atlas 1994 yields exactly the same relationship as before (U^K’ ₃₇ = 0.033 SST + 0.044; Müller et al. 1998). A slightly different equation is obtained using temperature data of World Ocean Atlas 1998 (U^K’ ₃₇ = 0.032 SST + 0.073) but both relationships yield similar temperature estimates (within 1°C) as the 60Prahl et al. (1988) calibration. Core-top as well as sediment trap results do not indicate a systematic deviation from linearity at the warm and cold ends of the calibrations. The linear relationships may therefore be used to determine paleotemperatures in the range from 0 to 29°C, with an uncertainty of about ±1°C. They also produce reasonable temperature estimates for periods that predate the first occurrence of E. huxleyi, suggesting that the Gephyrocapsa species contributing alkenones to Quaternary and Pliocene sediments responded similarly to temperature changes.