Alkenone unsaturation in surface sediments from the eastern equatorial Pacific: Implications for SST reconstructions

Significant uncertainties persist in the reconstruction of past sea surface temperatures in the eastern equatorial Pacific, especially regarding the amplitude of the glacial cooling and the details of the post‐glacial warming. Here we present the first regional calibration of alkenone unsaturation in surface sediments versus mean annual sea surface temperatures (maSST). Based on 81 new and 48 previously published data points, it is shown that open ocean samples conform to established global regressions of U 37 K′ versus maSST and that there is no systematic bias from seasonality in the production or export of alkenones, or from surface ocean nutrient concentrations or salinity. The flattening of the regression at the highest maSSTs is found to be statistically insignificant. For the near‐coastal Peru upwelling zone between 11–15°S and 76–79°W, however, we corroborate earlier observations that U 37 K′ SST estimates significantly over‐estimate maSSTs at many sites. We posit that this is caused either by uncertainties in the determination of maSSTs in this highly dynamic environment, or by biasing of the alkenone paleothermometer toward El Niño events as postulated by Rein et al. (2005).