Article révisé par les pairs
Résumé : The Uk’ 37 proxy for past sea surface temperature (SST) is based on the unsaturation ratio of C37 alkenones. It is considered a diagenetically robust proxy, but biases have been invoked because the index can be altered by preferential degradation of the C37:3 alkenone, resulting in higher reconstructed SST. However, alkenone degradation rate constants are poorly constrained, making it difficult to evaluate the plausibility of such a bias. Therefore, we quantitatively assessed the effect of (1) different alkenone degradation rate constants; (2) differential degradation factors between diunsaturated and triunsaturated C37 alkenones; (3) and initial Uk’ 37 values on the Uk’ 37 paleothermometer for two depositional environments (shelf and upper slope), by means of a reaction-transport model (RTM). RTM results reveal that preferential degradation of C37:3 can potentially alter the original signal of the Uk’ 37 paleothermometer, but SST biases (ΔSST) are largely within Uk’ 37 calibration error (ΔSST <1.5°C) assuming realistic model parameters. The magnitude of ΔSST is largely determined by the degradation rate constant, but it also increases with higher differential degradation factors. Additionally, initial Uk’ 37 values exert a nonlinear influence on the extent of potential SST bias, with midrange values (0.4 < Uk’ 37 < 0.6) being most sensitive. The most significant changes occur in the shallowest sediment layers and are attenuated with burial time/depth. Scenarios where ΔSST >1.5°C are associated with marked downcore decreases in alkenone concentration. Consequently, we caution against the interpretation of Uk’ 37 indices when extensive degradation results in very low alkenone concentrations (<5 ng g−1).