The impact of alkenone degradation on UK'37 paleothermometry: a model-derived assessment

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 di- and tri-unsaturated 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 ºC) 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 non-linear influence on the extent of potential SST bias, with mid-range 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 ºC are associated with marked downcore decreases in alkenone concentration. Consequently, we caution against the interpretation of U_37^K' indices when extensive degradation results in very low alkenone concentrations (< 5 ng g-1).