The impact of alkenone degradation on U 37 K ′ paleothermometry: A model‐derived assessment

The U 37 K ′proxy for past sea surface temperature (SST) is based on the unsaturation ratio of C 37 alkenones. It is considered a diagenetically robust proxy, but biases have been invoked because the index can be altered by preferential degradation of the C 37:3 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 C 37 alkenones; (3) and initial U 37 K ′values on the U 37 K ′paleothermometer for two depositional environments (shelf and upper slope), by means of a reaction‐transport model (RTM). RTM results reveal that preferential degradation of C 37:3 can potentially alter the original signal of the U 37 K ′paleothermometer, but SST biases (ΔSST) are largely within U 37 K ′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 U 37 K ′values exert a nonlinear influence on the extent of potential SST bias, with midrange values (0.4 <U 37 K ′< 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 U 37 K ′indices when extensive degradation results in very low alkenone concentrations (<5 ng g −1 ).