Evidence for differential degradation of alkenones under contrasting bottom water oxygen conditions: implication for paleotemperature reconstruction

Abstract

The successful reconstruction of sea surface temperatures using alkenone paleothermometry (U 37 k′) has relied on the premise that there is no significant differential degradation of alkenones with different states of unsaturation during diagenetic processes. To test this assumption, we conducted a comparative study of contemporary sediments in oxic and anoxic bottom waters from the Santa Monica Basin, offshore California. Long-chain alkenones were quantified and sea surface temperature were calculated using the calibrated U 37 k′–T relationship of Prahl et al. (1988). Our results show that temperature record from the oxic sediments is higher by as much as 4°C compared to those from time-equivalent anoxic sediments as a result of differential degradation of long-chain unsaturated alkenones and bioturbation mixing in the oxic sediments. The differential degradation of C 37:3 vs. C 37:2 alone could account for up to 2.5°C difference between these two records. This finding has significant implication in the interpretation of paleo–sea surface temperature data using alkenone paleothermometry.