U38MEK Expands the linear dynamic range of the alkenone sea surface temperature proxy

The alkenone paleothermometer, U37K , has been used to estimate past sea surface temperatures (SST) for nearly forty years. Despite its popularity, U37K is conventionally limited to water temperatures between 5 and 28.5~\textdegreeC. At both low and high temperature ranges [0\textendash8; >24~\textdegreeC], U37K displays nonlinear behavior and significant scattering, leading to major uncertainties in reconstructed SSTs. With data from 171 surface sediments across the global ocean, we demonstrate that U38MEK , an unsaturation index based on the ratio of di and tri unsaturated methyl alkenones with 38 carbon atoms (C38ME), extends the linear dynamic range of alkenone SST estimates from 0 to 29.5~\textdegreeC SST. Accurate measurements of U38MEK are made possible by recent advances in chromatographic techniques that provide excellent resolution of C38 alkenones, as well as improved separation of all alkenone homologues. Our newly calibrated U38MEK permits more accurate SST reconstruction in warmer ocean waters and climates. U38MEK also carries less risk than U37K of biased SST estimates in regions (e.g. estuaries, closed seas) where both open ocean and coastal alkenone producers (e.g., Isochrysis galbana) are present because the latter do not produce C38 methyl ketones but do produce C37 methyl ketones. The efficacy of the U38MEK paleothermometer is shown by downcore analyses of early Pliocene age sediments from ODP sites 806 and 1143, as well as Holocene age sediments from the Black Sea. The U38ETK paleothermometer is also evaluated and found to be unsuitable for paleoceanographic applications.