<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"><mml:mrow><mml:msubsup><mml:mi mathvariant="normal">U</mml:mi><mml:mrow><mml:mn>38</mml:mn><mml:mi mathvariant="normal">M</mml:mi><mml:mi mathvariant="normal">E</mml:mi></mml:mrow><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">K</mml:mi></mml:mrow><mml:mo>′</mml:mo></mml:msup></mml:mrow></mml:msubsup></mml:mrow></mml:math> 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 water between 5 and 28.5 °C. At both low high temperature ranges [0–8; >24 °C], displays nonlinear behavior significant scattering, leading major uncertainties in reconstructed SSTs. With data from 171 sediments across the global ocean, we demonstrate that U38MEK′, an unsaturation index based on ratio of di tri unsaturated methyl alkenones with 38 carbon atoms (C38ME), extends linear dynamic range SST estimates 0 29.5 °C SST. Accurate measurements U38MEK′ are made possible by recent advances chromatographic techniques provide excellent resolution C38 alkenones, as well improved separation all homologues. Our newly calibrated permits more accurate reconstruction warmer ocean waters climates. also carries less risk than biased regions (e.g. estuaries, closed seas) where open coastal producers (e.g., Isochrysis galbana) present because latter do not produce ketones but C37 ketones. efficacy paleothermometer shown downcore analyses early Pliocene age ODP sites 806 1143, Holocene Black Sea. U38ETK′ evaluated found be unsuitable paleoceanographic applications.