U38MEK′ Expands the linear dynamic range of the alkenone sea surface temperature proxy. (1st July 2022)

• Record Type: Journal Article
• Authors: Novak, Joseph; McGrath, Sarah M.; Wang, Karen Jiaxi; Liao, Sian; Clemens, Steven C.; Kuhnt, Wolfgang; Huang, Yongsong
• Is Part Of: Geochimica et cosmochimica acta. Volume 328(2022)
• Page Start: 207
• Page End: 220
• Publication Date: 2022-07-01
• Subjects: Alkenones -- Paleothermometer -- U37K′ -- U38MEK′; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2022.04.021
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: The alkenone paleothermometer, U 37 K ′, has been used to estimate past sea surface temperatures (SST) for nearly forty years. Despite its popularity, U 37 K ′ is conventionally limited to water temperatures between 5 and 28.5 °C. At both low and high temperature ranges [0–8; >24 °C], U 37 K ′ 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 U 38 M E K ′, an unsaturation index based on the ratio of di and tri unsaturated methyl alkenones with 38 carbon atoms (C38 ME), extends the linear dynamic range of alkenone SST estimates from 0 to 29.5 °C SST. Accurate measurements of U 38 M E K ′ 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 U 38 M E K ′ permits more accurate SST reconstruction in warmer ocean waters and climates. U 38 M E K ′ also carries less risk than U 37 K ′ 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 U 38 M E K ′ 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 U 38 E T K ′ paleothermometer is also evaluated and found to be unsuitable for paleoceanographic applications.