The upper end of the U^K′₃₇ temperature calibration revisited

Being able to decipher paleo sea surface temperatures for warm oceanic waters is of the utmost importance to further evaluate the role of the tropics in global climate change. The U^K′₃₇ index is one of such paleothermometers, although the warm range of temperatures often poses serious analytical challenges. This paper discusses and reviews several important aspects of the U^K′₃₇index calibration for waters warmer than 26°-27°C. Amongst them, insights into the possible existence of a limit above which the U^K′₃₇ index is not able to accurately record temperature variations are presented. Some warnings are also given on the misleading practice and expected consequences of reporting U^K′₃₇ index values of unity in the cases when abundances of the minor triunsaturated alkenone below detection limit are encountered. Future directions toward an improved knowledge of the behaviour of the U^K′₃₇ calibration for warm waters are suggested.