In this paper, we present 50 surface water samples collected during the IMAGES III cruise (June-July 1997) along a transect from New Zealand to the China Sea (42 o S-178 o E, 21 o N-120 o E) covering a temperature range from 13.3 to 30.4 o C. A very worthwhile aspect of this study is a coupling of both biomarker (alkenone) and coccolithophorid counting. We show that the U 3 7 k '-temperature relationship is very similar to the Prahl et al. [Geochim. Cosmochim. Acta 52 (1988) 2203] culture calibration and to the global core top calibration of Muller et al. [Geochim. Cosmochim. Acta 62 (1998) 1757]. However, in the warmest surface waters of the Western Pacific ocean (>26.4 o C) where Gephyrocapsa oceanica is likely the most widespread species, the associated U 3 7 k ' has a constant value of 1.0. The consequence is that above this temperature threshold, U 3 7 k ' cannot be used as an accurate paleothermometer.