Globigerinoides ruber has recently received much attention for paleoceanography in the subtropical and tropical ocean.
G. ruber has two morphotypes (
G. ruber s.s. and
G. ruber s.l.) which dwell at the surface layer and at deeper depth in the surface water, respectively. In order to establish a proxy for the quantitative reconstruction of sea surface temperature, oxygen and carbon isotopes were analyzed in
G.ruber collected at 30°N, 175°E in the Pacific. Based upon the fluxes of foraminifers and organic matter (OM), the entire duration of the sediment trap experiment was divided into two periods: Period 6A (May to December) characterized by low fluxes of organic matter and foraminifers under a stratified condition of the surface ocean, and 6B (January to April) characterized by high fluxes of OM and foraminifers under a well developed deep-mixing layer. With respect to seasonal variability, δ
18O values of both
G. ruber s.s. and
G. ruber s.l. decreased in summer with a minimum around September-October and increased in winter and spring, which was also the case with their δ
13C values. The mean difference of δ
18O values between both morphotypes was 0.25‰ in August-October, corresponding to a 1°C difference in water temperature. Consistency with field observation confirms that
G. ruber s.s. and
G. ruber s.l. dwell at the surface and at 30-50 m water depth, respectively. In contrast, the difference in δ
18O and and δ
13C values between the two morphotypes was not significant in early April due to deep-mixing. Together with foraminiferal assemblage, δ
18O values of the two morphotypes of
G. ruber can be a good proxy for quantitative reconstruction of vertical seawater temperature in the surface ocean.
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