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Oxygen isotopic compositions (δ 18 O) of benthic foraminifer tests are widely used for reconstructing paleoceanographic changes, such as global ice volumes during glacial–interglacial cycles. Although deep-sea benthic foraminifers have been well characterized and are considered reliable indicators, little attention has been paid to the geochemistry of shallow-water benthic foraminifers. In this study we evaluated δ 18 O in the shallow-water benthic foraminifer  Hanzawaia nipponica  Asano, which lives in surface sediments on continental shelves and upper slopes under the influence of two warm currents, the Kuroshio and Tsushima currents, in the East China Sea, northwest Pacific, and southwestern Japan Sea. To evaluate oxygen isotope equilibrium, we analyzed δ 18 O of  H. nipponica  and ambient seawater on the continental shelf in Tosa Bay, southwest Japan. Seawater δ 18 O and salinity in Tosa Bay are similar to those of surface and subsurface waters in the Kuroshio region in the Okinawa Trough and the northwest Pacific. Vertical profiles of seawater δ 18 O show no variation with water depth (0–200 m) in Tosa Bay. However, tests of living  H. nipponica  (as determined by staining with Rose Bengal) and fossil (non-stained)  H. nipponica , picked from samples of the top centimeter of seafloor sediment, yielded carbonate δ 18 O values that clearly increase with water depth, suggesting a temperature-dependent relationship. A comparison of carbonate δ 18 O values in living  H. nipponica  and those predicted on the basis of seawater δ 18 O and annual mean bottom temperature shows that  H. nipponica  tests are in oxygen isotopic equilibrium with ambient seawater. We determined the linear equations of δ 18 O–temperature relationship, and the slope of −5.26 (0.19‰°C −1 ) for living and −4.50 (0.22‰°C −1 ) for the fossil  H. nipponica , respectively. The carbon isotopic compositions (δ 13 C) of  H. nipponica  also closely match seawater δ 13 C. Thus, we propose that the carbonate δ 18 O and δ 13 C of  H. nipponica  are useful proxies to reconstruct shallow-water paleoenvironmental changes in the northwest Pacific and its marginal seas.

Oxygen Isotope Equilibrium of the Shallow-Water Benthic Foraminifer Hanzawaia nipponica Asano in Tosa Bay, Southwest Japan