[M-IS16] Paleoclimatology and paleoceanography

[MIS16-P02] Oxygen isotope equilibrium of the shallow-water benthic foraminifer Hanzawaia nipponica (Asano) in Tosa Bay, southwest Japan

*Minoru Ikehara^1,2, Shigetaka Kita², SHUNGO KAWAGATA³ (1.Center for Advanced Marine Core Research, Kochi University, 2.Graduate School of Integrated Arts and Sciences, Kochi University, 3.Faculty of Education, Yokohama National University)

Keywords:benthic foraminifer, oxygen isotope equilibrium, Tosa Bay, Kuroshio

Oxygen isotopic compositions (δ¹⁸O) 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 δ¹⁸O 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 δ¹⁸O of H. nipponica and ambient seawater on the continental shelf in Tosa Bay, southwest Japan. Seawater δ¹⁸O 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 δ¹⁸O 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 δ¹⁸O values that clearly increase with water depth, suggesting a temperature-dependent relationship. A comparison of carbonate δ¹⁸O values in living H. nipponica and those predicted on the basis of seawater δ¹⁸O and annual mean bottom temperature shows that H. nipponica tests are in oxygen isotopic equilibrium with ambient seawater. This result additionally suggests that the life cycle of this species is longer than one year. Thus, we propose that the carbonate δ¹⁸O of H. nipponica are useful proxies to reconstruct shallow-water paleoenvironmental changes in the northwest Pacific and its marginal seas.