Abstract
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. We determined the linear equations of δ18O–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 (δ13C) of H. nipponica also closely match seawater δ13C. Thus, we propose that the carbonate δ18O and δ13C of H. nipponica are useful proxies to reconstruct shallow-water paleoenvironmental changes in the northwest Pacific and its marginal seas.
Highlights
The oxygen isotopic composition (δ18O) of the carbonate tests of benthic foraminifers is widely used in studying the geologic history of the oceans, especially the deep ocean
Early researchers argued that the deep-sea benthic foraminifer Uvigerina spp. appears to record δ18O of calcite in isotopic equilibrium with seawater (Shackleton, 1974) and that Cibicidoides wuellerstorfi deviates from equilibrium by −0.64‰ in the Pacific (Shackleton and Opdyke, 1973)
The main findings of this study are as follows: 1) The oxygen isotopic composition (δ18Osw) and salinity of seawater samples from Tosa Bay generally overlap with those of the Kuroshio, which flows from the Okinawa Trough past Tosa Bay to the northwest Pacific
Summary
The oxygen isotopic composition (δ18O) of the carbonate tests of benthic foraminifers is widely used in studying the geologic history of the oceans, especially the deep ocean. Bemis et al (1998) countered their argument by compiling published δ18O data for Uvigerina and Cibicidoides species from core tops in the Atlantic, Pacific, and Indian oceans and comparing them with predictions from the paleotemperature equation established by experimental results from planktic foraminifers, finding that Cibicidoides δ18O data were in excellent agreement with the equation whereas Uvigerina data were scattered They concluded that the relative 18O enrichment of Uvigerina over Cibicidoides tests reflects the habitat differences of the genera: Uvigerina is infaunal and experiences low pH and decreased [CO32−] in pore waters, whereas Cibicidoides inhabits the sediment surface where bottom water pH and [CO32−] are higher (Bemis et al, 1998). Δ18O records from shallow-water settings are very important evidence of past environmental changes of the coastal ocean, there is presently no useful species of shallow-water benthic foraminifer to aid in reconstructing past δ18O changes
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