Glacial‐age hydrography of the far northwest Pacific Ocean

Abstract

Stable isotope data on benthic foraminifera from more than 30 cores on the northern Emperor Seamounts and in the Okhotsk Sea are synthesized in paleohydrographic profiles spanning the depth range 1000–4000 m. Holocene (core‐top) benthic foraminiferal δ18O and δ13C data are calibrated to modern hydrographic properties through measurements of δ13C of ΣCO2 and δ18O of seawater. Cibicidoides stable isotope ratios are close to the δ13C and equilibrium δ18O of seawater, whereas Uvigerina δ18O and δ13C are variably offset from Cibicidoides. Glacial maximum δ13C of Cibicidoides displays a different vertical profile than that of the Holocene. When results are adjusted by +0.32‰ to account for the secular change in δ13C during the last glacial maximum, the data coincide with the modern seawater and foraminiferal curves deeper than ∼2 km. However, at shallower depths δ13C gradually increases by as much as 1‰ above the modern value. Furthermore, above 2 km the benthic δ18O decreases by ∼0.5‰. These results are consistent with a benthic front at ∼2 km in the North Pacific [see Herguera et al., 1992], but they differ from interpretations based on trace metal data which indicate a source of nutrient‐depleted deep water during glaciation. The isotopic data suggest that during glaciation there was a better ventilated watermass at intermediate depths in the far northwestern Pacific, it was relatively fresher than deep waters there, and deep waters were as nutrient‐rich as today.