Bottom water variability in the subtropical northwestern Pacific from 26 kyr BP to present based on Mg/Ca and stable carbon and oxygen isotopes of benthic foraminifera

To understand bottom water variability in the subtropical northwestern Pacific, bottom water temperatures (BWTs), carbon isotopes (delta C-13), and oxygen isotopes of seawater (delta O-18(w)) at a water depth of 1166 m were reconstructed from 26 kyr BP to present. A new regional Mg / Ca calibration for the benthic foraminifera Cibicidoides wuellerstorfi (type B) was established to convert the benthic Mg / Ca value to BWT, based on 26 surface sediment samples and two core-top samples retrieved around Okinawa Island. During the Last Glacial Maximum (LGM), the delta O-18(w) in the intermediate water in the northwestern South Pacific was similar to 0.4 parts per thousand lower than in the deep South Pacific, indicating a greater vertical salinity gradient than at present. This salinity (and probably density) structure would have led to stratification in the intermediate and deep Pacific, which would, in turn, have greatly influenced carbon storage during the glacial time. The benthic Mg / Ca and delta O-18(w) records suggest changes that seem to follow Heinrich event 1 (H1) and the Bolling-Alleod (B/A) and Younger Dryas (YD) intervals, with BWT higher during H1 (similar to 17 kyr BP) and YD (similar to 12 kyr BP) and lower during B/A (similar to 14 kyr BP). The warming in the bottom water during H1 suggests increased contribution of North Pacific Intermediate Water (NPIW) to the subtropical northwestern Pacific and decreased upwelling of cooler waters from the abyssal North Pacific. During the interval from 17 to 14.5 kyr BP, the BWT tended to decrease successively in association with a decrease in delta C-13 values, presumably as a result of increased upwelling of the abyssal waters to the intermediate depths of the North Pacific caused by shoaling and enhancement of the southward return flow of Pacific Deep Water (PDW). During the Holocene, the millennial- to sub-millennial-scale variations in the BWT generally correlate with the sea surface temperatures in the Okhotsk Sea, the source region of the NPIW, suggesting that changes in the BWT are linked to changes in the NPIW production rate.