Abstract
Abstract. To understand bottom water variability in the subtropical northwestern Pacific, bottom water temperatures (BWTs), carbon isotopes (δ13C), and oxygen isotopes of seawater (δ18Ow) 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 δ18Ow in the intermediate water in the northwestern South Pacific was ~0.4‰ 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 δ18Ow records suggest changes that seem to follow Heinrich event 1 (H1) and the Bølling–Alleød (B/A) and Younger Dryas (YD) intervals, with BWT higher during H1 (~17 kyr BP) and YD (~12 kyr BP) and lower during B/A (~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 δ13C 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.
Highlights
Intermediate and deep water circulation in the Pacific is of particular interest because the deep Pacific is potentially an area in which a large amount of carbon was stored during glacial times (Broecker et al, 2004, 2008)
Previous studies have revealed that Mg / Ca of the benthic foraminifera C. wuellerstorfi are controlled by both the temperature and carbonate chemistry of seawater, especially at lower [CO23−] (Elderfield et al, 2006; Yu and Elderfield, 2008; Raitzsch et al, 2008; Healey et al, 2008)
We found a significant correlation between Mg / Ca and bottom water temperatures (BWTs) for type B at [CO23−] of approximately 10 μmol kg−1 (0.1 mmol mol−1 per ◦C), corresponding to BWT below 5 ◦C, and applied it to reconstruct BWT and bottom water δ18O of bottom water (δ18Ow) of core GH08-2004
Summary
Intermediate and deep water circulation in the Pacific is of particular interest because the deep Pacific is potentially an area in which a large amount of carbon was stored during glacial times (Broecker et al, 2004, 2008). Several studies have been conducted regarding the subarctic Pacific, including the western North Pacific (Ahagon et al, 2003; Sagawa and Ikehara, 2008), the Okhotsk Sea (Ohkushi et al, 2003), and the Bering Sea (Horikawa et al, 2010; Rella et al, 2012) Together, this body of work has improved our understanding of intermediate and deep water formation and ocean circulation in the subarctic North Pacific since the Last Glacial Maximum (LGM). Regardless of the conclusion of the debate regarding how deep the ventilated water reached during H1, the upper 1400 m of the water column appears to have been well ventilated in the subarctic North Pacific, likely owing to greater NPIW production in association with a shift of its production area from the Okhotsk Sea to the Bering Sea (Rella et al, 2012) or the open subarctic Pacific (Jaccard and Galbraith, 2013). In contrast to the intensive research that has focused on the deglaciation, there are fewer studies dealing with Holocene variability of the intermediate/deep Pacific circulation system
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