Interior hydrography and circulation of the glacial Pacific Ocean

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Abstract

The deep water of the Pacific Ocean is a key component of the global climate system on the time scale of late-Pleistocene glaciation and deglaciation. Despite its importance, the deep Pacific during the last glacial maximum has received relatively little attention compared to the deep Atlantic, in part, because the Pacific poorly preserves carbonate sediments on the sea floor. Here, we review the current state of knowledge of the deep hydrography and circulation of the glacial Pacific by examining available nutrient-proxy data, including some new δ13C and δ18O data measured on benthic foraminifera Planulina wuellerstorfi from the vicinity of Japan. Available benthic δ13C and δ18O and radiocarbon data from the Pacific support the presence of a deep hydrographic boundary at around 2000 m during the Last Glacial Maximum (Paleoceanography 3 (1988) 343; Paleoceanography 7 (1992) 273; Paleoceanography 13(4) (1998) 323). The deep hydrographic divide in the glacial Pacific is similar to what is inferred in the Atlantic (Quaternary Research 18 (1982) 218; Paleoceanography 3 (1988) 317; Paleoceanography 3 (1988) 343; Annual Reviews of Earth Planetary Sciences 20 (1992) 245; Science 259 (1993) 1148), the Indian (Nature 333 (1988) 651; Paleoceanography 13 (1998) 20), and the Southern Ocean (Paleoceanography 11 (1996) 191), suggesting that this is a global phenomenon during the glacial time. The upper water mass has a distinctly enriched δ13C compared to the deeper water mass, whose possible origins are discussed.

Introduction

Deep-water circulation and hydrography have been the foci of many paleoceanographic investigations over the years. Because of its sheer size, the deep ocean has a large capacity to store heat and various dissolved species of gases and salts. In addition, the slow movement of water gives the deep ocean one of the longest memories in the climate system. For these reasons, the deep ocean is believed to have played a central role in the large late-Pleistocene climate oscillations.

The Pacific deep water is volumetrically the most important, which has significant climatic implications, including the global carbon cycle on a glacial–interglacial time scale. On time scales much shorter than that for plate tectonics, the total surface carbon reservoir in the atmosphere–ocean–biosphere system can be assumed to be largely in steady state. Therefore, any significant change in atmospheric CO2 concentration over glacial–interglacial time scale as recorded in polar ice cores (e.g., Barnola et al., 1987) must involve a redistribution of CO2 amongst the carbon reservoirs (Broecker, 1982), of which the deep Pacific is by far the largest. Despite its obvious importance, the glacial Pacific deep water has received comparatively little attention than the Atlantic counterpart. One reason for this is that the carbonate compensation depth is shallower and carbonate preservation is poorer in the Pacific than in the Atlantic Ocean. The deficiency of carbonate material hinders paleoceanographic investigations, which have traditionally made use of the deep-sea sediment's carbonate fraction, from which foraminiferal δ18O chronology are constructed (e.g., Hays et al., 1976; Martinson et al., 1987). Commonly used seawater nutrient proxies, δ13C and cadmium concentration (Cd/Ca ratio) in benthic foraminiferal calcite tests, are also derived from the carbonate fraction.

For this and other reasons, our state of knowledge of the glacial Pacific deep water is limited. This was apparent, for example, in the work of Duplessy et al. (1988a), who produced a two-dimensional depth–latitude distribution of benthic foraminiferal δ13C in the eastern Atlantic Ocean during the Last Glacial Maximum (LGM) and graphically identified the Glacial North Atlantic Intermediate Water (GNAIW). In the same work, only a δ13C depth profile for the entire glacial Pacific was generated, since two-dimensional reconstruction was “impossible because of data limitation” (Duplessy et al., 1988a).

Since 1988, there have been few studies that have produced new glacial δ13C data from the Pacific (Herguera et al., 1992; Keigwin, 1998; Matsumoto and Lynch-Stieglitz, 1999; Matsumoto et al. 2001). Some of these and other works have shown that the water mass above approximately 2000 m is distinct in its nutrient content from waters below it but have yet to produce a consensus on its circulation even on whether deep waters were flowing northward or southward.

In this paper, we review the current state of knowledge of the deep Pacific hydrography and circulation during the LGM focusing on the meaning of deep-water nutrient proxies. As part of the review, we present a vertical profile of new benthic foraminiferal δ13C measurements from the vicinity of Japan. The new data clearly confirm the deep hydrographic boundary at around 2000 m in the glacial Pacific Ocean.

Section snippets

New benthic foraminiferal isotope data

Here, we briefly describe the new δ18O and δ13C measured on benthic foraminifera Planulina (Cibicidoides) wuellerstorfi from sediment cores raised from the vicinity of Japan between latitudes 28°N and 36°N and from a range in water depth from 740 to 3320 m (Table 1 and Fig. 1). The subtropical western boundary current, Kuroshio Current, overlies most of these core sites. The east coast of Japan where the nutrient-depleted Kuroshio and the nutrient-rich, subpolar western boundary current Oyashio

Hydrography

The Duplessy et al. (1988a) one-dimensional vertical profile of glacial benthic foraminiferal δ13C suggested importantly the presence of a high δ13C water mass at 700–2600 m. If one takes their vertical profile at face value (i.e., reflecting nutrient content), the high δ13C water mass at 700–2600 m would indicate a low nutrient, well-ventilated water mass. This interpretation is in sharp contrast with the modern macronutrient distribution of the Pacific (Fig. 5). Today, the most nutrient-rich

Conclusions

Examination of available nutrient proxy and other data from the Pacific Ocean indicates the presence of two different water masses in the glacial Pacific that were separated at approximately 2000 m water depth. The upper water mass has distinctly depleted δ18O and enriched δ13C, which may either reflect a water mass formed locally in the North Pacific, a modified GNAIW reaching the Pacific Ocean via the Southern Ocean, or some combination of the two as they are not necessarily mutually

Acknowledgements

Discussions with R.F. Anderson, A. Gordon, J.C. Herguera, L. Keigwin, and D. Sigman were very helpful. M. Oda provided samples for KH82-4-14 and BO94-20 PN3PC. Reviews by an anonymous referee and particularly D. Oppo and guidance provided by P. Clark helped improve the manuscript measurably. US National Science Foundation—Monbusho (Japanese Ministry of Education) Young Researcher Summer Exchange Program supported research at Hokkaido University by KM, who appreciates the support of “Hensen”

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