Hirofumi Asahi

Deepwater Formation in the North Pacific During the Last Glacial Termination

Switching Basins Most of the densest, deepest water at the bottom of the oceans comes from two regions, the North Atlantic and the circum-Antarctic. Have other regions been able to produce significant quantities of deep water in the past? For decades, researchers have looked, with limited success, for evidence of deepwater formation in the North Pacific since the time of the Last Glacial Maximum, about 23,000 years ago. Okazaki et al. (p. 200) combine published observational evidence from the North Pacific with model simulations to suggest that deep water did form in the North Pacific during the early part of the Last Glacial Termination, between about 17,500 and 15,000 years ago. The switch between deep-water formation in the North Atlantic and the North Pacific is likely to have had an important effect on heat transport and climate. The Atlantic was not the only ocean in the Northern Hemisphere in which deep water formed during the last deglaciation. Between ~17,500 and 15,000 years ago, the Atlantic meridional overturning circulation weakened substantially in response to meltwater discharges from disintegrating Northern Hemispheric glacial ice sheets. The global effects of this reorganization of poleward heat flow in the North Atlantic extended to Antarctica and the North Pacific. Here we present evidence from North Pacific paleo surface proxy data, a compilation of marine radiocarbon age ventilation records, and global climate model simulations to suggest that during the early stages of the Last Glacial Termination, deep water extending to a depth of ~2500 to 3000 meters was formed in the North Pacific. A switch of deepwater formation between the North Atlantic and the North Pacific played a key role in regulating poleward oceanic heat transport during the Last Glacial Termination.

Land—sea linkage of Holocene paleoclimate on the Southern Bering Continental Shelf:

Detailed diatom records within surface and core sediments from the Southern Bering Continental Shelf (SBCS) reveal that the Holocene evolution of sea-ice distribution is associated with low pressure patterns. Holocene sea-ice distribution over the SBCS was mainly controlled by the location of the Aleutian Low. The corresponding paleoceanographic and paleoclimate conditions can be divided into three stages: (1) the early Holocene (before 7000 cal. yr BP) was characterized by extensive sea-ice distribution under two low-pressure cells, which covered the western Bering Sea and the Gulf of Alaska, respectively. (2) Between 3000 and 7000 cal. yr BP, the low-pressure system over the Gulf of Alaska became weak, causing total sea-ice mass over the SBCS to retreat. (3) In the past 3000 years, prevailing southwesterly winds over the SBCS due to the developing Aleutian Low have reduced further sea-ice cover on the SBCS. These paleoclimatic changes were probably a response to ENSO variation. The timings of water mass exchanges on the SBCS coincided with sea-level change along the Alaskan Peninsula. As a result, subsequent morphologic alterations have also influenced the paleoceanographic condition of the SBCS. The effect of the surface coastal water and bottom marine water on the SBCS intensified about 6000 cal. yr BP when sea level increased.

Evolution of the North Pacific Subtropical Gyre during the past 190 kyr through the interaction of the Kuroshio Current with the surface and intermediate waters

The North Pacific Subtropical Gyre (NPSG) has two important functions, i.e., one in ocean-heat transfer and another as a driving force for circulation of the surface and intermediate waters on the basin scale. In the present study, we describe records of the vertical thermal structures and distributions of water masses in the upper ocean of the subtropical northwest (NW) Pacific for the past 190 kyr, using two sediment cores collected from the Kuroshio Current area in the East China Sea and the NPSG area. During the two glacial periods, the Kuroshio Current was weakened owing to changes in ocean–atmosphere circulation and eustasy. The differences in the Mg/Ca-derived temperatures between surface and thermocline waters show the changes of depth and temperature (warming) of thermocline during glacial periods. Conversely, the planktonic foraminiferal assemblages demonstrate that the indicator of the intermediate water from the central area of the NPSG increased synchronously with thermocline warming during Marine Isotope Stage (MIS) 6. These results suggest that warm intermediate water strongly affected the changes in the water-column structure of the subtropical NW Pacific during MIS 6. However, during MIS 2, cold water had precedence over intermediate water probably owing to the southward shift of the subtropical front associated with the reduced transport of the Kuroshio Current. Thus, the NPSG has evolved differently during the two glacial periods (MIS 2 and MIS 6) through interactions between the Kuroshio Current, surface water, and intermediate water.

Seasonal variability of δ18O and δ13C of planktic foraminifera in the Bering Sea and central subarctic Pacific during 1990–2000

We evaluated a 10 year time series of δ18O and δ13C records from three planktic foraminifers (Neogloboquadrina pachyderma, Globigerina umbilicata, and Globigerinita glutinata) in the Bering Sea and central subarctic Pacific with a focus on their responses to environmental changes. Foraminiferal δ18O followed the equilibrium equation for inorganic calcite, with species-specific equilibrium offsets ranging from nearly zero (−0.02‰ for N. pachyderma and −0.01‰ for G. umbilicata) to −0.16‰ (G. glutinata). Equilibrium offsets in our sediment trap samples were smaller than those from plankton tow studies, implying that foraminiferal δ18O was modified by encrustation during settling. Habitat/calcification depths varied from 35–55 m (N. pachyderma and G. umbilicata) or 25–45 m (G. glutinata) during warm, stratified seasons to around 100 m during winter, when the mixed layer depth increases. Unlike δ18O, foraminiferal δ13C showed species-specific responses to environmental changes. We found a dependency of δ13C in G. umbilicata on CO32− concentrations in ambient seawater that agreed reasonably well with published laboratory results, suggesting that δ13C of G. umbilicata is subject to vital effects. In contrast, δ13C of N. pachyderma and G. glutinata are likely affected by other species-specific biological activities. Seasonal flux patterns reveal that fossil records of N. pachyderma and G. glutinata represent annual mean conditions, whereas that of G. umbilicata most likely indicates those of a specific season. Because none of these three taxa was abundant from December to February, their fossil records likely do not reflect isotope signals from cold seasons.