Water masses in the Gulf of Aden

Hydrographic data collected from Gulf of Aden since 1920 have been compiled to identify and refine the definitions of water masses in the Gulf of Aden (GA) and to describe their spatio-temporal variability. Four water masses have been identified based on their θ-S characteristics. The Red Sea Water (RSW) that flows from the Red Sea is the most prominent water in the GA; this occupies about 37% of the total volume of Gulf of Aden. The Gulf of Aden Surface Water (∼3%) forms as a mixture of local water and the water from western Arabian Sea during winter and Red Sea surface water during summer. The intermediate water, identified as Gulf of Aden Intermediate Water (GAIW), occupies about 9% of the total volume of GA; a characteristic salinity minimum is associated with it at σ_θ=26.50 kg m⁻³. The northward spread of sub-tropical subsurface water from the south appears to be the major source of GAIW. The bottom water, named Gulf of Aden Bottom Water, showed the least variability. It was formed due to the mixing of Red Sea Water and water of southern origin. Mixing triangles have been used to analyze the composition of water in the GA.     

Possible Density Change of the Origin of North Pacific Intermediate Water Due to Double Diffusion in the Mixed Water Region

In this study we test Talley's hypothesis that Oyashio winter mixed-layer water (26.5–26.6σ _θ) increases its density to produce the North Pacific Intermediate Water (NPIW) salinity minimum (26.7– 26.8σ_θ) in the Mixed Water Region, assuming a combination of cabbeling and double diffusion. The possible density change of Oyashio winter mixed-layer water is discussed using an instantaneous ratio of the change of temperature and salinity along any particular intrusion (R _l ). We estimate the range of R _l ^DD required to convert Oyashio winter mixed-layer water to the NPIW salinity minimum due to double diffusion, and then assume double-diffusive intrusions as this conversion mechanism. A double-diffusive intrusion model is used to estimate R _l ^DD in a situation where salt fingering dominates vertical mixing, as well as to determine whether Oyashio winter mixed-layer water can become the NPIW salinity minimum. Possible density changes are estimated from the model R _l ^DD by assuming the amount of density change due to cabbeling. From these results, we conclude that Oyashio winter mixed-layer water contributes to a freshening of the lighter layer of the NPIW salinity minimum (around 26.70σ_θ) in the MWR.     

Possibility of recent changes in vertical distribution and size composition of chlorophyll-a in the western North Pacific region

Our analysis of the last three decades of retrospective data of vertical distributions and size composition of chlorophyll-a (Chl-a) over the western North Pacific has revealed significant changes of three indices related to Chl-a during summer season, as follows: (1) decreasing linear trend of the proportion of Chl-a in surface layer to that of the whole water column by 0.4 and 2.3% year⁻¹ in the subtropical area along 137°E (STA₁₃₇) during 1972 to 1997 and in the Kuroshio Extension area along 175°E (KEA₁₇₅) during 1990 to 2001; (2) increasing linear trend of the depth of subsurface Chl-a maximum (DCM) by 0.4 and 2.6 m year⁻¹ in STA₁₃₇ and KEA₁₇₅; and (3) decreasing linear trend of larger-size Chl-a (>3 μm) by 0.1 and 2.5% year⁻¹ in STA₁₃₇ and KEA₁₇₅, respectively. Water density (σ _θ ) at 75 m depth had also decreased by 0.006 and 0.05 year⁻¹ in STA₁₃₇ and KEA₁₇₅, respectively. The ratio of biogenic opal to biogenic CaCO₃ in the sinking flux decreased by 0.015 year⁻¹ in the subtropical region from 1997 to 2005. These findings may indicate that the subsurface chlorophyll maximum is deepening and larger phytoplankton such as diatoms has been decreasing during the past decade, associated with the decreasing density of surface water caused by warming in the western North Pacific, especially in the summer.     

Water mass exchange and diapycnal mixing at Bussol’ Strait revealed by water mass properties

Intensive CTD observations that resolve the mean and tidal components were done with a total of 129 casts in summer of 2001 at Bussol’ Strait. Based on these data and all the available historical data, we have revealed the outflow from Bussol’ Strait to the Pacific and the significant diapycnal mixing in the strait. In the range 27.0−27.3σ _θ , the water property in Bussol’ Strait is almost identical to that of the Kuril Basin Water (KBW). The KBW out of Bussol’ Strait forms a water mass front with the East Kamchatka Current Water (EKCW). This front also corresponds to the front of the Oyashio Current. In the lower part of the intermediate layer (27.3−27.6σ _θ ), part of the water in the strait is characterized by lower temperature, lower salinity, and higher dissolved oxygen than that of KBW and EKCW, which can be explained only by the diapycnal mixing. The strong diapycnal mixing in the strait can also be shown by the density inversion, occurrence frequency of which corresponds well to the amplitude distribution of the diurnal current. In the density range 26.7−26.8σ _θ , the water in Bussol’ Strait has the lowest potential vorticity, suggesting that it is a source region of the low potential vorticity water. Seasonal change of the water can reach up to a density of 26.8σ _θ around Bussol’ Strait. This leads us to propose that the combination of winter convection and local tidal mixing leads to effective ventilation of the intermediate layer.     

Formation of the salinity minimum in the Mixed Water Region between the Oyashio and Kuroshio Fronts

The salinity minimum frequently occurring in the Mixed Water Region between the Oyashio and Kuroshio Fronts seems to originate from the salinity minimum at the density of 26.8σ_θ called the North Pacific Intermediate Water. We examined water exchange of this region with the Oyashio and the Kuroshio Extension using mixing ratio R_K defined as (θ - θ_OY)/(θ_K - θ_OY) × 100, where θ_OY, θ_K, and θ represent potential temperature of the Oyashio and Kuroshio Waters and their mixture on the isopycnal surfaces, respectively. CTD data were obtained by repeated observation from January 1990 to May 1991. R_K increases southward from the Oyashio Front to the Kuroshio Front with the range of −20 to 120%. The gradient of R_K on the isopycnal surfaces is large around the Oyashio Front above the 26.8σ_θ surface, while it is large around the Kuroshio Front below it. This agrees with the average R_K in the Mixed Water Region decreasing greatly with the increase of density at densities less dense than 26.8σ_θ. We calculated thickness and volume transport of the Oyashio between the isopycnal surfaces near the coast of Hokkaido. They increase largely with density at densities less dense than 26.8σ_θ. It is supposed that the salinity minimum in the Mixed Water Region is the upper limit of the water largely influenced by the Oyashio Water. Its density could depend only on the density structure of the Oyashio.     

Variations of the oxygen minimum zone of the Okhotsk Sea during the last 50 ka as indicated by benthic foraminiferal and biogeochemical data

Benthic foraminiferal and sediment biogeochemical data (total organic carbon, calcium carbonate and biogenic opal contents) in two cores (1265 and 1312 m water depths) from the southeastern Sakhalin slope and one core (839 m water depth) from the southwestern Kamchatka slope were investigated to reconstruct variations of the oxygen minimum zone during the last 50 ka in the Okhotsk Sea. The oxygen minimum zone was less pronounced during cooling in the MIS 2 that is suggested to be caused by a maximal expansion of sea ice cover, decrease of marine productivity and increase of production of the oxygenated Okhotsk Sea Intermediate Water (OSIW). A two-step-like strengthening of oxygen minimum zone during the warmings in the Termination 1a and 1b was linked to (1) enhanced oxygen consumption due to degradation of large amount of organic matter in the water column and bottom sediments, originated from increased marine productivity and supply of terrigenous material from the submerged northern shelves; (2) sea ice cover retreat and reduction of OSIW production; (3) freely inflow of the oxygen-depleted intermediate water mass from the North Pacific.     

Seasonal change of oceanographic conditions and chlorophyll <Emphasis Type="Italic">a</Emphasis> vertical distribution in the southwestern Okhotsk Sea during the non-iced season

Seasonal changes in oceanographic conditions related to primary productivity was investigated in the southwestern Okhotsk Sea during non-iced seasons, using the observation data conducted in 2000∼2006. Based on hydrographic characteristics, the studied area could be classified into two regions, the Coastal Region which is influenced under the Soya Warm Current and the Forerunner Water of the Soya Warm Current, and the Offshore Region where the Intermediate Cold Water was located in the subsurface layer. This study is the first report on seasonal change of nutrient and chlorophyll a concentrations in the offshore region of the southwestern Okhotsk Sea. Variability of concentrations of chlorophyll a and nutrients is temporally and regionally high in the Coastal Region. The maximum chlorophyll a concentration in April was observed at the surface layer of both regions. The most remarkable feature on the vertical structure in the Offshore Region was the consistent existence of the Intermediate Cold Water and the development of seasonal thermocline in the subsurface layer during summer and autumn. The stratification formed within the euphotic zone in the Offshore Region resulted in the formation of the subsurface chlorophyll a maximum (SCM) from May to October. Throughout the research period, although less amplitude of nutrients at the surface was observed in the Coastal Region than that in the Offshore Region, comparable amplitude of chlorophyll a concentration was observed between regions. These results suggested differences of environmental conditions for primary production between the two regions. Depending on the presence of SCM, relationships between chlorophyll a concentration at the sea surface and chlorophyll a standing stock within the euphotic layer were different. At most stations with SCM, the surface chlorophyll a concentration was lower than 0.6 mg m-3. This suggests that the presence of SCM and the chlorophyll a standing stock within the euphotic layer may be estimated using the surface chlorophyll a concentration from spring to autumn in the studied area.     

Intermediate Waters in the East/Japan Sea

Properties of the intermediate layer in the East/Japan Sea are examined by using CREAMS data taken mainly in summer of 1995. Vertical profiles of potential temperature, salinity and dissolved oxygen and relationships between these physical and chemical properties show that the dissolved oxygen concentration of 250 μmol/l, roughly corresponding to 0.6°C at the depth of about 400 db, makes a boundary between intermediate and deep waters. Water colder than 0.6°C has a very stable relationship between potential temperature and salinity while salinity of the water warmer than 0.6°C is lower in the western Japan Basin than that in the eastern Japan Basin. The low salinity water with high oxygen corresponds to the East Sea Intermediate Water (ESIW; <34.06 psu, >250 μmol/l and >1.0°C) which was previously identified by Kim and Chung (1984) and the high salinity water with high oxygen found in eastern Japan Basin is named as the High Salinity Intermediate Water (HSIW; >34.07 psu, >250 μmol/l and >0.6°C). Spatial distribution of salinity and acceleration potential on the surface of σ_ϑ = 27.2 kg/m³ shows that the ESIW prevailing in the western Japan Basin is transported eastward by a zonal flow along the polar front near 40°N and a cyclonic gyre in the eastern Japan Basin is closely related to the HSIW. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temporal variability in physicochemical properties,phytoplankton standing crop and primary production for 7 years (2002–2008) in the neritic area of Sagami Bay,Japan

Seasonal and interannual variations in physicochemical properties (i.e., temperature, salinity, dissolved oxygen and dissolved inorganic nutrients), chlorophyll a (Chl-a), particulate carbon and nitrogen (PC and PN, respectively), and primary production were investigated in the neritic area of Sagami Bay, Kanagawa, Japan, from January 2002 to December 2008. These abiotic/biotic variables, except for NH₄ ⁺–N, repeated similar seasonal variations for all 7 years. On the basis of the analysis of data obtained on 167 sampling dates, depth-integrated primary production in this water can be easily estimated from Chl-a at the surface using the regression equations obtained in the present study. Intermittently high values of dissolved inorganic nutrients, Chl-a, PC, PN and primary productivity at the surface during the summer stratified period were induced by high freshwater discharge from the rivers after rainfalls and by the expansion of nutrient-rich Tokyo Bay Water. Temperature, salinity and dissolved inorganic nutrients showed drastic variations within a scale of a few days and/or weeks, and these variations were related to sea levels that represent the intrusion of the Kuroshio Water, Intermediate Oyashio Water or deep water from the continental slope. However, there was no consistent trend in the variations in Chl-a, PC, PN and primary production due to the complex effects of these waters.     

Circulation of Intermediate and Deep Waters in the Philippine Sea

The circulation of intermediate and deep waters in the Philippine Sea west of the Izu-Ogasawara-Mariana-Yap Ridge is estimated with use of an inverse model applied to the World Ocean Circulation Experiment (WOCE) Hydrographic Program data set. Above 1500 m depth, the subtropical gyre is dominant, but the circulation is split in small cells below the thermocline, causing multiple zonal inflows of intermediate waters toward the western boundary. The inflows along 20°N and 26°N carry the North Pacific Intermediate Water (NPIW) of 11 × 10⁹ kg s⁻¹ in total, at the density range of 26.5σ_θ–36.7σ₂ (approximately 500–1500 m depths), 8 × 10⁹ kg s⁻¹ of the NPIW circulate within the subtropical gyre, whereas the rest is conveyed to the tropics and the South China Sea. The inflow south of 15°N carries the Tropical Salinity Minimum water of 35 × 10⁹ kg s⁻¹, nearly half of which return to the east through a narrow undercurrent at 15–17°N, and the rest is transported into the lower part of the North Equatorial Countercurrent. Below 1500 m depth, the deep circulation regime is anti-cyclonic. At the density range of 36.7σ₂, – 45.845σ₄ (approximately 1500–3500 m depths), deep waters of 17 × 10⁹ kg s⁻¹ flow northward, and three quarters of them return to the east at 16–24°N. The remainder flows further north of 24°N, then turns eastward out of the Philippine Sea, together with a small amount of subarctic-origin North Pacific Deep Water (NPDW) which enters the Philippine Sea through the gap between the Izu Ridge and Ogasawara Ridge. The full-depth structure and transportation of the Kuroshio in total and net are also examined. It is suggested that low potential vorticity of the Subtropical Mode Water is useful for distinguishing the net Kuroshio flow from recirculation flows. This revised version was published online in August 2006 with corrections to the Cover Date.     

N2 fixation variability in the oligotrophic Sulu Sea, western equatorial Pacific region over the past 83 kyr

N₂ fixation is an important biological process that adds new nitrogen to oceans and plays a key role in modulating the oceanic nitrate inventory. However, it is not known how, when, and where N₂ fixation rates have varied in response to past climate changes. This study presents a new record of nitrogen isotopic composition (δ¹⁵N) over the last 83 kyr from a sediment core (KH02-4 SUP8) taken in the Sulu Sea in the western equatorial Pacific region; data allow the N₂ fixation variability in the sea to be reconstructed. Sediments, sinking, and suspended particulate organic matter (POM) all have lighter isotopic values compared to the δ¹⁵N values of substrate nitrate (av. 5.8‰) in North Pacific Intermediate Water. These lighter δ¹⁵N values are regarded as reflecting N₂ fixation in the Sulu Sea surface water. A δ¹⁵N mass balance model shows that N₂ fixation rates were significantly enhanced during 54–34 kyr in MIS-3 and MIS-2. It has been speculated that higher interglacial denitrification rates in the Arabian Sea and the eastern tropical Pacific would have markedly decreased the global oceanic N inventory and contributed to the increase in N₂ fixation in oligotrophic regions, but such a model was not revealed by our study. It is possible that changes in N₂ fixation rates in the Sulu Sea were regional response, and accumulation of phosphate in the surface waters due to enhanced monsoon-driven mixing is thought to have stimulated enhancements of N₂ fixation during MIS-3 and MIS-2.     

Effects of sea ice formation and diapycnal mixing on the Okhotsk Sea intermediate water clarified with oxygen isotopes

Processes relating to the formation of dense shelf water and intermediate water in the Okhotsk Sea were studied by examining oxygen isotope ratios (δ¹⁸O), salinity, and temperature. The salinity and δ¹⁸O of the cold dense shelf water on the northern continental shelf showed peculiar relationship. The relationship indicates that 3% of the mixed-layer water, having salinity of 32.6, froze and the remaining 97% became dense shelf water of salinities of more than 33.2 (σ_θ>26.7) during the sea ice formation. The salinity–δ¹⁸O relationship also shows that 20% of the Okhotsk Sea Intermediate Water at the σ_θ=26.8 level was derived from the dense shelf water. The remaining 80% came from the Western Subarctic Pacific water modified by diapycnal mixing of water affected by the surface cooling and freshening within the Okhotsk Sea. The mixing with dense shelf water contributes to only 26% of the temperature difference or 8% of the salinity difference between the original Pacific water and the Okhotsk Sea Intermediate Water at σ_θ=26.8. This result suggests that the cold and less saline properties of the Okhotsk Sea Intermediate Water are produced mainly by diapycnal mixing, rather than by mixing of the Pacific water with the dense shelf water.     

Tidal Mixing in the Kuril Straits and Its Impact on Ventilation in the North Pacific Ocean

A numerical study using a 3-D nonhydrostatic model has been applied to baroclinic processes generated by the K ₁ tidal flow in and around the Kuril Straits. The result shows that large-amplitude unsteady lee waves are generated and cause intense diapycnal mixing all along the Kuril Island Chain to levels of a maximum diapycnal diffusivity exceeding 10³ cm²s⁻¹. Significant water transformation by the vigorous mixing in shallow regions produces the distinct density and potential vorticity (PV) fronts along the Island Chain. The pinched-off eddies that arise and move away from the fronts have the ability to transport a large amount of mixed water (∼14 Sv) to the offshore regions, roughly half being directed to the North Pacific. These features are consistent with recent satellite imagery and in-situ observations, suggesting that diapycnal mixing within the vicinity of the Kuril Islands has a greater impact than was previously supposed on the Okhotsk Sea and the North Pacific. To examine this influence of tidal processes at the Kurils on circulations in the neighboring two basins, another numerical experiment was conducted using an ocean general circulation model with inclusion of tidal mixing along the islands, which gives a better representation of the Okhotsk Sea Mode Water than in the case without the tidal mixing. This is mainly attributed to the added effect of a significant upward salt flux into the surface layer due to tidal mixing in the Kuril Straits, which is subsequently transported to the interior region of the Okhotsk Sea. With a saline flux into the surface layer, cooling in winter in the northern part of the Okhotsk Sea can produce heavier water and thus enhance subduction, which is capable of reproducing a realistic Okhotsk Sea Mode Water. The associated low PV flux from the Kuril Straits to the open North Pacific excites the 2nd baroclinic-mode Kelvin and Rossby waves in addition to the 1st mode. Interestingly, the meridional overturning in the North Pacific is strengthened as a result of the dynamical adjustment caused by these waves, leading to a more realistic reproduction of the North Pacific Intermediate Water (NPIW) than in the case without tidal mixing. Accordingly, the joint effect of tidally-induced transport and transformation dominating in the Kuril Straits and subsequent eddy-transport is considered to play an important role in the ventilation of both the Okhotsk Sea and the North Pacific Ocean. This revised version was published online in July 2006 with corrections to the Cover Date.     

Long-term variations of surface and intermediate waters in the southern Indian Ocean along 32°S

Variations of water properties in surface and intermediate layers along 32°S in the southern Indian Ocean were examined using a 50-year (1960–2010) time series reproduced from historical hydrographic and Argo data by using optimum interpolation. Salinity in the 26.7–27.3σ_θ density layer decreased significantly over the whole section, at a maximum rate of 0.02 decade⁻¹ at 26.8–26.9σ_θ, for the 50-year average. Three deoxygenating cores were identified east of 75°E, and the increasing rate of apparent oxygen utilization in the most prominent core (26.9–27.0σ_θ) exceeded 0.05 ml l⁻¹ decade⁻¹. The pycnostad core of Subantarctic Mode Water (SAMW) and the salinity minimum of Antarctic Intermediate Water shifted slightly toward the lighter layers. Comparisons with trans-Indian Ocean survey data from 1936 suggest that the tendencies found in the time series began before 1960. Interestingly, cores of many prominent trends were located just offshore of Australia at 26.7–27.0σ_θ, which is in the SAMW density range. Spectrum analysis revealed that two oscillation components with time scales of about 40 and 10 years were dominant in the subsurface layers. Our results are fairly consistent with, and thus support, the oceanic responses in the southern Indian Ocean to anthropogenic climate change predicted by model studies.     

基于Argo浮标和历史资料的热带西太平洋次表层与中层水年代变化分析

利用20世纪80年代和90年代WOD01(World Ocean Database2001)中的CTD温盐剖面资料和2000年以后Argo资料,对比分析了热带西太平洋次表层和中层水团分布的年代变化特征。分析结果表明,在这两个时期,起源于南北太平洋中高纬度海域的各次表层水和中层水,在热带西太平洋分布特征和交织在一起的总体态势基本一致,水团性质的年代变化不大。这与上述两个时段全球海洋-大气耦合系统趋于正常状态相吻合。通过辨识和跟踪表征次表层水性质的盐度极大值,发现南太平洋热带水沿西边界向北扩散程度有所加大,由前一时期的5°N,进一步扩散到6°~7°N;北太平洋热带水在西边界附近的向南扩散程度有所削弱,在2002-2005年间只向南扩散到4°N,而前一个时期则可向南扩散到2°N。通过辨识表征中层水性质的盐度极小值,南极中层水在西边界附近向北扩散程度有所加大,在2002-2005年到达13°N附近,而前一个时期只到达11°N;同期,北太平洋中层水在西边界附近的向南扩散程度有所削弱。上述年代变化与全球水循环强度的变化之间有何关系有待进一步研究。     

Hydrographic Structure and Transport of Intermediate Water in the Kuroshio Region off the Boso Peninsula, Japan

Hydrographic structure and transport of intermediate water were observed in the Kuroshio region south of Japan, focusing on the 26.6–27.5σ_θ density in six cruises from May 1998 through September 2001. In the section off the Boso Peninsula where the Kuroshio exfoliates eastward, the intermediate water was clearly clustered into three groups meridionally composed of the coastal water, the Kuroshio water and the offshore water. Compared with the Kuroshio water characterized by warm, salty water transported by the Kuroshio, the coastal and offshore waters significantly degenerated due to mixing with cold, fresh waters originated from the subarctic region: the former was affected by alongshore spread of the coastal Oyashio and the latter by direct intrusion of the new North Pacific Intermediate Water (NPIW) into the southern side of the Kuroshio current axis. Particularly the offshore water showed higher apparent oxygen utilization (AOU) in layers deeper than 26.9σ_θ while it showed lower AOU in layers shallower than 26.9σ_θ, which indicated that colder, fresher and higher AOU water was distributed on the southeastern side of the Kuroshio in deeper layers. In May 1998, the Oyashio-Kuroshio mixing ratio was estimated to be typically 2:8 for the offshore water on the assumption of isopycnal mixing. Moreover, northeastward volume transport of the Kuroshio water was obtained from geostrophic velocity fields adjusted to lowered acoustic Doppler current profiler (LADCP) data to yield 6.1 Sv at 26.6–26.9σ_θ and 11.8 Sv at 26.9–27.5 σ_θ. This revised version was published online in July 2006 with corrections to the Cover Date.     

Re-Estimation of Annual Anthropogenic Carbon Input from Oyashio into North Pacific Intermediate Water

The annual transport of anthropogenic carbon (C_anth) to the North Pacific Intermediate Water (NPIW) from the Western Subarctic Gyre (WSG) has been re-estimated by using newly estimated Oyashio transport and C_anth concentration, the latter calculated by the recently-established “ΔC*” method with some modifications. Estimated annual C_anth transport through the nearshore Oyashio west of 146°E was 0.020 ± 0.010 GtC y⁻¹, closely approximating the previous estimation based on a 1-D model calibrated with the CFC vertical distribution. The present study, however, found that an additional 0.025 ± 0.010 GtC y⁻¹ of C_anth was transported into NPIW in the region east of 146°E. Total C_anth transport, 0.045 GtC y⁻¹, contributes about 35% of annual C_anth accumulation of the whole temperate North Pacific. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Western North Pacific Playing a Key Role in Global Biogeochemical Fluxes

The oceanic biogeochemical fluxes in the North Pacific, especially its northwestern part, are discussed to prove their importance on a global scale. First, the air-sea exchange processes of chemical substances are considered quantitatively. The topics discussed are sea salt particles transported to land, sporadic transport of soil dust to the ocean and its role in the marine ecosystem, the larger gas transfer velocity of CO₂ indicating the effect of bubbles, and DMS and greenhouse gases other than CO₂. Next, chemical tracers are utilized to reveal the water circulation systems in the region, which are the Pacific Deep Water including its vertical eddy diffusivity, the North Pacific Intermediate Water and the Japan Sea Deep Water. Thirdly, the particulate transport process of chemical substances through the water column is clarified by analyzing the distribution of insoluble radionuclides and the results obtained from sediment trap experiments. Fourthly, the northern North Pacific is characterized by stating the site decomposing organic matter and Si playing a key role in the marine ecosystem. Both are induced by the upwelled Pacific Deep Water. Fifthly, the oceanic CO₂ system related to global warming is presented by clarifying the distribution of anthropogenic CO₂ in the western North Pacific, and roles of the upwelled Pacific Deep Water and the continental shelf zone in the absorption of atmospheric CO₂. Finally, Mn and other chemical substances in sediments are discussed as recorders of the early diagenesis and indicators of low biological productivity during glacial ages in the northwestern North Pacific. It is concluded that the western North Pacific is characterized mainly by the Pacific Deep Water bringing nutrients to the northern North Pacific, located at the exit of the global deep water circulation and, therefore, the region plays a key role in the global biogeochemical fluxes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Late Pleistocene-Holocene Paleoceanography and Ventilation of the Gulf of California

Sediment cores collected in 1990 from the Gulf of California have been studied using stable isotope and radiocarbon techniques to reconstruct the climate and ventilation histories since the last glacial maximum. Benthic foraminiferal δ¹⁸O from core tops in a water depth range of 145 to 1442 m increases by about 2% with increasing depth. This is consistent with a composite temperature profile constructed from several hydrocasts in the various gulf basins. However, the δ¹⁸O water/salinity relationship is not sufficiently linear in gulf locations or in nearby open Pacific Geochemical Ocean Sections Study (GEOSECS) stations to be useful in solving paleotemperature equations. Of the most common benthic foraminifera, only Planulina ariminensis has δ¹³C that is consistent with the measured δ¹³C of ΣCO₂. Several cores in the depth range 500 to 900 m have the laminated Holocene and Bolling/Allerod sediments, and the nonlaminated glacial age and Younger Dryas sediments that are typical of the gulf and other locations such as Santa Barbara Basin. The best of those, Jumbo Piston Core (JPC) 56 from 818 m water depth on the western margin of Guaymas Basin, was sampled for intensive study. Oxygen isotope ratios in benthic and planktonic foraminifera show little evidence for deglacial temperature oscillations. Carbon isotope ratios are generally lower during warm epochs, but the most striking result is strongly lowered benthic and planktonic δ¹³C about 9500 years ago. This may reflect water column oxidation of locally released methane. Neither benthic δ¹³C in depth section nor paired benthic and planktonic ¹⁴C data in JPC56 are consistent with increased intermediate water ventilation during the glacial maximum and Younger Dryas. Likewise, ¹⁴C data from 5 pairs of foraminifera from the Okhotsk Sea fail to support better ventilation in that basin during the last glacial maximum. This revised version was published online in August 2006 with corrections to the Cover Date.     

Transport of Oyashio Water across the Subarctic Front into the Mixed Water Region and Formation of NPIW

Oyashio water flowing into the Mixed Water Region (MWR) and the Kuroshio Extension region that forms North Pacific Intermediate Water (NPIW) has been examined, based on four Conductivity-Temperature-Depth profiler (CTD)/Lowered Acoustic Doppler Current Profiler (L-ADCP) surveys of water masses and ocean currents. There are two processes by which the Oyashio water intrudes across the Subarctic Front (SAF): one is a direct cross-nearshore-SAF transport near Hokkaido along the western boundary, and the other is a cross-offshore-SAF process. Seasonal variations were observed in the former process, and the transport of the Oyashio water across SAF near Hokkaido in the density range of 26.6–27.4σ_θ was 5–10 Sv in spring 1998 and 2001, and 0–4 Sv in autumn 2000, mainly corresponding to the change of the southwestward Oyashio transport. Through the latter process, 5–6 Sv of the Oyashio water was entrained across the offshore SAF from south of Hokkaido to 150° in both spring 2001 and autumn 2000. The total cross-SAF Oyashio water transport contributing to NPIW formation is more than 10 Sv, which is larger than previously reported values. Most of the Oyashio water formed through the former process was transported southeastward through the Kuroshio Extension. It is suggested that the Oyashio intrusion via the latter process feeds NPIW in the northern part of the MWR, mainly along the Subarctic Boundary and SAF. This revised version was published online in July 2006 with corrections to the Cover Date.