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The Lombok Strait, as one of the outlet straits, is part of the ITF route, which is directly adjacent to the Indian Ocean. There is a sill in the Lombok Strait, which is a place for internal wave generation. Leg-1 data from the Japan Agency for Marine-Earth Science and Technology in collaboration with the Agency for the Assessment and Application of Technology which is part of the Tropical Ocean Climate Study Expedition including CTD Yoyo and ADCP taken using ship vehicles R/V Kaiyo. CTD Snapshot from PUSHIDROSAL using the KRI Spica 934 vehicle part of the Opssurta Baruna Jaya 2 Expedition . Determination of seawater mass stratification with the criteria for the thermocline layer is ≥ 0.05 °C.m -1 . Four types of water masses were identified, Java Sea, mixed seawater mass (Java Sea - ITF) which occurred diapycnal mixing, North Pacific Subtropical Water (NPSW) and North Pacific Intermediate Water (NPIW). The seawater mass stratification in the Lombok Strait based on temperature, salinity and density which are seen to follow the internal tidal pattern. The average values for energy dissipation and vertical diffusivity for each layer and replication were 5.73 x 10 -7 W.Kg -1 and 3.67 x 10 -2 m 2. s -1 for CTD Yoyo and 2.25 x 10 -6 W.Kg -1 and 7.38 x 10 -2 m 2 .s -1 for CTD Snapshot . The value obtained is greater than the open ocean and straits in other studies. The high shear value confirms this in the thermocline layer. The Richardson gradient value> 0.25 is relatively constant in the thermocline layer.

An investigation was carried out in the Y3 seamount area of the Western Pacific Ocean in December 2014, and the distribution of dissolved inorganic carbon (DIC) and its relationship with environmental factors in this area were explored. The results show that DIC concentration was higher in the adjacent waters of the Y3 seamount area, and the uplift of DIC isolines at the stations was close to the seamount. Meanwhile, interaction between the North Equatorial Current (NEC) and the Y3 seamount aff ected the DIC distribution, i.e., the upwelling in the same direction of the NEC was obvious, resulting in a decreasing trend of average concentration of DIC in the 200 m water column from the top to the two sides in this direction but in the cross direction. The DIC concentration increased with the water depth increase, and its distribution was aff ected by various environmental factors. In the surface water, high temperature was a decisive factor for the decrease of the DIC concentration, but the photosynthesis of phytoplankton showing only a weak influence. In the North Pacific Tropic Water (NPTW), DIC production rate from organic matter decomposition was higher than that of DIC consumption by phytoplankton photosynthesis, leading to a continual increase of DIC. In the North Pacific Intermediate Water (NPIW), organic matter decomposition played a leading role in the increase of DIC. In the deep water, decomposition of organic matter weakened, and the dissolution of CaCO3 controlled the carbonate system, and DIC had the smallest variation range.

The deep-ocean carbon cycle, especially carbon sequestration and outgassing, is one of the mechanisms to explain variations in atmospheric CO2 concentrations on millennial and orbital timescales. However, the potential role of subtropical North Pacific subsurface waters in modulating atmospheric CO2 levels on millennial timescales is poorly constrained. An increase in the respired CO2 concentration in the glacial deep-ocean due to biological pump generally corresponds to deoxygenation in the ocean interior. This link thus offers a chance to study oceanic ventilation and coeval export productivity based on redox-controlled sedimentary geochemical parameters. Here, we investigate a suite of geochemical proxies in a sediment core from the Okinawa Trough to understand sedimentary oxygenation variations in the subtropical North Pacific over the last 50 000 years (50 ka). Our results suggest that enhanced mid-depth western subtropical North Pacific (WSTNP) sedimentary oxygenation occurred during cold intervals and after 8.5 ka, while oxygenation decreased during the Bölling-Alleröd (B/A) and Preboreal. The enhanced oxygenation during cold spells is linked to the North Pacific Intermediate Water (NPIW), while interglacial increase after 8.5 ka is linked to an intensification of the Kuroshio Current due to strengthened northeast trade winds over the tropics. The enhanced formation of the NPIW during Heinrich Stadial 1 (HS1) was likely driven by the perturbation of sea ice formation and sea surface salinity oscillations in the high-latitude North Pacific. The diminished sedimentary oxygenation during the B/A due to a decreased NPIW formation and enhanced export production, indicates an expansion of the oxygen minimum zone in the North Pacific and enhanced CO2 sequestration at mid-depth waters, along with the termination of atmospheric CO2 concentration increase. We attribute the millennial-scale changes to an intensified NPIW and enhanced abyss flushing during deglacial cold and warm intervals, respectively, closely related to variations in North Atlantic Deep Water formation.

The Okhotsk Sea, which connects the high latitude Asian continent and the North Pacific, plays an important role in modern and past climate changes due to seasonal sea ice coverage and as a precursor of the North Pacific Intermediate Water. The long-term glacial-interglacial changes of sea ice coverage and its impacts on terrigenous transport and surface primary productivity in the Okhotsk Sea remain, however, not well constrained. Base on the paleomagnetic, rock magnetic, micropaleontological (diatom), and geochemical studies of the marine sediment core MD01-2414 (53°11.77′N, 149°34.80′E, water depth: 1,123 m) taken in the central Okhotsk Sea, we reconstruct the terrigenous sediment transport and paleoceanographic variations during the past 1550 thousand years (kyr). Seventeen geomagnetic excursions are identified from the paleomagnetic directional record. Close to the bottom of the core, an excursion was observed, which is proposed to be the Gilsa event ∼1550 thousand years ago (ka). During glacial intervals, our records reveal a wide extension of sea ice coverage and low marine productivity. We observed ice-rafted debris from mountain icebergs composed of coarse and high magnetic terrigenous detritus which were derived from the Kamchatka Peninsula to the central Okhotsk basin. Still during glacial intervals, the initiation (i.e., at ∼900 ka) of the Mid-Pleistocene Transition marks the changes to even lower marine productivity, suggesting that sea-ice coverage became larger during the last 900 ka. During interglacial intervals, the central Okhotsk Sea was either devoid of sea-ice or the ice was at best seasonal; resulting in high marine productivity. The weaker formation of Okhotsk Sea Intermediate Water, lower ventilation, and microbial degradation of organic matter depleted the oxygen concentration in the bottom water and created a reduced environment condition in the sea basin. The freshwater supplied by snow or glacier melting from Siberia and Kamchatka delivered fine grain sediments to the Okhotsk Sea. During the stronger interglacial intervals after the Mid-Brunhes Transition (i.e., Marine Isotope Stages 1, 5e, 9, and 11), strong freshwater discharges from Amur River drainage area are in association with intensified East Asian Summer Monsoon. This process may have enhanced the input of fine-grained terrigenous sediments to the central Okhotsk Sea.

Processes controlling environmental change in the subarctic Pacific Ocean on millennial to orbital timescales are not well understood. Here we use a 230-kyr sedimentary record from the northwest Pacific Ocean to assess the response of late Pleistocene sediment dynamics to orbital forcing. Combining a source-to-sink perspective based on sedimentological records with climate model reanalysis, we reveal that fluctuations in sediment provenance were closely linked to obliquity-forced changes in atmospheric circulation modes. Specifically, the position of the Aleutian Low controlled sediment transport from the Bering Sea and Aleutian Arc sources. Furthermore, a distinct shift in North Pacific ocean circulation during the Last Glacial Maximum may have been related to a strengthened Siberian High. The coincidence of atmospheric mode switches with changes in sea-ice extent and North Pacific Intermediate Water formation in the marginal seas suggests that this coupled ocean-atmosphere system may have acted as a regional amplifier of global climate variability. (Less)

On glacial-interglacial and millennial timescales, sea ice is an important player in the circulation and primary productivity of high latitude oceans, affecting regional and global biogeochemical cycling. In the modern North Pacific, brine rejection during sea-ice freezing in the Sea of Okhotsk drives the formation of North Pacific Intermediate Water (NPIW) that ventilates the North Pacific Ocean at 300 m–1000 m water depth. Glacial intervals of the late Quaternary, however, experienced a deepening of glacial NPIW to at least 2000 m, with the strongest ventilation observed during cold stadial conditions of the last deglaciation. However, the origin of the shifts in NPIW ventilation is poorly understood. Numerical simulations suggest an atmospheric teleconnection between the North Atlantic and the North Pacific, in response to a slowdown or shutdown of the Atlantic meridional overturning circulation. This leads to a build-up of salinity in the North Pacific surface ocean, triggering deep ventilation. Alternatively, increased sea-ice formation in the North Pacific and its marginal seas may have caused strengthened overturning in response to enhanced brine rejection. Here we use a multi-proxy approach to explore sea-ice dynamics, sedimentary redox chemistry, and benthic ecology at Integrated Ocean Drilling Program Site U1343 in the eastern Bering Sea across the last 40 ka. Our results suggest that brine rejection from enhanced sea-ice formation during early Heinrich Stadial 1 locally weakened the halocline, aiding in the initiation of deep overturning. Additionally, deglacial sea-ice retreat likely contributed to increased primary productivity and expansion of mid-depth hypoxia at Site U1343 during interstadial conditions, confirming a vital role of sea ice in the deglacial North Pacific carbon cycle.

Satellite data products and high-resolution in situ observations were combined to investigate the evolution and structure of the Kuroshio Extension Front in Spring 2019. The former reveals the variation of the front is influenced by the northward movement of the Kuroshio Extension through transporting warm and saline water to a cold and brackish water region. The latter indicates steep upward slopes of the isopycnals, tilting northward in the frontal zone, as well as several ~300 m thick blobs of North Pacific Intermediate Water between 26.25 and 26.75 kg/m3, where conspicuous thermohaline intrusions occur. Further analysis indicates these thermohaline intrusions prefer to alternate salt fingering and diffusive convection interfaces, and are affected by strong shears.

The first direct measurements of vertical turbulent nitrate flux, FluxNO3, were conducted in the Kuroshio across the Izu Ridge and revealed large FluxNO3 near the sill crest at the 1% light depth. This large FluxNO3 possibly enhances downstream phytoplankton growth. Extremely large FluxNO3 was also observed at 26.0–26.6 σθ between the core of the Kuroshio nutrient stream and the North Pacific Intermediate Water. We hypothesize that strong vertical mixing in the Izu Ridge draws sufficient nitrate upward from the North Pacific Intermediate Water to the nutrient stream, which eventually impacts the downstream biogeochemistry in the western North Pacific.

Paleo-redox proxies are crucial for reconstructing past bottom water oxygen concentration changes brought about by ocean circulation and marine productivity shifts in response to climate forcing. Carbonate I/Ca ratios of multiple benthic foraminifera species from ODP Hole 1017E – a core drilled within the Californian oxygen minimum zone (OMZ), on the continental slope – are employed to re-examine the transition from the well oxygenated last glacial into poorly oxygenated modern conditions. The redox and export productivity history of this site is constrained by numerous proxies, used to assess sensitivity of I/Ca ratios of benthic foraminifera to changes in bottom- and pore water O2 concentrations. Reconstructed iodate (IO3-) availability from the I/Ca ratio of epifaunal (Cibicidoides sp.), shallow infaunal (Uvigerina peregrina), and deep infaunal (Bolivina spissa) foraminifera. The reconstructed IO3- availability profile is used to determine the contribution of bottom water O2 relative to oxidant demand on pore water O2 concentrations. These results suggest that high export productivity on the California Margin drove pore low water O2 concentrations during the Bolling. In contrast low bottom water O2 concentrations at 950 m water depth only contributed to reduced sediments during the Allerod. Increased contribution of modified North Pacific Intermediate Water to the California Current System ventilated the California OMZ during the late glacial and the Younger Dryas such that water overlying the site was oxygenated. These results highlight the promising potential of this new proxy for understanding the relative influence of bottom water O2 concentration and pore water oxidant demand on OMZs.

A central theme of the ongoing GEOTRACES program is to improve the understanding of processes occurring at ocean interfaces with continents, sediments, and ocean crust. In this context, we studied the distributions of Al, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb around the Juan de Fuca Ridge (JdFR) in total dissolvable (td), dissolved (d), and labile particulate (lp) fractions, which represent a fraction in unfiltered samples, filtered samples through an AcroPak capsule filter, and the difference between td and d, respectively. Al and Fe were dominated by lp-species, while Ni, Zn, and Cd were dominated by d-species with undetectable amounts of lp-species. Major findings in this study are as follows: (1) The continental margin (CM) provided large sources of Al, Mn, Fe, and Co from the surface to ~2000 m in depth. The supply from CM caused high surface concentrations of dMn and dCo, a subsurface (100–300 m depth) maximum of dCo, and intermediate (500–2000 m depth) maxima of lpAl and lpFe. The supply of dFe from CM was ~10 times that from the high-temperature hydrothermal activity at station BD21, which is located at ~3 km from the Middle Valley venting site and ~ 200 km from Vancouver Island. (2) DPb was maximum at the top layer of North Pacific Intermediate Water, probably owing to isopycnal transport of anthropogenic Pb via advection of subducted surface waters. Although dCo and dPb had different sources in the upper water, they showed a strong linearity below 300 m (r 2 = 0.95, n = 38), indicating concurrent scavenging. (3) A high-temperature hydrothermal plume occurred at a depth of 2300 m at BD21, accounting for maxima of dAl, dMn, dFe, lpCu, and lpPb and a minimum of dCu. (4) Strong bottom maxima of lpAl, lpMn, lpFe, lpCo, and lpPb occurred above the abyssal plain at the western foot of the JdFR, indicating resuspension of sediments. However, bottom maxima of d-species were apparent only for dAl and dCu.

The Mid-Brunhes Transition (MBT) saw an increase in the amplitude of glacial cycles expressed in ice core and deep ocean records from about 400 ka, but its influence on high-latitude climates is not fully understood. The Arctic Ocean is thought to have warmed and exhibited reduced sea ice, but little is known of sea ice marginal locations such as the Bering Sea. The Bering Sea is the link between the Arctic and Pacific Ocean and is an area of high productivity and CO2 ventilation; it hosts a pronounced oxygen minimum zone (OMZ) and is thought to be the location of Glacial North Pacific Intermediate Water (GNPIW) formation in the Pleistocene. To understand palaeoceanographic change in the region, we analysed benthic foraminiferal faunas from Bowers Ridge (Site U1342, 800 m of water depth) over the past 600 kyr, as they are uniquely well preserved and sensitive to changes in deep and surface ocean conditions. We identified and imaged 71 taxa and provide a full taxonomy. Foraminiferal preservation is markedly higher during glacials, indicating the presence of less corrosive GNPIW. The most abundant species are Bulimina exilis, Takayanagia delicata, Alabaminella weddellensis, Gyroidina sp. 2, Cassidulina laevigata, Islandiella norcrossi, and Uvigerina bifurcata, consistent with broadly high net primary production throughout the last 600 kyr. Correspondence analysis shows that the most significant Assemblage 1 comprises B. exilis, T. delicata, Bolivina spissa, and Brizalina, which occur sporadically within intervals of laminated, biogenic-rich sediment, mostly during glacials and also some deglacials, and are interpreted as indicating very high productivity. Other assemblages contain the phytodetritivore species A. weddellensis, I. norcrossi, and C. laevigata, indicative of seasonal phytoplankton blooms. Before the MBT, more numerous intervals of the very high-productivity Assemblage 1 and A. weddellensis occur, which we suggest reflect a time of more sea-ice-related seasonal stratification and ice edge blooms. Our inference of a decrease in sea ice meltwater stratification influence in the central Bering Sea after the MBT is consistent with records showing that the Arctic and Pacific Ocean warmed during glacials and suggests that high-latitude productivity and sea ice changes were an important feature of this climate event.

High latitude deep water upwelling has the potential to control global climate over glacial timescales through the biological pump and ocean-atmosphere CO2 exchange. However, there is currently a lack of continuous long nutrient upwelling records with which to assess this mechanism. Here we present geochemical proxy records for nutrient upwelling and glacial North Pacific Intermediate Water (GNPIW) formation in the Bering Sea over the past 850 kyr, which demonstrates that glacial periods were characterised by reduced nutrient upwelling, when global atmospheric CO2 and temperature were also lowered. We suggest that glacial expansion of sea ice in the Bering Sea, and the simultaneous expansion of low nutrient GNPIW, inhibited vertical mixing and nutrient supply across the subarctic Pacific Ocean. Our findings lend support to the suggestion that high latitude sea ice and the resultant intermediate water formation, modulated deep water upwelling and ocean-atmosphere CO2 exchange on glacial-interglacial timescales.

The margin of the western North Pacific Ocean releases redox-active elements like Mn, Co, and Ce into the water column to undergo further transformation through oxide formation, scavenging, and reductive dissolution. Near the margin, the upper ocean waters enriched in these elements are characterized by high dissolved oxygen, low salinity, and low temperature, and are a source of the North Pacific Intermediate Water. High dissolved concentrations are observed across the Western Subarctic Gyre, with a rapid decrease in concentrations away from the margin and across the subarctic-subtropical front. The particulate concentrations of Mn, Co, and Ce are also high in the subarctic surface ocean and enriched relative to Ti and trivalent rare earth elements. Furthermore, the particles enriched in Mn, Co, and Ce coincide at the same depth range, suggesting that these elemental cycles are coupled through microbial oxidation in the subarctic gyre as the waters travel along the margin before being subducted at the subarctic-subtropical front. Away from the margin, the Mn, Co, and Ce cycles decouple, as Mn and Ce settle out as particles while dissolved Co is preserved and transported within the North Pacific Intermediate Water into the central North Pacific Ocean.

Dissolved neodymium isotopes (143Nd/144Nd, expressed as eNd) and rare earth elements (REEs) have the potential to trace the provenance of lithogenic material as well as water masses. The central North Pacific is poorly investigated with respect to its Nd isotope signature and REE cycling, and little is known about the contributions of volcanic islands, such as Hawaii, relative to dust input from Asian deserts to the surface water REE budgets. Here we present dissolved Nd isotope and REE data along with long-lived radium isotope activities from Hawaii Ocean Time-Series Station ALOHA and coastal waters from Oahu, sampled for a GEOTRACES process study in February 2011. The data are supplemented with seasonal samples from ALOHA. Our results show a clear influence of the Hawaiian Islands on the coastal ocean and surface waters at ALOHA during February, expressed by higher surface water Ra activities, radiogenic surface eNd (eNd = +1.4 to −1.0), and elevated Eu anomalies (Eu/Eu∗ ⩾ 1.3). Seasonal cycles of Asian dust deposition most likely contribute to the seasonal eNd variability of surface waters at ALOHA, as suggested by more negative eNd and the lack of Eu anomalies in summer. Neodymium isotopes in the intermediate and deep water column at ALOHA trace typical North Pacific water masses, such as North Pacific Intermediate Water and North Pacific Deep Water. We suggest that a radiogenic eNd excursion in 1000–2000 m water depth, observed in various North Pacific profiles, is controlled by advection of a modified Upper Circumpolar Deep Water or North Equatorial Pacific Intermediate Water. We further present an updated average eNd signature of −3.5 ± 0.5 for North Pacific Deep Water and show that REE patterns of deep waters at ALOHA are dominantly controlled by vertical processes.

Modern North Pacific intermediate water (NPIW) is formed in the mixed water region where the Oyashio and Kuroshio currents meet. The source for cooling and freshening of NPIW is intermediate water in the Okhotsk Sea. The Okhotsk intermediate water outflows to the open Pacific, forming the Oyashio intermediate water by mixing with the subarctic gyre water. In the Oyashio region, the intermediate water originating from the Okhotsk Sea flows mainly at depths shallower than 500 m. On the other hand, ventilation of intermediate water in the subarctic Pacific during the deglaciation remains a topic of debate. In this study, foraminiferal δ18O and δ13C signatures were determined in a dated sediment core collected at 777 m water depth to evaluate the intensity and depth distribution of the source of NPIW since the last deglaciation in the Oyashio region. Benthic foraminiferal δ18O increased by 0.3–0.4‰ from the end of the Bolling/Allerod warm episode to the Younger Dryas cold episode, suggesting intermediate water cooling. Consistent with this trend, benthic δ13C values point to decreased nutrient contents during the cold event. Conversely, benthic δ18O signatures from a nearby core site at a water depth of 1,366 m did not show such cooling. These results suggest that cold intermediate water originating from the north was actively ventilated at depths of at least 700–800 m, and possibly even 1,200 m during the Younger Dryas, implying that NPIW ventilation was thicker and deeper than under modern conditions.

Oceanic deoxygenation observed since the 1950s has drawn increasing concern as dissolved oxygen (DO) regulates marine biodiversity and biogeochemical cycles. Yet the DO time series from direct instrumental measurements is insufficiently long (several decades) to constrain the centennial to millennial variability. Here we present redox-sensitive metal (Re, U, and Mo) records from Santa Barbara Basin (SBB), California to reconstruct subsurface DO through the Holocene. Significantly higher authigenic metal fluxes and coherent benthic foraminifera faunal responses along a published depth transect reveal vertical expansion and intensification of the Southern California oxygen minimum zone (OMZ) prior to ∼6 ka. Comparison with post-Industrial trace metal records suggests low-oxygen conditions were more extreme during early to mid-Holocene (MH), highlighting the magnitude of OMZ variability in the absence of anthropogenic influences. Inconsistencies between authigenic metal fluxes and local oxygen control indicate a remote weakening of the North Pacific Intermediate Water (NPIW) formation prior to the MH. Reduced ventilation of the mid-depth North Pacific is supported by state-of-the-art Earth system model simulations with realistic MH (6 ka) forcing, highlighting the importance of brine rejection during sea ice formation in changing the buoyancy of NPIW and its basin-wide influences on North Pacific OMZ. Evidence from Holocene marine/lake records and model simulations further suggests a southward Intertropical Convergence Zone (ITCZ) migration and a consequent deepening of the Aleutian Low (AL) towards late Holocene, which favors more sea ice brine rejection. Thus, mid-depth ventilation in the North Pacific was closely associated with atmospheric circulation during the Holocene.

This work was funded by NERC studentship NE/I528185/1 awarded to W.R.G., NERC studentship NE/1492942/1 to B.T., NERC grant NE/N011716/1 awarded to J.W.B.R and A.B., and NERC grant NE/I013377/1 awarded to A.E.S. The interplay between ocean circulation and biological productivity affects atmospheric CO2 levels and marine oxygen concentrations. During the warming of the last deglaciation, the North Pacific experienced a peak in productivity and widespread hypoxia, with changes in circulation, iron supply and light limitation all proposed as potential drivers. Here we use the boron-isotope composition of planktic foraminifera from a sediment core in the western North Pacific to reconstruct pH and dissolved CO2 concentrations from 24,000 to 8,000 years ago. We find that the productivity peak during the Bølling–Allerød warm interval, 14,700 to 12,900 years ago, was associated with a decrease in near-surface pH and an increase in pCO2, and must therefore have been driven by increased supply of nutrient- and CO2-rich waters. In a climate model ensemble (PMIP3), the presence of large ice sheets over North America results in high rates of wind-driven upwelling within the subpolar North Pacific. We suggest that this process, combined with collapse of North Pacific Intermediate Water formation at the onset of the Bølling–Allerød, led to high rates of upwelling of water rich in nutrients and CO2, and supported the peak in productivity. The respiration of this organic matter, along with poor ventilation, probably caused the regional hypoxia. We suggest that CO2 outgassing from the North Pacific helped to maintain high atmospheric CO2 concentrations during the Bølling–Allerød and contributed to the deglacial CO2 rise. Postprint

The Pacific hosts the largest oxygen minimum zones (OMZs) in the world ocean, which are thought to intensify and expand under future climate change, with significant consequences for marine ecosystems, biogeochemical cycles, and fisheries. At present, no deep ventilation occurs in the North Pacific due to a persistent halocline, but relatively better-oxygenated subsurface North Pacific Intermediate Water (NPIW) mitigates OMZ development in lower latitudes. Over the past decades, instrumental data show decreasing oxygenation in NPIW; however, long-term variations in middepth ventilation are potentially large, obscuring anthropogenic influences against millennial-scale natural background shifts. Here, we use paleoceanographic proxy evidence from the Okhotsk Sea, the foremost North Pacific ventilation region, to show that its modern oxygenated pattern is a relatively recent feature, with little to no ventilation before six thousand years ago, constituting an apparent Early–Middle Holocene (EMH) threshold or “tipping point.” Complementary paleomodeling results likewise indicate a warmer, saltier EMH NPIW, different from its modern conditions. During the EMH, the Okhotsk Sea switched from a modern oxygenation source to a sink, through a combination of sea ice loss, higher water temperatures, and remineralization rates, inhibiting ventilation. We estimate a strongly decreased EMH NPIW oxygenation of ∼30 to 50%, and increased middepth Pacific nutrient concentrations and carbon storage. Our results (i) imply that under past or future warmer-than-present conditions, oceanic biogeochemical feedback mechanisms may change or even switch direction, and (ii) provide constraints on the high-latitude North Pacific’s influence on mesopelagic ventilation dynamics, with consequences for large oceanic regions.

The Bering Strait provides a shallow connection that allows freshwater to flow from the North Pacific into the North Atlantic, but this passage was closed during past glacials when sea level was at least 50 m lower than at present. Climate models investigating Bering Strait closure predict that this mechanism increases the salinity in the North Atlantic and reduces the salinity in the North Pacific, inducing a Pacific-Atlantic seesaw in meridional overturning circulation and poleward heat flux. However, the Pacific circulation response to Bering Strait closure, and thus the seesaw theory, has not been tested by long paleoceanographic records. We present long records of foraminiferal δ18O and δ13C from Integrated Ocean Drilling Program Site U1342 in the Bering Sea, which provide the first evidence of enhanced North Pacific Intermediate Water when the Bering Strait was closed during each of the extreme glacials of the last 1.2 Myr. These results suggest that orbital-scale variations in North Pacific Intermediate Water are coherent and in phase with variations in upper North Atlantic Deep Water, but are unrelated to changes in lower North Atlantic Deep Water. Together, these results provide evidence for systematic, orbital-scale variability in North Pacific Ocean circulation and may challenge the idea of an orbital-scale Pacific-Atlantic seesaw.

The variability of the intermediate water (IW) east of Luzon and Taiwan is investigated using data acquired from moored instrumented lines and shipboard hydrographic and current velocity surveys. The IW is defined as the water mass with a local salinity minimum along the Kuroshio path. An empirical formula is developed to estimate the IW salinity minimum east of Taiwan using temperature measurements around 580 m depth. Properties of the IW east of Taiwan vary greatly as a result of variable contributions from three water masses including the high-salinity South China Sea Intermediate Water (SCSIW), the low-salinity North Pacific Intermediate Water (NPIW), and the intermediate-salinity Kuroshio Intermediate Water (KIW). Our analysis concludes that NPIW is predominantly found east of Taiwan, and the northward transport of KIW from northeast of Luzon to east of Taiwan is not a steady process. Concurrent mooring measurements at these two locations enable us to correlate the variations of the layer thickness of the Kuroshio near its origin (KLTo) northeast of Luzon to the nature of the IW east of Taiwan. When the Kuroshio is deep, i.e., large KLTo, KIW is transported northward across the Luzon Strait, where its salinity increases presumably due to turbulence mixing with SCSIW. This modified KIW is then transported to the east of Taiwan. When the Kuroshio is shallow, i.e., small KLTo, the KIW transport east of Luzon is nil or southward. East of Taiwan, NPIW feeds in below the Kuroshio and is transported northward beyond the I-Lan Ridge.

The glacial-to-Holocene evolution of subarctic Pacific surface water stratification and silicic acid (Si) dynamics is investigated based on new combined diatom oxygen (δ18Odiat) and silicon (δ30Sidiat) isotope records, along with new biogenic opal, subsurface foraminiferal δ18O, alkenone-based sea surface temperature, sea ice, diatom, and core logging data from the NE Pacific. Our results suggest that δ18Odiat values are primarily influenced by changes in freshwater discharge from the Cordilleran Ice Sheet (CIS), while corresponding δ30Sidiat are primarily influenced by changes in Si supply to surface waters. Our data indicate enhanced glacial to mid Heinrich Stadial 1 (HS1) NE Pacific surface water stratification, generally limiting the Si supply to surface waters. However, we suggest that an increase in Si supply during early HS1, when surface waters were still stratified, is linked to increased North Pacific Intermediate Water formation. The coincidence between fresh surface waters during HS1 and enhanced ice-rafted debris sedimentation in the North Atlantic indicates a close link between CIS and Laurentide Ice Sheet dynamics and a dominant atmospheric control on CIS deglaciation. The Bolling/Allerod (B/A) is characterized by destratification in the subarctic Pacific and an increased supply of saline, Si-rich waters to surface waters. This change toward increased convection occurred prior to the Bolling warming and is likely triggered by a switch to sea ice-free conditions during late HS1. Our results furthermore indicate a decreased efficiency of the biological pump during late HS1 and the B/A (possibly also the Younger Dryas), suggesting that the subarctic Pacific has then been a source region of atmospheric CO2.

Using oceanographic observations and an eddy-resolving ice–ocean coupled model simulation from 1955 to 2004, the effects of the wind-driven ocean circulation change that occurred in the late 1970s during multidecadal-scale freshening of the North Pacific Intermediate Water (NPIW) at salinity minimum density (~26.8 σθ) were investigated. An analysis of the observations revealed that salinity decreased significantly at the density range of 26.6–26.8 σθ in the western subtropical gyre, including the mixed water region (MWR). The temporal variability of the salinity is dominated by the marked change in the late 1970s. With results similar to the observations, the model, selectively forced by the interannual variability of the wind-driven ocean circulation, simulated significant freshening of the intermediate layer over the subtropical gyre. The significant freshening is related to the increase in southward transport of the Oyashio associated with the intensification of the Aleutian low. Accompanying these changes, the intrusion of fresh and low potential vorticity water, originating in the Okhotsk Sea, to the MWR increased, and the freshening signal propagated farther southward in the western subtropical gyre during the subsequent 6 yr, crossing the Kuroshio Extension. These results indicate that the multidecadal-scale freshening of the NPIW is partly caused by intensification of the wind-driven cross-gyre transport of the subarctic water to the subtropical gyre.

A comparison of micropaleontological data on the distribution of the Cycladophora davisiana radiolarian species in the surface sediment layer and the Late Quaternary sediments from the Subarctic Pacific and Far East marginal seas allowed conclusions concerning the possible conditions and occurrence of intermediate waters during the last glacial maximum. We used the modern data on the C. davisiana species, which is a micro-paleontological indicator of the cold oxygen-rich upper intermediate water mass, which is now forming only in the Sea of Okhotsk. The high amount of C. davisiana in sediments of the last glacial maximum may point to the possible formation and expansion of the ventilated intermediate water in the most part of the Subarctic paleo-Pacific: the Bering Sea, the Sea of Okhotsk, within the NW Gyre, and in the Gulf of Alaska.

Highlights • Multi-proxy, multi-site reconstruction of Okhotsk Sea palaeo-productivity and mid-depth ventilation changes from 8 to 18 ka. • Link between hinterland river discharge and downstream Okhotsk Sea Intermediate Water (OSIW) ventilation/nutrient signatures. • Surplus Fe, Si(OH)4 export in OSIW during Bolling-Allerod to pelagic Pacific supported transient nutrient-replete conditions. • Subarctic and subtropical Pacific gyres disconnected during Bolling-Allerod, with restricted OSIW flow to lower latitudes. • Deglacial OSIW export and mid-depth Pacific biogeochemistry modulate millennial-scale regional CO2 source/sink conditions. The modern North Pacific plays a critical role in marine biogeochemical cycles, as an oceanic sink of CO2 and by bearing some of the most productive and least oxygenated waters of the World Ocean. The capacity to sequester CO2 is limited by efficient nutrient supply to the mixed layer, particularly from deeper water masses in the Pacific's subarctic and marginal seas. The region is in addition only weakly ventilated by North Pacific Intermediate Water (NPIW), which receives its characteristics from Okhotsk Sea Intermediate Water (OSIW). Here, we present reconstructions of intermediate water ventilation and productivity variations in the Okhotsk Sea that cover the last glacial termination between eight and 18 ka, based on a set of high-resolution sediment cores from sites directly downstream of OSIW formation. In a multi-proxy approach, we use total organic carbon (TOC), chlorin, biogenic opal, and CaCO3 concentrations as indicators for biological productivity. C/N ratios and XRF scanning-derived elemental ratios (Si/K and Fe/K), as well as chlorophycean algae counts document changes in Amur freshwater and sediment discharge that condition the OSIW. Stable carbon isotopes of epi- and shallow endobenthic foraminifera, in combination with 14C analyses of benthic and planktic foraminifera imply decreases in OSIW oxygenation during deglacial warm phases from c. 14.7 to 13 ka (Bolling-Allerod) and c. 11.4 to 9 ka (Preboreal). No concomitant decreases in Okhotsk Sea benthic-planktic ventilation ages are observed, in contrast to nearby, but southerly locations on the Japan continental margin. We attribute Okhotsk Sea mid-depth oxygenation decreases in times of enhanced organic matter supply to maxima in remineralization within OSIW, in line with multi-proxy evidence for maxima in primary productivity and supply of organic matter. Sedimentary C/N and Fe/K ratios indicate more effective entrainment of nutrients into OSIW and thus an increased nutrient load of OSIW during deglacial warm periods. Correlation of palynological and sedimentological evidence from our sites with hinterland reference records suggests that millennial-scale changes in OSIW oxygen and nutrient concentrations were largely influenced by fluvial freshwater runoff maxima from the Amur, caused by a deglacial northeastward propagation of the East Asian Summer Monsoon that increased precipitation and temperatures, in conjunction with melting of permafrost in the Amur catchment area. We suggest that OSIW ventilation minima and the high lateral supply of nutrients and organic matter during the Allerod and Preboreal are mechanistically linked to concurrent maxima in nutrient utilization and biological productivity in the subpolar Northwest Pacific. In this scenario, increased export of nutrients from the Okhotsk Sea during deglacial warm phases supported subarctic Pacific shifts from generally Fe-limiting conditions to transient nutrient-replete regimes through enhanced advection of mid-depth nutrient- and Fe-rich OSIW into the upper ocean. This mechanism may have moderated the role of the subarctic Pacific in the deglacial CO2 rise on millennial timescales by combining the upwelling of old carbon-rich waters with a transient delivery of mid-depth-derived bio-available Fe and silicate.

Gas hydrates, pervasive in continental margin sediments, are expected to release methane in response to ocean warming, but the geographic range of dissociation and subsequent flux of methane to the ocean are not well constrained. Sediment column thermal models based on observed water column warming trends offshore Washington (USA) show that a substantial volume of gas hydrate along the entire Cascadia upper continental slope is vulnerable to modern climate change. Dissociation along the Washington sector of the Cascadia margin alone has the potential to release 45–80 Tg of methane by 2100. These results highlight the importance of lower latitude warming to global gas hydrate dynamics and suggest that contemporary warming and downslope retreat of the gas hydrate reservoir occur along a larger fraction of continental margins worldwide than previously recognized.

Based on historical observations, ventilation of the Sulu Sea (SS) is investigated and, its interbasin exchange is also partly discussed. The results suggest that near the surface the water renewal process not only occurs through the Mindoro Strait (MS) and the Sibutu Passage, but also depends on the inflows through the Surigao Strait and the Bohol Sea from the Pacific and through the Balabac Strait from the South China Sea (SCS). Both inflows are likely persistent year round and their transports might not be negligible. Below the surface, the core layer of the Subtropical Lower Water (SLW) lies at about 200 m, which enters the SS through the Mindoro Strait not hampered by topography. Moreover, there is no indication of SLW inflow through the Sibutu Passage even though the channel is deep enough to allow its passage. The most significant ventilation process of the SS takes place in depths from 200 m to about 1 200 m where intermediate convection driven by quasi-steady inflows through the Mindoro and Panay straits (MS-PS) dominates. Since the invaded water is drawn from the upper part of the North Pacific Intermediate Water (NPIW) of the SCS, it is normally not dense enough to sink to the bottom. Hence, the convective process generally can only reach some intermediate depths resulting in a layer of weak salinity minimum (about 34.45). Below that layer, there is the Sulu Sea Deep Water (SSDW) homogeneously distributed from 1 200 m down to the sea floor, of which the salinity is only a bit higher (about 34.46) above the minimum. Observational evidence shows that hydrographic conditions near the entrance of the MS in the SCS vary significantly from season to season, which make it possible to provide the MS-PS overflow with denser water of higher salinity sporadically. It is hence proposed that the SSDW is derived from intermittent deep convection resulted from property changes of the MS-PS inflow.

The Okhotsk Sea is considered the only ventilation source area for North Pacific Intermediate Water (NPIW), which is widely distributed in the low and middle latitudes of the North Pacific. Previous studies have confirmed high levels of dissolved organic carbon (DOC) in NPIW, yet the amounts and the processes driving DOC export from the Okhotsk Sea are poorly understood. In this study, concentrations of DOC and dissolved organic nitrogen (DON) were measured in the western Okhotsk Sea during the summer of 2006, and additional DOC measurements were made during the late spring of 2010. Results indicate that DOC transport to the intermediate waters (26.7–27.0 σθ) occurs through two processes. The first process involves the spread of water discharged from the continental shelf (Dense Shelf Water), which contributes to a DOC and turbidity maxima in the 250–300 m layer of Okhotsk Sea Mode Water (OSMW) located off the eastern Sakhalin coast. The second process involves diapycnal mixing in the Kuril Basin and the Bussol’ Strait, where DOC is transported to a depth greater than 800 m. The ratio of DOC:DON in OSMW was significantly higher in the Kuril Basin and Bussol’ Strait than off of the Sakhalin coast, which suggests that the transport of terrigenous organic matter from the bottom occurs in the former regions. DOC and DON efflux from the Okhotsk Sea to the intermediate layer in the North Pacific water (26.7–27.0 σθ) were estimated to be 68–72 Tg C yr−1 and 5.4 Tg N yr−1, respectively, for which the DOC transported by diapycnal mixing accounts for 37%. We conclude that diapycnal mixing in the Kuril Basin and Bussol’ Strait regions could play a significant role in regulating the quality and quantity of DOC exported to the intermediate water in the North Pacific.

A high-recovery technique of dissolved organic matter (DOM) isolation – reverse osmosis coupled with electrodialysis (RO/ED) – was used to isolate DOM from the North Atlantic Senegal-Mauritanian upwelling area surface water (5 m), North Atlantic oxygen minimum water (415 m) and deep water (3000 m), North Pacific subtropical gyre surface water (5 m), and North Pacific intermediate water (674 m) and deep water (3500 m). Samples were characterized by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry with electrospray ionization (ESI FTICR-MS). RO/ED isolated DOM samples share a significant number of common formulas accounting for 54–79% of formulas in each sample. Total dissolved carbohydrate (TCHO) concentrations in RO/ED isolated DOM were specifically measured using a colorimetric method, and were found to have higher contribution to DOC than estimated by FTICR-MS data. Percentages of TCHO-C in DOC are in the range of 3.7–19.6% in all samples, with the North Pacific deep (3500 m) water having the lowest % and the North Atlantic upwelling core surface water having the highest %. Principal component analysis (PCA) using the relative magnitudes of MS peaks facilitated identification of specific peaks that are enriched in different samples. Peaks enriched in surface samples have higher H/C values than peaks enriched in deep samples, in both the North Atlantic DOM and the North Pacific DOM. This enrichment pattern is likely due to the selective photo-degradation of aromatic compounds and the bio-production of aliphatic and carbohydrate-like compounds in surface waters, and the selective bio-degradation of aliphatic and carbohydrate-like compounds with increasing depth. In further support of a photo-degraded signature for DOM in surface waters, photo-resistant and photo-produced molecular formulas were present in the highest numbers in the surface North Pacific subtropical gyre DOM. Peaks enriched in the North Pacific intermediate and deep DOM have significantly higher O/C values than the North Atlantic oxygen minimum layer and deep DOM, for both CHO formula compounds and CHON formula compounds. This difference in O/C values observed for the deep Pacific vs. Atlantic suggests oxidation of DOM, possibly via microbial activity during the ageing of DOM or the preferential remineralization of DOM from sinking particles at depth in the Pacific.

[1] Water mass formation in the intermediate and deep layers of the Okinawa Trough is investigated using two distinct data sets: a quasi-climatological data set of the water properties of the minimum salinity surface produced from Argo float profiles and historical CTD data, and a velocity data set in the Kerama Gap measured by moored current meters during June 2009 to June 2011. The formation process of Okinawa Trough Intermediate Water is explained on the basis of horizontal advection and mixing of North Pacific Intermediate Water (NPIW) and South China Sea Intermediate Water (SCSIW). The salinity-minimum water intruding into the Okinawa Trough through the channel east of Taiwan is approximately composed of 45% NPIW and 55% SCSIW, while that through the Kerama Gap is 75% NPIW and 25% SCSIW. Salinities of these water masses increase in the Okinawa Trough due to strong diapycnal diffusion; its coefficient is estimated as 6.8–21.5 × 10−4 m2 s−1 based on a simple advection-diffusion equation. On the other hand, deep water in the Okinawa Trough, below the sill depth of the Kerama Gap (∼1100 m), is ventilated by overflow in the bottom layer of the Kerama Gap down to the deepest layer (∼2000 m) in the southern Okinawa Trough. A simple box model predicts that this bottom overflow (0.18–0.35 Sv) causes strong upwelling (3.8–7.6 × 10−6 m s−1) in the southern Okinawa Trough, which must be maintained by buoyancy gain of the deep water due to strong diapycnal diffusion (4.8–9.5 × 10−4 m2 s−1).

[1] This study develops a strategy for tracing a target water mass, and applies it to analyzing the pathway of the North Pacific Intermediate Water (NPIW) from the subarctic gyre to the northwestern part of the subtropical gyre south of Japan in a simulation of an ocean general circulation model. This strategy estimates the pathway of the water mass that travels from an origin to a destination area during a specific period using a conservation property concerning tangent linear and adjoint models. In our analysis, a large fraction of the low salinity origin water mass of NPIW initially comes from the Okhotsk or Bering Sea, flows through the southeastern side of the Kuril Islands, and is advected to the Mixed Water Region (MWR) by the Oyashio current. It then enters the Kuroshio Extension (KE) at the first KE ridge, and is advected eastward by the KE current. However, it deviates southward from the KE axis around 158°E over the Shatsky Rise, or around 170°E on the western side of the Emperor Seamount Chain, and enters the subtropical gyre. It is finally transported westward by the recirculation flow. This pathway corresponds well to the shortcut route of NPIW from MWR to the region south of Japan inferred from analysis of the long-term freshening trend of NPIW observation. Copyright © 2013 John Wiley & Sons, Ltd.

Mesoscale eddies in sub-tropical gyres physically perturb the water column and can introduce macronutrients to the euphotic zone, stimulating a biological response by which phytoplankton communities can become dominated by large phytoplankton. Mesoscale eddies are therefore important in driving export in oligotrophic regions of the modern ocean. The character and magnitude of the biological response sustained by eddies are, however, variable. Here we present data from mesoscale eddies in the Sargasso Sea (Atlantic) and the waters off Hawai'i (Pacific), alongside mesoscale events that affected the Bermuda Atlantic Time Series (BATS) over the past decade. From this analysis, we suggest that the phytoplankton community structure sustained by mesoscale eddies is predetermined by the relative abundance of silicate over nitrate (Si*) in the upwelled waters. We present data that demonstrate that mode-water eddies (MWE) in the Sargasso Sea upwell locally formed waters with high Si* to the euphotic zone, and that cyclonic eddies in the Sargasso Sea introduce waters with low Si*, a signature that originated in the iron-limited Southern Ocean. We propose that this phenomenon can explain the observed abundance of large-diatom species in MWE and small prokaryotic phytoplankton in cyclonic features. In contrast to the Atlantic, cyclonic eddies in waters off Hawai'i induce North Pacific Intermediate Water (NPIW) that has high Si* and therefore also appears capable of establishing diatom populations. These observations suggest that the structure of phytoplankton communities sustained by eddies may not be directly related to the physical nature of the eddy but rather to the chemical composition of the upwelled waters. This paper links the biological production and export efficiency of mesoscale eddies to events in spatially and temporally disparate locations.

We detected temperature (salinity) decreases on the neutral density surfaces above the salinity minimum and increases below throughout the western North Pacific subtropical gyre ( 25 ∘ N – 40 ∘ N , west of 170 ∘ W ) by comparing World Ocean Circulation Experiment (WOCE) hydrographic sections from the 1980s, 1990s, and 2000s. The temperature (salinity) changes around the salinity minimum were associated with an apparent oxygen utilization (AOU) increase, and could not be explained by an increase of the subpolar input. These property changes were due to a density decrease of the salinity minimum. Furthermore, similar changes in temperature (salinity) were detected along 165 ∘ E as a 12-year trend from 1996 to 2007. This suggests that the changes evident in the comparison of the WOCE sections are the representative of changes at the intermediate depths in the last decade. This pattern was not detected in the eastern subtropical gyre ( 25 ∘ N – 40 ∘ N , east of 170 ∘ W ) and along 47 ∘ N , where the AOU increase due to ventilation reduction has been reported. Along 47 ∘ N , changes in temperature, salinity and AOU from 1999 to 2007 were opposite to those from 1985 to 1999.

The formation of North Pacific Intermediate Water (NPIW) requires a densification of ∼0.2-0.3 σθ through interior transformation processes. Evidence of this density anomaly is discovered on neutral density surfaces in the subarctic-tropical frontal zone (SATFZ) showing an eastward tongue of positive density anomaly coincident with a cabbeling maximum tongue. The northward displacement of the SATFZ with depth east of dateline enables the transformed dense water from overlying thermocline to sink into the northeast subtropical NPIW domain. The further southward and southwestward spreading across the subtropical gyre yields the observed distribution of NPIW.

A steady quasi-geostrophic 2.5-layer model, forced by both Ekman pumping and a mass source/sink situated at the western boundary has been constructed to investigate the effect of diapycnal transport due to convection in the Okhotsk Sea and tidal mixing at the Kuril Straits on the intermediate layer in the North Pacific. The model illustrates a combined effect of the wind-driven and mass-driven circulations. First, net mass input induces a “barotropic” mode inter-gyre flow along the western boundary through the dynamical influence of Kelvin waves. This flow creates characteristic curves (geostrophic contours) that facilitate inter-gyre communication through the western boundary layer from the location of the mass source to the subtropical gyre. Due to the effect of wind-driven circulation, the offshore part turns eastward into the interior, encircles the outer rim of the region (which would otherwise be the pool region in the absence of mass input), and then encounters the western boundary. Eventually, the water fed into the lower layer flows mostly along this path and later flows away to the equatorial region. Conversely, in the upper layer, water is fed from the equator to the subtropics, and to the subpolar interior region through the western boundary current. The water then circulates along the outer rim and is absorbed into the mass sink. The model is controlled mainly by three nondimensional parameters: (1) the ratio of net mass input rate to the maximum Sverdrup transport (Q/T Sv max ), which affects the inter-gyre communication by altering the paths of geostrophic contours, (2) the ratio of a mass input rate into the lower layer to that in total (Q 2/Q), which controls the vertical structure of the inter-gyre flow, and (3) the measure of the wind forcing effect relative to the β effect, which determines the horizontal extent of the area influenced by the mass input. The other parameter regimes with respect to Q/T Sv max and Q 2/Q are also presented.

This paper describes a reanalysis of the variability of water mass properties and currents in the western North Pacific using an ocean forecast system, Japan Coastal Ocean Predictability Experiment 2 (JCOPE2), to provide the basic description and information about the quality for data users. We have created the reanalysis data with high horizontal resolution of 1/12° to describe the oceanic variability associated with the Kuroshio-Kuroshio Extension, the Oyashio, and the mesoscale eddies from 1993 to 2007. The products made by an eddy-resolving ocean model combined with the three-dimensional variational data assimilation well reproduced the mean water mass property in the western North Pacific and the interannual variations of the Kuroshio-Kuroshio Extension and the Oyashio coastal branch. From the reanalysis data, we found that both the mean kinetic energy of the Kuroshio Extension axis at the first meandering crest and southward intrusion of the Oyashio coastal branch were closely related with the horizontal distribution of both the Oyashio Water and North Pacific Intermediate Water within the appropriate interannual time scale. The reanalysis data also indicated that the north-south migration of the Kuroshio Extension associated with its regime transitions affected the decadal modulation of the Subtropical Mode Water formation in the recirculation gyre of the Kuroshio Extension.

Recent in situ observations of chromophoric dissolved organic material (CDOM) in the Pacific Ocean reveal the biogeochemical controls on CDOM and indicate predictive potential for open-ocean CDOM in diagnosing particulate organic matter (POM) remineralization rates within ocean basins. Relationships between CDOM and concentrations of dissolved oxygen, nutrients and inorganic carbon in the subthermocline waters of the Pacific reflect the relative influences of water mass ventilation and water-column oxidative remineralization. Apparent in situ oxygen utilization (AOU) accounts for 86% and 61% of variance in CDOM abundance, respectively, in Antarctic Intermediate Water and North Pacific Intermediate Water. In the deep waters of the Pacific below the zone of remineralization, AOU explains 26% of CDOM variability. The AOU–CDOM relationship results from competing biogeochemical and advective processes within the ocean interior. Dissolved organic carbon (DOC) is not statistically linked to the CDOM or AOU distributions, indicating that the majority of CDOM production occurs during the remineralization of sinking POM and thus potentially provides key information about carbon export. Once formed in the ocean interior, CDOM is relatively stable until it reaches the surface ocean where it is destroyed by solar bleaching. Susceptibility to bleaching confers an additional tracer-like quality for CDOM in water masses with active convection, such as mode waters that appear as subsurface CDOM minima. In the surface ocean, atypically low CDOM abundance highlights a region of unusually extreme oligotrophy: the subtropical South Pacific gyre. For these hyper-oligotrophic waters, the present CDOM observations are consistent with analysis of in situ radiometric observations of light attenuation and reflectance, demonstrating the accuracy of the CDOM spectrophotometric observations. Overall, we illustrate how CDOM abundance in the ocean interior can potentially diagnose rates of thermohaline overturning as they affect regional biogeochemistry and export. We further show how relative surface ocean CDOM abundances are driven in large part by processes occurring in the deep layers of the ocean. This is particularly significant for the interpretation of the global surface distribution of CDOM using satellite remote sensing.

With high-resolution conductivity-temperature-depth (CTD) observations conducted in Oct.–Nov. 2005, this study provides a detailed quasi-synoptic description of the North Pacific Tropic Water (NPTW), North Pacific Intermediate Water (NPIW) and Antarctic Intermediate Water (AAIW) in the western North Pacific. Some novel features are found. NPTW enters the western ocean with highest-salinity core off shore at 15°–18°N, and then splits to flow northward and southward along the western boundary. Its salinity decreases and density increases outside the core region. NPIW spreads westward north of 15°N with lowest salinity off shore at 21°N, but mainly hugs the Mindanao coast south of 12°N. It shoals and thins toward the south, with salinity increasing and density decreasing. AAIW extends to higher latitude off shore than that in shore, and it is traced as a salinity minimum to only 10°N at 130°E. Most of the South Pacific waters turn northeastward rather than directly flow northward upon reaching to the Mindanao coast, indicating the eastward shift of the Mindanao Undercurrent (MUC).

Modification and distribution of North Pacific Intermediate Water (NPIW) in the area north of 30°N and east of 150°E are discussed based on an isopycnal particle tracking experiment with the velocity field estimated from a β-spiral inverse method based on climatological data, annual climatological data analysis, and isopycnal-tracking profiling float data. NPIW emanating from 149.5°E between 35.5° and 37.5°N retains its salinity minimum structure along the eastward flow of the northern subtropical gyre, and these pathways occupy the northern part of the NPIW distribution. On the other hand, NPIW from 149.5°E between 37.5° and 40.5°N loses its salinity minimum as it flows eastward to the “transition domain.” The extinction of the salinity minimum structure is caused by a salinity decrease in the upper layer (26.5–26.6 σθ) in the region at 40°–45°N, 155°–165°E where a winter mixed layer is developed to a depth of 300 m and a density of 26.7 σθ. This extinction was observed by a profiling float tracking a 26.7-σθ isopycnal density surface. Low-salinity subarctic water advected southward across the Subarctic Front by wind-induced Ekman drift from strong westerlies in winter is suggested as the source of the low-salinity mixed layer. After the extinction of the salinity minimum structure, a state without salinity minimum is maintained during transport eastward in the transition domain because the winter mixed layer in the area east of the deep mixed layer region reaches a density surface shallower than 26.5 σθ.

To clarify the sources and transformation of NO3− on the Pacific coast of Japan, observations over the continental shelf were conducted during the summer in 2005 and 2006 when the Kuroshio flowed close to and away from the coastal area, respectively. Below the halocline, there are two prominent salinity peaks that originated in two different waters. In the subsurface layer, the salinity maximum (Smax) was indicative of the Kuroshio Water (KW), while the salinity minimum (Smin) in the middle layer at ∼400 m depth was indicative of the North Pacific Intermediate Water (NPIW). δ15NNO3 ranged from 4.1‰ to 5.1‰ with a mean of 4.8±0.4‰ in the deeper water around Smin. Below 50 m depth over the shelf break, δ15NNO3 values (3.1±0.8‰ in 2005 and 4.6±0.3‰ in 2006) clearly increased as contribution of NPIW increased in 2006. On the contrary, subsurface δ15N of NO3− values (−1.1±0.1‰) remained unchanged in both years, although the contribution of the KW to the subsurface water changed significantly. This suggests that the source of NO3− has little effect on the δ15N of NO3− in this layer. The negative δ15N values also coincided with the base of the chlorophyll maximum layer suggesting that these isotopic signals must be evidence of active nitrification in the upper layer.

An examination of model water masses in the North Pacific Ocean is performed in the Community Climate System version 3 (CCSM3) and its ocean-only counterpart. While the surface properties of the ocean are well represented in both simulations, biases in thermocline and intermediate-water masses exist that point to errors in both ocean model physics and the atmospheric component of the coupled model. The lack of North Pacific Intermediate Water (NPIW) in both simulations as well as the overexpression of a too-fresh Antarctic Intermediate Water (AAIW) is indicative of ocean model deficiencies. These properties reflect the difficulty of low-resolution ocean models to represent processes that control deep-water formation both in the Southern Ocean and in the Okhotsk Sea. In addition, as is typical of low-resolution ocean models, errors in the position of the Kuroshio, the North Pacific subtropical gyre western boundary current (WBC), impact the formation of the water masses that form the bulk of the thermocline as well as the properties of the NPIW. Biases that arise only in the coupled simulation include too-salty surface water in the subtropical North Pacific and too deep a thermocline, the source of which is the too-strong westerlies at midlatitudes. Biases in the location of the intertropical convergence zone (ITCZ) and the southern Pacific convergence zone (SPCZ) lead to the opposite hemispheric asymmetry in water mass structure when compared to observations. The atmospheric component of the coupled model acts to compound most ocean model biases.

The glacial water properties and circulation changes in the North Pacific Ocean are investigated using a coupled ocean‐atmosphere‐sea‐ice climate model. With glacial boundary conditions, an increase in potential density in the upper layers of the northern North Pacific makes the water column highly unstable and eventually results in the enhancement of North Pacific Intermediate Water (NPIW) production, consistent with proxy evidence. The NPIW outflow reaches deeper layers than in the present‐day ocean, but remains largely confined to the North Pacific. The increase in potential density is predominantly caused by the increase in salinity and, to a lesser extent, by decreases in temperature. The increase in surface salinity is especially high in the Sea of Okhotsk and the western Bering Sea, which are possible source areas of glacial NPIW production. In these regions, an increase in brine release due to a marked increase in sea ice, evaporation exceeding precipitation, and a reduction in river disc...

Okhotsk Sea Intermediate Water (OSIW), the source water for ventilation of North Pacific Intermediate Water, exhibits a multidecadal warming trend. Historical data show that OSIW temperatures increased by 0.28, 0.57, 0.31 and 0.10°C during 1955 to 2003 at potential densities of 26.8, 27.0, 27.2 and 27.4σθ, at depths of approximately 250, 500, 700 and 900 m, respectively. This rate of warming is much faster than that of the global ocean. This OSIW warming is likely linked to the reduced ventilation of cold Dense Shelf Water associated with brine rejection during sea ice formation.

Like most other deep basins in Southeast Asia, the deep Sulu Sea (SS) basin is isolated from the neighboring seas by surrounding topography. While the near-surface circulation is mainly governed by the seasonally reversing monsoon winds, below the warm and fresh surface layer, the core of the incoming Subtropical Lower Water from the West Philippine Sea (WPS), by way of the South China Sea (SCS), can be seen, at a depth of around 200 m, to have a distinct salinity maximum. It lies well above the sill depth (420 m) in the Mindoro Strait and thus, its spreading is not hampered by topography. The deep circulation is forced by an inflow of upper North Pacific Intermediate Water (NPIW) from the SCS through the Mindoro Str. Below 1000 m, the physico-chemical properties are remarkably homogeneous. The higher temperature, but lower salinity, oxygen and nutrients, of the deep SS waters, compared to those of the SCS, is indicative of the intrusion of NPIW above the sill depth. The excess, anthropogenic CO2 penetrates the entire water column, because of the over-spill of the excess CO2-laden water from the SCS. It has been reported that the bottom of the SS is CaCO3 rich, relative to the SCS. Previous investigators attribute this to the higher θ in the SS. Indeed, the aragonite does not become undersaturated in the SS until below 1400 m, compared to 600 m in both the WPS and SCS; and the calcite does not become undersaturated until below 3800 m in the SS, compared to 2500 m in the SCS and around 1600 m in the WPS. However, the temperature effect is relatively small. These large differences are, in fact, largely a result of higher CO32− concentrations in the SS, relative to the WPS and SCS. The higher CO32− concentration in the SS, in turn, is mainly caused by the smaller amounts of organic carbon decomposition.

A basinwide ocean general circulation model of the North Pacific Ocean is used to study the sensitivity of the simulated distributions of water masses, chlorofluorocarbons (CFCs), and bomb carbon-14 isotope (14C) to parameterizations of mesoscale tracer transports. Five simulations are conducted, including a run with the traditional horizontal mixing scheme and four runs with the isopycnal transport parameterization of Gent and McWilliams (GM). The four GM runs use different values of isopycnal and skew diffusivities. Simulated results show that the GM mixing scheme can help to form North Pacific Intermediate Water (NPIW). Greater isopycnal diffusivity enhances formation of NPIW. Although greater skew diffusivity can also generate NPIW, it makes the subsurface too fresh. Results from simulations of CFC uptake show that greater isopycnal diffusivity generates the best results relative to observations in the western North Pacific. The model generally underestimates the inventories of CFCs in the western North Pacific. The results from simulations of bomb 14C reproduce some observed features. Greater isopycnal diffusivity generates a longitudinal gradient of the inventory of bomb 14C from west to east, whereas greater skew diffusivity makes it reversed. It is considered that the ratio of isopycnal diffusivity to skew diffusivity is important. An increase in isopycnal diffusivity increases storage of passive tracers in the subtropical gyre.

Seven piston cores were obtained from the Bering Sea and the western subarctic Pacific during Cruise KH99-3 of R.V. Hakuho-maru in August 1999: Cores BOW-8A, BOW-9A, BOW-12A, AB, UMK-3A, GAT-3A in the Bering Sea, and Core ES in the western subarctic Pacific. The quantitative high-resolution analyses of radiolarians in the cores revealed significant variations in both their faunal assemblages and accumulation rates (AR). A trend of conspicuous turn-over of the surface dwellers and intermediate dwellers (increase of one at expense of the other) after the last glacial maximum (LGM) is clearly visible in the Bering Sea data set. However, this turn-over is not present at Site ES in the western subarctic Pacific core. These changes in radiolarian assemblages around the LGM suggest that the surface waters had extremely low temperature and low salinity and that the subsurface dichothermal layer (temperature minimum layer) was thickened all the way across the entire east–west span of the Bering Sea. Furthermore, sea-ice coverage extended to the central Aleutian Basin around the LGM. This suggests that the water-mass structure in the western subarctic Pacific around the LGM was basically the same as that of the eastern Bering Sea based on the species composition of the surface dwellers. However, the detailed development of intermediate water masses differed from that of the eastern Bering Sea. During Marine Isotope Stage (MIS) 5 to 3, such species turn-over events did not occur and species abundance stayed relatively uniform in the studied cores. Therefore, the present water-mass structure is interpreted to have been formed after the LGM. Based on the relative abundances of Cycladophora davisiana (the most dominant species dwelling in the intermediate water, and a useful tracer of cold and well-oxygenated intermediate-water in the North Pacific), the past North Pacific Intermediate Water (NPIW) had the following source regions during the last 100 kyr: (1) both the Okhotsk Sea and the Bering Sea during MIS 5 to 3; (2) the Bering Sea around the LGM; and (3) shifting from the Bering Sea to the Okhotsk Sea after the LGM. Relatively low radiolarian AR at Site BOW-8A, which is the shallowest site studied here (present water depth: 884 m), were observed during the glacial periods. This is due to decrease of radiolarian production in the thinned upper water layer above the dicothermal layer caused by sea-level drop. Cycladophora davisiana probably lived deeper during the glacials than today, as it was absent in the shallow piston core. r 2005 Elsevier Ltd. All rights reserved.

We investigated the variation of the North Pacific Intermediate Water (NPIW) distribution in the western North Pacific, focusing on the intermediate salinity minimum (S < 34.2) core observed along the meridional hydrographic sections including the 137°E repeat section by the Japan Meteorological Agency. This core is a cross-section of a low salinity tongue extending westward along the recirculation in the subtropical gyre. The core size shows remarkable variabilities in interannual and decadal time scales. The salinity change in the density layer during the period of core expansion (shrinking) represents the spatial salinity change in the tongue toward the west (east). Thus, we conclude that the core size variation is associated with the zonal wobble of the tongue having thicker distribution to the east, rather than temporal changes of the water mass itself. The core size at 137°E is well correlated with the meridional gradient of the depth in the isopycnal surface at the salinity minimum representing the recirculation intensity, suggesting a relation with the intensity of the subtropical gyre. A significant lag-correlation between the gradient and the wind forcing over the North Pacific suggests that the first mode baroclinic Rossby waves excited in the central North Pacific propagated westward to change the intensity of the recirculation in interannual time scales. In decadal time scales, it is found that the wind stress curl and heat flux fields in the North Pacific precede the recirculation by about 11 years.

This study discusses branching of the Kuroshio Current including North Pacific Intermediate Water (NPIW) into the South China Sea (SCS). The spreading path of the subtropical salinity minimum of NPIW is southwestward pointing to the Luzon Strait between Taiwan and Luzon islands. Using a large collection of updated hydrography, results show that the SCS is a cul-de-sac for the subtropical NPIW because even the NPIW’s upper boundary neutral density surface σ N = 26.5 is completely blocked by the Palawan sill and partly blocked by the southern Mindoro Strait. In autumn, NPIW is driven out of the Luzon Strait by the preceding anticyclonic summer monsoon due to an intraseasonal variation and seasonal phase lag response to the weaker summer monsoon. Stronger inflow under winter monsoon than outflow under summer monsoon results in a net annual transport of NPIW of about 1.1 ± 0.2 Sv (1 Sv = 106 m3s−1) into the SCS. This net transport accounts for the anomaly in NPIW transport across the World Ocean Circulation Experiment section P8 (130° E). An earlier study estimated a large westward NPIW transport of about 3.9 ± 0.2 Sv, resulting in a difference of 1.2 ± 0.2 Sv from the basin-wide mean of 2.7 ± 0.2 Sv. Observations are generally in agreement with numerical results although the intraseasonal signal seems to cause a slight bias and remains to be simulated by future model experiments.

The intrusion of western boundary currents (WBC) across the wind-driven gyre boundary and associated cross-gyre transport are examined using layered numerical models with realistic topography and annual mean wind stress, with special emphasis on the Oyashio southward intrusion and North Pacific Intermediate Water (NPIW) formation. Diapycnal transport due to strong tidal mixing around the Kuril Islands in the subarctic North Pacific Ocean is modeled by source/sink thickness restoring terms in the continuity equation to form the deep pycnocline observed in the Okhotsk Sea. About 3 Sv (Sv ≡ 106 m3 s−1) of the model diapycnal upwelling occurring along the Kuril Islands induces a meridional–diapycnal overturning circulation, and the transport is consistent with the estimates from recent observations with chemical tracers, GCM results, and the present study using observed density fields with enhanced diapycnal diffusivity around the Kuril Islands. The model with the diapycnal transport reproduced the r...

Sedimentary, geochemical and paleontological data, extracted from a sediment core taken from the central Okhotsk Sea, record climatic and sedimentary changes over the past 100 ky. Benthic foraminiferal oxygen isotope record and accelerator mass spectrometry (AMS) 14C data provide the basis for the core age chronology. Planktonic and benthic foraminiferal δ18O and δ13C measurements, magnetic susceptibility, ice rafted debris and coarse fraction content, carbonate and organic carbon content, and diatom and pollen spectra show major changes in regional climate, sea surface conditions, ice extent and sedimentary regime, which are correlated with the global glacial–interglacial changes of marine isotope stages (MIS) 1 through 5.3. Regional cooling and intensification of winter sea ice formation during the last glaciation increased the northern shelf surface water density and Sea of Okhotsk Intermediate Water (SOIW) formation. In addition to major Milankovitch-scale changes, lithological, geochemical and paleontological indices also show suborbital oscillations in Okhotsk Sea hydrology, sedimentation and regional climate. These shorter-term oscillations are characterized by coupled maxima in ice rafted debris and sediment coarse fraction (lithodynamic indices, LDI) values and most are associated with cold events involving enhanced winter ice formation and intensification of SOIW and North Pacific intermediate water (NPIW) formation. The terminations of LDI maxima were induced by climate warming and reductions in ice formation, and were accompanied by sharp decreases in planktonic foraminiferal δ18O. During the glacial terminations of MIS 2 and 4, however, the onset of suborbital-scale LDI maxima occurs close to sharp negative δ18O shifts in planktonic foraminifera (δ18Opf) associated with climate warming. In these special cases, the LDI maxima occurred under warm climate and rising sea levels and did not lead to increases in SOIW or NPIW ventilation.

Four vertical profiles of the concentration and isotopic composition of Nd in seawater were obtained in the western North Pacific. Two profiles from the Kuroshio Current regime showed congruently that although the Nd concentration increases gradually with depth, its isotopic composition varies significantly with depth depending upon the water mass occupying the water column. The high-salinity Kuroshio waters originating from the North Pacific Tropical Water (NPTW) carry the least radiogenic Nd (ϵ Nd = −7.4 to −8.7) to this region at ∼250 m from the western margin continental shelves, most likely from the East China Sea. The Nd isotopic compositions in the North Pacific Intermediate Water (NPIW) that occurs at 600 to 1000 m in the subtropical region are fairly uniform at ϵ Nd = −3.7. The profile data from the ∼38° to 40°N Kuroshio/Oyashio mixed water region off Sanriku of Honshu, Japan, also suggest that the newest NPIW with ϵ Nd = −3.2 is formed there by the mixing of various source waters, and the radiogenic component of Nd is derived mainly from the Oyashio waters. In the Pacific Deep Water (PDW) below ∼1000 m, the Nd isotopic composition is neither vertically nor horizontally homogeneous, suggesting that it serves as a useful tracer for sluggish deep water circulation as well. Two profiles from the Izu-Ogasawara Trench showed a minimum ϵ Nd value at ∼2000 m, suggesting that there exists a horizontal advective flow in the vicinity of Honshu, Japan. There is some evidence from other chemical properties to support this observation. The waters below 4000 m including those within the trench in the subtropical region have ϵ Nd values of around −5, suggesting that the deep waters are fed from the south along the western boundary, ultimately from the Antarctic Bottom Water (AABW) in the South Pacific. This extends up to ∼40°N along the Japanese Islands. In the subarctic region (>∼42°N), the waters have more radiogenic Nd with ϵ Nd > −4.0 throughout the water column, presumably due to the supply of Nd by weathering in such igneous provinces as the Kuril-Kamchatska-Aleutian Island chain. The lateral inhomogeneity of the Nd isotopic composition in PDW suggests that there may be different circulation and mixing regimes in the North Pacific Basin.