Pacific Deep Water Research Articles

Mid Holocene origin of the sea-surface salinity low in the subarctic North Pacific

IMAGES core MD01-2416 (51°N, 168°E) provides the first centennial-scale multiproxy record of Holocene variation in North Pacific sea-surface temperature (SST), salinity, and biogenic productivity. Our results reveal a gradual decrease in subarctic SST by 3–5 °C from 11.1 to 4.2 ka and a stepwise long-term decrease in sea surface salinity (SSS) by 2–3 p.s.u. Early Holocene SSS were as high as in the modern subtropical Pacific. The steep halocline and stratification that is characteristic of the present-day subarctic North Pacific surface ocean is a fairly recent feature, developed as a product of mid-Holocene environmental change. High SSS matched a salient productivity maximum of biogenic opal during Bølling-to-Early Holocene times, reaching levels similar to those observed during preglacial times in the warm mid-Pliocene prior to 2.73 Ma. Similar productivity spikes marked every preceding glacial termination of the last 800 ka, indicating recurrent short-term events of mid-Pliocene-style intense upwelling of nutrient-rich Pacific Deepwater in the Pleistocene. Such events led to a repeated exposure of CO 2-rich deepwater at the ocean surface facilitating a transient CO 2 release to the atmosphere, but the timing and duration of these events repudiate a long-term influence of the subarctic North Pacific on global atmospheric CO 2 concentration.

Northward Intrusion of Antarctic Intermediate Water in the Western Pacific*

Abstract The northward intrusion of Antarctic Intermediate Water (AAIW) is examined using historical data combined with synoptic observations from a repeated hydrographic section in the western Pacific Ocean. The results of this analysis suggest that AAIW is traced as a salinity minimum to only about 15°N via the New Guinea Coastal Undercurrent and the Mindanao Undercurrent. There is no northward extension of AAIW farther to the north along the western boundary. Although relatively high oxygen water does exist in the Okinawa Trough, it is connected with high-oxygen water in the South China Sea (SCS) through the Luzon Strait but not from the south as an extension of AAIW. Local circulation seems to play an essential role in localizing the oxygen maximum in the SCS. Evidence exists to suggest that high-oxygen water enters the SCS as part of the Pacific deep water around the still depth (∼2000 m) of the Luzon Strait; from there, part of it upwells and is entrained into shallower isopycnal surfaces by vertica...

Mercury distributions in the North Pacific Ocean—20 years of observations

Vertical mercury (Hg) distributions for the North and Central Pacific Ocean are reported here for three different cruises over a time period of 20 years: N. Pac (1980), VERTEX (1986–87) and IOC (2002). The vertical distribution was not controlled solely by the hydrographic characteristics or by internal biogeochemical “nutrient type” recycling and mixing processes. Rather, Hg distribution appeared to be regulated by the local magnitude of external sources and the intensity of water column processes. During the 2002 IOC cruise, the total mercury (Hg TOT) concentrations averaged 1.15±0.86 pM with the highest concentrations found within the Japanese coastal waters. The overall upper-water Hg concentration average, calculated for the IOC study (0.64±0.26 pM), was similar to the earlier VERTEX cruise (0.58±0.37 pM) but was lower compared to the N. Pac cruise (1.40±0.34). Variance in Hg concentrations in the upper water, generally within or close to the main thermocline, was observed among several stations and for both VERTEX and IOC campaigns. Horizontal advection of water along isopycnals, vertical mixing, ventilation, the presence of a thermocline associated with remobilization of mercury as a result of remineralization of settling particles, the influence of diagenetic processes from continental margin sediments and seasonal stratification of the euphotic zone all appeared to be factors that can account for the distribution of Hg in the upper water. The deep and bottom waters of the North Pacific Ocean averaged 1.10±0.31 pM and were characterized by comparable Hg concentrations among the different cruises, which reinforced the homogeneous characteristics of this particular water mass. In addition, the comparison between the Atlantic and the Pacific deep water showed the presence of inter-ocean Hg fractionation resulting in Hg concentrations in the deep North Pacific Ocean being three- to sixfold lower than in the deep Atlantic.

Glacial intermediate water ventilation in the northwestern Pacific based on AMS radiocarbon dating

14C ages of benthic foraminifera and planktonic foraminifera in sediment core PC4 (41°07.10′N, 142°24.17′E) collected from the northwestern Pacific were measured by accelerator mass spectrometry. The purpose of this study was to investigate the changes in intermediate water ventilation in the northwestern Pacific during the last glacial period. We used age differences between benthic foraminifera and planktonic foraminifera from the same sediment horizons to estimate the ventilation time of intermediate water. Planktonic 14C ages show that the core recorded paleoenvironmental changes during the last 35 kyr. Glacial benthic–planktonic age differences were smaller in the early deglacial period (15000–17000 calyrBP) than in the period (19000–30000 calyrBP) before and during the last glacial maximum. The results at intermediate depths are consistent with those from a Pacific deep-water site.

Tracing the history of submarine hydrothermal inputs and the significance of hydrothermal hafnium for the seawater budget—a combined Pb–Hf–Nd isotope approach

Secular variations in the Pb isotopic composition of a mixed hydrogenous-hydrothermal ferromanganese crust from the Bauer Basin in the eastern Equatorial Pacific provide clear evidence for changes in hydrothermal contributions during the past 7 Myr. The nearby Galapagos Rise spreading center provided a strong hydrothermal flux prior to 6.5 Ma. After 6.5 Ma, the Pb became stepwise more radiogenic and more similar to Equatorial Pacific seawater, reflecting the westward shift of spreading to the presently active East Pacific Rise (EPR). A second, previously unrecognized enhanced hydrothermal period occurred between 4.4 and 2.9 Ma, which reflects either off-axis hydrothermal activity in the Bauer Basin or a late-stage pulse of hydrothermal Pb from the then active, but waning Galapagos Rise spreading center. Hafnium isotope time-series of the same mixed hydrogenous-hydrothermal crust show invariant values over the past 7 Myr. Hafnium isotope ratios, as well as Nd isotope ratios obtained for this crust, are identical to that of hydrogenous Equatorial Pacific deep water crusts and clearly indicate that hydrothermal Hf, similar to Nd, does not travel far from submarine vents. Therefore, we suggest that hydrothermal Hf fluxes do not contribute significantly to the global marine Hf budget.

Glacial ventilation rates for the deep Pacific Ocean

A key constraint in attempts to reconstruct the patterns and rates of the ocean's thermohaline circulation during the last glacial period is the difference between the 14C to C ratio in surface and deep water. While imperfect, it is our best index of past deep‐sea ventilation rates. In this paper we review published ventilation rate estimates based on the measured radiocarbon age difference between coexisting benthic and planktic foraminifera from glacial‐age Pacific sediments. We also present new results from a series of eastern equatorial Pacific sediment cores. The conclusion is that the scatter in these results is so large that the apparent 14C age of glacial deep Pacific water could lie anywhere between double and half today's. Further, it is not clear what is responsible for the wide scatter in the radiocarbon results.

Oceanography: Deep Pacific waters warmed in recent years

Oceanography: Deep Pacific waters warmed in recent years

Deep and bottom water export from the Southern Ocean to the Pacific over the past 38 million years

The application of radiogenic isotopes to the study of Cenozoic circulation patterns in the South Pacific Ocean has been hampered by the fact that records from only equatorial Pacific deep water have been available. We present new Pb and Nd isotope time series for two ferromanganese crusts that grew from equatorial Pacific bottom water (D137‐01, “Nova,” 7219 m water depth) and southwest Pacific deep water (63KD, “Tasman,” 1700 m water depth). The crusts were dated using10Be/9Be ratios combined with constant Co‐flux dating and yield time series for the past 38 and 23 Myr, respectively. The surface Nd and Pb isotope distributions are consistent with the present‐day circulation pattern, and therefore the new records are considered suitable to reconstruct Eocene through Miocene paleoceanography for the South Pacific. The isotope time series of crusts Nova and Tasman suggest that equatorial Pacific deep water and waters from the Southern Ocean supplied the dissolved trace metals to both sites over the past 38 Myr. Changes in the isotopic composition of crust Nova are interpreted to reflect development of the Antarctic Circumpolar Current and changes in Pacific deep water circulation caused by the build up of the East Antarctic Ice Sheet. The Nd isotopic composition of the shallower water site in the southwest Pacific appears to have been more sensitive to circulation changes resulting from closure of the Indonesian seaway.

Reconstruction of paleoproductivity in the Sea of Okhotsk over the last 30 kyr

Marine‐ and terrestrial‐derived biomarkers (alkenones, brassicasterol, dinosterol, and long‐chainn‐alkanes), as well as carbonate, biogenic opal, and ice‐rafted debris (IRD), were measured in two sediment cores in the Sea of Okhotsk, which is located in the northwestern Pacific rim and characterized by high primary productivity. Down‐core profiles of phytoplankton markers suggest that primary productivity abruptly increased during the global Meltwater Pulse events 1A (about 14 ka) and 1B (about 11 ka) and stayed high in the Holocene. Spatial and temporal distributions of the phytoplankton productivity were found to be consistent with changes in the reconstructed sea ice distribution on the basis of the IRD. This demonstrates that the progress and retreat of sea ice regulated primary productivity in the Sea of Okhotsk with minimum productivity during the glacial period. The mass accumulation rates of alkenones, CaCO3, and biogenic opal indicate that the dominant phytoplankton species during deglaciation was the coccolithophorid,Emiliania huxleyi, which was replaced by diatoms in the late Holocene. Such a phytoplankton succession was probably caused by an increase in silicate supply to the euphotic layer, possibly associated with a change in surface hydrography and/or linked to enhanced upwelling of North Pacific Deep Water.

Deep-Water Flow in the Mariana and Caroline Basins*

Abstract Two major water masses dominate the deep layers in the Mariana and Caroline Basins: the Lower Circumpolar Water (LCPW), arriving from the Southern Ocean along the slopes north of the Marshall Islands, and the North Pacific Deep Water (NPDW) reaching the region from the northeastern Pacific Ocean. Hydrographic and moored observations and multibeam echosounding were performed in the East Mariana and the East Caroline Basins to detail watermass distributions and flow paths in the area. The LCPW enters the East Mariana Basin from the east. At about 13°N, however, in the southern part of the basin, a part of this water mass arrives in a southward western boundary flow along the Izu–Ogasawara–Mariana Ridge. Both hydrographic observations and moored current measurements lead to the conclusion that this water not only continues westward to the West Mariana Basin as suggested before, but also provides bottom water to the East Caroline Basin. The critical throughflow regions were identified by multibeam echosounding at the Yap Mariana Junction between the East and West Mariana Basins and at the Caroline Ridge between the East Mariana and East Caroline Basins. The throughflow is steady between the East and West Mariana Basins, whereas more variability is found at the Caroline Ridge. At both locations, throughflow fluctuations are correlated with watermass property variations suggesting layer-thickness changes. The total transport to the two neighboring basins is only about 1 Sverdrup (1Sv ≡ 106 m3 s−1) but has considerable impact on the watermass structure in these basins. Estimates are given for the diapycnal mixing that is required to balance the inflow into the East Caroline Basin. Farther above in the water column, the high-silica tongue of NPDW extends from the east to the far southwestern corner of the East Mariana Basin, with transports being mostly southward across the basin.

Neodymium isotopic variations in Northwest Pacific waters

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.

Radium redux in the Dead Sea: profiles and transient Ra/Ba models

Two radium-226 profiles were measured on 31 Dead Sea water samples collected in February, 1978 before the 1979 overturn. These two profiles, located in the central Northern Basin, are practically identical and show a distinct two-layer feature. The Upper Water Mass (UWM) has a uniform Ra activity at 114.5 dpm/kg and the Lower Water Mass (LWM) has a fairly constant value at 97.8 dpm/kg, being separated by a 25 m transition layer (150–175 m depth). Radon measurements in the LWM indicate that there is no radon in excess over radium except for the very bottom samples at 300 m depth. These Ra activity levels are at least two orders of magnitude greater than that of the Pacific deep water. That the UWM has significantly higher Ra activity than the LWM is a unique feature, suggesting major inputs into the UWM with very limited mixing between the two water masses. This feature was used to estimate duration of the meromixis since the last overturn based on Ra decay in a closed system [Stiller and Chung, Limnol. Oceanogr. 29 (1984) 574–586]. A first-order removal rate or particulate scavenging process was invoked later to interpret the observed barium and radium data [Chan and Chung, Earth Planet. Sci. Lett. 85 (1987) 41–53]. These two studies assumed that no mixing occurred between the two water masses and the LWM was thus in a closed system. In this study we apply a transient box model in a closed system with varying scavenging rates of Ra and Ba to evaluate the isolation time since the previous overturn based on the Ra and Ba data measured in 1978 and 1979 as reported in this paper and in Chan and Chung [Earth Planet. Sci. Lett. 85 (1987) 41–53]. We also evaluate the fluid residence time of the LWM with respect to mixing and exchange with the UWM, τ m, which characterizes the mixing rate between the two water masses in an open system with particulate scavenging, assuming that the 1978 data represent a steady-state condition and that the 1979 data represent the previous overturn condition. The transient Ra/Ba model in a closed system shows that the isolation time of the LWM is a linear function of the scavenging rate ratio of Ra to Ba with a maximum of 317 years as the ratio reaches zero and 207 years as the ratio becomes 1, and the scavenging residence time for Ba is about 20 times the isolation time at any scavenging rate ratio. The scavenging residence time for Ra increases exponentially as the scavenging rate ratio decreases. In the open-system steady-state model, τ m is also a linear function of the scavenging rate ratio with a maximum of 339 years as the ratio vanishes. The scavenging residence time for Ba is also about 20 times greater than τ m. Similarly, the scavenging residence time for Ra increases exponentially as the ratio decreases. Since the same set of data are used in these calculations, the isolation time in a closed system is quite comparable to but slightly less than τ m in an open system at any given scavenging rate ratio. Both the isolation time and τ m are quite sensitive to the changes of Ra and Ba values in the LWM and the overturned water. If the mean Ra value of the 1979 overturned water is increased by 2% and that of the 1978 LWM is decreased by 2% while the Ba values remain unchanged, this 4% deviation in Ra values between the overturned water and the LWM leads to an increase of 28–30% in the isolation time and τ m, respectively. In comparison with the earlier calculations [Stiller and Chung, Limnol. Oceanogr. 29 (1984) 574–586; Chan and Chung, Earth Planet. Sci. Lett. 85 (1987) 41–53], the isolation time calculated with no Ra scavenging is about 55 years longer, probably because the concentrations of Ba and Ra are used for the present calculations with no constraint on the inventories and the lake volume changes.

Dissolved selenium species in the Sulu Sea, the South China Sea and the Celebes Sea

We have measured the vertical profiles of selenite, selenate and organic selenide in the Southeast Asian Basins (the Celebes Sea, the Sulu Sea and the South China Sea) for the two years, 1996 and 2002. Total selenium, selenite and selenate showed nutrient-like profiles. In particular, the vertical profiles of selenate and organic selenide in the Sulu Sea differed greatly between 1996 and 2002. The salinity and selenite/selenate ratio in the surface layer showed that the Sulu Sea surface water in these two years was derived from different sources. The vertical profiles of selenite, selenate and organic selenide in the South China Sea in 1996 and 2002 were almost the same. Both selenite and selenate were slightly elevated at the surface in the South China Sea. These inorganic selenium species seemed to be derived from atmospheric input, admixture of the East China Sea surface water, West Philippine Sea surface water, and fluvial, coastal and shelf waters of the South China Sea. The relationship between the concentration of organic selenide and the relative fluorescent intensity of fluorescent organic matter in the South China Sea suggested the organic selenide exists in the refractory DOC fraction possibility as humic-like substances. The total dissolved selenium concentration and the selenite/selenate ratio in deep water indicated that the water mass age of the Celebes Sea deep water was similar to that of the western North Pacific deep water. On the other hand, the water mass age of the Sulu Sea is younger than that of the western North Pacific. The selenite/selenate ratio in the South China Sea deep water was two times higher than that of the western North Pacific. This result suggested that a large amount of organic detritus was supplied to the South China Sea, and then the organic selenide in the organic detritus rapidly oxidized to selenite.

Rheinheimera pacifica sp. nov., a novel halotolerant bacterium isolated from deep sea water of the Pacific.

An aerobic, Gram-negative, non-fermentative, rod-shaped, motile, non-pigmented bacterium, KMM 1406(T), was isolated from a sample of Pacific deep sea water and investigated for phenotypic characteristics, chemotaxonomic features and phylogenetic relationships. The deep-sea isolate exhibited growth in 0-8 % (w/v) NaCl and at 4-37 degrees C, hydrolytic activity on gelatin, Tween 80 and starch and lack of D-glucose utilization. The major fatty acids were C(16 : 0), C(16 : 1)omega9c, C(17 : 1)omega8c and C(18 : 1)omega7c. The DNA G+C content was 49.6 mol%. 16S rRNA gene sequence analysis revealed that strain KMM 1406(T) was related closely to Rheinheimera baltica DSM 14885(T) within the gamma-Proteobacteria, with 96.8 % sequence similarity. On the basis of phenotypic and molecular data, a novel species, Rheinheimera pacifica sp. nov., is proposed. The type strain is KMM 1406(T) (=IAM 15043(T)=JCM 12090(T)=NRIC 0539(T)=CCUG 46544(T)).

Data-Based Meridional Overturning Streamfunctions for the Global Ocean

The meridional overturning circulation for the Atlantic, Pacific, and Indian Oceans is computed from absolute geostrophic velocity estimates based on hydrographic data and from climatological Ekman transports. The Atlantic overturn includes the expected North Atlantic Deep Water formation (including Labrador Sea Water and Nordic Sea Overflow Water), with an amplitude of about 18 Sv through most of the Atlantic and an error of the order of 3–5 Sv (1 Sv ≡ 106 m3 s−1). The Lower Circumpolar Deep Water (Antarctic Bottom Water) flows north with about 8 Sv of upwelling and a southward return in the South Atlantic, and 6 Sv extending to and upwelling in the North Atlantic. The northward flow of 8 Sv in the upper layer in the Atlantic (sea surface through the Antarctic Intermediate Water) is transformed to lower density in the Tropics before losing buoyancy in the Gulf Stream and North Atlantic Current. The Pacific overturning streamfunction includes 10 Sv of Lower Circumpolar Deep Water flowing north into the South Pacific to upwell and return southward as Pacific Deep Water, and a North Pacific Intermediate Water cell of 2 Sv. The northern North Pacific has no active deep water formation at the sea surface, but in this analysis there is downwelling from the Antarctic Intermediate Water into the Pacific Deep Water, with upwelling in the Tropics. For global Southern Hemisphere overturn across 30°S, the overturning is separated into a deep and a shallow overturning cell. In the deep cell, 22–27 Sv of deep water flows southward and returns northward as bottom water. In the shallow cell, 9 Sv flows southward at low density and returns northward just above the intermediate water density. In all three oceans, the Tropics appear to dominate upwelling across isopycnals, including the migration of the deepest waters upward to the thermocline in the Indian and Pacific. Estimated diffusivities associated with this tropical upwelling are the same order of magnitude in all three oceans. It is shown that vertically varying diffusivity associated with topography can produce deep downwelling in the absence of external buoyancy loss. The rate of such downwelling for the northern North Pacific is estimated as 2 Sv at most, which is smaller than the questionable downwelling derived from the velocity analysis.

An additional deep-water mass in Drake Passage as revealed by 3He data

We present 3He data from a repeat section across Drake Passage, from three sections off the South American continent in the Pacific, at 28°S, 35°S, and 43°S, and from three sections in the Atlantic, eastward of the Malvinas, close to 35°W, and near the Greenwich Meridian. In Drake Passage, a distinct high- 3He signal is observed that is centered just above the boundary of the Lower and the Upper Circumpolar Deep Water (LCDW, UCDW), and is concentrated towards the northern continental slope. 3He concentrations in the Antarctic Circumpolar Current (ACC) upstream of Drake Passage (World Ocean Circulation Experiment section P19 at 88°W) are markedly lower than those found in Drake Passage, and a regional source of primordial helium in the path of the ACC that might cause the high- 3He feature can be ruled out. We explain the feature by addition of high- 3He waters present at the 43°S Pacific section. This supports a previous, similar interpretation of a low-salinity anomaly in Drake Passage (Naveira Garabato et al., Deep-Sea Research I 49 (2002) 681), that is strongly related to the high- 3He feature. Employing multiparameter water mass analysis (including 3He as a parameter), we find that deep waters as met at the 43°S Pacific section, flowing south along the South American continental slope, contribute substantially to the ACC waters in Drake Passage (fractions exceed 50% locally). Lesser, but laterally more extended contributions are found east of the Malvinas, and still smaller ones are present at 35°W and at the Greenwich Meridian. Using velocity measurements from one of the two Drake Passage sections, we estimate the volume transport of these waters to be 7.0±1.2 Sv, but the average transport may be somewhat lower as the other realization had a less pronounced signal. The enhanced 3He signature in Drake Passage is essentially confined north of the Polar Front. Further downstream the signature crosses this front, to the extent that at 35°W the contributions south and north of it are of similar magnitude. At the same time, the 3He levels north of the front are reduced due to a substantial admixture of low- 3He North Atlantic Deep Water, such that 3He becomes highest south of the front. The flow of Southeast Pacific deep slope waters entering the ACC constitutes the predominant exit pathway of the primordial helium released in the deep Pacific, and represents a considerable fraction of the deep water return flow from the Pacific into the ACC. Therefore and also because the density range of the added deep slope waters is intermediate between those of UCDW and LCDW, they must be considered a distinct water mass.

Neon distribution in South Atlantic and South Pacific waters

We present and assess the distribution of neon in South Atlantic and South Pacific waters, on the basis of more than 3000 mostly new neon data which were obtained primarily under the hydrographic program of the World Ocean Circulation Experiment (predominantly southern summer to fall cruises). Data precision is better than±0.5%, and the set is internally consistent within±0.3% and partly better, and compatible with reported high-quality neon values. Using suitably averaged data (precision 0.1–0.3%), we find that the total range of neon anomalies relative to a solubility equilibrium with atmospheric neon at the observed potential temperature and salinity (using the solubilities of Weiss, J. Chem. Eng. Data 16 (1971) 235) is approximately 0–4%, and below 2000 m depth, 3–4% only. We consistently observe two types of neon depth profiles, one for the temperate-latitudes ocean, which is characterized by a near-surface maximum and a minimum in Antarctic Intermediate Water, and one for the Southern Ocean that essentially displays a steady increase with depth. The neon distribution reflects the influence of air injected by submerged air bubbles, the areal distribution of atmospheric pressure, seasonal temperature changes in the mixed layer and solar heating below it, and interaction with sea ice and glacial ice, largely in keeping with previous work. However, it appears that interaction with sea ice reduces neon anomalies distinctly less than the literature suggests. The temperate-ocean shallow maxima point to widespread subsurface heating in the course of the summer season by roughly 1 K. Among the major source water masses of the deep waters, the neon anomalies are lowest in Antarctic Intermediate Water (∼1.5%), intermediate in North Atlantic Deep Water (∼3%, confirming previous work) and similarly in Circumpolar Deep Water, and highest in Antarctic Bottom Water (∼3.8%). The anomalies in Southeast Pacific deep waters (>2500 m) are comparatively less (only∼3.3%), as a result of the contribution of Antarctic Intermediate Water. The present study is the first attempt to deal with the oceanic distribution of neon in a systematic fashion. The results can serve to assist assessments of the oceanic distributions of other dissolved gases.

Deep-water circulation at low latitudes in the western North Pacific

Full-depth conductivity-temperature-depth-oxygen profiler (CTDO 2) data at low latitudes in the western North Pacific in winter 1999 were analyzed with water-mass analysis and geostrophic calculations. The result shows that the deep circulation carrying the Lower Circumpolar Water (LCPW) bifurcates into eastern and western branch currents after entering the Central Pacific Basin. LCPW colder than 0.98°C is carried by the eastern branch current, while warmer LCPW is carried mainly by the western branch current. The eastern branch current flows northward in the Central Pacific Basin, supplying water above 0.94°C through narrow gaps into an isolated deep valley in the Melanesian Basin, and then passes the Mid-Pacific Seamounts between 162°10′E and 170°10′E at 18°20′N, not only through the Wake Island Passage but also through the western passages. Except near bottom, dissolved oxygen of LCPW decreases greatly in the northern Central Pacific Basin, probably by mixing with the North Pacific Deep Water (NPDW). The western branch current flows northwestward over the lower Solomon Rise in the Melanesian Basin and proceeds westward between 10°40′N and 12°20′N at 150°E in the East Mariana Basin with volume transport of 4.1 Sv (1 Sv=10 6 m 3 s −1). The current turns north, west of 150°E, and bifurcates around 14°N, south of the Magellan Seamounts, where dissolved oxygen decreases sharply by mixing with NPDW. Half of the current turns east, crosses 150°E at 14–15°N, and proceeds northward primarily between 152°E and 156°E at 18°20′N toward the Northwest Pacific Basin (2.1 Sv). The other half flows northward west of 150°E and passes 18°20′N just east of the Mariana Trench (2.2 Sv). It is reversed by a block of topography, proceeds southward along the Mariana Trench, then detours around the south end of the trench, and proceeds eastward along the Caroline Seamounts to the Solomon Rise, partly flowing into the West Mariana and East Caroline Basins. A deep western boundary current at 2000–3000 m depth above LCPW (10.0 Sv) closes to the coast than the deep circulation. The major part of it (8.5 Sv) turns cyclonic around the upper Solomon Rise from the Melanesian Basin and proceeds along the southern boundary of the East Caroline Basin. Nearly half of it proceeds northward in the western East Caroline Basin, joins the current from the east, then passes the northern channel, and mostly enters the West Caroline Basin (4.6 Sv), while another half enters this basin from the southern side (>3.8 Sv). The remaining western boundary current (1.5 Sv) flows over the middle and lower Solomon Rise, proceeds westward, then is divided by the Caroline Seamounts into southern (0.9 Sv) and northern (0.5 Sv) branches. The southern branch current joins that from the south in the East Caroline Basin, as noted above. The northern branch current proceeds along the Caroline Seamounts and enters the West Mariana Basin.

Lead isotopes in North Pacific deep water – implications for past changes in input sources and circulation patterns

The sources of non-anthropogenic Pb in seawater have been the subject of debate. Here we present Pb isotope time-series that indicate that the non-anthropogenic Pb budget of the northernmost Pacific Ocean has been governed by ocean circulation and riverine inputs, which in turn have ultimately been controlled by tectonic processes. Despite the fact that the investigated locations are situated within the Asian dust plume, and proximal to extensive arc volcanism, eolian contributions have had little impact. We have obtained the first high-resolution and high-precision Pb isotope time-series of North Pacific deep water from two ferromanganese crusts from the Gulf of Alaska in the NE Pacific Ocean, and from the Detroit Seamount in the NW Pacific Ocean. Both crusts were dated applying 10Be/ 9Be ratios and yield continuous time-series for the past 13.5 and 9.6 Myr, respectively. Lead isotopes show a monotonic evolution in 206Pb/ 204Pb from low values in the Miocene (≤18.57) to high values at present day (≥18.84) in both crusts, even though they are separated by more than 3000 km along the Aleutian Arc. The variation exceeds the amplitude found in Equatorial Pacific deep water records by about three-fold. There also is a striking similarity in 207Pb/ 204Pb and 208Pb/ 204Pb ratios of the two crusts, indicating the existence of a local circulation cell in the sub-polar North Pacific, where efficient lateral mixing has taken place but only limited exchange (in terms of Pb) with deep water from the Equatorial Pacific has occurred. Both crusts display well-defined trends with age in Pb–Pb isotope mixing plots, which require the involvement of at least four distinct Pb sources for North Pacific deep water. The Pb isotope time-series reveal that eolian supplies (volcanic ash and continent-derived loess) have only been of minor importance for the dissolved Pb budget of marginal sites in the deep North Pacific over the past 6 Myr. The two predominant sources have been young volcanic arcs, one located in the northeastern part and one located in the northwestern part of the Pacific margin, from where material has been eroded and delivered to the ocean, most likely via riverine pathways.

Pliocene–Pleistocene stable isotope and paleoceanographic changes in the northern South China Sea

Based on the stable isotopic analysis of planktonic and benthic foraminifers from Ocean Drilling Program Core 1148 of the northern South China Sea (SCS), Pliocene–Pleistocene isotope stratigraphy and events have been reconstructed. The benthic foraminiferal δ 18O record shows that the Pacific intermediate water had a greater influence upon the SCS or the Pacific deep water above ∼2600 m was warmer before ∼3.2 Ma than at present. After that, the benthic δ 18O conspicuously increased during the ∼3.2–2.5 Ma period, in correspondence to the formation of the Northern Hemisphere ice sheet, whereas the planktonic δ 18O signal suggests a stepwise overall decrease of sea surface temperature during the ∼2.2–0.9 Ma period. Compared to the equatorial Pacific records, the decrease in planktonic ( Globigerinoides ruber) δ 13C during the ∼3.2–2.2 Ma period is particularly striking, suggesting that fertility of surface water increased noticeably. According to the modern δ 13C distribution of G. ruber in the northern SCS, it is inferred that the East Asian winter monsoon strengthened during this interval. Afterwards, there were several conspicuous decreases of G. ruber δ 13C at ∼1.7, 1.3, 0.9, 0.45 and 0.15 Ma BP, that is, about every 0.4 Ma, suggesting that the East Asian winter monsoon became episodically stronger. This is confirmed by changes in relative abundance of planktonic foraminifer species Neogloboquadrina dutertrei, a typical East Asian winter monsoon proxy. The deep-water δ 13C of the SCS is close to that of the Pacific, but lighter than that of the Atlantic, implying that the pattern of deep water originating mainly from the Atlantic and through the Pacific entering the SCS existed at least since the early Pliocene. After 1.4 Ma, the benthic δ 13C signal decreased conspicuously but with a periodicity of ∼100 ka, suggesting that the deep-water ventilation of the SCS was reduced, probably corresponding to a decrease of the North Atlantic Deep Water and/or further isolation of the SCS deep basin from the Pacific during glaciations.