North Pacific Intermediate Water Research Articles

Annual variability of dissolved manganese in Northeast Pacific along Line-P: 2010–2013

The distribution of dissolved manganese (dMn) in the northeast Pacific across the Line-P transect was evaluated to investigate the mechanisms responsible for the spatial and temporal variability of dMn. A total of 299 filtered seawater samples (< 0.4 μm) collected in August at the five major stations each year from 2010 to 2013 were analyzed. Vertical profiles of dMn showed a clear distinction between onshore and offshore stations. Within the Summer Mixing layer (SML), we observed high dMn concentrations in all years driven by external sources, such as river water, coastal sediments or eolian dust, as well as photo-reduction. At the onshore stations, the absolute concentration of dMn at the surface was annually variable, depending on the strength of the Ekman transport. Within the subsurface layer, dMn decreased rapidly with depth down to 150 m due to particle scavenging. Within the Oxygen Minimum Zone (OMZ), near the continental margin, we observed elevated and annually variable dMn. The high dMn concentration is likely due to mixing of remobilized Mn from the continental margin, which varies year-to-year depending on the intensity of sedimentary Mn reduction process. At the offshore stations, dMn showed subsurface maxima, within the Winter Mixing Layer (WML) rather than surface maxima. We attribute this to the combined effect of biological drawdown of dMn in the surface during the spring and summer, where iron (Fe) is depleted in this High Nutrient and Low Chlorophyll (HNLC) region, and remnant dMn deeper in the WML, from earlier in the year. Using a simple one-dimensional advection and diffusion model, we identified that dMn found in North Pacific Intermediate Water (NPIW) at the western end of the transect can be advected as far as the outermost of the onshore stations in 2011 and 2013, while dMn in the central area of the transect is likely diluted by low dMn water from the south in 2010 and 2012. Within the OMZ, dMn showed elevated and annually variable concentrations, yet the enhancement of dMn of this study area was found to be less intense than in other regions. Lastly, an eddy found in 2010 enhanced dMn concentrations within the surface mixing layers.

The Bacterial Community in the Western Region of North Pacific Intermediate Water.

The bacterial composition in North Pacific Intermediate Water (NPIW) was investigated in three different years and compared with that in other seawaters around Japan. The results indicated that bacterial composition was surprisingly stable at the same point in a mesopelagic water mass throughout the years and supported previous physicochemical observations that NPIW is distributed to Kumejima, Japan.

Coupled climate and subarctic Pacific nutrient upwelling over the last 850,000 years

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.

Composition of Indonesia throughflow in North Sulawesi water masses and heat content

In this research the composition of water masses in the North Sulawesi (Sangihe Talaud) waters as part of the Indonesian Throughflow is investigated and analysed by using the Optimum Multi Parameter Analysis (OMP). The data used is the World Ocean Atlas data (WOA 13) in 1° x 1° latitude longitude resolution. Main data are the salinity, temperature and pressure. The Optimum Multiparameter analysis was used to calculate the percentage of each water mass contribution in the study waters. The contributions are 20-60% at 200-500 m depth for The North Pacific Intermediate Water (NPIW), 90-100%at around 0-100 meters depth for The North Pacific Subtropical Water (NPSW),100%at 900-1500 meters depth for the South Pacific Intermediate Water (SPIW)and 80-100% at 1100-1500 meters depth for TheAntartic Intermediate Water (AAIW). Surprisingly, the South Pacific Subtropical Water (SPSW) does not have any contribution and largest heat content comes from the AAIW. Magnitudes for each heat content of water types calculated in this study for NPIW, NPSW, AAIW, SPSW, and SPIW are in the range of 1.5 × 1014∼ 1.7 × 1014 J/m2, 2 × 1012∼ 9.9 × 1012 J/m2, 2.7 × 1014∼ 3.1 × 1014 J/m2, 0 ∼ 4.3 × 1011 J/m2, and 1.8 × 1014∼ 2 × 1014 J/m2, respectively. Eventhough the AAIW has the lowest temperature range but its large contribution in the waters outnumbers the warmer water masses.

Distinct basin-scale-distributions of aluminum, manganese, cobalt, and lead in the North Pacific Ocean

Abstract Aluminum (Al), manganese (Mn), cobalt (Co), and lead (Pb) are key trace elements in seawater and thus significant in chemical oceanography research. However, although all of these elements are highly scavenged in the ocean, only a few studies focus on the intercomparison of their distributions. Here, we report the basin-scale and full-depth sectional distributions of these elements observed during three GEOTRACES Japan cruises in the North Pacific. We confirmed that a surface maximum of the dissolved (d) species is not a common feature for the four elements and that the d species have the lowest concentrations in the Pacific Deep Water (PDW) as compared to other oceans. The elements showed different speciations and distributions. The fraction of labile particulate (lp) species was calculated as the difference between the total dissolvable (td) species and d species. The lpM/tdM ratio, where M refers to an element, is highest for Al, at 0.66 ± 0.31 (average ± sd, n = 489), and lowest for Pb, at 0.02 ± 0.08 (n = 575). Further, the distribution of each element is uniquely related to ocean circulation. The tdAl concentration is high in the Equatorial Under Current (EUC), the North Equatorial Current (NEC), and the Lower Circumpolar Deep Water (LCDW). Manganese is supplied from reductive sources such as sediments on the continental shelves around the northern boundary. Cobalt is concentrated in the North Pacific Intermediate Water (NPIW) and in the Equatorial Pacific Intermediate Water (EqPIW) owing to the combined effects of supply from the continental shelves, biogeochemical cycling, and scavenging. Lead shows a subsurface maximum centered at ∼35°N and ∼200 m depth, implying an association with the formation of the Subtropical Mode Water (SMW) and the Central Mode Water (CMW). Although the subsurface Pb maximum in the Atlantic has diminished over the last three decades owing to the ban on leaded gasoline use, it has been sustained in the North Pacific through the growth of other anthropogenic sources in Asia and Russia. We propose that the enrichment factor of dM, defined as EF(dM) = (dM/dAl)seawater/(M/Al)upper crust, where (M/Al)upper crust is the molar ratio in upper crustal abundance, can be a good parameter for the sources. The median is 1.3 × 102 (n = 436) for EF(dMn), 3.2 × 102 (n = 430) for EF(dCo), and 1.2 × 103 (n = 413) for EF(dPb). The EF(dPb) found in this study is on the same order of magnitude as the EF values for aerosols found in the literature, suggesting that the deposition of aerosols is a major source for dPb. Because EF(dMn) and EF(dCo) are ten to hundred times higher than the EF for aerosols, sources other than the aerosol deposition are more significant contributors to the concentrations of Mn and Co.

Enhanced North Pacific deep-ocean stratification by stronger intermediate water formation during Heinrich Stadial 1

The deglacial history of CO2 release from the deep North Pacific remains unresolved. This is due to conflicting indications about subarctic Pacific ventilation changes based on various marine proxies, especially for Heinrich Stadial 1 (HS-1) when a rapid atmospheric CO2 rise occurs. Here, we use a complex Earth System Model to investigate the deglacial North Pacific overturning and its control on ocean stratification. Our results show an enhanced intermediate-to-deep ocean stratification coeval with intensified North Pacific Intermediate Water (NPIW) formation during HS-1, compared to the Last Glacial Maximum. The stronger NPIW formation causes lower salinities and higher temperatures at intermediate depths. By lowering NPIW densities, this enlarges vertical density gradient and thus enhances intermediate-to-deep ocean stratification during HS-1. Physically, this process prevents the North Pacific deep waters from a better communication with the upper oceans, thus prolongs the existing isolation of glacial Pacific abyssal carbons during HS-1.

The Oyashio Nutrient Stream and Its Nutrient Transport to the Mixed Water Region

AbstractThe Oyashio, a subarctic western boundary current in the North Pacific, transports cool, fresh, and nutrient‐rich water equatorward. The Oyashio nutrient stream was assessed using long‐term mean hydrographic observation data. Its nitrate transport (about 350 kmol/s) is comparable to that of the Kuroshio but has a much shallower subsurface core at 250‐m depth. In the layer ranging from 26.6 to 27.4 σθ, the Oyashio nitrate transport to the mixed water region is about 110 kmol/s. This corresponds to the nitrate transport of the Oyashio to the North Pacific Intermediate Water in the mixed water region. Along with this process, the subarctic gyre nitrate export to the subtropic gyre reaches 24 kmol/s. This work first quantifies the Oyashio nutrient transport and its intrusion into the mixed water region and suggests its importance in local/basin‐scale nutrient cycles.

Plutonium in the western North Pacific: Transport along the Kuroshio and implication for the impact of Fukushima Daiichi Nuclear Power Plant accident

This study examined Pu source terms in the western North Pacific (WNP) based on data collected in 2014 and 2015. The basin wide 240Pu/239Pu atom ratios ranged from 0.227 to 0.263 with an average value of 0.244 ± 0.011, consistently higher than that of global fallout (~0.180). The spatial distribution of 240Pu/239Pu atom ratios showed higher values within the Kuroshio region, the main western ocean boundary current, as compared to the zone off of the Kuroshio. There was also an overall decreasing trend of 240Pu/239Pu along the Kuroshio path to its extensions. 239+240Pu activities in surface seawater exhibited a wide range from 1.15 to 4.30 mBq m−3 and their spatial distribution showed an increasing trend with latitude. Unlike the 240Pu/239Pu atom ratios, which had heavier isotopic compositions in the Kuroshio mainstream compared to the zone off of the Kuroshio, the 239+240Pu activities were higher outside the Kuroshio than within the Kuroshio. These patterns in both Pu isotopic ratios and activities point towards a unique close-in fallout source, which levels down in its source term and has a high degree of scavenging during its transport along the Kuroshio, and can be traced back to a precursor, the North Equatorial Current, which originates near the Pacific Proving Grounds (PPG) with characteristically higher 240Pu/239Pu atom ratios. High 240Pu/239Pu atom ratios found in the zone outside the Kuroshio were transported from the Kuroshio via the formation and circulation of North Pacific Intermediate Water. We further revealed, using a simple two end-member mixing model, that the PPG source contributed 60 ± 13% of the Pu in the Kuroshio zone and 45 ± 10% in the zone off of the Kuroshio. Both the comparison of Pu isotopic composition in the WNP within a radius of 500 km or 1000 km off the Fukushima Daiichi Nuclear Power Plant (FDNPP) between prior to and post the accident and a simple first order mass balance calculation in terms of atmospheric deposition and release suggest that the Pu originating from the FDNPP accident, if any, was either negligible, or the input flux of 239+240Pu was too small to significantly alter the Pu isotopic composition in the ambient seawater.This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Closure of the Bering Strait caused Mid-Pleistocene Transition cooling

The Mid-Pleistocene Transition (MPT) is characterised by cooling and lengthening glacial cycles from 600–1200 ka, thought to be driven by reductions in glacial CO2 in particular from ~900 ka onwards. Reduced high latitude upwelling, a process that retains CO2 within the deep ocean over glacials, could have aided drawdown but has so far not been constrained in either hemisphere over the MPT. Here, we find that reduced nutrient upwelling in the Bering Sea, and North Pacific Intermediate Water expansion, coincided with the MPT and became more persistent at ~900 ka. We propose reduced upwelling was controlled by expanding sea ice and North Pacific Intermediate Water formation, which may have been enhanced by closure of the Bering Strait. The regional extent of North Pacific Intermediate Water across the subarctic northwest Pacific would have contributed to lower atmospheric CO2 and global cooling during the MPT.

North-south difference of water mass properties across the Lembeh Strait, North Sulawesi, Indonesia

Two field observations were conducted around the Lembeh Strait in September 2015 and 2016, respectively. Evidences indicate that seawater around the Lembeh Strait is consisted of North Pacific Tropical Water (NPTW), North Pacific Intermediate Water (NPIW), North Pacific Tropical Intermediate Water (NPTIW) and Antarctic Intermediate Water (AAIW). Around the Lembeh Strait, there exist some north-south differences in terms of water mass properties. NPTIW is only found in the southern Lembeh Strait. Water mass with the salinity of 34.6 is only detected at 200–240 m between NPTW and NPTIW in the southern Lembeh Strait, and results from the process of mixing between the saltier water transported from the South Pacific Ocean and the lighter water from the North Pacific Ocean and Sulawesi Sea. According to the analysis on mixing layer depth, it is indicated that there exists an onshore surface current in the northern Lembeh Strait and the surface current in the Lembeh Strait is southward. These dramatic differences of water masses demonstrate that the less water exchange has been occurred between the north and south of Lembeh Strait. In 2015, the positive wind stress curl covering the northern Lembeh Strait induces the shoaling of thermocline and deepening of NPIW, which show that the north-south difference of airsea system is possible of inducing north-south differences of seawater properties.

Enhanced vertical turbulent nitrate flux in the Kuroshio across the Izu Ridge

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.

Population genetic structure of the deep-sea mussel Bathymodiolus platifrons (Bivalvia: Mytilidae) in the Northwest Pacific.

Studying population genetics of deep‐sea animals helps us understand their history of habitat colonization and population divergence. Here, we report a population genetic study of the deep‐sea mussel Bathymodiolus platifrons (Bivalvia: Mytilidae) widely distributed in chemosynthesis‐based ecosystems in the Northwest Pacific. Three mitochondrial genes (i.e., atp6, cox1, and nad4) and 6,398 genomewide single nucleotide polymorphisms (SNPs) were obtained from 110 individuals from four hydrothermal vents and two methane seeps. When using the three mitochondrial genes, nearly no genetic differentiation was detected for B. platifrons in the Northwest Pacific. Nevertheless, when using SNP datasets, all individuals in the South China Sea (SCS) and three individuals in Sagami Bay (SB) together formed one genetic cluster that was distinct from the remaining individuals. Such genetic divergence indicated a genetic barrier to gene flow between the SCS and the open Northwest Pacific, resulting in the co‐occurrence of two cryptic semi‐isolated lineages. When using 125 outlier SNPs identified focusing on individuals in the Okinawa Trough (OT) and SB, a minor genetic subdivision was detected between individuals in the southern OT (S‐OT) and those in the middle OT (M‐OT) and SB. This result indicated that, although under the influence of the Kuroshio Current and the North Pacific Intermediate Water, subtle geographic barriers may exist between the S‐OT and the M‐OT. Introgression analyses based on these outlier SNPs revealed that Hatoma Knoll in the S‐OT represents a possible contact zone for individuals in the OT‐SB region. Furthermore, migration dynamic analyses uncovered stronger gene flow from Dai‐yon Yonaguni Knoll in the S‐OT to the other local populations, compared to the reverse directions. Taken together, the present study offered novel perspectives on the genetic connectivity of B. platifrons mussels, revealing the potential interaction of ocean currents and geographic barriers with adaption and reproductive isolation in shaping their migration patterns and genetic differentiation in the Northwest Pacific.

Changes in Deep Water Oxygenation of the South China Sea Since the Last Glacial Period

AbstractThe Pacific meridional overturning circulation is thought to have a significant influence on global climate. However, the extent to which intermediate and deep circulations have changed in the Pacific Ocean since the Last Glacial Maximum (LGM) is not well known. At present, the South China Sea Deep Water (SCSDW) is fed by the upper Pacific Deep Water. Here we present new benthic foraminiferal δ13C and redox‐sensitive elemental data from a sediment core retrieved from the southern deep SCS to reconstruct the oxygenation history of the SCSDW since the LGM. Oxygenation records from the deep SCS and intermediate and deep waters in the Pacific Ocean demonstrate that the SCSDW deeper than 1,600 m has been sourced by the Pacific Deep Water since the LGM. Our data suggest that the well‐ventilated North Pacific Intermediate Water would not have influenced the SCSDW during cold stadials of the last deglacial period.

Spatial Distribution of Diffusivity Coefficients and the Effects on Water Mass Modification in the North Pacific

AbstractThree‐dimensional velocity, diapycnal mixing, and isopycnal mixing fields were estimated based on long‐term mean (2001–2015) salinity distributions obtained with Argo float arrays on neutral‐density (γ) surfaces. Strong estimated isopycnal mixings were associated with the Kuroshio Extension, subtropical boundary, and the west coast of North America, and strong estimated diapycnal mixing was associated with the Izu‐Ogasawara and Hawaiian ridges, which are internal wave‐generation spots. Patterns of the mixing coefficient distributions were consistent with previous estimates by other methods. We used the mixing coefficient fields to reveal the spatial patterns of volume transports and salinity budgets on isopycnal layers. Whereas relatively large eddy transports associated with thickness diffusivity contributed to the spatial patterns of volume transports, the contributions of spatial changes in diapycnal mixing were small. Spatial changes of diapycnal mixing strongly affected those of salinity, especially in the layers around North Pacific Intermediate Water (26.5–27.0γ and 27.0–27.5γ), whereas spatial changes of isopycnal mixing were important for water mass modification in all layers. This analysis suggests that the clear salinity minimum formed regionally along the Kuroshio Extension decays because of locally strong diapycnal mixing as well as isopycnal mixing. Although the results support the utility of using the spatial distributions of water properties to infer water mass modifications, assessments based on direct measurements of mixing and fine‐scale parameterization are needed to clarify the localized effects of mixing on water mass modifications because the inversion errors in this study were not small enough to enable detection of high‐resolution spatial changes.

Rapid shift and millennial-scale variations in Holocene North Pacific Intermediate Water ventilation

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.

Deep Hydrography of the South China Sea and Deep Water Circulation in the Pacific Since the Last Glacial Maximum

AbstractThe oxygen (δ18O) and carbon (δ13C) isotopic compositions of benthic foraminifer Cibicidoides wuellerstorfi as reliable proxies of deep water properties were compiled from 50 core‐top and down‐core sites in the South China Sea (SCS) for two time slices of the late Holocene and Last Glacial Maximum (LGM) to reconstruct the glacial deep hydrographic structure. The bathymetric profiles of both δ18O and δ13C in the SCS show similar trends between the LGM and the late Holocene, but the δ18O gradients between intermediate (∼500–1,000 m) and deep (&gt;1,500 m) waters obviously increased during the LGM, indicating an intensified bathyal stratification and weakened vertical mixing between intermediate and deep waters in the SCS during the glacial period. A spatial comparison of bathymetric δ18O and δ13C profiles was also made for the Southern Ocean (Indo‐Pacific sector) and southwest Pacific, western equatorial Pacific (i.e., Ontong Java Plateau), off‐Japan margin as well as the SCS. The meridional δ13C profiles indicate a decreasing trend of δ13C (“aging effect”) in the deep layer (∼1,200–2,600 m) from south to north, reflecting a northward flow pathway of deep waters from the southwest Pacific (upstream) to the low‐latitude northwest Pacific and the SCS (downstream) during the LGM. The gradients of deep water δ18O (∼1,200–2,600 m) between these upstream and downstream regions were higher during the LGM relative to the Holocene, implying that a greater contribution of the northern‐sourced North Pacific Intermediate Water (NPIW) with a relatively low δ18O signature to the deep waters in the low‐latitude Pacific during the LGM.

Deglacial upwelling, productivity and CO2 outgassing in the North Pacific Ocean

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 Bolling–Allerod 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 Bolling–Allerod, 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 Bolling–Allerod and contributed to the deglacial CO2 rise.

Sea ice dynamics across the Mid-Pleistocene transition in the Bering Sea

Sea ice and associated feedback mechanisms play an important role for both long- and short-term climate change. Our ability to predict future sea ice extent, however, hinges on a greater understanding of past sea ice dynamics. Here we investigate sea ice changes in the eastern Bering Sea prior to, across, and after the Mid-Pleistocene transition (MPT). The sea ice record, based on the Arctic sea ice biomarker IP25 and related open water proxies from the International Ocean Discovery Program Site U1343, shows a substantial increase in sea ice extent across the MPT. The occurrence of late-glacial/deglacial sea ice maxima are consistent with sea ice/land ice hysteresis and land−glacier retreat via the temperature−precipitation feedback. We also identify interactions of sea ice with phytoplankton growth and ocean circulation patterns, which have important implications for glacial North Pacific Intermediate Water formation and potentially North Pacific abyssal carbon storage.

Trace element (Mn, Zn, Ni, V) and authigenic uranium (aU) geochemistry reveal sedimentary redox history on the Juan de Fuca Ridge, North Pacific Ocean

Changes in meridional overturning circulation and water mass chemistry can be recorded by oxygen concentrations in the deep ocean. Because the deep Pacific is the largest ocean reservoir, its oxygen concentrations may be related to global climate change. In this study, oxygen conditions in the past are reconstructed by contrasting the sedimentary geochemistry of multiple redox-sensitive trace elements (Mn, Ni, Zn, V corrected for terrigenous and hydrothermal inputs) and authigenic U (aU) from six sediment cores on the Juan de Fuca Ridge from 2.7-2.8 km depth. We find that Mn and Ni are indicators for oxygen-rich conditions, while Zn, V, and aU are indicators for oxygen-poor conditions. Relative Redox Potentials (RRPs) for each core are calculated by converting excess metal fluxes into binary presence/absence designations, weighting each element by the strength and direction of its redox indication, summing the five elements, and then averaging the data in 5kyr bins. Metal depositional histories from all six cores demonstrate low oxygen conditions during interglacial periods, particularly during 100–120 ka (MIS5) but also 200–250 ka (MIS7), and high oxygen conditions during glacial periods (MIS2-4 and MIS6). This redox pattern does not appear to be driven by organic matter flux to the sediment, as reconstructed by three different paleo-productivity proxies (organic carbon, opal, and excess barium). Instead higher oxygen concentrations on the Juan de Fuca Ridge may be a result of better ventilation during glacial periods, possibly due to enhanced North Pacific Intermediate Water formation. Alternatively, sedimentary redox conditions on the Juan de Fuca Ridge may be locally controlled by the deposition of hydrothermal sulfides from nearby vent fields.

Rare earth element distributions in the West Pacific: Trace element sources and conservative vs. non-conservative behavior

Recent studies suggest that transport and water mass mixing may play a dominant role in controlling the distribution of dissolved rare earth element concentrations ([REE]) at least in parts of the North and South Atlantic and the Pacific Southern Ocean. Here we report vertically and spatially high-resolution profiles of dissolved REE concentrations ([REE]) along a NW–SE transect in the West Pacific and examine the processes affecting the [REE] distributions in this area. Surface water REE patterns reveal sources of trace element (TE) input near South Korea and in the tropical equatorial West Pacific. Positive europium anomalies and middle REE enrichments in surface and subsurface waters are indicative of TE input from volcanic islands and fingerprint in detail small-scale equatorial zonal eastward transport of TEs to the iron-limited tropical East Pacific. The low [REE] of North and South Pacific Tropical Waters and Antarctic Intermediate Water are a long-range (i.e., preformed) laterally advected signal, whereas increasing [REE] with depth within North Pacific Intermediate Water result from release from particles. Optimum multiparameter analysis of deep to bottom waters indicates a dominant control of lateral transport and mixing on [REE] at the depth of Lower Circumpolar Deep Water (≥3000 m water depth; ∼75–100% explained by water mass mixing), allowing the northward tracing of LCDW to ∼28°N in the Northwest Pacific. In contrast, scavenging in the hydrothermal plumes of the Lau Basin and Tonga-Fiji area at 1500–2000 m water depth leads to [REE] deficits (∼40–60% removal) and marked REE fractionation in the tropical West Pacific. Overall, our data provide evidence for active trace element input both near South Korea and Papua New Guinea, and for a strong lateral transport component in the distribution of dissolved REEs in large parts of the West Pacific.