Upper ocean vertical supply: A neglected primary factor controlling the distribution of neodymium concentrations of open ocean surface waters?

Neodymium (Nd) isotopes are an important geochemical tool to trace the present and 
past water mass mixing as well as continental inputs. The distribution of Nd concentrations 
in open ocean surface waters (0–100 m) is generally assumed to be controlled by lateral 
mixing of Nd from coastal surface currents and by removal through reversible particle 
scavenging. However, using 228Ra activity as an indicator of coastal water mass influence, 
surface water Nd concentration data available on key oceanic transects as a whole do not 
support the above scenario. From a global compilation of available data, we find that more 
stratified regions are generally associated with low surface Nd concentrations. This implies 
that upper ocean vertical supply may be an as yet neglected primary factor in determining 
the basin-scale variations of surface water Nd concentrations. Similar to the mechanism of 
nutrients supply, it is likely that stratification inhibits vertical supply of Nd from the 
subsurface thermocline waters and thus the magnitude of Nd flux to the surface layer. 
Consistently, the estimated required input flux of Nd to the surface layer to maintain the 
observed concentrations could be nearly two orders of magnitudes larger than riverine/dust 
flux, and also larger than the model-based estimation on shelf-derived coastal flux. In 
addition, preliminary results from modeling experiments reveal that the input from shallow 
boundary sources, riverine input, and release from dust are actually not the primary factors 
controlling Nd concentrations most notably in the Pacific and Southern Ocean surface 
waters.

Related papers:

title: Long-lived Isotopic Tracers in Oceanography, Paleoceanography, and Ice-sheet Dynamics
abstract: This chapter focuses on the basis for using authigenic neodymium, lead, and hafnium isotopes in oceanography and paleoceanography. In particular, it reviews in detail the currently available data on neodymium isotopes in the oceans in order to evaluate its strengths and weaknesses as an oceanographic tracer and a proxy to investigate paleocirculation. Neodymium isotope ratios are highlighted because lead isotopes in the present day oceans are contaminated by anthropogenic input, and dissolved hafnium thus far has not been measured. This chapter only gives a cursory summary of the results of studies on Fe–Mn crusts. This chapter also summarizes the application of strontium, neodymium, and lead isotopes for tracing the sources of continental detritus brought to the oceans by icebergs and implications for the history of the North Atlantic ice sheets.

title: Neodymium isotopes as a new tool for quantifying exchange fluxes at the continent-ocean interface
abstract: Continental margins are, via river sediment discharges, the major source of a number of elements to the ocean. They are also, for several reactive elements, sites of preferential removal from the water column, due to enhanced scavenging  [M.P. Bacon, Tracers of chemical scavenging in the ocean: Boundary effects and large-scale chemical fractionation, Philos.  A 325 (1988) 147–160.]. They can therefore be understood as sources of elements for the ocean, sinks or both. Although exchanges of matter are suspected to occur at the continent/ocean interface  [P.H. Santschi, L. Guo, I.D. Walsh, M.S. Quigley, M. Baskaran, Boundary exchange and scavenging of radionuclides in continental margin waters of the Middle Atlantic Bight: implications for organic carbon fluxes, Cont. Shelf Res. 19 (1999) 609–636.] and despite their probable importance for the ocean chemistry, closed budgets have still yet to be determined. Here, based on neodymium isotopic composition data obtained during the past 6 yr, we document and quantify significant neodymium exchange at ocean boundaries, in areas covering a large spectra of hydrographical, biological and geochemical characteristics : Eastern Indian Ocean, Western Equatorial Pacific, Western Tropical Pacific and Northwestern Atlantic, with neodymium removal fluxes accounting for 74F23%, 100F38%, 62F54% and 84F45% of the neodymium input fluxes, respectively. Recognition of boundary exchange and its potential globalization have important implications for (1) our understanding of margin/ocean interactions and their influence on the oceanic isotopic chemistry, and (2) geochemical cycling of reactive elements (including pollutants) at ocean margins.

title: Modeling the neodymium isotopic composition with a global ocean circulation model
abstract: For the first time, modeling of the neodymium (Nd) isotopic composition (expressed as eNd) is performed in an Ocean Global Circulation Model (OPA-ORCA2.0 model). In the interest of simplicity, Nd concentration is not explicitly included and eNd is modeled as a passive tracer. Recent studies suggest that dissolved/particulate exchanges between continental margin sediments and seawater (termed boundary exchange, BE), could be the dominant source-sink terms that determine the distribution of neodymium isotopes in the global ocean. To test this hypothesis, we only included this BE source-sink term in the model. BE was parameterized as a relaxing term that forces eNd of seawater to the isotopic composition of the continental margin over a characteristic exchange time. Sensitivity tests were made using different exchange times and parameterizations. The model reproduces most of the general trends of eNd composition in seawater at global scale (inter-basin gradient, composition of some of the main water masses). These results suggest that BE process may account for the major sources and sinks of oceanic Nd, while indicating that the characteristic time of interaction between seawater and margins varies from about half a year in surface waters to several years at depth. The disparities between the model and observations might reflect the typical shortcomings of utilizing a coarse resolution ocean model circulation fields or neglected sources of neodymium such as dissolution of aeolian inputs or dissolved river loads.

title: Reconstructing the Nd oceanic cycle using a coupled dynamical – biogeochemical model
abstract: The decoupled behaviour observed between Nd isotopic composition (Nd IC, also referred as eNd) and Nd concentration cycles has led to the notion of a "Nd paradox". While eNd behaves in a quasi-conservative way in the open ocean, leading to its broad use as a water-mass tracer, Nd concentration displays vertical profiles that increase with depth, together with a deep-water enrichment along the global thermohaline circulation. This non-conservative behaviour is typical of nutrients affected by scavenging in surface waters and remineralisation at depth. In addition, recent studies suggest the only way to reconcile both concentration and Nd IC oceanic budgets, is to invoke a "Boundary Exchange" process (BE, defined as the co-occurrence of transfer of elements from the margin to the sea with removal of elements from the sea by Boundary Scavenging) as a source-sink term. However, these studies do not simulate the input/output fluxes of Nd to the ocean, and therefore prevents from crucial information that limits our understanding of Nd decoupling. To investigate this paradox on a global scale, this study uses for the first time a fully prognostic coupled dynamical/biogeochemical model with an explicit representation of Nd sources and sinks to simulate the Nd oceanic cycle. Sources considered include dissolved river fluxes, atmospheric dusts and margin sediment re-dissolution. Sinks are scavenging by settling particles. This model simulates the global features of the Nd oceanic cycle well, and produces a realistic distribution of Nd concentration (correct order of magnitude, increase with depth and along the conveyor belt, 65% of the simulated values fit in the ±10 pmol/kg envelop when compared to the data) and isotopic composition (inter-basin gradient, characterization of the main water-masses, more than 70% of the simulated values fit in the ±3 eNd envelop when compared to the data), though a slight overestimation of Nd concentrations in the deep Pacific Ocean may reveal an underestimation of the particle fields by the biogeochemical model. Our results indicate 1) vertical cycling (scavenging/remineralisation) is absolutely necessary to simulate both concentration and eNd, and 2) BE is the dominant Nd source to the ocean. The estimated BE flux (1.1×1010 g(Nd)/yr) is much higher than both dissolved river discharge (2.6×108 g(Nd)/yr) and atmospheric inputs (1.0×108 g(Nd)/yr) that both play negligible role in the water column but are necessary to reconcile Nd IC in surface and subsurface waters. This leads to a new calculated residence time of 360 yrs for Nd in the ocean. The BE flux requires the dissolution of 3 to 5% of the annual flux of continental weathering deposited via the solid river discharge to the continental margin.

title: Modeling the distribution of Nd isotopes in the oceans using an ocean general circulation model
abstract: Nd isotopic compositions of authigenic phases in marine archives are increasingly used as a tracer of past changes in ocean circulation. Such applications assume that water masses are “tagged” with distinct Nd isotope compositions in source regions and that away from these regions the Nd isotope ratios observed in deep waters mainly reflect water mass mixing. However, the degree to which the simple assumption of “quasi-conservative mixing” reflects the complicated reality of general circulation in the oceans is not well understood. 
 
Here, we present numerical simulations of the Nd isotope distribution in seawater using an offline ocean general circulation model (OGCM), treating the Nd isotopic composition of seawater as a conservative tracer. Nd isotope data from modern surface waters are used to generate an interpolated map of Nd isotope compositions for the entire surface ocean. This map is treated as a fixed surface boundary condition that is propagated into the interior ocean by the advective–diffusive transport of the ocean model until a steady state is reached. This treatment of Nd isotopes was chosen because it mimics the behavior of well-known and well-understood conservative tracers, such as potential temperature and salinity. The model results for all non-surface grid points are then compared to published data. 
 
This simple approach produces Nd isotope estimates for North Atlantic Deep Water that are consistent with observations, but produces values lower than observed in the deep Pacific and Southern Oceans, indicating the need for a source of radiogenic Nd directly to the deep Pacific Ocean. Sensitivity tests show this to be a robust result. By fixing the return flow of the Pacific Ocean to the Southern Ocean with Nd isotope ratios that are consistent with modern observations, the model results are brought into close agreement with a majority of the available data. With the addition of a deep Pacific source of radiogenic Nd, more than half of the simulated values from the interior ocean fall within one e-unit of the corresponding observations. Our results thus indicate that the deep Pacific Ocean requires a source of radiogenic Nd in addition to inputs to the surface ocean, and that when the deep waters of the North Atlantic and North Pacific are set with realistic values, a majority of intermediate areas are consistent with quasi-conservative mixing of these two end-members.

title: Towards explaining the Nd paradox using reversible scavenging in an ocean general circulation model
abstract: The isotopic composition of the rare earth element neodymium (Nd) has the potential to serve as water-mass tracer, because it is naturally tagged by continental sources with distinct ages and lithologies. However, in order to understand the limitations of this approach we need to know more about the physical and biogeochemical processes controlling the distribution of Nd in the modern ocean. For example, Nd isotope ratios behave quasi-conservatively, while concentrations in the water column generally increase with depth, showing a broadly nutrient-like behaviour. We define this decoupling of Nd concentrations and isotopic compositions as the “Nd paradox”. For the first time we model Nd concentrations and isotopic compositions simultaneously and address the hypothesis that the Nd paradox can be explained by a combination of lateral advection and reversible scavenging. We impose a reversible-scavenging model of Nd removal from the ocean on the ocean circulation fields from the MIT general circulation model using the transport matrix method. We conclude that reversible scavenging is an active and important component in the cycling of Nd in the ocean. In the absence of an adequate alternative explanation, reversible scavenging should be considered a necessary component in explaining the Nd paradox.

title: Neodymium isotopes in seawater from the Barents Sea and Fram Strait Arctic–Atlantic gateways
abstract: The neodymium concentration, C Nd , and isotopic composition, e Nd , in seawater have been determined in the water column at five sites in the Barents Sea–Fram Strait area where most of the water exchange between the Arctic Ocean and the North Atlantic takes place. In the main Arctic Ocean inflow branch across the Barents Sea the concentration and isotopic composition ( C Nd  = 15.5 pmol/kg and e Nd  = −10.8) are similar to those reported for the northeastern Nordic Seas, which is consistent with this region being a source area for the Arctic inflow. Due to the addition of Nd from Svalbard shelf sediments, the C Nd in the surface waters above 150 m, in the Fram Strait inflow branch is higher by a factor of 2 and the e Nd is shifted to lower values (−11.8). In the stratified Nansen Basin, where cold low salinity water overlies warmer Atlantic water the C Nd and e Nd do not vary with the vertical temperature–salinity structure but are essentially constant and similar to those of the Atlantic inflow throughout the entire water column, down to 3700 m depth, which indicates that the Nd is to a large extent of Atlantic origin. Compared to the Atlantic inflow water, the Nd in the major Arctic Ocean outflow, the Fram Strait, show higher C Nd in the surface waters above 150 m, and a higher e Nd (−9.8) throughout the entire water column down to 1300 m depth. Sources for the more radiogenic Nd isotopic composition in deep water of the Fram Strait outflow most likely involve boundary exchange with sediments on the shelf and slope as the water passes along the Canadian archipelago. River water is a possible source in the surface water but it also seems likely that Pacific water Nd, modified by interactions on the shelf, is an important component in the Fram Strait surface outflow. Changes in the relative proportions of inflow of river water and flow of Pacific water through the Arctic Ocean could thus influence the isotopic composition of Nd in the North Atlantic.

title: The distribution of neodymium isotopes in Arctic Ocean basins
abstract: Nd concentration and isotope data have been obtained for the Canada, Amundsen, and Makarov Basins of the Arctic Ocean. A pattern of high Nd concentrations (up to 58 pM) at shallow depths is seen throughout the Arctic, and is distinct from that generally seen in other oceans where surface waters are relatively depleted. A range of isotopic variations across the Arctic and within individual depth profiles reflects the different sources of waters. The dominant source of water, and so Nd, is the Atlantic Ocean, with lesser contributions from the Pacific and Arctic Rivers. Radiogenic isotope Nd signatures (up to epsilon(Nd) = -6.5) can be traced in Pacific water flowing into the Canada Basin. Waters from rivers draining older terrains provide very unradiogenic Nd (down to epsilon(Nd) = -14.2) that can be traced in surface waters across much of the Eurasian Basin. A distinct feature of the Arctic is the general influence of the shelves on the Nd concentrations of waters flowing into the basins, either from the Pacific across the Chukchi Sea, or from across the extensive Siberian shelves. Water-shelf interaction results in an increase in Nd concentration without significant changes in salinity in essentially all waters in the Arctic, through processes that are not yet well understood. In estuarine regions other processes modify the Nd signal of freshwater components supplied into the Arctic Basin, and possibly also contribute to sedimentary Nd that may be subsequently involved in sediment-water interactions. Mixing relationships indicate that in estuaries, Nd is removed from major river waters to different degrees. Deep waters in the Arctic are higher in Nd than the inflowing Atlantic waters, apparently through enrichments of waters on the shelves that are involved in ventilating the deep basins. These enrichments generally have not resulted in major shifts in the isotopic compositions of the deep waters in the Makarov Basin (epsilon(Nd) similar to -10.5), but have created distinctive Nd isotope signatures that were found near the margin of the Canada Basin (with epsilon(Nd) similar to -9.0). The deep waters of the Amundsen Basin are also distinct from the Atlantic waters (with epsilon(Nd) = -12.3), indicating that there has been limited inflow from the adjacent Makarov Basin through the Lomonosov Ridge.

title: Study on vertical profiles of rare earth elements by using an ocean general circulation model
abstract: Observed vertical profiles of rare earth elements (REEs) in the North Pacific Ocean show a systematic change from lighter to heavier REEs. Their source and sink consist of external fluxes coming from the surface and the bottom of the ocean and internal recycling within the ocean. In order to evaluate the role of each source and sink in controlling the vertical profiles of REEs, we conduct numerical simulations of REEs by using an ocean general circulation model. These simulations demonstrate that the shape of their vertical profiles is not directly related to the external sources from river runoff, atmospheric dust deposition, and coastal regions but is sensitive to the internal recycling processes within the ocean. Nutrient-like treatment where REEs are removed at the surface and released in the deep ocean is suitable for simulating the vertical profiles of lighter REEs but not for heavier REEs. When the irreversible scavenging is considered as a sink term and particle dissolution is taken into consideration as a source term, the vertical profiles of heavier REEs can be reproduced but those of lighter REEs cannot. The observed difference in the vertical profiles among REEs can be reproduced only when the reversible scavenging process is incorporated into the model. Only the reversible scavenging process accounts for the observed vertical profiles of different REEs in comprehensive manner.

title: Nd isotopic composition in the central North Pacific
abstract: Three vertical profiles of seawater concentration and isotopic composition of Nd were determined for the western to central North Pacific Ocean. In the subarctic oceanic region, at depths greater than 500 m, one vertical profile of Nd isotopic composition was indistinguishable from most previously reported profiles from here. The data indicate a rather homogeneous Nd isotopic composition in the subarctic oceanic region at middle to deep depths (>500 m). Two stations in the subtropical oceanic region exhibited similar Nd isotopic composition profiles to those previously reported. The maxima eNd values at depths of 800– 1000 m (eNd = � 3.4 to � 2.7), which correspond to the North Pacific Intermediate Water (NPIW), are found at both subtropical stations. This implies a ubiquitous distribution of NPIW showing a radiogenic eNd value in the North Pacific. The subsurface minimum at a depth of � 200 m, which indicates the penetration of the North Pacific Tropical Water (NPTW) with an unradiogenic Nd isotopic signal, was observed at one station in the western Pacific. This station had much lower eNd than the central station at depths around 5000 m, suggesting the greater prominence of Antarctic Bottom Water (AABW) in the western subtropical Pacific than in the central to eastern subtropical Pacific. Results of a model calculation assuming boundary exchange indicate that the Hawaiian Islands play an important role in supplying radiogenic Nd to the central Pacific, similar to some continental margins. We show that Nd isotopic composition is a versatile tracer for ocean circulation and the geochemical cycle of Nd in the North Pacific. Further studies on the distribution of Nd isotopic composition in the Pacific Ocean, including the Southern Pacific, will better elucidate the circulation and geochemical cycle of Nd in the Pacific.

title: Hafnium and neodymium isotopes in surface waters of the eastern Atlantic Ocean: Implications for sources and inputs of trace metals to the ocean
abstract: We present hafnium (Hf) and neodymium (Nd) isotopic compositions and concentrations in surface waters of the eastern Atlantic Ocean between the coast of Spain and South-Africa. These data are complemented by Hf and Nd isotopic and concentration data, as well as rare earth element (REE) concentrations, in Saharan dust. Hafnium concentrations range between a maximum of 0.52 pmol/kg in the area of the Canary Islands and a minimum value of 0.08 pmol/kg in the southern Angola Basin. Neodymium concentrations also show a local maximum in the area of the Canary Islands (26 pmol/kg) but are even higher between ~20N and ~4N reaching maximum concentrations of 35 pmol/kg. These elevated concentrations provide evidence of inputs from weathering of the Canary Islands and from the partial dissolution of dust from the Sahara/Sahel region. The inputs from ocean island weathering are also reflected in radiogenic Hf and Nd isotopes. The Hf isotopic compositions of dust samples themselves are highly variable, ranging between eHf = 20 and 0.6. The combined Hf and Nd isotopic compositions of dust plot close to the “terrestrial array” during periods of appreciable dust load in the atmosphere. During low atmospheric dust loading combined Hf and Nd isotopic compositions similar to seawater are observed. Most of the variability can be explained in terms of variable degrees of zircon loss from the dust samples, which in turn is linked to sorting during atmospheric transport to the eastern Atlantic Ocean and possibly presorting by sedimentary redistribution on the continent. In addition, increasing relative proportions of radiogenic clay minerals with decreasing grain size may contribute to the radiogenic Hf isotopic compositions observed. While the Nd isotopic composition in the surface ocean reflects the Nd isotopic composition of the Saharan dust adjacent to the Sahara/Sahel region, the release of Hf from that dust appears to be incongruent and results in surface ocean Hf isotopic compositions which are ~10 eHf more radiogenic than the bulk dust. Radiogenic Hf appears to be released from clays and possibly from trace apatite. Rare earth element patterns of dust samples indicate the presence of apatite but provide no evidence for ferromanganese grain coatings, suggesting that such coatings are insignificant in the release of Hf and Nd from Saharan dust to the surface ocean. The Nd isotopic composition of the surface waters becomes less radiogenic south of the equator, most likely reflecting the release of Nd from Congo river sediments. The release of Hf from Saharan dust and the Congo river sediments, however, does not produce distinct Hf isotopic signatures in the surface ocean, implying that the mobile fraction of Hf integrated over large continental areas is isotopically uniform. The Hf isotopic uniformity in the surface ocean means that the limited variability in deep water isotopic compositions is consistent with a short deep water residence time and reflects homogenous continental inputs rather than efficient deep water homogenization.

title: Modelling Nd-isotopes with a coarse resolution ocean circulation model: Sensitivities to model parameters and source/sink distributions
abstract: The neodymium (Nd) isotopic composition (ϵNd) of seawater is a quasi-conservative tracer of water mass mixing and is assumed to hold great potential for paleoceanographic studies. Here we present a comprehensive approach for the simulation of the two neodymium isotopes 143Nd, and 144Nd using the Bern3D model, a low resolution ocean model. The high computational efficiency of the Bern3D model in conjunction with our comprehensive approach allows us to systematically and extensively explore the sensitivity of Nd concentrations and ϵNd to the parametrisation of sources and sinks. Previous studies have been restricted in doing so either by the chosen approach or by computational costs. Our study thus presents the most comprehensive survey of the marine Nd cycle to date.

Our model simulates both Nd concentrations as well as ϵNd in good agreement with observations. ϵNd covaries with salinity, thus underlining its potential as a water mass proxy. Results confirm that the continental margins are required as a Nd source to simulate Nd concentrations and ϵNd consistent with observations. We estimate this source to be slightly smaller than reported in previous studies and find that above a certain magnitude its magnitude affects ϵNd only to a small extent. On the other hand, the parametrisation of the reversible scavenging considerably affects the ability of the model to simulate both, Nd concentrations and ϵNd. Furthermore, despite their small contribution, we find dust and rivers to be important components of the Nd cycle. In additional experiments, we systematically varied the diapycnal diffusivity as well as the Atlantic-to-Pacific freshwater flux to explore the sensitivity of Nd concentrations and its isotopic signature to the strength and geometry of the overturning circulation. These experiments reveal that Nd concentrations and ϵNd are comparatively little affected by variations in diapycnal diffusivity and the Atlantic-to-Pacific freshwater flux. In contrast, an adequate representation of Nd sources and sinks is crucial to simulate Nd concentrations and ϵNd consistent with observations. The good agreement of our results with observations paves the way for the evaluation of the paleoceanographic potential of ϵNd in further model studies.

title: The neodymium isotopic composition of waters masses in the eastern Pacific sector of the Southern Ocean
abstract: The Antarctic Circumpolar Current is one of the key components of ocean circulation, and a knowledge of its isotopic composition is essential to the use of neodymium (Nd) isotopes to trace circulation now and in the past. Here we present 57 new analyses of the Nd isotopic composition of the water column in the eastern Pacific sector of the Southern Ocean, documenting both the variation in three dimensions as well as the controls on that variability.

Nd isotopic data for the middle of the water column demonstrate the homogeneity of Circumpolar Deep Water (CDW) at an eNd value of −8.7 ± 0.1. This homogeneity reflects the large Nd inventory in the ACC flow, and the degree to which this large inventory buffers the Nd characteristics of the ACC against extra-oceanic inputs from the continents, either via dust from the atmosphere or through dissolved and particulate material from the adjacent continents. CDW upwells onto the Amundsen Sea shelf and, even here, its Nd isotopic properties are close to conserved in the middle of the water column (eNd = −8.0 ± 0.2 at 600 m). At the top and bottom of the shelf water column, however, the Nd isotopic and concentration characteristics are strongly modified (to eNd as high as −4.5). All the shelf water column data obtained here are consistent with net addition of Nd to bottom and surface waters with a contrasting isotopic composition that is matched by local sediment (eNd = −1 to −2), followed by conservative mixing of that water into intermediate levels. Mixing with this shelf composition also leads to significant modification of open ocean surface water (Nd isotopic shift around 1 epsilon unit) in the ACC as it flows eastwards. Modification of open ocean bottom waters by interaction with sediment is more subtle, but there is marked non-conservative removal of Nd accompanied by significant changes in isotopic composition in waters within 10 m of the seabed.

The new data demonstrate the conservativity of Nd in the middle water column, especially for such large volume flows as the ACC. Though boundary exchange-type processes are clearly important in this region, and their imprint on both the shelf and open-ocean surface water is significant, there is no observable impact on the main core of CDW. This finding augurs well for the use of Nd isotopes as a conservative water-mass tracer now and in the past. For example, these data suggest that the only likely control on the temporal variability of southern-component deep water exported northwards into the Atlantic through the last glacial cycle is variations in the input of North Atlantic Deep Water in the Atlantic sector. On the other hand, the data also suggest caution in the use of sedimentary archives of bottom water Nd as records of deep water Nd isotopic characteristics, given the dramatic modification of bottom waters on the shelf and the more subtly non-conservative behaviour in the open ocean bottom water that are both suggestive of modification of the Nd characteristics of bottom waters by sediment.

title: The distribution of neodymium isotopes and concentrations in the Eastern Equatorial Pacific: Water mass advection versus particle exchange
abstract: The radiogenic isotope composition of the Rare Earth Element (REE) neodymium (Nd) is a powerful water mass proxy for present and past ocean circulation. The processes controlling the Nd budget of the global ocean are not quantitatively understood and in particular source and sink mechanisms are still under debate. 
 
In this study we present the first full water column data set of dissolved Nd isotope compositions and Nd concentrations for the Eastern Equatorial Pacific (EEP), where one of the globally largest Oxygen Minimum Zones (OMZ) is located. This region is of particular interest for understanding the biogeochemical cycling of REEs because anoxic conditions may lead to release of REEs from the shelf, whereas high particle densities and fluxes potentially remove the REEs from the water column. Data were obtained between 1°40′N and 16°S along a nearshore and an offshore transect. Near surface zonal current bands, such as the Equatorial Undercurrent (EUC) and the Subsurface Countercurrent (SSCC), which are supplying oxygen-rich water to the OMZ are characterized by radiogenic Nd isotope signatures (eNd=−2). Surface waters in the northernmost part of the study area are even more radiogenic (eNd=+3), most likely due to release of Nd from volcanogenic material. Deep and bottom waters at the southernmost offshore station (14°S) are clearly controlled by advection of water masses with less radiogenic signatures (eNd=−7) originating from the Southern Ocean. Towards the equator, however, the deep waters show a clear trend towards more radiogenic values of up to eNd=−2. The northernmost station located in the Panama basin shows highly radiogenic Nd isotope signatures in the entire water column, which indicates that particle scavenging, downward transport and release processes play an important role. This is supported by relatively low Nd concentrations in deep waters (3000–6000 m) in the EEP (20 pmol/kg) compared to locations in the Northern and Central Pacific (40–60 pmol/kg), which suggests enhanced removal of Nd in the EEP.

title: The hafnium and neodymium isotope composition of seawater in the Atlantic sector of the Southern Ocean
abstract: We present the first combined dissolved hafnium (Hf) and neodymium (Nd) concentrations and isotope compositions of deep water masses from the Atlantic sector of the Southern Ocean. Eight full depth profiles were analyzed for Hf and twelve for Nd. Hafnium concentrations are generally depleted in the upper few hundred meters ranging between 0.2 pmol/kg and 0.4 pmol/kg and increase to relatively constant values of around 0.6 pmol/kg in the deeper water column. At the stations north of the Polar Front (PF), Nd concentrations increase linearly from about 10 pmol/kg at depths of ~ 200 m to up to 31 pmol/kg close to the bottom indicating particle scavenging and release. Within the Weddell Gyre (WG), however, Nd concentrations are essentially constant at 25 pmol/kg at depths greater than ~ 1000 m. The distributions of both elements show a positive correlation with dissolved silicon implying a close linkage to diatom biogeochemistry. 
 
Hafnium essentially shows invariant isotope compositions with values averaging at eHf = + 4.6, whereas Nd isotopes mark distinct differences between water masses, such as modified North Atlantic Deep Water (NADW, eNd = − 11 to − 10) and Antarctic Bottom Water (AABW, eNd = − 8.6 to − 9.6), but also waters locally advected via the Agulhas Current can be identified by their unradiogenic Nd isotope compositions. Mixing calculations suggest that a small fraction of Nd is removed by particle scavenging during mixing of water masses north of the PF. Nevertheless, the Nd isotope composition has apparently not been significantly affected by uptake and release of Nd from particles, as indicated by mixing calculations. A mixing envelope of an approximated North Pacific and a North Atlantic end-member shows that Nd isotope and concentration patterns in the Lower Circumpolar Deep Water (LCDW) can be fully explained by ~ 30:70 percentage contributions of these respective end-members.

title: Spatial distribution of dissolved neodymium and εNd in the Bay of Bengal: Role of particulate matter and mixing of water masses
abstract: The concentration and isotope composition of dissolved Nd have been measured in the water column along an 87°E transect (GIO1 section of International GEOTRACES Program) in the Bay of Bengal (BoB) to investigate the effect of water mass mixing and Nd release from particulate matter in determining these properties. The concentration of Nd in surface waters of the BoB shows a North–South decreasing non-linear trend (∼46 to ∼22 pmol/kg) with salinity, whereas its depth profiles typically show a high value in surface waters, a minimum (∼15 to ∼23 pmol/kg) in the shallow subsurface (∼50–200 m), followed by a gradual increase with depth. The Nd concentration of BoB waters is generally higher than that in nearby oceanic basins. On the other hand, the e Nd values in the BoB are less radiogenic compared to those reported for other regions of the global oceans (except Baffin Bay, the North Atlantic Subpolar Gyre and the Niger delta margin), and show a greater variation in the upper water column. Surface waters of the southernmost profile (∼6°N) show a more radiogenic e Nd value ∼−8, which decreases to −15 in the northernmost profile (∼20°N), close to the values for dissolved and particulate phases of the Ganga–Brahmaputra (G–B) Rivers. This latitudinal trend is most likely a result of variations in mixing proportion between the more radiogenic Indonesian Throughflow surface waters (IW) and unradiogenic BoB low salinity water; the former’s signature being more clearly discernible in surface waters of the two southernmost profiles (∼6°N and ∼8.5°N). Attempts to balance the Nd budget in the water column based on an inversion model, suggest that in addition to water masses other source(s) is required, the strength of which is estimated to vary from 1% to 65% of the measured Nd concentration. The calculations also show that the e Nd of this additional source(s) has to be in the range of ∼−16 ± 2, typical of G–B river sediments. These observations, coupled with the North–South distribution of dissolved Nd and e Nd , indicate that this additional source is release from particulate phases supplied by the G–B river system. The calculations also bring out the presence of “hot-spots” of Nd release (excess Nd) near the sediment–water interface along the northern slope of the bay, indicating supply of Nd from continental margin sediments. This study underscores the significant role of dissolved/particulate Nd from the Ganga–Brahmaputra river system in contributing to the dissolved Nd budget of the global oceans.

title: From the subtropics to the central equatorial Pacific Ocean: neodymium isotopic composition and rare earth element concentration variations
abstract: Neodymium isotopic compositions (eNd) and rare earth element (REE) concentrations were measured for filtered surface to deep waters (112 samples) in the Southern Tropical Pacific. The relatively detailed picture of these tracer distributions allowed us to refine the areas where oceanic eNd variations occur. eNd values increase for most of the water masses flowing from Samoa to the Solomon Sea and in the Papua New Guinea (PNG) area, as already observed. Furthermore, water masses arriving from the eastern equatorial Pacific (200–550 m depth) also revealed radiogenic values, possibly acquired in the vicinity of the South American coasts and Galapagos Islands. These eNd variations affect the whole water column. The most likely process causing such variations is “boundary exchange” between the numerous radiogenic slopes/margins located in this area and seawater flowing past. Dissolution of atmospheric deposition and/or diffuse streaming of volcanic ash are also suggested to explain the radiogenic eNd observed at the surface in the PNG area. Interestingly, a positive europium (Eu) anomaly characterizes the normalized REE patterns of most of the studied water masses. This anomaly is consistent with the REE patterns of sediment and rock samples that are potential sources for the local waters. Such consistency reinforces the hypothesis that lithogenic sources play a major role in the oceanic REE budget, thanks to “boundary exchange.” The data set presented here is a good basis for further sampling that will be realized in the framework of the ongoing GEOTRACES program (www.geotraces.org).

title: Sensitivity of Nd isotopic composition in seawater to changes in Nd sources and paleoceanographic implications
abstract: 2012; revised 12 October 2012; accepted 14 October 2012;  (1) It has been argued that past changes in the sources of Nd could hamper the use of the Nd isotopic composition (� Nd) as a proxy for past changes in the overturning of deep water masses. Here we reconsider uncertainties associated withNd in seawater due to potential regional to global scale changes in the sources of Nd by applying a modeling approach. For illustrative purposes we describe rather extreme changes in the magnitude of source fluxes, their isotopic composition or both. We find that the largest effects onNd result from changes in the boundary source. Considerable changes also result from variations in the magnitude orNd of dust and rivers but are largely constrained to depths shallower than 1 km, except if they occur in or upstream of regions where deep water masses are formed. From these results we conclude that changes in Nd sources have the potential to affectNd. However, substantial changes are required to generate large-scale changes inNd in deep water that are similar in magnitude to those that have been reconstructed from sediment cores or result from changes in meridional overturning circulation in model experiments. Hence, it appears that a shift inNd comparable to glacial-interglacial variations is difficult to obtain by changes in Nd sources alone, but that more subtle variations can be caused by such changes and must be interpreted with caution.

title: Neodymium isotopic composition of the oceans: A compilation of seawater data
abstract: A global compilation of the neodymium isotopic composition of seawater is presented. With 880 data points, it confirms the gradual eNd increase for intermediate/deep water masses from the northwest North Atlantic, via the Austral and Indian oceans, to the Pacific. This confirms the usefulness of this tracer for studying large scale oceanic circulation. The compilation stresses the need for documenting the oceans south of 30°S, from which less than 4% of the data are derived. The associated neodymium concentrations display heterogeneous vertical gradients among major oceanic basins. In addition to particle remineralization along the global thermohaline circulation, the database suggests that basin size differences also contribute to that heterogeneity.

title: Denmark Strait water circulation traced by heterogeneity in neodymium isotopic compositions
abstract: Seawater neodymium isotopic composition ( e Nd ) and rare earth element (REE) concentrations were measured along four hydrologic sections within the East Greenland Current (EGC), between the Fram and the Denmark Straits, during summer 1999. EGC intermediate waters, between 77°N and 70°N, displaying similar hydrological characteristics as Denmark Strait Overflow Water (DSOW), had very constant REE characteristics and e Nd value of −10.9. This constancy reveals the absence of lithogenic input, from the East Greenland margin, into intermediate waters in this area, corroborating previous dissolved aluminum data. The DSOW was characterized by e Nd =−8.4±1.4. This value can be explained by the imprint of lithogenic formations, mainly basaltic, bordering the Denmark Strait on the intermediate waters described above. However, granitic Precambrian formations seem to contribute, although more slightly, to defining the DSOW Nd signature. This double influence could explain the heterogeneity of the Denmark Strait waters. These results provide a better understanding of the DSOW Nd signature, allowing a better use of this tracer in the study of present and past North Atlantic Deep Water dynamics. Atlantic Water was present at a station located at the mouth of Nansen Fjord, on the western side of Denmark Strait. We suggest that this water reaches the fjord intermittently as isolated water lenses or eddies detached from the northward flowing branch of the Irminger current.

title: Rare earth elements and yttrium in seawater: ICP-MS determinations in the East Caroline, Coral Sea, and South Fiji basins of the western South Pacific Ocean
abstract: Using inductively coupled plasma mass spectrometry (ICP-MS), vertical profiles of yttrium and all the rare earth elements (REEs) in seawater were obtained at three locations in the western South Pacific (the East Caroline, Coral Sea, and South Fiji Basins). Based on the data, we have analyzed the inter-REE(III) relationships and found that REE(III)s heavier than Dy and particularly close neighbors show strong coherencies in their oceanic behavior. Considering that the heavy REE(III) s are less particle-reactive than the light and middle REE (III) s and that they have very tight correlations with regressions passing almost through the origin, the neighboring element ratios of the heavy REE(III)s behave virtually conservatively and are suitable as tracers of water masses. The Dy-Ho-Er system is practically good, because its dynamic range (signal-to-noise ratio) is large in the ocean. The highest Ho/Dy ratios (∼0.31) are found in intermediate and deep waters throughout the western South Pacific which overlie the Antarctic Bottom Water with a low Ho/Dy ratio (∼0.27). These high Ho/Dy waters are probably formed in the Polar and Subantarctic Frontal Zone and advect northward. REE(III) data provide better insights into the deep water recirculation in the South Pacific than those discussed based upon the regular oceanographic properties alone.

title: Neodymium isotopes in the Mediterranean Sea: Comparison between seawater and sediment signals
abstract: Nine depth-profiles of dissolved Nd concentrations and isotopic ratios (e Nd ) were obtained in the Levantine Basin, the Ionian, the Aegean, the Alboran Seas and the Strait of Gibraltar. Thirteen core-top sediments and Nile River particle samples were also analyzed (leached with 1 N HCl, acetic acid or hydroxylamine hydrochloride). The seawater e Nd values become more radiogenic during the eastward circulation in the Mediterranean Sea. The relationship between salinity and the seawater e Nd shows that the Nd isotopic signature is more conservative than salinity in the Mediterranean Sea. The water mass with the highest e Nd (−4.8) is found at about 200 m in the easternmost Levantine basin. The average e Nd value for deep waters is −7.0 in the eastern basin, 2.5 e-units higher than in the western basin. By examining the sensitivity of seawater e Nd to Nd inputs from the Nile, we conclude that the most significant radiogenic Nd source is partially dissolved Nile River particles. The Nd flux from the Nile River water has a minor influence on the Mediterranean seawater e Nd . Except for the easternmost Levantine Basin, the leachate e Nd values are consistent with the seawater values. In the easternmost Levantine Basin, the leachate e Nd values obtained with HCl leaching are systematically higher than the seawater values. The relationship between leachate and residual e Nd values indicates that the HCl leaching partially dissolves lithogenic Nd, so the dissolution of Nile River particles is the cause of the observed shift. Some e Nd values obtained with hydroxylamine hydrochloride leaching are higher than those obtained with HCl leaching. Although the reason for this shift is not clear, 87 Sr/ 86 Sr successfully detects the presence of a nonmarine component in the leachate. Our results suggest that leaching performance may vary with the mineralogy of marine sediments, at least in the case of the Mediterranean Sea.

title: A boundary exchange influence on deglacial neodymium isotope records from the deep western Indian Ocean
abstract: The use of neodymium (Nd) isotopes to reconstruct past water mass mixing relies upon the quasi-conservative behaviour of this tracer, whereas recent studies in the modern oceans have suggested that boundary exchange, involving the addition of Nd from ocean margin sediments, may be an important process in the Nd cycle. Here we suggest that the relative importance of water mass advection versus boundary exchange can be assessed where the deep western boundary current in the Indian Ocean flows past the Madagascan continental margin; a potential source of highly unradiogenic Nd. Foraminiferal coatings and bulk sediment reductive leachates are used to reconstruct bottom water Nd isotopic composition (eNd) in 8 Holocene age coretops, with excellent agreement between the two methods. These data record spatial variability of ∼4 eNd units along the flow path of Circumpolar Deep Water; eNd≈−8.8 in the deep southern inflow upstream of Madagascar, which evolves towards eNd≈−11.5 offshore northern Madagascar, whereas eNd≈−7.3 where deep water re-circulates in the eastern Mascarene Basin. This variability is attributed to boundary exchange and, together with measurements of detrital sediment eNd, an isotope mass balance suggests a deep water residence time for Nd of ≤400 yr along the Madagascan margin. Considering deglacial changes, a core in the deep inflow upstream of Madagascar records eNd changes that agree with previous reconstructions of the Circumpolar Deep Water composition in the Southern Ocean, consistent with a control by water mass advection and perhaps indicating a longer residence time for Nd in the open ocean away from local sediment inputs. In contrast, sites along the Madagascan margin record offset eNd values and reduced glacial–interglacial variability, underlining the importance of detecting boundary exchange before inferring water mass source changes from Nd isotope records. The extent of Madagascan boundary exchange appears to be unchanged between the Holocene and Late Glacial periods, while a consistent shift towards more radiogenic eNd values at all sites in the Late Glacial compared to the Holocene may represent a muted signal of a change in water mass source or composition.

title: The geochemical balance of the rare earth elements and neodymium isotopes in the oceans
abstract: Profiles of REE concentrations and neodymium isotope measurements in the western Indian Ocean are consistent with published data from the other major oceans. REE concentrations show a strong hydrographic control. The influence of intermediate water masses is particularly significant. Surface water concentrations vary with salinity. Paniculate REEs are less than 5% of dissolved concentration, except for Ce, where up to ~20% of total REEs are present as >0.4 μm particles. In contrast to dissolved profiles, concentration profiles of paniculate REEs, including Ce, all increase with depth. REE concentrations correlate with silica over all but the deepest waters, although, in detail, correlations are complex. The neodymium isotopic composition of Indian Ocean seawater (avg. ϵNd(0) = −8.3) is intermediate between that for the Atlantic and Pacific oceans. Comparison of REE data from the major oceans shows that the HREEs behave more like Si than do the LREEs. Sm/Nd ratios are near-constant for all oceans, whereas ErNd ratios vary between the Atlantic, Indian, and Pacific oceans. The fractionation of the REEs in surface seawaters can be explained by fractionation factors relative to silica from ~0.12 for La to 0.05 for Er. Use of a seven-box model shows that the interoceanic variations in neodymium isotopic compositions and Nd concentrations cannot be reconciled unless particle-water exchange is invoked. The degree of exchange required is consistent with particle residence times of ~1–10 yr.

title: Nd isotopic composition and REE pattern in the surface waters of the eastern Indian Ocean and its adjacent seas
abstract: The Nd isotopic composition and dissolved rare earth elements (REEs) have been measured in the surface waters along the 1996/97 R.V. Hakuho-Maru Expedition route from Tokyo to the Southern Ocean, southwest of Australia, through the Philippine and Indonesian Archipelago, the eastern Indian Ocean, the Bay of Bengal and the South China Sea. The radiogenic eNd values of −1.3 and −1.4 were found in the Sulu Sea and near the Lombok Strait, indicating the strong influence of surrounding volcanic islands, whereas non-radiogenic eNd values of less than −10 were found in the Southern Ocean and the Bay of Bengal suggesting Nd of continental origin. The dissolved Nd concentrations also showed a wide range of variation from 2.8 to 19.6 pmol/kg and the trivalent REE patterns exhibited characteristic features that can be grouped into each different oceanic province. The geographical distribution of dissolved Nd is different from that of atmospherically derived 210Pb, but generally resembles that of coastally derived 228Ra. This strongly suggests that fluvial and coastal input predominates over eolian input for dissolved Nd in the surface ocean. However, the riverine dissolved Nd flux appears to be relatively minor, and remobilization of Nd from coastal and shelf sediments may play an important role in the total Nd input to the ocean. 
 
By modeling the distributions of the isotopic composition and concentration of Nd together with the activity ratio of 228Ra/226Ra in the southeastern Indian Ocean, we estimate a mean residence time of Nd in the surface mixed layer to be 1.5–2.6 years. The short mean residence time is comparable with, or slightly longer than that of 210Pb suggesting similar chemical reactivity.

title: Importance of iron for plankton blooms and carbon dioxide drawdown in the Southern Ocean
abstract: THE iron hypothesis1–3—the suggestion that iron is a limiting nutrient for plankton productivity and consequent CO2 drawdown— has been tested by small-scale experiments in incubation bottles in the subarctic Pacific2,4 and Southern5 –7 Oceans, and by a recent large-scale experiment in the equatorial Pacific Ocean8,9. Here we test the idea by looking at natural levels of productivity in regions of the Southern Ocean with differing iron abundance. In the southerly branch of the Antarctic circumpolar current (ACC), upwelling of deep waters supplies sufficient iron to the surface to sustain moderate primary production but not to permit blooms to develop. In contrast, within the fast-flowing, iron-rich jet of the polar front (PF), spring blooms produced phytoplankton biomass an order of magnitude greater than that in southern ACC waters, leading to CO2 undersaturation. The plankton-rich PF waters were sharply delineated from adjacent iron-poor waters, indicating that iron availability was the critical factor in allowing blooms to occur.

title: High-latitude controls of thermocline nutrients and low latitude biological productivity
abstract: 1 £ 1) surface unit cell. For the interfaces we used a grid corresponding to 16 k-points per (1 £ 1) unit cell. For metallic structures we used the Mermin functional with a temperature of 1,000 K and the extrapolation to zero Kelvin proposed in ref. 28. The band offsets have been derived by relating the plane wave part of the potential, averaged laterally, to band edges. The relative displacement between potential and band edges has been obtained from the epitaxially strained bulk materials. The phase diagram in Fig. 2 was obtained from the total energies as a function of the oxygen chemical potential. A number of different, stoichiometric and non-stoichiometric, interface structures with varying oxygen content have been considered. The regions where one of the stoichiometric interfaces (A, B or the dimer-oxidized variant of B) is most stable are shaded. Tests for the upper boundary of interface A with larger, that is (4 £ 4), unit cells confirmed that the phase boundaries obtained in (2 £ 2) unit cell are reliable to about 0.03 eV. growth of SrTiO3 films on Si(001) substrates using a focused electron beam evaporation method. Process for depositing an oxide epitaxially onto a silicon substrate and structures prepared with the process. Physical structure and inversion charge at a semiconductor interface with a crystalline oxide. First step towards the growth of single-crystal oxides on Si: formation of a two-dimensional crystalline silicate on Si(001). structure having a crystalline alkaline earth metal oxide interface with silicon. Band offsets of wide-band-gap oxides and implications for future electronic devices. Parts of the calculations have been performed on the computers of the 'Norddeutscher Verbund für Hoch-und Höchstleistungsrechnen' (HLRN).

title: Fueling export production: nutrient return pathways from the deep ocean and their dependence on the Meridional Overturning Circulation
abstract: In the Southern Ocean, mixing and upwelling in the presence of heat and freshwater surface fluxes transform subpycnocline water to lighter densities as part of the upward branch of the Meridional Overturning Circulation (MOC). One hypothesized impact of this transformation is the restoration of nutrients to the global pycnocline, without which biological productivity at low latitudes would be significantly reduced. Here we use a novel set of modeling experiments to explore the causes and consequences of the Southern Ocean nutrient return pathway. Specifically, we quantify the contribution to global productivity of nutrients that rise from the ocean interior in the Southern Ocean, the northern high latitudes, and by mixing across the low latitude pycnocline. In addition, we evaluate how the strength of the Southern Ocean winds and the parameterizations of subgridscale processes change the dominant nutrient return pathways in the ocean. Our results suggest that nutrients upwelled from the deep ocean in the Antarctic Circumpolar Current and subducted in Subantartic Mode Water support between 33 and 75% of global export production between 30° S and 30° N. The high end of this range results from an ocean model in which the MOC is driven primarily by wind-induced Southern Ocean upwelling, a configuration favored due to its fidelity to tracer data, while the low end results from an MOC driven by high diapycnal diffusivity in the pycnocline. In all models, nutrients exported in the SAMW layer are utilized and converted rapidly (in less than 40 years) to remineralized nutrients, explaining previous modeling results that showed little influence of the drawdown of SAMW surface nutrients on atmospheric carbon concentrations.

title: The Ekman transfer of nutrients and maintenance of new production over the North Atlantic
abstract: The maintenance of new production requires a supply of nutrients to the euphotic zone to o⁄set the loss through biological export. The dynamical supply of nutrients is usually discussed in terms of the vertical transfer from nutrient-rich, deep waters. However, the horizontal transfer is important in regions of downwelling over subtropical gyres, where nutrients may be transported across the intergyre boundaries by the surface Ekman drift or geostrophic eddies from the neighbouring nutrient-rich, upwelling regions. The Ekman transfer of nitrate to the euphotic layer is diagnosed from climatology over the North Atlantic. The vertical Ekman supply of nitrate is found to be significant over the subpolar gyre, the tropics and eastern boundary, whereas the horizontal transfer is found to be dominant at the intergyre boundaries. On the northern flank of the subtropical gyre, the Ekman transfer provides a source of nitrate from 0.03 to 0.06 mol N m~2 yr~1, corresponding to a contribution to new production of between 0.4 and 0.8 mol C m~2 yr~1. This estimate represents a significant fraction of the total new production of typically 1 mol C m~2 yr~1 suggested by both remote chlorophyll and sediment trap observations. A simplified nitrogen cycle model is used to assess the role of the Ekman supply over the North Atlantic. In the model the Ekman supply of nitrate leads to a plume of nitrate and enhanced productivity extending up to 1000 km into the subtropical gyre from the intergyre boundaries. This lateral scale is controlled by the seasonal cycle of the mixed layer and the remineralisation of the particulate organic fallout.

title: Eddy/Wind Interactions Stimulate Extraordinary Mid-Ocean Plankton Blooms
abstract: Episodic eddy-driven upwelling may supply a significant fraction of the nutrients required to sustain primary productivity of the subtropical ocean. New observations in the northwest Atlantic reveal that, although plankton blooms occur in both cyclones and mode-water eddies, the biological responses differ. Mode-water eddies can generate extraordinary diatom biomass and primary production at depth, relative to the time series near Bermuda. These blooms are sustained by eddy/wind interactions, which amplify the eddy-induced upwelling. In contrast, eddy/wind interactions dampen eddy-induced upwelling in cyclones. Carbon export inferred from oxygen anomalies in eddy cores is one to three times as much as annual new production for the region.

title: Nutrient streams and their induction into the mixed layer
abstract: Nutrients are transferred from the nutricline into the winter mixed layer through a combination of vertical and lateral advection, referred to here as induction, the reverse of the subduction process. This advective supply of nutrients maintains the high productivity at high latitudes over timescales of several years and longer. Climatological diagnostics over the North Atlantic reveal that the induction of nutrients into the winter mixed layer is dominated by lateral advection even in the subpolar gyre where there is significant Ekman upwelling. The induction flux of nutrients is itself sustained by nutrient streams, strong nutrient transport associated with the western boundary currents. In the North Atlantic basin, the integrated induction flux accounts for typically 40% of the nitrate flux in the nutrient stream at 36°N, while the remaining fraction is recirculated in the thermocline of the subtropical gyre. The relationship between the nutrient stream and induction flux is illustrated using a numerical model of the circulation and biogeochemistry of the North Atlantic. The model studies suggest that the nutrient stream preferentially supplies nutrients to isopycnals outcropping in the subpolar gyre. The nutrient sources for the nutrient stream are either mode waters originating from the Southern Ocean or the tropics, where the eastern upwelling circulation and particle fallout focuses nutrients onto the lighter isopycnal surfaces of the nutrient stream.

title: VERTICAL MIXING, ENERGY, AND THE GENERAL CIRCULATION OF THE OCEANS
abstract: ▪ AbstractThe coexistence in the deep ocean of a finite, stable stratification, a strong meridional overturning circulation, and mesoscale eddies raises complex questions concerning the circulation energetics. In particular, small-scale mixing processes are necessary to resupply the potential energy removed in the interior by the overturning and eddy-generating process. A number of lines of evidence, none complete, suggest that the oceanic general circulation, far from being a heat engine, is almost wholly governed by the forcing of the wind field and secondarily by deep water tides. In detail however, the budget of mechanical energy input into the ocean is poorly constrained. The now inescapable conclusion that over most of the ocean significant “vertical” mixing is confined to topographically complex boundary areas implies a potentially radically different interior circulation than is possible with uniform mixing. Whether ocean circulation models, either simple box or full numerical ones, neither explic...

title: Enhanced upper ocean stratification with climate change in the CMIP3 models
abstract: Changes in upper ocean stratification during the second half of the 21st century, relative to the second half of the 20th century, are examined in ten of the CMIP3 climate models according to the SRES-A2 scenario. The upper ocean stratification, defined here as the density difference between 200 m and the surface, is larger everywhere during the second half of the 21st century, indicative of an increasing degree of decoupling between the surface and the deeper oceans, with important consequences for many biogeochemical processes. The areas characterized by the largest stratification changes include the Arctic, the tropics, the North Atlantic, and the northeast Pacific. The increase in stratification is primarily due to the increase in surface temperature, whose influence upon density is largest in the tropical regions, and decreases with increasing latitude. The influence of salinity upon the stratification changes, while not as spatially extensive as that of temperature, is very large in the Arctic, North Atlantic and Northeast Pacific. Salinity also significantly contributes to the density decrease near the surface in the western tropical Pacific, but counteracts the negative influence of temperature upon density in the tropical Atlantic.

title: Eddy-induced enhancement of primary production in a model of the North Atlantic Ocean
abstract: In steady state, the export of photosynthetically fixed organic matter to the deep ocean has to be balanced by an upward flux of nutrients into the euphotic zone. Indirect geochemical estimates of the nutrient supply to surface waters have been substantially higher than direct biological and physical measurements, particularly in subtropical regions. A possible explanation for the apparent discrepancy is that the sampling strategy of the direct measurements has under-represented episodic nutrient injections forced by mesoscale eddy dynamics, whereas geochemical tracer budgets integrate fluxes over longer time and space scales. Here we investigate the eddy-induced nutrient supply by combining two methods potentially capable of delivering synoptic descriptions of the ocean's state on a basin scale. Remotely sensed sea-surface height data from the simultaneous TOPEX/Poseidon and ERS-1 satellite missions are assimilated into a numerical eddy-resolving coupled ecosystem–circulation model of the North Atlantic Ocean. Our results indicate that mesoscale eddy activity accounts for about one-third of the total flux of nitrate into the euphotic zone (taken to represent new production) in the subtropics and at mid-latitudes. This contribution is not sufficient to maintain the observed primary production in parts of the subtropical gyre, where alternative routes of nitrogen supply will have to be considered.

title: Closure of the meridional overturning circulation through Southern Ocean upwelling
abstract: The meridional overturning circulation of the ocean plays a central role in the climate and its variability. This Review of recent studies emphasizes the importance of wind-driven upwelling in the Southern Ocean for global ocean circulation.

title: Ventilation of the North Atlantic and North Pacific: Subduction Versus Obduction
abstract: Ventilation in the North Atlantic and North Pacific is examined by analyzing the Levitus climatological data and the Hellerman and Rosenstein wind stress data. Ventilation between the permanent pycnocline and the overlying seasonal pycnocline and mixed layer consists of two physical processes: subduction and obduction. Subduction takes place mainly in the subtropical basin where surface water is irreversibly transferred into the permanent pycnocline below. Obduction takes place in the subpolar basin where water from the permanent pycnocline is irreversibly transferred into the mixed layer above. Veatilation in the North Atlantic and North Pacific can be clarified into four physically different regions: the subductive region, the obductive region, the ambiductive region where both subduction and obduction take place, and the insulated region where neither subduction nor obduction occurs. Although the total subduction rates in these two oceans are comparable, the total obduction rates are considera...

title: Neodymium budget in the modern ocean and paleo‐oceanographic implications
abstract: The oceanic Nd budget is calculated using a steady state 10-box model and a compilation of field data. This is the first attempt to propose consistent estimates of the Nd fluxes entering the ocean, as well as indicating possible Nd sources and the proportion of Nd fluxes exchanged between dissolved and particulate fractions. With presently available Nd data the best estimates give a total Nd influx of 9 x 10(9) g/yr, which leads to an oceanic Nd residence time of 500 years. From modeling tests we suggest that the authigenic Nd scavenged by particulates is 100% remineralized in the deep ocean. The total exchanged Nd flux may be as high as 2 x 10(10) g/yr. The epsilon(Nd(0)) values of the influxes are -22, -11, +1, and -4 for the North Atlantic, surface Atlantic, North Pacific, and surface Indo-Pacific regions, respectively. Atmospheric and riverine Nd fluxes are insufficient to explain the magnitude and regional variability of calculated Nd influxes and epsilon(Nd(0)). We propose continental margins as an additional source supplying Nd to the ocean. Using the model calibrated for Nd, we examine the sensitivity of deep water epsilon(Nd(0)) to variations of Nd inputs to the ocean. Deep water Nd concentrations and epsilon(Nd(0)) vary with the changes in Nd influxes and their epsilon(Nd(0)). As Nd sources to the ocean may change during glacial/interglacial periods, the epsilon(Nd(0)) shifts recorded in ferromanganese nodules and crusts do not necessarily reflect changes in paleoceanic circulation. The effects of continental erosion should be considered in reconstructing patterns of ocean circulation using Nd isotopes.

title: Concentrations and isotopic compositions of neodymium in the eastern Indian Ocean and Indonesian straits
abstract: Four profiles of Nd concentration and isotopic composition were determined at two stations in the eastern Indian Ocean along a north/south section between Bali and Port-Hedland and at two others in the Timor and Sumba straits. Neodymium concentrations increase with depth, between 7.2 pmol/L at the surface to 41.7 pmol/L close to the bottom. The eNd of the different water masses along the section are −7.2 ± 0.2 for the Indian Bottom Waters and −6.1 ± 0.2 for the Indian Deep Waters. The intermediate and thermocline waters are less radiogenic at st-10 than at st-20 (−5.3 ± 0.3 and −3.6 ± 0.2, respectively). In the Timor Passage and Sumba Strait, eNd of the Indonesian waters is −4.1 ± 0.2 and that of the North Indian Intermediate Waters is −2.6 ± 0.3. These distinct isotopic signals constrain the origins of the different water masses sampled in the eastern Indian Ocean. They fix the limit of the nonradiogenic Antarctic and Indian contributions to the southern part of the section whereas the northern part is influenced by radiogenic Indonesian flows. In addition, the neodymium isotopic composition suggests that in the north, deep waters are influenced by a radiogenic component originating from the Sunda Arch Slope flowing deeper than 1200 m, which was not documented previously. Mixing calculations assess the conservativity of eNd on the scale of an oceanic basin. The origin of the surprising radiogenic signal of the NIIW is discussed and could result from a remobilization of Nd sediment-hosted on the Java shelf, requiring important dissolved/particulate exchange processes. Such processes, occurring in specific areas, could play an important role in the world ocean Nd budget.

title: Biogeochemical Controls and Feedbacks on Ocean Primary Production
abstract: Changes in oceanic primary production, linked to changes in the network of global biogeochemical cycles, have profoundly influenced the geochemistry of Earth for over 3 billion years. In the contemporary ocean, photosynthetic carbon fixation by marine phytoplankton leads to formation of approximately 45 gigatons of organic carbon per annum, of which 16 gigatons are exported to the ocean interior. Changes in the magnitude of total and export production can strongly influence atmospheric CO2 levels (and hence climate) on geological time scales, as well as set upper bounds for sustainable fisheries harvest. The two fluxes are critically dependent on geophysical processes that determine mixed-layer depth, nutrient fluxes to and within the ocean, and food-web structure. Because the average turnover time of phytoplankton carbon in the ocean is on the order of a week or less, total and export production are extremely sensitive to external forcing and consequently are seldom in steady state. Elucidating the biogeochemical controls and feedbacks on primary production is essential to understanding how oceanic biota responded to and affected natural climatic variability in the geological past, and will respond to anthropogenically influenced changes in coming decades. One of the most crucial feedbacks results from changes in radiative forcing on the hydrological cycle, which influences the aeolian iron flux and, in turn, affects nitrogen fixation and primary production in the oceans.

title: Lateral mixing and advection of reactive isotope tracers in ocean basins: observations and mechanisms
abstract: The global mapping of dissolved non-conservative isotopic tracers (Be, Pb, Nd) in the world's oceans has revealed relative homogeneities in isotope ratios that by far exceed those of the tracer's isotopic input ranges. For tracers with continental sources such as Pb and Nd, a first possible mechanism to account for such an isotopic homogenisation is the mixing within their source areas, such as rivers, deltas, or continental shelves. However, a further mixing mechanism within ocean water is likely, because in the Pacific Ocean the two isotopes of Be are mixed to within only 13% dispersion, despite their entirely different sources (cosmogenic 10 Be entering the oceans interior by rain, stable 9 Be entering the ocean margins from the continents). A second possibility is that well-mixed tracers are imported from the Antarctic Circumpolar Current (ACC) into the Pacific by Antarctic Bottom Water (AABW). However, since the area of the ACC is also one of intensive tracer scavenging, for Pb and most of the Pacific's Be imported sources are not dominant by mass balance. Hence a third mechanism, lateral intra-basin mixing between coastal and central gyre waters is suggested to be the most important mixing mechanism. We perform a numerical assessment of the feasibility of tracer mixing by lateral advection and eddy mixing, in two dimensions in a basin-width subtropical ocean gyre with laterally varying scavenging and input intensity. For Be, strong cross-streamline diffusion is required to achieve homogenisation of the 10 Be derived from the interior with the margin-sourced 9 Be. This is favoured by long scavenging residence times ( τ ) of Be, which is the case in both thermocline waters and deep waters. For Pb, most of which is entirely margin-sourced, isotopic homogenisation is favoured by high velocities (Peclet Numbers) in the gyre and τ of at least 20 y. This is the case in thermocline waters. If short-residence time tracers are dispersed in such an efficient way over large sections of an ocean basin important implications for the use of these isotopes in paleoceanography are: (1) sites of tracer input (e.g. dust) do not necessarily spatially correlate with the distribution of such input in either seawater or marine sediments; (2) gyre-wide, or even basin-wide responses might arise from relatively local changes in the provenance of isotopes, and need not necessarily result from global climatic changes, or variations of the deep thermohaline circulation.

title: Rare earth element (REE)–silicic acid complexes in seawater to explain the incorporation of REEs in opal and the “leftover” REEs in surface water: New interpretation of dissolved REE distribution profiles
abstract: In light of a new finding that diatom opal carries rare earth elements (REEs) at an amount exceeding that which would explain the REEs/Si ratio in open ocean columns, the complex formation of REEs with silicic acid and subsequent incorporation of the complex into diatom opal has been explored. Reported complex formation constants of EuH3SiO42+ and an empirical linear relationship of the constants with hydrolysis constants were employed in the species calculation. The calculation reveals that REE–silicic acid complex is the dominant species of dissolved REEs especially in deep layers of the Pacific and the Atlantic Oceans. The degree of complex formation varies depending on dissolved silica concentration, pH, and pCO2. Assuming that only REEs in the form of REEH3SiO42+, which is diffused/advected from deep water to surface water together with dissolved silica, are incorporated into diatom opal, a REE incorporation theory has been developed. The theory links the intake of REEs and silica by diatoms in surface water and their concentrations in deep water and presents two distribution coefficients (Ds) of REEs in diatom opal, conventional D against surface water and columnar D against deep water as functions of pH and dissolved silica concentration. The theory successfully explains the significant REE concentration level (“leftover REEs”) in the surface water of the oceans and reproduces a concentration of REEs in diatom opal that is consistent with that observed in diatom opal in the North Pacific Ocean. The REE composition of oceanic deep water is largely explained by two fractionation processes: REE incorporation into opal and carbonate/oxide scavenging of REEs.