Distributions of dissolved and particulate rare earth elements (REEs) and seawater neodymium isotopic composition (εNd) were established in samples from the BONUS GoodHope (BGH) IPY-GEOTRACES cruise ...in the SE Atlantic sector of the Southern Ocean (36°S-13°E to 57°S-0°, Feb.–Mar. 2008). Close to the South African continent in the subtropical domain, particulate REEs show the highest concentrations and flat PAAS-normalized patterns, clearly tracing their lithogenic origin. Active cerium oxidation onto suspended particles is evidenced by the mirror-image relationship of the cerium anomaly between dissolved and particulate phases. Unradiogenic dissolved neodymium in surface waters (εNd= -17.1) traces the influence of old sedimentary material brought by the Agulhas current and rings to the Cape Basin area. A mass balance calculation suggests that the release of Nd from dissolution of lithogenic material corresponds to a remobilization of 154×106T of sediment per year, i.e., 5% of the total sediment delivered to the southeast African coast annually. At open ocean stations, both dissolved and particulate REEs present negative cerium anomalies, indicating that particles have acquired a marine signature. The increasing REE concentrations with depth, and the strong linear correlations of dissolved REE with silica, indicate that surface removal and deep re-mineralisation of REEs are partially related to the biogeochemical cycle of silicate, which involves biogenic silica (diatoms). Combined with marine carbonates, these authigenic phases could explain the observed REE patterns in suspended particles, except for La. We suggest that the positive La anomalies in both phases are linked to the oceanic barium cycle and the partial dissolution of barite crystals, especially in the Polar Frontal Zone.
The εNd composition behaves conservatively in intermediate and deep waters, while input processes affect the isotopic signal of subtropical surface waters and Weddell Gyre bottom waters. An Indian Ocean and an Atlantic variety of AAIW have been isotopically differentiated (εNd=−9.3±0.3 and εNd=−8.0±0.5, respectively). Homogeneous signatures characterize circumpolar waters (εNd from −8.2 to −8.4 for CDW). A binary mixing model has been used to assess the contribution of undiluted NADW reaching southern latitudes.
This work presents iron isotope data in the western equatorial Pacific. Marine aerosols and top core margin sediments display a slightly heavy Fe isotopic composition (δ56Fe) of 0.33 ± 0.11‰ (2SD) ...and 0.14 ± 0.07‰, respectively. Samples reflecting the influence of Papua New Guinea runoff (Sepik River and Rabaul volcano water) are characterized by crustal values. In seawater, Fe is mainly supplied in the particulate form and is found with a δ56Fe between −0.49 and 0.34 ± 0.07‰. The particulate Fe seems to be brought mainly by runoff and transported across continental shelves and slopes. Aerosols are suspected to enrich the surface Vitiaz Strait waters, while hydrothermal activity likely enriched New Ireland waters. Dissolved Fe isotopic ratios are found between −0.03 and 0.53 ± 0.07‰. They are almost systematically heavier than the corresponding particulate Fe, and the difference between the signature of both phases is similar for most samples with Δ56FeDFe – PFe = +0.27 ± 0.25‰ (2SD). This is interpreted as an equilibrium isotopic fractionation revealing exchange fluxes between both phases. The dissolved phase being heavier than the particles suggests that the exchanges result in a net nonreductive release of dissolved Fe. This process seems to be locally significantly more intense than Fe reductive dissolution documented along reducing margins. It may therefore constitute a very significant iron source to the ocean, thereby influencing the actual estimation of the iron residence time and sinks. The underlying processes could also apply to other elements.
Key Points
Isotopic composition of dissolved and particulate Fe in seawaterIsotopic composition of Fe in marine aerosol, Sepik, and margin sedimentsNonreductive release would be an important source of dissolved Fe
Three vertical profiles of rare earth element concentrations and Nd isotopic compositions have been measured in the remote southeast Pacific Ocean. The three stations represent contrasting ...environments: the oligotrophic center of the gyre (station GYR), the “transition zone” east of the South Tropical Front (station EGY), and the Peru‐Chile upwelling marked by a pronounced oxygen minimum (station UPX). Rare earth concentrations display nutrient like vertical profiles except at UPX where surface waters are enriched. At this station Nd isotopic compositions are clearly more radiogenic than in the open ocean, suggesting that boundary exchange process is releasing lithogenic rare earth element from the volcanic Andes. Unexpected radiogenic values (εNd reaching –3.7) are also observed at 2000 m at station GYR in the Upper Circumpolar Deep Water that commonly have εNd values around –6. Exchange processes related to hydrothermal activity are suspected to produce this increase in εNd in the vicinity of the East Pacific Rise. These results provide some guidance for higher resolution studies planned in this region by the international GEOTRACES program.
Key Points
New REE and Nd isotope data in the remote South‐East PacificHydrothermal vents might affect Nd isotopes and concentrations of deep watersData allow characterizing the source of subtropical waters flowing westward
This work demonstrates for the first time the feasibility of the measurement of the isotopic composition of dissolved iron in seawater for a typical open ocean Fe concentration range (0.1–1 nM). It ...also presents the first data of this kind. Iron is preconcentrated using a Nitriloacetic Acid Superflow resin and purified using an AG1x4 anion exchange resin. The isotopic ratios are measured with a MC‐ICPMS Neptune, coupled with a desolvator (Aridus II), using a 57Fe‐58Fe double spike mass bias correction. Measurement precision (0.13‰, 2SD) allows resolving small iron isotopic composition variations within the water column, in the Atlantic sector of the Southern Ocean (from δ57Fe = −0.19 to +0.32‰). Isotopically light iron found in the Upper Circumpolar Deep Water is hypothesized to result from organic matter remineralization. Shallow samples suggest that, if occurring, an iron isotopic fractionation during iron uptake by phytoplankton is characterized by a fractionation factor, such as: ∣Δ57Fe(plankton‐seawater)∣ < 0.48‰.
Multi-collector inductively coupled plasma mass spectrometers (MC-ICPMS) are widely used for Fe isotope measurements. The latter may be perturbed by interferences (notably from Cr and Ni) and matrix ...effects (notably from major elements), caused by elements remaining in the samples after purification. We quantified some of these perturbations and our ability to correct them whenever possible, using Thermo Neptune and Neptune Plus MC-ICPMS with a
57-58
Fe double-spike mass bias correction.
54
Cr and
58
Ni isobaric interference corrections were found to be extremely efficient up to Cr/Fe=0.12 and Ni/Fe=0.04 (g/g natural Fe). Matrix effects were found negligible up to at least Na/Fe=175, Mg/Fe=10, K/Fe=1.5, and Mo/Fe=75 (g/g natural Fe).
28
Si
2
+
interference was found negligible up to Si/Fe=50. Finally, we found that calcium and aluminum could cause significant interferences (e.g.,
40
Ca
16
O and
27
Al
2
+
), for Ca/Fe ≥ 2.5 and Al/Fe ≥ 2.5. The perturbation intensity relative to the Ca/Fe ratio was found dependent on the measurement conditions (plateau width). While working with samples with potentially high calcium or aluminum contents (such as calcite minerals or tests, bones and teeth, or marine samples and crustal rocks), we recommend to carefully take into account Ca and Al while tuning the instrument and checking the measurement accuracy with isotopic standards (i.e., doping the isotopic standard with Ca and Al levels comparable to those of the samples).
The exchange of material between particulates and seawater along the continental margins, a process commonly referred to as boundary exchange, is thought to play a significant role in controlling the ...neodymium (Nd) isotope and Rare Earth Element (REE) composition of the oceans. This study provides experimental verification of this concept by quantifying the effect of particulate dissolution in seawater on dissolved εNd and REE compositions. Three closed-system experiments were performed using basaltic particulate material of riverine, estuarine and marine origin. The release of Nd from this basaltic material increased the εNd composition of seawater in all three experiments, with a εNd value close to that of the associated sediment being achieved within 80 days in all experiments. Mass balance indicates that up to 0.4% of Nd from the particulate phase was released to the seawater over the duration of these experiments, and that the rate of release varied according to particulate origin and surface area. Progressive variations in the PAAS normalised REE patterns, as well as the Eu and Ce anomalies and La/Yb ratio, demonstrate that REEs were also transferred from the basaltic particulates to seawater during the experiments. Despite evidence for the release of REEs from the particulate material, dissolved REE abundances decreased during the experiments, and are thought to reflect incorporation into the REE-phosphate mineral rhabdophane. Together these experimental results confirm that elemental release from basaltic sediments on the ocean margins is a significant marine flux that can have a major control on the composition of seawater.
•We experimentally assess the nature of basaltic particulate dissolution in seawater.•We quantify the importance of this process for marine εNd and REE compositions.•Particulate released Nd dominates seawater εNd by the end of the experiment.•REE patterns reflect particulate dissolution and scavenging into secondary phases.•These results verify the importance of boundary exchange on the ocean margins.
The natural river water certified reference material SLRS‐5 (NRC‐CNRC) was routinely analysed in this study for major and trace elements by ten French laboratories. Most of the measurements were made ...using ICP‐MS. Because no certified values are assigned by NRC‐CNRC for silicon and 35 trace element concentrations (rare earth elements, Ag, B, Bi, Cs, Ga, Ge, Li, Nb, P, Rb, Rh, Re, S, Sc, Sn, Th, Ti, Tl, W, Y and Zr), or for isotopic ratios, we provide a compilation of the concentrations and related uncertainties obtained by the participating laboratories. Strontium isotopic ratios are also given.
Le matériau de référence certifié d'eau de rivière naturelle SLRS‐5 (NRC‐CNRC) est analysé régulièrement comme contrôle qualité par dix laboratoires français étudiant les éléments majeurs et en trace dans les solutions naturelles. La plupart des mesures sont réalisées par ICP‐MS. Le silicium et 35 éléments en trace (terres rares, Ag, B, Bi, Cs, Ga, Ge, Li, Nb, P, Rb, Rh, Re, S, Sc, Sn, Th, Ti, Tl, W, Y et Zr) ne sont pas certifiés par NRC‐CNRC. Aucun rapport isotopique n'est disponible. Nous proposons, pour ces éléments, des valeurs moyennes et leurs incertitudes associées obtenues par les différents laboratoires participants. Le rapport isotopique de Sr est aussi mesuré.
Radiogenic isotopes of hafnium (Hf) and neodymium (Nd) are powerful tracers for water mass transport and trace metal cycling in the present and past oceans. However, due to the scarcity of available ...data the processes governing their distribution are not well understood. Here we present the first combined dissolved Hf and Nd isotope and concentration data from surface waters of the Atlantic sector of the Southern Ocean. The samples were collected along the Zero Meridian, in the Weddell Sea and in the Drake Passage during RV Polarstern expeditions ANTXXIV/3 and ANTXXIII/3 in the frame of the International Polar Year (IPY) and the GEOTRACES program. The general distribution of Hf and Nd concentrations in the region is similar. However, at the northernmost station located 200km southwest of Cape Town a pronounced increase of the Nd concentration is observed, whereas the Hf concentration is minimal, suggesting much less Hf than Nd is released by the weathering of the South African Archean cratonic rocks. From the southern part of the Subtropical Front (STF) to the Polar Front (PF) Hf and Nd show the lowest concentrations (<0.12pmol/kg and 10pmol/kg, respectively), most probably due to the low terrigenous flux in this area and efficient scavenging of Hf and Nd by biogenic opal. In the vicinity of landmasses the dissolved Hf and Nd isotope compositions are clearly labeled by terrigenous inputs. Near South Africa Nd isotope values as low as εNd=−18.9 indicate unradiogenic inputs supplied via the Agulhas Current. Further south the isotopic data show significant increases to εHf=6.1 and εNd=−4.0 documenting exchange of seawater Nd and Hf with the Antarctic Peninsula. In the open Southern Ocean the Nd isotope compositions are relatively homogeneous (εNd∼−8 to −8.5) towards the STF, within the Antarctic Circumpolar Current, in the Weddell Gyre, and the Drake Passage. The Hf isotope compositions in the entire study area only show a small range between εHf=+6.1 and +2.8 support Hf to be more readily released from young mafic rocks compared to old continental ones. The Nd isotope composition ranges from εNd=−18.9 to −4.0 showing Nd isotopes to be a sensitive tracer for the provenance of weathering inputs into surface waters of the Southern Ocean.
One of the key activities during the initial phase of the international GEOTRACES program was an extensive international intercalibration effort, to ensure that results for a range of trace elements ...and isotopes (TEIs) from different cruises and from different laboratories can be compared in a meaningful way. Here we present the results from the intercalibration efforts on neodymium isotopes and rare earth elements in seawater and marine particles. Fifteen different laboratories reported results for dissolved 143Nd/144Nd ratios in seawater at three different locations (BATS 15 m, BATS 2000 m, SAFe 3000 m), with an overall agreement within 47 to 57 ppm (2σ standard deviation of the mean). A similar agreement was found for analyses of an unknown pure Nd standard solution carried out by 13 laboratories (56 ppm), indicating that mass spectrometry is the main variable in achieving accurate and precise Nd isotope ratios. Overall, this result is very satisfactory, as the achieved precision is a factor of 40 better than the range of Nd isotopic compositions observed in the global ocean. Intercalibration for dissolved rare earth element concentrations (REEs) by six laboratories for two water depths at BATS yielded a reproducibility of 15% or better for all REE except Ce, which seems to be the most blank‐sensitive REE. Neodymium concentrations from 12 laboratories show an agreement within 9%, reflecting the best currently possible reproducibility. Results for Nd isotopic compositions and REE concentrations on marine particles are inconclusive, and should be revisited in the future.
This work demonstrates the feasibility of the measurement of the isotopic composition of dissolved iron in seawater for an iron concentration range, 0.05−1 nmol L−1, allowing measurements in most ...oceanic waters, including Fe depleted waters of high nutrient low chlorophyll areas. It presents a detailed description of our previously published protocol, with significant improvements on detection limit and blank contribution. Iron is preconcentrated using a nitriloacetic acid superflow resin and purified using an AG 1-×4 anion exchange resin. The isotopic ratios are measured with a multicollector-inductively coupled plasma mass spectrometer (MC-ICPMS) Neptune, coupled with a desolvator (Aridus II or Apex-Q), using a 57Fe−58Fe double spike mass bias correction. A Monte Carlo test shows that optimum precision is obtained for a double spike composed of approximately 50% 57Fe and 50% 58Fe and a sample to double spike quantity ratio of approximately 1. Total procedural yield is 91 ± 25% (2SD, n = 55) for sample sizes from 20 to 2 L. The procedural blank ranges from 1.4 to 1.1 ng, for sample sizes ranging from 20 to 2 L, respectively, which, converted into Fe concentrations, corresponds to blank contributions of 0.001 and 0.010 nmol L−1, respectively. Measurement precision determined from replicate measurements of seawater samples and standard solutions is 0.08‰ (δ56Fe, 2SD). The precision is sufficient to clearly detect and quantify isotopic variations in the oceans, which so far have been observed to span 2.5‰ and thus opens new perspectives to elucidate the oceanic iron cycle.
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