Using a simple parameterisation that resolves the first order global Nd isotopic composition (hereafter expressed as εNd in an Ocean Global Circulation Model, we have tested the impact of different ...circulation scenarios on the εNd in the Atlantic for the Last Glacial Maximum (LGM), relative to a modern control run. Three different LGM freshwater forcing experiments are performed to test for variability in the εNd oceanic distribution as a function of ocean circulation. Highly distinct representations of the ocean circulation are generated in the three simulations, which drive significant differences in εNd, particularly in deep waters of the western part of the basin. However, at the LGM, the Atlantic is more radiogenic than in the modern control run, particularly in the Labrador basin and in the Southern Ocean. A fourth experiment shows that changes in Nd sources and bathymetry drive a shift in the εNd signature of the basin that is sufficient to explain the changes in the εNd signature of the northern end-member (NADW or GNAIW glacial equivalent) in our LGM simulations. All three of our LGM circulation scenarios show good agreement with the existing intermediate depth εNd paleo-data. This study cannot indicate the likelihood of a given LGM oceanic circulation scenario, even if simulations with a prominent water mass of southern origin provide the most conclusive results. Instead, our modeling results highlight the need for more data from deep and bottom waters from western Atlantic, where the εNd change in the three LGM scenarios is the most important (up to 3 εNd. This would also aid more precise conclusions concerning the evolution of the northern end-member εNd signature, and thus the potential use of εNd as a tracer of past oceanic circulation.
The establishment of the Antarctic Circumpolar Current (ACC) is one of the most important events of the Cenozoic for both global oceanic circulation and climate. The onset of this major current ...hinges on the opening of two major oceanic passages, the Drake Passage and the Tasmanian gateways that connect Pacific, Atlantic and Indian oceans, allowing a modern-like thermohaline circulation. For decades, the ACC onset has been considered as the trigger of the Oligocene glaciation at 33.7 Ma, which marks the beginning of the modern icehouse climate. Today, this scenario is debated. The main obstacle to evaluate the ACC influence on the Oligocene glaciation remains the ill-constrained timing of the Drake Passage gateway opening. Here, we analyse the geochemical composition and Sr isotope ratio of dated planktonic and benthic foraminifera from two IODP and ODP legs in the Southern Atlantic and Pacific oceans (SAO and PO, respectively) to assess the variability of seawater masses' chemical composition through time and to better constrain the timing of the Drake Passage gateway opening along the Eocene-Oligocene interval. These results, based on seawater paleo temperature (Mg/Ca molar ratios), redox (Ce/Ce* anomaly) and provenance (87Sr/86Sr) proxies, highlight a gradual seawater mass mixing between the SAO and PO from 31 Ma to 26 Ma. Combined with a reconsideration of the fossil fish teeth Neodymium isotope records, these geochemical tracers evidencing the SAO-PO interconnection depicts the Drake Passage gateway opening and deepening during this 31–26 Ma interval and thus, the timing of the ACC onset. Hence, antecedence of the Oligocene glaciation onset (at 33.7 Ma) relative to the ACC onset (31–26 Ma) implies that the ACC did not trigger the Oligocene glaciation and that the role of atmospheric pCO2 should be further considered.
•The Drake gateway probably opened to significant eastward deep seawater flow from 31 to 26 Ma.•Antarctic Circumpolar Current likely started during this 31 to 26 Ma time interval.•The Antarctic Circumpolar Current onset did not trigger the Oligocene glaciation at 33.7 Ma.•The Oligocene glaciation enigma likely hinges on atmospheric pCO2 threshold effect.
We report detailed sections of radium-226 (226Ra, T1/2= 1602 years) activities and barium (Ba) concentrations determined in the North Atlantic (Portugal–Greenland–Canada) in the framework of the ...international GEOTRACES program (GA01 section – GEOVIDE project, May–July 2014). Dissolved 226Ra and Ba are strongly correlated along the section, a pattern that may reflect their similar chemical behavior. Because 226Ra and Ba have been widely used as tracers of water masses and ocean mixing, we investigated their behavior more thoroughly in this crucial region for thermohaline circulation, taking advantage of the contrasting biogeochemical patterns existing along the GA01 section. We used an optimum multiparameter (OMP) analysis to distinguish the relative importance of physical transport (water mass mixing) from nonconservative processes (sedimentary, river or hydrothermal inputs, uptake by particles and dissolved–particulate dynamics) on the 226Ra and Ba distributions in the North Atlantic. Results show that the measured 226Ra and Ba concentrations can be explained by conservative mixing for 58 and 65 % of the samples, respectively, notably at intermediate depth, away from the ocean interfaces. 226Ra and Ba can thus be considered conservative tracers of water mass transport in the ocean interior on the space scales considered here, namely, on the order of a few thousand kilometers. However, regions in which226Ra and Ba displayed nonconservative behavior and in some cases decoupled behaviors were also identified, mostly at the ocean boundaries (seafloor, continental margins and surface waters). Elevated 226Ra and Ba concentrations found in deepwater in the West European Basin suggest that lower Northeast Atlantic Deep Water (NEADWl) accumulates 226Ra and Ba from sediment diffusion and/or particle dissolution during transport. In the upper 1500 m of the West European Basin, deficiencies in 226Ra and Ba are likely explained by their incorporation in planktonic calcareous and siliceous shells, or in barite (BaSO4) by substitution or adsorption mechanisms. Finally, because Ba and 226Ra display different source terms (mostly deep-sea sediments for 226Ra and rivers for Ba), strong decoupling between 226Ra and Ba were observed at the land–ocean boundaries. This is especially true in the shallow stations near the coasts of Greenland and Newfoundland where high 226Ra / Ba ratios at depth reflect the diffusion of 226Ra from sediment and low 226Ra / Ba ratios in the upper water column reflect the input of Ba associated with meteoric waters.
Oxygen minimum zones (OMZs) cover extensive areas of eastern boundary ocean regions and play an important role in the cycling of the essential micronutrient iron (Fe). The isotopic composition of ...dissolved Fe (dFe) in shelf and slope waters on the Senegalese margin was determined to investigate the processes leading to enhanced dFe concentrations (up to 2 nM) in this tropical North Atlantic OMZ. On the shelf, the δ56Fe value of dFe (relative to the reference material IRMM-014) was as low as −0.33‰, which can be attributed to input of dFe from both reductive and nonreductive dissolution of sediments. Benthic inputs of dFe are subsequently upwelled to surface waters and recycled in the water column by biological uptake and remineralisation processes. Remineralised dFe is characterised by relatively high δ56Fe values (up to +0.41‰), and the contribution of remineralised Fe to the total dFe pool increases with distance from the shelf. Remineralisation plays an important role in the redistribution of dFe that is mainly supplied by benthic and atmospheric inputs, although dust inputs, estimated from dissolved aluminium concentrations, were low at the time of our study (2–9 nmol dFe m−2 d−1). As OMZs are expected to expand as climate warms, our data provide important insights into Fe sources and Fe cycling in the tropical North Atlantic Ocean.
This study presents a water mass analysis along the JC150 section in the subtropical North Atlantic, based on hydrographic and nutrient data, by combining an extended optimum multiparameter analysis ...(OMPA) with a Lagrangian particle tracking experiment (LPTE). This combination, which was proposed for the first time, aided in better constraining the OMPA end-member choice and providing information about their trajectories. It also enabled tracing the water mass origins in surface layers, which cannot be achieved with an OMPA. The surface layers were occupied by a shallow type of Eastern South Atlantic Central Water (ESACW) with traces of the Amazon plume in the west. Western North Atlantic Central Water dominates from 100 to 500 m, while the 13 °C-ESACW contribution occurs marginally deeper (500–900 m). At approximately 700 m, Antarctic Intermediate Water (AAIW) dominates the west of the Mid-Atlantic Ridge (MAR), while Mediterranean Water dominates the east with a small but non-negligible contribution down to 3500 m. Below AAIW, Upper Circumpolar Deep Water (UCDW) is observed throughout section (900–1250 m). Labrador Sea Water (LSW) is found centered at 1500 m, where the LPTE highlights an eastern LSW route from the eastern North Atlantic to the eastern subtropical Atlantic, which was not previously reported. North East Atlantic Deep Water (encompassing a contribution of Iceland-Scotland Overflow Water) is centered at ~2500 m, while North West Atlantic Bottom Water (NWABW, encompassing a contribution of Denmark Strait Overflow Water) is principally localized in the west of the MAR in the range of 3500–5000 m. NWABW is also present in significant proportions (>25%) in the east of the MAR, suggesting a crossing of the MAR possibly through the Kane fracture zone. This feature has not been investigated so far. Finally, Antarctic Bottom Water is present in deep waters throughout the section, mainly in the west of the MAR. Source waters have been characterized from GEOTRACES sections, which enables estimations of trace elements and isotope transport within water masses in the subtropical North Atlantic.
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•Optimum multiparameter analysis combined with a Lagrangian particle tracking experiment.•Impact of Amazon River plume identified as north as 22°N.•Mediterranean Water impact on North Atlantic Deep Water salinity as deep as 3500 m.•Un-previously described Labrador Sea Water route in the eastern North Atlantic.•North West Atlantic Bottom Water transport across the Mid-Atlantic Ridge.
Neodymium isotopic composition and rare earth element concentrations were measured in seawater samples from eleven stations in the Nordic Seas. These data allow us to study how the Iceland Scotland ...Overflow Water (ISOW) acquires its neodymium signature in the modern ocean. The waters overflowing the Faroe Shetland channel are characterized by ɛNd = −8.2 ± 0.6, in good agreement with the only other data point, published 19 years ago. In the Greenland and Iceland Seas the water masses leading to the formation of the ISOW display lower neodymium isotopic composition, with ɛNd around −11 and −9, respectively. Since no water masses in the Nordic Seas are characterized by ɛNd > −8, the radiogenic signature of the ISOW likely reflects inputs from the highly radiogenic Norwegian Basin basaltic margins (Jan‐Mayen, Iceland, Faroe, with ɛNd ≈ +7). In addition to the neodymium isotopic composition, the rare earth element patterns suggest that these inputs occur via the remobilization (which includes resuspension and dissolution) of sediments deposited on the margins. Whereas the neodymium isotopic composition behaves conservatively in the oceans in the absence of lithogenic inputs, and can be used as a water mass tracer, these results emphasize the role of interactions, between sediments deposited on margins and seawater, in the acquisition of the neodymium isotopic composition of water masses. These results should allow a better use of this parameter to trace the present and the past circulation in the North Atlantic.
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.
The oceanic distributions of 231Pa and 230Th are simulated with the global coupled biogeochemical‐ocean general circulation model NEMO‐PISCES. These natural nonconservative tracers, which are removed ...from the water column by reversible scavenging processes onto particles, have been used to study modern and past ocean circulation. Our model includes three different types of particles: particulate organic matter (POM), calcium carbonate (CaCO3), and biogenic silica (BSi). It also considers two particle classes: small particles (POM) that sink slowly (3 m/d) and large particles (POM, CaCO3, BSi) that sink much more rapidly (50 m/d to 200 m/d) in the water column. 231Pa and 230Th are simulated with a reversible scavenging model that uses partition coefficients between dissolved and particulate phases that depend on particle type and size. Model results are then compared with 231Pa and 230Th observations in the water column and modern sediments. A preliminary evaluation of the particle fields simulated by the PISCES model has revealed that particle concentrations are reasonable at the surface but largely underestimated in the deep ocean. Largely to compensate for this, we find it necessary to use partition coefficients that vary as a function of particle size by significantly more that observed to obtain relatively realistic results. In the water column, 231Pa and 230Th fluxes are mainly controlled by the slowly sinking particles and partition coefficients need to be parameterized as a function of particle flux, as suggested by observations. Considering discrepancies between the modeling particle fields and those observed, we were forced to use exaggerated values for partition coefficients in order to get realistic tracer distributions. These 231Pa and 230Th simulations have provided an opportunity to propose some future developments of the PISCES model, in order to make progress in the simulation of trace elements. Assigning calcium carbonate, biogenic silica, and aluminosilicates to the small particle pool represents a credible approach to increase its concentration and subsequently simulate realistic tracer distributions in the water column using reasonable values for the partition coefficients, as well as a realistic fractionation in the sediments at all depths.
We report data on the isotopic composition of cadmium, copper, iron, lead, zinc, and molybdenum at the GEOTRACES IC1 BATS Atlantic intercalibration station. In general, the between lab and within‐lab ...precisions are adequate to resolve global gradients and vertical gradients at this station for Cd, Fe, Pb, and Zn. Cd and Zn isotopes show clear variations in the upper water column and more subtle variations in the deep water; these variations are attributable, in part, to progressive mass fractionation of isotopes by Rayleigh distillation from biogenic uptake and/or adsorption. Fe isotope variability is attributed to heavier crustal dust and hydrothermal sources and light Fe from reducing sediments. Pb isotope variability results from temporal changes in anthropogenic source isotopic compositions and the relative contributions of U.S. and European Pb sources. Cu and Mo isotope variability is more subtle and close to analytical precision. Although the present situation is adequate for proceeding with GEOTRACES, it should be possible to improve the within‐lab and between‐lab precisions for some of these properties.
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