Search

An extensive compilation of published neodymium (Nd) concentrations and isotopic compositions (Nd IC) was realized in order to establish a new database and a map (using a high resolution geological map of the area) of the distribution of these parameters for all the Mediterranean margins. Data were extracted from different kinds of samples: river solid discharge deposited on the shelf, sedimentary material collected on the margin or geological material outcropping above or close to a margin. Additional analyses of surface sediments were done, in order to improve this dataset in key areas (e.g., Sicilian strait). The Mediterranean margin Nd isotopic signatures vary from non-radiogenic values around the Gulf of Lions, (εNd values ~ -11) to radiogenic values around the Aegean and the Levantine sub-basins up to +6. Using a high resolution regional oceanic model (1/12° of horizontal resolution), εNd distribution was simulated for the first time in the Mediterranean Sea. The high resolution of the model provides the unique opportunity to represent a realistic thermohaline circulation in the basin and thus apprehend the processes governing the Nd isotope distribution in the marine environment. Results reinforce the preceding conclusions on boundary exchange “BE� as an important process in the Nd oceanic cycle. Nevertheless the present approach simulates a slightly too radiogenic value in the Med Sea, this bias will likely be corrected once the dust and river inputs will be included in the model. This work highlights that a significant interannual variability of εNd distribution in seawater could occur. In particular, important hydrological events such as the Eastern Mediterranean Transient (EMT), associated with deep water formed in the Aegean sub-basin, could induce a shift in εNd at deep/intermediate depths that could be noticeable in the Eastern part of the basin. This underlines that the temporal and geographical variations of εNd could represent an interesting insight of Nd as tracer of the Mediterranean Sea circulation, in particular in the context of paleo-oceanographic applications. [ABSTRACT FROM AUTHOR]

The Mediterranean Sea is one of the most oligotrophic regions of the oceans, and nutrients have been shown to limit both phytoplankton and bacterial activities, resulting in a potential major role of dissolved organic carbon (DOC) export in the biological pump. Strong DOC accumulation in surface waters is already well documented, though measurements of DOC stocks and export flux are still sparse and associated with major uncertainties. This study provides the first basin-scale overview and analysis of organic carbon stocks and export fluxes in the Mediterranean Sea through a modeling approach based on a coupled model combining a mechanistic biogeochemical model (Eco3MMED) and a high-resolution (eddy-resolving) hydrodynamic simulation (NEMO-MED12). The model is shown to reproduce the main spatial and seasonal biogeochemical characteristics of the Mediterranean Sea. Model estimations of carbon export are also of the same order of magnitude as estimations from in situ observations, and their respective spatial patterns are mutually consistent. Strong differences between the western and eastern basins are evidenced by the model for organic carbon export. Though less oligotrophic than the eastern basin, the western basin only supports 39% of organic carbon (particulate and dissolved) export. Another major result is that except for the Alboran Sea, the DOC contribution to organic carbon export is higher than that of particulate organic carbon (POC) throughout the Mediterranean Sea, especially in the eastern basin. This paper also investigates the seasonality of DOC and POC exports as well as the differences in the processes involved in DOC and POC exports in light of intracellular quotas. Finally, according to the model, strong phosphate limitation of both bacteria and phytoplankton growth is one of the main drivers of DOC accumulation and therefore of export. [ABSTRACT FROM AUTHOR]

Helium isotopes (3He, 4He) are useful tracers for investigating the deep ocean circulation and for evaluating ocean general circulation models, because helium is a stable and conservative nuclide that does not take part in any chemical or biological process. Helium in the ocean originates from three different sources, namely, (i) gas dissolution in equilibrium with atmospheric helium, (ii) helium-3 addition by radioactive decay of tritium (called tritiugenic helium), and (iii) injection of terrigenic helium-3 and helium-4 by the submarine volcanic activity which occurs mainly at plate boundaries, and also addition of (mainly) helium-4 from the crust and sedimentary cover by -decay of uranium and thorium contained in various minerals. We present the first simulation of the terrigenic helium isotope distribution in the whole Mediterranean Sea using a high-resolution model (NEMO-MED12). For this simulation we build a simple source function for terrigenic helium isotopes based on published estimates of terrestrial helium fluxes. We estimate a hydrothermal flux of 3.5 mol3 He yr-1 and a lower limit for the crustal flux at 1.6×10-7 4He molm-2 yr-1. In addition to providing constraints on helium isotope degassing fluxes in the Mediterranean, our simulations provide information on the ventilation of the deep Mediterranean waters which is useful for assessing NEMO-MED12 performance. This study is part of the work carried out to assess the robustness of the NEMO-MED12 model, which will be used to study the evolution of the climate and its effect on the biogeochemical cycles in the Mediterranean Sea, and to improve our ability to predict the future evolution of the Mediterranean Sea under the increasing anthropogenic pressure. [ABSTRACT FROM AUTHOR]

This numerical study provides the first simulation of the anthropogenic tritium invasion and its decay product helium-3 (³He) in the Mediterranean Sea. The simulation covers the entire tritium (³H) transient generated by the atmospheric nuclear weapons tests performed in the 1950s and early 1960s and is run till 2011. Tritium, helium-3 and their derived age estimates are particularly suitable for studying intermediate and deep-water ventilation and spreading of water masses at intermediate/deep levels. The simulation is made using a high-resolution regional model NEMO (Nucleus for European Modelling of the Ocean), in a regional configuration for the Mediterranean Sea called MED12, forced at the surface with prescribed tritium evolution derived from observations. The simulation is compared to measurements of tritium and helium-3 performed along large-scale transects in the Mediterranean Sea during the last few decades on cruises of R/V Meteor: M5/6, M31/1, M44/4, M51/2, M84/3, and R/V Poseidon: 234. The results show that the input function used for the tritium generates a realistic distribution of the main hydrographic features of the Mediterranean Sea circulation. In the eastern basin, the results highlight the weak formation of Adriatic Deep Water in the model, which explains its weak contribution to the Eastern Mediterranean Deep Water (EMDW) in the Ionian sub-basin. It produces a realistic representation of the Eastern Mediterranean Transient (EMT) signal, simulating a deep-water formation in the Aegean subbasin at the beginning of 1993, with a realistic timing of deep-water renewal in the eastern basin. [ABSTRACT FROM AUTHOR]

Constraints on the Mediterranean Sea's storage of anthropogenic CO2 are limited, coming only from data-based approaches that disagree by more than a factor of two. Here we simulate this marginal sea's anthropogenic carbon storage by applying a perturbation approach in a high-resolution regional model. Our model simulates that, between 1800 and 2001, basin-wide CO2 storage by the Mediterranean Sea has increased by 1.0 Pg C, a lower limit based on the model's weak deep-water ventilation, as revealed by evaluation with CFC-12. Furthermore, by testing a data-based approach (transit time distribution) in our model, comparing simulated anthropogenic CO2 to values computed from simulated CFC-12 and physical variables, we conclude that the associated basin-wide storage of 1.7 Pg, published previously, must be an upper bound. Out of the total simulated storage of 1.0 Pg C, 75% comes from the air--sea flux into the Mediterranean Sea and 25% comes from net transport from the Atlantic across the Strait of Gibraltar. Sensitivity tests indicate that the Mediterranean Sea's higher total alkalinity, relative to the global-ocean mean, enhances the Mediterranean's total inventory of anthropogenic carbon by 10 %. Yet the corresponding average anthropogenic change in surface pH does not differ significantly from the global-ocean average, despite higher total alkalinity. In Mediterranean deep waters, the pH change is estimated to be between -0.005 and -0.06 pH units. [ABSTRACT FROM AUTHOR]

The distribution of dissolved aluminium in the West Atlantic Ocean shows a mirror image with that of dissolved silicic acid, hinting at intricate interactions between the ocean cycling of Al and Si. The marine biogeochemistry of Al is of interest because of its potential impact on diatom opal remineralisation, hence Si availability. Furthermore, the dissolved Al concentration at the surface ocean has been used as a tracer for dust input, dust being the most important source of the bio-essential trace element iron to the ocean. Previously, the dissolved concentration of Al was simulated reasonably well with only a dust source, and scavenging by adsorption on settling biogenic debris as the only removal process. Here we explore the impacts of (i) a sediment source of Al in the Northern Hemisphere (especially north of ~40 ° N), (ii) the imposed velocity field, and (iii) biological incorporation of Al on the modelled Al distribution in the ocean. The sediment source clearly improves the model results, and using a different velocity field shows the importance of advection on the simulated Al distribution. Biological incorporation appears to be a potentially important removal process. However, conclusive independent data to constrain the Al/Si incorporation ratio by growing diatoms are missing. Therefore, this study does not provide a definitive answer to the question of the relative importance of Al removal by incorporation compared to removal by adsorptive scavenging. [ABSTRACT FROM AUTHOR]

The distribution of dissolved aluminium (Al) in the ocean is of interest because of its potential impact on diatom remineralisation and the use of surface ocean Al as a tracer for dust. Previously, the ocean Al concentration has been simulated reasonably well with only a dust source and scavenging as the removal process. In this study the simulation has been significantly improved by a more refined circulation and the addition of a sediment resuspension source. The latter confirms that the most significant sources of Al to the ocean are dust deposition and sediment resuspension. Simulations with biological incorporation have been performed as well. These show that this can be an important removal process. However, this study does not provide a definitive answer to the question what the relative amount of incorporation is compared to scavenging. [ABSTRACT FROM AUTHOR]

Boundary Exchange (BE - exchange of elements between continental margins and the open ocean) has recently been emphasized as a key process in the oceanic cycle of neodymium (Nd). We here use a regional eddy-permitting resolution Ocean General Circulation Model (1/4°) of the North Atlantic basin to simulate the distribution of the Nd isotopic composition, considering BE as the only source. Results show good agreement with the data, confirming previous results obtained using the same parameterization of the source in a coarse resolution global model (Arsouze et al., 2007), and therefore the major control played by the BE processes in the Nd cycle on the regional scale. We quantified the exchange rate of the BE, and found that the time needed for the continental margins to significantly imprint the chemical composition of the surrounding seawater (further referred as characteristic exchange time) is of the order of 0.2 years. However, the timescale of the BE may be subject to large variations as a very short exchange time (a few days) is needed to reproduce the highly negative values of surface waters in the Labrador Sea, whereas a longer one (up to 0.5 years) is required to simulate the radiogenic influence of basaltic margins and distinguish the negative isotopic signatures of North Atlantic Deep Water from the more radiogenic southern origin water masses. This likely represents geographical variations in erosion fluxes and the subsequent particle load onto the continental marings. These exchange times are significantly lower than the previous evaluations using a low resolution model (6 months to 10 years), but however in agreement with the available seawater Nd isotope data, highlighting the importance of the model dynamics in simulating the BE process. [ABSTRACT FROM AUTHOR]

The decoupled behaviour observed between Nd isotopic composition (Nd IC, also referred as ϵNd) and Nd concentration cycles has led to the notion of a "Nd paradox". While ϵNd 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 ϵNd 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 ϵNd, and 2) BE is the dominant Nd source to the ocean. The estimated BE flux (1.1x1010 g(Nd)/yr) is much higher than both dissolved river discharge (2.6x108 g(Nd)/yr) and atmospheric inputs (1.0x108 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. [ABSTRACT FROM AUTHOR]

Estimates of the ocean's large-scale transport of anthropogenic CO2 are based on one-time hydrographic sections, but the temporal variability of this transport has not been investigated. The aim of this study is to evaluate how the seasonal and mesoscale variability affect data-based estimates of anthropogenic CO2 transport. To diagnose this variability, we made a global anthropogenic CO2 simulation using an eddy-permitting version of the coupled ocean seaice model ORCA-LIM. As for heat transport, the seasonally varying transport of anthropogenic CO2 is largest within 20° of the equator and shows secondary maxima in the subtropics. Ekman transport generally drives most of the seasonal variability, but the contribution of the vertical shear becomes important near the equator and in the Southern Ocean. Mesoscale variabilty contributes to the annual-mean transport of both heat and anthropogenic CO2 with strong pole-ward transport in the Southern Ocean and equatorward transport in the tropics. This "rectified" eddy transport is largely baroclinic in the tropics and barotropic in the Southern Ocean due to a larger contribution from standing eddies. Our analysis revealed that most previous hydrographic estimates of meridional transport of anthropogenic CO2 are severely biased because they neglect temporal fluctuations due to non- Ekman velocity variations. In each of the three major ocean basins, this bias is largest near the equator and in the high southern latitudes. In the subtropical North Atlantic, where most of the hydrographic-based estimates have been focused, this uncertainty represents up to 20% and 30% of total meridional transport of heat and CO2. Generally though, outside the tropics and Southern Ocean, there are only small variations in meridional transport due to seasonal variations in tracer fields and time variations in eddy transport. For the North Atlantic, eddy variability accounts for up to 10% and 15% of the total transport of heat and CO2. This component is not accounted for in coarse-resolution hydrographic surveys. [ABSTRACT FROM AUTHOR]

Estimates of the ocean's large-scale transport of anthropogenic CO2 are based on one-time hydrographic sections, but the temporal variability of this transport has not been investigated. We have evaluated this variability with the aid of an eddy-permitting version of the coupled ocean sea-ice model ORCA-LIM. As for heat transport, the seasonally varying transport of anthropogenic CO2 is largest within 20° of the Equator and shows secondary maxima in the subtropics. Ekman transport generally drives most of the seasonal variability, but the contribution of the vertical shear becomes important near the Equator and in the Southern Ocean. Mesoscale variabilty contributes to the annual-mean transport of both heat and anthropogenic CO2 with strong poleward transport in the Southern Ocean and equatorward transport in the tropics. This "rectified" eddy transport is largely baroclinic in the tropics and barotropic in the Southern Ocean due to a larger contribution from standing eddies. Our analysis revealed that most previous hydrographic estimates of meridional transport of anthropogenic CO2 are severely biased because they neglect temporal fluctuations due to non-Ekman velocity variations. In each of the three major ocean basins, this bias is largest near the Equator and in the high southern latitudes. In the subtropical North Atlantic, where most of the hydrographic-based estimates have been focused, this uncertainty represents up to 20% and 30% of total meridional transport of heat and CO2. Generally though, outside the tropics and Southern Ocean, there are only small variations in meridional transport due to seasonal variations in tracer fields and time variations in eddy transport. For the North Atlantic, eddy variability accounts for up to 10% and 15% of the total transport of heat and CO2. This component is not accounted for in coarse-resolution hydrographic surveys. [ABSTRACT FROM AUTHOR]

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 "Nd 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 ϵ Nd 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 "Nd signature, and thus the potential use of ϵNd as a tracer of past oceanic circulation. [ABSTRACT FROM AUTHOR]

The oceanic neodymium isotopic composition (hereafter expressed as εNd) is modeled for the Last Glacial Maximum (LGM) using the coarse resolution Ocean Global Circulation Model NEMO-ORCA2°. This study focuses on the impact of changes in the overturning cell and circulation patterns between LGM and Holocene on εNd in the Atlantic basin. Three different LGM freshwater forcing experiments are performed to test the variability in εNd oceanic distribution as a function of ocean circulation. Highly distinct representations of 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, mean Atlantic LGM εNd values are remain half a unit more radiogenic than for the modern control run. A fourth experiment shows that changes in Nd sources and bathymetry drive a shift in the εNd signature of Northern end-members (NADW or GNAIW glacial equivalent) that is sufficient to explain the shift in mean εNd during our LGM simulations. None of our three LGM circulation scenarios gives a better agreement with the existing εNd paleo-data, as the model fails in reproducing the dynamical features of the area. Therefore, this study cannot indicate the likelihood of a given LGM oceanic circulation scenario. Rather, our modeling results highlight the need for data from western Atlantic deep waters, where the εNd gradient in the three LGM scenarios is the most important (up to 3 εNd). This would also aid more precise conclusions concerning the north end-member εNd signature evolution, and thus the potential use of εNd as a tracer of past oceanic circulation. [ABSTRACT FROM AUTHOR]

Global-scale tracer simulations are typically made at coarse resolution without explicitly modelling eddies. Here we ask what role do eddies play in ocean uptake, storage, and meridional transport of transient tracers. We made global anthropogenic transient-tracer simulations in coarse-resolution (2° cos φ × 2°, ORCA2) and eddypermitting ( ½° cos φ×½°, ORCA05) versions of the ocean general circulation model OPA9. Our focus is on surfaceto- intermediate waters of the southern extratropics where air-sea tracer fluxes, tracer storage, and meridional tracer transport are largest. Eddies have little effect on global and regional bomb Δ14C uptake and storage. Yet for anthropogenic CO2 and CFC-11, refining the horizontal resolution reduced southern extratropical uptake by 25% and 28%, respectively. There is a similar decrease in corresponding inventories, which yields better agreement with observations. With higher resolution, eddies strengthen upper ocean vertical stratification and reduce excessive ventilation of intermediate waters by 20% between 60° S and 40° S. By weakening the residual circulation, i.e., the sum of Eulerian mean flow and the opposed eddy-induced flow, eddies reduce the supply of tracer-impoverished deep waters to the surface near the Antarctic divergence, thus reducing the air-sea tracer flux. Thus in the eddy permitting model, surface waters in that region have more time to equilibrate with the atmosphere before they are transported northward and subducted. As a result, the eddy permitting model's inventories of CFC- 11 and anthropogenic CO2 are lower in that region because mixed-layer concentrations of both tracers equilibrate with the atmosphere on relatively short time scales (15 days and 6 months, respectively); conversely, bombΔ14C's air-sea equilibration time of 6 years is so slow that, even in the eddy permitting model, there is little time for surface concentrationsto equilibrate with the atmosphere, i.e., before surface waters are subducted. [ABSTRACT FROM AUTHOR]