Upper North Atlantic Deep Water Research Articles

Propagation and Transformation of Upper North Atlantic Deep Water From the Subpolar Gyre to 26.5°N

AbstractBecause new observations have revealed that the Labrador Sea is not the primary source for waters in the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) during the Overturning in the Subpolar North Atlantic Programme (OSNAP) period, it seems timely to re‐examine the traditional interpretation of pathways and property variability for the AMOC lower limb from the subpolar gyre to 26.5°N. In order to better understand these connections, Lagrangian experiments were conducted within an eddy‐rich ocean model to track upper North Atlantic Deep Water (uNADW), defined by density, between the OSNAP line and 26.5°N as well as within the Labrador Sea. The experiments reveal that 77% of uNADW at 26.5°N is directly advected from the OSNAP West section along the boundary current and interior pathways west of the Mid‐Atlantic Ridge. More precisely, the Labrador Sea is a main gateway for uNADW sourced from the Irminger Sea, while particles connecting OSNAP East to 26.5°N are exclusively advected from the Iceland Basin and Rockall Trough along the eastern flank of the Mid‐Atlantic Ridge. Although the pathways between OSNAP West and 26.5°N are only associated with a net formation of 1.1 Sv into the uNADW layer, they show large density changes within the layer. Similarly, as the particles transit through the Labrador Sea, they undergo substantial freshening and cooling that contributes to further densification within the uNADW layer.

Lagrangian Overturning Pathways in the Eastern Subpolar North Atlantic

Abstract The strength of the Atlantic meridional overturning circulation (AMOC) at subpolar latitudes is dominated by water mass transformation in the eastern subpolar North Atlantic Ocean (SPNA). However, the distribution of this overturning across the individual circulation pathways of both the Subpolar Gyre (SPG) and the Nordic seas overflows remains poorly understood. Here, we introduce a novel Lagrangian measure of the density-space overturning to quantify the principal pathways of the time-mean overturning circulation within an eddy-permitting ocean model hindcast. By tracing the trajectories of water parcels initialized from the northward inflows across the Overturning in the Subpolar North Atlantic Program (OSNAP) East section, we show that water mass transformation along the pathways of the eastern SPG accounts for 55% of the mean strength of the eastern subpolar AMOC. Water parcels following the dominant SPG pathway, sourced from the Subarctic Front, form upper North Atlantic Deep Water by circulating horizontally across sloping isopycnals in less than 2 years. A slower SPG route, entrained by overflow waters south of the Iceland–Faroes Ridge, is a crucial conduit for subtropical-origin water masses to penetrate the deep ocean on subdecadal time scales. On reproducing our findings using time-averaged velocity and hydrographic fields, we further show that the Nordic seas overflow pathways integrate multiple decades of water mass transformation before returning across the Greenland–Scotland Ridge. We propose that the strong disparity between the overturning time scales of the SPG (interannual) and the Nordic seas overflows (multidecadal) has important implications for the propagation of density anomalies within the eastern SPNA and hence the sources of AMOC variability.

Air‐Sea Interactions and Water Mass Transformation During a Katabatic Storm in the Irminger Sea

AbstractWe use a global 5‐km resolution model to analyze the air‐sea interactions during a katabatic storm in the Irminger Sea originating from the Ammassalik valleys. Katabatic storms have not yet been resolved in global climate models, raising the question of whether and how they modify water masses in the Irminger Sea. Our results show that dense water forms along the boundary current and on the shelf during the katabatic storm due to the heat loss caused by the high wind speeds and the strong temperature contrast. The dense water contributes to the lightest upper North Atlantic Deep Water as upper Irminger Sea Intermediate Water and thus to the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). The katabatic storm triggers a polar low, which in turn amplifies the near‐surface wind speed due to the superimposed pressure gradient, in addition to acceleration from a breaking mountain wave. Overall, katabatic storms account for up to 25% of the total heat loss (20 January 2020 to 30 September 2021) over the Irminger shelf of the Ammassalik area. Resolving katabatic storms in global models is therefore important for the formation of dense water in the western boundary current of the Irminger Sea, which is relevant to the AMOC, and for the large‐scale atmospheric circulation by triggering polar lows.

Oxygen export to the deep ocean following Labrador Sea Water formation

Abstract. The Labrador Sea in the North Atlantic Ocean is one of the few regions globally where oxygen from the atmosphere can reach the deep ocean directly. This is the result of wintertime deep convection, which homogenizes the water column to a depth of up to 2000 m and brings deep water undersaturated in oxygen into contact with the atmosphere. In this study, we analyze how the intense oxygen uptake during Labrador Sea Water (LSW) formation affects the properties of the outflowing deep western boundary current, which ultimately feeds the upper part of the North Atlantic Deep Water layer in much of the Atlantic Ocean. Seasonal cycles of oxygen concentration, temperature, and salinity from a 2-year time series collected by sensors moored at 600 m nominal depth in the outflowing boundary current at 53∘ N show a cooling, freshening, and increase in oxygen content of the water flowing out of the basin between March and August. Analysis of Argo float data suggests that this is preceded by an increased input of LSW into the boundary current about 1 month earlier. This input is the result of newly ventilated LSW entering from the interior, as well as LSW formed directly within the boundary current. Together, these results imply that the southward export of newly formed LSW primarily occurs in the months following the onset of deep convection, from March to August, and that this direct LSW export route controls the seasonal oxygen increase in the outflow at 600 m depth. During the rest of the year, properties of the boundary current measured at 53∘ N resemble those of Irminger Water, which enters the basin with the boundary current from the Irminger Sea. The input of newly ventilated LSW increases the oxygen concentration from 298 µmol L−1 in January to a maximum of 306 µmol L−1 in April. As a result of this LSW input, an estimated (1.60 ± 0.42) × 1012 mol yr−1 of oxygen are added to the outflowing boundary current, mostly during spring and summer, equivalent to 50 % of the wintertime uptake from the atmosphere in the interior of the basin. The export of oxygen from the subpolar gyre associated with this direct southward pathway of LSW is estimated to supply 42 %–71 % of the oxygen consumed annually in the upper North Atlantic Deep Water layer in the Atlantic Ocean between the Equator and 50∘ N. Our results show that the formation of LSW is important for replenishing oxygen to the deep oceans, meaning that possible changes in its formation rate and ventilation due to climate change could have wide-reaching impacts on marine life.

Mechanisms for Late 20th and Early 21st Century Decadal AMOC Variability

AbstractRecent studies using data from the OSNAP observational campaign and from numerical ocean models suggest that the Iceland Basin and the Irminger Sea may be more significant for formation of upper North Atlantic Deep Water than the Labrador Sea. Here, we present a set of hindcast integrations of a global 1/4° NEMO simulation from 1958 until nearly the present day, forced with three standard forcing data sets. We use the surface‐forced stream function, estimated from surface buoyancy fluxes, along with the overturning stream function, similarly defined in potential density space, to investigate the causal link between surface forcing and decadal variability in the strength of the Atlantic meridional overturning circulation (AMOC). We use the stream functions to demonstrate that watermasses in the simulations are transformed to higher densities as they propagate around the subpolar gyre from their formation locations in the north‐east Atlantic and the Irminger Sea, consistent with the picture emerging from observations. The surface heat loss from the Irminger Sea is confirmed to be the dominant mechanism for decadal AMOC variability, with the heat loss anomaly from the Labrador Sea having about half the magnitude. A scalar metric based on the surface‐forced stream function, accumulated in time, is found to be a good predictor of changes in the overturning strength. The AMOC variability is shown to be related to that of the North Atlantic Oscillation (NAO), primarily through the surface heat flux, itself dominated by the air‐sea temperature difference, but also with some local feedback from the SST to the surface fluxes.

Reversible scavenging and advection – Resolving the neodymium paradox in the South Atlantic

Significant gaps in our understanding of the oceanic cycling of neodymium (Nd) and the other rare earth elements (REEs) remain despite decades of research. One important observation which has not been adequately explained is that the concentration of dissolved Nd typically increases with depth, similar to nutrient profiles, while Nd isotopes appear to reflect conservative water mass mixing in the intermediate and deep ocean; this has been termed the “Nd paradox”. Here we present a detailed study of the dissolved Nd isotopic composition across a section at 40°S in the South Atlantic, collected by UK GEOTRACES cruise (section GA10). The South Atlantic represents a natural laboratory for our understanding of spatial controls on ocean geochemistry, because of the large variability of inputs, spatial differences in particulate cycling, and horizontal advection and mixing at depth between major northern- and southern-sourced water masses. This variability has also made the South Atlantic a critical region subject to intense investigations that aim at reconstructing past changes in ocean processes, such as changes in biological productivity and deep ocean circulation.Our Nd isotope results from the GA10 section provide observational data show the signal of water mass mixing and reversible scavenging. In the surface ocean (0–600 m), Nd isotopic compositions are distinct between different surface ocean currents and spatially can be tied to various continental sources. In the intermediate ocean (600–2500 m), the vertical Nd isotope distribution exhibits distinct signals of different water masses by horizontal advection, including upper North Atlantic Deep Water and Antarctic Intermediate Water formed in the Atlantic Ocean or the Indian Ocean. The Nd isotope distribution also reflects influence of reversible scavenging that smears the signals downwards in the water column (i.e., offset to more radiogenic values). In the deep ocean below 2500 m, Nd isotope distribution largely follows conservative water mass mixing model. Nd concentration in the deep ocean, however, deviates from conservative mixing and increases constantly with depth. We also observe that Nd isotopes appear to be shifted towards the composition of overlying water masses. These observations suggest that reversible scavenging of Nd onto organic and other types of particles is a major vertical process throughout the water column. We also suggest that this process can resolve the “Nd paradox” of decoupling of Nd concentration and isotopic composition due to mixing dynamics. Because abyssal water masses already have a high Nd concentration, a given amount of Nd added from the vertical process has less of an effect on Nd isotopic compositions in deep water masses than it does for intermediate water masses which have comparatively low Nd concentration.

Can We Better Constrain the Timing of GNAIW/UNADW Variability in the Western Equatorial Atlantic and Its Relationship to Climate Change During the Last Deglaciation?

AbstractWe have revisited the well‐trod VM28‐122 core retrieved from the deep Colombian Basin, which includes sediments that reflect modern Upper North Atlantic Deep Water and extends through the last deglaciation into the last glacial period when the site was bathed in Glacial North Atlantic Intermediate Water. Here, we leverage the nearby Cariaco Basin's surface water radiocarbon reconstruction (reservoir age, and ΔR) on the IntCal20 timescale to recast the period of the last deglaciation with a newly constrained age model. Based on the revised age model, we observe that the multimillennial decrease in benthic δ13C and B/Ca (which record δ13C of dissolved inorganic carbon and Δ[CO3−2], respectively) began at 18,100 ± 240 (95% CI) calibrated years BP, synchronous with Termination 1, as identified by changes in the Antarctic Ice Sheet composite and by the onset of rapid glacier recession in the Southern Hemisphere (Denton et al., 2010, https://doi.org/10.1126/science.1184119; Denton et al., 2021, https://10.1016/j.quascirev.2020.106771). The beginning of the decrease in benthic δ18O is concurrent with the changes in carbon chemistry or at most, a few hundred years later. It is no later than 17,700 ± 300 (95% CI) yrs BP in our record, at the putative start of Heinrich Stadial 1. With sufficient data density (more than 2–3 control points per kyr) and an independent record of past surface water radiocarbon variations, it is possible to achieve late glacial and deglacial chronologies with fidelities similar to those of ice cores. Doing so in more oceanographic locations should shed light more broadly on the mechanisms instrumental to abrupt climate change.

Dynamics of Deep Recirculation Cells Offshore of the Deep Western Boundary Current in the Subtropical North Atlantic (15°–30°N)

AbstractThe dynamics of the deep recirculation offshore of the deep western boundary current (DWBC) between 15° and 30°N within the upper North Atlantic Deep Water layer (1000 ≤ z ≤ 3000 m) is investigated with two different eddy-resolving numerical simulations. Despite some differences in the recirculation cells, our assessment of the modeled deep isopycnal circulation patterns (36.77 ≤ σ2 ≤ 37.06 kg m−3) shows that both simulations predict the DWBC flowing southward along the continental slope, while the so-called Abaco Gyre and two additional cyclonic cells recirculate waters northward in the interior. These cells are a few degrees wide, located along the DWBC path, and characterized by potential vorticity (PV) changes occurring along their mean streamlines. The analysis of the mean PV budget reveals that these changes result from the action of eddy forcing that tends to erode the PV horizontal gradients. The lack of a major upper-ocean boundary current within the study region, and the fact that the strongest eddy forcing is constrained within a few hundreds of kilometers of the western boundary, suggest that the DWBC is the primary source of eddy forcing. Finally, the eddies responsible for forcing the recirculation have dominant time scales between 100 and 300 days, which correspond to the primary observed variability scales of the DWBC transport at 26.5°N.

Antarctic Bottom Water Warming in the Brazil Basin: 1990s Through 2020, From WOCE to Deep Argo

AbstractA warming trend of 2.1 (±0.4) m °C yr−1 in bottom waters (4,500 to 5,900 dbar) spreading north from Antarctica through the Brazil Basin is quantified by comparing 2019–2020 data from a new Deep Argo regional pilot array to 1989–1995 data from full‐depth zonal and meridional hydrographic sections occupied across the basin before and during the World Ocean Circulation Experiment (WOCE). Additionally, float temperatures are about 0.046°C warmer than those from a long‐term climatology in those same bottom waters. Lower North Atlantic Deep Water in the basin shows no detectable warming, but Upper North Atlantic Deep Water exhibits evidence of warming in both analyses. The bottom‐intensified warming results in a reduction in vertical density stratification between the bottom and deep waters of about 1% per decade. This change is in contrast with the effects of surface‐intensified warming, which tends to increase vertical density stratification.

Characterization and Mapping of a Deep-Sea Sponge Ground on the Tropic Seamount (Northeast Tropical Atlantic): Implications for Spatial Management in the High Seas

Ferromanganese crusts occurring on seamounts are a potential resource for rare earth elements that are critical for low-carbon technologies. Seamounts, however, host vulnerable marine ecosystems (VMEs), which means that spatial management is needed to address potential conflicts between mineral extraction and the conservation of deep-sea biodiversity. Exploration of the Tropic Seamount, located in an Area Beyond National Jurisdiction (ABNJ) in the subtropical North Atlantic, revealed large amounts of rare earth elements, as well as numerous VMEs, including high-density octocoral gardens, Solenosmilia variabilis patch reefs, xenophyophores, crinoid fields and deep-sea sponge grounds. This study focuses on the extensive monospecific grounds of the hexactinellid sponge Poliopogon amadou (Thomson, 1878). Deep-sea sponge grounds provide structurally complex habitat, augmenting local biodiversity. To understand the potential extent of these sponge grounds and inform spatial management, we produced the first ensemble species distribution model and local habitat suitability maps for P. amadou in the Atlantic employing Maximum Entropy (Maxent), General Additive Models (GAMs), and Random Forest (RF). The main factors driving the distribution of the sponge were depth and maximum current speed. The sponge grounds occurred in a marked bathymetric belt (2,500 – 3,000 m) within the upper North Atlantic Deep Water mass (2.5∘C, 34.7 psu, O2 6.7–7 mg ml-1), with a preference for areas bathed by moderately strong currents (0.2 – 0.4 ms-1). GAMs, Maxent and RF showed similar performance in terms of evaluation statistics but a different prediction, with RF showing the highest differences. This algorithm only retained depth and maximum currents whereas GAM and Maxent included bathymetric position index, slope, aspect and backscatter. In these latter two models, P. amadou showed a preference for high backscatter values and areas slightly elevated, flat or with gentle slopes and with a NE orientation. The lack of significant differences in model performance permitted to merge all predictions using an ensemble model approach. Our results contribute toward understanding the environmental drivers and biogeography of the species in the Atlantic. Furthermore, we present a case toward designating the Tropic Seamount as an Ecologically or Biologically Significant marine Area (EBSA) as a contribution to address biodiversity conservation in ABNJs.

Deep water circulation in the Hunter Channel (Southwest Atlantic) in a late Pleistocene and Holocene by benthonic foraminifera

Was reconstructed deep-sea water circulation near the Hunter Channel (Rio Grande Rise – South-West Atlantic) in a late Pleistocene and Holocene (MIS 4-MIS 1) by benthonic foraminifera. Was studied three cores of bottom sediment. Now moves the upper North Atlantic deep water (NADW) through the Hunter Channel from the North to the South. The lower NADW in the same direction came in MIS 2 and in MIS 4. There was the lower Circumpolar deep water (CPDW), NADW and Antarctic bottom water (AnBW) in MIS 3 periodically. CPDW prevail in a near bottom layer and in Holocene and in the late Pleistocene before the Hunter Channel sidewise the Argentine Basin. So in the Hunter Channel and on the way to it from south side for all studied period AnBW was almost not. Dissolution of carbonates during the Holocene happens in the deepest east part of the Hunter Channel. In Ice Ages processes of dissolution amplified and affected east part of the channel. Dissolution happen and happened not at the expense of AnBW, and at the expense of NADW which becomes there aggressive in relation to a calcium carbonate.

How Is Meridional Coherence Maintained in the Lower Limb of the Atlantic Meridional Overturning Circulation?

AbstractDespite a strong focus on latitudinal continuity of the Atlantic Meridional Overturning Circulation (AMOC) variability, transport continuity in different layers that constitute the AMOC lower limb has received considerably less attention. In this study, we investigate the transport connectivity of Upper North Atlantic Deep Water (UNADW) and Lower NADW (LNADW), with both defined by density. Using two ocean circulation models and an ocean reanalysis, we find that subpolar‐originated transport anomalies, particularly for UNADW, do not propagate to the subtropics over a period of five decades. We also find that transports in both layers are linked to AMOC at subpolar latitudes, yet only LNADW transport shows linkage to AMOC in the subtropical gyre. Thus, latitudinal AMOC continuity is likely unrelated to transport continuity in any single layer, but rather a result of connection between subpolar‐AMOC and subtropical‐LNADW transport. An exception to this generalization is possible with strong LNADW transport events.

Deep-Water Circulation in the Hunter Channel (Southwest Atlantic) in the Late Pleistocene and Holocene Based on Benthic Foraminifera

Was reconstructed deep-sea water circulation near the Hunter Channel (Rio Grande Rise – South-West Atlantic) in a late Pleistocene and Holocene (MIS 4-MIS 1) by benthonic foraminifera. Was studied three cores of bottom sediment. Now moves the upper North Atlantic deep water (NADW) through the Hunter Channel from the North to the South. The lower NADW in the same direction came in MIS 2 and in MIS 4. There was the lower Circumpolar deep water (CPDW), NADW and Antarctic bottom water (AnBW) in MIS 3 periodically. CPDW prevail in a near bottom layer and in Holocene and in the late Pleistocene before the Hunter Channel sidewise the Argentine Basin. So in the Hunter Channel and on the way to it from south side for all studied period AnBW was almost not. Dissolution of carbonates during the Holocene happens in the deepest east part of the Hunter Channel. In Ice Ages processes of dissolution amplified and affected east part of the channel. Dissolution happen and happened not at the expense of AnBW, and at the expense of NADW which becomes there aggressive in relation to a calcium carbonate.

The distribution of neodymium isotopes and concentrations in the eastern tropical North Atlantic

Dissolved neodymium (Nd) and its radiogenic isotope composition (143Nd/144Nd, expressed as εNd) belong to the key parameters of the international GEOTRACES program, which aims to investigate the processes controlling the distribution of trace elements and their isotopes in the global ocean. We present Nd isotope and concentration ([Nd]) data from eleven full depth water column profiles from the eastern (sub)tropical Atlantic Ocean between 2°N and 29°N and from the Romanche Fracture Zone sampled during Meteor cruise M81/1 (GEOTRACES cruise GA11) in February/March 2010.The εNd signatures range from −12.9 in upper North Atlantic Deep Water (UNADW) at the equator to −8.1 in the mixed layer near the Canary Islands. Nd concentrations range from 11.9 pmol/kg observed within the Equatorial Undercurrent to 40.2 pmol/kg in Antarctic Bottom Water (AABW) in the Romanche Fracture Zone. Large variations in surface water Nd isotope compositions (−12.7 ≤ εNd ≤ −8.1) and concentrations (15.7 pmol/kg ≤ [Nd] ≤ 27.7 pmol/kg) are caused by partial dissolution of Saharan dust particles between 2°N and 13°N and volcanic island weathering (Canaries) between 25°N and 29°N. In contrast to dust inputs, which predominantly affect the Nd concentrations and isotope compositions of surface waters and underlying South Atlantic Central Water (SACW), contributions originating from the Canary Islands affect the Nd isotope composition of the entire surrounding water column without significantly altering Nd concentrations, thus confirming the concept of boundary exchange in its strictest sense.The results confirm that the composition of lower North Atlantic Deep Water (LNADW, εNd = −12.1) in the abyssal plains of the eastern North Atlantic is exclusively set by the mixing ratio of LNADW and Antarctic Bottom Water (AABW). Upper North Atlantic Deep Water (UNADW), in contrast, is characterized by εNd signatures between −12.7 and − 12.0 between 2°N and 10°N, whereas further north it is clearly affected by admixture of Mediterranean Water (MW) and radiogenic inputs from the Canary Islands and likely also the Cape Verde Islands.This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Interhemispheric teleconnections: Late Pliocene change in Mediterranean outflow water linked to changes in Indonesian Through-Flow and Atlantic Meridional Overturning Circulation, a review and update

The ultimate, possibly geodynamic control and potential impact of changes in circulation activity and salt discharge of Mediterranean outflow waters (MOW) on Atlantic meridional overturning circulation have formed long-standing objectives in paleoceanography. Late Pliocene changes in the distal advection of MOW were reconstructed on orbital timescales for northeast Atlantic DSDP/ODP sites 548 and 982 off Brittany and on Rockall Plateau, supplemented by a proximal record from Site U1389 west off Gibraltar, and compared to Western Mediterranean surface and deep-water records of Alboran Sea Site 978. From ~3.43 to 3.3 Ma, MOW temperatures and salinities form a prominent rise by 2–4 °C and ~3 psu, induced by a preceding and coeval rise in sea surface and deep-water salinity and increased summer aridity in the Mediterranean Sea. We speculate that these changes triggered an increased MOW flow and were ultimately induced by a persistent 2.5 °C cooling of Indonesian Through-Flow waters. The temperature drop resulted from the northward drift of Australia that crossed a threshold value near 3.6–3.3 Ma and led to a large-scale cooling of the eastern subtropical Indian Ocean and in turn, to a reduction of African monsoon rains. Vice versa, we show that the distinct rise in Mediterranean salt export after ~3.4 Ma induced a unique long-term rise in the formation of Upper North Atlantic Deep Water, that followed with a phase lag of ~100 ky. In summary, we present evidence for an interhemispheric teleconnection of processes in the Indonesian Gateways, the Mediterranean and Labrador Seas, jointly affecting Pliocene climate.

North Pacific Intermediate Water circulation enhanced by the closure of the Bering Strait

AbstractThe Bering Strait provides a shallow connection that allows freshwater to flow from the North Pacific into the North Atlantic, but this passage was closed during past glacials when sea level was at least 50 m lower than at present. Climate models investigating Bering Strait closure predict that this mechanism increases the salinity in the North Atlantic and reduces the salinity in the North Pacific, inducing a Pacific‐Atlantic seesaw in meridional overturning circulation and poleward heat flux. However, the Pacific circulation response to Bering Strait closure, and thus the seesaw theory, has not been tested by long paleoceanographic records. We present long records of foraminiferal δ18O and δ13C from Integrated Ocean Drilling Program Site U1342 in the Bering Sea, which provide the first evidence of enhanced North Pacific Intermediate Water when the Bering Strait was closed during each of the extreme glacials of the last 1.2 Myr. These results suggest that orbital‐scale variations in North Pacific Intermediate Water are coherent and in phase with variations in upper North Atlantic Deep Water but are unrelated to changes in lower North Atlantic Deep Water. Together, these results provide evidence for systematic, orbital‐scale variability in North Pacific Ocean circulation and may challenge the idea of an orbital‐scale Pacific‐Atlantic seesaw.

Rare earth element distribution in Caribbean seawater: Continental inputs versus lateral transport of distinct REE compositions in subsurface water masses

The rare earth element (REE) concentrations of full water column profiles from ten stations in the Caribbean, the Gulf of Mexico, and the Florida Straits, as well as of surface waters close to the mouth of the Orinoco River have been analyzed. The results show a high concentration of REEs in surface waters, in particular close to the mouth of the Orinoco, and a middle-REE-enrichment that is strongest in the south and east of the study area suggesting that fluvial inputs are the main sources rather than REEs supplied by Saharan dust. The surface waters close to the Orinoco are heavy REE enriched compared to the potential fluvial sources, emphasizing the importance of processes in the estuaries and the surface ocean that preferentially remove light- and middle-REEs. Relatively small heavy-REE enrichments in near-bottom water samples at the mouth of the Orinoco may be the result of preferential release of light REEs from river-transported sediments. The REE patterns of distinct subsurface water masses are largely coherent across the Caribbean basin, suggesting that the lateral transport of preformed compositions is not significantly influenced by vertical processes of scavenging and release. In particular, low Dy/Er molar ratios are associated with Antarctic Intermediate Water and have most likely been advected into the Caribbean from the Southern Ocean. In contrast, deep waters in the Caribbean are enriched in light and middle REEs compared to incoming Upper North Atlantic Deep Water suggesting that release from sinking particles or from sediments is an important source of these REEs in the deep ocean, in particular when deep water residence times are long.

Anthropogenic CO2 changes in the Equatorial Atlantic Ocean

Methods based on CO2 and chlorofluorocarbon (CFC) data are used to describe and evaluate the anthropogenic CO2 (Cant) concentrations, Cant specific inventories, and Cant storage rates in the Equatorial Atlantic Ocean. The Cant variability in the water masses is evaluated from the comparison of two hydrographic sections along 7.5°N carried out in 1993 and 2010. During both cruises, high Cant concentrations are detected in the upper layers, with values decreasing progressively towards the deep layers. Overall, the Cant concentrations increase from 1993 to 2010, with a large increment in the upper North Atlantic Deep Water layer of about 0.18±0.03μmolkg−1y−1. In 2010, the Cant inventory along the whole section amounts to 58.9±2.2 and 45.1±2.0molm−2 using CO2 and CFC based methods, respectively, with most Cant accumulating in the western basin. Considering the time elapsed between the two cruises, Cant storage rates of 1.01±0.18 and 0.75±0.17molm−2y−1 (CO2 and CFC based methods, respectively) are obtained. Below ∼1000m, these rates follow the pace expected from a progressive increase of Cant at steady state; above ∼1000m, Cant increases faster, mainly due to the retreat of the Antarctic Intermediate Waters.

Neodymium isotopes and concentrations in Caribbean seawater: Tracing water mass mixing and continental input in a semi-enclosed ocean basin

We present the first full water column Nd isotope (εNd) and concentration data for Caribbean seawater, as well as for stations close to the Orinoco River mouth and in the Florida Straits. The surface inflow into the southeastern Caribbean via the Guyana Current is characterized by an εNd signature of −10.9, which is a consequence of the mixing of relatively unradiogenic εNd signatures (−13.6) supplied by the Orinoco River with contributions from the Amazon River (∼−10). Despite the proximity to land, sub-surface and intermediate waters within the Caribbean largely retain the εNd signatures of their source water masses in the Atlantic. In contrast, the deep waters of the Caribbean show εNd signatures at least 3 εNd units more radiogenic than the inflowing Upper North Atlantic Deep Water (UNADW). A εNd shift from −13 to −9.7 can be explained by addition of radiogenic Nd to the deep Caribbean through weathering inputs from land. However, in order to balance such large shifts in εNd with at the same time modest increases in Nd concentrations, Nd must also be removed from seawater within the basin. It is suggested that the long residence time of deep waters in the Caribbean allows significant interaction of seawater with sinking particles and seafloor sediments resulting in more radiogenic values. These findings have implications for the use of εNd as a proxy for paleocirculation in restricted basins, in which the residence times of the deep waters are long.

Meridional overturning transports at 7.5N and 24.5N in the Atlantic Ocean during 1992–93 and 2010–11

Transatlantic hydrographic sections along latitudes 7.5N and 24.5N have been repeated with about 20years difference, at the beginning of the 1990s and 2010s. For each period, an inverse model is applied to the closed box bound by both sections. The model imposes mass conservation for individual layers, defined by isoneutral surfaces, and the whole water column, using surface Ekman transport and several transport constraints for specific ranges of longitudes and depths. As a result, the velocities at the reference layer for each station pair and the dianeutral velocities between layers are estimated, and the horizontal velocity fields and the water, heat and freshwater transports are calculated; in particular, we find that mass transport per stratum at 24.5N in 2011 is in good agreement with the transport estimates from the RAPID-Watch array. During both realizations the dianeutral velocities downwell from the Upper North Atlantic Deep Water (UNADW) to the Lower North Atlantic Deep Water (LNADW) strata, resulting in the merging of the two southward flowing strata at 24.5N into one deep southward-moving stratum at 7.5N. At 24.5N, there is an increase in southward UNADW transport between 1992 and 2011, compensated by a decrease of southward LNADW transport; a descent in the upper limit of the Antarctic Bottom Water (AABW) from 1992 to 2011 is also inferred. The Atlantic Meridional Overturning Circulation (AMOC) is larger in 1992–93 than in 2010–11, decreasing from 24.7±1.7 to 20.1±1.4Sv at 24.5N and from 29.2±1.7 to 16.9±1.5Sv at 7.5N. Much of this decrease arises because of the northward flow of Antarctic Intermediate Water (AAIW), which was much more intense in 1992–93 than in 2010–11. As a consequence, heat transport at 24.5N is not significantly different in 1992 (1.4±0.1 PW) and 2011 (1.2±0.1 PW). The estimation of heat transport at 7.5N strongly depends on the magnitude of the North Brazil Current over the American continental platform. The freshwater flux into the box bounded by the two transoceanic sections decreases significantly from 1992–93 (−0.45±0.05Sv) to 2010–11 (−0.36±0.3Sv), meaning a decrease in net evaporation over this same area.