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Significant progress has been made in the last decade on the understanding of the role of the Coral and Solomon Seas as major suppliers of waters and chemical elements to the equatorial Pacific. Yet, the location, depth, and processes of chemical enrichment of these waters remain poorly constrained. Neodymium (Nd) isotopic compositions (εNd)) and rare earth element concentrations (REE) are powerful tracers of land-ocean chemical exchanges. Combined, they can greatly refine the characterization of these exchanges. Here we report profiles of εNd at 21 stations located in the Coral and Solomon Seas as part of the GEOTRACES GP-12 cruise that complement the rare earth element concentration (REE) profiles of Pham (Chemical Geology, 2019, 524 (May), 11-36). Waters exiting the Solomon Sea are generally slightly more radiogenic than the incoming ones, suggesting inputs of radiogenic material along their pathways across the Solomon Sea. This radiogenic material is brought to the surface waters via natural processes (rivers, volcanic dusts) and likely local mining activities. Noticeable εNd increases are also observed in subsurface and intermediate layers. All these processes indicate the occurrence of local Boundary Exchange (BE) processes, which are estimated to occur within a few days. Coupling hydrological and chemical tracers allows highlighting the land-ocean interactions affecting some water layers and quantifying the exchanged fluxes of Nd. Modifications of the Nd concentration and isotopic composition in the lower thermocline layer require an external flux of 7.9 ± 2.0 t(Nd)/yr only partly balanced by a scavenging flux of 1.8 ± 2.3 t(Nd)/yr, leading to a net influx of 6.1 ±1.7 t(Nd)/yr. Regarding the Upper Circumpolar Deep Water, a total net flux of 105 ± 50 t(Nd)/yr is estimated, the external flux is relatively high (86 ± 31 t(Nd)/yr while the scavenging flux remains. These results refine the role of the Solomon Sea as a supplier of continental chemical elements to the Pacific equatorial waters. [ABSTRACT FROM AUTHOR]

Variations in Mediterranean thermohaline circulation of the Quaternary are still not well constrained whereas they have been considered to have an influence on the Atlantic Meridional Overturning Circulation and on the oxygenation of waters in the deep basins of the Mediterranean Sea. εNd analyses have been carried out on planktonic foraminifera of cores collected in the central Mediterranean Sea to constrain water mass exchange between the Eastern and Western Mediterranean Sea (EMS and WMS) during the last climatic cycle. εNd records from the WMS and EMS display similar higher values during warm substages of interglacial Marine Isotopic Stage (MIS) 1 and 5. This suggests an efficient connection between the two Mediterranean sub‐basins and the transfer of radiogenic waters to the Tyrrhenian Sea via the Levantine Intermediate Water (LIW). Conversely, during glacial MIS, εNd of the intermediate depth of the Tyrrhenian Sea are less radiogenic than the EMS, implying limited hydrological connection between sub‐basins during low sea‐level stands. Superimposed on these glacial‐interglacial variations, increased εNd occurred during Heinrich Stadial events. This suggests a reduction in the formation of unradiogenic WIW in the Gulf of Lions due to the input of relatively fresh surface Atlantic water to the WMS and/or the inflow of radiogenic glacial LIW and upper EMDW to the Tyrrhenian Sea as a result of an active EMS convection related to saltier and colder conditions. Such potential millennial‐scale pulses of LIW intrusion into the Tyrrhenian Sea may have led to an enhanced Mediterranean Outflow Water intensity in the Gibraltar Strait. Key Points: First εNd record of the central Mediterranean Sea during the last climatic cycleReconstruction of water exchanges variations between the eastern and western Mediterranean basins during the last climatic cycleEvidence of millennial‐scale variations of εNd during the last glacial time indicating pulses of LIW intrusion into the Tyrrhenian Sea [ABSTRACT FROM AUTHOR]

Sapropels are sediments rich in black, pelagic organic matter which occur mainly in the Eastern Mediterranean, documenting anoxic environments and high biological productivity. The quasiperiodicity of deposition of sapropels ‐over millions of years‐ relates to the Earth's precession cycle, which directly enhances the African monsoon, ultimately increasing freshwater input from the Nile. The last sapropel event, S1, occurred about 10,000 years ago (Early Holocene), when the Mediterranean region was warmer and wetter than today. Several modeling studies reflect the impact of this climate and a stronger Nile influx on Mediterranean oceanic circulation, but the regional models used lacked the spatial resolution necessary to simulate winter intermediate and deep convection. Here, we investigate recently occurring changes in the convective areas in the Eastern Mediterranean, using a regional ocean–atmosphere coupled climate model of high spatial resolution, essential to the simulation of a realistic Mediterranean circulation for present‐day conditions. We focused on the thermohaline circulation and the simulation of neodymium isotopic composition to compare our modeling results to modern data and paleo‐proxies. A sensitivity experiment shows a radical response of the Mediterranean to enhanced Nile discharge, creating the appropriate conditions for sapropel formation. We thus demonstrate that increased discharge of the Nile River can trigger the shutdown of Eastern Mediterranean convection and create conditions favorable to the development of anoxic events. Key Points: High‐resolution numerical modeling of environment of deposition of sapropel S1Nile river outflow enhancement triggers important changes in oceanic circulationBoundary exchanges εNd modeling depicts reduced circulation, and records for S1 show a similar anomaly [ABSTRACT FROM AUTHOR]

Rare Earth Element (REE) concentrations and Nd isotopic compositions (εNd) of five new seawater profiles scattered across the Rockall Trough (North-East Atlantic) were investigated in order: (1) to establish the seawater εNd distribution in the Rockall Trough, which is poorly known so far; (2) to identify the southward-flowing Wyville-Thompson Ridge Overflow Water (WTOW) originating from the Nordic Seas; (3) to evaluate the potential impact of lithogenic sources on the seawater εNd in the Rockall Trough. Combined with two previously published seawater stations, the results reveal a spatial εNd gradient in the upper layer (roughly the first 900 m) of the Rockall Trough with εNd values ranging from −14.9 ± 0.3 to −11.6 ± 0.2. The influence of Nd lithogenic inputs through seawater-sediment interactions have been found to be likely negligible, so the spatial gradient has been attributed to the relative influence of unradiogenic Subpolar Gyre (SPG) water carried by the North Atlantic Current (NAC) and more radiogenic water derived from the Subtropical Gyre (STG) flowing into the Shelf Edge Current (SEC) along the European margin. Freshwater from the interior seas (Northern UK) is an additional source of radiogenic εNd, although it seems restricted to the shallow continental shelf. Below the upper layer, WTOW has been characterized for the first time with εNd (mean value of −13.8 ± 0.5; n = 9) as a homogeneous intermediate water body between 600 and 1200 m. The results reveal a strong modification of the outflowing water from the Norwegian Sea that cascades through the Wyville-Thompson ridge, as WTOW in the Rockall Trough is composed of more than 80 % Atlantic water. On the western flank of the Rockall Trough, a deep WTOW component has been identified that is responsible for a modification of the Labrador Sea Water (LSW) εNd values (-13.0 ± 0.3 to −12.1 ± 0.3). Two stations separated by only a few kilometers from each other but sampled four years apart (2012 and 2016) show a multi-annual variation of water properties and εNd values (∼1 ε-unit) for the first 800 m, pointing to a change in the relative contribution of both SPG and STG to the composition of water flowing in the Rockall Trough to the Nordic Seas. Overall, these results confirm that εNd can be used to assess the water mass composition in the Rockall Trough, allowing to reconstruct North Atlantic gyre dynamics in paleoceanographic studies. [ABSTRACT FROM AUTHOR]

This study focuses on the neodymium isotopic composition (εNd) of mixed planktonic foraminifera species in sediment cores collected on the north-western margin of the South China Sea (SCS) and in the western equatorial Pacific. The aim was to identify temporal changes in the εNd of the Upper Circumpolar Deep Water (UCDW) during its northward penetration in the western subtropical Pacific and to re-evaluate the significance of deep-water εNd changes at the northern margin of the SCS over the past 25 kyr. The past seawater εNd record derived from sediments of the subtropical western Pacific displays a narrow range from − 2.9 ± 0.3 to − 1.6 ± 0.4. This suggests that the εNd of the UCDW is greatly modified during its northward penetration through the process of boundary exchange along the western volcanic margin of the South Pacific and that the unradiogenic εNd signature of the UCDW observed in the South Pacific (about − 6) has not been propagated beyond the equatorial Pacific, at least not over the last 25 kyr. Seawater εNd values obtained from the northern SCS display large variations with values that are lower (− 6.8 ± 0.3 to − 5.6 ± 0.2) for the last glacial period than for the late Holocene (− 5.7 ± 0.3 to − 4.1 ± 0.2). Such glacial εNd cannot be attributed to a higher proportion of southern-sourced water (SSW) as εNd values of the UCDW in the equatorial western Pacific are more radiogenic than those observed in the SCS. The evolution of the seawater εNd values of the northern SCS during the last glacial period and the last climatic transition is coeval with variations in the contribution of sediments from the Pearl River. The low sea level stand of the last glacial period resulted in a more proximal position of the Pearl River mouth relative to core MD05-2904, which a higher input of freshwater and, in particular, detrital sedimentary particles originating from the Pearl River that partially dissolved during sinking led to a decrease in the seawater εNd along the north-western margin of the SCS. Enhanced rainfalls associated with intensified monsoon and more frequent typhoon activities during the early Holocene Climatic Optimum (from 9 to 7 cal kyr BP) are linked to a decrease in sea surface salinity (lower δ 18 O values of planktonic foraminifera) and a decrease in εNd values (− 6.2 ± 0.3) induced by lithogenic Nd input from pronounced freshwater and sediment discharge from distal rivers. [ABSTRACT FROM AUTHOR]

This paper reports the measurement of the Neodymium isotopic composition by Neptune Multiple Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) over the last two years. Although there is concomitant Cerium in the chemical separation process, this has no significant influence on the Neodymium analysis. As for the sample containing small amounts of Samarium (Sm/Nd < 0.04), direct calibration for isobaric interference and mass discrimination by the exponential law can be obtained by assuming that Samarium mass discrimination is the same as that of Neodymium. Geological samples after traditional chemical separation were measured by Neptune MC-ICP-MS and Thermal Ionization Mass Spectrometry (TIMS) respectively. The results show that Neptune MC-ICP-MS can measure Neodymium isotopic composition as precisely the TIMS does and is even more effective and less time-consuming than the TIMS Method. [ABSTRACT FROM AUTHOR]

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

Abstract: We succeeded in determining the Ce isotopic composition (138Ce/142Ce) in seawater with an error of 2σ m =0.3–0.7 of ε unit. In this study, 1000–3000 L of seawater samples were passed through MnO2 fibers to concentrate Ce and Nd for precise measurement of their isotope ratios. Four surface seawater samples of the northwestern Pacific and a coastal sample in Tokyo Bay were analyzed. Most Ce isotope ratios in the surface water showed positive ε Ce values (+0.8 to +1.4) in the northwestern Pacific Ocean. These values indicate that Ce in the surface water originates from the continental crust preferentially over mantle-derived materials. We examined binary mixing model between the continental crust and mid-ocean ridge basalt. However the model could not explain both isotopic compositions and concentrations, which implies that the atmospheric input was a possible pathway for Ce into the ocean. A negative ε Ce value was observed in Tokyo Bay, suggesting mantle-derived sources. [Copyright &y& Elsevier]

Abstract: Continental margins are, via river sediment discharges, the major source of a number of elements to the ocean. They are also, for several reactive elements, sites of preferential removal from the water column, due to enhanced scavenging [1] [M.P. Bacon, Tracers of chemical scavenging in the ocean: Boundary effects and large-scale chemical fractionation, Philos. Trans. R. Soc. Lond., A 325 (1988) 147–160.]. They can therefore be understood as sources of elements for the ocean, sinks or both. Although exchanges of matter are suspected to occur at the continent/ocean interface [2] [P.H. Santschi, L. Guo, I.D. Walsh, M.S. Quigley, M. Baskaran, Boundary exchange and scavenging of radionuclides in continental margin waters of the Middle Atlantic Bight: implications for organic carbon fluxes, Cont. Shelf Res. 19 (1999) 609–636.] and despite their probable importance for the ocean chemistry, closed budgets have still yet to be determined. Here, based on neodymium isotopic composition data obtained during the past 6 yr, we document and quantify significant neodymium exchange at ocean boundaries, in areas covering a large spectra of hydrographical, biological and geochemical characteristics : Eastern Indian Ocean, Western Equatorial Pacific, Western Tropical Pacific and Northwestern Atlantic, with neodymium removal fluxes accounting for 74±23%, 100±38%, 62±54% and 84±45% of the neodymium input fluxes, respectively. Recognition of boundary exchange and its potential globalization have important implications for (1) our understanding of margin/ocean interactions and their influence on the oceanic isotopic chemistry, and (2) geochemical cycling of reactive elements (including pollutants) at ocean margins. [Copyright &y& Elsevier]