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Abstract The distribution of barium (Ba) in seawater was determined in the main basins and straits of the Mediterranean Sea during the GEOTRACES GA-04S MedSeA cruise. In addition, the concentrations of Ba and radium (Ra) isotopes (228Ra) were determined in groundwaters discharging in the Mediterranean Sea. The dissolved Ba concentration increases from the inflowing Atlantic surface water (Ba ~ 45 nmol/kg) to the intermediate (Ba ~ 70 nmol/kg) and deep (Ba ~ 70–75 nmol/kg) Mediterranean Sea waters in general agreement with previously published profiles. We use these data to build a Ba budget of the Mediterranean Sea and to evaluate the role of different sources of Ba (dusts, rivers, sediments, submarine groundwater discharges, Messinian evaporites). Evaporation alone cannot account for the Ba concentration increase because salinity increases only by 8% between the Gibraltar Strait and the Eastern Basin, whereas the Ba concentration increases up to 75% between the Atlantic Water and the deep waters. Thus, the particulate Ba flux that maintains the Ba concentration gradient between the surface and the deep waters must be 10–100 times stronger than previously estimated and the net Ba loss at the Gibraltar Strait or out of the Western Basin must be balanced by external inputs to the Mediterranean Sea. Newly available data from Mediterranean rivers confirm that rivers are insufficient Ba sources to balance this budget. Using recent estimates of Saharan dust fluxes, we show that dust deposition is also a negligible Ba source, whereas it was previously considered as the main external source. Using Ba-228Ra data, we show for the first time that submarine groundwater discharges can significantly contribute to the Ba budget, but large uncertainties exist due to the high variability of the chemical composition of these groundwaters. The inputs from Messinian evaporites require to be further investigated. Ba transfer from the surface water to the intermediate and deep waters occurs through active particulate Ba cycling and/or Ba dissolution in the sediment. Our study suggests that the mean particulate Ba cycling is more intense than previously estimated. 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. [ABSTRACT FROM AUTHOR]

231 Pa, 230 Th and 232 Th were analyzed in unfiltered seawater samples (n = 66) and suspended particles (n = 19) collected in the Mediterranean Sea during the MedSeA-GA04-S cruise along the GEOTRACES section GA04S and used to investigate mechanisms controlling the distribution and fractionation of Pa and Th in an ocean margin environment. 231 Pa and 230 Th are particle reactive radionuclides and are often used as tracers of processes such as boundary scavenging, particle transport and ocean circulation. The depth profiles of total 231 Pa and 230 Th concentrations in the Mediterranean Sea displayed non-linear shapes. Higher total 232 Th concentrations were observed at the straits and in deep waters pointing at lithogenic sources. Fractionation factors F Th/Pa ranged from 1.4 to 9. Application of a box-model illustrated that 94% of the 231 Pa and almost all of the 230 Th (99.9%) produced in the Mediterranean Sea is removed to the sediment by scavenging. The negligible export of 230 Th to the Atlantic Ocean, leads to a reevaluation of the mean settling speed of the filtered particles, which is now estimated to 500–1000 m/y. The low F Th/Pa fractionation factors are attributed to the efficient scavenging and lack of transport of 231 Pa to the Atlantic Ocean. [ABSTRACT FROM AUTHOR]

Abstract: Four sediment cores collected in the Seine River basin and dated between 1916 and 2003 were analyzed for lead concentrations and isotopic composition. In all four cores, the measured Pb concentration (up to 460mgkg−1) lies significantly above the natural background (27–40mgkg−1), although a significant decrease (down to 75mgkg−1) was observed during the second half of the 20th century which can be explained by the reduction of lead emissions. The 206Pb/207Pb ratio measured in these samples indicates that the main source of Pb used in the Paris conurbation is characterized by a “Rio Tinto” signature (defined as 206Pb/207Pb=1.1634±0.0001). A high contribution, up to 25%, from the leaded gasoline (characterized by 206Pb/207Pb=1.08±0.02) is revealed in the Seine River downstream Paris, indicating that lead from the leaded gasoline is preferentially released to the river. The dominating Pb signature in the Paris conurbation that is currently sampled through incinerators fumes (206Pb/207Pb=1.1550±0.0005) and waste water treatment plant (206Pb/207Pb=1.154±0.002), represents a mixture of highly recycled lead from the Rio Tinto mine and lead from leaded gasoline (imprinted by the low 206Pb/207Pb of the Broken Hill mine). This signature is called “urban” rather than “industrial”, because it is clearly distinct from the Pb that is found in areas contaminated by heavy industry, i.e. the heavy industries located on the Oise River which used lead from European ores characterized by high 206Pb/207Pb ratios (∼1.18–1.19) and possibly a minor amount of North American lead (206Pb/207Pb ratios>1.20). The “urban” signature is also found in a rural area upstream of Paris in the 1970’s. At the Seine River mouth in 2003, Pb with an urban signature represents 70% of the total Pb sediment content, with the 30% remaining corresponding to natural Pb. [Copyright &y& Elsevier]

231Pa, 230Th and 232Th were analyzed in filtered seawater (n = 70) and suspended particles (n = 39) collected along a shelf-basin transect from the Barents shelf to the Makarov Basin in the Arctic Ocean during GEOTRACES section GN04 in 2015. The distribution of dissolved 231Pa and 230Th in the Arctic Ocean deviates from the linear increase expected from reversible scavenging. Higher 232Th concentrations were observed at the shelf, slope and in surface waters in the deep basin, pointing at lithogenic sources. Fractionation factors (F Th/Pa) observed at the Nansen margin were higher compared to F Th/Pa in the central Nansen Basin, possibly due to the residual occurrence of hydrothermal particles in the deep central Nansen Basin. Application of a boundary scavenging model quantitatively accounts for the dissolved and particulate 230Th distributions in the Nansen Basin. Modelled dissolved 231Pa distributions were largely overestimated, which was attributed to the absence of incorporation of water exchange with the Atlantic Ocean in the model. 231Pa/230Th ratios of the suspended particles of the Nansen Basin were below the 231Pa/230Th production ratio, but top-core sediments of the Nansen margin and slope have high 231Pa/230Th-ratios, suggesting that scavenging along the Nansen margin partly acts as a sink for the missing Arctic 231Pa. [ABSTRACT FROM AUTHOR]

The distributions of dissolved and particulate thorium isotopes (230Th and 232Th) were established in samples from the BONUS GoodHope (BGH) IPY-GEOTRACES cruise in the SE Atlantic sector of the Southern Ocean (36°S-13°E to 57°S-0°, Feb.–Mar. 2008). The distribution of total (dissolved + particulate) 232Th is dominated by the inputs from continental margins. The non-linear profiles of dissolved 230Th are interpreted as due to the southward upwelling of the isopycnal surfaces. However, total 230Th and 232Th versus salinity plots illustrate departures from binary mixing and provides evidence for non-conservative behavior of both isotopes along the section. We propose a model for total 230Th and 232Th scavenging and mixing along isopycnal surfaces. We use this model to estimate particle settling speeds and isopycnal eddy diffusion coefficients along the BGH section. Data-model comparison suggests particle settling velocities in the range of 400–700 m/y and isopycnal eddy diffusivity of the order of 2000 m2/s. • Small deviations of thorium isotopes from a conservative behavior occurs during isopycnal mixing through the ACC. • An advection-diffusion-scavenging model along neutral density surface is applied to thorium isotopes. • Combining thorium isotopes allows constraining particle settling speeds and isopycnal mixing coefficients. [ABSTRACT FROM AUTHOR]

In this paper we set forth a 3-D ocean model of the radioactive trace isotopes 230Th and 231Pa. The interest arises from the fact that these isotopes are extensively used for investigating particle transport in the ocean and reconstructing past ocean circulation. The tracers are reversibly scavenged by biogenic and lithogenic particles. Our simulations of 230Th and 231Pa are based on the NEMO-PISCES ocean biogeochemistry general circulation model, which includes biogenic particles, namely small and big particulate organic carbon, calcium carbonate and biogenic silica. Small and big lithogenic particles from dust deposition are included in our model as well. Their distributions generally compare well with the small and big lithogenic particle concentrations from recent observations from the GEOTRACES programme, except for boundary nepheloid layers for which, as of today, there are no non-trivial prognostic models available on a global scale. Our simulations reproduce 230Th and 231Pa dissolved concentrations: they compare well with recent GEOTRACES observations in many parts of the ocean. Particulate 230Th and 231Pa concentrations are significantly improved compared to previous studies, but they are still too low because of missing particles from nepheloid layers. Our simulation reproduces the main characteristics of the 231Pa=230Th ratio observed in the sediments and supports a moderate affinity of 231Pa to biogenic silica as suggested by recent observations relative to 230Th. Future model development may further improve understanding, especially when this will include a more complete representation of all particles, including different size classes, manganese hydroxides and nepheloid layers. This can be done based on our model as its source code is readily available. [ABSTRACT FROM AUTHOR]

In this paper, we set forth a 3-D ocean model of the radioactive trace isotopes 230Th and 231Pa. The interest arises from the fact that these isotopes are extensively used for investigating particle transport in the ocean and reconstructing past ocean circulation. The tracers are reversibly scavenged by biogenic and lithogenic particles. Our simulations of 230Th and 231Pa are based on the NEMO-PISCES ocean biogeochemistry general circulation model, which includes biogenic particles, namely small and big particulate organic carbon, calcium carbonate and biogenic silica. Small and big lithogenic particles from dust deposition are included in our model as well. Their distributions generally compare well with the small and big lithogenic particle concentrations from recent observations from the GEOTRACES programme, except for boundary nepheloid layers for which, as up to today, there are no non-trivial, prognostic models available on a global scale. Our simulations reproduce 230Th and 231Pa dissolved concentrations: they compare well with recent GEOTRACES observations in many parts of the ocean. Particulate 230Th and 231Pa concentrations are significantly improved compared to previous studies, but they are still too low because of missing particles from nepheloid layers. Our simulation reproduces the main characteristics of the 231Pa / 230Th ratio observed in the sediments, and supports a moderate affinity of 231Pa to biogenic silica as suggested by recent observations, relative to 230Th. Future model development may further improve understanding, especially when this will include a more complete representation of all particles, including different size classes, manganese hydroxides and nepheloid layers. This can be done based on our model, as its source code is readily available. [ABSTRACT FROM AUTHOR]

The first record of urban speleothems used to reconstruct the history of heavy metal pollution of shallow groundwaters is presented. Two speleothems grew during the last 300 years in an underground aqueduct in the north-eastern part of Paris. They display high Pb, Mn V, Cu, Cd and Al concentrations since 1900 due to the urbanization of the site which triggered anthropogenic contamination of the water feeding the speleothems. Surprisingly, these heavy metal concentrations are also high in the oldest part. This early pollution could come from the use of Parisian waste as fertilizers in the orchards and vineyards cultivated above the aqueduct before urbanization. Lead isotopes were measured in these carbonates as well as in lead artifacts from the 17th–18th centuries ( 206 Pb/ 207 Pb = 1.180 +/− 0.003). The mean 206 Pb/ 207 Pb ratio, for one of the speleothems is 1.181 +/− 0.003 unvarying with time. These lead signatures are close to those of coal and old lead from northern European mines, lower than the natural background signature. It confirms that the high metal concentrations found come from anthropogenic pollution. Conversely, the lead isotopic composition of the second speleothem presents two temporal trends: for the oldest levels, the mean value (1.183 +/− 0.003) is similar to the first speleothem. For the youngest part, a lower value (1.172 +/− 0.005) is recorded, evidencing the contribution of a new lead source at the beginning of the industrial revolution. Pb isotopes were also measured in recent samples from a nearby superficial site. The first sample is a recent (AD 1975 +/− 15 years) deposit ( 206 Pb/ 207 Pb = 1.148 +/− 0.003), and the second, a thin subactual layer ( 206 Pb/ 207 Pb = 1.181 +/− 0.002). These data are compatible with the adding of anthropogenic sources (leaded gasoline and industrial lead from Rio Tinto ore). [ABSTRACT FROM AUTHOR]

The 135Cs/137Cs isotopic ratio allows to characterize the potential sources of radioactive contamination in the environment. This ratio is usually determined in highly contaminated environmental samples by combining selective radiochemistry with mass spectrometry measurement. In contrast, few data are available at the environmental level of 137Cs (< 1 kBq.kg−1) due to the occurrence of interferents (Ba, Mo, Sb and Sn) in the mass range of interest and due to the very few radiocesium atoms. To obtain enough radiocesium from an environmental material with low levels of radioactivity, a large sample must be analyzed. In the present work, a new method of Cs extraction and purification has been developed allowing for the first time to analyze 100 g of ash of soil or sediment in only one week. The achieved chemical performances are excellent with a 137Cs recovery yield from soil and sediment samples, deduced from gamma spectrometry, above 80% and decontamination factors of 1.4 × 107, 5.7 × 104, 2.3 × 105 and 1.5 × 104 for Ba, Mo, Sb and Sn respectively. [ABSTRACT FROM AUTHOR]

Neodymium isotopic compositions ( 143 Nd/ 144 Nd or ε Nd ) have been used as a tracer of water masses and lithogenic inputs to the ocean. To further evaluate the faithfulness of this tracer, we have updated a global seawater ε Nd database and combined it with hydrography parameters (temperature, salinity, nutrients and oxygen concentrations), carbon isotopic ratio and radiocarbon of dissolved inorganic carbon. Archive ε Nd data are also compiled for leachates, foraminiferal tests, deep-sea corals and fish teeth/debris from the Holocene period (< 10,000 years). At water depths ≥ 1500 m, property-property plots show clear correlations between seawater ε Nd and the other variables, suggesting that large-scale water mass mixing is a primary control of deepwater ε Nd distribution. At ≥ 200 m, basin-scale seawater T-S-ε Nd diagrams demonstrate the isotopic evolution of different water masses. Seawater and archive ε Nd values are compared using property-property plots and T-S-ε Nd diagrams. Archive values generally agree with corresponding seawater values although they tend to be at the upper limit in the Pacific. Both positive and negative offsets exist in the northern North Atlantic. Applying multiple regression analysis to deep (≥ 1500 m) seawater data, we established empirical equations that predict the main, large-scale, deepwater ε Nd trends from hydrography parameters. Large offsets from the predicted values are interpreted as a sign of significant local/regional influence. Dominant continental influence on seawater and archive ε Nd is observed mainly within 1000 km from the continents. Generally, seawater and archive ε Nd values form gradual latitudinal trend in the Atlantic and Pacific at depths ≥ 600 m, consistent with the idea that Nd isotopes help distinguish between northern/southern sourced water contributions at intermediate and deep water depths. [ABSTRACT FROM AUTHOR]

Particles determine the residence time of many dissolved elements in seawater. Although a substantial number of field studies were conducted in the framework of major oceanographic programs as GEOSECS and JGOFS, knowledge about particle dynamics is still scarce. Moreover, the particulate trace metal behavior remains largely unknown. The GEOSECS sampling strategy during the 1970s focused on large sections across oceanic basins, where particles were collected by membrane filtration after Niskin bottle sampling, biasing the sampling toward the small particle pool. Late in this period, the first in situ pumps allowing large volume sampling were also developed. During the 1990s, JGOFS focused on the quantification of the “exported carbon flux” and its seasonal variability in representative biogeochemical provinces of the ocean, mostly using sediment trap deployments. Although scarce and discrete in time and space, these pioneering studies allowed an understanding of the basic fate of marine particles. This understanding improved considerably, especially when the analysis of oceanic tracers such as natural radionuclides allowed the first quantification of processes such as dissolved-particle exchange and particle settling velocities. Because the GEOTRACES program emphasizes the importance of collecting, characterizing and analyzing marine particles, this paper reflects our present understanding of the sources, fate and sinks of oceanic particles at the early stages of the program. [ABSTRACT FROM AUTHOR]

Abstract: The abundance, size and composition of micron-size U–Th–REE–Hf rich phases of marine clayey silt sediments from the Western Mediterranean Sea were determined using an automated scanning electron microscope equipped with an energy dispersive spectrometer. The minerals found in the sediment were monazite, allanite, florencite, xenotime and zircon. The size distribution and chemical composition of each phase were used to determine their contribution to the total content of the sediment in U, Th, REE, Y, Zr and Hf. Zircon accounts for most of the Zr and Hf of the sample. Xenotime and zircon account for most Y and HREE of the sample. However, the high Y–HREE–U–Th content of the zircons analyzed in this study (possibly due to xenotime overgrowth on zircon surface and alteration processes) contrasts with previous works and cannot be extended to any type of zircons. By contrast, only a small fraction (≈20–30%) of U, Th and LREE (such as Nd) are carried by U–Th–REE–Hf rich minerals (monazite mostly). It reflects the higher alterability of U–Th and LREE bearing phases compared to zircon and xenotime and implies that most U–Th and LREE are present in other phases at lower concentrations (possibly clays, organic matter or Fe–Mn oxides). The different size distributions of the different carriers can contribute to U–Th and LREE/HREE fractionation observed during the transport of these elements. Due to its very high physical and chemical resistance, zircon appears to be the main Hf carrier even in fine grained “zircon free type” sediments. By storing Lu and possibly releasing extremely radiogenic Hf, xenotime has an unforeseen role in the Lu–Hf systematics in marine sediment. [Copyright &y& Elsevier]

Abstract: Over the past few decades, the radioisotope pair of 238U/ 234Th has been widely and increasingly used to describe particle dynamics and particle export fluxes in a variety of aquatic systems. The present paper is one of five review articles dedicated to 234Th. It is focused on the models associated with 234Th whereas the companion papers (same issue) are focused on present and future methodologies and techniques (Rutgers van der Loeff et al.), C/ 234Th ratios (Buesseler et al.), 234Th speciation (Santschi et al.) and present and future applications of 234Th [Waples, J.T., Benitez-Nelson, C.R., Savoye, N., Rutgers van der Loeff, M., Baskaran, M., Gustafsson, Ö., this issue. An Introduction to the application and future use of 234Th in aquatic systems. Marine Chemistry, FATE special issue]. In this paper, we review current 234Th scavenging models and discuss the relative importance of the non-steady state and physical terms associated with the most commonly used model to estimate 234Th flux. Based on this discussion we recommend that for future work the use of models should be accompanied by a discussion of the effect that model and data uncertainty have on the model results. We also suggest that future field work incorporate repeat occupations of sample sites on time scales of 1–4 weeks in order to evaluate steady state versus non-steady state estimates of 234Th export, especially during high flux events (>ca. 800 dpm m−2 d−1). Finally, knowledge of the physical oceanography of the study area is essential, particularly in ocean margins and in areas of established upwelling (e.g., Equatorial Pacific). These suggestions will greatly enhance the application of 234Th as a tracer of particle dynamics and flux in more complicated regimes. [Copyright &y& Elsevier]