Radium 228

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Virginie Sanial - One of the best experts on this subject based on the ideXlab platform.

  • Timescales of hydrothermal scavenging in the South Pacific Ocean from 234 Th, 230 Th, and 228 Th
    Earth and Planetary Science Letters, 2019
    Co-Authors: Frank Pavia, Robert Anderson, Erin Black, Lauren Kipp, Sebastian Vivancos, Martin Fleisher, Matthew Charette, Virginie Sanial, Willard Moore, Mikael Hult
    Abstract:

    Hydrothermal activity in the deep ocean generates plumes of metal-rich particles capable of removing certain trace elements from seawater by adsorption and sedimentation. This removal process, known as scavenging, can be probed using the insoluble radiogenic isotopes of thorium (Th), which are produced at a known rate in the water column via the decay of soluble uranium (234 Th, 230 Th) and Radium (228 Th) isotopes. We present dissolved and particulate measurements of these three thorium isotopes in a hydrothermal plume observed in the southeast Pacific Ocean on the GEOTRACES GP16 section. Since their half-lives vary from days (234 Th) to years (228 Th) to tens of thousands of years (230 Th), the combination of their signals can be used to understand scavenging processes occurring on a wide range of timescales. Scavenging is a multi-step process involving adsorption and desorption onto particles, followed by particle aggregation, sinking, and eventual sedimentation. We use thorium isotopes to study how hydrothermal activity affects these steps. The rate constants for net adsorption of 234 Th determined here are comparable to previous estimates from hydrothermal plumes in the Atlantic and North Pacific Oceans. The partitioning of 234 Th and 230 Th between large and small particles is more similar in the hydrothermal plume than above it, indicating faster aggregation of particles within the hydrothermal plume at stations nearby the East Pacific Rise than in waters outside the plume. In addition to rapid scavenging and aggregation near the ridge axis, we also infer continuous off-axis scavenging from observations and modeling of 228 Th/ 228 Ra activity ratios. The degree of depletion of the three thorium isotopes increases in order of half-life, with total 234 Th activity close to that of its parent 238 U, but 230 Th showing nearly 70% depletion compared to expected values from reversible scavenging. By modeling the variations in depletion for the different isotopes, we show that much of the 230 Th removal is inherited from scavenging events happening long before the most recent hydrothermal inputs.

  • Radium-228 as a tracer of dissolved trace element inputs from the Peruvian continental margin
    Marine Chemistry, 2018
    Co-Authors: Virginie Sanial, Lauren E. Kipp, Paul B. Henderson, P. Van Beek, J.-l. Reyss, Douglas E. Hammond, Nicholas J. Hawco, Mak A. Saito, Joseph A. Resing, Peter N. Sedwick
    Abstract:

    Abstract Continental margins play a central role in the composition of seawater by being an important source of trace element essentials to the functioning of the ocean ecosystems. Here, we measured long-lived Radium isotopes ( 226 Ra, 228 Ra) along a zonal transect at 12°S (US GEOTRACES GP16) in the eastern tropical South Pacific Ocean. We used 228 Ra to quantify the trace element and isotope (TEI) fluxes (DMn, DFe, and DCo) delivered from the Peruvian continental i) shelf and ii) slope. First, elevated 228 Ra activities were measured in surface water over the entire transect (~ 8500 km), evidence that the continental shelf is an important source of sediment-derived TEIs not only to coastal areas, but to central Pacific Ocean waters. Modeled 228 Ra shelf fluxes combined with water column dissolved TEI/ 228 Ra ratios were used to quantify the shelf-ocean input rates (normalized to shelf-area) for DMn (3.3 × 10 3  μmol m − 2  y − 1 ), DFe (1.5 × 10 3  μmol m − 2  y − 1 ), and DCo (1.0 × 10 2  μmol m − 2  y − 1 ). Second, co-occurring plumes of 228 Ra, DFe, and DMn extended over 1800 km from the margin at 1000–2500 m depth, indicative of a continental slope sediment TEI input to the intermediate water column. The 228 Ra gradient allowed us to derive an effective horizontal eddy diffusion coefficient (K h ) of 46 m 2  s − 1 , which in turn permitted the calculation of slope sediment DMn (6.4 μmol m − 2  y − 1 ) and DFe (5.9 × 10 2  μmol m − 2  y − 1 ) fluxes based on their offshore concentration gradients. On the scale of the South Pacific continental margin between 0–20°S, the DMn shelf flux is approximately 2–3 orders of magnitude higher than the slope flux, while the DFe shelf/slope flux is ~ 3:1. Both shelf and slope sediment derived DMn was transported over a significant distance towards the ocean interior, while DFe concentration gradients were steep, consistent with longer water column residence time for DMn as compared to DFe in marine systems. These findings highlight the importance of considering the continental slope-ocean boundary in the oceanic budgets of biologically-important trace elements.

  • Radium isotopes as tracers of hydrothermal inputs and neutrally buoyant plume dynamics in the deep ocean
    Marine Chemistry, 2017
    Co-Authors: Virginie Sanial, Willard S Moore, Lauren E. Kipp, Paul B. Henderson, Douglas E. Hammond, Jean-louis Reyss, Pieter Van Beek, Matthew A. Charette
    Abstract:

    Abstract Radium (Ra) isotopes are enriched in hydrothermal fluids, have a wide range of half-lives and are minimally impacted by scavenging removal processes; therefore, they have the ability to provide key information on the fate of trace elements in both near- and far- field hydrothermal plumes. To expand our understanding of Ra isotopes and their usefulness as tracers of hydrothermal activity, we measured Ra isotopes in both high and low temperature vent fluids, and in neutrally buoyant plumes. The time scales of plume transport were derived from Ra isotopes in neutrally buoyant plumes emanating from 15°S on the East Pacific Rise (EPR) and the TAG vent field on the Mid Atlantic Ridge (MAR) relative to their source ratio in high-temperature vent fluids. In near-field ( 223 Ra: t 1/2  = 11.4 d, 224 Ra: t 1/2  = 3.66 d) suggest plume ages of ~ 1 month and ~ 5 d over the EPR and MAR, respectively. Farther afield (> 100 km from vent), the distribution of the longer-lived 228 Ra (t 1/2  = 5.75 y) is indicative of hydrothermal plumes with different points of origin along the EPR. Radium-228 derived plume ages were used to calculate the residence time of hydrothermal dissolved Fe, which was in the range of 9–20 y (with an upper limit of 50 y depending on model assumptions). These are shorter than typical deep ocean Fe residence times, a result of scavenging removal of Fe from the neutrally buoyant plume during transport. This study highlights the utility of Ra isotopes in providing time-scales of hydrothermally derived trace element cycling in the deep ocean.

Laure Resplandy - One of the best experts on this subject based on the ideXlab platform.

  • Improving the inverse modeling of a trace isotope: how precisely can Radium-228 fluxes toward the ocean and submarine groundwater discharge be estimated?
    Biogeosciences, 2017
    Co-Authors: Guillaume Le Gland, Laurent Mémery, Olivier Aumont, Laure Resplandy
    Abstract:

    Radium-228 (228 Ra), an almost conservative trace isotope in the ocean, supplied from the continental shelves and removed by a known radioactive decay (T 1/2 = 5.75 years), can be used as a proxy to constrain shelf fluxes of other trace elements, such as nutrients, iron, or rare earth elements. In this study, we perform inverse modeling of a global 228 Ra dataset (including GEOSECS, TTO and GEO-TRACES programs, and, for the first time, data from the Arc-tic and around the Kerguelen Islands) to compute the total 228 Ra fluxes toward the ocean, using the ocean circulation obtained from the NEMO 3.6 model with a 2 • resolution. We optimized the inverse calculation (source regions, cost function) and find a global estimate of the 228 Ra fluxes of 8.01-8.49 × 10 23 atoms yr −1 , more precise and around 20 % lower than previous estimates. The largest fluxes are in the western North Atlantic, the western Pacific and the Indian Ocean, with roughly two-thirds in the Indo-Pacific Basin. An estimate in the Arctic Ocean is provided for the first time (0.43-0.50 × 10 23 atoms yr −1). Local misfits between model and data in the Arctic, the Gulf Stream and the Kuroshio regions could result from flaws of the ocean circulation in these regions (resolution, atmospheric forcing). As Radium is enriched in groundwater, a large part of the 228 Ra shelf sources comes from submarine groundwater discharge (SGD), a major but poorly known pathway for terrestrial mineral elements , including nutrients, to the ocean. In contrast to the 228 Ra budget, the global estimate of SGD is rather uncon-strained, between 1.3 and 14.7 × 10 13 m 3 yr −1 , due to high uncertainties on the other sources of 228 Ra, especially diffusion from continental shelf sediments. Better precision on SGD cannot be reached by inverse modeling until a proper way to separate the contributions of SGD and diffusive release from sediments at a global scale is found.

  • An updated estimate of Radium 228 fluxes toward the ocean: how well does it constrain the submarine groundwater discharge?
    2017
    Co-Authors: Guillaume Le Gland, Laurent Mémery, Olivier Aumont, Laure Resplandy
    Abstract:

    Radium 228 ( 228 Ra), an almost conservative trace isotope of the ocean, supplied from the continental shelves and removed by a known radioactive decay ( T 1/2  = 5.75 yr), can be used as a proxy to constrain shelf fluxes of other trace elements, such as nutrients, iron, or rare earth elements. In this study, we perform inverse modeling of a global 228 Ra dataset (including GEOSECS, TTO and GEOTRACES programs, and, for the first time, data from the Arctic and around the Kerguelen islands) to compute the total 228 Ra fluxes toward the ocean, using the ocean circulation obtained from the NEMO 3.6 model with a 2° resolution. We optimized the inverse calculation (source regions, cost function) and find a global estimate of the 228 Ra fluxes of 8.01–8.49 × 10 23  atoms yr −1 , lower and more precise than previous estimates. The largest fluxes are in the western North Atlantic, the western Pacific and the Indian Ocean, with roughly two thirds in the Indo-Pacific basin. A first estimate in the Arctic Ocean is assessed (0.20–0.50 × 10 23  atoms yr −1 ). Local misfits between model and data in the Arctic, the Gulf Stream and the Kuroshio regions could result from flaws of the ocean circulation in these regions (resolution, atmospheric forcing). As Radium is enriched in groundwater, a large part of the 228 Ra shelf sources comes from submarine groundwater discharge (SGD), a major but poorly known pathway for terrestrial mineral elements, including nutrients, to the ocean. In contrast to the 228 Ra budget, the global estimate of SGD is rather unconstrained, between 1.3 and 14.7 × 10 13  m 3  yr −1 , due to high uncertainties on the other sources of 228 Ra, especially diffusion from continental shelf sediments. Better precision on SGD cannot be reached by inverse modeling until a proper way to separate the contributions of SGD and diffusion at a global scale is found.

Lauren E. Kipp - One of the best experts on this subject based on the ideXlab platform.

  • Increased fluxes of shelf-derived materials to the central Arctic Ocean
    Science advances, 2018
    Co-Authors: Lauren E. Kipp, Willard S Moore, Paul B. Henderson, Matthew A. Charette, Ignatius Rigor
    Abstract:

    Rising temperatures in the Arctic Ocean region are responsible for changes such as reduced ice cover, permafrost thawing, and increased river discharge, which, together, alter nutrient and carbon cycles over the vast Arctic continental shelf. We show that the concentration of Radium-228, sourced to seawater through sediment-water exchange processes, has increased substantially in surface waters of the central Arctic Ocean over the past decade. A mass balance model for 228Ra suggests that this increase is due to an intensification of shelf-derived material inputs to the central basin, a source that would also carry elevated concentrations of dissolved organic carbon and nutrients. Therefore, we suggest that significant changes in the nutrient, carbon, and trace metal balances of the Arctic Ocean are underway, with the potential to affect biological productivity and species assemblages in Arctic surface waters.

  • Radium-228 as a tracer of dissolved trace element inputs from the Peruvian continental margin
    Marine Chemistry, 2018
    Co-Authors: Virginie Sanial, Lauren E. Kipp, Paul B. Henderson, P. Van Beek, J.-l. Reyss, Douglas E. Hammond, Nicholas J. Hawco, Mak A. Saito, Joseph A. Resing, Peter N. Sedwick
    Abstract:

    Abstract Continental margins play a central role in the composition of seawater by being an important source of trace element essentials to the functioning of the ocean ecosystems. Here, we measured long-lived Radium isotopes ( 226 Ra, 228 Ra) along a zonal transect at 12°S (US GEOTRACES GP16) in the eastern tropical South Pacific Ocean. We used 228 Ra to quantify the trace element and isotope (TEI) fluxes (DMn, DFe, and DCo) delivered from the Peruvian continental i) shelf and ii) slope. First, elevated 228 Ra activities were measured in surface water over the entire transect (~ 8500 km), evidence that the continental shelf is an important source of sediment-derived TEIs not only to coastal areas, but to central Pacific Ocean waters. Modeled 228 Ra shelf fluxes combined with water column dissolved TEI/ 228 Ra ratios were used to quantify the shelf-ocean input rates (normalized to shelf-area) for DMn (3.3 × 10 3  μmol m − 2  y − 1 ), DFe (1.5 × 10 3  μmol m − 2  y − 1 ), and DCo (1.0 × 10 2  μmol m − 2  y − 1 ). Second, co-occurring plumes of 228 Ra, DFe, and DMn extended over 1800 km from the margin at 1000–2500 m depth, indicative of a continental slope sediment TEI input to the intermediate water column. The 228 Ra gradient allowed us to derive an effective horizontal eddy diffusion coefficient (K h ) of 46 m 2  s − 1 , which in turn permitted the calculation of slope sediment DMn (6.4 μmol m − 2  y − 1 ) and DFe (5.9 × 10 2  μmol m − 2  y − 1 ) fluxes based on their offshore concentration gradients. On the scale of the South Pacific continental margin between 0–20°S, the DMn shelf flux is approximately 2–3 orders of magnitude higher than the slope flux, while the DFe shelf/slope flux is ~ 3:1. Both shelf and slope sediment derived DMn was transported over a significant distance towards the ocean interior, while DFe concentration gradients were steep, consistent with longer water column residence time for DMn as compared to DFe in marine systems. These findings highlight the importance of considering the continental slope-ocean boundary in the oceanic budgets of biologically-important trace elements.

  • Radium isotopes as tracers of hydrothermal inputs and neutrally buoyant plume dynamics in the deep ocean
    Marine Chemistry, 2017
    Co-Authors: Virginie Sanial, Willard S Moore, Lauren E. Kipp, Paul B. Henderson, Douglas E. Hammond, Jean-louis Reyss, Pieter Van Beek, Matthew A. Charette
    Abstract:

    Abstract Radium (Ra) isotopes are enriched in hydrothermal fluids, have a wide range of half-lives and are minimally impacted by scavenging removal processes; therefore, they have the ability to provide key information on the fate of trace elements in both near- and far- field hydrothermal plumes. To expand our understanding of Ra isotopes and their usefulness as tracers of hydrothermal activity, we measured Ra isotopes in both high and low temperature vent fluids, and in neutrally buoyant plumes. The time scales of plume transport were derived from Ra isotopes in neutrally buoyant plumes emanating from 15°S on the East Pacific Rise (EPR) and the TAG vent field on the Mid Atlantic Ridge (MAR) relative to their source ratio in high-temperature vent fluids. In near-field ( 223 Ra: t 1/2  = 11.4 d, 224 Ra: t 1/2  = 3.66 d) suggest plume ages of ~ 1 month and ~ 5 d over the EPR and MAR, respectively. Farther afield (> 100 km from vent), the distribution of the longer-lived 228 Ra (t 1/2  = 5.75 y) is indicative of hydrothermal plumes with different points of origin along the EPR. Radium-228 derived plume ages were used to calculate the residence time of hydrothermal dissolved Fe, which was in the range of 9–20 y (with an upper limit of 50 y depending on model assumptions). These are shorter than typical deep ocean Fe residence times, a result of scavenging removal of Fe from the neutrally buoyant plume during transport. This study highlights the utility of Ra isotopes in providing time-scales of hydrothermally derived trace element cycling in the deep ocean.

Guillaume Le Gland - One of the best experts on this subject based on the ideXlab platform.

  • Contraindre les échanges côte-large et la pompe biologique de carbone par modélisation inverse de deux radio-isotopes (Radium228 et thorium234)
    2018
    Co-Authors: Guillaume Le Gland
    Abstract:

    Les cycles océaniques du carbone et des principaux nutriments sont mal connus car ils sont affectés par de nombreux puits et sources physiques, chimiques ou biologiques difficiles à estimer par des mesures directes. Une manière de mieux contraindre ces processus importants est d’utiliser l’information contenue dans des traceurs plus simples : les proxies. Le Radium 228 (228Ra), émis par les plateaux continentaux, est utilisé comme proxy des flux d’eau et d’éléments minéraux de la côte vers l’océan ouvert. Il permet en particulier d’estimer les flux d’eau souterraine ou SGDs (Submarine Groundwater Discharge). Le thorium 234(234Th), insoluble, permet quant à lui de contraindre la dynamique des particules par lesquelles il est adsorbé. Il est régulièrement utilisé pour estimer la pompe biologique du carbone (PBC), c’est-à-dire le flux de carbone de la surface vers l’océan profond.Au cours de cette thèse, un modèle numérique à une résolution de 2° a été construit pour chacun de ces deux radio-isotopes, en s’appuyant sur la circulation du modèle NEMO-OPA et les champs de particules du modèle PISCES. Plusieurs paramètres inconnus des modèles ont été contraints par des observations dans le cadre d’une méthode inverse.La modélisation inverse du 228Ra a permis d’estimer les flux de 228Ra venant de 38 régions côtières. En revanche, l’estimation des SGDs est imprécise, car les SGDs sont difficiles à distinguer d’une autre source de 228Ra: la diffusion par les sédiments.La modélisation inverse du 234Th a permis d’estimer les coefficients de partage du 234Th, qui représentent l’affinité de différents types de particules pour cet isotope. Elle a aussi permis d’estimer les erreurs associées à quelques simplifications courantes dans les études de la PBC fondées sur le 234Th.

  • Improving the inverse modeling of a trace isotope: how precisely can Radium-228 fluxes toward the ocean and submarine groundwater discharge be estimated?
    Biogeosciences, 2017
    Co-Authors: Guillaume Le Gland, Laurent Mémery, Olivier Aumont, Laure Resplandy
    Abstract:

    Radium-228 (228 Ra), an almost conservative trace isotope in the ocean, supplied from the continental shelves and removed by a known radioactive decay (T 1/2 = 5.75 years), can be used as a proxy to constrain shelf fluxes of other trace elements, such as nutrients, iron, or rare earth elements. In this study, we perform inverse modeling of a global 228 Ra dataset (including GEOSECS, TTO and GEO-TRACES programs, and, for the first time, data from the Arc-tic and around the Kerguelen Islands) to compute the total 228 Ra fluxes toward the ocean, using the ocean circulation obtained from the NEMO 3.6 model with a 2 • resolution. We optimized the inverse calculation (source regions, cost function) and find a global estimate of the 228 Ra fluxes of 8.01-8.49 × 10 23 atoms yr −1 , more precise and around 20 % lower than previous estimates. The largest fluxes are in the western North Atlantic, the western Pacific and the Indian Ocean, with roughly two-thirds in the Indo-Pacific Basin. An estimate in the Arctic Ocean is provided for the first time (0.43-0.50 × 10 23 atoms yr −1). Local misfits between model and data in the Arctic, the Gulf Stream and the Kuroshio regions could result from flaws of the ocean circulation in these regions (resolution, atmospheric forcing). As Radium is enriched in groundwater, a large part of the 228 Ra shelf sources comes from submarine groundwater discharge (SGD), a major but poorly known pathway for terrestrial mineral elements , including nutrients, to the ocean. In contrast to the 228 Ra budget, the global estimate of SGD is rather uncon-strained, between 1.3 and 14.7 × 10 13 m 3 yr −1 , due to high uncertainties on the other sources of 228 Ra, especially diffusion from continental shelf sediments. Better precision on SGD cannot be reached by inverse modeling until a proper way to separate the contributions of SGD and diffusive release from sediments at a global scale is found.

  • An updated estimate of Radium 228 fluxes toward the ocean: how well does it constrain the submarine groundwater discharge?
    2017
    Co-Authors: Guillaume Le Gland, Laurent Mémery, Olivier Aumont, Laure Resplandy
    Abstract:

    Radium 228 ( 228 Ra), an almost conservative trace isotope of the ocean, supplied from the continental shelves and removed by a known radioactive decay ( T 1/2  = 5.75 yr), can be used as a proxy to constrain shelf fluxes of other trace elements, such as nutrients, iron, or rare earth elements. In this study, we perform inverse modeling of a global 228 Ra dataset (including GEOSECS, TTO and GEOTRACES programs, and, for the first time, data from the Arctic and around the Kerguelen islands) to compute the total 228 Ra fluxes toward the ocean, using the ocean circulation obtained from the NEMO 3.6 model with a 2° resolution. We optimized the inverse calculation (source regions, cost function) and find a global estimate of the 228 Ra fluxes of 8.01–8.49 × 10 23  atoms yr −1 , lower and more precise than previous estimates. The largest fluxes are in the western North Atlantic, the western Pacific and the Indian Ocean, with roughly two thirds in the Indo-Pacific basin. A first estimate in the Arctic Ocean is assessed (0.20–0.50 × 10 23  atoms yr −1 ). Local misfits between model and data in the Arctic, the Gulf Stream and the Kuroshio regions could result from flaws of the ocean circulation in these regions (resolution, atmospheric forcing). As Radium is enriched in groundwater, a large part of the 228 Ra shelf sources comes from submarine groundwater discharge (SGD), a major but poorly known pathway for terrestrial mineral elements, including nutrients, to the ocean. In contrast to the 228 Ra budget, the global estimate of SGD is rather unconstrained, between 1.3 and 14.7 × 10 13  m 3  yr −1 , due to high uncertainties on the other sources of 228 Ra, especially diffusion from continental shelf sediments. Better precision on SGD cannot be reached by inverse modeling until a proper way to separate the contributions of SGD and diffusion at a global scale is found.

Mikael Hult - One of the best experts on this subject based on the ideXlab platform.

  • Timescales of hydrothermal scavenging in the South Pacific Ocean from 234 Th, 230 Th, and 228 Th
    Earth and Planetary Science Letters, 2019
    Co-Authors: Frank Pavia, Robert Anderson, Erin Black, Lauren Kipp, Sebastian Vivancos, Martin Fleisher, Matthew Charette, Virginie Sanial, Willard Moore, Mikael Hult
    Abstract:

    Hydrothermal activity in the deep ocean generates plumes of metal-rich particles capable of removing certain trace elements from seawater by adsorption and sedimentation. This removal process, known as scavenging, can be probed using the insoluble radiogenic isotopes of thorium (Th), which are produced at a known rate in the water column via the decay of soluble uranium (234 Th, 230 Th) and Radium (228 Th) isotopes. We present dissolved and particulate measurements of these three thorium isotopes in a hydrothermal plume observed in the southeast Pacific Ocean on the GEOTRACES GP16 section. Since their half-lives vary from days (234 Th) to years (228 Th) to tens of thousands of years (230 Th), the combination of their signals can be used to understand scavenging processes occurring on a wide range of timescales. Scavenging is a multi-step process involving adsorption and desorption onto particles, followed by particle aggregation, sinking, and eventual sedimentation. We use thorium isotopes to study how hydrothermal activity affects these steps. The rate constants for net adsorption of 234 Th determined here are comparable to previous estimates from hydrothermal plumes in the Atlantic and North Pacific Oceans. The partitioning of 234 Th and 230 Th between large and small particles is more similar in the hydrothermal plume than above it, indicating faster aggregation of particles within the hydrothermal plume at stations nearby the East Pacific Rise than in waters outside the plume. In addition to rapid scavenging and aggregation near the ridge axis, we also infer continuous off-axis scavenging from observations and modeling of 228 Th/ 228 Ra activity ratios. The degree of depletion of the three thorium isotopes increases in order of half-life, with total 234 Th activity close to that of its parent 238 U, but 230 Th showing nearly 70% depletion compared to expected values from reversible scavenging. By modeling the variations in depletion for the different isotopes, we show that much of the 230 Th removal is inherited from scavenging events happening long before the most recent hydrothermal inputs.