35 research outputs found
The export flux of particulate organic carbon derived from 210Poâ210Pb disequilibria along the North Atlantic GEOTRACES GA01 transect: GEOVIDE cruise
© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 License. The definitive version was published in Biogeosciences 16(2), (2019): 309-327, doi:10.5194/bg-16-309-2019.The disequilibrium between 210Po activity and 210Pb activity in seawater samples was determined along the GEOTRACES GA01 transect in the North Atlantic during the GEOVIDE cruise (MayâJune 2014). A steady-state model was used to quantify vertical export of particulate 210Po. Vertical advection was incorporated into one version of the model using time-averaged vertical velocity, which had substantial variance. This resulted in large uncertainties for the 210Po export flux in this model, suggesting that those calculations of 210Po export fluxes should be used with great care. Despite the large uncertainties, there is no question that the deficits of 210Po in the Iberian Basin and at the Greenland Shelf have been strongly affected by vertical advection. Using the export flux of 210Po and the particulate organic carbon (POC) to 210Po ratio of total (>â1â”m) particles, we determined the POC export fluxes along the transect. Both the magnitude and efficiency of the estimated POC export flux from the surface ocean varied spatially within our study region. Export fluxes of POC ranged from negligible to 10âmmolâCâmâ2âdâ1, with enhanced POC export in the Labrador Sea. The cruise track was characterized by overall low POC export relative to net primary production (export efficiency <â1â%â15â%), but relatively high export efficiencies were seen in the basins where diatoms dominated the phytoplankton community. The particularly low export efficiencies in the Iberian Basin, on the other hand, were explained by the dominance of smaller phytoplankton, such as cyanobacteria or coccolithophores. POC fluxes estimated from the 210Poâ210Pb and 234Thâ238U disequilibria agreed within a factor of 3 along the transect, with higher POC estimates generally derived from 234Th. The differences were attributed to integration timescales and the history of bloom events.We thank the captain (Gilles Ferrand) and crew of the R/V Pourquoi Pas? and the chief scientists (GerĂĄldine Sarthou and Pascale Lherminier) of the GEOVIDE cruise. We also thank Pierre Branellec, Floriane Desprez de GĂ©sincourt, Michel Hamon, Catherine Kermabon, Philippe Le Bot, StĂ©phane Leizour, Olivier MĂ©nage, Fabien PĂ©rault, and Emmanuel de Saint-LĂ©ger for their technical support during the GEOVIDE expedition; Catherine Schmechtig for the GEOVIDE database management, and Phoebe Lam for providing two modified McLane in situ pumps; FrĂ©dĂ©ric Planchon, Virginie Sanial, and Catherine Jeandel for their assistance with pump deployments and particulate sample collection. The authors also thank Arnout Roukaerts, Debany Fonseca-Batista, Florian Deman, and Frank Dehairs for providing primary production data. Funding for the GEOVIDE cruise was provided by the French National Research Agency (ANR-13-BS06-0014, ANR-12-PDOC-0025-01), the French National Center for Scientific Research (CNRS-LEFE-CYBER), the LabexMER (anr-10-LABX-19), and Ifremer. Gillian Stewart and Yi Tang were supported by NSF award #OCE 1237108. The Generalitat de Catalunya also helped through its grant 2017 SGR-1588. This work is contributing to the ICTA âUnit of Excellenceâ (MinECo, MDM2015-0552). Maxi Castrillejo and Montserrat Roca-Marti were funded by an FPU PhD studentship (AP-2012-2901 and AP2010-2510, respectively) from the Ministerio de EducaciĂłn, Cultura y Deporte of Spain. Maxi Castrillejo was also supported by the ETH Zurich Postdoctoral Fellowship Program (17-2 FEL-30), co-funded by the Marie Curie Actions for People COFUND Program. We also thank Gary Hemming (Queens College) and Troy Rasbury (Stony Brook University) for laboratory assistance with the ICP-MS analyses. Finally, we thank the associate editor and the anonymous reviewers for their helpful comments on how to improve the manuscript
The export flux of particulate organic carbon derived from 210Po/210Pb disequilibria along the North Atlantic GEOTRACES GA01 transect: GEOVIDE cruise
The disequilibrium between 210Po activity and 210Pb activity in seawater samples was determined along the GEOTRACES GA01 transect in the North Atlantic during the GEOVIDE cruise (May-June 2014). A steady-state model was used to quantify vertical export of particulate 210Po. Vertical advection was incorporated into one version of the model using time-averaged vertical velocity, which had substantial variance. This resulted in large uncertainties for the 210Po export flux in this model, suggesting that those calculations of 210Po export fluxes should be used with great care. Despite the large uncertainties, there is no question that the deficits of 210Po in the Iberian Basin and at the Greenland Shelf have been strongly affected by vertical advection. Using the export flux of 210Po and the particulate organic carbon (POC) to 210Po ratio of total (\u3e1Όm) particles, we determined the POC export fluxes along the transect. Both the magnitude and efficiency of the estimated POC export flux from the surface ocean varied spatially within our study region. Export fluxes of POC ranged from negligible to 10mmolCm-2d-1, with enhanced POC export in the Labrador Sea. The cruise track was characterized by overall low POC export relative to net primary production (export efficiency \u3c1%-15%), but relatively high export efficiencies were seen in the basins where diatoms dominated the phytoplankton community. The particularly low export efficiencies in the Iberian Basin, on the other hand, were explained by the dominance of smaller phytoplankton, such as cyanobacteria or coccolithophores. POC fluxes estimated from the 210Po210Pb and 234Th238U disequilibria agreed within a factor of 3 along the transect, with higher POC estimates generally derived from 234Th. The differences were attributed to integration timescales and the history of bloom events. © 2019 Author(s)
Distributions of total and size-fractionated particulate 210Po and 210Pb activities along the North Atlantic GEOTRACES GA01 transect: GEOVIDE cruise
Vertical distributions of total and particulate polonium-210 (210Po) and lead-210 (210Pb) activities in the water column were measured at 11 stations in the North Atlantic during the GEOTRACES GA01 transect: GEOVIDE cruise in MayâJune 2014. Total 210Po activity was on average 24% lower than 210Pb activity in the upper 100m, and it was closer to unity in the mesopelagic (100â1000m). The partitioning coefficients (Kd) along the transect suggest the preferential association of 210Po relative to 210Pb onto particles. The prominent role of small particles in sorption was confirmed by the observation that over 80% of the particulate radionuclide activity was on small particles. To account for the observed surface water 210Poâ210Pb disequilibria, particulate radionuclide activities and export of both small (1â53”m) and large (â\u3eâ53”m) particles must be considered. A comparison between the GEOVIDE total particulate 210Poâ210Pb activity ratios (ARs) and the ratios in previous studies revealed a distinct geographic distribution, with lower particulate ARs in the high-latitude North Atlantic (including this study) and Arctic in relation to all other samples. For the samples where apparent oxygen utilization (AOU) was calculated at the same depth and time as the 210Poâ210Pb AR (40 stations including this study), there was a two-phase correlation between the total particulate AR and AOU, likely reflecting the nature of the particles and demonstrating the forces of remineralization and radionuclide decay from particles as they age
The export flux of particulate organic carbon derived from 210Po/210Pb disequilibria along the North Atlantic GEOTRACES GA01 transect: GEOVIDE cruise
The disequilibrium between 210Po activity and 210Pb activity in seawater samples was determined along the GEOTRACES GA01 transect in the North Atlantic during the GEOVIDE cruise (May-June 2014). A steady-state model was used to quantify vertical export of particulate 210Po. Vertical advection was incorporated into one version of the model using time-averaged vertical velocity, which had substantial variance. This resulted in large uncertainties for the 210Po export flux in this model, suggesting that those calculations of 210Po export fluxes should be used with great care. Despite the large uncertainties, there is no question that the deficits of 210Po in the Iberian Basin and at the Greenland Shelf have been strongly affected by vertical advection. Using the export flux of 210Po and the particulate organic carbon (POC) to 210Po ratio of total (\u3e1Όm) particles, we determined the POC export fluxes along the transect. Both the magnitude and efficiency of the estimated POC export flux from the surface ocean varied spatially within our study region. Export fluxes of POC ranged from negligible to 10mmolCm-2d-1, with enhanced POC export in the Labrador Sea. The cruise track was characterized by overall low POC export relative to net primary production (export efficiency \u3c1%-15%), but relatively high export efficiencies were seen in the basins where diatoms dominated the phytoplankton community. The particularly low export efficiencies in the Iberian Basin, on the other hand, were explained by the dominance of smaller phytoplankton, such as cyanobacteria or coccolithophores. POC fluxes estimated from the 210Po210Pb and 234Th238U disequilibria agreed within a factor of 3 along the transect, with higher POC estimates generally derived from 234Th. The differences were attributed to integration timescales and the history of bloom events. © 2019 Author(s)
The export flux of particulate organic carbon derived from 210Po/210Pb disequilibria along the North Atlantic GEOTRACES GA01 transect : GEOVIDE cruise
Unidad de excelencia MarĂa de Maeztu MdM-2015-0552The disequilibrium between 210Po activity and 210Pb activity in seawater samples was determined along the GEOTRACES GA01 transect in the North Atlantic during the GEOVIDE cruise (May-June 2014). A steady-state model was used to quantify vertical export of particulate 210Po. Vertical advection was incorporated into one version of the model using time-averaged vertical velocity, which had substantial variance. This resulted in large uncertainties for the 210Po export flux in this model, suggesting that those calculations of 210Po export fluxes should be used with great care. Despite the large uncertainties, there is no question that the deficits of 210Po in the Iberian Basin and at the Greenland Shelf have been strongly affected by vertical advection. Using the export flux of 210Po and the particulate organic carbon (POC) to 210Po ratio of total (>1”m) particles, we determined the POC export fluxes along the transect. Both the magnitude and efficiency of the estimated POC export flux from the surface ocean varied spatially within our study region. Export fluxes of POC ranged from negligible to 10 mmolCmâ2 dâ1, with enhanced POC export in the Labrador Sea. The cruise track was characterized by overall low POC export relative to net primary production (export efficiency <1%-15%), but relatively high export efficiencies were seen in the basins where diatoms dominated the phytoplankton community. The particularly low export efficiencies in the Iberian Basin, on the other hand, were explained by the dominance of smaller phytoplankton, such as cyanobacteria or coccolithophores. POC fluxes estimated from the 210Po/210Pb and 234Th/238U disequilibria agreed within a factor of 3 along the transect, with higher POC estimates generally derived from 234Th. The differences were attributed to integration timescales and the history of bloom events
Differentiating Lithogenic Supplies, Water Mass Transport, and Biological Processes On and Off the Kerguelen Plateau Using Rare Earth Element Concentrations and Neodymium Isotopic Compositions
Distributions of dissolved rare earth element (REE) concentrations and neodymium isotopic compositions (expressed as ΔNd) of seawater over and off the Kerguelen Plateau in the Southern Ocean are presented. The sampling took place during the austral spring bloom in OctoberâNovember 2011 (KEOPS2 project, GEOTRACES process study) and aimed to further the investigations of the KEOPS1 austral summer study in terms of sources and transport of lithogenic material, and to investigate the impact of local biogeochemical cycles on the REE distributions. The REE signature of the coastal eastern Kerguelen Islands waters was characterized by negative europium anomalies (Eu/Eu*) and negative ΔNd in filtered samples. By contrast, the unfiltered sample showed a positive Eu/Eu* and more radiogenic ΔNd. These distinct signatures could reflect either differential dissolution of the local flood basalt minerals or differential leaching of local trachyte veins. The dissolved Kerguelen coastal REE patterns differ from those observed close to Heard Island, these latter featuring a positive Eu/Eu* and a less radiogenic ΔNd (Zhang et al., 2008). These differences enabled us to trace the transport of waters (tagged by the Kerguelen REE signature) 200 km downstream from the coastal area, north of the Polar Front. Northward transport of the central Plateau shallow waters, enriched by both local vertical supplies and lateral advection of inputs from Heard Island, was also evident. However, the transport of Kerguelen inputs southeastward across the Polar Front could not be discerned (possibly as a result of rapid dilution or scavenging of REE signatures), although evidence for such transport was found previously using Ra isotopes (Sanial et al., 2015). Comparison of the REE patterns at stations sampled prior, during and at the demise of the bloom revealed diverse fractionations, including production of significant lanthanum and europium anomalies, which are tentatively ascribed to chemical reactions with various inorganic and biogenic phases, including surface coatings, barite crystals, and biogenic silica
Multi-proxy approach (Thorium-234, excess Barium) of export and remineralization fluxes of carbon and biogenic elements associated with the oceanic biological pump
Lâobjectif principal de cette thĂšse est de mieux comprendre les diffĂ©rents facteurs contrĂŽlant la pompe biologique de carbone en Atlantique Nord et dans lâOcĂ©an Austral, Ă proximitĂ© des Ăźles Kerguelen, en utilisant notamment deux approches: le Thorium-234 (234Th) et le baryum biogĂ©nique (Baxs).En Atlantique Nord, les flux dâexport de carbone organique particulaire (POC) augmentent lorsquâils sont associĂ©s Ă des minĂ©raux biogĂ©niques (silice biogĂ©nique et carbonate de calcium) et lithogĂšnes, capable de lester les particules. LâefficacitĂ© dâexport, gĂ©nĂ©ralement plus faible que prĂ©cĂ©demment supposĂ© (< 10%), est inversement corrĂ©lĂ©e Ă la production, soulignant un dĂ©calage temporel entre production et export. La plus forte efficacitĂ© de transfert, i.e. la fraction de POC atteignant 400m, est reliĂ©e Ă des particules lestĂ©es par du carbonate de calcium ou des minĂ©raux lithogĂšnes.Les flux de reminĂ©ralisation mĂ©sopĂ©lagique sont similaires ou parfois supĂ©rieurs aux flux dâexports et dĂ©pendent de lâintensitĂ© du dĂ©veloppement phytoplanctonique, de la structure en taille, des communautĂ©s phytoplanctoniques et des processus physiques (advection verticale).Comme observĂ© pour le POC, lâexport des Ă©lĂ©ments traces est influencĂ© par les particules lithogĂšnes provenant des marges ocĂ©aniques, mais aussi des diffĂ©rentes espĂšces phytoplanctoniques.Dans lâOcĂ©an Austral, la zone Ă proximitĂ© de lâĂźle de Kerguelen est naturellement fertilisĂ©e en fer, augmentant les flux dâexport de fer, dâazote et de silice biogĂ©nique. Il a Ă©tĂ© dĂ©montrĂ© que la variabilitĂ© des flux dĂ©pendait des communautĂ©s phytoplanctoniques dans la zone fertilisĂ©e.The main objective of this thesis is to improve our understanding of the different controls that affect the oceanic biological carbon pump. Particulate export and remineralization fluxes were investigated using the thorium-234 (234Th) and biogenic barium (Baxs) proxies.In the North Atlantic, the highest particulate organic carbon (POC) export fluxes were associated to biogenic (biogenic silica or calcium carbonate) and lithogenic minerals, ballasting the particles.Export efficiency was generally low (< 10%) and inversely related to primary production, highlighting a phase lag between production and export. The highest transfer efficiencies, i.e. the fraction of POC that reached 400m, were driven by sinking particles ballasted by calcite or lithogenic minerals.The regional variation of mesopelagic remineralization was attributed to changes in bloom intensity, phytoplankton cell size, community structure and physical forcing (downwelling). Carbon remineralization balanced, or even exceeded, POC export, highlighting the impact of mesopelagic remineralization on the biological pump with a near-zero, deep carbon sequestration for spring 2014.Export of trace metals appeared strongly influenced by lithogenic material advected from the margins. However, at open ocean stations not influenced by lithogenic matter, trace metal export rather depended on phytoplankton activity and biomass.A last part of this work focused on export of biogenic silica, particulate nitrogen and iron near the Kerguelen Island. This area is characterized by a natural iron-fertilization that increases export fluxes. Inside the fertilized area, flux variability is related to phytoplankton community composition
Approche multi-proxy (Thorium-234, Baryum en excÚs) des flux d'export et de reminéralisation du carbone et des éléments nutritifs associés à la pompe biologique océanique
The main objective of this thesis is to improve our understanding of the different controls that affect the oceanic biological carbon pump. Particulate export and remineralization fluxes were investigated using the thorium-234 (234Th) and biogenic barium (Baxs) proxies.In the North Atlantic, the highest particulate organic carbon (POC) export fluxes were associated to biogenic (biogenic silica or calcium carbonate) and lithogenic minerals, ballasting the particles.Export efficiency was generally low (< 10%) and inversely related to primary production, highlighting a phase lag between production and export. The highest transfer efficiencies, i.e. the fraction of POC that reached 400m, were driven by sinking particles ballasted by calcite or lithogenic minerals.The regional variation of mesopelagic remineralization was attributed to changes in bloom intensity, phytoplankton cell size, community structure and physical forcing (downwelling). Carbon remineralization balanced, or even exceeded, POC export, highlighting the impact of mesopelagic remineralization on the biological pump with a near-zero, deep carbon sequestration for spring 2014.Export of trace metals appeared strongly influenced by lithogenic material advected from the margins. However, at open ocean stations not influenced by lithogenic matter, trace metal export rather depended on phytoplankton activity and biomass.A last part of this work focused on export of biogenic silica, particulate nitrogen and iron near the Kerguelen Island. This area is characterized by a natural iron-fertilization that increases export fluxes. Inside the fertilized area, flux variability is related to phytoplankton community composition.Lâobjectif principal de cette thĂšse est de mieux comprendre les diffĂ©rents facteurs contrĂŽlant la pompe biologique de carbone en Atlantique Nord et dans lâOcĂ©an Austral, Ă proximitĂ© des Ăźles Kerguelen, en utilisant notamment deux approches: le Thorium-234 (234Th) et le baryum biogĂ©nique (Baxs).En Atlantique Nord, les flux dâexport de carbone organique particulaire (POC) augmentent lorsquâils sont associĂ©s Ă des minĂ©raux biogĂ©niques (silice biogĂ©nique et carbonate de calcium) et lithogĂšnes, capable de lester les particules. LâefficacitĂ© dâexport, gĂ©nĂ©ralement plus faible que prĂ©cĂ©demment supposĂ© (< 10%), est inversement corrĂ©lĂ©e Ă la production, soulignant un dĂ©calage temporel entre production et export. La plus forte efficacitĂ© de transfert, i.e. la fraction de POC atteignant 400m, est reliĂ©e Ă des particules lestĂ©es par du carbonate de calcium ou des minĂ©raux lithogĂšnes.Les flux de reminĂ©ralisation mĂ©sopĂ©lagique sont similaires ou parfois supĂ©rieurs aux flux dâexports et dĂ©pendent de lâintensitĂ© du dĂ©veloppement phytoplanctonique, de la structure en taille, des communautĂ©s phytoplanctoniques et des processus physiques (advection verticale).Comme observĂ© pour le POC, lâexport des Ă©lĂ©ments traces est influencĂ© par les particules lithogĂšnes provenant des marges ocĂ©aniques, mais aussi des diffĂ©rentes espĂšces phytoplanctoniques.Dans lâOcĂ©an Austral, la zone Ă proximitĂ© de lâĂźle de Kerguelen est naturellement fertilisĂ©e en fer, augmentant les flux dâexport de fer, dâazote et de silice biogĂ©nique. Il a Ă©tĂ© dĂ©montrĂ© que la variabilitĂ© des flux dĂ©pendait des communautĂ©s phytoplanctoniques dans la zone fertilisĂ©e
The essential bioactive role of nickel in the oceans: Evidence from nickel isotopes
The role of nickel (Ni) in ocean biogeochemical cycles is both under-studied and controversial. Strong correlations between Ni and organic carbon in modern and ancient marine sediments suggest a prominent biogeochemical role over a substantial portion of Earth history. Addition of Ni to culturing and seawater incubation experiments produces strong responses in terms of cell growth, particularly of nitrogen-fixing organisms. But the implied limiting role for phytoplankton growth is inconsistent with observations in the real ocean, specifically that photic zone Ni concentrations never descend to the very low values that characterise other bioactive, and often bio-limiting, metals like iron. These two observations can be reconciled if a large portion of the total dissolved Ni present in open-ocean surface waters is not bio-available on short timescales. Here we present new Ni concentration and stable isotope data from the GEOVIDE transect in the North Atlantic. We interpret these new data in the light of the growing database for Ni stable isotopes in the modern ocean, with implications for the biogeochemical importance of Ni. In the new North Atlantic dataset, the lowest Ni concentrations (1.8-2.6 nmol/L) and highest ÎŽ60Ni (up to +1.67â°) are associated with low nitrate, south of the subarctic front (SAF). By contrast, stations at latitudes north of the SAF, with higher surface nitrate, show very subdued variation in Ni concentrations throughout the entire depth of the water column (3.6±0.3 nmol/L, mean and 2SD), and no variation in ÎŽ60Ni beyond the narrow global deep-ocean range (+1.33±0.13 â°). These North Atlantic Ni isotope data also show relationships with nitrogen isotope effects, observed in the same samples, that are suggestive of a link between Ni utilisation, isotope fractionation and nitrogen fixation. The global dataset, including the new data presented here, reveals a biogeochemical divide with Ni isotope fractionation only occurring in low latitude surface waters. A simple observationally constrained three-dimensional model of Ni cycling suggests that the creation of this isotopically heavy, Ni-poor, end-member, together with the physical circulation and remineralisation at depth, can explain the global Ni-ÎŽ60Ni systematics. Taken together, these findings hint at Ni-N co-limitation in the modern ocean. We advocate for more extensive and detailed culturing/incubation studies of this neglected metal in order to elucidate its potentially crucial biogeochemical role.ISSN:0012-821XISSN:1385-013
The essential bioactive role of nickel in the oceans: Evidence from nickel isotopes
The role of nickel (Ni) in ocean biogeochemical cycles is both under-studied and controversial. Strong correlations between Ni and organic carbon in modern and ancient marine sediments suggest a prominent biogeochemical role over a substantial portion of Earth history. Addition of Ni to culturing and seawater incubation experiments produces strong responses in terms of cell growth, particularly of nitrogen-fixing organisms. But the implied limiting role for phytoplankton growth is inconsistent with observations in the real ocean, specifically that photic zone Ni concentrations never descend to the very low values that characterise other bioactive, and often bio-limiting, metals like iron. These two observations can be reconciled if a large portion of the total dissolved Ni present in open-ocean surface waters is not bio-available on short timescales. Here we present new Ni concentration and stable isotope data from the GEOVIDE transect in the North Atlantic. We interpret these new data in the light of the growing database for Ni stable isotopes in the modern ocean, with implications for the biogeochemical importance of Ni.
In the new North Atlantic dataset, the lowest Ni concentrations (1.8-2.6 nmol/L) and highest
Ni (up to +1.67â°) are associated with low nitrate, south of the subarctic front (SAF). By contrast, stations at latitudes north of the SAF, with higher surface nitrate, show very subdued variation in Ni concentrations throughout the entire depth of the water column (3.6±0.3 nmol/L, mean and 2SD), and no variation in
Ni beyond the narrow global deep-ocean range (+1.33±0.13 â°). These North Atlantic Ni isotope data also show relationships with nitrogen isotope effects, observed in the same samples, that are suggestive of a link between Ni utilisation, isotope fractionation and nitrogen fixation.
The global dataset, including the new data presented here, reveals a biogeochemical divide with Ni isotope fractionation only occurring in low latitude surface waters. A simple observationally constrained three-dimensional model of Ni cycling suggests that the creation of this isotopically heavy, Ni-poor, end-member, together with the physical circulation and remineralisation at depth, can explain the global Ni-
Ni systematics. Taken together, these findings hint at Ni-N co-limitation in the modern ocean. We advocate for more extensive and detailed culturing/incubation studies of this neglected metal in order to elucidate its potentially crucial biogeochemical role