The export flux of particulate organic carbon derived from 210Po∕210Pb 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 (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 µm) 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 mmol C m−2 d−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 < 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.We thank the captain (Gilles Ferrand) and crew of the R/V Pourquoi Pas? and the chief scientists (Geráldine Sarthou and Pascale Lherminier) of the GEOVIDE cruise. We also thank Pierre Branellec, Floriane Desprez de Gésincourt, Michel Hamon, Catherine Kermabon, Philippe Le Bot, Stéphane Leizour, Olivier Ménage, Fabien Pérault, and Emmanuel de Saint-Léger 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; Frédéric 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 “Unit of Excellence” (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 Educación, 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 (\u3e1μm) 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

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 (\u3e1μm) 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 May–June 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 (100–1000m). 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 210Po∕210Pb disequilibria, particulate radionuclide activities and export of both small (1–53µm) and large ( \u3e 53µm) particles must be considered. A comparison between the GEOVIDE total particulate 210Po∕210Pb 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 210Po∕210Pb 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

Quantifying 210Po/210Pb disequilibrium in seawater: A comparison of two precipitation methods with differing results

The disequilibrium between lead-210 (210Pb) and polonium-210 (210Po) is increasingly used in oceanography to quantify particulate organic carbon (POC) export from the upper ocean. This proxy is based on the deficits of 210Po typically observed in the upper water column due to the preferential removal of 210Po relative to 210Pb by sinking particles. Yet, a number of studies have reported unexpected large 210Po deficits in the deep ocean indicating scavenging of 210Po despite its radioactive mean life of ∼ 200 days. Two precipitation methods, Fe(OH)3 and Co-APDC, are typically used to concentrate Pb and Po from seawater samples, and deep 210Po deficits raise the question whether this feature is biogeochemically consistent or there is a methodological issue. Here, we present a compilation of 210Pb and 210Po studies that suggests that 210Po deficits at depths \u3e 300 m are more often observed in studies where Fe(OH)3 is used to precipitate Pb and Po from seawater, than in those using Co-APDC (in 68 versus 33% of the profiles analyzed for each method, respectively). In order to test whether 210Po/210Pb disequilibrium can be partly related to a methodological artifact, we directly compared the total activities of 210Pb and 210Po in four duplicate ocean depth-profiles determined by using Fe(OH)3 and Co-APDC on unfiltered seawater samples. While both methods produced the same 210Pb activities, results from the Co-APDC method showed equilibrium between 210Pb and 210Po below 100 m, whereas the Fe(OH)3 method resulted in activities of 210Po significantly lower than 210Pb throughout the entire water column. These results show that 210Po deficits in deep waters, but also in the upper ocean, may be greater when calculated using a commonly used Fe(OH)3 protocol. This finding has potential implications for the use of the 210Po/210Pb pair as a tracer of particle export in the oceans because 210Po (and thus POC) fluxes calculated using Fe(OH)3 on unfiltered seawater samples may be overestimated. Recommendations for future research are provided based on the possible reasons for the discrepancy in 210Po activities between both analytical methods

The export flux of particulate organic carbon derived from 210Po/210Pb disequilibria along the North Atlantic GEOTRACES GA01 transect : GEOVIDE cruise

Unidad de excelencia María 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 (>1µm) 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 mmolCm−2 d−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 <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

Quantifying 210Po/210Pb Disequilibrium in Seawater: A Comparison of Two Precipitation Methods With Differing Results

The disequilibrium between lead-210 (210Pb) and polonium-210 (210Po) is increasingly used in oceanography to quantify particulate organic carbon (POC) export from the upper ocean. This proxy is based on the deficits of 210Po typically observed in the upper water column due to the preferential removal of 210Po relative to 210Pb by sinking particles. Yet, a number of studies have reported unexpected large 210Po deficits in the deep ocean indicating scavenging of 210Po despite its radioactive mean life of ∼ 200 days. Two precipitation methods, Fe(OH)3 and Co-APDC, are typically used to concentrate Pb and Po from seawater samples, and deep 210Po deficits raise the question whether this feature is biogeochemically consistent or there is a methodological issue. Here, we present a compilation of 210Pb and 210Po studies that suggests that 210Po deficits at depths &gt;300 m are more often observed in studies where Fe(OH)3 is used to precipitate Pb and Po from seawater, than in those using Co-APDC (in 68 versus 33% of the profiles analyzed for each method, respectively). In order to test whether 210Po/210Pb disequilibrium can be partly related to a methodological artifact, we directly compared the total activities of 210Pb and 210Po in four duplicate ocean depth-profiles determined by using Fe(OH)3 and Co-APDC on unfiltered seawater samples. While both methods produced the same 210Pb activities, results from the Co-APDC method showed equilibrium between 210Pb and 210Po below 100 m, whereas the Fe(OH)3 method resulted in activities of 210Po significantly lower than 210Pb throughout the entire water column. These results show that 210Po deficits in deep waters, but also in the upper ocean, may be greater when calculated using a commonly used Fe(OH)3 protocol. This finding has potential implications for the use of the 210Po/210Pb pair as a tracer of particle export in the oceans because 210Po (and thus POC) fluxes calculated using Fe(OH)3 on unfiltered seawater samples may be overestimated. Recommendations for future research are provided based on the possible reasons for the discrepancy in 210Po activities between both analytical methods

Introduction to the French GEOTRACES North Atlantic transect (GA01): GEOVIDE cruise

© 2018 Author(s). The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this special issue (https://www.biogeosciences.net/special-issue900.html), results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among 18 articles. Here, the scientific context, project objectives, and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue

Sources and distribution of artificial radionuclides in the oceans: from Fukushima to the Mediterranean sea

El objetivo de esta tesis es investigar las fuentes y distribuciones de una serie de radionúclidos artificiales (90Sr, 129I, 134Cs, 137Cs, 236U, 237Np, 239Pu and 240Pu) en dos zonas oceánicas que han recibido un gran impacto debido a actividades humanas. En primer lugar, el Océano Pacífico, que recibió el impacto de las fugas desde la central nuclear de Fukushima Dai-ichi, inmediatamente después del accidente y en los años siguientes. El estudio se centró en el 90Sr, sobre el cual había poca información a pesar de ser uno de los radionúclidos presentes en mayor cantidad el agua contaminada almacenada en los tanques y la central. En 2013, recolectamos muestras de agua de mar y aguas subterráneas a una distancia de entre 0.8 and ~100 km de la central nuclear. Además, evaluamos el cambio temporal en las concentraciones de 134Cs, 137Cs y 90Sr demostrando la importancia de las contribuciones parciales de cada fuente. Las muestras de agua recolectadas en Septiembre de 2013 presentaban concentraciones de 8.9 ± 0.4 Bq·m-3 de 90Sr, 124 ± 3 Bq·m-3 de 137Cs and 54 ± 1 Bq·m-3 de 134Cs. Estas concentraciones eran significativamente mayores que las medidas antes del accidente nuclear en 2011 debidas a las antiguas pruebas atmosféricas de bombas nucleares: ~1 Bq·m-3, 1-2 Bq·m-3 y negligible, respectivamente. Los resultados confirmaron mediante la relación 137Cs/90Sr la existencia de vertidos continuados de 90Sr desde la central, que estimamos en 2.3-8.5 GB·d-1 en Septiembre de 2013. Si bien loes vertidos continuados son menores que las estimadas para los vertidos que ocurrieron justo después del accidente, existe el riesgo de que sean mayores y por lo tanto, recomendamos continuar con el muestreo y el control de radionúclidos artificiales en el Océano Pacífico. En segundo lugar, estudiamos el Mar Mediterráneo que recibió los aportes de diferentes fuentes desde 1950: i) a nivel global, las pruebas atmosféricas con bombas nucleares entre 1950 y 1960, y el accidente nuclear de Chernobyl en 1986; ii) a nivel regional, las descargas de agua radioactiva desde la planta de reprocesamiento de Marcoule entre 1958 y 1997; iii) y a nivel local, el accidente de Palomares en 1966. Los resultados muestran que las distribuciones de 129I, 236U, 237Np y 137Cs, estaban controladas por la circulación termohalina. Los resultados también resaltan la importancia relativa de cada una de las fuentes. En particular, estimamos la contaminación derivada de la planta de reprocesamiento nuclear de Marcoule, que fue en comparación con el global fallout, del mismo orden de magnitud para el 236U (10-20 kg) y el 237Np (~7 kg), y dos órdenes de magnitud superior para el 129I (70-90 kg). Marcoule representa entorno al 30% del 237Np presente en el Mar Mediterráneo en 2013. Los cambios temporales en la distribución del 137Cs mostraron la influencia de las fuentes y la circulación oceánica. La relación atómica 240Pu/239Pu era ~0.19 indicando que la fuente principal de este radionúclido en el Mar Mediterráneo es el global fallout. La distribución del 239,240Pu estaba influenciada además, por el proceso de scavenging y la remineralización del material particulado. Uno de los resultados a destacar es el hecho que de las concentraciones e inventarios del plutonio en la cuenca oriental eran menores que en la cuenca occidental. Ello puede ser debido a: i) la mayor importancia del ‘boundary scavenging’ en los sedimentos de la plataforma oriental o ii) a la reducida actividad biológica (que conllevaría una menor exportación del plutonio por las partículas), unida a un mayor transporte lateral por las Levantine Intermediate Waters.This PhD dissertation aimed to understand the sources and distributions of a diverse set of artificial radionuclides (90Sr, 129I, 134Cs, 137Cs, 236U, 237Np, 239Pu and 240Pu) in two contrasting oceanic regions. Firstly, the Pacific Ocean close to Japan, largely impacted by the accident that occurred at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in 2011, which led to the release of radioactive contaminants to the environment immediately after the accident and during the following years. We focused on 90Sr, which was largely understudied despite being of major concern due to its large presence in cooling waters stored in tanks and in the nuclear facility since 2011. We collected seawater, groundwater and surface beach water samples between 0.8 and ~100 km off FDNPP in September 2013, two and a half years after the accident. In this work we also evaluated the temporal evolution of 134Cs, 137Cs and 90Sr concentrations, highlighting the relative importance of various sources in the coast off Japan over time.The radionuclide concentrations in water samples collected in September 2013 were up to 8.9 ± 0.4 Bq·m-3 for 90Sr, 124 ± 3 Bq·m-3 for 137Cs and 54 ± 1 Bq·m-3 for 134Cs in 2013, while their typical concentrations prior to the 2011 accident due to the nuclear tests were ~1 Bq·m-3, 1-2 Bq·m-3 and neglible, respectively. This confirmed the ongoing 90Sr releases from the FDNPP, that we estimated in 2.3-8.5 GB·d-1 in September 2013. Although substantially lower than the rates estimated to have occurred soon after the accident, the continuing releases and the potential larger leaks from the FDNPP evidenced the importance of continuous surveillance of artificial radionuclides in the Pacific Ocean. Secondly, the semi-enclosed Mediterranean Sea, impacted by various sources since the mid-twentieth century: i) on a global scale by atmospheric nuclear weapon tests carried during the 1950s-1960s and the Chernobyl nuclear accident in 1986; ii) at a regional level, by the low level radioactive discharges from the Marcoule nuclear reprocessing plant between 1958 and 1997; iii) and locally by the Palomares accident in 1966. Knowledge on their respective contributions to the presence of 129I, 236U, 237Np and Pu isotopes and on the radionuclide distributions in the Mediterranean Sea was limited or completely unknown. Our data on 129I and 236U showed that their distributions are governed by the thermohaline circulation, and highlighted the importance of constraining each radionuclide source. In particular, we estimated that the contamination derived from the Marcoule nuclear reprocessing plant was likely comparable to that from global fallout for 236U (10-20 kg) and about 2 orders of magnitude larger for 129I (70-90 kg). We also reported for the first time a comprehensive transect of 237Np. The distributions of both 237Np and 137Cs are driven by the circulation of the water masses, as expected for conservative radionuclides. The principal sources of 237Np were found to be global fallout and Marcoule, the later probably contributing to about 30% of the 237Np (~7 kg) present in the Mediterranean Sea in 2013. The distribution of 137Cs showed the influence of inputs and recent changes in circulation. The main source of Pu was confirmed to be global fallout, as shown by the 240Pu/239Pu atom ratios of about 0.18 throughout the Mediterranean Sea. The distribution of 239,240Pu was affected by, in addition to circulation, particle scavenging and remineralization. A notable result of this study was the lower levels of 239,240Pu in the water column of the Eastern Basin than in the Western Basin that were possible due to enchanced boundary scavenging on shelf sediments or the limited scavenging by particles in open waters and enhanced westward transport by water dynamics

Rapidly Increasing Artificial Iodine Highlights Pathways of Iceland-Scotland Overflow Water and Labrador Sea Water

Iceland-Scotland Overflow Water (ISOW) and Labrador Seawater (LSW) are major water masses of the lower Atlantic Meridional Overturning Circulation (AMOC). Therefore, the investigation of their transport pathways is important to understand the structure of the AMOC and how climate properties are exported from the North Atlantic to lower latitudes. There is growing evidence from Lagrangian model simulations and observations that ISOW and LSW detach from boundary currents and spread off-boundary, into the basin interior in the Atlantic Ocean. Nuclear fuel reprocessing facilities of Sellafield and La Hague have been releasing artificial iodine (129I) into the northeastern Atlantic since the 1960ies. As a result, 129I is supplied from north of the Greenland-Scotland passages into the subpolar region labelling waters of the southward flowing lower AMOC. To explore the potential of 129I as tracer of boundary and interior ISOW and LSW transport pathways, we analyzed the tracer concentrations in seawater collected during four oceanographic cruises in the subpolar and subtropical North Atlantic regions between 2017 and 2019. The new tracer observations showed that deep tracer maxima highlighted the spreading of ISOW along the flanks of Reykjanes Ridge, across fracture zones and into the eastern subpolar North Atlantic supporting recent Lagrangian studies. Further, we found that 129I is intruding the Atlantic Ocean at unprecedented rate and labelling much larger extensions and water masses than in the recent past. This has enabled the use of 129I for other purposes aside from tracing ISOW. For example, increasing tracer levels allowed us to differentiate between newly formed 129I-rich LSW and older vintages poorer in 129I content. Further, 129I concentration maxima at intermediate depths could be used to track the spreading of LSW beyond the subpolar region and far into subtropical seas near Bermuda. Considering that 129I releases from Sellafield and La Hague have increased or levelled off during the last decades, it is very likely that the tracer invasion will continue providing new tracing opportunities for 129I in the near future