Search

Report on Tracers, Carbon and ADCPs: Report on transient tracer, carbonate system variables and ADCP data from hydrographic sections and a system to sustain these observations past AtlantOS on key hydrographic sections

Refined description from AtlantOS work of the societal imperatives for sustained Atlantic Ocean observations, the phenomena to observe, EOVs, and contributing observing networks.

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., Biogeosciences, 15 (23), ISSN:1726-4170

The highest anthropogenic CO2 (Cant) inventories of all the oceans are found in the subpolar North Atlantic (SPNA). The OVIDE section crosses the SPNA from Greenland to Portugal and is repeated biennially since 2002. Therefore, OVIDE data allow evaluating the evolution of inventories of anthropogenic CO2 (Cant) in the SPNA. This oceanic region is known to host water formation and ventilation processes that change the water mass volumetric census year by year. Changes in the volumetric census cause differences in the Cant inventory estimates (Pérez et al., 2008). Consequently, Pérez et al. (2010) proposed a method for computing Cant inventory in the SPNA taking into account the volumetric census variability and a bathymetric adjustment. This method has been recently revised. The initial methodology and the recent improvements are detailed in this report., Les inventaires de CO2 anthropique (Cant) les plus hauts de tous les océans sont trouvés en Atlantique Nord subpolaire. La section OVIDE traverse l’Atlantique Nord Subpolaire du Groenland au Portugal ; elle est visitée tous les deux ans depuis 2002. Les données acquises pendant les campagnes OVIDE sont donc un outil très intéressant pour étudier l’évolution des inventaires de Cant dans l’Atlantique Nord subpolaire. Dans cette région océanique il y a des processus de ventilation et de formation de masses d’eau qui provoquent des changements de volume de masses d’eau d’une année à l’autre, et ces derniers causent des différences sur le calcul d’inventaires de Cant (Pérez et al., 2008). Donc Pérez et al. (2010) ont proposé une méthode pour calculer des inventaires de Cant en corrigeant la variabilité du volume des masses d’eau et en tenant compte de la bathymétrie du bassin. Cette méthode a était récemment révisée, et les améliorations de la méthode initiale sont détaillées et quantifiées dans ce rapport.

The BOCATS cruise (Biennial Observation of Carbon, Acidification, Transport and Sedimentation) was carried out in the North Atlantic from Vigo (June 18) to Reykjavik (July 31). It was composed of 2 main sections: the OVIDE section coast to coast between Portugal and the south tip of Greenland, followed by the Reykjanes Ridge section, from 55 °N to Iceland. The classical hydrographic rosette was cast 137 times along the route. While the CTD-O2 probe acquired continuous profiles of the “physical” variables (pressure, temperature, salinity and dissolved oxygen), 28 Niskin bottles were closed at different levels during the upcast to provide samples for biogeochemical analysis. We report here an overview of the cruise operations and team, followed by a focus on the acquisition and calibration steps of the physical variables measured by the hydrographic CTD-O2 probe. After calibration, we find precisions for pressure, temperature, salinity and dissolved oxygen that fit the GO-SHIP international quality requirements. Pressure and temperature were calibrated at the laboratory before and after the cruise, leading to a precision of 1 dbar and 0.002°C respectively. The calibration of salinity and dissolved oxygen data is obtained by applying polynomial correcting functions that are calculated to statistically minimize the differences between the probe data and the sample data analyzed on board in the chemistry container. These chemical data are also evaluated by the comparison of replicates. After calibration, the differences in salinity and oxygen follow a zero-centered Gaussian-like distribution which standard deviation is used to evaluate the probe precision for each variable. For salinity, we find a standard deviation of 0.001, and for oxygen, 0.027 ml/l (or 1.3 μmol/kg). Those numbers correspond to a 68 % confidence interval and must be doubled to reach a precision with a 95 % confidence interval., La campagne BOCATS s’est déroulée en Atlantique Nord, de Vigo (18 juin) à Reykjavik (31 juillet). Elle se compose de 2 sections principales: la section OVIDE du Portugal à la pointe sud du Groenland, suivie de la section sur la Ride de Reykjanes, de 55 °N à l’Islande. La rosette hydrographique a été mise à l’eau à 137 stations sur cette route. Alors que la sonde CTD-O2 enregistrait en continu les variables dites « physique » (pression, température, salinité et oxygène dissous), 28 bouteilles Niskin étaient fermées à la remontée à différentes profondeurs afin de disposer d’échantillons d’eau de mer pour les analyses biogéochimiques. Ce rapport expose tout d’abord une présentation des opérations et de l’équipe de la campagne, suivie par les étapes d’acquisition et d’ajustage des variables physiques mesurés par la sonde CTD-O2. Après ajustage, la précision estimée pour la pression, la température, la salinité et l’oxygène dissous est conforme aux exigences internationales des campagnes GO-SHIP. La pression et la température ont été ajustées grâce à une vérification au laboratoire de métrologie avant et après la campagne, et les précisions de 1 dbar et 0.002°C sont obtenues respectivement. L’ajustage des données de salinité et oxygène dissous se fait par application de polynômes de correction calculés pour statistiquement minimiser l’écart entre les données de la sonde et les analyses chimiques faites sur les prélèvements, ces dernières étant validées par l’analyse régulière de doublons. La différence entre les données de la sonde après ajustage et les données de chimie suit une distribution quasi gaussienne centrée sur zéro pour les deux variables, et la précision des données est estimée par le calcul de l’écart-type de cette distribution, qui donne une valeur de 0.001 en salinité et 0.027 ml/l (soit 1.3 μmol/kg) en oxygène, correspondant à un intervalle de confiance de 68%. Une précision à 95 % nécessite de doubler la valeur de l’écart-type.

The aim of the GEOVIDE cruise (May–June 2014, R/V Pourquoi Pas?) was to provide a better understanding of trace metal biogeochemical cycles in the North Atlantic Ocean. As marine particles play a key role in the global biogeochemical cycle of trace elements in the ocean, we discuss the distribution of particulate iron (PFe), in relation to the distribution of particulate aluminium (PAl), manganese (PMn), and phosphorus (PP). Overall, 32 full vertical profiles were collected for trace metal analyses, representing more than 500 samples. This resolution provides a solid basis for assessing concentration distributions, elemental ratios, size fractionation, and adsorptive scavenging processes in key areas of the thermohaline overturning circulation. Total particulate iron concentrations ranged from as low as 9 pmol L−1 in surface waters of the Labrador Sea to 304 nmol L−1 near the Iberian margin, while median PFe concentrations of 1.15 nmol L−1 were measured over the sub-euphotic ocean interior. Within the Iberian Abyssal Plain, the ratio of PFe to PAl was identical to the continental crust molar ratio (0.21 mol mol−1), indicating the important influence of crustal particles in the water column. Overall, the lithogenic component explained more than 87% of PFe variance along the section. Within the Irminger and Labrador basins, the formation of biogenic particles led to an increase in the PFe∕PAl ratio (up to 0.64 mol mol−1) compared to the continental crust ratio. Continental margins induce high concentrations of particulate trace elements within the surrounding water masses (up to 10 nmol L−1 of PFe). For example, horizontal advection of PFe was visible more than 250 km away from the Iberian margin. Additionally, several benthic nepheloid layers were observed more than 200 m above the seafloor along the transect, especially in the Icelandic, Irminger, and Labrador basins, suspending particles with high PFe content of up to 89 nmol L−1., Biogeosciences, 16 (7), ISSN:1726-4170

Particulate concentrations of the 14 Rare Earth Elements (PREE), yttrium, and 232-thorium were measured in 200 samples collected in the epipelagic (ca. 0–200 m) and mesopelagic (ca. 200–1500 m) zones of the North Atlantic during the GEOVIDE cruise (May/June 2014, R/V Pourquoi Pas?, GEOTRACES GA01), providing the most detailed snapshot of the PREE distribution in the North Atlantic so far. Concentrations of particulate cerium (PCe) varied between 0.2 and 16 pmol L−1, while particulate neodymium (PNd) concentrations ranged between 0.1 and 6.1 pmol L−1. Particulate ytterbium (PYb) concentrations ranged between 0.01 and 0.50 pmol L−1. In addition, this study showed that PREE distributions were also controlled by the biological production in the upper sunlit ocean and by remineralization processes in the mesopelagic area. Low surface concentrations combined with normalized PREE patterns displaying a negative Ce anomaly and HREE enrichments pointed to freshly formed biogenic particles imprinting the seawater signature. A significant relationship between biogenic silica (BSi) and PHREE was also observed in the Labrador and Irminger seas, due to the occurrence of strong diatom blooms at the sampling time. In order to identify dissolved-particulate processes independent of the ionic radius, we used PHo∕PY ratios and showed that absorption processes were predominant in the upper ocean, while adsorption processes dominated at deeper depths. This study highlighted different lithogenic fractions of PREE and dispersion depending on the shelf: off the Iberian margin, up to 100 % of the PREE were determined to have a lithogenic origin. This lithogenic input spread westward along an intermediate nepheloid layer (INL), following isopycnals up to 1700 km away from the margin. In contrast, along the Greenland and Newfoundland margins, the circulation maintained lithogenic inputs of PREE along the coasts., Biogeosciences, 17 (22), ISSN:1726-4170