Physical and remineralization processes govern the cobalt distribution in the deep western Atlantic Ocean

International audienceThe distributions of the bio-essential trace element dissolved cobalt (DCo) and the apparent particulate Co (PCo) are presented along the GEOTRACES-A02 deep section from 64° N to 50° S in the western Atlantic Ocean (longest section of international GEOTRACES marine environment program). PCo was determined as the difference between total cobalt (T Co, unfiltered samples) and DCo. DCo concentrations ranged from 14.7pM to 94.3 pM, and PCo concentrations from undetectable values to 18.8 pM. The lowest DCo concentrations were observed in the subtropical domains, and the highest in the low-oxygenated Atlantic Central Waters (ACW), which appears to be the major reservoir of DCo in the western Atlantic. In the Antarctic Bottom Waters, the enrichment in DCo with aging of the water mass can be related to suspension and redissolution of bottom sediments a well as diffusion of DCo from abyssal sediments. Mixing and dilution of deep water masses, rather than scavenging of DCo onto settling particles, generated the meridional decrease of DCo along the southward large-scale circulation in the deep western Atlantic. Furthermore, the apparent scavenged profile of DCo observed in the deep waters likely resulted from the persistence of relatively high concentrations in intermediate waters and low DCo concentrations in underlaying bottom waters. We suggest that the 2010 Icelandic volcanic eruption could have been a source of DCo that could have been transported into the core of the Northeast Atlantic Deep Waters. At intermediate depths, the high concentrations of DCo recorded in the ACW linearly correlated with the apparent utilization of oxygen (AOU), indicating that remineralization of DCo could be significant (representing up to 37% of the DCo present). Furthermore, the preferential remineralization of phosphate (P) compared to Co in these low-oxygenated waters suggests a decoupling between the deep cycles of P and Co. The vertical diffusion of DCo from the ACW appears to be a significant source of DCo into the surface waters of the equatorial domain. Summarizing, the dilution due to mixing processes rather than scavenging of DCo and the above-mentioned remineralization could be the two major pathways controlling the cycling of DCo into the intermediate and deep western Atlanti

The role of humic-type ligands in the bioavailability and stabilization of dissolved iron in the Western Tropical South Pacific Ocean

The high N2 fixation rate observed in the Lau Basin of the western tropical South Pacific Ocean (WTSP) is fueled by iron (Fe) released from shallow hydrothermal systems. Understanding Fe bioavailability is crucial but the controls on the stability and bioavailability of hydrothermal Fe inputs are still poorly understood. Here, we provide new data on the spatial and vertical distribution of the soluble ubiquitous humic-like ligands (LFeHS) and their associated dissolved Fe (DFe) in the WTSP, including in samples near hydrothermal vents. Our data show that LFeHS are heterogenous ligands with binding sites of both strong and intermediate strengths. These ligands are primarily produced in surface waters and partially mineralized in mesopelagic waters. A substantial fraction of DFe was complexed by LFeHS (mean ~30%). The DFe complexed by LFeHS is likely bioavailable to phytoplankton and LFeHS stabilized Fe released by the mineralization of sinking biomass. However, unsaturation of LFeHS by Fe suggest that part of DFe is not available for complexation with LFeHS. Possible reasons are competition between DFe and other metals, such as dissolved copper, or the inability of LFeHS to access colloidal DFe. The study of two volcanic sites indicates that LFeHS were not produced in these hydrothermal systems. At the active site (DFe ~50 nmol L-1), LFeHS can only partially solubilize the hydrothermal DFe released in this area (1~5.5% of the total DFe). We performed controlled laboratory experiments which show that the observed low solubilization yield result from the inability of LFeHS to solubilize aged Fe oxyhydroxides (FeOx - a kinetically mediated process) and to form stable complexes with Fe(II) species. Our study provides new understanding of the role of LFeHS on the bioavailability and stabilization of hydrothermal DFe

Iron-binding by dissolved organic matter in the Western Tropical South Pacific Ocean (GEOTRACES TONGA cruise GPpr14)

Iron (Fe) is an essential micronutrient for phytoplankton growth, but its scarcity in seawater limits primary productivity across much of the ocean. Most dissolved Fe (DFe) in seawater is complexed with Fe-binding organic ligands, a poorly constrained fraction of dissolved organic matter (DOM), which increase Fe residence time and impact Fe bioavailability. Here, we present the conditional concentration (LFe) and binding-strength (log KFe'Lcond) of Fe-binding ligands in the Western Tropical South Pacific (WTSP) Ocean during the GEOTRACES TONGA cruise (GPpr14). The transect crossed the Lau basin, a region subject to shallow hydrothermal Fe inputs that fuel intense diazotrophic activity, the oligotrophic South Pacific gyre, and the Melanesian basin. Organic speciation was analyzed by competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-AdCSV) using salicylaldoxime at 25 µM. We found a high mean LFe of 5.2 ± 1.2 nMeqFe (n = 103) across the entire transect, predominantly consisting of intermediate strength L2 ligands (84%; mean log KFe'Lcond of 11.6 ± 0.4), consistent with humic-like substances. DFe correlated with the humic-like component of the fluorescent DOM (HS-like FDOM), yet the electroactive Fe-binding humic-like substances (LFeHS) accounted for only 20 ± 13% of LFe in the mixed layer and 8 ± 6% in deep waters. Ligands were in large excess compared to DFe (mean excess ligand eLFe = 4.6 ± 1.1 nMeqFe), suggesting poor stabilization of DFe inputs. High LFe (up to 9 nMeqFe) in samples close to hydrothermal sites could be due to detoxification strategies from plankton communities toward hydrothermally-fueled toxic trace metals other than Fe, with an apparent dilution of the DOM from the Lau basin into neighboring regions. We also observed a different peak potential of the Fe salicylaldoxime complex detected by CLE-AdCSV between the Lau and Melanesian basins, and between surface and deep waters. To our knowledge, this change in potential has not previously been reported; whether this represents a novel detection of specificities in DOM composition merits further investigation. Competition between Fe and competing metals for ligand binding sites could favor DFe oxidation and precipitation near hydrothermal vents and explain the absence of strong Fe stabilization in the WTSP

Return of naturally sourced Pb to Atlantic surface waters

Anthropogenic emissions completely overwhelmed natural marine lead (Pb) sources during the past century, predominantly due to leaded petrol usage. Here, based on Pb isotope measurements, we reassess the importance of natural and anthropogenic Pb sources to the tropical North Atlantic following the nearly complete global cessation of leaded petrol use. Significant proportions of up to 30-50% of natural Pb, derived from mineral dust, are observed in Atlantic surface waters, reflecting the success of the global effort to reduce anthropogenic Pb emissions. The observation of mineral dust derived Pb in surface waters is governed by the elevated atmospheric mineral dust concentration of the North African dust plume and the dominance of dry deposition for the atmospheric aerosol flux to surface waters. Given these specific regional conditions, emissions from anthropogenic activities will remain the dominant global marine Pb source, even in the absence of leaded petrol combustion

The GEOTRACES Intermediate Data Product 2014

The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEIs) as well as classical hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing a strongly inter-linked on-line atlas including more than 300 section plots and 90 animated 3D scenes. The IDP2014 covers the Atlantic, Arctic, and Indian oceans, exhibiting highest data density in the Atlantic. The TEI data in the IDP2014 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at cross-over stations. The digital data are provided in several formats, including ASCII spreadsheet, Excel spreadsheet, netCDF, and Ocean Data View collection. In addition to the actual data values the IDP2014 also contains data quality flags and 1-? data error values where available. Quality flags and error values are useful for data filtering. Metadata about data originators, analytical methods and original publications related to the data are linked to the data in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2014 data providing section plots and a new kind of animated 3D scenes. The basin-wide 3D scenes allow for viewing of data from many cruises at the same time, thereby providing quick overviews of large-scale tracer distributions. In addition, the 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of observed tracer plumes, as well as for making inferences about controlling processes

No

Le cobalt est un métal de transition essentiel pour la croissance du phytoplancton, et en particulier pour les cyanobactéries qui ont un besoin absolu pour cet élément. En étant l'atome central de la cobalamine (vitamine B12), le cobalt est aussi indirectement essentiel aux eucaryotes marins qui ne synthétisent pas cette vitamine. Cet élément peut se substituer au zinc et au cadmium au sein de la carbonique anhydrase, l’enzyme permettant la fixation du dioxyde de carbone dans la cellule phytoplanctonique. Il pourrait intervenir également dans l’activation de l’alcaline phosphatase. De par ses implications biologiques, le cobalt pourrait jouer un rôle important dans le cycle océanique du carbone. Cependant, les connaissances du cycle biogéochimique du cobalt en milieu marin sont encore largement limitées. Ce travail de thèse de doctorat s’inscrit dans le cadre du programme international GEOTRACES au sein duquel le cobalt y est désigné comme un élément clé de la biogéochimie marine. Au cours de ces travaux, l’un des plus larges jeux de données, rapporté à ce jour, incluant les différentes fractions du cobalt (soluble, dissous, particulaire, spéciation organique) a été produit. Les données recueillies proviennent d’échantillons collectés au sein de domaines océaniques contrastés. Une stratégie de prélèvement à haute résolution et à grande échelle a été mise en place dans diverses régions océaniques du monde lors de campagnes à la mer. Ainsi pour la première fois, une cartographie du cobalt dissous (DCo) et particulaire (PCo) a pu être définie pour l’ensemble de l’Atlantique Ouest, ainsi que des bassins Méditerranéens et de la Mer Noire. Ce jeu de données a pu être produit par l’utilisation de différentes techniques d’analyses (Flow-Injection-Analysis and Chemiluminescence detection ; Voltamétrie, SF-ICP-MS) aux limites de détections basses permettant la détermination de cet élément, présent dans l’eau de mer à des concentrations de l’ordre du pico-molaire (10-12 M). Le cobalt est en effet l’un des micro-nutritifs le moins abondant dans l’eau de mer. Les concentrations les plus faibles en DCo ont été observées dans les eaux oligotrophes de l’Atlantique Ouest ( 5 nM). La distribution verticale du cobalt dissous variait selon les systèmes biogéochimiques. Ainsi, le profil vertical est de type nutritif comme les phosphates dans les eaux de surface de l’océan Atlantique. Les concentrations y augmentent avec la profondeur, jusqu’à un maximum relatif dans les eaux intermédiaires, puis décroissent dans l’océan profond. Ce comportement contraste avec le profil observé pour l’ensemble des bassins de la Mer Méditerranée. Dans cette mer, les fortes concentrations en DCo mesurées en surface (100-300 pM) diminuent en effet avec la profondeur. En Mer Noire, la distribution verticale varie selon les conditions d’oxygénation des eaux. Les concentrations y sont extrêmement élevées par comparaison aux autres systèmes marins. […]N

Cycle biogéochimique du cobalt en domaines océaniques contrastés : l'Atlantique Ouest, la Mer Méditerranée et la Mer Noire

NoLe cobalt est un métal de transition essentiel pour la croissance du phytoplancton, et en particulier pour les cyanobactéries qui ont un besoin absolu pour cet élément. En étant l'atome central de la cobalamine (vitamine B12), le cobalt est aussi indirectement essentiel aux eucaryotes marins qui ne synthétisent pas cette vitamine. Cet élément peut se substituer au zinc et au cadmium au sein de la carbonique anhydrase, l’enzyme permettant la fixation du dioxyde de carbone dans la cellule phytoplanctonique. Il pourrait intervenir également dans l’activation de l’alcaline phosphatase. De par ses implications biologiques, le cobalt pourrait jouer un rôle important dans le cycle océanique du carbone. Cependant, les connaissances du cycle biogéochimique du cobalt en milieu marin sont encore largement limitées. Ce travail de thèse de doctorat s’inscrit dans le cadre du programme international GEOTRACES au sein duquel le cobalt y est désigné comme un élément clé de la biogéochimie marine. Au cours de ces travaux, l’un des plus larges jeux de données, rapporté à ce jour, incluant les différentes fractions du cobalt (soluble, dissous, particulaire, spéciation organique) a été produit. Les données recueillies proviennent d’échantillons collectés au sein de domaines océaniques contrastés. Une stratégie de prélèvement à haute résolution et à grande échelle a été mise en place dans diverses régions océaniques du monde lors de campagnes à la mer. Ainsi pour la première fois, une cartographie du cobalt dissous (DCo) et particulaire (PCo) a pu être définie pour l’ensemble de l’Atlantique Ouest, ainsi que des bassins Méditerranéens et de la Mer Noire. Ce jeu de données a pu être produit par l’utilisation de différentes techniques d’analyses (Flow-Injection-Analysis and Chemiluminescence detection ; Voltamétrie, SF-ICP-MS) aux limites de détections basses permettant la détermination de cet élément, présent dans l’eau de mer à des concentrations de l’ordre du pico-molaire (10-12 M). Le cobalt est en effet l’un des micro-nutritifs le moins abondant dans l’eau de mer. Les concentrations les plus faibles en DCo ont été observées dans les eaux oligotrophes de l’Atlantique Ouest ( 5 nM). La distribution verticale du cobalt dissous variait selon les systèmes biogéochimiques. Ainsi, le profil vertical est de type nutritif comme les phosphates dans les eaux de surface de l’océan Atlantique. Les concentrations y augmentent avec la profondeur, jusqu’à un maximum relatif dans les eaux intermédiaires, puis décroissent dans l’océan profond. Ce comportement contraste avec le profil observé pour l’ensemble des bassins de la Mer Méditerranée. Dans cette mer, les fortes concentrations en DCo mesurées en surface (100-300 pM) diminuent en effet avec la profondeur. En Mer Noire, la distribution verticale varie selon les conditions d’oxygénation des eaux. Les concentrations y sont extrêmement élevées par comparaison aux autres systèmes marins. […

Influence of the conservation mode of seawater for dissolved organic carbon analysis

International audienceMarine dissolved organic matter (DOM) is one of the largest exchangeable organic carbon reservoir on the planet. The main proxy to track the distribution of DOM in the aquatic environments remains dissolved organic carbon (DOC). Thereby the optimal protocol for long-term DOC preservation in seawater samples must be defined. In this context, we monitored bulk DOC concentrations and its size class distribution in filtered seawater samples during yearlong experiments. With different conservation mode, we tested two types of commonly used materials (borosilicate brown glass and high-density polyethylene, HDPE) and three conditioning protocols (untreated, acidified at pH 2 and frozen at −20 °C). Offshore samples collected along the entire water column of the Pacific Ocean and stored in HDPE bottles were also analysed after 2 years of storage at pH of 2 and compared to frozen samples. Results demonstrated that bulk DOC concentrations can be accurately determined in untreated samples for one month and for years in frozen samples as well as in acidified samples, when samples are stored in acid cleaned HDPE bottles or flame sealed glass ampoules. Storage in brown glass vials with Bakelite caps seems more uncertain. The study of the size class distribution of DOC reveals the possibility to study DOM for 1 month in filtered samples with no additional treatment and for years in frozen samples when stored in acid cleaned HDPE bottles. Significant changes in DOC size fractionation were observed when samples were acidified. The high molecular weight (HMW) compounds and the humic substances from the upper 1000 m were significantly degraded at pH 2, incorporating DOC in the low molecular weight (LMW) fractions. These experiments provide preservation guidelines for future studies that aim either to study bulk DOC or the chemical properties marine DOM. It is recommended to store seawater in HDPE vials at −20 °C for DOM study, or at pH 2 for bulk DOC measurements

Trace metal limitations (Co, Zn) increase PIC/POC ratio in coccolithophore Emiliania huxleyi

International audienceThe effect of cobalt (Co), zinc (Zn) or cadmium (Cd) availability on the growth and particulate inorganic (PIC) to organic (POC) carbon ratio of the cosmopolitan coccolithophore Emiliania huxleyi was examined using batch cultures. Growth co-limitation by Co and Zn occurred at low free metal ion concentrations below 10–12 pM. The two metals could replace one another in biochemical functions: full replacement of Zn by Co was possible, but the 20% higher growth rate observed with high Co levels in the absence of Zn than observed in the presence of high Zn but no added Co suggested that there was an absolute growth requirement of Co that could only partly be met by Zn. In contrast, Cd had no effect on growth in the absence of Co and Zn, indicating that this strain cannot use Cd under these conditions. The co-limitation by inorganic Co and Zn led to highly calcified cells containing a large quota of nitrogen, but diminished organic carbon content, likely due to a lower photosynthetic production rate. Deficiency of Zn and Co cofactors could have dramatically decreased the activity of carbonic anhydrase, lowering CO2 availability for photosynthesis. In contrast to the large decreases in carbon fixation rates at low Zn and Co ion concentrations, calcification rates were unaffected at low Zn levels and were only minimally decreased by low Co. As a result the PIC:POC ratios in E. huxleyi increased by up to 8-fold under Zn limitation and 3–5-fold under Co limitation to values where coccolithophores act as a source of atmospheric CO2. As a result, low Co and Zn availability in seawater could affect the efficiency of the biological carbon pump, and atmospheric CO2 levels. Thus, the impact of Co and Zn limitation may need to be considered to improve assessments of the impact of E. huxleyi and other biogenic calcifiers on oceanic sequestration of CO2