Science Highlights


Some recent GEOTRACES science findings are reported below.  
When getting older they are compiled in the Science Highlights Archive where the "Title Filter" search box will allow you to filter them by words in title (please note that only one-word search queries are allowed e.g. iron, Atlantic, etc.).

Radium quartet reveals no less than four main processes along the GEOTRACES North Atlantic Ocean section (30°N)

The four radium (Ra) isotopes (224Ra, 223Ra, 228Ra, 226Ra, "radium quartet") are produced in situ via decay of their insoluble  thorium isotope parents in sediments from the continental margins and deep-sea and then released to the ocean. In the ocean, their distributions are controlled by particle removal (226Ra) and radioactive decay with four different half-lives. These properties make the "quartet" an invaluable tracer of coast-to-ocean processes and a water mass spreading chronometer.

Thanks to a dense and beautiful data set documenting the radium quartet along the 30°N GEOTRACES US section (GA03), Charette and co-authors (2015, see reference below) were able to identify:

  • a Mediterranean outflow spreading rate of 0.52-0.60 cm/s derived from 228Ra,
  • evidence of substantial sediment/water interaction in the benthic boundary layer along the oxygen minimum zones
  • decoupling between 223Ra and the other Ra isotope sources over the mid-Atlantic Ridge, and
  • significant continental inputs (e.g. submarine groundwater discharge) in the western Atlantic.

Last but not least, they conclude that the 228Ra inventories in the upper water column have remained constant over the past 40 years, which suggests that submarine groundwater discharge (the primary 228Ra source) is steady-state for the North Atlantic on decadal time scales.

15 Charette l
Figure. Box average 0-1000 m inventories (15° x 15°) of 228Ra (x 1010 atoms m-2) for samples collected on the GEOTRACES Atlantic section (GT) (2010-2011) and the Transient Tracers in the Ocean cruises (TTO) (1981-1986). Each solid black dot is a TTO station, each red x is a GT station. Modified from Charette et al. (in press) Click here to view the figure larger.

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When direct mapping of diatoms reveals unexpected fate of trace metals in the twilight zone

Twining and co-authors (2014, see reference below) used synchrotron x-ray fluorescence mapping to measure macronutrients such phosphorus (P), sulphur (S), and silicon (Si), and also trace metals like iron (Fe), nickel (Ni) and zinc (Zn), in individual cells of a diatom specie during a spring bloom off New Zealand. They clearly show that P, S, Zn and Ni are released faster than Fe and Si from sinking cells in the upper 200 m. Although the metals are co-located with P and S at the surface, the scheme changes deeper. The relationships with P and S become weak while an association of Fe with Si appears, suggesting re-adsorption when particles are settling. Exciting results revealing that ratios of dissolved Fe to macronutrients in the water column likely underestimate stoichiometries in sinking cells.

14 twining l
Figure.
Element maps (P, S, Fe ,and Zn) and associated scatterplots of Fe and S concentrations in each pixel of the scans for two diatom cells collected from 30m or 200m following a spring bloom off New Zealand.  The scatterplots show that Fe and S are spatially decoupled from each other when the diatom cells degrade as they sink through the upper water column.  S is lost more readily from the cells, while Fe appears to be retained or is re-scavenged.  Scale bar indicates 10um for each cell.  Adapted from Twining et al. (2014). Click here to view the figure larger.

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Iron isotopes in the Equatorial Pacific Ocean: when the dissolved phases are heavier than the particulate ones

 A detailed study of the dissolved and particulate iron (Fe) concentrations and isotopes off the Papua New Guinea coast is proposed. Regarding the sources, iron isotopic composition (δ56Fe) values reveal that the aerosols are heavier than the average crustal value while the sediments and river/volcano waters display δ56Fe similar to crustal values.

Surprisingly, all along the vertical profiles (down to 1000m) the dissolved phase is almost systematically heavier than the particulate one (see figure below). This likely reflects equilibrium exchanges between the dissolved and particulate iron. These interactions seem to result in a net non reductive release of dissolved iron. The intensity of this release suggests this process could play an important role on the global scale.

2015 Labatut l
Figure. Dissolved (triangles) and particulate (circles) Fe isotopic composition profiles in ‰ in three different stations in the Western Equatorial Pacific Ocean: (a) in Vitiaz Strait, (b) close to the Papua New Guinea coast downstream the Sepik river, and (c) close to New Ireland coast. The difference between dissolved and particulate iron isotopic composition (Δ56FeDFe - PFe) is represented by the colored area. It shows that the dissolved phase is almost systematically heavier (more positive values) than the particulate one with Δ56FeDFe - PFe = + 0.27 ± 0.25‰ (2SD, n = 11). Click here to view the figure larger.

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Unprecedented set of dissolved manganese data in the North Atlantic Ocean (US GEOTRACES cruise)

Manganese (Mn) is an essential nutrient for biological growth. In the ocean, manganese distribution is sensitive to several processes: redox conditions, photochemistry, biological activity, abiotic scavenging, and also eolian, hydrothermal and sedimentary sources. All of them are conditioning the concentration of dissolved Mn in the ocean vertical profiles as shown in the east-west ocean section proposed by Wu and co-workers across the Subtropical North Atlantic Ocean (see figure below).

Their simple model calculation suggests that the main actors determining the distribution are:

  • In the surface waters (0-40m): eolian Mn(II) deposition and in-situ photochemical reduction of dioxide of manganese (MnO2).
  • Below the mixed layer (40-200m): the intensity of sunlight available for in-situ MnO2 photochemical reduction.
  • Between 200 and 700 m: regeneration preformed Mn in the source water and lateral inputs from hydrothermal and sedimentary sources.
  • Below 700 m: lateral inputs from hydrothermal and sedimentary sources become predominant.

2014 Wu lFigure. Vertical distribution of manganese (Mn) along a section across Subtropical North Atlantic. Warm colours (red, orange, etc.) indicate high concentrations. Click here to view the figure larger.

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What is controlling the copper isotopic composition in oceanic waters?

Takano and co-workers (2014, see reference below) strongly suggest that the isotopic composition of dissolved copper (δ65Cu) in surface seawater is mainly controlled by supply from rivers, the atmosphere and deep seawater. This is the conclusion of a study involving six vertical profiles of copper (Cu) concentration and isotopes measured in the Indian (1) and North Pacific (5). The finding contradicts preceding interpretations suggesting a strong role of the biological activity in δ65Cu fractionation.

At depth, δ65Cu  values are becoming heavier with the age of deep seawater, likely due to preferential scavenging of the light isotope (63Cu). The authors built a box-model to quantify the oceanic budgets of both Cu concentrations and δ65Cu. Unbalance in this model suggests that Cu fluxes from continental shelf sediment might affect Cu distribution in the open ocean.

14 Takano lFigure:  A box-model of Cu in the ocean based on both Cu concentration and isotopic composition. Click here to view the figure larger.

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Seasonal iron supply in the Southern Ocean is dominated by winter mixing

An international team of researchers analysed the available dissolved iron data taken from all previous studies of the Southern Ocean, together with satellite images taken of the area, to quantify the amount of iron supplied to the surface waters of the Southern Ocean. They found that in contrast to the processes that supply so-called macronutrients in the tropics, seasonal iron supply is dominated by winter mixing with little iron input afterwards. This is because the vertical profile of iron is distinct from other nutrients, with subsurface reserves located much deeper in the water column and therefore only accessible by the deeper mixing that occurs in winter. This means that after this input pulse, intense iron recycling by the 'ferrous wheel' is necessary to sustain biological activity. This unique aspect of iron cycling is yet to be explained but places important constraints on how climate models represent the iron distribution and how changes in ocean physics impact iron limitation.

14 Tagliabue2 lFigure. This diagram represents the seasonal variability in Southern Ocean iron (Fe) cycling.
Click here to view the figure larger.

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Why is the deep ocean zinc isotopic signature so heavy?

The oceanic balance of the micronutrient zinc (Zn) is getting more puzzling –although better documented- while the Zn isotope data set increases. Presenting the first high resolution section of seawater dissolved Zn concentration and Zn isotope ratios (δ66Zn) from the North Atlantic, Conway and John (2014, see reference below) confirm that the deep ocean is fairly homogeneous for δ66Zn, close to +0.5‰, except near local North Atlantic sources of Zn (margins, the Mediterranean Sea, hydrothermal vents) where it is isotopically lighter.

Balancing the Zn isotopic budget raises questions. Indeed, the known inputs of this element (continental, riverine, and aerosol) display δ66Zn ranging between 0 and +0.3‰ while the known outputs (carbonates, ferromanganese nodules, and ferromanganese crusts) are isotopically heavier (+0.9 to +1‰). As previously suggested by Little et al. (2014, see also highlight about it), an isotopically light sink is therefore missing. The authors suggest that burial of biogenic Zn in sediments might be this important sink. They also suggest a potential role for zinc sulfide (ZnS) precipitation in low-oxygen open-ocean waters as a possible light sink, analogous to recent studies on Cd (Janssen et al., 2014, see also highlight about it).

14 Conway2 l
Figure.
Distribution of dissolved stable Zn isotope ratios (δ66Zn) for the US GEOTRACES North Atlantic GA03 and GA03_e sections. δ66Zn is expressed relative to JMC Lyon standard. Station numbers are shown for 2010 (USGT10, red) and 2011 (USGT11, blue). The black vertical line denotes the crossover between the cruises at USGT10-12 and USGT11-24. Click here to view the figure larger.

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Present day neodymium isotopic composition of the Caribbean Sea deep waters questions the paleo-application of this tracer in restricted basins

The first profiles of neodymium (Nd) concentration and isotopic composition in the Carribbean Sea have been published. They show that surface and intermediate waters flow through the Caribbean with essentially unchanged Nd isotopes ratios (εNd), whereas deep waters are strongly modified. Indeed, they likely receive radiogenic Nd released from the local sediments, of volcanic origin. Osborne and co-authors (2014, see reference below) suggest that this important shift is facilitated by the long residence time of these deep waters (150 years). This finding has general implications for paleoceanographic studies in restricted basins, where the composition of seawater is sensitive to its residence time within the basin.

14 Osborne l
Figure. Schematic cross-section of the Caribbean and Gulf of Mexico basins, showing the Nd isotopic composition and concentration of inflowing Atlantic water and how it changes to more radiogenic compositions and higher concentrations within the deep Caribbean. Interaction of seawater with radiogenic Nd from the margins of the Caribbean may be responsible for this change, aided by the slow replenishment rate and long residence time of deep waters in the Caribbean and Gulf of Mexico. Click here to view the figure larger.

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