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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.).

Scandium: a new oceanic tracer with surprising properties

The first modern data set of dissolved scandium (Sc) in the open ocean is presented. Thanks to three GEOTRACES cruises, its distribution is compared between the different oceanic basins. This work also compares the reactivity of Sc with its trivalent periodic table group mates: yttrium (Y) and lanthanum (La) on the one hand, and aluminium (Al) and gallium (Ga) on the other hand.

These distributions and comparisons reveal that scandium is a hybrid-type metal with a residence time on the order of 1000 years. In addition, Sc displays similar particle reactivity to iron, which makes the authors suggest that Sc could help to constrain the non-biogenic part of the iron cycle.

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Figure: Comparing the concentrations of elements from deep water in different ocean basins is a classic way to categorize their reactivity and understand something about how they cycle through the ocean. Scandium, yttrium and lanthanum all occupy the same column in the periodic table, meaning that they are likely to have similar reactivity. However, this figure shows that while yttrium and lanthanum have higher concentrations in the North Pacific deep water than in the North Atlantic, scandium has similar deep-water concentrations in both ocean basins. From this we conclude that scandium is more reactive than yttrium and lanthanum in the ocean, and that scandium is in fact similar to iron, an important element that perhaps can be better understood through its similarity to scandium. Please click here to view the figure larger.

Reference:

Parker, C. E., Brown, M. T., & Bruland, K. W. (2016). Scandium in the open ocean: A comparison with other group 3 trivalent metals. Geophysical Research Letters, 43, 2758–2764. doi:10.1002/2016GL067827

Gadolimium, a Rare Earth Element becoming a human contaminant and tracer of wastewater discharge in the ocean

Gadolinium (Gd) is increasingly used in contrast agents for magnetic resonance imaging. It is therefore released in the wastes of hospitals and research centres.

As a consequence, Hatje and collaborators (2016, see reference below) showed that anthropogenic Gd concentrations in San Francisco Bay increased by an order of magnitude over the past 2 decades, even reaching the northeast Pacific coastal waters. Beyond the fact that such input might be used as tracers of wastewater discharges and hydrological processes, such impressive environmental change suggests that more effective treatment technologies may be necessary to minimise future contamination by chemical elements specially rare earth elements (REE, such as Gd) that are critical for the development of new technologies.

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Figure: Evolving concentrations of Gd from anthropogenic sources (Gdanth) in San Francisco Bay is a clear example of the changing scenario of REE cycles in coastal environments.

Reference:

Hatje, V., Bruland, K. W., & Flegal, A. R. (2016). Increases in Anthropogenic Gadolinium Anomalies and Rare Earth Element Concentrations in San Francisco Bay over a 20 Year Record. Environmental Science & Technology, 50(8), 4159–4168. doi:10.1021/acs.est.5b04322

Onboard analysis of dissolved zinc everywhere in the open ocean with a Lab on Valve (LOV) system of the size of a bottle of wine is becoming possible

Thanks to the work of Maxime Grand and collaborators (2016, see reference below), it is now possible to analyse dissolved zinc (DZn) on board, from any kind of seawater using a "Lab On Valve (LOV)" method. For the first time, automated matrix removal, extraction of the target element, and fluorescence detection have been performed within a miniaturized flow manifold. The original flow programming is designed to pass sample through a minicolumn where the target analyte and other complexable cations are retained, while the seawater matrix is washed out. Once eluted, Zn is merged with a Zn selective fluorescent probe (FluoZin-3) prior to fluorescence detection in the LOV flow cell. This new shipboard method features a detection limit of 0.02 nM and a reagent consumption of 150 microliters per sample.

Successful comparison with GEOTRACES reference standards and analytical comparison with inductively coupled plasma mass spectrometry (ICPMS) eventually validate this beautiful, tiny and robust method.

16 GrandetalFigure: A close up of the Lab-On-Valve (LOV) module, where microliter volumes of fluids are manipulated prior to fluorescence detection in the LOV flow cell (red circle). Right: Comparison of DZn profiles from the South Indian Gyre analysed via LOV and ICPMS. Click here to download the figure.

Reference:

Grand, M. M., Chocholouš, P., Růžička, J., Solich, P., & Measures, C. I. (2016). Determination of trace zinc in seawater by coupling solid phase extraction and fluorescence detection in the Lab-On-Valve format. Analytica Chimica Acta, 923, 45–54. doi:10.1016/j.aca.2016.03.056

 

Are the dissolved iron distributions well represented by the global ocean biogeochemistry models?

Alessandro Tagliabue and co-workers (2016, see reference below) have conducted the first intercomparison of 13 global ocean iron models against the latest datasets emerging from GEOTRACES.

A large disparity in the residence times for iron across the different models was found, which reflects a lack of agreement in how to represent the iron cycle in such models. Many models perform relatively poorly in their representation of the observed trends, but those who reflect the emerging insights into new sources and cycling pathways are better able to reproduce observed features.

A key challenge for the future is to reduce uncertainties in the iron sources and especially the magnitude of scavenging losses.

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Figure:
 The range of iron residence times (in years) for the global ocean across the thirteen Iron Model Intercomparision Project (FeMIP) models.

Read more: Are the dissolved iron distributions well represented by the global ocean biogeochemistry models?

An example of a fruitful international intercomparison

The reliability of the GEOTRACES data products including the eGEOTRACES Electronic Atlas is strongly related to the quality of the data acquired by the different laboratories contributing to this international effort. A key aspect for assessing this quality relies on the good intercomparison of the trace metal data. Iron (Fe) concentrations and moreover Fe isotopes count among the most delicate parameters to be measured in seawater.

Conway (Switzerland), John (USA) and Lacan (France) (2016, see reference below) present the first comparison of dissolved Fe stable isotope ratio profiles in the oceans, analyzed at different depths at 3 different GEOTRACES crossover stations in the Atlantic Ocean (Bermuda Atlantic Time Series Station, off Cape Verde and in the Cape Basin, south Atlantic).

Having assessed the strong agreement between data and profiles measured by 5 different laboratories at Bermuda Atlantic Time Series (BATS), the authors discuss the temporal variability observed at the three locations, taking advantage of reoccupation of the stations by multiple cruises on a 1-3 year timescale. The authors find that the deep ocean at these locations is largely invariant for Fe isotopes on these timescales, but that there is variability in surface waters and near low-oxygen margins.

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Figure:
Comparison of δ56Fe (relative to IRRM-014) and Fe data from Bermuda Atlantic Time Series (BATS) in the subtropical North West Atlantic (31.75°N 64.17°W) from the U.S. GEOTRACES IC1 (June 2008) and GA03 cruises (USGT11, Nov. 2011). Data are reproduced from Boyle et al. (2012); Conway and John (2014a); Conway et al. (2013a; 2013b); John and Adkins (2012). Please click here to view the figure larger.

Read more: An example of a fruitful international intercomparison

All mercury species measured along the GEOTRACES-UK section, South Atlantic Ocean

Total mercury (THg), methylated mercury (MeHg), and dissolved gaseous mercury (DGM) concentrations were determined along 40°S in the Atlantic Ocean. All Hg species had higher concentrations in western than in eastern basin, although they are at the ultra-low femtomolar level.

MeHg increases with lower oxygen concentration, substantiating that microbial respiration of organic matter stimulates MeHg formation. Sediment is generally a likely source of MeHg to the water column; however it stays a modest contributor in this ocean region. At the surface, photo-demethylation might be responsible of the low MeHg concentration.

DMeHg was measured only at one station. As expected, it was higher in the intermediate and deep waters below 1000m and very low above them, especially in the surface waters. It reached its highest concentration in the Upper Circumpolar Deep Water (UCDW), similarly to DGM, probably due to lower oxygen concentrations and hence lower oxidation potential.

Altogether, results show very dynamic Hg processes in this ocean region, which are of global importance for Hg cycling.

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Figure:
The figure shows total mercury (THg), monomethyl mercury (MeHg) and dissolved gaseous mercury (DGM) concentrations in the water column. Mercury is ubiquitous (THg), DGM is stratified, which means that different water masses have different DGM concentrations, and MeHg is very low, but is sometimes higher in regions with plenty of chlorophyll. Blank spaces show areas, where the concentrations are too low to be reliably measured. DGM is lowest at surface, indicating that it escapes from water to the atmosphere. Click here to view the figure larger.

Read more: All mercury species measured along the GEOTRACES-UK section, South Atlantic Ocean

Amazingly detailed compilation of the silicon cycle, with an emphasis on the oceanic silicon isotope budget

Although this article is not resulting from GEOTRACES activity, its content is definitely GEOTRACES relevant. The authors constructed an up-to-date compilation of the continental silicon (Si) cycle, including the fate of Si isotopic composition in the different continental but also estuarine and marine solid and solutions.

This is a paper highly recommended by Catherine Jeandel, the GEOTRACES IPO science director.

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Figure:
Cartoon schematic of the modern day global Si cycle. The values show the magnitudes of the fluxes (in 1012 mol yr− 1) and their associated δ30Si values (in ‰). Typical fractionations (ε, ‰) associated with production of biogenic silica (BSi) and clay minerals are shown in the inset panels. Dotted lines indicate particulate fluxes; solid lines indicate solute fluxes or transformations. Source: Frings, et al., 2016, Chemical Geology.

Reference:

Frings, P.J., Fontorbe, G., Clymans, W., De La Rocha, C.L., Conley, D.J., 2016. The continental Si cycle and its impact on the ocean Si isotope budget. Chemical Geology 425, 12-36.doi:10.1016/j.chemgeo.2016.01.020

Water masses traced by neodymium isotopic compositions at an unprecedented level in the North Atlantic Ocean

As part of the Dutch GEOTRACES GA02 section, Myriam Lambelet analysed neodymium (Nd) isotopic compositions and concentrations at 12 profiles in the North West Atlantic Ocean, extending from the south of Iceland down to the Sargasso Sea.

The detailed discussion, allowed by the good quality of Lambelet's data, reveals many new features, among them 3 are selected:

1) εNd values of the surface waters provide insight into the unradiogenic continental Nd inputs from Greenland and North America while the subtropical gyre is influenced by dust input from Africa (see figure, upper panel).

2) Exported NADW can be separated into upper- and lower-NADW, based on their distinct Nd isotopic compositions, which was never demonstrated before (see figure, lower panel).

3) Comparing dissolved seawater Nd concentrations and isotopic compositions confirms that the two parameters are decoupled, one of the most striking feature being that in the middle of the water column (1000–3000 m), strong lateral advection dominates the cycling of Nd in the western North Atlantic Ocean.

As a whole, their data support the idea that Nd isotopes can serve as an excellent water mass tracer, if sampled in areas away from oceanic margin, and particularly in areas of strong advection (i.e. deep western boundary current).

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Figure:
 Upper panel: neodymium (Nd) isotopic composition for the surface North Atlantic Ocean (dots with black rim = this study). The coloured outlines of the coastlines represent the approximate Nd isotopic signature of the continents (purple = old; red = young). Lower panel: section of Nd isotopic composition for the western North Atlantic Ocean. The black lines are CFC concentrations (= water mass tracer). Click here to view the figure larger.

Read more: Water masses traced by neodymium isotopic compositions at an unprecedented level in the North...

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