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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.).

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.

16 Tagliabue l
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.

16 Bratkic l
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.

16 Frings
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).

16 Lambelet ls
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...

Important warning about the uncertainties affecting results of dissolved iron concentration measurements in seawater using flow-injection with chemiluminescence detection

Flow-Injection with Chemiluminescence (FI-CL) is a procedure commonly applied in the framework of the GEOTRACES cruises because of its portability and hence suitability for shipboard deployment.

Following the Guide for Uncertainty Measurement (GUM) approach, Floor and colleagues propose dedicated mathematical equations allowing the estimation of measurement uncertainties. They apply their model to estimate combined uncertainties obtained for analyses of seawater reference materials (SAFe and GEOTRACES).

This thorough and rigorous examination shows that the final uncertainty of the measurement results using FI-CL in the present protocol configuration cannot be better than 10–15% for seawater samples containing 0.5–1 nmol/kg of dissolved iron (Fe).

This uncertainty might be larger at sea, under more challenging conditions. The most influential sources of uncertainty are the uncertainty on the calibration slope and the lack of stability during the analytical sequence, see figures below).

Authors clearly consider that uncertainty estimations based on the intensity repeatability alone, as is often done in FI-CL studies, is not a realistic estimation of the overall uncertainty of the measurement procedure.

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Figures: 
Combined uncertainty budget estimated for measurements corresponding to signal peak height integration (left, rel. U = 12%, k = 2)
and to signal peak area integration (right, rel. U = 10%, k = 2). Click here to download the figures.

Read more: Important warning about the uncertainties affecting results of dissolved iron concentration...

When a multi-parameter end-member mixing model allows a quantitative deconvolution of the dissolved rare earth elements behaviour

The dissolved Rare Earth Elements (dREE) data discussed by Zheng and co-workers (2016, see reference below) have been collected along a full depth section at 12°S in the South Atlantic Ocean, using a new high-precision analytical protocol (Zheng et al., 2015).

Results show that more than 75% of the dREE concentrations are preformed, explaining the strong correlation often observed in deep waters between dREEs and dissolved silicon (Si).

Minor addition of up to 10% of dREE in Antarctic Bottom Water (AABW) in the deep Brazil Basin is observed, reflecting particle remineralization, while dREE addition of up to 25% is found at 1500 m and below 4000 m in the Angola Basin near the continent–ocean interface. These latest inputs are divided in 2 plumes: based on evidence from cerium anomalies, the shallow plume is attributed to release of dREEs from dissolution of sedimentary iron oxides on the continental margin, and the deep one to remineralization of calcite.

...if you wish to know more about the process identification, don't hesitate to read the paper!!!

16 Zheng lFigure: The multi-parameter mixing model reveals that >75% of dissolved REEs in the deep South Atlantic along ~12ºS is explained by mixing of different water masses (“preformed”), and significant (up to 25%) non-preformed REEs occur at ~1500 m and below 4000 m at the ocean-continent interface in the Angola Basin (eastern section) resulting from different REE sources. Click here to view the figure larger.

Read more: When a multi-parameter end-member mixing model allows a quantitative deconvolution of the...

What do the first 236-Uranium data reveal in the Arctic Ocean?

Casacuberta and co-authors (2016, see reference below) propose the first set of data for the artificial radionuclide 236-Uranium (236U) in the Arctic Ocean. The novelty of this study compared to the first comprehensive dataset they published in the western North Atlantic Ocean (GEOTRACES GA02 section), is the combination of 236U with 129-Iodine (129I). The 236U/238U and 129I/236U atomic ratios allow them to distinguish the sources of these two artificial radionuclides to the Arctic Ocean, an approach that would not be possible if only using individual concentrations of 236U, 129I or any other anthropogenic radionuclide. For example, using these ratios in a binary mixing model, they could identify Siberian rivers as potential source of artificial radionuclides in the Arctic Ocean, other than the global fallout and the European Reprocessing plants of Sellafield and La Hague. The highly sensitivity of the measurements of these two radionuclides using Accelerator Mass Spectrometry, also allows the detection of very low concentrations of both radionuclides. This dual tracer approach could therefore become an extremely sensitive tool to study isolation ages of deep and bottom waters of the Amerasian Basin.

16 CasacubertaFigure: 129I/236U atom ratio in surface waters of the Arctic Ocean (2011/2012). Atlantic Waters (dashed black line) have higher ratios showing a greater influence of Reprocessing Plants signal. Pacific Waters (dashed purple line) are more influenced by global fallout.

Read more: What do the first 236-Uranium data reveal in the Arctic Ocean?

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