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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.).

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...

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?

Using chromium isotopes to reconstruct the oxygenation history of the oceans is challenged by modern data

Analyses of seawater chromium (Cr) concentrations and isotopes in diverse marine environments (Arctic, Pacific, and Atlantic Oceans) reveal a strong correlation between Cr isotope composition (δ53Cr) and Cr concentration. High δ53Cr values and low Cr concentration reflect losses of isotopically light Cr in neritic environments. Contrastingly, open ocean waters with low δ53Cr values and high Cr concentration are hypothesized to reflect the addition of seawater-derived Cr released from marine sediments or settling particles. Although reductive removal of Cr in oxygen minimum zones (OMZs) may explain low Cr concentrations in subsurface north Pacific waters, the heterogeneity in δ53Cr values in the modern (oxic) ocean entails that redox cycling of Cr isotopes in the ocean should be considered in future research. 

16 Scheiderich lFigure: Correlation between chromium concentration (as ln[Cr]) and δ53Cr (‰) illustrates that samples from several locations around the world plot on a line that is consistent with closed-system Raleigh fractionation. This fractionation is probably the result of reduction of Cr(VI) is shallow and surface waters, and oxygen minimum zones, and possible reoxidation of Cr(III) at depth; observationally, Cr is "added" at depth, resulting in higher concentrations and lower δ53Cr values, and is "removed" in the surface, resulting in lower concentrations and higher d53Cr values. Click here to view the figure larger.

Read more: Using chromium isotopes to reconstruct the oxygenation history of the oceans is challenged by...

Impressive set of data reveal new features on the modern cadmium–phosphate relationship

Xie and co-authors (2015, see reference below) report vertical profiles of dissolved cadmium (Cd) in the western South Atlantic Ocean (GEOTRACES section GA02), which show nutrient-like distributions similar to those of the macronutrient phosphate (PO4). A close look at the data reveal:

  • In the surface ocean, preferential uptake of Cd over PO4 by phytoplankton occurs along the transect, regardless of ambient iron (Fe) concentrations, suggesting Fe availability is not critical for biological Cd utilization in the southwest Atlantic;
  • In addition, horizontal advection of Cd depleted low oxygen waters originating from the Angola Basin and brought across the Atlantic Ocean via the Benguela and Equatorial currents imparts a Cd-depleted signature to equatorial intermediate waters distinguishing them from southerly intermediate waters.
  • This new dataset provides further evidence that Subantarctic Mode Water plays an important role in generating the non-linearity of the global Cd-PO4 correlation.

16 Xie l
Figure:
Evaluation of Cd–PO4 systematics using new data from GEOTRACES GA02 Leg 3 (colored circles; see inset for location) and literature data (Southern Ocean: Abouchami et al., 2014; Baars et al., 2014; Boyé et al., 2012; Xue et al., 2013; Indian Ocean: Vu and Sohrin, 2013; Angola Basin: Waeles et al., 2013) at the scale of the South Atlantic Basin. The Cd–PO4 relationship for samples with PO4 >1.3 μmol kg-1 in this study exhibits two parallel linear correlations. The influence of low-oxygen waters originating in the Angola Basin (grey shading) is noticeable in intermediate waters at the equatorial stations. The clear kink at PO4 ~ 1.3 μmol kg-1 in the South Atlantic is attributed to northward flowing, nutrient-rich Subantarctic Mode Water. Click here to view the figure larger.

Read more: Impressive set of data reveal new features on the modern cadmium–phosphate relationship

Lead isotopes tracks leakage of Indian Ocean seawater into the Atlantic Ocean

Stable lead isotopes have been measured by Maxence Paul and co-workers along 40°S during the first and second legs of the UK GEOTRACES cruise (GA10) in the Atlantic Ocean. They clearly help to identify three different end-members and mixing between them: open ocean South Atlantic seawater, Indian Ocean seawater, and coastal inputs from South Africa. Lead isotopes and concentrations are excellent fingerprints of the occurrence of Agulhas rings, as confirmed by satellite observations and in situ hydrographic data. This study reveals that the Agulhas Leakage is not only a key pathway for heat but also impacts biogeochemical cycles...

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Figure:
Maps showing the location of surface seawater samples collected during Leg A (left) and Leg B (right) of UK GEOTRACES GA10 cruise D357. Shaded areas highlight the position of Agulhas rings on 19 October 2010 (left) and 17 November 2010 (right), as suggested by sea surface height (SSH) anomalies obtained from satellite data (AVISO, http://www.aviso.oceanobs.com/duacs/). Middle panels: Variations of Pb isotope compositions (colored circles) and Pb concentrations (grey circles) reflect the different sources present in the Cape Basin. Sample denoted as coastal, Agulhas, transitional, and open ocean are represented in blue, red, yellow and green, respectively. The Agulhas samples are marked by low 206Pb/207Pb and and their locations are in accord with the position of the Agulhas rings, as identified by satellite observations and salinity anomalies (bottom panels). Click here to view the figure larger.

Read more: Lead isotopes tracks leakage of Indian Ocean seawater into the Atlantic Ocean

Helium isotopes help to constrain high-resolution model dynamics in the Mediterranean Sea

The terrigenic helium isotope distribution was simulated for the first time in the whole Mediterranean Sea, using a high-resolution model (NEMO-MED12) at one-twelfth of a degree horizontal resolution (6–8 km).

Together with providing valuable constraints on the crustal and mantle helium fluxes, the simulations and their comparisons with observations provide a new technique for improving the dynamical regional model. This is clearly illustrated by the confirmation of the shortcomings of the model dynamics in representing the deep ventilation of the Ionian sub-basin...

16 Ayache
Figure: Total δ 3He (sum of terrigenic, tritiugenic and atmospheric helium) model–data comparison along the Meteor M5 (September 1987) section. (a) Colour-filled contours indicate simulated δ 3He (%), whereas colour-filled dots represent in situ observations. (b) and (c): comparison of average vertical profiles for the Levantine and Ionian sub-basins, respectively; model results are in blue; red indicates in situ data. Click here to view the figure larger.

Read more: Helium isotopes help to constrain high-resolution model dynamics in the Mediterranean Sea

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