Two papers describe findings on Rare Earth Elements in the North Atlantic Ocean (GEOVIDE cruise)

Science Highlights

26 September 2024

Margin inputs and deep-waters scavenging are the predominant features of Rare Earth Element cycle in the North Atlantic Ocean

Lagarde and co-authors (2024, see reference below) investigated the Rare Earth Element (REE) cycle along the GEOTRACES transect GA01 (French GEOVIDE cruise). High REE concentrations are observed in the upper 500m of the eastern part of the transect, close to the Iberian margin, clearly reflecting lithogenic inputs (low fractionation among REE series). The dissolved REE profiles tend to display rather constant or slightly decreasing concentrations with depth below 500 m below surface.

Using an Optimum multiparameter analysis (García-Ibáñez et al., 2015, 2018), the non-conservative signal (dREEXS) produced by inputs or subtractions along the water pathways was deconvoluted from the conservative signal brought by water masses. Results for neodymium (Nd), are shown in figure below, with two main outcomes:

–        For the two easternmost stations of the transect displaying the highest concentrations, the Iberian margin has been identified as a major source of particles whose partial dissolution leads to a dREE input, and probably other trace metals. Thus, the Iberian margin is a dissolved trace elements source for the North Atlantic.

–        The rather constant or decreasing concentrations with depth do not proceed from mixing, but from scavenging that leads to a net dREE depletion, as shown by the negative NdXS in the figure below.

Figure: Non-conservative fractions of dNd (Ndxs) determined using the results of the eOMPA from García-Ibáñez et al. (2018) along the GA01 section. Black dots represent samples to which the eOMPA was applied. Pale colors delimited by the “0” contour indicate concentrations resulting from source water types (SWT) mixing and being thus not influenced by any other source or sink of dREE. Warm orange and blue colors indicate inputs and subtractions of dREE, respectively. Interpolated and plotted with Ocean Data View.

The combination of margin inputs followed by scavenging at greater depth evidences that mechanisms occurring at the land-ocean interface explain part of the observations of boundary exchange.

These results raise the question of the importance of particulate phases in influencing dissolved REE distributions, which is investigated in the companion paper (Lagarde et al., 2024b), whose Science Highlight can be read below:

Rare earth element partition coefficients illustrates the major role of (hydr)oxides in their scavenging

In this other paper, Lagarde and co-workers propose a basin-scale section of Rare Earth element (REE) partition coefficients (Kd) established along the GA01 transect from the surface to 1500 m (GEOVIDE cruise, subpolar North Atlantic), considerably enriching a very limited data set.

Kd profiles displayed minimum values at the surface, variations in the upper 400 m, and an increase with depth below 400 m. The expression of the ratio of particulate to dissolved REE concentrations as a function of suspended particulate matter (SPM) concentrations showed that manganese oxides (MnO2) were the main driver of REE scavenging, followed by the lithogenic phase and iron hydroxides (Fe(OH)3). In addition, REE scavenging appeared to preferentially depend on the relative proportion of these three phases rather than on their absolute concentrations.

Kd is a key parameter of element cycle modeling, controlling the scavenging efficiency by particles. Because of the scarcity of this parameter data, modelling the oceanic REE cycles is challenging. Two Kd(Nd) parameterizations as a function of particulate phase fractions were proposed. The first one included MnO2 and Fe(OH)3, phases that are currently not represented in REE cycle models, and showed a good agreement between observed and calculated Kd(Nd) (R²=0.84, p<0.05). The second parameterization did not include MnO2 and Fe(OH)3 phases, as in existing Nd and REE cycle models, leading to a weaker correlation between observed and calculated Kd(Nd) (R²=0.71, p<0.05). This parametrization confirms the major role of MnO2 and Fe(OH)3, whose associated coefficients are significantly greater than for other particulate phases.

Given that scavenging is an important process in the water column, increasing the Kd data set is urgently needed to better constrain the REE cycle models, and more generally the trace metal ones.

References:

Lagarde, M., Pham, V.Q., Lherminier, P., Belhadj, M., Jeandel, C., 2024a. Rare earth elements in the North Atlantic, part I: Non-conservative behavior reveals margin inputs and deep waters scavenging. Chemical Geology 664, 122230. Access the paper: https://doi.org/10.1016/j.chemgeo.2024.122230

Lagarde, M., Pham, V.Q., Lemaitre, N., Belhadj, M., Jeandel, C., 2024b. Rare earth elements in the north atlantic, part II: Partition coefficients. Chemical Geology 664, 122298. Access the paper: https://doi.org/10.1016/j.chemgeo.2024.122298

García-Ibáñez, M.I., Pardo, P.C., Carracedo, L.I., Mercier, H., Lherminier, P., Ríos, A.F., Pérez, F.F., 2015. Structure, transports and transformations of the water masses in the Atlantic Subpolar Gyre. Progress in Oceanography 135, 18–36. Access the paper: https://doi.org/10.1016/j.pocean.2015.03.009

García-Ibáñez, M.I., Pérez, F.F., Lherminier, P., Zunino, P., Mercier, H., Tréguer, P., 2018. Water mass distributions and transports for the 2014 GEOVIDE cruise in the North Atlantic. Biogeosciences 15, 2075–2090. Access the paper: https://doi.org/10.5194/bg-15-2075-2018

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