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).
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.
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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!!!
Figure: 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...