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Rapid neodymium release to marine waters from lithogenic sediments in the Amazon estuary.

Rousseau TC, Sonke JE, Chmeleff J, van Beek P, Souhaut M, Boaventura G, Seyler P, Jeandel C - Nat Commun (2015)

Bottom Line: Although models suggest that REE release from lithogenic sediment in river discharge may dominate all other REE inputs to the oceans, the occurrence, mechanisms and magnitude of such a source are still debated.At mid-salinities, total dissolved REE levels slightly increase, while ɛNd values are shifted from the dissolved Nd river endmember (-8.9) to values typical of river suspended matter (-10.6).Combining a Nd isotope mass balance with apparent radium isotope ages of estuarine waters suggests a rapid (3 weeks) and globally significant Nd release by dissolution of lithogenic suspended sediments.

View Article: PubMed Central - PubMed

Affiliation: GET, Université de Toulouse, CNRS, IRD, CNES, 14 Avenue Edouard Belin, Toulouse F-31400, France.

ABSTRACT
Rare earth element (REE) concentrations and neodymium isotopic composition (ɛNd) are tracers for ocean circulation and biogeochemistry. Although models suggest that REE release from lithogenic sediment in river discharge may dominate all other REE inputs to the oceans, the occurrence, mechanisms and magnitude of such a source are still debated. Here we present the first simultaneous observations of dissolved (<0.45 μm), colloidal and particulate REE and ɛNd in the Amazon estuary. A sharp drop in dissolved REE in the low-salinity zone is driven by coagulation of colloidal matter. At mid-salinities, total dissolved REE levels slightly increase, while ɛNd values are shifted from the dissolved Nd river endmember (-8.9) to values typical of river suspended matter (-10.6). Combining a Nd isotope mass balance with apparent radium isotope ages of estuarine waters suggests a rapid (3 weeks) and globally significant Nd release by dissolution of lithogenic suspended sediments.

No MeSH data available.


REE concentrations and Nd isotopic composition in the Amazon River estuary.(a) Amazon estuary Nd versus salinity gradient in the dissolved fraction in the study by Sholkovitz29 (blue diamonds), and this study (red triangles). (b) Amazon estuary ɛNd versus salinity in the dissolved (blue dots) and particulate (orange squares) fractions; the theoretical dissolved Nd conservative mixing curve is shown (blue-dashed line). The apparent radium ages estimated for the different water samples (expressed in days) are also reported.
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f3: REE concentrations and Nd isotopic composition in the Amazon River estuary.(a) Amazon estuary Nd versus salinity gradient in the dissolved fraction in the study by Sholkovitz29 (blue diamonds), and this study (red triangles). (b) Amazon estuary ɛNd versus salinity in the dissolved (blue dots) and particulate (orange squares) fractions; the theoretical dissolved Nd conservative mixing curve is shown (blue-dashed line). The apparent radium ages estimated for the different water samples (expressed in days) are also reported.

Mentions: Detailed observations of the Amazon Estuary obtained during the AmasSeds 1 campaign illustrated the so called colloidal flocculation for the REE29. The fraction of REEs that are removed from the dissolved phase in the salinity gradient must be calculated relative to the REE concentrations in the river endmember. Sholkovitz29 noted that the lowest AmasSeds 1 salinity samples (0.3) taken on 4–10 August 1989, and which have Nd concentrations of 83.5 and 67.9 ng kg−1, do not represent the true river endmember. Using equation (1) and the Amazon discharge at this time in 1989 (237,814 m3 sec−1) yields corresponding Nd concentration at Obidos of 157±40 ng kg−1. Figure 3a shows the Nd concentration gradient observed during both AmasSeds 1 and Amandes 3 campaign (this study and the study by Sholkovitz29). Similar to Sholkovitz observations, we find that total dissolved Nd<0.45 μm concentrations rapidly decrease in the low-salinity region (0–10) from 123 ng kg−1 to a minimum of 4.3 ng kg−1. Using the estimated river endmember of 157 ng kg−1 (1.1±0.3 μmol kg−1) and the observed Nd minima of 3.8 ng kg−1 (26.3 pmol kg−1) for the AmasSeds I campaign, both the Sholkovitz29 and this study show that >90% of dissolved Nd was removed from solution at the first stages of the Amazon Estuary. The dissolved Nd removal is strongest at a salinity of 17.5, a value which is higher than the AmasSeds 1 study (6.6) (Fig. 3a). This feature could be due to the use of different filter pore sizes in the two studies (that is, 0.45 μm here versus 0.22 μm during AmasSed) but more likely represents true natural variability, possibly related to seasonality (AmasSeds-1 in August 1989; Amandes-3 in April 2008). We observe that 98% of Amazon River Nd is in the >1 kDa colloidal fraction, and that the fine colloidal Nd<10 kDa fraction gradually increases from the river endmember to the seawater endmember (that is, 18%, 53%, 73% and 90% for salinities 0.03, 1.5, 10.5 to 34.9 respectively, Supplementary Table 1). This trend confirms that large REE carrying colloids (between 10 kDa and 0.45 μm) flocculate out of solution along the estuarine mixing gradient as illustrated in Supplementary Fig. 1.


Rapid neodymium release to marine waters from lithogenic sediments in the Amazon estuary.

Rousseau TC, Sonke JE, Chmeleff J, van Beek P, Souhaut M, Boaventura G, Seyler P, Jeandel C - Nat Commun (2015)

REE concentrations and Nd isotopic composition in the Amazon River estuary.(a) Amazon estuary Nd versus salinity gradient in the dissolved fraction in the study by Sholkovitz29 (blue diamonds), and this study (red triangles). (b) Amazon estuary ɛNd versus salinity in the dissolved (blue dots) and particulate (orange squares) fractions; the theoretical dissolved Nd conservative mixing curve is shown (blue-dashed line). The apparent radium ages estimated for the different water samples (expressed in days) are also reported.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4510642&req=5

f3: REE concentrations and Nd isotopic composition in the Amazon River estuary.(a) Amazon estuary Nd versus salinity gradient in the dissolved fraction in the study by Sholkovitz29 (blue diamonds), and this study (red triangles). (b) Amazon estuary ɛNd versus salinity in the dissolved (blue dots) and particulate (orange squares) fractions; the theoretical dissolved Nd conservative mixing curve is shown (blue-dashed line). The apparent radium ages estimated for the different water samples (expressed in days) are also reported.
Mentions: Detailed observations of the Amazon Estuary obtained during the AmasSeds 1 campaign illustrated the so called colloidal flocculation for the REE29. The fraction of REEs that are removed from the dissolved phase in the salinity gradient must be calculated relative to the REE concentrations in the river endmember. Sholkovitz29 noted that the lowest AmasSeds 1 salinity samples (0.3) taken on 4–10 August 1989, and which have Nd concentrations of 83.5 and 67.9 ng kg−1, do not represent the true river endmember. Using equation (1) and the Amazon discharge at this time in 1989 (237,814 m3 sec−1) yields corresponding Nd concentration at Obidos of 157±40 ng kg−1. Figure 3a shows the Nd concentration gradient observed during both AmasSeds 1 and Amandes 3 campaign (this study and the study by Sholkovitz29). Similar to Sholkovitz observations, we find that total dissolved Nd<0.45 μm concentrations rapidly decrease in the low-salinity region (0–10) from 123 ng kg−1 to a minimum of 4.3 ng kg−1. Using the estimated river endmember of 157 ng kg−1 (1.1±0.3 μmol kg−1) and the observed Nd minima of 3.8 ng kg−1 (26.3 pmol kg−1) for the AmasSeds I campaign, both the Sholkovitz29 and this study show that >90% of dissolved Nd was removed from solution at the first stages of the Amazon Estuary. The dissolved Nd removal is strongest at a salinity of 17.5, a value which is higher than the AmasSeds 1 study (6.6) (Fig. 3a). This feature could be due to the use of different filter pore sizes in the two studies (that is, 0.45 μm here versus 0.22 μm during AmasSed) but more likely represents true natural variability, possibly related to seasonality (AmasSeds-1 in August 1989; Amandes-3 in April 2008). We observe that 98% of Amazon River Nd is in the >1 kDa colloidal fraction, and that the fine colloidal Nd<10 kDa fraction gradually increases from the river endmember to the seawater endmember (that is, 18%, 53%, 73% and 90% for salinities 0.03, 1.5, 10.5 to 34.9 respectively, Supplementary Table 1). This trend confirms that large REE carrying colloids (between 10 kDa and 0.45 μm) flocculate out of solution along the estuarine mixing gradient as illustrated in Supplementary Fig. 1.

Bottom Line: Although models suggest that REE release from lithogenic sediment in river discharge may dominate all other REE inputs to the oceans, the occurrence, mechanisms and magnitude of such a source are still debated.At mid-salinities, total dissolved REE levels slightly increase, while ɛNd values are shifted from the dissolved Nd river endmember (-8.9) to values typical of river suspended matter (-10.6).Combining a Nd isotope mass balance with apparent radium isotope ages of estuarine waters suggests a rapid (3 weeks) and globally significant Nd release by dissolution of lithogenic suspended sediments.

View Article: PubMed Central - PubMed

Affiliation: GET, Université de Toulouse, CNRS, IRD, CNES, 14 Avenue Edouard Belin, Toulouse F-31400, France.

ABSTRACT
Rare earth element (REE) concentrations and neodymium isotopic composition (ɛNd) are tracers for ocean circulation and biogeochemistry. Although models suggest that REE release from lithogenic sediment in river discharge may dominate all other REE inputs to the oceans, the occurrence, mechanisms and magnitude of such a source are still debated. Here we present the first simultaneous observations of dissolved (<0.45 μm), colloidal and particulate REE and ɛNd in the Amazon estuary. A sharp drop in dissolved REE in the low-salinity zone is driven by coagulation of colloidal matter. At mid-salinities, total dissolved REE levels slightly increase, while ɛNd values are shifted from the dissolved Nd river endmember (-8.9) to values typical of river suspended matter (-10.6). Combining a Nd isotope mass balance with apparent radium isotope ages of estuarine waters suggests a rapid (3 weeks) and globally significant Nd release by dissolution of lithogenic suspended sediments.

No MeSH data available.