Revelations from the dissolved 226Ra-228Ra pair distribution in the South East Pacific Ocean
While it is confirmed that radium-226 (226Ra) is an interesting tracer of the water masses encountered along the GP16 US East Pacific Zonal Transect (EPZT) section cruise, 228Ra data coupled to the dissolved iron (Fe), cobalt (Co) and manganese (Mn) ones provide evidence that lateral transport of sediments from continental margins, including shelves and slopes, play an important role in open ocean trace elements and isotopes (TEI) budgets and biogeochemistry.
Indeed, elevated 228Ra activities were measured in the upper 200 m over the entire transect, a distance of 8500 km, as a result of sedimentary inputs from the continental shelf. In addition, a deep 228Ra plume was observed at ~1000–2500 m as far as 600 km away from the margin.
Linear dissolved Mn/228Ra relationship is observed both in shelf and offshore surface waters, suggesting that shelf sediments were likely the main source of dissolved Mn to the upper ocean. A linear dissolved Co/228Ra relationship was also observed in surface waters off Peru but no specific dissolved Co/228Ra trend was seen in shelf waters underlining the more complex behavior of Co in this area. Finally, the dissolved Fe/228Ra gradient suggests a rapid removal of Fe.
These results evidence again the important yet underappreciated role of continental slopes as sedimentary TEI sources to the deep ocean.
Figure: The US GEOTRACES GP16 cruise took place between Peru and Tahiti on Oct-Dec 2013. This paper focused on the first half of the transect highlighted by the red box. The concentrations of oxygen, dissolved manganese (Mn), dissolved cobalt (Co), and dissolved Fe (Fe) of the first 500 m of the water column are shown below. The radium-228 activities represented by the black contours were elevated at the surface and coincided with high concentrations of trace elements, especially of Mn, suggesting a common sedimentary source from the Peruvian continental shelf. Click here to view the figure larger.
Sanial, V., Kipp, L. E., Henderson, P. B., van Beek, P., Reyss, J.-L., Hammond, D. E., Hawco, N.J., Saito, M.A., Resing, J.A., Sedwick, P., Moore, W.S., Charette, M. A. (2018). Radium-228 as a tracer of dissolved trace element inputs from the Peruvian continental margin. Marine Chemistry, 201, 20–34. http://doi.org/10.1016/j.marchem.2017.05.008
Estuary solid loads and solid-solution exchanges yield considerable dissolved trace metal enrichments
Based on a thorough investigation of water and suspended sediment samples collected over two years and a six seasons, Samanta and Dalai (2018, see reference below) show that the annual dissolved fluxes of metals from the Ganga (Hooghly) River are enhanced by up to 230–1770% when compared to the conservative mixing. They clearly demonstrate that this enrichment results from exchange processes between the large solid load (suspended particles) and the waters of the middle and lower estuary. Groundwater and direct anthropogenic flux are negligible in these estuary segments.
On a broader scale, their work suggests that solute-particle interaction is a globally significant process in the estuarine production of dissolved metals. The authors estimate that although South Asian Rivers account for only ~ 9% of the global riverwater flux, their high sediment loads results in contributing a far higher proportion of the global supply of the dissolved metals from the rivers: 40 ± 2% of nickel (Ni) and 15 ± 1% of copper (Cu).
Figure: A plot of dissolved flux of nickel (Ni) and copper (Cu) vs. the sediment flux, after normalizing with the corresponding water flux, for the estuaries of some of the major rivers around the world where production of the metals is documented. The strong positive correlation is suggestive of the direct link between the solute-particle interaction and the estuarine production of the metals. The data of the river Scheldt is excluded from regression analysis. Click here to view the figure larger.
Samanta, S., & Dalai, T. K. (2018). Massive production of heavymetals in the Ganga (Hooghly) River estuary, India: Global importance of solute-particle interaction and enhancedmetal fluxes to the oceans. Geochimica et Cosmochimica Acta, 228, 243–258. http://doi.org/10.1016/J.GCA.2018.03.002