Science Highlights

Gulf stream eddies are fertilizing the Western Atlantic Ocean

Tim Conway and co-authors (2018, see reference below) show that Gulf Steam eddies can provide an extra supply of iron, and nutrients such as phosphate and nitrate to the iron-starved Western Atlantic Ocean. Gulf stream eddies form when the northward fast-flowing Gulf Stream meanders and pinches off coastal water, spinning these 'rings' out into the ocean. This coastal water is rich in iron. The authors used satellite and ocean datasets to show that these eddies may be just as important than dust in supplying iron to this area of the ocean!

18 Conway lFigure:  Cruise track (left) and dissolved iron (Fe) concentrations (right) from a North Atlantic GEOTRACES dataset (GA03). The northward flowing Gulf Stream (labelled GS) can be clearly picked out as the boundary between the coastal Slope Water which is enriched in Fe, and the open gyre which is Fe-depleted. A gulf steam eddy (labelled) was serendipitously sampled on the cruise, and can be seen as carrying a column of water enriched in Fe across the Gulf Stream and out into the gyre. The authors used this chemical dataset, together with satellite data to calculate how much iron eddies carry into the gyre each year. Click here to view the image larger.


Conway, T. M., Palter, J. B., & de Souza, G. F. (2018). Gulf Stream rings as a source of iron to the North Atlantic subtropical gyre. Nature Geoscience, 1.

When hydrothermal iron fertilizes the surface ocean in the Western Tropical South Pacific Ocean

For the first time, iron measurements in the Western Tropical South Pacific reveal that shallow hydrothermal inputs from the Tonga Arc region fertilize the euphotic waters of this vast region. These results shed new light on the functioning of pelagic ecosystems in this region.  Indeed, blooms in this area are dominated by N2-fixing organisms (or diazotrophs), with iron requirements higher than those of non-diazotrophic organisms. The measurements of trace elements along a 4000 km ~ 19 ° S transect during the OUTPACE campaign ( together with Argo Float data and  circulation modeling, allowed Guieu and co-authors to establish that the important mesoscale activity of the region makes it possible to transport, disperse and maintain high iron concentrations several hundred kilometers from the source.

 18 Guieu
Figure: Surface Chlorophyll-a concentration (mg m−3) during the 45-day transect of the OUTPACE cruise (A) (The ocean color satellite products are produced by CLS. Figure courtesy of A. De Verneil). (B) Cross-section of dissolved Fe nM (0–500 m). Click here to view the figure larger.


Guieu, C., Bonnet, S., Petrenko, A., Menkes, C., Chavagnac, V., Desboeufs, K. Maes, C., Moutin, T. (2018). Iron from a submarine source impacts the productive layer of the Western Tropical South Pacific (WTSP). Scientific Reports, 8(1), 9075.

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.

18 SanialFigure: 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.

Labile particulate iron isotopic signatures trace hydrothermal and margin inputs down to the benthic layers in the Eastern Pacific Ocean

For the first time, labile and total particulate iron (pFe) isotopic signatures were measured along a full depth oceanographic section (more than 200 0.8-51 µm-sized particles) in the Eastern Pacific Ocean (GP16 East Pacific Zonal Transect cruise). Marsay and co-authors revealed that:

  1. a hydrothermal plume emanating from the South East Pacific Rise was identified as a source of labile pFe to the deep ocean, its isotopic composition tracing a quantitative precipitation of iron while the plume mixes with the surrounding seawater,
  2. export of labile pFe from the Peruvian shelf is isotopically lighter than the hydrothermal one, the negative signature being likely linked to redox cycling within shelf sediments and the Oxygen Minimum Zone,
  3. far from these two areas, upper ocean [Felabile] was very low, with variable δ56Felabile. Interestingly, the labile pFe isotopic signature are also observed in the sediments of the Benthic Boundary Layer, suggesting that they could help tracing hydrothermal and reduced oxygen conditions in the past ocean.

18 Marsay
Iron concentrations and stable isotope data from the GP16 section. a) elevated concentrations of labile particulate Fe (pFelabile; dots) broadly mirror those of dissolved Fe (dFe; shading), with hydrothermal and Peruvian margin sources; contours indicate dFe of >1nM and <0.1nM. b) iron stable isotope data for pFelabile (dots) is typically lighter than for dFe (shading), with both notably lighter near the Peruvian margin; contour indicates d56Fe of 0‰ for dFe. A map indicating where in situ pumps were used to collect particulate material is embedded in figures a) and b). Click here to view the figure larger.

Reference :

Marsay, C. M., Lam, P. J., Heller, M. I., Lee, J.-M., & John, S. G. (2018). Distribution and isotopic signature of ligand-leachable particulate iron along the GEOTRACES GP16 East Pacific Zonal Transect. Marine Chemistry, 201, 198–211.

High particulate organic carbon export in the Arabian Sea

In the framework of Indian GEOTRACES cruise GI02, high-resolution sampling was carried out in the Arabian Sea and the Indian Ocean during April–May 2014. The first 234Th-based carbon export flux ever made in the southern Arabian Sea are presented. In contrast to episodic decrease of primary production from 17°N to 16°S, POC export fluxes show high values  (up to 9 mmol m-2 d-1), definitely larger than what was observed in the Bengal Bay at the same season. 234Th deficit in subsurface depths is due to the diel vertical migration, grazing, and fecal pellet production by mesozooplankton. Interestingly, the modelled POC export fluxes from in situ and satellite-derived primary production are higher than the field values (234Th-Based) for two models and are comparable for a third one, discrepancies discussed in the paper.

18 Subha 2Figures: (A) Depth-latitude vertical profile of oxygen saturation (%) (white contour lines) is shown relative to 234Th/238U activity ratio (A.R.) along the 65°E meridional section of the Arabian Sea and the Indian Ocean. (B) Plot showing relation between primary production and POC export flux. The POC export flux sinking out of the euphotic zone was derived by Laws, Dunne and Henson’s models. Dashed trend lines represent export efficiencies of 2, 5, 10 and 20%. Click here to view the figure larger.


Subha Anand, S., Rengarajan, R., & Sarma, V. V. S. S. (2018). Th-234 Based Carbon Export Flux along the Indian GEOTRACES GI02 Section in the Arabian Sea and the Indian Ocean. Global Biogeochemical Cycles, 32(3), 417–436.

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