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Science Highlights


Some recent GEOTRACES science findings are reported below.  
When getting older they are compiled in the Science Highlights Archive where the "Title Filter" search box will allow you to filter them by words in title (please note that only one-word search queries are allowed e.g. iron, Atlantic, etc.).

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Has the role of atmospheric dust as a control on productivity in oligotrophic regions been overestimated?

Dust particles settling into the surface of open ocean environments are for years assumed to provide nutrients to these distant nutrient-limited areas.

Torfstein and Kienast (2018, see reference below) present a unique high-resolution coupling between dust concentrations (hourly resolution) and chlorophyll-a concentrations (daily time scale resolution) across a 4-year period in the deep, nutrient-poor water column of the north Red Sea, which seriously questions this hypothesis.

This long time series study reveals that there is no correlation between dust and surface chlorophyll-a concentrations, regardless of the time of year, or the possible lags between the dust settling and the oceanic response.

The authors conclude that the role of atmospheric dust as a control on productivity could have been previously overestimated.

18 Torfstein

Figure: The study took place in (a) the Gulf of Aqaba, northern Red Sea, and combined monthly and daily resolved records of  chlorophyl-a concentrations sampled at (b) the Interuniversity Institute for Marine Sciences (IUI) and station A (29°280N, 34°560E, water depth 700 m), respectively. The distance between the two sites is approximately 4 km. Dust time series were recorded at the IUI and its vicinity at weekly, daily and hourly resolution.  (c) A comparison between water temperatures and vertical chlorophyll-a (chl-a) concentrations at station A (monthly resolution), daily and monthly chl-a surface concentrations (μg/L), and dust concentrations (μg/m3) at a weekly, 6 hour and 1 hour time resolution, between January 2012 and August 2016, imply that no statistically significant correlation exists between dust patterns and chl-a concentrations. Click here to view the figure larger.

Reference:

Torfstein, A., & Kienast, S. S. (2018). No correlation between atmospheric dust and surface ocean chlorophyll-a in the oligotrophic Gulf of Aqaba, northern Red Sea. Journal of Geophysical Research: Biogeosciences, 123. https://doi.org/10.1002/2017JG004063

Influence of particle composition on the rate constants of thorium adsorption

Chemical species are constantly exchanged between seawater (solution, D) and particles (solid material, P). This continuous D-P exchange is a key process determining the chemical composition of the ocean. Particles are heterogeneous materials, made of (i) biological material from the surface ocean, (ii) lithogenic material from external inputs to the ocean, and (iii) authigenic (oxyhydr)oxides precipitation in the water column. Understanding the role of each of these phases in driving the D-P exchange is therefore a major issue.

Lerner and co-workers (2018, see reference below) propose to disentangle the particle composition effect on the thorium adsorption rate constant k1 using two different regression models. Model I considers biogenic particles, lithogenic particles, Mn (oxyhydr)oxides, and Fe (oxyhydr)oxides as regressors, and k1 as the regressand. Model II considers ln(biogenic particles), ln(lithogenic particles), ln(Mn (oxyhydr)oxides), and ln(Fe (oxyhydr)oxides) as regressors, and ln(k1) as the regressand, where ln() denotes the natural logarithm. Thus, models I and II posit that the effects of particle phases on k1 are, respectively, additive and multiplicative. Regressions are considered separately in two regions of the North Atlantic: an upwelling region off the western margin of Mauritania, and an open-ocean region east of Bermuda.

The authors find that model II better describes the effect of particle composition on k1. Based on this regression model, the authors find that Mn (oxyhydr)oxides have a stronger effect on k1 in the open-ocean region, and biogenic particles have a stronger effect on k1 in the upwelling region.

18 Lerner

Figure: Relative Importance (RI) of particle phases for influencing the thorium adsorption rate constant, k1, under the additive model (upper panels) and the multiplicative model (lower panels). Results shown at all stations (a,d), open-ocean stations (b,e), and Mauritanian upwelling stations (c,f). The red and blue bars show two different methods to obtain RI values. Biogenic particles and Mn (oxyhydr)oxides have the strongest relationship to k1, depending on the model and stations considered. Click here to view the figure larger.

Reference:

Lerner, P., Marchal, O., Lam, P. J., & Solow, A. (2018). Effects of particle composition on thorium scavenging in the North Atlantic. Geochimica et Cosmochimica Acta, 233, 115–134. http://doi.org/10.1016/J.GCA.2018.04.035

Coupling and decoupling of barium and radium-226 along the GEOVIDE GEOTRACES section

Because radium-226 (226Ra) and barium (Ba) are both alkaline earth metals and thus display similar chemical behaviours, studying their fate in contrasting environments is of prime interest. With this aim, Le Roy and co-workers (2018, see reference below) realised a high resolution description of the distribution of these tracers along the GEOVIDE section (GA01, see section map here). Using an optimum multi parameter analysis of their data, they figured out that:

- both tracers can be considered as conservative of water mass transport in the deep open ocean part of the section;

- non conservative and decoupled behaviours are observed at the ocean boundaries, namely:

  • in the eastern part of the section, the deepest waters (North East Atlantic Deep Water) display high Ba and 226Ra concentrations, which could reflect either an accumulation in this "old" water mass or diffusion from the sediment below;
  • contrastingly, depletion of both tracers in the upper layers of the West European basin likely reflects biological stripping;
  • at the land-ocean contact, as for example close to the Greenland and Newfoundland coasts, marked decoupled behaviours reflect the different main input sources of both tracers (rivers for Ba, sediment for 226Ra).

18 LeRoy
Figure:
Distribution of dissolved 226Ra/Ba ratio (a) measured along the GEOVIDE section and the difference between the measured concentrations and those calculated by the optimum multiparameter (OMP) analysis for 226Ra (b), Ba (c) and  226Ra/Ba ratio (d) along the GA01 section. Positive anomalies (in Red) reflect recent tracer addition, while negative ones (in blue) reflect recent tracer removal. Station numbers are found above the panels. Click here to view the figure larger.

Reference:

Le Roy, E., Sanial, V., Charette, M. A., van Beek, P., Lacan, F., Jacquet, S. H. M., Henderson, P. B., Souhaut, M,. García-Ibáñez, Maribel I., Jeandel, C.,Pérez, F., F., Sarthou, G. (2018). 226Ra–Ba relationship in the North Atlantic during GEOTRACES-GA01. Biogeosciences, 15(9), 3027–3048. http://doi.org/10.5194/bg-15-3027-2018

A new model for protactinium/thorium couple in the Atlantic ocean

The authors present an ocean model of the natural radioactive isotopes thorium-230 and protactinium-231. They are compared with many concentration measurements, both dissolved and particulate; most are from the GEOTRACES transects GA02, GA03 (Atlantic) and GIPY5 (Southern Ocean). The figure below shows the modelled dissolved thorium-230 activity at four depth levels; the discs present the observations. The activities are simulated well, based on an improved model of the scavengers, biogenic and lithogenic particles. This is an important result, because these isotopes are often used to investigate past ocean circulation and particle transport. The model may be used by anyone to make such investigations.

18 vanHulten l

Figure: Modelled dissolved thorium-230 activity at four depth level (mBqm−3 ); observations are represented as discs on the same colour scale. Click here to view the figure larger.

Reference: 

van Hulten, M., Dutay, J.-C., & Roy-Barman, M. (2018). A global scavenging and circulation ocean model of thorium-230 and protactinium-231 with improved particle dynamics (NEMO–ProThorP 0.1). Geoscientific Model Development, 11(9), 3537–3556. DOI: http://doi.org/10.5194/gmd-11-3537-2018

Contrasting fates of the cadmium-cadmium isotopes in the Kuroshio and Oyashio environmental systems

Yang et al. (2018, see reference below) studied the relative importance of physical and biogeochemical processes on controlling the isotopic composition of dissolved and particulate cadmium (Cd) in a GP18 transect, crossing over the relatively cold and eutrophic Oyashio Extension region and the relatively warm and oligotrophic Kuroshio Extension region. Particulate samples revealed preferential uptake of light Cd isotopes by the biological activity. However, the fractionation effect varied dramatically in the surface water of the two regions, larger fractionation factors being observed in the Oyashio Extension region. The cycling in the Kuroshio Extension region was found to follow a closed system fractionation model, whilst the cycling of the Oyashio Extension region fits a steady-state open system fractionation model better. The findings are consistent with the hydrographic contrast in the two regions. In terms of the deep water, physical mixing controls the variations of dissolved Cd concentrations and isotopic composition.

2018 Yang
Figure:
Locations of sampling stations and averaged chlorophyll a concentrations in 2011. Stations TR13, TR15 and TR16 are located in Oyashio extension region, whereas the other stations are located in Kuroshio extension region. Fig. B and C:
Transects of dissolved Cd concentrations and isotopic composition of the studied region, showing comparable distribution in the deep water and contrasting vertical gradient in the thermocline and surface water among Kuroshio and Oyashio stations. Click here to view the figure larger.

Reference:

Yang, S.-C. C., Zhang, J., Sohrin, Y., & Ho, T.-Y. Y. (2018). Cadmium cycling in the water column of the Kuroshio-Oyashio Extension region: Insights from dissolved and particulate isotopic composition. Geochimica et Cosmochimica Acta, 233, 66–80. http://doi.org/10.1016/j.gca.2018.05.001

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.

Reference: 

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. http://doi.org/10.1038/s41561-018-0162-0

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 (https://outpace.mio.univ-amu.fr) 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.

Reference:

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. http://doi.org/10.1038/s41598-018-27407-z

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
Figure:
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. http://doi.org/10.1016/J.MARCHEM.2017.07.003

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