scor

FacebookTwitter

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.).

What constrains the hydrothermal dissolved iron isotopic signatures?

Assessing the processes leading to dissolved iron (dFe) isotope fractionation in a hydrothermal plume is a key question, because it allows a better characterization of this specific source of dFe in the deep ocean. For the first time, Fe isotope composition of dissolved and total dissolvable Fe fractions was determined and compared to the bulk chemical composition of Fe particles.

This work, conducted on the same hydrothermal vents on the East Scotia Ridge, yielded two articles simultaneously published this month (February 2017, see references below).

These complementary papers demonstrate that the dFe isotopic composition observed at the end of the plume dispersion in the deep seawater is quite different from that of the pure fluid. Changes of this signature reflect redox processes, ligand complexation, exchanges with labile particulate Fe. They more specifically reveal that the proportions of authigenic Fe-sulfide and Fe-oxyhydroxide minerals that precipitate in the buoyant plume exert opposing control on the resultant isotopic signature of dFe found in the neutrally buoyant plume.

Although the isotopic composition of stabilized hydrothermal dFe in the East Scotia Sea is distinct from background seawater and may be used to quantify the hydrothermal dFe input to the ocean interior, these studies underline the fact that the multiple processes occurring during the early stages of the plume depend on the nature of the ridge substrate, more specifically its sulfur (S) content. The potentially highly variable isotopic signature of hydrothermal dFe is an important consideration for the mass balance of dFe in the modern ocean and for using Fe isotopes to infer changes in the Fe cycle throughout past Earth history...

17 Klar lFigure: Evolution of the isotopic composition of dissolved (δ56dFe) and particulate iron (δ56pFe) during plume dispersion. During the initial stages of venting the precipitation of iron-sulfide (FeS2) leads to the removal of light isotopes from the buoyant plume, and the precipitation of iron-oxyhydroxides leads to the removal of heavy isotopes. The resultant isotopic composition of Fe exported from the buoyant plume depends on the Fe/H2S ratio in the vent fluid. During dispersal in the neutrally buoyant plume (NBP), the isotopic composition of Fe becomes heavier (positive values), most likely due to exchange of Fe between particulate and dissolved phases, and formation of iron-ligand and nano-particulate Fe.

References:

Klar, J. K., James, R. H., Gibbs, D., Lough, A., Parkinson, I., Milton, J. A., Hawkes, Jeffrey A., Connelly, D. P. (2017). Isotopic signature of dissolved iron delivered to the Southern Ocean from hydrothermal vents in the East Scotia Sea. Geology, G38432.1. http://doi.org/10.1130/G38432.1

Lough, A. J. M., Klar, J. K., Homoky, W. B., Comer-Warner, S. A., Milton, J. A., Connelly, D. P., James, R.H, Mills, R. A. (2017). Opposing authigenic controls on the isotopic signature of dissolved iron in hydrothermal plumes. Geochimica et Cosmochimica Acta, 202, 1–20. http://doi.org/10.1016/j.gca.2016.12.022

 

What is generating the benthic nepheloid layers?

How ubiquitous, variable or persistent are nepheloid layers? What is the main process generating these "clouds at the bottom of the sea"? Gardner and co-workers (2017, see reference below) explore these two critical questions, with a focus on the western North Atlantic for which numerous time series and survey data exist. They piece together a detailed review of the mechanisms and provide important new insights into the creation, persistence, and decay of nepheloid layers, a major issue for the geochemistry of particle-reactive elements. Their main results are: Deep western boundary currents are too weak to create benthic storms and therefore to generate intense nepheloid layers; benthic storms are created primarily by deep cyclones beneath Gulf Stream meanders; benthic storms erode the seafloor and maintain the benthic nepheloid layer; and finally, benthic nepheloid layers are weak to non-existent in areas of low eddy kinetic energy.

17 GardnerFigure 1: Contours of integrated benthic particle load (red lines, in μg cm− 2) and abyssal eddy kinetic energy (EKE, dashed green lines, in cm2 s− 2). Numbers by stars and triangles are related to the mean time-series particle concentration and standard deviation of particle concentration (in parentheses). Click here to view the figure larger.

17 Gardner2
Figure 2: Map of surface EKE based on satellite observations during 2002–2006 (Dixon et al., 2011). Time-series stations are indicated. Click here to view the figure larger.


References:

Gardner, W. D., Tucholke, B. E., Richardson, M. J., & Biscaye, P. E. (2017). Benthic storms, nepheloid layers, and linkage with upper ocean dynamics in the western North Atlantic. Marine Geology. DOI:10.1016/j.margeo.2016.12.012 Open Access

K.W. Dixon, T.L. Delworth, A.J. Rosati, W. Anderson, A. Adcroft, V. Balaji, R. Benson, S.M. Griffies, H.-C. Lee, R.C. Pacanowski, G.A. Vecchi, A.T. Wittenberg, F. Zeng, R. Zhang Ocean circulation features of the GFDL CM2.6 & CM2.5 high-resolution global coupled climate models. WCRP Open Science Conference, October 2011, Denver, Colorado (2011)

The coupled zinc-silicon cycle paradox solved

The strong similarities between zinc (Zn) and silicon (Si) vertical profiles have led many studies to suggest the uptake of Zn in diatom frustules, followed by simultaneous remineralisation at depth. However, recent lab experiments have demonstrated that Zn, although essential for diatoms, is located in the organic part of the cell. These cells are characterized by particularly high Zn/P ratios in the Southern Ocean (up to 8). Such contrasting observations has raised the question as to what processes could lead to such consistent Si-Zn relationship, given that Zn and Si uptake are obviously not controlled by the same biological process. Vance and co-workers (2017, see reference below) demonstrate that the oceanic zinc distribution is the result of the interaction between the specific uptake stoichiometry in Southern Ocean surface waters and the physical circulation through the Southern Ocean hub.

Their approach couples in situ data collected in the different oceanic basins, experimental results from the literature and physical-biogeochemical coupled modelling on a global scale. This work emphasizes how the consideration of 1D cycling only can bias the understanding of (macro and micro) nutrient behaviours, and therefore their paleo-applications.

17 Vance lFigure: Depth profiles of dissolved zinc, silica and phosphate in three different ocean basins (bottom), with the locations of each profile shown on the map (top). Both zinc and silicate show deep maxima whereas phosphate has a much shallower maximum, despite the fact that the oceanic biogeochemical cycle of Zn is dominated by uptake into the organic parts of diatom cells with phosphate. Vance et al. explain these features in terms of biological and physical processes in the Southern Ocean. Modified from Nature Geoscience. Please click here to view the figure larger.

Reference:

Vance, D., Little, S. H., de Souza, G. F., Khatiwala, S., Lohan, M. C., & Middag, R. (2017). Silicon and zinc biogeochemical cycles coupled through the Southern Ocean. Nature Geoscience. DOI: 10.1038/ngeo2890

Dissolved neodymium isotopes and Rare Earth Elements combined with oxygen isotopes trace water masses in the Fram Strait

Georgi Laukert and co-workers (2017, see reference below) provide new insights into the sources, distribution and mixing of water masses passing the Fram Strait, the gateway between the Arctic Ocean and the Nordic Seas, based on a detailed geochemical tracer inventory including dissolved neodymium isotope (εNd), rare earth element (REE) and stable oxygen isotope (δ18O) data (see figure). They show that Nd isotope and REE distributions in the open Arctic Mediterranean (Arctic Ocean and Nordic Seas) primarily reflect lateral advection of water masses and their mixing and that the pronounced gradients in εNd signatures and REE characteristics in the upper water column can be used to assess shallow hydrological variability within the Arctic Mediterranean.

In particular the advection of northward flowing warm Atlantic Water, shallow southward flowing Arctic-derived waters, and intermediate and deep waters is clearly reflected by distinct εNd signatures, Nd concentrations ([Nd]) and REE distributions. Waters with hydrographic characteristics similar to those of Arctic-derived waters have different εNd and [Nd] values in the upper ~100 m on the NE Greenland Shelf (see figure: NEGSSW), which suggests local addition of Greenland freshwater.

17 Laukert lFigure: Upper panel: Bathymetric map of the Arctic Mediterranean with the inset representing the Fram Strait region. The surface Nd isotopic composition (εNd) of seawater from literature and this study is shown as color-coded symbols. The literature εNd values of rocks and beach sediments are shown in addition. Lower panel: Distribution of salinity (all CTD data), εNd and Nd concentrations ([Nd]ID, in pmol/kg) along the latitudinal section at 78.8° N. Click here to view the figure larger.

Reference:

Georgi Laukert, Martin Frank, Dorothea Bauch, Ed C. Hathorne, Benjamin Rabe, Wilken-Jon von Appen, Carolyn Wegner, Moritz Zieringer, Heidemarie Kassens, Ocean circulation and freshwater pathways in the Arctic Mediterranean based on a combined Nd isotope, REE and oxygen isotope section across Fram Strait, Geochimica et Cosmochimica Acta 202, 385-209, DOI: http://dx.doi.org/10.1016/j.gca.2016.12.028.

Siderophores facilitate microbial adaptation to iron limitation in the eastern tropical Pacific Ocean

Siderophores are organic compounds secreted by microbes to facilitate iron uptake. Using new methods to characterize trace metal organic ligands in seawater, Boiteau and colleagues (2016, see reference below) measured the distribution of siderophores along the US East Pacific Zonal Transect (EPZT; GEOTRACES GP16). The cruise track crossed from the highly productive Peruvian coastal upwelling region into the oligotrophic central gyre. The study revealed important changes in siderophore composition and concentration across different nutrient regimes (see figure below). Siderophores were found to be nine times more abundant in the most iron-depleted areas of the transect compared to the iron-rich coastal zone. Companion phylogenetic analysis of siderophore synthesis genes in the TARA Oceans metagenomic catalogue led Boiteau and co-workers to suggest that lateral transfer of siderophore synthesis genes help microbes adapt to low iron conditions found in many regions of the ocean.

17 RepetaFigure: Distribution of siderophores across the GEOTRACES EPZT. Amphibactin siderophores (top panel) appear as peaks in the trace of organic iron isolated from seawater (middle panels). Each peak represents a different molecular form of amphibactin. Concentrations of amphibactins across the ETPZ were low in the high iron coastal region, high in the HNLC region, and low again in the low iron oligotraphic region (lower panel). Please click here to view the figure larger.

Reference :

Boiteau, R. M., Mende, D. R., Hawco, N. J., McIlvin, M. R., Fitzsimmons, J. N., Saito, M. A., Sedwick, P. N., DeLong, E. F., Repeta, D. J. (2016). Siderophore-based microbial adaptations to iron scarcity across the eastern Pacific Ocean. Proceedings of the National Academy of Sciences of the United States of America, 113(50), 14237–14242. DOI: 10.1073/pnas.1608594113

Dissolved iron isotopes reveal that distinct processes are controlling this micronutrient distribution in the ocean

Abadie and co-workers propose new dissolved iron concentration and isotopic composition distributions (DFe and ICFe respectively) along the Bonus-Goodhope IPY section (GIPY4), in the Atlantic sector of the Southern Ocean. DFe vertical profiles display a continuous increase with depth (see figure B), classically interpreted as due to biological uptake at the surface followed by remineralization at depth. However, heterogeneous profiles of ICFe (see figure B) suggest a more complicated story driven by distinct processes, discussed here for the first time. Indeed, the authors demonstrate that in the intermediate waters, DFe primarily originates from remineralization of organic matter and the redistribution of this regenerated DFe through mixing. Moreover it is also due to horizontal advection of DFe released by reducing sediments of the nearby South African coast. The scheme changes deeper, where abiotic processes are dominating the DFe distribution as for example non-reductive release of DFe from lithogenic particles. This last process would add an additional source to the global oceanic DFe budget, which should be considered in the biogeochemical models. In addition, it suggests that the oceanic DFe budget could be more sensitive than previously thought to continental erosion, particle transport, and dissolved/particle interactions.

17 Lacan l2

Figure: (A) Position of the 5 stations sampled for iron isotopes during the Bonus-Goodhope IPY GEOTRACES cruise; (B) Examples of profiles for dissolved iron concentration (DFe) and dissolved iron isotopic composition (expressed by δ56DFe) obtained in one of the 5 stations sampled (other stations show similar patterns); Iron isotopes show a very sharp minimum in intermediate depths (i.e. between about 200 and 1500 m below the surface). The contrast between these intermediate depths and the deep ocean (3000-5000 m) demonstrates that two different processes dominate dissolved iron sources in the ocean at these two levels. (C) Dissolved iron isotopic composition (δ56DFe) along the Bonus-Goodhope section. Negative values (in cold colours, blue, green) indicate iron that is naturally enriched in light isotopes, while high values (in warm colours, red, orange) indicate heavy iron isotopes enrichment. Click here to view the figure larger.

 Reference:

Abadie, C., Lacan, F., Radic, A., Pradoux, C., Poitrasson F. (2017) Iron isotopes reveal distinct dissolved iron sources and pathways in the intermediate versus deep Southern Ocean, PNAS, DOI: 10.1073/pnas.1603107114

Up to four mercury species measured along the GEOTRACES East Pacific Zonal Transect

Total mercury (HgT), elemental mercury (Hg0), monomethyl-mercury (MMHg), and dimethyl-mercury (DMHg) were measured at an unprecedent resolution so far along the East Pacific Zonal Transect (EPZT) GEOTRACES cruise (GP16). Knowing that MMHg is a neurotoxin that accumulates in the trophic chain, establishing the full speciation of this element is required in order to better understand oceanic Hg cycle and more specifically where and how MMHg is formed and accumulated.

The other exciting feature of this article is that contrasted biogeographic oceanic provinces have been sampled during EPZT (Peru upwelling region, a suboxic OMZ, and an expansive submarine hydrothermal vent plume).

Bowman and co-authors results show that while HgT is not enriched in the hydrothermal plume, total Hg is accumulated with age and that DMHg is the dominant methylated species in deep waters. Importantly, filtered HgT is accumulated in upwelled waters near the coast of Peru where oxygen concentrations were the lowest, and MMHg represented 10–20% of the total Hg upwelling flux. This is the first identification of Hg behaviour along the Peru coast, a region that supports the largest fisheries of the world...

16 Bowman
Figures:
Images depict mercury concentrations in the water column from the coast of Peru (right), across the Eastern Pacific Rise (EPR), to Tahiti (left), measured during the 2013 U.S. GEOTRACES Eastern Pacific Zonal Section. Total mercury (HgT), gaseous elemental mercury (Hg0), methylmercury (MMHg), and dimethylmercury (DMHg) in filtered water is shown, in addition to methylmercury in suspended particles (MMHgpart.). Click here to view the figure larger.

Reference:

Bowman, K. L., Hammerschmidt, C. R., Lamborg, C. H., Swarr, G. J., & Agather, A. M. (2016). Distribution of mercury species across a zonal section of the eastern tropical South Pacific Ocean (U.S. GEOTRACES GP16). Marine Chemistry, 186, 156–166. DOI: 10.1016/j.marchem.2016.09.005

Water mass circulation and weathering inputs in the Labrador Sea based on coupled hafnium-neodymium isotope compositions and rare earth element distributions

Filippova and co-authors (2017, see reference below) show distinct water mass signatures in the Labrador Sea revealed by combined dissolved hafnium (Hf) and neodymium (Nd) isotope compositions and REE distribution patterns along the AR7W transect in May 2013.

The new data show that in a semi-enclosed basin such as the Labrador Sea, the radiogenic Hf isotope signatures can serve as a highly sensitive tracer of water mass mixing processes given that they allow distinction of particular water masses that do not differ in their Nd isotope compositions. Based on the new data, the authors suggest that the residence time of Hf in the Labrador Sea can only be on the order of decades in order to sustain the observed variability. The high sensitivity of Hf isotopes to decadal ocean circulation changes in the Labrador Sea suggests a potential prospect for their application in other restricted basins with similar geological settings and pronounced short-term hydrographic variability.

17 Filippova l

 17 Filipovafig2Figures: (top) Schematic map of the study area. Blue arrows represent cold deep currents and red arrows denote warm surface currents. Red dots indicate the positions of the stations occupied during CCGS Hudson Cruise 2013. A schematic representation of the geology of the surrounding landmasses is shown and includes average ɛHf and ɛNd values of the rocks. Please click here to view the figure larger. (bottom) Water masses distribution versus depth in the Labrador Sea based on their ɛNd (A) and ɛHf (B) signatures. Please click here to view the figure larger.

Reference:

Filippova, A., Frank, M., Kienast, M., Rickli, J., Hathorne, E., Yashayaev, I.M., and Böning, P. (2017): Water mass circulation and weathering inputs in the Labrador Sea based on coupled Hf-Nd isotope compositions and rare earth element distributions.- Geochimica et Cosmochimica Acta 199, 164-184. DOI: 10.1016/j.gca.2016.11.024

 Data Product (IDP2014)

eGEOTRACES Atlas

 Data Assembly Centre (GDAC)

 Outreach

Subscribe Mailing list

Contact us

To get a username and password, please contact the GEOTRACES IPO.

This site uses cookies to offer you a better browsing experience. Find out more on how we use cookies and how you can change your settings.