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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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The Colloidal Hourglass: North Atlantic Iron Distribution Controlled by Dynamic Colloidal Phase

K. Kunde and colleagues (2019, reference below) show that a highly dynamic colloidal iron phase in the upper ocean and at the seafloor boundary controls the distribution of dissolved iron at key sources and sinks across the subtropical North Atlantic Ocean, while the ocean interior is marked by an equilibrium partitioning between the soluble and colloidal phases. These processes result in size-fractionated iron distribution resembling an hourglass shape of the colloidal iron fraction (%cFe) of the dissolved pool against depth. In addition, high-resolution surface sampling revealed a strong, dust-derived gradient of surface dissolved iron concentrations that resulted in a “high dust, high %cFe” and “low dust, low %cFe” pattern across the basin. A global comparison of colloidal and dissolved iron reveals that this effect extends to the mesopelagic, where the lithogenic nature of a region is imprinted on the colloidal partitioning signature.

Kunde et al. 

Figure: (Left) The fraction of colloidal Fe to the dissolved Fe pool across the GApr08 section forms an hourglass shape against depth. Dynamic Fe cycling in the upper ocean and near the seafloor is reflected in the large range of %cFe, while the tight partitioning in the less dynamic ocean interior forms the neck of the hourglass. The four data points outside this pattern are from the Snakepit hydrothermal source on the Mid-Atlantic Ridge (Station 4) with almost 100% cFe. (Right) Map of the subtropical North Atlantic showing the cruise track and station locations of the GApr08 section (research cruise JC150).

Reference:

Kunde, K., Wyatt, N. J., González‐ Santana, D., Tagliabue, A., Mahaffey, C., & Lohan, M. C. (2019). Iron distribution in the subtropical north Atlantic: the pivotal role of colloidal iron. Global Biogeochemical Cycles, 33, 1532 1547. https://doi.org/10.1029/2019GB006326

A unique insight into the properties of iron aerosols in the Arctic Ocean


Gao and co-workers (2019, see reference below) collected size-segregated aerosols over 8 segments of the 2015 US GEOTRACES cruise (GN01) across the Arctic Ocean. They show that:

  1. aerosol iron (Fe) had a single-mode size distribution, peaking at 4.4 μm in diameter, suggesting regional dust sources of Fe around the Arctic Ocean,
  2. Fe in the coarse-mode particles accounted for ~68% of the aerosol Fe from all samples and 59% if sample M7 is excluded,
  3. the low Fe/Ti ratio indicate that Fe was not enriched relative to average upper continental crust,
  4. although the less abundant fraction, the fine dusts (<1µm) contain the most dissolvable fraction of iron but abundant organic ligands are also suspected to increase the Fe solubility and,
  5. dry deposition rates of aerosol Fe decreased from 6.1 μmol m−2 yr−1 in the areas of ~56°N–80°N to 0.73 μmol m−2 yr−1 in the areas north of 80°N.

20 Gao l

Figure caption: (a) Cruise tracks showing the coverage of eight sets (M1 – M8) of aerosol samples collected in the Arctic Ocean aboard the US Coast Guard Ice Breaker Healy during the period from 9th August 2015 to 12th October 2015. (b) Spatial variation of atmospheric concentrations of aerosol Fe in fine- and coarse-mode particles and total (coarse + fine) particles. Higher concentrations of aerosol Fe in samples M2 and M7 likely resulted from dust materials transported from nearby continental areas. (c) Atmospheric concentrations of total dissolvable Fe in fine aerosols were higher than those in coarse aerosols. One explanation is that smaller aerosols may provide larger surface areas relative to their volume for chemical reactions with other substances that promote Fe dissolution from dust. Click here to view the figure larger.

Reference:

Gao, Y., Marsay, C. M., Yu, S., Fan, S., Mukherjee, P., Buck, C. S., & Landing, W. M. (2019). Particle-Size Variability of Aerosol Iron and Impact on Iron Solubility and Dry Deposition Fluxes to the Arctic Ocean. Scientific Reports, 9(1), 16653. https://doi.org/10.1038/s41598-019-52468-z 

A review constituting the half-way mark of GEOTRACES

After a brief reminder on the motivation and foundation processes of the international and ambitious programme GEOTRACES, Bob Anderson (2020, see reference below) proposes an overview of many results of GEOTRACES activities related to the three guiding themes of the programme: (1) fluxes and processes at ocean interfaces (2) internal cycling of TEIs, and 3) the development of proxies for past change. It is beyond the scope of a highlight to summarise the main results obtained with the programme so far, knowing that most of them are already covered as highlights in this website!

Thus your favourite International Project Office (IPO) encourages colleagues, teachers and students who wish to discover how fruitful is the modern marine geochemistry to open this review. As a produit d'appel we propose the illustration below: the contrasting distributions of phosphate (PO4), aluminium (Al), iron (Fe), cobalt (Co) and zinc (Zn) along a meridional Atlantic section illustrates how the acquisition of clean and reliable data at high resolution questions established paradigms... Although all micronutrients, Fe, Zn and Co fates are readily different; although both of lithogenic origin, Al and Fe distributions are quite contrasting... so many exciting research fields opened!

 20 Anderson l

Figure: Meridional sections down the length of the Atlantic Ocean created by splicing data from multiple GEOTRACES sections (see inset in panel C). Data are available in the GEOTRACES Intermediate Data Product IDP2017. A) Phosphate data from IDP2017. B) Dissolved Zn (Middag et al 2019) and unpublished data from P. Croot, available in IDP2017. C) Dissolved Co (Dulaquais et al 2014a, Dulaquais et al 2014b) and unpublished data from M. Boye available in IDP2017. D) Dissolved Fe (Klunder et al 2011, Rijkenberg et al 2014). E) Dissolved Al (Middag et al 2015, Middag et al 2011) and unpublished data from Peter Croot available in IDP2017. Figure produced using Ocean Data View <http://odv.awi.de>.

References:

Anderson, R. F. (2020). GEOTRACES: Accelerating Research on the Marine Biogeochemical Cycles of Trace Elements and Their Isotopes. Annual Review of Marine Science, 12(1), DOI: https://doi.org/10.1146/annurev-marine-010318-095123

Dulaquais G, Boye M, Middag R, Owens S, Puigcorbe V, et al. (2014a). Contrasting biogeochemical cycles of cobalt in the surface western Atlantic Ocean. Glob. Biogeochem. Cycles 28:2014GB004903

Dulaquais G, Boye M, Rijkenberg MJA, Carton X. (2014b). Physical and remineralization processes govern the cobalt distribution in the deep western Atlantic Ocean. Biogeosciences 11:1561–80

Klunder MB, Laan P, Middag R, de Baar HJW, Ooijen JV. (2011). Dissolved iron in the Southern Ocean (Atlantic sector). Deep-Sea Res. II 58:2678–94

Middag R, van Hulten MMP, Van Aken HM, Rijkenberg MJA, Gerringa LJA, et al. (2015). Dissolved aluminium in the ocean conveyor of the West Atlantic Ocean: effects of the biological cycle, scavenging, sediment resuspension and hydrography. Mar. Chem. 177:69–86

Middag R, van Slooten C, de Baar HJW, Laan P. (2011). Dissolved aluminium in the Southern Ocean. Deep-Sea Res. II 58:2647–60

Rijkenberg MJA, Middag R, Laan P, Gerringa LJA, van Aken HM, et al. (2014). The distribution of dissolved iron in the West Atlantic Ocean. PLOS ONE 9:e101323

Unprecedented iron delivery from the Congo River margin to the South Atlantic Gyre

The Congo River is the world’s second largest river by discharge volume and the only major river discharging into an eastern boundary region. Despite of its size, the Congo River plume and its influence on trace element cycles and ocean primary productivity is poorly constrained. During GEOTRACES cruise GA08, Vieira and co-workers (2020, see reference below) used radium isotopes to demonstrate that a combination of high river discharge, coastal sediments or submarine groundwater discharge make the Congo the most significant riverine source of iron (Fe) to the South Atlantic. Rapid off-shelf transport close to the equator results in the delivery of extremely high fluxes of trace elements from the Congo River outflow into the Southeast Atlantic Ocean. This large trace element input relieves micro-nutrient limitation (iron and cobalt) across the eastern South Atlantic and provides an unusual example of a very efficient riverine Fe source.

 20 Vieira l

Figure: Comparison of dissolved iron (dFe) concentrations vs. distance from the river mouth in other riverine systems globally. The presence of dFe several hundreds of kilometres off-shelf indicates more rapid horizontal mixing of the river plume compared to other systems. TPD indicates Transpolar Drift, and NY Bight indicates New York Bight.

Reference:

Vieira, L. H., Krisch, S., Hopwood, M. J., Beck, A. J., Scholten, J., Liebetrau, V., & Achterberg, E. P. (2020). Unprecedented Fe delivery from the Congo River margin to the South Atlantic Gyre. Nature Communications, 11(1), 556. DOI: https://doi.org/10.1038/s41467-019-14255-2

Diatoms use a stolen bacterial gene to commit iron piracy


Joint Science Highlight with US-Ocean Carbon & Biogeochemistry (US-OCB).

Much of the primary production in low-iron marine environments is carried out by diatoms, and therefore the details of how these phytoplankton acquire the iron they need can have major impacts of biogeochemical cycles. The proteins involved in this process are largely unknown, but in 2018 a carbonate-dependent uptake protein was described that enables diatoms to access inorganic iron dissolved in seawater. As increasing atmospheric CO2 results in decreased seawater carbonate iron concentration, the future prospects for this iron uptake strategy are uncertain. In a recent study published in PNAS, CRISPR technology was used to characterize a parallel uptake system that requires no carbonate and is therefore unimpacted by ocean acidification. This system targets an organically complexed form of iron (siderophores), that must be produced by co-occurring microbes. Two genes are required to turn siderophores from a potent toxicant to an essential nutrient, and one of these (FBP1) is a receptor which was horizontally acquired by diatoms from siderophore-producing bacteria. The other (FRE2) is a eukaryotic reductase which facilitates the dissociation of iron-siderophore complexes. Ocean acidification may not result in exacerbation of iron limitation in marine ecosystems as long as diatoms and bacteria can co-exist. Are diatoms really pirating siderophores from hapless bacteria? The true nature of this interaction is unknown and may be at times mutualistic. When iron availability is limiting the carbon supply to a microbial community, heterotrophic bacteria may benefit from using siderophores to direct iron to diatom companions.      

20 Coale

Figure: (A) Growth curves of diatom cultures in low iron media. (B) Growth in same media with siderophores added. (C) Diatoms under 1000x magnification, brightfield. (D) mCherry-FBP1. (E) Plastid autofluorescence. (F) YFP-FRE2. (G) Phylogenetic tree of FBP1 and related homologs.


Reference:

Coale, T. H., Moosburner, M., Horák, A., Oborník, M., Barbeau, K. A., & Allen, A. E. (2019). Reduction-dependent siderophore assimilation in a model pennate diatom. Proceedings of the National Academy of Sciences of the United States of America, 116(47), 23609–23617. https://doi.org/10.1073/pnas.1907234116

Icebergs; a huge but highly variable source of iron to the ocean

Icebergs have been speculated to constitute one of the largest fluxes of iron (Fe) into the polar oceans since the 1930s and thus recent increases in ice discharge around the world could potentially change Fe availability in the ocean. But how much Fe is in an iceberg? As part of an international collaboration involving several cruises over the past 5 years including the GEOTRACES Fram Strait GN05 section Hopwood et al., (2019, see reference below) report the concentrations of Fe in ice from over 10 glaciated regions around the world. The global mean iceberg Fe content is found to be similar to, or slightly higher than, limited earlier estimates. However, a critical insight is the highly uneven distribution of this Fe with the ‘dirtiest’ 4% of samples collected accounting for over 90% of the cumulative Fe. Investigating how these ‘dirty’ layers are formed and their fate in the ocean is therefore essential to determining the significance of icebergs for marine primary production.

19 Hopwood

Figure: Ice from around the world is found to have a highly variable total dissolvable Fe content ranging from 2 nM to 2 mM. Click here to view the figure larger.

Mark J. Hopwood, Dustin Carroll, Juan Höfer, Eric P. Achterberg, Lorenz Meire, Frédéric A. C. Le Moigne, Lennart T. Bach, Charlotte Eich, David A. Sutherland & Humberto E. González, (2019) High variability is evident even within individual geographic regions. Reference: Highly variable iron content modulates iceberg-ocean fertilisation and potential carbon export, Nature Communications, 10, 5261 DOI: https://doi.org/10.1038/s41467-019-13231-0

Modern neodymium isotopic signatures of the South West Atlantic eventually documented!

The South West Atlantic is a critical place for studying the present and past Meridional Overturning Circulation pathways. Numerous studies conducted by research teams at Lamont (US) or Cambridge (UK) on the neodymium (Nd) isotopic signatures imprinted in sediment cores of this area attest of its great interest. Thus, establishing what control the Nd isotopic distribution in this area today is of prime importance.

This was done by Rahlf and his co-workers (2019, see reference below) who extensively sampled the Cape and Angola basins. They revealed unradiogenic signatures reaching −17.6 in the uppermost 200 m of the Angola and Cape basins. Interestingly, these negative values are attributed to different mechanisms: in the Angola basin, they likely reflect the admixture of a coastal plume originating near 13ºS and carrying an unradiogenic Nd signal attributed to the dissolution of iron-manganese coatings of particles formed in river estuaries or near the West African coast. In the Northern Cape Basin, they are likely due to the contamination of the Mozambic channel by weathering products of Archean terrains of southern Africa later advected via the Alghulas current, allowing tracing the advection of shallow waters via the Agulhas and Benguela currents into the southeastern Atlantic Ocean. Deeper, the Nd isotope signatures are primarily explained by conservative mass mixing despite local overprinting by terrestrial and/or sediment inputs.

19 Rahlf

Figure: Nd isotope distributions in the western Angola Basin (left section) and the northern Cape Basin (right section) along the cruise track of GA08, combined with isotope data obtained further south from cruise ANT-XXIIV/3 (Stichel et al., 2012). Dashed lines indicate approximate boundaries of the prevailing water masses. 
Negative neodymium values (blue-violet) in surface waters of the Angola Basin are primarily caused by dissolution of sediments originating from the west African coast. Negative neodymium values in surface waters of the Cape Basin originate from sediments of southern Africa (Mozambique Channel) and are transported by the Agulhas Current into the basin. The figure is adopted from Rahlf et al. (2020). Click here to view the figure in new window.

References:

Rahlf, P., Hathorne, Ed., Laukert, G., Gutjahr, M., Weldeab, S., Frank, M. (2020). Tracing water mass mixing and continental inputs in the southeastern Atlantic Ocean with dissolved neodymium isotopes. Earth Planet. Sci. Lett., in press, DOI: 10.1016/j.epsl.2019.115944.

Stichel, T., Frank, M., Rickli, J., Haley, B.A. (2012). The hafnium and neodymium isotope composition of seawater in the Atlantic sector of the Southern Ocean. Earth Planet. Sci. Lett. 317–318, 282–294.

Neural network as tools to replace oceanic data deficiencies

The importance of the cycle and speciation of nitrate and its isotopes (δ15N) in the ocean does not have to be demonstrated anymore. In an attempt to overcome the difficulty to compare the results of N/δ15N cycle models to a sparse set of data, Rafter and co-workers propose an original approach, based on artificial intelligence (AI) methods.

They use a compilation of 12,277 published δ15N measurements together with climatological maps of physical and biogeochemical tracers to create a surface to-seafloor map of δ15N using an ensemble of artificial neural networks (EANN). In other words, they train the seawater parameters to deduce a δ15N value at a given location and depth taking into accounts the climatological values. The strong correlation (R2 > 0.87) and small mean difference (< 0:05 ‰) between EANN-estimated and observed nitrate δ15N indicate that the EANN provides a good estimate of climatological nitrate δ15N without a significant bias. This climatology reveals large-scale spatial patterns in nitrate δ15N and allows the quantification of regional and basin-average oceanic values of nitrate δ15N. This work demonstrates how AI tools could help to address the unavoidable deficiency of data inherent to oceanic studies, keeping in mind that they require ab initio reasonable data coverage and mostly a good understanding of the parameter fate.

19 Rafter

Figure: (Top) Available nitrate δ15N (N isotopic composition) measurements at the time of publication. (Bottom) View of nitrate δ15N at 3500 m from two perspectives: the observed value (circles) and the model value (the contours). Click here to view the figure larger.

Reference:

Rafter, P. A., Bagnell, A., Marconi, D., & DeVries, T. (2019). Global trends in marine nitrate N isotopes from observations and a neural network-based climatology. Biogeosciences, 16(13), 2617–2633. https://doi.org/10.5194/bg-16-2617-2019

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