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Science Highlights

Drawing the future of phytoplankton in a changing ocean

Phytoplankton development is strongly linked to the dissolved iron availability in the surface waters. Iron’s behavior is sensitive to warming, stratification, acidification and de-oxygenation. In a changing ocean, these processes in addition to nutrient co-limitation interactions with iron biogeochemistry will all strongly influence phytoplankton dynamics. This paper establishes the potential future shifts in multiple facets of iron biogeochemistry, from cellular physiology to ocean circulation. Possible impacts of these multiple changes on diatoms and trichodesmium are illustrated in the figure below. This work warns us on the urgent need to improve our present knowledge of the micronutrient cycle forcing, in order to better predict their future behaviors.


17 Hutchins
Figure: Interactive influences of the changing ocean iron cycle on diatoms and nitrogen-fixing cyanobacteria.
Iron biogeochemistry will respond to global change-related warming (red), increased light (yellow), acidification (black), loss of oxygen (green), and lowered inputs of the nutrients nitrate (white), silicate (grey) and phosphate (blue). This will have direct consequences for the growth and physiology of both phytoplankton groups, as well as indirect effects on critical resource supply ratios (boxes). Important components of the marine iron cycle responding to environmental change include inputs from dust, complexation by organic ligands, redox chemistry, and biological availability (orange). Click here to view the figure larger. (adapted from Hutchins and Boyd 2016, with thanks to J. Brown for graphics)

Reference:

Hutchins, D. A., & Boyd, P. W. (2016). Marine phytoplankton and the changing ocean iron cycle. Nature Climate Change, 6(12), 1072–1079. DOI: 10.1038/nclimate3147

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

 

The coupled zinc-silicon cycle paradox illuminated

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 times greater than at low latitudes). Such contrasting observations have 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) infer 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 1-D cycling only can bias the understanding of (macro and micro) nutrient behaviours, and therefore

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

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)

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.

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