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Science Highlights

Dissolved zinc and silicate decoupling in the North Pacific Ocean

Large-scale distributions of dissolved zinc (Zn) in the western and central subarctic North Pacific Ocean was established as part of the North Pacific GEOTRACES section GP02. Kim and co-workers could figure out interesting Zn behavior in this area:

  • Decoupling between Zn and Silicate in the intermediate water,

  • Zn* values (used to show the variability in the relationship between dissolved Zn and silicate in the ocean) are strongly positive in the intermediate waters of the western and central subarctic North Pacific,

  • Dissolved Zn and soluble reactive phosphorus (SRP) concentrations were relatively high in the intermediate water.

The authors suggest that these particular values observed in the intermediate waters of the subarctic North Pacific result of remineralization from Zn-rich biogenic particles, weak reversible scavenging onto sinking biogenic particles, and sedimentary Zn sources. 

17 KimlFigure: (A) Locations of sampling stations in the subarctic North Pacific. (B) Relationships between dissolved Zn and silicate. (C) Zn* as a function of density. The shaded bar represents density range of the intermediate water (26.6–27.5 σθ). (D) Relationships between dissolved Zn and SRP. In (B) and (D), green, red, and black indicate shallow water, intermediate water, and deep water in the subarctic North Pacific, respectively, while blue indicates data from the subtropical North Pacific. Click here to view the figure larger.

Reference:

Kim, T., Obata, H., Nishioka, J., & Gamo, T. (2017). Distribution of Dissolved Zinc in the Western and Central Subarctic North Pacific. Global Biogeochemical Cycles, 31(9), 1454–1468. DOI: http://doi.org/10.1002/2017GB005711

New neodymium concentration and isotopic data from the West Pacific Ocean

In the framework of the GEOTRACES TransGeoBioOc process study (GPpr04), seawater samples were collected at 12 stations (and up to 17 depths per stations) along the SO223T cruise transect from Busan (South Korea) to Suva (Fiji) (Figure a).

This beautiful set of Nd concentration ([Nd]) and isotopic (εNd) signature data allow Behrens and co-workers (2017, see reference below) to clarify the relative importance of different Nd sources (rivers, volcanic islands), vertical (bio)geochemical processes and lateral water mass transport on these parameter distributions in this area. Interesting unradiogenic inputs from South Korea and Chinese rivers are identified in the northern part of the section, while volcanic islands of the tropical West Pacific supply radiogenic Nd to surface and subsurface waters (Figure a, b).

A thorough discussion of these data reveals:

  1. Clear differences in εNd signatures of east- and westward flowing surface and subsurface currents of the zonal equatorial current system that allow tracing of small-scale (sub)surface circulation at unprecedented detail (Figure b);
  2. Lateral transport of extremely low preformed [Nd] from the oligotrophic North/South Pacific subtropical gyres into the study area (Figure c);
  3. Conservative behaviour of Nd concentrations within AAIW, southern LCDW and upwelling of UCDW in the Philippine Sea.
  4. Non-conservative behaviour of εNd due to boundary exchange processes, mostly along the volcanic island shelves and submarine ridges.

17 Bahrens
Figure: 
(a) Map of stations (GeoB170-) 01-05 (SKO, South Korea - open ocean section) and 11-19 (TWP, tropical West Pacific section) with Nd sources (rivers, volcanic islands) and surface (solid arrows) and subsurface (dashed arrows) westward (white) and eastward (black) currents. Upper water column (b) εNd signatures with (sub)surface currents as in (a), and (c) Nd concentrations with salinity contours along the transect and different water masses (for abbreviations see figures 1 and 4 in original article). Please click here to view the image larger.

Reference:

Behrens, M.K., Pahnke, K., Schnetger, B., & Brumsack, H.-J. (2018). Sources and processes affecting the distribution of dissolved Nd isotopes and concentrations in the West Pacific. Geochimica et Cosmochimica Acta, 222, 508–534. http://doi.org/10.1016/j.gca.2017.11.008 (Unlimited download until January 16, 2018 at: https://authors.elsevier.com/a/1W7Sj3p4Z8lDE)

Widespread nutrient co-limitation discovered on GEOTRACES cruise

Browning and co-workers (2017, see reference below) find that multiple nutrients must be supplied to stimulate phytoplankton growth on the southeast Atlantic GEOTRACES GA08 cruise. The paper has been published in Nature.

Experiments to date have suggested that across most of the ocean surface marine phytoplankton are limited by either nitrogen or iron. But simultaneously low concentrations of these and other nutrients have been measured over large extents of the open ocean, raising the question: are phytoplankton in these waters only limited by one nutrient?

Browning and co-workers tested this by conducting experiments throughout the SE Atlantic GEOTRACES GA08 cruise, where seawater samples were amended with nitrogen, iron, and cobalt—alone and in all possible combinations. They found that adding both nitrogen and iron in combination was needed to stimulate any significant phytoplankton growth over 1000s of kilometres of ocean. Furthermore, addition of cobalt in combination with nitrogen and iron further enhanced phytoplankton growth in a number of experiments.

17 BrowningFigure: Experiments were conducted throughout the SE Atlantic GEOTRACES cruise transect (lines and dots on the map) and demonstrated that nitrogen and iron had to be added to significantly stimulate phytoplankton growth. Supplementary addition of cobalt (or cobalt-containing vitamin B12) stimulated significant additional growth. Experimental responses illustrated in the right panel are from the site indicated by the red point on the map. Click here to view the figure larger.

Reference:

Browning, T.J., Achterberg, E.P., Rapp, I., Engel, A., Bertrand, E.M., Tagliabue, A. and Moore, C.M., 2017. Nutrient co-limitation at the boundary of an oceanic gyre. Nature. 551, 242–246 doi:10.1038/nature24063.

Benthic nepheloid layer global compilation: an invaluable resource for GEOTRACES researchers

Models simulating any oceanic tracer biogeochemistry require a good depiction of the particle distribution, key to incorporating properly scavenging-remineralization processes. However, this kind of description is still rare.

Gardner and co-workers (2018, see reference below) are providing an exceptional compilation of the Benthic Nepheloid Layers (BNL) around the world.

BNLs have been mapped using 6,392 full-depth profiles of beam attenuation made during 64 cruises using their transmissometers mounted on CTDs in multiple national/international programs including WOCE, SAVE, JGOFS, CLIVAR-Repeat Hydrography, and GO-SHIP during the last four decades. Not surprisingly, intense BNLs are observed where eddy kinetic energy (EKE, see figure below) in overlying waters, mean kinetic energy 50 m above bottom, and energy dissipation in the bottom boundary layer are the highest. Therefore, intense BNLs are observed in the Western North Atlantic, the Argentine Basin, parts of the Southern Ocean and areas around South Africa. Contrastingly, most of the Pacific, Indian, and Atlantic central basins do not display strong sediment resuspension.

 18 Gardner
Figure: Map of log of surface eddy kinetic energy (EKE) based on satellite observations during 2002–2006 with transmissometer station locations superimposed.
Please click here to view the figure larger.

Reference:

Gardner, W.D., M.J. Richardson, A.V. Mishonov. Global Assessment of Benthic Nepheloid Layers and Linkage with Upper Ocean Dynamics. Earth and Planetary Science Letters 482 (2018) 126–134. https://doi.org/10.1016/j.epsl.2017.11.008   (Unlimited download until early January 2018 athttps://authors.elsevier.com/a/1W32w,Ig49OX9,Ig49OX9)

Tracing scavenging intensity with the original coupling between scandium, yttrium and lanthanum in surface waters

The first basin-wide surface scandium (Sc), yttrium (Y) and lanthanum (La) concentration data in a section across the North Atlantic subtropical gyre (2011 GEOTRACES GA03) is reported by Till and co-authors (2017, see reference below). First, comparison between dissolved Sc and La concentrations in aerosols and surface waters allow these authors to estimate their residence time in the upper layers. Then, they establish that Sc, which is particle reactive, is less concentrated at the gyre boundaries, likely reflecting that it could be drawn down by elevated particle flux in these areas. They propose to normalize the Sc distribution to that of Y or La, two elements much less particle reactive and which display constant ratios with Sc in the dust inputs over the North Atlantic. This trick allows them to get rid of variable inputs of Sc to the surface ocean and to propose that the variations in dissolved Y/Sc and La/Sc ratios may be due to preferential Sc scavenging and could therefore indicate scavenging intensity.

17 TillFigure: Distribution of surface dissolved La/Sc (a, dark circles) and Y/Sc (b) concentration ratios across the North Atlantic Gyre (cruise GA03). Panel (a) also shows the La/Sc ratios in soluble aerosols (open circles). The dissolved ratios are substantially higher than the source aerosol ratios, indicating that there is some process occurring in the seawater that elevates the ratios. The temperature distribution along the same cruise track (c) shows that the shape of the gyre as inferred by isotherm depth generally corresponds with the distribution of dissolved La/Sc and Y/Sc), suggesting the possibility that elevated scavenging and preferential drawdown of Sc at the gyre boundaries could leave a signature in the surface dissolved La/Sc and Y/Sc ratios. Click here to view the figure larger.

Reference:

Till, C. P., Shelley, R. U., Landing, W. M., & Bruland, K. W. (2017). Dissolved scandium, yttrium, and lanthanum in the surface waters of the North Atlantic: Potential use as an indicator of scavenging intensity. Journal of Geophysical Research: Oceans, 122(8), 6684–6697. http://doi.org/10.1002/2017JC012696

 

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