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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Why did the concentration of atmospheric carbon dioxide rise so much and so quickly during the last deglaciation? 

During the Last Glacial Maximum, the deep southern Pacific waters were stratified, efficiently accumulating old, CO2 rich waters. Basak and co-authors (2018, see reference below) measured neodymium isotopes in sediment cores that clearly show that when these deep waters became less stratified as the climate warmed they released their carbon which could escape to the atmosphere...what a tempting prospect and beautiful teaser for the forthcoming PAGES-GEOTRACES workshop of December 2018!

 18 Pahnke l
Figure: View from RV Polarstern while collecting sediment samples used in the study by Basak et al.
Read more at:
Credit: Dr. Katharina Pahnke


Basak, C., Fröllje, H., Lamy, F., Gersonde, R., Benz, V., Anderson, R. F., Molina-Kescher, M., Pahnke, K. (2018). Breakup of last glacial deep stratification in the South Pacific. Science, 359(6378), 900–904.

Read more also at:


Particle distribution in repeated ocean sections and sediment resuspension

This is the first compilation of an expansive data base of transmissometer data on a decadal period of time. More than 7376 stations have been analyzed for “cp” value, a proxy for particle concentrations, by Gardner and co-workers (2018, see reference below). Full water-column sections confirm that particle concentrations are higher in surface waters, decrease rapidly below 200 m, most often down to the seafloor. However, cloudy near-bottom waters, known as “benthic nepheloid layers”, are generated by sediment erosion and resuspension at specific geographic areas. These locations are directly linked to energetic surface dynamics that produce regions of high Eddy Kinetic Energy (EKE). More fascinating, however, is the decadal persistence of this close surface-to-deep connection. We invite you to read this very interesting article!

18 Gardner2Figures: (A) Map of log of surface eddy kinetic energy (modified from Dixon et al., 2011) with cp transects indicated. (B) Section of cp (proxy for particle concentration) along 53° W in spring, 2012 in the Western North Atlantic. Black contours are dissolved oxygen (µmol kg-1). Click here to view the figure larger.


Gardner, W. D., Mishonov, A. V., & Richardson, M. J. (2018). Decadal Comparisons of Particulate Matter in Repeat Transects in the Atlantic, Pacific, and Indian Ocean Basins. Geophysical Research Letters.

Where, how and which trace elements are released from dust at the sea surface?

Alex Baker and Tim Jickells (2017, see reference below) propose to answer to this question thanks to analysis of aerosols collected in the framework of the Atlantic Meridional Transect (AMT). They established the soluble concentrations of a range of trace metals (iron, aluminium, manganese, titanium, zinc, vanadium, nickel and copper) and major ions. They reveal much higher inputs to the North Atlantic Ocean compared to the South Atlantic Ocean, reflecting stronger land based emission sources in the Northern Hemisphere. Comparison of these inputs with the surface water contents of the same trace metals compiled in the GEOTRACES intermediate data product show surprising features that you will discover if you read this paper…

18 Baker lowFigures: (A) Approximate tracks of the AMT cruises (dots and triangles) and general flow directions of the seven major atmospheric transport routes encountered during the cruises (arrows). Abbreviations for the air transport regimes are: continental Europe (EUR), North Africa including the Sahara and Sahel: (SAH), Southern Africa impacted by biomass burning emissions (SAB), Southern Africa not impacted by biomass burning (SAF), South America (SAM), remote North or South Atlantic i.e. not crossing land for at least 5 days prior to collection (RNA and RSA respectively). (B) Box and whisker plots showing the variations in the concentrations of iron, aluminium, manganese, titanium, zinc, vanadium, nickel and copper with air transport/source type for the AMT transect. They reveal much higher inputs to the North Atlantic Ocean, reflecting stronger land based emission sources in the Northern Hemisphere. Please click here to view the figure larger. (Figures modified from Progress in Oceanography)


Baker, A. R., & Jickells, T. D. (2017). Atmospheric deposition of soluble trace elements along the Atlantic Meridional Transect (AMT). Progress in Oceanography, 158, 41–51.

Climate change induced spectacular increase of the land-ocean inputs in the Arctic Ocean

Measurements of radium-228 (228Ra) in the framework of the 2015 U.S. GEOTRACES Arctic Transect (GN01), revealed that the surface water content of this tracer has almost doubled over the last decade, specifically in the Transpolar Drift near the North Pole.

Radium isotopes are excellent tracers of land-ocean inputs. A mass balance model for 228Ra allowed Kipp and co-workers (2018, see reference below) to suggest that this increase is due to an intensification of shelf-derived material inputs to the central basin. These coastal changes, in turn, could also be delivering more nutrients, carbon, and other chemicals into the Arctic Ocean and lead to dramatic impacts on Arctic food webs and animal populations.

18 Kipp

Figure: Diminishing sea ice near the Arctic coast leaves more open water near the coast for winds to create waves. The increased wave action reaches down and stirs up sediments on shallow continental shelves, releasing radium and other chemicals that are carried up to the surface and swept away into the open ocean by currents such as the Transpolar Drift. Artwork: Natalie Renier, Woods Hole Oceanographic Institution. Please click here to view the figure larger.


Kipp, L. E., Charette, M. A., Moore, W. S., Henderson, P. B., & Rigor, I. G. (2018). Increased fluxes of shelf-derived materials to the central Arctic Ocean. Science Advances, 4(1), eaao1302. DOI:

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.


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:

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
(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.


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. 

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.


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.   

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.


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.

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