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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.).

GEOTRACES publishes the motivations and description of its first Intermediate Data Product

GEOTRACES proposes an article that describes its first integrated and quality controlled Intermediate Data Product 2014 (IDP2014): the digital data package and the eGEOTRACES electronic Atlas that provides section plots and animated 3D scenes of the data. The 3D scenes provide geographical and bathymetric context crucial for tracer assessment and interpretation.

With this release, GEOTRACES seeks to promote intensified collaboration within the marine geochemical community and beyond. The availability of a large integrated and quality controlled dataset, such as the IDP2014, will allow a much wider range of studies than would be possible with individual cruise data alone. Such open access is also an opportunity for other communities as for example physical and biological oceanographers, modellers but also policy makers... 

eGEOTRACES dissolved Zn Atlantic OceanFigure: 3D scene showing the distribution of dissolved zinc (Zn) in the Atlantic Ocean.

Read more: GEOTRACES publishes the motivations and description of its first Intermediate Data Product

Metal contents of North Atlantic phytoplankton across environmental gradients

Twining and co-workers (2015, see reference below) provide the first direct measurements of metal quotas in phytoplankton from across the North Atlantic Ocean (US GEOTRACES GA03), known to be subjected to aeolian Saharan inputs and anthropogenic inputs from North America and Europe.

The authors combine chemical leaches (to extract biogenic and otherwise labile partiulate phases) together with synchrotron X-ray fluorescence (SXRF) analyses of individual micro and nanophytoplankton to discern sptial trends across the basin. Among the numerous results of this study:

  • Mean cellular metal quotas were similar to those measured in the Pacific and Southern Oceans except for iron (Fe), which was approximately 3-fold higher in North Atlantic cells.
  • Cellular Fe quotas did not increase in step with aeolian dust inputs suggesting that the dust inputs have low bioavailability.
  • Although particulate aluminium (Al) was primarily associated with lithogenic material, the labile Al fraction was highly correlated with phosphorus (P) and biogenic silica suggesting that perhaps 20% of particulate Al may be associated with biogenic material.
  • Externally scavenged Fe was not a significant fraction of the metal associated with live phytoplankton, in contrast to the importance of adsorbed or precipitated phases on detrital material.

15 Twining l
Map of stations sampled during GEOTRACES cruise GA03 plotted over MODIS surface chlorophyll for November and December (2010-2011). Stations with SXRF samples are shown with filled symbols. Cellular iron quotas measured for cells at these stations are shown in the overlaid figure. Phytoplankton in the western part of the transect contain approximately four-fold more Fe than cells in the eastern part of the transect. Click here to view the figure larger.

Read more: Metal contents of North Atlantic phytoplankton across environmental gradients

New insights into global barium cycling from the first barium isotope data for seawater

A new method developed by Horner and co-workers (2015, see reference below) allows for the determination of dissolved barium (Ba) isotopic compositions in seawater using nanogram quantities of Ba.

The formation of BaSO4 (barite) in open ocean seawater is largely driven by the microbial remineralization of sinking organic matter. By analysing a 4500 m depth profile of Ba-isotopic compositions from the South Atlantic, Horner et al. find that BaSO4 precipitation removes isotopically light Ba from seawater, which renders residual, Ba-depleted seawater with heavier Ba-isotopic compositions. The distinct patterns of Ba isotopic variation in the deep ocean are clearly related to the mixing of distinct water masses formed at higher latitudes. These data indicate that water masses obtain their Ba-isotopic signatures when at or near the near-surface, and that regional circulation exerts a strong control over Ba-isotopic distributions in the South Atlantic, and possibly elsewhere.

More broadly, these new data point to several important linkages between C remineralization and the Ba cycle, suggesting that Ba-isotopes may develop into a powerful new means to study marine biogeochemical cycles.

15 Horner Barium lFigure: Horner et al. present the first Ba-isotopic analyses of seawater for samples collected as part of UK GEOTRACES from the South Atlantic (GA10E/D357; see inset for location). The profile shows substantial Ba-isotopic variation (left) that correlates with the major changes in Ba concentrations (right). The concentrations and isotopic data reflect a combination of both barite cycling in the shallow subsurface and large-scale oceanic circulation. Click here to view the figure larger.

Read more: New insights into global barium cycling from the first barium isotope data for seawater

Large fluxes of dissolved aluminium exported from the coast to the ocean

In the Eastern China Sea (ECS), the continental shelf serves as an important source of dissolved aluminium (DAl) for the overlaying waters via resuspension of sediments and benthic fluxes. This was demonstrated by Ren et al. (2015, see reference below), who identified cross-shelf transport in the subsurface water over the ECS. The DAl export from the 100 m isobath is 1.67 x 1010 g yr.

Thanks to the Kuroshio current, more than half of this Al is transported northward within the region enclosed by the 100 m and 200 m isobaths to the Japan Sea/East Sea. The remaining flux is transported out of the shelf across the 200 m isobath. This highlights the importance of coastal processes and subsurface cross-shelf transport as a source of dissolved trace elements to the open ocean.

15 Ren 2Figure: Horizontal distributions of temperature, salinity, SPM (mg/L), and dissolved Al (nM) in the surface water (a, c, e, g) and bottom water (b, d, f, h, with water depth ranging from 13 m to 1200 m) of the East China Sea. Click here to view the figure larger.
Changjiang Diluted Water (CDW) expanded southeastward in the surface, and was restricted to the coastal area by the incursion of Kuroshio Waters (KW). The incursion of Kuroshio Subsurface Water (KSSW) in the bottom layer can reach 30 °N near the Changjiang Estuary. The concentration of dissolved Al decreased gradually from the coastal area to the central shelf, and then decreased sharply at the shelf break. The bottom layer had higher concentrations of dissolved Al than in the surface layer in the coastal and middle shelf, consistent with their higher concentrations of SPM.

Read more: Large fluxes of dissolved aluminium exported from the coast to the ocean

Do you know that coastal phytoplankton is able to adapt to iron limitation?

Oceanographers have long understood that phytoplankton in the open ocean tend to have lower iron (Fe) requirements and/or more strategies for obtaining Fe from the environment. Adding a new twist to the established paradigm, Mackey and co-workers (2015, see reference below) show that coastal Synechococcus from the New England shelf is capable of dynamic, multitiered Fe adaptation. Their original approach was to compare growth, photophysiology, and quantitative proteomics of two Synechococcus strains from different Fe regimes.

The revealed competence of the coastal phytoplankton specie allow it to thrive over a broad range of Fe concentrations by partitioning Fe among different uptake and storage proteins as seawater Fe levels change. Conversely, a Synechococcus strain from the open Atlantic Ocean does not have this multitiered protein-based response. The ability of the coastal strain to acclimate to a wide range of Fe conditions comes with a cost: cells must invest nitrogen to make the proteins. Persistently low nitrogen levels in the open ocean may prove too costly for the oceanic Synechococcus strain, which has eliminated many Fe response genes from its genome. In contrast, higher nutrient availability closer to land affords coastal Synechococcus the flexibility to retain these proteins and so thrive even when and where Fe levels are low.

 15 Mackey
Figure: A) Locations of two strains of Synechococcus, the coastal WH8020 and Southern Sargasso WH8102, relative to the U.S. North Atlantic zonal section GEOTRACES (GA03) iron, nitrate and phosphorus concentrations. B-C) Fe concentrations were high in the central north Atlantic Ocean due to dust deposition, while coastal waters can have lower amounts of Fe relative to nitrogen. D) This variability is reflected in the highly dynamic protein response of the coastal phytoplankter Synechococcus WH8020, with Fe uptake, storage, and regulators varying with Fe abundance, allowing it to survive over a greater range of Fe concentrations. The oceanic strain has lost the ability to make many of these proteins, which may help it conserve nitrogen (as protein) in the open ocean where nitrogen is scarce. Click here to view the figure larger.

Read more: Do you know that coastal phytoplankton is able to adapt to iron limitation?

Decoupling between dissolved zinc and silicon in the North Atlantic Ocean driven by mixing of end-members

Roshan and Wu (2015, see reference below) reveal that the correlation between dissolved zinc (Zn) and silicon (Si) is relatively weak in the North Atlantic Ocean (GA03 US section). They use the results of an Optimum Multi-Parameter Water Mass Analysis to establish which parameter is mainly controlling the Zn distribution. Surprisingly, they evidence that remineralization might have an insignificant effect on the zinc distribution in this region. They conclude that dissolved zinc in the North Atlantic Ocean is mainly controlled by water mass mixing, although some water mass end-members exhibit deviations in the Zn-Si correlation, as for example the Mediterranean Outflow Waters. Unexpected large Zn inputs of hydrothermal origin are also perturbing the game...

15 RoshanFigure: Top panel shows the distribution of dissolved Zn along the GA03. Bottom panel shows the Zn-Si relationship for the zonal (left) and meridional (right) transect. Disappearance of a linear correlation is evident, particularly for the zonal transect where the Mediterranean waters have a great influence. Click here to view the figure larger.

Read more: Decoupling between dissolved zinc and silicon in the North Atlantic Ocean driven by mixing of...

Changing the cadmium : phosphorus paradigm?

The well-established strong linear relationship linking dissolved cadmium (Cd) and phosphorus (P) concentrations in seawater is at the origin of the attraction of Cd as a proxy for PO4 in the paleocean. However, exploring the dissolved Cd and PO4 distributions in the ocean, Quay and co-workers (2015, see reference below) show that the Cd/P of particles exported from the surface ocean doubles in high-nutrients low chlorophyll (HNLC) regions. They also demonstrate that Cd/PO4 variations in surface ocean and deep sea depend on Cd/P of degraded particles. Using a box model, they evidence that past changes in HNLC conditions would change Cd-PO4 relationship in deep sea... which has to be considered in paleo-reconstructions!

15 2 Quay
The meridional and interbasin trends in the estimated Cd/P ratio of particles exported from the surface ocean. The Cd/P of exported particles is a primary factor controlling the spatial variations dissolved Cd/P in the surface ocean and in the deep sea where particles are degraded.

Read more: Changing the cadmium : phosphorus paradigm?

Nitrate isotope distributions on the US GEOTRACES North Atlantic cross-basin section

The distribution of the δ15N and the δ18O in the U.S. GEOTRACES North Atlantic zonal transect (GA03 section) provides an insight on the interaction between aspects of the circulation of the Atlantic basin and features of the N cycle in the absence of a regional signal of water column denitrification. In the absence of denitrification, the elevation of the δ15N at intermediate depths (red dots on the eastern side of the transect as shown by the OMPA analyses of Jenkins et al., 2014) must derive from outside the basin, most likely entering from the Southern Ocean. The lack of a similar elevation in δ18O implies that the nitrate in southern-sourced intermediate waters has been diluted by the addition of nitrate from regeneration/nitrification in the low latitudes. The high nitrate δ15N in northward flowing intermediate waters is almost entirely erased by the input of low-δ15N newly fixed nitrogen to the Atlantic (including that from the Mediterranean) prior to the incorporation of those intermediate depth waters in southward flowing North Atlantic Deep Water. The data appear consistent with a nitrogen isotopic balance between northward and southward nitrate transports across the GEOTRACES section, suggesting that most Atlantic nitrogen fixation occurs south of the section.

15 Marconi

Figure: Depth profiles (top panels) and depth sections (bottom panels) of the δ15N and the δ18O of nitrate from the US GEOTRACES North Atlantic cross-basin transect. Colored circles are water type tags indicating samples with a high contribution of water masses relevant to the circulation of the Atlantic basin (from the OMPA analyses of Jenkins et al.).

Read more: Nitrate isotope distributions on the US GEOTRACES North Atlantic cross-basin section

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