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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.).

Impressive set of data reveal new features on the modern cadmium–phosphate relationship

Xie and co-authors (2015, see reference below) report vertical profiles of dissolved cadmium (Cd) in the western South Atlantic Ocean (GEOTRACES section GA02), which show nutrient-like distributions similar to those of the macronutrient phosphate (PO4). A close look at the data reveal:

  • In the surface ocean, preferential uptake of Cd over PO4 by phytoplankton occurs along the transect, regardless of ambient iron (Fe) concentrations, suggesting Fe availability is not critical for biological Cd utilization in the southwest Atlantic;
  • In addition, horizontal advection of Cd depleted low oxygen waters originating from the Angola Basin and brought across the Atlantic Ocean via the Benguela and Equatorial currents imparts a Cd-depleted signature to equatorial intermediate waters distinguishing them from southerly intermediate waters.
  • This new dataset provides further evidence that Subantarctic Mode Water plays an important role in generating the non-linearity of the global Cd-PO4 correlation.

16 Xie l
Figure:
Evaluation of Cd–PO4 systematics using new data from GEOTRACES GA02 Leg 3 (colored circles; see inset for location) and literature data (Southern Ocean: Abouchami et al., 2014; Baars et al., 2014; Boyé et al., 2012; Xue et al., 2013; Indian Ocean: Vu and Sohrin, 2013; Angola Basin: Waeles et al., 2013) at the scale of the South Atlantic Basin. The Cd–PO4 relationship for samples with PO4 >1.3 μmol kg-1 in this study exhibits two parallel linear correlations. The influence of low-oxygen waters originating in the Angola Basin (grey shading) is noticeable in intermediate waters at the equatorial stations. The clear kink at PO4 ~ 1.3 μmol kg-1 in the South Atlantic is attributed to northward flowing, nutrient-rich Subantarctic Mode Water. Click here to view the figure larger.

Read more: Impressive set of data reveal new features on the modern cadmium–phosphate relationship

Lead isotopes tracks leakage of Indian Ocean seawater into the Atlantic Ocean

Stable lead isotopes have been measured by Maxence Paul and co-workers along 40°S during the first and second legs of the UK GEOTRACES cruise (GA10) in the Atlantic Ocean. They clearly help to identify three different end-members and mixing between them: open ocean South Atlantic seawater, Indian Ocean seawater, and coastal inputs from South Africa. Lead isotopes and concentrations are excellent fingerprints of the occurrence of Agulhas rings, as confirmed by satellite observations and in situ hydrographic data. This study reveals that the Agulhas Leakage is not only a key pathway for heat but also impacts biogeochemical cycles...

16 Paul l
Figure:
Maps showing the location of surface seawater samples collected during Leg A (left) and Leg B (right) of UK GEOTRACES GA10 cruise D357. Shaded areas highlight the position of Agulhas rings on 19 October 2010 (left) and 17 November 2010 (right), as suggested by sea surface height (SSH) anomalies obtained from satellite data (AVISO, http://www.aviso.oceanobs.com/duacs/). Middle panels: Variations of Pb isotope compositions (colored circles) and Pb concentrations (grey circles) reflect the different sources present in the Cape Basin. Sample denoted as coastal, Agulhas, transitional, and open ocean are represented in blue, red, yellow and green, respectively. The Agulhas samples are marked by low 206Pb/207Pb and and their locations are in accord with the position of the Agulhas rings, as identified by satellite observations and salinity anomalies (bottom panels). Click here to view the figure larger.

Read more: Lead isotopes tracks leakage of Indian Ocean seawater into the Atlantic Ocean

Helium isotopes help to constrain high-resolution model dynamics in the Mediterranean Sea

The terrigenic helium isotope distribution was simulated for the first time in the whole Mediterranean Sea, using a high-resolution model (NEMO-MED12) at one-twelfth of a degree horizontal resolution (6–8 km).

Together with providing valuable constraints on the crustal and mantle helium fluxes, the simulations and their comparisons with observations provide a new technique for improving the dynamical regional model. This is clearly illustrated by the confirmation of the shortcomings of the model dynamics in representing the deep ventilation of the Ionian sub-basin...

16 Ayache
Figure: Total δ 3He (sum of terrigenic, tritiugenic and atmospheric helium) model–data comparison along the Meteor M5 (September 1987) section. (a) Colour-filled contours indicate simulated δ 3He (%), whereas colour-filled dots represent in situ observations. (b) and (c): comparison of average vertical profiles for the Levantine and Ionian sub-basins, respectively; model results are in blue; red indicates in situ data. Click here to view the figure larger.

Read more: Helium isotopes help to constrain high-resolution model dynamics in the Mediterranean Sea

Tracking the sea ice exported from the Siberian Shelf to the Arctic Ocean

An original method based on the use of the couple salinity – δ18O allows determining the fractions of water that had been removed as ice across the shelf and the consequent net export of sea ice from Laptev and East Siberian Sea towards the remaining Arctic Ocean (860 km3/yr).

An important point is that this method does not assume an ice end-member of fixed composition, very powerful when applied on samples with large differences in salinity....

15 Rosen lFigure: The map shows three geographical lines, sections, in the Laptev and East Siberian Sea. The sections stretch from the mouth of the Lena-, Indigirka- and Kolyma rivers and northward onto the shallow shelf. Based on the δ18O in the water, the vertical profiles indicate the amount of river water on the shelf (b, d and f) and also the fraction of water exported from the shelf as ice (a, c and e). Click here to view the figure larger.

Read more: Tracking the sea ice exported from the Siberian Shelf to the Arctic Ocean

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
Figure:
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

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