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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.).

Multiple controls on the dissolved aluminium fate in the western Atlantic Ocean

Thanks to the most impressive set of dissolved aluminium (Al) and silicon (Si) data ever published in the Atlantic Ocean, Middag and co-workers (2015, see reference below) are thoroughly scanning the processes determining their oceanic distribution. They reveal that i) atmospheric inputs are affecting only the surface and subsurface waters, ii) there is an elusive but obvious coupling between Si-containing biogenic particles and Al, iii) scavenging is occurring faster than the horizontal advective transports preventing the use of Al as quantitative water mass tracer, and iv) not observed at a basin-wide scale before, suspended sediments are a significant source for dissolved Al in the deep waters.

15 Middag lFigure: The distribution of Aluminium (Al) is depicted in colour scale overlain with neutral density isopycnals and main water masses labelled for the upper 1000m and the deep ocean. The effects on the Al concentrations of sediment resuspension in the deep ocean and atmospheric deposition in the surface ocean are clearly visible.

Read more: Multiple controls on the dissolved aluminium fate in the western Atlantic Ocean

Dissolved gallium in the Beaufort Sea: a promising conservative water mass tracer

Mc Allister and Orians (2015, see reference below) present the first dissolved Ga (DGa) data ever reported in the Arctic. DGa behaves very conservatively, helping to constrain the mixing of water masses of Pacific and Atlantic origins. Its conservative behaviour helps distinguishing seasonal processes that are influencing temperature and Nitrate:Phosphate of Pacific Water entering the Arctic; it also supports the interpretation that nitrogen fixation rates are associated with Pacific source waters and not to freshwater sources (i.e. Mackenzie River or sea ice melt). Comparing DGa and dissolved aluminium (DAl) profiles reveals that differential scavenging rates are affecting the two tracers.

15 McAlisterFigure: a. Dissolved gallium (Ga) was measured in the Beaufort Sea of the Western Arctic Ocean, b. the transect comprising stations L1 – L3 represent waters of the Canadian Basin, S stations represent the shelf and are discussed further in the manuscript, c) concentrations of dissolved Ga in the Canadian Basin are shown to provide a conservative tracer of Pacific and Atlantic source waters. Low concentrations of dissolved Ga associated with Pacific waters increase smoothly through the thermocline to comparatively high concentrations of Atlantic source waters. Note log depth scale to assist visualization of shallow depths.

Read more: Dissolved gallium in the Beaufort Sea: a promising conservative water mass tracer

Coupling Rare Earth Elements concentrations, neodymium and radium isotopes: a powerful tool to decode environmental processes

For the first time, neodymium (Nd) isotopic compositions have been measured together with dissolved and colloidal Rare Earth Elements (REE) concentrations in the Amazon estuary salinity gradient, as part of the GEOTRACES process study AMANDES (Chief scientist: Catherine Jeandel). The sharp drop of REE concentrations in the low-salinity area (already observed in several estuaries) is clearly driven by the coagulation of colloidal material. While dissolved REE concentrations are increasing again at mid-salinities, Nd isotopic ratios allow tracing that these REE are released by the lithogenic material, weathered and transported by the river to the Atlantic Ocean. The original coupling with the radium (Ra) isotopes demonstrates that these dissolution processes are occurring within three weeks in the Amazon plume.

15 rousseau l
Figure:
Nd concentrations and isotopic composition in the Amazon River estuary. Click here to view the image larger.

Upper panel: Amazon estuary [Nd] from Sholkovitz’ 1993 study (grey circles) and this study (Blue diamonds) are reported against the salinity gradient. We observe the non conservative mixing between the Amazon river and the atlantic waters. The sharp drop in [Nd] in the low salinity region is attributed to the coagulation of colloids, the main REE carriers within river. This Drop in concentrations is followed by an increase with salinity before reaching typical low Nd levels of marine waters.
Lower panel: Amazon estuary dissolved (Red triangles), particulate (Green squares) εNd and apparent radium ages (in days) values are reported against the salinity gradient. A simple two endmembers (Amazon and Atlantic dissolved Nd) mixing model (red dashed line) is not sufficient to explain εNd variation within the salinity gradient. The dissolved Nd phase rapidly (19 days) homogeneizes with a third source the suspended sediments.

Read more: Coupling Rare Earth Elements concentrations, neodymium and radium isotopes: a powerful tool to...

Unexpected magnitude of the hydrothermal iron inputs in the deep Pacific

Data from the US GEOTRACES Eastern Pacific Zonal Transect (EPZT, GP16) demonstrate that lateral transport of hydrothermal iron, manganese and aluminium is extending up to 4000 km west of the southern East Pacific Ridge, therefore crossing a significant part of the deep Pacific Ocean. The dissolved iron behaves conservatively, the resulting flux is more than four times what was assumed before. A coupled ocean circulation/biogeochemical modelling demonstrates that this hydrothermal iron input is sustaining a large fraction of the Southern Ocean export production.

Nature decided to largely promote this work by reporting a GEOTRACES 3D view of the bottom Pacific showing the hydrothermal vent as a cover. Congrats to the EPZT team!

15 Resing l
Figure:
The top three panels show concentrations of dissolved iron, manganese and aluminum measured during the voyage. The bottom panel shows concentration of a form of helium that marks the water as coming from a hydrothermal vent, and its decreasing concentration away from the ridge reflects mixing rather than a chemical reaction. Click here to view the image larger. Credit: J. Resing / Univ. of Washington.

Read more: Unexpected magnitude of the hydrothermal iron inputs in the deep Pacific

The potential of coupling high-resolution circulation models and geochemical tracers: example of the Mediterranean Sea

For the first time, a numerical study proposes the simulation of the anthropogenic tritium (3H) invasion as well as the distribution of its decay product helium-3 (3He) using a high-resolution regional circulation model (NEMO 1/12). Comparison with the numerous set of data acquired in the framework of different Mediterranean Sea transects conducted on the last 50 years underline the good performances of the model in simulating the Mediterranean Sea thermohaline circulation, and more specifically, the Eastern Mediterranean Transient (EMT), the intermediate water ventilation and the transports following winter convection in the Gulf of Lions. Contrastingly, the Adriatic Deep Water formation, as well as, the Western Deep Water propagation are too weakly reproduced while the Levantine Intermediate Water is too strong in the eastern basin. These results demonstrate how fruitful are such coupling between geochemical tracers and circulation models.

15 Ayache
Figure:
The tracer simulation together with observations made during the Poseidon 234 and Meteor 84/3 cruises in 1997 and 2011, has allowed to evaluate the formation of deep water and its associated temporal and spatial variability, and to study its impact on the deep water mass renewal over the basin: The same vertical gradient of tritium observed in 1997 is marked in 2011 in the WMed, with lower concentrations in the water column following the drastic reduction of tritium in the atmosphere after the stopping of the atmospheric nuclear weapons testing. Helium-3 maximum in LIW is deeper (about 1000 m) in 2011, with a significant accumulation of helium-3 in the deep water compared to 1997, which is due to continued tritium decay on the way. This evolution of the tracer signals was successfully simulated by the model indicating that the conversion of LIW in the western basin is well simulated.

Reference:

Ayache, M., Dutay, J.-C., Jean-Baptiste, P., Beranger, K., Arsouze, T., Beuvier, J., Palmieri, J., Le-vu, B., Roether, W. (2015). Modelling of the anthropogenic tritium transient and its decay product helium-3 in the Mediterranean Sea using a high-resolution regional model. Ocean Science, 11(3), 323–342. doi:10.5194/os-11-323-2015. Click here to access the paper.

The sedimentary flux of dissolved rare earth elements to the ocean

This work highlights the importance of the sedimentary source of dissolved Rare Earth Elements (REE) to the oceanic waters. Indeed, strong subsurface REE concentration maxima are evidenced in pore fluids in core tops collected along the Californian and Oregon margins (above, within and below the oxygen minimum zone of the North-East Pacific). Diffusive flux of neodymium (Nd) out of the sediments matches preceding estimates of the “missing term” of Nd in modeled global Nd budgets. Also interesting, the decoupling between REE and iron fates in these pore fluids...

15 Abbott l
Figure:
Site locations and the associated pore fluid profiles. Neodymium (Nd) and iron (Fe) profiles plotted against sediment depth in pore fluids from (clockwise from top left) the Oregon shelf, the Oregon slope, and the California shelf. Filled symbols represent sites unique to this study and open symbols are sites from prior expeditions. Rivers are indicated in blue and labeled. Click here to view the figure larger.

Reference:

Abbott, A. N., Haley, B. A., McManus, J., & Reimers, C. E. (2015). The sedimentary flux of dissolved rare earth elements to the ocean. Geochimica et Cosmochimica Acta, 154, 186–200. doi:10.1016/j.gca.2015.01.010. Please click here to access the paper.

Neodymium isotopic signature of the Ross Sea Water characterized

The first seawater neodymium isotopic compositions (εNd) and neodymium concentrations [Nd] profiles across the South Pacific circum-Antarctic fronts have been published recently (Basak et al., 2015, see reference below). Thanks to this exceptional GEOTRACES-compliant data, collected on R/V Polarstern cruise PS75, authors characterize the εNd signature of Ross Sea Bottom Water (εNd ~ -7) and show that meridional Nd concentrations changes follow the density structure of the South Pacific. The latter suggests a lateral transport component for the processes controlling Nd concentrations in the Southern Ocean rather than vertical processes.

15 Basak l
Figure: Distribution of εNd and [Nd] across the South Pacific frontal system (map) from the Ross Sea into the southeast Pacific. Left: Distribution of dissolved εNd; right: Distribution of [Nd] with neutral density contours. Click on the following links to view the figures larger: map, distribution of dissolved εNd and distribution of [Nd].

Reference:

Basak, C., Pahnke, K., Frank, M., Lamy, F., & Gersonde, R. (2015). Neodymium isotopic characterization of Ross Sea Bottom Water and its advection through the southern South Pacific. Earth and Planetary Science Letters, 419, 211–221. doi:10.1016/j.epsl.2015.03.011

First major ocean sections of silicon isotopes

The GEOTRACES programme is providing the first major ocean sections of silicon isotopes, δ30Si, aiding efforts to use this proxy to reconstruct diatom silica production in both the modern and paleo ocean. In the May, 2015 issue of Global Biogeochemical Cycles Holzer and Brzezinski explore the links between δ30Si within silicic acid and the meriodional overturning circulation using a restoring-type model of the silicon cycle with a data-assimilated, annual mean circulation model. They find the Southern Ocean to be the primary origin of both performed and regenerated δ30Si. The Southern Ocean Si trap sets the light isotopic signature of bottom waters, with heavy isotopes distilled out of the trap through mode water formation. Hydrographic control of δ30Si is important in the Atlantic where the horizontal gradients are dominated by preformed silicic acid. Regenerated silicic acid influences vertical gradients in the Atlantic and the overall δ30Si distributions in other ocean basins.

15 brzezinski l
Figure:
(Left panels) The isotope ratios, δ30Sipre and δ30Sireg, of the preformed and regenerated Si(OH)4 at ~2000m. (Right Panels) The preformed and regenerated Si(OH)4 fractions, fpre and freg. Note fpre δ30Si pre + freg δ30Sireg reconstructs the observed isotope value δ30Si (not shown). Click here to view the figure larger.

Read more: First major ocean sections of silicon isotopes

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