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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.).

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.

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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...

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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.

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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.

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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

Insights into Particle Cycling from Thorium and Particle Data

Phoebe Lam and Olivier Marchal (2015, see reference below) propose to describe, with the same model, the dynamics of particles in the oceanic water column and its effects –on four different tracers characterized by very distinct sources and sinks. The considered tracers are: Particulate Organic Carbon (POC) (a biogenic compound), barium (Ba) (an authigenic mineral), titanium (Ti) (mainly lithogenic), and thorium isotopes (a particle-reactive radionuclide). Thorium isotopes are used for the estimation of exchange rates between small and large particles and for the estimation of particle settling velocities.  Lessons learned from thorium isotopes may be applied to understand other classes of particle tracers such as POC, Ba, and Ti.

Main results:

  • The separation of oceanic particles in two distinct classes (small, suspended particles and large, sinking particles), which interact throughout the water column via aggregation and disaggregation processes, remains a useful description of particle cycling, provided its limitations are fully appreciated.
  • The simple models currently used in marine particle research (small particles are suspended and interact with seawater, large particles are removed by sinking, small and large particles interact throughout the water column, ...) allow one to reproduce the observed vertical distributions of a range of chemical substances in the ocean, such as POC, Ba, Ti, and 230Th, in spite of their distinct sources and sinks as well as reactivities.

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Figure: Schematic depiction of the biological carbon pump, emphasizing the important particle dynamics processes: aggregation (red arrows ), sinking (black arrows), disaggregation (dark blue arrows), and remineralization (light blue arrows). Particles in the small, suspended size fraction (brown) comprise phytoplankton, authigenic particles, and lithogenic particles and do not sink  or sink very slowly. Particles in the large, sinking size fraction (green) comprise fecal material and aggregates of smaller particles and do sink. Aggregation can be abiotic or mediated by zooplankton packaging through fecal pellet production. Click here to view the figure larger.

Read more: Insights into Particle Cycling from Thorium and Particle Data

Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean

Understanding persistent high levels of mercury in arctic biota has been an elusive goal for nearly two decades. Little is known about where exactly inorganic Hg inputs into the Arctic generate the toxic methylmercury (MeHg) form that bioaccumulates in biota. Lars-Eric Heimbürger and colleagues (2015, see reference below) present the first full-depth high resolution profiles (> 5200 m-depth) of total mercury (tHg) and MeHg in the central Arctic Ocean (79-90°N). MeHg maxima occur in the pycnocline waters, although noticeably shallower than in the other oceans (150 m in the Arctic versus roughly 1000 m in the Atlantic). These shallow maxima are probably due to the accumulation of settling biogenic particles slowed down by the strong density barrier of the arctic pycnocline, which in turn will favor their microbial degradation and MeHg production. The shallow MeHg maxima likely result in enhanced biological uptake at the base of the marine food web, yielding elevated MeHg levels in Arctic wildlife. For this study the authors developed a new double isotope-dilution MeHg detection method with exceptional precision and low detection limit. These new findings will be guiding future Arctic Hg research, notably the international Arctic GEOTRACES multi-ship survey planned for summer 2015 by American, Canadian and German teams.

15 Heimburger lFigure: Total mercury (tHg) and methylmercury (MeHg) profiles in picomoles per litre (pM) at the coastal influenced open water Laptev Sea station (PS78/280:79°N; brown triangles), the open water Amundsen Basin station at the sea ice edge (PS78/273:81°N; red dots), the > 75% sea ice covered Makarov Basin station (PS78/245:85°N; green squares), and the permanently sea ice-covered North Pole station (PS78/218:90°N, purple diamonds). The white line indicates the sea ice extent during the time of sampling. The blue line shows the general oceanic circulation of intermediate and Atlantic waters after Rudels, 2012. Click here to view the figure larger.

Read more: Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean

Thorium and protactinium radionuclides reveal marine particles processes

Thanks to the compilation of thorium 230 (230Th) and protactinium 231 (231Pa) data obtained along north-south and east-west transoceanic sections (in the Atlantic and Pacific Oceans), Hayes and co-workers (2015, see reference below) enlighten the processes at play among the lifestyles of marine particles, namely the adsorption to particles (scavenging) of these tracers. This systematic study is of first importance because both tracers are useful chronometers of particle dynamics and can be used to predict the adsorption of other insoluble trace metals in the water column. They find a widespread impact of authigenic Fe/Mn hydroxides as major scavenging phases (be it of hydrothermal or deposited sediment origin). They also show that biogenic opal is not a major scavenging phase due to the low abundance of opal in the water columnThey eventually discuss why the distribution coefficients are surprisingly low in surface water, but if you wish to know more…read it!

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Figure: With samples from the U.S. North Atlantic Zonal Transects (upper map), we investigated how the radionuclides Th and Pa “stick” onto particles by analyzing their dissolved (bottom section) and particulate (middle section) phases in seawater, along with the amount and chemical composition of the particles. The ratio of Pa to Th in ocean sediments can be used as a proxy for properties of the ocean’s geologic past, but this ratio fundamentally depends on the relative intensity of each metal’s “stickiness” or tendency to adsorb onto various particles. Among many observations, we found the Pa/Th ratio was largely driven by the total amount of particles (top section) but was also significantly affected by a relatively minor mineral phase, the iron-manganese oxides forming at the Mid-Atlantic ridge and in West African margin sediments. Click here to view the image larger.

Read more: Thorium and protactinium radionuclides reveal marine particles processes

Submarine Groundwater Discharge as a major source of nutrients in the Mediterranean Sea

The authors are proposing a radium-228 (228Ra) isotope mass balance for the upper Mediterranean Sea to estimate the magnitude of Submarine Groundwater Discharge (SGD) to this basin. Radium isotopes have been widely used as tracers of SGD, mainly because they are enriched in coastal groundwater relative to seawater and behave conservatively once released to the ocean. Naturally decaying with time, they are also good chronometers of processes at play at the continent-ocean interface. 228Ra was also recently used to establish the amount of SGD to the Atlantic and world oceans (Kwon et al., 2014; see here the highlight about this paper). Here, the authors conclude that SGD is a volumetrically important process in the Mediterranean Sea and of larger magnitude than riverine discharge. Importantly, they also demonstrate that SGD is a major source of dissolved inorganic nutrients to the Mediterranean basin, with fluxes comparable to riverine and atmospheric inputs.

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Figure: Interpolated concentrations of 228Ra in surface waters of the Mediterranean Sea (high concentrations are indicated in warm colours red, orange, etc.). Click here to view the figure larger.

Read more: Submarine Groundwater Discharge as a major source of nutrients in the Mediterranean Sea

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