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.).

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.

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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.

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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.

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Two GEOTRACES works question the present ocean chemical elements budgets

New revelations on the land-ocean flux of chemical elements are presented in two different papers highlighting the value of the ocean isotopic data. Both works reveal that the present quantification of the oceanic cycle of the chemical elements need to be revisited...a big question to dig in!

When radiogenic and trace metal isotopes suggest that the land-ocean flux of elements have to be revisited

An extensive review of our present understanding and quantification of the oceanic budgets of the elements is proposed by Jeandel & Oelkers (2015, see reference below). Thanks to the international GEOTRACES programme (among others) isotopic data are currently acquired in the ocean and in its tributaries (rivers, estuaries...). Isotopes provide additional constraints to the oceanic budgets of the chemical species, and often reveal that these budgets are imbalanced. While this was already demonstrated for radiogenic Sr and Nd isotopes, recent data on Mo, Zn, Ni, Fe isotopes show that their budgets are also imbalanced. Since the processes yielding this disequilibrium are not clear yet, the possibility of an under-estimated input term has to be considered. The authors suggest that the term that would close the ocean isotope budgets could be, as for Nd and Sr, the dissolution of particles of terrigeneous origin.

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Figure: 
Dissolved Si budget of the Mediterranean Sea, in 1010 mol/year (Ribera d'Alcala et al., 2003; Durrieu de Madron et al., 2009; Jeandel et Oelkers, 2015). It is estimated that 28.8 to 42.4 x 1010 mol/year of Si are released from the Mediterranean Sea to the Atlantic Ocean, but only 5.1 to 12.7 x 1010 mol/year are returned. It is not possible to balance this flux by considering dissolved river and atmospheric inputs alone. The authors suggest it requires the input from the dissolution of continentally derived particulates.

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A new method to measure lead isotopes in the ocean with an outstanding precision

A new method for the determination of seawater lead (Pb) isotope compositions and concentrations was developed, which combines and optimizes previously published protocols for the separation and isotopic analysis of this element. It involves 1 to 2 L of seawater, double spike, magnesium hydroxide coprecipitation, anion exchange chromatography and thermal ionization mass spectrometry. Ratios involving the minor 204Pb isotope are a factor of five more precise than previously published data, yielding uncertainties better than ±3‰. Results are presented on GEOTRACES intercalibration samples and a first depth profile from the eastern South Atlantic Ocean.

15 Paul lFigure: Methodology to separate and analyse Pb isotopes and concentrations from seawater samples using a 207Pb-204Pb double spike and thermal ionisation mass spectrometry (TIMS). Click here to view the figure larger.

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How to constrain the biogeochemical cycle of cobalt in the surface Western Atlantic Ocean?

Cobalt (Co) is an important micronutrient for many species living in the surface waters (bacteria, cyanobacteria, different types of phytoplancton...). Interestingly, the Co requirements vary between the different species and these variations are influencing the dissolved Co distribution in the surface waters. This is clearly demonstrated by the set of data published by Dulaquais and co-authors (2014, see reference below) along a GEOTRACES north-south section in the western Atlantic Ocean (GA02). This work also shows that recycling sustains the biological requirement for cobalt in subtropical domains, and that both atmospheric and Amazon inputs affect the Co distribution.

15 Dulaquais lFigure: Interpolated concentrations of dissolved cobalt overlaid with phosphate (µM; white contours) in the upper 1000 m of the West Atlantic during the GA02 section. Click here to view the figure larger.

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New neodymium data in the North East Atlantic Ocean allow progressing on the behaviour of this geochemical tracer

High-resolution vertical profiles of neodymium (Nd) concentrations and isotopic compositions were measured at the eastern part of the US GEOTRACES North Atlantic Zonal Transect (GA03, Gulf of Cadiz – Mauritanian Shelf – Cape Verde Islands). This allowed documenting impacts of different environmental settling on the tracer's behaviours: the Mediterranean Outflow Waters (MOW), the Saharan dust plume, the Mauritanian margin with nepheloid layers and an Oxygen Minimum Zone (OMZ).

Distinguishing water masses in this oceanic area was a difficult task due to the relative uniformity of their Nd isotopic composition. Except in the Gulf of Cadiz, where MOW displays distinctively more radiogenic values than the surrounding waters (indicating the advection from the western North Atlantic Central Water from the West Atlantic).

Other striking features are: highest Nd concentrations below the Saharan dust plume with non radiogenic Nd; first documented release of mostly scavenged Nd within an OMZ; and, a prominent benthic nepheloid layer at the bottom of the Mauritanian margin, which favours the release of non radiogenic Nd. Thorough treatments of these data allow disentangling horizontal transports from biogeochemical processes in this area.

15 StichelFigure: Map of the cruise track and stations with a section along GA03 (Cape Verde Islands – Mauritanian Shelf – Gulf of Cadiz, from left to right) showing measured Nd concentrations (colour shading) overlain by the ratio of expected to actual Nd concentrations (iso-lines). Values >1 indicate excess of Nd with respect to water mass mixing, which is particularly the case within the OMZ at ~500m between section distance 0 and ~2500km. Click here to view the figure larger.

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What drives the silicon budget in the Bay of Bengal? The isotope composition clues...

The first data set of dissolved silicon isotope composition (δ30Si) along with concentrations (DSi) in seawater of the northern Indian Ocean is presented from the Bay of Bengal (BoB) region.

Elevated Si (>3 µmol/kg) in surface waters of coastal stations indicates the continental supply, whereas a spike of Si (~30 µmol/kg) and a salinity maxima at depth 60 m of the southernmost station hint at intrusion of the Arabian Sea High Salinity waters. In the central bay, higher δ30Si in surface waters indicates greater utilisation of the available Si via diatom production. DSi and δ30Si in surface waters of the BoB vary dramatically in response to the Si supply and its consumption through biological production.

Modelling δ30Si in the deep/bottom waters of the BoB hints at dissolution of diatoms rather than lithogenic clays at/near the sediment­–water interface as the main cause of the elevated Si concentrations in the bay.

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Figure.
Depth profiles of dissolved Si concentrations (left panel) and δ30Si (right panel) are shown for the coastal stations 0814–0820 (upper panel) and open ocean stations 0806–0813 (lower panel). Locations of coastal and open ocean stations are also given. Click here to view the figure larger.

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