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.).

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.

15 Jeandel l
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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How iron isotopes offer a new window on the oceanic biogeochemical cycling of iron

Two process studies (2008 and 2012) designed to study temporal changes in the biogeochemical cycling of iron (Fe), provide in situ results for dissolved and particulate Fe (DFe and PFe) cycling during the annual spring bloom.

These field data have been complemented by a shipboard 700-L mesocosm incubation experiment. Later, a conceptual model helps figuring out the key chemical and biological processes involved in Fe isotope fractionation.

This works demonstrates that DFe is isotopically lighter than PFe at the beginning of the spring, likely reflecting the reduction of PFe by photochemistry and bacteria-mediated reduction processes. As the bloom develops, DFe within the surface mixed layer becomes isotopically heavier, consistent with the preferential uptake of light iron by phytoplankton while scavenging appears to play a minor role in fractionating Fe.

15 Ellwood l
Figure.
This figure shows the increase in chlorophyll concentration across three different stages of the subtropical phytoplankton bloom (upper panels), along with depth profiles for dissolved iron and nitrate (middle panels), and the iron isotope composition of dissolved and particule iron (lower panels). Disssolved iron (DFe) is isotopically lighter (negative values) than particulate iron (PFe) at the beginning of the spring (stage I). As the bloom develops, DFe within the surface mixed layer becomes isotopically heavier (more positive values), consistent with the preferential uptake of light iron by phytoplankton. Click here to view the image larger.

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Radium quartet reveals no less than four main processes along the GEOTRACES North Atlantic Ocean section (30°N)

The four radium (Ra) isotopes (224Ra, 223Ra, 228Ra, 226Ra, "radium quartet") are produced in situ via decay of their insoluble  thorium isotope parents in sediments from the continental margins and deep-sea and then released to the ocean. In the ocean, their distributions are controlled by particle removal (226Ra) and radioactive decay with four different half-lives. These properties make the "quartet" an invaluable tracer of coast-to-ocean processes and a water mass spreading chronometer.

Thanks to a dense and beautiful data set documenting the radium quartet along the 30°N GEOTRACES US section (GA03), Charette and co-authors (2015, see reference below) were able to identify:

  • a Mediterranean outflow spreading rate of 0.52-0.60 cm/s derived from 228Ra,
  • evidence of substantial sediment/water interaction in the benthic boundary layer along the oxygen minimum zones
  • decoupling between 223Ra and the other Ra isotope sources over the mid-Atlantic Ridge, and
  • significant continental inputs (e.g. submarine groundwater discharge) in the western Atlantic.

Last but not least, they conclude that the 228Ra inventories in the upper water column have remained constant over the past 40 years, which suggests that submarine groundwater discharge (the primary 228Ra source) is steady-state for the North Atlantic on decadal time scales.

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Figure. Box average 0-1000 m inventories (15° x 15°) of 228Ra (x 1010 atoms m-2) for samples collected on the GEOTRACES Atlantic section (GT) (2010-2011) and the Transient Tracers in the Ocean cruises (TTO) (1981-1986). Each solid black dot is a TTO station, each red x is a GT station. Click here to view the figure larger.

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When direct mapping of diatoms reveals unexpected fate of trace metals in the twilight zone

Twining and co-authors (2014, see reference below) used synchrotron x-ray fluorescence mapping to measure macronutrients such phosphorus (P), sulphur (S), and silicon (Si), and also trace metals like iron (Fe), nickel (Ni) and zinc (Zn), in individual cells of a diatom specie during a spring bloom off New Zealand. They clearly show that P, S, Zn and Ni are released faster than Fe and Si from sinking cells in the upper 200 m. Although the metals are co-located with P and S at the surface, the scheme changes deeper. The relationships with P and S become weak while an association of Fe with Si appears, suggesting re-adsorption when particles are settling. Exciting results revealing that ratios of dissolved Fe to macronutrients in the water column likely underestimate stoichiometries in sinking cells.

14 twining l
Figure.
Element maps (P, S, Fe ,and Zn) and associated scatterplots of Fe and S concentrations in each pixel of the scans for two diatom cells collected from 30m or 200m following a spring bloom off New Zealand.  The scatterplots show that Fe and S are spatially decoupled from each other when the diatom cells degrade as they sink through the upper water column.  S is lost more readily from the cells, while Fe appears to be retained or is re-scavenged.  Scale bar indicates 10um for each cell.  Adapted from Twining et al. (2014). Click here to view the figure larger.

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