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.).

What is controlling the copper isotopic composition in oceanic waters?

Takano and co-workers (2014, see reference below) strongly suggest that the isotopic composition of dissolved copper (δ65Cu) in surface seawater is mainly controlled by supply from rivers, the atmosphere and deep seawater. This is the conclusion of a study involving six vertical profiles of copper (Cu) concentration and isotopes measured in the Indian (1) and North Pacific (5). The finding contradicts preceding interpretations suggesting a strong role of the biological activity in δ65Cu fractionation.

At depth, δ65Cu  values are becoming heavier with the age of deep seawater, likely due to preferential scavenging of the light isotope (63Cu). The authors built a box-model to quantify the oceanic budgets of both Cu concentrations and δ65Cu. Unbalance in this model suggests that Cu fluxes from continental shelf sediment might affect Cu distribution in the open ocean.

14 Takano lFigure:  A box-model of Cu in the ocean based on both Cu concentration and isotopic composition. Click here to view the figure larger.

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Seasonal iron supply in the Southern Ocean is dominated by winter mixing

An international team of researchers analysed the available dissolved iron data taken from all previous studies of the Southern Ocean, together with satellite images taken of the area, to quantify the amount of iron supplied to the surface waters of the Southern Ocean. They found that in contrast to the processes that supply so-called macronutrients in the tropics, seasonal iron supply is dominated by winter mixing with little iron input afterwards. This is because the vertical profile of iron is distinct from other nutrients, with subsurface reserves located much deeper in the water column and therefore only accessible by the deeper mixing that occurs in winter. This means that after this input pulse, intense iron recycling by the 'ferrous wheel' is necessary to sustain biological activity. This unique aspect of iron cycling is yet to be explained but places important constraints on how climate models represent the iron distribution and how changes in ocean physics impact iron limitation.

14 Tagliabue2 lFigure. This diagram represents the seasonal variability in Southern Ocean iron (Fe) cycling.
Click here to view the figure larger.

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Why is the deep ocean zinc isotopic signature so heavy?

The oceanic balance of the micronutrient zinc (Zn) is getting more puzzling –although better documented- while the Zn isotope data set increases. Presenting the first high resolution section of seawater dissolved Zn concentration and Zn isotope ratios (δ66Zn) from the North Atlantic, Conway and John (2014, see reference below) confirm that the deep ocean is fairly homogeneous for δ66Zn, close to +0.5‰, except near local North Atlantic sources of Zn (margins, the Mediterranean Sea, hydrothermal vents) where it is isotopically lighter.

Balancing the Zn isotopic budget raises questions. Indeed, the known inputs of this element (continental, riverine, and aerosol) display δ66Zn ranging between 0 and +0.3‰ while the known outputs (carbonates, ferromanganese nodules, and ferromanganese crusts) are isotopically heavier (+0.9 to +1‰). As previously suggested by Little et al. (2014, see also highlight about it), an isotopically light sink is therefore missing. The authors suggest that burial of biogenic Zn in sediments might be this important sink. They also suggest a potential role for zinc sulfide (ZnS) precipitation in low-oxygen open-ocean waters as a possible light sink, analogous to recent studies on Cd (Janssen et al., 2014, see also highlight about it).

14 Conway2 l
Figure.
Distribution of dissolved stable Zn isotope ratios (δ66Zn) for the US GEOTRACES North Atlantic GA03 and GA03_e sections. δ66Zn is expressed relative to JMC Lyon standard. Station numbers are shown for 2010 (USGT10, red) and 2011 (USGT11, blue). The black vertical line denotes the crossover between the cruises at USGT10-12 and USGT11-24. Click here to view the figure larger.

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Present day neodymium isotopic composition of the Caribbean Sea deep waters questions the paleo-application of this tracer in restricted basins

The first profiles of neodymium (Nd) concentration and isotopic composition in the Carribbean Sea have been published. They show that surface and intermediate waters flow through the Caribbean with essentially unchanged Nd isotopes ratios (εNd), whereas deep waters are strongly modified. Indeed, they likely receive radiogenic Nd released from the local sediments, of volcanic origin. Osborne and co-authors (2014, see reference below) suggest that this important shift is facilitated by the long residence time of these deep waters (150 years). This finding has general implications for paleoceanographic studies in restricted basins, where the composition of seawater is sensitive to its residence time within the basin.

14 Osborne l
Figure. Schematic cross-section of the Caribbean and Gulf of Mexico basins, showing the Nd isotopic composition and concentration of inflowing Atlantic water and how it changes to more radiogenic compositions and higher concentrations within the deep Caribbean. Interaction of seawater with radiogenic Nd from the margins of the Caribbean may be responsible for this change, aided by the slow replenishment rate and long residence time of deep waters in the Caribbean and Gulf of Mexico. Click here to view the figure larger.

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The sources of the water soluble organic matter contained in the aerosols over the Atlantic ocean decoded

Organic matter is an important component of aerosols, which can absorb (or scatter) light, contributing a warming (or cooling) effect to the atmospheric radiative budget. However, this impact is tightly linked to the molecular characteristics of aerosol organic matter. It is, therefore, of prime importance to establish the organic matter molecular details of aerosols from different sources.

This characterization is the aim of Wozniak et al. paper (2014, see reference below). Based on aerosols samples collected in the framework of the North Atlantic US GEOTRACES GA03 cruise, the authors analyzed their water soluble organic matter (WSOM) molecular characteristics using an ultrahigh-resolution analytical method*. Multivariate statistics allowed the identification of five sources with very different distinguishing WSOM characteristics, enlightening the origin of these different aerosol components.

*electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. 

14 Wozniak l
Figure.
Three dimensional plots showing the principal component analysis a) scores for samples from the five aerosol emission sources and the b) molecular formula loadings that distinguish each source from the others. Click here to view the image larger.

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Organic copper complexation may stabilise seawater stable copper isotopic composition

Three deep-sea profiles were produced for the analysis of copper (Cu) concentration, along a transect covering very different biogeochemical regions: the oligotrophic North Tasman Sea (30ºS), the Tasman Front (40°S) and the productive waters of the Southern Ocean in the south (46°S).

Despite these differences, the Cu isotope composition of all three profiles was relatively homogenous. This homogeneity is attributed to the fact that more than 99% of the Cu is organically complexed, measured as part of the same study (Thompson et al, 2014; see references below). It is therefore argued that organic complexation stabilises heavy values of seawater stable copper isotopic composition (δ65Cu).

The authors also propose that decomposition of organic Cu complexes in environments such as anoxic basins may provide an isotopically heavy source of Cu for further scavenging and/or removal to the sediments. Such mechanism would help to balance the oceanic budget of δ65Cu, discussed in Little et al, 2014 (see reference below, and GEOTRACES science highlight).

14 Thompson lFigure: Three dissolved copper concentration profiles versus depth (left panel) along with the isotope composition for dissolved copper (right panel). Samples were collected from three stations (P1, P2 and P3) occupied in the Tasman Sea region.

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Overview of the dissolved iron, manganese and aluminium distributions along the North Atlantic GEOTRACES GA03 section

Three trace elements distributions (iron, manganese and aluminium) help to constrain the sources of chemical elements in the North Atlantic Ocean. Atmospheric, Mediterranean Sea and margin inputs are confirmed while the importance of the hydrothermal venting is revealed.

Hatta, Measures and co-workers (2014, see references below) established an overview of dissolved iron, manganese and aluminium (dFe, dMn and dAl respectively) distributions (see figures below) along the North Atlantic GA03 GEOTRACES section. Elevated dFe concentrations correlate with elevated dAl ones in the surface waters of the subtropical gyre, confirming a substantial atmospheric source for both tracers. But this is not the case for dMn. Sedimentary inputs from margins concern the three tracers but are mostly revealed from elevated dMn signals in the eastern basin, particularly near the African coast and in the western basin, along the advective flow path of the Upper Labrador Sea Water.

The most striking results are found in the neutrally buoyant hydrothermal plume sampled over the Mid-Atlantic Ridge. There the largest dFe anomaly (~68 nM), a dMn anomaly (up to ~33 nM) and large amount of Al (up to 40nM) are detected, with signals visible for ~500 km to the west of the ridge.

14 hatta measures final lFigure. Distribution of dissolved iron (upper), aluminiun (middle) and manganese (lower) along the North Atlantic GA03 GEOTRACES section. Warm colours (red, orange, etc.) indicate high concentrations. Click here to view the figure larger.

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Field data allow constraining total mercury budget

Thanks to recent measurements during several oceanographic expeditions, among them GEOTRACES cruises, estimates of the total amount and spatial distribution of anthropogenic mercury in the global ocean were substantially improved.  

Global budgets of total mercury suggest that there has been a tripling of the surface water mercury content and a ~150% increase in the amount of mercury in thermocline waters.

This study has been recently published in Nature Journal (1).

14 Lamborgv2
Figure: GEOTRACES researchers led by Carl Lamborg found that antropogenic mercury (primarily atmospheric emissions produced by coal burning and cement production, as well as gold mining) have caused ocean waters down to 100 meters depth being enriched in the toxic element up to 3.5 times the background level resulting from the natural breakdown, or weathering, of rocks on land. Once in the ocean, mercury adheres to organic particles and sinks or is consumed by progressively larger marine animals. One result is that intermediate levels of the ocean (between 100 and 1,000 meters depth) are also enriched in mercury up to 2.5 times the natural background rate. Even the deepest parts of the ocean have not escaped unscathed. Researchers found signs of pollution-derived mercury in the North Atlantic at depths below 1,000 meters, but those levels decreased as sampling efforts moved away from the North Atlantic basin. This is likely because pollution mercury has not yet moved with deep ocean currents throughout the global ocean, a process that can take as long as 1,000 years (extracted from WHOI's press release). Artwork: Jack Cook, WHOI. Click here to view the figure larger.

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