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.).

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).

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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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A new model of the oceanic aluminium distribution

Taking into account most of the parameters that govern any trace element's oceanic behaviour is a challenge, given their number and complexity.

Marco van Hulten and co-workers (2014, see reference below) propose here the most complete model ever written for the oceanic aluminium (Al) distribution. In addition to atmospheric input -which was the only term constraining the Al distribution in a preceding model, see van Hulten et al., 2013-, circulation, sediment re-suspension and biological incorporation by diatoms are considered in this new scheme.

These new sources and sinks are significantly improving the simulated distribution, more specifically a sediment source of Al in the bottom waters of the Northern Atlantic and the velocity fields.

14 vanHulten lowFigure: The dissolved aluminium concentration (nM) of a global model simulation of aluminium. The circles are the observations. The sources in the model comprise the release of Al from dust and from resuspended deep-ocean sediments, the latter depending on the bottom Si concentration. Al is removed by reversible scavenging by biogenic silica. (This figure may be reused, changed and redistributed according to Creative Commons BY-SA. Click here to view the figure larger.)

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Impact of volcanic ash on marine algae and the global carbon cycle

Volcanic ash fertilization of iron-limited phytoplankton in remote marine waters has been suggested to perturb global biogeochemical cycles and climate. For example, ash from the Pinatubo (Philippines) eruption in 1991 was suggested to have fertilized vast areas of the iron-limited Southern Ocean - potentially causing the drawdown in atmospheric carbon dioxide observed subsequently. However, until recently the impact of volcanic ash on phytoplankton communities in the Southern Ocean had never been directly tested.

Browning and co-authors conducted over 20 experiments in the South Atlantic and Southern Ocean where they added small quantities of volcanic ash to natural phytoplankton communities incubated in bottles. The responses they observed led to two important findings: (i) they conclusively demonstrated for the first time that volcanic ash deposition events strongly stimulated phytoplankton in the Southern Ocean; and (ii) at several experimental locations phytoplankton responded significantly to supply of volcanic ash, but not to iron only. This latter finding could be particularly important as it suggests phytoplankton at these sites may have been limited by another micronutrient other than iron. Manganese concentrations at these sites were amongst some of the lowest ever recorded in seawater and Browning and co-authors therefore suggested that the enhanced response to ash may have likely been a result of relieving manganese (co)limitation.

Both of these findings could both have important implications for our understanding of marine biogeochemistry in the Southern Ocean. Firstly, the Southern Atlantic and Drake Passage, where the experiments were conducted, are areas highly prone to ash deposition from explosive volcanic eruptions in South America - suggesting that ash-driven fertilization and potential carbon export from these waters could be an important control on the biogeochemistry of the region. Secondly, if manganese is (co)limiting marine algae in these waters, addition of this element alongside iron might be critical for stimulating phytoplankton blooms in the region.

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Figure: Maps showing the sites where experiments were conducted, highlighting the nutrient concentrations measured in seawater (warmer colors represent higher nutrient concentrations) and the response of phytoplankton to iron and ash additions (warmer colors represent larger phytoplankton responses). For (d-e) sites where the phytoplankton response was statistically significant (relative to bottles where no treatment was made) are shown with black outlines. Please click here to view the figure larger.

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A geochemical-physical coupled approach to study phytoplankton plume dynamics off the Crozet Islands (Southern Ocean)

Interaction of the currents with the sediments deposited on the margins of the Crozet Islands (Southern Ocean) contributes to the supply of iron and other micronutrients to marine waters. This natural fertilization feeds a phytoplankton bloom that was object of study of the KEOPS 2 GEOTRACES process study.

Sanial and co-authors (2014, see reference below) combined three independent methods - including geochemical and physical methods. This allowed them to assess the origin of the natural iron fertilization and the rates and times scale of the offshore transport in the phytoplankton bloom of Crozet. Shelf-water contact ages were determined using radium isotopes and were compared to in situ drifter data and modeling data based on altimetry.

This work highlights the key role played by the horizontal transport in the natural iron fertilization and provides constraints on the transit time of surface waters between the shelf and offshore waters.

14 Sanial lowFigure: Ages of surface waters derived from a Lagrangian model based on altimetry data. The drifter launched offshore Crozet Islands followed the numerical plume deduced from the model. White circles show the location of radium samples. Please click here to view the figure larger.

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The distribution of dissolved iron in the West Atlantic Ocean

Iron (Fe) is an essential trace element for marine life. Extremely low Fe concentrations limit primary production and nitrogen fixation in large parts of the oceans and consequently influence ocean ecosystem functioning. In a publication published on 30 June in Plos ONE, Rijkenberg and co-authors present dissolved Fe (DFe) values measured at an unprecedented high intensity (1407 samples) along the longest full ocean depth transect (17500 kilometers) covering the entire western Atlantic Ocean.

DFe measurements along this transect revealed details about the supply and cycling of Fe. External sources of Fe identified included off-shelf and river supply, hydrothermal vents and aeolian dust. Nevertheless, vertical processes, such as the recycling of Fe resulting from the remineralization of sinking organic matter and the removal of Fe by scavenging, dominated the distribution of DFe. Iron recycling and lateral transport of DFe from the eastern tropical North Atlantic Oxygen Minimum Zone (OMZ) were important sources of DFe to the northern West Atlantic Ocean.

Finally, this study showed that the North Atlantic Deep Water (NADW), the major driver of the so-called oceanic conveyor belt, contains excess DFe relative to phosphate after full biological utilization and is therefore an important source of Fe for biological production in the global ocean.

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Figure
: The distribution of DFe along the 17500 km long full depth transect in the western Atlantic Ocean. 
Click here to view the figure larger.

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Dissolved iron sources in the North Atlantic Ocean quantified

The relative importance of four different dissolved iron (Fe) sources in the North Atlantic Ocean have been precisely determined for the first time thanks to GEOTRACES.

Using a novel method based on the stable isotopic composition of dissolved Fe, Conway and John (2014, see reference below) have "fingerprinted" different sources of Fe along a section in the North Atlantic Ocean (GEOTRACES GA03 section). This has allowed the scientists to determine precisely the relative contribution of these sources to the North Atlantic Ocean. They found that the dominant sources were Saharan dust, which contributes 71-87 per cent of dissolved iron, followed by North American margin sediments (10-19 per cent). Smaller contributions were observed from the African margins (1-4 per cent) and hydrothermal venting at the Mid-Atlantic Ridge (2-6 per cent).

Since Fe is an essential marine micronutrient for phytoplankton, the scarcity of dissolved Fe in surface waters limits biological productivity over much of the oceans. Thus, changes in Fe inputs from different dissolved Fe sources have important implications for patterns of marine productivity and the global carbon cycle. This study therefore represents a significant contribution to our understanding of how dissolved Fe may influence past and future global change.

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Figure:
The figure shows the fraction of the seawater-dissolved Fe across the GA03 North Atlantic section that originates from each of four distinct sources : 1. Fe from oxygenated sediments on the North American margin (fnon-red); 2. Fe released by dissolution of atmospheric dust (fdust);  3. Fe from reducing sedimentry porewaters on the West African Margin (fred); and 4. Fe from hydrothermal venting on the Mid-Atlantic Ridge (fhyd). Click here to view it larger.

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