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.).

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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The impact of the different sources of iron on the ability of the ocean to absorb atmospheric carbon dioxide: reversing the paradigm?

Using model simulations, Tagliabue and co-authors (2014, see reference below) tested the sensitivity of the ocean to absorb the atmospheric carbon dioxide (CO2) in response to variable supply of iron. They found that while atmospheric CO2 is sensitive to sedimentary iron input, it is relatively insensitive to dust and hydrothermal iron input.

The weak reaction of atmospheric CO2 to dust input, which completely change previous paradigms, is due to the fact that dust is not the major iron input to the remote Southern Ocean, while sediment supply plays an overwhelming role in regulating export production in this oceanic area.

This works also shows that while hydrothermal input is crucial in governing the iron inventory for ~25% of the ocean, it remains restricted to the deep ocean, and has small effect on atmospheric CO2.

14 Tagliabue lFigure: A map of the dominant iron source in controlling the dissolved iron inventory (upper) and biological carbon export flux (lower panel). Please click here to view the figure larger.

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Undocumented cadmium, zinc and copper sink in oxygen minimum zones

Cadmium (Cd) is a micronutrient for marine algae and has a marine distribution similar to the macronutrients nitrate and phosphate. The use of sedimentary microfossil records of Cd, thus, allow reconstructions of past ocean nutrient distributions that facilitate the understanding of the role of the oceans in the carbon cycle and climate change. However, this proxy is limited by the incomplete knowledge of processes that control the addition and removal of Cd in the ocean, and Cd's variability relative to major nutrients.

Janssen and co-authors (2014, see reference below) present coupled data of Cd concentration and isotopic composition in seawater and suspended marine particles. They found that in oxygen-deficient waters, Cd is removed directly via coprecipitation with sulfide. They also underline that, together with Cd, concurrent decoupling of zinc (Zn) and copper (Cu) from corresponding macronutrients are observed in the northeast Pacific Ocean. These results suggest that the marine Cd cycle (but also Zn and Cu ones) may be highly sensitive to the extent of global oceanic oxygen depletion.

14 Janssen lFigure: This figure shows particulate cadmium and phosphorus concentrations and cadmium stable isotope ratios (δ114Cd) from the US GEOTRACES North Atlantic Transect (GA03), along with oxygen concentrations and fluorescence which is an estimate of algal biomass.  In blue, station USGT 11-14 from the central North Atlantic (27.6°N, 49.6°W) and, in purple, station USGT 10-09 from the Mauritanian upwelling (17.4°N, 18.25°W). The upper panel shows the depth range of 0-2000 m while the lower panel focuses on the 0-500 m (oxygen deficient zone). Click here to view the figure larger.

In both the oxygen depleted waters of the North Atlantic (purple) and the higher dissolved oxygen waters of the central basin (blue), particulate phosphorus concentrations show a decreasing trend from the near surface waters to deep waters, typical of nutrient type elements such as phosphorus and cadmium in particles. In the central basin (blue), particulate cadmium and the particulate cadmium to phosphorus ratio show low and nearly constant values with depth; however there is a pronounced subsurface increase in particulate cadmium and the cadmium to phosphorus ratio in particles from the Northwestern Atlantic oxygen deficient waters (in purple, at a depth of 100 – 500 m).

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Latest discoveries about zinc concentrations and isotopes in the ocean

P1090745 lZinc (Zn) is an essential micronutrient for phytoplankton and plays a key role in the productivity of the oceans. Despite the importance of this element, the processes which govern its cycling in the ocean are poorly understood. Thanks to GEOTRACES, an unprecedentedly large volume of data has been reported, revealing fascinating results published in four recent papers. 

Click on the links below to access science highlights about these papers:

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