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.).

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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Copper and zinc oceanic mass balance revisited

Little and co-workers (2014; see reference below) propose an update of the oceanic copper (Cu) and zinc (Zn) mass balance, with the original approach that takes into account the hitherto ignored constraint of their isotopes. They establish an up-to-date inventory of the input fluxes of these tracers with their isotopic signatures, discuss the internal processes that might fractionate both tracers and evaluate one major sedimentary sink: sediments deposited beneath an oxic water column. Although the Cu oceanic mass balance appears to be roughly in balance, the Zn one is far from being constrained... isotopes reveal that either an "isotopically light sink" or "isotopically heavy source" is missing.

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Figure: This figure illustrates the global ocean isotopic mass balance of Zn. Click here to view the figure larger.

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What does the largest data set of zinc concentration ever reported tell us?

Zinc (Zn) is one of the key micronutrients that are measured during the GEOTRACES cruises. N.J. Wyatt and colleagues (2014; see reference below) largely contributed to this collective effort by measuring Zn distribution at high resolution (556 discrete samples) between Cape Town and Montevideo in the South Atlantic Ocean (40°S) on board the UK GEOTRACES GA10 cruise.

The reported surface Zn concentrations are among the lowest reported for the world's oceans (0.015 – 0.39 nM). An intriguing result was the fact that Zn concentrations were very low down to depths of 500m, which was similar to that of silicate (Si) concentrations. Using the strong relationship between Zn and Si, the authors present a new tracer Zn* (Zn* = Zn - 0.065 x Si + 0.209), which illustrates that Zn is removed from surface waters in the Southern Ocean and remineralised deeper in the water column. This results in very low Zn concentrations in Sub-Antarctic-Mode Water (SAMW), which is the main pathway for supplying nitrate and phosphate to the thermocline waters of the South and North Atlantic. These low Zn concentrations in SAMW may result in growth-limiting Zn concentrations in the surface waters of both the South and North Atlantic sub-tropical gyres.

14 Wyatt lFigure: Concentrations of Zn along GA10 section (~40°S). Warm colours indicate high concentrations. 
Click here to view the figure larger or here to view a map of GEOTRACES Sections in the Atlantic.

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Sinking organic matter: a major driver of the oceanic zinc cycle?

Zinc (Zn) is a marine micronutrient, important for phytoplankton and involved in biogeochemical cycling throughout the oceans. Despite the importance of this element, the processes that control the oceanic Zn cycle are still poorly understood. As with other trace elements, this lack of knowledge is being addressed by high-resolution sampling as part of the GEOTRACES program.

A new study by John and Conway (2014; see reference below), presenting the first high-resolution coupled profiles of both dissolved Zn and cadmium (Cd) concentration and isotope ratios from the GEOTRACES North Atlantic (GA03) section, suggests that scavenging of isotopically heavy Zn onto organic matter plays an important role in the surface marine cycling of Zn, and may be important for understanding why Zn, like silicon (Si), has a deeper regeneration in the oceans than nitrogen (N), phosphorus (P) and Cd. The new GEOTRACES data is supported by modelling and culture experiments, which show that whilst Cd and major nutrients are quickly released as phytoplankton degrade, a significant portion of the Zn is instead scavenged back onto organic matter.

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Figure: Zn and Cd concentration and stable isotope profiles along the North Atlantic GA03 section.
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