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Iron chemistry in Southern Ocean waters

Iron is an essential element for many biological functions of marine organisms, and in particular for the microscopic algae composing phytoplankton. In half of the oceans, the production of living matter (primary production) by these algae is limited by its paucity or its chemical form. Yet many aspects of iron chemistry in seawater are still unknown. It can take two oxidized forms: Fe(III) which is stable but is highly insoluble and forms colloidal Fe coagulating into particles, and Fe(II) which is much more soluble but is very quickly oxidized into Fe(III).

How is Fe(II) distributed in the oceans? What are its sources and which mechanisms influence its oxidation into Fe(III)? These questions are of crucial importance in the huge oceanic regions where iron availability is the limiting factor of phytoplanktonic production. This study was undertaken in the largest of these regions, the Southern Ocean, during the "Bonus Good Hope" cruise (February-March 2008) aboard the French research vessel Marion Dufresne. Water samples were taken along a line track between South Africa (34°S) to Weddel Sea (58°S).

It is an understatement to say that dissolved iron is rare in Southern Ocean: its concentration is about a few grams per million tons of water. Sampling and analytic techniques therefore must be able to measure concentrations that small and to prevent sample contamination, owing to extreme precautions (like cleaning sampling bottle with acid) and equipments devoid of any iron component (for example a Kevlar cable to hold the bottles).

 

Analyse_Keops

Sample analyses aboard R/V Marion-Dufresene during Keops cruise (photo IPEV / V. Pasquero)


In the surface mixed layer (a few tens of meters deep), Fe(II) concentrations are always greater than the detection threshold; in intermediary layers, high concentration areas were locally observed, particularly in the subtropical latitudes; in deep waters (deeper than 1500 m), concentrations were very low, close to or lower than the detection limit of the analytic techniques, except for two stations.

 

Sarthou_fig3

Vertical section of Fe(II) concentration (top) and proportion within dissolved iron (bottom) along the section, from north (left) to south (right). Seabed is in grey and measured values shown by the colors.

Different processes are acting within these three layers.

In the surface layer, Fe(II) concentration and percentage in total dissolved iron are highest at midday, suggesting that light play an important role. Photochemical reactions leading to Fe(II) as well as biological processes related to photosynthesis and reducing Fe(III) into Fe(II) contribute to this high production, even though the inverse reaction is very fast in this layer (a few minutes are enough to oxidize into Fe(III) half of Fe(II) present at a given time).

These light-related processes cannot explain the maxima observed in intermediate depths, where light is almost absent. Neither does this Fe(II) come from the African continental margin, as it would have been oxidized into Fe(III) long before being transported to the study area by the currents. In this 100-600 m, a probable source of Fe(II) could be the disaggregation and the remineralization of particles of biological origin which sink from surface waters.

In the deeper waters, the origin of the few measurable concentrations seems to be related to the bottom, either from sediments or hydrothermal activity.

Fe(II) → Fe(III) oxidation rates measured in seawater was compared with two theoretical models considering environmental conditions within each layer: the differences found between observations and models results suggest in particular that the association of iron atoms with organic compounds to form complexes could play an important role, although still largely unknown, on the oxidation dynamics of Fe(II).

This study is the first to measure Fe(II) oxidation rates in the deep sea environmental conditions, and one of the few which measured its concentrations on a large-scale oceanic section. The study of the various chemical forms of iron in seawater is continued within the GEOTRACES and KEOPS international projects. The KEOPS 2 cruise (October 8 – November 29) focuses on natural fertilization of iron in the vicinity of Kerguelen islands shelf.

 

The paper

Sarthou G., Bucciarelli E., Chever F., Hansard S. P., González-Dávila M., Santana-Casiano J. M., Planchon F., Speich S., 2011. Labile Fe(II) concentrations in the Atlantic sector of the Southern Ocean along a transect from the subtropical domain to the Weddell Sea Gyre. Biogeosciences, 8, 2461-2479.

 

The authors

This paper is co-authored by scientists from France (IUEM: Lemar, LPO), USA (Florida) ans Spain (Canary Islands)

 

The journal

Biogeosciences is an international journal devoted to interactions between biological, chemical and physical processes involved into life in geosphere, hydrosphere and atmosphere. The journal is published by a private publisher on behalf of a scientific professional association (the European Geosciences Union); it is original in two ways. It is published under the open access model (free of charge and without subscription) and the review of manuscripts is open to anybody. After a rapid access-review by one of the editors, manuscripts are subject to interactive public discussion within the scientific community, while the classical peer-review process is completed. Comments, referees' evaluation and answers by authors are published on Biogeosciences Discussionswebsite

 

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