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Identifying sources and transport pathways of iron in the Southern Ocean
Stockholm University, Faculty of Science, Department of Geological Sciences.ORCID iD: 0000-0003-0008-1886
Stockholm University, Faculty of Science, Department of Geological Sciences.
(English)In: Deep Sea Research Part I: Oceanographic Research Papers, ISSN 0967-0637Article in journal (Refereed) Submitted
Abstract [en]

Over large regions of the global ocean primary productivity is limited by the availability of dissolved iron. Changes in the supply of iron to these regions could have major impacts on primary productivity and the carbon cycle. One of the largest sources of dissolved iron to the ocean is thought to be from shelf sediments, and this source is often parameterized in biogeochemical models as a depth dependent iron flux through the seafloor. Using the knowledge that Southern Ocean surface waters are iron limited, we infer source regions of iron to the Southern Ocean by identifying where the most intense chlorophyll blooms develop. We further derive surface current patterns from satellite sea surface height fields to assess the role of the ocean circulation in transporting iron away from these source regions. We find a tight relationship between satellite chlorophyll concentrations and sea surface height. Large chlorophyll blooms develop on the shelf and where the western boundary currents detach from the continental shelves and turn eastward into the Southern Ocean. This is likely due to shelf supplied iron becoming entrained into western boundary currents and advected into the Southern Ocean along the Dynamical Subtropical Front. The most intense chlorophyll blooms are located along coastal margins of islands and continents. Blooms do not develop over submerged seamounts or plateaus in the open ocean. This suggests that shelf sediments in coastal regions act as large bioavailable iron sources to the Southern Ocean. We recommend that a more accurate method of parameterizing the shelf sediment iron flux could be to prescribe this flux only through grid cells neighboring coastlines. Finally, we hypothesize how changes in sea level during glacial-interglacial cycles may have altered the distribution of shelf sediment iron sources in the Southern Ocean and helped to drive export production anomalies in the Sub-Antarctic Zone.

National Category
Geochemistry Geosciences, Multidisciplinary Oceanography, Hydrology, Water Resources
Research subject
Marine Geology
URN: urn:nbn:se:su:diva-108739OAI: diva2:760330
Available from: 2014-11-03 Created: 2014-11-03 Last updated: 2014-11-03
In thesis
1. The role of Southern Ocean fronts in the global climate system
Open this publication in new window or tab >>The role of Southern Ocean fronts in the global climate system
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The location of fronts has a direct influence on both the physical and biological processes in the Southern Ocean. However, until recently fronts have been poorly resolved by available data and climate models. In this thesis we utilise a combination of high resolution satellite data, model output and ARGO data to improve our basic understanding of fronts.

A method is derived whereby fronts are identified as local maxima in sea surface height gradients. In this way fronts are defined locally as jets, rather than continuous-circumpolar water mass boundaries. A new climatology of Southern Ocean fronts is presented. This climatology reveals a new interpretation of the Subtropical Front. The currents associated with the Subtropical Front correspond to the western boundary current extensions from each basin, and we name these the Dynamical Subtropical Front. Previous studies have instead suggested that the Subtropical Front is a continuous feature across the Southern Ocean associated with the super gyre boundary.

A comprehensive assessment of the relationship between front locations and wind stress is conducted. Firstly, the response of fronts to a southward shift in the westerly winds is tested using output from a 100 year climate change simulation on a high resolution coupled model. It is shown that there was no change in the location of fronts within the Antarctic Circumpolar Current as a result of a 1.3° southward shift in the westerly winds. Secondly, it is shown that the climatological position of the Subtropical Front is 5-10° north of the zero wind stress curl line, despite many studies assuming that the location of the Subtropical Front is determined by the zero wind stress curl.

Finally, we show that the nutrient supply at ocean fronts is primarily due to horizontal advection and not upwelling. Nutrients from coastal regions are entrained into western boundary currents and advected into the Southern Ocean along the Dynamical Subtropical Front. 

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University, 2014. 41 p.
Meddelanden från Stockholms universitets institution för geologiska vetenskaper, 355
Southern Ocean, fronts, jets, Antarctic Circumpolar Current, wind stress, chlorophyll, iron, Last Glacial Maximum
National Category
Climate Research Oceanography, Hydrology, Water Resources Geosciences, Multidisciplinary
Research subject
Marine Geology
urn:nbn:se:su:diva-108736 (URN)978-91-7447-991-1 (ISBN)
Public defence
2014-12-05, Ahlmannsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 13:00 (English)

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Submitted.

Available from: 2014-11-13 Created: 2014-11-03 Last updated: 2014-11-04Bibliographically approved

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Graham, Robert M.De Boer, Agatha M.
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