Iron biogeochemistry in the waters surrounding the Crozet Islands, Southern Ocean

The aim of this thesis was to improve our understanding of the natural iron fertilisation processes<br/>that can alleviate the High Nutrient Low Chlorophyll conditions normally associated with the<br/>Southern Ocean. The annual phytoplankton bloom occurring north of the Crozet Plateau (46°26’S<br/>- 52°18’E) provided a good opportunity to study these phenomena during CROZEX, a<br/>multidisciplinary study performed in austral summers 2004/2005 and 2005/2006. Particular<br/>emphasis has been placed on the sources of Fe to the upper water column, and on the different<br/>processes that drive the distribution of iron, such as mixing with deeper waters, advection of Fe<br/>rich waters from the islands, or particles and atmospheric inputs.<br/>A flow-injection analyser with DPD catalytic spectrophotometric detection (FIA-DPD) was first<br/>set up for the determination of total dissolved Fe (DFe, ? 0.2 µm) and careful assessment of data<br/>demonstrated the high quality of the information obtained in this study. Analytical measurements<br/>of DFe were performed in twenty vertical profiles from the North to the South of the islands that<br/>show evidence of a range of processes influencing the iron distributions. Most significantly, an<br/>enrichment of DFe (&gt;1 nM) has been identified at close proximity to the islands, which suggests<br/>that the plateau and the associated sediments are a source of iron. Waters further north also appear<br/>to be affected by this input of both coastal and shelf origin, although dissolved iron concentrations<br/>decrease as a function of distance to the north of the plateau with a gradient of 0.07 nM.km-1 as a<br/>result of dispersion and mixing. This gradient was then combined with short-lived Radium<br/>isotopes profiles, allowing the determination of a horizontal advective flux of Fe. Estimates of<br/>atmosphere and vertical fluxes of Fe to surface waters were also calculated. It was then possible to<br/>estimate a pre-bloom concentration of ~ 0.44 nM, which is sufficient to drive the inferred level of<br/>the new production in the bloom area.<br/>Labile iron has been distinguished from the refractory fraction of iron in the suspended particulate<br/>matter exported from the mixed layer by developing a two-stage leaching technique. The resulting<br/>solutions were analyzed using ICP-MS. By using associated 234Th fluxes, vertical fluxes of<br/>particulate iron in both phases were determined as well as their residence times. Data clearly show<br/>that carbon export was linked to the iron residence time and enhanced in the northern region<br/>where iron inputs occur.<br/>This thesis therefore provides a comprehensive data set of DFe and particulate iron in waters<br/>surrounding the Crozet Islands. It also presents the first attempt to establish an iron budget that<br/>can be developed into a model and provides a good reference point for subsequent studies of<br/>natural iron fertilization processes that occur around these islands.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2008

Author dc:creator

• Planquette, Hélène