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

Abstract

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

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