Objective 1:    Identification of the mechanisms of natural iron fertilisation of the Kerguelen plateau

( 21-Jan-2004 / sB/mpT)  


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General context

  General Objectives
    The Southern Ocean and oceanic CO2 pump
  A natural laboratory in the Southern Ocean: the Kerguelen plateau


    Objective 1
  Objective 2
  Objective 3
KEOPS in the national context
International co-operations /
KEOPS and the international research programs
  References cited

Objective 1
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  Understand which mechanisms are responsible for the enrichment of deep waters with iron
  Understand what is the major mechanism of the upward transfer of iron, from deep waters to the surface layer?



The Kerguelen plateau is a unique natural laboratory in the ocean. It is located in the core of a key oceanic area in the context of global climate change: the Southern Ocean (Sarmiento et al., 1998) . The French community has a good knowledge of this area, supported by previous or ongoing programs (ANTARES, KERFIX, OISO, CLIOKER). Recent experiments have given first insights into the concentration of dissolved iron in the water column above the Kerguelen plateau and the response of the phytoplankton community to natural iron enrichment (Blain et al., 2001; Bucciarelli et al., 2001) . In addition, available SeaWiFS data provide a good description of the spatial and temporal variability of bloom events helping to choose the appropriate time of the year and location for field studies. Examination of the 5 years of monthly SeaWiFS images (Fig 2) suggests two distinct regions of high primary productivity in the austral summer (December, January, February) over the Kerguelen Plateau. The one lays on the shelf (of depths < 200 m) north of Kerguelen and the other over the shallow plateau (between the 500 and 1000 m isobaths) widely developed south of Kerguelen. The two regions are often well separated in SeaWiFS images by the Polar Front or a branch of the Antarctic Circumpolar Current (ACC) which are strongly attached to the shelf edge just south and east of Kerguelen (between the 200 and 500 m isobaths) (Park et al., 1998). According to the available buoy drift information (Projet Marisonde, 1979), this narrow branch of the current has a relatively strong speed of 20 to 40 cm/s, respectively, south and east of Kerguelen. The former region has been studied for biogeochemical parameters by Blain et al. (2001) who relate the high primary productivity of the region to natural iron fertilization that has two possible sources: 1) lithogenic inputs from the islands via atmosphere and 2) inputs from the deep waters. Atmospheric deposition of iron (see discussion in objective 2.2) is probably not a major source of iron for the surface water of the southern ocean. considering a general northeastward surface wind drift at the KEOPS site, this assumption is also probably valid for in the studied area. However this will be checked. Therefore, the most likely source of iron in the latter region south of Kerguelen (which will be the major KEOPS site) is  the deep water of the Kerguelen plateau.



Our first  objective is  to understand which mechanisms are responsible for the enrichment of deep waters with iron.


The following hypotheses will be tested 1) Before reaching the site where deep waters are upwelled, they circulate around the plateau in contact with the margin. Sediments from the continental margin rich in organic matter and characterised by a thin oxygenated layer could favour the release of  iron to the water column directly from interstitial waters or by resuspension in the water colum. 2) The remineralisation of sinking biogenic material after a massive bloom in the surface waters of the plateau contributes to iron enrichment of deep waters. The release from the shelf sediment which is highly enriched with organic matter can also be a major source of iron. 3) The lithogenic particles issued from the weathering of the island sink and dissolve in the water column.



Our second objective is to understand what is the major mechanism of the upward transfer of iron, from deep waters to the surface layer?


Before addressing this important topic, we  summarise first the background dynamics over the Kerguelen Plateau. According to the KERFIX currentmeter data (Park et al, 1997), currents near the western escarpment south-west of Kerguelen are dominated by semidiurnal tidal currents and inertial currents (15.7h), with an amplitude of about 20 cm/s. In the lower frequencies, shelf waves (of periods of 4-5 days) and mesoscale waves (of periods of 2 to 6 months) are present, although they are much less energetic than tidal or inertial currents. The time-mean current there at 200 m is of the order of 5 cm/s. Around the Kerguelen Plateau escarpment, the general circulation seems to be anticyclonic (or counterclockwise in the SH), which is discernible from the subsurface property distribution. This anticyclonic circulation is most clear along the eastern escarpment where exists an along-slope, north-westerly directed cold tongue (Park et al., 1997). This cold tongue, indicative of advection of cold subsurface water originating from the south (Park et al., 1998), has recently been confirmed by a high-resolution temperature map at 100 m (Fig 4) using data from instrumented penguins (Charrassin et al., 2002).


Figure 4 : Surface plot of sea water temperature over the Kerguelen plateau at two depths (100m (left) and 0 m (right)) during summer 1988-2001. The temperature have been measured using sensors attached to penguins. The location, latitude and longitude of the animals was provided by GPS system and depth using  pressure sensors.  J.-B. Charrassin, Y.-H. Park et C.-A. Bost, , 2002.



The observed northwestward current, which is opposed to the dominant westerlies and thus to the general eastward flow of the ACC, cannot be explained by a wind-driven current mechanism. Rather, it hints at the forcing dynamics coming from a tidal current-bottom topography interaction that generates along-isobath residual currents flowing counterclockwise in the SH (NW direction in our eastern escarpment). This leads to the hypothesis:


There is a strong interaction between tidal currents and the bottom topography. especially over the steep eastern escarpment of the Kerguelen Plateau, generating the northwestward tide-induced residual current. Theories and observations (e.g., Robinson, 1983; Holloway and Merrifeld, 1999; Merrifeld et al., 2001) show that such an interaction over a steep escarpment can generate also internal tides that propagate both towards the shallow shelf and the deep abyssal region. Strong activity of internal tides concentrated over the shallow part (< 1000 m) of the Kerguelen Plateau has recently been suggested both from satellite altimetry and numerical tide modelling (Le Provost et al., 2001), which leads to the hypothesis:


Internal tides above the shallow Kerguelen plateau transfer Fe enriched bottom water into the surface layer. The internal tide activity above kerguelen plateau and the importance of internal tides in transporting Fe to the surface mixed layer will be investigated. This hypothesis is supported by the fact that internal tides eventually cascade into turbulence, thus providing enhanced vertical mixing within the water column. It is also expected that the strongest vertical excursion of internal tides occurs in the permanent pycnocline that is observed between the 200 and 300 m depths, or just below the winter mixed layer depth (that is found at < 200 m on the average, compared to the summer mixed layer depth at about 50 m) (Park et al., 1998). Although internal tides should exist all the year round, it is likely that those in the austral winter will be most efficient for eddy fluxing of near bottom water across the permanent pycnocline into the mixed layer, simply because of the closeness of the base of the winter mixed layer and the top of the permanent pycnocline where internal tide activity should be strongest. The transferred iron into the winter mixed layer could therefore be used for summer phytoplanctonic blooms






so the specific questions related to the objective 1 are


F Is there a northward tide induced resulting current along the eastern slope of the Kerguelen plateau?

F Is there internal tide activity above the Kerguelen plateau and what is its importance for iron transport to the surface layer?

F What are the processes which result in iron enriched deep water?

F What is the magnitude of the iron flux from deep water  to the surface layer?



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