1.3.1 Historical development
The concentrations of iron changed dramatically when around 2.5 billion years ago the oxygen levels on earth started to increase due to the establishment of photosynthesizing organisms that produced oxygen as a waste product on Earth (Barber, 2008). Since iron was readily available at that time and can have multiple electrical potentials, organisms
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Figure 1.1: Schematic view on the biological pump illustrating the role of phytoplank-ton, zooplankton and bacteria (from Herndl and Reinthaler (2013)). Particulate organic carbon is denoted by POC.
had based their physiology strongly on iron and incorporated it into electron transport systems of photosynthesis and in essential enzymes (Behrenfeld and Milligan, 2013). How-ever, the rising oxygen concentrations had negative consequences for the availability of iron in the ocean. Iron is present in two oxidative states in seawater, ferric iron (Fe(II)) and ferrous iron (Fe(III)). While Fe(II) is easy soluble in water and readily available for phytoplankton, Fe(III) is not very soluble. At the presence of oxygen, Fe(II) is rapidly oxidized to Fe(III) which then quickly precipitates, coagulates and adsorbs to particles (Rose and Waite, 2003). The result is that dissolved and thus bioavailable iron concen-trations are reduced strongly at oxygen concenconcen-trations that organisms experience today.
It is assumed that the reduced availability of iron lead to iron limitation in large areas of the ocean, i.e. the High-Nutrient Low-Chlorophyll (HNLC) regions described in the next section.
1.3.2 High-Nutrient Low-Chlorophyll (HNLC) regions
HNLC regions are a phenomenon that scientists struggled to explain until the discovery of widespread iron limitation in these regions (Raiswell and Canfield, 2012). Low con-centrations of phytoplankton (chlorophyll) seemed to be in a logical conflict with high concentrations of the nutrients nitrate and phosphate that should allow phytoplankton to grow. Before the wide-spread iron limitation was discovered, strong grazing and light
5 1.3 The abundance of iron limitation were discussed as possible reasons for the HNLC phenomenon, which was found in the Southern Ocean (Boyd et al., 2000), equatorial Pacific (Martin et al., 1994) and North Pacific (Tsuda et al., 2003). The area around the Kerguelen Plateau in the South-ern Ocean is naturally iron fertilized because of iron release from sediments close to the ocean surface. The Kerguelen Plateau is an ideal place to study the marine biogeochem-istry under iron limiting and iron replete conditions (Blain et al., 2007). In addition to studies at the Kerguelen Plateau, numerous artificial iron fertilization experiments were conducted, both, in shipboard bottle incubations and by directly fertilizing the ocean surface. In almost all experiments phytoplankton chlorophyll and macronutrient uptake increased significantly after the addition of iron to surface waters (de Baar et al., 2005;
Boyd et al., 2007). However, the fate of the added iron is not very clear as in some experiments multiple iron additions were necessary to increase the surface iron concen-trations and to stimulate phytoplankton growth (Bowie et al., 2001). Bowie et al. (2001) suggest that horizontal dispersion and scavenging strongly are likely to be responsible for the rapid loss of the fertilized iron. It was also rarely measured during the fertilization experiment to what quantity the organic matter, which was build up by phytoplankton in response to the iron fertilization, sinks in the water column and exports carbon to the deep ocean (de Baar et al., 2005; Aumont and Bopp, 2006). While, strong grazing and colimitation of iron with other nutrients and light are also still discussed to contribute to the limited growth of phytoplankton in the HNLC regions, it is widely accepted today that iron limitation is the main reason for the HNLC phenomenon (Moore and Doney, 2007; Breitbarth et al., 2010).
1.3.3 Modern global distribution
In the late 1980s trace metal clean bottles and highly sensitive analysis methods dras-tically improved the accuracy of measurements of iron concentrations in the ocean (e.g.
Martin and Fitzwater (1988)). Different chemical forms of iron are operationally defined by the different pore filter sizes used during iron concentration measurements. Soluble iron is defined to be smaller than 0.02 µm, colloidal iron to have a size between 0.02 and 0.4 µm, and particulate iron is defined to be larger than 0.4 µm (Wu et al., 2001).
Dissolved iron comprises soluble and colloidal iron and is usually assumed to be avail-able for phytoplankton. However, some measurements of dissolved iron also use a pore filter size of 0.2 µm and thus do not include the whole fraction of colloidal iron, which complicates the interpretation of observations (Raiswell and Canfield, 2012). A compila-tion of dissolved iron observacompila-tions by Tagliabue et al. (2012) shows that the number of measurements is still low as there are large areas of the ocean that remain completely unsampled (Fig. 1.2a). However, the observations at the ocean surface clearly show low
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Figure 1.2: Observed iron concentrations: Surface iron concentrations averaged over the top 50 m and binned in 3.6◦x1.8◦ boxes from the observation compilation by Tagliabue et al. (2012) are shown in a). All observations plotted versus depth (gray dots) with horizontally averaged iron concentrations discretized in 200 m depth intervals (solid line) and a strongly smoothed curve (dashed line) plotted on top are shown in b).
concentrations of dissolved iron (< 0.2 µmol m−3) in the Southern Ocean and elevated iron concentrations (> 0.6 µmol m−3) in regions influenced by the atmospheric deposi-tion of iron originating from nearby deserts. For example in the tropical Atlantic where dust from the Sarahan desert is deposited, iron concentrations are clearly elevated. The globally averaged vertical profile of dissolved iron observations reveals an approximate nutrient like profile with low concentrations at the surface and increasing concentrations at the midwater maximum (Fig. 1.2b). The shape of the profile is caused by biological uptake at the surface and remineralization of iron from organic particles at subsurface depths. However, below 2000 m iron concentrations decrease again because of scavenging, the chemical transformation of dissolved iron to particulate iron. Scavenging is explained in more detail in section 1.4.2.