Biologically driven export from ocean

In their hypothesis of a glacial decrease in Southern Ocean CO2 leak, early workers considered two causes (i) an increase in biological export production and (ii) a decrease in the exposure rate of deep waters at the polar surface (Figure 7). One can imagine processes that would have reduced the evasion of CO2 from the Southern Ocean by either of the two mechanisms mentioned above. For instance, an increase in the input of dust and its associated trace metals to the Southern Ocean might have driven an increase in the rate of nutrient and carbon uptake by phytoplankton (Martin, 1990). Alternatively, an increase in the salinity-driven stratification of the Antarctic and/or a decrease in wind-driven upwelling could have lowered the rate of nutrient and carbon dioxide supply to the surface... 

The vertical distribution of dissolved iron in the vast majority of the ocean is similar to that of other biologically-assimilated nutrients, notably nitrate and phosphate, and characterized by the following key features (a) depletion at the surface, driven by biological uptake (b) a mid-depth maximum, correlated to the rate of export production from the photic zone and the result of microbial regeneration of exported material (c) reasonably stable, invariant concentrations at depth [71]. Iron, however, dilfers from the macronutrients in notable, sometimes surprising ways, which raise basic questions about how its nutrientlike profile is maintained (Fig. 3). 

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