Climatic effect of Southern Ocean Fe fertilization: Is the jury still out?

Abstract

[1] One of the great puzzles in oceanography has been the reason for the existence of certain areas of the world ocean like the Southern Ocean and equatorial and north Pacific where there is an abundance of unused macronutrients essential for phytoplankton growth (such as nitrate and silicic acid) [de Baar and Boyd, 2000]. Although physical (solar insolation and ocean surface mixing) and grazing regimes must play a part in controlling phytoplankton standing stocks in these so-called ` high-nitrate low-chlorophyll'' (HNLC) regions, Martin suggested that growth limitation through insufficient availability of the micronutrient iron (Fe) might also exert an important control [Martin and Fitzwater, 1988]. Furthermore, he proposed that stimulation of biological productivity through increased aeolian Fe delivery to the Southern Ocean at the height of the last ice age (the Last Glacial Maximum (LGM)) might have led to the concurrently low levels of atmospheric CO2 observed [Martin, 1990]. The ` IronEx II'' open ocean Fe fertilization experiment demonstrated unequivocally the role that iron played in limiting phytoplankton growth (particularly of larger diatoms) in the equatorial Pacific [Coale et al., 1996]. More recently, a similar experiment (Southern Ocean Iron Release Experiment (SOIREE)) was carried out in the Southern Ocean, a region suspected to play a key role in controlling variability in the concentration of atmospheric CO2 over glacial-interglacial timescales [Knox and McElroy, 1984]. Here, too, an unambiguous response of the phytoplankton standing stock to the addition of Fe was observed [Boyd et al., 2000]. However, for these changes to produce a significant response in the concentration of atmospheric CO2, there must also be an increase in the export of carbon to the deep ocean. It is here that our understanding of biogeochemical cycling in the Southern Ocean and in particular of the potential relationships between iron supply and climate now appears to falter.