Effect of iron supply on Southern Ocean CO2 uptake and implications for glacial atmospheric CO2

Photosynthesis by marine phytoplankton in the Southern Ocean, and the assoc iated uptake of carbon, is thought to be currently limited by the availabil ity of iron(1,2). One implication of this limitation is that a larger iron supply to the region in glacial times(3) could have stimulated algal photos ynthesis, leading to lower concentrations of atmospheric CO2. Similarly, it has been proposed that artificial iron fertilization of the oceans might i ncrease future carbon sequestration. Here we report data from a whole-ecosy stem test of the iron-limitation hypothesis in the Southern Ocean(4), which show that surface uptake of atmospheric CO2 and uptake ratios of silica to carbon by phytoplankton were strongly influenced by nanomolar increases of iron concentration. We use these results to inform a model of global carbo n and ocean nutrients, forced with atmospheric iron fluxes to the region de rived from the Vostok(3) ice-core dust record. During glacial periods, pred icted magnitudes and timings of atmospheric CO2 changes match ice-core reco rds well. At glacial terminations, the model suggests that forcing of South ern Ocean biota by iron caused the initial similar to 40 p. p. m. of glacia l-interglacial CO2 change, but other mechanisms must have accounted for the remaining 40 p. p. m. increase. The experiment also confirms that modest s equestration of atmospheric CO2 by artificial additions of iron to the Sout hern Ocean is in principle possible, although the period and geographical e xtent over which sequestration would be effective remain poorly known.