Diatom physiology controls silicic-acid leakage in response to iron fertilization

We explore how the iron dependence of the Si:P uptake ratio R^Si:P of diatoms controls the response of the global silicon cycle and phytoplankton community structure to Southern Ocean iron fertilization. We use a data-constrained model of the coupled Si-P-Fe cycles that features a mechanistic representation of nutrient co-limitations for three phytoplankton classes and that is embedded in a data-assimilated global ocean circulation. We consider three parameterizations of the iron dependence of R^Si:P, all of which allow equally good fits to the observed nutrient climatology but result in very different responses to iron fertilization: Depending on how sharply R^Si:P decreases with increasing iron concentration, iron fertilization can either cause increased silicic-acid leakage from the Southern Ocean or increased Southern Ocean silicon trapping. Silicic-acid leakage drives a floristic shift in favor of diatoms in the subtropical gyres and stimulates increased low-latitude opal export. The diatom contribution to global phosphorus export increases, but the lower diatom silicon requirement under iron-replete conditions reduces the global opal export. Regardless of R^Si:P parameterization, the global response of the biological phosphorus and silicon pumps, as well as the implications for oceanic carbon uptake, are dominated by the Southern Ocean. The Si-isotope signature of opal flux becomes systematically lighter with increasing iron-induced silicic-acid leakage, consistent with sediment records from iron-rich glacial periods.