Silicon uptake and supply during a Southern Ocean iron fertilization experiment (EIFEX) tracked by Si isotopes

We studied the evolution of the isotopic composition of dissolved Si (d30SiDSi) and biogenic Si (d30SiBSi) during a diatom bloom induced by an iron fertilization experiment in a Southern Ocean (SO) eddy. Dissolved Si (DSi) consumption was clearly boosted as a result of iron addition. We estimate an apparent Si isotopic fractionation factor of 21.36% 6 0.11%, not significantly different from estimates based on in vitro incubations under Fereplete conditions and in agreement with previous SO studies under Fe-depleted conditions. The temporal variations of the d30Si are best reconstructed considering the fertilized surface waters as an open system, i.e., DSi is continuously supplied from the winter water. Using vertical diffusivity, we estimate a supply of 6.7 6 1.2 mmol Si m22 d21 representing 30% 6 5% of total DSi demand. We estimate biogenic silica (BSi) production at 22.1 6 1.8 mmol Si m22 d21, similar to BSi production under natural conditions in the SO. The DSi use preceding the start of the fertilization was conservatively estimated at 49% 6 4%, and as a result of the Fe-fertilization, a further 31% 6 4% of the initial DSi reservoir was consumed. The BSi export was estimated to be 17.4 6 1.7 mmol Si m22 d21. Our results suggest that for this study three main processes were acting significantly on d30Si signatures, biological uptake, Si supply from subsurface, and export of BSi, and that the magnitude of production and export in such a Fe-fertilized experiment are similar to those for natural diatom blooms occurring in the Southern Ocean.