| Summary: | We studied the evolution of the isotopic composition of dissolved Si (d 30 Si DSi ) and biogenic Si (d 30 Si BSi ) 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 -1.36 parts per thousand +/- 0.11 parts per thousand, not significantly different from estimates based on in vitro incubations under Fe-replete conditions and in agreement with previous SO studies under Fe-depleted conditions. The temporal variations of the d 30 Si 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 +/- 1.2 mmol Si m -2 d -1 representing 30% +/- 5% of total DSi demand. We estimate biogenic silica (BSi) production at 22.1 +/- 1.8 mmol Si m -2 d -1 , similar to BSi production under natural conditions in the SO. The DSi use preceding the start of the fertilization was conservatively estimated at 49% +/- 4%, and as a result of the Fe-fertilization, a further 31% +/- 4% of the initial DSi reservoir was consumed. The BSi export was estimated to be 17.4 +/- 1.7 mmol Si m -2 d -1 . Our results suggest that for this study three main processes were acting significantly on d 30 Si 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.
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