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

Abstract

We studied the evolution of the isotopic composition of dissolved Si (δ 30 Si DSi ) and biogenic Si (δ 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 21.36% ± 0.11%, 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 δ 30 Si 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 δ 30 Si 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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