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

• Published in: Limnology and Oceanography
• Main Authors: Cavagna, A.J., Fripiat, F., Dehairs, F., Wolf-Gladrow, D., Cisewski, B., Savoye, N., André, L., Cardinal, D.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2011
• Subjects: Southern Ocean
• Online Access: http://www.vliz.be/nl/open-marien-archief?module=ref&refid=211192

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.