Unveiling the Si cycle using isotopes in an iron fertilized zone of the Southern Ocean: from mixed layer supply to export

A massive diatom-bloom is observed annually in the surface waters of the naturally Fe-fertilized Kerguelen Plateau (Southern Ocean). In this study, silicon isotopic signatures (δ30Si) of silicic acid (DSi) and suspended biogenic silica (BSi) were investigated in the whole water column with an unprecedented spatial resolution in this region, during the KEOPS-2 experiment (spring 2011). We use δ30Si measurements to track the silicon sources that fuel the bloom, and investigate the seasonal evolution of Si biogeochemical cycle in the iron fertilized area. We compare the results from a HNLC reference station with stations characterized by different degrees of iron enrichment and bloom conditions. Dissolved and particulate δ30Si signatures were generally highly variable in the upper 500 m, reflecting the effect of the intense silicon utilization in spring, while they were quite homogeneous in deeper waters. The Si-isotopic and mass balance identified a unique WW Si-source for the iron-fertilized area originating from the southeastern Kerguelen Plateau and spreading northward. However, when reaching a retroflection of the Polar Front (PF), the δ30Si composition of WW silicic acid pool was getting progressively heavier. This would result from sequential diapycnal mixings between these initial WW and ML water masses, highlighting the strong circulation of surface waters that defined this zone. When comparing the results from the two KEOPS expeditions, the relationship between DSi depletion, BSi production and their isotopic composition appears decoupled in the iron fertilized area. This seasonal decoupling could help to explain the low apparent fractionation factor observed here in the ML at the end of summer. Taking into account these considerations, we refined the seasonal net BSi production in the ML of the iron-fertilized area to 3.0 ± 0.3 mol Si m−2 y−1, that was exclusively sustained by surface water phytoplankton populations. These insights confirm that the isotopic composition of dissolved and particulate silicon is a promising tool to improve our understanding on the Si-biogeochemical cycle since the isotopic and mass balance allows resolving the processes involved i.e. uptake, dissolution, mixing

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Closset Ivia, Cardinal Damien, Rembauville Mathieu, Thil Francois, Blain Stephane (2016). Unveiling the Si cycle using isotopes in an iron fertilized zone of the Southern Ocean: from mixed layer supply to export. Biogeosciences. 13 (21). 6049-6066. https://doi.org/10.5194/bg-13-6049-2016, https://archimer.ifremer.fr/doc/00344/45528/

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