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

Ivia Closset,Damien Cardinal,François Thil,Mathieu Rembauville,Stéphane Blain

doi:10.5194/bg-13-6049-2016

Abstract

Abstract. A massive diatom bloom forms annually in the surface waters of the naturally iron-fertilized Kerguelen Plateau (Southern Ocean). In this study, silicon isotopic signatures (δ30Si) of silicic acid (DSi) and suspended biogenic silica (BSi) were investigated through the whole water column with unprecedented spatial resolution, during the KEOPS-2 experiment (spring 2011). We used δ30Si measurements to track the sources of silicon that fuelled the bloom, and investigated the seasonal evolution of the Si biogeochemical cycle in the iron-fertilized area. We compared the results from stations with various degrees of iron enrichment and bloom conditions to an HNLC reference station. Dissolved and particulate δ30Si signatures were highly variable in the upper 500 m, reflecting the effect of intense silicon utilization in spring, while they were quite homogeneous in deeper waters. The Si isotopic and mass balance identified a unique Winter Water (WW) Si source for the iron-fertilized area that originated from southeast of the Kerguelen Plateau and spread northward. When the WW reached a retroflection of the Polar Front (PF), the δ30Si composition of the silicic acid pool became progressively heavier. This would result from sequential diapycnal and isopycnal mixings between the 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 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 yr−1, which 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 of the Si biogeochemical cycle since the isotopic and mass balance allows resolution of processes in the Si cycle (i.e. uptake, dissolution, mixing).

Highlights

The Southern Ocean plays a crucial role in the regulation of the global climate as it contributes significantly to the world’s ocean primary production and represents a major carbon sink (Takahashi et al, 2009)
These blooms are often associated with high export events when diatoms are removed from the mixed layer (ML) via aggregation, settling, or grazing processes (Boyd and Trull, 2007; Buesseler, 1998), and contribute to the extensive biogenic silica deposition that characterizes the sediments of this region known
The KEOPS-2 expedition was conducted in the Kerguelen Plateau region (Indian sector of the Southern Ocean) from 10 October to 20 November 2011 on board of the R/V Marion Dufresne

Summary

Introduction

The Southern Ocean plays a crucial role in the regulation of the global climate as it contributes significantly to the world’s ocean primary production and represents a major carbon sink (Takahashi et al, 2009). In the Southern Ocean, diatoms, a phytoplankton group that produces an opaline cell wall called a frustule (amorphous SiO2 nH2O), are responsible for more than 75 % of the annual primary production. Brzezinski et al, 2001) These blooms are often associated with high export events when diatoms are removed from the mixed layer (ML) via aggregation, settling, or grazing processes (Boyd and Trull, 2007; Buesseler, 1998), and contribute to the extensive biogenic silica (hereafter referred to as BSi) deposition that characterizes the sediments of this region known. Closset et al.: Unveiling the Si cycle using isotopes as the “opal belt” (Tréguer and De la Rocha, 2013; Ragueneau et al, 2000)

Methods

Results

Conclusion