An iron budget during the natural iron fertilisation experiment KEOPS (Kerguelen Islands, Southern Ocean)

Abstract

Abstract. Total dissolvable iron (TDFe) was measured in the water column above and in the surrounding of the Kerguelen Plateau (Indian sector of the Southern Ocean) during the KErguelen Ocean Plateau compared Study (KEOPS) cruise. TDFe concentrations ranged from 0.90 to 65.6 nmol L −1 above the plateau and from 0.34 to 2.23 nmol L −1 offshore of the plateau. Station C1 located south of the plateau, near Heard Island, exhibited very high values (329-770 nmol L −1 ). Apparent particulate iron (Fe app ), calculated as the difference between the TDFe and the dissolved iron measured on board (DFe) represented 95±5% of the TDFe above the plateau, suggesting that particles and refractory colloids largely dominated the iron pool. This paper presents a budget of DFe and Fe app above the plateau. Lateral advection of water that had been in contact with the continental shelf of Heard Island seems to be the predominant source of Fe app and DFe above the plateau, with a supply of 9.7±3.6×10 6 and 8.3±11.6×10 3 mol d −1 , respectively. The residence times of 1.7 and 48 days estimated for Fe app and DFe respectively, indicate a rapid turnover in the surface water. A comparison between Fe app and total particulate iron (TPFe) suggests that the total dissolved fraction is mainly constituted of small refractory colloids. This fraction does not seem to be a potential source of iron to the phytoplankton in our study. Finally, when taking into account the lateral supply of dissolved iron, the seasonal carbon sequestration efficiency was estimated at 154 000 mol C (mol Fe) −1 , Correspondence to: G. Sarthou (geraldine.sarthou@univ-brest.fr) which is 4-fold lower than the previously estimated value in this area but still 18-fold higher than the one estimated during the other study of a natural iron fertilisation experiment, CROZEX.

Iron biogeochemistry across marine systems – progress from the past decade

“…The artificial experiments have indicated an efficiency of biological carbon export into deeper water (100-250 m) ranging from 650 (SERIES, Boyd et al, 2004) to 3300 (mol C/mol Fe) (SOFEX -south, Buesseler et al, 2004). The seasonal sequestration efficiencies estimated for natural Fe fertilization are much higher, 8640 for CROZEX (Pollard et al, 2009) and 154 000 for KEOPS (Chever et al, 2010). The discrepancies in effectiveness between natural and purposeful fertilizations might be partly due to the ∼75% immediate loss of added Fe in artificial fertilisations .…”

“…In the open ocean, a large variety of naturally iron-fertilized sites exist, which could allow for improved forecasting of the oceanic response to Fe fertilization and a better knowledge of Fe sources to the open ocean. Chever et al (2010) provide a Fe budget for the naturally fertilized area above the Kerguelen Plateau, using total dissolvable Fe as an additional tracer to better constrain the Fe cycle in this area. They show that horizontal advection of water from South of the Plateau seems to be the predominant source of apparent particulate and dissolved iron above the plateau, over atmospheric and vertical inputs.…”

“…We follow the structure of the workshop topics, which were: Natural Fe fertilization (Sect. 2, articles: Ardelan et al, 2010;Chever et al, 2010;Duggen et al, 2010;Ye et al, 2009); artificial Fe fertilization (3: Bucciarelli et al, 2010;Chever et al, 2010); Fe inputs into coastal and estuarine systems (4: Breitbarth et al, 2009); Colloidal iron and organic matter (5); Linking biological processes to iron chemistry (6: Breitbarth et al, 2009;Bucciarelli et al, 2010;Hassler and Schoemann, 2009;Steigenberger et al, 2010); and Iron and Climate Change (7: Breitbarth et al, 2010;Rose et al, 2009). Each section concludes with recommendations for future research.…”

Sourcing the iron in the naturally fertilised bloom around the Kerguelen Plateau: particulate trace metal dynamics

“…Blain et al (2007) and Chever et al (2010) demonstrated that dFe fertilisation from the plateau increased primary production in the area. From the data gathered it was proposed that resuspension of plateau-derived sediments and entrainment into the mixed layer during increased wind mixing that deepened the mixed layer, was the primary source of particulate and subsequently, dissolved Fe to the downstream blooms.…”

Dissolved greenhouse gases (nitrous oxide and methane) associated with the naturally iron-fertilized Kerguelen region (KEOPS 2 cruise) in the Southern Ocean

“…The determination of the role of the Southern Ocean in CH 4 and N 2 O air-sea exchange may be critical in understanding the factors that influence GHG cycling. This includes dFe which comes from different sources within the KPR, inducing mesotrophic conditions associated with the coastal waters of the Kerguelen Islands and of the central Kerguelen Plateau, an area within the central plateau area of the KPR that demonstrates an annually recurrent phytoplanktonic bloom in the (Blain et al, 2008;Chever et al, 2010), and with the Antarctic PF and other mesoscale structures (Mongin et al, 2008;Lasbleiz et al, 2014).…”