Labile Fe(II) concentrations in the Atlantic sector of the Southern Ocean along a transect from the subtropical domain to the Weddell Sea Gyre

BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Biogeosciences Discuss., 8, 4163–4208, 2011 www.biogeosciences-discuss.net/8/4163/2011/ doi:10.5194/bgd-8-4163-2011 © Author(s) 2011. CC Attribution 3.0 License. Biogeosciences Discussions This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Labile Fe(II) concentrations in the Atlantic sector of the Southern Ocean along a transect from the subtropical domain to the Weddell Sea Gyre G. Sarthou1,2, E. Bucciarelli1,2, F. Chever1,2,*, S. P. Hansard3, M. Gonzalez-Davila4, J. M. Santana-Casiano4, F. Planchon1,2, and S. Speich1,5 1Universit´ e Europ´ eenne de Bretagne, France 2Universit´ e de Brest, CNRS, IRD, UMR 6539 LEMAR, IUEM; Technopˆ ole Brest Iroise, Place Nicolas Copernic, 29280 Plouzan´ e, France 3Florida Geological Survey, 903 W. Tennessee Street, Tallahassee FL 32301, USA 4Facultad de Ciencias del Mar. Departamento de Quimica, Universidad de Las Palmas de Gran Canaria, Campus de Tafira, 35017, Las Palmas de Gran Canaria, Spain 5Laboratoire de Physique des Oc´ eans, UMR 6523, IFREMER, CNRS, IRD, UBO, 29280 Plouzan´ e, France 4163 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ∗current address: Universit´ e de Brest, CNRS, IRD, UBO, UMR 6538 LDO, IUEM ; Technopˆ ole Brest Iroise, Place Nicolas Copernic, 29280 Plouzan´ e, France Received: 1 April 2011 – Accepted: 8 April 2011 – Published: 29 April 2011 Correspondence to: G. Sarthou (geraldine.sar[email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. 4164 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract Labile Fe(II) distributions were investigated in the Sub-Tropical South Atlantic and the Southern Ocean during the BONUS-GoodHope cruise from 34 to 57◦S (February– March 2008). Concentrations ranged from below the detection limit (0.009 nM) to values as high as 0.125 nM. In the surface mixed layer, labile Fe(II) concentrations were5 always higher than the detection limit, with values higher than 0.060 nM south of 47◦S, representing between 39% and 63% of dissolved Fe (DFe). Biological production was evidenced. At intermediate depth, local maxima were observed, with the highest values in the Sub-Tropical domain at around 200 m, and represented more than 70% of DFe. Remineralization processes were likely responsible for those sub-surface max-10 ima. Below 1500 m, concentrations were close to or below the detection limit, except at two stations (at the vicinity of the Agulhas ridge and in the north of the Weddell Sea Gyre) where values remained as high as ∼0.030–0.050 nM. Hydrothermal or sediment inputs may provide Fe(II) to these deep waters. Fe(II) half life times (t1/2) at 4 ◦C were measured in the upper and deep waters and ranged from 2.9 to 11.3 min, and15 from 10.0 to 72.3 min, respectively. Measured values compared quite well in the upper waters with theoretical values from two published models, but not in the deep waters. This may be due to the lack of knowledge for some parameters in the models and/or to organic complexation of Fe(II) that impact its oxidation rates. This study helped to considerably increase the Fe(II) data set in the Ocean and to better understand the Fe20 redox cycle. 1 Introduction Iron (Fe) is an essential micronutrient for all marine organisms, playing a key role in many metabolic processes, such as photosynthesis, respiration, nitrate reduction, and nitrogen fixation (Sunda, 1988, 1989). Its low concentrations have been shown to limit25 primary production in more than 50% of the ocean (Boyd and Ellwood, 2010). All 4165 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | natural and artificial Fe fertilization experiments unequivocally showed the importance of Fe for the carbon cycle, particularly for the growth and composition of the phytoplanktonic community (Boyd et al., 2000; Coale et al., 1996; Gervais et al., 2002; Tsuda et al., 2003; Boyd, 2004; Coale et al., 2004; Blain et al., 2007; Pollard et al., 2007). Despite numerous studies on Fe cycling the last 25 years, lots of unknowns persist, in5 particular because Fe chemistry in seawater is very complex. Fe has been observed to occur in two redox states (Fe(III) and Fe(II), Waite and Morel, 1984). In oxic seawater, the thermodynamically most stable state is Fe(III), but is highly insoluble (0.011 nM in 0.7 NaCl solution, Liu and Millero, 2002) and is rapidly hydrolyzed resulting in the formation of various Fe(III) oxyhydroxide (de Baar and de Jong, 2001). These species,10 with Fe(OH)3being the dominant one in seawater at pH ∼8, have the tendency to form colloidal Fe (Kuma et al., 1996) which coagulate and form particulate Fe (Johnson et al., 1997). In contrast to Fe(III), Fe(II) is more soluble but is rapidly oxidized by oxygen (O2) and hydrogen peroxide (H2O2) (Millero et al., 1987; Millero and Sotolongo, 1989; Santana-Casiano et al., 2004, 2005; Gonzalez-Davila et al., 2005, 2006). Although15 Fe(II) in seawater is less stable than Fe(III), recent models of Fe acquisition by eukaryotic phytoplankton suggest that the reduction of Fe(III) to Fe(II), with subsequent re-oxidation to Fe(III), is a possible mechanism by which Fe is made more bioavailable to phytoplankton (Shaked et al., 2004; Salmon et al., 2006; Maldonado et al., 2006; Morel et al., 2008). Numerous studies have investigated the oxidation of Fe(II) by O2 20 and H2O2in different aqueous solutions to understand the behavior of Fe(II) in natural waters (Santana-Casiano et al., 2006 and references herein). The most widely accepted mechanism to describe Fe oxidation with O2and H2O2is the Haber-Weiss mechanism, with reactions 1 or 3 limiting the overall oxidation rate (King et al., 1995). Fe(II)+O2→Fe(III)+O•− 2(1)25 2H++Fe(II)+O•− 2→Fe(III)+H2O2(2) Fe(II)+H2O2→Fe(III)+OH−+HO•(3) 4166 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Fe(II)+HO•→Fe(III)+OH−(4) The rates for Eq. (1–4) strongly depend on the relative concentrations of the individual Fe(II) species in solution, mainly Fe2+, Fe(OH)+, Fe(OH)2, FeHCO+ 3, Fe(CO3)2− 2, and FeCO3(OH−) (Millero, 1989; King, 1998; Santana-Casiano et al., 2006; Trapp and Millero, 2007), as well as on the concentrations of O2and H2O2, pH, temperature (T)5 and salinity (S). In warm oxygenated seawater, the half-life of Fe(II) can be as low as few seconds (King, 1998), whereas in cold surface or suboxic waters it can be on the order of hours to days (Croot et al., 2001, 2008; Hansard et al., 2009; Moffett et al., 2007). Several mechanisms provide Fe(II) in the dissolved phase and are reviewed by10 Hansard et al., 2009). They consist of in-situ processes (both abiotic and biotic) and external sources. The abiotic in-situ processes are mainly photochemical reactions. They include photoreduction of dissolved Fe(III) (oxy)hydroxides, and photoreduction or photolysis of organic, colloidal and particulate Fe (Rich and Morel, 1990; Wells et al., 1991; Kuma et al., 1992a, b; King et al., 1993; Barbeau et al., 2001; Rijkenberg et15 al., 2006). Extracellular reduction of Fe(III) by photochemically-produced superoxide or reductive dissolution of particulate Fe(III) may also occur (Voelker and Sedlak, 1995; Rose and Waite, 2002, 2003; Kustka et al., 2005; Rose et al., 2005; Salmon et al., 2006). Biotic in-situ processes include bioreduction of organic Fe(III) at cell surface (Maldonado and Price, 2001, 1999; Shaked et al., 2004; Morel et al., 2008), reduc-20 tion by biogenic superoxide (Rose and Waite, 2002, 2003; Salmon et al., 2006), as well as remineralization via microbial activity (Alldredge and Cohen, 1987), cell lysis (Gobler et al., 2002), and grazing (Hutchins and Bruland, 1994; Hutchins et al., 1995; Sarthou et al., 2008). The external sources are atmospheric inputs (Kieber et al., 2001; Journet et al., 2007; Ozsoy and Saydam, 2001), sediment inputs (Elrod et al., 2004;25 Lohan and Bruland, 2008), submarine groundwater discharge (Windom et al., 2006), and hydrothermal vents (Coale et al., 1991; Chin et al., 1994; Field and Sherrell, 2000; Statham et al., 2005; Bennett et al., 2008). These sources supply Fe(II), which can then be transported by advective and/or diffusive mixing. 4167 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Although it is now evident that Fe(II) plays a key role in Fe chemistry and bioavailability in the ocean, there are relatively few open-ocean measurements of Fe(II) available due to the difficulty to measure such an ephemeral species at subnanomolar concentrations (Bruland and Rue, 2001). To our knowledge, the most comprehensive data set of Fe(II) was published by Hansard et al. (2009) in the Pacific Ocean along a zonal5 transect at 30◦N and a meridional one at 152◦W, within the CLIVAR/CO2Repeat Hydrography Program. In this paper, we present results of the Bonus-GoodHope (BGH) cruise, carried out in February-March 2008 during the International Polar Year in the Sub-Tropical South Atlantic and the Southern Ocean. Fe(II) distributions as well as oxidation rates are presented and results are discussed considering different processes10 such as photoreduction, oxidation, biological production, and different inputs, such atmospheric, sediment, and hydrothermal inputs, and/or advection and mixing. 2 Materials and methods 2.1 Study area Sampling and ship board measurements were done aboard R/V Marion Dufresne from15 8 February to 24 March 2008 in the Atlantic sector of the Southern Ocean during the BGH cruise. Figure 1 shows the cruise track together with the main oceanographic fronts and domains crossed during the cruise, from north to south: (i) the subtropical domain and the southern subtropical front (S-STF), (ii) the Antarctic Circumpolar Current (ACC) domain with 3 fronts crossed, the subantarctic front (SAF), the polar front20 (PF) and the southern ACC front (SACCF), and (iii) the eastern part of the Weddell Sea gyre with the southern boundary (SBdy) separating this domain from the ACC. Twelve stations were sampled for Fe(II), among which seven were sampled between 0 and 2000 m (Large stations L1 to L7) and five between 0 to 4000 m (Super stations S1 to S5). The position of each station is reported in Fig. 1 and Table 1.25 4168 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2.2 Sample processing and analytical methods Samples were collected using acid-cleaned 12 L Go-Flo bottles. When not in use, the Go-Flo bottles were stored inside a clean van with plastic bags covering the top and the bottom including the spigots. On station, the Go-Flo bottles were transferred to the sampling deck and mounted on a Kevlar cable. Plastic bags were removed just5 after attachment to the Kevlar cable. When the expected depths were reached, bottles were tripped by a Teflon® messenger. Once back on board, the bottles were directly transferred to the clean van for sub-sampling. All sub-samples for Fe(II) measurements were immediately collected in previously 60 mL acid-cleaned high density brown polyethylene (HDPE) bottles. The maximum time between sub-sampling time from10 the Go-Flo bottle and analysis was 3 min. In order to minimize this time, no filtration was carried out, thus avoiding an underestimation of Fe(II) concentrations due to rapid oxidation. However, Fe(II) produced by fast-kinetic processes involving lithogenic or biogenic particles, such as reductive dissolution of particulate Fe(III) (Rich and Morel, 1990) or bioreduction of organic Fe(III) at cell surface (Maldonado and Price, 1999,15 2001; Shaked et al., 2004; Morel et al., 2008) is measured and tends to overestimate Fe(II) concentrations. On the other hand, filtration step can also bring some artifacts. A first one is the damage and explosion of the cells that may release Fe(II) in the dissolved phase (Hutchins et al., 1993). A second one is related to the stress of the cells during filtration that may increase the production of superoxide (Godrant et al., 2009).20 This reactive oxygen species is involved in the redox Fe cycle, but it also initiates the three-step oxidation of luminol (Ussher et al., 2005; Rose and Waite, 2001), thus potentially inducing an overestimation of the Fe(II) concentrations in the dissolved phase. In the following, the term “labile” Fe(II) is then used, since the measurement is operationally defined and the exact speciation of the measured fraction is not known (Ussher25 et al., 2007). Labile Fe(II) concentrations were determined by chemiluminescence flow injection analysis following the method of Croot and Laan (2002). As in Croot and Laan (2002), 4169 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | there was no preconcentration prior to reaction with luminol, allowing a minimal analytical time (∼80–90 s). The percentage of labile Fe(II) was calculated as the ratio of labile Fe(II) over the dissolved Fe concentration (i.e. (Fe(II)/DFe) ·100). The instrument was calibrated by standard addition using peak height measurements and freshly prepared acidified Fe(II) standards added to a surface (20–300 m) and a deep (500–2000 m)5 sample. Samples were stored at 4 ◦C in the dark for 24 h to enable complete decay of ambient Fe(II). Non-linear calibration curves were observed, due to the kinetics of luminol oxidation and free-radical generation (Rose and Waite, 2001), and a polynomial 2nd degree curve-fitting technique was used to quantify the results. The blank was determined by running an aged seawater sample (4 ◦C for 24 h). The detection limit10 was calculated as three times the standard deviation of the blank value and was equal to 0.009±0.006 nM (n=29). After each Fe(II) spike, the change in Fe(II) signal was recorded over 10 min every 80–90 s, and allowed us to estimate Fe(II) oxidation rates, similarly to Roy et al (2008). The oxidation rates may be overestimated by up to ∼10% due to pH decrease after15 standard addition (∼0.1 pH unit/standard addition). In addition to the samples used for the calibrations, for the four super stations S2 to S5, analyses were also performed with deeper samples (2300–3600 m). 2.3 Ancillary measurements Samples for dissolved Fe (Fe(III) +Fe(II), DFe) and hydrogen peroxide (H2O2) anal-20 yses were collected from the same Go-Flo bottles as for labile Fe(II) measurements in acid-washed low density polyethylene (LDPE) and high density brown polyethylene (HDPE) bottles, respectively. DFe concentrations were determined by FIA with on line preconcentration onto 8-HQ resin and chemiluminescence detection (Obata et al., 1993; modified by Sarthou et al., 2003). The comprehensive data set is published25 elsewhere (Chever et al., 2010). H2O2samples were analyzed on board within 3 h after collection using a flow injection method with chemiluminescent detection (Yuan and Shiller, 1999). A comprehensive data set will be available elsewhere (Bucciarelli 4170 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | et al., 2011). The other ancillary parameters were measured from the closest (15– 50 min) CTD cast. In-situ Tand Swere acquired from a CTD SEABIRD SBE 911+ mounted in a Niskin-rosette. Oxygen (O2) concentrations were measured on board by Winkler titration. The pH was measured in total scale at a constant temperature of 25 ◦C (pHT,25) using an automated spectrophotometric technique with m-cresol purple5 as indicator (Gonzalez-D´ avila et al., 2003). A VINDTA 3C system (Mintrop et al., 2000), with coulometer determination was used for the titration of the total dissolved inorganic carbon (CT) after phosphoric acid addition. Carbonate concentration were estimated from pHT,25, total alkalinity (potentiometrically titrated, Mintrop et al., 2000) and CT, and computed by using CO2sys.xls v12 (Lewis and Wallace, 1998).10 3 Results 3.1 Hydrography The hydrography of the area is detailed in Chever et al. (2010), based on Gladyshev et al. (2008) and using the Sand Tdata measured during the BGH cruise (Fig. 2). The subtropical domain (STZ) extended southward to the S-STF (about 42◦S, between15 station L2 and S2). Although station S2 is located south of the S-STF, its surface waters exhibit Sand Tsignatures of subtropical waters. This station will be considered in the following as a Sub-Tropical station. Further south, the domain of the ACC extended to the Southern Boundary (SBdy) (∼42◦S to ∼55◦S, stations S2 to L7). The SAF, PF and SACCF were found at ∼44◦S, 50◦S, and 51◦S, respectively. South of the Sbdy (station20 S5), waters were entrained in the large scale cyclonic flow of the Weddell gyre. Along the transect, several major water masses were sampled. They are described elsewhere (Arhan et al., 2011; Speich et al., 2011) and briefly summarized here and on Fig. 2. In the subtropical domain, the central water layer was mostly occupied by waters of Indian Ocean origin (Boebel et al., 2003). Below, the Antarctic Intermediate Water25 (AAIW), the Upper Circumpolar Deep Water (UCDW), the diluted North Atlantic Deep 4171 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | the mixed layer and may rely on small scale remineralisation of sinking material (Cai et al., 2008, Maiti et al., 2010). During the BGH cruise, high resolution depth profiles were also obtained for 234Th/238U ratio and a more detailed data set is available from the surface to 1000 m depth (Planchon et al., 2011). Except at station L6, our labile Fe(II) maxima also coincided with increases in 234Th/238U ratios in the subsurface5 (Fig. 7). 234Th/238U ratios ranging from 1 to maximum 1.3 clearly indicated fast and intense remineralization of sinking material in the mesopelagic zone (100–600 m), which was further confirmed by parallel biogenic particulate barium data (Fig. 7). Therefore one of the most likely sources of labile Fe(II) in the subsurface might be remineralization/disaggregation of biogenic particles settling from above. Moreover, the very large10 sub-surface maxima in the STZ were consistent with a bloom in a senescent stage (Ras and Claustre, pers. com.). The value of %Fe(II)/DFe at these depths can be as high as 50–70%, suggesting that biogenic Fe is mainly regenerated as Fe(II) species, as already observed in other studies (Hutchins and Bruland, 1994; Sarthou et al., 2008). The remineralization of organic matter (OM) is associated with the consumption of oxy-15 gen and the apparent oxygen utilization (AOU) can also provide a quantitative estimate of the amount of material that has been remineralized (Sarma et al., 2007). In our study, no coincidence between the sub-surface maxima of labile Fe(II) and AOU was observed (not shown). Indeed, the maxima of AOU were around 1000–1200 m in the STZ and in the ACC north of the PF, and around 300–800 m south of the PF. The very20 particle reactive 234Th has a short half-life (24.1 days), and may then help to trace remineralization processes with a much shorter half-life than when using the AOU, and which occur locally. In the southern part of our transect (stations L6, S4, L7, S5, Fig. 8), the winter waters were strongly visible on the T data and were associated with local minima of labile Fe(II)25 concentrations. At the end of the winter, the stratification of the water column divides the Antarctic surface waters into two water masses that evolve differently: the Antarctic surface summer waters (AASSW) and the Antarctic winter waters (AAWW). The cold AAWW are less affected by phytoplankton activity and keep the signal of the winter 4178 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | time. Thus, when the concentrations of a given element in the AAWW is higher than the ones in the AASSW, the variation of concentrations can be assumed to represent the biological uptake (Sarthou et al., 1997; Blain et al., 2007). In our case, labile Fe(II) concentrations are lower in the AAWW. This could reflect processes involving a biological source of Fe(II) in the AASSW and confirm results observed in the SML.5 4.2.3 Below 1500 m Only two stations showed labile Fe(II) concentrations higher than the detection limit below 1500 m. Station S2 was located at the vicinity of the Agulhas Ridge and local maxima of DFe were observed there (Chever et al., 2010), suggesting hydrothermal or sediment inputs (Elrod et al., 2004; Boyle and Jenkins, 2008; Bennett et al., 2008;10 Tagliabue et al., 2010). %Fe(II)/DFe was not very high (1.4–5%) and Fe(II) half-life was the lowest of the section (Fig. 3b), suggesting that Fe(II) is continuously provided to the deep waters but reoxidized quite fast. During the ANT XXIV/3 expedition, Klunder et al. (2010) and Middag et al. (2010) evidenced hydrothermal inputs of Fe and Mn in the Bouvet region (52–56◦S). The hydrothermal signal was not clearly seen on our DFe15 (Chever et al., 2010) nor on our labile Fe(II) data, likely due to a lower resolution of our sampling. At station S5, concentrations of labile Fe(II) as high as 0.050 nM were observed at 3500 m, with %Fe(II)/DFe equal to 13%. Along the zero meridian, DFe concentrations in the deep waters north of the Weddell Gyre (0.47±0.16 nM, n=98, Klunder et al., 2010, and 0.42±0.07, n=4, Chever et al., 2010) were higher than south20 of the Weddell Gyre (0.33±0.14 nM, n=98, Klunder et al., 2010). North of the Weddell Sea Gyre, the deep waters flow eastward, and might have had a recent contact with the northern limit of the Weddell Basin (Orsi et al., 1993; Meredith et al., 2000; Klatt et al., 2005), flowing along the North Weddell Ridge. A local reductive dissolution of particles coming from the slope sediments of the ridge may explain the high values of25 labile Fe(II) at this station. 4179 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 4.3 Oxidation rates The Fe(II) half-life values in the surface waters were similar to previous values estimated in natural surface seawater at near-ambient concentrations (6–28 min, Croot et al., 2008). Our values were higher in the deep than in the surface waters. To our knowledge, our study is the first one to measure Fe(II) oxidation rates in natural deep5 seawater at near-ambient concentrations. The deep values were never as high as the ones estimated by Hansard et al. (2009, up to 690 min) in the Pacific Ocean. However, their [O2] at ∼1000 m were as low as 13 µM, whereas [O2] was never lower than 100 µM along the BGH transect. The overall oxidation rate of Fe(II) is a function of oxidant concentrations (e.g. oxygen, hydrogen peroxide, superoxide, etc., Gonzalez-Davila et al.,10 2006), T, pH, as well as Fe(II) chemical speciation (mainly Fe2+, Fe(OH)+, Fe(OH)2, FeCO3, Fe(CO3)2− 2, Fe(CO3)(OH)−, Millero, 1989; King, 1998; Santana-Casiano et al., 2006; Trapp and Millero, 2007). To compare our data with theoretical ones, we used two published models of Fe(II) oxidation kinetics and in situ physical-chemical conditions (Model I: Santana-Casiano et al., 2005; Model II: Trapp and Millero, 2007;15 see supplementary material for detailed calculations). Theoretical values are given in Table 3. The two models differ in the equations used for the calculations of the oxidation rate constants of the individual species for oxidation by oxygen (see Supplement). Moreover, Model I considers the Fe(CO3)(OH)−species, as well as the oxidation by the superoxide. None of the two models considers organic matter effects, and differences20 in oxidation rates among samples are only related to T, pH, S, and carbonate effects. In the upper waters, for Model I, the Fe(II) half-life times ranged from 7.5 to 18.4 min, with a mean value of 12.4±3.0 min. For Model II, values ranged from 5.5 to 13.6 min, with a mean value of 9.5±2.3 min. The ranges of variations of the two theoretical data sets were similar, although a paired-t test showed that the two data sets were signifi-25 cantly different (P < 0.01, n=12), with values from Model I always higher than values from Model II (up to 5 min). The measured t1/2(2.9–11.3 min, mean value 6.7±2.6 min) showed systematically lower values than the theoretical ones of both models (by up to 10–15 min at station L2), except at station S5 where the measured value was slightly 4180 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | higher than the Model II one. A much larger difference was observed in the deep waters between the two models. The Model I values ranged from 86 to 138 min (mean value 112±19 min), whereas the Model II ones were about 4 times lower, ranging from 19 to 32 min (mean value 26±5 min). This difference may come from the uncertainties in the model parameterization and/or on superoxide concentrations which could5 vary at depth more than oxygen concentration (see Supplement). However, what both models indicated was that in the deep waters, the half life times were almost constant (less than a factor of two) compared to the measured values which varied by a factor of 7 (10–72 min). Indeed, in the deep waters, the pH and Tranges of variation are relatively small, inducing a small range of variation of theoretical values. The largest10 range of variation of the measured values could be induced by a change in oxygen concentrations. Indeed, oxygen may have been consumed by micro-organisms in the sample during the 24 h storage or increase if an oxygen contamination occurred. Also, when the oxidation rates were measured at 4 ◦C, the deep samples and some of the surface samples were heated while most of the surface samples were cooled. These15 differences can affect the intermediate species and equilibrium processes. Moreover, after heating, changes in pH due to CO2dissolution-exchange can modify the Fe(II) speciation. Another explanation for the variability of the measured t1/2and the discrepancy between measured and theoretical is organic complexation. Although dissolved Fe(III) is20 now well known to be strongly bound by organic chelators in seawater (Gledhill and van den Berg, 1994; Rue and Bruland, 1997; Gerringa et al., 2006, 2008; Thur´ oczy et al., 2010), organic complexation of Fe(II) has been suggested but never directly measured (Croot et al., 2007, 2008; Roy et al., 2008). In the Subarctic Pacific, Roy et al. (2008) observed a significant difference between the measured Fe(II) oxidation rates in natural25 surface water and the ones in UV-treated surface water, which strongly suggested that organic ligands influenced Fe(II) speciation in seawater. Like for Fe(III) species, the Fe(II) organic speciation may help maintaining Fe(II) in the dissolved phase. However, numerous studies on the effect of Fe organic complexation on the oxidation kinetics 4181 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | showed that the organic complexation can either increase or decrease the Fe(II) oxidation rates (Rijkenberg et al., 2006 and references herein). Variability of organic compounds within the water column could thus induce variability in the observed oxidation rates. 5 Conclusions5 Concentrations of labile Fe(II) in the surface waters were systematically higher than the detection limit of our analytical method. The highest values were observed where sampling was done between 12:00 and 16:00, suggesting a biological production of Fe(II) in the SML linked to photosynthesis. South of the section, local minima coinciding with the Winter Waters confirm that direct biological reduction of Fe(III) may occur in the10 SML. This would explain why our nighttime surface samples have concentrations higher than the detection limit. At intermediate depths, sub-surface maxima were observed all along the section, although more pronounced in the STZ. A bloom at a senescent stage in the STZ, together with a good consistency between the maxima of labile Fe(II) and the increase in 234Th/238U towards values over 1 suggested that Fe remineralization15 occurred at those depths and that Fe was mainly regenerated as Fe(II) species. In the deep waters, labile Fe(II) concentrations were higher than the detection limit at two stations: one located at the vicinity of the Agulhas ridge and another one in the north of the Weddell Gyre. Here we propose that this was likely due to hydrothermal and/or sediment inputs. Fe(II) oxidation rates were measured in the surface and deep20 waters. Our study is the first one, to our knowledge, to measure Fe(II) oxidation rates in natural deep seawater at near-ambient concentrations. In the deep waters, t1/2 values were on average 6 times higher than in the surface waters. The comparison of our measured t1/2with theoretical ones using two different models suggested that organic complexation may strongly influence the oxidation rates, although more studies25 are needed to better constrain the organic speciation of Fe(II) and its influence on the half-lifes of Fe(II). The global data set of Fe(II) also needs to be increased and this will be done in the framework of the GEOTRACES programme and the associated cruises. 4182 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Supplementary material related to this article is available online at: http://www.biogeosciences-discuss.net/8/4163/2011/ bgd-8-4163-2011-supplement.pdf. Acknowledgements. We are grateful to Frank Dehairs, the cochief scientist with S. S., the captain and the crew of the R. V. Marion Dufresne for their excellent support and commitment5 onboard. This work was supported by the Institut National des Sciences de L’Univers of the Centre National de la Recherche Scientifique, the French Polar Institute (Institut Polaire Emile Victor), the French Research Institute for Exploitation of the Sea, and the National Agency for Research Funding (ANR-07-BLAN-0146). 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M.: Measurements of superoxide radical30 concentration and decay kinetics in the Gulf ofAlaska, Deep-Sea Res. Pt. I, 57, 1111–1119, 2010. Hong, H. and Kester, D. R.: Redox state of iron in the offshore waters of Peru, Limnol. 4187 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Table 1. Location of the stations sampled during the cruise in relation to the domains and fronts crossed. Mixed layer depth observed from the vertical profile of temperature for nearby CTD station, as well as day and time of sampling are indicated. Dates are in DD/MM/YYYY format (D =day, M =month and Y =year). Oceanographic Fronts Station Position MLD (m) Day of Time of Domain crossed sampling sampling Subtropical L1 34.43◦S, 14.40◦E 50–60 17/02/2008 10:50 S1 36.50◦S, 13.10◦E 40–50 20/02/2008 00:15 L2 41.18◦S, 09.92◦E 25 25/02/2008 23:15 S-STF 42.2◦S ACC S2 42.47◦S, 08.93◦E 50–80 27/02/2008 18:30 SAF 44.2◦S L3 44.88◦S, 06.88◦E 60–80 01/03/2008 17:45 L4 46.02◦S, 05.87◦E 80 03/03/2008 05:40 S3 47.55◦S, 04.37◦E 80-100 05/03/2008 15:50 L5 49.03◦S, 02.84◦E 100–110 07/03/2008 16:30 PF 50.2◦S L6 50.38◦S, 01.33◦E 60–80 09/03/2008 04:05 SACCF 51.5◦S S4 51.85◦S, 00.00◦E 120–150 10/03/2008 12:10 L7 55.23◦S, 00.03◦E 80–110 13/03/2008 22:30 Sbdy 55.5◦S Eastern part of the S5 57.55◦S, 00.03◦W 100 16/03/2008 16:00 Weddell Sea Gyre (EWSG) 4194 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Table 2. Labile Fe(II) concentrations and percentage of labile Fe(II) over dissolved Fe (DFe, Chever et al., 2010). Uncertainties on the concentrations correspond to standard deviation of a same sample measured 3 times. EWSG =eastern part of the Weddell Sea Gyre. nd =not determined, when no DFe data were available. Domain Station Position Bottom Depth Fe(II) STD Fe(II)/DFe STD depth (m) (m) (nM) (nM) (%) (%) Subtropical L1 34.43◦S, 14.40◦E 4505 20 0.017 0.001 7.2 0.6 40 0.016 0.001 3.2 0.2 60 0.025 0.001 nd 80 0.063 0.003 30.1 1.8 200 0.076 0.004 10.2 0.5 700 0.033 0.002 2.8 0.2 800 0.049 0.002 7.7 0.5 1000 0.035 0.002 5.4 0.3 1200 0.017 0.001 2.7 0.2 2100 0.014 0.001 2.1 0.1 S1 36.50◦S 13.10◦E 4915 20 0.038 0.002 5.5 0.3 30 0.039 0.002 5.5 0.4 40 0.099 0.005 nd 70 0.107 0.005 nd 300 0.112 0.006 nd 500 0.107 0.005 nd 700 0.091 0.005 nd 1000 0.039 0.002 3.9 0.3 1200 0.029 0.001 3.1 0.2 1400 0.015 0.001 2.0 0.1 1600 0.009 0.000 1.3 0.3 2000 0.009 0.000 nd 2700 0.009 0.000 1.4 0.1 3050 0.009 0.000 1.0 0.0 3500 0.009 0.000 1.6 0.1 3800 0.009 0.000 0.8 0.0 4000 0.010 0.000 0.6 0.0 L2 41.18◦S 09.92◦E 4525 15 0.040 0.002 25.4 1.3 35 0.009 0.000 nd 45 0.009 0.000 1.5 0.2 95 0.094 0.005 32.3 3.1 300 0.102 0.005 nd 600 0.047 0.002 6.4 0.4 800 0.025 0.001 2.3 0.2 1200 0.022 0.001 2.7 0.2 1400 0.015 0.001 2.0 0.2 2100 0.009 0.000 1.0 0.1 4195 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Table 2. Continued. Domain Station Position Bottom Depth Fe(II) STD Fe(II)/DFe STD depth (m) (m) (nM) (nM) (%) (%) ACC S2 42.47◦S 08.93◦E 4070 15 0.024 0.001 13.3 1.6 30 0.019 0.001 14.6 2.1 35 0.021 0.001 21.5 2.8 45 0.116 0.006 nd 196 0.125 0.007 70.4 5.8 314 0.105 0.006 62.3 4.9 461 0.102 0.006 nd 598 0.093 0.005 nd 809 0.088 0.005 20.2 1.3 1029 0.078 0.004 18.0 1.3 1250 0.071 0.004 15.5 1.1 1441 0.051 0.003 7.9 0.5 1764 0.032 0.002 5.0 0.3 2156 0.033 0.002 3.2 0.2 2548 0.027 0.001 3.4 0.2 2891 0.020 0.001 1.4 0.1 3234 0.025 0.001 3.2 0.3 3626 0.020 0.001 2.8 0.2 3940 0.023 0.001 3.7 0.2 L3 44.88◦S, 06.88◦E 4315 30 0.043 0.003 nd 100 0.019 0.002 9.3 1.7 150 0.017 0.002 14.4 2.7 270 0.023 0.002 11.6 1.4 400 0.016 0.002 4.7 0.7 600 0.013 0.001 3.1 0.4 1200 0.009 0.001 0.9 0.1 1400 0.009 0.001 1.1 0.1 2100 0.011 0.001 1.8 0.2 L4 46.02◦S 05.87◦E 4147 30 0.035 0.002 21.0 3.2 60 0.030 0.002 nd 100 0.046 0.002 22.3 2.3 150 0.023 0.001 10.1 1.1 270 0.029 0.001 10.4 0.7 480 0.024 0.001 6.3 0.7 800 0.018 0.001 5.3 0.3 1600 0.018 0.001 2.5 0.2 2050 0.016 0.001 2.1 0.2 S3 47.55◦S 04.37◦E 4480 20 0.065 0.003 40.2 7.0 30 0.066 0.003 38.7 2.8 40 0.032 0.002 nd 70 0.021 0.001 11.5 1.4 100 0.018 0.001 9.6 1.6 200 0.048 0.002 33.8 3.9 300 0.063 0.003 22.6 1.4 450 0.055 0.003 19.1 1.0 600 0.050 0.002 12.2 0.9 800 0.041 0.002 7.3 0.5 1070 0.037 0.002 5.6 0.4 1500 0.028 0.001 4.6 0.3 2020 0.018 0.001 1.6 0.1 4196 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Table 2. Continued. Domain Station Position Bottom Depth Fe(II) STD Fe(II)/DFe STD depth (m) (m) (nM) (nM) (%) (%) 2500 0.015 0.001 1.9 0.2 3000 0.010 0.001 1.7 0.1 3500 0.009 0.000 1.2 0.1 3980 0.010 0.001 1.3 0.1 L5 49.03◦S 02.84◦E 4025 40 0.018 0.001 12.6 1.4 80 0.060 0.001 nd 150 0.065 0.002 49.2 1.7 170 0.087 0.002 nd 250 0.055 0.002 25.6 1.2 350 0.048 0.003 13.6 1.4 700 0.036 0.004 6.0 1.0 1000 0.039 0.003 7.7 0.7 1600 0.026 0.003 5.5 0.8 2200 0.023 0.001 3.2 0.2 L6 50.38◦S 01.33◦E 3576 30 0.039 0.002 15.8 1.8 60 0.047 0.002 nd 100 0.021 0.001 9.6 1.5 135 0.015 0.001 nd 180 0.018 0.001 8.4 0.8 300 0.018 0.001 4.3 1.0 600 0.014 0.001 1.6 0.1 850 0.012 0.001 2.8 0.2 1600 0.010 0.000 2.3 0.2 2100 0.011 0.001 1.0 0.1 S4 51.85◦S 00.00◦E 2632 30 0.116 0.006 63.5 4.6 60 0.081 0.004 nd 130 0.060 0.003 44.4 4.8 160 0.064 0.003 50.6 4.9 180 0.054 0.003 29.4 1.8 250 0.066 0.003 34.5 2.6 300 0.050 0.003 24.9 2.7 350 0.060 0.003 29.6 1.9 400 0.047 0.002 21.9 1.6 500 0.057 0.003 12.8 0.7 700 0.035 0.002 8.3 0.5 900 0.049 0.002 nd 1117 0.026 0.001 5.6 0.3 1950 0.021 0.001 nd 2300 0.012 0.001 nd 2500 0.009 0.000 1.1 0.1 L7 55.23◦S 00.03◦E 2770 30 0.016 0.001 14.6 1.0 60 0.012 0.001 18.7 2.1 100 0.017 0.001 26.0 3.7 120 0.024 0.001 26.1 2.4 200 0.037 0.002 17.4 1.1 300 0.031 0.002 8.3 0.5 650 0.020 0.001 4.5 0.2 1000 0.012 0.001 2.5 0.2 1500 0.019 0.001 4.0 0.3 4197 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Table 2. Continued. Domain Station Position Bottom Depth Fe(II) STD Fe(II)/DFe STD depth (m) (m) (nM) (nM) (%) (%) EWSG S5 57.55◦S 00.03◦W 3932 30 0.063 0.003 66.6 8.4 60 0.042 0.002 44.4 2.5 120 0.017 0.001 8.3 0.6 140 0.018 0.001 24.4 3.4 190 0.014 0.001 16.0 1.0 250 0.025 0.001 22.5 1.8 350 0.011 0.001 11.8 0.7 550 0.014 0.001 12.9 1.0 750 0.021 0.001 4.4 0.5 800 0.024 0.001 7.1 0.5 1250 0.009 0.000 2.2 0.1 1700 0.009 0.000 3.5 0.2 2150 0.035 0.002 12.2 0.7 2600 0.036 0.002 10.0 0.9 3050 0.047 0.002 11.2 0.6 3500 0.050 0.003 13.1 0.8 3840 0.044 0.002 8.5 0.8 4198 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Table 3. Theoretical values of Fe(II) half lifes (in min) for surface and deep water samples at 4◦C using models of Fe(II) oxidation kinetics (Santana-Casiano et al., 2005: Model I; Trapp and Millero, 2007: Model II; see Supplement). The ionic strength has been calculated from salinity using the equation I=(19.9201*S)/(1000–1.00488*S) (Millero, 1995). Theoretical values Measured Stations Latitude (◦S) I(M) [O2] [H2O2] pH [CO2− 3] Model I Model II values (µM) (nM) (µM) t1/2 (min) t1/2 (min) t1/2 (min) Upper waters L1 34.43 0.74 233.7 27.0 8.09 216.1 7.5 5.5 2.9 S1 36.50 0.73 245.4 19.6 8.12 209.1 7.7 5.6 2.9 L2 41.18 0.71 251.3 2.4 7.99 110.3 18.4 13.6 3.8 S2 42.47 0.71 273.5 2.1 8.04 117.9 15.4 11.4 7.6 L3 44.88 0.70 293.2 8.07 128.0 12.6 9.7 6.4 L4 46.02 0.70 300.9 21.5 8.07 125.0 11.3 9.0 6.2 S3 47.55 0.70 311.3 15.1 8.08 119.8 12.0 9.4 9.0 L5 49.03 0.70 313.5 19.8 8.07 120.0 11.6 9.2 8.3 L6 50.38 0.70 326.2 22.6 8.08 112.1 12.2 9.6 5.9 S4 51.85 0.69 340.7 9.0 8.05 106.1 13.4 10.4 8.8 L7 55.23 0.70 349.2 31.1 8.06 98.8 13.1 10.3 7.0 S5 57.55 0.70 353.3 31.5 8.06 93.9 14.1 10.9 11.3 Deep waters S1 36.50 0.72 218.0 7.93 90.5 91.1 20.4 43.8 L2 41.18 0.71 187.7 1.1 7.85 74.6 127.0 30.8 63.4 S2 42.47 0.71 185.4 7.86 75.3 132.6 31.0 11.8 S2 42.47 0.72 227.8 7.88 91.3 85.6 19.2 14.6 L3 44.88 0.71 185.5 7.85 76.2 130.5 30.5 10.0 L4 46.02 0.72 187.0 7.87 85.5 114.8 26.1 39.2 S3 47.55 0.71 181.2 7.86 74.5 137.6 32.3 36.7 S3 47.55 0.72 218.7 7.84 81.7 101.3 23.3 37.6 L5 49.03 0.71 180.5 1.4 7.85 77.6 125.5 30.3 28.2 S4 51.85 0.71 183.9 7.88 79.5 126.7 29.2 33.6 S4 51.85 0.72 218.5 7.87 84.0 98.8 22.6 40.5 L7 55.23 0.72 201.9 6.3 7.92 84.4 89.9 23.4 30.9 S5 57.55 0.72 209.1 7.91 76.8 115.8 26.8 72.3 S5 57.55 0.72 252.4 7.85 79.7 89.8 20.8 47.2 4199 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Table 4. Fe(II) concentration ranges in previous published studies. Latitude/ Longitude Environments/ Experiments Depth range (m) [Fe(II)] range Reference 33.43◦S–57.55◦S/14.40◦E– 00.03◦W Open Ocean South Atlantic/Southern Ocean 15–4000 m 9–125 pM This study 30◦N/135◦E–118◦W 14◦S–56◦N/152◦W Open Ocean Pacific Ocean and northern Philippine Sea 13–1010 m <12–280 pM <12–76 pM Hansard et al. (2009) 3.0◦S–9.1◦N/140◦W Open Ocean Equatorial Pacific 0–100 m <120–530 pM O’Sullivan et al.( 1991) 178.72◦E/46.24◦S Open Ocean, South West Pacific, FeCycle experiment. 2 m Up to 46 pM (during nighttime) Croot et al. (2007) 23◦180S–24◦480S/8◦390E– 9◦590E Open Ocean, South Atlantic 1–2 m <12–45 pM Bowie et al. (2002) 50◦920S–51◦250S/143◦380E– 143◦030E Open Ocean, Southern Ocean, Subantarctic Front 1–2 m <12–29 pM Bowie et al. (2002) 42–51◦N/23◦E–2◦W Eastern North Atlantic, European continental shelf and English Channel 2 m <160 pM (oceanic waters) Up to 250 pM (shelf waters) 500–1800 pM (coastal waters). Boye et al. (2003) 37–42◦N/23◦W Eastern North Atlantic 0–2000 m <100–550 pM Boye et al. (2006) 46◦N–52.4◦N/ 8◦W–4.3◦E European continental margin. Open Ocean and shelf waters 3–4000 m <12 →200pM Ussher et al. (2007) 9.02◦S–12.27◦S/127.43◦E– 144.19◦E Northern Australian shelf waters 2–3 m Up to 3 nM Waite et al. (1995) 31.53◦N-h56.50◦N/ 0.39◦E–0.83◦E Northern North Sea 0–70m Up to 1.2 nM Gledhill and van den Berg (1995) 9.5◦S–10.9◦S/78.1◦W–79.1◦W Suboxic zone, near the coast of Peru 0–2300 m Up to 40 nM Hong and Kester (1986) 17◦N–23.5◦N/ 57◦E–74◦E Suboxic zone, Arabian Sea 0–1000m Up to 600 pM Moffett et al. (2007) 15◦N–18◦N/ 105◦W–115◦W Oxic-suboxic zone, Eastern Tropical North Pacific 0–300 m Up to 150 pM Hopkison and Barbeau (2007) 61◦S/140◦E Southern Ocean SOIREE fertilization experiment 2–3 m Up to 1 nM Croot et al. (2001) 48◦S/21◦E Southern Ocean EISENEX fertilization experiment 0–100 m Up to 1 nM Croot and Laan (2002); Croot et al. (2005) 56.2◦S-6-6◦S/172◦W Southern Ocean SOFEX fertilization experiment 2–3 m Up to 300 pM in patch Croot et al. (2008) 50◦S/2◦E Southern Ocean EIFEX fertilization experiment 2–3 m Up to 800 pM in patch Croot et al. (2008) 46.7◦N/165.8◦E Sub-Arctic Pacific Ocean SEEDS II fertilization experiment 0–80 m UP to >200 pM in patch Roy et al. (2008) 4200 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Figure 1 Fig. 1. Location of the stations sampled during the BONUS-GoodHope cruise along with the three main oceanographic provinces encountered. The three domains crossed were the subtropical domain (stations L1, S1 and L2), the ACC domain (stations S2, L3, L4, S3, L5, L6, S4 and L7) and the eastern part of the Weddell Sea Gyre (station S5). Five fronts were crossed: the southern-subtropical front (S-STF), the sub Antarctic front (SAF), the polar front (PF), the southern ACC front (SACCF) and the southern boundary (Sbdy). 4201 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 34 34.5 35 35.5 PSU Salinity 0 1000 2000 3000 0 1000 2000 3000 4000 5000 Depth (m) Distance (km) S-STF SAF PF SACCF SBdy 35°S 40°S 50°S 55°S 45°S SE-NADW SW-NADW AABW I-AAIW A-AAIW LCDW UCDW L1 S1 L2 S2 L3 L4 S3 L5 L6 S4 L7 S5 (a) (b) 150 200 250 300 μmol/kg Oxygen 0 1000 2000 3000 0 1000 2000 3000 4000 5000 Depth (m) Distance (km) S-STF SAF PF SACCF SBdy 35°S 40°S 50°S 55°S 45°S SE-NADW SW-NADW AABW I-AAIW A-AAIW LCDW UCDW L1 S1 L2 S2 L3 L4 S3 L5 L6 S4 L7 S5 (c) Figure 2 Fig. 2. Vertical distribution of salinity (a), theta (b), and oxygen (c) measured along the transect from the north (left) to the south (right) of the section. Water masses are indicated: AAIW: Antarctic Intermediate Water. This water mass is coming from the Indian Ocean through the Agulhas Current (I-AAIW) north of ∼37◦S and from the Atlantic sector (A-AAIW) south of 37◦S (Gordon et al., 1992). NADW: North Atlantic Deep Water. The highest salinity values close to the African continental slope reflect advection by a southeastward deep boundary current (SE-NADW,Arhan et al., 2003). LCDW: Lower Circumpolar Deep Water and AABW: Antarctic Bottom Water. 4202 BGD 8, 4163–4208, 2011 Labile Fe(II) concentrations in the South Atlantic G. Sarthou et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | (a) (b) Figure 3 Fig. 3. Vertical section of (a) labile Fe(II) in nM and (b) %Fe(II)/DFe from the north (left) to the south (right) of the section. DFe data are from Chever et al. (2010). 4203