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

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 were 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). Apparent biological production of Fe(II) 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 maxima. 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 (<i>t</i><sub>1/2</sub>) at 4°C were measured in the upper and deep waters and ranged from 2.9 to 11.3 min, and 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 Fe redox cycle

Iron isotopes reveal distinct dissolved iron sources and pathways in the intermediate versus deep Southern Ocean

“…Measurements of DFe redox speciation (DFe II and total DFe from which DFe III can be computed) during the BGH cruise provide the opportunity to evaluate the effect of the iron redox state on its isotope signature in the open ocean. These data show that Fe(II) accumulation mostly occurs close to the surface, but not at intermediate depths (26). This mismatch between measured iron redox state and isotopic composition demonstrates that local iron reduction-at the time and location of sampling-was not responsible for the observed intermediate depth δ 56 DFe minimum.…”

Atmospheric trace elements in aerosols observed over the Southern Ocean and coastal East Antarctica

“…Sarthou et al (2011) showed that higher values of labile Fe(II) were found in the surface mixed layer than in deep waters toward high latitudes, which could be attributed to atmospheric Fe(II) deposition . Gao et al (2013) reported that total dissolvable Fe airÁsea deposition fluxes were 0.007Á0.52 mg m (2 yr (1 over the SO.…”

A Comparison between Four Analytical Methods for the Measurement of Fe(II) at Nanomolar Concentrations in Coastal Seawater

“…One exception was the humic/fulvic spiked experiment (Table 2), where 32 nM TdFe remained in suspension at the end of the experiment which most likely indicated the presence of more stable Fe(III)-organic complexes or colloids (Kuma et al, 1998) following Fe(II) oxidation. All the experiments used relatively high spikes of Fe(II) (5.2 to 31 nM) which was necessary to keep the Fe(II) concentration above the detection limit ( (Ussher et al, 2007;Sarthou et al, 2011), our experimental conditions ( Table 2) form a reasonable representation of conditions within tropical OMZs where Fe(II) concentrations on the order of 10-100 nM can be found in coastal waters (Hong and Kester, 1986;Lohan and Bruland, 2008;Vedamati et al, 2014).…”

“…As discussed further in prior work Sarthou et al, 2011), there are multiple reasons why these calculated rate constants are for indicative purposes only (e.g., O 2 may have changed over the duration of the experiments, the effect of DOC is not accounted for in this simple rate equation, the contribution of H 2 O 2 to the oxidation rate was likely nonnegligible, and the values derived from Roy and Wells (2011) are not corrected for small discrepancies in salinity or temperature). Yet, the differences between the luminol and ferrozine based methods are striking nonetheless (Figure 5).…”

“…Such extensive exercises, however, have not yet been widely conducted with transient species such as Fe(II) which, due to their short-lived and highly reactive nature, present many challenges to the analyst. The rapid oxidation rate of Fe(II) in oxic seawater (King et al, 1995;Santana-Casiano et al, 2005;Sarthou et al, 2011), and the sensitivity of Fe(II) concentration and oxidation/reduction rates to multiple variables including pH, temperature, light intensity, dissolved oxygen (O 2 ), reactive oxygen species (ROS) and natural organic material (NOM; Davison and Seed, 1983;Millero et al, 1987;Rose and Waite, 2002) make sample collection logistically challenging and mean that the accuracy of Fe(II) measurements is difficult to verify.…”