|Comments 2nd version Jilbert et al.|
The authors have made very substantial changes to the first version and the revised version has been much improved. The authors have also thoroughly reinterpreted their data and have drawn new conclusions compared to the first version. Sedimentary iron species concentration data were recalculated and corrected and Mössbauer spectra at Station A were re-interpreted as inorganic iron oxides instead of organic Fe. Altogether, this is a very far-reaching re-write, which casts the data in a new light. I would like to thank the authors for their pro-active attitude. I still have some remaining concerns about the interpretation of the organic Fe (II) phase, which is still prominent in the abstract, because the authors rely on an operational Fe extraction without more direct evidence (see below). Since the manuscript is new in many parts, I have some comments that arise from this revised version that I would like to see addressed:
What I find confusing in this version and which I hope the authors can improve is that the extensive methods description to extract labile and refractory Fe infers inorganic Fe phases, but not one organic Fe phase, although their suggested presence is one of the major conclusions of the manuscript. Mössbauer spectra are only presented for Fe-oxalate. The fit of the spectra, however, does not suggest that Fe-oxalate is abundant in the sediment, not really a surprise. The way the authors conclude that organic Fe(II) are dominant is thus by negative evidence and mainly based on their calculation procedure to determine non-sulfidized Fe (II) i.e., based on the correct assessment of AVS-Fe from the sulfide extraction and the fact that ALL 1M extracted Fe can be classified as labile. Since this is an operational definition, the interpretation of a non-accounted difference in Fe as organic Fe remains tentative.
I am also concerned that the freeze-drying process alters the mineralogy and leads to some oxidation. This could have reduced the AVS-sulfide concentration and could be an explanation why non-sulfide Fe (II) is so high (and, consequently, interpreted as organic Fe (II) ). Finally, the preservation of these substantial amount of organic Fe (II) in light of the observation that DOM and Fe are decoupled during estuarine mixing and not deposited with each other seem like a contradiction with the assessment of abundant organic Fe, in particular since concentration profiles of Fe (II) interpreted as organic fe (II) show an increase below the sediment surface. There are likely also reactive silicate phases involved in the early diagenetic Fe cycling.
Station A is inferred to contain a substantial portion of smelter-derived Fe. This needs to be made clearer in the methods description part, since it is also presented as a potentially extracted component with 1M HCl. In Figure 5 Station A shows concentrations of Fe up to 28% by weight of sediment (> 5000 µmol Fe/g sediment). This is unusually high, as noted by the authors. A simple calculation: If the Fe mineral phase were the most Fe-rich mineral possible, i.e., FeO (MW = 88 g/mol; with atom Fe proportion = 0.7), then 28% Fe by weight translates to 36% of the weight of the whole sediment represented by the FeO mineral at this depth. For Magnetite the proportion is 39%. It seems very unusual that more than every third mineral in this sediment is one of the two minerals, even if they are smelter-derived. Was material dumped in the estuary? I think it is noteworthy that the apparent dilution of the other sediment components by this smelter-Fe is not visible in the total S or organic C profiles. Further, I advise against using any of the Station A sediment data – solid-phase or porewater to generalize overall Fe dynamics, because of this contamination anomaly.