Fe‐Binding Organic Ligands in the Humic‐Rich TransPolar Drift in the Surface Arctic Ocean Using Multiple Voltammetric Methods

Abstract

Samples inside and outside the Arctic Ocean's TransPolar Drift (TPD) have been analyzed for Fe‐binding organic ligands (Lt) with Competitive Ligand Exchange Adsorptive Stripping Voltammetry (CLE‐AdCSV) using salicylaldoxime (SA). This analysis is compared to prior analyses with CLE‐AdCSV using 2‐(2‐thiazolylazo)‐p‐cresol (TAC). The TPD's strong terrestrial influence is used to compare the performance of both CLE‐AdCSV methods in representing the nature of natural organic ligands. These measurements are compared against direct voltammetric determination of humic substances (HS) and spectral properties of dissolved organic matter. The relationship between the two CLE‐AdCSV derived [Lt] versus HS in the TPD has a comparable slope, with a 40% offset toward higher values obtained with SA. Higher [Lt] values inside the TPD, most probably due to HS, explain high dissolved Fe concentrations transported over the Arctic Ocean by the TPD. Outside of the TPD in the surface Arctic Ocean HS occur as well but at lower concentrations. Here changes in HS relate to changes in dissolved Fe concentration and to [Lt] obtained with SA, whereas [Lt] obtained with TAC remain constant. Moreover, with decreasing HS the offset between the methods using TAC and SA decreases. We surmise that in the presence of HS, the TAC method detects HS only either at higher concentrations or of specific composition. On the other hand, the SA method might overestimate [Lt], as an offset with the TAC method that remains constant where HS are not detected. Regardless, HS are the dominant type of Fe‐binding organic ligand in the surface of the Arctic Ocean.

The organic complexation of iron in the Ross sea

“…By convention, a number 1 is assigned to the strongest ligand group and a number 2 to the weaker ligand group. (Slagter et al, 2019). In the present study, 51 out of 72 samples (70%) gave a good correspondence between the two models, as indicated by the ratio [L1] + [L2]/[Lt] being close to 1 ( Table 2).…”

“…In humic rich surface waters of the Arctic [Lt] estimated with TAC did increase with the humic content, but [Lt] was lower than DFe and lower than [Lt] obtained with salicylaldoxime (SA, Rue and Bruland, 1995;Buck et al, 2015). According to Slagter et al (2019) the method using TAC detects humic-like substances only at high concentrations or only of specific composition. Thus in this research we cannot exclude the BNL as a possible source of DFe-binding organic ligands, we can only conclude that the BNL is probably not a large source.…”

“…However, these maxima do not coincide with high DFe-binding organic ligand concentrations, implying that the BNL is only a source of Fe and not of ligands. Our analysis method used TAC as competing ligand, which can miss part of the humic DFe-binding ligands (Laglera et al, 2011, Slagter et al, 2019Dulaquais et al, 2018). Therefore it is possible that the ligands are underestimated if humic-like substances are present, which might be the case in the BNL.…”

“…The ability to detect two ligand groups may be related to the specific analytical method used. Multiple ligand groups are found much more often using SA as competing ligand in the CLE-CSV technique than when using TAC (Bundy et al 2015;Buck et al 2016;Slagter et al, 2019). One of the reasons might be that TAC misses part of the humic-like substances as discussed above.…”

The Transpolar Drift as a Source of Riverine and Shelf‐Derived Trace Elements to the Central Arctic Ocean

“…Hence, increased macronutrient and micronutrient concentrations delivered to the central Arctic Ocean via the TPD may play an important role in upper ocean productivity in the coming decades, since, for example, nitrate already limits primary production in some Arctic locations (Tremblay & Gagnon, 2009), as did Fe in the case of one large under ice bloom, though this was located outside of the TPD (Rijkenberg et al, 2018). In the case of Fe, whether limitation will occur in the changing Arctic will depend on the interplay between nutrient utilization ratios (Rijkenberg et al, 2018) and projected increases of ligand-borne, specifically humic-borne, terrestrial dFe (Slagter et al, 2017(Slagter et al, , 2019. 10.1029/2019JC015920…”