River-derived humic substances as iron chelators in seawater

Regina Krachler; Rudolf F Krachler; Gabriele Wallner; Stephan Hann; Monika Laux; Maria F Cervantes Recalde; Franz Jirsa; Elisabeth Neubauer; Frank von der Kammer; Thilo Hofmann; Bernhard K Keppler

doi:10.1016/j.marchem.2015.05.009

River-derived humic substances as iron chelators in seawater

Mar Chem. 2015 Aug 20:174:85-93. doi: 10.1016/j.marchem.2015.05.009.

Affiliations

¹ Institute of Inorganic Chemistry, University of Vienna, Währingerstraße 42, 1090 Vienna, Austria.
² Division of Analytical Chemistry, University of Natural Resources and Life Sciences, Vienna, Muthgasse 18, 1190 Vienna, Austria.
³ Department of Environmental Geosciences, University of Vienna, Althanstraße 14, 1090 Vienna, Austria.

Abstract

The speciation of iron(III) in oxic seawater is dominated by its hydrolysis and sedimentation of insoluble iron(III)-oxyhydroxide. As a consequence, many oceanic areas have very low iron levels in surface seawater which leads to iron deficiency since phytoplankton require iron as a micronutrient in order to grow. Fortunately, iron solubility is not truly as low as Fe(III) solubility measurements in inorganic seawater would suggest, since oceanic waters contain organic molecules which tend to bind the iron and keep it in solution. Various iron-binding organic ligands which combine to stabilize dissolved iron have been detected and thoroughly investigated in recent years. However, the role of iron-binding ligands from terrestrial sources remains poorly constrained. Blackwater rivers supply large amounts of natural organic material (NOM) to the ocean. This NOM (which consists mainly of vascular plant-derived humic substances) is able to greatly enhance iron bioavailability in estuaries and coastal regions, however, breakdown processes lead to a rapid decrease of river-derived NOM concentrations with increasing distance from land. It has therefore been argued that the influence of river-derived NOM on iron biogeochemistry in offshore seawater does not seem to be significant. Here we used a standard method based on ⁵⁹Fe as a radiotracer to study the solubility of Fe(III)-oxyhydroxide in seawater in the presence of riverine NOM. We aimed to address the question how effective is freshwater NOM as an iron chelator under open ocean conditions where the concentration of land-derived organic material is about 3 orders of magnitude smaller than in coastal regions, and does this iron chelating ability vary between NOM from different sources and between different size fractions of the river-borne NOM. Our results show that the investigated NOM fractions were able to substantially enhance Fe(III)-oxyhydroxide solubility in seawater at concentrations of the NOM ≥ 5 μg L^{- 1}. Terrigenous NOM concentrations ≥ 5 μg L^{- 1} are in no way unusual in open ocean surface waters especially of the Arctic and the North Atlantic Oceans. River-derived humic substances could therefore play a greater role as iron carriers in the ocean than previously thought.

Keywords: Dissolved organic matter; Iron; Ligands; Sea water.