Abstract

The biogeochemical cycles of iron (Fe) and organic carbon (OC) are closely interconnected. The concentration of reactive Fe is tightly controlled by soluble organic ligands in oceanic waters. In soils, Fe stabilizes OC by forming aggregates that shield OC from degradation. In sediments however; the role of Fe in the preservation of OC is only starting to be explored. To quantify the amount of OC retained in the solid phase through its interaction with Fe, an approach based on the reductive dissolution of reactive solid phase Fe, was applied. In this work, we investigated Fe-OC interactions in lake sediments, using sediments collected from Lake Tantaré, a system with two basins characterized by contrasting redox conditions in the summer. These contrasting redox conditions provided an opportunity to assess the importance of oxic/anoxic interfaces in the formation of stable OC-Fe complexes. We found 30.1 ± 6.4% of OC directly associated with Fe minerals. We characterized the Fe-associated and the non-Fe-associated OC pools at the elemental (OC, TN), isotopic (δ13C, δ15N) and functional group (FTIR) levels. We found large differences in OC:Fe and TN:Fe ratios among the two basins which were not related to differences in OM chemical composition but rather to differences in reactive iron concentrations stemming from the higher abundance of mackinawite (FeS). Since the affinity of OM for mackinawite is much lower than for iron hydroxides, using OC:Fe and TN:Fe ratios as a diagnostic tool for the type of OM-Fe interactions should be done with care in anoxic environment.