Thesis defense of Elaheh Lotfi-Kalahroodi

A new isotopic approach to the relationships between iron and organic matter in wetlands


Wetlands are well recognized to control the biogeochemical cycle of many major and trace elements such as iron (Fe). They are submitted to temporarily soil saturation involving redox conditions variations. If several studies precise the dominant mineral, colloidal and soluble Fe species under redox variations, many uncertainties are still remained on their formation and evolution. Iron isotopic composition is a potential tool to investigate and elucidate these mechanisms. However, before be used as proxy, the exact Fe isotopic signature of a precise process has to be identified. We thus performed a series of experiment to assess to this signature in Fe-OM aggregates. The first step of this work was to verify that ultrafiltration, used to separate the Fe species, does not fractionate Fe isotopes. After the separation technique validation, a series of experi-ments was performed to elucidate the Fe isotopic signature of Fe-OM particles, colloids and soluble species and their impact on the wetland soil solution. The results demonstrated that abiotic hydrolysis does not fractionate Fe isotopes and that the complexation of Fe to OH- or OM ligands, that preferentially bind heavy Fe isotopes, controls the Fe isotopic composition of large Fe-OM colloids and particles. The processes were then tested in a wetland soil submitted to 3 successive oxic/anoxic cycles and to field dataset. Experimental data confirm that preferential Fe binding to bacterial organic ligands controls long-term Fe isotopic signature. However confrontation to field data displayed, that water flow in wetland subsurface could minimize the impact of this mechanism on the Fe isotopic composition of the wetland soil solution.