Iron isotope pathways in the boreal landscape: Role of the riparian zone

Abstract Stable Fe isotope compositions have been measured in water samples of the subarctic Kalix River, a first-order stream, and soil water samples from a riparian soil profile adjacent to the first-order stream (Northern Sweden). In the first-order stream, dominated by forest, both the particulate (>0.22 µm) and dissolved ( 56 Fe values (relative to IRMM-014) during base flowand meltwaterdischarge in May (−0.97 to −0.09‰). The Fe isotope composition in the water from the riparian soil profile varied between −0.20 and +0.91‰ with sharp gradients near the groundwater table. A linear correlation between the δ 56 Fe values and the TOC/Fe bulk ratio was measured during snowmeltin the unfilteredriver waters (δ 56 Fe from −0.02 to +0.54‰), suggesting mixing of two Fe components. Two groups of Fe aggregates, with different Fe isotope compositions, are formed in the boreal landscape. We propose that carbon-rich aggregates, Fe(II)(III)-OC, have negative δ 56 Fe values and Fe-oxyhydroxides have positive δ 56 Fe values. A mixture of these two components can explain temporal variations of the Fe isotope composition in the Kalix River. This study suggests that stable Fe isotopes can be used as a tool to track and characterize suspended Fe-organic carbon aggregates during transport from the soil, via first-order streams and rivers, to coastal sediment. Furthermore, the differences in Fe isotope values in the Kalix River and the first-order stream during base flowconditions suggest that the primary Fe sources for river water change throughout the year. This model is combining the Fe isotope composition of first-order streams and rivers to weathering and transport processes in the riparian soil.