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Research article - Peer-reviewed, 2018

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

Ingri, Johan; Conrad, Sarah; Lidman, Fredrik; Nordblad, Fredrik; Engstrom, Emma; Rodushkin, Ilia; Porcelli, Don


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 mu m) and dissolved (<0.22 mu m) phase showed negative delta Fe-56 values (relative to IRMM-014) during base flow and meltwater discharge in May (-0.97 to -0.09 parts per thousand). The Fe isotope composition in the water from the riparian soil profile varied between -0.20 and +0.91 parts per thousand with sharp gradients near the groundwater table. A linear correlation between the delta Fe-56 values and the TOC/Fe-bulk ratio was measured during snowmelt in the unfiltered river waters (8 56 Fe from -0.02 to +0.54 parts per thousand), 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 delta Fe-56 values and Fe-oxyhydroxides have positive delta Fe-56 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 flow conditions 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. (C) 2018 The Author(s). Published by Elsevier Ltd.


Fe aggregates; Fe isotope; soilwater; Krycklan Catchment; first-order stream; snowmelt; spring flood

Published in

Geochimica et Cosmochimica Acta
2018, Volume: 239, pages: 49-60

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