Iron in the Lena River basin, NE Russia : Insights from microscopy, spectroscopy and isotope analysis

Abstract: Iron is an important mediator of biotic and abiotic processes on the Earth’s surface, being an electron acceptor in organic matter degradation, a surface for organic matter and trace element adsorption, and a required element for enzymatic processes during primary production. Yet, the role of iron as a mediator of carbon and trace element cycling in high latitude, permafrost-dominated regions remains poorly investigated. The aim of this study was to characterise the chemical reactivity (by size separation, microscopy and spectroscopy) and sources (by isotope composition) of Fe in the Lena River and major tributaries, spanning a wide range in lithology, topography and climate. The Fe transported in the Lena River and major tributaries carries an integrated signal of Fe weathering processes across the permafrost-dominated terrain.A spatial sample set was collected during the post-spring flood period (July 2012, June 2013), from the main channel and tributaries draining contrasting topography and permafrost extent. Across the basin, Fe is mainly transported as chemically reactive ferrihydrite that spans the particulate (> 0.22 µm) and colloidal (0.22 µm – 1 kDa) fractions. The remaining Fe transported as poorly reactive detrital Fe in clays and crystalline oxides. Fe is transported in larger size fractions than the dissolved OM showing that Fe is not a major carrier of DOM. Nano-sized ferrihydrite was attached to OM in the particulate fraction, evidence of a Fe – OM particle association in the Lena River basin.Ferrihydrite shows distinct isotope values in particulate and colloidal fractions, showing that there is a difference in isotopic composition between different size fractions of the same mineral. A conceptual model was developed to understand ferrihydrite formation in the riparian zone of the Lena River and tributaries.  Particulate ferrihydrite has isotope values lower than crustal values resulting from redox and organic-ligand promoted mineral dissolution and precipitation of Fe(II)aq to form coatings of ferrihydrite on particles in the riparian zone.  Ferrihydrite colloids span a wider range of isotope values, higher than Fe particles, resulting from variations in the size and isotope composition of the Fe(II)aq pool transported in soil groundwaters, and the isotope fractionation factor for Fe oxidation and organic complexation.A temporal sample set was collected in the main channel between September 2012 – March 2013 and every three days during May 2015. The colloidal Fe shows distinct seasonal Fe isotope signatures and Fe fluxes, with isotope values lower than crustal during winter baseflow, overlying crustal values during spring flood and higher than crustal values during summer, attributed to changing sources and thus conditions for isotope fractionation. The combined understanding of Fe reactivity and isotope composition allows us to isolate the dominant sources of Fe entering the Lena River.