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Characterisation of Fe-bearing particles and colloids in the Lena River basin, NE Russia
Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History, Sweden.
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Number of Authors: 102017 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 213, p. 553-573Article in journal (Refereed) Published
Abstract [en]

Rivers are significant contributors of Fe to the ocean. However, the characteristics of chemically reactive Fe remain poorly constrained, especially in large Arctic rivers, which drain landscapes highly susceptible to climate change and carbon cycle alteration. The aim of this study was a detailed characterisation (size, mineralogy, and speciation) of riverine Fe-bearing particles (> 0.22 mu m) and colloids (1 kDa-0.22 mm) and their association with organic carbon (OC), in the Lena River and tributaries, which drain a catchment almost entirely underlain by permafrost. Samples from the main channel and tributaries representing watersheds that span a wide range in topography and lithology were taken after the spring flood in June 2013 and summer baseflow in July 2012. Fe-bearing particles were identified, using Transmission Electron Microscopy, as large (200 nm(-1) mu m) aggregates of smaller (20-30 nm) spherical colloids of chemically-reactive ferrihydrite. In contrast, there were also large (500 nm(-1) mu m) aggregates of clay (illite) particles and smaller (100-200 nm) iron oxide particles (dominantly hematite) that contain poorly reactive Fe. TEM imaging and Scanning Transmission X-ray microscopy (STXM) indicated that the ferrihydrite is present as discrete particles within networks of amorphous particulate organic carbon (POC) and attached to the surface of primary produced organic matter and clay particles. Together, these larger particles act as the main carriers of nanoscale ferrihydrite in the Lena River basin. The chemically reactive ferrihydrite accounts for on average 70 +/- 15% of the total suspended Fe in the Lena River and tributaries. These observations place important constraints on Fe and OC cycling in the Lena River catchment area and Fe-bearing particle transport to the Arctic Ocean.

Place, publisher, year, edition, pages
2017. Vol. 213, p. 553-573
Keywords [en]
iron particles, Arctic reactivity, Transmission Electron Microscopy, X-ray Absorption Spectroscopy
National Category
Earth and Related Environmental Sciences
Research subject
Marine Geology
Identifiers
URN: urn:nbn:se:su:diva-146961DOI: 10.1016/j.gca.2017.07.012ISI: 000407259200031OAI: oai:DiVA.org:su-146961DiVA, id: diva2:1142406
Available from: 2017-09-19 Created: 2017-09-19 Last updated: 2018-10-12Bibliographically approved
In thesis
1. Iron in the Lena River basin, NE Russia: Insights from microscopy, spectroscopy and isotope analysis
Open this publication in new window or tab >>Iron in the Lena River basin, NE Russia: Insights from microscopy, spectroscopy and isotope analysis
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University, 2018. p. 49
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper ; 375
Keywords
iron, reactivity, isotopes, permafrost, river, Arctic
National Category
Geochemistry
Research subject
Marine Geology
Identifiers
urn:nbn:se:su:diva-160797 (URN)978-91-7797-414-7 (ISBN)978-91-7797-415-4 (ISBN)
Public defence
2018-11-23, Högbomsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript.

Available from: 2018-10-30 Created: 2018-10-05 Last updated: 2018-10-29Bibliographically approved

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