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Distribution of Fe isotopes in particles and colloids in the salinity gradient along the Lena River plume, Laptev Sea
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.ORCID iD: 0000-0002-7313-5833
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.ORCID iD: 0000-0003-2276-0564
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
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2019 (English)In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 16, no 6, p. 1305-1319Article in journal (Refereed) Published
Abstract [en]

 Riverine Fe input is the primary Fe source for the ocean. This study is focused on the distribution of Fe along the Lena River freshwater plume in the Laptev Sea using samples from a 600 km long transect in front of the Lena River mouth. Separation of the particulate ( >  0.22 μm), colloidal (0.22 μm–1 kDa), and truly dissolved (<  1 kDa) fractions of Fe was carried out. The total Fe concentrations ranged from 0.2 to 57μM with Fe dominantly as particulate Fe. The loss of >  99% of particulate Fe and about 90% of the colloidal Fe was observed across the shelf, while the truly dissolved phase was almost constant across the Laptev Sea. Thus, the truly dissolved Fe could be an important source of bioavailable Fe for plankton in the central Arctic Ocean, together with the colloidal Fe. Fe-isotope analysis showed that the particulate phase and the sediment below the Lena River freshwater plume had negative δ56Fe values (relative to IRMM-14). The colloidal Fe phase showed negative δ56Fe values close to the river mouth (about -0.20 ‰) and positive δ56Fe values in the outermost stations (about +0.10 ‰). We suggest that the shelf zone acts as a sink for Fe particles and colloids with negative δ56Fe values, representing chemically reactive ferrihydrites. The positive δ56Fe values of the colloidal phase within the outer Lena River freshwater plume might represent Fe oxyhydroxides, which remain in the water column, and will be the predominant δ56Fe composition in the Arctic Ocean.

Place, publisher, year, edition, pages
European Geosciences Union (EGU), 2019. Vol. 16, no 6, p. 1305-1319
Keywords [en]
iron isotopes, estuarine mixing, iron particles, truly dissolved iron
National Category
Natural Sciences Geochemistry
Research subject
Applied Geochemistry
Identifiers
URN: urn:nbn:se:ltu:diva-73352DOI: 10.5194/bg-16-1305-2019ISI: 000462793900001Scopus ID: 2-s2.0-85063632617OAI: oai:DiVA.org:ltu-73352DiVA, id: diva2:1300578
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council for Environment, Agricultural Sciences and Spatial Planning, 621-2004-4039Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning, 211-621-2007Swedish Polar Research SecretariatSwedish Research Council for Environment, Agricultural Sciences and Spatial Planning, 2017-05687EU, European Research Council, ERC-AdG CCTOP project #695331
Note

Validerad;2019;Nivå 2;2019-04-03 (johcin)

Available from: 2019-03-29 Created: 2019-03-29 Last updated: 2019-04-25Bibliographically approved
In thesis
1. Iron isotopes in aquatic systems
Open this publication in new window or tab >>Iron isotopes in aquatic systems
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Järnisotoper i akvatiska system
Abstract [en]

The cycling of iron (Fe) is a key component for understanding water quality and biogeochemical processes. It serves as mediator during biotic and abiotic processes, as electron acceptor during the degradation of organic matter, as surface for trace element and organic matter adsorption, and is necessary for primary production processes. Since the beginning of Fe isotope studies, researchers focussed on the ratios in soils, rivers and oceans in various environments. The aim of this study was to characterize the Fe isotope ratios from the source (e.g. soils), along the river course, through the estuaries and into the adjacent sea within the boreal landscape. Therefore, seasonal sampling of water from Swedish headwater streams (2016/2017), rivers (2016), estuaries (2013/2014) and the Baltic Sea (2013/2014) were conducted, with the purpose to better understand the role and fate of riverine Fe export. Fe is transported in two main phases from the headwater streams into the oceans: organic Fe complexes and Fe(oxy)hydroxide. It has been proposed that these Fe phases varies in response to seasonal differences in hydrology.

                      This thesis includes the first Fe isotope dataset describing seasonal variations of headwater streams on a regional scale. In the headwater streams positive and negative Fe isotopes ratios can be used to distinguish between different Fe phases. Furthermore, Fe isotope ratios in headwater streams could verify regional drought periods and the subsequent rewetting of the subsurface soils.

Within the rivers and estuaries, we found positive Fe isotopes in the dissolved phase (< 0.22µm) and negative Fe isotopes (> 0.22µm) in the particulate phase during high discharge. The correlation between different chemical parameters, Fe and DOC showed that the Fe isotope composition during spring flood is evolving in the upper soil layers of headwater streams. Therefore, the lighter Fe isotope signal is correlated to the organic-rich soil layers of the riparian zones in forested catchments. During baseflow, particulate Fe has a positive Fe isotope signal. This shows that the Fe has different origin throughout the season within one catchment.

Salt-induced flocculation in the estuaries and under experimental conditions, is removing about 80 % of the dissolved and particulate Fe. Newly formed colloids and particles aggregate and sediment due to small changes in salinity. This major flocculation at low salinities might cause an underestimation of riverine Fe flux. Interestingly, salinity-induced aggregation experiments revealed that Fe(oxy)hydroxide, which dominated aggregates, displayed lower Fe isotope ratios than in the river samples Fe, while organic Fe complexes in the suspension had higher Fe isotope values. The seasonal variability in Fe isotope values could not be simply linked to Fe phases but was probably also influenced by variation in source areas of Fe and processes along the flow-path that alter both Fe phases and isotopic composition.

Within the estuarine mixing zone, no Fe isotope fractionation was observed. The Fe isotope signal is constant over time and space, which excludes fractionation processes for example by oxidation. The Fe isotope signal within the Bothnian Bay was positive showing that different surface properties of Fe-OC and Fe(oxy)hydroxide aggregates lead to the flocculation of negative Fe aggregates.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2019
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Geochemistry
Research subject
Applied Geochemistry
Identifiers
urn:nbn:se:ltu:diva-73763 (URN)978-91-7790-376-5 (ISBN)978-91-7790-377-2 (ISBN)
Public defence
2019-06-20, F531, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2019-04-25 Created: 2019-04-25 Last updated: 2019-06-05Bibliographically approved

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Conrad, SarahIngri, JohanGelting, JohanNordblad, FredrikEngström, EmmaRodushkin, IliaÖhlander, Björn

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