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Iron isotopes in aquatic systems
Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik. (Applied Geology)ORCID-id: 0000-0002-7313-5833
2019 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)Alternativ titel
Järnisotoper i akvatiska system (Svenska)
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.

Ort, förlag, år, upplaga, sidor
Luleå: Luleå University of Technology, 2019.
Serie
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Nationell ämneskategori
Geokemi
Forskningsämne
Tillämpad geokemi
Identifikatorer
URN: urn:nbn:se:ltu:diva-73763ISBN: 978-91-7790-376-5 (tryckt)ISBN: 978-91-7790-377-2 (digital)OAI: oai:DiVA.org:ltu-73763DiVA, id: diva2:1306856
Disputation
2019-06-20, F531, Luleå, 10:00 (Engelska)
Opponent
Handledare
Tillgänglig från: 2019-04-25 Skapad: 2019-04-25 Senast uppdaterad: 2019-06-05Bibliografiskt granskad
Delarbeten
1. Distribution of Fe isotopes in particles and colloids in the salinity gradient along the Lena River plume, Laptev Sea
Öppna denna publikation i ny flik eller fönster >>Distribution of Fe isotopes in particles and colloids in the salinity gradient along the Lena River plume, Laptev Sea
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2019 (Engelska)Ingår i: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 16, nr 6, s. 1305-1319Artikel i tidskrift (Refereegranskat) 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.

Ort, förlag, år, upplaga, sidor
European Geosciences Union (EGU), 2019
Nyckelord
iron isotopes, estuarine mixing, iron particles, truly dissolved iron
Nationell ämneskategori
Geokemi
Forskningsämne
Tillämpad geokemi
Identifikatorer
urn:nbn:se:ltu:diva-73352 (URN)10.5194/bg-16-1305-2019 (DOI)000462793900001 ()2-s2.0-85063632617 (Scopus ID)
Forskningsfinansiär
Knut och Alice Wallenbergs StiftelseSwedish Research Council for Environment, Agricultural Sciences and Spatial Planning, 621-2004-4039Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning, 211-621-2007PolarforskningssekretariatetSwedish Research Council for Environment, Agricultural Sciences and Spatial Planning, 2017-05687EU, Europeiska forskningsrådet, ERC-AdG CCTOP project #695331
Anmärkning

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

Tillgänglig från: 2019-03-29 Skapad: 2019-03-29 Senast uppdaterad: 2021-10-24Bibliografiskt granskad
2. The stability of Fe isotope signatures during low salinity mixing in subarctic estuaries
Öppna denna publikation i ny flik eller fönster >>The stability of Fe isotope signatures during low salinity mixing in subarctic estuaries
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2019 (Engelska)Ingår i: Aquatic geochemistry, ISSN 1380-6165, E-ISSN 1573-1421, Vol. 25, nr 5-6, s. 195-218Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

We have studied iron (Fe)-isotope signals in particles (> 0.22 µm) and the dissolved phase (< 0.22 µm) in two subarctic, boreal rivers, their estuaries and the adjacent sea in northern Sweden. Both rivers, the Råne and the Kalix, are enriched in Fe and organic carbon (up to 29 µmol/L and up to 730 µmol/L, respectively). Observed changes in the particulate and dissolved phase during spring flood in May suggest different sources of Fe to the rivers during different seasons. While particles show a positive Fe-isotope signal during winter, during spring flood, the values are negative. Increased discharge due to snowmelt in the boreal region is most times accompanied by flushing of the organic-rich sub-surface layers. These upper podzol soil layers have been shown to be a source for Fe-organic carbon aggregates with a negative Fe-isotope signal. During winter, the rivers are mostly fed by deep groundwater, where Fe occurs as Fe(oxy)hydroxides, with a positive Fe-isotope signal. Flocculation during initial estuarine mixing does not change the Fe-isotope compositions of the two phases. Data indicate that the two groups of Fe aggregates flocculate diversely in the estuaries due to differences in their surface structure. Within the open sea, the particulate phase showed heavier δ56Fe values than in the estuaries. Our data indicate the flocculation of the negative Fe-isotope signal in a low salinity environment, due to changes in the ionic strength and further the increase of pH.

Ort, förlag, år, upplaga, sidor
Springer, 2019
Nyckelord
Fe-isotopes, Fe geochemistry, Dissolved and particulate Fe, Organically complexed Fe, Fe(oxy)hydroxides, Salinity gradient, Spring flood
Nationell ämneskategori
Geokemi
Forskningsämne
Tillämpad geokemi
Identifikatorer
urn:nbn:se:ltu:diva-73760 (URN)10.1007/s10498-019-09360-z (DOI)000494758500001 ()2-s2.0-85074857809 (Scopus ID)
Anmärkning

Validerad;2019;Nivå 2;2019-12-06 (johcin)

Tillgänglig från: 2019-04-25 Skapad: 2019-04-25 Senast uppdaterad: 2021-10-24Bibliografiskt granskad
3. Spring flood induced shifts in Fe speciation and fate at increased salinity
Öppna denna publikation i ny flik eller fönster >>Spring flood induced shifts in Fe speciation and fate at increased salinity
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2019 (Engelska)Ingår i: Applied Geochemistry, ISSN 0883-2927, E-ISSN 1872-9134, Vol. 109, artikel-id 104385Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Rivers have traditionally been viewed as negligible sources of iron (Fe) to marine waters, as most Fe gets lost during estuarine mixing. However, recent findings demonstrate that Fe from boreal rivers display a higher resistance towards salinity-induced aggregation, presumably due to stabilizing interactions with organic matter. Previous studies have shown that Fe (oxy)hydroxides are selectively removed by aggregation processes, and that organic Fe complexes are less affected by increasing salinity. It has been further proposed that Fe speciation varies in response to seasonal differences in hydrology. In this study X-ray absorption spectroscopy (XAS) was used to determine the temporal variation in Fe speciation and the connection to Fe stability in response to increasing salinity in two boreal rivers (Kalix and Råne River), with the purpose to better understand the fate of riverine Fe export. Sampling was done from winter pre-flood, over the spring flood, to post-flood conditions (early April until mid June). In addition, parallel analyses for Fe speciation and isotope composition (δ56Fe relative to IRMM-14) were made on river samples, as well as salinity-induced aggregates and the fraction remaining in suspension, with the main objective to test if δ56Fe reflect the speciation of Fe.

The contribution of organically complexed Fe increased during spring flood compared to the pre- and post-flood, as did Fe transport capacity. However, since Fe (oxy)hydroxides were dominating throughout the sampling period, the seasonal variability was small. Interestingly, salinity-induced aggregation experiments revealed that Fe (oxy)hydroxides, which dominated aggregates, displayed lower δ56Fe than in the river samples Fe, while organic Fe complexes in suspension had higher δ56Fe values. The seasonal variability in Fe isotope signature could not be simply linked to Fe speciation, but was probably also influenced by variation in source areas of Fe and processes along the flow-path that alter both Fe speciation and isotopic composition.

Ort, förlag, år, upplaga, sidor
Elsevier, 2019
Nyckelord
Fe geochemistry, Fe speciation, Fe isotopes, Organically complexed Fe, Fe (oxy)hydroxides, Boreal, Sub-arctic, Transport capacity, Salinity gradient, XAS
Nationell ämneskategori
Geokemi
Forskningsämne
Tillämpad geokemi
Identifikatorer
urn:nbn:se:ltu:diva-73761 (URN)10.1016/j.apgeochem.2019.104385 (DOI)000490982400019 ()2-s2.0-85070319579 (Scopus ID)
Anmärkning

Validerad;2019;Nivå 2;2019-08-20 (johcin);

Artikeln har tidigare förekommit som manuskript i avhandling.

Tillgänglig från: 2019-04-25 Skapad: 2019-04-25 Senast uppdaterad: 2021-10-24Bibliografiskt granskad
4. Iron isotope pathways in the boreal landscape: Role of the riparian zone
Öppna denna publikation i ny flik eller fönster >>Iron isotope pathways in the boreal landscape: Role of the riparian zone
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2018 (Engelska)Ingår i: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 239, s. 49-60Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

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 (<0.22 µm) phase showed negative δ56Fe values (relative to IRMM-014) during base flow and meltwater discharge 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 δ56Fe values and the TOC/Febulk ratio was measured during snowmelt in the unfiltered river waters (δ56Fe 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 δ56Fe values and Fe-oxyhydroxides have positive δ56Fe 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.

Ort, förlag, år, upplaga, sidor
Elsevier, 2018
Nationell ämneskategori
Geokemi
Forskningsämne
Tillämpad geokemi
Identifikatorer
urn:nbn:se:ltu:diva-70367 (URN)10.1016/j.gca.2018.07.030 (DOI)000445036400003 ()2-s2.0-85051391721 (Scopus ID)
Anmärkning

Validerad;2018;Nivå 2;2018-08-14 (andbra)

Tillgänglig från: 2018-08-14 Skapad: 2018-08-14 Senast uppdaterad: 2019-04-25Bibliografiskt granskad
5. Iron isotopes in headwater streams
Öppna denna publikation i ny flik eller fönster >>Iron isotopes in headwater streams
(Engelska)Manuskript (preprint) (Övrigt vetenskapligt)
Nationell ämneskategori
Geokemi
Forskningsämne
Tillämpad geokemi
Identifikatorer
urn:nbn:se:ltu:diva-73762 (URN)
Tillgänglig från: 2019-04-25 Skapad: 2019-04-25 Senast uppdaterad: 2021-10-24

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