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The role of iron and sulfur in the visual appearance of lake sediment varves
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Kemiska institutionen.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Kemiska institutionen.
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2009 (Engelska)Ingår i: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 42, nr 1, s. 141-153Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Easily discernible sediment varves (annual laminations) may be formed in temperate zone lakes, and reflect seasonal changes in the composition of the accumulating material derived from the lake and its catchment (minerogenic and organic material). The appearance of varves may also be influenced by chemical processes. We assessed the role of iron (Fe) and sulfur (S) in the appearance of varves in sediments from Lake Nylandssjön in northern Sweden. We surveyed Fe in the lake water and established whether there is internal transport of Fe within the sediment. We used a unique collection of seven stored freeze cores of varved sediment from the lake, collected from 1979 to 2004. This suite of cores made it possible to follow long-term changes in Fe and S in the sediment caused by processes that occur in the lake bottom when the sediment is ageing. We compared Fe and S concentrations using X-ray fluorescence spectroscopy (XRF) in specific years in the different cores. No diagenetic front was found in the sediment and the data do not suggest that there is substantial vertical transport of Fe and S in the sediment. We also modeled Fe and S based on thermodynamic, limnological, and sediment data from the lake. The model was limited to the five components H+, e-, Fe3+, SO42-, H2CO3 and included the formation of solid phases such as Fe(OH)3 (amorphous), FeOOH (aged, microcrystalline), FeS and FeCO3. Modeling showed that there are pe (redox) ranges within which either FeS or Fe(OH)3/FeOOH is the only solid phase present and there are pe ranges within which the two solid phases co-exist, which supports the hypothesis that blackish and grey-brownish layers that occur in the varves were formed at the time of deposition. This creates new possibilities for deciphering high-temporal-resolution environmental information from varves.

Ort, förlag, år, upplaga, sidor
Springer Netherlands , 2009. Vol. 42, nr 1, s. 141-153
Identifikatorer
URN: urn:nbn:se:umu:diva-23348DOI: 10.1007/s10933-008-9267-6OAI: oai:DiVA.org:umu-23348DiVA, id: diva2:223419
Tillgänglig från: 2009-06-12 Skapad: 2009-06-12 Senast uppdaterad: 2018-06-08Bibliografiskt granskad
Ingår i avhandling
1. Varved lake sediments and diagenetic processes
Öppna denna publikation i ny flik eller fönster >>Varved lake sediments and diagenetic processes
2009 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

Varved (annually laminated) sediments are of great interest for inference of past environmental conditions, as they provide dated records with high time resolution. After deposition, the sediment varves are affected by diagenesis; i.e., chemical, physical and biological changes that occur within the sediment. An important premise when reconstructing past environmental conditions using lake sediments is that the signal of interest is preserved in the sediment. In this thesis I have used a unique collection of ten stored freeze cores of varved lake sediment from Nylandssjön in northern Sweden, collected from 1979 to 2007. The suite of cores made it possible to follow long-term (up to 27 years) changes in iron (Fe), sulfur (S), carbon (C), nitrogen (N), δ13C and δ15N in the sediment caused by processes that occur in the lake bottom as the sediment ages. The sediment geochemistry and resulting changes were followed in years for which there are surface varves in the core series. Fe and S concentrations analyzed by X-ray fluorescence spectroscopy showed no diagenetic front in the sediment and the data do not suggest a substantial vertical transport of Fe and S in the sediment. A model based on thermodynamic, limnological, and sediment data from the lake, showed that there are pe (redox) ranges within which either FeS (reduced specie) or Fe(OH)3/FeOOH oxidized species) is the only solid phase present and there are pe ranges within which the two solid phases co-exist. This supports the hypothesis that blackish and grey-brownish Fe-layers that occur in the varves were formed at the time of deposition. C and N analyzed with an elemental analyzer showed that within the first five years after deposition the C concentration of the sediment decreased by 20% and N by 30%, and after 27 yr in the sediment, there was a 23% loss of C and 35% loss of N. The C:N ratio increased with increasing age of the sediment; from ~ 10 in the surface varves to ~12 after 27 years of aging. δ13C and δ15N analyzed on a mass spectrometer showed that δ13C increased by 0.4-1.5‰ units during the first five years, after that only minor fluctuations in δ13C were recorded. Another pattern was seen for δ15N, with a gradual decrease of 0.3-0.7‰ units over the entire 27-year-period. The diagenetic changes in the stable isotope values that occur in Nylandssjön are minor, but they are of about the same magnitude as the variation in the isotopic signal in the varves deposited between 1950-2006.

My results show that diagenesis does not change the visual appearance of the varves, except for varve thickness; the varves get thinner as the sediment ages. As the color of Fe in the varves likely reflects the environmental conditions at the time of deposition this creates possibilities for deciphering high-temporal-resolution information of past hypolimnetic oxygen conditions from varves. My findings on C, N, δ13C and δ15N will have implications for interpretations of paleolimnological data. The diagenetic effects should be carefully taken into consideration when C, N, δ13C and δ15N in sediment cores are used to study organic matter sources or paleoproductivity, in particular when dealing with relatively small and recent changes.

In addition to the significance of diagenetic effects on sediment parameters, a comparison of the varves in Nylandssjön and the adjacent lake Koltjärnen, and the two deep basins of Nylandssjön show that subtle features in the lakes and their catchments affect the appearance of the varves, which make interpretation of varves complicated.

Ort, förlag, år, upplaga, sidor
Umeå: Ekologi, miljö och geovetenskap, 2009. s. 18
Nyckelord
Varved (annually laminated) lake sediment, diagenesis, varve appearance, iron, sulfur, chemical speciation, iron cycling, carbon, nitrogen, stabile isotopes, δ13C, δ15N
Nationell ämneskategori
Geovetenskap och miljövetenskap
Identifikatorer
urn:nbn:se:umu:diva-5724 (URN)978-91-7264-694-0 (ISBN)
Disputation
2009-02-27, KB3A9, KBC-huset, Linneaus vägUmeå, Umeå, 10:00 (Engelska)
Opponent
Handledare
Tillgänglig från: 2009-02-06 Skapad: 2009-02-06 Senast uppdaterad: 2011-03-25Bibliografiskt granskad

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Gälman, VeronikaRydberg, JohanShchukarev, AndreySjöberg, StaffanBindler, RichardRenberg, Ingemar

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