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Varved lake sediments and diagenetic processes
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Umeå: Ekologi, miljö och geovetenskap , 2009. , 18 p.
Keyword [en]
Varved (annually laminated) lake sediment, diagenesis, varve appearance, iron, sulfur, chemical speciation, iron cycling, carbon, nitrogen, stabile isotopes, δ13C, δ15N
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:umu:diva-5724ISBN: 978-91-7264-694-0 (print)OAI: oai:DiVA.org:umu-5724DiVA: diva2:145344
Public defence
2009-02-27, KB3A9, KBC-huset, Linneaus vägUmeå, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2009-02-06 Created: 2009-02-06 Last updated: 2011-03-25Bibliographically approved
List of papers
1. A comparison of sediment varves (1950–2003 AD) in two adjacent lakes in northern Sweden
Open this publication in new window or tab >>A comparison of sediment varves (1950–2003 AD) in two adjacent lakes in northern Sweden
2006 (English)In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 35, 837-853 p.Article in journal (Refereed) Published
Abstract [en]

Koltjärnen and Nylandssjön are two closely situated lakes (<2 km apart) in northern Sweden. During the past century, distinct varved sediments have formed in these lakes. Nylandssjön has two varved, deep basins. Since lake and catchment characteristics superficially appear very similar for the two lakes and they are exposed to the same climate, one would expect the sediment varves to be similar. This investigation compares the varves in the two deep basins of Nylandssjön, and in the two lakes. The comparison of basins of Nylandssjön shows that varve thickness, water content and annual accumulation rates of organic matter and nitrogen are correlated for the period (1970–2003). The grey-scale curves are only clearly similar in about 50% of the varves. In the between-lake comparison varve thickness, water content and annual accumulation rates of organic matter and nitrogen are correlated for the period (1950–1996). However, the annual accumulation rates of dry mass, minerogenic matter and biogenic silica differ between the lakes, as well as within-varve successions in grey-scale. A general explanation to the differences is that the prerequisites for varve formation are not totally similar because of differences in catchment size, catchment- to-lake material fluxes, lake productivity and land-use influence. This study illustrates the complex relationships that exist between a lake, its catchment, in-lake productivity and formation of sediment varves. As a consequence, we must not apply a too simplistic view of the potential of varves as past climatic indicators, especially if the lakes are affected by land-use.

Place, publisher, year, edition, pages
Dordrecht: Kluwer, 2006
Keyword
Climate change, Grey-scale, Image analysis, Land-use, Sediment accumulation rate, Varve thickness
Identifiers
urn:nbn:se:umu:diva-7420 (URN)10.1007/s10933-005-5952-x (DOI)
Available from: 2008-01-09 Created: 2008-01-09 Last updated: 2011-03-11Bibliographically approved
2. The role of iron and sulfur in the visual appearance of lake sediment varves
Open this publication in new window or tab >>The role of iron and sulfur in the visual appearance of lake sediment varves
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2009 (English)In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 42, no 1, 141-153 p.Article in journal (Refereed) 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.

Place, publisher, year, edition, pages
Springer Netherlands, 2009
Identifiers
urn:nbn:se:umu:diva-23348 (URN)10.1007/s10933-008-9267-6 (DOI)
Available from: 2009-06-12 Created: 2009-06-12 Last updated: 2010-12-16Bibliographically approved
3. Carbon and nitrogen loss rates during aging of lake sediment: Changes over 27 years studied in varved lake sediment
Open this publication in new window or tab >>Carbon and nitrogen loss rates during aging of lake sediment: Changes over 27 years studied in varved lake sediment
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2008 (English)In: Limnology and Oceanography, ISSN 0024-3590, Vol. 53, no 3, 1076-1082 p.Article in journal (Refereed) Published
Abstract [en]

We used a collection of ten freeze cores of annually laminated (varved) lake sediment from Nylandssjön in northern Sweden collected from 1979 to 2007 to follow the long-term loss of carbon (C) and nitrogen (N) due to processes that occur in the lake bottom as sediment ages. We compared specific years in the different cores. For example, the loss of C from the surface varve of the 1979 core (sediment deposited during 1978) was followed in the cores from 1980, 1985, 1989, and so on until 2006. The C concentration of the sediment decreased by 20% and N decreased by 30% within the first five years after deposition, and after 27 yr in the sediment, there was a 23% loss of C and 35% loss of N. Because the relative loss of C with time was smaller than loss of N, the C:N ratio increased with increasing age of the sediment; the surface varves start with a ratio of ~10, which then increases to ~12.

Place, publisher, year, edition, pages
Waco, Tex.: American Society of Limnology and Oceanography, 2008
Identifiers
urn:nbn:se:umu:diva-11453 (URN)
Available from: 2009-01-08 Created: 2009-01-08 Last updated: 2011-03-25Bibliographically approved
4. Decadal diagenetic effects on δ13C and δ15N studied in varved lake sediment
Open this publication in new window or tab >>Decadal diagenetic effects on δ13C and δ15N studied in varved lake sediment
2009 (English)In: Limnology and Oceanography, ISSN 0024-3590, Vol. 54, no 3, 905-916 p.Article in journal (Refereed) Published
Abstract [en]

The Livingstone and Imboden (1996) model for the oxygen depletion rate in lake hypolimnia was evaluated with field results in 32 lakes. The volumetric oxygen consumption rate (JV) was strongly related to lake trophic state, as represented by the growing season mean chlorophyll a (Chl a) and annual mean total phosphorus concentration, and might reach a maximum of 0.23–0.24 g O2 m-3 d-1 in eutrophic lakes. The areal oxygen consumption rate (JA) was not related to trophic state and was lower than the value estimated by the sediment core method, particularly in eutrophic lakes; the mean was 0.0816 6 0.0150 g O2 m-2 whole sediment d-1. We modified the model and it was used to confirm that the field JA, the average for the hypolimnion for the deoxygenation period, is influenced by JV and the range of sediment area to water volume ratio, (Z) (Z is water depth), in the hypolimnion, as well as the oxygen consumption rate in whole sediment (R). It was suggested that this is the reason for the difference between field and sediment core JA values. The R in 10 lakes varied from 7 to 460 g O2 m-3 whole sediment d-1. Overall, the Livingstone and Imboden model with constant JA and with a JV value derived from an empirical relationship with either measure of trophic state was considered to be a suitable condensed model for oxygen depletion in lake hypolimnia.

Place, publisher, year, edition, pages
Association for the Sciences of Limnology and Oceanography, 2009
National Category
Earth and Related Environmental Sciences Ecology
Identifiers
urn:nbn:se:umu:diva-5728 (URN)10.4319/lo.2009.54.3.0905 (DOI)
Available from: 2009-02-06 Created: 2009-02-06 Last updated: 2012-05-30Bibliographically approved

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