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Combining limnology and paleolimnology: a refined understanding of environmental sediment signal formation in a varved lake
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Paleolimnology)
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Paleoclimatic archives, such as lake sediments, extend our understanding of terrestrial and aquatic ecosystem dynamics in relation to climate variability beyond the period covered by instrumental data. In this context, annually laminated (i.e. varved) lake sediments are particularly valuable, as they offer high temporal resolution and undisturbed sediment. However, in order to extract reliable climate information from lake sediments, a careful calibration with the processes controlling the sediment formation is essential. This thesis combines limnological and paleolimnological data from a varved, boreal lake in northern Sweden (Nylandssjön, Nordingrå) collected over different time scales. The main aim of the thesis is to gain a more refined insight into which processes are reflected in the sedimentary diatom assemblage. More specifically, sequential sediment trap records were coupled with physical, chemical and biological lake monitoring and environmental data for comparison and validation with the varved sediment record. The main result of the thesis is that timing, succession and inter-annual variability of key limnological and environmental processes (e.g. ice-cover duration, lake over-turn or catchment run-off) are of major importance for the sedimentary diatom assemblage formation. Continuous monitoring of physico-chemical parameters over three consecutive years identified varying winter air temperature as a major factor influencing in-lake processes and hence the diatom record. Timing of lake over-turn and catchment run-off seemed to be the driver for monospecific diatom blooms, which are reflected in the annual sediment signal. The integrated annual diatom signal in the sediment was dominated by spring or autumn blooms, resulting either from a Cyclotella glomerata dominated spring bloom after relatively warm winter conditions, or a Asterionella formosa dominated autumn bloom after relatively cold winter conditions. The analysis of the diatom stratigraphy in the varved sediment over several decades corroborated the importance of climatic variables (late winter air temperature and NAO), even though the variables with the most predictive power for variance in the diatom data were associated with sediment composition (C, N and sedimentation rate) and pollution (Pb and Cu). Overall, the analysis of the drivers of inter-annual and decadal diatom assemblage fluctuations emphasizes the importance of winter air temperature, indicating that weather extremes may be disproportionately represented in annual sediment records in contrast to nutrient concentrations or sedimentation rate.

Place, publisher, year, edition, pages
Umeå: Umeå University , 2017. , 29 p.
Keyword [en]
varved lake sediments, diatom sediment signal formation, sequential sediment trap, seasonal process timing, ice thinning, varve compaction, climate impact, catchment properties
National Category
Earth and Related Environmental Sciences
Research subject
Earth Sciences with Specialization Environmental Analysis
Identifiers
URN: urn:nbn:se:umu:diva-135231ISBN: 978-91-7601-603-9 (print)OAI: oai:DiVA.org:umu-135231DiVA: diva2:1097345
Public defence
2017-06-15, KB3A9, KBC, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2017-05-24 Created: 2017-05-22 Last updated: 2017-05-24Bibliographically approved
List of papers
1. Inter-annual variation of seasonal diatom dynamics reveals importance of process timing for sediment signal formation
Open this publication in new window or tab >>Inter-annual variation of seasonal diatom dynamics reveals importance of process timing for sediment signal formation
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Disentangling the process information contained in a diatom sediment signature is crucial for reliable future predictions based on paleolimnological records. The aim of this study was to identify the hierarchical importance of seasonal processes for the formation of an annual diatom sediment record. We combined annual vertical water column chemistry, lake thermal structure, sequential sediment trap samples and annually resolved varved sediment diatom records from a boreal lake in northern Sweden. Whole-year water column monitoring indicates that the entire annual diatom sediment signal in different years can be driven by winter air temperature-induced timing of ice and snow melt and persistent under-ice stratification, which promote an early diatom bloom under the ice before spring lake over-turn. In a year of late ice thinning and development of a chlorophyll maximum after spring lake over-turn, an annually integrated diatom sediment signal was built by a continuous diatom flux, with important implications for diatom-based model predictions.

Keyword
Diatoms, limnological process timing, sediment trap, varve lake sediment, seasonal air temperature
National Category
Climate Research
Identifiers
urn:nbn:se:umu:diva-135326 (URN)
Available from: 2017-05-23 Created: 2017-05-23 Last updated: 2017-05-23
2. Using a decadal diatom sediment-trap record to unravel seasonal processes important for the formation of the sedimentary diatom signal
Open this publication in new window or tab >>Using a decadal diatom sediment-trap record to unravel seasonal processes important for the formation of the sedimentary diatom signal
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Sediment trap studies are of great importance to develop a deeper understanding of how environmental processes are influencing sediment deposition. We analyzed diatom assemblages from 2002 to 2015 in a sediment-trap record from a boreal lake (Nylandssjön, Sweden) together with environmental parameters, and compared the diatom record in the trap with the corresponding annually laminated sediments. In general, our data set indicates a large year-to-year variability of diatom succession and abundance patterns, which is well reflected in the varved sediments. The relative abundance of major taxa seems primarily controlled by seasonal characteristics, such as above-average winter air temperature and autumn runoff. Specifically, Cyclotella glomerata dominated the annual sediment trap record (and also the corresponding sediment varves) in years with warmer air temperatures in March-April, and Asterionella formosa dominated the annual sediment assemblages in years characterized by higher runoff before autumn lake over-turn. This study highlights that short-term environmental conditions and extreme events are of major importance for interpreting annual sediment signals.

Keyword
Diatoms, sediment trap, varved lake sediment, seasonal air temperature, discharge
National Category
Climate Research
Identifiers
urn:nbn:se:umu:diva-135329 (URN)
Available from: 2017-05-23 Created: 2017-05-23 Last updated: 2017-05-23
3. Annually resolved impact of climate and catchment properties on the sediment diatom assemblage in a varved, boreal lake since AD 1930
Open this publication in new window or tab >>Annually resolved impact of climate and catchment properties on the sediment diatom assemblage in a varved, boreal lake since AD 1930
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Assessing ecosystem dynamics beyond experimental time-scales is a prerequisite to understand effects of ongoing environmental change and essential to improve predictions of future conditions. In this study, we analyze a time series of an annually resolved sediment record from a boreal lake (Nylandssjön) in northern Sweden, consisting of diatom data since AD 1931 and geochemistry data since AD 1953. The aim of this paper is to quantify to which extent the main primary producers (i.e., diatom assemblage) in a typical boreal lake are related to processes in the lake catchment, sediment properties, and climate variables (temperature, precipitation, circulation patterns). The diatom assemblage consists mainly of Cyclotella taxa (C. glomerata, C. radiosa, and C. rossii), Asterionella formosa, Fragilaria delicatissima and other small Fragilaria taxa. Although the dominant taxa remain the same over the entire sediment record, their relative abundances exhibit some distinct shifts. We use ordination techniques and variance partitioning to quantify the explanatory power of environmental variables to explain shifts in the diatom assemblage composition. A series of redundancy analyses (RDA) using one sole constraining variable showed that both geochemical proxies and climate variables explained a statistically significant amount of the variance in the diatom data. The most powerful sediment proxy variables were Pb (ppm, 17.9%), C (%, 16.8%), N (%, 15.7%) and sedimentation rate (g cm-2 yr-1, 13.2%). For climate variables, temperature during winter/spring (April 6.4%, May 5.0%, February 4.6%) and the Northern Atlantic Oscillation index (NAO 6.4%) showed the highest predictive power, whereas precipitation data did not turn out to be a powerful predictor. Variance partitioning showed that sediment proxies (sedimentation rate, C, N; 20.8%), variables associated with pollution (Pb, Cu, P; 23.1%) and climate (winter/spring temperature, NOA; 10.4%) explained significant amounts of the variance in the diatom data. This quantification indicates to which extent sediment properties and climate conditions are jointly responsible for shaping the ecosystem properties in boreal lakes, highlighting in particular the importance of winter climate.

Keyword
Diatoms, geochemistry, boreal lake, varves, climate, variance partitioning
National Category
Climate Research
Identifiers
urn:nbn:se:umu:diva-135330 (URN)
Available from: 2017-05-23 Created: 2017-05-23 Last updated: 2017-05-23
4. Compaction of recent varved lake sediments
Open this publication in new window or tab >>Compaction of recent varved lake sediments
2013 (English)In: GFF, ISSN 1103-5897, E-ISSN 2000-0863, Vol. 135, no 3-4, 231-236 p.Article in journal (Refereed) Published
Abstract [en]

To assess the rates of compaction in recent, varved (annually laminated) lake sediments, we used a collection of 13 freeze cores sampled from 1979 to 2012 in Nylandssjon (northern Sweden). This unique series of stored freeze cores allowed us to measure how the thickness of individual varves changed when they were overlain by new varves. The compaction rate was greatest during the first few years after deposition; varve thickness decreased by 60% after 5 years (sediment depth 5cm). Thereafter, the compaction rate declined, but after 33 years (the study period) there was still a weak trend of continued compaction (sediment depth 12cm). The rate of compaction is clearly linked to loss of water along with an increase in dry-bulk density of the sediment. Despite compaction causing considerable varve thickness changes over time, the year-to-year variation in varve thickness is preserved while sediment ages. Understanding compaction processes is of fundamental importance to paleolimnology, for example in studies involving calibration of varve thicknesses of recent varves against environmental parameters.

Place, publisher, year, edition, pages
Taylor & Francis, 2013
Keyword
varved lake sediment, compaction, varve thickness, bulk density, diagenesis, post-depositional changes, paleolimnology
National Category
Geology
Identifiers
urn:nbn:se:umu:diva-85095 (URN)10.1080/11035897.2013.788551 (DOI)000327571800002 ()
Note

Special Issue: Varve Genesis, Chronology and Paleoclimate

Available from: 2014-01-28 Created: 2014-01-28 Last updated: 2017-05-23Bibliographically approved

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