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The sensitivity of Swedish alpine lakes to hydro-climatic changes during the Late Holocene
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The thesis objective is to use lake sediments to investigate the relationships between chironomids, as proxy for temperature, and δ18O and minerogenic in-wash, as precipitation proxies, in northern Sweden during the Late Holocene. Three lakes along a N–S transect in the Scandes were studied using biological, geochemical and physical sediment properties, together with numerical methods. Emphasis was put on changes in hydrological and erosional regimes and the configurations of lake catchments in an attempt to avoid misinterpretation of the climate signals. Mean July air temperatures were inferred from the chironomid composition in sediment records covering the last 5000 yrs in Vindelfjällen Mountains in Mid Lapland and the last 1000 yrs in Jämtland. The temperature reconstructions underwent significance tests, suggesting reliable results for the Vindelfjällen reconstruction but not the Jämtland reconstruction. Data from XRF core scanning and grain size variations, interpreted together with catchment geomorphology, suggested periods with increased fluvial erosion. A major flood event occurred at the Vindelfjällen site c. 2800 cal. yr BP. Furthermore, this record indicates stream flow and wetland surface erosion during the last 1200 yrs. Seasonal variations in the amount of precipitation, reflecting changes in atmospheric circulation during the last 1000 yrs, were reconstructed based on δ18O in diatoms from one lake in Jämtland and one in northern Lapland. Relatively higher summer precipitation was inferred between AD 1000 and 1080, AD 1300 and 1440, and during the early 19th century as a result of a positive Arctic Oscillation mode. Changes in chironomid composition in the Jämtland lake were concurrent with shifts in δ18Odiatom suggesting indirect responses to circulation changes. Changes in precipitation are suggested to be an important factor that may cause the spatially variable results among chironomid-based July air temperature reconstructions in Scandinavia.

Place, publisher, year, edition, pages
Stockholm: Department of Physical Geography and Quaternary Geology, Stockholm University , 2014. , 34 p.
Series
Dissertations from the Department of Physical Geography and Quaternary Geology, ISSN 1653-7211 ; 44
Keyword [en]
alpine lake sediments, temperature reconstruction, precipitation, catchment erosion, chironomids, oxygen isotopes, XRF core scanning, geomorphology, grain size analysis
National Category
Geology
Research subject
Quaternary Geology
Identifiers
URN: urn:nbn:se:su:diva-107073ISBN: 978-91-7447-978-2 (print)OAI: oai:DiVA.org:su-107073DiVA: diva2:743854
Public defence
2014-10-09, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2014-09-18 Created: 2014-09-03 Last updated: 2014-10-30Bibliographically approved
List of papers
1. Late-Holocene temperature and precipitation changes in Vindelfjallen, mid-western Swedish Lapland, inferred from chironomid and geochemical data
Open this publication in new window or tab >>Late-Holocene temperature and precipitation changes in Vindelfjallen, mid-western Swedish Lapland, inferred from chironomid and geochemical data
2014 (English)In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 24, no 1, 78-92 p.Article in journal (Refereed) Published
Abstract [en]

In this article, we present results from a palaeolimnological study from Lake Vuoksjavratje in the mountain tundra region in the Vindelfjallen Mountains, northwest Sweden. We suggest that the influence of precipitation may be one of the factors causing discrepancies between chironomid-based late-Holocene July temperature (JulyT) reconstructions from Fennoscandia. We combine quantitative temperature reconstruction using chironomids for the last 5100 years with qualitative analysis of chironomid composition and geochemical analyses, such as x-ray fluorescence (XRF), total organic carbon (TOC) and C/N analysis. The studied sequence is dated by Pb-210, Cs-137 and 11 C-14 datings from terrestrial macrofossils. The aim of the study was to use chironomids to reconstruct late-Holocene summer temperature variation on a multi-centennial to centennial timescale and to use geochemical data to identify periods during which the changes in chironomid composition might have been forced by environmental variables other than temperature, such as within lake processes or precipitation. Based on ordination techniques, and a comparison between chironomid-inferred JulyTs and changes in minerogenic sedimentation with regional temperature and wetness records, it is concluded that the JulyT signal was modulated by precipitation. The proxies indicate that both JulyT and annual precipitation have influenced the chironomid communities in Lake Vuoksjavratje, and that catchment-related processes caused by enhanced precipitation have overridden the summer temperature signal between 3000 and 2200 cal. yr BP, and between 1050 and 100 cal. yr BP.

Keyword
chironomids, late Holocene, northern Sweden, precipitation, temperature reconstruction, x-ray fluorescence
National Category
Geology Physical Geography
Identifiers
urn:nbn:se:su:diva-99867 (URN)10.1177/0959683613512167 (DOI)000328828800007 ()
Note

AuthorCount:3;

Available from: 2014-01-24 Created: 2014-01-20 Last updated: 2017-12-06Bibliographically approved
2. Shifts in precipitation during the last millennium in northern Scandinavia from lacustrine isotope records
Open this publication in new window or tab >>Shifts in precipitation during the last millennium in northern Scandinavia from lacustrine isotope records
Show others...
2013 (English)In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 66, 22-34 p.Article in journal (Refereed) Published
Abstract [en]

Here we present delta P-18(diatom) data from two high-latitude lakes; one has short residence time and a water isotopic composition (delta O-18(lake)) that fluctuate due to seasonal variations in precipitation and temperature, and the other has delta O-18(lake) that is influenced by longer lake water residence times and evaporation. The delta O-18(diatom) records reveal common responses to precipitation forcing over the past millennium. Relatively wet summers are inferred from delta O-18(diatom) between 1000 and 1080 AD, 1300 and 1440 AD, and during the early 19th century, coincided with periods of high cloud cover inferred from tree-ring carbon isotopes, and other data for high Arctic Oscillation index. While relatively dry summers with increasing influence of winter snow are indicated between 1600 and 1750 AD. The co-response between carbon isotopes in trees and oxygen isotopes in diatoms strengthens the relationship between cloud cover and precipitation and the hypothesis that these changes were the result of significant regional shifts in atmospheric circulation.

Keyword
Last millennium, Lakes, Isotope, Diatoms, Scandinavian climate
National Category
Physical Geography Geology
Identifiers
urn:nbn:se:su:diva-89989 (URN)10.1016/j.quascirev.2012.10.030 (DOI)000317448500003 ()
Funder
EU, European Research Council, 017008
Note

AuthorCount:11;

Available from: 2013-05-21 Created: 2013-05-20 Last updated: 2017-12-06Bibliographically approved
3. An evaluation of a chironomid-based summer temperature reconstruction from west 1 central Sweden covering the last millennium
Open this publication in new window or tab >>An evaluation of a chironomid-based summer temperature reconstruction from west 1 central Sweden covering the last millennium
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(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
Abstract [en]

Here we present a lake sediment study from west central Sweden that covers the last thousand years. The study site Lake Spåime is a shallow, hydrologically open lake situated above the present tree line. We applied chironomid analysis with the aim to reconstruct mean July air temperature at a high temporal resolution and we evaluated our reconstruction with significance tests and comparison with observed meteorological data. We examined the relationship between changes in chironomid-inferred July temperature, isotope hydrology and minerogenic detrital input. We also assessed the influence of running water and semi-terrestrial/terrestrial chironomids. The inferred July air temperatures ranged from 8.5˚C to 11.6˚C (mean 9.6˚C) over the past millennium. The temperatures inferred with running water- and terrestrial taxa removed was similar, but with a 0.6 ˚C higher amplitude. The results of significance testing were inconclusive. Comparison with meteorological data showed that the chironomid-based July air temperatures were similar during the last 110 years. Individual years when chironomid-inferred July air temperatures were lower than the instrumental data were characterized by low June air temperatures and high amounts of winter precipitation. From this, we assume that late snowmelt led to cool melt water input in July which in turn caused a decoupling between water and air temperatures causing too low inferred temperatures. Inferred July temperatures show similarities and discrepancies with reconstructions derived from other proxies in the region, such as tree-ring and sea surface temperature records, and fail to reconstruct the cold summer conditions characteristic of the years between AD 1650 and 1750. We conclude that the Spåime July temperature reconstruction was partly biased, possibly because of major changes in the Spåime catchment system that influenced the chironomids. For example, changes in hydrology caused by shifts in seasonality and amount of precipitation may have triggered changes in sediment erosion and deposition rates, affecting the chironomid composition to a larger extent than mean July air temperatures. Future challenges include examination of the temperature sensitivity of chironomid taxa and continued critical assessment of individual chironomid inferred temperature reconstructions, not only against other temperature proxies but also against proxies for variations in hydrology.

Keyword
chironomids, temperature sensitivity, alpine lake, temperature, precipitation, critical 5 assessment, XRF core scanning
National Category
Geology
Research subject
Quaternary Geology
Identifiers
urn:nbn:se:su:diva-107068 (URN)
Available from: 2014-09-03 Created: 2014-09-03 Last updated: 2014-09-05
4. Late Holocene high precipitation events recorded in lake sediments and catchment geomorphology, Lake Vuoksjávrátje, NW Sweden
Open this publication in new window or tab >>Late Holocene high precipitation events recorded in lake sediments and catchment geomorphology, Lake Vuoksjávrátje, NW Sweden
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this paper, we highlight the importance of combining multi-proxy analysis of lake sediments with associated catchment geomorphology to better understand the late Holocene palaeoenvironmental evolution in a high latitude Alpine lake in N Sweden. Previous studies have suggested that such lakes may be highly sensitive to variations in catchment erosion and variations in precipitation, and that this sensitivity may influence ecologically-based reconstructions of past temperature changes. Here we have analysed lake sediments covering the last 5100 years from Lake Vuoksjávrátje in NW Sweden to identify different erosional regimes in the lake catchment and to identify sediment sources and lake sedimentary processes, which ultimately affect the palaeoecological record. Methods that were used include XRF core scanning, grain size analysis and geomorphological mapping, supported by previously published chironomid, total organic carbon and carbon/nitrogen data. From the integrated results we identify time intervals when increased amounts/intensity of precipitation altered sedimentation and lake catchment erosional processes. The most prominent event in our record occurred between 3090 and 2750 cal. a BP and is interpreted to be the result of excessive precipitation in relation to the 2.8 ka event. By combining the multi-proxy analysis of a lake sediment core with a detailed catchment characterisation it is possible to reach a better understanding of the processes active within the lake catchment, the factors governing the erosional regimes and the way these are recorded in lake sediments. For future palaeoclimatological research based on lake sediments we recommend increasing the integration of catchment geomorphology, sedimentology and palaeoecology.

Keyword
Holocene, Precipitation, Catchment erosion, Solifluction, Lake sediments, Geomorphology, XRF core scanning, Grain size
National Category
Geology
Research subject
Quaternary Geology
Identifiers
urn:nbn:se:su:diva-107071 (URN)
Available from: 2014-09-03 Created: 2014-09-03 Last updated: 2014-09-05

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