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An evaluation of a chironomid-based summer temperature reconstruction from west 1 central Sweden covering the last millennium
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
University of Bergen.
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
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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 [en]
chironomids, temperature sensitivity, alpine lake, temperature, precipitation, critical 5 assessment, XRF core scanning
National Category
Geology
Research subject
Quaternary Geology
Identifiers
URN: urn:nbn:se:su:diva-107068OAI: oai:DiVA.org:su-107068DiVA: diva2:743044
Available from: 2014-09-03 Created: 2014-09-03 Last updated: 2014-09-05
In thesis
1. The sensitivity of Swedish alpine lakes to hydro-climatic changes during the Late Holocene
Open this publication in new window or tab >>The sensitivity of Swedish alpine lakes to hydro-climatic changes during the Late Holocene
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
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:nbn:se:su:diva-107073 (URN)978-91-7447-978-2 (ISBN)
Public defence
2014-10-09, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
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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

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