The potential of non-alpine lakes for quantitative palaeotemperature reconstructions based on subfossil chironomids [Elektronische Ressource] : a comparative palaeolimnological study from southern Norway / presented by Britta Lüder

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Informations

• Publié par: universitat_bremen
• Publié le: 01 janvier 2007
• Nombre de lectures: 27
• Langue: English
• Poids de l'ouvrage: 4 Mo

Extrait

The potential of non-alpine lakes for quantitative
palaeotemperature reconstructions based on
subfossil chironomids:
A comparative palaeolimnological study
from southern Norway










A dissertation submitted to
Fachbereich 8
University of Bremen

for the degree
Doctor of Natural Sciences (Dr. rer. nat.)

presented by
Dipl.-Geogr. Britta Lüder




Bremen, February 2007
















Reviewer

Prof. Dr. Bernd Zolitschka
GEOPOLAR, Institute of Geography
Universtity of Bremen
Germany

Dr. Felix Bittmann
Niedersächsisches Institut für historische Küstenforschung
Wilhelmshaven
Germany


Public defence: 11.05.2007
Table of Contents

Abstract i
Zusammenfassung iii


Chapter 1 Introduction 1
Chironomids and palaeolimnology 2 id-inferred palaeotemperatures 4
Concept, background, and goals of the study 7

Chapter 2 Regional overview and study sites 9
Geographical and geological overview 9
Climate 9
Vegetation 10
Deglaciation history and Holocene climatic oscillations 12
Archaeological evidence for prehistoric human activity 13
Study sites 15

Chapter 3 Methods 21
Coring and sampling 21
Dating, age-depth modelling, and sedimentation rates 21
Physical and chemical proxy data 21
Biological proxy data 23
Numerical data analysis 25

Chapter 4 Catchment histories and sedimentary changes 31
Introduction 31
Results 32
Lithology32
Chronology, sedimentation rates, and sample resolution 34
Dry density and influx rates 39
Geochemical data 39
Pollen analyses 41
Interpretation and discussion 48
Lake genesis, sedimentation history, and trophic state 48
Local vegetation development 55
Conclusions 58
CONTENTS
Chapter 5 Holocene successions of chironomid assemblages and their
palaeoecological implications 61
Introduction 61
Results 62
Chironomid analyses 62
Non-chironomid remains 72
Ordination 72
Rate-of-change analyses 76
Interpretation and discussion 78
Palaeoecological development during the Holocene 78
Sensitivity of the chironomid fauna to environmental changes 86
Conclusions 89

Chapter 6 Reconstruction and evaluation of Holocene mean
July temperatures 91
Introduction 91
Results 92
Chironomid- and pollen-inferred mean July temperatures 92
Numerical evaluation criteria 97
Interpretation and discussion 98
Palaeoecological consideration of the ’less reliable’ temperature
reconstructions 98
Site-specific validation of the chironomid-inferred temperatures 100
Inter-site comparison 104
Regional and supra-regional comparisons with independent
palaeoclimatological records 106
Comparison with Holocene chironomid-inferred temperature
reconstructions from southern Norway 112
Conclusions 113

Chapter 7 Conclusions and Outlook 115
Initial questions and hypotheses 115
Implications for future research 119


References 121
Appendix 139
Acknowledgements 149 i
Abstract
In this study, Holocene mean July temperatures were reconstructed quantitatively
based on subfossil chironomid remains. The specific goal was to evaluate whether lakes
located below the modern tree line are appropriate sites for the chironomid-based
reconstruction of small-scale temperature variations. On this account, the main
questions were whether the water chemistry and physical conditions of the chosen lakes
can be regarded as stable during the Holocene, whether the chironomid assemblages
react sensitively to Holocene temperature fluctuations, and whether they record human
impact. To answer these questions, the sediments of the lakes Reiarsdalsvatnet
(245 m asl, maximum water depth: 9.2 m, boreo-nemoral) and Isbenttjønn (787 m asl,
maximum water depth: 9.4 m, northern boreal) were studied in a multi-proxy approach,
including age-depth modelling based on radiocarbon dating, chironomid and pollen
analyses, and the analyses of carbon, nitrogen, sulphur and biogenic silica.
The reconstructed environmental conditions can be summarised as follows: By
9,900 cal. BP, the region around Reiarsdalsvatnet was covered with a mixed deciduous-
coniferous forest, and Betula-Pinus woodland was present around Isbenttjønn after
9,300 cal. BP. The onset of the formation of organic sediment in the lakes occurred in
the Preboreal and Boreal chronozone, respectively. By 9,600 cal. BP in Reiarsdalsvatnet
and by 8,600 cal. BP in Isbenttjønn, soil stability in the catchments was reached. A
distinct opening of the landscape together with other signs of human impact is seen
from 2,700 cal. BP at Reiarsdalsvatnet. In the Isbenttjønn profile, signs of human
activity are evident from 3,300 cal. BP, but a distinct opening of the vegetation occurs
not until 1,400 cal. BP. Though human activity is indicated at both lakes during the last
millennia, humans seem to have changed the vegetation around the lakes only
gradually. Both lakes were generally oligotrophic during most of the Holocene and only
in the youngest sediments increasing productivity at low levels is indicated. Well
oxygenated conditions prevailed in the hypolimnion of Reiarsdalsvatnet and good to
moderate hypolimnetic oxygen conditions in the one of Isbenttjønn. Concluding, both
lakes and their catchments have undergone only minor changes during the Holocene and
the water chemistry and physical conditions are regarded as stable.
In a further step, Holocene mean July temperatures were reconstructed quanti-
tatively based on the chironomid data using transfer functions. By the application of
several evaluation procedures, it was possible to differentiate between phases of reliable
and phases of less reliable reconstructions. For this, mean July temperatures were also
inferred from the pollen data to enable a comparison with independent temperature
reconstructions. The results of this comparison show very similar centennial to
millennial-scale trends during most of the studied period in the chironomid- and the ii ABSTRACT
pollen inferred temperatures of both lakes. Only in the youngest sediments of
Isbenttjønn, human impact could be accounted to be responsible for unreliably high
chironomid-inferred temperatures, even though the impact was only moderate. Apart
from this period, the chironomid-inferred temperatures of both lakes generally show
similar Holocene temperature trends: Temperatures increase until c. 8,000 cal. BP and
maximum Holocene temperatures are reached between 7,600 cal. BP and 7,200 cal. BP.
Until c. 4,500 cal. BP, stable temperatures at high level are recorded. Thereafter,
temperatures decrease slowly until they drop clearly after 1,500 cal. BP. These trends
resemble widely known features of the Holocene summer temperature development in
Scandinavia, as assessed by the comparison to records of Norwegian Holocene glacier
variations, of Holocene changes in net precipitation in southern Sweden, and of inferred
air temperatures in central Greenland.
In this study is demonstrated that subfossil chironomids from the studied non-alpine
lakes do react sensitive to small-scale summer temperature fluctuations. The results
point to the importance of a cautious selection of study sites and to the fact that a
thorough knowledge about the catchment-lake system and a thorough evaluation of the
reconstructed temperatures is crucial to obtain reliable results. In summary, these
suggest that non-alpine lakes are high potential sites for the reconstruction of small-
scale temperature variations based on chironomids. iii
Zusammenfassung
In dieser Arbeit wurden mittlere Julitemperaturen für das Holozän anhand von
paläolimnologischen Untersuchungen der Sedimente zweier südnorwegischer Seen
rekonstruiert. Die Rekonstruktionen basieren auf der Analyse von subfossilen Zuck-
mückenlarven (Chironomidenlarven). Ziel dieser Studie war herauszufinden, ob
subfossile Chironomiden aus Seen, die unterhalb der heutigen Waldgrenze liegen, für
eine quantitative Rekonstruktion von holozänen Temperaturschwankungen geeignet
sind. Den Schwerpunkt bildeten die Fragen, ob die wasserchemischen und physika-
lischen Bedingungen in den untersuchten Seen während des Holozäns als stabil
bezeichnet werden können, ob die Chironomiden sensibel auf holozäne Temperatur-
schwankungen reagieren und ob menschlicher Einfluss im Einzugsgebiet der Seen
durch Veränderungen in den Chironomidengesellschaften aufgezeichnet wird.
Untersucht wurden dafür die mittels Radiokarbonmethode datierten Sedimente des
Reiarsdalsvatnet (245 m ü. NN, maximale Wassertiefe: 9.2 m, Mischwaldzone) und des
Isbenttjønn (787 m ü. NN, maximale Wassertiefe: 9.4 m, nördliches Boreal). Die
Multiproxy-Analysen der Sedimente umfassten die Bestimmung der subfossilen
Chironomiden, Pollenanalysen und Analysen des Kohlen- und Stickstof