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The Potential of Non-Alpine Lakes for Quantitative Palaeotemperature

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The Potential of Non-Alpine Lakes for Quantitative Palaeotemperature 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 Chironomid-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 Lithology 32 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 Stickstoffs, des Schwefels und des biogenen Siliziums. Die wichtigsten Ergebnisse der Rekonstruktion der Umweltgeschichten der Seen sind folgende: Durch die Pollenuntersuchungen konnte Waldbedeckung in der Umgebung des Reiarsdalsvatnet ab 9,900 cal. BP und am Isbenttjønn ab 9,300 cal. BP nachgewiesen werden. Die Ablagerung organischer Sedimente in den Seen begann im Präboreal bzw. Boreal. Die Böden im Einzugsgebiet des Reiarsdalsvatnet sind seit 9,600 cal. BP und im Einzugsgebiet des Isbenttjønn seit 8,600 cal. BP stabil. Nach 2,700 cal. BP konnten eine Auflichtung der Vegetation und erste Hinweise auf anthropogenen Einfluss im Einzugsgebiet des Reiarsdalsvatnet festgestellt werden. Im Profil des Isbenttjønn sind Anzeichen menschlicher Aktivität seit 3,300 cal. BP zu finden, jedoch wird eine Auflichtung der Vegetation erst nach 1,400 cal. BP deutlich. Obwohl es in den Sedimenten beider Seen Hinweise auf die lokale Präsenz des Menschen während des letzten Jahrtausends gibt, hat dieser die Vegetation in den Einzugsgebieten nur graduell verändert. Beide
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