Call number:
AWI G3-19-92384
Description / Table of Contents:
Widespread landscape changes are presently observed in the Arctic and are most likely to accelerate in the future, in particular in permafrost regions which are sensitive to climate warming. To assess current and future developments, it is crucial to understand past environmental dynamics in these landscapes. Causes and interactions of environmental variability can hardly be resolved by instrumental records covering modern time scales. However, long-term environmental variability is recorded in paleoenvironmental archives. Lake sediments are important archives that allow reconstruction of local limnogeological processes as well as past environmental changes driven directly or indirectly by climate dynamics. This study aims at reconstructing Late Quaternary permafrost and thermokarst dynamics in central-eastern Beringia, the terrestrial land mass connecting Eurasia and North America during glacial sea-level low stands. In order to investigate development, processes and influence of thermokarst dynamics, several sediment cores from extant lakes and drained lake basins were analyzed to answer the following research questions:
1. When did permafrost degradation and thermokarst lake development take place and what were enhancing and inhibiting environmental factors?
2. What are the dominant processes during thermokarst lake development and how are they reflected in proxy records?
3. How did, and still do, thermokarst dynamics contribute to the inventory and properties of organic matter in sediments and the carbon cycle?
Methods applied in this study are based upon a multi-proxy approach combining sedimentological, geochemical, geochronological, and micropaleontological analyses, as well as analyses of stable isotopes and hydrochemistry of pore-water and ice. Modern field observations of water quality and basin morphometrics complete the environmental investigations.
The investigated sediment cores reveal permafrost degradation and thermokarst dynamics on different time scales. The analysis of a sediment core from GG basin on the northern Seward Peninsula (Alaska) shows prevalent terrestrial accumulation of yedoma throughout the Early to Mid Wisconsin with intermediate wet conditions at around 44.5 to 41.5 ka BP. This first wetland development was terminated by the accumulation of a 1-meter-thick airfall tephra most likely originating from the South Killeak Maar eruption at 42 ka BP. A depositional hiatus between 22.5 and 0.23 ka BP may indicate thermokarst lake formation in the surrounding of the site which forms a yedoma upland till today. The thermokarst lake forming GG basin initiated 230 ± 30 cal a BP and drained in Spring 2005 AD. Four years after drainage the lake talik was still unfrozen below 268 cm depth.
A permafrost core from Mama Rhonda basin on the northern Seward Peninsula preserved a full lacustrine record including several lake phases. The first lake generation developed at 11.8 cal ka BP during the Lateglacial-Early Holocene transition; its old basin (Grandma Rhonda) is still partially preserved at the southern margin of the study basin. Around 9.0 cal ka BP a shallow and more dynamic thermokarst lake developed with actively eroding shorelines and potentially intermediate shallow water or wetland phases (Mama Rhonda). Mama Rhonda lake drainage at 1.1 cal ka BP was followed by gradual accumulation of terrestrial peat and top-down refreezing of the lake talik. A significant lower organic carbon content was measured in Grandma Rhonda deposits (mean TOC of 2.5 wt%) than in Mama Rhonda deposits (mean TOC of 7.9 wt%) highlighting the impact of thermokarst dynamics on biogeochemical cycling in different lake generations by thawing and mobilization of organic carbon into the lake system.
Proximal and distal sediment cores from Peatball Lake on the Arctic Coastal Plain of Alaska revealed young thermokarst dynamics since about 1,400 years along a depositional gradient based on reconstructions from shoreline expansion rates and absolute dating results. After its initiation as a remnant pond of a previous drained lake basin, a rapidly deepening lake with increasing oxygenation of the water column is evident from laminated sediments, and higher Fe/Ti and Fe/S ratios in the sediment. The sediment record archived characterizing shifts in depositional regimes and sediment sources from upland deposits and re-deposited sediments from drained thaw lake basins depending on the gradually changing shoreline configuration. These changes are evident from alternating organic inputs into the lake system which highlights the potential for thermokarst lakes to recycle old carbon from degrading permafrost deposits of its catchment.
The lake sediment record from Herschel Island in the Yukon (Canada) covers the full Holocene period. After its initiation as a thermokarst lake at 11.7 cal ka BP and intense thermokarst activity until 10.0 cal ka BP, the steady sedimentation was interrupted by a depositional hiatus at 1.6 cal ka BP which likely resulted from lake drainage or allochthonous slumping due to collapsing shore lines. The specific setting of the lake on a push moraine composed of marine deposits is reflected in the sedimentary record. Freshening of the maturing lake is indicated by decreasing electrical conductivity in pore-water. Alternation of marine to freshwater ostracods and foraminifera confirms decreasing salinity as well but also reflects episodical re-deposition of allochthonous marine sediments.
Based on permafrost and lacustrine sediment records, this thesis shows examples of the Late Quaternary evolution of typical Arctic permafrost landscapes in central-eastern Beringia and the complex interaction of local disturbance processes, regional environmental dynamics and global climate patterns. This study confirms that thermokarst lakes are important agents of organic matter recycling in complex and continuously changing landscapes.
Type of Medium:
Dissertations
Pages:
XII, 128 Seiten
,
Illustrationen
,
1 DVD
URL:
http://www.gbv.de/dms/tib-ub-hannover/877835284.pdf
URL:
https://nbn-resolving.org/urn:nbn:de:kobv:517-opus4-101364
Language:
English
Note:
Enthält 5 Publikationen:
1) Mid Wisconsin to Holocene permafrost and landscape dynamics based on a drained lake basin core from the northern Seward Peninsula, northwest Alaska / Josefine Lenz, Guido Grosse, Benjamin M. Jones [und 5 weitere]
2) Evidence of multiple thermokarst lake generations from an 11,800-year old permafrost core on the northern Seward Peninsula, Alaska / Josefine Lenz, Sebastian Wetterich, Benjamin M. Jones [und 3 weitere]
3) Impacts of shore expansion and catchment characteristics on lacustrine thermokarst records in permafrost lowlands, Alaska Arctic Coastal Plain / Josefine Lenz, Sebastian Wetterich, Benjamin M. Jones [und 5 weitere]
4) Periglacial landscape dynamics in the western Canadian Arctic: Results from a thermokarst lake record on a push moraine (Herschel Island, Yukon) / Josefine Lenz, Michael Fritz, Lutz Schirrmeister [und 4 weitere]
5) Regional environmental change versus local signal preservation in Holocene thermokarst lake sediments: A case study from Herschel Island, Yukon (Canada) / Michael Fritz, Ingmar Unkel, Josefine Lenz [und 6 weitere]
,
Table of contents
Abstract
Zusammenfassung
Abbreviations and nomenclature
1 Thesis organization
1.1 Overview of chapters
1.2 Author contribution
2 Introduction
2.1 Scientific background
2.1.1 Arctic environments and permafrost in the study region of Beringia
2.1.2 Permafrost degradation and its global feedbacks
2.1.3 Thermokarst lakes and basins as paleoenvironmental archives
2.2 Aims and approaches
3 Mid Wisconsin to Holocene permafrost and landscape dynamics based on a drained lake basin core from the northern Seward Peninsula, northwest Alaska
3.1 Abstract
3.2 Introduction
3.3 Study area
3.4 Material and methods
3.5 Results
3.5.1 Core stratigraphy
3.5.2 Cryostratigraphy
3.5.3 Grain size distribution
3.5.4 Magnetic susceptibility
3.5.5 Biogeochemical characteristics
3.5.6 Tephra
3.5.7 Palaeoecology
3.5.8 Geochronology
3.6 Discussion
3.7 Conclusions
4 Evidence of multiple thermokarst lake generations from an 11,800-year old permafrost core on the northern Seward Peninsula, Alaska
4.1 Abstract
4.2 Introduction
4.3 Study area
4.4 Material and methods
4.5 Results
4.5.1 Geochronology
4.5.2 Cryolithological description
4.5.3 Geochemical results
4.5.4 Bioindicators
4.5.5 Characteristics of intra-sedimentary ice and comparison with modern waters
4.6 Discussion
4.6.1 Thermokarst lake dynamics
4.6.2 Regional lake dynamics and global environmental change
4.6.3 Carbon cycling
4.7 Conclusions
5 Impacts of shore expansion and catchment characteristics on lacustrine thermokarst records in permafrost lowlands, Alaska Arctic Coastal Plain
5.1 Abstract
5.2 Introduction
5.3 Study area
5.4 Material and methods
5.5 Results
5.5.1 Sedimentological results of near-shore core P1
5.5.2 Sedimentological and palynological results of lake center-cores P2 (and P3)
5.5.3 Lake age estimation
5.6 Discussion
5.6.1 Thermokarst lake development
5.6.2 Impact of catchment genesis and morphology on the lake sediment record
5.6.3 Carbon degradation
5.7 Conclusions
6 Synthesis
6.1 Study sites in central-eastern Beringia: Similarities and differences
6.2 Permafrost degradation and thermokarst development in central-eastern Beringia
6.2.1 Timing of thermokarst development
6.2.2 Environmental factors supporting and inhibiting thermokarst
6.3 Processes of thermokarst lake development and their imprint in proxy records
6.4 Contribution of thermokarst dynamics to the carbon cycle
6.5 Potentials/limitations of thermokarst lake archives and outlook
Bibliography
Appendix I: Periglacial landscape dynamics in the western Canadian Arctic: Results from a thermokarst lake record on a push moraine (Herschel Island, Yukon)
I-1 Abstract
I-2 Introduction
I-3 Study area
I-4 Material and methods
I-5 Results
I-5.1 Core lithology
I-5.2 Radiography
I-5.3 Magnetic susceptibility and water content
I-5.4 Grain size distribution
I-5.5 Biogeochemical characteristics
I-5.6 Geochronology
I-6 Discussion
I-6.1 Evolution of Lake Herschel
I-6.2 Paleoenvironmental implications of the Lake Herschel record
I-7 Conclusions
Appendix II: Regional environmental change versus local signal preservation in Holocene thermokarst lake sediments: A case study from Herschel Island, Yukon (Canada)
II-1 Abstract
II-2 Introduction and study area
II-3 Material and methods
II-3.1 Sediment core
II-3.2 Radiocarbon dating and age modelling
II-3.3 Pore-water chemistry
II-3.4 X-ray fluorescence (XRF) scanning
II-3.5 Micropaleontology
II-4 Results
II-4.1 Chronostratigraphy: The revised age model
II-4.2 XRF chemistry
II-4.3 Pore-water chemistry
II-4.4 Calcareous microfossils
II-4.5 Pollen
II-5 Discussion
II-5.1 Sedimentation history of Lake Herschel
II-5.2 Limnological, sedimentary and geochemical properties predefine the habitat
I-5.3 Autochthonous versus allochthonous deposition of calcareous microfossils
II-5.4 Regional pollen-based reconstruction of vegetation and climate
II-6 Conclusions
Acknowledgements
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