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Temporal and spatial dynamics in subarctic peat plateaus and thermokarst lakes
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Permafrost peatlands are widespread at high northern latitudes and are important soil organic carbon reservoirs. A future warming in these areas, as suggested by global climate models, can cause thawing and increased ground subsidence (thermokarst), resulting in changes in surface hydrology and ecosystem functioning. The aim of this thesis is to increase our knowledge of temporal and spatial dynamics in subarctic peat plateaus with interspersed thermokarst lakes in order to better understand how these ecosystems respond to climate change. Detailed plant macrofossil and carbon/nitrogen ratio analyses of two peat plateaus located in the continuous and northern discontinuous permafrost zones in west-central Canada show that permafrost conditions have been stable since permafrost developed around 5600–4500 cal yr BP. Peat plateaus act as carbon sinks over time. The lack of wet phases since the plateaus formed, despite several local fires, suggests that this type of peatlands have been negligible as methane sources throughout most of their history, representing a negative net radiative forcing on climate. Thermokarst lakes are common features in peat plateaus across the northern permafrost region. A time-series analysis of aerial photographs and high resolution satellite images in three peat plateau/thermokarst lake complexes along a climatic and permafrost gradient shows that where the mean annual air temperature (MAAT) is below -5ºC and ground temperatures are -2ºC or colder, only minor changes in thermokarst lake extent have occurred from the mid 1970s until the mid 2000s. During the same time interval extensive lake drainage and new lake formation has taken place where the MAAT is ca -3ºC and the ground temperature is close to 0ºC. In a future progressively warmer and wetter climate, permafrost degradation can cause significant impacts on landscape pattern and greenhouse gas exchange also in the vast peat plateaus presently experiencing stable permafrost conditions.

Place, publisher, year, edition, pages
Stockholm: Department of Physical Geography and Quaternary Geology (INK), Stockholm University , 2010. , 36 p.
Series
Dissertations from the Department of Physical Geography and Quaternary Geology, ISSN 1653-7211 ; 23
Keyword [en]
Peatlands, permafrost, subarctic, peat plateau, thermokarst lake, macrofossil analysis, carbon/nitrogen ratio, peat accumulation, carbon accumulation, remote sensing, binary encoding, time-series analysis
National Category
Natural Sciences
Research subject
Physical Geography
Identifiers
URN: urn:nbn:se:su:diva-42115ISBN: 978-91-7447-106-9 (print)OAI: oai:DiVA.org:su-42115DiVA: diva2:343989
Public defence
2010-10-01, 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 4: Manuscript.Available from: 2010-09-09 Created: 2010-08-17 Last updated: 2011-12-27Bibliographically approved
List of papers
1. Long-term stability of permafrost in subarctic peat plateaus, west-central Canada
Open this publication in new window or tab >>Long-term stability of permafrost in subarctic peat plateaus, west-central Canada
2008 (English)In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 18, no 4, 589-601 p.Article in journal (Refereed) Published
Abstract [en]

Long-term vegetation succession and permafrost dynamics in subarctic peat plateaus of west-central Canada have been studied through detailed plant macrofossil analysis and extensive AMS radiocarbon dating of two peat profiles. Peatland inception at these sites occurred around 5800-5100 yr BP (6600-5900 cal. BP) as a result of paludification of upland forests. At the northern peat plateau site, located in the continuous permafrost zone, palaeobotanical evidence suggests that permafrost was already present under the forested upland prior to peatland development. Paludification was initiated by permafrost collapse, but re-aggradation of permafrost occurred soon after peatland inception. At the southern site, located in the discontinuous permafrost zone, the aggradation of permafrost occurred soon after peatland inception. In the peat plateaus, permafrost conditions have remained very stable until present. Sphagnum fuscum-dominated stages have alternated with more xerophytic communities characterized by ericaceous shrubs. Local peat fires have occurred, but most of these did not cause degradation of the permafrost. Starting from 2800-1100 yr BP (2900-1000 cal. BP) consistently dry surface conditions have prevailed, possibly related to continued frost heave or nearby polygon crack formation.

Keyword
peatlands, peat plateau, permafrost, fire, macrofossils, vegetation succession, radiocarbon dating, subarctic, Canada, Holocene
Identifiers
urn:nbn:se:su:diva-16208 (URN)10.1177/0959683608089658 (DOI)000256882300008 ()
Available from: 2009-01-26 Created: 2009-01-26 Last updated: 2017-12-13Bibliographically approved
2. Holocene peat growth and decay dynamics in sub-arctic peat plateaus, west-central Canada
Open this publication in new window or tab >>Holocene peat growth and decay dynamics in sub-arctic peat plateaus, west-central Canada
2009 (English)In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 38, no 1, 13-24 p.Article in journal (Refereed) Published
Abstract [en]

Peat and net carbon accumulation rates in two sub-arctic peat plateaus of west-central Canada have been studied through geochemical analyses and accelerator mass spectrometry (AMS) radiocarbon dating. The peatland sites started to develop around 6600–5900 cal. yr BP and the peat plateau stages are characterized by Sphagnum fuscum peat alternating with rootlet layers. The long-term peat and net carbon accumulation rates for both profiles are 0.30–0.31 mm/yr and 12.5–12.7 gC/m2yr, respectively. These values reflect very slow peat accumulation (0.04–0.09 mm/yr) and net carbon accumulation (3.7–5.2 gC/m2yr) in the top rootlet layers. Extensive AMS radiocarbon dating of one profile shows that accumulation rates are variable depending on peat plateau stage. Peat accumulation rates are up to six times higher and net carbon accumulation rates up to four times higher in S. fuscum than in rootlet stages. Local fires represented by charcoal remains in some of the rootlet layers result in very low accumulation rates. High C/N ratios throughout most of the peat profiles suggest low degrees of decomposition due to stable permafrost conditions. Hence, original peat accretion has remained largely unaltered, except in the initial stages of peatland development when permafrost was not yet present.

Identifiers
urn:nbn:se:su:diva-18787 (URN)10.1111/j.1502-3885.2008.00048.x (DOI)000262635900002 ()
Available from: 2009-02-03 Created: 2009-02-03 Last updated: 2017-12-13Bibliographically approved
3. High-resolution remote sensing identification of thermokarst lake dynamics in a subarctic peat plateau complex
Open this publication in new window or tab >>High-resolution remote sensing identification of thermokarst lake dynamics in a subarctic peat plateau complex
2010 (English)In: Canadian journal of remote sensing, ISSN 0703-8992, E-ISSN 1712-7971, Vol. 36, no 1, S26-S40 p.Article in journal (Refereed) Published
Abstract [en]

Methods to quantify the dynamics of thermokarst lake margins in subarctic permafrost peatlands have been examined using historical aerial photographs and QuickBird imagery from the Hudson Bay Lowlands in west-central Canada, spanning a time period of 52 years (1954-2006). The goal of this study was to develop a method for detection of metre-scale changes in thermokarst lake extent using a time series of high resolution imagery. The method should be applicable to a variety of lake forms, transferable to other locations and sufficiently robust as to support different data types. Semi-automatic remote sensing techniques such as unsupervised and supervised classification and texture and high-pass filtering were tested, evaluated and rejected. According to an experiment of manual digitalization of shorelines by multiple operators the relative uncertainty for lakes surrounded by peat plateau was ±1.5 m. The uncertainty was reduced to ±0.6 m when binary encoding of transects perpendicular to the shoreline was used to refine the manual delineation. This proved to be the most accurate method to detect small-scale changes in lake extent. An increased understanding and quantification of thermokarst dynamics in permafrost peatlands is important for predicting future scenarios of greenhouse gas emissions from these ecosystems under changing climatic conditions and our method supports such goals.

National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-42152 (URN)10.5589/m10-010 (DOI)000282371000004 ()
Available from: 2010-08-18 Created: 2010-08-18 Last updated: 2017-12-12Bibliographically approved
4. Warming-induced destabilization of peat plateau/thermokarst lake complexes
Open this publication in new window or tab >>Warming-induced destabilization of peat plateau/thermokarst lake complexes
2011 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, G03035- p.Article in journal (Refereed) Published
Abstract [en]

Peat plateaus are widespread at high northern latitudes and are important soil organic carbon reservoirs. A warming climate can cause either increased ground subsidence (thermokarst) resulting in lake formation or increased drainage as the permafrost thaws. A better understanding of spatiotemporal variations in these landforms in relation to climate change is important for predicting the future thawing permafrost carbon feedback. In this study, dynamics in thermokarst lake extent during the last 35-50 years has been quantified through time series analysis of aerial photographs and high-resolution satellite images (IKONOS/QuickBird) in three peat plateau complexes, spread out across the northern circumpolar region along a climatic and permafrost gradient. From the mid-1970s until the mid-2000s there has been an increase in mean annual air temperature, winter precipitation, and ground temperature in all three study areas. The two peat plateaus located in the continuous and discontinuous permafrost zones, respectively, where mean annual air temperatures are below -5 degrees C and ground temperatures are -2 degrees C or colder, have experienced small changes in thermokarst lake extent. In the peat plateau located in the sporadic permafrost zone where the mean annual air temperature is around -3 degrees C, and the ground temperature is close to 0 degrees C, lake drainage and infilling with fen vegetation has been extensive and many new thermokarst lakes have formed. In a future progressively warmer and wetter climate permafrost degradation can cause significant impacts on landscape composition and greenhouse gas exchange also in areas with extensive peat plateaus, which presently still experience stable permafrost conditions.

National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-66512 (URN)10.1029/2010JG001635 (DOI)000295130400002 ()
Note
authorCount :2Available from: 2011-12-27 Created: 2011-12-20 Last updated: 2017-12-08Bibliographically approved

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