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Permafrost carbon in a changing Arctic: On periglacial landscape dynamics, organic matter characteristics, and the stability of a globally significant carbon pool
Stockholm University, Faculty of Science, Department of Physical Geography.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Organic matter (OM) in arctic permafrost ground contains about twice as much carbon (C) as is currently present in the atmosphere. Climate change is particularly strong in the Arctic, and could cause a considerable part of the OM in permafrost to thaw out, decompose, and be released as greenhouse gases; further enhancing global warming. The exact size of the northern circumpolar C pool remains unclear, and processes that control decomposition and mineralization rates are even more uncertain. Superimposed on the long-term release of C through microbial decomposition of OM in the gradually deepening active layer, is the rapid release of currently sequestered OM through geomorphological processes. This thesis considers the quantity, quality, and availability of permafrost C, and explores interactions and common controls.

To better understand the potential effects of thawing permafrost, it is vital to: i) obtain more accurate size and distribution estimates of permafrost C stocks, and develop methods to accurately and efficiently implement these in models, ii) identify OM characteristics that control decomposition, specifically for permafrost material, and iii) determine and quantify key geomorphological processes that cause large amounts of OM to become available for rapid decomposition.

Detailed C quantifications are valuable to increase our fundamental understanding of permafrost soil processes and C sequestration, but including high levels of heterogeneity in models is challenging. Simple upscaling tools based on e.g. elevation parameters (Paper I) can help to bridge the gap between detailed field studies and global C models.

Permafrost OM quality is controlled by different factors than those commonly observed in temperate soils (without permafrost). We observed an unexpected (significant) correlation in upper permafrost samples, where material that is generally considered more recalcitrant showed the highest CO2 production rates per g C, indicating high lability (Paper II). In ancient Pleistocene permafrost, labile samples related significantly to OM that was enriched in decomposed microbial remains, whereas less-decomposed plant material was more stable (Paper III). Investigation of multiple incubation datasets revealed that the unusual relationship between %C and CO2 production occurred in contrasting field sites throughout the Arctic, indicating important permafrost-specific controls over OM quality (Paper IV). We discuss several possible explanations for the observed high lability of permafrost OM, such as a pool of labile dissolved organic C in the upper permafrost, or increased lability caused by past decomposition. In order to conclusively identify causal relationships, and to answer the question whether or not the same mechanisms control OM quality in different environments, further investigation of permafrost-specific OM quality is required.

Geomorphology plays a key role in C reworking and OM decomposition. Vast amounts of OM can be released abruptly (e.g. in thaw slumps and thermokarst lakes, Paper II), resulting in C turnover that will likely outweigh decomposition through gradual active layer deepening. Climate change could enhance this rapid release of C, and changes in surface hydrology and increased fire activity are expected to become the largest contributors to C loss from permafrost regions. Together with C quantity and quality, availability through gradual and abrupt processes must be parameterized and included in models in order to accurately assess the potential permafrost C climate feedback.

Place, publisher, year, edition, pages
Stockholm: Department of Physical Geography, Stockholm University , 2017. , 43 p.
Series
Dissertations from the Department of Physical Geography, ISSN 1653-7211 ; 65
Keyword [en]
Permafrost, Carbon, Climate, Arctic, Soil, Organic Matter, Cryosphere, Geomorphology
National Category
Physical Geography
Research subject
Physical Geography
Identifiers
URN: urn:nbn:se:su:diva-142586ISBN: 978-91-7649-738-8 (print)ISBN: 978-91-7649-739-5 (electronic)OAI: oai:DiVA.org:su-142586DiVA: diva2:1092649
Public defence
2017-06-16, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, 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 1: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2017-05-22 Created: 2017-05-03 Last updated: 2017-07-03Bibliographically approved
List of papers
1. Elevation-based upscaling of organic carbon stocks in high arctic permafrost terrain: a storage and distribution assessment for Spitsbergen, Svalbard
Open this publication in new window or tab >>Elevation-based upscaling of organic carbon stocks in high arctic permafrost terrain: a storage and distribution assessment for Spitsbergen, Svalbard
(English)Manuscript (preprint) (Other academic)
National Category
Physical Geography
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-142581 (URN)
Available from: 2017-05-03 Created: 2017-05-03 Last updated: 2017-05-17Bibliographically approved
2. Thermokarst dynamics and soil organic matter characteristics controlling initial carbon release from permafrost soils in the Siberian Yedoma region
Open this publication in new window or tab >>Thermokarst dynamics and soil organic matter characteristics controlling initial carbon release from permafrost soils in the Siberian Yedoma region
Show others...
2016 (English)In: Sedimentary Geology, ISSN 0037-0738, E-ISSN 1879-0968, Vol. 340, 38-48 p.Article in journal (Refereed) Published
Abstract [en]

This study relates soil organic matter (SOM) characteristics to initial soil incubation carbon release from upper permafrost samples in Yedoma region soils of northeastern Siberia, Russia. Carbon (C) and nitrogen (N) content, carbon to nitrogen ratios (C:N), delta C-13 and delta N-15 values show clear trends that correspond with SOM age and degree of decomposition. Incubation results indicate that older and more decomposed soil material shows higher C respiration rates per unit incubated C than younger and less decomposed samples with higher C content. This is important as undecomposed material is often assumed to be more reactive upon thawing. Large stocks of SOM and their potential decomposability, in combination with complex landscape dynamics that include one or more events of Holocene thaw in most of the landscape, are of consequence for potential greenhouse gas release from permafrost soils in the Yedoma region.

Keyword
Permafrost, Yedoma, Thermokarst, Carbon, SOM decomposition, Soil incubation
National Category
Earth and Related Environmental Sciences
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-132571 (URN)10.1016/j.sedgeo.2015.12.004 (DOI)000378439500006 ()
Available from: 2016-08-18 Created: 2016-08-15 Last updated: 2017-05-17Bibliographically approved
3. Characterization of labile organic matter in Pleistocene permafrost (NE Siberia), using Thermally assisted Hydrolysis and Methylation (THM-GC-MS)
Open this publication in new window or tab >>Characterization of labile organic matter in Pleistocene permafrost (NE Siberia), using Thermally assisted Hydrolysis and Methylation (THM-GC-MS)
(English)Manuscript (preprint) (Other academic)
National Category
Physical Geography
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-142583 (URN)
Available from: 2017-05-03 Created: 2017-05-03 Last updated: 2017-05-17Bibliographically approved
4. Vulnerabilityof organic matter in upper permafrost from contrasting northern circumpolar regions
Open this publication in new window or tab >>Vulnerabilityof organic matter in upper permafrost from contrasting northern circumpolar regions
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Physical Geography
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-142584 (URN)
Available from: 2017-05-03 Created: 2017-05-03 Last updated: 2017-05-17Bibliographically approved

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