| 1. |
- Sannel, A. Britta K., 1968-
(författare)
-
Holocene dynamics in subarctic peat plateaus of west-central Canada : Vegetation succession, peat accumulation and permafrost history
- 2007
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Dynamics in vegetation, permafrost and peat and net carbon accumulation rates throughout the Holocene have been studied in two subarctic peat plateaus of west-central Canada through plant macrofossil analysis, geochemical analyses and AMS radiocarbon dating. Peatland formation at the studied sites began around 6600-5900 cal yr BP as a result of paludification of upland forests. Permafrost aggradation probably occurred 5600-4500 cal yr BP when Sphagnum fuscum became established and rootlet layers started to appear. Alternating layers of Sphagnum fuscum and rootlet peat throughout most of the peat profiles are indicating relatively dry surface conditions, suggesting that permafrost conditions have remained stable since the peat plateau stages were initiated. Local fires have occurred in the peatlands, but most fires did not cause degradation of the permafrost. However, lower peat and net carbon accumulation rates are recorded from rootlet layers containing charcoal. The long-term peat and net carbon accumulation rates for both studied peat profiles are 0,30-0,31 mm/yr and 12,5-12,7 gC/m2yr. Accumulation rates are variable depending on peat plateau stage. Peat accumulation rates are in general 4-5 times higher in S. fuscum than in rootlet stages, and net carbon accumulation rates are 3-4 times higher. Therefore even though Sphagnum peat makes up a majority of the peat profile depth, rootlet peat stages can represent most of the time since the peatland was initiated. The gross stratigraphy and plant macrofossil analyses show that there have been no wet phases, indicating permafrost collapse, since the peat plateau stages were initiated. This suggests that subarctic peat plateaus with alternating Sphagnum fuscum and rootlet peat layers have been acting as long-term net carbon sinks, accumulating carbon which has been incorporated into the permafrost, throughout most of the Holocene. High and stable carbon/nitrogen ratios throughout most of the profiles suggest that decomposition has not occurred in the perennially frozen peat. Since the peat plateaus are characterized by no decay in the permafrost and dry surface conditions, methane emissions are negligible from these ecosystems. In a future warmer climate carbon that has been stored under permafrost conditions can be remobilized. The warming may cause drier surface conditions resulting in increased emissions of carbon dioxide or, alternatively, permafrost collapse resulting in wetter surface conditions and increased methane emissions.
|
|
| 2. |
|
|
| 3. |
- Hugelius, Gustaf, 1980-
(författare)
-
Soil organic carbon in permafrost terrain : Total storage, landscape distribution and environmental controls
- 2009
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High latitude terrestrial ecosystems are considered key components in the global carbon (C) cycle and hold large reservoirs of soil organic carbon (SOC). To a large degree, this SOC is stored in permafrost soils and peatlands and is vulnerable to remobilization under future global warming and permafrost thawing. Recent studies estimate that soils in permafrost regions store SOC equivalent to ~ 1.5 times the global atmospheric C pool. Ecosystems and soils interact with the atmospheric C pool; photosynthesis sequesters CO2 into SOC whereas microbial decomposition releases C based trace gases (mainly CO2 and CH4). Because of the radiative greenhouse properties of these gases, soil processes also feedback on the global climate system. Recent studies report increases in permafrost temperatures and under future climate change scenarios permafrost environments stand to undergo further changes. As permafrost thaws and surface hydrology changes, there is concern that periglacial tundra and peatland ecosystems will switch from being sinks for atmospheric C into sources, creating a potential for positive feedbacks on global warming. The magnitude of change in C fluxes resulting from climate warming and permafrost thawing depends on the remobilization processes affecting SOC stores, the size of SOC stores that become available for remobilization and the lability of the SOM compounds in these stores. While the large size and potential vulnerability of arctic SOC reservoirs is recognized, detailed knowledge on the landscape partitioning and quality of this SOC is poor. Paper I of this thesis assesses landscape allocation and environmental gradients in SOC storage in the Usa River Basin lowlands of northeastern European Russia. The Russian study area ranges from taiga region with isolated permafrost patches to tundra region with nearly continuous permafrost. Paper II of this thesis investigates total storage, landscape partitioning and quality of soil organic carbon (SOC) in the tundra and continuous permafrost terrain of the Tulemalu Lake area in the Central Canadian Arctic. Databases on soil properties, permafrost, vegetation and modeled climate are compiled and analyzed. Mean SOC storage in the two study regions is 38.3 kg C m-2 for the Usa River Basin and 33.8 kg C m-2 for Tulemalu Lake (for 1m depth in mineral soils and total depth of peat deposits). Both estimates are higher than previous estimates for the same study areas. Multivariate gradient analyses from the Usa Basin show that local vegetation and permafrost are strong predictors of soil chemical properties, overshadowing the effect of climate variables. The results highlight the importance of peatlands, particularly bogs, in bulk SOC storage in all types of permafrost terrain. In the Tulemalu Lake area significant amounts of SOC is stored in cryoturbated soil horizons with C/N ratios indicating a relatively low degree of decomposition. As this pool of cryoturbated SOC is mainly stored in the active layer, no dramatic increases in remobilization are expected following a deepening of the active layer. However, recent studies have demonstrated the importance of SOC storage in deep (>1m) cryoturbated horizons. Perennially frozen peat deposits in permafrost bogs constitute the main vulnerable SOC pool in the investigated regions. Remobilization of this frozen C can occur through gradual but widespread deepening of the active layer with subsequent talik formation, or through more rapid but localized thermokarst erosion.
|
|
| 4. |
- Kaislahti Tillman, Päivi
(författare)
-
Holocene climate change in high latitudes recorded by stable isotopes in peat
- 2010
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A key to the understanding of natural and human induced climate variations is to reconstruct past changes from different environments. No outstanding method for general use has been pinpointed, instead, a need of multi-proxy studies is often stressed and the reconstructions are under constant improvement by new techniques. The aim of my PhD project is to test a relatively new method, stable carbon and oxygen isotopes isolated from single moss species, and develop climate reconstructions based on them. The main interest is to implement the method in records from northern peatlands where permafrost conditions prevail, and contribute to the discussion about the warming Arctic. The first part of the Licentiate thesis is a method study about the variation of stable carbon and oxygen isotope ratios in different moss plant components. Modern isotopic values were calibrated against instrumental climate records from the study region in west-central Canada. The impact of peat decay on proxies was investigated by colorimetric and chemical (C/N) methods. The results indicate that the isotope signal is well preserved in peat that started to accumulate c. 6000 years ago. Furthermore, statistical analyses imply that the variation of stable carbon isotope ratios in Sphagnum fuscum is significantly correlated to the variation of summer temperatures. A temperature reconstruction was developed in the second part of the thesis, based on stable carbon isotope ratios. Wet/dry periods were derived from the stable oxygen isotope record, macrofossil analysis, and the peat humification record. The results were compared with other proxy records from the vicinity of the study area. The main climate periods, such as The Mediaeval Warm Period and The Little Ice Age were registered in the temperature record. The amplitude of the temperature change was similar to especially those in chironomid based reconstructions, showing c. 6.5±1°C variation in July temperatures during the past 6.2 ka.
|
|
| 5. |
- Palmtag, Juri, 1980-
(författare)
-
Storage, landscape partitioning and lability of soil organic matter in permafrost terrain
- 2015
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Recent estimates indicate that soils in the northern circumpolar permafrost region store substantial amounts of soil organic carbon (SOC). This reservoir has accumulated over 10-100.000 years and is often preserved in a relatively undecomposed state because frozen and often water-logged conditions prevented microbial degradation. Under a projected future climate change caused by rising greenhouse gases, permafrost thaw and rapid decomposition of vulnerable soil organic matter (SOM) could provide a positive feedback on global warming by releasing large amounts of carbon dioxide and/or methane into the atmosphere.SOC pools have large regional and landscape-level variability depending on topographic, ecoclimatic and edaphic factors. As a consequence, large scale maps and even regional data sets describing SOC storage should be taken with caution since they are highly simplified. The purpose of this thesis is to improve our knowledge on quantity and quality of SOM in different areas of continuous permafrost and provide regional high quality data from hitherto under-sampled regions for future assessment of the potential remobilization of SOC under global warming. A special focus is put on SOC partitioning within the landscape and soil horizon levels as well as on soil forming processes under periglacial conditions. Throughout the five different study areas presented in this thesis the landscape mean SOC storage ranges between 8 and 30 kg C m-2, while site differences are in the order of 0 to 80 kg C m-2. Paper I presents new SOC data from contrasting areas in continuous permafrost: a mountainous High Arctic site in Zackenberg (NE Greenland) and lowland sites in Shalaurovo and Cherskiy lower Kolyma (NE Siberia). The main difference is that about 60% of the Zackenberg area is higher elevation terrain with mostly barren ground and very low SOC content, resulting in a much lower landscape-level mean SOC storage compared to the Siberian sites. In addition, Paper II shows that even when comparing two lowland sites located only 150 km apart in Taymyr Peninsula (N Siberia) the mean SOC storage differs with 40% between the areas. This emphasizes that even in lowlands on a regional scale not only different landforms and land cover but also microrelief, soil moisture and especially parent material play a very important role for obtaining more accurate SOC storage estimates.Throughout this thesis a special emphasis is put on understanding the role of cryoturbation for SOC storage. Signs of cryoturbation were observed at all sites and 14C dates show that this process is occurring since at least the early Holocene. On average, 30% of all SOC in the top meter of soil is located in buried C-enriched pockets. The only exception is Zackenberg, with only 12%, where slope processes were the dominant mechanism for burying C-enriched material into deeper layers.We use the weight ratio of Carbon/Nitrogen (C/N) to gain information about SOM decomposability. Generally, all sites show the same trend that the C/N ratio decreases with soil depth. Top organic soil and peat samples have always the highest C/N ratios, suggesting little decomposed SOM. Except for the Zackenberg site, the buried C-enriched pockets have significantly higher C/N ratios than the adjacent mineral subsoil samples. We assume that this C-enriched material was exposed over longer time periods to aerobic decomposition and was therefore relatively well decomposed before it was buried by reactivated slope processes.
|
|
