Cyclic Development and Thermokarstic Degradation of Palsas in the Mid‐Alpine Zone at Leirpullan, Dovrefjell, Southern Norway
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The palsas of Leirpullan are developed in deltaic sands and silts in the zone of alpine permafrost at an altitude of 1437 m (estimated mean annual air temperature −2.7°C). Five mounds that represent a chronosequence are analysed with particular reference to initiation, aggradation and degradation phases. It is inferred that geoecological factors control initial mound formation beneath areas of thin (< 20 cm) moss-peat, which provide summer insulation for developing permafrost. Low summer temperatures and low summer precipitation appear to allow the survival of permafrost, the existence of stable mounds, and limited height growth, after the gradual successional replacement of moss-peat by fellfield vegetation growing directly on the mineral substrate. Degradation occurs owing to thermokarstic processes, which result in the formation of rim-ridge ramparts. ‘Exogenous’ processes associated with lateral thermal erosion by lake and river water are important at the site. Regeneration and the possible effects of climatic change are also discussed. Palsas with only a thin peat cover in the initial stage of formation but no peat cover during subsequent development and degradation are considered to be near one extreme of a morphological and genetic continuum of pure peat palsas to mineral permafrost mounds. The rim-ridge ramparts characteristic of the degraded palsas of Leirpullan appear to provide appropriate modern analogues for relict Pleistocene forms in Wales and elsewhere in Europe formerly interpreted as pingo scars. © 1997 John Wiley & Sons, Ltd. Les palses de Leirpullan sont développées dans des sables et limons deltaïques de la zone de pergélisol alpin, à une altitude de 1437 m (moyenne annuelle estimée de température −2.7°C). Cinq buttes qui représentent une chronoséquence sont analysées en étudiant spécialement les phases d'apparition, de croissance et de dégradation des formes. Il en est conclu que des facteurs géoécologiques contrôlent l'apparition des buttes: celles-ci apparaissent en des endroits où la tourbe qui protège le pergélisol en été, a une faible épaisseur (< 20 cm). De faibles températures et de faibles précipitations d'été permettent la conservation du pergélisol, et autorisent l'existence de buttes stables avec une croissance en hauteur limitée après le remplacement graduel de la tourbe par une végétation croissant directement sur le substrat minéral. La dégradation se produit par des processus thermokarstiques qui provoquent la formation de remparts. Des processus exogènes associés à une érosion thermique latérale par les eaux de lac ou de rivière sont importants. Une régénération et les effets possibles du changement climatique sont aussi discutés. Des palses avec seulement une fine couverture de tourbe au moment de leur apparition et sans couverture de tourbe au moment de leur croissance et de leur dégradation sont considérées comme un extrême d'un continuum morphologique et génétique entre des palses organiques et des palses minérales. Les remparts caractéristiques des palses dégradées de Leirpullan fournissent des analogues modernes pour les formes pléistocènes reliques du Pays de Galles et d'autres formes qui, en Europe, ont été interprétées comme des traces de pingos. © 1997 John Wiley & Sons, Ltd.Keywords:
Chronosequence
Aggradation
Alpine climate
Aggradation
Active layer
Thermokarst
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A chronosequence of three calcareous soils from the Punjab plains of Pakistan was studied with respect to total, organic, and inorganic P fractions. Changes in these parameters and previously reported indices of development for these soils reflected their weakly weathered status.
Chronosequence
Calcareous soils
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Describes use of air temperatures to determine permafrost presence for engineering purposes. Annual mean temperature and thawing index (a yearly summation of daily mean temperature over 32 F) in 61 localities of northern Canada were compared with reported permafrost occurrences. The latter are divided into four categories: free of permafrost; discontinuous permafrost; continuous permafrost in forest, and in tundra. Correlations were found in all but twelve of the localities; the latter are considered individually and in some, permafrost was uncertain.
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Aggradation at bridges causes the bridge waterway opening to be reduced, possibly resulting in upstream flooding and increased contraction scour. Aggradation results when the sediment load supplied to a reach of river from upstream exceeds its capacity to transport sediment. Solutions to aggradational problems at bridges are often complex and expensive. Solutions include increasing sediment transport through the bridge by modifying the channel, constructing an upstream sediment trap, redesigning the bridge, dredging, and treating the cause of the aggradation. At many bridges, aggradation problems can be severe. As an example, aggradation at a bridge in northern Pennsylvania is described. The benefits, disadvantages, and costs for various possible solutions to the example problem are compared and the most cost-effective solution is presented.
Aggradation
Dredging
Bridge (graph theory)
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Permafrost contains a large (1700 Pg C) terrestrial pool of organic matter (OM) that is susceptible to degradation as global temperatures increase. Of particular importance is syngenetic Yedoma permafrost containing high OM content. Reactive iron phases promote stabilizing interactions between OM and soil minerals and this stabilization may be of increasing importance in permafrost as the thawed surface region ("active layer") deepens. However, there is limited understanding of Fe and other soil mineral phase associations with OM carbon (C) moieties in permafrost soils. To elucidate the elemental associations involved in organomineral complexation within permafrost systems, soil cores spanning a Pleistocene permafrost chronosequence (19,000, 27,000, and 36,000 years old) were collected from an underground tunnel near Fairbanks, Alaska. Subsamples were analyzed via scanning transmission X-ray microscopy–near edge X-ray absorption fine structure spectroscopy at the nano- to microscale. Amino acid-rich moieties decreased in abundance across the chronosequence. Strong correlations between C and Fe with discrete Fe(III) or Fe(II) regions selectively associated with specific OM moieties were observed. Additionally, Ca coassociated with C through potential cation bridging mechanisms. Results indicate Fe(III), Fe(II), and mixed valence phases associated with OM throughout diverse permafrost environments, suggesting that organomineral complexation is crucial to predict C stability as permafrost systems warm.
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Understanding patterns of soil C and N accumulation in restored prairies is necessary to determine the sequestration potential of these systems. Estimates of C and N accumulation following prairie restoration are generally based on chronosequence studies. However, few studies have compared a chronosequence approach to direct measures of accumulation over time. Restored prairies in the Cowling Arboretum of Carleton College, Northfield, MN, were established annually from 1995 to 2007. In 2000 and 2010–2011, prairies were sampled for soil %C and %N and soil bulk density, allowing for a comparison of the effects of prairie age using a chronosequence approach to direct measures of C and N accumulation rates. Soil C and N content increased in all restorations, although there was no effect of field age across the chronosequence on C or N pools. However, rates of soil C and N accumulation decreased with restoration age, suggesting a rapid increase in the years immediately following agricultural conversion. This rapid accumulation was likely due to the turnover of annual and biennial species that dominated plant cover in the early in succession. This suggestion was supported by an observed increase in δ13C values with increasing prairie age. Thus, contributions from early successional species may result in rapid recovery of soil pools following restoration from agriculture. Direct measures of soil C and N accumulation may be necessary when initial variability among sites limit the ability of a chronosequence approach to detect significant changes in soil C and N content over short time periods.
Chronosequence
Soil carbon
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One method of studying the effects of time as a soil-forming factor is to recognize and investigate a chronosequence, wherein four out of five soil-forming factors are constant or vary ineffectively. Thus, observed differences between soils of different ages forming a sequence are deemed to be the result of the lapse of varying intervals of time since the initiation of soil formation. In this paper theoretical considerations underlying the chronosequence concept are examined and soil development is discussed. Several non-strict chronosequences are reviewed, and a number of chronosequence studies are examined. The significance of such studies is assessed, and general desiderata for future investigations set out.
Chronosequence
Sequence (biology)
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Using continuous data obtained from 17 monitoring sites, the permafrost temperature profiles and the depths of zero annual amplitude (DZAA) on the Qinghai-Tibet Plateau are examined. Permafrost thermal trumpet curves are generally narrow and the DZAAs are generally shallow in warm permafrost regions, especially at sites where the permafrost temperature is close to 0 °C. The observed DZAAs in warm permafrost regions are indeed generally less than 7.0 m and for three sites less than 4.0 m. In low-temperature permafrost areas, the situation is reversed: the thermal trumpet curves are generally wide and the DZAAs are generally deep. Theoretical and numerical analyses clearly show there is a causal relationship between permafrost warming and the decrease of the DZAA. Latent heat effects are buffering the increase of permafrost temperature and result in narrow thermal trumpet curves and shallow DZAAs. Based on observations and numerical analyses, this research suggests that most of the permafrost on the Qinghai-Tibet Plateau is undergoing internal thaw and the latent heat effects have important implications on the permafrost thermal regime. The temperature-dependent adjustments in permafrost will promote both the downward and upward degradation of permafrost as a result of climate warming.
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Aggradation
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Chronosequence
Soil carbon
Carbon fibers
Carbon sink
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