Article

Thermal regime through a sorted circle and stone-banked lobe, Drakensberg, southern Africa

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Abstract

Ground and air temperature data are presented for the high Drakensberg, southern Africa, representing the period Jan-Dec, 2000. The ground temperatures were measured at depths of 10, 20, 30, 40 and 50 cm through a north-facing sorted circle and a nearby south-facing stone-banked lobe tread, thus providing data for comparative assessments. The primary focus is on determining contemporary cryogenic thermal parameters to 50 cm depth and ascertaining associated environmental implications. Whilst the mean annual ground temperatures are similar for the two sites, considerable variations of summer and winter ground temperatures occur on opposing slopes at ∼ 3,400 m a.s.l. The amplitude of ground freeze varied from -6.5 °C at 10 cm to -0.1 °C at 50 cm through the sorted circle, whilst temperatures dropped to -7.8 °C at 20 cm and - 2.8 °C at 50 cm in the stone-banked lobe. The rate of freeze penetration and freeze duration is considerably more pronounced through the stone-banked lobe, where a maximum freeze duration of 98 days was recorded at 40 cm. The differences in the thermal regimes at the two sites are controlled by site-specific differences in sedimentary properties and ground moisture, microrelief (microclimate) and snowpack characteristics. Given that seasonal freeze may continue into late spring at the stone-banked lobe site, contemporary gelifluction may be possible during periods of snowmelt or spring rainfall. In contrast, the sub-surface of the north-facing sorted circle site is already thawed by the time sufficient moisture is added to the soil profile, thus restricting the potential for contemporary frost heave at this site during years of mild to average climatic conditions. It is suggested that winter ground temperatures are directly affected by the longevity and depth of snow cover in the Drakensberg and thus snow is an important factor determining the distribution of periglacial landforms in the region. © 2004 Gebrüder Borntraeger, D-14129 Berlin · D-70176 Stuttgart.

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... The Mafadi-Njesuthi summit region ( Figure 1) hosts a variety of sorted patterned ground forms developed in weathered debris of the underlying basalts. These features range from active miniature stone stripes (diameter 13-16 cm; maximum sorting depth 7 cm) and 'fine earth flags' (Grab 1996) to very large sorted circles ( Figure 3) with centers <1.34 m in diameter, borders <1.23 m in width, and which are sorted to >0.85 m depth (Grab 2002b(Grab , 2004. The Mafadi interfluve (at ∼3400 m asl) is the only known site in southern Africa where such relatively well-developed and preserved large sorted patterned ground phenomena occur, and hence these are of considerable geoheritage and scientific value. ...
... These have treads averaging 8.8 m in length and frontal stone risers 4.4 m in width and 0.5 m in height (Grab 2000). Based on past (from the single year 2000) continuous records of ground temperature measurements, regolith may freeze to over 0.5 m depth in sorted circle centers and turf-/stonebanked lobe treads, and remain frozen for two to three months duration, respectively (Grab 2004). Gentler north-facing slopes host broad, poorly drained basins with deep sediment infills that are rich in diatomite and peat (Fitchett et al. 2017). ...
... This is also consistent with visual evidence from the discoloration of clasts by biological and chemical weathering in different parts of the sorted circles. In addition, ground thermal data from Mafadi-Njesuthi during the year 2000 confirm recent seasonal freeze to beyond 50 cm depth, and that the ground can remain frozen for >100 days at 20 cm depth (Grab 2004). In sum, these observations strongly support at least episodic contemporary cryogenic activity on the Mafadi interfluve and adjacent south-facing upper convex slope (at ∼3400 m asl). ...
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Here, we aim to establish possible morphogenetic associations between periglacial phenomena (sorted circles and turf-/stone-banked lobes) by examining (a) rock surface weathering rinds by thin section microscopy and SEM-EDX analysis, and (b) alpine shrub growth patterns of Helichrysum trilineatum supported by AMS 14 C age determination. The study area is the Mafadi-Njesuthi summit zone, one of the highest alpine regions of southern Africa. Rock weathering results indicate that clasts in large sorted circle centres experience high chemical weathering rates on both exposed (upper) and non-exposed (lower) surfaces, whereas clasts in lobes and very large apparently 'relict' sorted circles have been inactive for a prolonged period of time, based on strongly contrasting weathering rates on both exposed and non-exposed clast surfaces. Large sorted circles originating from a previous (possibly Last Glacial Maximum or late-glacial) colder period are still marginally or episodically active, thus restricting plant succession on these landforms. Even where very large sorted circles and turf-/stone-banked lobes are inactive, their legacy continues, as expressed in today's surface morphology, and still influence ground abiotic conditions through the generation of microrelief and microclimate which in turn have impacts on ecosystem patterns including the distribution of H. trilineatum. Such shrub growth patterns are influenced by microscale site morphology, associated abiotic controls, and ongoing seasonal cryogenic activity. Both active and inactive periglacial landforms on highest summits of the Drakensberg represent microscale environments with distinct fine-scale abiotic and geomorphic settings, which in turn are manifested through differences in plant ecology and rock surface weathering, respectively.
... There are considerable diurnal air temperature variations, with surficial soil freezing typically occurring from late March to mid-October (Grab, 2000). In the Mafadi summit region, contemporary frozen ground may persist from mid-May to early October and to depths exceeding 50 cm (Grab, 1996(Grab, , 2004. ...
... yr BP, this sequence does not span the entire Holocene but rather commences at a period coincident with the final disappearance of the major Northern Hemisphere ice sheets (Mayewski et al., 2004). The very slow sedimentation rate of 100 yr cm −1 is attributed to the particularly low temperatures throughout the late Quaternary to present (Grab, 1996(Grab, , 2004. Climate would have been an important control at high altitude, given the peat inclusion in the sediment record, which required relatively mild temperatures to facilitate plant growth and sufficient humidity for peat to develop under water-logged conditions (Meadows, 1988;Van Zinderen Bakker, 1955). ...
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The eastern Lesotho Highlands experience climate patterns distinct from those of surrounding lower altitude regions, representing a niche environment with a unique biodiversity, leading to well-adapted but restricted vegetation. This study explores changes in the Holocene composition of diatoms and pollen at southern Africa’s highest altitude wetland (Mafadi: 3390 m a.s.l.). The palaeoenvironmental record for Mafadi Wetland indicates fluctuations between cold, wet conditions, prevalent between ~8140 and 7580 cal. yr BP and between ~5500 and 1100 cal. yr BP, and warmer, drier periods between ~7520 and 6680 cal. yr BP and between ~6160 and 5700 cal. yr BP. Marked climatic variability is noted from ~1100 cal. yr BP with colder conditions at ~150 kyr BP. Notably, the first of these cold periods occurs soon after the Northern Hemisphere 8.2 kyr event, while a second period of notably cold conditions occurs around 1100 cal. yr BP. Variability exists between the moisture reconstructions presented in this study and those from adjacent lower altitude sites, which is hypothesised to reflect variations in the strength and extent of the Westerlies throughout the Holocene.
... This study hence uses the 2001 data to scrutinize fine-scale trends in temperature and lapse rates. According to Grab (2004), the year 2001 had a mean annual air temperature of 0.3 ЊC below the 20yr mean, and winter temperatures (May-August) coincided exactly with the 20-yr mean. However, the year had an exceptionally cold January (ca. ...
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The objective of this study is to determine fine-scale spatial and temporal trends in screen temperature, lapse rates, and inversions associated with the Great African Escarpment wall. Particular attention is focused on the characteristics of near-surface-temperature lapse rates, their variability associated with particular synoptic conditions, and environmental implications such as for modeling climate projections in topographic- and climatic-specific high mountain regions. Hourly temperature data were logged on northeast- and southeast-facing slopes over 12 months in 2001, in the high Drakensberg Escarpment of southern Africa, at a range of elevations including: 2650, 2900, and 3200 m a.s.l. High spatial and temporal variations in lapse rates are recorded and illustrated. A relatively low mean annual lapse rate of -0.42 degrees C 100 m between 2650 and 2900 m a.s.l. doubles to 0.84 degrees C 100 m(-1) between 2900 and 3200 m a.s.l. The frequent westerlies and tropical temperate troughs provide for high lapse rates (0.91 degrees C 100 m(-1)), which are likely responsible for the relatively high annual average lapse rate of -0.63 degrees C 100m(-1). Quantifying the fine-scale temperature trends has important implications for better understanding site-specific paleoenvironmental signatures and for projecting future climate scenarios and associated bio- and geosystem responses.
... At this elevation, approximately 200–225 annual frost days occur (Schulze, 1997). Ground freezing to N 50 cm depth and over a 3-month period has been locally recorded (Grab, 2004). As such the high Drakensberg has been classified as marginally periglacial, and a range of active and inactive periglacial landforms has been identified including earth hummocks (thufur), sorted patterned ground and stone-and turf-banked lobes (see Grab et al., 2012 for a review). ...
Article
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A 12 month study on the thermal regime for a thufa apex and its adjoining depression was undertaken in the Mashai Valley (∽2950 m ASL), Lesotho Highlands, during 1993–94. Pronounced temperature differentials occurred during the winter months when apexes were frozen for several weeks and depressions remained predominantly unfrozen. The data show that existing micro-topography is an important factor controlling the maintenance or further development of thufur in marginal periglacial environments. © 1997 John Wiley & Sons, Ltd. Une étude du régime thermique du sommet d'un thufa et de la dépression qui le borde a été poursuivie dans la vallée de Mashai (2950 m au-dessus du niveau de la mer) dans les hautes terres du Lesotho pendant 12 mois en 1993–94. Des différences de température importantes existent pendant les mois d'hiver quand les sommets des thufurs sont gelés pendant plusieurs semaines et que les dépressions restent non gelées. Les données montrent que la microtopographie existante est un facteur important contrôlant la conservation et le développement des thufurs dans les environnements périglaciaires marginaux.
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Small needle-ice mounds are described from the High Drakensberg. They develop infrequently during periods of several days of uninterrupted cold conditions. Ice growth incorporates sediment and stones. An accumulated segregated ice length of >41 cm was measured, with clasts heaved ∼14 cm. During subsequent thaw phases, the ice ablates from the top downwards, forming sediment veneers and cappings. These mounds contribute towards turf exfoliation, thereby exposing sediment to deflation. Copyright © 2002 John Wiley & Sons, Ltd.
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Single-year air and ground temperature records from 1995 to 1996 in Karkevagge (approximately 68°26'N, 18°18'E), a glaciated trough in northern Sweden, are compared using reduced major axis analysis (RMAA) to demonstrate the limitations in using air temperatures as indicators of ground temperatures in an Arctic-alpine environment. While air temperatures predict some 10 and 50 cm level ground temperatures with as much as 95% explanatory power at daily and monthly scales, other ground sites are predicted with as little as ~20% explanatory power. Such variability, found over sites differing in elevation, aspect and vegetation cover, is assigned to the influence of snow cover, which was not measured. As would be expected, the smoothing of data from daily to monthly scales generally improves the correlation between air and ground temperatures; this phenomenon is also due, in part, to lag effects associated with heat conductance. The 1995-96 record appears to be drawn from a period that is probably somewhat warmer than the average over the last 47 years; a conclusion drawn by examining the 1951-97 record from two nearby weather stations.
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Turf-banked solifluction lobes are common landforms of the Canadian Rocky Mountains. Observations made 1980–3 at 45 sites reveal a wide range in the rate of solifluction. The most rapid rates occur on moderately sloping, northeast-facing sites, in close proximity to the local treeline. However, statistical comparison indicates these variations are insignificant and that the mean rate of solifluction is similar throughout the area. Solifluction has an average rate of surface movement of 0.61 ± 0.21 cm/year and is responsible for annually transporting 8.76 ± 3.57 cm3/cm of sediment in the southern Canadian Rockies. La topographie des Rocheuses canadiennes prend souvent la forme de lobes de solifluxion gazonner. Des observations faites entre 1980 et 1983 dans 45 sites de ces lobes révèlent que le processus de solifluxion varie grandement d'un endroit à l'autre dans cette région. Les taux les plus rapides se retrouvent dans des sites modérément inclinés et orientés vers le nord-ouest, situés à proximité de la lisière des bois. Pourtant, des comparaisons statistiques indiquent que ces variations sont d'importance minime et que le taux moyen de solifluxion recte semblable à travers toute la région. La solifluxion se faut à un taux moyen de mouvement à la surface de 0,61 cm ± 0,21 cm par année et on peut lui imputer le fait de transporter annuellement de 8,76 cm3/ cm ± 3,57 cm3/cm de sediments dans la partie sud des Rocheuses canadiennes.
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Near-surface ground thermal conditions are a delayed response to changing air temperature and may result in needle ice development when air temperatures are above 0 °C and ground temperatures below 0 °C. It is difficult, therefore, to predict needle ice development based only on climatological data. Observations indicate that temporally restricted growth phases produce shorter needle ice lengths than longer growth phases. Copyright © 2001 John Wiley & Sons, Ltd.RÉSUMÉLes conditions thermiques proches de la surface du sol évoluent avec retard après un changement de la température de l'air, aussi des pipkrakes peuvent se développer quand la température de l'air est supérieure à 0 °C et que la température du sol est inférieure à 0 °C. Il est difficile, en conséquence, de prédire le développement des pipkrakes d'après les seules données climatologiques. Des observations indiquent que de courtes phases de croissance produisent des pipkrakes de longueur plus réduite que des phases de croissance de plus grande durée. Copyright © 2001 John Wiley & Sons, Ltd.
Article
Past evaluation of high altitude slope development in Lesotho, southern Africa, is largely based on hypothetical or macro-scale geomorphic approaches. Consequently, the information pertaining to high altitude southern African Quaternary slope environments has remained rather rudimentary. The present study describes the morphology and discusses the likely palaeogeomorphic processes of blockstreams and debris deposits on the Popple Peak and Njesuthi-Mafadi south-facing-slopes in the Drakensberg. The geomorphic evidence provides much needed information to help improve the understanding of south-facing slope processes during past colder periods. A model for high altitude Drakensberg south facing slopes is presented and used to challenge and expand on recent models and ideas on southern African valley asymmetry. It is found that solifluction and debris flows/avalanches were operative on south-facing slopes during past cold periods and thereby contributed to past slope development at some high altitude sites in Lesotho. However, the geomorphological observations do not support the valley asymmetry hypothesis and it is suggested that greater caution be exercised in valley-form interpretations, particularly where geomorphological ground-truthing has been absent.
Article
Field data on the rates of solifluction and associated parameters are compiled from the literature, in an attempt to evaluate factors controlling the spatial variability in solifluction processes and landforms, with special attention on the climate–solifluction relationship. The analyzed data originate from 46 sites over a wide range of periglacial environments, from Antarctic nunataks to tropical high mountains. Solifluction, broadly defined as slow mass wasting resulting from freeze–thaw action in fine-textured soils, involves several components: needle ice creep and diurnal frost creep originating from diurnal freeze–thaw action; annual frost creep, gelifluction and plug-like flow originating from annual freeze–thaw action; and retrograde movement caused by soil cohesion. The depth and thickness of ice lenses and freeze–thaw frequency are the major controls on the spatial variation in solifluction processes. Near the warm margin of the solifluction-affected environment, diurnal freeze–thaw action induces shallow but relatively rapid movement of a superficial layer 5–10 cm thick on average, often creating the thin stone-banked lobes typically seen on tropical high mountains. In addition to diurnal movement, annual frost creep and gelifluction may occur on slopes with soil climates of seasonal frost to warm permafrost, dislocating a soil layer shallower than 60 cm at a rate of centimeters per year and eventually producing medium-size solifluction lobes. In High-Arctic cold permafrost regions, two-sided freezing can induce plug-like flow of a soil mass 60 cm or thicker. The correlation between process and landform suggests that the riser height of lobes is indicative of the maximum depth of movement and prevailing freeze–thaw type. Climate change may result in new different ground freezing conditions, thereby influencing the surface velocity and maximum depth of soil movement. Soil moisture and topography also control solifluction. High moisture availability in the seasonal freezing period enhances diurnal freeze–thaw action and subsequent seasonal frost heaving. The latter contributes to raising the moisture content of the thawed layer and promotes gelifluction during the thawing period. The slope angle defines the upper limit of the surface velocity of solifluction. A diagram correlating the potential frost creep with the actual surface velocity permits an inter-site comparison of the relative magnitude of solifluction components. Physically based modelling of periglacial slope evolution requires synthetic and more detailed field monitoring and laboratory simulations of solifluction processes.
Article
A method of accurately estimating and mapping patterns of incoming solar radiation fluxes on sloping terrain, using data from standard meteorological instruments, is described. The results of the radiation maps produced are then related to a specific example of vegetation successional changes in the Cathedral Peak area of the Natal Drakensberg mountain range, indicating that these changes occur most readily on cooler aspects. For the same region, the differences in the major plant communities at two other sites, attributable largely to variations in incoming solar radiation loads, are discussed.Une mthode d'estimation fiable et d'chantillons cartographiques d'entre de flux de radiation solaire sur des terrains en pente, utilisant des donnes partir d'instruments mtorologiques standard, est dcrite. Les rsultats des cartes de radiation obtenus sont ensuite relis un exemple spcifique de changements de vgtation successifs dans la rgion de Cathedral Peak, de la Chane de montagnes Natal Drakensberg, indiquant que ces changements apparaissent beaucoup plus sur des pentes aux directions plus fraches. Pour la mme rgion, les diffrences dans les groupements vgtaux principaux deux autres sites, que l'on peut largement attribuer aux variations des charges de radiation solaire, sont discutes.
Article
Soil temperature has been monitored continuously at hourly intervals to a depth of 1 m since 1993 at a site near Barrow, AK. Time series of soil moisture from the active layer and upper permafrost have been collected since 1996 at the same location. These records are supplemented by meteorological data from NOAA's Barrow Climate Monitoring and Diagnostics Laboratory facility and detailed description of depth-dependent soil properties at the site. Soil sensors are situated within a low-centered ice-wedge polygon characterized by meadow tundra vegetation. A thin (7 cm) organic layer grades into reworked marine silts at depth. The soil temperature and moisture are used in a site-specific, multiyear thermal analysis of the atmosphere/snow/active-layer/permafrost system. Fusion retards soil freezing during early winter as soil water is converted to ice. Soil heat transfer is dominated by conduction in winter. Infiltration of snow meltwater in spring produces a series of thermal pulses in the active layer, causing rapid warming of the upper several decimeters by about 1°C. The thermal impact is limited because the soil tends to be nearly saturated at the time of freezeback. Volumetric soil water content in summer is generally around 35–40% at a depth of 15 cm, while the base of the thawed zone remains saturated near 50%. The near-surface soil exhibits drying from evapotranspiration and rewetting from precipitation events. During the period of thaw, the apparent thermal diffusivity is around 2–3×10−7 m2 s−1 and increases with depth to reflect the greater soil water content. The maximum thaw depth at the site is typically around 35 cm. However, end-of-season thaw depth has been monitored near Barrow since 1994 and has increased between 1994 and 1998. This warming trend is also reflected in the thawing degree days calculated for the thawed soil volume. A strong correlation exists between maximum annual thaw depth and annual thawing degree days at this site over the period of record.
Article
Relict large sorted periglacial circles are described from the Drakensberg plateau of southern Africa. Morphological and sedimentological data for three populations of patterns occurring on high summit (>3400 m a.s.l.) interfluves are presented. The patterns range between 85 and 904 cm in diameter, with centres encircled by primary and, in some cases, secondary borders. Patterns are sorted to maximum depths of 85 cm or more, with borders occupying trough-like depressions to a depth of 26 cm. Although it is found that contemporary freeze to 40 cm depth still permits some cryogenic activity within smaller patterns, it is argued that the larger patterns are degrading owing to limited moisture and freeze duration and intensity. The paper provides a literature summary of contemporary environmental parameters from 10 northern hemisphere localities where active/relict large periglacial patterns occur. Findings suggest that patterns over 80 cm in diameter develop where the MAAT is −1.6°C or less. It is demonstrated that the Drakensberg palaeo-MAAT reached such low temperatures during the LGM. It is thus argued that the patterns developed during the LGM and that smaller varieties became reactivated during Holocene cold phases. The relict sorted circles provide confirmation for Late Quaternary permafrost above 3400 m a.s.l. in southern Africa. The results further suggest pronounced palaeo-climatic/environmental variation over short spacial scales over southern Africa's high plateau.
Article
A working hypothesis for the development and maintenance of sorted circles, which are finegrained soil domains delineated by borders of coarser material, involves clast motion relative to soil and intermittent circulatory motion of soil in the active layer. The hypothesis is compatible with the observed size, regularity, sorting, microtopography, surface soil motion, vegetation cover, and subsurface organic carbon content of soil patterns studied in Spitsbergen. Theoretical considerations of buoyancy-driven convection suggest that intermittent convection of thawed soils is plausible, and that the requisite bulk density decrease with depth arises naturally from moisture gradients that develop during the thaw consolidation of ice-rich soils. These considerations also help in understanding the dependence of sorted circles on soil characteristics and on moisture. Moreover, they point to the possible use of sorted patterns as paleoclimatic indicators based on the expected relation between pattern size and active layer depth.