Thermal Contraction Cracks Research Articles

Present-Day Periglacial Environments in Central Spitsbergen, Svalbard

This report overviews observations on periglacial geomorphology in central Spitsbergen, which have been undertaken by the Japanese geomorphological groups. Focus is given to permafrost-related processes, including solifluction, thermal contraction cracking and pingo growth. Annual freeze-thaw action dominates the ground, resulting in extensive occurrence of solifluction and shallow landslides on soil slopes. Solifluction shows low surface velocity but large volumetric transport, which respectively reflects infrequent diurnal frost creep and deep movement. The latter partly originates from plug-like flow in the basal active layer, where permafrost temperature is low and muddy sediment is thick. Non-sorted polygons with a wide range of diameters develop on lowlands. In colder inland terrains, large polygons (>7m) have ice wedges, whereas smaller polygons have only soil wedges or cracks confined to the active layer. Significant ice-wedge cracking occurs during rapid and intensive cooling in midwinter. In warmer coastal terrains, ice wedges are not common even below large polygons, because higher winter temperature can produce only shallow cracks. Open-system pingos occur in valley bottoms and near shores. Some pingos are still growing under a low artesian pressure fed by constant supply of sub-permafrost water. Following lateral river erosion, a new frost mound emerged at a side of a pingo, reaching 3m high during three years. The observations demonstrate that central Spitsbergen is situated in a High Arctic but relatively warm permafrost environment. The transitional condition between cold and warm permafrost allows diverse periglacial features to coexist within a small area. Even minor climatic change can switch the two thermal regimes, affecting significantly the type and magnitude of periglacial processes.

Thermal‐contraction‐crack networks as evidence for late‐Pleistocene permafrost in Inner Mongolia, China

AbstractNumerous wedges on the Ordos Plateau show typical characteristics of periglacial sand wedges that enable them to be distinguished from desiccation cracks in clayey illuviation soil horizons. The sand wedges are organized in two generations of polygonal networks. The older generation is a large‐scale network with a diameter of 8 to 9 m and wedge depths up to more than 2 m. The younger generation has a mean diameter of 3 to 4 m and is formed within the large networks. It consists of shallow (0.6 to 1 m deep) but relatively wide wedges. In contrast to the typical sand wedges that form within continuous permafrost, the shallow wedges formed probably as ‘ground wedges’ by seasonal freezing. Ice wedges and cryoturbations developed only rarely, and exclusively in the most humid areas. Thermal‐contraction cracking occurred mainly between 26 and 20 ka BP, indicating mean annual temperatures at least 13°C lower than present. Before and after that period mean annual temperatures were at least 7 to 8.5° lower than today. The presence of these periglacial phenomena show that the southern limit of continuous permafrost in Inner Mongolia occurred south of 38°N during the Last Glacial Maximum. Copyright © 2004 John Wiley & Sons, Ltd.

Neoproterozoic sand wedges: crack formation in frozen soils under diurnal forcing during a snowball Earth

Thermal contraction cracking of permafrost produced sand-wedge polygons at sea level on the paleo-equator during late Neoproterozoic glacial episodes. These sand wedges have been used as evidence for high (≥54°) paleo-obliquity of the Earth’s ecliptic, because cracks that form wedges are hypothesized to require deep seasonal cooling so the depth of the stressed layer in the ground reaches ≥1 m, similar to the measured depths of cracks that form wedges. To test the counter hypothesis that equatorial cracks opened under a climate characterized by a strong diurnal cycle and low mean annual temperature (snowball Earth conditions), we examine crack formation in frozen ground subject to periodic temperature variations. We derive analytical expressions relating the Newtonian viscosity to the potential crack depth, concluding that cracks will form only in frozen soils with viscosities greater than ∼10 14 Pa s. We also show numerical calculations of crack growth in frozen soils with stress- and temperature-dependent rheologies and find that fractures may propagate to depths 3–25 times the depth of the thermally stressed layer in equatorial permafrost during a snowball Earth because the mean annual temperature is low enough to keep the ground cold and brittle to relatively great depths.

Formation of patterned ground and sublimation till over Miocene glacier ice in Beacon Valley, southern Victoria Land, Antarctica

A thin glacial diamicton, informally termed Granite drift, occupies the floor of central Beacon Valley in southern Victoria Land, Antarctica. This drift is 40 Ar/ 39 Ar analyses of presumed in situ ash-fall deposits that occur within Granite drift. At odds with the great age of this ice are high-centered polygons that cut Granite drift. If polygon development has reworked and retransported ash-fall deposits, then they are untenable as chronostratigraphic markers and cannot be used to place a minimum age on the underlying glacier ice. Our results show that the surface of Granite drift is stable at polygon centers and that enclosed ash-fall deposits can be used to define the age of underlying glacier ice. In our model for patterned-ground development, active regions lie only above polygon troughs, where enhanced sublimation of underlying ice outlines high-centered polygons. The rate of sublimation is influenced by the development of porous gravel-and-cobble lag deposits that form above thermal-contraction cracks in the underlying ice. A negative feedback associated with the development of secondary-ice lenses at the base of polygon troughs prevents runaway ice loss. Secondary-ice lenses contrast markedly with glacial ice by lying on a δD versus δ 18 O slope of 5 rather than a precipitation slope of 8 and by possessing a strongly negative deuterium excess. The latter indicates that secondary-ice lenses likely formed by melting, downward percolation, and subsequent refreezing of snow trapped preferentially in deep polygon troughs. The internal stratigraphy of Granite drift is related to the formation of surface polygons and surrounding troughs. The drift is composed of two facies: A nonweathered, matrix-supported diamicton that contains >25% striated clasts in the >16 mm fraction and a weathered, clast-supported diamicton with varnished and wind-faceted gravels and cobbles. The weathered facies is a coarse-grained lag of Granite drift that occurs at the base of polygon troughs and in lenses within the nonweathered facies. The concentration of cosmogenic 3 He in dolerite cobbles from two profiles through the nonweathered drift facies exhibits steadily decreasing values and shows the drift to have formed by sublimation of underlying ice. These profile patterns and the 3 He surface-exposure ages of 1.18 ± 0.08 Ma and 0.18 ± 0.01 Ma atop these profiles indicate that churning of clasts by cryoturbation has not occurred at these sites in at least the past 10 5 and 10 6 yr. Although Granite drift is stable at polygon centers, low-frequency slump events occur at the margin of active polygons. Slumping, together with weathering of surface clasts, creates the large range of cosmogenic-nuclide surface-exposure ages observed for Granite drift. Maximum rates of sublimation near active thermal-contraction cracks, calculated by using the two 3 He depth profiles, range from 5 m/m.y. to 90 m/m.y. Sublimation rates are likely highest immediately following major slump events and decrease thereafter to values well below our maximum estimates. Nevertheless, these rates are orders of magnitude lower than those computed on theoretical grounds. During eruptions of the nearby McMurdo Group volcanic centers, ash-fall debris collects at the surface of Granite drift, either in open thermal-contraction cracks or in deep troughs that lie above contraction cracks; these deposits subsequently lower passively as the underlying glacier ice sublimes. The fact that some regions of Granite drift have escaped modification by patterned ground for at least 8.1 Ma indicates long-term geomorphic stability of individual polygons. Once established, polygon toughs likely persist for as long as 10 5 –10 6 yr. Our model of patterned-ground formation, which applies to the hyperarid, cold-desert, polar climate of Antarctica, may also apply to similar-sized polygons on Mars that occur over buried ice in Utopia Planitia.

The first 20 years (1978-1979 to 1998–1999) of ice-wedge growth at the Illisarvik experimental drained lake site, western Arctic coast, Canada

In August 1978, a large tundra lake was drained to study the aggradation of permafrost into newly exposed lake-bottom sediments. Ice-wedge growth, which started in the first winter following drainage, had ceased in most of the lake bottom within about twelve years. The gradual cessation of thermal contraction cracking can be attributed to rapid vegetation growth, snow entrapment, an increase in winter ground temperatures, and a decrease in the linear coefficient of thermal contraction associated with freeze–thaw consolidation of the initially saturated lake-bottom sediments. The tilt and separation of markers in the active layer revealed gradual convergence towards the troughs even after ice-wedge growth had ceased. For the first few years the ice-wedge growth rate was up to 3 cm/a as determined by excavation, drilling, separation of the bottoms of benchmarks installed into permafrost, and divergence of free-floating inductance coils placed on the sides of ice wedges well below the bottom of the active layer. The vertical extent of most ice wedges was probably about 2 m, as deduced from the depths of ice-wedge cracks and the geometries of the wedge tops. Many thermal contraction cracks propagated upward to the ground surface from the tops of the ice wedges rather than downward from the ground surface. Small, upward facing, horizontal steps and vertical slickensided surfaces in permafrost on both sides of an excavated ice wedge near its top indicated that the adjacent permafrost had moved upward, relative to the wedge, from thermal expansion during the warming period.

Fracture toughness of frozen base and subbase soils in pavement

Temperature distribution in the pavement structure, moisture distribution in granular soils, modulus of the asphalt concrete, and fracture toughness of material in the pavement structure strongly influence the propagation and spacing of thermal contraction cracks. Fracture toughness was determined for frozen sand (subbase layer) and frozen crushed stone (base layer) by adapting established fracture mechanics test procedures recommended in American Society for Testing and Materials standard test method E399-83 for metals. It was established that fracture toughness increases with decreasing temperature and increasing volumetric ice content. For a temperature of –5°C, the fracture toughness of frozen crushed stone increased almost linearly from 0.05 to 0.40 MPa·m0.5 when the volumetric ice content increased from 6 to 14%. For frozen sand, the fracture toughness KIC in a wedge-opening mode increased from 0.04 to 0.70 MPa·m0.5 when the volumetric ice content increased from 8 to 28%. It was also established that the fracture toughness of frozen soil decreases with decreasing soil average grain size according to a logarithmic law.Key words: fracture, toughness, experimental, frozen, granular soil, pavement.

Non-conductive heat transfer associated with frozen soils

The assertion that pure conductive heat transfer always dominates in cold climates is at odds with decades of research in soil physics which clearly demonstrate that non-conductive heat transfer by water and water vapor are significant, and frequently are for specific periods the dominant modes of heat transfer near the ground surface. The thermal regime at the surface represents the effective boundary condition for deeper thermal regimes. Also, surface soils are going to respond more quickly to any climatic fluctuations; this is important to us because most facets of our lives are tied to earth's surface. To accurately determine the surface thermal regime (for example, the detection of climate change), it is important to consider all potential forms of heat transfer. Gradients that have the potential to alter the thermal regime besides temperature include pore water pressure, gravitational, density, vapor pressure and chemical. The importance of several non-conductive heat transport mechanisms near the ground surface is examined. Infiltration into seasonally frozen soils and freezing (release of latent heat) of water is one mechanism for the acceleration of warming in surficial soils in the spring. Free convection due to buoyancy-induced motion of fluids does not appear to be an important heat-transfer mechanism; estimates of the Rayleigh number (the ratio of buoyancy to viscous forces) are generally around 2, which is too low for effective heat transfer. The Peclet number (ratio of convective to conductive heat transfer) is on the order of 0.25 for snowmelt infiltration and up to 2.5 for rainfall infiltration for porous organic soils. In mineral soils, both vertical and horizontal advection of heat can be neglected (Peclet number is approximately 0.001) except for snowmelt infiltration into open thermal contraction cracks. The migration of water in response to temperature or chemical gradients from unfrozen soil depths to the freezing front, and the redistribution of moisture within the frozen soil from warmer depths to colder depths, can also result in heat transfer although this effect has not been quantified here. Many of these processes are seasonal and effective only during periods of phase change when the driving gradient near the ground surface is relatively large.

Frozen ground phenomena in the vicinity of terra nova bay, northern victoria land, antarctica: a preliminary report

Frozen ground phenomena in the Northern Foothills, Northern Victoria Land, Antarctica, include large–scale polygons, 15–20 m in diameter, and small frost mounds, 1–5 m high. The polygons are most widespread on terrain formed upon Younger Drift and are usually surrounded by interpolygon furrows or troughs, 10–30 cm deep and 10–100 cm wide. The troughs contain shallow wedges of sandy gravel (sand wedges) near the surface but excavations into underlying permafrost indicate that small ice wedges or ice veins are locally present. Field and anecdotal evidence suggest that thermal contraction cracking is active under today's climate. Frost mounds occur in association with a number of perennially frozen lakes in the region. In most cases they appear related to frost and icing blister activity caused by the episodic injection of free water from below. The debris–covered nature of the centre of Enigma Lake is best explained in terms of basal ice accretion beneath the lake–ice cover.

Sand veins and wedges in cold aeolian environments

Sedimentary structures formed by the progressive primary infilling of thermal contraction cracks with sand are termed primary sand veins and sand wedges. In addition to simple vein- or wedge-shapes irregularities can be caused by sand veins branching from their sides and toes. Primary sand wedges form widely in sandy polar deserts, locally in sandy areas of tundra and probably in seasonally frozen ground. There are no unique criteria for distinguishing primary sand veins and wedges from sand veins and wedges of other origins. Identification of the former depends on the occurrence of distinctive features not always present and on evaluation of their lithofacies and palaeoenvironmental contexts. Care and caution are advocated in the use of ancient/relict primary sand veins and wedges as quantitative palaeoenvironmental indicators because modern active wedge distribution is poorly known and hence inferred thermal climatic threshold values are questionable. It is suggested that ancient/relict primary sand wedges exceeding 2 m in depth and with well-developed vertical lamination probably indicate the former occurrence of continuous permafrost, whereas sand veins and narrow sand wedges (frost cracks) are potentially ambiguous as they may form not only in the active layer above and within continuous permafrost but also in seasonally frozen ground in non-permafrost areas.

Late Devensian and Holocene depositional environments associated with the coversand around Caistor, north Lincolnshire, UK

Sand deposits described at three sites near Caistor, north Lincolnshire (UK), provide a record of Late Devensian (Late Weichselian) to Holocene palaeoenvironments at the western margin of the European sand belt. Thermoluminescence (TL) and radiocarbon analyses provide for the first time a chronological framework for the demise of proglacial Lake Humber and the onset of coversand deposition. The reconstructed palaeoenvironmental history suggests that proglacial Lake Humber had receded from its initial high‐level stand before c. 18 ka, exposing the lake floor to periglacial conditions marked by the development of thermal contraction cracks. In the period between c. 18 and 14 ka, sand‐depositional processes changed from dominantly fluvial to aeolian. The fluvial activity was possibly a consequence of ameliorating winter climates between c. 17 and 16 ka. The aeolian coversand deposition in this period has not been previously recognized in Britain and correlates with the Older Coversand II and Younger Coversand I deposits elsewhere in the European sand belt. Peat accumulation followed during the Windermere (Bølling/Allerød) Interstadial and early part of the Loch Lomond Stadial (Younger Dryas) before regionally extensive coversand deposition took place in the later part of the Loch Lomond Stadial. This coversand correlates with the widespread Younger Coversand II deposits found both within the UK and across the European sand belt. The Holocene has been characterized by widespread stability with the development of soils on the coversand punctuated with periods of localized reworking through to the present day.

Structure and Tectonics of Intermediate-Spread Oceanic Crust Drilled at DSDP/ODP Holes 504B and 896A, Costa Rica Rift

Abstract Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) Hole 504B, situated on the southern flank of the intermediate-spreading Costa Rica Rift in the eastern equatorial Pacific, penetrates 2.1 km into 5.9 Ma old oceanic crust and provides an in situ reference section for the physical and chemical structure of the upper oceanic lithosphere. Hole 896A, located on a basement high nearly 1 km south of Hole 504B, penetrates 290 m into the upper volcanic sequence, and together with Hole 504B provides an opportunity to examine variations in basement lithostratigraphy and structure. The sea-floor bathymetry in the vicinity of Holes 504B and 896A is defined by east-west trending asymmetric elongated ridges and troughs with wavelengths of 5–6 km and relief of 100–150 m. Hole 504B penetrates two major fault zones at about 800 metres below sea floor (mbsf) in the lower volcanic rocks and at 2111 mbsf in the lower dykes. The fault zone at 800 mbsf coincides with the lithological boundary between the volcanic sequence (layer 2B) above and the transition zone (pillow and massive lava flows, dykes) below, separates contrasting domains of magnetic properties, and marks a south-dipping (outward-facing) normal fault. The fault zone at 2111 mbsf in the bottom of the hole represents a dip-slip fault defined by closely spaced east-northeast striking microfractures with steep dips and steeply plunging lineations. The dyke margins that have been reoriented palaeomagnetically have strikes subparallel to the ridge axis of the Costa Rica Rift and steep dips towards both the north (inward-dipping) and the south (outward-dipping). The main deformation of the dyke complex has been fracturing and veining as suggested by the analyses of core samples and geophysical downhole measurements. Chlorite and/or actinolite veins represent extension fractures and form two orthogonal vein sets. E-W to ESE-WNW striking, steeply dipping extensional veins are commonly parallel to the dyke margins and to the orientation of the Costa Rica Rift axis; N-S to NNE-SSW striking veins are dyke-orthogonal, probably representing thermal contraction cracks. The apparent increase in grain size of the diabasic dyke rocks around 2 kmbsf coincides with an increase in abundance of actinolite at the expense of clinopyroxene and plagioclase, and a sharp decrease in the occurrence of chlorite in the core. These changes in the core mineralogy are accompanied by a steady increase in compressional wave velocity, which reaches a value of 6.8 km s −1 within the sheeted dyke complex nearly 1.4–1.6 km into the basement. The boundary between seismic layers 2 and 3 thus corresponds to changes in physical properties of the rocks, rather than a lithological boundary, and occurs over a finite depth interval within the sheeted dyke complex. The upper oceanic crustal rocks recovered from Hole 896A include massive and pillow lava flows with interlayered breccias. An intensely fractured zone that coincides with sharp changes in acoustic velocity, electrical resistivity and magnetic properties in the volcanic strata occurs at 360 mbsf and marks a fault zone. The thickness of the volcanic strata, which have the same lithostratigraphy and magnetic properties, increases from Hole 896A in the south to Hole 504B in the north, suggesting off-axis ponding of lava(s) in the vicinity of Hole 504B. The sea-floor bathymetry defined by linear hills and intervening troughs in the vicinity of Sites 504 and 896 is interpreted to reflect a basement topography defined by fault-bounded asymmetric volcanic hills that developed within the crustal accretion zone in the spreading environment; it is inferred that the volcanic section drilled into Hole 896A represents one of these abyssal hills in the region

THE CONCEPT OF GELIC MATERIALS IN THE NEW GELISOL ORDER FOR PERMAFROST-AFFECTED SOILS

By taxonomic definition, soils in the recently adopted Gelisol order in Soil Taxonomy must contain permafrost within 100 cm of the soil surface or gelic materials within 100 cm of the surface and permafrost within 200 cm of the surface. Five pedons representative of Gelisols are used to illustrate the nature and properties of gelic materials. Gelic materials may be either mineral or organic and are affected by cryoturbation(frost churning), ice segregation, and/or thermal contraction cracking. Gelic materials may occur in any horizons of the active (seasonally frozen) layer and/or the upper permafrost. Cryoturbation is manifested by irregular and broken horizons, involutions, organic matter accumulation on top of and within the permafrost, oriented coarse fragments, and silt-enriched layers. Ice segregation is manifested by ice crystals, ice lenses, vein ice, and ice wedges. Thermal cracks may be filled with ice or soil material. In surface mineral horizons, gelic materials contain granular, platy, and vesicular structures; subsurface horizons have blocky and prismatic structures or structureless, massive conditions. There are distinct soil fabrics in gelic materials that are visible in thin sections, e.g., orbiculic, suscitic, conglomeric, banded, and granulitic, that reflect the cryopedogenic processes of cryodesiccation, displacement, and pore formation. Cryopedogenic processes that lead to gelic materials are driven by the physical volume change of water to ice, moisture migration along a thermal gradient in the frozen system, or thermal contraction of the frozen material by continued rapid cooling.

Small‐scale polygonal features on Mars: Seasonal thermal contraction cracks in permafrost

A time‐dependent viscoelastic model of thermal stress in Martian ice‐rich permafrost is developed to test the hypothesis that small‐scale polygonal features observed from orbit and by the Viking Lander 2 are the result of thermal contraction cracking, as commonly occurs in terrestrial permafrost. Results indicate that significant tensile stress occurs in Martian ice‐rich permafrost as a result of seasonal cycles in the ground temperature. Using conservative rheological parameters appropriate for ice at low temperatures, tensile stresses poleward of about 20° to 30° latitude easily exceed the tensile strength (assumed to be 2 to 3 MPa) and fractures should readily form. In the equatorial regions, special conditions may allow tensile stresses to approach the tensile strength. These results support a thermal contraction origin of observed small‐scale polygonal features and emphasize the utility of these features as valuable morphological indicators of ground ice.

Observations on “cryoplanation” benches in Antarctica

A series of benches on nunataks of Alexander Island (Antarctica) are described. An increase in bench size with distance away from the retreating glacier suggests an age spectrum. The benches have thermal contraction cracks (in bedrock) on shallower, upper sections of the risers as well as salt encrusted runnels on the steeper lower section of the tread. The benches also show a distinct orientational preference (orientated to the north through to west) and, from first principles, these seem to be the aspects with optimal freeze-thaw cycles and temperatures conducive to thermal stress fatigue. The extensive dilatation associated with the retreating glaciers is thought to play a significant role in the origin and development of the benches as the combination of extensive jointing and optimal process conditions are thought to constrain where benches begin. The jointing, aided by the thermal contraction cracking, then facilitates extension and continued weathering of the treads. It would appear that these benches are examples of so called “cryoplanation terraces” that have been reported as fossil forms in Europe and North America. The study of such active forms in the Antarctic may provide good analogues for fossil features found in the Northern Hemisphere.

Prediction of the spacing between thermal contraction cracks in asphalt pavements

This article presents a simplified approach for the prediction of the spacing between thermal cracks in asphalt pavements based on a finite element formulation of linear elastic fracture mechanics applied to an idealized layered system in which the properties of the frozen layers are temperature dependent. The numerical model was valid for the case of a homogeneous semi-infinite medium and used to investigate the influence of various factors such as depth of frost front, modulus of deformation of frozen base material and of asphalt, and critical energy release rate for crack propagation on crack spacing. Moisture content in the base and subbase plays an important role by providing bonding between grains when frozen. Once cracks can propagate through bonded and brittle base material, calculated crack spacing is found between 25 and 90 m. The model gives support to a new hypothesis of low-temperature cracking which considers that a weaker asphalt strip, which is micro-cracked and bonded with ice wedges, associated with a gradual increase in the moisture content in the base and subbase can account for the field observations that the time where cracks are noticed does not necessarily correspond to the time of year when the pavement is at its lowest temperature.

Prediction of the spacing between thermal contraction cracks in asphalt pavements

Prediction of the spacing between thermal contraction cracks in asphalt pavements

Subaqueous eruption and emplacement of the Shinjima Pumice, Shinjima (Moeshima) Island, Kagoshima Bay, SW Japan

The Shinjima Pumice is a fines-depleted pumice lapilli tuff emplaced several thousands years ago at about 100–140 m below sea level. This 40-m-thick deposit comprises many poorly defined flow units, which are 1–10 m thick, diffusely stratified and showing upward-coarsening of pumice clasts with a sharp to transitional base. Parallel to wavy diffuse stratifications are commonly represented by alignment of pumice clasts, especially in the lower half of the flow units. Pumice clasts of block to coarse-lapilli size commonly have thermal-contraction cracks best developed on the surfaces, demonstrating that they were hot but cooled down to the ambient temperatures prior to their emplacement. These features are suggestive of the direct origin of the Shinjima Pumice from subaqueous eruptions. A theoretical consideration on the behavior of subaqueous eruption plumes and hot and cold pumice clasts suggests that subaqueous eruption plumes commonly collapse by turbulent mixing with the ambient water and are transformed into water-logged mass flows.

Morphology and Paleoenvironmental Significance of Quaternary Sand Veins, Sand Wedges, and Composite Wedges, Tuktoyaktuk Coastlands, Western Arctic Canada

Sand and sand-ice fillings of Quaternary thermal contraction cracks on Summer and Hadwen Islands, Western Arctic Canada, comprise sand veins, sand wedges, and composite wedges. Sand wedges in diamicton-poor ice, diamicton-rich ice, and ice-rich sand generally have simple V shapes, whereas those in ice-poor sand vary from V-shaped to irregular forms and may contain inclusions of host sand. These morphological differences are explained in terms of the relative tensile strength of the wedge and host materials. Bundles of sand veins within sand wedges indicate discrete stages of wedge growth. Criteria previously proposed for identifying relict sand wedges are reevaluated: (1) Not all wedges are V.shaped; some are irregular forms with offshoot sand veins. (2) A vertical or steeply dipping lamination is not apparent within all wedges; some appear to have a massive fill, suggesting that the sand source can be texturally and mineralogically very uniform. (3) Individual sand veins and groups of veins can be just as common within sandy host materials as the better-known sand wedges. Composite sand-ice wedges at Crumbling Point, Summer Island, commenced growth as composite wedges, continued as sand wedges, were modified by thermokarst, and, in some cases, recommenced at a stratigraphically higher level as ice wedges. The sand-wedge and ice-wedge stages reflect environmental change from cold, arid, and windy proglacial conditions during Oxygen Isotope Stage 2 to warmer and wetter interglacial conditions during OI Stage 1.

Late Pleistocene deposits at Block Fen, Cambridgeshire, England

AbstractOrganic sediments in a gravel quarry at Block Fen, Cambridgeshire, form a sheet dividing lower from upper gravels. Analyses of pollen, macroscopic plant remains and molluscs from these organic sediments are presented. They indicate the presence of temperate freshwater and slightly brackish fine floodplain sediments, which, on the basis of the palaeobotany, are correlated with the temperate Ipswichian Stage. The freshwater sediments, ascribed to Ipswichian substage IIb, occur at ca. −3 m OD. Marine‐influenced tidal sediments, ascribed to Ipswichian substage III, occur at ca. −6 m OD. No evidence was found for the presence of more than one temperate stage in the sequence. The lower gravels are then correlated with the cold Wolstonian Stage and the upper gravels with the cold Devensian Stage. In contrast to the woodland environments indicated by the palaeobotany of the Ipswichian organic sediments, post‐Ipswichian pollen diagrams and macroscopic plant remains in the upper suite of sands and gravels indicate open tree‐less vegetation typical of the cold Devensian Stage. They also contain a typical cold‐stage mollusc fauna. The sediments containing these floras and faunas are associated with thermal contraction cracks, indicating the presence of permafrost. The final sand and gravel aggradation in the Devensian forms the Block Fen Terrace, near 0 m OD. The evidence indicates that it is younger than the lacustrine sediments resulting from the blocking of the Fenland at the Wash by Late Devensian ice at ca. 18.5 ka BP.The sequence at Block Fen is related to nearby Ipswichian and Devensian sediments at Chatteris, March, Wimblington and Mepal, and to deposits at Wretton on the east margin of Fenland. The correlation permits an outline reconstruction of the history of the valley carrying the River Great Ouse between the Isle of Ely and the Chatteris and March ‘islands’ from the time of a gravel aggradation before the Ipswichian to the Flandrian. The reconstruction shows the time and level of the Ipswichian marine incursion into the Middle Level of Fenland and the extent of aggradation and erosion in the Devensian.

Climate controls and high‐altitude permafrost, qinghai‐xizang (tibet) Plateau, China

AbstractThe high altitude‐induced permafrost of the Tibet Plateau reflects a geothermal regime characterized by a strong diurnal pattern, high solar radiation inputs at the surface, and a high geothermal gradient. As a result, the permafrost is relatively [warm] and sensitive to thermal disturbance. Typical permafrost‐related processes, such as ice segregation and thermal contraction cracking, are not widespread, and patterned ground phenomena (circles and nets) are restricted to sites of locally high moisture and variable grain size. Ice‐rich sediments on north‐facing slopes occur mainly as silty colluviums derived from local argillaceous bedrock. Mean annual ground temperatures are usually warmer than −3.5°C. Temperatures at 20 m depth have increased by an average of 0.2 to 0.3°C during the past 15 years. If one assumes continued warming, permafrost is likely to become relict within the next 200 years, and an increase in degradation (i.e. thermokarst processes) is to be expected.