Julian B. Murton

Bedrock Fracture by Ice Segregation in Cold Regions

The volumetric expansion of freezing pore water is widely assumed to be a major cause of rock fracture in cold humid regions. Data from experiments simulating natural freezing regimes indicate that bedrock fracture results instead from ice segregation. Fracture depth and timing are also numerically simulated by coupling heat and mass transfer with a fracture model. The depth and geometry of fractures match those in Arctic permafrost and ice-age weathering profiles. This agreement supports a conceptual model in which ice segregation in near-surface permafrost leads progressively to rock fracture and heave, whereas permafrost degradation leads episodically to melt of segregated ice and rock settlement.

Fifty thousand years of Arctic vegetation and megafaunal diet

Although it is generally agreed that the Arctic flora is among the youngest and least diverse on Earth, the processes that shaped it are poorly understood. Here we present 50 thousand years (kyr) of Arctic vegetation history, derived from the first large-scale ancient DNA metabarcoding study of circumpolar plant diversity. For this interval we also explore nematode diversity as a proxy for modelling vegetation cover and soil quality, and diets of herbivorous megafaunal mammals, many of which became extinct around 10 kyr bp (before present). For much of the period investigated, Arctic vegetation consisted of dry steppe-tundra dominated by forbs (non-graminoid herbaceous vascular plants). During the Last Glacial Maximum (25–15 kyr bp), diversity declined markedly, although forbs remained dominant. Much changed after 10 kyr bp, with the appearance of moist tundra dominated by woody plants and graminoids. Our analyses indicate that both graminoids and forbs would have featured in megafaunal diets. As such, our findings question the predominance of a Late Quaternary graminoid-dominated Arctic mammoth steppe.

Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean

The melting Laurentide Ice Sheet discharged thousands of cubic kilometres of fresh water each year into surrounding oceans, at times suppressing the Atlantic meridional overturning circulation and triggering abrupt climate change. Understanding the physical mechanisms leading to events such as the Younger Dryas cold interval requires identification of the paths and timing of the freshwater discharges. Although Broecker et al. hypothesized in 1989 that an outburst from glacial Lake Agassiz triggered the Younger Dryas, specific evidence has so far proved elusive, leading Broecker to conclude in 2006 that “our inability to identify the path taken by the flood is disconcerting”. Here we identify the missing flood path—evident from gravels and a regional erosion surface—running through the Mackenzie River system in the Canadian Arctic Coastal Plain. Our modelling of the isostatically adjusted surface in the upstream Fort McMurray region, and a slight revision of the ice margin at this time, allows Lake Agassiz to spill into the Mackenzie drainage basin. From optically stimulated luminescence dating we have determined the approximate age of this Mackenzie River flood into the Arctic Ocean to be shortly after 13,000 years ago, near the start of the Younger Dryas. We attribute to this flood a boulder terrace near Fort McMurray with calibrated radiocarbon dates of over 11,500 years ago. A large flood into the Arctic Ocean at the start of the Younger Dryas leads us to reject the widespread view that Agassiz overflow at this time was solely eastward into the North Atlantic Ocean.

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.

Ice-wedge casts as indicators of palaeotemperatures: precise proxy or wishful thinking?

The use of ice-wedge casts to reconstruct palaeotemperatures involves three stringent assumptions: (1) the influence of air temperature on ice-wedge cracking and the distribution of growing ice wedges are well known in contemporary permafrost environments; (2) contemporary and former permafrost environments are sufficiently similar for the same quantitative relationships between air temperature and ice-wedge cracking to apply to both environments; and (3) the history of ice-wedge growth and decay can confidently be inferred from ice-wedge casts. We propose that the validity of these assumptions has been overestimated in terms of the Weichselian of northwest Europe because of (i) limited knowledge of the frequency of ice-wedge cracking in contemporary permafrost environments; (ii) the complex and incompletely understood natural controls on cracking; (iii) probable differences between former cold environments in mid latitudes and contemporary cold environments in high latitudes; (iv) limited understanding of ice-wedge growth and decay histories, and of the natural controls on and mechanisms of ice-wedge casting; and (v) different time perspectives. Given all these uncertainties, it is timely to critically re-evaluate the use of Weichselian ice-wedge casts for palaeoclimatic and environmental reconstructions.

Thermokarst sediments and sedimentary structures, Tuktoyaktuk Coastlands, western Arctic Canada

Abrupt climate warming during glacial–interglacial transitions promotes regional thermokarst activity in areas of ice-rich permafrost. The ensuing thaw-related processes of melt-out, soft-sediment deformation and resedimentation may produce widespread thermokarst sediments and sedimentary structures. Examples of the most distinctive thermokarst sediments and sedimentary structures from the Tuktoyaktuk Coastlands, western Arctic Canada, comprise: (1) soft-sediment deformation structures (thermokarst involutions) in a palaeoactive layer; (2) ice-wedge casts and composite-wedge casts; (3) peaty to sandy diamicton deposited mainly by debris flows in retrogressive thaw slumps; and (4) a basal unit of diamicton and/or impure sand in some thermokarst-basin sequences, deposited by progradation of resedimented materials in thermokarst lakes. Many of the thermokarst sediments and sedimentary structures in the Tuktoyaktuk Coastlands formed as a result of rapid climate warming during the last glacial–interglacial transition, although some continue to form at present due to local (non-climatic) factors. Identification of thermokarst sediments and sedimentary structures in the geological record requires evidence for the thaw of excess ice. Direct evidence for the former occurrence of excess ice includes: (1) ice-wedge casts; (2) composite-wedge casts; (3) lenticular platy microstructures in frost-susceptible sediment; (4) certain near-surface brecciation of frost-susceptible bedrock; and (5) ramparted depressions attributed to the decay of frost mounds. Indirect evidence for former excess ice results where thaw consolidation initiates soft-sediment deformation or gelifluction.

Near‐surface brecciation of chalk, isle of thanet, south‐east England: a comparison with ice‐rich brecciated bedrocks in Canada and Spitsbergen

Chalk on the Isle of Thanet, Kent, is brecciated to depths of a few metres beneath the ground surface. The brecciation commonly comprises (i) an undeformed layer of angular, platy blocks more or less parallel to the surface overlain by (ii) a deformed layer containing small open folds, typically with vertical axial planes. Above the brecciated chalk is an involuted layer (∼0.5 to 2.0 m thick) of chalk diamicton and brickearth. By analogy with brecciated ice-rich limestones, arkoses and shales in areas of continuous permafrost in Arctic Canada and Spitsbergen, it is suggested that brecciation of the Chalk resulted primarily from ice segregation in perennially frozen bedrock, and repeated segregation formed an ice-rich layer just beneath the former permafrost table. Subsequent thaw consolidation of this layer is thought to have formed an involuted layer through soft-sediment deformation. Three implications arise from this study: (i) near-surface brecciation of the Chalk probably took place during conditions of continuous permafrost; (ii) the growth and thaw of the ice-rich layer in chalk was probably an important element in the geomorphological evolution of the English Chalklands, heaving and brecciating the Chalk during permafrost conditions, and deforming or redepositing the overburden during periods of active-layer deepening; and (iii) repeated ice segregation near the top of permafrost may have brecciated other bedrocks in the British Isles.

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.

Stratigraphy and glaciotectonic structures of permafrost deformed beneath the northwest margin of the Laurentide ice sheet, Tuktoyaktuk Coastlands, Canada

The upper 5-20 m of ice-rich permafrost at three sites overridden by the northwest margin of the Laurentide ice sheet in the Tuktoyaktuk Coastlands, western Arctic Canada, comprise massive ice beneath ice-rich diamicton or sandy silt. The diamicton and silt contain (1) truncated ice blocks up to 15 m long, (2) sand lenses and layers, (3) ice veins dipping at 20-30°, (4) ice lenses adjacent and parallel to sedimentary contacts, and (5) ice wedges. The massive ice is interpreted as intrasedimental or buried basal glacier ice, and the diamicton and silt as glacitectonite that has never thawed. Deformation of frozen ground was mainly ductile in character. Deformation was accompanied by sub-marginal erosion of permafrost, which formed an angular unconformity along the top of the massive ice and supplied ice clasts and sand bodies to the overlying glacitectonite. After deformation and erosion ceased, postglacial segregated ice and ice-wedge ice developed within the deformed permafrost.

Experimental design for a pilot study on bedrock weathering near the permafrost table.

An experimental design is described to test the hypothesis that ice segregation near the top of permafrost and in the lower part of the active layer can brecciate frost-susceptible bedrock. Seasonal temperature cycles in the active layer and the top of permafrost were simulated to a first approximation within a block of chalk measuring 30 m × 31 m wide and 33 m high and insulated around the sides to minimize lateral heat transfer. The block, moistened first by capillary rise, was initially frozen from the surface downwards, simulating permafrost aggradation. Thereafter, the upper half of the block was cycled above and below 0°C (simulating seasonal freezing and thawing of the active layer) while the lower half remained below 0°C (simulating permafrost). During thaw cycles water was supplied to the surface and base of the simulated active layer. Temperature, unfrozen water content, surface frost heave and porewater pressure within the chalk were monitored during the experiment.