Stuart A. Harris

A 10-yr record of stable isotope ratios of hydrogen and oxygen in precipitation at Calgary, Alberta, Canada

Short-term (0.5–3 d) precipitation samples were collected from January 1992 to December 2001 in Calgary, Alberta, Canada, and the stable isotope ratios of hydrogen (2H/1H) and oxygen (18O/16O) for these samples were determined. The 10-yr amount-weighted average δ2H and δ18O values of precipitation were −136.1° and −17.9°, respectively. Consistent with International Atomic Energy Agency (IAEA) established practice, the following local meteoric water line (LMWL) for Calgary was derived using amount-weighted monthly average δ2H and δ18O values: δ2H = 7.68 δ18O −0.21 (r2= 0.96, n= 104). The correlation equation between δ2H and δ18O values from individual samples was found to be δ2H = 7.10 δ18O −13.64 (r2= 0.95, n= 839), which is different from the LMWL, exhibiting lower slope and intercept values. A comparison of δ2H and δ18O correlation equations with temperature during precipitation events showed a trend of decreasing slopes and intercepts with increasing temperature. Our data suggest that this is caused by incorporation of moisture derived from evaporation from water bodies and soils along the storm paths and by secondary evaporation between the cloud base and the ground during precipitation events. These processes compromise the usefulness of d-excess values as an indicator for the meteorological conditions in the maritime source regions. The δ18O temperature dependence at Calgary was found to be ⊼ 0.44°¼C−1. The study shows that short-term sampling of individual precipitation events yields valuable information, which is not obtainable by the widely used monthly collection programs.

The influence of below-cloud secondary effects on the stable isotope composition of hydrogen and oxygen in precipitation at Calgary, Alberta, Canada

Stable isotope compositions of hydrogen (δ2H) and oxygen (δ18O) for short-term precipitation samples (n = 436) collected at Calgary, Alberta, Canada, between January 1997 and December 2001 were determined. Linear regression between δ2H and δ18O values of snow and large amount rain samples (≥4 mm) yielded correlation equations δ2H = 7.72 × δ18O + 5.02 and δ2H = 7.50 δ18O + 0.27, respectively. In contrast, correlation equations between δ2H and δ18O values for small amount rain samples (≤4 mm) resulted in progressively lower slope and intercept values with decreasing precipitation. Correlations of isotope data with parameters such as local temperature, relative humidity, and precipitation amount provided evidence that small amount rain samples undergo secondary evaporation accompanied by mass dependent isotope fractionation during their descent from the cloud base to the ground. Hence, the isotope compositions of precipitation at Calgary, and likely also at other locations in the North American Great Plains region, are influenced by below-cloud secondary effects. Since about one-third of the precipitation events in the 5-yr observation period were rain samples accumulating less than 4 mm, below-cloud secondary effects resulted in a slight decrease of slope and intercept values of the local meteoricwater line (δ2H=7.43 × δ18O-2.79) calculated using amount-weighted monthly average δ2H and δ18O values compared to equations based on isotope data for snow and large amount rain events only. The correlation equation (δ2H = 7.11 δ18O-11.60) calculated using δ2H and δ18O values of individual samples (non-amount weighted) yielded the lowest slope and intercept values caused by the significant influence of small amount rain samples.

Recent research on the nature, origin and climatic relations of blocky and stratified slope deposits

A selection of subaerial slope processes is discussed together with the morphological and sedimentological traces that are left by the processes. Emphasis is on mass transfers related to (coarse) blocky slopes and to scree accumulations, either on steep or on gentle slopes. New developments in the interpretation of more or less clearly stratified slope deposits are discussed in the light of the findings of research focusing on present-day process-form (process-material) relationships. The question of the climatic (and, more specifically, the periglacial) significance of the different processes and their morpho-sedimentary expression is a returning theme in this paper. It is concluded that many deposits are formed by azonal processes, although their activity (in terms of magnitude-frequency combinations) is often relatively high under periglacial conditions. Some of the deposits point to (cold-climate) extreme events. This especially is the case for frost-coated clast flows, aeolian transport of large platy clasts, and to a lesser extent debris flows.

Thermal history of the Arctic Ocean environs adjacent to North America during the last 3.5 Ma and a possible mechanism for the cause of the cold events (major glaciations and permafrost events)

At 3.5 Ma, the Arctic Ocean was unfrozen, and only during the second Pliocene cold event (Californian - 3.0 Ma) did an extensive glaciation occur in Alaska-Yukon and Iceland. The sea froze during the third (Alaskan - 2.58 Ma) event as the western Arctic cooled rapidly. Baffin Island and Labrador were the centres of ice sheets, and ice-rafted debris appeared in the North Atlantic. Shrub-tundra replaced boreal forest in the west during the next warm episode but forested-shrubtundra persisted in north Greenland during the next (Wyoming - 2.2 Ma) cold event. During the last Pliocene (Montanan - 1.9 Ma) cold event, tundra surrounded the Arctic basin with widespread permafrost in unglaciated areas. Quaternary cold events were more frequent, with tundra persisting on land during warm episodes, although coastal seas usually thawed seasonally. There was a continuous cooling trend due to the demise of the Tethyan sea, but the 18O marine curve shows about 130 fluctuations compared with 14 major cold events on land. The cause of the terrestrial changes seems to be the interaction of many cyclical controls with different periodicities. When enough cycles are synchronized for air temperature to cross a critical threshold, a climatic change occurs. The critical thresholds are dependent on local environments and latitude.

Twenty Years of Data on Climate–Permafrost–Active Layer Variations at the Lower Limit of Alpine Permafrost, Marmot Basin, Jasper National Park, Canada

In March 1979, a hole was drilled to a depth of 16.8 m through 1 m of till and loess into the clast‐supported boulders of a fossil Pre‐Late Wisconsin rock glacier just above tree line (2195 m elevation) at the Marmot Basin Ski Area, Jasper National Park (latitude 52∘47′36.5′′N, 118∘06′45.9′′W). Vegetation cover consists of a moist alpine meadow, on a bench which slopes gently to the south on the otherwise steep mountainside. The mean daily temperature for the site has been recorded continuously, and monthly readings have been made of ground temperatures using a thermistor string. Depth of the snow pack has been recorded on each visit, while November to April snowfall data are measured daily at 1985 m elevation. The mean annual air temperature (−2.38∘C) has decreased by about 0.7∘C since 1979, while the average winter snowfall (c. 310 cm) has increased by about 70 cm. A water table occurs at about 12 m depth with a water temperature of about 1.1∘C. Permafrost up to 6.8 m thick was present for almost all the period of study, with the active layer ranging from 2.2 to 4.3 m in depth. Temperatures in the permafrost varied seasonally, ranging down to −2.5∘C. The ground temperatures are constantly changing and are more closely related to thickness and duration of snow cover rather than air temperatures. The geothermal gradient averages 0.45∘ per metre in the upper 10 m, and there is a ‘thermal offset’ of about 4.1∘. Heat conduction is dominant at the surface, being replaced by closed‐cell convection between the blocks. In the early summer, warmer snow melt from the adjacent mountainside increases the thickness of the active layer and sometimes pierces the permafrost so that the warm water descends to the main water table below.

Chronostratigraphy of glaciations and permafrost episodes in the Cordillera of western North America

Glaciations in the Cordillera of western North America began during the Late Miocene in the St Elias Range and coastal ranges near Anchorage, Alaska. Radiometric dating of the tephra and lava flows intercalated in the succession of older tills, loesses and outwash deposits permits the reconstruction of the probable early glacial sequence along the Cordillera. No one site shows the complete sequence, but the available data suggest synchroneity of the major glacial events throughout the region. The first evidence for cold conditions at low elevations at midlatitudes is from 3.5 Ma BP. By 2.8 Ma, alpine glaciations may have occurred in the Sierra Nevada and ice wedges had formed in bedrock near Fairbanks, Alaska. Three more major glaciations complete with contemporary periglacial and permafrost landforms had occurred by 1.65 Ma, while at least six more major cold events can be recognized during the Quaternary period. Once again, expansion of permafrost conditions occurred during each event and forms an integral part of the evidence for climatic change.

Petrology and origin of stratified scree in New Zealand

Stratified scree in the Porters Pass area of the South Island New Zealand consists of up to 30 m of alternating bands of angular gravel and gravelly silt. The gravel consists of frost-shattered bedrock, while the silt appears to have originated from weathered till that entered the area as loess. Every 10 m of stratified scree actually contains at least 5 m of loess. Glaciers left oversteepened slopes on which screes subsequently formed, probably during later nearby glacial advances. The more gently sloping land may have been covered in vegetation as dense as today, but every lo118 yr, widespread destruction of the vegetation resulted in a marked period of loess deposition. Thus the stratified scree of Soons (1962) has a different origin to that postulated for the grezes lit&es of European workers. The scree is overlain unconformably by 50 cm of cumulic soil. Deposition of loess with 70-79% gravel was much slower in postglacial time but the loess is much less weathered. Soil formation has resulted in appreciable rounding of the pebbles in the soil horizons.

Index of structure: evaluation of a modified method of determining aggregate stability

Abstract By determining total and natural clay by the pipette method, it is possible to obtain a more reproducible measure of aggregate stability in soils. This index has been called the “index of structure” and is reported on a scale from 0 to 100. High values represent water stable materials. Where total clay contents are below 25%, multiple determinations are needed to produce reliable results. The index of structure can be used to differentiate between parent materials since it does not vary appreciably in the drainage catenas examined so far. It gives a good measure of the relative water stability of aggregates, especially when the time on the mixer is altered during the natural clay determination. The index is significantly related to porosity but there are appreciable differences between major soil groups. Humid region soils appear to give a more consistent relationship than those of seasonally dry areas.

Global heat budget, plate tectonics and climatic change

For the past 2000 Ma, the temperature of the Earths surface has fluctuated around a mean similar to that of today, although individual locations have undergone long‐term changes of ∼30°C at different times in different places. Water bodies absorb at least five times as much solar radiation as land surfaces, and ocean currents transport the excess heat absorbed in the tropics towards the poles. Changes in the distribution of land and sea due to plate tectonics explain the major temperature fluctuations (>25°C) around the globe in the last 350 Ma, and are first‐order controls. Large‐scale changes in ocean currents and thermohaline circulations are probably second‐order controls (15–25°C). The Milankovitch orbital cycles are third‐order controls producing variations in air temperature of the order of 10°C, while massive volcanic eruptions and changes in carbon dioxide are amongst the fourth‐order controls producing minor perturbations (<5°C). The major climatic fluctuations are continuous but regional in effect and not global. Extraterrestrial factors may not cause major changes in climate when viewed from a geological perspective.

Palaeoecology and palaeoceography near Cochrane, Alberta, Canada, just after the last major high stand of glacial Lake Calgary

Abstract This paper describes a fossiliferous section through a mound of Lake Calgary sediments at 1201 m elevation at Grid Reference 6671, 6701 south of Cochrane, Alberta, Canada. The section lies just west of the zone of interaction of Cordilleran and Laurentide ice sheets. These lake deposits can be traced laterally to sections where they overlie gravels of the “Bighill Creek Formation” date previously at about 10,200–11,300 radiocarbon years B.P. The lake maximum is therefore younger. The sediments indicate that forest occurred around the glacial lake until shortly after 9650 yr B.P. and that contemporary forest fires produced charcoal. As air temperatures rose, grassland replaced the forest, and the ice melted. The lake had drained by 8145 yr B.P.