Lacustrine sediments

One of the best archives of terrestrial palaeo-climate information is lake sediment. Annual and decadal climate shifts influence sediment production and deposition in lakes. During the winter, lakes, at least in mountainous regions, are normally frozen and little clastic sediment enters the lakes. Organic production is effectively reduced due to little sunlight and snow-covered lake ice. In spring and early summer, strengthened sunlight, ice-free lake conditions, and nutrients released from...

Calving glaciers

Several mechanisms of glacier calving have been described however, the fundamental control and relationship with calving rate is poorly known. This makes it difficult to explain the lack of climatic sensitivity of different glaciers, in particular the order-of-magnitude difference in calving rate between tidewater and lake-calving glaciers (Fig. 4.49). The mechanism for 'normal' slab calving includes the development of an overhang before failure occurs as a result of deformation of the glacier...

Gas content in ice cores

Below the surface of a glacier or ice sheet, air bubbles become isolated and trapped during the transformation of snow into ice. If no diffusion occurs, the bubbles contain 'fossil' air from the time of the inclusion. The composition of the bubbles in ice cores therefore provides a way to analyse changes in the atmospheric composition several thousands of years back in time. The reconstruction of atmospheric C02 concentration from ice cores is, however, not straightforward, due to (a) possible...

Supraglacial ice morphology

Supraglacial structures reflect glacier formation, deformation and flow. Glacier ice exhibits a wide variety of internal and superficial structures, such as crevasses, icefalls, ogive banding, and layering. Crevasses form where ice is pulled apart by tensile stresses that exceed the strength of the ice, and they are commonly oriented at right angles to the main stress direction (e.g. Paterson, 1994). Crevasses are therefore a reflection of the stress orientation in a glacier. Chevron crevasses...

Outlet glaciers and ice streams

The ice movement in an ice sheet can be divided into sheet flow in the central dome areas, and stream flow in outlet glaciers and ice streams, characterized by rapidly moving, channelled ice flow. Outlet glaciers draining plateau glaciers normally have the form of valley glaciers. The accumulation area may be difficult to delimit. The longest glacier in the world, the 700 km Lambert Glacier in Antarctica, and the world's fastest moving glacier, Jacobshavn Glacier in western Greenland, are both...

The astronomical Milankovitch theory of climate variation

During at least the last million years, climate has fluctuated in a distinctive way. In recent years there have been attempts to explain the causes of the long-term climatic fluctuation (e.g. Imbrie and Imbrie, 1979 Bradley, 1985). The astronomical theory or the Milankovitch theory has undoubtedly attracted the greatest attention. This theory was in fact developed by Croll about 100 years ago, but was later elaborated by the Serbian geophysicist Milutin Milankovitch. The theory is based on the...

Variation in atmospheric gas content and climate change

Direct measurements of the atmospheric concentration of the greenhouse gas carbon dioxide (C02) began in 1958, and this figure has shown a significant increase from 315 parts per million by volume (ppmv) to 364 ppmv in 1997 (data from Keeling et al, 1995 Keeling and Whorf, 1998 Fig. 2.2). The increase in atmospheric CO2 is caused by burning of fossil fuels and a change in land use. Previous investigations showed that the atmospheric C02 concentration was about 280 ppmv 315310 1 1 1 1 1 1 1 * 1...

Environments of lamina formation and preservation

There are two fundamental requirements for the development of laminated sediment sequences (1) Variation in input chemical conditions biological activity that will result in compositional changes in the sediment. (2) Environmental conditions that will preserve the laminated sediment fabric from bio-turbation. In lakes, strong seasonal signals are dominant while preservation is effected by bottom-water anoxia resulting from stratification, and high sedimentation rates. Varves are defined as...

Surging and tidewater glaciers

A surging glacier commonly shows little sign of unusual activity for several years. Then it starts to move rapidly and the glacier surface, especially in the lower part, is transformed from a fairly smooth surface into deep crevasses and ice pinnacles. The frontal part may move several kilometres in a few months or years before it suddenly stops. Ice velocities during most surges are 10-100 times those in normal glaciers. Observed velocities vary from about 100 m per day over short periods, to...

Climate records from ice cores

Palaeoclimatic records can be obtained from ice cores drilled from glaciers and ice sheets, containing information on accumulation and atmospheric composition through time. In the upper part of ice cores, annual layers are commonly preserved as alternating bands of clear and bubbly ice. Deeper in the ice cores, however, annual layers are usually not discernible and dating is made indirectly. Short ice cores can be drilled manually, but long cores have to be retrieved by sophisticated mechanical...

Holocene glacier and climate variations

The Little Ice Age was the most extensive Neo-glacial glacier advance in the Canadian Rocky Mountains (Luckman et al, 1993). Evidence of earlier, less extensive Neoglacial advances is based on wood recovered from several glacier forefields. Three radiocarbon dates, ranging between 8230 and 7550 yr bp, obtained from wood flushed out of Athabasca Glacier, and two dates from Dome Glacier ranging between 6380 and 6120yrbp, indicate that forests occurred upvalley of present glacier fronts during the...

Seaice ice shelves

Sea-ice ice shelves are normally formed by a combination of thickening by snow accumulating on the surface, and freezing of seawater. Movement and deformation in such a shelf is entirely determined by the weight of the ice mass. Sea-ice ice shelves exist where annual temperatures are sufficiently low for ice to form and where embayments and islands Figure 4.1 Tentative temperature profiles through polar, subpolar and temperate glaciers. (Adapted from Skinner and Porter, 1987) -25 -20 -15 -10 -S...

Temperature distribution in glaciers and ice sheets

Temperatures in glaciers and ice sheets vary in space and time. Temperate glaciers have temperatures at or close to 0 C, while the upper part of the Antarctic ice sheet may be as cold as -40 to -60 C (Fig. 4.2). The melting temperature of ice decreases with increasing pressure at a rate of 0.072 C per 106 pascals (MPa lPa INm-2). As an example, the pressure at the base of a 2000 m thick glacier or ice sheet is about 17.6 MPa, corresponding to a lowering of the melting point to 1.27 C. Therefore...

Glacier hydrology

Meltwater is an important component in glacier systems, and glacial meltwater exerts a strong influence on the hydrology of proglacial areas. Within and underneath glaciers and ice sheets, water affects glacier behaviour, controlling rates of glacier flow and influencing processes and rates of erosion and deposition. Surface and basal melting of glacier ice can produce large volumes of meltwater. In regions with low summer precipitation, but with glaciers in their catchment, glacial meltwater...

Svein Olaf Dahl

Department of Geography, University of Bergen, Norway A member of the Hodder Headline Group LONDON Co-published in the United States of America by Oxford University Press Inc., New York First published in Great Britain in 2000 by Arnold, a member of the Hodder Headline Group, 338 Euston Road, London NWI 3BH Co-published in the United States of America by 198 Madison Avenue, New York, NY10016 2000 Atle Nesje and Svein Olaf Dahl All rights reserved. No part of this publication may be reproduced...

Reconstruction of icesurface profiles and calculation of basal shear stress

From studies of modern glaciers, the basal shear stress is known to be related to the surface profile and thickness of glaciers, expressed by the equation where tb is basal shear stress, r and g the ice density and gravitational constant, respectively, a the surface slope of the glacier, and h the glacier thickness (Paterson, 1994). The value of tb (the basal shear stress) can be calculated from measurements of ice thickness h and surface slope a. Most calculated values of basal shear stress...

Sea ice

Iceland is located where the warm water from the Atlantic and the cold water from the Arctic meet. The Irminger current, a branch of the North Atlantic Drift, is deflected westwards by a submarine ridge and flows along the south and west coast before it sinks below the East Greenland current. A branch of the East Greenland current flows around the north and east coast of Iceland and sometimes brings drifting sea ice close to the coast. On some occasions, ice drifts further west along the east...

Mass balance

Glaciers and ice sheets are stores of water, exchanging mass with other components involved in the global hydrological system. Glaciers and ice sheets grow by snow and ice accumulation, and lose mass by different ablation processes. The difference between accumulation and ablation over a given time span is the mass balance, which can be either positive or negative. The mass balance reflects the climate of the region, together with glacier morphology and local topographic conditions. Mass...

Global circulation models GCMs

To gain a better understanding of atmospheric conditions, we simulate global-scale climatic processes using global circulation models (GCMs), which are computer simulations of mathematical models of how the atmosphere operates and interacts with the hydrological cycle (Wright et ah, 1993). GCM experiments have provided pertinent information about relationships between atmospheric circulation and mechanisms of climate forcing such as land uplift (Ruddiman and Kutzbach, 1990), vegetation cover...

Determination of the equilibrium line altitude ELA

The equilibrium line altitude marks the area or zone on the glacier where accumulation is balanced by ablation. The ELA is sensitive to variations in winter precipitation, summer temperature, and wind transport of dry snow. When the annual net mass balance is negative, the ELA rises, and when the annual net balance is positive, the ELA drops. The steady-state ELA is defined as the ELA when the annual net balance is zero, and can be calculated by linear regression analysis of annual net balance...

Reconstruction of the equilibrium line altitude

Fluctuations in the ELA provide an important indicator of glacier response to climate change which may allow reconstructions of palaeoclimate, but also of future glacier response to given climate change. reconstruction of the equilibrium line altitude 59 Figure 4.6 The principle of calculating the depression of the equilibrium line altitude on a glacier based on the maximum elevation of lateral moraines. The previous extent (a) is compared with the modern extent (b) for an idealized glacier....

Variations of local glaciers during the last glaciation

For the Colombian Andes, a radiocarbon-dated framework of pre-Late-glacial maximum was presented by Helmens et al (1997). During each of three pre-LGM advances, glaciers extended beyond the LGM. In other parts of the Andes, in New Zealand, and in some Eurasian mountain ranges, glaciers advanced farther earlier in the last glacial cycle than at the LGM (Gillespie and Molnar, 1995 Bon-darev et al, 1997 Clapperton et al, 1997 Fitzsimons, 1997 Clapperton, 1998). The interval of greatest glacier...

Relative sealevel changes

A wide range of sediments and landforms may be used to reconstruct sea-level histories, such as deltas, beaches, shore ridges and erosional platforms marking former shorelines. More accurate sea-level data can be obtained from sediment cores retrieved from small lakes and bogs situated at different elevations in a small area or along the same isobase (e.g. Svendsen and Mangerud, 1987). In northwest Europe, the sea-level rise since deglaciation has been reconstructed at numerous sites along the...

Box I I The concept of ice ages historical background

Environmental change is a continuous process where dynamic systems of energy and material operate on a global scale to cause gradual and sometimes catastrophic changes in the atmosphere, hydrosphere, lithosphere and biosphere. During most of the Earth's history the agents in the environmental system have been the natural elements (wind, ice, water, plants and animals). Some 2-3 million years ago, however, a new and perhaps the most powerful generator of environmental change, the hominids,...

Lateglacial glacier and climate variations in NW Europe

High-frequency climatic fluctuations during the last d glaciation (ca. 14,000-9000 14cyrbp) are well documented in terrestrial data, marine records and ice cores from the North Atlantic region (e.g. Lowe and Walker, 1997, and references therein). These climatic changes occurred at a time of maximum solar radiation receipt in the northern hemisphere and cannot therefore be explained by orbital forcing. The cause(s) of these climatic fluctuations must therefore be sought in the ocean atmosphere...

Ice aprons ice fringes glacierets and niche glaciers

Ice aprons are thin accumulations of snow and ice commonly lying on relatively steep mountain sides. Ice fringes are similar ice and snow accumulations occupying small depressions along coasts. Glacierets are referred to as thin patches of ice occupying terrain depressions formed by snow drifting and avalanching. Glacierets have been called fall glaciers or regenerated glaciers where they exist below the ELA as a result of ice avalanches from overlying ice falls. Niche glaciers are controlled...

Glacioeustasy and glacioisostasy

The basic idea of isostasy is that the Earth's crust, with a mean density of approximately 2800 kg m 3, is floating on the underlying plastic mantle with a mean density of about 3300 kg m 3 The amount of crustal depression resulting from ice sheet loading is a function of ice thickness and the ratio between the densities of ice and rock. The density of ice is about a third that of the crust, and therefore the crustal depression beneath an ice sheet is about a third of the ice thickness....

Little Ice Age glacier variations

During the last few centuries glaciers advanced on all continents, indicating that the Little Ice Age was a global phenomenon. In the European Alps, the main advance around ad 1850 was preceded by another of similar magnitude around ad 1300. Thirteenth to fourteenth-century advances in Scandinavia and North America are not well documented however, the evidence for such advances in the Himalayas and New Zealand are better documented (Grove, 1988). In the European Alps, the initiation of the main...

Glacialinterglacial cycles during the Quaternary

Quaternary Sea Level Curve

The Quaternary is conventionally subdivided into glacials and inter glacials, with further subdivision into stadials (shorter cold periods within interstadial or interglacial stages) and interstadials (shorter mild episodes within a glacial phase). Glacial stages are normally considered as cold phases of major expansion of glaciers and ice sheets. Interglacials are considered as warm periods when temperatures were at least as high as during the present Holocene interglacial. These terms,...

Glacier movement

Mass Movements Wind And Glaciers

Information about glacier movement has been obtained from mathematical and numerical modelling, laboratory experiments, measurements in boreholes, observations in natural and artificial ice cavities, and interpretation of landforms and sediments in formerly glaciated regions. Snow and ice are transferred from the accumulation area to the ablation area by glacier flow. Flow takes place as sliding, deformation of the ice, and deformation of the bed under the glacier. The deformation and sliding...

Response timetime lag

Advance and retreat of the glacier front normally lag behind the climate forcing because the signal must be transferred from the accumulation area to the snout. This is referred to as the time lag, or preferably the response time, which is longest for long, low-gradient and slowly moving glaciers, and shortest on short, steep and fast-flowing glaciers (e.g. Johannesson et al., 1989 Paterson, 1994). Kinematic wave theory has been applied Figure 4.35 The annual (upper panel) and cumulative (lower...

References

Aber, J.S., Croot, D.G. and Fenton, M.M. 1989 Glacio-tectonic Landforms and Structures. Kluwer, Dordrecht. Adam, S., Pietroniro, A. and Brugman, M.M. 1997 Glacier snow line mapping using ERS-1 SAR imagery. Remote Sensing of Environment 61, 46-54. Ahlmann, H.W. 1927 Physio-geographical research in the Horung Massif, Jotunheim. 4 Ablation. Geografiska Annaler 9, 35-66. Alley, R.B. and Anandakrishnan, S. 1995 Variations in melt-layer frequency in the GISP2 ice core implications for the Holocene...