Another important resource for reconstructing climes of the quaternary is through the analysis of fossilised insects, predominantly beetles, which are commonly preserved as disarticulated skeletal fragments. Their analysis has many advantages over other proxies such as pollen because they are the most diverse group of organisms, accounting for approximately 25% of all species on earth. Most beetles can only survive in specific climates, giving researchers an accurate, detailed map of their distribution. They also fill a vast range of ecological roles and habitats from the desert to the rainforest, offering great potential for palaeoclimatic research because of their ability to provide information on short-term environmental fluctuations.
Compared to large mammals (megafauna), which have a large roaming area, smaller mammals are good ecological indicators. They provide details on migration, local habitats and can be used to deduce climate.
Other fossil remains used to aid in the reconstruction of the quaternary include diatoms, animal bones/teeth and fungal remnants. Dating techniques currently used, all of which can only be applied to certain periods of the quaternary such as dendrochronlogy, amino-acid diagenesis and some radiometric methods are still relatively unreliable, with scientists constantly striving to improve them.
Work by the Yugoslavian astronomer and geophysicist Milutin Milankovitch (1879 – 1958), following up that of nineteenth century James Croll (1842), resulted in the creation of the Milankovitch curve (1920). From periodic fluctuations known to be intrinsic in the orbital elements of the world, Milankovitch calculated the alterations in intensity of solar radiation that were believed to lend themselves to the timing of quaternary climatic events.
The variability of the earths orbit causes seasons which are not set lengths, but duration varies from ~82.5 – 100 days. The eccentricity of the orbit does not have a great deal of effect on insolation due to the fact that there is only a 6/7% difference between the time when the earth is closest to the sun (perihelion) and the time when it is at its furthest (aphelion). This distance varies from 91.4-94.6 million miles. A more influential cause of the varying distribution of insolation (and thus the variation of atmospheric and ocean circulation patterns) occurs through obliquity. With increased obliquity (the tilting of the axis of the earth) radiation is concentrated more on the higher latitudes and decreased at the poles and equator. The maximum effect is seen at 66°N.
Milankovitch argued that glaciations in the past were principally a result of variations in the Earths orbital parameters, and the ensuing redistribution of solar radiation reaching the earth. Evidence to support this can be found through examining coral terrace formation using GCM (general circulation model) experiments. Dates of formation indicating a higher sea level (hence lower global ice volumes) were shown to be closely related to times of insolation maxima. This data suggested that sea level fell by approx. 50m from ~115-105ka B.P. Due to a combination of greater eccentricity, lower obliquity and perihelion closer to the northern winter solstice, incoming solar radiation at 115ka was reduced by ~7% during the northern hemisphere summer, but was higher at other times. This particular model showed that temperatures in North America were reduced and precipitation increased, suggesting that conditions at 115ka BP. were indeed more favourable for ice sheet growth. The model was, however, too crude to determine whether an ice sheet could be sustained or not. So, using a GFDL (geophysical fluid dynamics lab) GCM, Rind et al found that with orbital changes alone, ice growth could not be achieved in the model - a conclusion also reached by Philips and Held (1994).
Heys et al (1976) was the first to attempt to assess the evidence for orbital changes in paleoclimatic data. Using two ocean core records, dating back ~450,000, from the Indian Ocean he studied three parameters, each providing different indexes. The first produced an index of global, but primarily northern hemisphere ice volume; the second showed an index of sub-Antarctic temperatures; thirdly an index of Antarctic surface water structure. By manipulating data, Hays showed that much of the variance was concentrated at frequencies corresponding closely to those expected from an orbital forcing function. Moreover, not only are the proxy and orbital series closely matched solely through frequency, but also an examination of the time domain of each periodic component showed consistent phase relationships (back to 300,000 – 400,000) between orbital parameters and “resultant” climatic signals. However lags in time were observed where ice melt occurred before peak solar radiation suggesting differences in ice thickness.
Hays’ work provided the first strong evidence to show that changes in the earth’s orbital geometry played an important role in causing glacial-interglacial variations over the past 300,000-400,000 years.
A study examining deep-sea sediment cores (published in Science) found that Milankovitch’s theory corresponded to periods of climate change showing, finally, that ice ages had occurred when the earth was going through different stages of orbital variation. However, it must be remembered that although Milankovitch cycles do explain long term climatic change, they can not account for changes made by humans, which appear to have an even greater effect than variations in earth-sun interaction.
Geomorphic evidence can be used to reconstruct former ice sheets, flow directions and overall glacial movements. This evidence can be seen in physical formations and landscapes in many areas, on a range of scales from local to global. The most obvious regions of the world to assess this evidence are mountain ranges, for example, the Rockies in North America and The Southern Alps in New Zealand.
Evidence showing former ice flow direction is present in many forms within mountainous areas. The landscapes carved by the movement of ice until the Last Glacial Maximum (LGM), ~18ka, are varied in distribution, scale and frequency, however it is relatively clear to model the directions of flow.
In formerly glaciated areas, morphological feature and associated deposits can be used to determine former ice flow direction, splitting into two categories. The first is end moraines, also known as terminal moraine, which forms a traverse to the ice direction. Secondly, drumlins and other pertinent structures orientated parallel to the ice flow. Thus, an overall idea of direction is immediately provided upon discovery of such deposits.
End moraines are composed of varying amounts of ablation till, lodgement till and other eroded sediments and can be sub-divided into push moraines and dump moraines. Push moraines form when ice bulldozes into existing sediment at the glacier margin raising it to form a ridge. They often contain evidence of glaciotectonic deformation within them. Major push moraines represent the mark of maximum extent of the glacier, allowing scientists to map ice coverage. For example, this is how the maximum extent of the Laurentide ice sheet covering a vast area of North America was discovered. Dump moraines are ridges formed traverse to ice flow direction also. They mark the stationary position of an ice margin. It should be noted that moraine ridges orientated traverse to the ice flow direction are not always marginal structures.
Drumlins are small, elongated, streamlined hills that are composed primarily of lodgement till usually occurring in swarms. The direction of ice flow can be determined from the shape of the drumlins. Typically, these features form with a blunt end (‘Stross end’) pointing upstream, tailing off (‘Lee end’) down-valley and can be up to tens of meters thick. This is similar to the orientation and shape of roche moutonnee. Lateral and medial moraines are formed parallel with the ice flow direction as glacial debris is deposited on the edge or, in the case of the former, in the middle of a U-shaped valley.
There are also many small-scale features to show ice flow direction. One of the most reliable sources are striations. Rocks embedded either in the basal ice, or the side, of the glacier form these by scraping across the bedrock leaving scratch marks. For example, on Shetland, stiae have been the most important type of evidence used in the reconstruction of former ice flow patterns. In this example, striae is crossing, suggesting that the last ice to move across each area was sufficiently erosive to remove earlier markings. They also give an indication of the relative hardness of the bedrock compared to the load of the glacier. Non-uniform slip over the bedrock results in features such as chatter marks and crescentric gouges, scars and fractures. Eskers (straight or sinuous ridges) also provide an indication of ice flow direction because they align perpendicular to the ice margin. They can reach lengths of hundreds of kilometres long, for example in Central Sweden.
Larger scale, more obvious landforms such as U-shaped valleys also provide evidence as to previous flow directions, but do not show detailed movements and subtle directions.
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References:
Holden, J, (2005) An Introduction to Physical Geography and the Environment Pearson, London
Goudie, A, (1992) Environmental Change 3rd Edition. Oxford Press, United States
Nilsson, T, (1983) The Pleistocene - Geology and Life in the Quaternary Ice Age D.Reidel, West Germany
Bradley, R.S, (1999) Paleoclimatology - Reconstructing Climates of the Quaternary 2nd Edition. Harcourt, United States
