Unit 4 Glaciation, surficial deposits, soils and landslides 4.1 The Pliocene - Pleistocene glaciations Many aspects of glaciation, including glacial erosion, glacial landforms and glacial deposits are relevant to environmental geology issues such as slope Importance of stability, soil formation and groundwater. Furthermore, an understanding of the glaciation to causes of glacial and inter-glacial periods is very important from an environmental environmental geology point of view because the process has implications for geology climate change, its geological origins and its impact on geological processes. Average surface temperatures over the past 100 million years, based on analyses Drop in of various marine and terrestrial deposits as summarized by Crowley (1996)1, are temperature shown on the figure below. It is evident that there has been an overall drop of during the between 6 and 8º C since the Cretaceous, a change that is largely ascribed to a Tertiary reduction in atmospheric carbon dioxide levels over that time Variations in global temperatures over the past 100 million years (note that the time scale is not linear) The generally low temperatures experienced during the past 2 to 2.5 million years represent the Pliocene - Pleistocene glaciation2. During the Plio-Pleistocene there Plio- have been 5 distinct glacial periods, each lasting around one hundred thousand Pleistocene years, and 5 inter-glacial periods. Actually, its 6 if you count the present warm glacial events stretch as an “interglacial” and take the view that there is likely to be more ice to come! Temperature fluctuations over the past 160,000 years are shown on the figure below. These values are derived from hydrogen isotope measurements on Antarctic ice-cores, and they show that during the glacial interval that lasted from 1 From Crowley, 1996. The atmospheric temperatures values are based largely on oxygen and hydrogen isotope data. Crowley’s 1996 paper is included in full at the website of the journal Consequences. (see link in the references) 2 Although it is commonly referred to as the Pleistocene glaciation, its actually Plio-Pleistocene because glacial periods started in the Pliocene, well before the beginning of the Pleistocene at 1.6 m.y. Malaspina University-College – Geology 312 – Environmental Geology – Steven Earle - 2005 2 around 120,000 to 15,000 years b.p. (what we call the Wisconsin Glaciation) the temperature was commonly 3 to 4º C cooler, and as much as 8º C cooler than it is now. The Eemian interglacial period may have been a little warmer than the present interglacial, but it didn’t stay warm for very long. Estimated earth-average temperature variations over the past 160,000 years Apart from the critical role of greenhouse gases (particularly carbon-dioxide), The impact of there are several factors, which can contribute to the onset of a glacial period (ie. continental a period of much colder than average temperatures, and of continental glaciation distribution on in areas which are not glaciated at normal times). One has to do with continental glaciations drift, and the concentration of landmasses near to the poles. It has been argued that unless there is a large continental mass near to one or other of the poles, there 3 cannot be a widespread glaciation 3. At present the only two large landmasses very near to the poles are Antarctica and Greenland, and while these areas are almost completely glaciated, the extent of their glaciation is restricted by the surrounding oceans. Except under extreme conditions true glacier ice will not extend far out into the oceans. Northern Canada, Alaska, Siberia and northern Europe are also sufficiently close to the north pole to be ice-covered during a glacial period (as they were during the most recent glaciation, which ended only about 10,000 years ago). On the other hand, areas such as Australia and southern Africa, which do not extend south of 40º S, are too far from Antarctic landmass to be glaciated. The Plio-Pleistocene glaciations cannot be directly attributed to continental drift Plate because the difference in continental positions between 3 m.y. ago, when there tectonics and was no glaciation and 2 m.y. ago, when there was, only adds up to about 50 the Plio- kilometres. On the other hand, plate tectonics may have played a different but Pleistocene very significant role in contributing to the onset of the Plio-Pleistocene glaciations glaciation. Much of the existing data show that the cooling of the climate was very fast in geological terms, perhaps only a few hundred years, possibly even less. One possibly important event is the connection of North and South America 3 Except in exceptional circumstances, such as the “snowball earth” of the Proterozoic (http://www.mala.bc.ca/~earles/snowball-jan00.htm) Unit 4 – Glaciation, surficial deposits, soils and landslides Malaspina University-College – Geology 312 – Environmental Geology – Steven Earle - 2005 3 along the Isthmus of Panama. The completion of the isthmus - by subduction- related volcanism - is dated at around 2.5 m.y. ago, and this would have affected the circulation of ocean waters. A significant reduction in the amount of warm equatorial Pacific water moving through the Caribbean towards the northern Atlantic polar region could have led to a sufficient increase in climate extremes to trigger a period of glaciation. While changes in the patterns of ocean currents may have been a trigger for the Volcanism Plio-Pleistocene glaciation, a period of intense volcanism, which has been shown and the Plio- to have occurred at around 2.6 m.y., might also have been a factor. As discussed Pleistocene in Unit 3, it would have to have been a series of very significant events to provide glaciations sufficient cooling. The major eruptions of the past 200 years produced, at most, only one or two years of significantly reduced temperatures. On the other hand, if the atmosphere had already been cooled substantially by a drop in carbon-dioxide levels, it might not have taken much extra cooling to start a glacial period Glaciation provides its own positive feedback. A period of cooling will lead to an Ice albedo as increase in build-up of snow and ice, and this will increase the amount of the a positive sun’s energy reflected back to space, thus adding to the cooling effect. This is feedback particularly the case for sea-ice, since snow covered ice is one of the most mechanism reflective of earth’s surfaces, while open water is one of the least reflective. An increase in the extent of Arctic and Antarctic sea ice might also inhibit deep circulation of cold polar water towards tropical regions (and circulation of warm tropical water towards polar regions) and would thus lead to more extreme temperatures in the polar regions. Another positive feedback mechanism, which may be important in controlling the The possible climate, is the existence of vast quantities of methane stored in methane hydrate role of 4 within sea-floor sediments and in permafrost areas 4. The stability of these methane materials is controlled by ocean water temperature and also by the pressure - hydrate which is largely a function of ocean-water depth. Warming of ocean water - perhaps simply because of a change in ocean current patterns - could trigger the release of vast quantities of methane gas, which would have a major greenhouse effect. Conversely, cooling of seawater could lead to greater methane hydrate stability in some areas and hence to a reduction of atmospheric carbon (in the form of carbon dioxide and methane), resulting in cooling. The cyclic nature of the Plio-Pleistocene glaciation can be explained in terms of Milankovitch the periodicity of several aspects of the earth’s motion, both on its axis and in its cycles orbit around the sun. As summarized in the table below, there are periodic variations in the shape of the earth’s orbit (from close to circular to strongly elliptical), in the tilt of the axis of rotation, and in the direction of that tilt (axial precession). Milutin Milankovitch (1879-1958), a Yugoslavian mathematician, argued that combinations of these factors can result in sufficient cooling to trigger a glacial period. He also demonstrated a relationship between these factors and the periodicity of the Pleistocene glacial and inter-glacial periods. 4 See http://www.mala.bc.ca/~earles/mh-instability-apr00.htm, http://www.mala.bc.ca/~earles/m-hydrate-nov99.htm and http://www.mala.bc.ca/~earles/methane-wipeouts-dec02.htm Unit 4 – Glaciation, surficial deposits, soils and landslides Malaspina University-College – Geology 312 – Environmental Geology – Steven Earle - 2005 4 While it is clear that variations in the orbital and rotational elements do not change the net amount of the earth’s insolation (incoming solar radiation) they do change the amount of light that strikes different parts of the earth. Insolation at around 65º N is interpreted to be important to the growth and decay of ice sheets, so orbital variations that lead to less insolation at that latitude can promote the onset of glaciation. The Milankovitch theory was largely ignored until the publication of a detailed analysis of climate change, based on 450,000 years of sedimentary records, showed a remarkable correspondence between paleo- climate cycles and the estimated climate influences of the orbital variations (Hays, Imbrie and Shackleton, 1976). Diagrams of the orbital cycles that can impact climate (Figures from Scott Rutherford, Roger Williams University) Element Period Explanation Elements of During periods of low orbital eccentricity the Orbital ~100,000 the earth’s winters tend to be warmer and the summers cooler. eccentricity years motion cited Ice accumulation is enhanced by cool summers. by Seasonal contrast is correlated with axial tilt.