Land that is now free from permanent ice was glaciated for rather more than half of the Pleistocene.
It is reasonable to suppose that the global climate now fluctuates between glacial and interglacial conditions and that the present interglacial may be near its end, unless the subsequent cooling is overridden by warming induced by the greenhouse effect. The alternation between glacial and interglacial is the most extreme climate change imaginable, but the historical evidence suggests that living organisms adapt to it fairly robustly. As conditions deteriorate enough of them migrate to more favourable environments for their species to survive, from which they return when opportunity allows. In certain local areas, called refugia, communities survive even without migrating, because the older conditions prevail. There are several Pleistocene refugia in Britain, Upper Teesdale being probably the best-known example. Although undoubtedly inconvenient for humans, rapid climate change does not necessarily imply the extinction of species, merely their absence until the next change allows them to return.
fossil assemblages can be used to identify particular strata wherever those strata are found. In other words, animal species have come into existence, lived for a while, and then have disappeared and their places have been taken by others, newly arrived: we would say evolved. From this it is possible to construct a ‘stratigraphic column’, a vertical section through sedimentary rocks in which each stratum is shown in chronological order. The Cuvier and Brogniart study inspired geologists all over Europe to apply the method to their own localities and eventually to divide geologic time into distinct episodes on the basis of the animals associated with them. Though much amended, the geologic time-scale used today is derived from this work, as are most of the names.
Geologic time is divided into eons, eras, suberas, periods, and epochs. Table 2.4 shows the present arrangement, starting with the oldest, although some of the dates are revised from time to time (Ma means millions of years ago). Priscoan, Archaean, and Proterozoic together comprise the eon formerly known as the Precambrian. The term ‘Precambrian’ is still widely used, but not in a formal sense (all it means, after all, is ‘before the Cambrian’).
The order in which historical episodes should be arranged having been established, the next step is to allot dates to them. The thickness of strata is no help with this. Sedimentation is an Table 2.4 Geologic time-scale
irregu-lar process, so a thick layer may have accumulated rapidly, a thin one more slowly, and there is no way to tell. Some sediments, however, build more regularly, and it was the record they left that allowed the retreat of Scandinavian ice sheets, starting about 10000 years ago, to be traced. Each spring, as the ice melts, an assortment of mineral particles is washed into a lake by the meltwater. Heavier particles, such as sand grains, settle quickly. Later in the year, as water freezes again, the supply to the lake ceases and the finer particles, of silt and clay, gradually settle on top of the sandy layer. Year after year the process is repeated, each pair of layers, one pale and coarse, one dark and fine, being known as a ‘varve’. These can be counted, each varve representing one year, and if varves are forming at the edge of a retreating glacier they will follow it, so that its progress can be traced and dated. The study of varves is known as varve analysis, varve chronology, or a varve count.
Varves resemble tree rings, which provide another method of measuring time. In spring, woody plants grow rapidly by producing large, thin-walled cells in the xylem, just below the bark of stems and branches. Growth slows in summer, ceasing in late summer, and consists of smaller cells with thicker walls. The large cells of spring are pale in colour, the smaller ones of summer dark, and so each year the plant produces a ring of pale wood separated by a thin, dark ring from the pale wood of the following year. A count of the rings is a count of years, but there are some risks. If conditions are very severe, a plant may produce no growth for a whole year, and if conditions are unusually favourable it may produce two or more sets of rings. For this reason, tree-ring dating (called dendrochronology) must be based on as many specimens as is practical, obtained from widely scattered locations. The fact that rings are strongly affected by growing conditions has advantages. The width of rings can be used to infer weather (dendroclimatology) and environmental (dendroecology) conditions at the time they formed.
Obviously, the study of tree rings can provide dates only up to the age of the living plant from which they are taken, but trees can live a surprisingly long time. There are bristlecone pines (Pinus longaeva), found in California, more than 4600 years old, and correlating rings from them (taken as cores, without destroying the tree) with rings from dead pines has allowed scientists to construct a chronology for arid zones going back 8600 years and, at the upper tree limit on mountains, one going back 5500 years.
These chronologies are used to calibrate radiocarbon (14C) dates. Bombardment by cosmic radiation generates neutrons, a few of which collide with atoms of nitrogen (14N), displacing a proton and converting the 14N to 14C. Chemically, 14C behaves just like ordinary 12C and living organisms exchange both with their surroundings. When they die, however, carbon exchange ceases. Carbon-14 is radioactive, half of any amount of it decaying to 12C in 5730±30 years (its half-life), so the ratio of
12C:14C in dead organic matter is directly related to the time that has elapsed since it died. Radiocarbon dating rests, however, on the assumption that the rate of 14C formation in the atmosphere is constant.
This is now known not to be so, because the intensity of cosmic-ray bombardment is variable, but, when correlated with tree-ring series from bristlecone pine, radiocarbon analysis makes it possible to date material up to about 70000 years old.
Dating material older than this requires other methods. These, too, are based on the decay of radioactive elements, but ones with much longer half-lives. The first to be exploited were uranium (U) and thorium (Th). Uranium occurs naturally as a mixture of two isotopes, 238U and 235U in the constant proportions 137.7:1; both decay to stable isotopes of lead (Pb). Uranium-238, with a half-life of 4510 million years, decays to 206Pb, and 235U, with a half-life of 713 million years, to 207Pb. Thorium-232, with a half-life of 13900 million years, decays to 208Pb. Lead also occurs naturally as the stable isotope 204Pb, so this must be deducted from lead isotopes resulting from radioactive decay before an age can be calculated.
Potassium-40, a radioactive isotope of potassium with a half-life of 1300 million years, also occurs naturally (and, because of its presence in our food, is the principal source of our own exposure to radiation). Most 40K decays to 40Ca, which cannot be used because calcium is so common, but about 11 per cent decays by a different route to 40Ar (argon). This decay is used to date rocks more than 250000 years old.
A radioactive isotope of rubidium, 87Rb, decays in a single step to strontium (87Sr) and this decay is used to date certain rocks, especially those containing mica and potassium, but there is some doubt about the half-life of 87Rb. Two values are used: 4.88×1010 and 5.0×1010 years. A more recent method uses the decay of samarium (147Sm) to neodymium (143Nd). Samarium-147 has a half-life of 2.5×1011 years and this decay is used in studies of the formation of rocks in the Earth’s crust and mantle (and can also be used on materials of extraterrestrial origin).
It is impossible to predict when an individual unstable atom will decay, but it is possible to calculate the probability that the atom will decay within a certain period. This is called the ‘decay constant’
for the isotope, from which the half-life can be calculated as the time taken for the decay of half the unstable atoms present. The process is exponential: half the atoms decay in the first half-life period, half the remainder in the second period, half of that remainder in the third, and so on (e.g. 100; 50;
25; 12.5, etc.). Most of the decays used are based on half-lives much longer than the age of the Earth, but it is not necessary to wait until a complete half-life has elapsed before calculating an age. What matters is the ratio of isotopes.
Since radioactive decay involves only the nucleus of the atom, its rate is not affected by temperature, pressure, or any other outside influence. This makes it a very reliable measure of the age of materials.
Radiometric dating has allowed scientists to reconstruct the history of the Earth in some detail.