QUATERNARY PERIOD

Table 4: Geologic time scale. To see more information about a period, select one from the chart. interval of geologic time, the youngest of the 11 periods in the Earth's history. The Quaternary is both the shortest and most recent period. It is the second period of the Cenozoic Era, following the Tertiary Period, and began about 1.6 million years ago (see Table). The Quaternary is subdivided into two epochs, the Pleistocene (1,600,000 to 10,000 years ago) and the Holocene (10,000 years ago to the present). The Pleistocene Epoch thus comprises almost all of Quaternary time. The term Quaternary originated early in the 19th century when it was applied to the youngest deposits in the Paris Basin in France. The designations Pleistocene and Holocene also date from the 19th century, when they were defined with respect to strata containing certain fossils or a certain percentage of fossils of plants and animals that were still living. Since their introduction, these terms have undergone a complex and confusing evolution with respect to their usage, and neither is used as originally defined. In 1948 a decision was made at the 18th International Geological Congress in London that the PliocenePleistocene boundary should be fixed in marine rocks exposed in the coastal areas of Calabria in southern Italy. As ratified by the International Commission on Stratigraphy in 1985, the type section for the PliocenePleistocene boundary occurs in a sequence of marine strata at Vrica in Calabria. A type section for the PleistoceneHolocene boundary has yet to be agreed upon, but most investigators concur that it should be placed about 10,000 years ago. The Quaternary is best characterized as a time of many cycles of climatic change. Some of these cycles resulted in the episodes of extensive glaciation of the Earth for which the Quaternary is well known. Although earlier studies suggested that there were four major glaciations, it is now recognized that many more occurred and that the climate of the Quaternary has alternated between periods of extensive ice buildup on land and periods of warmer climate, like today, when only about 10 percent of the land area is covered by glacial ice. The low-latitude regions of the Earth became increasingly arid during the Quaternary, and they were subject to fluctuating arid and more humid conditions. The climatic cycles and their resultant glaciations and periods of aridity had a dramatic effect on geologic processes, sedimentological regimes, the morphology of the terrestrial surface, and the fauna and flora on land and in the oceans. In addition, it was during the Quaternary that much of human evolution occurred, and these climatic cycles must have exerted a strong impact on the activities and distribution of early humans. Understanding the Quaternary and its environments and climatic conditions is particularly important for interpreting past geologic time and for considering the future. According to the premise underlying the principles of uniformitarianism, studies of modern natural processes are the basis on which the past geologic record can be interpreted. Thus, studies of modern Holocene environments and processes provide the data base upon which inferences can be made on the origin and environments of older rocks, structures, landforms, and other Earth features. Of equal and probably greater importance are concerns regarding the current situation and the future. Present-day climatic conditions, ocean and continent configurations, and environments are similar to conditions that occurred during past Quaternary climatic cycles. Therefore, an understanding of the characteristics of these cycles and their cause provides a basis for predicting future climatic change and the environmental consequences of such change. The significance of the Quaternary as a period of time was recognized more than a century ago. Grove K. Gilbert, an early prominent geologist of the United States Geological Survey, wrote in 1890: When the work of the geologist is finished and his final comprehensive report written, the longest and most important chapter will be upon the latest and shortest of the geologic periods. W. Hilton Johnson interval of geologic time, the youngest of the 11 periods in Earth history. The Quaternary follows the Tertiary Period and is part of the Cenozoic Era. Its boundaries have been defined as 1,600,000 years ago to the present day. It is subdivided into the Pleistocene Epoch (q.v.; 1,600,000 to 10,000 years ago) and the Holocene Epoch (q.v.; 10,000 years ago to the present). The Quaternary is characterized by major cyclical changes of climate on a global scale. Because these led to repeated invasions of vast areas of mid-latitude North America and northwestern Eurasia by ice sheets, the period is frequently referred to as the Great Ice Age. The onset of this ice age was marked by a major climatic cooling, which is now placed within the preceding Pliocene Epoch of the Tertiary Period at about 2,500,000 years ago. In the Quaternary Period world sea levels fell more than 100 m (330 feet) during at least one episode of glacial expansion. There was also a sequence of major oscillations between warmer and cooler climatic conditions, occurring during the last 1,700,000 years, that affected the whole planet. On land these oscillations corresponded with cycles of alternating interglacial and glacial stages recorded from continental geologic sequences in middle latitudes. Interglacial stages of these cycles were relatively short (10,000 to 30,000 years) and were characterized by temperate climates and vegetation conditions similar to those of today. The Holocene is regarded as the latest interglacial stage of the Quaternary Period. During the much longer glacial stages (70,000 to 80,000 years), large areas of North America and Eurasia, as well as southern South America and New Zealand, were affected either by massive advances of glaciers and ice sheets or by dry but cold periglacial climates similar to those found in parts of Arctic Canada and Siberia today. As a result of these climatic changes, there were large and repeated shifts of vegetation belts and their associated faunas, particularly in the Northern Hemisphere. The most recent episode of ice expansion (about 30,000 to 10,000 years ago) is well marked in both North America and northern Europe. A succession of older, sometimes more extensive ice advances, occurring in Europe as far back as 500,000 years ago and in North America before 700,000 years ago, has been recognized. The most important climatic and environmental changes in the tropics and subtropics during the Quaternary Period were associated with changes in rainfall. Intervals of wet, pluvial climate alternating with drier interpluvial conditions were accompanied by shifts in the boundaries between tropical forest, savanna, and more arid vegetation zones. Changes in rainfall were clearly recorded by the rise and fall of lake levels during the Quaternary, not only in the tropics but in other areas such as Australia, Central Asia, and the Great Basin of the western United States. These changes in rainfall regime, however, were not apparently synchronous within the glacial-interglacial cycles in the middle latitudes. The underlying causes of Quaternary climatic changes were complex and are still much disputed. The effects of continental drift on ocean and atmospheric currents, the influence of variations in solar radiation, and the effects of volcanic eruptions probably all played major or minor roles, but the periodicity of the major oscillations that are recorded in the deep-sea sediments does seem to be closely related to long-term geometric variations in the Earth's orbit around the Sun. Biologically, the major feature of the Quaternary Period has been the evolution and dispersion of humans. Indeed, Russian scientists refer to the Quaternary as the Anthropogene. During the early and middle Pleistocene, early hominids (Homo erectus) spread from Africa to Asia and Europe. Modern humans (Homo sapiens) reached the Americas and Australia during the last glacial stage. The drastic changes of climate and environment in the Quaternary Period led to rapid rates of evolution and extinction, particularly among the mammals. The extinction of large mammals in North and South America and in Eurasia toward the end of the last glacial stage may also be related to the rapid territorial expansion of humans at that time. Additional reading General summaries of the physical and biological record of the Pleistocene Epoch are found in Richard F. Flint, Glacial and Quaternary Geology (1971); Tage Nilsson, The Pleistocene: Geology and Life in the Quaternary Ice Age (1983); and V. ibrava, D.Q. Bowen, and G.M. Richmond (eds.), Quaternary Glaciations in the Northern Hemisphere (1986). Regional surveys include W.F. Ruddiman and H.E. Wright, Jr. (eds.), North America and Adjacent Oceans During the Last Deglaciation (1987); Stephen C. Porter and H.E. Wright, Jr. (eds.), Late-Quaternary Environments of the United States, 2 vol. (1983); N.J. Shackleton, R.G. West, and D.Q. Bowen (eds.), The Past Three Million Years: Evolution of Climatic Variability in the North Atlantic Region (1988); and A.A. Velichko, H.E. Wright, Jr., and C.W. Barnosky (eds.), Late Quaternary Environments of the Soviet Union, trans. from Russian (1984). R.S. Bradley, Quaternary Paleoclimatology: Methods of Paleoclimatic Reconstruction (1985), is a well-written reference source. A historical account of the development of the glacial theory, the Quaternary climatic record, and hypotheses of climatic change is given in John Imbrie and Katherine Palmer Imbrie, Ice Ages: Solving the Mystery (1979, reissued 1986). Megafaunal extinctions at the end of the Pleistocene are explored in Paul S. Martin and Richard G. Klein (eds.), Quaternary Extinctions: A Prehistoric Revolution (1984). The biological record is emphasized in Antony J. Sutcliffe, On the Track of Ice Age Mammals (1985); and Michael O. Woodburne (ed.), Cenozoic Mammals of North America: Geochronology and Biostratigraphy (1987). W. Hilton JohnsonA chronological survey of the Holocene Epoch is provided by Ernst Antevs, Geologic-Climatic Dating in the West, American Antiquity, 20(4):317335 (April 1955). O.K. Davis et al., The Pleistocene Dung Blanket of Bechan Cave, Utah, in H.H. Genoways and M.R. Dawson (eds.), Contributions in Quaternary Vertebrate Paleontology (1984), pp. 267282; Russell W. Graham, Holmes A. Semken, Jr., and Mary Ann Graham (eds.), Late Quaternary Mammalian Biogeography and Environments of the Great Plains and Prairies (1987); and J.I. Mead et al., Dung of Mammuthus in the Arid Southwest, North America, Quaternary Research 25(1):121127 (1986), are paleoecological and paleontological studies. Human ecology is the subject of Neil Roberts, The Holocene: An Environmental History (1989); and Ian Tattersall, Eric Delson, and John Van Couvering (eds.), Encyclopedia of Human Evolution and Prehistory (1988). W. Hilton Johnson