Paleoclimatology(redirected from History of climate)
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a branch of science dealing with the climates of past ages and with the earth’s climatic history. Ancient climates are reconstructed indirectly, on the basis of such evidence as the fossilized remains of organisms and the composition and texture of sedimentary rocks. By re-creating the climates of the past, paleoclimatology becomes an important part of paleogeography. It is also closely related to stratigraphy, paleontology, geomorphology, and the theory of useful minerals. Information obtained from geological findings is analyzed and generalized on the basis of the theoretical principles of climatology, meteorology, geography, geophysics, and astronomy.
The first attempt at paleoclimatic interpretation of fossilized organic remains was made by the English physicist and mathematician R. Hooke, who in 1686 determined that the earth’s climate had been warmer at some earlier time. He explained that this was due to a change in the position of the earth’s axis. The discovery of traces of Quaternary glaciation in Europe provided impetus for the development of paleoclimatology; these traces became the primary object of study. Scientific paleoclimatology began only in the 1880’s, when investigators first used lithologi-cal findings, in addition to paleontological data, as indicators of ancient climates. Lithological data provide valuable climatic indicators: salt indicates an arid climate; bauxites and bean ore, an alternation of humid and dry warm climates; peat, anthracite, and kaolin, a humid climate; limestone, a warm climate; and glacial moraines, a cold climate.
In the 20th century, numerous monographs on the history of ancient climates have been published in which the development of climate is considered dependent on some single factor (the French scientist E. Dacqué, 1915; the Germans W. Köppen and A. Wegener, 1924; the American C. Brooks, 1926; the German M. Schwarzbach, 1950). For example, Brooks maintained that the change in climate is due to paleogeographic conditions, whereas Köppen and Wegener attributed the change to shifts in the poles and continental drift.
Virtually all methods of paleoclimatology rely on a study of different effects of climate (lithological, paleontological). In the mid-20th century, the use of various geochemical and geophysical research methods became widespread. The temperature of the waters of ancient sea basins is estimated by means of the quantitative ratios of isotopes of oxygen l8O and 16O in the calcite of fossilized invertebrate shells (belemnites, pelecypods) and the ratios of Ca to Mg and of Ca to Sr in carbonate sediments and the skeletons of fossil organisms. Also important is the paleomagnetic method, which determines the position of ancient latitudes by using the residual magnetization of certain igneous and sedimentary rocks containing ferromagnetic minerals (magnetites, hematites, and titanomagnetite). This magnetization occurred under the influence of the magnetic field that existed at the time the rocks formed.
The three basic groups of indicators of ancient climate are the lithological, the paleobotanical, and the paleozoological.
Lithological indicators, which are found virtually everywhere, reflect the climatic conditions of the past through the nature and intensity of weathering, the degree of sedimentary differentiation, and the scale of autogenous mineral formation. In hot and humid climates the weathering of native rocks was intensive and occurred year-round. It consisted primarily of chemical changes in mineral composition. Typical of hot and humid climates are lithogenous (climatic) formations of sediments that are extremely variegated by composition, possess maximally expressed sedimentary differentiation, and contain many new mineral formations (pure quartz sands, kaolin clays, siliceous rocks, limestones, ferromagnesian sediments).
In temperate climates, where the processes of weathering were less intensive and occurred seasonally, sediments formed that were composed primarily of quartz-feldspar and graywacke sandstones with admixtures of hydromicaceous and montmoril-lonite clays. The rocks are distinguished by the least amount of weathering and a minimal degree of sedimentary differentiation. There are no carbonate sediments, and the amount of autogenous mineral formations is insignificant. Arid regions that in the past were in the tropical zone are characterized by formations of carbonate red beds (in continental basins of sedimentation), carbonate-sulfate formations (the zones of shallow seawaters and lagoons), and extracarbonate formations (under open sea conditions). Indicators of an arid climate include high carbonate and salt content in the sediments and widespread distribution of low-hydrate and completely waterless compounds (hematite, anhydrite, boehmite).
Paleobotanical indicators are fossilized remains of plants, which reflect the influence of climate, time, and locality. Such factors determine generic and specific composition, a plant’s ecological characteristics, life forms and their morphology, and the differentiation of ancient vegetation into zonal and provincial types. For example, a hot, humid climate is reconstructed according to the formation of tropical forests, whereas a hot, dry climate follows the distribution of the savanna formations and xerophyllic scrub growth. Deciduous forests indicate a temperate climate. The imprints of the annual growth rings of woody plants are also paleobotanical indicators. Such imprints are studied by dendroclimatologists.
Paleozoological indicators are the fossilized remains of ancient organisms, which reflect the climate of the time of their existence as part of communities in specific habitats. Beginning with the Carboniferous period, marine fauna was differentiated into the tropical and the boreal biogeographical belts, with a broad transitional zone between them. The weakly differentiated temperature conditions of the past were reflected in these belts. Periodic changes in the structure and position of the boundaries of the biogeographical belts attest to historical changes in climate.
Land vertebrates appeared in the Devonian. During subsequent periods, the generic composition of ecological types varied in accordance with the alternations of arid and humid climates on earth. The level of adaptation to the environment among Paleozoic and Mesozoic vertebrates was lower, thus explaining their limited ecological diversity. Cenozoic mammals were able to endure a broader range of climates and, thus, had a greater variety of habitats. These mammals made up the fauna complexes of the tropical forests and savannas and of the deciduous forests and steppes of temperate regions.
Reconstructions of past climates based on all three groups of indicators produce the most reliable results. This method involves compiling maps of natural zones of a particular time and provides not only qualitative characteristics of past climates (hot and humid, hot and dry) but also rough quantitative estimates of the temperature and precipitation for distinct natural zones. Interpretation of past climates is based on a comparison of past and present climatic types of weathering and sediment accumulation, as well as past and present ecological and thermal types of plant and animal life.
Ancient climates are known only in general outline and only beginning with the Paleozoic. We have only insignificant information about the climates of earlier times, in particular the Archean era, because these climates existed under conditions of a denser atmosphere that contained many vapors of water, CO2, H3CH4, and no oxygen. Land was nonexistent. The climate of the Early and Middle Paleozoic was isothermal. Latitudinal zonality in tropical and boreal (southern and northern) regions was established only in the second half of the Carboniferous. In the late Paleozoic, the Mesozoic, and the Paleogene the climate remained only slightly differentiated. The difference between winter temperatures in the high and low latitudes was not more than 12°-14°C. Until the end of the Paleogene, changes in climate were related primarily to fluctuations in humidity and manifested themselves in an alternation of arid and humid periods. The principal arid periods occurred in the Early Cambrian, the Late Ordovician, the Late Silurian, the Early Devonian, the Late Permian, a significant part of the Triassic, the Late Jurassic to the Early Cretaceous, the end of the Cretaceous to the first half of the Paleogene, and the Middle Miocene. The major humid periods were the Early Silurian, Early Carboniferous, Early Jurassic, and Late Oligocene.
The composition of the earth’s atmosphere changed in each geological period: the content of water vapors and CO2 decreased, and the role of oxygen increased. For this reason the “warming effect” diminished, and the temperature contrast between the poles and the equator grew stronger, promoting the development of the interlatitudinal atmospheric circulation.
A significant cooling began in the second half of the Oligocene, encompassing the high latitudes of both hemispheres and manifesting itself most strongly in the polar regions where the climates became first moderate and then arctic. With the passage of time, the continentality and seasonality of the climate intensified, the total amount of atmospheric precipitation decreased, and the distribution of precipitation became increasingly differentiated. The cooling became more intense in the Anthropogene. Numerous fluctuations in temperature and humidity led to an alternation of glacial and interglacial epochs in the high latitudes and pluvial and xerothermic climates in the low latitudes.
The changes in the earth’s ancient climates were caused by a number of extremely diverse factors. A number of astronomical hypotheses link climatic changes to fluctuations in the amount and composition of solar radiation and to changes in the earth’s orbit. Geological-geographical hypotheses consider the following factors to be primary: the inconstant composition of the atmosphere (cloudiness, CO2 content, presence of volcanic ash), the varied nature of the earth’s surface (distribution of land and water, elevation of land above sea level, mountains), the salinity of the ocean, the shifting of the poles, and continental drift. Present-day geological evidence indicates that no one of these hypotheses fully explains the causes of changes in the climates of the past.
Paleoclimatology broadens our knowledge of the past processes of weathering and sediment accumulation and the formation of useful mineral deposits associated with them. It shows the conditions of existence of vegetation and the animal world in past geological periods and is useful in forecasting future climatic changes.
REFERENCESBrooks, C. Klimaty proshlogo. Moscow, 1952. (Translated from English.)
Sinitsyn, V. M. Drevnie klimaty Ewazii, parts 1–3. Leningrad, 1965–70.
Sinitsyn, V. M. Vvedenie v paleoklimatologiiu. Leningrad, 1967.
Strakhov, N. M. Tipy litogeneza i ikh evoliutsiia v istorii Zemli. Moscow, 1963.
Problemy paleoklimatologii. Moscow, 1968. (Translated from English.)
Schwarzbach, M. Das Klima der Vorzeit, 2nd ed. Stuttgart, 1961.
Bowen, R. Paleotemperature Analysis. Amsterdam-London-New York, 1966.
V. M. SINITSYN