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Paleoclimatology

Paleoclimatology is the study of past climates throughout geological time and of the causes of their variations. Climate involves complex interrelationships between the atmosphere, the oceans (ocean-atmosphere interaction; ocean and sea), and the continents (particularly continental ice) over time scales from thousands to several hundred million years. Climatic change involves alterations in the long-term mean values, and variations about the means, of such atmospheric elements as temperature (paleotemperature), precipitation, and pressure. The magnitude of variance from the norm that constitutes a climatic change depends on the time scale being considered. Some scientists restrict the term climatic change to variations occurring over periods of tens of thousands to millions of years, such as the Quaternary glacial and interglacial events. In such a hierarchy of terminology, variations of longer duration may be designated climatic revolutions (such as cycles of orogeny) and those of shorter duration as fluctuations (such as changes that have occurred during the Recent Epoch, and sunspot cycles) and iterations (such as annual and quasibiennial cycles).

OVERVIEW

Viewed in the context of the past 600 million years of Earth history, the widespread glaciation of the Pleistocene Epoch is a rare event. During about 95 percent of geologic time, the Earth experienced warmer average temperatures than those prevailing today; equator-pole temperature gradients were more modest, and circulation was less vigorous. The Paleozoic Era, cold and glacial to begin with, increased in warmth until the Silurian and Devonian periods, and then cooled to the carboniferous-permian glaciation (Carboniferous Period; Permian Period). The Mesozoic Era was generally warm and equable but moderated somewhat during the early Cenozoic Era. The Late Tertiary Period experienced marked cooling, estimated at 5 to 10 C degrees (9 to 18 F degrees) or more on the average, presaging the Quaternary glaciations. Within the Quaternary Era at least four major glacial-interglacial cycles are recognized, with multiple minor fluctuations.

Variations within the Recent Epoch are recognized, but their number and nature are still debated. Some scientists have proposed a threefold division: Anathermal, an interval, directly following the Pleistocene Period, that experienced increasing warmth; Altithermal, between about 7,000 and 5,000 years ago, distinctly warmer than today and dry; and Medithermal, much like the present. Others have proposed a more complex sequence of episodes involving regional, rather than globally applicable, variations.

Even finer degrees of variation can be recognized from information recorded over the past 1,000 years. A relatively warm period--the Little Climatic Optimum--between about AD 900 and 1150 was superseded by about 250 years of mid-latitude cooling. A century or so of returned warmth then followed. Between about 1550 and 1850, general and progressive cooling occurred (during the Little Ice Age, or Neo-Borea). Data gathered during the period of instrumental records suggest Northern Hemisphere warming from about 1880 to 1940, and cooling since then.

CLIMATIC EVIDENCE AND ITS INTERPRETATION

The nature of the evidence is such that the farther into the past one looks, the less information is obtained. The geologic record consists of incomplete data accumulated and integrated over long periods of time. Interpretations, therefore, tend to be qualitative and of low resolution, and sometimes ambiguous. Closer to the present, much ephemeral (that is, short time-scale) evidence is still in existence, and it is possible to make out more detail. Thus, while an early Paleozoic glaciation might be identified and its extent roughly estimated, Cenozoic glacial-interglacial events can be clearly recognized and the character of fluctuations within them elucidated. The apparent increase in variability closer to the present is a result of the preservation of high-resolution data. Past geological periods and epochs probably experienced variable climates throughout their durations.

Evidence of Cold Climates

Most types of evidence of glaciation tend to be transitory on geologic time scales. Alpine landforms can disappear within the hundred million years or so of an erosion cycle. Drumlin fields, eskers, and moraines may be eroded away as well. Periglacial evidence of cold climate, such as patterned ground, permafrost, and solifluction deposits, may be even more impermanent. Only deposits that are incorporated into the stratigraphic record may persist through geologic time as markers of cold climates. The most common such deposit is tillite, or lithified glacial till, such as ground moraine. The interpretation of a deposit as tillite is not without ambiguities, but association with striated bedrock or ice-rafted erratics may assure the identification.

Evidence Of Warm Climates

Numerous deposits have been interpreted as indicative of warm paleoclimates, but not all are unequivocal. Coal deposits have been considered as representative of warm, humid forests, such as modern tropical mangrove swamps. Extensive mid-latitude peats and subbituminous deposits, however, suggest that coal measures can only be interpreted as indicative of a humid, equable, perhaps warm--but not necessarily tropical--climate. The world's major coal deposits date from the Late Paleozoic, during which time extensive glaciation also occurred.

Coal measures are often succeeded in the geologic column by red beds of Permian-Triassic age. About a dozen theories for the origin and environmental significance of the red sandstones and siltstones have been proposed. Red beds have been interpreted as representing conditions varying from hot, arid deserts to warm, humid deltas. The prevailing view seems to be that the hematitic red pigment developed in upland soils in warm, moist climates with seasonally distributed rainfall, somewhat analogous to the process by which laterites are now forming. Deposition as detrital sediment occurred in oxidizing environments in which the coloration of the soil was preserved. The stratigraphic relation to coal measures suggests that a change from reducing to oxidizing environments took place as swamps dried up.

Carbonate rocks formed from coral reefs represent warm tropical seas. Although shifts in position of coral deposits through time might best be interpreted in terms of continental drift, changes in the width of individual coral belts can indicate temperature changes in the waters that once surrounded them.

Evidence of Aridity

Evaporite sequences are certain indicators of arid climates in which evaporation exceeded precipitation. Lake brines tend to be too highly variable in physical and chemical properties to allow anything more than qualitative interpretations of their deposits. Marine evaporites, however, may serve as paleothermometers, based on geochemical studies of the sequence and intensity of precipitation of various salts as functions of temperature. Widespread, thick evaporite sequences, suggesting warmth and aridity, were deposited in the Early Cambrian, Late Silurian, Middle and Late Devonian, Permian, Late Triassic, Early Cretaceous, and Middle Tertiary periods. Aeolian sandstones, when they can be confidently identified, are certain indicators of arid conditions. Analysis of cross-bedding structures can yield evidence of paleowind directions.

Use of Fossils

The use of fossils of flora and fauna to determine paleoclimates requires the assumption of uniformitarianism. Given the facts of evolution and extinction, however, "no-analog" situations often arise in which no modern counterpart exists to suggest environmental relationships for a given fossil. Large ectothermic reptiles, for example, are often cited as evidence for elevated temperatures during the Mesozoic. (The recent theory of warm-blooded dinosaurs, however, may modify that interpretation.) Where generic continuity to the present exists, such as is the case with the Tertiary, interpretations are more certain.

Close to the present, several techniques allow high-resolution quantitative estimates of paleoclimate. Estimation of a variety of paleoclimatic parameters is achieved with multivariate statistical-transfer functions, whereby the relationship between modern faunal and floral assemblages and their environments is used to interpret fossil assemblages. The technique has proved successful in studies of oceanic foraminifera and of terrestrial pollen (pollen stratigraphy), and it provides resolution of a few years to a century or so for the past few thousand to few hundred thousand years. For example, revegetation of and vegetation change in eastern North America can be mapped from pollen studies, and climatic changes can in turn be estimated. Similar techniques also permit the analysis of annual tree-ring series (dendrochronology). A year-by-year record for nearly the entire Recent Epoch now exists for bristlecone pine (Pinus longaeva) in the White Mountains of California. Among the parameters estimated in addition to temperature and precipitation are DROUGHT indices and pressure fields.

Isotope Chemistry

The fractionation in oceans of heavy and light isotopes of oxygen in water, carbon dioxide, and calcium carbonate is temperature- and salinity-dependent. Measurement of the isotope ratios in the carbonate tests of fossil foraminifera provides a measure of sea temperatures and of ocean volumes, both of which correspond to changes in ice-sheet and glacier volumes. Isotope ratios in glacial ice provide a measure of average air temperatures. Continuous records have now been constructed for nearly the entire Quaternary Period.

Historic Data

The few centuries of the historic period provide a wealth of qualitative and quantitative data. For the period prior to the keeping of instrumental records of meteorological parameters, indices of climatic severity or temperance have been constructed from such social records as private diaries, tithe and tax accounts, harvest records, cherry-blossom festival dates, travelers' notes, and freeze-thaw dates for lakes and rivers. When such qualitative data are supplemented by quantitative data, some calibration is possible.

Although the first meteorological instruments were invented in the 17th century, networks of stations were only widely established in the middle to late 19th century. Meteorological records provide very high-resolution data, but only for short periods of time. The detail is not always transferable to longer and older geological time scales, because of the generalizations that must be made.

MECHANISMS OF CLIMATIC CHANGE

Many mechanisms have been proposed for climatic change. The theory of continental drift removes many previously intractable problems. For example, a major principle of paleoclimatology is that the poles have never been very warm. continental drift can explain coal deposits in Antarctica that are synchronous with glaciation of lands that are now near the equator, without recourse to improbable climatic constructs.

Most theories of climatic change aim to explain the phenomenon of glaciation. Most promising of the many theories seem to be those of orogeny and of continental masses in polar positions (see paleogeography). Mountains perturb the mid-latitude atmospheric circulation and are associated in geologic time with steepened hemispheric temperature gradients. Continents around or at a pole can isolate the pole from heat flux from neighboring regions and can collect sufficient snow and ice to form glaciers. Long-period variations in the parameters of the Earth's orbit (eccentricity, obliquity, and precession) can alter the seasonal values of solar insolation and may act as pacemakers of glacial events when the requisite geography is present.

Concern over possible human alterations of climate arises from considerations of atmospheric transparency. The addition of carbon dioxide, a product of fossil-fuel combustion, can increase the amount of terrestrial radiation trapped by the atmosphere, thereby heightening the greenhouse effect. On the other hand, the addition to the atmosphere of particulates generated by agriculture and industry may increase the amount of solar radiation reflected back into space, thus imparting a cooling effect. Analysis of the recent meteorological records suggests possible support for both theories. The evidence available is inconclusive, and trends cannot be predicted with confidence, but the sensitivity of mechanisms involved, as well as humanity's great susceptibility to minor climatic fluctuations, gives cause for cautious concern.

Paul A. Kay

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