GEOCHRONOLOGY

scientific study concerned with the sequence of geologic events in the history of the Earth. Geologic history provides a conceptual framework and overview of the evolution of the Earth. An early development of the subject (initiated in the early 1800s) was stratigraphy, the systematic study of order and sequence, including faunal succession, in bedded sedimentary rocks. Applying the general principle of stratigraphy that different sedimentary strata contain different and distinctive fossils, investigators of the 19th and early 20th centuries were able to correlate strata with similar fossils over large areas. More recently, specialists in biostratigraphy have employed fossil remains to characterize successive intervals of geologic time, but only as relatively precise time markers to about 570 million years ago. Since the mid-20th century, radiometric dating has provided absolute age data to supplement the relative dates obtained from the fossil record and made it possible to quantify the geologic time scale as far back as the oldest rocks more than 3.9 billion years ago. Radiometric-dating techniques are based on the principle that radioactive isotopes in geologic material decay at constant, known rates to daughter isotopes. Using such techniques, investigators have been able to calculate the ages for various rock-forming minerals and thereby date the rocks and strata that contain such minerals. These dates have furnished an absolute age framework for the stratigraphic time scale that had been developed by matching the evolutionary changes exhibited by fossil remains in successive rock beds. field of scientific investigation concerned with determining the age and history of the Earth's rocks and rock assemblages. Such time determinations are made and the record of past geologic events is deciphered by studying the distribution and succession of rock strata, as well as the character of the fossil organisms preserved within the strata. The Earth's surface is a complex mosaic of exposures of different rock types that are assembled in an astonishing array of geometries and sequences. Individual rocks in the myriad of rock outcroppings (or in some instances shallow subsurface occurrences) contain certain materials or mineralogic information that can provide insight as to their age. For years investigators determined the relative ages of sedimentary rock strata on the basis of their positions in an outcrop and their fossil content. According to a long-standing principle of the geosciences, that of superposition, the oldest layer within a sequence of strata is at the base and the layers are progressively younger with ascending order. The relative ages of the rock strata deduced in this manner can be corroborated and at times refined by the examination of the fossil forms present. The tracing and matching of the fossil content of separate rock outcrops (i.e., correlation) eventually enabled investigators to integrate rock sequences in many areas of the world and construct a relative geologic time scale. Scientific knowledge of the Earth's geologic history has advanced significantly since the development of radiometric dating, a method of age determination based on the principle that radioactive atoms in geologic materials decay at constant, known rates to daughter atoms. Radiometric dating has provided not only a means of numerically quantifying geologic time but also a tool for determining the age of various rocks that predate the appearance of life-forms. Additional reading Overviews are presented in Claude C. Albritton, Jr., The Abyss of Time, Changing Conceptions of the Earth's Antiquity After the Sixteenth Century (1980); Don L. Eicher, Geologic Time, 2nd ed. (1976); William B.N. Berry, Growth of a Prehistoric Time Scale: Based on Organic Evolution, rev. ed. (1987); Henry Faul and Carol Faul, It Began with a Stone: A History of Geology from the Stone Age to the Age of Plate Tectonics (1983); Robert H. Dott, Jr., and Roger Lyman Batten, Evolution of the Earth, 4th ed. (1988); and Reed Wicander and James S. Monroe, Historical Geology: Evolution of the Earth and Life Through Time (1989). A. Hallam, Great Geological Controversies, 2nd ed. (1989), traces the development of the history of geology and of various, often contradictory, concepts. For the early recognition of the geologic cycle and the promulgation of uniformitarianism, see the classics themselves: James Hutton, Theory of the Earth, With Proofs and Illustrations, 2 vol. (1795, reissued 1972); John Playfair, Illustrations of the Huttonian Theory of the Earth (1802, reprinted 1964); and Charles Lyell, Principles of Geology, 3 vol. (183033), available also in many later editions. Relevant developments in modern geologic sciences are discussed in Donald R. Prothero, Interpreting the Stratigraphic Record (1989); Ruth E. Moore, Man, Time, and Fossils: The Story of Evolution, 2nd rev. ed. (1961); Martin J.S. Rudwick, The Meaning of Fossils: Episodes in the History of Palaeontology, 2nd rev. ed. (1976, reprinted 1985); W. Lee Stokes, Essentials of Earth History: An Introduction to Historical Geology, 4th ed. (1982); Don L. Eicher and A. Lee McAlester, History of the Earth (1980); and Don L. Eicher, A. Lee McAlester, and Marcia L. Rottman, The History of the Earth's Crust (1984). Gary Dean Johnson Completion of the Phanerozoic time scale With the development of the basic principles of faunal succession and correlation and the recognition of facies variability, it was a relatively short step before large areas of Europe began to be placed in the context of a global geologic succession. This was not, however, accomplished in a systematic manner. Whereas the historical ideas of Lehmann and Arduino were generally accepted, it became increasingly clear that many diverse locally defined rock successions existed, each with its own unique fauna and apparent position within some sort of universal succession. As discussed above, Arduino's Tertiary was recognized in certain areas and was in fairly common use after 1760, but only rudimentary knowledge of other rock successions existed by the later part of the 18th century. The German naturalist Alexander von Humboldt had recognized the widespread occurrence of fossil-bearing limestones throughout Europe. Particular to these limestones, which formed large tracts of the Jura Mountains of Switzerland, were certain fossils that closely resembled those known from the Lias and Oolite formations of England, which were then being described by William Smith. Subsequently, Humboldt's Jura Kalkstein succession, as he described it in 1795, came to be recognized throughout Europe and England. By 1839, when the geologist Leopold Buch recognized this rock sequence in southern Germany, the conceptual development of the Jurassic System was complete. The coal-bearing strata of England, known as the Coal Measures, had been exploited for centuries, and their distribution and vertical and lateral variability were the subject of numerous local studies throughout the 17th and early 18th centuries, including those of Smith. In 1808 the geologist Jean-Baptiste-Julien d'Omalius d'Halloy described a coal-bearing sequence in Belgium as belonging to the Terrain Bituminifre. Although the name did not remain in common usage for long, the Terrain Bituminifre found analogous application in the work of two English geologists, William D. Conybeare and William Phillips, in their synthesis of the geology of England and Wales in 1822. Conybeare and Phillips coined the term Carboniferous (or coal-bearing) to apply to the succession of rocks from north-central England that contained the Coal Measures. The unit also included several underlying rock formations extending down into what investigators now consider part of the underlying Devonian System. At the time, however, the approach by Conybeare and Phillips was to encompass in their definition of the Carboniferous all of the associated strata that could be reasonably included in the Coal Measures succession. D'Omalius mapped and described a local succession in western France. While doing so, he began to recognize a common sequence of soft limestones, greensands (glauconite-bearing sandstones), and related marls in what is today known to be a widespread distribution along coastal regions bordering the North Sea and certain regions of the Baltic. The dominant lithology of this sequence is frequently the soft limestones or chalk beds so well known from the Dover region of southeast England and Calais in nearby France. D'Omalius called this marl, greensand, and chalk-bearing interval the Terrain Crtac. Along with their adoption of the term Carboniferous in 1822, Conybeare and Phillips referred to the French Terrain Crtac as the Cretaceous System. Clearly, surficial deposits and related unconsolidated material, variously relegated to the categories of classification proposed by Arduino, Lehmann, Werner, and others as alluvium or related formations, deserved a place in any formalized system of rock succession. In 1829 Jules Desnoyers of France, studying sediments in the Seine valley, proposed using the term Quaternary to encompass all of these various post-Tertiary formations. At nearly the same time, the important work of Lyell on the faunal succession of the Paris Basin permitted finer-scaled discrimination of this classic Tertiary sequence. In 1833 Lyell, using various biostratigraphic evidence, proposed several divisions of the Tertiary System that included the Eocene, Miocene, and Pliocene epochs. By 1839 he proposed using the term Pleistocene instead of dividing his Pliocene Epoch into older and newer phases. The temporal subdivision of the Tertiary was completed by two German scientists, Heinrich Ernst Beyrich and Wilhelm Philipp Schimper. Beyrich introduced the Oligocene in 1854 after having investigated outcrops in Belgium and Germany, while Schimper proposed adding the Paleocene in 1874 based on his studies of Paris Basin flora. Werner's quadripartite division of rocks in southern Germany was applied well into the second decade of the 19th century. During this time, rock sequences from the lower part of his third temporal subdivision, the Fltzgebirge, were subsequently subdivided into three formations, each having fairly widespread exposure and distribution. Based on his earlier work, Friedrich August von Alberti identified in 1834 these three distinct lithostratigraphic units, the Bunter Sandstone, the Muschelkalk Limestone, and the Keuper Marls and Clays, as constituting the Trias or Triassic System. Perhaps one of the most intriguing episodes in the development of the geologic time scale concerns the efforts of two British geologists and in large measure their attempts at unraveling the complex geologic history of Wales. Adam Sedgwick and Roderick Impey Murchison began working, in 1831, on the sequence of rocks lying beneath the Old Red Sandstone (which had been included in the basal sequence of the Carboniferous, as defined by Conybeare and Phillips, earlier in 1822). What started as an earnest collaborative attempt at deciphering the structurally and stratigraphically complicated rock succession in Wales ended in 1835 with a presentation outlining two distinct subdivisions of the pre-Carboniferous succession. Working up from the base of the post-Primary rock succession of poorly fossiliferous clastic rocks in northern Wales, Sedgwick identified a sequence of rock units defined primarily by their various lithologies. He designated this succession the Cambrian, after Cambria, the Roman name for Wales. Murchison worked downward in the considerably more fossiliferous pre-Old Red Sandstone rock sequence in southern Wales and was able to identify a succession of strata containing a well-preserved fossil fauna. These sequences defined from southern Wales were eventually brought into the context of Sedgwick's Cambrian. Murchison named his rock succession the Silurian, after the Roman name for an early Welsh tribe. In a relatively short time, Murchison's Silurian was expanding both laterally and temporally as more and more localities containing the characteristic Silurian fauna were recognized throughout Europe. The major problem created by this conceptual expansion of the Silurian was that it came to be recognized in northern Wales as coincident with much of the strata in the upper portion of Sedgwick's Cambrian. With Sedgwick's Cambrian based mainly on lithologic criteria, the presence of Silurian fauna created correlational difficulties. As it turned out, Sedgwick's Cambrian was of little value outside of its area of original definition. With it being superseded by the paleontologically based concept of the Silurian, some sort of compromise had to be worked out. This compromise came about primarily as a result of the work of Charles Lapworth, the English geologist who in 1879 proposed the designation Ordovician System for that sequence of rocks representing the upper part of Sedgwick's Cambrian succession and the lower (and generally overlapping) portion of Murchison's Silurian succession. The term Ordovician is derived from yet another Roman-named tribe of ancient Wales, the Ordovices. A large part of Lapworth's rationale for this division was based on the earlier work of the French-born geologist Joachim Barrande, who investigated the apparent Silurian fauna of central Bohemia. Barrande's 1851 treatise on this area of Czechoslovakia demonstrated a distinct succession from a second Silurian fauna to a third Silurian fauna. This divisible Silurian, as well as separate lines of evidence gathered by Lapworth in Scotland and Wales, finally enabled the individual character of the Cambrian, Ordovician, and Silurian systems to be resolved. While involved in their work on Welsh stratigraphic successions, Sedgwick and Murchison had the opportunity to compare some rock outcroppings in Devonshire, in southwest England, with similar rocks in Wales. The Devon rocks were originally thought to belong to part of Sedgwick's Cambrian System, but they contained plant fossils very similar to basal Carboniferous (Old Red Sandstone) plant fossils found elsewhere. Eventually recognizing that these fossil-bearing sequences represented lateral equivalents in time and perhaps temporally unique strata as well, Sedgwick and Murchison in 1839 proposed the Devonian System. During the early 1840s, Murchison traveled with the French paleontologist Edouard de Verneuil and the Latvian-born geologist Alexandr Keyserling to study the rock succession of the eastern Russian platform, the area of Russia west of the Ural Mountains. Near the town of Perm, Murchison and Verneuil identified fossiliferous strata containing both Carboniferous and a younger fauna at that time not recognized elsewhere in Europe or in the British Isles. Whereas the Carboniferous fossils were similar to those they had seen elsewhere (mainly from the Coal Measures), the stratigraphically higher fauna appeared somewhat transitional to the Triassic succession of Germany as then understood. Murchison coined the term Permian (after the town of Perm) to represent this intermediate succession. With continued refinement of the definition of the Carboniferous in Europe, particularly in England, what at one time comprised the Old Red Sandstone, Lower Coal Measures (Mountain Limestone and Millstone Grit), and Upper Coal Measures now stood as just the Lower and Upper Coal Measures. It was beginning to be recognized that certain rock sequences in the Catskill Mountains of eastern New York state in North America resembled the Old Red Sandstone of western England. Furthermore, coal-bearing strata exposed in Pennsylvania greatly resembled the similar coal-bearing strata of the Upper Coal Measures. Lying beneath these coal-bearing rocks of Pennsylvania was a sequence of limestones that could be traced over thousands of square kilometres and that occurred in numerous outcrops along various tributary streams to the Ohio and Mississippi rivers in Indiana, Kentucky, Missouri, Illinois, and Iowa. This subcarboniferous strata, identified by the American geologist David Dale Owen in 1839, was subsequently termed Mississippian in 1870 as a result of work conducted by another American geologist, Alexander Winchell, in the upper Mississippi valley area. Eventually the overlying strata, the coal-bearing rocks originally described from Pennsylvania, were formalized as Pennsylvanian in 1891 by the paleontologist and stratigrapher Henry Shaler Williams. The North American-defined Mississippian and Pennsylvanian systems were later correlated with presumed European and British successions. Although approximately similar in successional relationship, the MississippianPennsylvanian boundary in North America is now considered slightly younger than the LowerUpper Carboniferous boundary in Europe. By the 1850s, with the development of the geologic time scale nearly complete, investigators were beginning to recognize that a number of major paleontologically defined boundaries were common and recurrent regardless of where a succession was studied. By this time rock successions were being defined according to fauna they contained, and the relative time scale, which was being erected, was based on the principle of faunal succession; consequently, any major hiatus or change in faunal character was bound to be interpreted as important. In 1838 Sedgwick proposed that all pre-Old Red Sandstone sediments be included in the rock succession designated the Paleozoic Series (or Era) that contained generally primitive fossil fauna. John Phillips, another English geologist, went on to describe the Mesozoic Era to accommodate what then was the Cretaceous, Jurassic, Triassic, and partially Permian strata, and the Kainozoic (Cainozoic, or Cenozoic) era to include Lyell's Eocene, Miocene, and Pliocene. This subdivision of the generally fossiliferous strata that lay superpositionally above the so-called Primary rocks of many of the early workers resulted in the recognition of three distinct eras. Subsequent subdivision of these eras into specific geologic periods finally provided the hierarchy for describing the relative dating of geologic events. Development of radioactive dating methods and their application As has been seen, the geologic time scale is based on stratified rock assemblages that contain a fossil record. For the most part, these fossils allow various forms of information from the rock succession to be viewed in terms of their relative position in the sequence. Approximately the first 87 percent of Earth history occurred before the evolutionary development of shell-bearing organisms. The result of this mineralogic control on the preservability of organic remains in the rock record is that the geologic time scaleessentially a measure of biologic changes through timetakes in only the last 13 percent of Earth history. Although the span of time preceding the Cambrian periodthe Precambrianis nearly devoid of characteristic fossil remains and coincides with some of the primary rocks of certain early workers, it must, nevertheless, be evaluated in its temporal context. Early attempts at calculating the age of the Earth Historically, the subdivision of Precambrian rock sequences (and, therefore, Precambrian time) had been accomplished on the basis of structural or lithologic grounds. With only minor indications of fossil occurrence (mainly in the form of algal stromatolites), no effective method of quantifying this loosely constructed chronology existed until the discovery of radioactivity enabled dating procedures to be applied directly to the rocks in question. The quantification of geologic time remained an elusive matter for most human enquiry into the age of the Earth and its complex physical and biological history. Although Hindu teachings accept a very ancient origin for the Earth, medieval Western concepts of Earth history were based for the most part on a literal interpretation of Old Testament references. Biblical scholars of Renaissance Europe and later considered paternity as a viable method by which the age of the Earth since its creation could be determined. A number of attempts at using the begat method of determining the antiquity of an eventessentially counting backward in time through each documented human generationled to the age of the Earth being calculated at several thousand years. One such attempt was made by Archbishop James Ussher of Ireland, who in 1650 determined that the Creation had occurred during the evening of Oct. 22, 4004 BC. By his analysis of biblical genealogies, the Earth was not even 6,000 years old! From the time of Hutton's refinement of uniformitarianism, the principle found wide application in various attempts to calculate the age of the Earth. As previously noted, fundamental to the principle was the premise that various Earth processes of the past operated in much the same way as those processes operate today. The corollary to this was that the rates of the various ancient processes could be considered the same as those of the present day. Therefore, it should be possible to calculate the age of the Earth on the basis of the accumulated record of some process that has occurred at this determinable rate since the Creation. Many independent estimates of the age of the Earth have been proposed, each made using a different method of analysis. Some such estimates were based on assumptions concerning the rate at which dissolved salts or sediments are carried by rivers, supplied to the world's oceans, and allowed to accumulate over time. These chemical and physical arguments (or a combination of both) were all flawed to varying degrees because of an incomplete understanding of the processes involved. The notion that all of the salts dissolved in the oceans were the products of leaching from the land was first proposed by the English astronomer and mathematician Edmond Halley in 1691 and restated by the Irish geologist John Joly in 1899. It was assumed that the ocean was a closed system and that the salinity of the oceans was an ever-changing and ever-increasing condition. Based on these calculations, Joly proposed that the Earth had consolidated and that the oceans had been created between 80 and 90 million years ago. The subsequent recognition that the ocean is not closed and that a continual loss of salts occurs due to sedimentation in certain environments severely limited this novel approach. Equally novel but similarly flawed was the assumption that, if a cumulative measure of all rock successions were compiled and known rates of sediment accumulation were considered, the amount of time elapsed could be calculated. While representing a reasonable approach to the problem, this procedure did not or could not take into account different accumulation rates associated with different environments or the fact that there are many breaks in the stratigraphic record. Even observations made on faunal succession proved that gaps in the record do occur. How long were these gaps? Do they represent periods of nondeposition or periods of deposition followed by periods of erosion? Clearly sufficient variability in a given stratigraphic record exists such that it may be virtually impossible to even come to an approximate estimate of the Earth's age based on this technique. Nevertheless, many attempts using this approach were made. William Thomson (later Lord Kelvin) applied his thermodynamic principles to the problems of heat flow, and this had implications for predicting the age of a cooling Sun and of a cooling Earth. From an initial estimate of 100 million years for the development of a solid crust around a molten core proposed in 1862, Thomson subsequently revised his estimate of the age of the Earth downward. Using the same criteria, he concluded in 1899 that the Earth was between 20 and 40 million years old. Thomson's calculation was based on the assumption that the substance of the Earth is inert and thus incapable of producing new heat. His estimate came into question after the discovery of naturally occurring radioactivity by the French physicist Henri Becquerel in 1896 and the subsequent recognition by his colleagues, Marie and Pierre Curie, that compounds of radium (which occur in uranium minerals) produce heat. As a result of this and other findings, notably that of Ernest Rutherford (see below), it became apparent that naturally occurring radioactive elements in minerals common in the Earth's crust are sufficient to account for all observed heat flow. Within a short time another leading British physicist, John William Strutt, concluded that the production of heat in the Earth's interior was a dynamic process, one in which heat was continuously provided by such materials as uranium. The Earth was, in effect, not cooling. Nonradiometric dating In addition to radioactive decay, many other processes have been investigated for their potential usefulness in absolute dating. Unfortunately, they all occur at rates that lack the universal consistency of radioactive decay. Sometimes human observation can be maintained long enough to measure present rates of change, but it is not at all certain on a priori grounds whether such rates are representative of the past. This is where radioactive methods frequently supply information that may serve to calibrate nonradioactive processes so that they become useful chronometers. Nonradioactive absolute chronometers may conveniently be classified in terms of the broad areas in which changes occurnamely, geologic and biological processes, which will be treated here. Geologic processes as absolute chronometers Weathering processes During the first third of the 20th century, several presently obsolete weathering chronometers were explored. Most famous was the attempt to estimate the duration of Pleistocene interglacial intervals through depths of soil development. In the American Midwest, thicknesses of gumbotil and carbonate-leached zones were measured in the glacial deposits (tills) laid down during each of the four glacial stages. Based on a direct proportion between thickness and time, the three interglacial intervals were determined to be longer than postglacial time by factors of 3, 6, and 8. To convert these relative factors into absolute ages required an estimate in years of the length of postglacial time. When certain evidence suggested 25,000 years to be an appropriate figure, factors became yearsnamely, 75,000, 150,000, and 200,000 years. And, if glacial time and nonglacial time are assumed approximately equal, the Pleistocene Epoch lasted about 1,000,000 years. Only one weathering chronometer is employed widely at the present time. Its record of time is the thin hydration layer at the surface of obsidian artifacts. Although no hydration layer appears on artifacts of the more common flint and chalcedony, obsidian is sufficiently widespread that the method has broad application. In a specific environment the process of obsidian hydration is theoretically described by the equation D = Kt1/2, in which D is thickness of the hydration rim, K is a constant characteristic of the environment, and t is the time since the surface examined was freshly exposed. This relationship is confirmed both by laboratory experiments at 100 C (212 F) and by rim measurements on obsidian artifacts found in carbon-14 dated sequences. Practical experience indicates that the constant K is almost totally dependent on temperature and that humidity is apparently of no significance. Whether in a dry Egyptian tomb or buried in wet tropical soil, a piece of obsidian seemingly has a surface that is saturated with a molecular film of water. Consequently, the key to absolute dating of obsidian is to evaluate K for different temperatures. Ages follow from the above equation provided there is accurate knowledge of a sample's temperature history. Even without such knowledge, hydration rims are useful for relative dating within a region of uniform climate. Like most absolute chronometers, obsidian dating has its problems and limitations. Specimens that have been exposed to fire or to severe abrasion must be avoided. Furthermore, artifacts reused repeatedly do not give ages corresponding to the culture layer in which they were found but instead to an earlier time, when they were fashioned. Finally, there is the problem that layers may flake off beyond 40 micrometres (0.004 centimetre, or 0.002 inch) of thicknessi.e., more than 50,000 years in age. Measuring several slices from the same specimen is wise in this regard, and such a procedure is recommended regardless of age.