HOLOCENE EPOCH

Table 4: Geologic time scale. To see more information about a period, select one from the chart. formerly Recent Epoch, younger of the two epochs that comprise the Quaternary Period and the latest interval of geologic time, covering approximately the last 10,000 years of the Earth's history (see Table). The sediments of the Holocene, both continental and marine, cover the largest area of the globe of any epoch in the geologic record, but the Holocene is unique because it is coincident with the late and post-Stone Age history of mankind. The influence of humans is of world extent and is so profound that it seems appropriate to have a special geologic name for this time. formerly Recent Epoch younger of the two epochs that comprise the Quaternary Period, and the latest interval of the Earth's geologic history. The Holocene Epoch follows the Pleistocene Epoch, and it constitutes the last 10,000 years to the present. It follows the last glacial stage of the Pleistocene and is characterized by relatively warm climatic conditions. The Holocene is important in terms of human history; within a few thousand years after the Old Stone Age, man developed the skills that have led to the present-day level of civilization. The progress of deglaciation at the end of the last glacial stage was complex. A major climatic warming occurred about 13,000 years ago, and it can be detected in deep-ocean sediments and in the changes of fossils of flora and insect faunas in western Europe. This warming was discontinuous, and the final remnants of the vast North American ice sheet lingered in Labrador until less than 7,500 years ago. The date of the onset of the Holocene proved difficult to define on climatic grounds, but radiocarbon dating (a method of age determination based on the measurement of radioactive carbon decay) has now placed it at 10,000 radiocarbon years BP (before the present, the reference year being AD 1950). This date coincides closely with the end of the Younger Dryas interval of cooling in northwestern Europe and the final fragmentation of the dwindling Scandinavian ice sheet there. A direct result of the melting of the great ice sheets was a major rise of world sea levels during the early part of the Holocene. At 10,000 BP, ocean levels were still about 35 m (110 feet) below that of the present; they subsequently rose rapidly until about 6000 BP. Since that time the sea level has fluctuated within a few metres of its present level. Transgression of the sea over exposed continental shelves has had major paleogeographic effects. The land bridge from Asia to North America across the Bering Strait that had enabled early humans to migrate to the Americas was covered by water. So too were the connections between the British Isles and Europe, Japan and Siberia, Sri Lanka and India, and Tasmania and the Australian mainland. In areas that had actually been covered by thick ice sheets, the direct effects of ice melting on relative land and sea levels were even more complicated. Here the weight of the ice masses had depressed the underlying crust of the Earth, a process known as isostatic depression. Subsequent melting of the ice resulted in a release of pressure, followed by uplift of the crust, a process known as isostatic recovery. Heavily glaciated areas such as Scandinavia have undergone several hundred metres of isostatic uplift since the end of the last glaciation. The climatic development of the Holocene has been investigated chiefly through the technique of palynology (pollen analysis). Past vegetational conditions can be deduced from assemblages of fossil pollen and spores preserved in lake and bog sediments. Comparisons of these assemblages from different levels within deposits indicate vegetational changes that can in turn be interpreted in terms of ecological and climatic change. Furthermore, sediment from levels in which major changes occurred can be subjected to radiocarbon dating. Results from many parts of the world show that the beginning of the Holocene was characterized by rapid climatic warming and widespread vegetational change. In previously glaciated areas of western Europe, for example, tundra vegetation was replaced by boreal forest (birch and pine), which in turn gave way to broad-leaved deciduous trees, such as oak, elm, and hazel. This warming culminated in a period lasting from approximately 8000 to 5000 BP, known as the climatic optimum, or hypsithermal interval, when mean annual temperatures were generally 2 to 3 C (3.6 to 5.4 F) higher than at the present day. At this time broad-leaved trees reached farther north than today in both North America and northern Europe, and the prairie grasslands of what is now the midwestern United States expanded far to the east of their historic limits. In tropical regions it is particularly notable that between 10,000 and 5000 BP there were lakes with high water levels across areas of Africa, Arabia, and India that are now quite arid. This indicates a much wetter climate than that of today and a very different distribution of monsoonal rainfall in the first half of the Holocene. After 5000 BP the vegetational evidence is harder to interpret because of soil deterioration and the ever-increasing and widespread effects of human activities. Climates, however, certainly became cooler and have been characterized by small-scale oscillations between warmer and cooler intervals lasting a few hundred years. Such intervals are evident from historical records as well as from the meteorological records that have been kept since the 17th century. The favourable climate of the Little Climatic Optimum (AD 10001250) and the severe Little Ice Age (12501850) had extremely significant effects on human civilizations and their economies. Although the Earth is presently supposed to be in a long-term cooling of the glacial-interglacial cycles, reliable records of temperature readings from around the world (maintained since the mid-1800s) indicate that a global warming of nearly 0.5 C (0.9 F) has occurred over the past century. A number of scientists believe that the buildup of carbon dioxide and other so-called greenhouse gases (e.g., methane and nitrous oxide) in the atmosphere largely as a result of human activities may be primarily responsible for this warming trend. Additional reading A 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). Larry D. Agenbroad Holocene environment and biota In formerly glaciated regions, the Holocene has been a time for the reinstitution of ordinary processes of subaerial erosion and progressive reoccupation by a flora and fauna. The latter expanded rapidly into what was an ecological vacuum, although with a very restricted range of organisms, because the climates were initially cold and the soil was still immature. Floral change The most important biological means of establishing Holocene climate involves palynology, the study of pollen, spores, and other microscopic organic particles. Pollen from trees, shrubs, or grasses is generated annually in large quantities and often is well preserved in fine-grained lake, swamp, or marine sediments. Statistical correlations of modern and fossil assemblages provide a basis for estimating the approximate makeup of the local or regional vegetation through time. Even a crude subdivision into arboreal pollen (AP) and nonarboreal pollen (NAP) reflects the former types of climate. The tundra vegetation of the last glacial epoch, for example, provides predominantly NAP, and the transition to forest vegetation shows the climatic amelioration that heralded the beginning of the Holocene. The first standard palynological stratigraphy was developed in Scandinavia by Axel Blytt, Johan Rutger Sernander, and E.J. Lennart von Post, in combination with a theory of Holocene climate changes. The so-called BlyttSernander system was soon tied to the archaeology and to the varve chronology of Gerard De Geer. It has been closely checked by radiocarbon dating, establishing a very useful standard. Every region has its own standard pollen stratigraphy, but these are now correlated approximately with the BlyttSernander framework. To some extent this is even true for remote areas such as Patagonia and East Africa. Particularly important is the fact that the middle Holocene was appreciably warmer than today. In Europe this phase has been called the Climatic Optimum (zones Boreal to Atlantic), and in North America it has been called the hypsithermal (also altithermal and xerothermic). Like pollen, macrobotanical remains by themselves do not establish chronologies. Absolute dating of these remains does, however, provide a chronology of floral changes throughout the Holocene. Recent discoveries of the dung deposits of Pleistocene animals in dry caves and alcoves on the Colorado Plateau, including those of mammoth, bison, horse, sloth, extinct forms of mountain goats, and shrub oxen, have provided floristic assemblages from which temperature and moisture requirements for such assemblages can be deduced in order to develop paleoenvironmental reconstructions tied to an absolute chronology. Macrobotanical remains found in the digestive tracts of late Pleistocene animals frozen in the permafrost regions of Siberia and Alaska also have made it possible to build paleoenvironmental reconstructions tied to absolute chronologies. From these reconstructions, one can see warming and drying trends in the terminal Pleistocene ( 11,500 BP). Cold-tolerant, water-loving plants (e.g., birch and spruce) retreated to higher elevations or higher latitudes (as much as 2,500 metres in elevation) within less than 11,000 years. Detailed studies of late Pleistocene and Holocene alluvium, tied to carbon-14 chronology, have provided evidence of cyclic fluctuations in the aggradation and degradation of Holocene drainage systems. Although it is still too early in the analysis to state with certainty, it appears from the work of several investigators that there is a regional, or semicontinental cycle, of erosion and deposition that occurs every 250300, 500600, 1,0001,300, and possibly 6,000 years within the Holocene. Nature of the Holocene record The very youthfulness of the Holocene stratigraphic sequence makes subdivision difficult. The relative slowness of the Earth's crustal movements means that most areas which contain a complete marine stratigraphic sequence are still submerged. Fortunately, in areas that were depressed by the load of glacial ice there has been progressive postglacial uplift (crustal rebound) that has led to the exposure of the nearshore deposits. Deep oceanic deposits The marine realm, apart from covering about 70 percent of the Earth's surface, offers far better opportunities than coastal environments for undisturbed preservation of sediments. In deep-sea cores, the boundary usually can be seen at a depth of about 1030 centimetres, where the Holocene sediments pass downward into material belonging to the late glacial stage of the Pleistocene. The boundary often is marked by a slight change in colour. For example, globigerina ooze, common in the ocean at intermediate depths, is frequently slightly pinkish when it is of Holocene age because of a trace of iron oxides that are characteristic of tropical soils. At greater depth in the section, the globigerina ooze may be grayish because of greater quantities of clay, chlorite, and feldspar that have been introduced from the erosion of semiarid hinterlands during glacial time. During each of the glacial epochs the cooling of the ocean waters led to reduced evaporation and thus fewer clouds, then to lower rainfall, then to reduction of vegetation, and so eventually to the production of relatively more clastic sediments (owing to reduced chemical weathering). Furthermore, the worldwide eustatic (glacially related) lowering of sea level caused an acceleration of erosion along the lower courses of all rivers and on exposed continental shelves, so that clastic sedimentation rates in the oceans were higher during glacial stages than during the Holocene. Turbidity currents, generated on a large scale during the low sea-level periods, became much less frequent following the rise of sea level in the Holocene. Studies of the fossils in the globigerina oozes show that at a depth in the cores that has been radiocarbon-dated at about 10,00011,000 BP the relative number of warm-water planktonic foraminiferans increases markedly. In addition, certain foraminiferal species tend to change their coiling direction from a left-handed spiral to a right-handed spiral at this time. This is attributed to the change from cool water to warm water, an extraordinary (and still not understood) physiological reaction to environmental stress. Many of the foraminiferans, however, responded to the warming water of the Holocene by migrating poleward by distances of as much as 1,000 to 3,000 kilometres in order to remain within their optimal temperature habitats. In addition to foraminiferans in the globigerina oozes, there are nannoplankton, minute fauna and flora consisting mainly of coccolithophores. Research on the present coccolith distribution shows that there is maximum productivity in zones of oceanic upwelling, notably at the subpolar convergence and the equatorial divergence. During the latest glacial stage the subpolar zone was displaced toward the equator, but with the subsequent warming of waters it shifted back to the borders of the polar regions. The distribution of the carbonate plankton bears on the problem of rates of oceanic circulation. Is the Holocene rate higher or lower than during the last glacial stage? It has been argued that, because of the higher mean temperature gradient in the lower atmosphere from equator to poles during the last glacial period, there would have been higher wind velocities and, because of the atmosphereocean coupling, higher oceanic current velocities. There were, however, two retarding factors for glacial-age currents. First, the eustatic withdrawal of oceanic waters from the continental shelves reduced the effective area of the oceans by 8 percent. Second, the greater extent of floating sea ice would have further reduced the available airocean coupling surface, especially in the critical zone of the westerly circulation. According to climatic studies by the British meteorologist Hubert H. Lamb, the presence of large continental ice sheets in North America and Eurasia would have introduced a strong blocking action to the normal zonal circulation of the atmosphere, which then would be replaced by more meridional circulation. This in turn would have been appreciably less effective in driving major oceanic current gyres. Stratigraphy Chronology and correlation The Holocene is unique among geologic epochs because varied means of correlating deposits and establishing chronologies are available. One of the most important means is carbon-14 dating. Because the age determined by the carbon-14 method may be appreciably different from the true age in certain cases, it is customary to refer to such dates in radiocarbon years. These dates, obtained from a variety of deposits, form an important framework for Holocene stratigraphy and chronology. The limitations of accuracy of radiocarbon age determinations are expressed as a few tens or hundreds of years. In addition to this calculated error, there also is a question of error due to contamination of the material measured. For instance, an ancient peat may contain some younger roots and thus give a falsely young age unless it is carefully collected and treated to remove contaminants. Marine shells consist of calcium carbonate (CaCO3), and in certain coastal regions there is upwelling of deep oceanic water that can be 500 to more than 1,000 years old. An age from living shells in such an area can suggest that they are already hundreds of years old. The Table shows the comparative dates of radiocarbon years and those obtained by other means. Two sets of radiocarbon years are given because the half-life of carbon-14 was reassigned a value of 5,730 years by agreement of scientists. Many dates available in the literature, however, are based on the originally established half-life of 5,570 years. In certain areas a varve chronology can be established. This involves counting and measuring thicknesses in annual paired layers of lake sediments deposited in lakes that undergo an annual freeze-up. Because each year's sediment accumulation varies in thickness according to the climatic conditions of the melt season, any long sequence of varve measurements provides a distinctive signature and can be correlated for moderate distances from lake basin to lake basin. The pioneer in this work was the Swedish investigator Baron Gerard De Geer, who developed a long chronology on which that shown in the Table is partly based. In some relatively recent continental deposits, obsidian (a black glassy rock of volcanic origin) can be used for dating. Obsidian weathers slowly at a uniform rate, and the thickness of the weathered layer is measured microscopically and gauged against known standards to give a date in years. This has been particularly useful where arrowheads of obsidian are included in deposits. As noted elsewhere in this article, paleomagnetism is another phenomenon used in chronology. The Earth's magnetic field undergoes a secular shift that is fairly well known for the last 2,000 years. The magnetized material to be studied can be natural, such as a lava flow; or it may be man-made, as, for example, an ancient brick kiln or smeltery that has cooled and thus fixed the magnetic orientation of the bricks to correspond to the geomagnetic field of that time. Another form of dating is tephrochronology, so called because it employs the tephra (ash layers) generated by volcanic eruptions. The wind may blow the ash 1,5003,000 kilometres, and, because the minerals or volcanic glass from any one eruptive cycle tend to be distinctive from those of any other cycle, even from the same volcano, these can be dated from the associated lavas by stratigraphic methods (with or without absolute dating). The ash layer then can be traced as a time horizon wherever it has been preserved. When the Mount Mazama volcano in Oregon exploded at about 6600 BP (radiocarbon-dated by burned wood), 70 cubic kilometres of debris were thrown into the air, forming the basin now occupied by Crater Lake. The tephra were distributed over 10 states, thereby providing a chronological marker horizon. A comparable eruption of Thera on Santorin in the Aegean Sea about 3,400 years ago left tephra in the deep-sea sediments and on adjacent land areas. Periodic eruptions of Mount Hekla in Iceland have been of use in Scandinavia, which lies downwind. Finally, the measurement and analysis of tree rings (or dendrochronology) must be mentioned. The age of a tree that has grown in any region with a seasonal contrast in climate can be established by counting its growth rings. Work in this field by the University of Arizona's Laboratory of Tree-Ring Research, by selection of both living trees and deadwood, has carried the year-by-year chronology back more than 7,500 years. Certain pitfalls have been discovered in tree-ring analysis, however. Sometimes, as in a very severe season, a growth ring may not form. In certain latitudes the tree's ring growth correlates with moisture, but in others it may be correlated with temperature. From the climatic viewpoint these two parameters are often inversely related in different regions. Nevertheless, in experienced hands, just as with varve counting from adjacent lakes, ring measurements from trees with overlapping ages can extend chronologies back for many thousands of years. The bristlecone pine of the White Mountains in California has proved to be singularly long-lived and suitable for this chronology; some individuals still living are more than 4,000 years old, certainly the oldest living organisms. Wood from old buildings and even old paving blocks in western Europe and in Russia have contributed to the chronology. This technique not only offers an additional means of dating but also contains a built-in documentation of climatic characteristics. In certain favourable situations, particularly in the drier, low latitudes, tree-ring records sometimes document 11- and 22-year sunspot cycles. The PleistoceneHolocene boundary Arguments can be presented for the selection of the lower boundary of the Holocene at several different times in the past. Some Russian investigators have proposed a boundary at the beginning of the Allerd, a warm interstadial age that began about 12,000 BP. Others, in Alaska, proposed a Holocene section beginning at 6000 BP. Marine geologists have recognized a worldwide change in the character of deep-sea sedimentation about 10,00011,000 BP. In warm tropical waters, the clays show a sharp change at this time from chlorite-rich particles often associated with fresh feldspar grains (cold, dry climate indicators) to kaolinite and gibbsite (warm, wet climate indicators). Some of the best-preserved traces of the boundary are found in southern Scandinavia, where the transition from the latest glacial stage of the Pleistocene to the Holocene was accompanied by a marine transgression. These beds, south of Gteborg, have been uplifted and are exposed at the surface. The boundary is dated around 10,300 200 years BP (in radiocarbon years). This boundary marks the very beginning of warmer climates that occurred after the latest minor glacial advance in Scandinavia. This advance built the last Salpausselk moraine, which corresponds in part to the Valders substage in North America. The subsequent warming trend was marked by the Finiglacial retreat in northern Scandinavia, the Ostendian (early Flandrian) marine transgression in northwestern Europe.