Holocene

The Holocene is also called the Recent, and its boundary with the preceding Pleistocene epoch is not precisely agreed upon. However, it is defined as either when the continental glaciers in North America and Eurasia began their retreat, 15,000 years ago, or when most of the glaciated areas were free of ice, 10,000 years ago.

The Holocene is the span of time during which people have changed and dominated the environment of the world: at first by hunting while following herds of animals across the terrain, and later by settling into commu nities, cutting down forests for houses, and creating open spaces for farming. Later these communities expanded into large cities, necessitating large-scale mining operations, the burning of fossil fuels, and the interconnection of urban and rural areas with networks of communication and transportation.

One of the major events of the Holocene is the rapidity of climatic change and the resulting redistribution of plants and animals with migrating climatic zones. Glaciers probably reached their maximum distribution about 18,000 years ago, at the time that sea level was about 130 m lower than at the present and vast areas of the continental shelves were exposed. As the climate warmed about 12,000 years ago, glaciers shrank and sea level rose as torrents of glacial meltwater flowed into the oceans. About 12,000 years ago postglacial lakes formed over large parts of the North American continent, Lake Agassiz in the Midwest and Lake Bonneville and Lake Lahontan in Utah and Nevada. During this time the climate of the areas south of the glacial margin were much wetter. These disappeared as a result of the irregular uplift of the land surface caused by glacial rebound and changing climate. Glaciers widened and deepened river valleys, gouging deep basins that became lakes, such as the Finger Lakes in central New York and the Great Lakes of Central North America.

As the climate warmed the land was released from the grip of the ice and revege-tated. In eastern North America the overall succession of tundra to boreal forest to deciduous forest is revealed at numerous sites by the study of sediments and their entombed spores, pollen, and other vegetable matter.

However, the climatic change from glacial to postglacial times was not an even transition. It was marked by warming and cooling cycles, the data for which are derived from a number of lines of evidence. For example, using radiometric dating and the position of a reef-building coral, scientists have shown that the island of Barbados in the Caribbean was involved in three periods of rapid rise in sea level between about 15,000 and 7,000 years ago. The coral grows very close to sea level today, but dead coral is also found many meters above sea level and as fossils in cores several layers well below the surface. Using radioactive dating methods and subtracting the results of tectonic rise of the island, the ages of these corals have been determined to be 14,680, 11,600, and 7,600 years. The rises in sea level that these data represent also reflect a corresponding climatic shift.

As an example, the annual snowfall layers in Greenland about 14,680 years ago doubled in thickness in the space of ten years. The rise in sea level detected in Barbados, which represents a climatic warming, increased the supply of moisture for the North Atlantic Ocean, allowing for an increase in snow accumulation in Greenland. Starting about 13,000 years ago, the Northern Hemisphere began to shift back toward glacial conditions. This cold interval is called the Younger Dryas, named after the plant that spread southward with colder conditions. Sea ice expanded southward, and mountain glaciers extended farther down their valleys. This cold period suddenly ended 11,600 years ago. Study of ice cores from Greenland and elsewhere indicate that this shift took place within just three years. The last sudden rise in sea level took place 7,600 years ago, perhaps as a result of the melting of large amounts of ice from Antarctica, raising the sea level close to its present level.

The Younger Dryas was also the time that large numbers of people migrated to North America. Although some archaeologists believe that small numbers of people reached

North America 30,000 years ago, it wasn't until the glaciers began to melt on a large scale in Alaska and western Canada that a route opened to the unglaciated parts of North America. It was probably about 11,500 years ago that the Clovis people developed the first widespread culture of North America. These early hunters sought out elephants and bison, using spears to kill them for food and clothing. Three species of elephants lived in North America: the wooly mammoth in tundra and grassy environments that were adjacent to continental ice sheets, protected from the cold by their long, hairy coats; mastodons in the Eastern conifer forests; and the great Southern mammoth, living in the prairies of the Midwest and Southwest.

Another feature of the Holocene is the disappearance of many species of large mammals between 12,000 and 10,000 years ago. Their demise altered the terrestrial ecosystem and left behind an enduring controversy—namely, why did they disappear? Was it the result of climatic change or the wholesale slaughter by Clovis hunters, the early inhabitants of North America? In addition to the elephants, many other large mammals disappeared, such as a giant beaver, several species of horses, a camel, two species of oxen, three species of musk oxen, a large bison, two species of giant armadillos, several species of giant sloth, the dire wolf, and three species of saber-toothed cats.

It is believed that the cats and several species of eagles, vultures, and condors died out because the large mammals no longer provided prey or carcasses for them to feed on. The cause of their extinction continues to be debated, however, and although there are merits to both sides of the debate, neither side is conclusive. Perhaps, as some have suggested, the answer is a combination of both ideas. As the Clovis population and hunting expanded in conjunction with climatic change and vegetation shifts during the Younger Dryas, the interplay of these events led to their demise.

During the last 10,000 years, climate continued to fluctuate, with warming and cooling intervals. It was at the beginning of this time frame that people began to settle down in communities and domesticate plants and animals for food, between 9,000 and 6,000 years ago in Europe. It is thought that domestication and farming emerged in the Zagros Mountains near the borders of Iran, Iraq, and Turkey, and then spread to Greece and across Europe, reaching Scandinavia some 6,000 years ago. It is believed that this expansion correlated with the beginning phase of a temperate climatic regime and the eventual disappearance of all the continental ice sheets in North America and Europe.

This warmer period is designated the hyp-sithermal interval, or climatic optimum, a time when the evidence from pollen and plant material indicates that the mean annual temperature of North America and Europe was about 2 degrees centigrade warmer than today. Hemlocks, for example, lived farther up mountain slopes than they do now, and dwarf birches lived where there is today only tundra. It is believed that the great civilizations of Egypt, Mesopotamia, and the Indus Valley began to flourish at this time.

Study of pollen indicates that the hyp-sithermal interval came to an end when climate turned colder, about 5,800 to 4,900 years ago. Another cooling occurred 3,300 to 2,400 years ago, and then again from 700 C.E. to 900 C.E. Warming and the reduction of sea ice after the last cooling event allowed the Vikings to extend their base of operations along the European coast, maintain a community on Greenland, and gain a foothold on Newfoundland. In Greenland they raised cattle and sheep, but by 1500 the colony was aban doned because extensive sea ice prevented communications and the delivery of supplies.

This cooling marked the beginning of the so-called Little Ice Age, when climate turned cold in Europe and North America, causing crop failures and the introduction of crops that could grow in shorter summer seasons. During this time mountain glaciers enlarged and extended farther down their valleys, and new glaciers formed where they had previously existed during former cold periods.

One of the effects of the advance and retreat of glaciers is the corresponding rise and fall of sea level because of the addition of water to, or its subtraction from, glaciers. Masses of glacial ice also applied great stresses to the earth's surface, depressing it as they advanced and permitting it to rebound when they retreated. Taking into account tectonic movements, shorelines can portray a very complicated series of events. Many shoreline configurations, especially locally, have dramatically changed over time as a result of these phenomena. Former river valleys, such as those of the Chesapeake and the Hudson, were converted to wide estuaries.

Such changes, both physical and social, will occur as a result of climate change; they are why many scientists and other individuals are concerned about global warming. The geologic record shows that changes to climate can occur rapidly, during the lifetime of an individual; the consequences of these changes on the reorganization of climatic belts and the resulting drowning of coastal cities and probable increased desertification of the interior of continents will have direct effects on where people will live.

—Sidney Horenstein

See also: Climatology; Glaciation; Global Climate

Change; Pleistocene Epoch

Bibliography

Berner, Elizabeth K., and Robert A. Berner. 1996.

Global Environment: Water, Air, and Geochemical Cycles. Upper Saddle River, NJ: Prentice Hall; Dawson, Alastair G. 1992. Ice Age Earth: Late Quaternary Geology and Climate. London: Routledge; Lamb, H. H. 1995. Climate, History and the Modern World, London: Methuen; Levin, Harold L. 1999. The Earth Through Time, 6th ed. Fort Worth: Saunders College Publishing; Pielou, E. C. 1991. After the Ice Age: The Return of Life to Glaciated North America. Chicago: University of Chicago Press.

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