Holocene Epoch

I

Introduction

Holocene Epoch, fourth and most recent division of the Neogene Period on the geologic time scale. The Neogene Period is the youngest period in geologic time. The Holocene (Greek for “wholly recent”) Epoch began 11,500 years ago and continues into the present. It is also called the Recent Epoch. see Epoch.

Geologists roughly mark the boundary between the earlier Pleistocene Epoch and the Holocene Epoch with a climatic moderation that accompanied the melting of huge ice sheets that covered more than one-fourth of the earth’s land area during the Pleistocene. The ice sheets retreated at different times in different places, so their disappearance is only an approximate boundary. The universal boundary for the Holocene is 11,500 years before the present, as measured in calendar years.

II

Geologic Activity

At the beginning of the Holocene Epoch ice covered much of North America, Scandinavia, and other northern and southern latitude and high altitude regions of the world. The ice was melting rapidly, and substantial glacial lakes, including ancestors of the Great Lakes of the United States and Canada, formed at the edge of melting glaciers. The world’s climate warmed extremely rapidly in the early Holocene Epoch, and by 8000 years before the present temperatures in the middle latitudes of the earth neared today’s temperatures.

By about 6500 years before present the last remnants of the Laurentide Ice Sheet, which had covered most of Canada, finally vanished from either side of Hudson Bay. Afterward, the only continental ice existed at high altitudes and northerly latitudes. Plant and animal communities rapidly colonized the landscape revealed by retreating ice sheets. Humans have modified some of the natural landscape, most substantially in the last two hundred years.

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III

Biological Activity

Animals and plants have not evolved much during the Holocene Epoch. Most extinctions in the Holocene are the result of human interference. The retreat of the Pleistocene glaciers caused significant shifts in the areas that animals and plants occupied. In the earliest part of the Holocene Epoch, species re-colonized the recently uncovered land. As the climate warmed, some animal and plant species became separated from other populations. In some cases species thrived in their new habitats and in some cases they did not. Human hunting practices and new diseases carried by humans and other new animals probably made a difference in whether some of these species, particularly many of the large North American late Pleistocene mammals such as mastodons and saber-toothed tigers, became extinct.

Later in the Holocene and particularly in Europe, the Middle East, and parts of Asia, humans were responsible for great modifications in natural vegetation and changing animal ranges. This pattern of interference has become particularly pronounced in the last few hundred years as human numbers have grown. Human interference with natural systems has caused, and will continue to cause, many extinctions in both the animal and plant kingdoms. Some scientists suggest that this part of the Holocene will mark another major extinction event in the geologic record of the earth.

IV

Human Activity

The earth has experienced radical natural changes throughout its history. Even during the last 20,000 years, climate and sea levels have changed dramatically. Human activity has had an appreciable effect on the earth only in the last few centuries. People have modified the earth’s vegetation patterns, created conditions that led to the loss of soil in many parts of the world, and devastated entire ecosystems.

Signs of human alteration of the planet’s atmosphere are evident in carbon dioxide levels recorded in ice cores over the past 250,000 years. During ice ages natural carbon dioxide levels were 200 parts per million (ppm) by volume. During warm stages, such as the Holocene Epoch, the carbon dioxide levels were 280 ppm. When humans began to use vast quantities of fossil fuels to power machines, the carbon dioxide levels in the atmosphere began to rise. The atmospheric level of carbon dioxide is now 379 ppm, a rise that equals the full natural fluctuation between an ice age and a warm period. According to one theory, the buildup of carbon dioxide and other gases released from fossil fuels into the atmosphere has caused a warming of the planet (see Global Warming). If warming continues, it could lead to a rise in sea level, melting of permafrost (permanently frozen earth) and glaciers at high altitudes, changes in temperature and precipitation extremes, and new animal migration patterns and habitat changes.

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