About Historical Knowledge In reflecting upon its past, humanity is aware of times when life was different than it now is. The artifacts of those times are still to be seen in stone monuments, pottery shards, tools, and other physical remains. History is a collection of stories from the past which explain how the world that we know came to be.
Stratigraphy Pliocene—Pleistocene boundary Definition of the base of the Pleistocene has had a long and controversial history. Because the epoch is best recognized for glaciation and climatic changemany have suggested that its lower boundary should be based on climatic criteria—for example, the oldest glacial deposits or the first occurrence of a fossil of a cold-climate life-form in the sediment record.
Other criteria that have been used to define the Pliocene—Pleistocene include the appearance of humans, the appearance of certain vertebrate fossils in Europe, and the appearance or extinction of certain microfossils in deep-sea sediments.
These criteria continue to be considered locally, and some workers advocate a climatic boundary at about 2. Pre-Pleistocene intervals of time are defined on the basis of chronostratigraphic and geochronologic principles related to a marine sequence of strata. Following studies by a series of international working groups, correlation programs, and stratigraphic commissions, agreement was reached in to place the lower boundary of the Pleistocene series at the base of marine claystones that conformably overlie a specific marker bed in the Vrica section in Calabria.
The boundary occurs near the level of several important marine biostratigraphic events and, more significantly, is just above the position of the magnetic reversal that marks the top of the Olduvai Normal Polarity Subzone, thus allowing worldwide correlation.
Since evidence of Cenozoic glaciation was discovered in rocks laid down earlier than those of the Vrica section, some geologists proposed that the base of the Pleistocene be moved to an earlier time.
To many geologists, the most reasonable time coincided with the type section for the Gelasian Stage, the rock layer laid down during the Gelasian Age, found at Monte San Nicola near Gela, Sicily.
The base marker for the Gelasian—that is, the global stratotype section and point GSSP —was placed in rock dated to 2, years ago a notable point because it is within 20, years of the Gauss-Matuyama geomagnetic reversal.
In addition, the date of the rock is closely correlated with the timing of a substantial change in the size of granules found in Chinese loess deposits. Changes in loess grain size suggest regional climate changes. The Pleistocene is subdivided into four ages and their corresponding rock units: Of these, only the Gelasian and Calabrian are formal intervals, whereas others await ratification by the ICS.
The Calabrian, which was previously known as the early Pleistocene, extends to the Brunhes—Matuyama paleomagnetic boundary atyears ago.
The Ionian, also known as the middle Pleistocene, extends to the end of the next to the last glaciation at aboutyears ago. The Tarantian, also known as the late Pleistocene, includes the last interglacial—glacial cycle ending at the Holocene boundary about 11, years ago. Chronology and correlation The chronology of the Pleistocene originally developed through observation and study of the glacial succession, which in both Europe and the United States was found to contain either soils that developed under warm climatic conditions or marine deposits enclosed between glacial deposits.
From these studies, as well as studies of river terraces in the Alpsa chronology was developed that suggested the Pleistocene consisted of four or five major glacial stages which were separated by interglacial stages with climates generally similar to those of today.
Beginning with studies in the s, a much better chronology and record of Pleistocene climatic events have evolved through analyses of deep-sea sediments, particularly from the oxygen isotope record of the shells of microorganisms that lived in the oceans.
Marine oxygen isotope record The isotopic record is based on the ratio of two oxygen isotopesoxygen 16O and oxygen 18Owhich is determined on calcium carbonate from shells of microfossils that accumulated year by year on the seafloor.
The ratio depends on two factors, the temperature and the isotopic composition of the seawater from which the organism secreted its shell. Shells secreted from colder water contain more oxygen relative to oxygen than do shells secreted from warmer water.
The isotopic composition of the oceans has proved to be related to the storage of water in large ice sheets on land. Because molecules of oxygen evaporate less readily and condense more readily, an air mass with oceanic water vapour becomes depleted in the heavier isotope oxygen as the air mass is cooled and loses water by precipitation.
When moisture condenses and falls as snow, its isotopic composition is also dependent on the temperature of the air.
Snow falling on a large ice sheet becomes isotopically lighter i. As a result, large ice sheets store water that is relatively light has more oxygenand so during a major glaciation the ocean waters become relatively heavier contain more oxygen than during interglacial times when there is less global ice.
Accordingly, the shells of marine organisms that formed during a glaciation contain more oxygen than those that formed during an interglaciation. Although the exact relationship is not known, about 70 percent of the isotopic change in shell carbonate is the result of changes in the isotopic composition of seawater.
Because the latter is directly related to the volume of ice on land, the marine oxygen isotope record is primarily a record of past glaciations on the continents.
Long core samples taken in portions of the ocean where sedimentation rates were high and generally continuous and where water temperature changes were relatively small have revealed a long record of oxygen isotope changes that indicate repeated glaciations and interglaciations going back to the Pliocene.
The record is relatively consistent from one core sample to the next and can be correlated throughout the oceans. Warmer periods interglacials are assigned odd numbers with the current warm interval, the Holocene, being 1, while the colder glacial periods are assigned even numbers.
Subdivisions within isotopic stages are delineated by letters. The ages of the stage boundaries cannot be measured directly, but they can be estimated from available radiometric ages of the cores and from position with respect to both paleomagnetic boundaries and biostratigraphic markers, and also by using sedimentation rates relative to these data.
An isotopic record from the North Atlantic suggests the first major glaciation in that region occurred about 2, years ago. It also suggests that the first glaciation likely to have covered extensive areas of North America and Eurasia occurred aboutyears ago during oxygen isotope stage This timeline of the evolutionary history of life represents the current scientific theory outlining the major events during the development of life on planet Earth.
In biology, evolution is any change across successive generations in the heritable characteristics of biological populations.
Fifty-four scholars worked for four years on the project. Pilgrims coming to America sign the Mayflower Compact and commit themselves to seek the public good, uphold group solidarity and .
The history of the United States is vast and complex, but can be broken down into moments and time periods that divided, unified, and changed the United States into the country it is today: The Library of Congress has compiled a list of historic events for each day of the year, titled "This Day in.
Published in Philadelphia in , this book divided world history into three parts: “Ancient History”, “the Middle Ages”, and “Modern History”. Ancient history began with Adam’s life in the Garden of Eden and ended with the fall of the west Roman empire in A.D.
Instructions: Complete the matrix by providing the Time Period/Date(s) in column B, and the Description and Significance of the People/Event(s) to American History in column C.
Major Event/Epoch in American History Time Period/Date(s) Description and Significance of the People/Event(s) to American History 1) The evolution of the institution of slavery from the Colonial Period to the s.
When slavery first started, it was in all colonies, until the Revolutions.