Chapter 103: Pleistocene


    Definition of the base of the Pleistocene has had a long and controversial history. Because the epoch is best recognized for ciation and climatic change, many have suggested that its lower boundary should be based on climatic criteriafor example, the oldest cial deposits or the first urrence of a fossil of a cold-climate life-form in the sediment record.


    Other criteria that have been used to define the PliocenePleistocene 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 2.4 million years.


    Pre-Pleistocene intervals of time are defined on the basis of chronostratigraphic and geochronologic principles rted to a marine sequence of strata.


    Following studies by a series of international working groups, corrtion programs, and stratigraphicmissions, agreement was reached in 1985 to ce the lower boundary of the Pleistocene series at the base of marine ystones that conformably overlie a specific marker bed in the Vrica section in Cbria.


    The boundary urs near the level of several important marine biostratigraphic events and, more significantly, is just above the position of the maic reversal that marks the top of the Olduvai Normal Prity Subzone, thus allowing worldwide corrtion.


    Since evidence of Cenozoic ciation was discovered in rocksid 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 Gsian Stage, the rockyerid down during the Gsian Age, found at Monte San Nic near G, Sicily. The base marker for the Gsianthat is, the global stratotype section and point (GSSP)was ced in rock dated to 2,588,000 years ago (a notable point because it is within 20,000 years of the Gauss-Matuyama geomaic reversal). In addition, the date of the rock is closely corrted 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.) After years of discussion, the International Union of Geological Sciences (IUGS) and the International Commission on Stratigraphy (ICS) designated the Gsian as the lowermost stage of the Pleistocene Epoch.


    The Pleistocene is subdivided into four ages and their corresponding rock units: the Gsian (2.6 million to 1.8 million years ago), the Cbrian (1.8 million to 780,000 years ago), the Ionian (780,000 to 126,000 years ago), and the Tarantian (126,000 to 11,700 years ago). Of these, only the Gsian and Cbrian are formal intervals, whereas others await ratification by the ICS.


    The Cbrian, which was previously known as the early Pleistocene, extends to the BrunhesMatuyama paleomaic boundary at 780,000 years ago. The Ionian, also known as the middle Pleistocene, extends to the end of the next to thest ciation at about 130,000 years ago. The Tarantian, also known as thete Pleistocene, includes thest intercialcial cycle ending at the Holocene boundary about 11,700 years ago.


    The chronology of the Pleistocene originally developed through observation and study of the cial session, 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 cial deposits. From these studies, as well as studies of river terraces in the Alps, a chronology was developed that suggested the Pleistocene consisted of four or five major cial stages which were separated by intercial stages with climates generally simr to those of today.


    Beginning with studies in the 1950s, a much better chronology and record of Pleistocene climatic events have evolved through analyses of deep-sea sediments, particrly from the oxygen isotope record of the shells of microorganisms that lived in the oceans.


    The isotopic record is based on the ratio of two oxygen isotopes, oxygen-16 (16O) and oxygen-18 (18O), which is determined on calcium carbonate from shells of microfossils that umted year by year on the seafloor. The ratio depends on two factors, the temperature and the isotopicposition of the seawater from which the organism secreted its shell.


    Shells secreted from colder water contain more oxygen-18 rtive to oxygen-16 than do shells secreted from warmer water. The isotopicposition of the oceans has proved to be rted to the storage of water inrge ice sheets onnd. Because molecules of oxygen-18 evaporate less readily and condense more readily, an air mass with oceanic water vapour bes depleted in the heavier isotope (oxygen-18) as the air mass is cooled and loses water by precipitation.


    When moisture condenses and falls as snow, its isotopicposition is also dependent on the temperature of the air. Snow falling on arge ice sheet bes isotopically lighter (i.e., has less oxygen-18) as one goes higher on the cier surface where it is both colder and farther from the moisture source.


    As a result,rge ice sheets store water that is rtively light (has more oxygen-16), and so during a major ciation the ocean waters be rtively heavier (contain more oxygen-18) than during intercial times when there is less global ice. ordingly, the shells of marine organisms that formed during a ciation contain more oxygen-18 than those that formed during an interciation.


    Although the exact rtionship is not known, about 70 percent of the isotopic change in shell carbonate is the result of changes in the isotopicposition of seawater. Because thetter is directly rted to the volume of ice onnd, the marine oxygen isotope record is primarily a record of past ciations 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 rtively small have revealed a long record of oxygen isotope changes that indicate repeated ciations and interciations going back to the Pliocene.


    The record is rtively consistent from one core sample to the next and can be corrted throughout the oceans. Warmer periods (intercials) are assigned odd numbers with the current warm interval, the Holocene, being 1, while the colder cial 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 avable radiometric ages of the cores and from position with respect to both paleomaic boundaries and biostratigraphic markers, and also by using sedimentation rates rtive to these data.


    The record for thest 730,000 years indicates that eight major cial and intercial events or climatic cycles of about 100,000 years'' duration urred during this interval. An isotopic record from the North Antic suggests the first major ciation in that region urred about 2,400,000 years ago. It also suggests that the first ciation likely to have covered extensive areas of North America and Eurasia urred about 850,000 years ago during oxygen isotope stage 22. Thergest ciations appear to have taken ce during stages 2, 6, 12, and 16; the intercials with the least global ice, and thus possibly the warmest, appear to be stages 1, 5, 9, and 11. Thest interciation urred during all of stage 5 or just substage 5e, depending on location; thest ciation took ce during stages 4, 3, and 2; and the current interciation falls during stage 1.


    The marine isotopic record is a continuous record, unlike most terrestrial records, which contain gaps because of erosion orck of sedimentation and soil formation or abination of these factors.


    Because of its continuity and its excellent record of climatic events onnd (ciations), the marine oxygen isotope record is the standard to which the terrestrial and other stratigraphic records are corrted. Corrtions to it are based on avable chronometric ages, on paleomaic data where avable, and on attempts to match the terrestrial record and its interpretation with specific characteristics of the isotopic curve.


    Unfortunately, most terrestrial records contain few radiometric ages and are iplete, and specific corrtions, except for the most recent part of the record, are difficult and uncertain. A few terrestrial records, however, are exceptional and can be corrted with confidence.


    Central China is covered by deposits of windblown dust and silt, called loess. Locally the loess is more than 100 metres thick, mantling hillsides and forming loess teaus and tablnds. The loess umted primarily during times that were colder and drier than present, and most of it was derived from desert areas to the west. The loess session contains many colourful buried soils or paleosols that formed during periods which were both warmer and wetter than today.


    Thus, on stable tablnds with minimal erosion, the session provides an exceptional climatic and chronological record that extends back 2.4 million years to thete Pliocene. In total, up to 44 climatic cycles have been delineated, with more frequent cycles urring during the early Pleistocene. Although not directly rted to ciation, corrtion with the marine oxygen isotope record is excellent, and many of the specific loess and soil units have simr climatic inferences, as do their corrtive oxygen-18 stages.