NOVEL WEB Chapter 103: Pleistocene
Definition of the base of the Pleistocene has had a long and controversial history. Because the epoch is best recognized for glaciation and climatic change, many 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 2.4 million years.
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 1985 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 2,588,000 years ago (a notable point because it is within 20,000 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.) After years of discussion, the International Union of Geological Sciences (IUGS) and the International Commission on Stratigraphy (ICS) designated the Gelasian as the lowermost stage of the Pleistocene Epoch.
The Pleistocene is subdivided into four ages and their corresponding rock units: the Gelasian (2.6 million to 1.8 million years ago), the Calabrian (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 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 at 780,000 years ago. The Ionian, also known as the middle Pleistocene, extends to the end of the next to the last glaciation at about 130,000 years ago. The Tarantian, also known as the late Pleistocene, includes the last interglacial–glacial cycle ending at the Holocene boundary about 11,700 years ago.
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 Alps, a 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 1950s, 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.
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 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-18 relative to oxygen-16 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-18 evaporate less readily and condense more readily, an air mass with oceanic water vapour becomes 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 isotopic composition is also dependent on the temperature of the air. Snow falling on a large ice sheet becomes isotopically lighter (i.e., has less oxygen-18) as one goes higher on the glacier surface where it is both colder and farther from the moisture source.
As a result, large ice sheets store water that is relatively light (has more oxygen-16), and so during a major glaciation the ocean waters become relatively heavier (contain more oxygen-18) than during interglacial times when there is less global ice. Accordingly, the shells of marine organisms that formed during a glaciation contain more oxygen-18 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.
The record for the last 730,000 years indicates that eight major glacial and interglacial events or climatic cycles of about 100,000 years' duration occurred during this interval. An isotopic record from the North Atlantic suggests the first major glaciation in that region occurred about 2,400,000 years ago. It also suggests that the first glaciation likely to have covered extensive areas of North America and Eurasia occurred about 850,000 years ago during oxygen isotope stage 22. The largest glaciations appear to have taken place during stages 2, 6, 12, and 16; the interglacials with the least global ice, and thus possibly the warmest, appear to be stages 1, 5, 9, and 11. The last interglaciation occurred during all of stage 5 or just substage 5e, depending on location; the last glaciation took place during stages 4, 3, and 2; and the current interglaciation falls during stage 1.
The marine isotopic record is a continuous record, unlike most terrestrial records, which contain gaps because of erosion or lack of sedimentation and soil formation or a combination of these factors.
Because of its continuity and its excellent record of climatic events on land (glaciations), the marine oxygen isotope record is the standard to which the terrestrial and other stratigraphic records are correlated. Correlations to it are based on available chronometric ages, on paleomagnetic data where available, 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 incomplete, and specific correlations, except for the most recent part of the record, are difficult and uncertain. A few terrestrial records, however, are exceptional and can be correlated 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 plateaus and tablelands. The loess accumulated primarily during times that were colder and drier than present, and most of it was derived from desert areas to the west. The loess succession contains many colourful buried soils or paleosols that formed during periods which were both warmer and wetter than today.
Thus, on stable tablelands with minimal erosion, the succession provides an exceptional climatic and chronological record that extends back 2.4 million years to the late Pliocene. In total, up to 44 climatic cycles have been delineated, with more frequent cycles occurring during the early Pleistocene. Although not directly related to glaciation, correlation with the marine oxygen isotope record is excellent, and many of the specific loess and soil units have similar climatic inferences, as do their correlative oxygen-18 stages.