Bulletin of the Geological Society of Finland (Dec 1992)
Quaternary chronostratigraphy - a review
Abstract
Quaternary development is characterized by strong climatic fluctuations associated with repeated glacier expansions. Consequently the Quaternary chronostratigraphy is largely based on the dating of climatic fluctuations which have led to large-scale, often global, environmental changes. The cooling episode around 2.5 million years ago that swept forests away from large areas and rapidly increased the ice volume in the Northern Hemisphere marks the beginning of the Quaternary Period. The Milankovitch, or astronomical, theory of climatic change is today commonly accepted as the explanation for the rhythms of Quaternary climatic fluctuations. Climatic variability is controlled by small periodic or quasiperiodic changes in the Earth's orbit around the Sun, which in turn lead to changes in the intensity of the seasonal solar radiation reaching different latitudes. Small variations in the insolation trigger events which eventually lead to enormous climatic and environmental changes. Different orbital periodicities have dominated the climatic variability at different times during the Quaternary, for example, the 41-ka-long cycle of the tilt of the Earth's spinning axis and on the average 22-ka-long precession and 100-ka-long eccentricity cycles. During the last 800 ka, however, the eccentricity cycle of the orbit stands out clearly. It has set the pace for the largest Quaternary glaciations, all of which fall into the latter part of the Quaternary. The validity of the Milankovitch theory has been confirmed by many proxy records, including deep-sea oxygen isotope stratigraphy, Chinese loess records and many palaeobotanical studies on long lacustrine sequences; the present chronostratigraphical division of the Quaternary Period is largely based on these data. However, the lack of good dating methods still poses a major problem in many Quaternary stratigraphical studies. In some cases the shortcomings and methodical uncertainties can be avoided and accuracy improved by comparing dates obtained with different techniques. The comparison of K/Ar ages with climatic events dated by calculations of astronomical periodicities and the comparisons of radiocarbon ages with U/Th dates and varved sediment counts serve as examples of how the reliability and applicability of chronostratigraphy can be checked and further improved in this way.
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