Milankovitch Cyclicity In The Stratigraphic Record— A Review

Abstract

The Milankovitch or astronomical theory of paleoclimates relates climatic variation to the amount of solar energy available at the Earth's surface. The theory helps explain periodic, climatically related phenomena such as the Pleistocene ice ages. Identification of Milankovitch cyclicity within sediments demonstrates the influence of climate on sedimentation patterns and creates a time frame for the estimation of basin subsidence rates. Spectral analysis of deep sea and ice cores indicates periodic climatic fluctuations during Tertiary and Quaternary times. These fluctuations are strongly cyclical with low frequencies centered at periods around 400 ka and 100 ka together with shorter periodic components of approximately 41 and 21 ka. Lower frequencies reflect eccentricity of the Earth's orbit; 41- and 21-ka components are associated with periodic changes in the tilt of the Earth's axis and the precession of the equinoxes. Astronomically forced glacial eustasy results in distinct stratigraphic units or parasequences of widespread extent. Milankovitch band parasequences occur in both carbonate and clastic shelf systems, including cyclothemic Upper Paleozoic successions of North America. During the 1920's and 30's the Serbian mathematician Milutin Milankovitch studied cyclical variations in three elements of the Earth-Sun geometry: eccentricity, precession, and obliquity, and was able to calculate the Earth's solar radiation history for the past 650 ka (Milankovitch, 1969). Berger (1978, 1980) accurately determined the periodicities of the three orbital variations. Eccentricity—The Earth's orbit around the Sun is an ellipse; this results in the seasons. The eccentricity of the Earth's orbit periodically departs further from a circle and then reverts to almost true circularity. Periodicities are located around 413, 95, 123, and 100 ka. Secondary peaks appear to be located around 50 and 53 ka. There are further important periodicities at 1.23, 2.04, and 3.4 ma (Schwarzacher, 1991). Precession—Precession refers to variation in time of year at which the Earth is nearest the Sun (perihelion). This variation is caused by the Earth wobbling like a top and swiveling on its axis. Periodicities of 23,000, 22,400, 18,980, and 19,610 yr are recognized and often simplified to two periods of 19 and 23 ka. Secondary peaks are also located around 30 and 15 ka.