E Horizon

Abstract

Cycles in the major chronological sense of affecting the planet Earth are (a) rather strictly defined within an astronomic framework; (b) in an evolutionary sense, that is, taken loosely in a birth / death or youth / maturity / old age system; (c) in a stochastic or random sense within a window, with feedback between certain constraints; and (d) on a small scale dynamic sense, like a ball bouncing to and fro. Astronomic chronological cycles 200EARTH CYCLES role played by the dedication of a few in the face of daunting adversities. R. W. F.)The orbital variations demonstrated by Milankovitch could only be approximated at first, because each of the cycles was slightly different. Only very long-term series could establish the precise averages, and in the geological sequences, the dating precision falls off in time which is measured in up to 10 8 yr periods. The best-established value for the general precession is 25 694 yr (pers. comm. Berger, 1992), which the writer (R. W. F.) is able to determine is equivalent to 2 010.0 Â 12.78 297, the Neptune-Jupiter "Lap" (beat frequency). This demonstrates that the fundamental lunar orbital parameter is precisely in tune with two key planets. The general resonance of the planetary orbits is discussed in another Encyclopedia (Planetary Sciences: Shirley and Fairbridge, 1998).Very approximately, the principal Milankovitch periodicities are as follows:Eccentricity: 410 kyr 94 kyr Obliquity: 40-41 kyr Precession:23 kyr 19 kyr Evolutionary cyclesCycles in this sense are not defined mathematically but clearly exist in a growth and decline sense (Small, 1970). Thus, an oak tree starts with an acorn, sprouts, grows to a sapling, eventually to a full-grown tree, but eventually passes into an old-age state, which may last 500 years or more, until disease, decay, and lightning-strike or whatever, terminates its life cycle. A youth-maturity-old age evolutionary theory was developed for geomorphology by W. M. Davis, a celebrated Harvard professor of a century ago. In a two-volume compendium on landforms by Chorley, with Beckinsale and Dunn (1964, 1973) the volumes are divided into pre-Davis and post-Davis, which reflects his former importance. Since then, the popular thrust in geomorphology has turned to a quantitative, process-approach. Youthmaturity-old age was set aside. Nevertheless, it is clearly a conceptual tool, as in (a) the theoretical emergence of a new land surface from the ocean, (b) the initiation of a rill and drainage system, (c) evolution of "youthful" valleys with fast-running streams, rapids and waterfalls, (d) gradient reduction, flood plains, meandering water-courses, and (e) reduction of the entire landscape to a peneplain. It was a beautiful, idealized model, but no example could ever be found. The explanation is that the Earth's surface is immensely complex, its tectonics active, and lithology varied, while paleoclimates and their inherited traces are great in number. Accordingly, soil classifications cannot be fit into convenient generalizations, except in a non-...