PERIODIC CLIMATIC VARIATIONS IN THE EARTH'S GEOLOGICAL PAST Paleoclimatological data occupy a prominent position among new facts and generalizations in the fields of geology, geography, and related sciences. These data are of fundamental interest, and possibly character ize better and more completely (compared with some other more striking discoveries) the current developmental trend of the geological and geo graphical sciences.
It is characteristic of this trend that investigators working in different branches of science discover that a close interrelationship exists between various facts which appear to be outwardly quite dissimilar, and that these investigators are forced to turn to extraterrestrial, cosmic factors for an explanation of the vast group of phenomena which heretofore were the domain of the geographer and the geologist. In this connection data concerning periodic variations in climate during past geological periods become most important. Analysis of numerous observation and specialized paleogeographic reconstructions concerning deposits of various ages, beginning with Upper Archean and ending with Neogene and Quaternary formations, leads to several inferences which appear to be of fairly general significance. Some of these inferences are presented below.
1. Climatic variations (or fluctuations) were generally synchronous for large areas of the Earth's surface, i.e., they were of a planetary nature.
2. The general regularities of such climatic variations (their duration and trend) have been operative throughout the entire geological history accessible to specialized investigations, probably beginning with the Archean and certainly from the Middle Proterozoic, and continuing until the present time. The facts elicited by geologists and paleogeographers offer no suggestion of any fundamental changes in the general trend of the climatic evolution in the remote geological past in comparison with the thoroughly studied climate of the Anthropogenic or even with that of historical periods.
The general, universal scheme of variation in the conditions of the Earth's aerial and liquid envelopes during the development of our planet has not been disturbed by the specific climatic features of the individual geological periods, such as the sheet glaciation of the Upper Pre-Cambrian or Upper Paleozoic (Gondwana region), the hot and humid climate of the Lower and Middle Carboniferous periods, or the arid conditions in the Devonian, Upper Permian, and Lower Triassic periods.
3. Climatic variations constitute rhythms or cycles of different durations. Short periods of climatic fluctuations have left their distinct imprints on sediments of all the geological ages, both in the continental facies (lacustrine-glacial and proper lacustrine sediments) and in the marine facies, especially in shallow marine and lagoon sediments. These features include rhythms of saliferous and carbonate or argillaceous carbonate sediments (Figure 1), varved structure of lacustrine and marine oozes containing numerous intercalations found in the skeletal parts of microorganisms or algal residues, etc. Finally, short climatic rhythms are vividly reflected by the structure of organic matter, including annual rings of fossil wood as well as those in the xylem of present-day trees (Figure 2), structural regularities of accumulation layers of carbonate algae (Figure 3), structures of mollusk shells, etc.
Short climatic periods correspond to the principal seasons of the year (more rarely to shorter periods, i.e., parts of seasons), as well as to periods of 2 to 3, 5, 11, 20 to 22, 35, and 70 years (Figures 4 and 5).
As yet, cycles of 100, 1000,. 10,000 to 12,000 years, etc. can be established only with a small degree of certainty. The maximum duration of well established climatic cycles approaches 180 to 200 million years.
In studies of the banding of sediments and of the climatic rhythms, the reliability of inferences depends on proving the annual nature of the principal banding elements, i. e., of a conjugated pair of elementary layers corresponding to summer and winter, or more precisely to the autumn winter and the spring-summer seasons. This task is facilitated when the investigator deals with banding in organogenic structures, such as separate bioherms or entire reef structures, and even more so when he deals with fossil wood, Investigation of varved clays (varve sediments) in Sweden, Norway, and Finland and recently also in the USA demonstrated that this problem can be adequately solved for periglacial lacustrine sediments as well (Figure 6). In studies of the banded sediments of marine and continental origins, it °s sometimes very difficult to establish the seasonal nature of the banding.