THE CLIMATIC FACTOR IN THE GEOLOGICAL RECORD

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Abstract
The greater part of the preserved geological record represents marine environments; source and transport conditions are difficult to assess, and there may be a tendency to interpret the stratigraphy in terms of diastrophic change without adequate consideration of the more obscure factor of climatic change. Nevertheless, recent oceanographic research has emphasized the effect that polar ice-caps have had on marine sedimentation through their influence on oceanic circulation. Continents suffer extremes of climate and the character of the deposits often favours the preservation of climatic evidence. Correlation by radio-carbon dating of alternations of arid-phase evaporite formation and humid-phase clastic sedimentation in inland drainage areas with fluctuations in extent of ice-sheets is noteworthy. Study of the Pleistocene floras has stressed the magnitude, complexity, and rapidity of the climatic changes, especially in the best-known late-glacial and post-glacial record. It seems probable that the pattern of zonal climatic change, whether short-term, seasonal, within the historic record, during post-glacial time, or within the Pleistocene as a whole, is basically similar, varying in scale and duration. The theory that fluctuation in solar radiation is a major factor in climatic change finds increasing favour. In the late Pleistocene the speed of temperature amelioration led to the survival of ice-sheets out of phase with their climatic environment, to stagnation, extremely rapid melting, and a correspondingly sudden world-wide rise of sea-level. A comparable sudden rise of sea-level inferred for the Carboniferous has presented a major difficulty on the diastrophic theory, but would fit well the 'glacial control’ theory of Wanless and Shepard. The extent and perfection of the Permo-Carboniferous cycles mark them as abnormal in the geological record. They are probably related to the long period of intermittent glaciation of one or more of the southern continents. A diastrophic origin for contemporaneous cyclic sedimentation in local lacustrine or terrestrial environments is questionable. It is envisaged that climatic fluctuations may have been considerable in the non-glacial phases of the earth's history. The apparent anomaly that general extinction—conventionally associated with catastrophic climatic change linked with world-wide orogenies—has occurred during orogenic calms is largely removed ff climatic changes are not necessarily linked with crustal movement. The environmental stimulus to biological change may thus be always present.