34. THE PLEISTOCENE RECORD 



C. Emiliani and R. F. Flint 



1. Introduction 



The Earth is a geojjhysical, geochemical and biological system of such 

 complexity that less is known about its structure, composition and dynamics 

 than about those of most other celestial bodies. The complexity of the Earth as 

 a planet rests essentially upon a threefold basis: its mass, large enough to 

 have permitted the differentiation of core, mantle and crust, and the retention 

 of a substantial hydro-atmosphere; its heat balance, permitting surface water 

 to exist in all three phases; and its angular velocity and axis orientation 

 resulting in a diurnal cycle of convenient frequency. A perhaps unavoidable 

 consequence of these things has been the luxuriant development of life. 



Motions of a convective nature occur within the hydro-atmosphere, energized 

 almost entirely by the sun. There is some reason to believe that convective 

 motions take place also within the geosphere, energized by the decay of radio- 

 active nuclides and possibly by continuing growth of the core. Thus the Earth 

 seems always to have been in a mildly dynamic state, with a whole array of 

 geophysical, geochemical and biological processes operating concurrently, in 

 both cyclical and non-cyclical patterns, and interacting with each other in 

 complex ways. 



The various processes operate through an extreme range of time intervals. 

 Core growth and mantle degassing (two eminently non-cyclical processes) seem 

 to have begun soon after the final accumulation of the Earth, and are likely to be 

 still continuing and to continue for a long time in the future, even though their 

 rates may today be much smaller than in the early times when 235U and ^OK 

 were generating relatively large amounts of internal heat. Other non-cyclical 

 processes operating over extreme lengths of time may be the controversial 

 activities of continental drift or polar wandering. Cyclical or quasi-cyclical 

 processes, on the other hand, usually operate over much shorter time intervals. 

 Thus, orogenic cycles may involve tens or hundreds of millions of years; the 

 glacial-interglacial cycle involves only some tens of thousands of years ; and the 

 diurnal migration of plankton involves only a day. In addition, there are a 

 multitude of non-cychcal processes of relatively short duration, such as the life 

 of a volcano (a few years to several million years), the life of a marine molluscan 

 species (some ten million years), tectonic motions of various kinds, isostatic 

 phenomena, weathering, non-glacial eustatic changes of sea-level and short 

 term meteorological events. Some of these could be episodes in longer-range 

 cyclical or non-cyclical phenomena, but others are not. 



Whereas processes operating over short intervals (up to a few hundred years) 

 are easily studied by direct observation, those operating over longer intervals 

 can be understood only if, in addition to direct observation, their effects 

 through 2^eriods proportional to their durations are studied. In this context, 



[MS received March, 1961] 888 



