Milankovitch versus chaos: opposing or complementary theories?

Introduction The analytical phase of quantitative Palaeoecology Palaeoceanography started in the middle seventies with the development of an hypothetical-deductive model which seeks to interpret and explain rhythmical patterns on high resolution proxy time series as a linear response to a global mechanism known as orbital forcing. Directly related to the theory proposed in the thirties by the Serbian mathematician and astronomer Milutin Milankovitch, according to this ~echanism Pleistocene glacial cycles, were produced by variations of the solar radiation flux reaching our planet (Berger et al., 1984). The notion that these variations could be related to cyclic geometric changes in the earth's orbit began with J. Adhemar, in 1842, who suggested that the main cause for the "ice ages'' could be variations in the direction of earth's orbital axis and, consequently, in insolation (Berger & Pestiaux, 1984). Such theories usually assume that climate and the ecological systems in general have a linear response to the regular and periodic oscillatory behaviour of several Earth orbital parameters, i.e. the proxy data response from (palaeo )climate or (palaeo )oceanographic dynamics is more or less proportional to the magnitude of the orbitally induced periodic disturbance (lmbrie et al., 1984; Herterich & Sarnthein, 1984 ). This linear approach to Milankovitch theory will be referred to here as the ''Periodic behaviour model" (Cachao, 1992). The search for numerical solutions for the equations that rule our planetary system goes back to the first attempts of the French mathematician Pierre Simon de La place to solve an important issue: the Solar System's stability. By making some simplifications La place showed that his simplified system was integrable and that there where long-term (tens ofth<;msands of years) periodicities (Murray, 1992). The present day values for the periodic variations of the maximum solar energy that reaches our planet correspond to one of the trigonometric solutions of the Lagrangean equations for the planetary movements, which are (Berger, 1984): 400 and 100 ka, periodicities associated with sinusoidal variations of the earth orbit eccentricity around the Sun; 41 ka, a period related. to variations on Earth rotation axis obliquity; 23 and 19 ka, both periods related to variations in the precession of the equinoxes. These values are only valid for an interval of time that goes back to -5 Ma (earliest Pliocene) due to inherent limitations to the calculations involved ( op. cit.) and to the existence of multiple disturbance factors such as solid tides, solar wind, and meteoritic showers (Buys & Ghil, 1984 ). So several corrections have been introduced and new values proposed (e.g. Berger et al., 1989) for other time intervals. Milankovitch frequencies result from the nonlinear interference of the several gravitational forces involved in movement of our planetary system. The importance in recognising these frequencies in our time series data is obvious: it would increase our biostratigraphic resolution to a degree unthinkable a few years ago. But what does it strictly tell us about how the natural systems (climate, oceanic currents the "conveyor belt" of Broecker & Denton, 1990)-, upwelling and the " El Nifio" events, population dynamics, ecological succession) work? Not much. In fact the linear approach to the Milankovitch model assumes that the environment is ''transparent'' or passive to the orbital frequency signals. But our everyday experience suggests the opposite. There are no natural dynamic systems that can be illustrated by a straight line, or in other words, natural processes are all nonlinear• By nonlinear we mean ''with feedback' ' and the full consequences of this are only now being understood through the application of Chaos Theory, since the first discoveries of Eduard Lorenz, a meteorologist from the Massachusetts Institute of Technology, in the early 1960s (Lorenz, 1963). The main purpose of this essay is to contribute to the development of an alternative model that may find room to explain, as a gestalt theory, not only data series with rhythmic patterns but also alternating processes without a strict periodicity. To discuss whether there is ''Milankovitch'' or not is a false problem. Orbital forcing has always been present as a pumping mechanism on Earth environment. Whethe.r its frequencies are stable or change to some degree, is another problem which will not be addressed here. What we should discuss is under what conditions should we expect to recognise these periodic signals in our data and in which conditions should we expect not to see them, and why? ·