Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021 Orbital forcing timescales: an introduction MICHAEL R. HOUSE Department of Geology, The University, Southampton SO17 IBJ, UK Abstract: A brief review is given of orbital patterns affecting the Earth which may be of use in establishing, for long or short periods, orbital forcing timescales (OFT). The metronomic varia- tions of the Earth-Moon system and of the Earth-Sun orbital patterns produce gravitational and temperature effects which alter the physical environment on the Earth's surface. These give an interpenetrating effect of forcing cycles ranging from twice daily tides, day-night alternations, various tidal patterns and the annual solar pattern. All of these have been used palaeontologic- ally to give precision to short-term age determination in the past. It is cycles of the Milankovitch band which are showing promise of enabling new practical timescales to be established for parts of geological time. These depend on changes in the Earth-Sun distance (perihelion and precession cycles of 19 and 23 ka at the present time), changes in the tilt of the Earth's axis with respect to the Earth's orbit round the Sun (the obliquity cycles of 41 and 54 ka), and changes in the geometry of the Earth's orbit around the Sun (eccentricity cycles of 106 and 414 ka). Since the number of days in the year have changed through time; so have the periods of the perihelion and precession cycles. There is increasing evidence that small-scale sedimentary rhythmic couplets, often grouped into bundles, may repre- sent the effect of some of these; often the precessional couplets are grouped into bundles of five or so within the lower eccentricity period. The disentangling of the interpenetrating cycles to pro- duce an OFT is an exciting problem and challenge for palaeobiology and sedimentology. These should enable numerical dates to be given to biostratigraphic and chronostratigraphic timescales and eventually enable many earth processes to be analysed in real time. 26 Ma oscillations related to the Cosmic Year (c. 260 Ma) have been invoked to explain periodic mass extinctions in the fossil record. But evidence is presented to suggest such extinctions are not, in fact, periodic. The purpose of this contribution is to provide an complex, and in many ways poorly understood. introductory review of those orbital patterns which However, it is thought that resultant sedimentary have such an effect on the environment of the microrhythms result from changes of sea level, and Earth's surface that they give potential for the changes in the pattern of vegetation and erosion on establishment of orbital forcing timescales (OFT) adjacent land areas which are mainly driven by for parts, perhaps eventually much, of Earth climate. Figure 1 & Table 1 give the range of history. That the establishment of time in geology, orbitally forcing frequencies which may contribute for record of its past events and in the establishment to the development of timescales. of rates for processes is of major importance is self The recognition of the potential of orbitally- evident. At present we rely on biostratigraphic forced microrhythms for the construction of scales for the Phanerozoic, but these are relative, timescales was first most clearly stated by G.K. not absolute, scales. For the late Mesozoic and Gilbert (1895, 1900a, b) and developed further by Tertiary, in suitable circumstances, the radiometric Barrell (1917). Such ideas followed naturally from scales are extremely important, if incomplete; for the laws of planetary motion established in 1609 pre-Cretaceous rocks, however, their increasing and 1618 by Johannes Kepler, and the later recog- sparseness and unreliablity make them of limited nition, by Newton, of the role of gravitational practical use. The exciting possibility is that new attraction between planetary bodies. Adhrmar timescales can be constructed using microrhythmic (1842) and Croll (1875) gave an early summary of sequences which may show the effects of particu- such views and Charles Lyell considered them in larly precession, obliquity and eccentricity orbital detail in the later editions of his Principles of patterns, over frequencies usually referred to as of Geology. However, it was the calculations of the Milankovitch band, may provide timescales of Milutin Milankovitch (1920, 1941), using climatic considerable refinement. Such cycles affect the effects of orbital patterns to explain the ice ages, solar energy reaching the outer atmosphere because which was the major turning point; but such ideas the Earth-Sun distance is changed during them, or were not well received at the time. There followed seasonal distribution of insolation. The way in a long period when microrhythmic sequences which outer atmosphere changes are reflected in formed the basis for mathematical studies of series local changes on the Earth's surface is undoubtably analysis, but with little attempt to invoke the real From HOUSE, M. R. & GALE, A. S. (eds), 1995, Orbital Forcing Timescales and Cyclostratigraphy, Geological Society Special Publication No. 85, pp. 1-18. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021 2 M.R. HOUSE FREQUENCY YEARS ORBITALCYCLES Wells (1962), who recognized daily and annual banding in Devonian rugose corals and was able to estimate the number of days in the Devonian year at i 1.0 Ga GALACTIC YEAR rather over 400. The recognition of lunar effects 100 Ma GALACTIC [EXTINCTIONJ followed shortly after (Scrutton 1964), which BAND 10 Ma enabled the periods of the Earth-Moon orbits to be 1-0 Ma estimated for the Devonian. Since such motion ~3 2 ECCENTRIC/TY controls the perihelion and precession cycles, it has MILANKOWTCH I 100 Ka subsequently been shown by Berger et al. ~.__~1 OBLIQUITY (1989a, b) how these cycles have changed through BAND L 10 Ka PRECESSION PERIHEL ION geological time. The recognition of daily, lunar and 1.0 Ka annual effects in the shells of bivalves (House & HALE SOLAR Farrow 1968) was followed by many studies 100 a LUNAR NODAL (Scrutton 1978). The annual changes in tree rings BAND ~ POL E ECL IPTIC 10 a SOLAR YEAR have long been known and dendrochronology is CHANDLER now a discipline in its own right extending back ANNUAL over several thousand years. o,1 a EQUINOX Frequencies of orbital forcing cycles have been CA LENDA R t 1.o a LUNAR MONTH divided into the calendar band, solar band, BAND O,Ola SPRING TIDES Milankovitch band and galactic band (Fig. 1). DALLY o.001a TIDAL Imbrie (1985) used another system, which may be modified here by the inclusion of the highest Fig. 1. Logarithmic table of orbital periods which exert frequencies as follows: daily band (0-25 h), gravitational effects on Earth, or which which exert monthly band ( 25 h-0.5 a), annual band (0.5- orbitally forced changes in the temporal energy distribution reaching the outer atmosphere of the Earth 2.5 a), interannual band, (2.5-10 a), decadal band from the Sun. (10-400 a), millenial band (400-10 000 a), Milankovitch band (10 000-400 000 a) and tectonic band 400 000+ a). time dimension, with the exception of some elegant discussions on long records, such as in the late Trias of the Newark Basin (van Houten 1964; Olsen Annual and lesser orbital cycles (< 1.0 a) 1984; Anderson et al., 1984) dealing mainly with sub-Milankovitch band effects. These frequencies have been named the calendar The modern phase was undoubtably reached band (Fischer & Bottjer 1991). The principal lower- with the calculations of possible orbital forcing to order cycles may be separated into the tidal, whose produce documented evidence in ocean cores of effects result primarily from gravitational changes temperature changes that really established such in the Earth-Moon system and the solar, which theories indisputably (Hays et al. 1976; Imbrie & result from changes in the energy received from the Imbrie 1979; Covey 1984). The urgency to use such Sun resultant upon daily to annual changes. The tools to improve the timescale was pressed by second are well known and merit little attention House (1985a, 1986a, b) and is part of the theme of here, although it should be pointed out that for this symposium. There have been many symposia organisms, as for sedimentation, the interpene- and reviews in past years, both of biological rhyth- tration of these effects can be complex. Emphasis micity (Rosenberg & Runcorn 1975), sedimento- here will be on such factors as contribute to OFT logical and astronomical aspects (Merriam 1964; criteria. Einsele & Seilacher 1982; Berger et al., 1984; Cycles at frequencies of < 1 a are recognizable in Fischer & Bottjer 1991; De Boer & Smith 1994; both the sedimentary record, where they are Smith 1990a, b), and methods of mathematical embraced in the term rhythmites, and in the fossil analysis of microrhythmic sequences (Weedon record, where they show as growth banding where 1993; Schwarzacher 1964, 1975, 1987). The term the accretion of tissues reflects environmental cyclostratigraphy has been coined for sedimentary rhythms: under suitable circumstances these may phemomena, but there has been little concentration be preserved in both plant and animal tissues. It is on techniques to improve the geological time- unlikely that evidence from this source will ever be scales. integrated into a continuous timescale for the past. On the scale of daily, monthly and annual effects, Nevertheless, for short periods, the documentation the causation of tides essentially followed the of tidal, daily, monthly, equinoxial and annual recognition of the laws governing planetary cycles have already contributed much to short-term motion. A turning point was the classic paper by environmental analysis, quite apart from the contri- Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021 ORBITAL FORCING TIMESCALES 3 EARTH'S EOUA TORIAL PLANE FULL TO ~ J high tide L~ ~.~ high tide ~ equilibriu tide NEW ~ ~ J~J~ MOON'S' ORBIT MOON AXIS OF EARTH'S RO TA TION Fig.