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Home , Equinox, Galactic year, Geochronometry, Milankovitch cycles, Orbital forcing

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Orbital forcing timescales: an introduction

MICHAEL R. HOUSE Department of , The University, Southampton SO17 IBJ, UK

Abstract: A brief review is given of orbital patterns affecting the which may be of use in establishing, for long or short periods, timescales (OFT). The metronomic varia- tions of the Earth-Moon system and of the Earth- 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, - alternations, various tidal patterns and the annual solar pattern. All of these have been used palaeontologic- ally to give precision to short- 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 . These depend on changes in the Earth-Sun distance (perihelion and 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 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 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 . Figure 1 & Table 1 give the range of . 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 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 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 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 is CHANDLER now a discipline in its own right extending back ANNUAL over several thousand years. o,1 a Frequencies of orbital forcing cycles have been CA LENDA R t 1.o a LUNAR divided into the band, solar band, BAND O,Ola 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 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 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. 2. Diagram showing the Earth-Moon system and the changing axes and orbits which produce tidal changes from twice daily tides to the lunar nodal cycle of, at present, 18.6 a. bution to factual knowledge on the Earth-Moon makes a low (and changing) angle with the axis of orbital parameters in the past. the Earth's rotation (Fig. 2). This angle, or declina- tion, is not constant, and obviously the inequality is greatest when the is greatest. At the Tidal effects time when a tidal node is below the Earth's The around the Earth (Fig. 2) on one side of the Earth it will be above the Equator attracts a tidal wave around the Earth with a period on the other side. At a place in mid- (illus- of approximately once a day, the actual period trated in Fig. 2), therefore, the higher high water (24 h 54 min) being termed lunar day, since it is will be when a point is closest to the Moon. Local not quite of the same period as the day-night cycle, geographical effects of coastal shape and seafloor or solar day. It is fortuitous that, at the present time, morphology, and of storms, often far away, will the period of the Earth's rotation around it's spin modify these simple cycles, often considerably. axis is only slightly faster than the orbit of the There is a distinct solar tide which has an Moon around the Earth giving such similarity interval of 12 h. Although the gravitational attrac- between the solar day and the lunar day: this was tion due to the Sun is 177 stronger than that not so in the past. However, as Newton demon- of the Moon, the solar tides are smaller in effect strated, because of the centrifugal force resultant than lunar tides because there is a fundamentally upon the rotation of the Earth, not only is the different relationship between the two; the Earth is strongest tide developed around the Earth where the in orbit around the Sun and hence a state of non- plane of the Moon's orbit crosses the Earth (giving weightlessness occurs because the radial compo- the equilibrium tide of Fig. 1), but tides are nent of force due to the Earth's orbital momentum developed at opposite sides of the Earth in the form is almost cancelled out by the gravitational pull of of water heaping (tidal nodes) at points closest to the Sun. and furthest away from the Moon. Thus, given the It is easy to consider tidal effects solely in terms rotation of the Earth, the typical tidal cycle is of the sea, and the effects on the seashore, and formed which gives about two tides a day, a pattern limited to the changing distance between high which it is convenient to call semidiurnal (half-day, and low tide marks. This would be a mistake. or half-solar day) but which in reality is half of the Undoubtedly of greatest importance for tectonics is lunar day (Fig. 3A). the continual stresses on the deformable solid earth Usually, the two semi-diurnal tides are not equal, by tides ranging from the semidiurnal tides to the that is the alternation of levels reached by succes- tidal node cycles of c. 20 a. Rocks may give a less sive tides is not the same. Hence, there can be a obvious response, but the continual stresses pro- lower high tide (low high water) followed by a duced in this way will be an important factor in higher high tide (high high water) (Fig. 3A). This their ultimate relief by fracture, jointing, faulting diurnal inequality of the tide results from the fact and folding, and perhaps at much larger scales too. that the orbital plane of the Moon around the Earth Similarly, the effects caused by cyclical movements Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

4 M.R. HOUSE

A SEMI- DIURNAL I , I ! :! ' .

I...

0- _J >w -1-

-2- diurnal inequality

B MIXE D 1 0 v vvvt'vv'v, vv,v'vltvVvvvv v !/

C1_ D URNAL AA

~ i 1 0 l0 20 30 DAYS Fig. 3. Diagram showing: (A) a semidiurnal tidal system with diurnal inequality, (B) a mixed tidal system; and (C) a durnal tidal system (modified from De Boer et al. 1989, based on Lisiutzin 1974).

of the atmospheric gases will modify wind move- information on local tidal regimes and on the ments and pressure systems. But these we have no timing of astronomical controls. means, at present, of documenting. The founder of petrography, H.C. Sorby, over a hundred years ago, appears to have been the first to recognize petrographical differences between the Semidiurnal (0.00135 a) ebb and flood tidal characteristics in the Jurassic The typical twice-daily rise and fall of sea level can Forest Marble of England. He was able to distin- follow several patterns from one in which there is guish different directions of depositing flow an extreme of semidiurnal change (twice daily) to between the ebb and the flow sedimentary regimes. one which is essentially daily, or circadian, but in Examples of such rhythmites from the German reality reflects the period of one lunar day. Both coast were reported by Reinecke & Wtinderlich cases have been recorded from sedimentary (1967) (Fig. 4B), who found that fine sand was sequences and fossil shells from the past (see deposited at times of tidal current flow and clays below). This is a means of obtaining precise were deposited from suspension during times of Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

ORBITAL FORCING TIMESCALES 5

~ Flood tidal current T Low water slack Tide 2 Ebb tidal currentr

:+..:.,.,-,- - J----- High wateLter slack

T 5 Tide 1 ~Fine sand ITml ~ -- Mud

i~'ig. 4. Semidiurnal signatures in sediments.(A) Semidiurnal inequality and the progressive time progression between high-water slacks and low-water slacks (darker bands): an example from the Upper Carboniferous of Francis Creek, Illinois, USA. Photograph kindly provided by E Broadhurst. (B) A similar example, but from Present day North Sea sediments (modified from Reinecke & Wtinderlich 1967).

minimal current flow at either high or low slack shows the effects of changing diurnal inequality in tides. At that locality the ebb tide was much the tides (Fig. 5). A rather more systematic study of stronger than the flood tide and hence more sand such changes has recently been published by Ohno was deposited on the ebb. Note that such records (1989). An example in Fig. 6 illustrates an acetate only occur when the environment allows con- peel made from a polished radial section of a speci- tinuous deposition. Modern examples have been men of Cerastoderma edule from mid-tide level of discussed by Allen (1981, 1982). the Bury Inlet, South Wales. This shows a dominant An excellent example (Fig. 4A) from the diurnal pattern with only slight evidence of a semi- Carboniferous of Illinois has been illustrated by diurnal effect. Broadhurst (1988), in which a typical semidiurnal tide pattern is seen where progressive changes in Solar day (0.0028 a) the timing of ebb and flood results in a systematic changes during the neap-spring-neap cycle Because the lunar day so closely corresponds, at the (Fig. 4A). present time, to the solar day, it is understandable For fossils, elegant contributions on fine-scale that it is often difficult to disentangle the effects of banding in bivalve shells was given by Evans day-night rhythmicity from daily tide rhythmicity, (1972) and Pannella & MacClintock (1968). Evans in the past especially when modulation by a demonstrated for the Recent cockle, Clinocardium semidiurnal tide effect is not apparent. Thus, the nuttalli from the coast of Oregon, that there was a estimation of days in the , or Earth year, fine-scale cyclicity in the growth of the ostracum are dependent on whether tidal or insolation effects which matched the semidiurnal cycle and which are dominant. The authors of many such estimates Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

6 M.R. HOUSE

A 1970 J u n e I Jury

B 13 .. 14 15 16. f7 18 19 20 21 22 23 2~ 25 26 27 28 29 30 I 2 3 /~ 5 li 7 e 9 10 11 12 13 1/, 15 16

C ~ i , ~ / / s / 9 / / / ." ." .~ / / / e -" . / ,. . / , z i i r i i 1 / , / , / . . ~ ~ -" / t - .- / / z /

= = =~ One rnitlirnetre Fig. 5. (A) Progressive semidiurnal tidal signatures for Empire, Oregon, USA, for a period in June and July 1970. (B) Correlated growth pattern record of Clinocardium nuttalli from the Oregon coast for the same period (modified from Evans 1972). either do not mention this difficulty or assume that shells such patterns have been well documented the present day situation of near-identity held in (Figs 5 & 6), but there are not many good fossil the past. Scrutton (1978) drew attention to this examples partly because many shells, being made problem. of aragonite, recrystallize after fossilization. In the The importance of a circadian biorhythmicity to case of littoral shells, position relative to mean sea- living things is well known and well documented level is important since this controls modulation (Harker 1964; Neville 1967; Clarke II 1974). It of shell growth and low tidal forms show the operates physiologically, behaviourly and, when phenomenon least (Farrow 1971, 1972). approriate, in the deposition of organic tissues. Geological information is only obtainable when Lunar month (0.081 a) fossils record circadian growth increments in preservable hard parts. At the alignment of the Earth, Moon and Sun is most perfect and this gives a dominant tidal Spring tides (0.038 a) effect leading to enhanced spring tides. The period of the lunar month at present is 29.53 days, and Spring tides occur approximately every 14 days there are 12.37 lunar in the year. Tidal when there is an approximate alignment of the Sun, gauges show the greater amplitude of spring tides at Moon and Earth (Fig. 2). This occurs either at con- this time (Fig. 8A). junction or opposition of the Moon and Earth with Evidence for the lunar month in the past has been respect to the Sun. Neap tides occur between these claimed from the Precambrian to the Recent and when the Sun and Moon are in quadrature with the attempts have been made to show how the number Earth. Tidal records show this well (Fig. 8A). of lunar months per year has increased in the past, Sedimentological evidence of this effect in ancient reaching in excess of 13 in the Precambrian rocks has been recognized and discussed especially (Williams 1989a, b). Data has been drawn from by Williams (1989a, b). corals, bivalves, stromatolites and other groups. In the pattern of growth increments in modern The first such study was by Scrutton (1964),

Fig. 6. Semidiurnal, diurnal, lunar and annual () signatures in the growth of the cockle Cerastoaerrna eaute (Linnaeus) from the mid-shore level in the Bury Inlet, South Wales, UK, based on an acetate peel of a polished radical section, • 80 [by Farrow (1972, pl. 8B)]. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

ORBITAL FORCING TIMESCALES 7 who claimed lunar month periodicity in Devonian (0.5 a) using corals collected by the writer from the Bell Shale of Michigan (Scrutton 1964, Fig. 7). The These are the two times in the year when the Sun is exactly above the Equator and day and night are of calculation was based on fine-scale increments thought to be daily. But, as Scrutton (1978), equal duration. The vernal (spring) equinox for the pointed out, whether this referred to the lunar day is about March 21 and the or solar day is not clear. Thus, the calculation of autumnal equinox is about September 23: for the the lunar month could be in error. Figure 7 shows it is the reverse. Associated with this are the equinoxial tides which form their a well-developed banding of this type. Pompea annual maxima at this time and storms are often & Kahn (1979) made similar observations in nautiloids. The cause of the lunar banding is not associated with climatic changes. clear; is this caused by tidal interference at extreme spring tides or is it a lunar-related or biological Annual cycle (1.0 a) rhythm connected with spawning? Pannela et al. This is the frequency which has received most (1968) produced a graph of days per month show- study. There are 365 solar days in the Earth year. ing a rise from the present period of 29.53 to a Environmentally it represents the complete cycle of plateau just below 30 days during the Mesozoic and the dominant extremes of solar radiation reaching rising to c. 31.5 in the Cambrian. The more recent the outer atmosphere of the Earth. There are, how- review of Williams (1989b) shows greater error ever, lag effects in how this operates to affect bars on the data, but a similar trend. Calculations of climate at the surface of the Earth where highest the period of the Milankovitch band precession and or lowest temperatures are delayed. obliquity in the past depend on accurate knowledge For sediments the classical work of de Geer of the period of the lunar month in the past. (1928), on annual varving was on sediments in

Fig. 7. Diumal rhythms and lunar monthly bands shown on the epitheca of a rugose coral from the Bell Shale, Michigan, USA, collected by the author. (Photograph kindly supplied by Dr. C. T. Scrutton and figured Scrutton 1964). Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

8 M.R. HOUSE lakes near glacial regimes where spring melts and Chandler Wobble (1.17 a) organic debris classically comprise a couplet or two of laminae resulting in a single In 1891 S.C. Chandler noted that variations varve. Varves have led to the establishment of a contained two components with periods of 428 post-Glacial . A more general term for days (14 months) and 1 a. Known as the Chandler laminated sediments is laminites since their annual Wobble, it is considered to be a free wobble, the period is not often demonstrable. Lacustrine varves period being extended from an expected 304 days through time have been reviewed by Anderson & (on a rigid-body theory) by the response of the Dean (1988). deformable Earth to rotational forces (Munk & There are many examples of ancient varved MacDonald 1960; Smylie & Mansinha 1971; sediments. For the Tertiary, the Eocene Green River Chinnery 1971). This seems to have no OFT Formation Colorado (Bradley 1929, 1931; Fischer relevance. & Bottjer 1991). The Jurassic Todilto Formation (Anderson & Kirkland 1960) has long sequences of varves. Hallam (1960) has claimed occurences in El Ni~o or ENSO (1.0-9.9 a) black shales in the Jurassic. For the Trias long sequences have been established in the Lockatong Off the coast of Peru the cold Humboldt Current Formation of the Newark Basin (van Houten 1962). from the south usually gives way around Christmas For the Carboniferous Kvale et al. (1989) have (hence E1 Nifio) to tropical and warmer waters from recognized annual and higher-frequency cycles. the north. Changes in rainfall, sedimentation and Devonian annual varves have been claimed in the ecology result. Stronger changes occur about every Ireton Shale of western Canada (Anderson 1961) four years and there may be other peak frequencies and in the Achanarras Limestone of Scotland up to 9.9 a (Quinn & Neal 1987). More extreme (Rayner 1963; Trewin 1986). Precambrian varving E1 Nifio effects than over the previous has been claimed for the Mid-Proterozoic (c. occured in 1982 and 1984. It is now recognized that 1.75 Ga) by Jackson (1985). The Elatina laminites there are global effects climatic of an E1 Nifio type. (Williams 1989a, b) suggest that there were then Whilst these effects may be initiated by orbital c. 400 solar days to the year. factors their operation is complex Organisms commonly reflect the annual cycle in Sedimentary cycles at about this period are their deposited tissues. Annual growth rings in increasingly being interpreted as ENSO effects. plants have established dendrochronology as a Cycles at 4.8-5.6 in the Eocene Green River discipline (Creber 1977; Fritts 1976a, b) giving a Formation (Ripepe et al. 1991) may be of this type, chronology going back over five millenia [Suess confirming their recognition by Bradley (1929), (1970), following fundamental contributions of and there may be a weaker periodicity at 33 a. A.E. Douglas in his monumental Climatic Cycles and Tree Growth (1919-1936)]. Studies have been attempted on Mesozoic material (Creber & Lunar perigee (8.85 a) Chaloner 1985). Similarly for animals, annual bands have been The distance between the Earth and Moon varies used for dating fish otoliths and bivalves, and such and it is closest, at perigee, every 8.85 a. It has been groups, and in addition brachiopods, stromato- calculated for the Precambrian Elatina Formation poroids and stromatolites, but the time spans and Reynella Siltstone (65 Ma) at 9.7 + 0.1 a involved a very small. An early well-documented (Williams 1998a, b). study demonstrating annual banding in the cockle Cerastoderma edule was by Orton (1926), in which growth is very restricted during winter-forming winter bands (Fig. 6). Solar year (c. 11.0 a) Sunspots are localized vortices on the surface of the Orbital cycles between annual and the Sun thought to be produced by magnetic activity: Milankovitch band (1.0 a-10.0 ka) spots often appear in pairs with opposite magnetic polarity. The spots reduce of the Sun Frequencies within this span have been refered and hence the solar energy received by the outer to as the solar frequency band because solar atmosphere of the Earth (Dickey 1979). In 1843 phenomena and atmospheric and magnetospheric S.H. Schwale discovered that there is a cyclicity reactions to them dominate (Fischer & Bottjer when spots reach maximum of every c. 11 a, and 1991, p. 1065). But several gravitational elements this is termed the solar year (a term sometimes continue to be important. For convenience these are used, misleadingly, also for the earth year); spots again treated in order of increasing wavelength. move latitudinally during the cycle. Higher periods Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

ORBITAL FORCING TIMESCALES 9

E _ b- lunar month :E _ L9 w I LU "1- _

...J

r'~ _

b- , , I I ' l t I I I I 1 I I i 1 1 1 I I 20 40 60 80 100 DAYS

E [unar nodal cycle -- U 20- W Z rr 10- _J r-a

~--0 ' l ' I I I I I' 1 I 1930 1950 1970 YEAR

Fig. 8. (Top) Showing variation in the daily tidal height from January 1 to April 10 1966 for Townsville, Queensland, Australia, showing fortnightly tides of differing amplitude and the corresponding lunar month period (data from Flinders Institute for Atmospheric and Marine Sciences; after Williams 1989b). (Below) Annual mean tidal range for Boston, Massachusetts, USA, showing the 18.6 a lunar nodal cycle (modified from Kaye & Stucldey 1973; Williams 1989b). up to 60-120 a have been thought to be due to Lunar nodal (18.61 a) similar activity but the 22 year cycle (Hale cycle, see below) is the best attested. These are not, of The lunar orbital plane relative to the plane course, controlled by orbital factors but they have (Fig. 2) is not constant but precesses with a present some time relevance. day period of 18.61 a. This is the period of the Anderson (1965) reviewed Recent, lunar nodal cycle. There is a wobble on the plane of and pre-Pleistocene records of this cycle. It has the Moon's orbit around the Earth such that the been, in particular, long recognized in the Eocene direction of tilt changes through 360 ~ in Green River Formation, where Bradley (1929) on the over 18.61 a. Although the recognized an 11 year cycle: Crowley et al. (1986) orientation of the tilt changes, the angle between calculated a 10.4-year cycle and Ripepe et aI. the plane of the Moon's orbit and the ecliptic is (1991) a 10.4--14.7-year cycle. In the Jurassic constant at e. 5 ~ (Kaye & Stuckley 1973). The Todilto Formation, Anderson & Kirkland (1960) effect of this is to give very enhanced tides at this recognized this cycle, and also higher wavelength frequency (Fig. 8B). cycles. The cycle has been recognized in the Williams (1989a, b) has claimed to recognize Precambrian (Williams 1981). For the mid- this cycle in the Proterozoic Elatina Formation and Proterozoic Jackson (1985) considered a 7.0-11.0- Reynella Siltstone (650 Ga) when, he suggests, the year cycle to be recognizable. period was 19.5 + 0.5 a. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

10 M.R. HOUSE

NORTH ECLIPTIC POLE PRECESSION 1 I NORTHCELESTIAL I POLE I ECCENTRICITY l /

// / / \ /

J J

L NORMAL TO AXIS OF ANGULAR J EC LIP TIC MOMEN TUM

Fig. 9. Diagram illustarting the Earth-Moon-Sun system and the oscillations which produce changes in insolation and energy flux on the Earth;'s outer atmosphere such as may lead to orbitally forced signatures in the sedimemtary record. For explanation see text.

Hale cycle (c. 22.0 a) (Imbrie & Imbrie 1979; Imbrie 1985). The main cycles of precession, obliquity and eccentricity A sunspot cycle twice that of the solar year has (Fig. 9), although almost sinusoidal, combine to been been recognized both in historic time and in give quite complex patterns (Fig.10), and changes geological time. It is named after G.E. Hale who, of insolation flux of c. 5%, and of up to c. in 1908, recognized the magnetic character of 100W m -2. The reflection of these outer sunspots (Mitchell et al. 1979). atmosphere energy changes at sea level in climatic Early records at this frequency were reviewed by changes is undoubtedly very complex, not only Anderson (1965). This is the dominant periodicity affecting climate, but by thawing or freezing ice, in in the Miocene Sicilian anhydrites (Fischer 1986) changing sea level, and in changing climatic and has a powerful signature in the Devonian Ireton regimes, altering erosional and weathering pro- Shale (Anderson 1961). cesses so that the nature and rate of sedimentation will change. These various frequencies vary in Cycles of the Milankovitch band relation to the Equator. This geographical control is (10 ka to 1.0 Ma) apparently the reason why at various times in the geological past, when obliquity or precession Introduction influences may dominate as the major control of orbitally forced microrhythms, the precessional The complex orbital patterns of the Earth- effects are greater at low latitudes. Moon-Sun system presumably result from chance patterns on coalescence during their origin. These orbital effects operate through changing the seasonal distribution of insolation and the distance Precession cycle (19-23 ka) between the Earth and the Sun from time to time. Precession is usually used for the combination of Such changes alter the amount of solar energy, or the precession of the equinoxes and the movement insolation reaching the outer atmosphere of the of the perihelion. These cycles (or pseudocycles) Earth. They are appropriately named after Milutin refer to the movement of the axial projection of the Milankovitch who used such orbital changes to axis of rotation of the Earth with regard to the . produce a coherent explanation of the ice ages It was first recorded in 129 BC by . The Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

ORBITAL FORCING TIMESCALES 1 |

0.05-

0"02

24"5 OBLIQUITY

23'3

22'0

- O. 07 -1 PRECES SI ON .

-0.02

0-04

2.7 E TP

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- 2''7

" | I ' 26o ' 8do KILOYEARS

Fig. 10. Variations in oscillations of the Earth-Sun system due to eccentricity, obliquity and precession (19 ka) and the calculated resultant sum of all such oscillations which is a measure of the energy by the outer atmosphere (based on Imbrie et al. in Berger et al. 1984). projected axis today lies close to the Pole , perihelion, is indicated in Fig. 10, and is an , but through time it precesses through important high-frequency modulator. a celestial with a period of c. 26 ka. This wobble of the Earth's axis is caused by the pull of Obliquity or tilt (41 ka) the Moon and Sun on the Earth's equatorial bulge. Other introduce a slight additional preces- The projection on to the sky of the Earth's Equator sion. The fundamental periodicity varies relative to is the . This plane lies at 23.5 ~ from the ellipse of the orbit, i.e., to the equinoxes or the vertical to the ecliptic, the plane of the Earth's perihelion, with two peaks at the present time of orbit. This angle varies by c. 3.5 ~ fluctuating from 1900 and 23 000 a. The former, the time off 21.5 ~ to 24.4 ~ and back with a period of about Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

12 M.R. HOUSE

Fig. 11. Photograph of a Jurassic sequence in the Atlas Mountains north of Rich showing the small scale couplets groups as bundles of about five couplets and representing the interpenetration of orbital forcing due to the eccentricity and precession cycles (photograph M.R. House).

41 000 a. This changes insolation energy received suggests the operation of an approximate 5 : 1 ratio by the outer atmosphere in two ways; it changes the in agreement with the approximate ratio between intensity of the seasonal cycle and it alters the pole- the 23 ka precessional and 106ka eccentricity to-equator insolation gradient on which climatic cycles. Although orbital forcing was invoked as a and ocean circulation depend. For a given latitude causation of some sedimentary rhythmicity earlier, and , typical departures from present day there is no doubt that it was only the analysis of the values are of the order of • (Imbrie 1985). oxygen isotope records in ocean cores, now going back over 2.5 Ma (Fig. 12), which has convinced most sceptics of its importance. Eccentricity (54, 106 and 410 ka) Precession and obliquity are dependent upon the There are several wobbles on the orbit of the character of the Earth-Moon system. Since, as Earth-Moon system around the Sun (Fig. 9). Some demonstrated earlier, there is good evidence that are true eccentricity factors, but also the orbit itself the lunar day and lunar month have changed with changes from near-circular to an ellipse. The effect time, then so will both the precessional and of the latter is considerable, and variations in obliquity periods. A calculation to correct these insolation, which are minimal when the orbit is periods through geological time has been provided circular, can reach 30% of the total flux at extremes by Berger et al. (1989a, b) (Fig. 13). The eccen- of the ellipse. The most powerful of these are the tricity cycles are not changed by modifications of 106 and 410 ka cycles. Figure 12, based on the the Earth-Moon system. analysis of a 2.5 Ma record of oxygen isotope Many examples of the operation of Milankovitch records from ocean cores, shows the signature frequencies below 106 ka have been documented (labelled 100 ka). (see especially Berger et al. 1984; Fischer & Bottjer 1991, De Boer & Smith 1994). Particularly valuable have been the correlations between Interpretation rhythmic sequences and oxygen isotope measure- Sedimentary rhythmicity is common in the ments (Ditchfield & Marshall 1989) confirming stratigraphical record (Fig. 11) and is much temperature dependence and correlations with debated. The only striking suggestion that such carbonate percentages (Weedon & Jenkyns 1990), patterns are orbitally controlled is when couplets which might be expected to have a temperature- of rhythms are combined into groups of about control effect. The frequency of metre-scale five couplets as bundles (Fig. 11), since this rhythms in the stratigraphic record, in many facies, Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

ORBITAL FORCING TIMESCALES 13

0'12 DSDP 607 50 ..... jJ'J~ o 008, 41 ka Obtiq ui t~.,~.~ l > 106 ka 1 _~JC_ 0.04"

nr' 30~" o o 0"02, 0.04 , 0-6 b 0"08 Cycles per ka Precession 20 _~ J 0.12 400 3bo ' 2;0 ' 100 ' ' 0 L. 41 ka ODP 677 MILLION YEARSBEFORE PRESENT ~o 0"08" Fig. 13. Graphs showing the changes through time of Q" / 106 ka orbital periods for obliquity (top) and precession (below). (Based on Berger et al. 1989b).

9-> ffo4 term-cycles of the sort recognized by Heckel 3ka (1986) can form the basis for a framework, and we ~,,.~ .__:_~ _ are clearly not at that yet. Meanwhile, the best approach may be to concentrate on stage duration, o 0"02 0"04 0'06 0"08 and the duration of zones within stages, as has been Cycles per ka commenced for the Givetian (House 1992 and this volume) and Cenomanian (Gale 1989 and this Fig. 12. Fourier spectrum of oxygen isotope records for volume). For this purpose sequences from basinal the last 2.5 Ma (using data of Shakelton et al. 1990) or pelagic regions with little asymmetricity in based on a spectral analysis of Weedon (1993) and showing the eccentricity (106 ka), obliquity (41 ka) rhythms and with good biostratigraphic control and precession (23 ka) peaks (from Weedon 1993). are required, i.e. sequences of the sort illus- trated in Fig. 11. Sections such as those of the British Lias or Kimmeridge Clay suffer from too many hiatuses and are from too shallow a facies was emphasized by House (1986a, b), who argued to be very helpful. that a common interpretation was required. Anderson & Goodwin (1990) considered such an allocycle a fundamental stratigraphic unit, building Long period orbital cycles (>1.0 Ma) on their earlier work on Devonian rhythms Periodic extinctions (Anderson et al. 1984; Goodwin & Anderson 1985). Similar cycles have been recognized in There has been much interest in recent years in terrestrial sequences (Astin 1990), and in bitu- the possibility that orbital events of very long minous and evaporitic sequences (Bougersma- frequencies may have periodically affected the Sanders 1971). environment of the Earth. The reappearance of For operation of longer-term cycles a fascinating Halley's Comet every 76 a appeared to be a trigger study lies ahead. For example, Heckel (1986) for such thoughts, and especially the return in 1985 recognized a long series of Pennsylvanian sedi- and 1986. It was the publication of the claim by mentary cycles which he estimated might be Raup & Sepkoski (1982, 1984) that of 0.5 Ma each, and related to transgressive pulses fossil groups occurred at regular intervals during due to ice-melting. If these are the 410 ka cycles the post-Palaeozoic that led to the proposal of many then the framework for a precise timescale is, hypotheses to explain this. The period suggested by established. Raup & Sepkoski was c. 26 Ma, but 29 and 31 Ma It is unlikely that a continuous timescale at the periods were also proposed. Fischer & Arthur Milankovitch band level is possible unless long (1977) had made similar suggestions earlier but had Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

14 M.R. HOUSE

9 . : ...... ,, -'. -, . .

:'.". ." ". -.:.":'z"'" ;,,:-",L" : " ~" " " " " " '., ". -" .-_ ' 9 - .,'" : :" ".:..:.... " 9

. .. ,_ . . . ~.-/ . . :..-.-~ . ., ;..- .. -.. -... - . . 9 9 ....% -.

::" i t .'. ".'~.:..~":: -.'..-': : " -~:'-.'-"-'.-~';b".":,; :" : " :."..'- . - ".'...,. - -" "-~"::;." 9 . i ,'. ... ~7. .-,.? :;,;:.:- 9...",':r149 -:.-~.:-'"--, :".'.. ..'-,.. "C.q.'.. - " :.- '-.'.'.:., .' / J :.'... ~ :' "," . ~;~: . ~ : : .",.:z : - "-'-'.. " ":.'- ".. - " -" c:,

/ ~ ." :', " 9.'- :': .' ~"'~. :".~:.;~:::~:-.'-'.-: ." ", ':' "~'--~ " ) ~:,. "::-:-~'.:.'. 9 ",":.~ " :,'. ". "';':- "-P.~" /,,, - .. 'if:!.:;:.:.- -.. !::: 9.. "'% " ....'A ' -, .....' .". " " - 9 9 !; % " :~:~."'" "':" ": '.',..... --..,..;7...~. 9 ~ .- .. - ...... -: ".~.~,~.,t-." .'.. ?-.~,,'~..---..-,-:. -,.: -.!.:;i~'.:.~ 9 -" "~,. ". .'~'~':, '-. ~:;..t* ".'.':.': ~,,". 9 ~ .- " "t.: " *:" "'11'-" ..... :' , ..$..~ ..~ ,:' .. .. t.. . .,.j...... :: ;::.:.:/ ",;. ".~:,"-'-~..:,.., .'.:-_.'...':.-:..:''.-..-.:".:..'. ;.- . -.;~:':.'" ...::.,'..':~.,- ..'.:t{',

I 31 -33 x106____ t SYSTEM PLANE years I

Fig. 14. Diagrammatic cartoon showing the movement of the of the around the invok- ing oscillations along the orbit near the galactic plane. A model proposed to explain alleged extinction periodicities.

not assigned a period. Associated in timing with a period of 320 Ma, with 162 data points, for the these ideas was the claim by Alvarez et al. (1980) history of the marine Ammonoidea families from that the Iridium Clay associated with the their origin in the Devonian to their extinction at Cretaceous/Tertiary (K/T) boundary was caused by the KIT boundary (House 1989, 1993). This may cometary dust clouds. well represent the most detailed and closely docu- On the presumption that Raup & Sepkoski's mented such analysis ever undertaken for the fossil evidence was correct, many theorists suggested that record. A Fourier transform of the data (House the Earth periodically passed through regions of 1993) gave no evidence whatever of a 26 Ma, or cosmic dust clouds. The simplest models invoked other, long frequency extinction pattern (Fig. 15). the cosmic year, and suggested that the solar system Hence the Raup & Sepkoski proposals must be oscillated above and below the galactic plane at the rejected. Were a regular orbital effect proven then it required frequency (Fig. 14), taking it in and out of would be of enormous advantage in calibrating vulnerable space (Rampino & Stothers 1984). those time scales using the Milankovitch band but Others invoked a periodically returning Companion regrettably there appears to be no such modulator. Star (Whitmire & Jackson 1984), named the Nemesis Star (Davis et al. 1984), or the periodic Cosmic year (220-250 Ma) effects of a X (Whitmire & Matese 1985); these speculations have continued (Crawford 1985; The period for the solar system to move around the Clube & Napier 1982). Milky Way galaxy (Fig.15) has been variously But from the beginning, there were criticisms of estimated, but is thought to be c. 220-250 Ma. The the basic conclusions of Raup & Sepkoski. Some suggestion has been made that major glaciation claimed falsity in their statistical tests (Noma & periods in the past might be related to this cycle. Glass 1987; Stigler & Wagner 1987), or bias in the The Pleistocene (1.6 Ma), (c. 250Ma), selection of data (Patterson & Smith 1987), or the Ordovician (c. 440 Ma) and Vendian (600 Ma) do timescale definitions (Hoffmann 1985) 9That the not appear to be separated by equal amounts on K/T boundary was caused by cosmic events was current radiometric data, so this suggestion is also questioned (Clemens et al. 1981; Hallam highly speculative. 1987). The crudity of Raup & Sepkoski's data, with This contribution, given here as an introduction to only 55 data points used to assess extinctions, and this volume on Orbital Forcing Timescales and the assumption that the stages they used were of Cyclostratigraphy, is based essentially on the equal duration, raised problems. Accordingly the Special Invitation Lecture given by the author to writer assembled data, at 2 Ma time intervals, for the Geologists' Association in March 1990. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

ORBITAL FORCING TIMESCALES 15

150'

100- "0 0 5:

I,.. 50-

0 LL

! I I I ! 0 20 l,O 60 80 100 Frequency (Ka)

Fig. 15. Test for periodicity in the extinction of fossil ammonoids from the Devonian to the end of the Cretaceous. A fast Fourier transform shows no evidence for alleged extinction periods at 26, 33 Ma or other times (after House 1993).

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