History of the Earth's Obliquity

Earth-Science Ret'iews, 34 (1993) 1-45 Elsevier Science Publishers B.V., Amsterdam

History of the Earth's obliquity

George E. Williams Department of Geology and Geophysics, University of Adelaide, GPO Box 498, Adelaide, S.A. 5001, Australia (Received October 9, 1991; revised and accepted August 18, 1992)

ABSTRACT

Williams, G.E., 1993. History of the Earth's obliquity. Earth-Sci. Rev., 34: 1-45.

The evolution of the obliquity of the ecliptic (E), the Earth's axial tilt of 23.5 °, may have greatly influenced the Earth's dynamical, climatic and biotic development. For e > 54 °, climatic zonation and zonal surface winds would be reversed, low to equatorial latitudes would be glaciated in preference to high latitudes, and the global seasonal cycle would be greatly amplified. Phanerozoic palaeoclimates were essentially uniformitarian in regard to obliquity, with normal climatic zonation and zonal surface winds, circum-polar glaciation and little seasonal change in low latitudes. Milankovitch-band periodicity in early Palaeozoic evaporites implies ~ ~- 26.4 +_ 2.1 ° at ~ 430 Ma, suggesting that the obliquity during most of Phanerozoic time was comparable to the present value. By contrast, the paradoxical Late Proterozoic (~ 800-600 Ma) glacial environment--frigid, strongly seasonal climates, with permafrost and grounded ice-sheets near sea let,el preferentially in low to equatorial palaeolatitudes--implies glaciation with e > 54 ° (assuming a geocentric axial dipolar magnetic field). Palaeotidal data accord with a large obliquity in Late Proterozoic time. Indeed, Proterozoic palaeoclimates in general appear non-uniformitarian with respect to climatic zonation, consistent with e > 54 °. The primordial Earth's obliquity is unconstrained by the widely-accepted single-giant-impact hypothesis for the origin of the Moon; an impact-induced obliquity > 70 ° is possible, depending on the impact parameters. Subsequent evolution of E depends on the relative magnitudes of the rate of obliquity-increase i t caused by tidal friction, and the rate of decrease ip due to dissipative core-mantle torques during precession (~ < 90 ° is required for precessional torques to move ~ toward 0°). Proterozoic palaeotidal data indicate i t = 0.0003-0.0006"/cy (seconds of arc per century) during most of Earth history, only half the rate estimated using the modern, large value for tidal dissipation. The value of ~p resulting from the combined effects of viscous, electromagnetic and topographic core-mantle torques cannot be accurately determined because of uncertainties in estimating, at present and for the geological past, the effective viscosity of the outer core, the nature of magnetic fields at the core-mantle boundary (CMB) and within the lower mantle, and the topography of the CMB. However, several estimates of ~p approximate, or exceed by several orders of magnitude, the indicated value of ~. If ~p did indeed exceed ~t in the past, then the obliquity would have decreased during Earth history. It is postulated here that the primordial Earth acquired an obliquity of ~ 70 ° (54 ° < e < 90 °) from the Moon-producing single giant impact at ~ 4500 Ma (approach velocity = 5-20 km/s, impactor/Earth mass-ratio = 0.08-0.14). Secular decrease in g subsequently occurred under the dominant influence of dissipative core-mantle torques. From 4500-650 Ma, g slowly decreased to ~ 60 ° ((~) = -0.0009"/cy). g then decreased relatively rapidly from ~ 60 ° to ~ 26 ° between 650 and 430 Ma ((~) = -0.0556"/cy); climatic zonation changed from reverse to normal when g = 54 ° at - 610 Ma, and (~) and the rate of amelioration of global seasonality were maxima for g = 45 ° at - 550 Ma (the precessional rate 1) is maximum when E = 45 °, and ~p varies as f12). Since 430 Ma, (g) has been < --0.0025"/cy and g has remained near its Quaternary range. The postulated relatively rapid decrease in ~ between 650 and 430 Ma may partly reflect special conditions at the CMB which caused significant increase in dissipative core-mantle torques at that time. This inflection in the curve of ~ versus time centred at g = 45 ° also may be partly explained by the function ~p ~ (f~2/~o)(sin 2e), where w is the Earth's rate of rotation, and other dynamical effects on gp. The Proterozoic-Phanerozoic transition may record profound change in global state caused by reduction in g through the critical values of 54 ° and 45 °. The postulated flip-over of climatic zonation at ~ 610 Ma (g = 54 °) coincides with the widespread appearance of the Ediacaran metazoans at ~ 620-590 Ma, and the interval of most rapid reduction of obliquity

and seasonality at ~ 550 Ma (g = 45°) with the "Cambrian explosion" of biota at 550 ± 20 Ma. These two most spectacular radiations in the history of life thus may mark the passage from an inhospitable global state of reverse climatic zonation and extreme seasonality (the Earth's Precambrian "Uranian" obliquity state) to a relatively benign state of normal climatic zonation and moderate seasonality. Further geological, palaeomagnetic and geochronological studies of Precambrian glaciogenic and aeolian deposits can test the predictions of a large obliquity (e > 54°) and reverse climatic zonation and zonal surface winds during the pre-Ediacaran Precambrian.

CONTENTS

I. INTRODUCTION ...... 3

2. CLIMATIC EFFECTS OF MAJOR CHANGE OF OBLIQUITY ...... 5 2.1 Increase in obliquity (e >> 23 °) ...... 5 2.2 Decrease in obliquity (e < 23 °) ...... 7

3. EVIDENCE FOR OBLIQUITY IN THE GEOLOGICAL PAST ...... 7 3.1 Phanerozoic ...... 7 3.2 Late Proterozoic ( ~ 800-600 Ma) ...... 10 3.2.1 The paradoxical Late Proterozoic glacial climate ...... l0 3.2.2 Global refrigeration ...... 15 3.2.3 Equatorial ice-ring system ...... 15 3.2.4 Geomagnetic field non-axial ...... 16 3.2.5 Large obliquity (e > 54 °) ...... 17 3.2.6 Discrimination between hypotheses ...... 17 3.3 Prior to ~ 800 Ma ...... 21

4. ORIGIN AND LIMITS OF THE PRIMORDIAL EARTH'S OBLIQUITY ...... 24

Sahara and central Australia. Since then his main research interests have been Precambrian climatic history and the past dynamics of the Earth-Moon system as revealed by the sedimentary and pedological records, and the effects of major meteorite impact on the Earth. He was elected a Fellow of the Geological Society of America in 1979. His discovery of the Late Proterozoic Acraman impact structure, the largest George Williams received his PhD in 1966 from known impact structure on the Australian conti- the Sedimentology Research Laboratory, Univer- nent, was honoured by the International Astro- sity of Reading, where he held an 1851 Exhibition nomical Union in 1988. Williams was awarded Overseas Scholarship, and an MSc from the the 1991 Research Medal of the Royal Society of University of Melbourne in 1963. His PhD re- Victoria for research in the earth sciences in search investigated the palaeoenvironment and Australia during 1985-1990. Formerly Principal palaeoclimatology of the Late Proterozoic Torri- Geologist with BHP Minerals International in donian sediments and palaeosols of northwest Australia, he is now Senior Research Fellow in Scotland, after which he studied Quaternary the Department of Geology and Geophysics, sedimentation and palaeosols in the northern University of Adelaide. HISTORY OF THE EARTH'S OBLIQUITY 3

5. MECHANISMS FOR SECULAR CHANGE IN THE EARTH'S OBLIQUITY ...... 25 5.1 Tidal friction ...... 25 5.2 DissipatiL~e core-mantle torques ...... 27

6. A PROPOSED OBLIQUITY HISTORY ...... 31

7. THE PROTEROZOIC-PHANEROZOIC TRANSITION ...... 34

8. DISCUSSION AND POSSIBLE TESTS ...... 35

9. CONCLUSIONS ...... 36

ACKNOWLEDGMENTS ...... 37

10. REFERENCES ...... 37

1. INTRODUCTION Earth such as dissipative core-mantle coupling (e.g. Aoki, 1969; Bills, 1990a) or take The obliquity of the ecliptic (E)--the account of evidence from the geological Earth's axial tilt of 23.5 ° (Fig. 1) or the angle record concerning non-uniformitarian past between the equatorial plane and the plane climates and the history of the Earth's rota- of the Earth's orbit (the ecliptic plane)--ex- tion and lunar orbit (e.g. Williams, 1975a, erts a fundamental influence on terrestrial 1990). climate and dynamics. The obliquity controls Many of the planets have undergone or the seasonal cycle and strongly influences are subject to significant change in obliquity tidal rhythms. In addition, fluctuations of (Table 1), indicating that planetary obliquity obliquity between 21.5 ° and 24.5 ° over a pe- may be regarded as a potential variable. riod of 41 ka constitute an important orbital Venus, Uranus and Pluto have large obliqui- element which, together with precession and variations in eccentricity, drives glacial and Rotation other medium-term (10-100 ka) climatic cy- axis Pole of ~l~ cles (Berger et al., 1984). Indeed, the Earth's eclip precession is maintained largely because the obliquity permits luni-solar torques to act strongly on the Earth's equatorial bulge. As noted by Gold (1966), the obliquity of the ecliptic may represent a balance between the effects of tidal friction, which increases the obliquity, and internal dissipation within the Earth, which tends to erect the axis. \ " Dynamical calculations (MacDonald, 1964; Goldreich, 1966; Mignard, 1982), which assume that the modern, large value of tidal friction applies for much of the geological past, have suggested that the obliquity is very slowly increasing under the action of luni- solar tidal friction and that early in Earth Fig. 1. Schematic representation of the obliquity of the history the obliquity was perhaps ~ 10-15 °. ecliptic (E), the Earth's axial tilt of 23.5 ° or the angle Such calculations, however, do not consider between the equatorial plane and the ecliptic or orbital possible geophysical mechanisms within the plane. 4 G l~. WILlJAMS ties that could have resulted from several tention to the origin and evolution of the mechanisms including tidal torques, dissipa- Earth's obliquity. However, if the obliquity in tive core-mantle coupling, planetary impact, the geological past extended well beyond its resonant axial-orbital precession and a twist Quaternary limits, the implications would be of the orbital angular momentum vector. In vital for the Earth's climatic and biotic histo- addition, resonant axial-orbital precession ries as well as for the dynamical evolution of causes large fluctuations in the obliquities of the Earth-Moon system. Despite the numer- Mars and Pluto on time-scales of 10 6 years. ous mechanisms that may affect a planet's Axial-orbital precession causes only small obliquity, a commonly held view is that no variations in the Earth's obliquity (_+l.5°); potential mechanism exists for substantial however, Ward (1982) suggested that with change of the Earth's obliquity. This percep- continued tidal evolution of the Earth-Moon tion is without foundation. As discussed here, system, in less than 2000 Ma resonant axial- a recognised geophysical mechanism--dis- orbital precession will cause wider oscilla- sipative core-mantle coupling--may indeed tions of obliquity about an increased mean have accomplished geologically significant value (51.8 _+ 8.4°). secular change in obliquity during Earth his- Geologists in general have given little at- tory.

TABLE 1 Planetary obliquities (angles between equatorial and orbital planes)

Planet Present Range or past Mechanism Reference for variation obliquity obliquity for variation of obliquity (o) (o)

Mercury 0 < 90 tidal torque and Peale (1976) core-mantle dissipation Venus 177 0 to ~ 180 tidal torques and Goldreich and Peale (1970) core-mantle dissipation 0 to ~ 180 tidal torques and Lago and Cazenave (1979) core-mantle dissipation Earth 23.5 23 + 1.5 axial-orbital precession Berger et al. (1984) ~ 10 to 23 tidal friction Goldreich (1966)

- 13 to 23 tidal friction Mignard (1982) 70 to 23 core-mantle dissipation this study Moon 6.7 77 to 6.7 tidal torque Ward (1975) Mars 25.2 24.4 _+ 13.6 axial-orbital precession Ward (1979) 27.5 _+ 18.5 axial-orbital precession Ward et al. (1979) (prior to Tharsis uplift) 25.8 + 25.6 axial-orbital precession Bills (1990b) 0 to ~ 20 + polar ice-cap loading Rubincam (1990) Jupiter 3.1 Saturn * 26.7 0 to ~ 27 twist of orbital plane Tremaine (1991) Uranus 97.9 0 to ~ 98 planetary impact Safronov (1966) 0 to ~ 98 planetary impact Korycansky et al. (1990) 0 to ~ 98 tidal torque Greenberg (1974) 0 to ~ 98 twist of orbital plane Tremaine (1991) 0 to ~ 98 axial-orbital precession Harris and Ward (1982) Neptune * 28.8 0 to ~ 29 twist of orbital plane Tremaine (1991) Pluto 104.5 99 _+ 14 axial-orbital precession Dobrovolskis (1989) 118 0 to 118 twist of orbital plane Tremaine (1991)

* Krimigis (1992) gives post-Voyager 2 obliquities of Saturn and Neptune as 29.0 ° and 29.6 °, respectively. tIISTORY ()F THE EAR I'H'S OBLIQUITY 5

This review presents a new approach to seasonal oscillations of temperature would the history of the Earth's obliquity. Evidence extend into low latitudes. The strongly sea- from palaeoclimatology, geochronometry and sonal climate experienced at all latitudes geophysics, as well as the widely-accepted would likely be too stressful for all but rela- single-giant-impact hypothesis of lunar origin tively primitive organisms. (which does not constrain the impact-in- Moreover, a monotonic temperature gra- duced obliquity), leads to a model of an dient directed from the summer to the win- evolving obliquity that has slowly decreased ter pole should exist for a large obliquity from a primordial large value (54 ° < • < 90 °) (Hunt, 1982). Substantial atmospheric circu- and has played a vital role in the Earth's lation across the equator therefore would dynamical, climatic and biotic development. occur around solstices, when very cold air from the anticyclonic province in the winter 2. CLIMATIC EFFECTS OF MAJOR CHANGE OF hemisphere would flow toward the deep OBLIQUITY thermal depression in the summer hemisphere. At equinoxes the global climate 2.1 Increase in obliquity (e >> 23 °) would display a "normal" day-night cycle. Hence, low to equatorial latitudes would ex- As discussed by Williams (1975a), substan- perience frigid temperatures and very cold tial increase in the Earth's obliquity would winds around solstices, alternating with more cause major changes in global climate (Table benign equinoctial conditions. 2): (b) The ratio of solar radiation received (a) The amplitude of the global seasonal annually at either pole to that received at the cycle would be greatly increased. With an equator would be increased (Fig. 2). For obliquity of 60 °, for example, the tropics E = 54 °, all latitudes would receive equal ra- would be at 60 ° latitude and the polar circles diation annually and the climatic zones would at 30 ° latitude. Areas between 30-60 ° lati- disappear. For E > 54 °, low to equatorial latitude thus would be within both the tropics tudes would receive less radiation annually and the polar circle! All areas poleward of than high latitudes. Figure 2b shows that 30 ° latitude would endure greatly contrasting minimum insolation occurs at +30 ° latitude seasons, with dark winters of deep cold and for E = 60 ° , and at the equator for E > 60 ° . torrid summers under a continually circling Williams (1975a) obtained the critical obliq- Sun. Seasonal temperature contrasts would uity of 54 °, at which value the climatic zona- be most extreme in high latitudes, and large tion reverses, from Milankovitch (1930). This

TABLE 2 Relation between the obliquity of the ecliptic and global climate (modified from Williams, 1975a)

Obliquity Seasonality Annual insolation, Climatic Preferred latitudes (°) either pole : equator * zonation of glaciation (°) 23.5 moderate 0.4247 "normal", high ( > 50) strong 54 strong 1.0 nil < 40 * * 9(I very strong 1.5708 reverse, low to moderate equatorial ( < 30) * Values from Croll (1875) and Milankovitch (1930). ** Despite the lack of climatic zonation for e = 54 °, the very hot summers likely would prevent glaciation in high latitudes. (3 (LE. WILLIAMS

r~igh latitude Low latitude I~ glaciation ~ glaciation tion in that zone and allow the accumulation of permanent ice. !a i 80 b ] ,' / / ! The principle of potential glaciation in low E 60 / ,' J to equatorial latitudes for a large obliquity is ~; I 2 / I 40 applicable to other planets. Ward (1974) ob- 0 CL 10 , • .2 • 20 served that if Mars had an obliquity similar it ', ~ o to that of Uranus (~ 98°), the Martian de- £ = 90 75 60 posits of permanent CO 2 ice would be at the osi / ~.: i equator and not at the poles. Furthermore, : I . 1 -40 -- 04 / [I ' / Jakosky and Carr (1985) suggested that when 6(] the obliquity of Mars periodically was as high

80 as 45 ° early in that planet's history, ice could / 0 10 20 30 40 50 6(] 70 80 90 2 3 4 have accumulated in low latitudes by subli- Obliquity (degrees) Relative mean annual insolation mation of ice from the polar caps and trans- Fig. 2. (a) Relation between the obliquity of the ecliptic port of the water vapour equatorward; they E and the ratio of annual insolation at either pole to concluded that polar ice may have formed that at the equator (solid line); the dashed line at • = 54 ° separates the fields of potential low-latitude only for minimum obliquities. and high-latitude glaciation. After Williams (1975a). As noted by Williams (1975a), the area of (b) Latitudinal variation of relative mean annual inso- a low-latitude zone symmetrical about the lation of a planet for various values of obliquity (e, in equator is 4rrRE 2 sin,~, and the total area of degrees); solid line for e = 90% long-dashed line for two equal polar caps is 4n-RE2(1-sin,D, = 75 °, short-dashed line for • = 60 °. Adapted from Van Hemelrijck (1982). The plots in (a) and (b) to- where R E is the Earth's radius and ,~ the gether illustrate that for • > 54 °, glaciation would occur limiting latitude. Hence, 64% of the Earth's preferentially in low to equatorial latitudes. surface area occurs between +40 ° latitude, whereas a total of only 23% of the Earth's surface area occurs poleward of _+50° latitude. The area of potential glaciation there- critical value of 54 ° was confirmed by Ward fore is much larger for e > 54 ° than for e -- (1974). 23 ° . If continents moved through low lati- If an Earth with e > 54 ° were to enter a tudes of potential glaciation when • > 54 °, a glacial interval through some independent misleading impression of global glaciation cause such as decline in atmospheric CO 2 (albeit non-synchronous) could be gained partial pressures or moderate decrease in from the stratigraphic record. solar luminosity--a large obliquity per se is (c) The directions of zonal surface winds not a cause of glaciation--low to equatorial such as the tropical easterlies and mid-lati- latitudes (~ 30 °) would be glaciated prefer- tude westerlies would reverse for • > 54 ° as entially. In high latitudes the cold, arid win- the circulation in "Hadley cells" reversed ter atmosphere would allow only limited direction (Hunt, 1982). Westerly zonal sur- snowfall which would melt entirely during face winds would occur in low latitudes, the very hot summer. Permanent ice could, which would receive much less precipitation however, form in low to equatorial latitudes; than present low latitudes, and easterly zonal that zone, as well as receiving minimal solar winds in mid-latitudes. radiation annually, would experience the ad- (d) Climatic zonation would be weakened. ditional cooling effect of frigid winds during The stability of latitude-dependent climates each solstice and no extreme summer tem- therefore would be lessened, and any Mi- peratures. The increased albedo resulting lankovitch-band fluctuations in insolation due from a snow cover in low to equatorial lati- to orbital variations might cause large or tudes would further reduce effective insola- abrupt changes of climate over wide areas. HIS'FORY OF 'l'Hff EARTH'S ()BLIQUITY 7

Such climate changes might be recorded by occurrence of apparent evergreen floras, in- the stratigraphic proximity of cold- and cluding broad-leaved forms, in high latitudes warm-climate indicators. --subsequently shown to be high palaeolatitudes--during Mesozoic-early Cenozoic 2.2 Decrease in obliquity (E < 23 °) time (e.g. Belt, 1874; Warring, 1885; Allard, 1948; Wolfe, 1980). It was argued that such floras are dependent on annual equability of The effects of a reduced obliquity on inso- light, whatever the temperature, and that lation and climate have been investigated by only by a reduction in the Earth's obliquity Hunt (1982) and Barron (1984). Decrease in could the required equability of light for- obliquity (e < 23 °) causes an increase in mean merly have prevailed in high latitudes. How- annual insolation at low latitudes and a de- ever, a smaller obliquity and consequent crease at high latitudes, thus steepening the cooler poles conflict with the evidence for equator-to-pole gradient of mean annual in- warmer polar temperatures in Mesozoic and solation. Cooler polar temperatures and a early Cenozoic time (Barton, 1984). Alterna- reduction in amplitude of the seasonal cycle tively, the floras may have adapted to a po- would be expected. lar-light regime, given ambient temperatures much higher than those in such regions today 3. EVIDENCE FOR OBLIQUITY IN THE GEO- (Creber and Chaloner, 1984). The presence LOGICAL PAST of tree-rings in high palaeolatitude Mesozoic floras confirms the occurrence of seasonal 3.1 Phanerozoic changes (Jefferson, 1982). Hence the palaeobotanical evidence may not demon- The distribution of Phanerozoic palaeocli- strate a Mesozoic-early Cenozoic obliquity mate indicators such as glacial deposits, much different from the Quaternary value. evaporites and coral reefs with respect to The diurnal inequality of the tides (that is, palaeolatitudes (McElhinny, 1973; Drewry et the unequal amplitude of successive semidi- al., 1974; Frakes, 1979; Merrill and McE1- urnal tidal cycles), which is attributable hinny, 1983) and the frequency distribution largely to the obliquity of the ecliptic, can of palaeomagnetic inclination angles for the produce tidal laminae or cross-strata display- Phanerozoic (Evans, 1976) together strongly ing distinct thick-thin alternations (De Boer support the geocentric axial dipole model of et al., 1989). Such alternations are displayed the Earth's magnetic field and imply that by tidal deposits of Cenozoic, Mesozoic and normal climatic zonation has prevailed since Palaeozoic age (e.g. De Boer et al., 1989; Nio Precambrian time. Normal climatic zonation and Yang, 1989; Williams, 1989c) and by during the Palaeozoic and Mesozoic is indi- tidal increments of a Late Cretaceous mol- cated also by the prevalence of predicted lusc bivalve (Pannella, 1976). The evidence normal zonal palaeowind directions deter- of a clear diurnal inequality in an ancient mined from aeolian sandstones (Runcorn, deposit or fossil indicates that the orbital 1964a; Parrish and Peterson, 1988). It follows plane of the Moon was inclined to the Earth's that the obliquity of the ecliptic has been less equatorial plane. Since the inclination of the than the critical value of 54 ° (above which lunar orbital plane to the ecliptic plane dur- value reverse climatic zonation and direc- ing most of Earth history probably was little tions of zonal surface winds prevail) through- different from its present value of only 5.15 ° out the Phanerozoic. (Goldreich, 1966; Mignard, 1982), the palae- A long-standing, controversial hypothesis otidal data imply at least a moderate (al- in palaeobotany postulates that a former re- though unquantifiable) palaeo-obliquity dur- duced obliquity (e < 23 °) may explain the ing the Phanerozoic. G.E. WILLIAMS

A quantitative estimate of the obliquity in 1 0 113 Late Ordovician-Early Silurian time, when >, the Earth was experiencing circum-polar g .~ 22.1 glaciation (Hambrey and Harland, 1981), can o_ 05 354 197 be made from Milankovitch-band periodicity u~ in bedded halite deposits of the Canning o~ Basin, northwestern Australia (Williams, EL 1991b). Drill core of a 477-m stratigraphic O0 I ooo o.os Frequency (cyc/m) o.lo interval of the Late Ordovician-Early Sil- 100 50 3r0 2'0 ~5 110 urian Mallowa Salt, the largest halite deposit Period or waveJength (metres) in Australia, provided detailed geochemical Fig. 3. Fast Fourier transform smoothed spectrum of (Br, KzO, MgO, Na20) stratigraphic series the bromine geochemical stratigraphic series for the in which strong periods (wavelengths) are Late Ordovician-Early Silurian Mallowa Salt, Western revealed by Fourier spectral analysis (Fig. 3). Australia. Sample interval = 1 m, 477 points read; spectrum normalised to unity for the strongest peak The relative frequencies, amplitudes and and with linear frequency scale. The periods (wave- structure of spectral peaks suggest climatic lengths) are expressed as stratigraphic thicknesses in oscillations forced by orbital cycles. If the metres. Peaks that are significant at the 95% level of strongest spectral peak at 113 m is taken to confidence are labelled in bold type (sec Table 3). represent the relatively stable eccentricity After Williams (1991b). period of 100 ka and a constant net rate of accretion assumed for long sections, the other main periods recorded by the Mallowa Salt obliquity maintains a constant value of 23.4 ° as exemplified by the Br spectrum (Fig. 3) (see Berger et al., 1989a). Hence, the appar- would be 31.3 + 3.0 ka, 19.6+ 1.1 ka and ent good agreement between predicted and 17.4 + 1.1 ka (error estimates +1o'). These observed precessional periods for 430 Ma figures are consistent with predicted Late may imply that the mean obliquity at 430 Ma Ordovician-Early Silurian periods for obliq- was similar to that of today. uity (30.5 ka) and precession (19.3 and 16.4 This apparent close agreement of preces- ka) based on evolutionary change in the sional periods may, however, be partly fortu- Earth-Moon system (Table 3), assuming the itous. Berger et al. (1989a), in calculating

TABLE 3 Observed and postulated periodicities obtained from the fast Fourier transform spectrum of Br values for the Late Ordovician-Early Silurian Mallowa Salt (Fig. 3), compared with Milankovitch orbital periods predicted for 430 Ma (Early Silurian) and the present orbital periods. Values for the Mallowa Salt that are significant at the 95% level of confidence are shown in italics; error estimates are _+ l~r. Adapted from Williams (1991b)

Orbital element Eccentricity Obliquity Precession Stratigraphic wavelengths for the Mallowa Salt (m) 113 + 13 35.4 ± 3.0 22.1 +_ 1.2 19.7 _+ 1.2 Postulated climatic periods for the Mallowa Salt (ka) 100 + 12 31.3 + 3.0 19.6 +% 1.1 17.4 + 1.1 Predicted orbital periods for 430 Ma (Berger et al., 1989a) (ka) 100 30.5 19.3 16.4 Revised predicted precessional periods for 430 Ma (see text) (ka) 21.3 ~ 17.8 Present main orbital periods (ka) 100 41 23 19 HISTORY OF THE EARTH'S OBLIQUITY

TABLE 4 Late Proterozoic (~ 650 Ma) and modern tidal and rotational values

Parameter Late Proterozoic Modern Lambeck (1978, 1988) * Rhythmites * * (a) (b) solar days/lunar month 30.7 30.2 30.5 +_ 0.5 29.53 lunar months/year 14.4 13.2 13.1 ± 0.1 12.37 lunar apsides cycle (years) 9.7 _+ (I.1 8.85 lunar nodal cycle (years) 19.5 ± 0,5 18.61 solar days/year ~ 440 ~ 400 400 _+ 7 365.24 length of solar day (hours) 21.9 + 0.4 24.00 mean Earth-Moon distance in earth radii (R E) 58.28 +_ 0.30 60.27 * Values taken from plots in Lambeck (1978, 1988) based on Phanerozoic palaeontological data. (a) average equivalent phase lag = 6° (the present value); (b) average equivalent phase lag = 3°. Lambeck's plots assume that tidal friction is the only phenomenon responsible for secular changes in the Earth's rotation rate and the period of the Moon's revolution. ** Values indicated by tidal rhythmites of the Late Proterozoic Elatina Formation and Reynella Siltstone Member, South Australia (Williams, 1988, 1989a-c, 1990, 1991a; Deubner, 1990). The rhythmite data also display strong semi-annual and annual periods (see Figs. 7a and 8). orbital periods back to Early Silurian time dent estimates of mean Earth-Moon dis- (taken here as 430 Ma 1) for an evolving tance at ~ 650 Ma (Table 5) strongly en- Earth-Moon system, used Earth-Moon dis- dorses the accuracy of the Late Proterozoic tances determined from palaeontological tidal and rotational values. Two possible in- geochronometric data. They took 399 solar terpretations of the geochronometric data days/year and a relative mean Earth-Moon therefore are: distance (a/a o) of 0.966 for 380 Ma (Middle (a) Virtually no deceleration of the Earth's Devonian), values virtually the same as the rotation occurred during an interval of ~ 270 400 + 7 solar days/year and a/a o = 0.968 _+ Ma between ~ 650 and 380 Ma, and the 0.007 indicated for ~ 650 Ma by the orbital periods predicted by Berger et al. palaeotidal data of the Elatina Formation (1989a) for 430 Ma are accurate. (Williams, 1989c, 1990; Deubner, 1990; see (b) The estimate of 399 solar days/year at Tables 4 and 5). As concluded by Pannella 380 Ma is too large, and the early to middle (1975) and Scrutton (1978) and reviewed by Palaeozoic orbital periods predicted by Williams (1989c), the palaeontological data Berger et al. (1989a) require revision. It must be regarded as approximate only; in should be noted that the value of 399 solar particular, the suggested number of days per days/year for the Middle Devonian used by year for the early and middle Palaeozoic may Scrutton (1964, 1970) was not based on coral be too large. In contrast, Deubner (1990) and data but was, in fact, an estimate of Williams (1990) showed that the high degree days/year for that time obtained by extrapo- of internal consistency among three indepen- lating the modern rate of deceleration of the Earth's rotation. As it is difficult to accept that the tidal The age of the Late Ordovician-Early Silurian is evolution of the Earth-Moon system was here rounded-off to 430 Ma, based on the Ordovician- Silurian boundary age of 434 Ma given by Young and halted for an interval of ~ 270 Ma, a revi- Claoud-Long (1991), which is supported by the latest sion of Berger et al.'s (1989a) predicted or- zircon U-Pb SHRIMP data. bital periods for 380 Ma seems required. 10 (;.E. WILLIAMS

Adjusting the time-scale of the plot of pre- Ma would be 23.4 °- 0.7 °= 22.7 ° in the ab- cessional period against time (Berger et al., sence of other effects. Overall, the predic- 1989b), so that their 380 Ma = 650 Ma, gives tions of Berger et al. (1989a,b) and data from revised predicted precessional periods for 430 the Elatina Formation and Mallowa Salt to- Ma of ~21.3 ka and ~17.8 ka. It is these gether suggest that the Earth's mean obliq- revised predictions that should be compared uity at 430 Ma was ~ 26.4_+ 2.1 °, which is with the precessional periods of 19.6 _+ 1.1 ka ~ 3.7 _+ 2.1 ° larger than the expected value. and 17.4 ± 1.1 ka suggested for ~ 430 Ma by In summary, palaeoclimate data indicate periodicities in the Mallowa Salt (Table 3). normal climatic zonation and by inference an The dominant observed precessional period obliquity less than 54 ° throughout the of 19.6 ± 1.1 ka, which alone is significant Phanerozoic. Indeed, the geological record at the 95% level of confidence, is signifi- for most of that eon provides no evidence for cantly different from the revised prediction significant departures of obliquity beyond the of ~ 21.3 ka. This difference may be ex- Quaternary limits of 21.5-24.5 °, although the plained by the mean obliquity of the ecliptic obliquity may have been slightly larger than at 430 Ma being larger than the value of the present value in early Palaeozoic time. ,,3.49 o used by Berger et al. (1989a). As the The latter suggestion is contrary to models of rate of lunisolar precession ,Q varies as sin 2e a slowly increasing obliquity due to tidal fric- (Woolard and Clemence, 1966), the palaeo- tion (MacDonald, 1964; Goldreich, 1966; obliquity e~ at 430 Ma is given by Mignard, 1982), implying that previous stud- e~ = 0.5 arc sin (Pp,/P,)(sin 2El (1) ies on the evolution of the Earth's obliquity may be incomplete. where P~ is the observed precessional period of 19.6 ± 1.1 ka at 430 Ma, Pp is the revised predicted precessional period of ~ 21.3 ka at 3.2 Late Proterozoic (~ 800-600 Ma) 430 Ma, and E = 23.4 °. The data indicate E~ = 26.4 ± 2.1 ° at 430 Ma, a value signifi- 3.2.1 The paradoxical Late Proterozoic cantly larger than the assumed palaeo-ob- glacial climate liquity of 23.4 °. The difference is increased Late Proterozoic glaciation, which af- when the effect of tidal friction on the obliq- fected all continents (with the possible ex- uity is included; taking the rate of increase of ception of Antarctica) between about 800 obliquity due to tidal friction as 0.0006"/cy and 600 Ma, presents a major climatic (seconds of arc per century) for the Phanero- enigma. As reviewed by Embleton and zoic (see Section 5.1), the obliquity at 430 Williams (1986), early palaeomagnetic stud-

TABLE 5 Mean Earth-Moon distance at ~ 650 Ma and mean rate of lunar retreat for the past ~ 651J Ma indicated by tidal rhythmites of the Elatina Formation and Reynella Siltstone Member (from Williams, 1989a,c, 1990; Deubner, 1990)

Tidal/rotational Relative mean Earth- Mean Earth-Moon Mcan rate of lunar value Moon distance (a/a o) distance (R E) retreat (cm/year) * 19.5 + 0.5 years (lunar nodal period) 0.969 _+ 0.017 58.40 + 1.02 1.83 _+ 1.00 13.1 +O.1 lunar months/year 0.967 + 0.005 58.28 + 0.311 1.95 _+ 0.29 400 +_ 7 solar days/year 0.968 _+ 0.007 * * 58.34 + 0.42 1.89 + 1/.41

* The present rate of lunar retreat is 3.7 + 0.2 cm/year, determined by lunar laser ranging (Dickey et al., 1990). ** Makes allowance for the solar tides' contribution to the loss of angular momentum of the Earth's rotation (Deubner, 1990). HISTORY OF THE EARTH'S ()BLIQUFrY 1 l ies of Late Proterozoic glaciogenic rocks in et al., 1987; Schmidt et al., 1991). The Elatina Norway, Greenland, Scotland and Canada in palaeomagnetic data satisfy six of the seven general suggested low palaeolatitudes of reliability criteria of Van der Voo (1990); the glaciation, although some of the data are only criterion not fulfilled is the presence of equivocal or contentious. Since 1980, how- reversals because of the brief time-interval ever, new palaeomagnetic studies of Late sampled (60-70 years). Although represent- Proterozoic glaciogenic rocks and contiguous ing a virtual geomagnetic pole (an instanta- strata in Australia, South Africa, West Africa neous sample of the geomagnetic field, which and China have consistently indicated low to is generally subject to secular variation), the equatorial palaeolatitudes of glaciation pole position for the Elatina Formation is in (Kr6ner et al., 1980; McWilliams and McEI- close proximity to other Late Proterozoic hinny, 1980; Zhang and Zhang, 1985; Emble- poles for Australia, indicating that the low ton and Williams, 1986; Sumner et al., 1987; inclination for the Elatina data is not a record Perrin et al., 1988; Chumakov and Elston, of a geomagnetic excursion or reversal. The 1989; Li et al., 1991 and Schmidt et al., proximity to other Late Proterozoic pole po- 1991 ). Virtually all these recent studies, which sitions also is evidence that inclination error include rocks from the two major glaciations is not significant. Hence the Elatina palaeo- of the Late Proterozoic, indicate glaciation magnetic data do indeed indicate deposition between 0 ° and 12° palaeolatitude. Unequiv- in low palaeolatitudes. The Elatina data and ocal evidence for high-palaeolatitude glacia- palaeomagnetic data for contiguous formation on any continent during the Late Pro- tions in the Adelaide Geosyncline together terozoic is lacking, and attempts to relate indicate a mean palaeolatitude of ~ 12°N Late Proterozoic glaciation to movement of for Late Proterozoic glaciation in South Aus- continents through polar regions (e.g. Craw- tralia (Embleton and Williams, 1986; Schmidt ford and Daily, 1971) have been unsuccess- et al., 1991). ful. Late Proterozoic glaciation in Australia Glacial pavements occur below Late Pro- occurred between 0 ° and 30 ° palaeolatitude terozoic tillites in Western Australia, north (McWilliams and McElhinny, 1980), not the Africa, southern Africa, Scandinavia, Spits- 45-60 ° palaeolatitude incorrectly shown by bergen, Normandy, South America and Chumakov and Elston (1989). Furthermore, China (see Williams, 1975a; Hambrey and the North-China block occupied high palaeo- Harland, 1981; Guan et al., 1986). The excel- latitudes (57-62 °) during the Late Protero- lently preserved Late Proterozoic glacial zoic (800-700 Ma) yet affords no evidence of pavements in Western Australia indicate glaciation, whereas the Yangzi block experi- grounded ice-sheets close to the edge of ma- enced glaciation in low palaeolatitudes (0- rine basins, with transgression accompanying 20 °) during the same time interval (Zhang glacial retreat (Plumb, 1981). and Zhang, 1985). Structures interpreted as periglacial ice- The Elatina Formation, of the Late Pro- and sand-wedges that formed by seasonal terozoic (~ 650 Ma) Marinoan glacial suc- contraction-expansion, occur in partly ma- cession in South Australia, possesses a high- rine Late Proterozoic basins in South Aus- temperature, stable remanent magnetisation tralia, Mauritania, Scotland 2, northern and that indicates a palaeomagnetic latitude of southern Norway, and Spitsbergen (see re- 5 ° (inclination < 10°) (Embleton and view by Williams, 1986) and East Greenland Williams, 1986; Schmidt et al., 1991). Positive fold tests on soft-sediment slump folds in the Elatina Formation confirm that the rema- 2 The origin of sand wedges in the Port Askaig Forma- nence was acquired very soon after deposition in Scotland is contentious (sec Spencer, 1985; tion, prior to soft-sediment folding (Sumner Eyles and Clark, 1985). 12 ¢;.E. WILLIAMS

(Spencer, 1985). A fossil permafrost horizon summer expansion causes upturning of per- displaying numerous periglacial features, in- mafrost adjacent to wedges. cluding excellently preserved sand wedges Periglacial features, particularly ice- and with near-vertical diverging lamination (Fig. sand-wedge structures, can provide quantita- 4), formed near sea level on the cratonic tive palaeoclimatological information such as Stuart Shelf bordering the Adelaide Geosyn- mean annual air temperature, seasonal tem- cline during the Marinoan Glaciation (Wil- perature range and mean annual precipita- liams and Tonkin, 1985); the Stuart Shelf tion (Washburn, 1980; Karte, 1983). Indeed, subsequently was submerged by the Mari- periglacial structures are perhaps the most noah post-glacial marine transgression (Pre- reliable of palaeoclimate indicators, because iss, 1987). Identical sand wedges are forming they formed through processes of physical today in arid periglacial regions such as the weathering and their interpretation thus Arctic and the dry valleys of Antarctica avoids such uncertainties as the former na- (P6w6, 1959; Washburn, 1980; Karte, 1983); ture of the atmosphere and biosphere and the permafrost contracts and cracks vertically subsequent diagenetic modification. during severe winters and the cracks fill with Periglacial wedge structures indicate a windblown sand, and lateral pressure during strongly seasonal palaeoclimate, as only the

Fig. 4. Vertical exposure of a Late Proterozoic (~ 650 Ma) periglacial sand wedge 3 m deep (2) in brecciated mid-Proterozoic quartzite at the Cattle Grid copper mine, South Australia. A near-vertical, diverging lamination that parallels the wedge margins is discernible within the wedge. Deformed sand wedges of an earlier generation (1) occur within the breccia and a third-generation wedge (3) occurs within overlying Late Proterozoic aeolian sandstone (Whyalla Sandstone) and the uppermost part of the large wedge. Bedding in breccia and sandstone is turned upward adjacent to the wedges. Numerous such sand wedges formed near sea level in the area bordering the Adelaide Geosyncline during the Marinoan Glaciation at ~ 650 Ma. Identical sand wedges are forming today in the periglacial dry valleys of Antarctica and in the Arctic. The structures indicate a very cold, arid. strongly seasonal climate. H1S'['()RY OF TH['2 12ARTH'S ()BLIQU1TY l 3 seasonal temperature cycle, coupled with well-developed lacustrine varves may not be rapid drops of temperature, can produce the common, since many Late Proterozoic required contraction cracking. The presence glaciogenic sequences are largely of marine of primary sand wedges within Late Protero- origin (e.g. Preiss, 1987). Overall, strong sea- zoic partly-marine basins, best exemplified sonality is an important feature of the Late by the sand wedges on the Stuart Shelf, Proterozoic glacial climate and cannot be South Australia (Fig. 4), indicates that (a) ignored. mean annual air temperatures near sea level Arid, windy, Late Proterozoic glacial cli- were then as low as - 12 ° to -20°C or lower, mates also are indicated by the presence of (b) strongly seasonal palaeoclimates pre- widespread periglacial-aeolian sandstones vailed, with mean monthly temperature (Williams and Tonkin, 1985; Deynoux et al., ranges as great as ~ 40°C or more (mean 1989; Williams, 1993) and glacial Ioessites monthly temperatures from <-35°C in (Edwards, 1979). Curiously, dominant palae- midwinter to < +4°C in midsummer) and owinds indicated by cross-bedding in the (c) climates were arid (< 100 mm mean an- periglacial (Marinoan) Whyalla Sandstone in nual precipitation) and windy (see Karte, South Australia were westerlies relative to 1983; Williams and Tonkin, 1985; Williams, contemporary low (~ 12°N) palaeolatitudes 1986). In South Australia this harsh palaeo- (Williams, 1993), whereas expected zonal climate occurred in a coastal region. surface winds in such latitudes would be The presence of a powerful annual signal easterlies to northeasterlies. in rhythmites of tidal-climatic origin from the Another interesting feature of Late Pro- Marinoan glacial succession in South Aus- terozoic glaciogenic sequences is the strati- tralia, attributable to a large seasonal oscilla- graphic proximity of cold- and warm-climate tion of sea level (see Section 3.2.6), is consis- indicators. The presence of micritic, appar- tent with strong seasonality. The occurrence ently primary dolostones capping Late Pro- of varve-like laminites with dropstones in terozoic glaciogenic sequences in Australia certain Late Proterozoic glaciogenic se- (Williams, 1979) and immediately underlying quences (e.g. Spencer, 1971) also accords with such sequences in Spitsbergen (Fairchild and a strongly seasonal glacial climate. However, Hambrey, 1984) is consistent with relatively

TABLE 6 The Late Proterozoic Iow-palaeolatitude climate near sea level indicated by periglacial sand-wedge structures in fossil permafrost horizons, compared with low-latitude climates modelled for possible arrangement of Late Proterozoic continents and for Palaeozoic supercontinents (assuming an obliquity of the ecliptic the same as that of today), and with the modern surface climate in low latitudes

Agc, and Mean surface Seasonal temperature Reference(s) basis for temperature in low range in low latitudes palaeoclimate latitudes ( < 8-10 ° lat.) < 8-10 ° lat.) estimates (°C) (°C) Late Proterozoic < - 12 to < - 20 > 40 Karte (1983), Williams (1986) (periglacial structures) Late Proterozoic + 23.5 Worsley and Kidder (1991) (modelled) Carboniferous Gondwana + 22 Crowley et al. (1991) (modelled) Late Permian Pangaea + 25 to + 30 _< 5 Crowley et al. (1989) (modelled) Modern + 26.3 1.8 Oort (1983) 14 G.E. WII.LIAMS abrupt changes of mean temperature (al- peratures near sea level in low palaeolati- though detrital carbonates interbedded with tudes during Late Proterozoic glaciation (Ta- Late Proterozoic tillites are not of palaeocli- ble 6).The model of Worsley and Kidder matic importance; Fairchild et al., 1989). Sig- (1991) permits glaciation at > 46 ° latitude, nificant changes of mean annual surface whereas available palaeomagnetic data indi- temperature in low palaeolatitudes (fluctua- cate that Late Proterozoic glaciation oc- tions perhaps as great as 20°C or more, rang- curred mainly at 5 12 ° palaeolatitude. ing from as low as -20°C to >0°C) on a Moreover, climate modelling for Gondwana 103-year time-scale during Late Proterozoic during Carboniferous glaciation indicates a glaciation are supported by the presence of mean surface tropical temperature only at least four generations of periglacial sand- ~ 4°C lower than that of today caused by a wedge structures in South Australia (Wil- presumed lower solar luminosity (Crowley et liams and Tonkin, 1985; Williams, 1992). The al., 1991). Climate modelling also shows that implied temperature fluctuations greatly ex- Pangaea experienced an annual temperature ceed the changes of only 1-2°C in mean range < 5°C in low latitudes during the Per- surface temperature experienced in low lati- mian, whereas annual temperature ranges tudes between the last glacial maximum at 18 > 40°C were confined to middle and high ka and the present day (Crowley and North, southern latitudes (Crowley et al., 1989). 1991). Clearly, climates modelled for Palaeozoic and As shown in Table 6, the Late Proterozoic possible Late Proterozoic arrangements of low-palaeolatitude climate near sea level in- continents, assuming a mean obliquity of 23 °, dicated by periglacial sand-wedge structures do not duplicate the enigmatic Late Protero- in fossil permafrost horizons contrasts zoic glacial climate. markedly, in regard to mean surface temper- Overall, palaeomagnetic and palaeocli- ature and seasonal temperature range, with mate data thus present the enigma of frigid, the modern climate in low latitudes and with strongly seasonal climates, with permafrost and modelled low-latitude climates for Palaeo- grounded ice-sheets near sea lecel apparently zoic supercontinents and for possible ar- in preferred low to equatorial palaeolatitudes, rangement of Late Proterozoic continents during the Late Proterozoic. This climate is (climate modelling tacitly assuming an obliq- all the more extraordinary because the faster uity the same as that of today). In present low rotation of the Earth in the past would have latitudes (0 + 10°), the mean annual air tem- reduced poleward transport of heat, causing perature near sea level is 26.3°C and the warmer low latitudes and colder high lati- mean monthly temperature range is only tudes (see Hunt, 1979). The Late Proterozoic 1.8°C (Oort, 1983). Mean annual air temper- glacial climate is a major paradox in contem- atures near sea level < -20°C and seasonal porary Earth science, challenging conven- temperature ranges > 40°C, which occurred tional views on the nature of the geomag- in low palaeolatitudes during the Late Pro- netic field, climatic zonation and the Earth's terozoic, are found today only in high lati- orbital dynamics in Late Proterozoic time. tudes. Climate modelling for a possible Late Four hypotheses have been advanced to Proterozoic arrangement of continents at in- explain glaciation near sea level in low to termediate to low latitudes gives a mean equatorial palaeolatitudes during the Late annual surface temperature in tropical re- Proterozoic: gions (< 8 ° latitude) of +23.5°C (Worsley (a) Glaciation extended over all latitudes and Kidder, 1991). This mean value is only a during global refrigeration and severe ice few °C lower than mean surface tempera- ages (Harland 1964a,b). tures in present low latitudes, but is up to (b) The Earth then possessed an equato- 40°C or more higher than mean annual tern- rial ice-ring system that shielded low lati- FtlS'I'()R~ ()|: IHE EARTH'S OBIAQUITY 15 tudes from solar radiation and induced low- peratures inhibit precipitation and sublima- latitude glaciation (Sheldon, 1984). tion. Hence the large seasonal mean-monthly (c) The geocentric axial dipole model for temperature range (as much as -40°C or the Earth's magnetic field does not hold for more) in low to equatorial palaeolatitudes the Late Proterozoic (Embleton and indicated by Late Proterozoic periglacial Williams, 1986). sand-wedge structures (Fig. 4) argues strongly (d) Reverse climatic zonation and marked against the global-refrigeration hypothesis. global scasonality prevailed because of a large Furthermore, even with a 30% reduction obliquity of the ecliptic (E > 54 °) (Williams, in solar luminosity more than half of the 1975a: Embleton and Williams, 1986). Earth's land area between +20 ° latitude remains snow-free (Sellers, 199(/); hence, global 3,2.2 Global refrigeration refrigeration would not lead to preferential Since low palaeolatitudes experienced low-latitude glaciation for the present Earth. mean annual air temperatures as low as Indeed, a frozen-over Earth may be very -20°C or lower near sea level in coastal difficult to unfreeze; solar radiation is so areas, as much as 45°C or more lower than efficiently reflected by ice and snow that the mean annual air temperature near sea solar luminosity has to increase by ~ 35% level in present low latitudes, an average above its present value to remove the ice and global surface temperature less than - 1.9°C snow that covers a frozen-over Earth (North (the freezing point of seawater; Frakes, 1979) et al., 1981). Such an unlikely increase in and global freezing during the Late Protero- solar luminosity would exceed the total in- zoic are implied by the hypothesis of glacia- crease in luminosity expericnced by the Sun tion over all latitudes. However, the North in its 4700-Ma history (see Gough, 1981; China block occupied high palaeolatitudes Endal and Schatten, 1982). during the Late Proterozoic yet apparently The survival of long-evolved organisms, in- was not glaciated, while the Yangzi block cluding metazoans (Runnegar, 1991a) and experienced glaciation in low palaeolatitudes other shallow-water biota, through Late Pro- (Zhang and Zhang, 1985). The Chinese data terozoic time may present a further objection appear inconsistent with the concept of global to the concept of global refrigeration. All glaciation and show that the lack of evidence known organisms require liquid water during for high-palaeolatitude glaciation is not ade- at least some stage of their life cycles (Kast- quately explained by an alleged absence of ing, 1989), a requirement that would not be continents in high palaeolatitudes during met in paralic settings and all other shallow global refrigeration. and surface waters during thc implied global Crowell (1983, p. 254) noted that signifi- freezing. cantly greater iciness of the Earth accompa- For numerous reasons, therefore, the hy- nying world-wide glaciation would have pothesis of global refrigeration and glacia- caused drastically lowered sea level. How- tion over all latitudes during the Late Pro- ever, detailed palaeogeographic reconstruc- terozoic is difficult to support. Use of the tions for Late Proterozoic South Australia term Cryogenian, implying global glaciation, (Preiss, 1987) do not reveal any drastic lower- for the interval 850-650 Ma (Harland et al., ing of sea level during the major glaciations 1990, p. 17) appears premature. of the Late Proterozoic. Climate modelling demonstrates addi- tional difficulties with the concept of global 3.2.3 Equatorial ice-ring system refrigeration. Importantly, Sellers (1990) The hypothesis of' an equatorial ice-ring found there is little seasonal t'ariation with system in Late Proterozoic time also encoun- global refrigeration because the very low tern- ters major difficulties. Ice rings probably 16 (i.E. WII.LIAMS

42 angles together strongly support the geocen- 4.0 tric axial dipole model for the Phanerozoic,

~'38 and hence the model remains basic to >, out rings r~ 36 palaeomagnetic interpretation. Embleton and

c 34 Williams (1986) noted that any deviation from ~z cq this model for the Late Proterozoic could not ~ 32 have been short-lived, because of the group- ~30 o ing of Australian poles for the quasi-static N 2.8 wth n~ / ~ interval from ~ 800 Ma to Middle Cambrian ._o 2.6 time (see also Schmidt et al., 1991). They m 24 _c concluded that invalidation of the geocentric _~22 axial dipole model for such a long interval

2O would raise strong doubts concerning other

18 Precambrian palaeomagnetic data unsup- 0 20 30 40 50 60 70 80 90 ported by independent evidence of past lati- Latitude (degrees) Fig. 5. Latitudinal variation of the mean annual daily tude. insolation for Saturn. The equator-to-pole insolation The Earth, Jupiter and Saturn exhibit only gradient is modified by the shadows of Saturn's rings small dipole tilts with respect to spin axes for most latitudes <45 °, but is not inverted, and (11.4 °, 9.6 ° and 0 °, respectively; R~idler and insolation at the equator is unaltered. After Brinkman Ness, 1990) as well as relatively small and McGregor (1979). quadrupole contributions (< 10% of the dipole; Ness et al., 1989). The Voyager 2 could not exist so close to the Sun, even for a spacecraft discovered that the magnetic fields younger Sun of slightly lower luminosity. of both Uranus and Neptune are, by con- More importantly, mean annual daily insola- trast, tilted at surprisingly large angles (58.6 ° tion at the top of the atmosphere of Saturn, and 46.9 °, respectively) to the spin axes and a planet with a well developed equatorial that quadrupole moments are comparable to ring system and an obliquity (26.7 °) similar to dipole (Connerney et al., 1987, 1991; Ness et the Earth's, is maximum at the equator and al., 1989). The observation that the magnetic minimum at the poles (Fig. 5; Brinkman and fields of two planets are of such configura- McGregor, 1979), Saturn's equator-to-pole tion virtually rules out the possibility that insolation gradient is modified by the ring they are undergoing transient excursions or shadows for most latitudes less than 45 °, but reversals, and indicates that non-axial is not inverted, and insolation at the equator dipole-quadrupole planetary magnetic fields is unaltered. High latitudes of an Earth with are indeed possible. However, R~idler and an equatorial ring system, not low to equato- Ness (1990) concluded that simply reorient- rial latitudes, would be glaciated preferen- ing an Earthlike or Jupiterlike magnetic field tially. cannot produce the Uranian magnetic field. The magnetic fields of Uranus and Neptune 3.2.4 Geomagnetic field non-axial may result from the unique interior composi- The validity of the geocentric axial dipole tion and state of those planets (Connerney et model for the Earth's magnetic field in the al., 1987, 1991; Ness et al., 1989). geological past can be tested by comparing Idnurm and Giddings (1988) and Idnurm palaeomagnetic data with independent indi- (1990) suggested that the near-linear plots of cators of past latitude. As discussed in Sec- accumulated polar wander angle versus time tion 3.1, the distribution of Phanerozoic for 0-3500 Ma, and the low scatter of data palaeoclimate indicators and the frequency points, may indicate that the geocentric axial distribution of palaeomagnetic inclination dipole model has been a reasonably good HISTORY OF THE EARTH'S OBliQUITY 17 approximation over that long interval; how- stitial winds and reduced precipitation in low ever, such plots are based on the arguable to equatorial latitudes. assumption that Precambrian polar wander (c) The directions of zonal surface winds paths as presently known are substantially would be reversed. The dominant palae- complete (P.W. Schmidt, pers. commun., owinds indicated by the Late Proterozoic 1991). The tendency for local palaeomag- periglacial-aeolian Whyalla Sandstone in netic intensity to increase with distance from South Australia were indeed westerlies rela- the palaeomagnetic equator avoids such un- tive to contemporary low palaeolatitudes, certainty and suggests that the palaeomag- rather than the expected easterlies to north- netic field may indeed be approximated by a easterlies. geocentric dipole model as far back as 2500 (d) Climatic zonation would be weakened, Ma (Schwarz and Symons, 1969). The distri- and any medium-term Milankovitch-type bution of Precambrian palaeomagnetic incli- variations of insolation might cause large or nation angles also accords with a geocentric abrupt changes of climate over wide areas. dipole model for the interval 600-3000 Ma Large changes of mean temperature on a (Piper and Grant, 1989). Embleton and 103-year time-scale in low palaeolatitudes Williams (1986) noted, moreover, that the during the Late Proterozoic are indeed indi- hypothesis of a non-axial dipole field does cated by the formation of at least four gener- not predict an unusually large area of glacia- ations of periglacial sand-wedge structures in tion. South Australia. The stratigraphic proximity Hence, global palaeomagnetic data and of cold- and warm-climate indicators also is the widespread occurrence of Late Protero- consistent with abrupt changes of mean tem- zoic glaciation together argue against a non- perature. axial, non-dipole geomagnetic field during Considering all the above points, the hy- the Late Proterozoic. pothesis of an obliquity greater than 54 ° (together with a geocentric axial dipolar mag- 3.2.5 Large obliquity (e > 54 °) netic field) in explanation of the Late Pro- Of the four hypotheses, the climatic ef- terozoic paradox--frigid, strongly seasonal fects of a large obliquity (e > 54°; see Section climates, with permafrost and grounded ice- 2.1) may best account for the enigmatic fea- sheets near sea level apparently in preferred tures of the Late Proterozoic glacial climate low to equatorial palaeolatitudes--must be (assuming the Late Proterozoic geomagnetic given serious consideration. The complex na- field was a geocentric axial dipole) because: ture and distribution of the Late Proterozoic (a) Glaciation and periglacial climates glacial climate is not adequately explained by would occur preferentially in low to equato- a simple global refrigeration or icehouse ef- rial latitudes, which accords with available fect alone, whatever the former arrangement palaeoclimatic and palaeomagnetic data for of continents. the Late Proterozoic. (b) The global seasonal cycle would be 3.2.6 Discrimination between hypotheses greatly amplified, and low to equatorial lati- Palaeomagnetic and geochronological tudes would experience marked seasonal studies of Late Proterozoic glaciogenic rocks changes of temperature. Strong seasonality may discriminate between the hypotheses of in low to equatorial palaeolatitudes is indeed global refrigeration (synchronous low-lati- an important feature of the Late Proterozoic tude glaciation) and a large obliquity. As glacial climate. Furthermore, the occurrence discussed by Williams (1975a), diachronism of extensive glacial loessites and periglacial of glaciation could result from movement of aeolian sandstones of Late Proterozoic age is continents across low latitudes when E > 54°; consistent with the predictions of frigid sol- hence Late Proterozoic glaciogenic se- [~ (;.E. WILLIAMS quences, despite their low palaeolatitudes of geochronological data supporting diachro- deposition, may not be synchronous and cor- nism of Late Proterozoic glaciogenic se- relative over wide areas. Isotopic age deter- quences in Central and West Africa. Preiss minations and palaeomagnetic studies of Late (1987) argued, however, that Kr6ner's (1977) Proterozoic glaciogenic rocks therefore might geochronological data are insufficiently pre- distinguish between (a) global refrigeration, cise to demonstrate the diachronism he pro- which predicts synchronous glaciation in low posed, palaeolatitudes, and (b) glaciation preferen- Discrimination between the hypotheses of tially in low palaeolatitudes for a large obliq- a non-axial geocentric dipole model of the uity, which could be either synchronous or Earth's magnetic field, and a large obliquity, diachronous. As noted by Li et al. (1991), if requires palaeogeophysical data that are in- all continents were static in low latitudes one dependent of palaeomagnetism. Such data could not distinguish on palaeomagnetic evi- are provided by rhythmites of tidal-climatic dence between glaciation during global re- origin from the Late Proterozoic Marinoan frigeration and for a large obliquity. Kr6ner glacial sequence in South Australia (Fig. 6; (1977) indeed concluded that the apparent Williams, 1988, 1989a-c, 1990, 1991a). The diachronism of Late Proterozoic glaciations ~ 60-year long, unsurpassed palaeotidal in Africa supports the large-obliquity model, record of mixed/synodic type from the and Deynoux et al. (1978) presented much Elatina Formation, and palaeotidal data also

Fig. 6. Late Proterozoic (~ 650 Ma) tidal rhythmites, South Australia. Muddy material appears darker than sandy to silty layers. Scale bar 1 cm for both photographs. (a) Elatina Formation from Pichi Richi Pass, showing four fortnightly lamina-cycles each comprising ~ 12 + 2 graded (upward-fining), diurnal laminae of very fine-grained sandstone and siltstone. Dark bands delineating lamina-cycles are mud-drapes deposited at neaps. The lamina-cycles typically are abbreviated by the absence of several clastic laminae near neaps. (b) Reynella Siltstone Member from Hallett Cove, showing a thick, fortnightly lamina-cycle that contains 14 diurnal laminae of fine-grained sandstone each with a muddy top. Arrows indicate thinner, muddy laminae deposited at neaps. Most diurnal laminae in (b) comprise two semidlurnal sublaminae of unequal thickness, which record the diurnal inequality of the tides. HISTORY OF l'Hft~ LARTH'S OBIAQUITY 19

261 of mixed/synodic type from the Reynella 10 ~ a Siltstone Member of the same formation, contain features that together are consistent with a substantial obliquity in Late Protero- 05 zoic time and a low latitude of deposition. 13,1 215 The features are a clear diurnal inequality of the tides, strong semi-annual and annual sig- j 87 66 O0 k _J I i5 3. nals and marked abbreviation of fortnightly £o o.oo o'.1o o'.~c, o'.3o 0'40 o'.so tidal cycles at equinoctial neaps. Frequency (1/cycles) 1'o ; ~: ~ The diurnal inequality (see Section 3.1), *d Period or wavelength (lamina-cycles) 2,15 best displayed by the Reynella rhythmites 1.0- b (Fig. 6b) and recorded also by the Elatina rhythmites (Williams, 1991a), indicates at O_ least a moderate palaeo-obliquity. Although 0.5- the tidal forcing for the diurnal inequality vanishes at the equator and the poles (de 125 Boer et al., 1989), the arrangement of conti- o.o nents allows the inequality to occur over all 000 O. 10 020 030 0~40 050 latitudes. Hence this palaeotidal feature is Frequency (1/cycles) not latitude-dependent. Period (lunar fortnightly cycles) The semi-annual tidal period of solar de- Fig. 7. Fast Fourier transform smoothed spectra of clination (the angular distance of the Sun palaeotidal and tidal data, with power spectral densi- north or south of the celestial equator) is ties normalised to unity for the strongest peak in each conspicuous in the fast Fourier transform spectrum and with linear frequency scales. (a) Spec- (FFT) spectrum of the Elatina palaeotidal trum for the Late Proterozoic Elatina tidal-rhythmite sequence of 1580 fortnightly lamina-cycle thickness data (Fig. 7a). The very strong annual signal measurements ( ~ 60-year record). The very strong peak in the same spectrum is attributable to a at 26.1 lamina-cycles represents an annual signal, with large seasonal oscillation of sea level. Today, harmonics at 13.1 (semi-annual), 8.7, 6.6 and 5.3 lam- the dominant seasonal oscillation of sea level, ina-cycles. The peak near :2 lamina-cycles (the Nyquist and the less-important solar annual tidal frequency) reflects the monthly inequality of alternate constituent (S,) due to the eccentricity of the thick and thin fortnightly lamina-cycles, due to the eccentric palaeo-lunar orbit. (b) Spectrum for the max- Earth's orbit, both attain their maximum de- imum heights of 495 spring tides between January 1, velopment in low latitudes (Pariwono et al., 1966 and December 31, 1985, for Townsville, Queens- 1986; and unpublished data from the Na- land. The periods of 24.4 and 12.5 fortnightly cycles tional Tidal Facility, Flinders University). represent annual and semi-annual signals. The peak Hence the presence of a very strong annual near 2 fortnightly cycles (the Nyquist frequency) reflects the monthly inequality of alternate high and low signal in the Elatina data accords with the spring tides, due to the eccentric lunar orbit. Tidal indicated low palaeolatitude of deposition of data for Townsville supplied by the National Tidal the Elatina Formation. Facility, Flinders University. An idea of the relative power of the semi- annual and annual signals in the Elatina rhythmite data may be gained by comparing Fig. 7 are for similar, long sequences (~ 20- FFT spectra for the Elatina data and for 60 years) of fortnightly data and show an- modern tidal data from Townsville, Queens- nual, semi-annual and monthly peaks. Nor- land (Fig. 7). Comparing these spectra ap- malising the power spectral densities against pears justified because each tidal record is of the monthly peak for each spectrum shows mixed/synodic type and is from low latitudes that the annual and semi-annual signals in (Townsville is at 19°16'S). The two spectra in the Elatina spectrum (Fig. 7a) have ~ 15 20 G.E. WILl JAMS times and ~ 4 times more power, relative to -,~-~sola year--~- lamina thickness the related monthly peak, than do the annual ~,15 . SS AE WS VE SS 1"~ 1.0 r and semi-annual signals in the Townsville spectrum (Fig. 7b). A further difference be- m~ ~ ' ~ J ~ ' I tween the two spectra is the presence, only in the Elatina spectrum, of a sequence of higher harmonics of the annual and semi-an- laminae per lamina-cycle nual signals (peaks at 8.7, 6.6 and 5.3 fortnightly cycles). These higher harmonics are 25 50 75 100 attributable to beating among the annual and Lamina cycles semi-annual signals and their combination Fig. 8. The number of diurnal clastic laminae per tones. It would seem that the Late Protero- fortnightly lamina-cycle counted between lamina thickness minima (neaps), for 110 lamina-cycles from the zoic annual and semi-annual oscillations of Elatina tidal rhythmites (see Fig. 6a); lamina-cycle sea level in the Adelaide Geosyncline had number increases up-sequence. The unbroken curve sufficient power to generate a sequence of (smoothed by 5-point weighted filter) reveals a strong higher harmonics in sea-level height and/or modulation of lamina counts by a semi-annual signal of tidal range, which was recorded by the ~ 13 lamina-cycles; maximum abbreviation of lamina- cycles occurred near equinoxes. The dashed curve Elatina rhythmites. Although differences in shows lamina thickness for the same stratigraphic in- geographic settings may account for some of terval (smoothed by 111-point filter). The shaded bands the distinctions between these ancient and mark the stratigraphic positions of essentially unabbre- modern tidal records, the important point is viated lamina-cycles, which were deposited at solstices. that annual and semi-annual signals are very VE= vernal equinox, AE= autumnal equinox, SS = strongly developed in the Late Proterozoic summer solstice, WS = winter solstice. Adapted from Williams (1991a). data. The common abbreviation (absence of several diurnal laminae) near the neap part of the inclination of the lunar orbital plane to fortnightly lamina-cycles in the Elatina the ecliptic plane of 5.15 °. Hence, tidal ranges rhythmites (Fig. 6a) is attributable to small at and around equinoctial neaps, when lunar neap-tidal ranges (Williams, 1988, 1989a-c, declination is maximal, would be much re- 1990, 1991a). The number of diurnal laminae duced for e >> 23.5 °. The average equinoc- deposited per lamina-cycle, and by inference tial neap-tidal range of the M 2 constituent the range of palaeo-neap tides, were strongly for e=54 ° would be ~41% that of today, modulated by the semi-annual tidal period. for • = 60 ° only ~ 30%, and for • = 65 ° only As shown by Williams (1991a), the abbrevia- ~ 21% that of today. Extended intervals of tion of fortnightly lamina-cycles was very very small neap-tidal ranges thus would oc- marked at and near equinoxes, when deposi- cur at and near equinoxes for • > 54 °. tion of diurnal laminae ceased for up to 6-8 It is thus significant that the independent lunar days (Fig. 8). The palaeotidal pattern palaeogeophysical data provided by the indicated by the Elatina rhythmites--ex- Elatina and Reynella tidal rhythmites accord tended intervals of very small neap-tidal with a substantial obliquity of the ecliptic ranges at and around equinoxes--also is and a low palaeolatitude of deposition dur- consistent with a large obliquity in Late Pro- ing Late Proterozoic glaciation. terozoic time. The amplitude of fluctuations, Vanyo and Awramik (1982, 1985) sug- and range, of the semidiurnal part of the gested, from the apparent sinuosity of one lunar equilibrium tide (the principal tidal sample of an inclined columnar stromatolite constituent, M 2) vary as cos2d, where d is from central Australia, that the obliquity was the Moon's declination (Pillsbury, 1940); and 26.5 ° at ~ 850 Ma. However, in most in- maximum declination dma x = • ± i, where i is stances the causes of inclination of columnar HISTORY OF "I'HE EARTH'S OBt,IQUITY 2 [ stromatolites are unknown or only tentatively tralian Precambrian palaeolatitudes do not inferred. Current direction (Walter, 1976) fit uniformitarian concepts of palaeoclimate". and prevailing wind direction (Playford and Instances of non-uniformitarian palaeocli- Cockbain, 1976) appear to strongly influence mates mentioned by them, in addition to the such inclination. Moreover, Chivas et al. low palaeolatitudes of Late Proterozoic (1990) queried the use of stromatolites as glaciogenic deposits discussed here (Section possible indicators of heliotropism because 3.2), include an apparent glaciogenic drop- of the very high rates of stromatolite growth stone facies deposited at ~ 1400 Ma when (up to ~ 10 cm/year) assumed by Vanyo Australia occupied low palaeolatitudes; the and Awramik (1982, 1985). Indeed, it seems palaeogeography is uncertain, but Idnurm unlikely that columnar stromatolites could and Giddings (1988, p. 80) stated that "a faithfully track the Sun throughout the year general low latitude glaciation cannot be for large values of obliquity. That would re- ruled out". quire symmetrical growth, partly by sediment Early Proterozoic (--, 2300 Ma) glaciogenic trapping, on submerged algal columns tilted rocks occur in North America, South Africa, 45 ° or more (taking account of the refraction Australia and several other continents (see of light at the seawater surface), whose con- De Villiers and Visser, 1977; Hambrey and vex algal surfaces would range from near- Harland, 1981), but palaeomagnetic data are horizontal to near-vertical. Since a near- available only for the Gowganda Formation horizontal algal surface would trap settling (Coleman Member) and Chibougamau For- sediment more efficiently than would a mation of the Huronian Supergroup in steeply inclined surface, columnar stromato- Canada. The Gowganda Formation is partly lites probably have maintained a very strong of glaciomarine origin and overlies striated vertical component of growth throughout the basement rocks (Young, 1981). Morris (1977) year, whatever the obliquity. The apparent found that most specimens from the Gow- sinuosity of a few specimens thus could lead ganda and Chibougamau formations carried to large underestimates of palaeo-obliquity. either an A or a B remanence direction. Any annual sinuosi~ in the ,-, 850-Ma stro- Specimens carrying the A direction were from matolites may reflect the annual oscillation sites showing little sign of metamorphism of sea level, which strongly influenced the and contained no secondary overgrowths on Late Proterozoic paralic environment in detrital magnetite; fold tests of remanence South Australia and left a clear imprint on were positive and the A direction thus pre- the Elatina tidal rhythmites (Fig. 7a). Until a dates folding of the Huronian Supergroup. valid basis for interpreting stromatolite By contrast, sites giving the B direction show growth patterns is widely accepted, the sug- metamorphic grades from lowest to upper- gestion of Vanyo and Awramik (1982, 1985) most greenschist and specimens contained must be viewed as unsubstantiated. secondary euhedral overgrowths on magnetite grains; fold tests for the B direction 3.3 Prior to ~ 800 Ma were negative, indicating remagnetisation that postdates folding. As noted by Morris Interpretation of Precambrian palaeocli- (1977), Symons' (1975) remanence directions mates requires reliable palaeomagnetic data, indicate such secondary (reset) magnetisa- based on the tenet that the geomagnetic field tion. The A direction, which Morris (1977) was then a geocentric axial dipole (see Sec- implied was acquired at the time of deposition 3.2.4). Importantly, Idnurm and Gid- tion of the rocks, consistently gave an incli- dings (1988, p. 81), in reviewing Australian nation of 54 ° for the dip-corrected direction Precambrian palaeomagnetic data back to for all three areas sampled. Hence, the glacial 3500 Ma, concluded that overall "the Aus- sediments may be assigned a palaeolatitude -)~ G.E, WII.LIAMS of ~ 35°; Young (1981), in discussing the mean annual air temperature <-4°C to data of Morris (1977), evidently mistook in- < -8°C (see Karte, 1983; Williams, 1986). clination for palaeolatitude. Furthermore, The presence of laminated argillites with Kalam-Aldin (1983) showed that the matrix abundant dropstones in the Huronian de- of glaciogenic conglomerate from the Gow- posits, interpreted as or compared to glacial ganda Formation has a remanent magnetisa- varves (e.g. Lindsey, 1969; Long, 1974; tion, interpreted by him as a primary compo- Young, 1981; Mustard and Donaldson, 1987), nent, that indicates a palaeolatitude of depo- may provide supporting evidence for a sition of 26 ° (inclination = 44.3°). A palaeo- strongly seasonal glacial climate. Another latitude of 26-35 ° for the Huronian glacio- feature of the Huronian deposits is the puz- genic rocks is supported by the magnetisa- zling association of glaciogenic formations tion of the Firstbrook Member of the Gow- with aluminous quartzites that are suggestive ganda Formation (immediately overlying the of intense chemical weathering under warm- glaciogenic Coleman Member), which Roy climate conditions; the same facies associa- and Lapointe (1976) concluded was acquired tion occurs in Early Proterozoic rocks in early and which indicates a palaeolatitude of South Africa (Young, 1973), The evidence of 34 °. New palaeomagnetic data for the 2217- grounded ice sheets near sea level in moder- Ma Nipissing diabase in the Gowganda area ately low palaeolatitudes under a cold, ap- in Canada give a palaeolatitude of 28 ° (mean parently very strongly seasonal climate, to- inclination = 47 °) for igneous intrusion gether with the stratigraphic proximity of (Buchan, 1991), which accords with a moder- cold- and warm-climate indicators, suggest ately low palaeolatitude for the Gowganda non-uniformitarian glacial conditions compa- area during the Early Proterozoic. rable to those of the Late Proterozoic. A palaeolatitude of 26-35 ° for Huronian Early Proterozoic banded iron-formations glaciation near sea level is intriguing. Indeed, (BIFs) on several continents, best exempli- Donaldson et al. (1973, p. 11) had noted fied by the ~ 2500-Ma BIFs of the Hamers- previously "the somewhat puzzling anomaly" ley Group, Hamersley Basin, Western Aus- and Irving (1979) the "paradox" of a possible tralia (Trendall, 1983), typically show regu- low to moderately low palaeolatitude for lar, laterally persistent microband couplets of Huronian glacial deposition. Although the silica and iron oxide. The couplets usually Pleistocene continental ice sheet in the North are interpreted as annual varves (e.g. Tren- American interior at times expanded far be- dall, 1972, 1973a, 1983; Ewers and Morris, yond its usual areas in high latitudes to 1981); locally cyclic, very thin banding ~ 37-40°N latitude (Frakes, 1979), glaciation (Trendall, 1973b) may record subdivisions of near sea level would be expected preferen- yearly microbands (Williams, 1989c, 1990). tially in high latitudes for the Early Protero- An annual origin of the BIF microbands zoic because the faster rotation of the Earth implies a strong seasonal influence on basi- at that time would have increased the equa- nal sedimentation. Palaeomagnetic data tor-to-pole surface temperature gradient and (Clark and Schmidt, 1986) show that the caused warmer low latitudes and colder high BIFs of the Hamersley Group possess a con- latitudes (see Hunt, 1979). Importantly, the sistent, pre-folding magnetisation that indi- Huronian glaciogenic deposits contain struc- cates near-equatorial palaeolatitudes (< 5 °) tures regarded as sandstone casts after of deposition. A strong seasonal influence on periglacial ice-wedges (Young and Long, BIF deposition near the palaeo-equator im- 1976); the structures, if interpreted correctly, plies a large amplitude of the global seasonal would indicate a very strongly seasonal cycle (see also Trendall, 1972). palaeoclimate (seasonal mean-monthly tem- A mixtite containing striated and faceted perature range > 35°C to > 60°C) and a boulders occurs in the Early Proterozoic }tlST()RY OF I'Ht5 EARTH'S OBLIQUITY 23

Turee Creek Formation, which conformably (Taylor and Middleton, 1990). More palae- overlies the Hamersley Group in the Hamer- owind and related palaeomagnetic data are sley Basin (Trendall, 1976). As discussed by required for the Proterozoic to establish Trendall, the mixtite may record an Early whether non-uniformitarian palaeowind di- Proterozoic glaciation in Western Australia rections are typical for that con. that was broadly coeval with the Huronian Surface temperatures as high as 50-80°C Glaciation in Canada. It is noteworthy that during the Precambrian (~ 1300-3800 Ma) the apparent polar wander path for Australia and Phanerozoic surface temperatures of spanning the interval from ~ 1800 Ma to ~ 15-45 ° have been estimated from ~SO > 2860 Ma (Clark and Schmidt, 1986), which values for sedimentary cherts and phos- includes primary poles for the Hamersley phates (Knauth and Epstein, 1976; Knauth Basin, suggests that northwestern Australia and Lowe, 1978; Karhu and Epstein, 1986). occupied low palaeolatitudes at ~ 2300 Ma. Such proposed palaeotemperatures are sus- Palaeomagnetic study of the Turee Creek pect as absolute temperatures because of Formation is required to ascertain whether possible diagenetic modification of 3lSo val- glacial deposition occurred near the palaeo- ues. However, the good correlation of a equator. ~ 150-Ma periodicity in the Phanerozoic No palaeomagnetic data are available for palaeotemperature data (Karhu and Epstein, the Early Proterozoic glaciogenic Mak- 1986) with the apparent near periodicity of ganyene Formation in South Africa. Accord- ~ 150 Ma for major glaciations since the ing to Piper et al. (1973), Upper Ventersdorp Late Proterozoic (Williams, 1975b; Frakes volcanics (2300 + 100 Ma) gave a palaeolati- and Francis, 1988) gives confidence that the tude of ~ 40 ° and volcanics of the Cox Group 3mO values can be expressed as relatic'e (~ 2250 Ma), which overlie the Makganyene palaeotemperatures. One possible explana- Formation with regional unconformity (Kent, tion of apparent relatively high surface tem- 1980), a high palaeolatitude. The Gaberones peratures during the Precambrian is that they granite (2340 +_ 50 Ma) in Botswana gave a reflect high summer temperatures resulting palaeolatitude of 3-7 ° (McElhinny et al., from a strongly seasonal global climate. 1968). Further palaeomagnetic studies clearly The above review discusses some impor- are required. tant palaeoclimate indicators and events for Since the direction of zonal surface winds pre-Sinian time. Although more sedimento- such as the trade winds and mid-latitude logical and palaeomagnetic studies are of westerlies would be reversed for • > 54 °, course required, an image seems to be form- palaeowind directions indicated by Protero- ing of a strangely zoned and strongly sea- zoic aeolian deposits also may provide infor- sonal non-uniformitarian Precambrian world. mation on palaeo-obliquity. As mentioned in At different times the pre-Sinian environ- Section 3.2.5, the dominant direction of ment was marked by seasonal deposition of palaeowinds indicated by the Late Protero- banded iron-formations near the palaeozoic periglacial Whyalla Sandstone in South equator, grounded ice-sheets and glaciation Australia (mean palaeolatitude ~ 12 oN ; Em- near sea level in moderately low palaeolati- bleton and Williams, 1986; Schmidt et al., tudes under a cold, evidently very strongly 1991) is opposite to that expected for zonal seasonal climate, and the interbedding of surface winds in such latitudes (Williams, cold- and warm-climate indicators in moder- 1993). In addition, the mean direction of ately low palaeolatitudes. The apparent re- palaeowinds for the Keweenawan (1000-1200 verse climatic zonation (despite the climatic Ma) Copper Harbor Formation (palaeolati- effects of a faster-rotating Earth), very strong tude = 21 °) in Michigan is perpendicular to seasonality and abrupt changes of climate in the expected direction of palaeo-trade winds moderately low palaeolatitudes are consis- -34 G.E. WIIJIJAMS tent with a large obliquity (e > 54 °) during tle then went into orbit about the Earth as Precambrian time. the primordial Moon. The wide appeal of the single-giant-impact hypothesis arises from its 4. ORIGIN AND LIMITS OF THE PRIMORDIAL apparent explanation of the angular momen- EARTH'S OBLIQUITY tum and orbital characteristics of the Earth-Moon system and the distinctive lunar It has long been suggested that the pri- composition. The obliquity of the primordial mordial Earth acquired its obliquity by im- Earth also is attributed to this single giant pact with a large body (e.g. Safronov and impact. Zvjagina, 1969; Cameron, 1973; Singer, 1977). Hartmann and Vail (1986) showed that a This view is strongly supported by the now given impactor can produce a wide range of widely-accepted single-giant-impact hypothe- results, depending on the impact parameters. sis of lunar origin (Hartmann and Davis, The impact-induced obliquity of the target 1975; Cameron and Ward, 1976; Cameron, planet can range from 0 ° to ~ 180°, the most 1986; Hartmann, 1986; Taylor, 1987; New- likely value depending on the impactor/tar- sore and Taylor, 1989), which has become get-planet mass ratio and the velocity of ap- the "current consensus theory" of the Moon's proach. Importantly, Hartmann and Vail formation (Melosh, 1990). This hypothesis (1986) demonstrated that a most common holds that at ~ 4500 Ma the Protoearth, post-impact obliquity of up to - 70 ° for the when close to its present size, experienced primordial Earth could result from approach one very large glancing impact with a differ- velocities of 20-30 km/s and mass ratios of entiated body about the size of Mars or ~ 0.05-0.10 (Fig. 9a); the most likely obliq- slightly larger (the requisite mass of the im- uity increases with impactor mass and also pactor depends on its velocity); material from tends to increase with increasing approach the impactor's and/or the Protoearth's man- velocity. An obliquity of ~ 70-80 ° also could

180

• o 7O ~ i ~ I i • ~ ~ I a 150 ~" 60 APPROACH VELOCITY O • 30 km/s • O 20 km/s ~ 120 ~ 50 • 10 km/s • O x 5 km/s O •

"5 40 ~ 90 • O • o 30 0 0 • 0 ~) • • x x x * x ~ 60 E E 2O g o ~ x ~ x~o 30 ~o 10 • × 0 I~ i I r i J J i r I 0 0 0.05 0.10 0 0.05 010 Planetesimal mass / target planet mass Planetesimal mass / target planet mass Fig. 9. (a) Impact-induced obliquity of a target planet for a range of approach velocities and impactor/target-planet mass ratios. The target planet has physical parameters of the Earth, with pre-impact obliquity of 0 ° and pre-impact rotation period of 10 hours. Each point plotted is the most frequent value in a run of 500 impacts at specified impactor mass. Results applicable to any planet, but assume the Earth's specific density and coefficient of angular momentum. Adapted from Hartmann and Vail (1986). (b) Variation in obliquity occurring when the impactor approaches the Earth at 5 kin/s, with pre-impact obliquity of 0 ° and pre-impact rotation period of 10 hours. The plot, which is smoothed, represents the results of 500-impact runs at intervals of 0.01 mass ratio. Shaded zone includes two-thirds of the values, centred on the most frequent values (dashed line). The outer envelope includes 99.8% of the values. Adapted from Hartmann and Vail (1986). HISTORY OF THE EARFH'S OBLIQUITY 25 readily be produced by an approach velocity impact would produce a maximum shift of as low as 5 km/s and mass ratios of 0.08-0.11 axis of only 0°32'. Furthermore, the dynami- (Fig. 9b). Approach velocities ranging from 5 cal effects of terrestrial impact over time to 20 km/s and mass ratios from 0.08 to 0.14 would have been random and would not have have indeed been suggested for the Moon- led to cumulative change in obliquity. Slow producing impact (e.g. Cameron, 1986; Hart- change in obliquity, however, could have re- mann, 1986; Taylor, 1987; Newsom and Tay- suited from the action of luni-solar tidal and lor, 1989), and hence a large (~ 70 °) impact- precessional torques during Earth history. induced obliquity must be considered very possible to likely. Such impact parameters 5.1 Tidal friction give a post-impact rotation period for the Earth in the range of ~ 3-10 hours (Hart- The effect of lunar tidal friction on the mann and Vail, 1986). obliquity of the ecliptic is discussed by Mac- The large obliquity (~ 98 °) of Uranus has Donald (1964, 1966). Because the Earth is been attributed to an early impact with a not perfectly elastic, the tidal bulge raised by single large body of mass ratio ~ 0.07-0.14 the Moon lags behind the tide-raising force (Hartmann and Vail, 1986; Taylor, 1987; Ko- and so is carried forward by the Earth's rycansky et al., 1990). According to Goldre- rotation (Fig. 10). The angle between the ich and Peale (1970), the rapid rotation of Earth-Moon axis and the tidal bulge is Uranus, the negligible mass of its moons, and termed the phase lag. The lunar attraction its great distance from the Sun imply that on the tidal bulge facing the Moon exceeds tidal friction has not appreciably altered its that on the more distant bulge, because tidal primordial spin. The negligible tidal and pre- force decreases with distance. Consequently cessional torques acting on Uranus may im- the Moon exerts a net torque acting about ply that its large obliquity also is essentially the Earth's spin axis that tends to retard the primordial. As discussed below, the Earth, by Earth's rotation, and the tidal bulge exerts a contrast, has been subjected to substantial reciprocal torque that tends to accelerate the luni-solar tidal and precessional torques Moon's orbital motion. By this mechanism, throughout its history, providing mechanisms angular momentum is transferred from the for secular change not only in rotation rate Earth's rotation to the lunar orbital motion. but also obliquity. Because of the Earth's obliquity of 23.5 ° and the inclination of the lunar orbital plane 5. MECHANISMS FOR SECULAR CHANGE IN THE EARTH'S OBLIQUITY

Tidal bulge Collisions between the Earth and im- pactors since the formation of the Moon are most unlikely to have caused significant change in the Earth's obliquity. Dachille (1963) calculated that an impactor 32 km in diameter (about three times the diameter of Fig. 10. Tidal bulge for the present case where the ratc the envisaged K/T-boundary bolide), for ex- of the Earth's rotation exceeds the rate of the Moon's ample, with a density of 3.5 g/cm 3 and trav- revolution. The tidal bulge raised by the Moon is elling at the very high velocity of 72 km/s carried forward by the Earth's rotation; 'F represents would alter the obliquity by only 0°00'02 " the phase lag angle. The Moon exerts a net torque on under optimum conditions of impact. He also the tidal bulge that acts about the Earth's spin axis and tends to retard the Earth's rotation, and a reciprocal showed that an impactor 320 km in diameter torque tends to accelerate the Moon's orbital motion. and with the same density and conditions of Adapted from MacDonald (1964). 2(3 (;.E. WILI.IAMS to the ecliptic plane of 5.15 °, the Moon does 30 a not revolve in the equatorial plane of the s- 2s Earth. The Earth's rotation therefore carries ~,~- 2o the tidal bulge out of the Moon's orbital >, "5 I plane. The net effect of lunar torques acting Z15 on the tidal bulge tends to decrease the 0 I 10 ~_ J component of angular momentum of the 15100 10100 500 0 Earth that is perpendicular to the lunar or- Age (Ma) 50 bital plane and conserve the component in "~'40 the orbital plane. The Earth's obliquity thus tends to increase and, conversely, a torque ~o tends to decrease the Moon's orbital inclina- ~-2o o- tion. ~1o x- t t Employing the lunar tidal torque indicated i i i i k__ i i 10 20 30 40 50 60 70 by modern observations, MacDonald (1964; Distance (Earth radii) i i i i -~-- 1~- Fig. 1 la) calculated that the Earth's obliquity 24 was 20 ° at 1000 Ma. Based on MacDonald's 2o C obliquity~..~ expressions for the lunar tidal torque, Gol- a) dreich (1966; Fig. l lb) illustrated a slow de- ~12 crease in obliquity going back in Earth his- 8 tory (time expressed as Earth-Moon dis- 4 inclin~tion ...... 0 z i £ • tance), with a mean obliquity of ~ 10° for an 1 @ 20 ~0 40 50 60 Earth-Moon distance < 10 R E (earth radii). Distance (Earth radii) More recently, Mignard (1982; Fig. 1lc), pro- Fig. ll. Variation in the obliquity of the ecliptic due to tidal friction plotted againsl age or Earth-Moon dis- jecting the rate of tidal dissipation deduced tance. (a) Adapted from MacDonald (1964), showing from Palaeozoic coral data (which are of the obliquity back to 1600 Ma for a mean phase lag dubious reliability and give a rate of tidal equal to the present value. (b) Adapted from Goldre- dissipation similar to the modern rate; see ich (1966). (c) Adapted from Mignard (1982). Section 3.1 and Williams, 1989c) back to 1300 Ma and using a smaller value before that time, calculated that the obliquity was 1990; Deubner, 1990), that is, only about 13 ° when the lunar distance was ~ 10 Rp 0.0006"/cy. MacDonald's (1964) curve of obliquity- These computed values for (i,) of change (Fig. l la) indicates an average rate of ~ 0.0006-0.0012"/cy are at odds, however, increase in obliquity due to lunar tidal fric- with recently acquired geochronometric data. tion ((it)) of 0.0012"/cy (brackets ( ) indi- The palaeotidal record provided by the Late cate time-average) since 1000 Ma; this value Proterozoic Elatina Formation and Reynella is based on the assumption that the phase lag Siltstone Member in South Australia (Wil- remained equal to its present value. The liams, 1989a-c, 1990, 1991a) indicates an av- curve of Mignard (1982; Fig. llc) implies a erage equivalent phase lag since ~ 650 Ma similar value of (i t ) since 1300 Ma. The that is only half the present value (Table 4). curves of Goldreich (Fig. llb) and Mignard The palaeotidal data also indicate a mean further indicate that i~ has steadily increased rate of lunar retreat of 1.83-1.95 cm/year with time; (i,) for early Earth history since ~ 650 Ma (Table 5), only about half (Earth-Moon distance=20-35 RE: Lam- the value of 3.7 _+ 0.2 cm/year determined beck and Pullan, 1980; Webb, 1982) is only by lunar laser ranging (Dickey et al., 1990). about half the value of (i t) since the Late Tentative palaeotidal data for ~ 2500 Ma Proterozoic (~ 58-60 RE: Williams, 1989a, suggest that tidal dissipation during the Pro- HISTORY OF THE EARTH'S OBI_IQUITY 27 terozoic was even smaller (Williams, 1990). Pe These findings are consistent with increasing oceanic tidal dissipation as the Earth's rota- I/ \ tion slows (Hansen, 1982; Webb, 1982). Geo- Pc\.. / logical observations therefore imply that the rate of increase in obliquity during Earth history due to tidal friction was only about half that calculated by MacDonald (1964), Goldreich (1966) and Mignard (1982); rea- sonable estimates are (~t) ~ 0.0006"/cy since the Late Proterozoic and (it)--- 0.0003"/cy in earlier time. The total increase in obliquity during Earth history attributable to tidal friction is thus < 7.5 °.

5.2 Dissipati~e core-mantle torques

As discussed by Munk and MacDonald (1960), changes in attitude of the Earth's Fig. 12. The precessing Earth, showing the obliquity of rotation axis in space result largely from the the ecliptic e = 23.5 ° and the circle described about the gravitational pull of the Moon and Sun on pole of the ecliptic Pc by the celestial pole P,. (the the Earth's equatorial bulge. The equatorial extension of the Earth's spin axis on the celestial sphere) as a result of the astronomical precession. RE, plane and the lunar orbital plane are in- R l and R 2 are the radii of the Earth, the outer core clined at 23.5 ° and 5.15 ° to the ecliptic, re- and the inner core~ respectively. Adaptcd from Malkus spectively, and if the Earth did not rotate the (1968). action of the Moon and Sun would pull these planes into coincidence. Because of the gyro- scopic effect of the Earth's spin, however, the obliquity of the ecliptic remains near 23.5 ° and the celestial pole describes a circle core and mantle are, however, essentially about the pole of the ecliptic in 25.5 ka coupled and precess at virtually the same relative to the fixed perihelion (Fig. 12). This rate, although there may be a small non- motion is termed the astronomical preces- alignment of their figure axes. The mechani- sion of the equinoxes as distinct from cli- cal couples or torques at the core-mantle matic precession relative to the moving peri- boundary (CMB) most commonly discussed helion, which has main periods of 23 ka and in regard to the transfer of angular momen- 19 ka (Berger, 1984). The precessional torque tum between core and mantle (see Lambeck, is about four million times greater than the 1980; Merrill and McElhinny, 1983; tidal torque but acts solely in the ecliptic Rochester, 1984; Hide, 1989) are: plane (Hipkin, 1975). (a) t~iscous coupling~ a function of the vis- Because the mean density of the core is cous friction in either a laminar or turbulent much greater than that of the mantle, the boundary layer at the CMB, and dependent dynamic ellipticity of the core is only 3/4 the on either the kinematic or effective viscosity corresponding value for the mantle (Malkus, of the outer core; 1968; Lambeck, 1980). If the core and mantle (b) electromagnetic coupling, a function of were uncoupled, the core would thus precess magnetic field strengths and electrical con- at 3/4 the rate of mantle precession. The ductivity in the core and mantle; and 2~ (i.E. WILI.IAMS

(c) topographic coupling, in which and Peale, 1970; Lago and Cazenave, 1979) "bumps" or depressions at the CMB modify and Mercury (Peale, 1976). Bills (1990a) also the flow of fluid past the boundary and the recognised the potential importance of dissi- pressure distribution on the mantle. pative core-mantle viscous and electromag- Hide and Dickey (1991, p. 632) noted that netic coupling to the long-term obliquity his- limited knowledge of the motions, viscosity tories of the Earth and Mars. In addition, and electrical conductivity of the core, the Vanyo (1991) argued that viscous core-man- electrical conductivity and magnetic field of tle coupling may dominate over electromag- the lower mantle, and the topography of the netic and topographic coupling, causing some core-mantle interface "make it impossible to sizeable portion of the Earth's secular despin determine the torque acting at the CMB with (presumably with concomitant secular de- much certainty." Nonetheless, as discussed crease in obliquity). below, each of the above three mechanisms Aoki (1969) proposed that the rate of de- has been advanced as a cause of substantial crease in the Earth's obliquity (~p) caused by core-mantle torques. viscous core-mantle dissipation =-0.32"/ Any precession-induced differential mo- cy (the minus sign indicates decreasing obliq- tion between a fluid core and a non-aligned uity). Rochester (1976), however, argued that rigid mantle causes dissipation of rotational core-mantle coupling is provided largely by energy at the CMB. Since the precessional inertial torques (a view disputed by Vanyo, torque acts only in the ecliptic plane, the 1991) and that ~p is <; -0.0004"/cy. In de- component of spin angular momentum in riving his figure for ~p, Rochester (1976) that plane is reduced by core-mantle dissi- observed that the most uncertain parameter pation and the component perpendicular to is the kinematic viscosity of the Earth's outer the orbital plane is conserved (Peale, 1976). core (u). He showed that ~v varies as u ~/2 Dissipation at the CMB thus tends to drive for large values of u (/> 105 cm2/s), and took the obliquity toward 0 ° for • < 90 ° and to- an upper limit for u of 105 cm2/s (from ward 180 ° for • > 911°. Deceleration of the Toomre, 1974). Rochester (19761 acknowl- Earth's rotation accompanying decrease in edged that core-mantle coupling would be obliquity by core-mantle coupling involves important in the dynamical evolution of the no exchange of angular momentum between Earth-Moon system if u approaches 105 the Earth and the Moon, and may be small cm -/k.S compared to the tidally-produced decelera- The kinematic viscosity of the Earth's tion. Aoki and Kakuta (1971) obtained (o/w outer core is, however, "one of the least =--5.8X lO-14"/cy due to core-mantle known physical parameters of the Earth" coupling (where to is the rate of the Earth's (Jacobs, 1987, p. 55), postulated values dif- rotation) for a rate of obliquity-decrease of fering by many orders of magnitude and 11.0254"/cy, as against the observed rota- ranging from 10 -~ to 10 ~ cm2/s. High val- tional deceleration of -10-s"/cy. Accord- ues for ~, of 10 9 to 1011 cm2/s (which may be ing to Rochester (1976), the rate at which regarded as far upper limits) have been sug- rotational kinetic energy is dissipated by the gested from seismological observations (e.g. axial component of core-mantle coupling Sato and Espinosa, 1967, 1968; Suzuki and currently is < 10 ~' W, which is negligible Sato, 1970). Inferred upper limits are < 105 compared with the total rate of tidal energy cm2/s from the phase of the Earth's forced dissipation of 4.5 × 10 j2 W (Lambeck, 1980). nutation (Toomre, 1974), < 106 cm2/s from Secular change in planetary obliquity the amplitudes of the forced nutation through t,iscous core-mantle coupling has (Molodenskiy, 1981), < 10 7 cm2/s from tidal been proposed for the Earth (Aoki, 1969; variations in the Earth's rotation rate Aoki and Kakuta, 1971), Venus (Goldreich (Moiodenskiy, 1981) and < 106 cm2/s from HISTORY OF THE [-AR I'H'S OIaII.1QUITY 29 the estimated amplitude of topography at the concentration of solid particles is very high". CMB (Hide, 1971). The lowest values for v Hence the presence of a core slurry may of 10 3 to > 10 ° cm2/s are theoretical esti- permit the viscosity to be much larger than mates based on the behavior of liquid metals theoretical estimates, although any shear flow at high temperatures and pressures (e.g. in the outer core could reduce the slurry Gans, 1972; Bukowinski and Knopoff, 1976; viscosity by "shear thinning" (see Jeffrey and Poirier, 1988). Workers in geomagnetism Acrivos, 1976). usually have assumed v -- 10 -2 cm2/s as for Importantly, turbulent flow in the fluid liquid metals (Gans, 1972). Theoretical esti- core may greatly increase its effective viscos- mates of v were challenged by Officer (1986), ity and the drag on the mantle. The occur- who presented a model of core dynamics and rence of turbulent flow in a fluid gives rise to the Earth's magnetic field using v = 2 × 107 an "eddy" or "turbulence" viscosity that may cm2/s. Officer's model predicts the correct be up to several thousand times the "molecu- order of magnitude for the external magnetic lar" or "absolute" viscosity (Hinze, 1975; field and the westward drift of the non-di- Massey, 1979); the turbulence viscosity is not pole field. Toomre's (1974) upper limit for v a constant for a given fluid but is primarily a has been regarded as the best estimate yet function of the fluid turbulence. Melchior (e.g. Rochester, 1976), but the value of 1, (1986, pp. 216, 224) has applied this principle remains uncertain. to the Earth's outer core, observing that to- Further uncertainty regarding the viscosity pography at the CMB may produce turbu- of the core arises from the suggestion that, lence in the fluid core and a turbulence through consideration of the core's likely viscosity "several orders of magnitude higher temperature limits, the outer core may be a than v" which could increase viscous torques slurry comprising up to 60% or more of solid at the CMB by "several powers". If the ef- particles suspended in an iron-sulphur melt fective viscosity of the outer core resulting (Jeanloz, 1990; Williams and Jeanloz, 1990). from turbulent flow were ~ 105 cm2/s dur- The viscosity of a suspension of solid parti- ing the geological past, then viscous dissipa- cles increases rapidly with increase in the tion at the CMB would indeed have played volume fraction of suspended solids above an important role in the evolution of the 0.4-0.5 (Roscoe, 1952; Jeffrey and Acrivos, Earth's obliquity. 1976). Tonks and Melosh (1990) showed that The magnitude of electromagnetic core- the viscosity V~ur,~ of a mechanical suspen- mantle coupling depends on the assumed in- sion of solid crystals in a high-temperature tensity of the magnetic field at the core- melt may be expressed as mantle interface and the assumed electrical conductivity of the lower mantle (Hide and Dickey, 1991). Such coupling commonly is /)slurry' = 12 liquid 10 re,t, (2) viewed as the most likely mechanism causing decade fluctuations in the length of day (e.g. where /"liquid is the viscosity of the liquid Roden, 1963; Yukutake, 1972; Lambeck, component and • the fraction of solids. High 1980; Merrill and McElhinny, 1983). Indeed, concentrations of suspended solids would Stix and Roberts (1984) concluded that the greatly increase the viscosity of the outer average electromagnetic couple exceeds that core compared with that of an iron-rich liq- required for length of day fluctuations, and uid; using eq. 2, theoretical estimates of Vuqu~d so must be balanced by an equally important of 10 3 to > 10 ° cm2/s give /"slurry = 103 to electromagnetic couple of opposite sense. > 10" cm2/s for ~ = 0.6. Gubbins (1976, p. Variations in electromagnetic core-mantle 39) likewise concluded that the viscosity of coupling also have been associated with the core "may be drastically affected if the changes in the geomagnetic dipole moment 3() G.E. WILl JAMS with periods of ~ 102-104 years (Watanabe rill, 1984). Changes in core-mantle coupling and Yukutake, 1975). Secular change in the might arise also from chemical reactions be- Earth's obliquity by dissipative electromag- tween the silicate mantle and the liquid iron netic core-mantle torques has been pro- alloy of the Earth's core, which may cause posed by Aoki and Kakuta (1971), who found the electrical conductivity of the 200-300 ip=-0.0254"/cy (employing the value of km-thick D" layer at the base of the mantle the electromagnetic torque between core and to vary spatially (and also temporally?) by up mantle given by Rochester, 1968), and by to eight orders of magnitude (see Knittle and Kakuta and Aoki (1972), who found ip = Jeanloz, 1991). Furthermore, the topography - O. l"/cy. of the CMB may change with time through Topographic core-mantle coupling requires reactions between the outer core and mantle bumps no more than 1 km in vertical ampli- and changes in subduction or mantle convec- tude at the CMB, but such irregularities can- tion (Jacobs, 1972; Gubbins and Richards, not yet be resolved by seismology (Bloxham 1986); such topographic changes could affect and Jackson, 1991). Hide (1969, 1989) has the turbulence viscosity of the outer core and suggested that topographic coupling may be cause temporal variations in topographic and comparable to or even more effective than viscous torques at the CMB. electromagnetic coupling. Indeed, topogra- In summary, the present and past values phy at the CMB of a few hundred metres or of ip resulting from the combined effects of more may lead to a torque that is orders of viscous, electromagnetic and topographic magnitude larger than the torques inferred torques at the CMB cannot be accurately from irregularities in the length of day fluc- determined because of uncertainties in esti- tuations, implying that such fluctuations may mating, at present and for the geological be caused by changes of flow in the core with past, the effective viscosity of the outer core, respect to a stable flow-configuration the nature of magnetic fields at the CMB (Hinderer et al., 199{/; Jault and Le Mou61, and within the lower mantle, and the topog- 1990; Le Mou61 et al., 1992). Furthermore, raphy of the CMB. The above review indi- core-mantle topography and possible density cates, however, that several potential mecha- heterogeneities in the lower mantle may give nisms for substantial core-mantle dissipation rise also to a gravity torque which could be do indeed exist; if flow in the core were of similar magnitude to the topographic turbulent and hydromagnetic, large drag on torque (Jault and Le MouEl, 1989). Topo- the mantle could result even for a core fluid graphic core-mantle coupling may greatly of low molecular viscosity. Importantly, esti- affect the value of ip; Aoki and Kakuta mates of iv by Aoki and Kakuta (1971) and (1971) suggested that their figure of Kakuta and Aoki (1972) are two to three -0.0254"/cy for i~, caused by electromag- orders of magnitude greater than the mean netic core-mantle coupling may be amplified rate of increase in obliquity (i t) of by up to two orders of magnitude if topo- 0.0003-0.0006"/cy attributable to lunar graphic coupling at the CMB is taken into tidal friction, and the upper limit for ip of consideration. 0.0004"/cy proposed by Rochester (1976) Important changes in core-mantle cou- employing u = 105 cm2/s (which may under- pling may occur on geological time-scales. estimate the effective or turbulence viscosity Thermal perturbations at the CMB may cause of the core now and in the geological past) electromagnetic core-mantle coupling to vary approximates the likely value of (it) during significantly on time-scales of the order of Earth history. Given the range of uncertain- IIIs years and have expression in long-term ties and the likelihood that conditions at the variations in the geomagnetic field (Merrill CMB have varied with time, dissipative and McElhinny, 1983; McFadden and Mer- torques at the CMB may indeed have been HISIORY OF IHI LAI<'I'H'S ()BHQUITY 31 of great importance in the evolution of the (1) At 4500 Ma, when the primordial Earth's obliquity. Earth acquired a large obliquity (54°< e < 90 °) by a single giant impact with a Mars-sized 6. A PROPOSED OBLIQUITY HISTORY body that produced the Moon. As noted Since dissipative torques at the CMB could earlier, an obliquity < 90 ° is required for be substantial and may have varied in the dissipative core-mantle torques to drive the geological past, the evolution of the Earth's obliquity toward 0 °. A mean obliquity (g) of obliquity must be critically reviewed. The 70 ° arbitrarily chosen at 4500 Ma could have commonly held views that the obliquity has been induced by an impact velocity of 5-20 slowly increased during Earth history under km/s and an impactor/Earth mass-ratio of the sole influence of tidal friction and that 0.08-0.14 (Fig. 9). the obliquity of the primordial Earth was (2) At 650 (+_ 30) Ma, the time of the last < 10-15 ° are open to question. Indeed, as major glaciation of the Late Proterozoic. The discussed in Section 4 and shown in Fig. 9, preferred low palaeolatitudes of glaciation the primordial Earth's obliquity could have and the large amplitude of the seasonal cycle been as great as 70 ° or more. in low palaeolatitudes imply that e > 54 ° (see Accordingly, a proposed curve of obliquity Section 3.2.5), and g = 60 ° is taken as a mini- against time, based on the normal climatic mum value at 650 Ma (some alternative cal- zonation during the Phanerozoic, the para- culations are made in Section 7 also for doxical Late Proterozoic glacial climate, the g = 65 ° at 650 Ma). Milankovitch-band oscil- seeming reverse climatic zonation of the Pre- lations of obliquity about the mean value cambrian in general, and the single giant may have influenced the advance and retreat impact hypothesis for the origin of the Moon, of ice sheets. is shown in Fig. 13. The curve has four (3) At 430 Ma (Irate Ordovician-Early control points: Silurian). Palaeo-orbital and palaeotidal data

I I I I I I L I [ T-

80 80 1 Q 60 60

45"

.~r 40 40 o g 4 uOI w ~ 20 20 I Precambrian-Cambrian boundary ~'~l I 0 I I I I I I I I It 0 4500 3500 2500 1500 500 0 Age (Ma) Fig. 13. Proposed curve of mean obliquity of the ecliptic g against time, consistent with the single giant impact hypothesis for the Moon's origin and interpretation of the geological record. The curve's four control points are: (1) = 70 ° at 4500 Ma; (2) g = 60 ° at 650 Ma; (3) g = 26 ° at 430 Ma; and (4) ~ = 23 ° at 0 Ma. These data indicate that ~: = 45 ° (when gp is maximum) at ~ 550 Ma, which is taken as the Precambrian-Cambrian boundary. 32 (~.[:,. WILI,IAMS for the Mallowa Salt and Elatina Formation The postulated relatively rapid decrease in suggest that g: = 26 ° is a best estimate (see mean obliquity from g = 60 ° at 650 Ma to Section 3.11. g = 26 ° at 430 Ma may partly reflect special (4) At0Ma, f=23 ° . conditions at the CMB which caused signifi- An important feature of the curve is the cant increase in dissipative core-mantle overall decline in obliquity during Earth his- torques at that time. The palaeomagnetic tory, reflecting a postulated dominant influ- record may provide independent evidence of ence of dissipative core-mantle coupling. possible change in conditions at the CMB Furthermore, the curve has a strong inflec- between 650 and 430 Ma. Studies of palaeo- tion and thus is divisible into three distinct magnetic field intensity back to ~ 2700 Ma parts: suggest that the mean virtual dipole moment (a) Very slow decrease in mean obliquity, was abnormally low during the Late Protero- from g=70 ° at 4500 Ma to g=60 ° at 650 zoic and early Palaeozoic, with an apparent Ma, giving a mean rate of obliquity-change minimum < 10% of the present value at (~) of -0.0009"/cy: (~p)=(g)-(et) = ~ 500 Ma (Carmichael, 1967, 1970; Smith, -0.0012"/cy. The time of commencement 1967, 1970; McEIhinny, 1973; Stacey, 1977; of decrease in obliquity from the primordial Merrill and McElhinny, 1983, Pal, 1991). As large value, by means of dissipative torques noted by McElhinny (1973), the weak palae- at the CMB, is dependent on the time of ofield seems to be a real effect because the formation of the fluid outer core. The origin mean virtual dipole moment from - 1000- of the core is reviewed by Jacobs (19871; a 2700 Ma was similar to the present value. commonly held view is that core formation Stacey (1977, p. 274) stated that this pro- was essentially coeval with, or occurred longed interval of very weak field "may imply shortly after, the accretion of the Earth, al- a special condition in the Earth's interior at though it has also been suggested that core that time", and McElhinny (19791 concluded growth took place over much of Earth his- that long-term change in the geomagnetic tory or is still continuing (e.g. Runcorn, field would most likely be accomplished by a 1964b), The single giant impact hypothesis change in the conditions at the CMB. In view for the origin of the Moon assumes that the of the apparent connection between recent Earth at the time of impact had an iron core changes in the geomagnetic field and in elec- (e.g. Taylor, 1987; Newsom and Taylor, 1989). tromagnetic core-mantle coupling (see Sec- Any subsequent growth of the outer core tion 5.2), major change in electromagnetic would increase core-mantle dissipation and torques at the CMB may have occurred dur- ¢}p. ing the Late Proterozoic and early Palaeo- (b) Relatively rapid decrease in mean zoic. As discussed in Section 5.2, electromag- obliquity, from g = 60 ° at 650 Ma to g = 26 ° netic core-mantle coupling may indeed vary at 430 Ma; (~)= -0.0556"/cy and (~v) significantly on time-scales of ~ l0 s years -0.(1562"/cy. This postulated decrease in g and be expressed as long-term variations in actually is very slow; the required value of the geomagnetic field. Changes in the topog- (~) is three orders of magnitude less than raphy of the CMB and consequently in tur- the present rate of obliquity oscillation bulent flow and effective viscosity of the outer (~ 47"/cy) and is below the level of de- core also could greatly affect core-mantle tectability by current astronomical observa- dissipation and the nature of the geomag- tions. netic field. (c) Very slow reduction in mean obliquity, The inflection in the proposed curve of g from g=26 ° at 430 Ma to g=23 ° at 0 Ma; versus time at g = 45 + 15 ° also may partly (~) = -0.0025"/cy and (Ep) ~ -0.0031"/ reflect feedback effects arising from an in- cy. creased precessional rate and the value of HISTORY OF THE EARTH'S OBLIQUITY 33 the obliquity itself. A first-order approxima- space) would have resonated precisely with tion of ~p in terms of the Earth's motions is the Earth's retrograde annual nutation caused by solar torques. This annual reso- ~p cc (~2/~o)(sin 2e) (3) nance would have caused nutation of the mantle (obliquity oscillation) with an ampli- where fZ is the precessional rate of the tude of up to ~ 75", several orders of magni- mantle and w the rate of the Earth's rotation tude greater than the present upper limit of (from equations in Aoki, 1969, and any free nutation (see Rochester et al., 1974) Rochester, 1976). The function ~2 in eq. 3 and as much as an order of magnitude greater finds support in experiments with precessing than the observed 18.6-year principal nuta- and spinning liquid-filled spheroidal cavities tion. Intriguingly, palaeorotational data for (Vanyo, 1984), which imply that energy dissi- ~ 650 Ma (Table 4) imply that the resonance pation rates at the CMB vary as precessional would have occurred at ~ 530 Ma, about the rates squared. The function sin 2e provides time when the postulated value of ~p was the required change of sign for ~p at e = 90 °. maximal (see Section 7). Since f~ varies as sin 2e, ft is maximum The occurrence of glaciation preferentially when E = 45 °, and eq. 3 shows that ~p also is in low to equatorial palaeolatitudes during maximum when e = 45 °. the Late Proterozoic (see Section 3.2) would From eq. 3, the combined effect of em- have moved polar ocean water to equatorial ploying estimated increased values of ~ (see ice sheets and thus altered the mass distribu- Berger et al., 1992), w (see Table 4) and e tion of the Earth. The added mass of ice at for the latest Proterozoic increases ~p by a the equator would have increased slightly the factor of up to ~ 4 relative to ~p determined Earth's rate of precession, which may have from present-day values. Furthermore, ex- led in turn to further, small increase in ~p in periments with a rotating and precessing liq- Late Proterozoic time. uid-filled spheroidal cavity (Malkus, 1968) Special conditions at the CMB and dy- showed that turbulence produced in the pre- namical effects on ~p therefore may explain cessing fluid increased with increase in pre- the inflection in the curve of obliquity versus cessional rate, abruptly jumping to a "satura- time, centred on g = 45 °, during the Late tion" value at a critical rate. Malkus (1968, p. Proterozoic-early Palaeozoic. Interestingly, 262) concluded that "the core was very un- Lago and Cazenave (1979), in modelling the stable in the past when the moon was closer evolution of the obliquity of Venus under the to the earth than it is today." His findings influence of dissipative core-mantle viscous raise the possibility of increased turbulence coupling and tidal torques, showed a sharp in the core, and consequent increase in ef- inflection in the curve of obliquity versus fective (turbulence) viscosity and dissipative time for several cases where the obliquity core-mantle torques, as the obliquity ap- moved from 90 ° to 180 ° . proached the value of 45 ° at which the pre- Since the rate at which rotational kinetic cessional rate is maximal. Dynamical effects energy is dissipated by the axial component therefore may have significantly increased of core-mantle coupling varies as ~O2~p tan e the magnitude of ~p in latest Proterozoic (Rochester, 1976), the hypothesis presented time. here may imply that a sizeable portion of the The secular deceleration of the Earth's Earth's secular deceleration during the Pre- rotation may have contributed to instability cambrian was caused by core-mantle dissi- of the obliquity at ~ 530 Ma. Toomre (1974) pation; this possibility accords with Vanyo's showed that when the day was ~ 22.3 h (1991) model of viscous core-mantle cou- (~ 0.93 of the present day), the core's free pling. Palaeotidal data suggest, however, that nutation (periodic motion of the spin-axis in tidal friction has been the principal cause of 34 (u-. WILLIAMS the Earth's deceleration during the Phanero- Earth history: the widespread appearance of zoic (see Williams, 1989c, 1990, 1991a; Deub- soft-bodied metazoans in latest Proterozoic net, 1990). time at -620-590 Ma and the Cambrian explosion of biota commencing at around 7. THE PROTEROZOIC-PHANEROZOIC TRAN- 550 ± 20 Ma. These radiations during the SITION Proterozoic-Phanerozoic transition signal "a major change in the earth system" (Valen- An important implication of Fig. 13 is that tine, 1989, p. 145). the interval of largest and most rapid de- The postulated flip-over of climatic zona- crease in mean obliquity, from g = 60 ° at 650 tion at ~ 610 (or ~590) Ma would have Ma to g = 26 ° at 430 Ma, spans the Protero- occurred near the start of the Ediacaran zoic-Phanerozoic transition. Such decrease epoch that spans the interval between the in g, although actually very slow on a short last major glaciation of the Proterozoic and time-scale, may have caused relatively abrupt earliest Cambrian time; the age limits of the transitions between different climate states; Ediacaran have been given as ~ 590-540 Ma Crowley and North (1988) have shown that (Jenkins, 1984), 590-570 Ma (Harland et al., transitions between climate states at "critical 1990) and ~ 620-530 Ma (Conway Morris, points" can be rather sudden and can be 1988, 1990). The widespread appearance of caused by small changes in forcing. Ediacaran soft-bodied metazoans (Hofmann, Profound changes in global climate state 1987; Conway Morris, 1990) may in part re- may have occurred at two such "critical flect their spreading out from Late Protero- points" while g decreased from 60 ° to 26°: zoic equatorial havens of least stressful cli- (a) A change from reverse to normal climate and their proliferation in response to matic zonation would have occurred as the proposed change to normal climatic decreased past the critical value of 54 ° . Lin- zonation and accompanying reduction in sea- ear interpolation between the points g = 60 ° sonality. at 650 Ma and g = 26 ° at 430 Ma places this The "Cambrian explosion" of faunas has "flip-over" at ~ 610 Ma (taking g = 65 ° at been described as "a biological event of pro- 650 Ma puts the flip-over at ~ 590 Ma). found significance in the history of life" (b) Decrease in amplitude of the seasonal (Harland and Rudwick, 1964, p. 36) and "an cycle between 650 and 430 Ma, which evolutionary burst that was unprecedented changed a global climate state of intense and which has been unsurpassed" (Valen- seasonality to a state more like that of today, tine, 1989, p. 145). This fundamental event in would have been most rapid at g: = 45 + 5 ° the biotic record took place during the postu- when the rate of decrease of obliquity was lated time of most rapid decrease in obliq- maximal. The time of this critical point is uity centred at ~ 550 (or ~ 540) Ma and placed by linear interpolation at ~ 550 __% 30 hence the most rapid reduction of global Ma (or ~ 540 _+ 30 Ma, taking ~: = 65 ° at 650 seasonality. The substantial amelioration of Ma). This age-range is of particular impor- the environment through reduction in sea- tance because it includes the Precambrian- sonal temperature stresses during that inter- Cambrian boundary, variously placed at 530 val would have been a major catalyst to evo- to 570 Ma (Gale, 1982; Odin et al., 1983, lution; it also would have enabled faunas to 1985; Conway Morris, 1988; Harland et al., spread out over most of the Earth, including 1990). the vast vacant habitats of middle and high It is most significant, therefore, that the latitudes where former seasonal tempera- interval of postulated rapid and marked cli- ture-ranges had been too large to permit matic and seasonal amelioration spans the advanced forms of life. Furthermore, neap- two most spectacular radiation bio-events in tidal ranges would have become more uni- HISTORY OF THIz F2ARTH'S OBLIQUITY 35 form throughout the year and the long inter- oxygen is almost entirely the product of pho- vals of small neap-tidal ranges at and near tosynthesis by green plants, any rapid in- equinoxes would have contracted as the crease in atmospheric oxygen during the Pro- obliquity decreased (see Section 3.2.6). terozoic-Phanerozoic transition may have Hence, during the Proterozoic-Phanerozoic been a consequence of improt,ing era,iron- transition, paralic tidal environments would mental conditions which permitted the prolif- have become more hospitable to benthonic eration of photosynthesising biota. An in- fauna dependent on a continual oscillation of crease in atmospheric oxygen at that time the tides. could have provided a positive feedback that A causal connection has been suggested assisted the coeval development of the Meta- between deglaciation in latest Proterozoic zoa. time and the subsequent appearance of the It is postulated here that the first-order Ediacaran metazoans (e.g. Runnegar, 1982; cause of the abrupt evolution and radiation Sokoiov and Fedonkin, 1986) and the Cam- of the Metazoa after more than three billion brian fauna (e.g. Rudwick, 1964; Harland years of life on Earth was not deglaciation, and Rudwick, 1964). Such suggestions, how- rapid increase in atmospheric oxygen, tecton- ever, do not adequately explain how ism, or changes in oceanic circulation, but deglaciation at ~ 620-600 Ma alone could the transition from an inhospitable global have triggered profound biotic events over state of reverse climatic zonation and ex- the succeeding ~ 50-100 Ma. Data from treme seasonality to a benign state of normal molecular biology suggest that metazoans climatic zonation, moderate seasonality and first appeared prior to 1000 Ma (Conway more habitable tidal environments. These Morris, 1990). Indeed, burrow-like structures changes occurred during the key interval of apparent metazoan origin occur in 800- bounded by terminal Late Proterozoic low- 850 Ma sandstones stratigraphically below latitude glaciation at ~ 600 Ma and the start the two main glaciogenic sequences (equiv- of Late Ordovician circum-polar glaciation at alents of the Sturtian and Marinoan glacial ~ 450 Ma. deposits in South Australia) of the Late Pro- terozoic in central Australia (Lindsay, 1991), and rare Ediacaran remains occur between 8. DISCUSSION AND POSSIBLE TESTS the two Late Proterozoic glaciogenic horizons in Canada (Hofmann et al., 1990; Aitken, 1991). Hence it may be asked why Secular decrease in obliquity during Earth the disappearance of the earlier (Sturtian) history is envisaged as a progressive change ice sheets, which had occupied low palaeolat- in global state which had spectacular geologi- itudes, and the advent of interglacial warm cal expression only at the Proterozoic- climates and transgressive seas at ~ 750 Ma Phanerozoic transition. The progressive (see Preiss, 1987), did not then trigger major change has been punctuated by other events biotic events. Evidently the evolutionary trig- of independent origin such as icehouse and ger was not deglaciation per se. greenhouse conditions and mass extinctions. Another suggestion is that a significant The hypothesis of a large obliquity during increase in atmospheric oxygen at the end of the Precambrian--the Earth's prolonged the Proterozoic and the beginning of the "Uranian" obliquity state--requires rigorous Phanerozoic caused the appearance and testing. Palaeomagnetic and geochronologi- rapid development of the Metazoa (Knoll, cal studies of Late Proterozoic glaciogenic 1991), although Runnegar (1991b) concluded rocks in several continents may discriminate that Late Proterozoic-early Palaeozoic oxy- between glaciation during global refrigera- gen levels are uncertain. Since atmospheric tion and low-latitude glaciation for a large 3('t (;.E, WIIJ.IAMS obliquity. Age determinations of the glacial 9. CONCLUSIONS deposits, preferably by zircon U-Pb dating of coeval volcanics, may show whether glacia- Study of the Earth's palaeoclimate record tion was synchronous or diachronous; the indicates that Phanerozoic conditions were demonstration of diachronous glaciation in essentially uniformitarian with regard to cli- low palaeolatitudes would favour the large- matic zonation, whereas Precambrian envi- obliquity hypothesis (although the converse ronments in general appear to have been does not hold). The hypothesis presented non-uniformitarian in this regard. Most no- here predicts ice sheets and periglacial cli- tably, strongly seasonal glacial and periglacial mates preferentially in low to equatorial Late Proterozoic climates occurred near sea palaeolatitudes for all pre-Ediacaran glacia- level evidently in preferred low to equatorial tions. Hence, palaeomagnetic studies of Early palaeolatitudes, implying reverse climatic Proterozoic glaciogenic rocks in South Africa, zonation and a Late Proterozoic obliquity Australia and Europe are required to ascer- greater than 54 ° (assuming a geocentric axial tain palaeolatitudes of glacial and periglacial dipolar magnetic field). Palaeotidal data are deposition. consistent with a substantial obliquity during Evidence for the predicted large ampli- the Late Proterozoic. tude of the global seasonal cycle during the It is postulated here that the primordial Precambrian could be sought by the detailed Earth acquired a large obliquity (54°< E < study of varve-like deposits, rhythmites of 90 °) from a single giant impact with a Mars- tidal-climatic origin, and periglacial wedge- sized impactor at ~ 4500 Ma, which is widely structures attributed to seasonal contraction believed to have produced the Moon. Since and expansion, together with palaeomagnetic ~ 4500 Ma the obliquity has slowly de- determinations of their palaeolatitudes of creased under the dominant influence of dis- formation. The hypothesis presented here sipative core-mantle torques. Some 3900 Ma predicts that during the Precambrian high after the formation of the Earth-Moon sys- latitudes were subjected to extreme seasonal- tem an increase in core-mantle dissipation, ity, with very hot summers and severe win- apparently related to changes in conditions ters; hence the search for and study of Pre- at the core-mantle boundary and other dy- cambrian high-palaeolatitude deposits--such namical effects, caused an interval of rela- as the Late Proterozoic carbonates and shales tively rapid decrease in obliquity. This "ob- of the North China block (Zhang and Zhang, liquity revolution" caused a profound change 1985)--should be major objectives. Further- of global state recognised as the Protero- more, much palaeowind data that can be zoic-Phanerozoic transition. Before the related to apparent polar wander paths for obliquity revolution the Precambrian Earth the Precambrian are required to test the endured a "Uranian" obliquity state (g: > 54 °) prediction of non-uniformitarian palaeowind with reverse climatic zonation and a strongly directions such as low-latitude westerlies and seasonal global climate, and was subject to mid-latitude easterlies. episodic glaciation preferentially in low to Geophysical investigations also are re- equatorial latitudes. Subsequent to the revo- quired into the viability of the geocentric lution the obliquity has been similar to that axial dipole model of the Earth's magnetic of the Quaternary, resulting in a less-stress- field during Precambrian time. In addition, ful global state marked by normal climatic the role of core-mantle dissipation in the zonation, episodic circum-polar glaciation, dynamical history of the Earth requires criti- and a much-reduced amplitude of the global cal review, particularly for the vital interval seasonal cycle. The obliquity revolution trig- spanning the Proterozoic-Phanerozoic tran- gered the two most spectacular radiation sition. bio-events recognised: the widespread ap- |tlST()RY Of-: FHI: [:AI~/FH'S ()BLIQUrl'Y 37 pearance of the Ediacaran metazoans at Geological Survey for helpful discussions; ~ 610-590 Ma when climatic zonation and Sherry Proferes for drafting. Comments flipped from reverse to normal at g = 54 °, by several referees led to an improved and the "Cambrian explosion" of biota com- manuscript. The work is supported by the mencing at 550 +_ 20 Ma when the rates of Australian Research Council. obliquity-decrease and amelioration of global seasonality were maximal at g = 45 °. 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