Origins and Assessment of Snowball Earth Hypotheses

Home , Huronian glaciation, Precambrian, Snowball Earth, Sturtian glaciation

Geol. Mag. 144 (4), 2007, pp. 633–642. c 2007 Cambridge University Press 633 doi:10.1017/S0016756807003391 First published online 6 June 2007 Printed in the United Kingdom Origins and assessment of snowball Earth hypotheses

W. BRIAN HARLAND† Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK

Note from the Editors – Brian Harland was for many years an editor of this journal. He was also a seminal figure in the origins of the current ‘snowball Earth’ debate, having recognized in 1964 the significance of coupling emerging palaeomagnetic data on palaeolatitude with his interpretations of diamictites. Harland worked extensively in the Arctic and knew well many of the workers involved in the arguments surrounding the origin of diamictites. He thus had a unique perspective on the evidence and the disputes surrounding it. This was his last paper but he was not able to complete it before he died. However, with the help of Professor Ian Fairchild to whom we are indebted, the editors have lightly revised this work which is presented as the personal view of one of the key figures with a very broad stratigraphic appreciation of the problems of ‘snowball Earth’.

Abstract – Records of Precambrian glaciation onwards from the late nineteenth century led to the concept of one or more major ice ages. This concept was becoming well advanced by the mid 1930s, particularly through the compilation of Kulling in 1934. Even so tillite stratigraphy shows that glaciation was exceptional rather than typical of Earth history. Some Proterozoic tillites, sandwiched between warm marine facies, indicate low, even equatorial palaeolatitudes as determined magnetically, and more recently led to ideas of a snow- and ice-covered ‘snowball Earth’. However, interbedded non-glacial facies as well as thick tillite successions requiring abundant snowfall both militate against the hypothesis of extreme prolonged freezing temperatures referred to here as an ‘iceball Earth’ in which all oceans and seas were sealed in continuous ice cover. On the other hand tropical environments were interrupted by glaciation several times in the Proterozoic, something that did not recur in the Phanerozoic. The term ‘snowball Earth’ is consistent with the established view of extremely widespread Proterozoic glaciation, but the ‘iceball Earth’ version of this is not compatible with the geological record.

Keywords: snowball Earth, iceball Earth, glaciation, tillite.

1. Introduction were sealed by ice. The latter interpretation is here referred to as ‘iceball Earth’. However, a less extreme Thoroughly tested palaeomagnetic evidence has es- snowball hypothesis entailing low latitude (tropical) tablished that late Proterozoic glaciation extended to glaciations is now well established and may be typical equatorial palaeolatitudes (Harland & Herod, 1975; for Precambrian rather than Phanerozoic ice ages. In the Evans, 2000). This led to the hypothesis of a ‘snowball following text, the term tillite, implying a sedimentary Earth’ by Kirschvink (1992) who also inferred that rock with a recognizable glacial component, terrestrial the planetary cooling was stabilized by an ice-albedo or marine, is used here as in most of the literature feedback mechanism. Hoffman et al. (1998) developed quoted. this concept further to make a more all-embracing theory, but at the same time developed a more extreme interpretation whereby all ocean surfaces were sealed 2. Origins of the hypothesis of global glaciation by ice over a time period of 4 to 30 million years. Snow- 2.a. British Isles ball Earth has attracted wide attention as a stimulating popular science topic (Walker, 1999, 2003; Hoffman & The ﬁrst Precambrian glacial evidence to be so Schrag, 2000; Fairchild, 2001) as did its precursors interpreted was at Port Askaig in Islay, Scotland, by (Harland & Rudwick, 1964; Hambrey, 1992). In recent James Thomson (1871). It is appropriate that a most literature there has been a lack of emphasis on the thorough description of any glacial sequence was of this historical development of ideas, although this has been and related tillites (Spencer, 1971). Archibald Geikie partially corrected by Hoffman & Schrag (2002). This (1880) considered a Torridonian diamictite as possibly brief paper develops the historical element further glacial. James Croll in 1864 (1875) had suggested and shows that early literature had already established a genetic relationship of such an ice age with the the essential nature of the glaciation, but also had advent of Cambrian life, an idea that has recurred precluded the interpretation that all ocean surfaces and been progressively developed over time (Hicks, 1876; Ramsey, 1880; Coleman, 1926; Mawson, 1949; † deceased 1 November 2003. Harland & Wilson, 1956; Termier & Termier, 1956;

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Harland & Rudwick, 1964; Kirschvink, 1992; Hoffman Norwegian horizons (Harland, Hambrey & Waddams, et al. 1998). Hicks (1880) went on to consider the 1993; Harland, 1997). relationship between glaciation and volcanism. The best-preserved examples are in the Polaris- breen Group (Eastern Terrane) in the Hecla Hoek Complex. The upper tillite, Wilsonbreen Formation, 2.b. Norway correlates with Kulling’sSveanor tillite and Mortensnes In 1891 H. Reusch described a glacial moraine at Formation, of Finnmark. It is 200+ m below early Bigganjargga in Finnmark, north Norway, and referred Cambrian fossiliferous strata. A late Neoproterozoic to an ice age (istiden), then ?Cambrian (Strachan, age is consistent with acritarchs there (Knoll, 1981) 1897), now late Neoproterozoic. This small outcrop and in the Western Terrane (Knoll, 1992). Wilson & is now thought to be a slumped unit in a sandstone– Harland (1964) emphasized the conspicuous reddish quartzite formation with soft sediment sliding marks matrix of the tillite as evidence of erosion into a and boulder impressions (i.e. not a glaciated pave- lateritic basement, but Fairchild & Hambrey (1984) ment as he suggested). Other sliding lineations were found evidence that the reddening was secondary. observed at other levels in the formation (Harland, However these authors reinforced the earlier notion 1964b). This allochthonous tillite is no less indicative that profound climatic changes were represented at the of contemporary glaciation. Following a description of upper and lower boundaries of each of two glaciations, the Finnmark region by Holtedahl (1919), Føyn (1937) by the characteristics of the bounding dolomitic and Strand (1937), Bjørlykke (1967) established two carbonate strata. Together these observations reinforce distinctive tillite horizons: the lower (Smalfjord)˚ and the concept of a major Varanger glacial period in which upper (Mortensnes) formations. The lower unit was freezing temperatures interrupted dominantly warm correlated with Reusch’s moraine. They are separated environments (Harland, 1964a, b). Moreover, attempts by the reddish siltstone Nyborg Formation, all in the at palaeoclimatic maps as by Bain (1960) as well Vestertana Group (Vidal & Bylund, 1981). This tillite as more recent late Neoproterozoic–early Cambrian sequence was correlated with Sparagmite diamictites reconstructions (e.g. Unrug, 1997; Dalziel, 1997) place of southern Norway (Moelv Formation, Holtedahl Greenland on or near the palaeoequator. It was thus (1922)). no great surprise to obtain an early palaeomagnetic result for Greenland of 8◦ N (Bidgood & Harland, 1961; Harland, 1964a, b), although now known to be 2.c. East Greenland overprints (Evans, 2000). However, it was also clear in In East Greenland Poulsen (1930) and Kulling (1930) the 1960s that continental drift reconstructions could described the Tillite Group (Koch, 1929) with the Cape not accommodate all Neoproterozoic glacials as polar. Oswald Formation. They correlated the succession with In the Central Terrane of Spitsbergen a distinct two glacial horizons with those in Finnmark. This has stromatolite unit is interbedded with the later Varanger become a classic succession for research and detailed tillite. Stromatolites were regarded as exclusively correlation with Svalbard (Hambrey & Spencer, 1987; belonging to tropical carbonates, but Fairchild et al. Fairchild & Hambrey, 1995). (1989) showed that examples in the Wilsonbreen Formation resembled those in Antarctic lakes, recalling the earlier concept (Harland & Herod, 1975) of 2.d. Svalbard carbonate precipitation in extreme cold environments. A massive boulder unit was described in southwestern The Western Terrane, with two much thicker early Spitsbergen by Garwood & Gregory (1898). Kulling and late Varanger tillite formations, was then near (1934) described the Sveanor tillite northeast of Spits- to Ellesmere Island and adjacent to Trettin’s Pearya bergen (Nordaustlandet). He correlated it with East Terrane (1987) where Troelsen had identified tillite Greenland and Finnmark glacials and surveyed current (1950) and where these overall successions are well knowledge of late Precambrian tillites worldwide, matched (Harland, 1997). which is discussed further later. Reflecting on this and on other Scandinavian occurrences Kulling (1951) 2.e. Australia proposed a major Varanger Ice Age based on the Finnmark Varangerfjord succession. In South Australia H. P. Woodward in 1884 suspected The Svalbard tillites are now best known from a Precambrian tillite, which opinion T. W. E. David the main island of Spitsbergen where three distinct confirmed (Howchin, 1901, 1908) and it was described terranes make up the archipelago (except for Bear and named the Sturt Tillite. Further outcrops were Island). The Eastern, Central and Western terranes, identified by David (1907) and by Mawson (1912). each displaying two tillite horizons, were in Proterozoic Mawson correlated the Broken Hill glacials as Sturtian time respectively adjacent to East Greenland, North- tillites. In 1949 Mawson described a geosynclinal se- East Greenland, and eastern North Greenland beside quence at the Bibliando Dome, where tillites occurred Ellesmere Island. They correlate with Greenland and 3064 m below the newly described Pound Quartzite

fauna at Ediacara. Mawson thus suggested that it was The two scales cannot coincide and the inevitable ‘a long time before the Cambrian’. There the upper uncertainties of mutual calibration remains to be of two tillite formations (c. 2335 m) was separated by advanced (Harland et al. 1990). Evans (2000) noted interglacial beds (c. 2062 m) from the, newly identified, that Sturtian glaciations were younger than 777 ± 7Ma lower tillite (> 1344 m) which he named the Bibliando and the Marinoan younger than 650 Ma. tillite; the two distinct formations are characteristic Whereas the Varanger (Marinoan) glaciations may of Sturtian glaciations. In that paper Mawson, fol- be established as global, the Sturtian palaeolatitude lowing Kulling (1934), listed 22 Precambrian tillite in Australia is not known, but its correlative Rapitan occurrences worldwide as marking the intensity of Formation in Canada was given a ‘moderately reliable’ the ‘Greatest Ice Age’. That these ranged in age palaeolatitude status as within 10◦ of the equator from the early Palaeoproterozoic Gowganda to the (Evans, 2000). latest Neoproterozoic Varanger glaciations suggests that he was influenced by the geosynclinal scale of his 2.f. Africa Bibliando Dome Sturtian sucession of unknown age. The African continent excels in the widespread In 1950 Mawson identified and named a third glaci- variety of Precambrian and Phanerozoic glacial history. ation from the relatively thin Elatina tillite, of reddish Precambrian tillites were recorded in Griqualand by chocolate colour, and directly underlying fossiliferous A. W. Rogers in 1906 (Visser, 1981) and are approx- ‘Cambrian’ (Sprigg, 1949–1950; i.e. Ediacaran). This imately 2224–2300 ± 100 Ma (early Proterozoic). He has latterly assumed importance with a magnetic near- then described an older tillite in the Upper Witwater- equatorial palaeolatitude (Embleton & Williams, 1986; srand Series, late Archean or early Palaeoproterozoic Schmidt, Williams & Embleton, 1991). This result (Harland, 1981). triggered the snowball Earth hypothesis of Kirschvink The late Neoproterozoic tillites are abundant and (1992) and was confirmed by further work (Schmidt & rest, as in Australia, as cover strata on a typical shield Williams, 1995; Sohl, Christie-Blick & Kent, 1999). ◦ basement. Of these Namibia offers now the best known, A palaeolatitude of 7.5 N was uniquely accorded namely the lower Chuos (de Kock & Gevers, 1932) and ‘very reliable’ status by Evans (2000). The underlying the upper Ghaub tillites in the Otari Group. They have YaltepinaFormation yielded a consistent palaeolatitude ◦ been described by Kaufman et al. (1991) and applied in of 8.4 N. the snowball Earth hypothesis by Hoffman and others In the meantime further Neoproterozoic tillites were (Hoffman et al. 1998, 1999; Hoffman, Kaufman & found, not only in the Flinders Range geosyncline Halverson, 1999) especially with the unusually thick of South Australia, but in outlier outcrops notably cap dolostones (300 m) resting on each tillite with the Georgina and Amadeus–Ngalia basins and in the sharp sedimentary contact. These tillites were reported margins of the Kimberley Block. In almost every as earlier and later Sturtian, estimated age span 760– succession two glacial formations have been correlated 700 Ma. Estimated palaeolatitudes were said not to with the Sturt and Elatina glacials and referred to be reliable (Evans, 2000). An extreme ‘iceball Earth’ generally as Sturtian and Marinoan. A suggested age hypothesis was in part supported by negative δ13C range for the latter is 610–575 Ma but there is generally values. However, Kennedy, Christie-Blick & Prave a lack of reliable chronometric age determinations. The (2001) investigating the glacial facies did not support Egan tillite (Kimberley region) was tentatively argued their inference. to be post-Marinoan (Corkeron & George, 2001), and Other well-known late Neoproterozoic glacial re- so with other occurrences in other continents raises cords include the Numees diamictite of South Africa the possibility of late Varanger, inter- and/or post- (?Marinoan), and the Katangan sequence with the Ediacaran glaciations. Indeed although Marinoan and Grand and the Petit Conglomerat´ described in 1935 Varanger successions correlate in general the detailed (Cahen, 1963) respectively in the Lower and Upper glacial sequences are not known to be coeval. Kundelungu Group with an estimated age range c. The IUGS Commission on the Terminal Proterozoic 950–602 Ma. Kulling (1934) supported Beetz (1929) ‘system’ has recommended that the initial GSSP for as interpreting an ‘ice age greater than any later the terminal Proterozoic period be decided immediately glaciation’. Good correlation appears to be lacking but above the Elatina Formation. Thus the encompassing it would appear that Sturtian glaciation at least must Wilpena Group (from Nuccaleena to Uratana form- have obtained. ations) is approximately just post-600 Ma to about 543 Ma. The underlying Umberatana Group in the 2.g. China Flinders Ranges would thus begin with Sturtian glacial formations and end with the Elatina Formation. That Willis, Blackwelder & Sargeant (1907) identified the is the chronostratigraphic division, whereas the chro- Nantuo tillite in the Yangtze Gorge of China, described nometric Cryogenian interval was defined precisely at by J. S. Lee (Li Siguang) (1924) and Y. Y. Li (Lee, 850 to 650 Ma (Precambrian Subcommission of the 1936) and jointly in more detail in 1940. Norin (1924) International Commission on Stratigraphy in 1988). extended tillite discoveries to north Shansi and in 1930

and 1937 to the Kurugh Tagh in the Tarim Basin where 1971). Generally in each area there is only one he distinguished three glacial horizons. Since then horizon commonly conjectured as Sturtian but without tillite occurrences have been mapped in many parts radiometric evidence. However, in the Windermere of China. Supergroup of the Mackenzie Mountains two distinct Wang Hongzhen (1956) and Zhao Xiwen et al. tillite horizons are now known. The older in the Rapitan (1992) attempted to correlate three Precambrian ice Group is the Sayunei and Shezal formations (Eisbacher, ages. The oldest, Changan, was thought to be Sturtian, 1985); the younger is the Ice Brook Formation (Aitken, the Nantuo would be Varanger (Marinoan). The 1991). youngest Hangeroqioke in NW China was thought to The Ice Brook tillite overlies the Twitya Formation be one of the suspected evidences of a minor cold (containing fossils alleged by some to have Ediacaran episode near the initial Cambrian boundary. However, affinities) and is followed by the main Ediacaran later work summarized by Evans (2000) appears to have fauna, so implying a late Varanger glaciation. The resolved conflicting claims with an alternative scheme Rapitan tillites are younger than 760 Ma. Moreover, that makes the Changan (= Gucheng) and the Nantuo Evans (2000) awarded as ‘moderately reliable’ the tillites as coeval, both overlying the Liantuo Formation palaeolatitude determination of 0◦ to 10◦ (Park, 1997). dated at 748 ± 12 Ma. Uncertain in intercontinental Thus if the presumed Sturtian correlation is sound, then correlation, their estimated palaeolatitude of 30◦ to 40◦ some Sturtian tillites were equatorial and so qualify was regarded as ‘very reliable’ (Evans, 2000). with the Varanger (Marinoan) as ‘snowball’ events. In Whereas the international definition of Protero- both Rapitan and Ice Brook successions, members and zoic time is from 2500 Ma to the initial Cambrian beds suggest a succesion of distinct glacial episodes. boundary (c. 543 Ma), Chinese geologists have long Recent literature has also brought to the fore succes- inserted a latest Precambrian Sinian Era from about sions in other locations, for example in Newfoundland 850 Ma, so limiting Proterozoic time from 2500 to (e.g. Narbonne & Gehling, 2003) and in other regions 850 Ma. Harland et al. in 1982 and 1990 attempted such as Oman (Brasier et al. 2000), particularly because to acknowledge Australian, Chinese, European and of new radiometric age determinations. Russian contributions by proposing a chronostratic scale (alongside the chronometric Proterozoic scale 2.j. Kulling’s early synthesis defined in years) so that the Phanerozoic Eon would follow a Sinian Era comprising Sturtian and Vendian Kulling (1934) provided new data on late Proterozoic divisions, the latter comprising Varanger and Ediacara glaciation in Nordaustlandet, a careful analysis of periods. possible correlatives elsewhere in Svalbard, in East Greenland, and in Norway, but also a comprehensive overview of possible examples globally. He was 2.h. Russia particularly careful to group the occurrences into three Nicolaev (1929), Tchurakov (1932) and the Interna- categories: (1) those with conformity with overlying tional Geological Congress in Moscow (1937) recorded Cambrian strata, (2) those with minor disconformity, widespread Russian tillite occurrences and many west and (3) those in unfossiliferous successions that could of the Ural Mountains are subsurface. Within their not directly be dated. In category (1) he placed the Vendian period they recognize the Varanger glaciations northern North Atlantic examples together with others as ‘Lapplandian’ (Chumakov, 1981). in China, Australia and North America mentioned here above. In category (2) were examples from central Siberia and elsewhere in central Asia, and in category 2.i. North America (3) the Blaini tillite of northern India (Wadia, 1926) In North America, southwest of Lake Superior, Robert and a number of examples from central and southern Bell ‘reported boulders reaching three feet eight inches Africa. He stated: ‘In my earlier paper on the result in dimension and having grooves like glacial striae, of the 1931 expedition (1932), I suggested that the in a conglomerate with sandy matrix belonging to eo-Cambrian glaciation probably was comparable in the Keweenawan of Pointe aux Mines’ (Geology of magnitude to that of the Permo-Carboniferous and Canada, 1876–7, p. 214 in Coleman, 1926, p. 207). Quaternary glaciations... An ice-age...is so peculiar Agassiz had moved to America in about 1857 and and rare a phenomenon that there is every reason to applied his glacial theory (Agassiz, 1840) extensively. believe that such glacial formations as may be found, Lane & Searman (1907) made similar observations. in generally analogous positions, in successions of Coleman (1908) described Lower Huronian glacials, strata which within certain limits can be considered namely the Cobalt conglomerate in the Gowganda homotaxial to represent the same Ice Age.’ Here is well Formation, now perhaps the best-known early Palaeo- established the basis for correlation of tillites that has proterozoic tillite. now led the IUGS Terminal Proterozoic Commission Neoproterozoic tillites have since been described to the brink of defining a new Ediacaran geological throughout much of the Western Cordillera (e.g. Utah: ‘system’. It is true that Mawson (1949) declared for the Blackwelder, 1932; the Toby Conglomerate: Aalto, first time that ‘glaciations in Precambrian time were

probably the most severe of all in Earth history’, but and western Europe glacial and interglacial sequences he did so on the basis of a compilation which was less were established. So only at the margins of an ice critical than that of Kulling (1934) and contained few sheet would the fluctuating sedimentary record be additional locations, and none that would significantly preserved, whereas interior glaciation might compound alter the perception of scale of the glaciation. The tills in final deposits of boulder clay. Interglacial data compiled by Kulling (1934) already pointed to episodes, or at least episodes of glacial retreat, are an extraordinary widespread glaciation, although he evidenced by interdigitating strata of warm, or non- did not state a view on the likelihood or otherwise of glacial, facies. Concurrent higher latitude glaciation continental drift. might well persist. If Precambrian supercontinents were distributed in low latitudes, and polar latitudes were 3. Discussion oceanic with or without thick ice cover, there would be no sedimentary record of non-glacial facies, contrary 3.a. Tillite recognition and evidence of glaciation to the geological evidence. International Geological Congresses encouraged re- Carbonate strata, chemically precipitated, often ports of tillite discoveries, notably in 1906 (Mexico); dolomitic and with sharp conformable lower contacts 1929 (South Africa); and 1937 (Moscow). Attention on tillites, are characteristic of late Neoproterozoic has been drawn to widespread Precambrian tillites tillite successions. Such were described by Wilson in and with increasingly long lists of occurrences as by the Wilsonbreen Formation in eastern Spitsbergen, both Coleman (1926), Beetz (1929), Kulling (1934), Howell within and above the tillites as valuable marker horizons (1937), Mawson (1949), Schwarzbach (1963), Harland just a few metres thick (posthumously published (1964a), Hambrey & Harland (1981) and Evans (2000). in Wilson & Harland, 1964). The cap carbonates However, by 1937 slumping and gravity flow (Kennedy, 1996) and their carbon isotope composition processes were established by O. T. Jones, and by have later become a key plank in the arguments 1963 tectonic and other explanations of diamictites for the iceball version of snowball Earth (Hoffman were widely preferred to glacial interpretations (e.g. et al. 1998; Hoffman & Schrag, 2002). However, Crowell, 1957; Klitin, 1960; Schermerhorn & Stanton, Kennedy, Christie-Blick & Sohl (2001) have provided 1963). Bucher (1964) summed up the palaeoclimates an alternative model for their generation involving conference in 1963 (Nairn, 1964) by identifying the methane liberation from gas hydrate with rapid melting issue as the ‘third confrontation’, after the Age of the of permafrost. Earth and Continental Drift, and concluded against Fairchild et al. (1989) and Fairchild (1993) de- glacial interpretations. At that conference, the idea veloped a model, based on the NE Svalbard occur- of global glaciation (Harland, 1964a) was ridiculed. rences, whereby the carbonates outside glacial forma- To counter the new fashion, criteria for identifying tions were tropical, whereas those within were detrital a glacial sedimentary component were developed or glaciolacustrine. However, Kennedy, Christie- (Harland, 1964b; Harland, Herod & Krinsley, 1966; Blick & Prave (2001) also found evidence for precip- Hambrey & Harland, 1981), and the view eventually itated carbonates, resembling those normally thought has prevailed that glacial sediments can become to have formed in warm water, within glacial deposits redeposited readily and that marine sediments can have from several regions. widely varying glacial components to them. This view was already well established by Kulling (1934) who 3.c. Timing of glaciation and the global nature refers to ‘a two component sediment, composed of of Proterozoic glaciation one more or less local component assorted by waves and currents on its way to the sedimentation area, and Glacial environments, although familiar today in polar another long-transport component, which has joined regions and mountains, are known only from excep- the former by dropping to the bottom of the sea from tional records in Earth history. The norm for the last 4 melting ice-bergs ...’ billion years was of land and sea with a functioning hydrologic cycle. Glacial events appear in distinct clusters for which we may distinguish occasional 3.b. Sedimentological inconsistencies with the iceball glacial eras bracketing glacial periods each of which Precambrian tillite sequences have sometimes been may comprise more than one epoch, even occasionally suggested to show evidence of warm interglacials (e.g. resolved into glacial stages as predictable from orbital the Port Askaig glaciation: Spencer, 1971, cf. Fairchild, forcing. There are corresponding interglacial eras 1985), but in any case do typically display interbedded and periods when atmospheric temperatures did not glacial strata with sediments which do not require fall in the Milankovich cycle sufficiently to promote glacial action for their formation (e.g. Kulling, 1934; glaciation. Albedo feedback accounts for the orbital Leather et al. 2002). The iceball model would not glaciation and deglaciation being rapid. predict this. This recalls the analogous monoglacialist Archaean tillites are exceptional, the earliest record debate in Britain where in the north the Pleistocene ice being about 2.9 Ga (Young et al. 1998). Lack of age appeared as one event whereas in southern England older records suggests that there were no earlier

ice ages, but the Witwatersrand conglomerate was As already noted, low-latitude glaciation implies latest Archaean or earliest Proterozoic (boundary at widespread refrigeration and so increases confidence 2500 Ma). The Huronian glaciation may have been in correlating tillites that otherwise need not be global with at least three major tillite horizons around considered as coeval. Stable isotopes have played 2500 to 2300 Ma (e.g. Young, 1981). There was a key role in palaeoclimatic investigations. Because 13 then a long Proterozoic inter-glacial era (Harland, atmospheric CO2 is so critical a greenhouse gas, δ C 1983) followed by the Neoproterozoic glacial era with determinations of marine sediments reflect the oceanic two principal periods (Sturtian and Marinoan), both composition in response to volcanism, photosynthesis, probably global. With a possibly weak glaciation near and burial of organic carbon. Negative excursions are the initial Cambrian boundary, the Phanerozoic Eon related to glacial episodes (Kaufman & Knoll, 1995). glacials, with early Palaeozoic, later Palaeozoic and Similarly strontium isotopes in marine carbonates may Cenozoic glacial periods, were typically polar. be taken as an indication of terrestrial weathering. Neither can serve to distinguish chronometric ages. Each does, however, serve as facies indicator and so 3.d. Palaeolatitude of tillites and continental have reinforced the interpretation of discrete glacial reconstructions events separated by longer non-glacial intervals. Brasier et al. (2000) demonstrated a fortunate Lacking good age determinations for nearly all case where radiometric ages help to discriminate five Precambrian tillites it was not known how many, if diamictite horizons, the lower two being probably any, were synchronous. Otherwise they might all be the + Sturtian separated by a 723 16 Ma U–Pb zircon result of polar glaciation if each were brought to high −10 age; the other three are constrained as older than latitudes as by rapid polar wander. D. M. Williams, 544.5 ± 3.3 Ma and supposedly Marinoan and later. Kasting & Frakes (1998) suggested rapid changes in The lower two correlate with negative δ13C excursions. Earth’s oblateness with obliquity changes. However, In other cases, the assignment of glacials to particular palaeomagnetism offered a solution (Blackett, 1961). episodes is controversial and there has been increas- Early determinations suggested that the Varanger ing willingness to discard long-accepted correlations tillites were formed in tropical latitudes from East ◦ ◦ without new dating constraints (e.g. Kennedy et al. Greenland c. 8 N, south Norway c. 11 N, and North ◦ 1998; Brasier & Shields, 2000). Norway c. 4.5 N (Harland, 1964b). If true then global refrigeration would incur glaciation at other latitudes. The magnetic inclinations might have been acquired 3.f. Causes of glaciation later as was confirmed by subsequent work on the Croll (1875) was perhaps the first to consider the causes Norwegian rocks. Indeed the north Norwegian, Vester- of glaciation, resulting in his orbital theory. Chamberlin tana Group, was assessed as ‘moderately reliable’ at ◦ ◦ (1899) thoroughly assessed quantitatively the possible 30 to 40 by Evans (2000), and pre-Devonian Baltica roles of atmospheric CO for climate in his Part I, was more than 1000 km distant from East Greenland. 2 based in part on Arrhenius’ work in 1896. He applied Of the many hundreds of Precambrian palaeolatitude this in Part II to the Pleistocene Ice Age, but in Part III determinations only the Australian Elatina tillite ◦ ◦ could not explain the late Palaeozoic tillites of Africa, (0–10 ) and the Nantuo–Chang’an tillites (30–40 ) Australia and India at their present latitudes, he then were accepted by Evans as ‘very reliable’. However, it being innocent of continental drift and polar wander. seems that not only the Varanger and Marinoan glacials Coleman (1926) discussed constituent processes, qualify for a snowball Earth, but Sturtian glacials may ◦ none of which he considered adequate. These included: well qualify (6–21 , McWilliams & McElhinny, 1980), ◦ cooling of the Earth; changes in topographic heights; especially the Rapitan tillites (0–10 , Evans, 2000), and ocean currents breaking land barriers; continental even the Huronian (Williams & Schmidt, 1997; Evans, drift; changes in oceanic and atmospheric circulation; Beukes & Kirschvink, 1997). atmospheric CO2 drawdown; volcanic dust; Croll’s planetary changes causing glacial and interglacials; Drayson’s (1873) theory of greater obliquity of the 3.e. Correlation of Precambrian tillites spin axis (up to 35.5◦; compare with Williams’ Precambrian tillites generally lack radiometric age (1975) > 54◦); shifting the Earth’s axis, i.e. polar constraints sufficient for correlation of glacial episodes. wander; variation of Sun’sradiation; the temperature of When tillites occur in successions with Ediacaran or space. Cambrian biotas general correlation is feasible such In spite of greater knowledge of process in all as to distinguish Varanger glaciations from earlier departments and the unforeseen applications of pa- ones, but even in this time span the sequence of laeomagnetism and stable isotopes, we still face the glacial episodes is mostly not identified (Knoll & problem of the historical circumstances to cause a Walter, 1992). Pre-Ediacaran biostratigraphy serves to glacial period (Crowell, 1999). The story is one distinguish glaciations on a broad time scale. of global temperature turning exceptionally low so

that warm tropical environments cooled to freezing. BEETZ, W. 1929. Uber das Wahrscheinliche altkambrische Obliquity of 54◦ (Williams, 1975) does not account oder jungproterozoische Alter der Glazialschichten an for the prevailing warm rather than freezing tropical der Basis des Kundelungu Systems in Katanga und temperatures through Precambrian time. Geotectonic am unteren Kongo. Neues Jahrbuch fur¨ Mineralogie, Geologie und Palaontologie,¨ Beilagebande.¨ Abteilung changes as in the formation and break-up of super- B. Bd. LXI, 61–82. continents with drawdown of atmospheric CO2 from BIDGOOD,D.E.T.&HARLAND, W. B. 1961. Palaeomagnet- increased weathering may be sufficient as suggested by ism in some East Greenland sedimentary rocks. Nature Young (1995), the silicate weathering being monitored 189, 633–4. by strontium isotopes (Blum & Erel, 1995) BJØRLYKKE, K. 1967. 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