Global ocean—atmosphere change across the Precambrian—Cambrian transition

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    Geol. Mag. 129 (2), 1992, pp. 161-168. Printed in Great Britain 161

    Global ocean-atmosphere change across thePrecambrian-Cambrian transition

    M.D. BRASIER

    Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, U.K.

    (Received 5 November 1991; accepted 2 December 1991)

    Abstract-The late Precambrian and Cambrian world experienced explosive evolution of thebiosphere, including the development of biomineral skeletons, and notably of phosphate and siliceousskeletons in the initial stages of the adaptive radiation. Ongoing research indicates profound changesin climate and atmospheric carbon dioxide over this span of time. Glacial conditions of the Varangianepoch occur enigmatically at low latitudes, associated with carbonate rocks. Later changes inpalaeogeography, sea level rise and salinity stratification encouraged prolonged 'greenhouse'conditions in both latest Precambrian and Cambrian times, with indications of relatively low primaryproduction in the oceans. The Precambrian-Cambrian boundary interval punctuated this trendwith evaporites, phosphogenic events and carbon isotope excursions; these suggest widespreadeutrophication and conjectured removal of carbon dioxide from the atmosphere. Whatever the cause,nutrient-enriched conditions appear to have coincided with the development of phosphatic andsiliceous skeletons among the earliest biomineralized invertebrates.

    1. Introduction

    Were revolutionary biological changes across thePrecambrian-Cambrian transition related to majorenvironmental perturbations? In general terms, theCambrian System certainly provides a remarkablecontrast with the late Precambrian (see Table 1).Glacial conditions spread to tropical latitudes in theearly Vendian, followed by warm, greenhouse condi-tions that reached mid to high latitudes by middleCambrian times. The Precambrian-Cambrian bound-ary interval lies, of course, in the transition betweenthese two contrasting regimes. The boundary itself isnow to be taken at the relatively low level of thePhycodes pedum ichnofossil zone in southeast New-foundland, for the reasons discussed in Cowie &Brasier (1989). Many excellent sections are also foundalong the Gondwana margin from China to Iran, sothat it is necessary to refer to the stratigraphicnomenclature of the Yangtze Platform of China(Brasier & Gao, in press). Siberian and East Europeansections are also extremely important, and suggestedcorrelation with Chinese and other chronostrati-graphic names is given in Figure 1. A tentativepalaeogeographical reconstruction for the early Cam-brian is given in Figure 2.

    This paper looks first at the evidence for latePrecambrian glaciations and climate. The focus thenfalls upon the evidence for the Cambrian greenhouseclimate, and closes with an examination of thePrecambrian-Cambrian boundary interval.

    2. Precambrian glaciations

    Most continents yield evidence for a Varangian glacialepoch (Harland, 1983) that reached down to lowlatitudes during the initial stages of the terminalPrecambrian. Tilloids, dropstones, glacial striations,ice wedges and varves (e.g. Spencer, 1971) have allbeen noted as evidence for glacial and periglacialclimates at this time. Associations between tilloids,reddened and dolomitic rocks and even halite pseudo-morphs suggest that the ambient climate which thecold spells interrupted was not typically polar, a viewlargely supported by the many palaeomagnetic deter-minations which yield low or intermediate latitudes(Frakes, 1979; Harland, 1989). Although the strati-graphic control on these tilloids is relatively poor,there is reason to believe they were essentiallysynchronous markers for a nearly global Varangian

    Table 1. Summary of major geological and biological changesbetween late Precambrian and Cambrian times

    Plate tectonics

    Continents

    Sea levelGlaciationsClimatic

    gradientsFossils

    Trace fossils

    Biostratigraphy

    LatePrecambrian

    Supercontinents

    Mainly lowlatitudes?

    LowWidespreadStrong

    Soft bodied

    Small,superficial

    Poor

    Cambrian

    Opening oceanbasins

    Mainly lowlatitudes

    HighLackingWeak

    Soft bodied andskeletal

    Larger, deeper,bioturbating

    Fair to good

    GEO 129

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    162 M. D. BRASIER

    Era

    Palaeozoic

    Sinian

    Period

    Cambrian

    Vendian

    Sturtian

    Epoch

    Lenian

    Aldanian

    -

    Ediacarian

    Varangian

    -

    Age (U.S.S.R)

    Toyonlan

    Botomlan

    Atdabanian

    Tommotian

    Nemakit-Daldynlan

    Kotlinian

    Redkinian

    -

    Glaclations

    ? * Late Slnlan

    ^ Varangian(Nantuo?)

    A Sturtian

    Evaporites

    O China,Auatralia,Siberia

    i > i ' i >Blgotinld-Redllchldfauna

    Blgotinld-Redllchld-Olenellld fauna

    E Evaporites (Kotlinian to Tommotian) P Phosphorites (Meishucunian/Tommotlan) Archaeocyatha (Atdabanian)

    Figure 2. Continental reconstruction for the early Cambrian; modified from Pillola (1990). Superimposed on this are the twomain trilobitic realms (with two transitional faunas), and the distribution of Kotlinian to Tommotian evaporites,Meishucunian-Tommotian phosphorites and Atdabanian archaeocyathans.

    glacial epoch, tentatively dated at c. 650 Ma B.P.(Harland, 1989). The Nantuo tillite of south China isincluded here (Fig. 1).

    Climates became warm enough to prevent ice at sealevel in tropical or temperate climates during theensuing Redkinian Age. It was during this time thatthe Ediacara fauna of large, soft-bodied metazoansand small, superficial trace fossils evolved (Glaessner,1984). The earliest occurrences are discoidal impres-sions found between two successive tillites in northwest

    Canada, suggesting a Varangian or older age(Hoffman, Narbonne & Aitken, 1990). Their peak ofdevelopment is largely confined, however, to rocks ofthe Redkinian Stage in the U.S.S.R., followed bydecline and size diminution within the Kotlinian Stage(Sokolov & Fedonkin, 1986) at a time of widespreadregression (Brasier, 1985, 1989).

    Most intriguing is the evidence for a late Sinian (orLuoquan) glacial epoch, developed on the NorthChina Platform, but also traceable west into Xinjiang

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    Precambrian-Cambrian ocean-atmosphere change

    and east into North Korea (Guan Baode et al. 1986;Harland, 1989; Brasier & Gao, in press). These glacialdeposits appear well above supposed Varangian tillitesand (by projection) just above strata with worm tubesof Ediacarian type. Overlying strata include possiblelate Sinian and definite mid Lower Cambrian strata.Supposed glacial deposits of similar age may occurin Russia, Poland, Sweden, Alaska, British Columbiaand southern Africa (Harland, 1989).

    While some believe this late Sinian glaciation tookplace in the latest Precambrian or earliest Cambrian(Guan Baode et al. 1986; Harland, 1989; authors inBrasier & Gao, in press) this is still controversialbecause of poor biostratigraphic control.

    The paradox of a low-latitude Varangian glaciationhas attracted some non-uniformitarian explanations,such as an extreme obliquity of the ecliptic (Williams,1975) or the presence of Saturn-like ring systemsaround the equator (Sheldon, 1989). Neither of theseexplanations has yet found favour but a speculationby Roberts (1976) deserves consideration here: hesuggested these glaciations resulted from massiveremoval of atmospheric carbon dioxide into platformcarbonates in late Precambrian time. This wouldimply an adequate supply of limiting nutrients(nitrogen and phosphorus), of course, which is alsoindicated by the extremely heavy S13C of the precedingRiphean carbonates (Knoll et al. 1986).

    3. Towards the Cambrian 'greenhouse' climate

    For long it was believed that the Cambrian Period wasrelatively cool, with the Chinese name, 'Hanwu Ji',being literally the 'fiercely cold period' (Harland,1989). Various lines of evidence have transformed thispicture, however, to one of a world moving towardsextremely warm,' greenhouse' conditions (e.g. Fischer

    Depth

    High

    Cooler

    Temperature

    Warmer

    Low. - * HighCa:Mg

    Figure 3. Schematic diagram of the controls for aragoniteand calcite precipitation.

    163

    & Arthur, 1977). The current evidence is brieflyreviewed below.

    3.a. Carbonates and carbonate mineralogy

    After the Varangian glaciation, carbonate sedimen-tation reappeared on many platforms at low to midlatitudes, notably on the Siberian Platform, acrossChina and northern India to the Arabian Gulf and theCaspian Sea. Ediacarian carbonates also occur aroundthe margins of North America, in southern Africa,South America and Australia. Dolomites, micro-bialites, oolites and flat pebble breccias suggest warmdepositional conditions.

    Areas that seem to have experienced little or nocarbonate sedimentation in Vendian time include theBaltic Platform and Avalonia. These are usuallybelieved to have lain at higher, southern latitudesduring the Cambrian Period (e.g. Parrish et al. 1986).Biomicritic limestones first appear on the AvalonPlatform with Tommotian-type skeletal assemblages(e.g. Landing, Narbonne & Myrow, 1988) and inBaltica by about Botomian times (e.g. Bergstrom &Ahlberg, 1981). These may be taken to indicateclimatic amelioration in each area.

    Most interesting are changes in carbonate min-eralogy reported across the Precambrian-Cambriantransition. In Morocco and Australia, a shift fromaragonitic to calcitic ooids has been observed (Tucker,1989), while in Siberia dolomites are abruptly replacedby red nodular limestones (Rozanov, 1984). At leasttwo kinds of geochemical hypothesis have been putforward to explain these secular changes in carbonatemineralogy (Fig. 3). The first suggests an increase inthe ratio of Ca:Mg ions in seawater, perhaps inresponse to increased uptake of Mg2+ through midoceanic ridges (Tucker, 1989). The second invokesincreasing /?CO2, perhaps related to increased meta-morphic reactions at subduction zones and volcanicactivity at ocean spreading centres (e.g. Tucker, 1989).The increase in carbon dioxide is, at least, broadlyconsistent with evidence from stable isotopes andfossils but the tectonic explanation may be only partof the story.

    3.b. Reefs

    Skeletal reefs built by small tubular worms (Cloudina)have been reported from the Ediacarian of southernAfrica (Germs, 1983) but these are relatively unusual.Major reef-builders did not appear until the earlyCambrian with archaeocyathan sponges and thrombo-litic algae (Rowland & Gangloff, 1988). Such reefswere limited to the Siberian Platform during theTommotian Stage but spread progressively in theAtdabanian (Fig. 2) to reach their acme in Botomiantimes, declining to extinction in the Toyonian (Brasier,1981;Zhuravlev, 1986).

    12-2

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    164 M. D. BRASIER

    By Botomian and Toyonian times, archaeocyathanreefs had spread from Siberia and south China downto Antarctica and south Australia. Like modern coralreefs, this suggests distributions dominantly within30 of the Equator, but with a few occurrences up toaround 40 (McKerrow, Scotese & Brasier, 1992).Archaeocyathan reefs never developed in the moretemperate regions of Baltica and Avalonia, despiteepisodes of carbonate deposition.

    3.a. Evaporites, phosphorites and saline bottom waters

    Dolomitic carbonates, gypsum and halite are widelydeveloped over the latest Precambrian to Cambrian,particularly on the carbonate platforms of Siberia andGondwana (Figs 1, 2). Major episodes of evaporationlay close to the Precambrian-Cambrian and Lower-Middle Cambrian boundaries (Fig. 1).

    The first of these evaporitic intervals laid down upto 1.3 km of evaporites in the Hormuz Salt of theArabian Gulf (Wolfart, 1981), up to 2 km of evaporitesin the Salt Range Formation of Pakistan (Yeats &Lawrence, 1984) and lesser thicknesses on the SiberianPlatform (Khomentovsky, 1986).

    These saline deposits indicate the potential for thedevelopment of warm, saline bottom waters overshelves and ocean basins during the Cambrian,especially in the absence of glacial meltwaters. Suchbrines may help to explain the widespread devel-opment of phosphorites in Kotlinian to Cambriantimes (Figs 1, 2) by providing a source of unmixed,nutrient-enriched bottom waters. Phosphogenesispeaked in the Precambrian-Cambrian boundary in-terval (Cook & Shergold, 1986; Shergold & Brasier,1986; Brasier et al. 1990), forming massive depositsalong Gondwana from south China to northernIndia, Xinjiang, Pakistan, Kazakhstan and Iran.

    3.d. Metalliferous black shales, sulphur isotopes and anoxicbottom waters

    Warm, saline bottom waters may also explain themany indications of anoxia in latest Precambrian andCambrian strata. During rising sea levels, such watersmay result in oxygen depletion, leading to theformation of black shales (e.g. Arthur, Schlanger &Jenkyns, 1987). An absence of eutrophic planktonassemblages in comparable Toarcian and Cenomanianblack shales (e.g. Reigal et al. 1986; Thiersten, 1989)perhaps indicates that export productivity was low,owing to the removal of phosphorus into deep brines(cf. Thiersten, 1989) and/or to the expansion ofnitrate-reducing bacteria in association with an ex-panded oxygen minimum zone (cf. Codispoti, 1989).Both could explain the association between blackshales and presumed warm or greenhouse climates(Fischer & Arthur, 1977) since reduced productivity

    allows atmospheric carbon dioxide to build up througha system of positive feedbacks (e.g. Brasier, 1990a).

    Although black shales are locally common inCambrian shelf deposits, they are not widespread inlate Precambrian strata until the latest Redkinian toKotlinian times (Fig. 1). The latter interval marked aclimax in carbonaceous preservation of vendotaeniidribbons (sulphate-oxidizing bacteria? Vidal, 1989)and large globular vesicles of the Chuaria (bacterialenvelopes?) in metalliferous black shales on the EastEuropean Platform (Felitsyn, Sochava & Vaganov,1989). Associated heavy S3iS isotopes suggest extensivesulphate reduction on the sea floor (Vidal, 1989).Similar occurrences of vendotaeniids are known fromthe Siberian platform, northwest Canada, Newfound-land and Gondwana from Iran to China. Black oilshales of this age notably provide a source for theOmani Huqf oil fields, and their intimate associationwith the Hormuz Salt Formation (Husseini & Hus-seini, 1990) suggests they almost certainly formed inresponse to salinity stratification. Remarkably heavysulphur isotopes in these evaporites confirm theimplication of a great expansion of the sulphate-reducing microbial biotope (Holser, 1977) duringKotlinian to Nemakit-Daldynian times.

    Subsequent transgressive pulses left highly met-alliferous black shales over the carbonate shelves ofArabia, India, and south China. Black shales, dolo-mites and phosphorites are found particularly inthe Meishucunian to Qiongzhusian, with anoxic shalesin the latter giving rise to preservation of the Chenjiangsoft-bodied fauna. Black alum shales did not becomewidespre...

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