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 cal

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