Global ocean—atmosphere change across the Precambrian—Cambrian transition

http://journals.cambridge.org Downloaded: 23 Jun 2014 IP address: 143.167.2.135

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

http://journals.cambridge.org


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

<0 Siberia

O Salt RangeO HormuzO Yudoma

Y Hormuz

Phosphorltea

• Thorntonia

• XinJI

• Zhongylcun,Krol.Uchur-Maya

. Douehantuo

Black shales

• Sinska

p Shuljingtuo,Yuhucun

B Jiucheng,Hormuz,Chopoghlu

Age (China)

Maozhuangian +Longwangmiaoan

Canglangpulan

Qlongzhutian

Maishucunian

Dengyingxian

Figure 1. Stratigraphic nomenclature for the late Precambrian-early Cambrian, showing the distribution of glaciations,evaporites, phosphorites and black shales. Yudoma, Sinska (Siberian Platform); Salt Range (Pakistan); Hormuz (ArabianGulf); Sinian, Nantuo, Doushantuo, Zhongyicun, Badaowan, Xinji, Jiucheng, Yuhucun and Shuijingtuo (Yangtze Platform,south China); Tal (north India); Sturtian, Thorntonia (Australia); Chopoghlu (Iran).

Olenellld realm Redllchiid realm; ; ; ;

i i i i> 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



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



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 becomewidespread in Avalonia and Baltica until middle andlate Cambrian times (Thickpenny & Leggett, 1987),with a global climax of anoxic deposition beginning inthe Ptychagnostus gibbus Zone (Shergold & Brasier,1986). On Laurentia, the Burgess Shale fossil lager-stdtten was preserved under anoxic conditions duringearly middle Cambrian time (e.g. Conway Morris,1986) at the foot of a reef scarp c. 100 m below thesurface of the carbonates (Mcllreath, 1977). Thisclearly suggests anoxic water very close to the surfaceof the ocean.

So widespread were anoxic shales of middleCambrian times that Raiswell & Berner (1986)suggested ' normal marine carbonaceous shales' werelimited to shallow regions with turbulent circulation.This curious picture is supported by sedimentologicalstudies of the Alum Shales in Scandinavia (Thick-penny, 1985) which indicate high sea-level stands, lowdetrital influx, stratification of the water column andlow organic carbon production. Euxinic conditionsbelow wave base therefore led to preservation oforganic matter, even under conditions of very slowsedimentation. As outlined above, such anoxic condi-tions appear to have spread from lower to higherlatitudes through the course of the Cambrian, perhapsin response to global warming.



Precambrian-Cambrian ocean-atmosphere change 165

SIBERIAN PLATFORM Diversity of Suggestedphosphatic s e a | e v e |

skeletal species

10 20 30 O N L A P 0 F F L A P

Paaetiellus anabarus ZoneThickevaporitesin SW

Fallotaspis Zone

Protallotaspis jakutensis Zone• / / / / / / .

, Interval•o\ great'phosphateavailability

D. lenaicus Zone Aged and 02 -depleted bottomwaters overshelf

Dokidocyathus regularis Zone

Massive transgression

AldanocyatbussunnaginicusZone

-1 0813C %o PDB

10 20 30

Figure 4. The carbon isotope record across the Precambrian-Cambrian transition in Siberia. The peak of phosphatic skeletalspecies falls just after the isotopic maximum and indicates the influx of nutrient-enriched waters. These events appear to beclosely tied to sea-level changes. Data mainly from Brasier (19906), Kirschvink et al. (1991).

It is important to emphasize that these stagnantintervals lasted much longer than Mesozoic 'OceanicAnoxic Events', spanning many biozones withoutmajor interruption.

3.e. Carbon isotopes

That widespread metalliferous Kotlinian-Cambrianblack shales were associated with widespread'greenhouse' conditions is suggested by a study ofcarbon isotopes. These have proved to be usefulindices of atmospheric carbon dioxide levels throughglacial and interglacial cycles in the Quaternary(Shackleton & Pisias, 1985). It is known that primaryproducers preferentially fix lighter 12C into organicmaterial during photosynthesis, leaving surface watersrelatively enriched in heavier 13C, especially in areas ofhydrodynamic mixing and greater nutrient avail-ability. Planktonic carbonates therefore yield positive13C excursions (in relation to benthic §13C) during thestronger upwelling of glacial episodes.

Mesozoic pelagic carbonates associated with'Oceanic Anoxic Events' often bear such positivecarbon isotopic excursions (e.g. Jenkyns, 1985) and ithas hitherto seemed reasonable to conclude that pro-ductivity was raised during their formation. LatePrecambrian and Cambrian black shales, however,yield anomalously light carbon isotopes, as forexample in the latest Precambrian of Oman (Gran-

tham, Lijmbach & Posthuma, 1990), the basalCambrian of India (Banerjee, Schidlowski & Arneth,1986) and correlated strata in Siberia (Kontorovitchet al. 1990) and China (Brasier et al. 1990, and Brasier,1990a). The same depleted signature is present in theCambrian Alum Shales of Sweden (Thickpenny, 1985).

Because these very light carbon signatures occureven in the organic matter, they cannot be dismissedas diagenetic artefacts. They may indicate extensivefractionation by sulphate-reducing or methanogenicbacteria. They may also indicate low levels of primaryproductivity in surface waters owing to limitedavailability of phosphorus, since one atom is neededto fix 106 atoms of carbon and 16 atoms of nitrogen(e.g. Codispoti, 1989). Phosphorus supply may havebeen reduced by rising sea levels and reduced P-runoff(cf. Broecker, 1982), by massive removal in phos-phorites (cf. Codispoti, 1989) or into deep salinebottom waters.

A second limiting nutrient is nitrogen (as nitrate orammonia). This largely enters the biosphere throughnitrogen-fixing bacteria but it is also consumed bynitrate-reducing bacteria (Codispoti, 1989). Nitrogenlimitation can therefore occur where the latter expandin anoxic environments such as those discussed above.

Where nutrients are in sufficient supply, thenphotosynthesis can proceed to its limits. In effect, thiscauses 12C limitation and the primary producers aredriven to incorporate increasing proportions of 13C.



166 M. D. B R A S I E R

Table 2. Climatic changes inferred in relation to carbon and nutrient changes through late Precambrian and Cambrian time

Riphean Varangian Ediacarian Cambrian

Sea level High Low Rising Rising513C Positive Negative Rising FluctuatingCarbon dioxide High Low Rising HighPhosphorites Rare None Increasing ManyClimate Warm Glacial Warming 'Greenhouse'

Carbon isotope curves for the boundary intervaland the ensuing Cambrian (Fig. 4) are therefore ofextreme interest in this context. The long intervals ofnegative S13C imply low background levels of primaryproductivity from Kotlinian to late Cambrian time,perhaps under 'greenhouse' conditions. Shorter inter-vals of positive S13C provide conspicuous markersthat suggest episodes of raised productivity and,perhaps, climatic cooling. One such interval is clearlypresent close to the Precambrian-Cambrian boundary(see below). Some broad trends and climatic inter-pretations are summarized in Table 2.

4. The Precambrian-Cambrian boundary excursions

The Precambrian-Cambrian boundary appears tostraddle an interval between cool 'icehouse' con-ditions of the Varangian and its immediate after-math and the warmer 'greenhouse' conditions ofthe Cambrian. It remains, therefore, to interpret themajor positive and negative carbon isotope excursionsthat lie across this interval (Fig. 4). These have beenreported in low-latitude carbonate successions fromChina, India, Iran, Morocco and Siberia and arewidely believed to represent global changes in thecarbon budget (Brasier et al. 1990). Their analysis andinterpretation is one of the main objectives of IGCPProject 303.

The distinctive positive excursion known as theDahai or Yudoma S13C maximum (Fig. 4) may beexplained as a eutrophication event, related towidespread shallow carbonate conditions, with highprimary productivity favoured by abundant suppliesof phosphorus. This explanation is still partly specu-lative and a final solution must await further analysisof the carbon and phosphorus cycles over this interval.Several lines of evidence, however, indicate freelyavailable phosphorus driving photosynthesis towardseutrophication (Fig. 4): positive carbon isotopes,associated major phosphorites (Brasier et al. 1990)and the contemporaneous radiation of phosphaticand siliceous skeletons (Brasier, 19906).

Was this eutrophication driven up upwelling? Thishas been inferred by Cook & Shergold (1986) forphosphorites of this age but the evidence is stillslender. A possible scenario would imply climaticcooling, increased circulation of the oceans, turnoverof the water masses and delivery of old, metalliferous

and nutrient-enriched waters to the surface. Althoughsuch an 'Oxic Event' might seem to lend support tothe notion of a late Sinian glacial epoch referred toabove, a glacial interpretation is neither certain nornecessary to explain the phenomena and other modelsare being explored. For example, the increased flux ofnutrients seems to have broadly coincided withregressive and evaporite facies. Nutrients may havebeen drawn into shallow waters by evaporation andassociated recharge from offshore.

5. Conclusion

It has been shown that glacial conditions werewidespread in the late Precambrian Varangian epoch.The change towards 'greenhouse' conditions possiblybegan with the development of deep, saline bottomwaters, produced in evaporitic rift basins around theArabian Gulf, in Pakistan and on the SiberianPlatform. A series of major transgressions fromKotlinian to late Cambrian times brought salinity-stratified waters onto the shelf, laying down extensivemetalliferous black shales and phosphorites. Reducedrates of nutrient recycling during transgressions mayaccount for lowered primary productivity in surfacewaters (indicated by light carbon isotopes in bothcarbonates and organic matter). A massive expansionof the sulphate-reducing bacterial biotope underanoxic conditions is indicated by sulphur isotopes.This, and reduced primary productivity, may accountfor implied increases in atmospheric carbon dioxide,as suggested both by light carbon isotopes and changesfrom aragonitic to calcite mineralogy in carbonatesediments.

The Precambrian-Cambrian boundary interval per-haps indicates what happens when a salinity-stratifiedocean is perturbed by climatic change and/or sea levelrise. A rich broth of nutrients flooded the carbonateplatforms and eutrophication arguably led to a hugebut temporary rise in the biomass of primaryproducers. Such incursions of phosphorus- and silica-enriched water masses over the shallow shelves appearto have left their imprint on the invertebrate fossilrecord (Brasier, 1990 a, b). Biomineralization andmacroevolution were beginning over this interval, andmany of the earliest skeletal fossils secreted phosphatichard parts (e.g. hyolithelminthes, protoconodonts,tommotiids, conulariids, inarticulate brachiopods and



Precambrian-Cambrian ocean-atmosphere change 167

many microproblematica) or siliceous skeletons (e.g.hexactinellid sponges, worm tubes: see Fig. 4). Agradual switch to calcitic skeletons can be traced inbrachiopods and sponges from the Tommotian on-wards. The extent to which this extraordinary switchin biomineral secretion is related to other evidence fornutrient depletion in surface waters is a fertile area forfuture research.

Acknowledgements. The writer is grateful to the many whoencouraged the progress of this research and made availabletheir field sections and materials for analysis. I thankparticularly A. Yu. Rozanov, V. V. Khomentovsky, A.Zhuravlev, Xiang Liwen, Luo Huilin, Jiang Zhiwen, HeTinggui, Yue Zhao, A. J. Azmi, D. M. Banerjee, V. Tewari,B. Hamdi, M. M. Anderson and the stimulus provided bythe related IGCP and IUGS projects of P. J. Cook and J. H.Shergold, J. W. Cowie, K. J. Hsu, M. Schidlowki and O. H.Walliser. This paper is a contribution to IGCP Project 303on Precambrian-Cambrian Event Stratigraphy.

References

ARTHUR, M. A., SCHLANGER, S. O. & JENKYNS, H. C. 1987.

The Cenomanian-Turonian Oceanic Anoxic Event. II.Palaeoceanographic controls on organic matter pro-duction and preservation. In Marine Petroleum SourceRocks (eds J. Brooks and A. J. Fleet), pp. 401-20.Special Publication of the Geological Society, Londonno. 26.

BANERJEE, D. M., SCHIDLOWSKI, M. & ARNETH, J. D. 1986.

Genesis of upper Proterozoic-Cambrian phosphoritedeposits of India: isotope inferences from carbonatefluorapatite, carbonate and organic carbon. Pre-cambrian Research 33, 239-53.

BERGSTROM, J. & AHLBERG, P. 1981. Uppermost LowerCambrian biostratigraphy in Scania, Sweden. Geo-logiska Foreningens i Stockholm Forhandlingar 103,193-214.

BRASIER, M. D. 1981. Sea-level changes, facies changes andthe late Precambrian-early Cambrian evolutionaryexplosion. Precambrian Research 17, 105-23.

BRASIER, M. D. 1985. Evolutionary and geological eventsacross the Precambrian-Cambrian boundary. GeologyToday 1, 141-6.

BRASIER, M. D. 1989. On mass extinction and faunalturnover near the end of the Precambrian. In MassExtinctions: Processes and Evidence (ed. S. K.Donovan), pp. 73-88. New York: Columbia UniversityPress.

BRASIER, M. D. 1990 a. Phosphogenic events and skeletalpreservation across the Precambrian-Cambrianboundary interval. In Phosphorite Research and De-velopment (eds A. G. Notholt and I. Jarvis), pp. 282-303. Special Paper of the Geological Society, Londonno. 52.

BRASIER, M. D. 19906. Nutrients in the early Cambrian.Nature 347, 521-2.

BRASIER, M. D. & GAO SHUPING (eds). The Stratigraphy ofChina. Volume 4. The Cambrian System. Edinburgh:Scottish Academic Press (In press).

BRASIER, M. D., MAGARITZ, M., CORFIELD, R., LUO, H.,

Wu, X., OUYANG, L., JIANG, Z., HAMDI, B., HE, T. &

FRASER, A. G. 1990. The carbon- and oxygen-isotoperecord of the Precambrian-Cambrian boundary in-terval in China and Iran and their correlation.Geological Magazine 127, 319-32.

BROECKER, W. S. 1982. Ocean chemistry during glacial time.Geochimica et Cosmochimica Ada 46, 1689-705.

CODISPOTI, L. A. 1989. Phosphorus vs. nitrogen limitationof new export production. In Productivity of the Ocean:Present and Past (eds W. H. Berger, V. S. Smetacek,and G. Wefer), pp. 377-44. Dahlem Workshop Report44. Chichester: Wiley.

CONWAY MORRIS, S. 1986. The community structure of theMiddle Cambrian Phyllopod Bed (Burgess Shale).Palaeontology 29, 423-67.

COOK, P. J. & SHERGOLD, J. H. 1986. Proterozoic and

Cambrian phosphorites - nature and origin. In Phos-phate Deposits of the World. Volume 1. Proterozoicand Cambrian Phosphorites (eds P. J. Cook and J. H.Shergold), pp. 369-86. Cambridge: Cambridge Uni-versity Press.

COWIE, J. W. & BRASIER, M. D. (eds) 1989. The Pre-

cambrian-Cambrian Boundary. Oxford: ClarendonPress.

FELITSYN, S. B., SOCHAVA, A. V. & VAGANOV, P. A. 1989. Ir

anomaly at boundary of Ediacara fauna extinction.Abstracts of the 28th International Geological Congress,Washington 1, 178.

FISCHER, A. G. & ARTHUR, M. 1977. Secular variations inthe pelagic realm. In Deep Water Carbonate Environ-ments (eds H. F. Cook and P. Enos), pp. 19-50. SpecialPublication of the Society of Economic Paleontologistsand Mineralogists no. 25.

FRAKES, L. A. 1979. Climates throughout Geologic Time.Amsterdam: Elsevier, 310 pp.

GERMS, G. J. B. 1983. Implications of a sedimentary faciesand depositional environmental analysis of the NamaGroup in south west Africa/Namibia. Special Pub-lication of the Geological Society of South Africa 11,89-114.

GLAESSNER, M. F. 1984. The Dawn of Animal Life.Cambridge: Cambridge University Press, 244 pp.

GRANTHAM, P. J., LIJMBACH, G. W. M. & POSTHUMA, J.

1990. Geochemistry of crude oils in Oman. In ClassicPetroleum Provinces (ed. J. Brooks), pp. 317-28. SpecialPublication of the Geological Society, London no. 50.

GUAN BAODE, W U RUITANG, HAMBREY, M. J. & GANG

WUCHEN. 1986. Glacial sediments and erosional pave-ments near the Cambrian-Precambrian boundary inwestern Henan province. China. Journal of the Geo-logical Society, London 143, 311-23.

HARLAND, W. B. 1983. The Proterozoic glacial record.Memoir of the Geological Society of America 161,279-88.

HARLAND, W. B. 1989. Palaeoclimatology. In The Pre-cambrian-Cambrian Boundary (eds J. W. Cowie andM. D. Brasier), pp. 199-204. Oxford: Clarendon Press.

HOFMANN, H. J, NARBONNE, G. M. & AITKEN, J. D. 1990.

Ediacaran remains from intertillite beds in northwesternCanada. Geology 18, 1199-202.

HOLSER, W. T. 1977. Catastrophic chemical events in thehistory of the ocean. Nature 267, 403-7.

HUSSEINI, M. I. & HUSSEINI, S. I. 1990. Origin of the

Infracambrian salt basins of the Middle East. In ClassicPetroleum Provinces (ed. J. Brooks), pp. 279-92. SpecialPublication of the Geological Society, London no. 50.



168 Precambrian-Cambrian ocean-atmosphere change

JENKYNS, H. C. 1985. The early Toarcian and Cenomanian-Turonian anoxic events in Europe: comparisons andcontrasts. Geologische Rundschau 74/3, 505-18.

KHOMENTOVSKY, V. V. 1986. The Vendian System of Siberiaand a standard stratigraphic scale. Geological Magazine123, 333-48.

KlRSCHVINK, J. L., MAGARITZ, M., RlPPERDAN, R. L. &ROZANOV, A. Yu. 1991. The Precambrian-Cambrianboundary; magnetostratigraphy and carbon isotopesresolve correlation problems between Siberia, Morocco,and South China. GSA Today 1, pp. 69-71, 87, 91.

KNOLL, A. H., HAYES, J. M., KAUFMAN, A. J., SWETT, K. &

LAMBERT I. 1986. Secular variation in carbon isotoperatios from Upper Proterozoic successions of Svalbardand East Greenland. Nature 321, 832-8.

KONTOROVITCH, A. E., MANDEL'BAUM, M. M., SURKOV,

V. S., TROFIMUK, A. A. & ZOLOTOV, A. N. 1990. Lena-

Tunguska Upper Proterozoic-Palaeozoic petroleumsuperprovince. In Classic Petroleum Provinces (ed. J.Brooks), pp. 473-89. Special Publication of the Geo-logical Society, London no. 50.

LANDING, E., NARBONNE, G. M. & MYROW, P. (eds) 1988.

Trace Fossils, Small Shelly Fossils and the Pre-cambrian-Cambrian Boundary. Bulletin of the NewYork State Museum no. 463.

MCILREATH, I. A. 1977. Accumulation of Middle Cambrian,deep-water limestone debris apron adjacent to a verticalsubmarine carbonate escarpment, South RockyMountains, Canada. Special Publication of the Societyof Economic Paleontologists and Mineralogists, Tulsa25, 113-24.

MCKERROW, W. S., SCOTESE, C. R. & BRASIER, M. D. 1992.

Early Cambrian continental reconstructions. Journal ofthe Geological Society, London (In press)

PARRISH, J. T., ZEIGLER, A. M., SCOTESE, C. R., HUMPHRE-

VILLE, R. G. & KIRSCHVINK, J. K. 1986. Proterozoic

and Cambrian phosphorites - specialist studies. EarlyCambrian palaeogeography, palaeoceanography andphosphorites. In Phosphate Deposits of the World.Volume 1. Proterozoic and Cambrian Phosphorites (edsP. J. Cook and J. H. Shergold), pp. 280-94. Cam-bridge : Cambridge University Press.

PILLOLA, G. L. 1990. Lithologie et trilobites du Cambrieninfe'rieur du SW de la Sardaigne (Italie): implicationspaleobiogeographique. Comptes Rendus de TAcademiedes Sciences, Paris, Series II 310, 321-8.

RAISWELL, R. & BERNER, R. A. 1986. Pyrite and organicmatter in Phanerozoic normal marine shales. Geo-chimica et Cosmochimica Ada 50, 1967-76.

REIGEL, W., LOH, H., MAUL, B. & PRAUSS, M. 1986. Effects

and causes in a black shale event - the ToarcianPosidonia Shale of NW Germany. In Global Bio-Events(ed. O. J. Wallisser), pp. 267-76. Berlin: Springer-Verlag.

ROBERTS, J. D. 1976. Late Precambrian dolomites, Vendianglaciation and synchroneity of Vendian glaciations.Journal of Geology 84, 47-63.

ROWLAND, S. M. & GANGLOFF, R. A. 1988. Structure and

paleoecology of Lower Cambrian reefs. Palaios 3,111-35.

ROZANOV, A. Yu. 1984. The Precambrian-Cambrianboundary in Siberia. Episodes 1, 12-19.

SHACKLETON, N. J. & PISIAS, N. G. 1985. Atmospheric

carbon dioxide, orbital forcing and climate. AmericanGeophysical Union, Geophysics Monographs 32, 303-17.

SHELDON, R. P. 1989. Evidence for ring systems orbitingEarth in the geologic past. In Phosphorite in India (ed.D. M. Banerjee), pp. 139-60. Memoir of the GeologicalSociety of India no. 13.

SHERGOLD, J. H. & BRASIER, M. D. 1986. Biochronology of

Proterozoic and Cambrian phosphorites. In PhosphateDeposits of the World, Volume 1. Proterozoic andCambrian Phosphorites (eds P. J. Cook and J. H.Shergold), pp. 295-326. Cambridge: Cambridge Uni-versity Press.

SOKOLOV, B. S. & FEDONKIN, M. A. 1986. Global biologicalevents in the late Precambrian. In Global Bio-Events(ed. O. H. Wallieser), pp. 105-8. Berlin: Springer-Verlag.

SPENCER, A, M. 1971. Late Pre-Cambrian Glaciation inScotland. Memoir of the Geological Society, Londonno. 6, 100 pp.

THICKPENNY, A. 1985. 'Black Shales' and early Palaeozoicpalaeo-oceanography. Terra Cognita 5, 109.

THICKPENNY, A. & LEGGETT, J. K. 1987. Stratigraphic

distribution and palaeo-oceanographic significance ofEuropean early Palaeozoic organic-rich sediments. InMarine Petroleum Source Rocks (eds J. Brooks and A.Fleet), pp. 301-16. Special Publication of the GeologicalSociety, London no. 26.

THIERSTEN, H. R. 1989. Inventory of palaeoproductivityrecords: the mid Cretaceous enigma. In Productivity ofthe Ocean: Past and Present (eds W. H. Berger, V. S.Smetacek and G. Wefer), pp. 355-76. Dahlem Work-shop Report no. 44. Chichester: Wiley.

TUCKER, M. E. 1989. Carbon isotopes and Precambrian-Cambrian boundary geology, South Australia: oceanbasin formation, seawater chemistry and organicevolution. Terra Nova 1, 573-82.

VIDAL, G. 1989. Are late Proterozoic carbonaceous mega-fossils metaphytic algae or bacteria? Lethaia 22, 375-9.

WILLIAMS, G. E. 1975. Late Precambrian glacial climate andthe earth's obliquity. Geological Magazine 112, 441-4.

WOLFART, R. 1981. Lower Palaeozoic rocks of the MiddleEast. In Lower Palaeozoic of the Middle East, Easternand Southern Africa and Antarctica (ed. C. H. Holland),pp. 5-130. Chichester: Wiley.

YEATS, R. S. & LAWRENCE, R. D. 1984. Tectonics of the

Himalayan Thrust belt in Northern Pakistan. In MarineGeology and Oceanography of the Arabian Sea andCoastal Pakistan (eds B. U. Haq and J. D. Milliman),pp. 177-98. New York: Van Nostrand Reinhold.

ZHURAVLEV, A. YU. 1986. Evolution of archaeocyathansand palaeobiologeography of the early Cambrian.Geological Magazine 123, 377-85.


Documents

Global ocean—atmosphere change across the Precambrian—Cambrian transition