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The Precambrian-Cambrian Boundary: Seawater Chemistry, Ocean Circulation and Nutrient Supply in Metazoan Evolution, Extinction and Biomineralization

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The Precambrian-Cambrian Boundary: Seawater Chemistry, Ocean Circulation and Nutrient Supply in Metazoan Evolution, Extinction and Biomineralization Journal of the Geological Society, London, Vol. 149, 1992, pp, 655-668, 11 figs. Printed in Northern Ireland The Precambrian-Cambrian boundary: seawater chemistry, ocean circulation and nutrient supply in metazoan evolution, extinction and biomineralization MAURICEE. TUCKER Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK Abstract: This paper reviews the evidence for changesin the global environment from the late Precambrian into the Cambrian, against which the evolution of many metazoan groups and the development of bio- mineralization should be seen. With higher carbon dioxide levels, Precambrian seawater was more super- saturated with respect to CaCO, than Phanerozoic seawater and carbonateswere precipitated easily. From the late Precambrian to the early Cambrian, therewas a decrease in the Mg/Ca ratio and an increasein the aCO, of seawater. Changes in global climate (icehouseto greenhouse) and increased plate tectonic activity resulted in major changes in ocean circulation and nutrient levels, a rise in global temperature, and the formation of extensive shallow seas. The Vendian-Cambrian radiation events and onset of biomineraliz- ation must have been strongly influenced, if not driven, by these global environmental changes. The Precambrian-Cambrian boundary interval was a time of early-mid Vendian, around the time of the last great Precam- profound change in the biosphere, with the evolution of many brianglaciation. Beforethis, thebiota was dominated by new metazoan groups and the development of biomineraliz- unicellular procaryotes (from about 3500 Ma) and eucaryotes ation and skeletonization. It was also a time of global change, (from about 1800 Ma). especially the cyanobacteria and pro- withincreased plate movements and opening oceans and a tista. The latter include the acritarchs,which suffered a reduc- climatic progression from late Precambrian glaciation to Cam- tion in diversity around the time of the last Precambrianglaci- brian global warming. Many explanations have been put for- ation or just after(Vidal & Knoll 1982; Vidal & Moczydlowska ward to account for the Vendian-Cambrian radiation events 1992). Stromatolites are the only macroscopic recordof life to and these can be grouped into four main categories. (a) Bio- be seen in the field in Riphean and Archaean rocks, but some logical: evolutionary diversification intovacant niches and very simple trace fossils occur in lower Vendian strata. During evolutionary responses to predation and cropping pressures. themid-Vendian glaciation, the globally-occurring soft- (b) Environmental: new groups evolving in response to major bodied Ediacara faunaevolved (e.g. Hofmann et al. 1990), with transgressions, deglaciations, nutrient supply and access to ex- medusoids, cnidaria, worms and a few arthropods. This fauna tensive shallow-marinehabitats. (c) Geochemical:evolution mostly died out just before the end of the Vendian, in thefirst of groups forced through changesin atmosphere-hydrosphere major extinction event (Brasier 1989). Also coming and going chemistry, particularly pC0, and phosphate levels. (d) Extra- in the late Vendian were the vendotaenids (largemacrocel- terrestrial:the influence of large meteorites, bolides and lularalgae) (Gnilovskaya 1985). Morecomplicated trace asteroids (see papers in Cowie & Brasier 1989 and reviews by fossils began to evolve in the late Vendian, mostly in shallow- Brasier 1982,1985, 1990; McMenamin 1987; Sepkoski 1983; water niches (although some ofthese became extinct along with Tucker 1989, and other papersin this thematic set). Therewere theEdiacara fauna). In the early Cambrian (Atdabanian), clearly many factors involvedin the Vendian-Cambrian radia- shallow-water trace fossils diversified and deeper water ich- tion events and many of these are inter-related. It is unlikely nofaunas evolved (Fig. 1; Crimes this volume). that one particular cause was responsible, but rather that the Small shelly fossilsfirst appear inNemakit-Daldynian biosphere was responding to changes taking place on many time (late Vendian), risingto a peak in the Tommotian (Fig.1). fronts. This paper is concerned with the evidence for chemical There appears to have been a change in the skeletal composi- changes within the oceans as deduced from carbonate facies, tion of these organismsas they evolved, from phosphatic in the mineralogyand geochemistry. Precambrian carbonates in earliestNemakit-Daldynian to calcareousinthe latest generalare firstconsidered andthen boundary limestones. Nemakit-Daldynian to earliestTommotian (Lowenstam & Interpretations are discussed within the context of global en- Margulis 1980; Brasier 1986; Brasier this volume; Rozanov this vironmental changes from the Vendian into the Cambrian. A volume).Developing in the Tommotian were the molluscs, scenario is then presented for the Vendian through Cambrian with their aragonitic shells, and the archaeocyathans, sponge- radiation events. likecolonial organisms with skeletons of high-Mg calcite (James & Klappa 1983). The archaeocyathans diversified in Atdabanian time, when they werejoined by the trilobites, bra- The Vendian-Cambrian radiation events chiopods and echinoderms. The cyanobacteria, in existence The Vendian-Cambrian radiation may beviewed as a series of from the earliest times(3500 Ma ago),began to calcify heavily events taking place overa relatively short period of time, in the in the latest Vendian (Riding 1982). order of 2WO million years, from the mid-Vendian into the Thus the Vendian-Cambrian radiation comprises five epi- early Cambrian (see Fig. 1). In the latest radiometric dating sodes: (1) pre-Vendian-unicellular procaryotesand scheme (Harlandet U/. 1989), the base of the Vendianis placed eucaryotes;(2) mid-late Vendian-Ediacara fauna, vendo- at 610 Ma and the base of the Cambrian at 570 Ma. taenids and early tracefossils; (3) Nemakit-Daldynian-phos- Thebiological revolution appears to have begun in the phatic small shelly fossils andmany shallow-water trace 655 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/149/4/655/4892314/gsjgs.149.4.0655.pdf by guest on 29 September 2021 M . E. TUCKER656 E. M. xx xx X X glaciation X X xx xx Fig. 1. The evolution and extinction of metazoan groups in the late Precambrian to early Cambrian. Based on many sources, but especially Brasier (1982), Crimes (this volume), Hofmann et al. (1990), Lowenstam & Margulis (1980), Riding (1982), Rozanov (this volume) and Vidal & Moczydlowska (this volume). Abbreviations of stages: N-D, Nemakit-Daldynian; Tom, Tommotian; Atd, Atdabanian; Bot, Botomian; R, Riphean. The five stages of metazoan evolution are discussed in the text. The Precambrian-Cambrian boundary is around 570 Ma and the Vendian-Riphean boundary around 610 Ma (Harland et al. 1989). The boundary shown here is higher than that adopted in other contributions to this set (e.g. McKerrow et al. this volume), which place the base of the Cambrian at a low level in the Nemakit-Daldynian Stage fossils; (4) Tommotian-calcareous smallshelly fossils with glaciations of continents as they drifted rapidly over the polar the first archaeocyathans andmolluscs; (5)Atdabanian-abun- regions remains a possibility (Hambrey& Harland 1985; Piper dantarchaeocyathans, with trilobites, brachiopods, ech- 1985). inoderms and deeper-water trace fossils (see Fig. 1). Precambrian carbonates Plate movements, opening oceans and glaciation Carbonate rocks are common in the Proterozoic sedimentary In the early Vendian, a megacontinent (an early Gondwana) record and were deposited in the whole range of sedimentary was formed by China, Africa, South America, India and Aus- environments that exist today, allowing for the differences in tralia (e.g. Ilyin 1990). Attachedto this vast continent was biota (see the review of Grotzinger 1989). However, there are Avalonia, which included southern Britain. Laurentia, Baltica differences between Proterozoic and Phanerozoic carbonates andSiberia formed another large continental plate, which which are relevant to the Precambrian-Cambrian boundary began to break up around 600 Ma ago ora little later, leading problem. Theevidence comes from the natureof Precambrian to the formation of the Iapetus Ocean (McKerrow et al. this carbonate facies and from their mineralogy, both original and volume). This plate probably straddled the equator and was early diagenetic, andrelates to the saturation stateof seawater sufficiently close to Gondwana to have common fauna1 ele- withrespect to CaCO, (whichwas evidently higher during ments in the early Cambrian. Gondwanaitself began to break thePrecambrian), to the $0, atmosphere(and then the up in thelatest Vendian-early Cambrian. Apparently there aCO,seawater, a = activity)and to the Mg/Ca ratio of were no continents situated at the poles (McKerrowet al. this seawater. volume). This is consistent with the palaeomagnetic data from latePrecambrian glacial horizons whichsuggest low Carbonate precipitation in the Precambrian palaeolatitudes(e.g. Embleton & Williams 1986). However, interpretation of late Precambrian palaeomagnetic data has There is abundant evidence for a greater ease of CaCO, pre- proved difficult (e.g. Perrin & Prevot 1988) and successive cipitation in the Precambrian compared with the Phanerozoic. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/149/4/655/4892314/gsjgs.149.4.0655.pdf by guest on 29 September 2021 C AR BO NA TES AND ENVIRONMENTALCHANGECARBONATES AND 657 Fig. 2. Precambrian carbonate facies providing evidence
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