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

M A U R I C E E . T U C K E R Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK

Abstract: This paper reviews the evidence for changes in the global environment from the late Precambrian into the Cambrian, against which the evolution of many metazoan groups and the development of biomineralization should be seen. With higher carbon dioxide levels, Precambrian seawater was more supersaturated with respect to CaCO, than Phanerozoic seawater and carbonates were precipitated easily. From the late Precambrian to the early Cambrian, there was a decrease in the Mg/Ca ratio and an increase in the aCO, of seawater. Changes in global climate (icehouse to 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 biomineralization must have been strongly influenced, if not driven, by these global environmental changes.

The Precambrian-Cambrian boundary interval was a time of profound change in the biosphere, with the evolution of many new metazoan groups and the development of biomineralization and skeletonization. It was also a time of global change, with increased plate movements and opening oceans and a climatic progression from late Precambrian glaciation to Cam- brian global warming. Many explanations have been put forward to account for the Vendian-Cambrian radiation events and these can be grouped into four main categories. (a) Bio- logical: evolutionary diversification into vacant niches and evolutionary responses to predation and cropping pressures. (b) Environmental: new groups evolving in response to major transgressions, deglaciations, nutrient supply and access to extensive shallow-marine habitats. (c) Geochemical: evolution of groups forced through changes in atmosphere-hydrosphere chemistry, particularly pC0, and phosphate levels. (d) Extra- terrestrial: the influence of large meteorites, bolides and asteroids (see papers in Cowie & Brasier 1989 and reviews by Brasier 1982, 1985, 1990; McMenamin 1987; Sepkoski 1983; Tucker 1989, and other papers in this thematic set). There were clearly many factors involved in the Vendian-Cambrian radiation events and many of these are inter-related. It is unlikely that one particular cause was responsible, but rather that the biosphere was responding to changes taking place on many fronts. This paper is concerned with the evidence for chemical changes within the oceans as deduced from carbonate facies, mineralogy and geochemistry. Precambrian carbonates in general are first considered and then boundary limestones. Interpretations are discussed within the context of global environmental changes from the Vendian into the Cambrian. A scenario is then presented for the Vendian through Cambrian radiation events.

The Vendian-Cambrian radiation events

The Vendian-Cambrian radiation may be viewed as a series of events taking place over a relatively short period of time, in the order of 2 W O million years, from the mid-Vendian into the early Cambrian (see Fig. 1). In the latest radiometric dating scheme (Harland et U/ . 1989), the base of the Vendian is placed at 610 Ma and the base of the Cambrian at 570 Ma.

The biological revolution appears to have begun in the

early-mid Vendian, around the time of the last great Precam- brian glaciation. Before this, the biota was dominated by unicellular procaryotes (from about 3500 Ma) and eucaryotes (from about 1800 Ma). especially the cyanobacteria and pro- tista. The latter include the acritarchs, which suffered a reduc- tion in diversity around the time of the last Precambrian glaciation or just after (Vidal & Knoll 1982; Vidal & Moczydlowska 1992). Stromatolites are the only macroscopic record of life to be seen in the field in Riphean and Archaean rocks, but some very simple trace fossils occur in lower Vendian strata. During the mid-Vendian glaciation, the globally-occurring soft- bodied Ediacara fauna evolved (e.g. Hofmann et al. 1990), with medusoids, cnidaria, worms and a few arthropods. This fauna mostly died out just before the end of the Vendian, in the first major extinction event (Brasier 1989). Also coming and going in the late Vendian were the vendotaenids (large macrocel- lular algae) (Gnilovskaya 1985). More complicated trace fossils began to evolve in the late Vendian, mostly in shallow- water niches (although some of these became extinct along with the Ediacara fauna). In the early Cambrian (Atdabanian), shallow-water trace fossils diversified and deeper water ich- nofaunas evolved (Fig. 1; Crimes this volume).

Small shelly fossils first appear in Nemakit-Daldynian time (late Vendian), rising to a peak in the Tommotian (Fig. 1). There appears to have been a change in the skeletal composition of these organisms as they evolved, from phosphatic in the earliest Nemakit-Daldynian to calcareous in the latest Nemakit-Daldynian to earliest Tommotian (Lowenstam & Margulis 1980; Brasier 1986; Brasier this volume; Rozanov this volume). Developing in the Tommotian were the molluscs, with their aragonitic shells, and the archaeocyathans, sponge- like colonial organisms with skeletons of high-Mg calcite (James & Klappa 1983). The archaeocyathans diversified in Atdabanian time, when they were joined by the trilobites, brachiopods and echinoderms. The cyanobacteria, in existence from the earliest times (3500 Ma ago), began to calcify heavily in the latest Vendian (Riding 1982).

Thus the Vendian-Cambrian radiation comprises five episodes: (1) pre-Vendian-unicellular procaryotes and eucaryotes; (2) mid-late Vendian-Ediacara fauna, vendotaenids and early trace fossils; (3) Nemakit-Daldynian-phosphatic small shelly fossils and many shallow-water trace

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X X glaciation X X x x x x

x x x x

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 small shelly fossils with the first archaeocyathans and molluscs; ( 5 ) Atdabanian-abundant archaeocyathans, with trilobites, brachiopods, echinoderms and deeper-water trace fossils (see Fig. 1).

Plate movements, opening oceans and glaciation In the early Vendian, a megacontinent (an early Gondwana) was formed by China, Africa, South America, India and Aus- tralia (e.g. Ilyin 1990). Attached to this vast continent was Avalonia, which included southern Britain. Laurentia, Baltica and Siberia formed another large continental plate, which began to break up around 600 Ma ago or a little later, leading to the formation of the Iapetus Ocean (McKerrow et al. this volume). This plate probably straddled the equator and was sufficiently close to Gondwana to have common fauna1 elements in the early Cambrian. Gondwana itself began to break up in the latest Vendian-early Cambrian. Apparently there were no continents situated at the poles (McKerrow et al. this volume). This is consistent with the palaeomagnetic data from late Precambrian glacial horizons which suggest low palaeolatitudes (e.g. Embleton & Williams 1986). However, interpretation of late Precambrian palaeomagnetic data has proved difficult (e.g. Perrin & Prevot 1988) and successive

glaciations of continents as they drifted rapidly over the polar regions remains a possibility (Hambrey & Harland 1985; Piper 1985).

Precambrian carbonates Carbonate rocks are common in the Proterozoic sedimentary record and were deposited in the whole range of sedimentary environments that exist today, allowing for the differences in biota (see the review of Grotzinger 1989). However, there are differences between Proterozoic and Phanerozoic carbonates which are relevant to the Precambrian-Cambrian boundary problem. The evidence comes from the nature of Precambrian carbonate facies and from their mineralogy, both original and early diagenetic, and relates to the saturation state of seawater with respect to CaCO, (which was evidently higher during the Precambrian), to the $0, atmosphere (and then the aCO, seawater, a = activity) and to the Mg/Ca ratio of seawater.

Carbonate precipitation in the Precambrian There is abundant evidence for a greater ease of CaCO, precipitation in the Precambrian compared with the Phanerozoic.

CARBONATES AND ENVIRONMENTAL CHANGE 657

Fig. 2. Precambrian carbonate facies providing evidence for seawater carbonate supersaturation. (A) Finely-laminated basinal limestone originally composed of aragonite. (B) Photomicrograph of (A) showing coarse neomorphic spar crystals containing peloidal structures, the remains of microfossils. The crystal fabric, plus high (up to 3000 ppm) strontium, indicate an original aragonite precursor. Biri Formation, Vendian, Southern Norway. (C) Intraclast breccia formed by storm disruption of incipient hardgrounds in an outer shelf environment. Note the castellated shapes of the clasts. (D) Thin limestone bed in centre of view has been undercut and broken up to the right, indicating lithification on the seafloor. Biri Formation, Vendian, Southern Norway. (E) Stromatolite columns up to 1 m high, encased in mudrocks. Upper Belt Supergroup, lower Vendian, Montana. (F): Pisoids, up to 2cm in diameter. Trezona Formation, upper Precambrian, Flinders Ranges, South Australia.

(1) Many Proterozoic outer shelf or basinal successions Riphean-lower Vendian Kingston Peak Formation, Califor- contain carbonate laminites (e.g. Fig. 2A), which yield evi- nia (Tucker 1986); the upper Proterozoic of the Bambui Group, dence for an original aragonite mineralogy, e.g. the Vendian Brazil (Peryt et al. 1990); and the upper Proterozoic Biri Formation, southern Norway (Tucker 1983~); the upper Wumishang Formation, Jixian, northeast China (Ge, Meng &

658 M . E. TUCKER

Tucker pers. observ.). Evidence for an aragonitic precursor in all these cases is provided by neomorphic spar fabrics resulting from calcitization (e.g. Fig. 2B), and high strontium values (several 1000 ppm). Many of these laminites consist entirely of clots of micrite (peloids, Fig. 2B). In addition, fibrous marine cements have been described from toe-of-slope rhythmites deposited in at least 1000 metres of water in the lower Pro- terozoic Rocknest Formation of Northwest Territories ( N W ) , Canada (Grotzinger 1986~). These occurrences show that seawater was supersaturated with respect to aragonite to very significant depths (for calcite it would have been even deeper). At the present time, in low latitudes, seawater is saturated with respect to aragonite to depths of less than 300 metres in the Pacific, but to depths of around 2000m in the Atlantic (Broecker 1974).

(2) A very distinctive Proterozoic carbonate facies in platform successions, and one which occurs through to the Cambro-Ordovician (e.g. Whisonant 1987), is an intraclast conglomerate or flakestone, composed of slabs of micritic limestone (e.g. Fig. 2C). This facies is not the result of desic- cation of tidal-flat muds but the product of disruption of incipient hardgrounds, especially by storms (Tucker 1982a). The facies shows that CaCO, was readily precipitated in shoreface to offshore environments (water depths in the order of 10- 100 m). The shape of clasts, with castellated borders (Fig. 2C), demonstrates that they were at least partially lithified, and partial lithification is also evident where storms have undercut and excavated the limestone layers on the seafloor (Fig. 2D). Such extensive seafloor precipitation of fine-grained carbonate does not occur at the present time, nor did it through most of the Phanerozoic.

(3) Stromatolites were common throughout the Precam- brian, and in many cases there is abundant evidence for precipitation of CaCO, within the structures, rather than a simple trapping of carbonate grains as is commonly believed to have been the case in Phanerozoic stromatolites. Isotopic studies show that the carbonate has a normal marine 6I3C value, rather than a more negative value which would indicate some microbial involvement in the carbonate precipitation (e.g. Fairchild er al. 1990). The in situ precipitation of CaCO, is best shown by stromatolite columns encased within mudrocks (e.g. Fig. 2E).

(4) Another feature of many high-energy Proterozoic carbonates is the presence of large ooids or pisoids, 1 cm or more across (e.g. Fig. 2F, and Swett & Knoll 1989). These grains are very rare in Phanerozoic limestones and those large pisoids that do occur in younger rocks are usually oncoids (formed by cyanobacteria) or of vadose-pedogenic origin. Rapid precipitation of CaCO, is again implicated.

(5 ) Marine tufas are not a common carbonate facies in the Phanerozoic but they do occur in many peritidal Precambrian sequences, such as the shoal complex and inner shelf facies of the Rocknest Formation, NWT, Canada (Grotzinger 19866), the Beck Spring Dolomite of California and the Porsanger Dolomite of Arctic Norway (Tucker, pers. observ.). These must represent rapid carbonate precipitation on tidal flats and again support the notion of seawater over-saturated in CaC0,.

All these various points suggest that Precambrian seawater was supersaturated with respect to CaCO,, and to a greater extent than Phanerozoic seawater. Such a situation has also been suggested by Grotzinger (1989, 1990) and Knoll & Swett (1990). The reason for this is probably related to the higher p C 0 2 in the atmosphere at the time, as accepted in all current models for the early atmosphere. Higher temperatures, which are frequently proposed for the Precambrian (e.g. Karhu &

Epstein 1986), also increase CaCO, supersaturation, as well as precipitation rates, as discussed below.

The equation for carbonate equilibrium is well established (see standard geochemistry texts, e.g. Krauskopf 1979 and Morse & MacKenzie 1990):

CO, + H,O 11

CaCO, + H,CO, e Ca2+ + 2HCO; With increasingpC0, atmosphere, more CaCO, is dissolved to maintain the equilibrium. Extraction of CO2 from the system leads to CaCO, precipitation. Carbonate saturation in seawater is a very complicated business, but there appear to be three major controls: temperature, p C 0 2 atmos/aC02 Seawater and ionic strength of seawater.

Carbonate saturation is given by C2 = aCa2+ ~co:-

KSP

(a = activity, Ksp = solubility product, which itself varies with temperature and aC02seawater). If G is > 1 then the solution is supersaturated. A mineral can be precipitated from a solution as soon as it is saturated with respect to that mineral. However, in carbonate systems, CaCO, is frequently not precipitated immediately the solution becomes saturated, and so supersaturation is very common in natural waters. The supersaturation is maintained through kinetic effects.

The solubility of CaCO, decreases with increasing temperature, but it also decreases as a function of the lower solubility of CO, in warmer water. An increase in temperature will lower the solubility product and so lead to increased saturation. CaCO, precipitation may take place to lower the saturation. An increase in the amount of CO, of the atmosphere, as through increased volcanism, will cause CaCO, dissolution in the oceans and so increase the carbonate saturation. The degree of supersaturation is also strongly affected by ionic strength and is lowered by a high concentration of electrolytes in the solution, which reduces the activity of the ions. More saline seawater, with its higher ionic strength, will have a lower saturation state for CaCO, compared to less saline seawater with the same total Ca2+ and CO:- ions. Diluting seawater, through glacial melting for example, will increase the saturation state.

Precumbriun dolomites Another feature of Precambrian carbonate sequences in general is that there appears to be a predominance of dolomite. This has frequently been noted in the literature (e.g. Chilingar 1956; Ronov 1964; Tucker 19826; Grotzinger 1989), although no systematic study of their abundance has been carried out along the lines of Given & Wilkinson (1987) for Phanerozoic dolomites. Nevertheless, at least in this author's experience, dolomites genuinely are more common than limestones in the Precambrian. Precambrian dolomites show several features which appear to distinguish them from Phanerozoic dolomites.

(1) Dolomite facies are predominantly shallow subtidal and intertidal (e.g. Fairchild 1980; Fairchild & Spiro 1987; Fairchild e[ al. 1990; Grotzinger 1989; Tucker 1977, 19836). This contrasts with Precambrian limestones which are commonly of deeper-water facies (e.g. Tucker 1983a, 1986; Peryt er al. 1990; Herrington & Fairchild 1988), though shallow-water facies occur too (e.g. Tucker 1985).


(2) In many cases, shallow-water carbonate platforms are completely dolomitized.

(3) Many Precambrian dolomites have good fabric preservation of grains and marine cements (e.g. Figs 3A, 3B; Fairchild & Spiro 1987; Tucker 19826, 19836).

(4) Precambrian dolomites have primary dolomite spar cements, precipitated in syn-sedimentary or early diagenetic cavities, in a manner exactly analogous to calcite spar in Phan- erozoic limestones (e.g. Fig. 3A and B; Tucker 19836).

( 5 ) Dolomitization in most cases was a very early diagenetic process, if not synsedimentary (e.g. Tucker 19836; Zempolich er al. 1988).

Much has been written about dolomitization in general and Precambrian dolomites in particular; see Tucker & Wright (1990) for a recent review of both topics, The consensus view is against primary precipitation of dolomite on the seafloor at any time in geological history because of the kinetic obstacles to direct precipitation. However, dolomitization on the seafloor and just below by seawater could have been a major process, especially in the Precambrian. Three factors which promote

3

Fig. 3. Precambrian dolomites with well-preserved primacy textures. (A) Two ooids with strong radial fabric and aggregate-grain nuclei set in a drusy dolomite spar cement. Porsanger Dolomite, upper Riphean, Arctic Norway. (B) Peloidal texture and calcified cyanobacterial filaments in a drusy dolomite spar cement; from a planar stromatolite. Beck Spring Dolomite, upper Riphean, California.

dolomitization of carbonate sediments are: increased MgKa ratio, higher aCO, seawater and lower sulphate levels. Current thinking on seawater chemistry suggests that the M d C a ratio was high during the Precambrian, but decreased towards the end of the Precambrian as plate tectonic processes became important and Ca2+ levels increased (see next section and Kaz- mierczak et al. 1985). Higher aCOa in seawater has long been suggested for the Precambrian when compared with the Phan- erozoic in general, linked to the higherpC0, of the atmosphere at the time (previous section). The sulphate content of seawater may well have been lower in the Precambrian judging from the paucity of evaporite deposits (Grotzinger 1989), although there is the problem of preservation potential with such strata. Thus all three major factors promoting dolomitization by seawater would appear to be favourable during Precambrian time, and could account for the increased abundance of dolomites and their other features noted above.

One scenario to explain the predominance of shallow-water carbonates being dolomitized is that of the CO, interchange between the atmosphere and seawater. The diffusion of CO, from the atmosphere into the oceans is only effective down to depths of 5&75 m (e.g. Berner & Berner 1987). The constant exchange of CO, between the two could account for seawater dolomitization taking place chiefly in shallow seas. The lack of such exchange in deeper waters would allow limestones to be deposited there. Higher seawater temperatures have been postulated by many people for the Precambrian (e.g. Karhu & Epstein 1986) and if correct these would facilitate dolomite precipitation.

In summary then, the predominance and nature of dolomites in the Precambrian over limestones could be the result of higher pCOz in the atmosphere and higher M d C a ratio of seawater, perhaps coupled with higher temperatures and lower seawater sulphate. Higher pC0, and higher temperatures would also account for the postulated higher carbonate supersaturation state of Precambrian seawater and increased marine carbonate precipitation.

Changes in primary mineralogy of marine carbonate precipitates in boundary strata

Studies of the textural preservation of ooids and marine cements in limestones and dolomites allow their original mineralogy to be deduced (see Sandberg 1983, 1985; Tucker 1991 and Tucker & Wright 1990 for details and reviews). Aragonitic ooids are either calcitized, so that the ooids now consist of coarse neomorphic calcite spar crystals with only remnants of their original concentric or radial structure, or they are dissolved out and the voids filled with drusy calcite spar or left empty. Calcitic ooids, on the other hand, are normally well preserved, with radial or radial-concentric structures clearly present. Calcitic ooids and cements may have had low (W mole% MgCO,) or high (1 1-19 mole% MgCO,) magnesium contents. However, during diagenesis the Mg2+ content of high Mg-calcite is lost or drastically reduced to only a few mole% MgCO,; thus the original magnesium content of ancient calcitic ooids and cements is very difficult, and often imposs- ible, to determine. An original high Mg-calcite mineralogy can be suggested from the presence of microdolomite crystals, a slightly higher MgCO, content of the calcite (2-5 mole%) relative to grains and/or cements of originally low Mg-calcite, or some partial replacement and recrystallization. Calcite with 11-13 mole% MgCO, has a similar stability in seawater to aragonite, so that they could occur together (Walter & Morse

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1984). Higher Mg contents render calcite more soluble. One other type of ooid texture is micritic. This could be a primary fabric, the result of microbial alteration, or the product of diagenetic alteration (e.g. degrading neomorphism). It is normally not possible to deduce the original mineralogy of micritic ooids from textural evidence. The fabric of an ooid is a reflection of both mineralogy and environment. Aragonitic ooids have a concentric structure in high-energy environments and a radial structure in low energy settings (e.g. Bahamian ooids are concentric, whereas Great Salt Lake ooids are radial). Calcitic ooids are mostly radial in structure; larger calcitic ooids develop a radial-concentric structure.

There is abundant evidence for aragonite being the dominant precipitate in the late Precambrian, especially the Vendian, and calcite the more common precipitate in the early Cam- brian. Formerly aragonite ooids and cements are recorded from the Vendian of southern Norway (e.g. Fig. 4; Tucker 1985); from upper Precambrian-lowest Cambrian strata (the Serie Lie de Vin) of the Anti-Atlas Mountains, Morocco (Tucker unpub.); from the Vendian Islay Limestone of western Scotland (Tucker pers. observ.); from the Vendian Virgin Spring Limestone in the Kingston Peak Formation, Death Val- ley, California (Tucker 1986); from the lower Vendian Trezona Formation (Fig. 5A; Singh 1987; Tucker 1989) and lower to middle Vendian Wonoka Formation (Fig. 5B) of the Flinders Ranges of South Australia (Tucker 1989). Formerly aragonitic, high Mg-calcite and bimineralic ooids are promi- nent in the upper Riphean-lower Vendian part of the Belt Supergroup in Montana (Tucker 1984).

By way of contrast, early Cambrian ooids were most commonly composed of calcite (as they still are), and this probably had a low to moderate magnesium content. The change from aragonite-dominated to calcite-dominated oolites is well shown in the South Australian sequence of the Flinders Ranges, where ooids in the Atdabanian and younger Hawker Group limestones were clearly originally calcite, with well- preserved radial structure (Fig. 5E and F), contrasting greatly with those from the upper Precambrian (Tucker 1989). Also of note is the widespread occurrence of fibrous calcite as a marine cement in the lower Cambrian (present in Fig. 11C). It is particularly common in the archaeocyathan reefs which are present in many lower Cambrian sequences (e.g. James & Gravestock 1990; James & Klappa 1983).

In their review of ooid mineralogy through the Phanerozoic, Wilkinson et al. (1985) recorded solely calcitic ooids in the lower Cambrian (although there are only 3 data-points). The later Cambrianxarly Ordovician was one of the peaks of ooid precipitation in the Phanerozoic, and it appears from their data that about half of the oolites had aragonitic ooids and half had calcitic ooids (in 35 upper Cambrian formations studied). Some originally aragonitic ooids and cements do occur in lower Cambrian strata, but they are not as abundant as the marine calcite precipitates. Examples occur in the Atdabanian SCrie Schisto-Calcaire of Morocco (Tucker pers. observ.), Atdab- anian-Botomian archaeocyathan reefs of Labrador (James & Klappa 1983) and in the Parachilna Formation (Tommotian, Fig. 5C) of the Flinders Ranges of South Australia (Tucker 1989). Also in the Parachilna Formation are two-phase, bimineralic ooids (Fig. 5D), with calcitic inner parts and formerly aragonitic outer parts (now calcite). Similar ooids occur in the Serie Lie de Vin of Morocco.

In modern tropical seas, all ooids are composed of aragonite, and marine cements in reefs and carbonate sands comprise aragonite and high Mg-calcite. These two minerals

Fig. 4. Former aragonite ooids from the Biri Formation, Vendian, Southern Norway. (A) Calcite crystals with a brick texture produced through calcitization of an aragonite ooid. (B) Coarse, neomorphic calcite crystals replace original aragonite ooid.

have apparently been the marine abiotic precipitates since the early Tertiary, and they were also precipitated on the seafloor during the Permo-Triassic. Both are periods of relatively low sea-level. During the Jurassic-Cretaceous and mid- Palaeozoic, on the other hand, calcite, probably with a low magnesium content, was the dominant marine precipitate (see Sandberg 1983, 1985; Wilkinson et al. 1985). These were times of relatively high sea-level on the first-order global sea-level curve. It is generally believed that high aCO, seawater and/or a low Mg/Ca ratio favours the precipitation of low Mg-calcite, whereas low aCO, seawater and/or a high Mg/Ca ratio favours aragonite and high Mg-calcite precipitation. These two factors vary with seafloor spreading rates, which control the first-order sea-level curve, so that a geotectonic explanation has been put forward to account for the secular variation in mineralogy of marine precipitates (Wilkinson et al. 1985). At times of relatively high seafloor spreading rates, increased volcanism and subduction zone metamorphism serve to raise the pC02 of the atmosphere. Associated with this, the increased pumping of seawater through mid-ocean ridge basalts extracts Mg2+ in their low-grade metamorphism and releases Ca2+ so that the Mg/Ca ratio decreases. Thus calcite (low Mg) precipitation is


Fig. 5. Changes in texture of ooids from the upper Precambrian into the Cambrian, Flinders Ranges, South Australia. (A) Calcitized ooids from the Trezona Formation, lower Vendian. Ooids now composed of brown, pseudopleochroic calcite. Cement was originally aragonite too. (B) Calcitized ooids from the Wonoka Formation, upper Vendian. Coarse calcite crystals have replaced originally aragonitic ooids. (C) Dolomitized aragonitic ooids from the Parachilna Formation, Tommotian. Outer lamellae of ooids have spalled off indicating an original concentric structure and aragonitic mineralogy. (D) Dolomitized bimineralic ooids from the Parachilna Formation. The inner part was originally calcitic and the outer part aragonitic. (E) Primary calcitic ooids from the Wilkawillina Limestone, Hawker Group, Atdabdnian. The ooids have a strong radial fabric and perfect preservation. (F) Scanning electron microscope view of ooid from Wilkawillina showing original radial structure.

favoured at these times. When seafloor spreading rates slow high Mg-calcite precipitation when the appropriate thresholds down, these factors move in the other direction, pCOz falls and are crossed. the Mg/Ca ratio rises, to permit the change to aragonite and The controls are less certain on whether aragonite o r high

662 M . E. TUCKER

Mg-calcite is precipitated. It does seem though that higher Mg2+, higher SO:-, lower PO:-, a higher temperature and a higher carbonate supply rate all favour the precipitation of aragonite over high Mg-calcite (Burton & Walter 1987; Given & Wilkinson 1985; Mucci & Morse 1983; Railsback & Ander- son 1987; Walter 1986). Thus, if the evidence from ooid and marine cement occurrences is accepted, the change from aragonite-dominated seas in the late Precambrian to calcite- dominated seas in the early Cambrian is a reflection of increasing p C 0 , of the atmosphere and decreasing Mg/Ca ratio of seawater across the boundary.

Global environmental changes across the Precambrian-Cambrian boundary

The evidence from Proterozoic carbonates as a whole, outlined above, suggests that Precambrian seawater had a higher Mg/Ca ratio, was more saturated with respect to CaCO, and had a higher aCO, than Phanerozoic seawater in general. In addition, Precambrian seawater SO:- may have been lower and temperatures higher compared with the Phanerozoic. However, across the boundary itself, an increase in aCO, seawater, ac- companied by a decrease in the Mg/Ca ratio, can account for the change in mineralogy of marine carbonate precipitates. The Vendian may well have had lower temperatures, as a result of the glaciations, compared with the rest of the Proterozoic and the Cambrian. The Vendian to early Cambrian events should now be placed in the context of global environmental changes. There are two aspects here, global climatic, i.e. the change from Vendian icehouse to Cambrian greenhouse, and global tectonic, i.e. the opening of new oceans and seaways. Additional factors are introduced into the story, notably oceanic circulation patterns, carbon isotopes, phosphorites and nutrient levels. Although the Earth’s climate is linked to plate tectonics (e.g. Veevers 1990), they are discussed as separ- ate issues here. Indeed, it is quite possible that the late Precam- brian climate was divorced from plate tectonic processes and controlled more by changes in the Earth’s atmosphere or rotation (e.g. Williams 1989).

Lute Precumbriun icehouse-coldhouse to Cumbriun greenhouse The Varangian glaciation in early to mid-Vendian time must have had a profound effect on the oceans. A model for the oceans during an icehouse or coldhouse period is depicted in Fig. 6. ‘Icehouse’ refers to a period of extensive polar icecaps; a coldhouse is a time of no polar ice-caps, but high latitude temperatures are below 5°C. Global sea-level is low during icehouse times since much water is locked up in polar ice-caps, and may be as much as 100 metres below present level. During coldhouse times, sea-level will be a few metres below that of greenhouse times (see below). The changes in sea-level between coldhouse and greenhouse are simply the response of ocean volume to temperature changes.

During icehouse and coldhouse times, there is vigorous circulation within the oceans. This is largely the result of the temperature differences between the equator and the poles, and the formation of cold, dense water at the poles which sinks and moves at depth towards lower latitudes. Apparently, a temperature of less than 5°C in high latitude seas is required for this circulation to be established (Wilde & Berry 1986); hence the concept of coldhouse which is oceanographically similar to

ICEHOUSE/COLDHOUSE:circulation,nutrients,chemistry

high latitude 5%. low sea-level stand

c02 c02 nut r ien ts

increased run-off

cold, pC02-rich OXIC water

vigorous oceanic circulation from ventilation by cold polar

Increased C a C 0 3 dissolution, decreased C a C 0 3 saturation waters, decreased pc02 atmosphere, increased a C 0 2 seawater,

Fig. 6. Model for ocean circulation, chemistry and nutrient levels during an icehousexoldhouse episode.

the icehouse condition, although there are no polar ice-caps. In an icehouse-coldhouse episode, there is a temperature stratification of the oceans, and deep waters are oxic and rich in carbon dioxide. The lower global temperature leads to increased solubility of COz so that a transfer is set up of CO2 from the atmosphere to the oceans. Also, the carbonate compensation depth (CCD) rises, in part the result of the CO,-rich deeper waters. The important consequence of vigorous oceanic circulation is that the supply of nutrients (phosphate and nitrogen) to continental margins is high. Intense plankton productivity can be expected, and phosphorites may be deposited. Nutrients will also be supplied from continental run-off, which will have increased as a result of the lower sea-level stand.

In a greenhouse state (see Fig. 7), the climate is more equ- able generally and temperatures at high latitudes will be relatively high so that cold, dense waters are not produced and oceanic circulation is drastically reduced. A salinity stratification will develop within the oceans, and warm, saline, dysaerobic or anoxic waters will be established in deep basins where organic-rich sediments may be deposited. The warmer global climate will lead to CO2 loss from the oceans to the atmosphere as CO, solubility goes down. ThispCO, increase in the atmosphere will contribute to the global warming. The CCD will move to deeper levels in the oceans. Nutrient supply will be reduced as circulation of deep ocean waters is limited. On shallow-water shelves, generated by the relative sea-level rise, carbonates will be deposited in abundance as higher temperatures promote marine precipitation.

GREENHOUSE: circulation, nutrients, chemistry

high sea-level Stand c02 c02 f ~~~~

t CCD 4- s_ali_nily_ ~ ~

stratification low nu t r i en t avallability

warm, saline anoxic water

poor oceanlc clrculation, stagnant basins, increased pc02 atmosphere. decreased aC02 seawater Increased C a C 0 3 Saturation and precipltation

Fig. 7. Model for ocean circulation, chemistry and nutrient levels during a greenhouse episode.


It has been suggested that the Earth fluctuates from icehouse to greenhouse states on a timescale of the order of 100 Ma. In the Phanerozoic, greenhouse conditions existed in the mid- Palaeozoic and Mesozoic, occurring at times of relative sea- level highstand on the first order global sea-level curve (see Fischer 1980 and Veevers 1990). There are shorter time-scale climatic fluctuations of course, notably within icehouse times and involving Milankovitch cyclicities. During greenhouse episodes, similar changes must occur, but without polar ice- caps to wax and wane, the effects on relative sea-level and sedimentation will obviously be reduced. Jeppsson (1990) has used somewhat analogous models to the greenhouse-coldhouse models outlined here to account for 1-3 Ma sedimentary cycles in the Silurian. An important point to make here is that the icehouse-type conditions will continue after the melting of all ice-caps, until the high latitude temperatures have exceeded 5 "C, when greenhouse-type conditions will take over, the tran- sitional period being the coldhouse.

With the transition from icehousexoldhouse in the late Precambrian (Vendian) to greenhouse in the early Cambrian, various changes can be predicted. The carbon dioxide content of seawater will decrease, as CO, leaves the oceans for the atmosphere. The saturation state of seawater will increase and carbonate precipitation will be more extensive in shallow- water as global temperature rises. Relative sea-level will rise, rapidly at first through ice-cap melting, but continuing after the ice has gone through expansion of ocean water with rising temperature. The availability of nutrients will decrease as circulation within the oceans becomes more sluggish. Much organic matter may be preserved in sediments, on the basin floor and on slopes in the expanded oxygen minimum zone. In the early stages of deglaciation, when circulation is still vigorous but the ocean waters are warming up, plankton productivity will be high and phosphorites will be deposited.

There will be some lowering of salinity with change of states; the release of melt-waters will lower ocean salinity by a few parts per thousand. If the late Precambrian glaciations were low latitude, this would have made salinity changes much more significant in tropical waters where most carbonate sedimentation takes place. A decrease in ionic strength would have raised carbonate saturation levels.

Lute Precumbriun to Cumbriun plate tectonic activity As noted above, the Precambrian-Cambrian boundary time was one of continental extension, rifting and ocean formation. Increased rates of seafloor spreading and subduction have various consequences for circulation patterns within the oceans and for seawater chemistry. These are summarized in the model presented in Fig. 8. Opening oceans result in new circulation patterns being established, and this could lead to increased nutrient availability. Upwelling currents will supply phosphate-rich and nitrate-rich waters to shelves and slopes, and plankton productivity will increase in surface waters. Phos- phorites may be deposited on outer continental shelves and upper slopes. Relative sea-level will rise through the decrease in the ocean-basin volumes as mid-ocean ridge systems are established. The staggered opening of oceans during late Pre- cambrian-Cambrian time resulted in a long period of sea-level rise, with several major transgressions, until relative sea-level began to fall in the late early Cambrian. Increased plate tectonic activity also results in an increase in pC0 , atmosphere and K O , seawater through increased volcanism and subduction zone metamorphism. This will in turn lead to global

OPENING OCEANS: circulation, nutrients, chemistry

major transgressions c02

b A

circulation carbonates

phosphorites

_ _

spreading seafloor

increased pc02 atmosphere, increased aC02 seawater decreased MglCa ratio, CaC03 precipitation

Fig. 8. Model for ocean circulation, chemistry and nutrient levels during an episode of opening oceans and increased plate tectonic activity.

warming through a greenhouse effect. One important difference between the opening-ocean model (Fig. 8) and the icehouse-coldhouse and greenhouse models (Figs 6 and 7), is that there is a change in the Mg/Ca ratio of seawater as a result of increased hydrothermal activity at mid-ocean ridges. An increase in Ca2+ and decrease in Mg2+ leads to a decrease in the MgiCa ratio as seafloor spreading rates increase. Once all major oceans have opened and continents are drifting, then a steady state situation is established and the more climatically-driven, icehouse-greenhouse effects begin to exert their influence more strongly.

Thus, from a consideration of the global environmental changes taking place in the late Precambrian to Cambrian time (see Fig. 9), it can be postulated that:

1

2

3

4

5

6

7

8

9

10

total CO, will have increased from an already high level as a result of increased seafloor spreading rates; pC0 , atmosphere will have increased from the transfer of CO, from the oceans to the atmosphere as a result of the icehousexoldhouse to greenhouse change; global temperature will have risen as a result of the icehouse-coldhouse to greenhouse change; carbonate saturation will have increased from an already high level because of ( l ) , (2) and (3) above; the Mg/Ca ratio of seawater will have decreased through increased seafloor spreading rates; circulation within the oceans will have been vigorous during and soon after the Varangian glaciation and during the phases of opening oceans; nutrient levels will have increased during the times of increased circulation, when plankton productivity will have increased too and phosphorites could be deposited; nutrient levels would have declined as greenhouse effects took over and the opening oceans reached a steady state; major transgressions will have occurred through deglaciation and increased rates of seafloor spreading and; CaCO, precipitation would have been favoured in the newly-created, warm shallow seas.

Fitting in with these points is the information discussed above from the carbonate rocks: the high saturation state of Precambrian seawater permitting extensive precipitation of CaCO,, the change in nature of dolomitization from the Pre- cambrian into the Phanerozoic, and the change in the mineralogy of marine carbonate precipitates in boundary strata.

664 M . E . TUCKER

Also consistent with the models is the preferential development of phosphorites in uppermost Precambrian-Cambrian strata (Cook this volume; Cook & Shergold 1984, 1986).

The carbon isotope story In the last few years considerable interest has been attached to the carbon isotope stratigraphy of Precambrian-Cambrian boundary strata. Isotope excursions have been detected in carbonates in several boundary sections, and have been used as an aid to correlation (e.g. Brasier et al. 1990). Correlation is particularly difficult in strata near the Precambrian-Cambrian boundary because of problems with biostratigraphy and hiatuses in the sequences. Carbon isotopes of carbonates have also been used to give an indication of changes in oceanographic conditions (e.g. Tucker 1989), in terms of variations in organic productivity and biomass, and organic matter burial rates.

The carbon isotopic composition of modern seawater is around 0x00, and shallow-marine carbonate sediments have values that range from 0 to +4%oPDB as a result of vital effects of organisms and local and regional variations in 6I3CTDc seawater (TDC = total dissolved carbonate). Organic carbon has very negative &"C, in the range of -20 to-30%0, since the lighter "C is preferentially incorporated over I3C. Changes in oceanic 6'3CT,c in time and space are mostly attributed to variations in the carbon output to the organic carbon (Corg) and carbonate carbon (Ccarb) reservoirs (MacKenzie & Piggott 198 l ; Anderson & Arthur 1983). The output ratio, Corg/Ccarb, in modern oceans of about 1 :4 maintains the steady-state seawater 6"CTDC ofO%o. There are several ways in which this ratio can be altered to give isotopic excursions and trends, and they mostly relate to the organic carbon reservoir. Two processes can lead to positive isotopic excursions: (1) increased burial of organic matter (the result of increased stagnation in the basin and poor circulation) and (2) increased organic productivity (the result of increased nutrient supply and good circulation) and an increase in biomass. These processes preferentially extract the light ''C isotope out of seawater, so leaving seawater and the carbonate precipitates therefrom a little heavier. More negative carbonate values will be given by the above processes moving in the other direction, and by increased recycling of organic matter, brought about by good circulation in a basin and by major regression and reworking of organic carbon deposited on shallow shelves. Two other processes can affect

Fig. 9. Postulated schematic trends in eustatic sea-level, CO, in the atmosphere, magnesium and calcium in seawater, ocean nutrient levels and temperature.

613CTD,seawater. Long-term high rates of calcium carbonate precipitation will lead to a more negative value of the seawater reservoir, as will increased volcanism, supplying I3C-depleted CO,. However, neither of these processes can lead to very negative 613C signatures in carbonate precipitates.

Carbon isotope profiles are now available from at least nine boundary sections in Siberia, Iran, India, China, Morocco and South Australia (see Brasier et al. 1990 for references). The trend comprises low negative to low positive values in upper Vendian strata, a positive excursion close to the boundary in Nemakit-Daldynian and Tommotian strata, a swing back to more negative-less positive values in the Tommotian, and then a general upward trend to more positive values in the Atdab- anian (see Fig. 10).

As discussed above, there was a change from icehouse- coldhouse to greenhouse from the Precambrian into the Cam- brian and an increased rate of seafloor spreading with opening oceans. In terms of organic matter production, oxidation and burial, the processes which can influence carbon isotope excursions and trends most easily, a change from icehouse-cold-

i3c -," O%o +5

phosphorite occurrence

Fig. 10. Generalized trend in 6I3C and abundance of phosphorites in upper Precambrian-lower Cambrian strata (the latter after Cook, 1992). Abbreviations of stages: N-D, Nemakit-Daldynian; Tom, Tommotian; Atd, Atdabanian; Bot, Botomian.

C A R B O N A T E S A N D E N V I R O N M E N T A L C H A N G E 665

house to greenhouse is likely to have resulted in increased organic matter burial and decreased recycling of organic matter. Both of these would lead to a more positive trend. In- creased precipitation of carbonate under the warmer climate may lead to a more negative trend, but this probably would not be a major effect. Decreased organic productivity from icehouse to greenhouse may also have led to a more negative trend, but this could have been counter-balanced by the decreased recycling. With the opening ocean model, the increased organic productivity through increased circulation is likely to have led to more positive values; recycling of organic matter would lead to a negative trend but this may well have been short-lived, while a steady situation was being attained. Increased volcanism could have had a small effect, making 6'3C,,c seawater a little lighter. One further factor with this particular time interval is the possibility of an increase in biomass. With the incom- ing of new organisms and the rapid availability of shallow shelf seas, some increase is very likely to have occurred, leading to positive excursions.

Thus, returning to the generalized isotope profile (Fig. lO), the positive excursion that appears to be characteristic of the boundary strata fits in with both oceanic models, i.e. it is consistent with increased organic matter burial, as postulated by the greenhouse model, and with increased circulation, nutrient supply and organic productivity, as suggested by the opening ocean model. It is also consistent with the notion of an increase in biomass.

The predominance of phosphorites in this late Nemakit- Daldynian through Tommotian interval (see Fig. 10; Cook this volume; Cook & Shergold 1986) coincides with the positive carbon isotope excursion. Phosphorites are commonly associated with high organic productivity and vigorous circulation. This would tend to favour the opening ocean model as the major control at this time. A similar correlation of phosphogenesis and positive Si3C occurs in the Miocene (Compton et al. 1990). The return to less positive, more negative values after the boundary time could be the result of a decrease in organic productivity as nutrients (phosphates and nitrates) were used up and oceanic circulation patterns settled down. The gradual trend to more positive values in the Atdabanian appears to coincide with the extensive development of carbonate platforms as the Cambrian transgression continued. The isotopic trend could reflect a gradual change to increased burial of organic matter in the shallow-water limestones, rather than in deep-water sediments. An increase in biomass is again a possibility, perhaps more likely now, with the extensive shallow seas and most of the metazoan groups already evolved and now proliferating.

Short-lived excursions do occur within some isotope profiles but are absent in others. These could represent regional or intrabasinal events such as local periods of anoxia. One such sharp positive anomaly occurs within the South Australian profile (Tucker 1989).

The scenario It is now possible to bring together the various factors that have been discussed in this paper concerning global environmental changes from the late Precambrian into the Cambrian (see Fig. 9) and suggest how they may have been instrumental in the evolution of the metazoan groups and the development of biomineralization.

First, with the evolution of new groups, the main environmental factors are the development of extensive shallow seas,

global temperature rise and variations in nutrient supply. The melting of ice-caps and floating ice-shelves in the mid-Vendian and the opening of new oceans in the late Vendian and into the Cambrian resulted in the flooding of continental margins. The new niches and habitats provided here, plus the global warming, could well have induced new metazoan groups to evolve. Brasier (1982) has discussed the evolutionary succession of metazoan groups against the background of major transgression and development of new niches. The increase in ambient seawater temperature also encourages organisms to diversify and radiate,

To catalyse the evolution of new groups, nutrient supply is a major factor. Nutrient levels have a major influence on organisms; where they are high, plankton, cyanobacteria and bacteria are produced in vast numbers. This inhibits the growth and development of many shallow-water metazoans through the generation of dysaerobic and anoxic conditions on the seafloor (Hallock & Schlager 1986). Vigorous circulation within the oceans, as would have occurred during icehouse- coldhouse times (Fig. 6) and during the opening of new oceans, would have brought abundant supplies of nutrients into shallow waters through upwelIing. The rapid evolution of microplankton during the early Vendian could have been a response to high nutrient levels in the oceans during this icehouse-coldhouse time. The subsequent decline of the plankton could reflect general oceanic stagnation and loss of nutrients. The specialized soft-bodied Ediacara fauna may have evolved in response to the reduced nutrient levels in shallow silici- clastic seas. The radiation of the organisms of the small shelly fossils in the late Vendian could have been a response to the re-establishment of high nutrient levels, as circulation patterns were established following continental break-up. This would coincide with the major phosphorite occurrence and positive carbon isotope excursion. A return to lower nutrient levels in the early Cambrian, reflected in the decline in phosphorites and the return to more normal marine carbon isotope signatures, would be around the time of the evolution of many other metazoan groups, including many new benthic forms such as the archaeocyathans, brachiopods, trilobites and echinoderms. The warmer shallow-waters would have been conducive to this. By now of course, organisms had developed the ability to secrete skeletons.

The development of biomineralization in the latest Vendian apparently began with phosphatic skeletons, as possessed by many small shelly fossils, and this was followed in the Tommotian and Atdabanian by the development of calcareous skeletons (Lowenstam & Margulis 1980; Brasier 1986). How- ever, it appears that many of the phosphatic fossils were phos- phatized diagenetically, and that their shells were originally calcareous (Runnegar 1985; Brasier 1990). It is interesting to note that aragonite is more easily replaced by phosphate than calcite. Brasier (1990), however, has shown that some phosphatic shells are original. With the calcareous shells, it appears that the first to evolve in the latest Nemakit- Daldynian and Tommotian had aragonitic shells, the molluscs and hyolithids for example (e.g. Fig. l IA), whereas those evolving a little later in the Tommotian and Atdabanian, such as the echinoderms, brachiopods and trilobites, were calcitic (Fig. 11B and C). As discussed above, there appears to be a change from aragonite-dominated precipitation in the Vendian to calcite-dominated precipitation in the early Cambrian. The pattern in the calcareous fossils seems to be the same, especially if some of the Nemakit-Daldynian small shelly fossils were originally aragonitic before phosphatization. The occur-

666 M. E. TUCKER

Fig. 11. Skeletal elements of the lower Cambrian. (A) Small conical fossils preserved as coarse calcite spar. Shells originally of aragonite. Lowest Cambrian, Anti-Atlas, Morocco. (B) Archaeocyathan, with skeleton composed of micritic calcite. Originally microcrystalline calcite, probably high magnesium. Lowest Cambrian, Anti-Atlas, Morocco. (C) Trilobites, originally (and still) calcitic shells, encrusted with fibrous calcite. Atdabanian, Flinders Ranges, South Australia.

rence of primary phosphatic skeletons in the Nemakit- Daldynian could well be a response to the high levels of phosphate in the oceans at this time, as shown by the phosphorite peak. The change from aragonite to calcite skeletons could be following the changes in seawater chemistry discussed above (increasing CO, atmosphere and decreasing Mg/Ca ratio seawater), resulting from the increased rate of seafloor spreading at this time. It seems that once organisms have evolved the ability and apparatus to secrete skeletons of a particular mineralogy (aragonite or calcite), then this mineralogy does not change, even though seawater chemistry may subsequently change and the other mineral becomes the preferred marine precipitate (as occurred with corals, see Railsback & Anderson 1987).

As to why organisms should develop skeletons in the first place, biological reasons like predation, cropping pressures and scavenging have all been suggested. However, there could again be environmental reasons. Kazmierczak et al. (1985) have discussed the role of Ca2+ in metazoan metabolism and shown that it has a deleterious effect. They have suggested that a build-up of calcium through the Precambrian eventually forced many organisms to precipitate skeletons as a form of cell detoxification. One would certainly expect the Ca2+ content of the oceans to be increasing in the late Precambrian and early Cambrian as seafloor spreading rates increased. Further- more, carbonate saturation was already high during the Pre- cambrian, but could have increased further in the latest Pre- cambrian as a result of global warming and the increase in pC0, from the operation of the greenhouse and opening ocean models presented herein. The fact that the cyanobacteria began to calcify heavily in this Nemakit-Daldynian to Tommotian time (Riding 1982) would certainly strongly support an environmental rather than a biological cause for biomineralization.

Conclusions (1) The Vendian-Cambrian radiation took place over some

2 0 4 0 Ma and five episodes are distinguished. (2) The nature of Precambrian carbonate facies indicates

that seawater was highly saturated with respect to CaCO, and that precipitation was relatively easy compared with Phan- erozoic times.

(3) The nature of Precambrian dolomites, indicating very early and extensive dolomitization of shallow-water carbonates, suggests that seawater had a high aCO, and high MgiCa ratio.

(4) There is a change in the original mineralogy of marine precipitates with aragonite dominating in the Vendian and calcite in the lower Cambrian. This reflects a change in seawater chemistry in boundary strata with increasing aCO, and decreasing Mg/Ca ratio.

(5) Models for ocean circulation, chemistry, temperature and nutrient supply have been put forward for icehouse- coldhouse, greenhouse and opening-ocean episodes, and the predictions from the models are consistent with the geological evidence for changes in seawater chemistry and nutrient

(6) The carbon isotope record in boundary strata has excursions and trends which can be reconciled with the geological evidence and oceanic models.

( 7 ) The Precambrian-Cambrian boundary events in the biosphere, with the spaced evolution of new metazoan groups and the development of biomineralization can be explained as

supply.


a response to major changes in the environment. The most important are changes in temperature, seawater nutrient levels, pC0 , seawater-atmosphere and MgiCa ratio seawater, the result of deglaciation, the transition from an icehouse to a greenhouse and the increased rates of seafloor spreading due to the opening of new oceans and seaways leading to the development of extensive shallow seas.

Much of the work of the author discussed and reviewed here has been funded over the years by grants from the Natural Environment Re- search Council (NERC). These are gratefully acknowledged. Thanks are extended to 1. J. Fairchild, D. M. Hirst, J. A. Pearce, A. Peckett, M. D. Brasier and H. A. Armstrong for discussions on seawater chemistry and boundary problems. I am grateful to J. D. Hudson, H. C. Jenkyns, H. A. Armstrong and S. Molyneux for comments on the manuscript. Figures were drafted by K. Atkinson and photography was by G. Dresser and A. Carr.

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Received 18 March 1991; revised typescript accepted 6 January 1992.

Documents

The Precambrian–Cambrian boundary: seawater chemistry, ocean circulation and nutrient supply in metazoan evolution, extinction and biomineralization