G.M. Narbonne, S. Xiao and G.A. Shields, Chapter 18 With contributions by J.G. Gehling
The Ediacaran Period
Abstract: The Ediacaran System and Period was ratified in a pivotal position in the history of life, between the micro- 2004, the first period-level addition to the geologic time scale scopic, largely prokaryotic assemblages that had dominated in more than a century. The GSSP at the base of the Ediacaran the classic “Precambrian” and the large, complex, and marks the end of the Marinoan glaciation, the last of the truly commonly shelly animals that dominated the Cambrian and massive global glaciations that had wracked the middle younger Phanerozoic periods. Diverse large spiny acritarchs Neoproterozoic world, and can be further recognized world- and simple animal embryos occur immediately above the wide by perturbations in C-isotopes and the occurrence of base of the Ediacaran and range through at least the lower half a unique “cap carbonate” precipitated as a consequence of of the Ediacaran. The mid-Ediacaran Gaskiers glaciation this glaciation. At least three extremely negative isotope (584e582 Ma) was immediately followed by the appearance excursions and steeply rising seawater 87Sr/86Sr values of the Avalon assemblage of the largely soft-bodied Ediacara characterize the Ediacaran Period along with geochemical biota (579 Ma). The earliest abundant bilaterian burrows and evidence for increasing oxygenation of the deep ocean envi- impressions (555 Ma) and calcified animals (550 Ma) appear ronment. The Ediacaran Period (635e541 Ma) marks towards the end of the Ediacaran Period. 600 Ma Ediacaran
Tarim South China North Australia China Siberia Greater India Kalahari Seychelles Continental rifts E. Madagascar Congo Sao Francisco Rio de Laurentia Collision zones Arabia la Plata Sahara Nubia West Africa Amazonia Avalonia Baltica 8 l. 200 Li et a modified from
Chapter Outline
18.1. Historical Background 414 18.3.4.2. Strontium Isotopes (Figure 18.5) 423 18.2. Cap Carbonates and the Base of the Ediacaran System 415 18.3.4.3. Sulfur Isotopes 424 18.3. The Biostratigraphic Basis for the Ediacaran Period 415 18.3.4.4. Redox Proxies (Figure 18.5) 424 18.3.1. Ediacaran Megafossils and Trace Fossils 415 18.3.5. Radiometric Dating 425 18.3.2. Ediacaran Microfossils 419 18.4. Towards an Ediacaran Chronostratigraphy 425 18.3.3. Ediacaran Glaciations 422 18.5. Ediacaran e Last Period of the Proterozoic or 18.3.4. Chemical Evolution of Ediacaran Oceans 422 First Period of the Phanerozoic? 429 18.3.4.1. Carbon Isotopes (Figure 18.5) 422 References 429
The Geologic Time Scale 2012. DOI: 10.1016/B978-0-444-59425-9.00018-4 413 Copyright Ó 2012 Felix M. Gradstein, James G. Ogg, Mark Schmitz and Gabi Ogg. Published by Elsevier B.V. All rights reserved. 414 The Geologic Time Scale 2012
18.1. HISTORICAL BACKGROUND before the Cambrian. Evidence that these glacial deposits consistently predated Ediacara-type fossils in the same Designation of the Ediacaran Period in 2004 reflected the sections led Harland and Herod (1975; see also Harland et al., gradual solution of a problem that had vexed even Charles 1989) to propose an Ediacaran Epoch (with Ediacara-type Darwin in his writing of The Origin of Species (1859), the megafossils) in the upper half of a Vendian Period above basal apparently abrupt appearance of diverse groups of shelly diamictites. Cloud and Glaessner (1982) proposed that the fossils at the base of the Cambrian System without any “Ediacarian” instead be defined as a period with its base at the obvious pre-Cambrian ancestors. Darwin attributed this top of the Marinoan tillites and their equivalents worldwide. absence to massive record failure, a view formalized by Neoproterozoic strata are characterized by major, appar- Walcott (1914) in his designation of the “Lipalian Interval” of ently synchronous isotopic excursions in carbon (Knoll et al., erosion beneath the base of the Cambrian. The concept of 1986; Kaufman and Knoll, 1995; Halverson et al., 2005, a global period of erosion prior to the Cambrian was soon 2010), strontium (Kaufman et al., 1993; Halverson et al., contradicted by discovery of thick successions of largely 2007, 2010), and sulfur (Halverson et al., 2010), that provide unmetamorphosed strata concordantly beneath the base of the a third criterion for recognition and correlation of a terminal Cambrian in numerous localities worldwide, most notably Neoproterozoic system worldwide. Neoproterozoic isotope the “Sinian” in China (Grabau, 1922), the “Marinoan” in excursions are not unique in either shape or magnitude, but Australia (Mawson and Sprigg, 1950), and the “Vendian” in in conjunction with biostratigraphy (Kaufman et al., 1991; Russia (Sokolov, 1952). These names were originally Narbonne et al., 1994), climatic indicators (Kaufman et al., proposed as regional lithostratigraphic units that later 1997), and precise U-Pb dates (Grotzinger et al., 1995), assumed a chronostratigraphic significance, but scarcity of provide an exceptionally useful tool for global correlation of reliable radiometric dates and a lack of consistent criteria for Neoproterozoic strata. correlation frustrated early attempts to extend these divisions The International Commission on Stratigraphy established globally. a Working Group (later Subcommission) on the Terminal The first and perhaps most distinctive criteria recognized Proterozoic Period at the IGC in Washington in 1989. Over were Ediacara-type fossil impressions. Ediacaran megafossils th the succeeding decade and a half, the Working Group/ were first described in the late 19 century (Billings, 1872) Subcommission formally visited terminal Neoproterozoic but few paleontologists at that time were willing to accept the sections on five continents and held a series of international widespread occurrence of megascopic pre-Cambrian life, and symposia to discuss the features that could be used to define even the complex Ediacaran fossils reported from Namibia and correlate a terminal Neoproterozoic system. These and Australia in the 1930s and 1940s were tentatively scientific discussions corresponded with rapid discoveries of regarded as “Cambrian” by their discoverers (Gehling et al., new scientific techniques in geochronology, isotope chemo- 2000). Ford’s (1958) description of the unequivocally pre- stratigraphy, sequence stratigraphy, and paleontology that Cambrian frond Charnia from central England led Glaessner significantly elucidated the characteristics and history of the (1959) to propose a global “Ediacara Fauna” of large, soft- Neoproterozoic Earth. After 10 years of investigation, a series bodied, animal-like fossils that immediately preceded the of ballots increasingly focused the decision on the position of Cambrian, a concept that exists to the present day and has the GSSP and name for this new period (see Knoll et al., 2006 proven instrumental in subsequent recognition of a terminal for details of the ballots and opinions). The first ballot Proterozoic system. A year later, Termier and Termier (1960) (December 2000) asked whether the stratigraphic level of the proposed the “Ediacarien” as a pre-Cambrian chronostrati- GSSP should be placed at: graphic interval characterized by this distinctive fossil assemblage. Discovery of abundant and diverse microfossils, 1) The base of the Varanger/Marinoan glacial deposits including probable embryos and resting cysts of early animals (Sokolov and Fedonkin, 1982); (Xiao et al., 1998; Cohen et al., 2009a,b), significantly 2) The cap carbonate atop these deposits (Cloud and enhanced the prospects for Ediacaran biostratigraphy Glaessner, 1982); (Vorob’eva et al., 2009a). 3) A biostratigraphic level corresponding to the first Climatic indicators also provide critical information appearance of Ediacaran macrofossils in a local section for Neoproterozoic subdivision. Late pre-Cambrian glacial (Jenkins, 1981); or deposits were first recognized in the late 1800s (Thomson, 4) Some other level. 1871; Reusch, 1891) with steadily increasing number of Most of the voting members believed that a boundary at the reports throughout the 20th century (Hoffman and Li, 2009). top rather than the base of the Varanger/Marinoan glacial In a landmark paper, Harland and Rudwick (1964) summa- deposits would be more significant in Earth evolution, since rized studies showing evidence of late pre-Cambrian glacial most Neoproterozoic glacial deposits would be regarded as deposits on every continent except Antarctica, and proposed Cryogenian, whereas all known assemblages of diverse the “Infra-Cambrian” as a period of continental glaciation Ediacara-type fossils would fall into the succeeding period. Chapter | 18 The Ediacaran Period 415
It was also likely that the end of a glacial interval marked by distinguished from the dark, bituminous limestones that char- a distinctive cap carbonate would probably be more easily acterize older post-Sturtian caps (Kennedy et al., 1998)and correlated than the onset of glaciation, as the latter would from the white micritic (recrystallized to sparry calcite) lime- likely be highly diachronous both regionally and globally. stone cap on the top of the younger (mid-Ediacaran) Gaskiers There was little support for placing the system-level boundary diamictite (Myrow and Kaufman, 1999). Interpretations of at the first appearance of megafossils, but it was considered Neoproterozoic cap carbonates vary considerably, but most that this level might prove useful in later subdivision of the involve a perturbation in the saturation state of the oceans period into series or stages. The second ballot (March 2003) accompanying the rapid meltdown of continental ice sheets received a strong mandate (63% of votes cast) for placing (see Shields, 2005; Hoffman et al., 2007; Hoffman, 2011;and a GSSP defined using these agreed characteristics at the references therein). “Enorama Creek Section, Flinders Ranges, South Australia”. The final ballot (September 2003) received overwhelming approval (89% of votes cast) to establish the GSSP for the 18.3. THE BIOSTRATIGRAPHIC BASIS FOR Terminal Proterozoic Period “at the base of the Nuccaleena THE EDIACARAN PERIOD Formation cap carbonate, immediately above the Elatina diamictite in the Enorama Creek section, Flinders Ranges, 18.3.1. Ediacaran Megafossils and Trace South Australia” and a clear mandate (79% of votes cast) that Fossils the new period and system be named the “Ediacaran”. These decisions were ratified by the ICS on 20 February 2004. A Ediacara-type fossil impressions provide the most distinctive full-length description of the GSSP and the selection process and readily recognizable characteristic of the Ediacaran Period, was subsequently published in Lethaia (Knoll et al., 2006). and appear to be a reliable indicator of the upper part of this system worldwide (Figures 18.2 and 18.3). Ediacara-type fossils are centimeter- to meter-scale impressions of soft-bodied 18.2. CAP CARBONATES AND THE BASE organisms that typically were preserved at the bases of event OF THE EDIACARAN SYSTEM beds of sand or volcanic ash (see recent reviews in Waggoner, 2003; Narbonne, 2005; Gehling et al., 2005; Fedonkin et al., The GSSP for the Ediacaran System in Enorama Creek, 2007; Xiao and Laflamme, 2009). The affinities of the Ediacara Australia was fixed at the base of the Nuccaleena Dolomite, biota are contentious e some groups such as the rangeomorphs a 6-m-thick, distinctive dolostone that caps the Mar- and erniettomorphs may not be ancestral to any Phanerozoic or inoan glaciomarine diamictites of the Elatina Formation living life forms, whereas other forms such as Dickinsonia and (Figure 18.1(c)). The Nuccaleena Dolomite is composed of Kimberella preserving evidence of locomotion and feeding buff-weathering, creamy pink microcrystalline dolomite arguably represent stem-group animals (Narbonne, 2005; organized in centimeter-scale event beds. Meter-scale Gehling et al., 2005; Xiao and Laflamme, 2009). teepee-like structures and horizontal sheet cracks filled with Assemblages of Ediacara-type fossil impressions have been synsedimentary calcite cement are common. C isotopes reported from more than 30 regions worldwide. A few possible become increasingly negative, from about 2to 3.5&, Ediacaran precursors (Hofmann et al., 1990) and Ediacaran upward through the Nuccaleena Dolomite (see Knoll et al., survivors (Hagadorn et al., 2000) are known, but in general 2006 and references therein). Ediacara-type fossil impressions are strictly restricted to the Strikingly similar “cap carbonates” or “cap dolomites” upper half of the Ediacaran System. Some occurrences occur on the top of Marinoan glacial deposits (or an uncon- (e.g., Finnmark in northern Europe) are low in diversity and/or formity surface corresponding to this glaciation) worldwide contain only simple discs such as Aspidella, Nemiana, and (Figure 18.1(a) and (c), and serve as a superb global lithos- Heimalora that are of limited use in biostratigraphy. Diverse tratigraphic and chemostratigraphic marker for the base of the assemblages suitable for biostratigraphy have been described Ediacaran. Marinoan cap carbonates are typically less than from Australia (Flinders Ranges), Europe (White Sea, Urals, 10 m thick, and like the cap at the GSSP are buff-weathering Ukraine, and central England), North America (Newfound- dolostones with horizontal sheet cracks and meter-scale land, Mackenzie Mountains), and Africa (Namibia); low teepee-like structures. Other features typifying Marinoan cap diversity assemblages with one or more widespread and age- carbonates include macropeloids, unusual stromatolite facies, diagnostic taxa are known from Asia (central China, Oman), barite crystals, and cylindrical tubes of carbonate (Kennedy, and North America (British Columbia, Mojave Desert, and 1996; James et al., 2001; Hoffman and Shrag, 2002; Jiang North Carolina). Possible Ediacaran megafossils described et al., 2006). Marinoan cap dolostones are commonly overlain from India and South America require further study and by limestones rich in aragonite crystal fans and/or by deep- substantiation. In general, Ediacaran megafossils and trace water fine-grained siliciclastics. This suite of features permits fossils are relatively common in both shallow- and deep-water most or all basal Ediacaran (post-Marinoan) caps to be readily siliciclastics (see reviews in Waggoner, 2003; Narbonne, 2005) 416 The Geologic Time Scale 2012
(A) (B)
(C) (D)
(E) (F)
FIGURE 18.1 Key Ediacaran sections. (A) Overview of c. 1 km upper Cryogenianebasal Ediacaran strata near Shale Lake, NW Canada. The prominent light ridge and scree marks the basal Ediacaran cap carbonate overlying the (Marinoan) Ice Brook Tillite. After James et al., 2001, Figure 4A. (B) Overview of the Flinders Ranges succession at Wilpena Pound. The lip of the syncline marks the disconformable base of the Cambrian over shallow-water Ediacaran sandstones and shales. Ediacara-type fossil impressions occur in the banded sandstones below the massive quartzite ridge at the top of the bluff. Photo by J.G. Gehling. (C) Close-up of the basal Ediacaran GSSP at Enorama Creek. Jim Gehling’s right foot stands on the top of the uppermost bed of Marinoan diamictite; the GSSP is located midway between his two feet where siltstone is topped by pure buff-weathering dolomite marking the base of the Nuccaleena cap carbonate. A prominent tepee structure is visible on the right. After Knoll et al., 2006, Figure 7. (D) Interbedded black shale and argillaceous dolostone in the lower Doushantuo Formation in the Yangtze Gorges area, South China. Stratigraphic thickness in the photograph is about 7 m. (E) Mid-Ediacaran (582 Ma) Gaskiers Tillite from Harbour Main, Newfoundland. Strata strongly dipping, with stratigraphic top to the left of the photograph. Note abundant clasts and increasing red colour upwards through the Gaskiers Formation, white capping limestone (left of figure), and overlying thin turbidites of the Drook Formation. Inset shows a striated clast in the Gaskiers Formation. (F) Uppermost Ediacaran strata at Swartpunt, Namibia showing stratigraphic levels of key fossils and beds of volcanic ash. The section is approximately 125 m thick. See Narbonne et al. (1997) for details. Chapter | 18 The Ediacaran Period 417
Cambrian
Rangeomorphs Erniettomorphs Schwarzrand 545 Khatyspyt Nama Assemblage 550 Kuibis Bilateral Discoidal Triradial Octoradial Tetraradial Pentaradial Ediacara Ernietta Cloudina 555 Zimmie Gory Arkarua White Sea Assemblage Phyllozoon Yorgia Vendia Eoandromeda 560 Charnwood Rangea Spriggina
Fermeuse Kimberella Conomedusites Pteridinium Dickinsonia Ventogyrus Swartpuntia Parvancorina Trepassey Tribrachidium Ediacara biota 565 Mistaken Point Eoandromeda Ernietta Dickinsonia
570 Aspidella Palaeopascichnus
Hiemalora Cloudina Bradgatia Kimberella Charnia Fractofusus Avalon Assemblage Briscal Charniodiscus Ediacaran Period Tribrachidium 575 Drook Doushantuo biota
Charniodiscus 580 Fractofusus Gaskiers Glaciation (582 Ma)
Doushantuo-Pertatataka acritarchs Doushantuo embryos
635 Lantian biota Marinoan Snowball Earth Glaciation (635 Ma)
Cryogenian FIGURE. 18.2 Ediacaran megafossil zonation. After Xiao and Laflamme (2008). and have also been reported sporadically from deep-water for both environmental (Grazhdankin, 2004) and taphonomic carbonates (Blueflower Formation in NW Canada: Narbonne (Narbonne, 2005; Gehling et al., 2005) influences on their and Aitken, 1990, MacNaughton et al., 2000; Khatyspyt composition, but this problem is not unique to Ediacaran Formation in Siberia: Grazhdankin et al., 2008), enhancing megafossils and affects all fossil groups of all ages to varying global correlation based on megafossils and trace fossils. degrees. Calcified megafossils are common in latest Ediacaran shallow- The Avalon assemblage (579e559 Ma; Figures 18.2 water carbonates worldwide, but Ediacara-type impressions and 18.3(g) is known only from deep-water deposits in occur only rarely in these facies and this has severely hindered Newfoundland (Misra, 1969; Narbonne and Gehling, 2003; correlation of megafossil zones into the microfossil-rich Narbonne, 2004; Gehling and Narbonne, 2007; Hofmann phosphatic carbonates and shales of central Asia. et al., 2008; Narbonne et al., 2009), England (Ford, 1958; Available dates allow three broad assemblages Boynton and Ford, 1995; Brasier and Antcliffe, 2009), and (Figure 18.2) to be recognized (Waggoner, 2003; Narbonne, the Sheepbed Formation of NW Canada (Narbonne and 2005; Xiao and Laflamme, 2009). Each assemblage exhibits Aitken, 1990, 1995). Grazhdankin et al. (2008) showed a major evolutionary innovation in complex multicellularity, that some long-ranging, cosmopolitan taxa of the Avalon segmentation, mobility, or calcification that is unknown from assemblage (such as Charnia and Hiemalora) persist in deep- previous assemblages and is inferred to represent a significant water deposits to the end of the Ediacaran, but these younger development in the evolution of life. Use of these assem- deep-water assemblages typically also contain Ediacaran blages for biostratigraphy is complicated by obvious evidence fossils typical of the younger Ediacaran assemblages 418 The Geologic Time Scale 2012
FIGURE 18.3 Ediacaran macrofossils. (A) Dickinsonia tenuis;(B)Spriggina (A) (B) floundersi;(C)Kimberella quadrata;(D) Parvancorina minchami;(E)Eoandromeda octobrachiata;(F)Tribrachidium heraldicum; (G) Fractofusus misrai;(H)Charniodiscus arboreus.SpecimensAeE&Hfromthe Ediacara Member, Rawnsley Quartzite, South Australia; specimen G from Mistaken Point Formation, SE Newfoundland. Specimens A, E & G field specimens. Scale 1cm for AeG, 10 cm for H. Diagram provided by J.G. Gehling.
(C) (D)
(E) (F)
(G) (H)
with which they occur (e.g., Hofmann and Mountjoy, 2010). constructions such as Charnia, Bradgatia, and Fractofusus The Avalon assemblage (s.s.) consists largely of rangeo- (Narbonne, 2004). Palaeopascichnus and other body fossil morphs (Figure 18.2 and Figure 18.3G), fossils consisting of impressions consisting of serial arrangements of hollow centimeter-scale elements exhibiting self-similar branching chambers also make their first appearance at this time that were used as modules to build decimeter- to meter-scale (Gehling et al., 2000; Haines, 2000; Seilacher et al., 2003). Chapter | 18 The Ediacaran Period 419
The oldest Ediacara-type fossils are large rangeomorph Ediacaran burrows in both deep- and shallow-water facies of fronds (Trepassia wardae) and other fossil impressions that the White Sea assemblage may mark a significant evolu- occur approximately 150 m below an ash dated at 579 Ma tionary and biostratigraphic event. (Bowring in Van Kranendonk et al., 2008; Appendix 2 of this The Nama assemblage (549e542 Ma; Figure 18.2) is best book) in the Drook Formation of eastern Newfoundland known from the classic shallow-water occurrences in Namibia (Narbonne and Gehling, 2003). This is only 3 million years (Gu¨rich, 1933; Germs, 1972; Narbonne et al., 1997; Grot- after the end of the Gaskiers glaciation and also corresponds zinger et al., 2000; Grazhdankin and Seilacher, 2002, 2005) with geochemical evidence for a significant rise in deep-sea and also shallow-water carbonates in Oman (Amthor et al., oxygen levels (Canfield et al., 2007), implying a causal 2003) and the Dengying Formation of China (Sun, 1986; Zhao relationship between Neoproterozoic glaciation, oxygena- et al., 1988; Hua et al., 2005; Weber et al., 2007; Chen et al., tion, and the rise of complex eukaryotic life (Narbonne, 2008), and deeper-water siliciclastics and carbonates of the 2010). Ediacaran fossils have not been found in age- Khatyspyt Formation of eastern Siberia (Fedonkin 1987; equivalent shallow-water deposits, implying that these early Grazhdankin et al., 2008). These contain depauperate experiments in complex megascopic life originated in deep- assemblages of Ediacara-type fossil impressions, mainly sea settings (Narbonne, 2005). Liu et al., 2010 described rangeomorphs and erniettomorphs, most of them also known possible trace fossils from a single bed at Mistaken Point, but from the preceding assemblages (Xiao and Laflamme, 2009). these equivocal structures may represent cnidarian trails (Liu Trace fossils include the first simple treptichnids (Jensen et al., et al., 2010), trails of large coenocytic protists (Knoll, 2011), 2000). The worldwide appearance of calcified megafossils, or even stem impressions of poorly preserved fronds. In principally Cloudina and Namacalathus (Grant, 1990; Grot- general, trace fossils and other evidence of mobility are zinger et al., 2000; Hofmann et al., 1985), represents an either absent (Jensen et al., 2006) or exceedingly rare (Liu important evolutionary event of probable biostratigraphic et al., 2010) from the Avalon assemblage worldwide. significance. U-Pb dates and fossils from Namibia (Grotzinger The White Sea assemblage (558e550 Ma; Figures 18.2 et al., 1995; Narbonne et al., 1997) and Oman (Amthor et al., and 18.3(a)e(e), (g) is best known from shallow-water 2003) show that soft-bodied erniettomorphs and Ediacaran settings in the White Sea, Urals, and Ukraine in eastern calcified megafossils both became extinct at the end of the Europe (Fedonkin, 1981, 1990, 1992) and in the Flinders Ediacaran, 542 million years ago, immediately prior to the Ranges of Australia (Sprigg, 1949; Glaessner and Wade, 1966; Cambrian explosion of shelly fossils. Jenkins, 1992; Gehling et al., 2005); the Blueflower Formation Carbonaceous compressions interpreted as fleshy algae of NW Canada (Narbonne and Aitken, 1990; Narbonne, occur sporadically throughout the Ediacaran, especially in the 1994) is a probable equivalent from deep-water. The White phosphatic carbonates and shales of Asia. These exceptional Sea assemblage contains a depauperate assemblage of fossil assemblages are important in showing the level of rangeomorph taxa, many of them holdovers from the macroscopic complexity achieved in Ediacaran soft-bodied preceding Avalon assemblage. New developmental plans algae and potentially even animals, but the need for excep- include erniettomorphs (e.g., Pteridinium and Phyllozoon), tional preservation limits their value in biostratigraphy. The which show a modular construction of soda-straw-shaped Lantian assemblage of eastern China (Yuan et al., 1999, 2011), elements. Segmented forms (e.g., Dickinsonia, Windermeria), presently dated somewhere between 635 and 577 Ma based on some of which show polarity and possible cephalization correlation with the classic Yangtze sections, contains centi- (e.g., Spriggina, Kimberella), provide iconic images of the meter-scale branching algae along with problematic fossils Ediacara biota (Figure 18.3). These segmented fossils are informally compared with cnidarians and simple bilaterians. commonly regarded as stem-group bilaterians (Gehling, 1991; Several Lantian taxa also range into the younger Miaohe biota Fedonkin and Waggoner, 1997; Peterson et al., 2008), but of the Doushantuo Formation (Xiao et al., 2002) which is late other interpretations have also been suggested (see review in Ediacaran (approximately 551 Ma; Condon et al., 1995) and Sperling and Vinther, 2010). Rare and typically contentious contains a diverse flora of megascopic algae and problematica. burrows have been reported sporadically from older strata The presence of the eight-armed spiral body fossil Eoan- ranging back to the Paleoproterozoic, but the oldest abundant dromeda (Figure 18.3(e) in both the Miaohe biota of China and and reasonably unequivocal bilaterian animal burrows appear the Ediacara biota of Australia supports other evidence that the worldwide 555 million years ago, coincident with the White carbonaceous compressions of the Miaohe biota and the Sea assemblage (Martin et al., 2000). Simple, unbranched, Ediacara-style sandstone impressions of the White Sea biota subhorizontal burrows such as Planolites and Helminthoi- were contemporaneous (Zhu et al., 2008). dichnites dominate the assemblage, with some like Torro- wangea showing beaded fills implying peristalsis and active 18.3.2. Ediacaran Microfossils backfill (Narbonne and Aitken, 1990; Jensen et al., 2006). Most of these simple Ediacaran trace fossil genera also range The lower part of the Ediacaran System is characterized by into the Phanerozoic, but the appearance of abundant a group of relatively large acanthomorphic (spiny) acritarchs 420 The Geologic Time Scale 2012
(A) (B)
(C) (D)
(E) (F)
(G) (H)
FIGURE 18.4 Ediacaran microfossils. (A) Tianzhushania spinosa Yin and Li, 1978. Acanthomorph from the lower Doushantuo Formation at Wuhe, Guizhou Province, South China. Thin section WD-71-5. Coordinates 35 153. (B) Magnified view of (A) showing process morphology and multilaminate structure surrounding vesicle. (C) Wengania minuta Xiao, 2004. Multicellular alga from the Doushantuo Formation at Nantuocun, Yangtze Gorges, South China. Thin section NTC-4-5. Coordinates 3 131. (D) Parapandorina raphospissa Xue et al., 1995. Exposed animal embryo from the upper Doushantuo Formation at Weng’an, Guizhou Province, South China. Sample number 1001-WJY98-18E. (E) Meghystrichosphaeridium reticulatum Xiao and Knoll, 1999. Acanthomorph from the upper Doushantuo Formation at Weng’an, Guizhou Province, South China. Sample number 601-WJY98-18E. (F) Magnified view of (E) showing process morphology and reticulate pattern on vesicle wall. (G) Tanarium conoideum Kolosova, 1991. Acanthomorph from the upper Doushantuo Formation, Yangtze Gorges area, South China. Image courtesy of Pengju Liu. (H) Tanarium conoideum Kolosova, 1991. Acanthomorph from the Tanana Formation, Giles 1 drillhole, Officer Basin, South Australia. Image courtesy of Sebastian Willman. Reproduced with permission from Willman and Moczyd1owska (2008). White scale bar ¼ 100 mm. Black scale bar ¼ 10 mm. Chapter | 18 The Ediacaran Period 421
that are known as DoushantuoePertatataka acritarchs Australia, there is some encouraging evidence that a moderate (DPAs) (Zhou et al., 2001, 2007), Ediacaran Complex Acan- biostratigraphic resolution can be achieved in inter-regional thomorph-dominated Palynoflora (ECAP) (Grey et al., 2003), correlation based on Ediacaran acanthomorphs. For example, or large ornamented Ediacaran microfossils (LOEMs) (Cohen Vorob’eva and colleagues recognized three acritarch assem- et al., 2009b). These acritarchs are 50e1000 mm (typically blages from the Vychegda Formation in the northeastern 400e600 mm) in diameter and ornamented with processes of margin of Baltica (Vorob’eva et al., 2009b). These authors different morphologies (Figure 18.4). Although a few spiny interpreted the lower Vychegda assemblage as pre-Sturtian acritarch species are present in the Mesoproterozoic (Xiao et al., Cryogenian, but the middle and upper assemblages share 1997; Javaux et al., 2001; Nagovitsin, 2009) and early Neo- some broad similarity with Grey’s assemblage zone (1) and proterozoic (Butterfield et al., 1994; Butterfield, 2005), Edia- zones (2)e(5), respectively; their correlation predicts that the caran acanthomorphs tend to have a greater maximum diameter majority of the Cryogenian, including the Sturtian and Mar- and a greater taxonomic diversity. There are more than 200 inoan glaciations, is represented by a cryptic unconformity described species of Ediacaran acanthomorphs, and the list between the lower and middle Vychegda assemblages. In keeps growing. These taxonomically diverse and morphologi- South China, Ediacaran acanthomorphs are found in the cally complex acanthomorphs contrast sharply to late Ediacaran Doushantuo Formation. They first occur immediately after leiosphere acritarchs that lack processes and to basal Cambrian the basal Doushantuo cap dolostone atop the Marinoan-age acanthomorphs that are extremely small (typically <50 mmin Nantuo diamictite, and continue to be present until the onset diameter) (Moczyd1owska, 1991,1998;Yao et al., 2005; Dong of upper Doushantuo negative d13C excursion EN3 that is et al., 2009). equivalent to the Shuram negative d13C excursion (Zhou Ediacaran acanthomorphs provide a useful biostratigraphic et al., 2007). Viewed at the broadest scale, there is seemingly tool for Ediacaran subdivision and correlation. These acan- a discrepancy between the Australia and South China record; thomorphs have been described from Ediacaran shales, cherts, in Australia, Ediacaran acritarchs began with leiospheres and and phosphorites in South China (Yuan and Hofmann, 1998; it is not until after the Acraman impact that the first acan- Zhang et al., 1998; Zhou et al., 2001; Xiao, 2004; McFadden thomorphs appeared, whereas in South China there is no et al., 2009), Australia (Zang and Walter, 1992; Grey, 2005; leiosphere-dominated interval and acanthomorphs diversified Willman et al., 2006; Willman and Moczyd1owska, 2008), shortly after the Marinoan glaciations (Yin et al., 2007). This Siberia (Moczyd1owska et al., 1993; Nagovitsyn et al., 2004; discrepancy can be resolved if we accept some degree of Moczyd1owska, 2005; Vorob’eva et al., 2008), India (Tiwari regional variation in Ediacaran acanthomorph diversity. and Azmi, 1992; Tiwari and Knoll, 1994; Shukla et al., 2008), Recent analysis of the South China data shows that two Svalbard (Knoll, 1992), and Baltica (Vidal, 1990; Veis et al., acanthomorph biozones can be recognized, with the lower 2006; Vorob’eva et al., 2006, 2009a,b). In search for biozone characterized by Tianzhushania spinosa and the a biostratigraphic resolution, one is tempted to ask whether upper biozone by Ericiasphaera rigida (McFadden et al., biozones of Ediacaran acanthomorphs can be recognized and 2009). New data (Liu et al., in press) suggest that the upper used in biostratigraphic subdivision and correlation. Indeed, Doushantuo acanthomorph biozone is taxonomically more five Ediacaran acritarch assemblage zones have been distin- similar to Grey’s assemblage zones (2)e(5) and to the upper guished in Australia and these can be correlated in the Ade- Vychegda assemblage zone. Thus, it is possible that the laide Rift Complex, Officer sub-Basin, and Amadeus Basin Tianzhushania spinosa-dominated acanthomorph biozone in (Grey, 2005). The biozones are, in ascending order: the lower Doushantuo Formation is present only in South China and perhaps northern India (Tiwari and Knoll, 1994), (1) the Leiosphaeridia jacuticaeLeiosphaeridia crassa Ass- and elsewhere this interval is dominated by leiospheres. The emblage Zone; difference could be due to ecological, biogeographical, or (2) the Appendisphaera tabifica (¼Appendisphaera barbata)e taphonomical factors. If this is substantiated in the future, Alicesphaeridium medusoidumeGyalosphaeridium pul- then it is conceivable to divide the Ediacaran System into chra Assemblage Zone; three large-scale microfossil zones e a lower zone dominated (3) the Tanarium conoideumeSchizofusa risoriae by Tianzhushania spinosa and leiospheres, a middle zone Variomargosphaeridium litoschum Assemblage Zone; characterized by Ericiasphaera, Tanarium, Appendisphaera, (4) Tanarium irregulareeCeratosphaeridium glaberosume Variomargosphaeridium and many other acanthomorphs, and Multifronsphaeridium pelorium Assemblage Zone; and an upper zone characterized by simple leiospheres. (5) Ceratosphaeridium mirabileeDistosphaera australicae Although there are some promising data suggesting the Apodastoides verobturatus Assemblage Zone. biostratigraphic importance of Ediacaran acanthomorphs, Except for the first assemblage zone, which is characterized there are several challenges which limit the full potential of by smooth-walled leiospheres, the other four zones are all these microfossils. First, diagnostic acanthomorphic acri- characterized by large acanthomorphs. Although it is unclear tarchs are well known from shallow-water sediments in Asia, whether these assemblage zones can be recognized beyond Australia, and Europe but have not yet been reported from 422 The Geologic Time Scale 2012
truly deep-water deposits or any other continents, reducing Ediacaran glacial deposits are rare and typically isolated their effectiveness for global correlation and integration with despite their generally more polar positions. In their recent the early stages of Ediacaran macrofossils as recorded in compilation, Hoffman and Li (2009) listed 13 probable Edia- Avalon and NW Canada. Second, Ediacaran acritarchs can be caran glacial deposits located on eight paleocontinents. The preserved in shales (as two-dimensional carbonaceous best-known Ediacaran glacial deposit is the Gaskiers Forma- compressions) or in cherts and phosphorites (in three tion from Avalonian Newfoundland (Eyles and Eyles, 1989; dimensions). Thus, different methods (e.g., acid maceration Myrow and Kaufman, 1999) which is dated at 582e584 Ma by and thin sectioning) are used in the extraction and observation ash beds below, within, and immediately above the glacial of these microfossils. It has been shown that taphonomic deposits (Bowring in Hoffman and Li, 2009;Appendix2of alteration can introduce significant morphological noise in the this book). The Gaskiers Formation is a 250 m-thick, deep- taxonomic study of Ediacaran acanthomorphs (Grey and water, glaciomarine deposit that is similar to Cryogenian Willman, 2009). As such, there have been some taxonomic glacial deposits in showing significant iron enrichment inconsistencies between systematic treatments based on thin upwards and in locally exhibiting a cap carbonate (0.5 m thick section and maceration materials, although recent efforts have composed of white-weathering sparry calcite) in an otherwise successfully focused on resolving some of these taxonomic completely carbonate-free succession (Figure 18.1E).Simi- issues. Also, environmental factors and preservational bias larity in their sedimentary and paleontological succession can lead to variation in the geographic and stratigraphic and sparse U-Pb dates imply that the Squantum Diamictite of distribution of Ediacaran acanthomorphs (Zhou et al., 2007); Massachusetts and perhaps the Mortensnes Formation of this is especially evident in the smooth leiospheres, which northern Norway may be correlative with the Gaskiers. characterize uppermost Ediacaran strata worldwide but also There are some preliminary data implying that there may occur as default taxa in some early Ediacaran microfossil have been more than one glaciation in the Ediacaran Period. assemblages where acanthomorphs may have been excluded The age of the Gaskiers glaciations is tightly constrained by environmental or taphonomic variables. It should be noted, between 584 and 582 Ma, firmly placing it before the termi- however, that these challenges are not unique to Ediacaran nation of the Shuram negative d13C excursion. The Hankal- acanthomorphs; they are general problems that all bio- chough glaciation in the Quruqtagh area, the Hongtiegou stratigraphers have to face. Careful taphonomical and paleo- glaciations in North China, however, may post-date the environmental analyses will help us to maximize the Shuram negative d13C excursion (Shen et al., 2010). Also, biostratigraphic potential of Ediacaran acanthomorphs. several authors have argued for the presence of glacial dia- Ediacaran successions also yield several other groups of mictites near the EdiacaraneCambrian boundary in Kazakh- microfossils and mesofossils, but these fossils have limited stan, Kyrgyzstan, and Siberia (Chumakov, 2009; Kaufman biostratigraphic significance, either because of their restricted et al., 2009). geographic distribution or because of their rather long strati- graphic range. Of the former category are animal embryo fossils, tubular microfossils, and multicellular algal fossils from 18.3.4. Chemical Evolution of Ediacaran the Doushantuo Formation (Xiao et al., 1998, 2000, 2002, 2004; Oceans Liu et al., 2008). Of the latter category are various coccoidal and filamentous cyanobacteria (Venkatachala et al., 1990; Tiwari 18.3.4.1. Carbon Isotopes (Figure 18.5) and Azmi, 1992; Zhang et al., 1998; Shukla et al., 2005), as well Basal Ediacaran “cap dolostones” are characterized by 13 as Chuaria-, Tawuia-, and Longfengshania-like carbonaceous negative d Ccarb values (Knoll et al., 2006), which tend to compressions (Tang et al., 2006, 2007, 2008). Of potential but become more negative during cap carbonate deposition unproven biostratigraphic significance is Salome hubeiensis e reaching minima of about 5&, after which they return to a giant multisheathed filamentous cyanobacterium that has near 0& within about 3 million years (Condon et al., 2005). been found in the Krol Group in northern India (Shukla et al., The seawater d13C trend through the subsequent early Edia- 2008) and the Doushantuo Formation in South China (Zhang caran is poorly constrained due to age uncertainties. However, 13 et al., 1998), and Vendotaenia-like fossils that have been found a return to very high positive d Ccarb values (þ6to10&) has in upper Ediacaran (<555 Ma) successions in Russia, South been noted from many sections, e.g., NW Namibia and NE China, and Namibia (Cohen et al., 2009a; Gnilovskaya, 1990; Svalbard (Halverson et al., 2005) and Brazil (Misi and Veizer, Zhao et al., 1988). 1998), while the number of negative excursions recorded can be as many as five (Sawaki et al., 2010). A dominant feature in the Ediacaran d13Crecordis 18.3.3. Ediacaran Glaciations 13 an unusually negative d Ccarb excursion with values In contrast with the Cryogenian, in which abundant evidence of below 10&, commonly referred to as the Shuram (or continental glaciation dominates the sedimentary record ShurameWonoka) anomaly (Burns and Matter, 1993). Its despite largely equatorial positions for most of the continents, magnitude is large, but its precise duration, age, and origin are Chapter | 18 The Ediacaran Period 423
Deep water O2 Deep sea anoxia (stratified Fe/S ocean)
.6
3
0 0.709
-3 87Sr/86Sr δ13C(0/00) -6 EN1 EN2 0.708
-9 EN3 -12 0.707
635 Ma 580 Ma 540 Ma
Age, million years FIGURE 18.5 Summary figure of current understanding of ocean composition evolution during the Ediacaran Period showing 1) a generalized seawater 87Sr/86Sr curve (two possibilities are shown for the Ediacaran Period with the curve of Sawaki et al. (2008) for South China uppermost); 2) a simplified d13C curve based on various records (see text); and 3) ocean redox evolution based on Fe speciation studies (Canfield et al., 2007, 2008; Li et al., 2010) whereby dashed lines represent more intermittent and full lines relatively continuous redox conditions.
uncertain (Grotzinger et al., 2011). Despite challenges to (e.g., Magaritz et al., 1986; Narbonne et al., 1994; Knoll et al., a primary interpretation for this anomaly (Knauth and Kennedy, 1995; Saylor et al., 1998; Amthor et al., 2003) and serves as 2009; Derry, 2010), its global duplication at the same approx- a global geochemical marker of this boundary. imate level within successions exhibiting a diverse range of facies, including Sr-rich limestones in Siberia (Pokrovskii et al., 2006; Melezhik et al., 2009; Le Guerroue´,2010), argues for 18.3.4.2. Strontium Isotopes (Figure 18.5) a primary oceanographic origin. Although no unambiguous Reconstruction of the seawater 87Sr/86Sr curve before the evidence for both glaciation and the full anomaly occurs in any Ordovician Period suffers greatly from a lack of suitably well- single stratigraphic succession, the isotopic signature of dolo- preserved materials, which necessitates careful sample mite beds within Member E in the uppermost Nyborg Forma- selection and preparation (Kaufman et al., 1993; Bailey tion (Norway) implies that the Mortensnes glaciation post-dated et al., 2000). Nevertheless, strontium isotope stratigraphy the nadir of the negative anomaly (Halverson et al., 2005). can potentially resolve stratigraphic conflicts caused by However, others disagree on the basis of pre-anomaly dia- the ambiguity and circular reasoning inherent in matching mictites and evidence for cooling, respectively (Prave et al., otherwise identical d13C excursions because of the magnitude 2009; Sawaki et al., 2010). In South China, at least of the rise in seawater 87Sr/86Sr during the Ediacaran period three negative anomalies are present within the Doushantuo (w0.7071 to w0.7087). Formation with the upper anomaly EN3, perhaps coeval to the Although cap dolostones are not suitable for 87Sr/86Sr Shuram anomaly, ending by 551 Ma (Condon et al., 2005). Low studies (e.g., Yoshioka et al., 2003), immediately overlying 13 87 86 d Ccarb values of the Shuram anomaly (EN3) appear to be limestone units have provided consistent data. Sr/ Sr 13 decoupled from the coeval organic carbon (d Corg)record values for Sr-rich rocks of the Hayhook Formation (NW (Calver, 2000; Fike et al., 2006; McFadden et al., 2008; Bjerrum Canada) yielded 0.707 14 2(James et al., 2001)whichis and Canfield, 2011; Grotzinger et al., 2011). However, the consistent with data from post-glacial limestones of Nami- 13 significance of the middle Ediacaran d Corg record is debatable bia (Halverson et al., 2007) as well as pre-Ediacaran non- 13 13 as the relationship between d Ccarb and d Corg is not the same glacial strata from Mongolia, Canada and Namibia (Shields in every succession. et al., 2002; Halverson et al., 2007). In Namibia, least A significant and short-lived negative d13C anomaly altered 87Sr/86Sr values rise subsequently to c. 0.7080 as coincides with the EdiacaraneCambrian boundary worldwide d13C values recover from 4.4& to 0&. High-Sr samples 424 The Geologic Time Scale 2012
34 from NW Canada show an increase to 0.707 53, while least however, that seawater d Ssulfate had reached its Ediacaran altered 87Sr/86Sr data from South America trace a rise from c. norm of w20& by the end of the basal Ediacaran negative 0.7074 to 0.707 77 2 during the d13C recovery (de Alvar- d13C excursion (Shields et al., 2007). By the early Cambrian 34 enga et al., 2007; Nogueira et al., 2007). Identical values (c. d Ssulfate, as measured in a variety of minerals, including 0.7077e0.7078) have been reported for basal Ediacaran evaporites, francolite, and carbonate, was high (35e40&)in barite samples of NW Africa at a comparable point in the most basins (e.g., Strauss, 1993; Holser and Kaplan, 1966; post-glacial d13C curve (Shields et al., 2007). Taken together, Shields et al., 1999; Kampschulte and Strauss, 2004; 87 86 34 these data indicate a rise in seawater Sr/ Sr from c. 0.7071 Schro¨der et al., 2004). However, d Ssulfate values from the to c. 0.7077 or higher during the post-glacial d13C recovery to late Ediacaran in South China (McFadden et al., 2008) and positive values which lasted at least 3 million years (Condon southern Namibia (Ries et al., 2009) are generally lower, with et al., 2005). concomitant decreases in D34S. In order to constrain the early Ediacaran seawater The Ediacaran sulfur isotope record is filled in by 87 86 34 34 Sr/ Sr curve, the assumption must be made that the rise to a detailed data set of d Ssulfate (CAS) and d Spyrite data from high d13C after end-Cryogenian glaciation was a global trend. the Nafun Group, Oman (Fike et al., 2006) and pyrite data Several Brazilian studies indicate that 87Sr/86Sr decreased from the Pertatataka Formation, central Australia (Gorjan from its post-glacial peak of ~0.7078 to ~0.7074 during the et al., 2000). The Oman record, along with other fragmentary 13 87 86 34 rise to high d C(Misi et al., 2007). Decreasing Sr/ Sr data (Figure 18.1), indicate that more typical d Ssulfate values during an interval of high d13C finds further support from close to 20& are the norm for the Ediacaran Period, although South China (Sawaki et al., 2010), and may be followed by a here again CAS data appear to contradict evaporite results second rise in 87Sr/86Sr from ~0.7072 to ~0.7080 (Pokrovskii (e.g., cf. Fike et al., 2006 with Schro¨der et al., 2004). At the et al., 2006). The most complete 87Sr/86Sr record derives from same time, data from both Oman and central Australia 34 South China, and indicates a complicated, but as yet uncon- display a spike to positive d Spyrite values in the early firmed, trend with three 87Sr/86Sr peaks during the Ediacaran Ediacaran Period, followed by a gradual decrease through the Period (Sawaki et al., 2010). middle Ediacaran Period, resulting in a steady increase in Recent studies from Siberia (Pokrovskii et al., 2006; D34S e a predictable result of increasing seawater sulfate Melezhik et al., 2009) indicate that seawater 87Sr/86Sr concentrations (Halverson and Hurtgen, 2007). Detailed 34 increased from ~0.7080 to ~0.7086 during the major Shuram d Ssulfate (CAS) records have been produced across the excursion of the late Ediacaran. This late Ediacaran 87Sr/86Sr onset of the ShurameWonoka anomaly. In Oman, a spike in 13 34 peak is broadly consistent with published data for C- d Ssulfate to þ29& coincides with the anomaly (Fike et al., depleted strata from South China (Jiang et al., 2007; Sawaki 2006). Across the presumably equivalent interval in Death et al., 2010) situated below a horizon dated at 551 Ma Valley (Kaufman et al., 2007) and South China (McFadden 34 (Condon et al., 2005), and with contemporaneous data from et al., 2008), d Ssulfate decreases to values < 20&. Although Oman (Burns et al., 1994) and Australia (Calver, 2000). variability and inconsistency can possibly be explained by Several 87Sr/86Sr studies span the PrecambrianeCambrian unusually low sulfate concentrations in seawater (Halverson boundary, the most comprehensive being that of Brasier et al. and Hurtgen, 2007; Canfield et al., 2008), there is still no (1996). That study and other work (Derry et al., 1994; consensus Ediacaran d34S curve. It seems that seawater Kaufman et al., 1996; Nicholas, 1996; Valledares et al., 2006; sulfate was isotopically heterogeneous (Ries et al., 2009), Jiang et al., 2007) constrain latest Ediacaran and earliest inferred correlations are inaccurate, or the measured sulfur Cambrian 87Sr/86Sr to c. 0.708 45 5. Least altered samples isotope compositions in some or all of the sedimentary from Mongolia and Siberia (Brasier et al., 1996; Kaufman successions do not preserve reliable signatures of primary et al., 1996) indicate that seawater 87Sr/86Sr decreased seawater composition. through the PrecambrianeCambrian transition interval to reach a low of 0.708 05 5 during Cambrian Stage 2. 18.3.4.4. Redox Proxies (Figure 18.5) The redox state of the global ocean has gradually changed 18.3.4.3. Sulfur Isotopes from anoxic and ferruginous on the early Earth to fully Evaporite sulfate deposits are scarce during the Ediacaran oxygenated in our modern world. Ediacaran sediments Period and so, with few exceptions, knowledge of seawater sampled mainly shallow marine and near-continent deep- 34 d Ssulfate through this interval derives mainly from carbonate water regimes during a critical stage in this evolution (Can- associated sulfate (CAS), trace sulfate in phosphorite and field and Poulton, 2011). 34 34 from barite. According to CAS data, predominantly high The impressive range of d Spyrite and d Ssulfate values 34 34 d Ssulfate and low D S values near 0& prevail in the basa- which are typical of the Neoproterozoic are commonly inter- l Ediacaran Period although absolute values are variable preted in terms of a low sulfate ocean (Halverson and Hurtgen, 34 (Halverson et al., 2010). Barite d Ssulfate data indicate, 2007). This is consistent with the observation that seawater Chapter | 18 The Ediacaran Period 425
during both the Cryogenian and Ediacaran periods was occa- Key U-Pb dates within the Ediacaran Period include the sionally rich in dissolved ferrous iron (Hoffman and Schrag, beginning and end of the Gaskiers glaciation in Avalon, all 2002). Fe speciation studies shed further light on the redox three megafossil assemblages, and the end of the late acan- state of the oceans and are being carried out increasingly on thomorph acritarch assemblage (Appendix 2). These precise Neoproterozoic successions (Canfield et al., 2008). Sediments dates significantly constrain the subdivision and correlation deposited under an oxic water column tend to contain a lower within the upper half of the Ediacaran System. However, most ratio of reactive iron than those deposited under an anoxic Ediacaran U-Pb dates are from the upper part of the system, water column with a proposed cut-off at 38% reactive versus with only three dates constraining events in the first 50 total iron (Raiswell et al., 1988; Lyons and Severmann, 2006). million years of the Ediacaran. The often quoted dates for the Neoproterozoic sediments deposited beneath storm wave base Gaskiers glaciation and the Avalon macrofossil assemblage in typically exhibit high FeHR/FeT ratios indicating that anoxia Newfoundland are thus far available only in abstracts and on was common in the deeper marine environment. However, websites that have not yet been subjected to rigorous scien- sulfidic conditions are so far only indicated during the pre- tific review. glacial Neoproterozoic and post-glacial early Cambrian (Canfield et al., 2008), suggesting ferruginous rather than sulfidic (euxinic) anoxia during the early Ediacaran. Because 18.4. TOWARDS AN EDIACARAN ferruginous conditions can only arise when the molar flux of CHRONOSTRATIGRAPHY FeHR to the deep ocean is greater than half the flux of sulfide, their reappearance during the Neoproterozoic after an absence From the above, it is clear that megafossil biostratigraphy, of more than a billion years is consistent with a low sulfate microfossil biostratigraphy, C- and Sr-chemostratigraphy, ocean reservoir. precise U-Pb dates, and climatic events can be used to According to Canfield et al. (2007), anoxic ferruginous subdivide the Ediacaran into broad divisions that might form conditions in the deep marine environment give way to more the basis for series and stage level subdivisions. All of these oxic conditions after the Gaskiers glaciation in Newfound- indicators are strongly affected by facies and preservational land, but ferruginous and even sulfidic conditions may have factors (see the respective discussions above), requiring the persisted longer in some regions (Canfield et al., 2008; Li use of multiple biological and geochemical proxies from et al., 2010). If redox structure changes of the Ediacaran multiple regions to provide a robust Ediacaran subdivision. ocean represent a global oceanographic phenomenon, then Fe These problems are not currently critical in the lower third speciation and other redox-sensitive geochemical parameters of the Ediacaran (635e600 Ma), which contains a Tianz- such as Mo/TOC (Scott et al., 2008) may assist in global hushania-dominated microfossil assemblage in Australia and stratigraphic correlation as well as environmental interpreta- China and apparently lacks evidence for Ediacara-type tion of the Ediacaran Period. megafossils worldwide. A distinctive cap carbonate at the base of the Ediacaran overlying Marinoan glacial deposits, 87 86 13 18.3.5. Radiometric Dating low Sr/ Sr (0.7071e0.7080) and a rise to high d C ratios in marine carbonates, and evidence of widespread deep-water Key U-Pb dates from the Ediacaran System and immediately anoxia also characterize this interval worldwide. Similarly, adjacent Cryogenian and Cambrian strata are listed in there is little problem in large-scale subdivision and corre- Appendix 2. The base of the Ediacaran is well constrained by lation in the uppermost part of the Ediacaran (560e542 Ma), dates of 635.5 0.6 Ma from the top of the Cryogenian in which a wide variety of marine facies worldwide contain Ghaub Tillite in Oman and dates of 635.2 0.6 Ma within a leiosphere microfossil assemblage and numerous Ediacaran the Ediacaran cap carbonate overlying the Cryogenian Nan- megafossils. Abundant U-Pb dates, high 87Sr/86Sr ratios tuo Tillite and 632.5 0.5 Ma a few meters higher in the (>0.7080) and moderate positive d13C values in marine Doushantuo Formation in central China. The top of the carbonates, and proximity to the overlying Cambrian also Ediacaran in Namibia is constrained by dates of 540.61 significantly constrain uppermost Ediacaran correlations. 0.67 Ma immediately below and 538.18 1.11 Ma imme- Uncertainty in correlating siliciclastic and carbonate diately above the disconformable contact with the overlying facies in the middle part of the Ediacaran (600e560 Ma) Lower Cambrian (Appendix 2, recalculated from Grotzinger hinders global correlation and our understanding of the et al., 1995) and is constrained by dates of 542.37 0.28 Ma transition between the basal and uppermost Ediacaran. immediately below and 541.00 0.29 Ma immediately above Phosphatic carbonates and fine siliciclastics of this age in the top of the Ediacaran in Oman (Appendix 2, recalculated central Asia contain diverse acanthomorphic acritarchs from Bowring et al., 2007). The occurrence of effectively and embryos in carbonate strata that are amenable to an array identical dates from two different continents provides robust of chemostratigraphic techniques, but apparently lack either constraints for both the base (635 Ma) and top (542e540 Ma) radiometric age constraints or Ediacaran megafossils of the Ediacaran Period. (McFadden et al., 2008). In contrast, deep-water siliciclastics 426 (A) Microfossils Ediacara-type Fossils
Possible Complex Shells and Burrows Assemblage Divisions Abundant Simple Burrows Carbonaceous Algae Bilaterians Erniettomorphs Radiometric dates constraining macrofossils Series- Stage- C-isotope stratigraphy C2 Cambrian Radiometric dates constraining microfossils terminal Ediacaran leiosphere assemblage Rangeomorphs Palaeopascichnids 540 ? Cloudina C1 Level Level 540 E16, E17 E14, E15 E13 E12 4 550 E11 E11 550 E10 E10 EN3
Doushantuo embryos 3 Late acanthomorph assemblage
560 -dominated assemblage E9 560 ? Upper (E8) 570 2 570
(E7)
580 Tianzhushania (E6) 580 EN2 Gaskiers Glaciation (E5) (E4) 590 590
600 Ediacaran 600
610 Lower 1 610
E3
620 620 2012 Scale Time Geologic The
630 E2 630 EN1 E1
640 Cryo- 640 genian Marinoan Glaciation
FIGURE 18.6 Correlation and internal subdivision of the Ediacaran Period. Radiometric dates correspond to those in Appendix 2 of this volume; dates known only in abstract form are indicated by brackets. Fossil distributions and C isotope curve from phosphatic carbonates and shales of central Asia are in red, with data from mainly siliciclastic successions worldwide shown in blue. These two correlations and subdivisions are consistent with all known radiometric and biostratigraphic data. (A) Preferred correlation. (B) Alternative correlation. hpe 18 | Chapter
(B) Microfossils Ediacara-type Fossils h daaa Period Ediacaran The
Possible assemblage
Complex Shells and Burrows Divisions Carbonaceous Algae Abundant Simple Burrows Bilaterians Radiometric dates constraining macrofossils Erniettomorphs Series- Stage- C-isotope stratigraphy C2 Cambrian terminal Ediacaran leiosphere assemblage Radiometric dates constraining microfossils Rangeomorphs Palaeopascichnids 540 ? Cloudina C1 Level Level 540 E16, E17 E15E14, E13 5 E12 550 E11 E11 550 E10 E10 4 560 560 ? E9 Upper (E8) 570 3 570
(E7) (E6) Doushantuo embryos -dominated assemblage
580 Late acanthomorph assemblage 580 (E5) EN3 Gaskiers Glaciation (E4) 590 590 Middle 2 Ediacaran 600 600 Tianzhushania EN2 ? 610 610 E3 620 Lower 1 620
630 E2 630 E1 EN1
640 Cryo- 640 genian Marinoan Glaciation 427 FIGURE 18.6 (Continued). 428 The Geologic Time Scale 2012
in Newfoundland and England contain abundant Ediacara- plus abundant calcified metazoans including Cloudina and style megafossils of the Avalon assemblage beneath volcanic Namacalathus, characterize the uppermost division (Stage 4; ash beds suitable for radiometric dating, but carbonates 550e541 Ma) of the Ediacaran. suitable for chemostratigraphy are virtually absent and only An alternative and partly overlapping correlation nondiagnostic leiosphere microfossils are preserved in these (Figure 18.6(b) regards EN3 in the Doushantuo Formation of thermally mature strata (Hofmann et al., 1980; Myrow, central China as correlative with the Gaskiers glaciation in 1995). The Gaskiers glaciation (582e584 Ma) marks Avalonia (Xiao, 2004, 2008; Xiao et al., 2004; Fike et al., a significant divide that at least regionally separates Edia- 2006; Zhou et al., 2007). This correlation results in three caran strata with deep-water anoxia and apparently lacking series, each corresponding to one of the three major Ediacaran Ediacaran megafossils from upper Ediacaran strata with microfossil assemblages. The “lower series” would range from regionally developed deep-water oxic environments and the base of the Ediacaran (635 Ma) to EN2 (not yet dated but abundant Ediacaran megafossils (Canfield et al., 2007). probably about 600 Ma based on this correlation) and locally However, the Gaskiers glaciation is known only from Ava- exhibits a Tianzhushania-dominated microfossil assemblage lonia and possibly Baltica, and its global correlation and the and a lack of evidence for Ediacara-type megafossils anywhere synchroneity of the associated deep-sea oxygenation event in the world; it is geochemically characterized by depleted remain uncertain. Major changes in the microfossil assem- 87Sr/86Sr ratios in marine carbonates and evidence of wide- blages of central Asia coincide with prominent negative spread deep-water anoxia. The “middle series” (approximately C isotope excursions marking probable sequence boundaries 600 Ma to 582 Ma based on this correlation) is characterized (McFadden et al., 2008),andattemptstocorrelatethese by the more widespread late acanthomorph acritarch assem- excursions with the negative C isotope excursion marked by blage and the acme of Doushantuo phosphatized embryos; all the Gaskiers cap carbonate (Myrow and Kaufman, 1999) other biological and chemical characters are broadly similar to provide a potential vehicle for global correlation in this that of the underlying “lower series”. The “upper series” would critical interval. Two distinct correlations consistent with all be identical to that defined in our preferred correlation, and available radiometric dates and biostratigraphic and might similarly be divisible into three stages based primarily sequence-stratigraphic data emerge from this (Figure 18.6). on Ediacaran megafossils. This correlation is consistent with Our preferred correlation (Figure 18.6(a) regards EN2 in the view that there may be a distinct stratigraphic separation the Doushantuo Formation of central China as correlative between acanthomorphic acritarchs in the “lower” and “middle with the Gaskiers glaciation in Avalonia (Condon et al., 2005; series” and Ediacaran megafossils in the “upper series” (Knoll Cohen et al., 2009b; Sawaki et al., 2010). This correlation and Walter, 1992), but determination of the microfossil results in two series of approximately equal length, the upper assemblage coeval with the Avalon macrofossil assemblage is of which can be subdivided into three stages. The “lower needed to test this model. series” (635e582 Ma) comprises a single stage that locally Correlations 1 and 2 both satisfy all available biostrati- exhibits a Tianzhushania-dominated microfossil assemblage graphic and radiometric data (Figure 18.6(a) and (b), and and lacks evidence for Ediacara-type megafossils anywhere the bases of EN2 and EN3 in the Doushantuo Formation in the world. Geochemically it is characterized by low both coincide with flooding surfaces that can be interpreted 87Sr/86Sr ratios in marine carbonates and shows evidence of as a eustatic response to Gaskiers deglaciation. The widespread deep-water anoxia. The “upper series” (582e541 preferred correlation is driven by the realization that the Ma) begins at the top of the Gaskiers glaciation or its EN3, which terminated ~551 Ma, could not have lasted equivalent level outside of Avalonia, and is characterized by more than 10 myr (Condon et al., 2005), and is also abundant Ediacaran megafossil impressions, high 87Sr/86Sr consistent with slightly 18O-enriched carbonates that have ratios in marine carbonates, and local evidence of deep-water been interpreted as evidence for global cooling (Sawaki oxygenation. One testable implication of this correlation is et al., 2010). Carbonates suitable for isotopic analysis are that the Avalon rangeomorph-dominated megafossil assem- absent from the fossiliferous parts of the Avalon, White Sea, blage should be temporally equivalent to the late acantho- and Flinders Ranges successions, but equivalent fossil morph acritarch assemblage and the acme of the Doushantuo assemblages in NW Canada are associated with mainly phosphatized embryos (Condon et al., 2005), and that positive C isotope values and high 87Sr/86Sr ratios in marine together these might define the basal stage (Stage 2; 580e560 carbonates (Narbonne et al., 1994; Kaufman et al. 1997), Ma) of the “upper series”. The middle stage of this series also consistent with our preferred correlation. The alterna- (Stage 3; 560e550 Ma) exhibits the widespread but non- tive correlation is supported by a Pb-Pb isochron age of 599 diagnostic terminal Ediacaran leiosphere assemblage and 4 Ma on phosphorites from the upper Doushantuo more complex macrofossils including the erniettomorphs, Formation (above EN2 based on regional correlation) at bilaterian impressions, abundant bilaterian burrows, and Weng’an (Barfod et al., 2002). Both of these correlations the Miaohe biota of carbonaceous compressions. A similar make significant predictions, and obtaining additional but less diverse assemblage of Ediacaran megafossils, radiometric dates from key Ediacaran fossil successions or Chapter | 18 The Ediacaran Period 429
TABLE 18.1 GSSP of the Ediacaran Period, with location and primary correlation criteria
System/Period GSSP Location Latitude, Longitude Boundary Level Correlation Events Reference
Ediacaran Enorama Creek, 31 19’53.2“S Base of the Marinoan Beginning of Knoll et al., 2006 Flinders Ranges, 138 38’0.2”E cap carbonate a distinctive pattern South Australia of secular changes in carbon isotopes
discovery of Ediacaran megafossils associated with acan- Both stratigraphic arguments have merit, but in the greater thomorph acritarchs would provide critical tests of both part we concur with the traditional view that the Ediacaran correlations. Period is best regarded as the terminal period of the Prote- Both correlations recognize a similar “upper series” rich rozoic. All of the authors of this chapter support the view that in Ediacara-type fossil impressions overlying middle Edia- stem-group animals were present among the Ediacaran biota, caran glacial deposits (Gaskiers glaciation or its equivalent). but even this broad consensus is debated by some workers. This is a stratigraphic interval that is remarkably similar In contrast with the nearly pervasive abundance of animal to Sokolov’s (1952) original concept of the “Vendian Series” skeletons and burrows in most Phanerozoic marine strata, of western Russia. The most significant problem with Ediacaran megafossils occur only very rarely and highly formalizing an upper series for the Ediacaran System is the discontinuously and are not known from the lower half of the present uncertainty in the correlation of the lower boundary of Ediacaran. Similarly, in contrast with the massive changes in this series between the siliciclastic strata of Europe, Australia, engineering properties of Phanerozoic marine sediments and North America (rich in megafossil impressions) and the worldwide through “biological bulldozing” (Thayer, 1979; phosphatic carbonates of central Asia (rich in microfossils Seilacher, 1999) and the accumulation of hard parts (Kidwell and amenable to a wide array of chemostratigraphic tech- and Jablonski, 1983), megascopic Ediacaran organisms had niques). Resolution of this problem is likely to come from only minimal impact on their sedimentary environment. integrated macrofossil, microfossil, and chemostratigraphic Ediacaran glaciations and associated isotopic excursions studies of mixed siliciclasticecarbonate successions and/or provide a link with those of the Cryogenian. These points the discovery of additional datable horizons. support retention of the Ediacaran Period as the terminal period of the Proterozoic, and none justify the enormous logistical problems that would be associated with its transfer 18.5. EDIACARAN e LAST PERIOD OF THE to the Phanerozoic. PROTEROZOIC OR FIRST PERIOD OF THE The Ediacaran Period represents an important interval in PHANEROZOIC? Earth evolution, and exhibits a unique biological, chemical, and cryogenic character, fully justifying its definition as The Ediacaran Period represents the transition between the a distinct geological period (Knoll et al., 2004, 2006). In Proterozoic and the Phanerozoic, and as such shares attributes addition to these unique characters, the Ediacaran exhibits of both eons. The pre-Cambrian time interval presently rep- numerous characters transitional between the Proterozoic and resented by the Ediacaran Period was traditionally regarded Phanerozoic, and the nature of these transitions have and will as part of the Proterozoic, and this was reflected in its formal provide avenues for fruitful research on changes in the Earth designation as the terminal period of the Proterozoic (Knoll system through time. This ongoing research will also further et al., 2004, 2006). However, recognition of the “visible life” facilitate the correlation and future subdivision of the Edia- afforded by the iconic soft-bodied Ediacara biota and caran Period, the most recently named period of the geologic molecular clock evidence for Ediacaran and potentially even time scale. Cryogenian origins for some modern animal groups bring up the question whether it might alternatively be considered as the basal period of the Paleozoic and Phanerozoic, a view REFERENCES debated without resolution by Cloud and Glaessner (1982) de Alvarenga, C.J.S., Figueiredo, M.F., Babinki, M., Pinho, F.E.C., 2007. and later discussed by Butterfield (2007).Redefiningthebase Glacial diamictites of Serra Azul Formation (Ediacaran, Paraguay belt): of the Phanerozoic to include the Ediacaran would be a high Evidence of the Gaskiers glacial event in Brazil. Journal of South profile change that would require the revision of most American Earth Sciences 23, 236e241. geological textbooks, maps, stratigraphic columns, and cross- Amthor, J.E., Grotzinger, J.P., Schroeder, S., Bowring, S.A., Ramezani, J., sections worldwide. Is this change justified? Martin, M.W., Matter, A., 2003. Extinction of Cloudina and 430 The Geologic Time Scale 2012
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