Accepted Manuscript Environmental upheavals of the Ediacaran period and the Cambrian “explosion” of animal life Grant M. Young PII:
S1674-9871(14)00114-5
DOI:
10.1016/j.gsf.2014.09.001
Reference:
GSF 320
To appear in:
Geoscience Frontiers
Received Date: 7 August 2014 Revised Date:
30 August 2014
Accepted Date: 4 September 2014
Please cite this article as: Young, G.M., Environmental upheavals of the Ediacaran period and the Cambrian “explosion” of animal life, Geoscience Frontiers (2014), doi: 10.1016/j.gsf.2014.09.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Environmental upheavals of the Ediacaran period and the Cambrian “explosion” of animal life
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Grant M. Young *
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Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada N6A
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*Corresponding author. E-mail address:
[email protected]
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Abstract The second half of the Ediacaran period began with a large impact - the Acraman impact in
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South Australia, which was accompanied by a negative δ13Ccarb anomaly and an extinction-
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radiation event involving acritarchs. A few million years later (~570 Ma?) there was a second,
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deeper and longer-lived world-wide δ13Ccarb anomaly (the Shuram anomaly) which coincides
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with extinction of the acanthomorphic acritarchs. Wide distribution of the Shuram event is
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exemplified by stratigraphic sections from South Australia, Oman, southern California and South
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China. The widespread anomaly has been tentatively attributed to a marine impact. During
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recovery from the Shuram event the enigmatic Ediacaran biota achieved its zenith, only to be
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extirpated and replaced by a polyphyletic assemblage of shelly animals in what is known as the
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Cambrian “explosion”. This extinction-radiation cycle was preceded by glaciation, another
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δ13Ccarb excursion and the highest 87Sr/86Sr values known from marine carbonates. These high Sr
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ratios have been linked to weathering of extensive tracts of continental crust that were elevated
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during amalgamation of the supercontinent Gondwana. Introduction of essential nutrients to the
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oceans would have promoted biological production of oxygen and provided P and Ca for the
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important skeletonization that characterizes the Cambrian “explosion” and caused a quantum
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leap in the preservation potential of animal remains. Turbulent events of the last 50 million years
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of Precambrian time include three glaciations, two large impacts and a massive orogenic episode.
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These dramatic environmental upheavals are held responsible for three consecutive extinction-
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radiation cycles that culminated in the appearance of a diverse array of shelly fossils. Various
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lines of evidence suggest that the metazoans have deep roots so that they too may have been
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subjected to the environmental pressures of the late Ediacaran period clearly illustrated by
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acritarchs and the Ediacaran biota but the long-lived diversity of the metazoan population was
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“suddenly” revealed by the acquisition of biomineralization.
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Keywords: Ediacaran; Shuram; Impacts; Orogeny; Cambrian explosion
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1. Introduction
Despite being the last-named period of the geological time scale (Knoll et al., 2006) the
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Ediacaran, between about 635 and 542 Ma, was perhaps the most important in Earth history.
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During the latter half of the Ediacaran period, the brief span of time between about 580 and 541
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Ma, the Earth may have been completely changed. The atmosphere and oceans were oxygenated
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to a higher degree than previously and there were dramatic changes in the biological world that
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culminated in the so-called “Cambrian explosion” with the appearance of large, complex life
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forms whose ancestors remain shrouded in mystery but whose descendants went on to populate
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the planet. The Ediacaran was a pivotal period in the evolution of Earth and its biota. These
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events are widely recognised but there is little agreement concerning their precise timing or
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understanding of their root cause or causes. Many of the radical environmental and biotic
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changes may have been affected (effected?) by two large impacts – the well-documented
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terrestrial Acraman impact in South Australia (Gostin et al., 1986; Williams, 1986; Williams and
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Gostin, 2005) and the much more speculative Shuram impact proposed by Young (2013).
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Many important environmental and biological events have been attributed to the great
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glaciations that gripped the Earth in the Cryogenian period (e.g. Hoffman et al., 1998; Hoffman, 3
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2009) but the Ediacaran rock record lacks evidence of glaciations of comparable magnitude to
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the older Sturtian and Marinoan events of the Cryogenian period. Paleomagnetic results from
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glacial deposits suggest that, during the Ediacaran period there was a radical change from low to
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high paleolatitudes for deposits from sea-going continental ice sheets.
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The change from low to high paleolatitudes appears to have taken place at about 570 Ma
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when glaciations may have adopted their modern circum-polar distribution. Paleomagnetic data
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also indicate anomalous behaviour of the Earth’s geodynamic system during the Ediacaran
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period, including unusually rapid displacement of lithospheric plates, possible oscillatory plate
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movements and evidence of inertial interchange-related true polar wander (Levashova et al.,
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2014 and references therein). Unprecedented environmental changes may have been the catalyst
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for extinctions and remarkable biological changes including the dramatic proliferation and
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demise of two successive assemblages of acritarchs, the evolution and extirpation of the
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Ediacaran biota and the appearance of the ancestors of modern metazoans. Another unusual
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aspect of the Ediacaran is the occurrence of the world’s largest negative δ13Ccarb (hereafter δ13C)
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anomaly, known as the Shuram-Wonoka anomaly. This exceptionally deep δ13C excursion was
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first discovered in the Wonoka Formation in South Australia (Jansyn; 1990; Pell et al., 1993;
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Urlwin et al., 1993) and was later documented in the Shuram Formation in the Sultanate of
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Oman (Burns and Matter, 1993) and in the Siberian Aldan Shield (Pokrovsky and Gertsev,
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1993). These findings attracted considerable attention (e.g. Melezhik et al., 2005; Le Guerroué et
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al., 2006; Le Guerroué, 2010) and the deep isotopic excursion is now known throughout the
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world as the Shuram anomaly. The Shuram anomaly, unlike those associated with the Sturtian
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and Marinoan glaciations, does not appear to be related to widespread paleoclimatic changes.
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Over the last several decades evidence has accumulated in support of the existence, in
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sedimentary rocks belonging to what is now known as the latter part of the Ediacaran period, of a
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diverse and advanced biota, including early metazoans. Thus there is convergence, around the
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later part of the Ediacaran period, of several unusual events (Fig. 1) including aspects of isotopic
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geochemistry of the oceans, an apparent change in the distribution of the Earth’s climatic zones,
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poorly understood anomalous geophysical phenomena, and unprecedented biological changes.
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The idea that some of these events might be related to the c.580 Ma Acraman impact in South
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Australia was introduced by Grey et al. (2003), Grey and Calver (2007) and Gostin et al. (2011),
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supported by the discovery of changes in organic chemistry across the Acraman ejecta layer
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(McKirdy et al., 2006). In an effort to explain the world’s largest δ13C anomaly and associated
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physical and biological phenomena, a slightly younger (c.570 Ma?) large marine impact was
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proposed by Young (2013). The purpose of this contribution is to more fully explore the possible
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geological and biological consequences of the well documented Acraman impact and the more
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speculative marine impact thought to have been responsible for the Shuram anomaly.
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2. The Acraman impact
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At about the same time as the Acraman impact crater was discovered in the Gawler Ranges
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of South Australia (Gostin et al., 1986; Williams, 1986, 1987) anomalous occurrences of
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relatively large volcanic clasts were noted in the fine-grained Bunyeroo Formation in the
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northern part of the Flinders Ranges in the Adelaide “Geosyncline”, Fold Belt or Rift Complex
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(ARC) (Preiss et al., 2011 and references therein), about 300 km to the east of the impact site
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(Gostin et al., 1986). In an attempt to obtain the age of the Bunyeroo Formation, some of the
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volcanic fragments were dated but the ages were disappointingly old (about 1.6 Ga) compared to
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the anticipated age of the host Bunyeroo Formation (Gostin et al., 1986). Through detailed
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sedimentological and geochemical studies (Gostin et al., 1989) it was discovered that the
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volcanic fragments were derived from the Gawler Ranges (site of the Acraman impact), and
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weak Ir anomalies in the host sediments suggested that the rocks carried an extra-terrestrial
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chemical signal. Eventually the blanket of impact ejecta was extended to the Officer Basin about
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540 km to the NW (Gostin et al., 2010). The age of the Acraman impact is not precisely known
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but it is thought to be about 580 Ma, which is close to the age of the Gaskiers glaciation in
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Newfoundland. It was suggested by Hebert et al. (2010, fig. 1) and Narbonne et al. (2012, fig.
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18.6) that there is a negative δ13C anomaly associated with the Gaskiers glaciation and Gostin et
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al. (2010) reported what they interpreted as ice-rafted debris both above and below the Acraman
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impact layer in the ARC and Officer Basin to the west. Younger glacial deposits of the Fauquier
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glaciation in Virginia (USA) were considered by Hebert et al, (2010) to be about 570 Ma,
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apparently associated with a weak δ13C anomaly. It was also noted by Hebert et al. (2010, p.408)
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that δ13C values as low as -9‰ are present in limestones beneath the Fauquier Formation. They
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commented that these were similar to the low values associated with the Shuram anomaly
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elsewhere, which was considered by them to be older. As discussed later (see Section 5) the
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Fauquier glacial deposits may be of approximately the same age as (or slightly younger than) the
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Shuram event.
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3. Possible biological consequences of the Acraman impact
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Rocks of the Ediacaran period are rich in organic remains, including both microfossils such as
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acritarchs and the enigmatic structures known as the “Ediacaran biota”, although use of this
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name has recently come under critical scrutiny (MacGabhann, 2014). The micro-organisms
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(acritarchs) are of diverse origin and taxonomic affinity but many are thought to be resting cysts 6
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of chlorophyta (green algae). In the Ediacaran period they show remarkable changes in
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abundance, complexity and diversity so that some authors have used them in attempts to set up
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biozones (e.g. Grey et al., 2003; Grey and Calver, 2007; Willman, 2007; McFadden et al., 2008;
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Liu et al., 2014) that may eventually be used to establish global correlations comparable to those
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of the Phaneozoic.
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A significant change from small smooth spherical acritarchs (leiospheres) to larger forms with
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complex ornamentation (acanthomorphic acritarchs) was reported by Grey et al. (2003) in
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Ediacaran rocks of the Officer Basin in Australia. They noted that the change to acanthomorphic
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forms and their diversification occurred about 50 m above the ejecta layer attributed to the
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Acraman impact and speculated that the marked change in acritarch morphology, size and
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diversity might be related to environmental upheavals associated with the impact event. The
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changes in the palynoflora coincide with a short negative δ13C anomaly based on data from
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Calver (2000).
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The Three Gorges area of South China contains a well preserved succession of Ediacaran
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rocks that have also yielded remarkable acritarch assemblages (McFadden et al, 2008; Liu et al.,
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2014). Within the Doushantuo Formation, which is between about 635 and 551 Ma there are
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three important negative δ13C excursions, EN1 to EN3. The first is considered to have developed
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in response to the widespread Marinoan glaciation. The second (EN2) probably corresponds to
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that associated with the less widespread Gaskiers glaciation, which is thought to be about the
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same age as the Acraman impact so that its origin could also be linked to that event. As noted by
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Gostin et al. (2010), ice-rafted debris is associated with the ejecta blanket of the Acraman impact
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in the ARC and Officer Basin. In southern China, as was the case in Australia, the sedimentary
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succession above the negative δ13C anomaly (EN2) shows a remarkable diversification of
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acanthomorphic acritarchs (McFadden et al., 2008). In spite of problems associated with
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environmental and preservational differences among basins and the use of various extraction
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techniques, Grey et al. (2003), Grey and Calver (2007), McFadden et al, (2008) and Liu et al.
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(2014) were optimistic about the possibility of establishing global biozones using Ediacaran
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acritarchs and it was suggested by Dehler (2014) that such biostratigraphically based division
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and correlation may be extended down into the Cryogenian and Tonian periods.
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Interpretation of the δ13C anomaly (EN2) and ensuing changes in the acritarch palynoflora
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are rendered difficult because of the evidence of glaciation that may have been contemporaneous
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with the Acraman impact. Since the glaciation is not widespread it lends support to the
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suggestion of Grey and Calver (2003) and Gostin et al. (2011) that the extinction-radiation cycle
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that affected acritarchs, and associated changes in organic chemistry (McKirdy et al., 2006) were
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attributable to environmental changes related to the Acraman impact.
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4. Stratigraphic context of the Shuram anomaly
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In order to illustrate and compare the important stratigraphical characteristics of rock
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successions in some widely separated areas where the Shuram isotopic anomaly has been
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recognised, four simplified stratigraphic sections are shown in Fig. 1. Although the δ13C anomaly
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was first detected in South Australia and is sometimes described as the Shuram-Wonoka
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anomaly, it has become known in the literature as the Shuram anomaly and that short form of the
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name is retained here.
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The four sections shown in Fig. 1 are not drawn to the same vertical scale. The stratigraphic
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thicknesses of the columns have been adjusted to accommodate the idea that the Shuram isotopic 8
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excursion was essentially synchronous in all four areas. In fact the stratigraphic thickness
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displaying negative δ13C values varies from about 1000 m in the Shuram Formation of Oman (Le
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Guerroué et al., 2006; column 3, Fig. 1) to as little as 30 m in column 1 (Fig. 1) which is from
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the South China area (Liu et al., 2014). If the assumption of synchroneity is correct then average
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accumulation rates of sediment during development of the Shuram anomaly must have varied
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greatly from basin to basin. In spite of these probable differences in accumulation rates the
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Shuram anomaly retains a similar pattern – a rapid drop to values as low as -12‰ in some
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places, followed by a gradual recovery that is spread through a much greater stratigraphic
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thickness (Fig. 1). In all four areas there is a marked contrast between rapid development of the
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anomaly and a gradual return to more “normal” δ13C values.
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4.1. The Shuram Formation, Sultanate of Oman
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This area contains a well preserved mixed succession of fine siliciclastic-carbonate rocks
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that includes sedimentary rocks belonging to the period spanning the Marinoan glaciation to the
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Cambrian–Precambrian boundary. The succession is known from surface exposures in several
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areas but is also well documented from numerous drill cores recovered during oil exploration. As
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shown in Figs. 1 and 2 the succession includes the Shuram Formation for which the large
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negative δ13C excursion is named. As previously outlined (Burns and Matter, 1993; Le Gerroué,
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2010; Young, 2013 and references therein), the deep isotopic anomaly begins above a surface
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that locally displays evidence of exposure, then rapidly drops to its lowest value at a stratigraphic
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level interpreted as a maximum flooding surface, followed by a gradual and generally smooth
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recovery to slightly positive values in the basal portion of the Buah Formation (Fig. 2). Above
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the Marinoan-age glacial deposits there are two large scale shoaling upward cycles, the younger
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of which contains the Shuram anomaly (McCarron, 2000; Grotzinger et al., 2011) (Fig. 2).
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Facies changes in these sedimentary rocks were reported by Lee et al. (2013, fig. 1) who
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portrayed a NE-trending, deeper-water facies bordered by shallower water areas to the east (Huqf
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area) and possibly continuing to deepen to the west where relationships are obscured by uplift
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and erosion. Most investigations were carried out in rocks of the shallower, ramp areas, as
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illustrated in Fig. 2). The isotopic excursion begins above an ancient exposed surface and its
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nadir coincides with a maximum flooding surface, formed during a rapid rise of sea level,
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followed by a gradual return to more ‘normal’ seawater values during prolonged deposition of a
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sequence of marine sediments that occupied the accommodation space resulting from sea level
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rise, possibly accompanied by basin subsidence
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An investigation of the isotopic composition of carbon-rich rocks from a deeper water
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section of the Shuram Formation was carried out by Lee et al. (2013). It was shown by these
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authors that the same large isotopic excursion exists in the deeper-water facies and that the
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commonly observed co-variance of C isotopes in carbonates and organic material did not exist in
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the sediments of either environment during development of the Shuram anomaly, as was also
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reported by Tahata et al. (2014) from the Doushantuo Formation in South China. It was
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concluded by Lee et al. (2013) that pre-existing theories as to the origin of the Shuram excursion,
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such as diagenetic changes or gradual (episodic) oxidation of a large reservoir of dissolved
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organic carbon in the ocean were not viable. They tentatively ascribed the unusually large
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Shuram anomaly to, “systematic global changes in the fractionations between organic and
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inorganic carbon”. A similar conclusion was reached by Tahata et al. (2014) on the basis of
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modelling but these interpretations do not account for the geological phenomena that accompany
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development of the Shuram anomaly throughout the world, such as evidence of a rapid global
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drop in sea level that coincides with initiation of the isotopic excursion (Young, 2013) and the
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similarities between the geological and biological events surrounding the well documented
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Acraman impact (Grey et al., 2003) and those accompanying the Shuram event.
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4.2. The Wonoka Formation, South Australia
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The Shuram anomaly was discovered in the Flinders Ranges of South Australia, in the Wonoka
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Formation (Jansyn, 1990; Pell et al., 1993; Urlwin et al., 1993), which is partly preserved in deep
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(>1 km) steep-sided canyons cut into the underlying sedimentary rocks (Coats, 1964; von der
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Borch et al., 1989; Fig. 1). These canyons are present in a large part of the ARC and in the
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Officer Basin, about 700 km to the NW (Gostin et al., 2010). The generalised stratigraphic
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relationships between the canyons and their host rocks are shown in Fig. 1. Also depicted in Fig.
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1 is the large δ13C anomaly, the interpretation of which is complicated by the existence of the
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deeply eroded channels. The timing of the isotopic anomaly can, however be ascertained in
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“shoulder” areas which escaped incision. An attempt to depict these relationships is shown in
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Fig. 3). As in the Shuram Formation in Oman, the isotopic curve displays an extremely abrupt
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drop in δ13C values (Figs. 1 and 3) and a smooth, gradual recovery. There has been considerable
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debate about the genesis of these canyons but none of the theories (Coats, 1964; von der Borch et
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al., 1989; Williams and Gostin, 2000; Dyson, 2003) provides an explanation for the global
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isotopic anomaly nor for their occurrence in widely separated areas.
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In an important study of the isotopic composition of carbonate clasts in breccias from the
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Wonoka canyons (Husson et al., 2012) it was shown that canyon fill occurred after development
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of the full spectrum of isotopic variations, showing that the anomaly was early and therefore not
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diagenetic, as had been previously suggested. A similar early (syn-depositional) development of
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the Shuram anomaly was favoured by Lee et al. (2013). The canyons were probably subaerially cut although the fill includes both fluvial and marine
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deposits. The presence of basal fluvial deposits suggests a subaerial origin and since they are
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incised into a marine succession this in turn indicates a significant drop in base level, either due
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to a drop in sea level or to rapid thermo-tectonic elevation of the area (Williams and Gostin,
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2000). The global distribution of the Shuram anomaly and associated evidence of lowered sea
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level (Fig. 1) strongly suggest a sudden, short-lived eustatic sea-level fall of unprecedented
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magnitude, followed by a fast recovery (see sea level curve in Fig. 2). Falsification of the
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conclusion that the deep negative δ13C anomaly was a product of global changes in the chemistry
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of the world ocean system demands significant discrepancies among isotopic curves from
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different Ediacaran sections but the available data base continues to document their remarkable
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similarity (Le Guerroué et al., 2006; Verdel et al., 2011, fig.16; Fig. 1).
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4.3. The Rainstorm Member of the Johnnie Formation, California
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The Ediacaran Johnnie Formation (Figs. 1 and 4) is a mixed siliciclastic-carbonate unit that
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was deposited in a passive margin setting (Fedo and Cooper, 2001). The Rainstorm Member is
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the upper part of the formation and is made up of siltstones and sandstones with subordinate
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carbonates. A thin (~2 m) oolite bed, the “Johnnie oolite”, is overlain by siliciclastic rocks that
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contain evidence of storm activity (hummocky cross beds) together with carbonates containing
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some ooids and intraclasts. These carbonates are mainly pink but there are grey limestone beds
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containing calcite crystal fans thought to be derived from aragonite seafloor cements (Pruss et
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al., 2008). These beds were inferred to be older than 580 Ma by Kaufman et al. (2007) but they
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have not been directly dated. They are younger than the Noonday dolomite, which is thought to
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represent the end of the Marinoan glaciation at about 635 Ma, and older than Cloudina-bearing
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beds (~548 Ma) of the overlying Stirling quartzite. It has been suggested that the Rainstorm
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Member may be less than 575 Ma (Verdel et al., 2011), based on correlation with the Clemente
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oolite which is underlain by horizontal burrows and other fossils. The carbonate beds of the
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Rainstorm Member are incised by a regional erosional surface that formed paleovalleys with up
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to 150 m of relief (Fig. 4, right column). These valleys are commonly filled with breccias that
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include a huge fragment of oolitic carbonate thought to have been cannibalized from the Johnnie
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Formation itself (Trower and Grotzinger, 2010). Because the Johnnie oolite has an abrupt contact
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with underlying green and purple shales and because it overlies a weathered and karsted surface
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in places near to the craton margin, it was interpreted by Kaufman et al. (2007) to have been
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deposited above a regional disconformity related to a significant lowering of sea level. This rapid
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fall of sea level could be contemporaneous with that inferred by Young (2013) to have produced
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the deep canyons in the Adelaide Rift Complex, for both mark the onset of a deep δ13C excursion
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comparable to the Shuram anomaly in Oman. The younger valleys incised into the upper
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Rainstorm Member, which superficially resemble the Australian Wonoka canyons, but have a
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much greater width-to-depth ratio and are less steep-sided, may be a manifestation of syn-
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sedimentary normal faulting that affected the passive margin of the southern Cordilleran region
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(Clapham and Corsetti, 2005).
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Grey limestone beds above the Johnnie oolite contain calcite crystal fans considered by
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Bergmann et al. (2011) to represent former aragonitic seafloor cements. These are the youngest
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known Proterozoic seafloor cements of this kind and exhibit very strong negative δ13C values,
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possibly corresponding to the nadir of the worldwide Shuram excursion. The isotopic anomaly is 13
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shown as beginning in the oolite but the lowest values are in the crystal fan-bearing limestones
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(Fig. 4). As in Oman and South Australia (and elsewhere) there is a rapid descent to lowest
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values (in less than 20 m of stratigraphic section). These data were supplemented by Verdel et al.
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(2011) who obtained an extended δ13C curve which closely resembles the classical Shuram
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excursion (Fig. 4). They also reported several smaller negative δ13C anomalies, of unknown
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origin from the overlying rocks. They noted that these anomalies do not appear in some other
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studied sections and suggested that this might be due to breaks (sequence boundaries) in some
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areas. Another possibility is that the small anomalies may be related to more local events such as
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glaciations or they could reflect diagenetic alteration. The succession from the Johnnie oolite to
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the seafloor cement-bearing limestones is thought to represent a transgression. Slightly positive
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δ13C values from the giant oolite raft were interpreted to mean that the Rainstorm Member is
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younger than the Shuram anomaly but data published by Bergmann et al. (2011) (see Fig. 4)
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show a dramatic negative excursion in the lower part of the Rainstorm Member comparable to
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those displayed by the Shuram anomaly in other locations and the remarkable similarity between
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the large anomaly in the Johnnie Formation and the Shuram anomaly (Verdel et al., 2011, fig.
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16) leaves little doubt that it is the result of the same widespread and profound changes in ocean
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chemistry.
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Thus the Johnnie oolite and succeeding seafloor cements in the Rainstorm Member of the
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Johnnie Formation were deposited after a hiatus, first in shallow waters followed by a rapid sea
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level rise culminating in a maximum flooding surface. This transgressive, deepening succession
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is accompanied by development of a strong negative δ13C anomaly (Bergmann et al., 2011), in
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the same manner as depicted by Young (2013, fig. 4) for the Shuram anomaly in Oman (see Fig.
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2). Development of incised breccia-filled valleys in the same area may be related to 14
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contemporary fault activity (Clapham and Corsetti, 2005), as opposed to global sea level
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changes, for these erosive events appear to have occurred later than the isotopic excursion. The
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effects of sudden lowering of sea level are thus expressed differently in different locations
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(canyons vs. exposed and karsted surfaces) depending on the water depth, and possibly
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proximity to the putative marine impact site, that existed prior to the dramatic drop of sea level
312
accompanying development of the Shuram anomaly (Fig. 1).
313
4.4. The Doushantuo Formation, South China
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Among the world’s best-preserved carbonate-rich Ediacaran successions, the Three Gorges
315
area in South China has received some of the most detailed study (Liu et al., 2014 and references
316
therein). Although the section is condensed compared to some other Ediacaran sucessions such
317
as that in Oman, it is dominated by carbonates that are rich in chert and phosphate nodules so
318
that it is ideal for investigation of δ13C variations and also for tracing the distribution of
319
microfossils such as acritarchs (McFadden et al., 2008; Liu et al., 2014). The potential
320
importance of phosphate in early biomineralization was pointed out by Cook and Shergold
321
(1984) and Brasier and Lindsay (2000), who attributed the abundance of phosphorus to overturn
322
of an anoxic deep ocean and contemporaneous orogenic and plate tectonic activity. The isotopic
323
geochemistry and biostratigraphy of the Chenjiayuanzi section were studied by Liu et al. (2014).
324
A simplified depiction of their stratigraphic section, together with variations in δ13C, is shown in
325
Fig. 1). They recognised three negative excursions in the isotopic curve, separated by two
326
positive ones and pointed out that there is a close relationship between the distribution (number
327
of individuals and taxa) of acritarchs and the isotopic curve. Positive anomalies appear to
328
coincide with biotic blooms whereas extinctions took place at the time of initiation of two of the
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negative excursions. They recognised two contrasting zones of acanthomorphic acritarchs; a
330
rather sparse assemblage (seven known taxa) in the lower part of the section (Member II) and a
331
much richer and more diverse assemblage comprising more than 24 taxa in the upper part
332
(Member III) (see Fig. 1). Ediacaran acanthomorphic acritarchs disappear during the basal part
333
of the anomaly in the upper part of Member III of the Doushantuo Formation - at the onset of the
334
Shuram excursion. The lower negative excursion coincides with disappearance of the lower
335
assemblage of Liu et al. (2014) and the upper one is associated with loss of the more varied and
336
abundant acritarchs of the upper assemblage. The strong relationship between these chemical and
337
biological events suggests that they may be related in some way, possibly through environmental
338
changes, as suggested by Liu et al. (2014).
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The Doushantuo Formation of the Chenjiayuanzi section is constrained by U-Pb dates from
340
ash beds at the base (635 Ma) and top (551 Ma) but there are no available dates for subdivision
341
of the formation, so that the ages of the isotopic anomalies and biotic changes are not precisely
342
known. The lowest negative carbon isotopic anomaly is associated with glacial deposits
343
considered to be products of the widespread Marinoan glaciation. The highest and most severe
344
negative excursion is considered to be equivalent to the widely distributed Shuram anomaly in
345
Oman, South Australia and elsewhere (Le Guerroué et al., 2006; Young, 2013). The age and
346
origin of this isotopic anomaly are poorly understood but it possibly occurred at about 570 Ma
347
(see discussion and references in Young, 2013). The remaining small negative excursion has also
348
been identified in other parts of the world and has been attributed by some (Narbonne et al.,
349
2012, fig. 18.6; Hebert et al., 2010, fig. 1) to the Gaskiers glaciation, which has been dated at 580
350
Ma (Bowring et al., 2003). The picture is clouded, however, by the occurrence of a large impact
351
scar and an associated widespread ejecta blanket in Ediacaran sedimentary rocks of the Adelaide
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Rift Complex and in the Officer Basin to the NW (Gostin et al., 2011). Once again precise dates
353
are not available but the age of the Acraman impact was estimated to be about 580 Ma and
354
Gostin et al. (2010) presented evidence of ice-rafting in fine grained sedimentary rocks deposited
355
both before and after the impact layer. All of these lines of evidence, some admittedly
356
circumstantial, point towards a possible ~580 Ma age for the middle negative anomaly and
357
support the idea that it was contemporaneous with both the Gaskiers glaciation and the large
358
Acraman impact in South Australia. Evidence favouring a marine impact origin for the Shuram
359
anomaly was listed by Young (2013, p. 8). The oldest negative carbon isotopic anomaly in the
360
South China section appears to be related to the Marinoan glaciation (e.g. Halverson et al., 2005,
361
2010) which is thought to have terminated at about 635 Ma. The middle anomaly may be
362
associated with the Acraman impact and related to, or at least contemporaneous with the
363
development of glaciers attributed to the Gaskiers glaciation at about 580 Ma. The upper
364
(Shuram) anomaly may have occurred about ten million years later (~570 Ma) as a result of an
365
even larger marine impact (Young, 2013). The origin of a negative anomaly close to the
366
Precambrian–Cambrian boundary is uncertain but there is evidence of glaciations at that time in
367
several countries.
368
5. Comparison of the four areas
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In spite of highly contrasted tectonic settings thicknesses and facies the Ediacaran
370
sedimentary rocks of the four areas discussed above display some remarkable similarities in δ13C
371
patterns, suggesting that the Shuram event reflects a profound and rapid event that affected the
372
global ocean system and left a strong imprint, regardless of local environmental conditions. This
373
idea is echoed in the recent investigation by Lee et al. (2013) of a deeper water, organic C-rich 17
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facies of the Shuram Formation in Oman, where they documented the presence of the large δ13C
375
anomaly previously found in quite different (shallower-water) environments in Oman. In all four
376
currently widely separated parts of the world (South Australia, Oman, California and South
377
China) considered here there is evidence of an abrupt lowering of sea level. In the Adelaide Rift
378
Complex and Officer Basin in Australia this had the effect of causing incision of deep, steep-
379
sided canyons but in a craton-marginal setting in California or in a marine platformal area
380
(Oman) there is evidence of shallowing and local development of griked and karsted surfaces
381
that testify to some subaerial exposure. Sedimentological evidence from South China is less clear
382
but the presence of fine grained siliciclastic rocks, followed by dolomitic carbonates (Fig. 1,
383
section 1) is similar to that in Oman. Relative lowering of sea level could be explained locally,
384
by thermo-tectonic uplift (Williams and Gostin, 2000) but it is unlikely that the effects of such
385
uplift would have been so widespread, making eustatic sea level fall a more attractive hypothesis.
386
Large drops in sea level have also been attributed to Messinian-style evaporation (Christie-Blick
387
et al., 1990) but again this would presumably have affected a restricted area. It has also been
388
proposed that glaciations may have played a role in lowering sea levels (Gaucher et al., 2004;
389
McGee et al., 2013) but most of the incised valleys and exposed surfaces show no evidence of
390
glacial activity. However it is possible that, just as the ejecta layer of the Acraman impact is
391
associated with evidence of ice-rafting (Gostin et al., 2010), and is thought to be approximately
392
the same age as the Gaskiers glaciation, the proposed Shuram impact may be linked to a younger
393
cold climatic episode such as that responsible for the Fauquier glaciation of Hebert et al. (2010).
394
In both cases the glaciations do not appear to have been very widespread so that even if they
395
contributed to the development of the δ13C anomaly, such local glaciations are unlikely to have
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been the primary cause of the world’s greatest δ13C anomaly. Development of the Shuram
397
anomaly during a marine transgression also makes a glacial interpretation unlikely.
398
6. Extinctions and evolutionary innovations
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The Ediacaran period is unique in that its sedimentary rocks contain evidence of a variety of
400
complex life forms, some of which went on to colonize the planet following the “Cambrian
401
explosion”. The reality, or at least severity, of the Cambrian “explosion” was recently questioned
402
by Meert (2013) who noted that molecular clocks and evidence of early provincialism of
403
Ediacaran faunas (and Cambrian trilobites) suggest that these organisms have a pedigree that
404
extends deep into, or beyond the Ediacaran period (e.g. Hoffman, 2009, fig. 9). Some of these
405
early complex remains are known as the Ediacaran biota, although use and misuse of that term
406
have come under scrutiny recently (MacGabhann, 2014). It was claimed by Retallack (2012,
407
2013) that the so-called Ediacaran biota in the type area of the Ediacaran Range of South
408
Australia may have been mis-represented as including remains of marine organisms. He claimed
409
to have discovered evidence of paleosols in the succession and considered the “fauna” to be
410
lichens or microbial colonies. This radical departure from the general belief that the Ediacaran
411
biota includes marine animals was challenged by Xiao et al. (2013) who presented evidence that
412
forms claimed by Retallack (2013) to have existed in ancient soils are preserved in marine shales
413
elsewhere. They also stated that the morphology of the fossils was unsuited to subaerial survival.
414
Likewise Menon et al. (2013) described and figured sedimentary fabrics associated with the
415
taxon Aspidella, from the 560 Ma Fermeuse Formation in southern Newfoundland which were
416
interpreted as evidence of both vertical and horizontal movements of a cnidarian-grade animal. It
417
was suggested by Xiao et al. (2013) that further study of the depositional environments
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(subaerial or subaqueous) of the rocks in the Ediacara Member of South Australia is imperative.
419
As discussed by McFadden et al. (2008) and Narbonne et al. (2012) attempts to use Ediacaran
420
fossils for biostratigraphical correlation have been frustrated by variable depositional and
421
preservational environments among Ediacaran sedimentary basins.
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About 10 Ma after the Gaskiers glaciations, at about 570 Ma, there was an increase in the
422 87
Sr/86Sr ratio in carbonates (Sawaki et al., 2013, figs. 6, 9), which could correspond to the
424
beginning of the Shuram anomaly. A much more prominent rise in the Sr ratio near the
425
Precambrian–Cambrian boundary was interpreted by Sawaki et al. (2013) to reflect input of
426
continental weathering products from the Trans-Gondwana orogen, thus providing a source of
427
nutrients that promoted proliferation of marine photosynthetic organisms and ultimately
428
generated the oxygen required to fuel the metabolic activities of higher life forms. On the other
429
hand, evidence presented by Meert (2013) suggests that metazoan evolution began much earlier
430
and that the Cambrian “explosion” had a long (but largely cryptic) history extending deep into
431
Ediacaran time. The idea of an explosion of new life forms was also downplayed by Brasier
432
(2009) when he pointed out that the so-called “explosion” lasted for several millions of years. He
433
noted that the Cambrian “lagerstätten”, such as the Burgess Shale are much younger (about 520
434
Ma) than the beginning of the Cambrian (542 Ma) and expressed the opinion that there was a
435
sufficiently long time for the observed new forms to have evolved without involving an
436
explosion of life forms. On the other hand, the discovery of so many unknown and relatively
437
sophisticated life forms in such “lagerstätten”, rather than demonstrating the sudden appearance
438
of such forms, as claimed by some, opens a narrow window on the teeming millions of soft-
439
bodied organisms that populated the late Ediacaran oceans but vanished, leaving barely a trace.
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The idea of marine fertilization resulting from continental uplift was supported by Santosh et
441
al. (2014) who proposed that oxygenation and evolution of higher life forms were related to
442
crustal elevation associated with the Laurentian superplume (Mitchell et al., 2011).
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It was suggested by Rothman et al. (2003) and Fike et al. (2006) that the Ediacaran negative
444
δ13C isotopic excursions were related to progressive oxidation events in the world ocean, On the
445
basis of investigations of Ediacaran successions in the Mackenzie Mountains in the Canadian
446
Cordillera and in South China it was suggested by Shen et al. (2008) that oxygenation of the
447
deep oceans occurred at around 580 Ma. It was proposed by Fike et al. (2006) that maximum
448
diversity of acritarchs accompanied such oxidation events, but this assertion was challenged by
449
Grey and Calver (2007) who, on the basis of data from stratigraphic sections in South Australia,
450
pointed out that acritarch diversity was greatest before onset of the Shuram anomaly (and
451
putative oxidation event). This finding also seems to be confirmed by McFadden et al. (2008, fig.
452
1) who also concluded that diversification of acanthomorphic acritarchs was high before onset of
453
the Shuram anomaly and suggested that these organisms may have become extinct at that time,
454
as did Liu et al. (2014). One of the most striking aspects of the report by McFadden et al. (2008,
455
fig. 1) is the portrayal of the disappearance of 27 acritarch taxa at the onset of the EN3 δ13C
456
anomaly (presumed Shuram equivalent), and appearance of 20 new taxa (multicellular algae)
457
near its termination. These events were interpreted by McFadden et al. (2008) as reflecting
458
oxidation of the DOC-rich Ediacaran ocean but it was suggested (Young, 2013) that this
459
important change may have been brought about by a large marine impact. According to
460
McFadden et al. (2008) recovery from the Shuram event was followed by proliferation of a
461
variety of organisms including multicellular algae and possible metazoans, followed by frondose
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462
Ediacaran organisms and trace fossils that attest to the presence of bilaterian animals, and small
463
biomineralised animals such as Cloudina. Five assemblage zones based on the range of more than 60 species of acritarchs were
465
recognised by Grey (2005) in Australian Ediacaran rocks. She also noted that these zones may be
466
further refined and show promise for world-wide correlations. Although emphasizing taxonomic
467
and taphonomic problems in dealing with acritarchs, Willman (2007) was also optimistic about
468
their great potential for correlation of Ediacaran successions around the world, such as those in
469
Australia, Siberia, the East European Platform, South China, Svalbard and India. The enigmatic
470
Shuram event appears to have played an important role in significant geochemical and biotic
471
changesn around the world.
472
7. Are (at least) two Ediacaran δ13C anomalies related to impacts?
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If the approximate 580 Ma date for the Acraman impact is correct, then it is possible that it
474
coincided with the Gaskiers glaciation which has, in turn, been linked to a negative δ13C
475
excursion (Hebert et al, 2010). Recovery from a similar (contemporary?) isotopic anomaly in
476
South China (EN2 of McFadden et al., 2008, fig. 1; Liu et al., 2014; fig. 6) is marked by
477
proliferation and innovation among acanthomorphic acritarchs, as reported earlier from the post-
478
Acraman succession in the Officer Basin of Australia (Grey and Calver, 2007). Thus recovery
479
from the Acraman impact was associated with radiation of acanthomorphic acritarchs. Marked
480
changes in organic geochemistry (steranes) above the Acraman ejecta layer were reported by
481
McKirdy et al. (2006). A possible link between the Acraman impact and biotic changes was
482
predicted by McLaren and Goodfellow (1990) and documented by Grey et al. (2003) and Gostin
483
et al. (2011). As noted above there was near-extirpation of these new acritarch taxa at the onset
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of the EN3 anomaly (Fig. 5) and introduction of many new life forms assigned to the Ediacaran
485
biota during the return to “normal” δ13C values. The similarity between these two dramatic
486
episodes of biotic change may point towards a similar cause but whereas the evidence for the
487
Acraman impact and its widespread ejecta layer is compelling (Gostin et al., 2010) there is no
488
known impact scar that can be linked to the much stronger Shuram anomaly (EN3) and
489
associated biotic changes. This in no way precludes such an origin, for about two thirds of the
490
world’s surface is underlain by oceanic crust so that scars left by most ancient impacts (older
491
than the oldest existing oceanic crust) would have been lost by subduction. For a recent
492
discussion of this question see Johnson and Bowling (2014).
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It was proposed by Kaufman et al. (1997) that the Neoproterozoic isotopic anomalies were
494
related to ice ages and although there appears to be a fairly strong case for such a relationship for
495
the widespread Cryogenian glaciations it seems less certain for the Ediacaran period. The
496
world’s deepest δ13C excursion (the Shuram anomaly) might be expected to be associated with a
497
correspondingly great glaciation but neither it nor the slightly older and much weaker negative
498
anomaly present in rocks deposited around the time of the Acraman impact appears to have
499
developed during a time of widespread glaciation, although there is some evidence that both may
500
have been accompanied by local glacial activity (Gostin et al., 2010; Hebert et al., 2010). The
501
impact theory for the Shuram excursion is supported by its asymmetrical shape with a sudden
502
drop of δ13C values followed by a gradual recovery. If the deep and long-lived Shuram anomaly
503
were related to a severe glacial episode, as deduced by Halverson et al. (2005, 2010) for some
504
older, Cryogenian isotopic anomalies, it would presumably have occurred at a time of lowered
505
sea level rather than during a transgression as appears to be the case for the Shuram anomaly
506
(Fig. 1). It is possible that local glaciations were triggered or enhanced by environmental changes
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in the aftermath of an impact, but it was suggested by Gostin et sl. (2010) that some glacial
508
transport preceded emplacement of the ejecta layer in the ARC and Officer Basin in South
509
Australia. A diagenetic origin for the anomaly seems unlikely in view of its widespread (global?)
510
distribution and its occurrence in both shallow-water settings and in more basinal carbon-rich
511
facies of the Shuram Formation in Oman (Lee et al., 2013). Evidence of a widespread and
512
dramatic lowering of sea level immediately prior to onset of the Shuram anomaly is more
513
compatible with eustatic processes than thermo-tectonic uplift (Williams and Gostin, 2000) or
514
local Messinian-type drawdown by evaporation in isolated basins (Christie-Blick et al., 1990).
515
Coarse sedimentary breccias are present in several areas where the Shuram anomaly has been
516
identified. These coarse deposits may have been produced by tsunamis, an interpretation
517
consistent with their common association with finer grained siliciclastic rocks displaying
518
hummocky cross-bedding, suggesting the prevalence of storms – features that are likely in the
519
aftermath of a marine impact. The lowest δ13C values commonly coincide with a maximum
520
flooding surface – the opposite of what would be expected if the anomaly were related to a
521
glaciation. These relationships may be explained by removal of large volumes of DOC- and
522
particulate organic carbon-rich seawater during an impact. Rapid return of oxidised organic
523
carbon and incorporation into carbonate sediments (mineralization) would explain the unusually
524
low δ13C values that accompany the maximum flooding stage when oceanic waters drained back
525
from flooded continents or condensed from the atmosphere. After sea level was restored to close
526
to pre-impact levels there was a gradual return to “normal” marine isotopic ratios as
527
photosynthetic organisms became re-established and many new taxa appeared (Grey et al., 2003;
528
McFadden et al., 2008; Liu et al., 2014). Such evidence is of necessity circumstantial because of
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529
the absence of an impact scar but this must be the case with most ancient impacts since the
530
majority would have occurred in an oceanic setting. These considerations have potentially important relevance to the apparently sudden
532
appearance of most animal phyla that are recognised today. Many have noted the occurrence of
533
biotic extinctions and radiations throughout the fossil-rich Phanerozoic sedimentary record
534
(McLaren, 1983) but similar events also punctuated the biotic history of the Ediacaran period
535
(Fig. 5). Among a myriad of potentially important environmental factors (McLaren, 1983)
536
impacts of asteroids or comets present the possibility of extremely rapid and dramatic
537
environmental change. Escape and survival in the face of such rapid and dramatic environmental
538
changes were optimal for organisms that were mobile and capable of surviving in an
539
environment that had limited nutritional resources and was therefore highly competitive. Such
540
conditions would have favoured organisms that were mobile and predatory. Predation may have
541
spawned the introduction of body armour (exoskeletons) which not only provided protection but
542
also greatly enhanced the probability of preservation, introducing a quantum change in the fossil
543
record. This simple change was probably a major factor in “Darwin’s dilemma” concerning the
544
apparently sudden appearance of many phyla at the beginning of the Cambrian period. The
545
origin of these phyla probably extends far back into or beyond the Ediacaran period but
546
preservation of evidence was compromised by their soft-bodied nature so that only glimpses are
547
afforded by exceptional preservation in “lagerstätten” such as the Burgess shale and others in
548
South China that reveal a remarkably diverse assemblage of soft-bodied organisms. Darwin’s
549
dilemma does not point to “intelligent design” but rather can be compared to the difficulties
550
involved in solving a murder in the absence of a body. Resolution lies in the length of time
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available for evolution to have taken place and an understanding of the turbulent environmental
552
history that preceded the Cambrian “explosion”.
553
8. Discussion and conclusions
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There are many remaining problems in attempts to understand the meaning of events in the
555
Cryogenian and Ediacaran periods. Few sedimentary deposits have been precisely dated and
556
problems are emerging regarding some assumed inter-continental correlations. For example,
557
geochronological data from the type “Sturt” glaciation in Australia (Kendall et al., 2006; Fanning
558
and Link, 2008) suggested that these glacial deposits are significantly younger than supposed
559
correlatives in North America (e.g. Macdonald et al., 2013 or 2010), leading Hill et al. (2011) to
560
state that, “There is no firm evidence in Australia for the Sturt glaciations extending back much
561
beyond 660 Ma.”, whereas the “Sturt glaciations” in NW North America has been dated at about
562
716.5 Ma. This discrepancy leads to doubts about the age of the undated Elatina glaciation in
563
Australia, which is commonly assumed to be part of the Marinoan event at about 630 Ma. It is
564
becoming apparent that some negative δ13C anomalies in Cryogenian carbonate rocks are
565
followed by a rapid return to positive values before deposition of the first diamictites (Halverson
566
and Shields-Zhou, 2011). There is no obvious explanation for this trend but perhaps it could
567
reflect increased biological activity when deposition of glacial sediments began because of the
568
associated influx of nutrients. Repetition of similar patterns in the δ13C curve makes it difficult to
569
differentiate rock sequences of different ages. The 87Sr/86Sr curve (Halverson and Shields-Zhou,
570
2011) shows an overall rise during the Ediacaran period with small, steeper rises at times of
571
glaciation (and at the time of the Shuram anomaly). Thus there may be difficulty in separating
572
the effects of glaciations and impacts, for both the Acraman impact in South Australia and the
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putative Shuram impact may be associated with glacial episodes. The origin of a negative δ13C
574
anomaly near the base of the Cambrian period may be related to glaciations that have been
575
documented in a number of places (Caby and Fabre, 1981; Bertrand-Sarfati et al., 1995;
576
Deynoux et al., 2006; Chumakov, 2009, 2011a, b; Shen et al., 2010; Chumakov et al., 2011;
577
Jenkins, 2011; Vernheta et al., 2012).
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The δ13C record of the Ediacaran period is punctuated by a series of alternating positive and
579
negative anomalies (Fig. 5). The oldest negative excursion (EN1) follows, and is probably
580
related to the widespread Marinoan glaciation. The second is quite small, unlabelled on Fig. 5,
581
and of unknown origin. The third (EN2) is thought to have occurred at about 580 Ma and its
582
origin has been linked to the Acraman impact whose ejecta blanket is under- and overlain by
583
material interpreted as ice-rafted debris (Gostin et al., 2010). There is clear evidence of profound
584
biotic changes (Grey et al., 2003; Grey and Calver, 2007; McFadden, et al, 2008; Liu et al, 2014)
585
and changes in organic chemistry (McKirdy et al., 2006), which have been attributed by some to
586
environmental upheavals and biotic responses associated with the Acraman impact. Following
587
the striking diversification and increased abundance of acanthomorphic acritarchs after the
588
Acraman extinction there was a second extinction event that decimated these forms (Liu et al.,
589
2014; Fig. 5). The Shuram negative anomaly (EN3) is the deepest known from the geological
590
record and is now recognised around the world. This anomaly involved a rapid descent to its
591
lowest values which, in many places appear to have taken place during a rapid sea level rise that
592
followed closely on a significant drop in sea level. A fall in base level is perhaps best manifested
593
in the development of deep, canyons in South Australia and elsewhere but may also take the
594
form of karsted surfaces. The Shuram anomaly was recently identified by Macdonald et al.
595
(2013) in the June beds (informal name) and Gametrail Formation in the northern part of the
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Canadian Cordillera. They described the large δ13C anomaly as forming and reaching its nadir
597
during a transgression and recovering gradually during a high sea-level stand. They suggested
598
that local variations in intensity of the anomaly might indicate that there were variations in
599
abundance of authigenic carbonate but such a mechanism is unlikely to have produced the
600
strikingly similar profiles of the Shuram anomaly around the world (Verdel et al., 2011, fig. 16;
601
Macdonald et al., 2013, fig. 13). The inferred sea level history is closely comparable to what has
602
been suggested here (see Fig. 2) and by Young (2013) for conditions during development of the
603
Shuram anomaly in several widely separated parts of the world. The anomaly has previously
604
been ascribed to oxidation of the deep oceans, which were thought to have held a large reservoir
605
of organic carbon in both dissolved and particulate form but there was no satisfactory causal
606
mechanism. It is here suggested that oxidation of the oceans may have been brought about by a
607
large marine impact, causing tsunamis, unprecedented erosion of certain parts of the continents
608
(the canyons), expulsion of much of the ocean on to the continents and into the (oxygen-bearing)
609
atmosphere, and its return, together with a rich supply of oxidised organic carbon, to the world
610
oceans, giving rise to the rapid plunge in δ13C signatures, accompanied by evidence of a world-
611
wide dramatic rise in sea level.
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As the oceans gradually recovered from the chemical and physical upheavals set in motion
613
by the Shuram impact event, the enigmatic Ediacaran biota took advantage of new ecological
614
niches as it expanded and diversified, accompanied, like the earlier acritarch radiations, by a
615
positive δ13C anomaly (Fig. 5). Close to the Precambrian–Cambrian boundary, marked once
616
more by a negative δ13C anomaly (of uncertain origin) the Ediacaran biota collapsed and was
617
apparently replaced by a very diverse and successful group of metazoans that dominated the
618
megascopic biological world for the rest of its known history. As noted above, late Ediacaran
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glaciations may have played an important role in producing the negative δ13C excursion and in
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the demise of the Ediacaran biota near the beginning of the Cambrian period. The diversity of the early metazoans (almost all extant animal phyla appeared at that time)
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greatly troubled Charles Darwin for it appeared to contradict his gradualistic evolutionary theory
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and has come to be known as ‘Darwin’s dilemma’. Recent discoveries of fossil remains of
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animals older than the Marinoan glaciation (c. 635 Ma) by Maloof et al. (2010) and as old as 766
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Ma (Meert et al., 2011) suggest that complex animal life began long before the so-called
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“Cambrian explosion”. The reasons for the extirpation of the Ediacaran biota and the emergence
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of the highly successful metazoan population are unknown. When the magnitude of some of the
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Cryogenian glaciations was realised it was suggested (e.g. Hoffman et al., 1998; Hoffman, 2009)
629
that environmental perturbations associated with such significant glacial episodes may have been
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the catalyst for emergence of higher life forms but it appears that many of the greatest
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evolutionary events (extinctions and radiations) occurred long after these great ice sheets had
632
vanished, although the more restricted Ediacaran glaciations (Cawood and Hawkesworth, 2014,
633
fig. 1A) may also have played a role. The latter portion of the Ediacaran period contains
634
evidence of at least two major impacts whose massive global environmental influence may have
635
influenced the evolution of metazoans and contributed to the so-called “Cambrian explosion”. It
636
has been proposed (McLaren, 1983, and references therein) that rapid rates of environmental
637
change have a profound influence on biotic extinctions and radiations. Large-scale impacts
638
would have wreaked instantaneous havoc in almost all surface environments. Following
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inevitable extinctions, survival and recovery would have been facilitated by mobility. Extirpation
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of the Ediacran biota may be explained, in part by their sessile and slow-moving life styles. More
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mobile organisms would have been able to migrate to refugia. Competition for limited food
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resources could have promoted predation which would, in turn, have favoured survival of
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creatures with body armour (exoskeletons). The appearance of protective exoskeletons may have
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played an important role in the perception of an evolutionary explosion near the beginning of the
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Cambrian period. Perhaps resolution of “Darwin’s dilemma” – the apparently sudden appearance
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of an already highly diverse biota - lies, at least in part, in biotic changes that took place in
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response to tumultuous environmental catastrophes in the aftermath of two (or more?) major
648
impacts during the pivotal latter portion of the Ediacaran period. The dramatic biological
649
changes (extinction-radiation cycles) clearly manifested in acritarch populations and in the rise
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and demise of the Ediacaran biota must have been paralleled by similar extraordinary changes in
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the metazoan population, which may be older than 770 Ma (Meert et al., 2011), changes
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rendered cryptic by their soft-bodied nature and attendant poor preservation potential.
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In addition to environmental upheavals associated with impacts and glaciations the
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Precambrian–Cambrian transition was also a time of particularly widespread mountain building
655
as a result of the Trans-Gondwana orogeny (Squire et al., 2006) or associated uplift related to a
656
large mantle plume, either during the assembly of the Gondwana supercontinent or shortly
657
thereafter (Santosh et al., 2014). These elevated regions of continental crust have been held
658
responsible for flooding world oceans with essential nutrients that could have contributed to
659
increased atmospheric oxygen that fuelled the energy needs of the burgeoning metazoan
660
population of the Cambrian explosion. Clearly there was something very unusual about the
661
composition of seawater at around the Cambrian–Precambrian boundary. For example it was
662
shown by Shields (2007) that the continental component of
663
had its highest value in all of geological history at the beginning of the Phanerozoic eon. Similar
664
high Sr isotopic values were also noted by Sawaki et al. (2013, fig. 9) who illustrated
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Sr/86Sr values in carbonate rocks
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anomalously high and oscillating values in Ca isotope ratios at the same boundary. Unusually
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strong input of continental materials at the end of late Ediacaran glaciations and at a time of
667
exceptionally widespread orogenic or thermal uplift would have introduced a large supply of
668
nutrients to latest Ediacaran and early Cambrian oceans and raised the
669
seawater. The Ca content would also have been elevated (Sawaki et al., 2013), facilitating the
670
onset of widespread mineralized exoskeletons. There may also have been biological pressures
671
that moved the animal population towards development of a hard protective outer layer – the
672
proliferation of macrophagous predators (Bengston, 2002). The idea that protection from
673
predation may have been at the root of exoskeletal development was suggested at the beginning
674
of the twentieth century (Evans, 1912) but has subsequently fallen out of favour, although
675
Conway Morris (1986) demonstrated a high level of predation in part of the Burgess Shale
676
lagerstätte and it was suggested by Germs (1972) and Grotzinger et al. (2000) that the late
677
Ediacaran shelly fossil Cloudina might bear evidence of such predation. As with most geological
678
problems there is a tendency to focus on one causative mechanism, whereas the Earth is a
679
complex evolving and interactive system so that spectacular occurrences such as the relatively
680
abrupt appearance of exoskeletons are more likely to reflect convergence of a number of
681
disparate factors (the “perfect storm” theory) including multicellularity (safety in size), the need
682
for a rigid frame to support a larger body, protection from predators and other physical dangers,
683
extraordinary composition of seawater, as shown by high
684
biomineralization (see Porter, 2010 and references therein) and involving involuntary “hardening
685
of the arteries”). Convergence of climatic changes and unusual seawater composition, related to
686
large scale tectonic events near the beginning of the Phanerozoic eon, together with unusually
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severe selection pressures related to two large impact events may have produced ideal conditions
Sr/86Sr ratios in
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Sr/86Sr ratios, facilitating
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for simultaneous appearance of exoskeletons in a number of phyla, giving the false impression of
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an unparalleled explosive evolutionary advance that forever changed the nature of the fossil
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record and drew an artificial but indelible line between Precambrian and Cambrian life forms
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Acknowledgements
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I would like to express my gratitude to the Natural Sciences and Engineering Research
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Council of Canada for financial assistance in carrying out research on glacial deposits. These
696
investigations would have been impossible without the assistance of many individuals, who gave
697
generously of their time and expertise on field trips and at meetings in many parts of the world.
698
Particularly instructive have been the pioneering efforts of G. E. Williams, V. A. Gostin and K.
699
Grey in unravelling the complex story of the Acraman impact and its important environmental
700
and biological ramifications. I thank D. M. McKirdy for drawing my attention to important
701
literature on the Australian Ediacaran rocks. I am also grateful to anonymous reviewers for their
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time and help and thank Dr. M. Santosh for guiding the manuscript through the editorial process.
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Santosh, M., Maruyama, S., Sawaki, Y., Meert, J. G., 2014. The Cambrian Explosion: Plumedriven birth of the second ecosystem on Earth. Gondwana Research 25, 945-965. Sawaki, Y., Tahata, M., Ohno, Komiya, T. T., Hirata, T., Maruyama, S., Han, J., Shu, D., 2014.
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Figure captions
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Figure 1. Generalised stratigraphic columns from four currently widely separated areas (see
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map inset) containing sedimentary rocks of the Ediacaran period. The columns are
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numbered as follows: 1 – The Doushantuo Formation of the Chenjiayuanzi section in South
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China (after Liu et al., 2014). The δ13C curve is from the same reference; 2 – Rainstorm
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Member of the Johnnie Formation in California (after Bergmann et al., 2011, fig. 1 and
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Verdel et al., 2011); 3 – The Shuram Formation in the Sultanate of Oman, based on Le
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Cuerroué et al. (2006, fig. 2, and references therein); 4 – The Wonoka Formation in the
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Flinders Ranges of South Australia (after Christie-Blick et al., 1995, fig. 3). See text for
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source of isotopic curve. In spite of their present wide separation and disparate rock types
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the Ediacaran rocks of these areas all show similar deep asymmetrical negative δ13C
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excursions which appear to be related to a rapid fall and recovery of sea level, followed by
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deposition of a shoaling upward succession. The isotopic anomaly reaches its nadir rapidly
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but the return to more “normal” marine values is gradual. The columns are not drawn to the
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same scale but rather to facilitate comparison of the Shuram isotopic anomaly. The Wonoka
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Canyons (section 4) are up to 1 km in depth. If the inferred large sea level changes were
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contemporaneous then they, and the closely associated isotopic excursion, may provide a
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means of establishing precise correlation among these rocks. See text for fuller discussion.
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Figure 2. Simplified representation of the stratigraphy of Ediacaran rocks from Jabal Akhdar
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in northern Oman (after Le Guerroué et al., 2006, fig. 2). The δ13C curve is from Fike et al.
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(2006, fig. 1). An abrupt sea-level drop and subsequent rise are inferred from karst
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development in the Oman area — also expressed as deep canyon incision elsewhere (see 47
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column 4, Fig. 1), and identification of a maximum flooding surface. It was suggested
1009
(Young, 2013) that these sea level changes and the unusual deep isotopic anomaly resulted
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from a large marine impact at the time of initiation of the negative δ13C anomaly. A
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shoaling-upward succession then records a slow return to “normal” isotopic values. See text
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for discussion. M.F.S. – maximum flooding surface (after Le Guerroué et al., 2006, their fig.
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2).
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Figure 3. Schematic cross sections (after Christie-Blick et al., 1990) to illustrate how incision
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of the Wonoka canyons in the northern Flinders Ranges of South Australia may be related to
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a proposed marine impact. A – The impact occurs during deposition of unit Wonoka 3 (after
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Christie-Blick et al. (1990, fig. 3). B – Sea level temporarily falls as a result of the impact,
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causing subaerial excavation of the canyons (up to 1 km deep) and some fill by fluvial
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processes and mass flow. C – Sea level rises as expelled oceanic waters return and the
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canyons fall under marine influence. It is during this phase that the isotopic excursion
1022
reaches its nadir. D – Sediments of the upper part of the Wonoka Formation accumulate as a
1023
shallowing upward sequence as “normal” conditions are re-established and the negative
1024
isotopic anomaly slowly disappears. The left portions of the cross sections represent the
1025
situation in the Shuram Formation of Oman, which was a marine basin before the impact
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events and was not subject to canyon excavation when global sea level fell after the impact.
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The Wonoka Canyons are schematically shown in Fig. 1 (colum 4).
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Figure 4. Stratigraphic log on right side (after Bergmann et al., 2011, fig.1) to show the
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context of the Johnnie Formation and variations in δ13C (after Verdel et al., 2011). Note that
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a pronounced δ13C negative excursion begins at the base of the Johnnie oolite and reaches its
1032
lowest values about 20 m higher in limestones containing crystal fans interpreted as calcite
1033
pseudomorphs of aragonite seafloor cements. The severity and shape of the isotopic
1034
anomaly are comparable to those of the Shuram anomaly and it appears to have developed
1035
during a rapid transgression (see Fig. 1). Incised paleovalleys (right column) superficially
1036
resemble the Wonola canyons of South Australia but they are much shallower (up to 150 m)
1037
and less steep. They are also younger than development of the isotopic anomaly. They were
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ascribed by Clapham and Corsetti (2005) to erosion associated with syn-depositional normal
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faulting.
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Figure 5. Relationships between δ13C excursions and some biotic extinction-radiation events.
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The age of the proposed Shuram impact is poorly constrained but from stratigraphic
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relationships in the Flinders Ranges of South Australia, it is known to be slightly younger
1044
than the Acraman impact (Gostin et al., 2010) and possibly the Gaskiers glaciation (both
1045
~580 Ma). It was suggested by Bowring et al. (2009) that the Shuram δ13C excursion took
1046
place between about 570 Ma and 551 Ma but there is still considerable uncertainty about the
1047
time of its initiation. Biotic changes associated with the Acraman impact were noted by
1048
Grey et al. (2003), McFadden et al. (2008), Gostin et al. (2011) and Liu et al. (2014) who
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also showed that there were similar biotic changes at the time of the younger Shuram
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anomaly. Each of these anomalies appears to have been contemporaneous with an extinction
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event, followed by biotic recrudescence. The main emergence of the Ediacaran biota took
1052
place during the recovery from the Shuram anomaly but these organisms were replaced by
1053
the metazoans that went on to colonize the planet near the beginning of the Phanerozoic.
1054
Note that metazoan remains have been considered to be as old as 700 Ma (Maloof et al.,
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2010; Meert et al., 2011) but shelly animals are considered by most to have appeared only a
1056
few million years before the beginning of the Cambrian period. It is postulated that at least
1057
two large impacts – the terrestrial Acraman impact and the marine Shuram impact – played a
1058
role in “preparing the ground” for emergence of the diverse shelly metazoan biota by
1059
causing extinctions and creating harsh conditions that increased competition and selective
1060
pressures. The metazoans have a long history but their emergence is commonly confused
1061
with the appearance of shelly fossils. These dynamic life forms were fuelled by rising
1062
oxygen levels in the Earth’s fluid layers. EN1, EN2 and EN3 are negative δ13C excursions,
1063
after McFadden et al. (2008, and references therein). The demonstrated extinctions and
1064
radiations of acritarchs (Ac) and the Ediacaran biota may have been accompanied by similar
1065
events that (cryptically) affected the soft-bodied metazoan community, culminating in the
1066
appearance of exoskeletons that gave the false impression of a Cambrian “explosion”. The
1067
timing of the Pan-African orogeny is from Santosh et al. (2014) and the Sr ratio curve is
1068
from Sawaki et al. (2013). Increased size of metazoans, mobility, predation and especially
1069
the development of exoskeletons changed the nature of the fossil record and ushered in the
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Phanerozoic Eon. See text for discussion.
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Environmental upheavals of the Ediacaran period and the Cambrian “explosion” of animal life
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Grant M. Young *
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*Corresponding author. E-mail address:
[email protected]
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Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada N6A
• Two large Ediacaran impacts (Acraman and Shuram) are associated with negative δ13Ccarb anomalies and glaciations.
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• Associated biotic extinction-radiation (E-R) cycles strongly affected acritarchs and the Ediacaran biota.
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• Metazoans underwent the same E-R cycles; the Cambrian ‘explosion’ is an artifact of exoskeleton development.
