GR-01044; No of Pages 11
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The Nebula Winter: The united view of the snowball Earth, mass extinctions, andexplosive evolution in the late Neoproterozoic and Cambrian periods☆
Ryuho Kataoka a,⁎, Toshikazu Ebisuzaki b, Hiroko Miyahara c, g, Tokuhiro Nimura d, Takayuki Tomida b,
Tatsuhiko Sato e, Shigenori Maruyama faInteractive Research Center of Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, JapanRIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
Musashino Art University, 1-736 Ogawacho, Kodaira, Tokyo 187-8505, Japand
Okayama Astronomical Museum, 3037-5 Honjo, Kamogata, Asakuchi, Okayama 719-0232, Japane
Japan Atomic Energy Agency, Tokai, Naka, Ibaraki 319-1195, Japanf
Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japang
Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama 236-0001, Japanbca r t i c l e

i n f oArticle history:
Received 5 April 2013
Received in revised form 25 April 2013
Accepted 5 May 2013
Available online xxxx
Keywords:
Molecular cloudSupernova
Snowball Earth
Mass extinctions
Cambrian explosiona b s t r a c t
Encounters with nebulae, such as supernova remnants and dark clouds in the galaxy, can lead to an environmental
catastrophe on the Earth through the negative climate forcings and destruction of the ozone layer by enhanced
ﬂuxes of cosmic rays and cosmic dust particles. A resultant reduction in primary productivity leads to mass extinctions through depletion of oxygen and food starvations as well as anoxia in the ocean. The model shows threelevels of hierarchical time variations caused by supernova encounters (1–10 kyrs), dark cloud encounters (0.1–
10 Myrs), and starbursts (~100 Myrs), respectively. This “Nebula Winter” model can explain the catastrophic phenomena such as snowball Earth events, repeated mass extinctions, and Cambrian explosion of biodiversities which
took place in the late Proterozoic era through the Cambrian period. The Late Neoproterozoic snowball Earth event
covers a time range of ca. 200 Myrs long spanning from 770 Ma to the end of Cambrian period (488 Ma) with two
snowball states called Sturtian and Marinoan events. Mass extinctions occurred at least eight times in this period,
synchronized with large ﬂuctuations in δ13C of carbonates in the sediment. Each event is likely to correspond toeach nebula encounter. In other words, the late Neoproterozoic snowball Earth and Cambrian explosion are possibly driven by a starburst, which took place around 0.6 Ga in the Milky Way Galaxy. The evidences for a Nebula
Winter can be obtained from geological records in sediment in the deep oceans at those times.
© 2013 The Authors. Published by Elsevier B.V. on behalf of International Association for Gondwana Research.
All rights reserved.

1. Introduction
Multi-disciplinary geological investigations in the last decades produced three important concepts of natural history of the Earth. They
are snowball Earth, mass extinctions, and Cambrian explosion, all ofwhich remain enigmatic until now. These three phenomena, which appear independent at ﬁrst sight, are closely related to one another. In fact,
a number of normal glacial periods and mass extinctions occurred fromthe end of the last Snowball Earth event (Marinoan glaciation) throughEdiacaran to the Cambrian periods, as summarized by Kopp et al.
(2005), Zhu et al. (2007), and Maruyama et al. (in press).

☆ This is an open-access article distributed under the terms of the Creative Commons
Attribution-NonCommercial-ShareAlike License, which permits non-commercial use,
distribution, and reproduction in any medium, provided the original author and source
are credited.
⁎ Corresponding author.
E-mail address: ryuho@geo.titech.ac.jp (R. Kataoka).

First, a number of geological evidences support that the snowballEarth events occurred at 2.2–2.4 Ga and 0.55–0.77 Ga in the Proterozoiceon (Hoffman and Schrag, 2002; Kopp et al., 2005; Maruyama andSantosh, 2008). Glacial deposits left by the retreating ice are found inmany places (Hambrey and Harland, 1981). Paleomagnetic and geological data from these deposits suggest that they were emplaced at tropical low latitudes (Evans et al., 2000). In most locations, the glacial
deposits are overlain by “cap” carbonate sediments (Grotzinger andKnoll, 1995). The snowball Earth hypothesis (Kirschvink, 1992;
Hoffman et al., 1998) provides a single explanation for the following observations as follows. When the emplacement of glacial deposits
reached a critical latitude (30° North and South), a runaway ice–albedo
feedback (Budyko, 1968; Erikson, 1968; Sellers, 1969) took place,locking the Earth into a totally-frozen state, i.e. snowball, because of
the high planetary albedo. In order to explain deglaciation, extremely
high levels of atmospheric CO2 released through volcanic emissions
have been suggested (Caldeira and Kasting, 1992). The cap carbonates
were formed during the ultragreenhouse climate in the aftermath of
the glaciation (Hoffman et al., 1998). In fact, observations revealed1342-937X/$ – see front matter © 2013 The Authors. Published by Elsevier B.V. on behalf of International Association for Gondwana Research. All rights reserved.
http://dx.doi.org/10.1016/j.gr.2013.05.003

Please cite this article as: Kataoka, R., et al., The Nebula Winter: The united view of the snowball Earth, mass extinctions, and explosive evolution
in the late Neoproterozoic and..., Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.05.003

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R. Kataoka et al. / Gondwana Research xxx (2013) xxx–xxx

that a snowball-Earth event of about a few hundred Myrs is not a simple
contiguous super-cool period but rather is composed of several sets ofsuper-cool periods followed by a super-warm period (Hoffman andSchrag, 2002), though the details of the snowball Earth, such as the synchronicity or ice- or slush-covered ocean, have been discussed (e.g.,
Maruyama and Santosh, 2008; Sansjofre et al., 2011).
An unresolved question associated with the snowball-Earth hypothesis is what caused the Earth to trigger the ice–albedo instability.
However, the initiation of snowball-Earth events is difﬁcult to determine based on previous models, which have only included internal
forcings (e.g., the reduction of greenhouse gas in the atmosphere). A
large negative radiative forcing equivalent to a 10% decrease in thesolar constant, such as the reduction of p(CO2) to 0.01 mbar, was required to achieve a global-freezing solution. Maruyama and Liou
(2005) argued that a suppression of volcanic activity might cause asigniﬁcantly reduced p(CO2), driving the ice–albedo instability. Rinoet al. (2008), however, found that volcanism was most active in theProterozoic period, and thus, the atmosphere was most likely rich(by no means poor) in CO2. Furthermore, Shaviv and Veizer (2003)
found that there is no correlation between p(CO2) and ice-houseand green-house climates in the last 600 Myrs, suggesting external
forcings as the cause of changes in the climate of the Earth.

Second, by applying statistical methods to compiled fossil data, Raupand Sepkoski (1982) found “big ﬁve” mass extinction events, in whichmost of the species were exterminated in the Phanerozoic era. Mass extinction has played one of the critical roles in the entire history of theevolution of life. However, its cause has long been debated as either
geological catastrophic events such as global-scale volcanism recordedat the P/T boundary (e.g., Isozaki et al., 2007) or astronomical events,
such as asteroid impacts at the K/T boundary (Alvarez et al., 1980; Keller
et al., 2004; Renne et al., 2013). In addition to volcanic- and impactbased hypotheses, there have been others including the breakup of a supercontinent and oxygen depletion (Stanley, 1987; Erwin, 1993;
Hallam and Wignall, 1997; Erwin, 2006; Young, 2013a,b). Since the statistical analysis of mass extinctions through fossil records by Sepkoski(1981), several different models have been proposed, such as periodical
impacts of icy meteorites whose orbits are perturbed by unknown planet X or an assumed binary star called Nemesis, both hypothesized to beof our solar system (Davis et al., 1984; Whitmire and Jackson, 1984;
Whitmire and Matese, 1985). Aside from these ad-hoc researches on
speciﬁc topics, a more comprehensive and integrated approach has
awaited understanding of the inﬂuence of the galactic environmenton the environment of the Earth, taking into account the new results
based from investigations of the Milky Way Galaxy, which have greatly

Starburst
SFR ~100 SFR0

Normal Galaxy
SFR = SFR0

Supernova Remnant
Dark Cloud

Orbit of Solar System

Intermediate
SFR ~ SFR0

Fig. 1. Schematic illustration of the Milky Way Galaxy. Interaction with other galaxies or the accretion of a satellite galaxy triggers a starburst (top), in which the galactic disk is almost
totally occupied by many dark clouds (large and small dark brown circles) with supernova remnants (small red circles) embedded therein. Star formation rate (SFR) is enhanced by afactor of ~100 compared with the normal state (bottom left). It returns back to the normal state through the intermediate state (middle right) where almost all dark clouds are evaporated
by the heating of supernova explosions. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

Please cite this article as: Kataoka, R., et al., The Nebula Winter: The united view of the snowball Earth, mass extinctions, and explosive evolution
in the late Neoproterozoic and..., Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.05.003

R. Kataoka et al. / Gondwana Research xxx (2013) xxx–xxx

that paper. In Section 2, descriptions of the nebula encounters areshown. In Section 3, the model is compared with the recent data in
Ediacaran through Cambrian periods. In Section 4, we compare theother models and discuss how to obtain evidence of a Nebula Winter.
2. Nebula Winter model
2.1. Nebulae in the Milky Way Galaxy
The Milky Way Galaxy has a galactic disk with a radius of 10 kpc anda thickness of 200 pc. Our solar system is located at approximately
8.5 kpc from the center of the Galaxy, inside the galactic disk. Many
nebulae, such as dark clouds and supernova remnants, are distributedin the galactic disk. A dark cloud consists of high-density (100–1000 H cm−3) and low-temperature (10–100 K) neutral gas. Cosmic
dust also exists in a dark cloud, accounting for approximately 1% of
the mass of the dark cloud. The size of the dark cloud ranges from
1 pc to 100 pc. In contrast, a supernova remnant is a shell structure produced by a shock wave caused by a catastrophic explosion of a starheavier than eight solar masses. Galactic cosmic rays with energies
greater than ten GeV per nucleon (the super-GeV component) are accelerated in supernova remnants.
The Earth has a history extending back to 4.6 Gyrs, recording at leasta few nebulae encounters. For the present Milky Way Galaxy, it has
been estimated that a supernova occurs within 10 pc of the solar system approximately once per several hundred Myrs (Clark et al., 1977),
whereas an encounter with a dense dark cloud of 2000/cc likely occurs
once every billion years (Talbot and Newman, 1977). Furthermore, theMilky Way Galaxy is believed to have undergone several starbursts inthe past. Rocha-Pinto et al. (2000) and de la Fuente Marcos and de laFuente Marcos (2004) reconstructed the star-formation rate in thepast Milky Way Galaxy based on the ages of stars and star clusters, ﬁnding that the Milky Way Galaxy has experienced at least two starburst

Earth environment
HadeanArchianProterozoic

Paleoproterozoic
Snowball events

Neoproterozoic
Snowball events

Phanerozoic

Metazoa
Eukarya
Bacteria

Proterozoic
glacial gap

Banded iron formations

4.0 Ga3.0

2.0

Starburst I

Star formation rateadvanced in the last decades. For example, Isozaki (2009) has discussed
the effect of galactic cosmic rays on the P–T boundary in terms of
paleomagnetism.
Third, the importance of Ediacaran/Cambrian period was ﬁrst
pointed out by Gould (1989), who argued that all the extant animal
phyla or the crown group of metazoan appeared in this relatively
short time interval (70 Myrs, referred to as Cambrian explosion),while no new phyla have appeared since then (for more than
500 Myrs). As far as we know, there are no persuasive explanations
why this period is so special. For example, Hoffman et al. (1998) proposed that the recovery from the snowball Earth events provided newniches and drove the explosive increase of biodiversity in the Cambrian period. However, the interval between the last Snowball earth
events (Marinoan during 660–635 Ma) and the Cambrian explosion
around 540–520 Ma is as long as 115 Myrs, which is too long to explain the Cambrian explosion by the snowball Earth event, comparing
with the typical timescale of the evolution of biological systems.
Our approach presented in this paper is not geological but theoretical on the basis of the recent advancements in astronomy. The purposeof this work is to present a uniﬁed picture to explain the three important issues listed above through the natural history of the Earth in acomprehensive way, considering new data available during the periodof the last snowball-Earth events (Marinoan during 660–635 Ma) to
the Cambrian period (542–488 Ma), coupled with advanced information of the galactic environment, the latter having a major inﬂuenceon the former. This is a forward model or a working hypothesis to inspire researches in a new direction to understand the Earth's natural
history. Our theoretical basis is the starburst model, in which Kataoka
et al. (2012) (hereafter Paper I) ﬁrst conducted quantitative and systematic discussions on the external climate forcings on the Earth dueto the encounters with nebulae in the galactic disk. They found that extensive and frequent encounters with nebulae can explain many features of the snowball-Earth events (Figs. 1 and 2). First, the negative
radiative forcings during nebula encounters are strong enough to trigger the ice–albedo instability, leading to a snowball Earth during nebula
encounters as described above. Second, the starburst of the Milky Way
Galaxy provides a plausible explanation for the temporal pattern of theoccurrence of the snowball-Earth events, which have occurred only
twice (during the Early Paleoproterozoic period around 2.3 Ga, and thelate Neoproterozoic period, 0.8–0.6 Ga). Statistics of stars and star clusters imply that the Milky Way Galaxy has experienced at least two starburst events: Burst I — 2.0–2.4 Ga (Rocha-Pinto et al., 2000) and Burst II— 0.6–0.8 Ga (de la Fuente Marcos and de la Fuente Marcos, 2004), as
shown in Fig. 2. Bursts I and II correspond to the snowball-Earth eventsin the early Paleoproterozoic era and the late Neoproterozoic era, respectively. Third, it explains the hierarchical nature of the time variation ofsnowball Earth events. In the late Neoproterozoic era, two snowball
Earth events occurred (Sturtian and Marinoan), separated by 100 Myrs.
Recent observations revealed that the snowball Earth event is not a simple contiguous super-cool period but rather is composed of several sets ofsuper-cool periods followed by a super-warm period (Hoffman andSchrag, 2002). Such a hierarchical temporal structure in geological records of snowball-Earth events may correspond to the hierarchical natureof phenomena related to the Nebula Winter, or in other words, the timescale of three levels of hierarchy, i.e. supernova encounters (1–10 kyrs),
dark cloud encounters (0.1–10 Myrs), and starbursts of the entire galaxy(~100 Myrs), respectively (Table 1). After the super-cool period is triggered by a nebula encounter, the climate becomes super-warm becauseof the accumulation of CO2 in the atmosphere.
In the present paper, we provide an extended analysis by proposing a new uniﬁed model, referred to here as “Nebula Winter” of thesnowball Earth events during the Proterozoic era and mass extinctions in the Ediacaran to Cambrian periods, which was followed by
the explosive evolution of multi-cell organisms. We name “Nebula
Winter” after the monumental paper of “Nuclear Winter” by Turcoet al. (1983), since many of the processes reported here come from

3

1.0

0.0

Galaxy environment

Starburst II?
4.0 Ga

3.0

2.0

1.0

0.0

Fig. 2. (Top) Two snowball Earth events with other glaciations. (Bottom) Putative starformation rate in the Milky Way Galaxy is possibly enhanced by a large factor in two
starburst periods: Starburst I (~2.2 Ga) and Starburst II (~0.6 Ga). The starburst periods coincide with the two snowball Earth periods. See Paper I for details.

Please cite this article as: Kataoka, R., et al., The Nebula Winter: The united view of the snowball Earth, mass extinctions, and explosive evolution
in the late Neoproterozoic and..., Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.05.003

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Table 1Time scale of supernova encounter, dark cloud encounter, and starburst of the Galaxy.

Time scale

Supernova encounter

Dark cloud encounter

Starburst

1–10 kyrs

0.1–10 Myrs

~100 Myrs

events. A starburst is a phenomenon in which the star-formation rate ina galaxy is enhanced by dynamic interactions with nearby galaxies. A
starburst galaxy (e.g., M82) is completely covered by thick dark clouds
in which numerous supernova remnants are embedded. In such starburst periods, therefore, the frequency of nebula encounters is likely
to be as high as one event in every several ten Myrs, and thus a plausible
explanation for the snowball-Earth events.
A number of previous studies have suggested that an encounter witha nebula may lead to an environmental catastrophe (Whitten et al.,
1963; Ruderman, 1974; Begelman and Rees, 1976; Clark et al., 1977;
Talbot and Newman, 1977). An encounter of the solar system with a nebula, such as a dark cloud or a supernova remnant, enhances the ﬂux ofcosmic dust particles and cosmic rays, which leads to global coolingand destruction of the ozone layer. The cosmic dust particles remain in
the stratosphere for more than several years, working as sunscreens.
Sub-GeV cosmic rays, which penetrate into the stratosphere, produce
NOX that destroys the ozone layer. Largely enhanced NO2 can also reduce
the amount of radiation received at the ground (Reid et al., 1978). In contrast, super-GeV cosmic rays produce charged ions in the tropospherethat enhance aerosol nucleation (Svensmark et al., 2007; Kirkby et al.,2011), leading to greater cloud cover and increased Earth albedo(Svensmark and Friis-Christensen, 1997; Svensmark, 2007).

Fig. 3 illustrates the Nebula-Winter model, based on Paper I. “Threeshields” of the heliosphere, geomagnetic ﬁeld, and ozone layer protect
the Earth's environment against “three spears” of cosmic dust particles,
sub- and super-GeV cosmic rays, and UV-B radiation from the Sun. An encounter with a supernova remnant or a dark cloud causes a signiﬁcant enhancement in the “three spears” and breakdown of the “three shields”,
leading to catastrophic events such as snowball-Earth events and associated mass extinctions. In the following subsection, we will describe
what happens in the dark-cloud encounter and supernova encounter,
respectively.
2.2. Dark-cloud encounterFig. 3C and D depicts the Earth's environment during a dark-cloud
encounter when the heliospheric boundary shrinks by a factor of 100
and locates around the orbit of the Earth (breakdown of the ﬁrst
shield). The heliospheric boundary becomes unstable against the Rayleigh–Taylor instability, forming many cloudlets. These cloudlets orbit
the Sun independently, gradually sinking down to the Sun. The
shocks between these dense cloudlets and the solar wind constantly
accelerate the sub-GeV component of cosmic rays in the heliosphere,
unlike the present solar system, where such acceleration occurs only
after intensive solar-ﬂare events. The super-GeV cosmic rays also
freely penetrate into the Earth's orbit. The super-GeV cosmic rays enhance the radiation dose at the ground, possibly causing global
cooling via enhancing the cloud formation. The sub-GeV cosmic rays
penetrate the stratosphere of the polar region of the Earth and destroy the ozone layer through NOX formation (breakdown of the
40 kmA) Present Earth

B) Supernova

Ozone Destruction
20 km
NOx
10 km

VolcanoSuper-GeV Cosmic RaysDNA
Cloud Formation

Ozone Layer

40km 20km 10km

10km 20km 40km
Snowball Earth
Mass Extinctions
Reduced Photosynthesis

Dust Layer

Global Cooling

DNA
Strong UVB

Cosmic Dust Particles

Dust Layer

Visible Light + UV

NOx
Sub-GeV Cosmic Rays

10 km

Ozone Destruction

20 kmC) Dark Cloud

40 kmD) Dark Cloud (Weak Geomagnetic Field)

Fig. 3. Illustration of the Nebula Winter model. The large enhancement of “three spears” of cosmic rays, cosmic dust particles, and UV radiation from those at (A) the present Earthcan induce catastrophic events of the Earth encountering with (B) nearby supernova remnant and (C) dark cloud, especially during (D) weak geomagnetic ﬁeld.

Please cite this article as: Kataoka, R., et al., The Nebula Winter: The united view of the snowball Earth, mass extinctions, and explosive evolution
in the late Neoproterozoic and..., Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.05.003

R. Kataoka et al. / Gondwana Research xxx (2013) xxx–xxx

third shield), particularly during geomagnetic excursions or reversals(breakdown of the second shield, Fig. 3D). The largely enhanced NO2
may also contribute to the global cooling. Furthermore, a large
amount of cosmic dust particles in the dark cloud accretes on the
Earth and causes global cooling.
Fig. 4 shows the time proﬁles of factors inﬂuencing the Earth's environment during an encounter with a dark cloud with a central density of2500 H cm−3, a Gaussian density proﬁle with a scale of 10 pc across, anda relative velocity of 20 km s−1. Fig. 4b shows the radiative forcings due
to cosmic dust, cosmic rays, and NO2. The most important effect is the effect of cosmic dust. Pavlov et al. (2005) found that cosmic dust particles
with submicron size persist in the stratosphere for several years and thatthe radiative forcing of cosmic dust is as strong as −15 W m−2 when
the solar system passes the densest part of the dark cloud, which well exceeds the snowball forcing of −14 W m−2.
Furthermore, as illustrated in Fig. 5, a dark cloud perturbs the orbits of comets/asteroids orbiting in outer space or speciﬁcally in the5so-called Oort cloud of the solar system, which may cause multiple
impact events on the Earth (Matese et al., 1995; Mazeeva, 2004;
Jakubik and Neslusan, 2008). Assuming the radius of the Oort cloud(10 4 AU) and the radius of dark cloud (8 pc), velocity perturbationby the dark cloud is as high as:
2GMDC a 1
r vDC
r



−2 
−1
MDC
a
r
vDC
−1
:
¼ 29 ms
1035 kg 104 AU 8 pc
12 km s−1

dv ¼

Since Kepler velocity is about 290 ms −1 in the Oort cloud, thechange in the velocity of comet/asteroids can be as large as 10% during a dark-cloud encounter. The loss-cone around the Earth's orbit isreﬁlled by the perturbed comets and asteroids in the Oort cloud.
The frequency of the comet/asteroid impacts increases considerably
during the dark cloud encounter (Mazeeva, 2004).

(a) Dark Cloud Density(b) Radiative Forcing(c) Cosmic Ray Energy Density(d) Dose Rate at Ground(e) UV-B (280-300nm)

Fig. 4. Time proﬁle of an encounter with a dark cloud with a diameter of 10 pc and a central density of 2500 H cm−3; (a) interstellar gas density; (b) negative radiative forcings (solid curve:
total; dashed curve: cloud albedo by cosmic rays; dash–dot curve: NO2 effect; dash–dot–dot curve: cosmic dust effect; and dotted line: snowball forcing of −14 W m−2); (c) cosmic rayenergy densities outside of the Earth's magnetosphere; (d) radiation dose rate at the ground; and (e) UV-B intensity under a weak geomagnetic ﬁeld (after Paper I).

Please cite this article as: Kataoka, R., et al., The Nebula Winter: The united view of the snowball Earth, mass extinctions, and explosive evolution
in the late Neoproterozoic and..., Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.05.003

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R. Kataoka et al. / Gondwana Research xxx (2013) xxx–xxx

Oort Cloud

Comet

dV

Dark Cloud

Solar System

Fig. 5. A dark cloud encounter perturbs the orbits of comets/asteroids in the Oort cloud.

We used the PHITS code (Niita et al., 2010) to calculate the atmospheric impact of cosmic rays from a nearby supernova remnant withthe hardest (power of −2) spectra. Protons with energies from
100 MeV to 1 TeV were considered as the source particle inducing the
air shower. Details of the calculation procedure are given in Sato et al.
(2008). The relative amplitude of cosmic-ray ﬂux v.s. present levels is
assumed to be 1000 fold at 1 GeV, considering the nearby supernova
at a 10 pc distance. As shown in Fig. 6, it is found that the absorbed
dose rate in the air can be enhanced at most 104 times larger than thepresent level at the ground if the cosmic-ray spectrum is ﬂat enough.
The inﬂuence of a supernova encounter with the environment of the
Earth is summarized in Fig. 7. First, the maximum negative radiative

Atmospheric Impact of Cosmic Rays
50

40

Altitude (km)The ﬂux of the sub-GeV cosmic rays increases by a factor of approximately 1000 (Fig. 4). This increase in ﬂux leads to the destruction of theozone layer in the stratosphere via the production of NOX (in particular,
NO and NO2). Ozone depletion has been observed in the polar region
during large solar-proton events (Jackman et al., 2005). The destructionof the ozone layer is limited to the polar region when the geomagnetic
ﬁeld is as strong as that at present time (~10−5 T). The destruction, however, extends to lower latitudes during geomagnetic excursions when
the geomagnetic ﬁeld is weak (Fig. 4d). Because at least one geomagnetic
excursion or reversal is expected during a 10-Myr period (Cox, 1975), theencounter with a dark cloud, which extends for several Myrs, is sufﬁciently long to encompass several geomagnetic excursions and reversals.
Fig. 4e shows the UV-B radiation increase due to the destruction of
the ozone layer by enhanced sub-GeV cosmic rays, assuming an
ozone column density of 300 Dobson Units and a geomagnetic ﬁeld
as low as 10% of the present value. The UV-B radiation destroys thephotosynthesis mechanisms of phytoplankton, and the primary productivity decreases to 30–50% of the present level. In fact, in the present polar ocean exposed to the enhanced UV-B ﬂux due to the ozone
hole, a reduced primary productivity of phytoplankton (Smith andBaker, 1989) has been observed. Furthermore, the stratospheric NO2
enhancement drives global cooling by up to 3 K (Reid et al., 1978).
The reduced primary productivity together with the global coolingcauses a reduced oxygen density both in the atmosphere and in theocean (anoxia), negative excursions of the δ 13C, and mass extinction.

30

20

Present

2.3. Supernova remnant encounter10

Fig. 3B shows the Earth's environment during an encounter with asupernova remnant when the heliosphere shrinks to around the Earth's
orbit (breakdown of the ﬁrst shield). The cosmic rays of both the superand sub-GeV components increase by a large factor (100–1000 times).
The duration of the encounters is 10 3 to 104 yrs depending on the supernova distance and the surrounding gas density. The increased
cosmic-ray ﬂux also depletes the ozone layer through the productionof NOX, leading to an enhanced UV-B intensity (breakdown of thethird shield).

Supernova
0
10-2

100

102

104

106

Air Dose (uGy/h)Fig. 6. Dose rate due to cosmic rays during supernova encounter with a spectral index
of −2 (solid curve). The present proﬁle of cosmic ray impact is shown by dotted curve
as a reference.

Please cite this article as: Kataoka, R., et al., The Nebula Winter: The united view of the snowball Earth, mass extinctions, and explosive evolution
in the late Neoproterozoic and..., Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.05.003

R. Kataoka et al. / Gondwana Research xxx (2013) xxx–xxx

forcing due to the increased ﬂux of sub-GeV and super-GeV cosmic rays
is approximately −20 W m −2 for several thousand years and well exceeds the snowball forcing of −14 W m−2. Second, the dose rate at the
ground reaches 1 Sv yr−1, which has a signiﬁcant effect on the biological systems via the genome instability. Third, the increased cosmic-ray
ﬂux depletes the ozone layer through the production of NOX, leading toan enhanced UV-B intensity. The reduction of primary production due
to this enhanced UV-B radiation, together with global cooling, causesa catastrophic perturbation of the ecosystem at the surface of the
Earth during a supernova encounter, similar to the effect caused by
the encounter with a dark cloud.
3. Late Neoproterozoic snowball Earth and Cambrian explosion
Snowball-Earth events have been documented in the Neoproterozoic
based on glacial-deposit records at low-latitudes near equatorial regions(Hoffman and Schrag, 2002). In addition to the nearly complete snowball7

periods, Sturtian (710–685 Ma) and the Marinoan (660–635 Ma)(Fig. 7), local glacial periods have also been present, such as Kaigas(770–730 Ma), Gaskier (582 Ma), and Baikonur (542 Ma), but presumably more prevalent and smaller in scale in the Cambrian as seen by alocal parallel unconformity (see a summary by Maruyama et al., inpress).
Cloud (1948) ﬁrst pointed out that the Cambrian explosion
highlighted a sudden appearance of metazoans at the onset of the Cambrian, and the majority of the living metazoan phyla appeared in this period. It has been recently revealed that the Cambrian explosion was not asingle event but a complex event, composed of three distinct major
phases: (1) Ediacaran fauna, (2) small shelly fossils (SSFs), and (3)Chengjian faunas (mega-fossils such as ﬁrst ﬁsh) by the end of Early
Cambrian (Shu, 2008). Gould (1989) emphasized that it was an extremely unique event because there was no association to mass extinctions. On
the contrary, detailed analysis has revealed that the period from 635 Ma
to the end Cambrian was a time when the frequency of mass extinctions(a) Supernova Pressure(b) Radiative Forcing(c) Cosmic Ray Energy Density(d) Dose Rate at Ground(e) UV-B (280-300(nm)Fig. 7. Time proﬁles of the encounter with a supernova remnant that exploded at a distance of 10 pc and with an interstellar gas density of 0.5 H cm−3; (a) pressure of the post-shockedgas of the supernova remnant; (b) negative radiative forcings (solid curve: total; dashed curve: cloud albedo by cosmic rays; dash–dot curve: NO2 effect; dash–dot–dot curve: cosmic dust
effect; and dotted line: snowball forcing of −14 W m−2); (c) cosmic ray energy density; (d) radiation dose rate at the ground; and (e) UV-B intensity (after Paper I).

Please cite this article as: Kataoka, R., et al., The Nebula Winter: The united view of the snowball Earth, mass extinctions, and explosive evolution
in the late Neoproterozoic and..., Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.05.003

R. Kataoka et al. / Gondwana Research xxx (2013) xxx–xxx

of the occurrence of the snowball-Earth events that occurred only twice,during the early Paleoproterozoic era (around 2.3 Ga) and the late
Neoproterozoic era (0.8–0.6 Ga: Fig. 2). The statistics of stars and starclusters imply that the Milky Way Galaxy has experienced at least two
starburst events, i.e., Burst I — 2.0–2.4 Ga; (Rocha-Pinto et al., 2000)
and Burst II — 0.6–0.8 Ga (de la Fuente Marcos and de la Fuente
Marcos, 2004). Starbursts I and II correspond to the snowball Earthevents in the early Paleoproterozoic era and the late Neoproterozoic
era, respectively. Third, it also explains the hierarchical nature of the climate change in the Late Neoproterozoic era in general. Two snowball
Earth events occurred (Sturtian and Marinoan), separated by 100 Myrs.
Observations revealed that a snowball-Earth event is not a simple contiguous super-cool period but rather is composed of several sets ofsuper-cool periods followed by a super-warm period (Hoffman andSchrag, 2002). Such a hierarchical temporal structure in the geologicalrecords of snowball-Earth events may correspond to the hierarchical nature of phenomena related to the Nebula Winter; in other words, thetimescale of three levels of hierarchy, i.e., supernova encounters(1–10 kyrs), dark cloud encounters (0.1–10 Myrs), and starbursts ofthe entire galaxy (~100 Myrs), respectively.
Following the starburst, the Milky Way Galaxy gradually returned toa more normal state, which we refer to as an intermediate state (Fig. 1),
in which supernova remnants are predominant in the galactic disk,
since most of the dark clouds were evaporated by the heating of supernova explosions. The frequent encounters with nebulae, particularly
with supernova remnants, can lead to multiple mass-extinction events
reported in the Ediacaran through Cambrian periods.
The nebula encounters are likely to induce rapid evolution and speciation which leads to the explosive emergence of new types of organism after mass extinctions. There are four necessary conditions for rapid

Earth environment
Proterozoic
Crygenian Edia Cam O

Phanerozoic
S

D

C

J

T

K

Pg N

K-TB

P-TB

0.8Ga

0.7

0.6

Bikonur

GaskiasMarinoan

Mass Extinctions (Big 5 Level)

0.5

0.4

0.3

4. Discussions

0.2

0.1

0.0

Galaxy environment
Negative radiative forcing

A Nebula-Winter model has been developed to present a uniﬁed
picture in order to explain snowball-Earth events, mass extinctions,and Cambrian explosion in the Ediacaran through the Cambrian periods in terms of changes in the galactic environment by the starburstof the Milky Way Galaxy around 0.6 Ga and the gradual recovery to
the normal state through the intermediate state (Fig. 1).
The extensive and frequent encounters with nebulae due to the starburst of the Milky Way Galaxy can explain many features of thesnowball-Earth events (Paper I; Figs. 1 and 2). The previous snowballEarth models, in which only internal forcings were considered, cannot
explain the triggering mechanism nor occurrence pattern, while the
Nebula Winter model, which includes external forcing from outside of
the Earth, can explain both. First, the negative radiative forcing is strong
enough to trigger ice–albedo instability, leading to a snowball Earth during the solar-system encounters as described above. Second, the NebulaWinter model provides a plausible explanation for the temporal pattern

P

Snowball

Kaigaswas at least ﬁve times higher than in the Phanerozoic (Zhu et al., 2007).
For example, based on the compilation of all existing paleontological descriptions of this period, Zhu et al. (2007) recognized the presence of
frequent mass extinctions such as the mass extinctions of large
Acanthomorph Acritarchs (560 Ma), Ediacaran fossils (542 Ma), SSFs
(523 Ma), and Archaeocyathids (514 Ma). Moreover, based on the
curve of carbonate deposited on the continental shelf in the late
Neoproterozoic in the S. China craton, which is related to mass extinctions, they have further speculated that eight periods of mass extinctions
were present in the time span from 635 Ma to 488 Ma (Zhu et al., 2007),
observations of which are consistent with the recent chemostratigraphic
studies of multiple drilled cores (Tahata et al., in press; Ishikawa et al., inpress; Sawaki et al., this volume; Yamada et al., under review). Bambach(2006) also independently speculated the presence of more than fourmass extinctions in the Cambrian, and those being larger than the big
ﬁve events in post Ordovician mass extinctions (Fig. 8).
These numerous mass extinctions from Ediacaran through the Cambrian period are likely to be caused by the nebula encounters in the intermediate state after the starburst of the Milky Way Galaxy. In such anintermediate state, the solar system predominately encountered supernova remnants; though most of the dark clouds were evaporated, thesupernova explosions still remain frequent enough. Glacial deposits
due to the Nebula Winter of supernova remnants are difﬁcult to beformed or found since they are as short as 1–10 kyrs, although they
were long enough to drive mass extinctions.
This higher frequency of mass extinctions in this period may explain
the rapid evolution in the Cambrian period. Natural radiation level is
100–1000 times higher even on the ground during the supernova encounter than that in the present time; it is high enough to trigger genome
instability in biological organisms (Dubrova, 2006), which leads to a signiﬁcant increase in the frequency of chromosomal rearrangement in offsprings (Aghajanyan et al., 2011). It possibly accelerates the evolution
and adaptation of new species in the new environment through creationsof new genes by segmental or whole-genome duplication (Ohno, 1970)
and gene shufﬂing. In fact, as can be seen in Fig. 9, the new phyla of metazoan branched just after the mass extinction events. Furthermore, in the
Ediacaran period, the lineage of vertebrate underwent two-round,whole-genome duplications, which gave them vast freedom to try different body plans and metabolisms. These whole genome-duplication
events are likely to be related to the genome instability perturbed by
the encounters with supernova remnants.
Moreover, the Acraman impact crater was formed ca. 580 Ma(Grey et al., 2003; Meert and Lieberman, 2008), which may be related
to the Gaskias glaciation. The ultimate cause of the impact may be thegravitational perturbation exerted on asteroids/comets due to an encounter with a dark cloud (see Subsection 2.2). Such potential extraterrestrial phenomena merit further detailed investigation.

Startian8

0.8Ga

Starburst
Intermediate
Normal--2

Supernova

0.7

0.6

0.5

0.4

14Wm

Dark Cloud

0.3

0.2

0.1

0.0

Fig. 8. (Top) The Sturtian (710–685 Ma) and Marinoan (660–635 Ma) were nearly complete snowball states, while Kaigas (770–730 Ma), Gaskier (582 Ma), and Baikonur(542 Ma) were local glacial periods. (Bottom) Putative external negative forcings from
0.8 Ga to the present. Milky Way Galaxy underwent starburst in 0.8–0.6 Ga and thengradually returned back to the normal state (0.4 Ga to present) through the intermediate
state (0.8–0.4 Ga). See also Fig. 1.

Please cite this article as: Kataoka, R., et al., The Nebula Winter: The united view of the snowball Earth, mass extinctions, and explosive evolution
in the late Neoproterozoic and..., Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.05.003

R. Kataoka et al. / Gondwana Research xxx (2013) xxx–xxx

9Earth environment
Proterozoic
Ediacaran(a)Phanerozoic
Cambrian

GaskiasGlaciation

MarinoanGlaciation

Baikonur
Glaciation
542 Ma580 Ma635 Ma

(b)
-6
δ 13C

0
6

Mass extinction of
Large Acanthomorph Acritarchs
560

Ediacaran SSFs
fossils
523
542

Archaeocyathids
510

(c)

Achnitarchs
SpongeMulti-cellular algae
Miaohe Biota
Embryo & lavae
Biomineralzating metazoans
SSFs
Bilaterian
Trace and bioturbation
Archaeocyathids

(d)

526
573

538
560

615
653

Negative radiative forcing

542
548

600
Doushantuo Formation
Oldest metazoans

(e)

519

555
Oldest bilaterian
macrofossils

Echinodermata
Hemichordata
Annelida
Bivalvia
Platyhelminthes
Arthropoda
Priapulida
Anthozoa
Hydrozoa
Demospongia

520
Chengjian fauna

Galaxy environment

Dark Cloud

Starburst

Supernova

Intermediate

Fig. 9. When the last snowball Earth glaciation (Marinoan glaciation) ﬁnished, Ediacaran period started. (a) Since then, there were two local glaciations, Gaskier (582 Ma) andBaikonur (542 Ma) and presumably numerous glaciations but smaller in scale were likely to take place in Cambrian period as seen in local parallel unconformity (see a summary
by Maruyama et al., in press). (b) Some of them are synchronized with the strong excursions in δ13C carbonate deposited on the continental shelf in the S. China craton, (adaptedfrom Zhu et al. (2007)), which correspond to the mass extinction events, such as those of large Acanthomorph Acritarchs (560 Ma), of Ediacaran fossils (542 Ma), of SSFs (523 Ma),and of Archaeocyathids (514 Ma) as indicated in arrows (c). (d) A phylogenetic tree among the animal phyla is given by Peterson et al. (2004). The divergence times are estimatedby the genomic distances among the species. Note that they generally include considerable error (more than 10%). (e) Putative negative radiative forcing; Milky Way Galaxy was
gradually returning back to the normal state after the starburst around 0.6 Ga through the intermediate state, where supernova remnants dominate in the galactic disk. The encounters with the supernova remnants may drive environmental catastrophe to lead mass extinctions. The enhanced radiation triggers genome instability in the biological organisms on
the ground to accelerate their evolution.

speciation. These are as follows: a small population (Mayr, 1942, 1954;
Ohno, 1970), strict isolation (Kimura and Weiss, 1964), high mutation
rate, and availability of new niches. Mass extinction events of nebula
encounters satisfy all of them. First, a population group is naturally
expected to become small due to difﬁcult environmental conditions
during a nebula encounter. Secondly, the habitat drastically shrinks
and the immigration among the groups becomes difﬁcult during catastrophic events. The strict isolation of a small population group is naturally achieved during a nebula encounter. Third, the mutation rate
becomes considerably higher because of the genome instability throughthe enhanced natural radiation level during a nebula encounter. Fourth,many new niches become available for new species when a nebula encounter is over. Ediacaran to Cambrian periods were, therefore, in an
ideal situation for the rapid evolution of biological organizations, since
all of the four conditions were satisﬁed.

Noteworthy, Sepkoski (1981) found that the global speciation rateon the Earth in the last 600 Myrs has two distinct peaks, followed byexponential decreases in the Cambrian period and the post P/T mass
extinction event. We will discuss this matter in more detail in a separate paper.
The Nebula-Winter model is a working hypothesis. Evidence to support the model may be provided by detailed geochemical studies, including isotope studies of platinum-group elements or plutonium in
deep-sea sediments where the accumulation speed is sufﬁciently low.
These types of studies may corroborate the existence of a large amountof exosolar grains in the sediment during the snowball-Earth period. As
possible evidence of a very recent supernova, an excess of 60Fe has been
observed in a Fe/Mn crust in marine sediments (Knie et al., 2004; Fields
et al., 2005). This excess was interpreted to be a signature of a supernova explosion 2.8 ± 0.4 Myrs ago.

Please cite this article as: Kataoka, R., et al., The Nebula Winter: The united view of the snowball Earth, mass extinctions, and explosive evolution
in the late Neoproterozoic and..., Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.05.003

10

R. Kataoka et al. / Gondwana Research xxx (2013) xxx–xxx

Bodiselitsch et al. (2005) found iridium anomalies at the base of allcap carbonates both after the Marinoan and Sturtian glaciations; iridium and other platinum-group elements are typical proxies for extraterrestrial (including exosolar) materials and are much moreabundant than in the Earth's upper mantle and crust. These authorsinterpreted the anomalies to be the product of the rapid accumulationof extraterrestrial material once trapped in the surface ice and latertransported to the ocean ﬂoor via the melting of the ice at the endof the glaciation. Nevertheless, the excursion of iridium may also reﬂect the enhanced ﬂux of extraterrestrial material due to an encounter with a nebula.
Moreover, 244Pu is expected to remain in the Fe/Mn crusts of interest. Fields et al. (2005) estimated the 244Pu production by a supernova
explosion to approximately 0.9 × 10−7 solar masses. The expected
ﬂux of the 244Pu at the Earth is approximately 10–16 kg m−2 yr−1. A
signiﬁcant amount (0.1 Bq m −2) of the accreted 244Pu remains in thesedimentary rocks, even considering the half-life of 244Pu (80 Myrs).
The existence of abundant 100-μm spherical iron balls in pelagic
sediments is also possible direct evidence. For example, Miono et al.
(1993) argued that the spherules accumulate in Paleozoic–Mesozoic
bedded cherts. These spherules are generally believed to originate
from micro-meteorites. The Nebula Winter model can be tested bysimilar but further advanced investigations, including the analysis of
the abundance of the isotopes and platinum family elements of theminerals separated from spherules in the pelagic sediments, detailingtheir exosolar nature. In addition, mineralogical investigations on theages, the abundances of platinum family elements, and the isotope ratios of the minerals of the exosolar grains in the pelagic sediments atthat time, will clarify the inﬂuence of the galactic environment on thenatural history of the Earth.
Acknowledgments
We thank Prof. J. Dohm for constructive and helpful reviews which
aided in improving the manuscript. We acknowledge interdisciplinary discussions with the Interactive Research Center of Science,
Tokyo Tech. To understand this interdisciplinary ﬁeld at the interface
of geology, biology, and astrophysics, we read numerous papers. We
express sincere thanks to all the efforts of the librarians in Japan,
who supplied us with copies of papers through the Interlibrary
Copy Order Service. This work was supported by Grants-in-Aid for
Scientiﬁc Research (23224012) from the Japanese Ministry of Education, Science, Sports, Technology, and Culture.
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Please cite this article as: Kataoka, R., et al., The Nebula Winter: The united view of the snowball Earth, mass extinctions, and explosive evolution
in the late Neoproterozoic and..., Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.05.003