Supercontinent Tectonics and Biogeochemical Cycle: a Matter of `Life and Death

Geoscience Frontiers (2010) 1,21e30

available at www.sciencedirect.com China University of Geosciences (Beijing) Geoscience Frontiers

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ORIGINAL ARTICLE Supercontinent tectonics and biogeochemical cycle: A matter of ‘life and death’

M. Santosh

Division of Interdisciplinary Science, Faculty of Science, Kochi University, Kochi 780-8520, Japan

Received 3 June 2010; accepted 25 July 2010 Available online 25 September 2010

KEYWORDS Abstract The formation and disruption of supercontinents have significantly impacted mantle Supercontinents; dynamics, solid earth processes, surface environments and the biogeochemical cycle. In the early history Mantle dynamics; of the Earth, the collision of parallel intra-oceanic arcs was an important process in building embryonic Superplume; continents. Superdownwelling along Y-shaped triple junctions might have been one of the important Life evolution; processes that aided in the rapid assembly of continental fragments into closely packed supercontinents. Extinction; Various models have been proposed for the fragmentation of supercontinents including thermal blanket Cambrian explosion and superplume hypotheses. The reassembly of supercontinents after breakup and the ocean closure occurs through “introversion”, “extroversion” or a combination of both, and is characterized by either Pacific-type or Atlantic-type ocean closure. The breakup of supercontinents and development of hydrothermal system in rifts with granitic basement create anomalous chemical environments enriched in nutrients, which serve as the primary building blocks of the skeleton and bone of early modern life forms. A typical example is the rifting of the Rodinia supercontinent, which opened up an NeSorientedsea way along which nutrient enriched upwelling brought about a habitable geochemical environment. The assembly of supercontinents also had significant impact on life evolution. The role played by the Cambrian Gondwana assembly has been emphasized in many models, including the formation of ‘Trans- gondwana Mountains’ that might have provided an effective source of rich nutrients to the equatorial waters, thus aiding the rapid increase in biodiversity. The planet has witnessed several mass extinction events during its history, mostly connected with major climatic fluctuations including global cooling and warming events, major glaciations, fluctuations in sea level, global anoxia, volcanic eruptions, asteroid impacts and gamma radiation. Some recent models speculate a relationship between

Peer-review under responsibility of China University of Geosciences (Beijing). doi:10.1016/j.gsf.2010.07.001

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superplumes, supercontinent breakup and mass extinction. Upwelling plumes cause continental rifting and formation of large igneous provinces. Subsequent volcanic emissions and resultant plume-induced “winter” have catastrophic effect on the atmosphere that lead to mass extinctions and long term oceanic anoxia. The assembly and dispersal of continents appear to have inﬂuenced the biogeochemical cycle, but whether the individual stages of organic evolution and extinction on the planet are closely linked to Solid Earth processes remains to be investigated. ª 2010, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction during the evolutionary history of the Earth where continental differentiation, enzyme efficiency, atmospheric oxygen levels, and The early Earth was dominated by island arcs in an oceanic realm, solar luminosity are considered to represent increasing secular but after the formation of the proto-continents through arcearc trends. Worsley (ibid.) divided a supercontinent cycle into four collision and accretion, continental crust was assembled into large phases: fragmentation, maximum continent dispersal, continental land masses. These were again fragmented and re-assembled assembly, and supercontinental stasis. These recur at ca. 500 Ma within newer configurations throughout the latter half of Earth interval and correlate with tectonism, cratonic sediment preserva- history. The assembly and disruption of supercontinents has had tion, atmospheric and hydrospheric evolution, and the distribution significant impact on the mantle dynamics, solid earth processes, of marine platform stable isotopes. Non-recurring and irreversible surface environments and the biogeochemical cycle (Worsley events such as development of photosynthesis, formation of banded et al., 1986; Worsley and Nance, 1989; Condie, 2001; Condie iron formations, and deposition of detrital uraninite represent et al., 2001; Santosh, 2010, among others). Among surface envi- periods of rapid geochemical adjustment to biospheric evolution. ronmental changes is the well established relationship of super- This paper provides a synoptic overview of some of the current continents and sea level changes. When continents are packed speculative models on the tectonics of the assembly and disruption together, sea level is generally low, as against a high sea level of supercontinents and their impact on Earth’s biosphere. when they are in a dispersed state (Parsons and Sclater, 1977). An increased sea level leads to flooding of the continents, whereas 2. Conceptual models on the assembly and low sea level exposes the continental shelves. Global climatic disruption of supercontinents patterns are also markedly influenced by the formation and fragmentation of supercontinents. When continents are assembled 2.1. Supercontinent assembly together, an icehouse climate predominates and when they are dispersed, a greenhouse climate takes over. Life evolution is less In the early history of the Earth, the collision of parallel intra- extensive when continents are welded together, whereas isolated oceanic arcs was an important process in building embryonic marine environments during continental breakup accelerate continents. Recent studies from Archean terranes in different parts extensive diversification (Maruyama and Santosh, 2008). of the world have offered important clues for the process of When continents assemble into large land masses, the weath- amalgamation of composite arcs (Komiya et al., 2002; Santosh ering processes in the increased surface area consume more CO2 et al., 2009, and references therein). A modern analogue for the from the atmosphere, which is eventually transferred into the Archean process is the western Pacific domain where 60%e70% oceans through erosion. Increased weathering and erosion leads to of island arcs are concentrated. In a recent study, Maruyama et al. the release of more nutrients which promote enhanced biological (2009) described the characteristics and processes of subduction productivity. The enhanced CO2 sequestration also leads to cooler zone magma factories in relation to the age of the subducting plate climates with more intense ocean circulation, nutrient upwelling, and the mode of subduction in the western Pacific region. marine productivity and phosphate deposition. The drastic drop in Based on the topology of Y-shaped triple junctions in major CO2 concentration, together with the increasing albedo caused by supercontinental assemblies, Santosh et al. (2009) recognized two the high land/ocean ratio would ultimately result in widespread distinct categories of subduction zones on the globe: the Circum- glaciation. On the other hand, supercontinent breakup leads to the Pacific subduction zone and the Tethyan subduction zone. In the formation of rift basins, and the restricted circulation promotes scenario where the subduction is double sided, the triangular regions anoxic conditions in the deeper parts of the basins. Actively with Y-shaped topology selectively refrigerate the underlying eroding escarpments along new rift margins contribute sediments mantle and turn down the temperature in these domains as compared to rift basins, and marine transgressions increase the rate of burial to the surrounding regions. The Y-shaped domains also accelerate of organic and carbonate carbon on stable continental shelves. the refrigeration through larger amounts of subduction and thus An increase in length of the ocean ridge system during super- promote stronger downwelling as compared to other regions of the continent breakup would also promote mantle degassing and mantle. Once this process is initiated, a runaway growth of cold release of CO2 into the atmosphere (Condie, 2001). The rising CO2 downwelling occurs which eventually develop into a large zone of levels and elevated sea level generate warmer climates. Thus, the super-downwelling. Santosh et al. (2009) proposed that such super- Earth’s climatic evolution has been tectonically punctuated by downwelling could be one of the fundamental processes that drags alternating intervals of “greenhouse” drowning and “icehouse” the dispersed continental fragments through mantle convection and emergence. Such icehouse intervals may also correspond to close-packs them within a tight supercontinent assembly. episodes of biotic innovation, and phosphate deposition. Worsley The effects of supercontinents on the dynamics and structure of et al. (1986) summarized the recurring and non-recurring events the mantle have been addressed in several investigations (e.g., Gurnis, Supercontinent tectonics and biogeochemical cycle: A matter of ‘life and death’ 23

1988; Lowman and Jarvis, 1993; Yoshida et al., 1999; Coltice et al., in which exterior ocean floor is preferentially subducted. Murphy 2007; Phillips and Bunge, 2007; Zhang et al., 2009). These studies et al. (2009) speculated that superplumes, perhaps driven by slab identify that a mantle convection planform with short-wavelength avalanche events, could occasionally overwhelm topedown geo- structures and a large number of downwellings may not lead to the dynamics, imposing a geoid high over a pre-existing geoid low assembly of supercontinents. This is because continental blocks and causing the dispersing continents to reverse their directions to would be trapped by different downwellings, thus inhibiting collision. produce an introverted supercontinent. If the continental blocks floating on the Earth are smaller than the In a slightly different model, Silver and Behn (2008) proposed wavelength of mantle flow and their number is larger, it would take two modes of ocean closure during supercontinent formation which a longer time for them to form a supercontinent (Phillips and Bunge, they termed as P-type (Pacific-type) and A-type (Atlantic-type) 2007; Zhang et al., 2009). Thus, long-wavelength convection is (Fig. 1). In A-type closure, the subduction begins at a passive margin preferred for effective close packing of continental fragments into of the internal ocean and the internal ocean begins to close. In P-type a supercontinent assembly. Both Rodinia and Pangea were largely closure, the internal ocean continues to grow and becomes an surrounded by subduction zones suggesting downwellings and enlarged ocean basin. When a supercontinent is assembled through upwellings, and leading to kinematic models of supercontinent cycles A-type closure, the internal ocean closes, shutting down subduction which require mantle convection of very long-wavelengths. zones in the internal ocean, while subduction and sea-floor Whereas “supercontinent cycle” describes the periodic assembly spreading in the external ocean continues. In P-type closure, the and disruption of land masses, the “Wilson cycle” refers to the external ocean closes, shutting down all subduction. Similar to the opening and closing of large oceans. Although both are considered case of ‘introversion’ and ‘extroversion’, the P-type and A-type as complimentary, the processes associated with the assembly, closure represent two end-member cases, and the actual mode of dispersal and reorganization of supercontinents are more complex. closure may likely involve a combination of the two processes. Hoffman (1991) proposed the concept of “inside-out” process as a mechanism of assembling crustal fragments after the breakup of an earlier supercontinent. If the supercontinent was rifted on one side to bear a passive margin such as in the case of the Atlantic Ocean, then the opposite side becomes a consuming boundary along the continents, such as in the case of the Pacific margin. With time, the Pacific Ocean would shrink and finally close by the passive collision of two continents to generate a large continental mass. In the case of Atlantic, initially both continental margins are passive, but later turn to active margins; the ocean shrinks in size and may totally disap- pear. On the other hand, the Indian Ocean has two different types of margins simultaneously in operation, active and passive, trans- porting the northern continental margin of Gondwana by the Atlantic-type process and amalgamating the rifted continents to the southern margin of Asia by the Pacific-type process. Therefore, the Indian Ocean-type process illustrates simultaneous continental breakup and continental amalgamation exemplifying the “inside- in” mechanism (Murphy and Nance, 2005). Similar simultaneous rifting and accretion on different margins were also proposed in the case of the Paleoproterozoic supercontinent Columbia by Rogers and Santosh (2002, 2004). The Tethyan process started at least by the Permian to Triassic time and operated simultaneously with the Pacific process to the east. Subsequently, double-sided subduction started to define the frontier of the future supercontinent (Maruyama et al., 2007). The Tethyan region is an example for the “inside-in” reassembly of supercontinents (Hoffman, 1991; Murphy and Nance, 2003; Murphy and Nance, 2005; Murphy et al., 2009), whereas the Pacific region is related to the “inside-out” configuration. Thus, the process of completion of a supercontinent would involve a combination of both introversion and extroversion. Such a combination operated in the case of Rodinia assembly, and is also predicted in the amalgamation of the future supercontinent Amasia (see Maruyama et al., 2007; Santosh et al., 2009). In another recent work, Murphy et al. (2009) discussed the two geodynamically distinct tracts of oceanic lithosphere generated during the breakup of supercontinents: a relatively young interior ocean floor that develops between the dispersing continents, and Figure 1 Two types of ocean closure following supercontinent a relatively old exterior ocean floor, which surrounded the breakup as proposed by Silver and Behn (2008). Orange-shaded supercontinent before breakup. The geologic and Sm/Nd isotopic regions e supercontinents; black lines with dark blue triangles e record synthesized in their study suggests that supercontinents subduction zones; thick red lines e internal ocean; blue rectangles in may form by two end-member mechanisms: introversion, in which (d) and (f) e oceanic crustal materials including ophiolites trapped in interior ocean floor is preferentially subducted, and extroversion, the suture zone. See text for discussion. 24 M. Santosh

The complexity of ‘introversion’ and ‘extroversion’ and the P-type and A-type processes associated with supercontinent cycles discussed above also reﬂects upon the destruction and creation of varied environments which impact on the extinction or survival and diversiﬁcation of life. Future studies should integrate these conceptual models in unraveling the planet’s life history, which would lead to a better understanding of the geological control on environmental and biological processes.

2.2. Supercontinent breakup

Models on the fragmentation of supercontinents can be broadly divided into plume-absent and plume-related processes. The plume- absent models consider that the thermal insulation effect of supercontinents (Anderson, 1982) leads to a temperature increase beneath the large landmass, which results in continental rifting and breakup. The concept of a thermal blanket (Fig. 2) envisages the breakup of supercontinents through radiogenic heating (e.g., Gurnis, 1988). Granites contain higher K, U and Th compared to mantle peridotite. Recent studies recognize that substantial volume of crust of granitic Figure 3 Phase relations for continental crust represented by composition is subducted during the assembly of supercontinents anorthosite and TTG (tonalite-trondhjemite-granodiotite; after through arc subduction, sediment trapped subduction, and subduc- Komabayashi et al., 2009). MORBeMid ocean ridge basalt; Pve tion erosion. This TTG (tonalite-trondhjemite-granite) material is perovskite; PpVe post-perovskite. The typical mantle geotherm is dragged down and is thought to accumulate in the mantle transition also shown. See text for discussion. The study demonstrates that all zone (Senshu et al., 2009). It is possible that the radiogenic elements these materials can be subducted down to the core-mantle boundary, in the subducted TTG crust heat up the overlying mantle with time to leading to the development of a compositional stratification in the D” initiate continental rifting and dispersion leading to the opening of layer. oceans. Komabayashi et al. (2009) performed phase assemblage analysis in the mid-oceanic ridge basalt (MORB)-anorthosite-TTG system down to core-mantle boundary (CMB) conditions. Their and their convection modeling an internally heated led them to results show that all these materials can be subducted even to the conclude that the assembly of continents into supercontinents CMB, thus leading to the development of compositional stratifica- would naturally lead to mantle global warming without the tion in the D” layer (Fig. 3). involvement of hot active plumes. Coltice et al. (2007) tested the hypothesis that the assembly of Supercontinents effectively self-destruct in response to the supercontinents would force larger length scales and, therefore, buildup of heat and resultant magmatism, since these effects drive the underlying mantle to higher temperatures. They inves- significantly weaken the lithosphere and make it more susceptible to tigated the intrinsic temperature difference between superconti- breakup in response to regional tectonics. Vaughan and Storey nent and dispersed continents. The position of the continents was (2007) presented a conceptual model where, the subduction rela- fixed and an equilibrium temperature field was computed by tion to the formation of supercontinents focuses on narrow areas of stacking the temperature fields over several billion years to the 660 km mantle discontinuity, triggering superplume events remove the time-dependent features and in order to obtain which eventually lead to the fragmentation of the continental mass a statistical steady state. Their results show that subcontinental above. This supercontinent-triggered superplume mechanism for temperature correlates inversely with the number of continents, continental breakup was examined in the light of the Mesozoic fragmentation of Pangea-Gondwana. Vaughan and Storey (2007) summarized evidence for a superplume event that occurred between 227 and 183 Ma ago during the Late Triassic- Early Jurassic break, which is comparable in scale with those in the Late Protero- zoic (ca. 800 Ma) and during Cretaceous time (ca. 120e80 Ma). During the amalgamation of continental fragments, the subducted oceanic lithosphere of intervening oceans either moves down to the deep mantle or gets horizontally flattened as stagnant slabs in the mantle transition zone (Fukao et al., 1992, 2001, 2009; van der Hilst et al., 1997; Grand, 2002; Zhao, 2004, 2009). Blobs of these stagnant slabs sink down into the deep mantle and accumulate as slab graveyards at the core-mantle boundary. Zhao (2004) synthesized a P-wave tomographic image for the Western Pacific region, along a transect covering Beijing to Tokyo, where ca. 1200 km-long stagnant slabs are seen floating in the mantle transition zone between 410 and 660 km, also termed as the Figure 2 Schematic illustration of the thermal blanket hypothesis 660 km phase boundary. The image shows the presence of a high of supercontinent breakup (after Senshu et al., 2009). See text for P-wave velocity anomaly close to the bottom of the mantle and discussion. immediately above the core-mantle boundary (CMB) which has Supercontinent tectonics and biogeochemical cycle: A matter of ‘life and death’ 25 been interpreted as a ‘slab graveyard’ (Maruyama et al., 2007; the 8000 km-long ‘Transgondwanan Supermountains (Squire et al., Maruyama et al., 2009). The recycled oceanic lithosphere at the 2006; Campell and Squire, 2010) that might have provided an core-mantle boundary contributes potential fuel for generating effective source of rich nutrients including Fe and P to the equa- superplumes which rise from the core-mantle interface to the torial waters, thus aiding the rapid increase in biodiversity, partic- uppermost mantle (Fig. 4), penetrating the mantle transition zone ularly algae and cyanobacteria, which in turn produced a marked and eventually giving rise to hot spots (e.g., Maruyama et al., increase in the production rate of photosynthetic oxygen. Enhanced 2007). Highly resolved seismic tomography models clarify the sedimentation during these periods promoted the burial of a high configuration and amplitude of the velocity anomaly of two fraction of organic carbon and pyrite, thus preventing their reaction superplumes beneath the southern Africaesouthern Atlantic with free oxygen, and leading to sustained increases in atmospheric Ocean and the southern Pacific. oxygen (Campbell and Squire, 2010). Primitive bacteria which represent forerunners of modern cyanobacteria, developed the 3. Supercontinent assembly and life evolution ability to strip electrons from water through oxygenic photosynthesis, simultaneously generating an important by-product: oxygen gas. This evolutionary feat is one of the most important biological The Ediacara fauna documents an important evolutionary episode innovations in the history of life on Earth which set the stage for at the dawn of Cambrian, prior to the so-called Cambrian explosion, profound changes in the redox state of the oceans and atmosphere and holds critical information about the early evolution of macro- (Konhauser, 2009). scopic and complex multi-cellular life (Xiao and Laflamme, 2009). Brasier and Lindsay (2001) evaluated the effect of the assembly The Cambrian radiation is a key episode in the history of life when of supercontinents on the radiation of life, with specific reference to a large number of animal phyla appeared in the fossil record over the Cambrian explosion following the amalgamation of Gondwana a geologically short period of time. The formation of the Gondwana during the Ediacaran-early Cambrian period and the accompanying supercontinent in the Late Neoproterozoic-Cambrian marked the widespread development of foreland basins. A rise in the rate of final phase of a series of severe glaciations during the Neo- sediment accumulation between ca. 550 and 530 Ma suggests that proterozoic, culminating in the appearance of the Ediacara fauna rapid subsidence took place in cratonic margins and interior basins which served as a bridge for the subsequent Cambrian radiation around the globe. From a synthesis of sediment patterns and (Meert and Lieberman, 2008). It has been proposed that the poly- accumulation rates, and carbon, strontium, and neodymium phase assembly of Gondwana during the East Africa, Braziliano, isotopes, Brasier and Lindsay (2001) suggested that rates of Kuungan and Damaran orogenies resulted in an extensive mountain subsidence and uplift accompanied the dramatic radiation of animal chain (Squire et al., 2006), the weathering of which delivered life through the Neoproterozoic- Cambrian interval (ca. 600 to 500 nutrients into a shifting oceanic realm. In addition to the funda- Ma). Supercontinental assembly creates large marginal oceans with mental biological changes that promoted the appearance and old, oxygen-depleted and nutrient-enriched bottom waters. As the proliferation of modern life forms on Earth, it is believed that the shelf barriers begin to submerge, the nutrient-enriched bottom geochemical and tectonic changes which occurred during the waters begin to invade the shelves. This leads to the flourishing of Ediacaran-Cambrian time enhanced the complexity of the ecosys- eutrophic planktons and suspension feeders and the phosphatisation tems. Whether the “Cambrian explosion” was an extremely rapid preserves small shelly fossils. As the restricted carbonate platforms event which occurred within short, and unprecedented, time dura- are drowned, pandemic early fauna flourish. With the increasing tion, or a more gradual natural biological response to changing rate of space creation, condensation, winnowing and phospho- geological environment, is equivocal (Meert and Lieberman, 2008). genesis, sediment ingesters flourish and invertebrates burrow Nevertheless, the role played by the Cambrian Gondwana assembly deeper to retrieve buried organic matter. The high rate of space is emphasized in most of the models, including the formation of creation promotes the formation of organic-rich shales, cherts and thick halite beds in the interior basins. The rapid burial enhances the preservation of grazing traces and together with anoxia also preserves Burgess shale-type faunas (Butterfield, 1995). Maruyama and Santosh (2008) provided a slightly different model where they attempted to correlate the initiation of the biological changes culminating in Cambrian explosion to the history of fragmentation of the pre-Gondwana supercontinent Rodinia. In their synthesis, they identified a number of essential components necessary to build a habitable planet. Life evolution fundamentally requires the appropriate physical environment, and as various studies demonstrated, the creation of a wide platform by lowering of sea-level generates a photic zone on the continental shelf. The birth of the photic zone with a complex food chain and environment, and increased free oxygen promotes a rapidly evolving life. Such diversification of life occurs in the enlarged shelf and photic zone. An upwelling of cold currents with enriched Figure 4 Supercontinent breakup initiated by superplume rising nutrients (particularly derived from a tonalite-trondhjemite-gran- from the core-mantle boundary (after Maruyama et al., 2007; Senshu odioriteeTTGecontinental crust; Maruyama and Santosh, 2008) et al., 2009). The phase change of subducted Mid Ocean Ridge Basalt in isolated rift basins developed through continental fragmentation (MORB) into post-perovskite releases latent heat generating many provides added impetus. In addition, biomineralization aids doesn’t small scale hot plumes, which eventually gather to create a super- only enhance the size of the animal phyla, but also promotes plume. See text for discussion. fundamental changes such as the transformation from exoskeletons 26 M. Santosh to endoskeletons. Extraterrestrial factors such as prolonged cosmic radiation are also speculated to have contributed to the frequent mutation and the final evolution of metazoans that ultimately led to the Cambrian radiation. Maruyama and Santosh (2008) modeled South China as a typical example where many of the drastic fluctuations and oscillations in environmental factors considered above are well- preserved in fossil records (e.g., Shu, 2008), including the several ‘trial and error’ experiments to create modern life involving a number of extinction events. The rifting of Neoproterozoic Rodinia began sometime around 750 Ma, or even earlier in some places (Li et al., 1999). The supercontinent was fragmented mainly into three domains including the western Rodinia (with Australia), eastern Rodinia (with Laurentia), and South China. The western and eastern domains were probably not fully sepa- rated until the late Neoproterozoic, when fragmentation and reconstitution built the Cambrian Gondwana. East Rodinia was ripped apart into several small continental fragments including the San Francisco-Congo, Baltica, Amazonia and W. Africa, Laurentia, North China, Siberia and Tarim cratons. On the contrary, the South China block appears to have been isolated after its separation from Rodinia until its amalgamation with Gondwana at around 540 Ma. Because of this tectonic quiescence, and with a continuous subsidence triggered by a cooling lithosphere, the South China block retained one of the best preserved sedimentary records of biological evolution during a key time in Earth history. Since hydrothermal system in rifts with granitic basement create anomalous chemical environment enriched in nutrients which serve as the primary building blocks of the skeleton and bone of the early modern life forms, the birthplace of vertebrates must have been in lakes developed within continental rifts similar to the present day African Rift Valley and Dead Sea (Maruyama and Santosh, 2008). The rifting of the Rodinia supercontinent opened up an NeS oriented sea way along which nutrient- enriched upwelling brought about a habitable geochemical environment. The origin of metazoans, the ancestors of verte- 2þ Figure 5 A speculative model for the evolution of modern life brates is probably related to the large flux of Ca, Fe ,HCO3,P, Na þ,Kþ, V, Mo and other elements which aided to build their forms following supercontinent breakup. The breakup of the Neo- hard parts (Maruyama and Santosh, 2008). These elements are proterozoic Rodinia supercontinent (a, after Rino et al., 2008) predominantly found in rocks of granitic composition and the generated enlarged shallow marine environments (b, after Maruyama hydrothermal system in the rifts with granitic basement created and Santosh, 2008), with increased oxygen level and enrichment of anomalous chemical environment enriched in the above nutrients. nutrients. This isolated environment was conducive for the extensive These were the primary building blocks of the skeleton and bone development of body plans and enhancement in the size and diversity of the first modern animals. The content of enriched Ca2þ aided life forms which finally led to the so-called Cambrian explosion. The in the amalgamation of microorganisms and their transformation rift-related basins thus provided the ‘life soup’ for the rapid evolution to large multi-cellular animals. Following the rifting of Rodinia, of modern life. G1 to G5 in the Rodinia reconstruction shown in (a) the sea level had dropped since 750 Ma and led to the develop- correspond to the classification into various groups (North American, ment of a wide shallow marine platform environment, with African, Russian and Asian Grenville Groups) based on Rino et al. a photic zone that offered an environment for life diversification. (2008) on the basis of zircon age data. Cosmic radiation caused mutation at various levels leading to genome duplication and shuffling (Miyata and Suga, 2001). However, it took a long time, until around 560 Ma, to develop the 4. Supercontinent cycle and mass extinction adequate oxygen levels in the shallow marine environment and atmosphere for life to thrive, which is when ultimately the 4.1. Mass extinction events in earth history Ediacara fauna evolved. Thereafter, the mass extinctions and biomineralization coded by genome created almost all the body Extinction is considered as an integral and essential feature of life on plans of animals by the end of Cambrian. Thus, the model of Earth (Bradshaw and Brook, 2009). The Archean Earth was a hot Maruyama and Santosh (2008) traces the geological processes greenhouse with high level of methane and carbon dioxide marking leading to the generation of the ‘life soup’ for biological evolu- a prolonged 600 million year global warming event. This was fol- tion back to the history of fragmentation of the Rodinia super- lowed by a global freezing episode at ca. 2.3 Ga, and another continent (Fig. 5). warming event at ca. 1.8 Ga (Brocks et al., 2005; Canfield, 2005; Supercontinent tectonics and biogeochemical cycle: A matter of ‘life and death’ 27

Nisbett and Nisbett, 2008). Such extreme climatic fluctuations are during the Cretaceous period which began at 135 Ma and came to considered to have driven extinction events in the early Earth. a sudden and catastrophic end at 65 Ma. Almost 85% of all species Joseph (2009) and Elewa and Joseph (2009) provide recent concise on the globe was eliminated during the KT (Cretaceous/Tertiary) reviews on the mass extinction events in the history of our planet. extinction event (Raup, 1992). The first snowball Earth event that froze the entire planet at ca. 2.3 Among the geological reasons attributed for such mass Ga must have seriously damaged the prokaryote and eukaryote extinction events are major climatic fluctuations including global habitats from the low temperatures, and reduced levels of methane cooling and warming events, major glaciations, fluctuations in sea and enhanced oxygen concentration. Complex multi-cellular level, global anoxia, volcanic eruptions, asteroid impacts and eukaryotes emerged and proliferated by around 1.6 to 1.2 Ga (Xiao gamma rays (see Elewa and Joseph, 2009 and references therein). and Knoll, 1999; Butterfield, 2000). Acritarchs, as well as plankton, Obviously, many of these attributes are related to the history of coccoid and filamentous cyanobacteria, protozoa, fungi, amoebo- continents, supercontinents and oceans on the globe. Mass zoans, cercozoans, and eukaryotic and marine algae had proliferated extinction events are generally unknown for the Precambrian, throughout the oceans and inland seas by 800 Ma (Butterfield, except some speculative correlations, whereas these were common 2005). The process of photosynthesis started the oxygen pump in the Phanerozoic. The assembly and dispersal of continents have leading to a drastic reduction in the concentration of CO2 (Holland, influenced some of these events, but whether the individual stages 2006). The breakup of the Neoproterozoic supercontinent Rodinia of organic evolution and extinction on the planet are closely linked led to enhanced silicate weathering rates, which, coupled with to Solid Earth processes remains to be investigated. increase in the levels of oxygen impacted the climate significantly. Methane and CO2 levels dropped, taking the Earth into the major 4.2. Relationship to supercontinent breakup Sturtian global ice age (Hoffman et al., 1998; Harland, 2007). During this period, which lasted up to ca. 670 Ma, a vast number of Yale and Carpenter (1998) synthesized information on the global species became extinct (Fanning and Link, 2004). Between 640 and distribution of large igneous provinces (LIPs) and giant dyke 580 Ma, the planet witnessed yet another snowball event, the Mar- swarms (GDS) which suggest that both occur periodically and inoan glaciation (Hoffman et al., 1998, 2004; Hyde et al., 2000) might be related to the mantle insulation following supercontinent followed by a less extreme period of cooling referred to as Gaskiers, assembly. The periodicity of these events (ca. 300 to 500 Ma) also which came to a close around 580 Ma (Eyles and Eyles, 1989). The broadly coincides with the supercontinent cycles. A 475 Myr gap Marinoan glaciation probably witnessed the extinction of several in the LIP records between 725 and 250 Ma was explained to be early microscopic life forms. Following the termination of the related to a relatively dispersed state of continents on the globe. Marinoan/Gaskiers glaciation and the warming of the Earth, an This period is argued to coincide with a period of Earth history explosion of life ensued (Peterson and Butterfield, 2005) including characterized by oxygen, carbon, strontium and sulfur isotope the evolution of megascopic Ediacarans. However, by 540 Ma, the ratios indicative of relatively small mantle fluxes. Ediacaran fauna vanished, probably serving as an evolutionary Fig. 6 shows the cumulative percentage of the global occurrence bridge to the Cambrian explosion that followed. of GDS, carbonatites and kimberlites, together with the model for Beginning around 540 Ma, there was an explosive evolution of periodic production of GDS as proposed by Yale and Carpenter life within a period of less than 10 million years, and characterized (1998). The approximate time spans of the assembly and dispersal by the body plans of modern animals (Conway Morris, 2000; of the major supercontinents on the globe are also superimposed. Peterson and Butterfield, 2005; Meert and Lieberman, 2008). Although Yale and Carpenter (1998) originally defined the step- The Cambrian era witnessed four major extinctions during the wise steep patterns in their model GDS production curve to correlate interval 540e510 Ma including trilobites, archaeocyathids, brachiopods and conodonts. The Cambro-Ordovician history also set the stage for another two-stage mass extinction, which is considered as the second most devastating tragedy of animal life in the history of our planet. Over one hundred families of marine invertebrates perished and others were driven to near extinction (Sheehan, 2001), probably related to another cycle of global cooling and glaciation. During Devonian (410e360 Ma) many new species evolved including amphibians, insects, and a new generation of reef builders, but the Paleozoic era ended with another mass extinction event wiping out over 70% of life forms (Raup, 1992). The Permian (290e248 Ma) also culminated with a great mass extinction event destroying multiple life forms including some amphibians, reptiles, and repto-mammals, and erasing nearly 95% of all species of marine animals (Raup, 1992). The Triassic period which extended from ca. 250e200 Ma witnessed the evolution of the mammal-like therapsids, the first flying vertebrates, and the pterosaurs. This period witnessed the close- packing of the world’s land masses into one supercontinent Pan- Figure 6 Cumulative percentage of the occurrence of giant dyke gea, located in the temperate and tropical regions of Earth (Rogers swarms, carbonatites and kimberlites and the model periodic and Santosh, 2004). The closing of the Triassic was marked production of giant dyke swarms (after Yale and Carpenter, 1998). by another mass extinction which terminated nearly half of the The approximate timings of assembly and disruption of the major marine fauna and most marine reptiles (Tanner et al., 2004). supercontinents through Earth history are also shown. See text for The dinosaurs were the only survivors, but were also eliminated discussion. 28 M. Santosh

LIP eruptions with the assembly of supercontinents, the data fit better with a scenario where the steep curves for production correlate with supercontinent disruption and with the intermittent quiescent periods linked with supercontinent assembly. The maximum increase in LIP volume occurred between 150 and 70 Ma and this rapid volume increase coincides with mid Cretaceous superplume event (Larson, 1991). The link between LIP and GDS event and breakup of supercontinents might result from either mantle insulation following the amalgamation of supercontinental assemblies or rising mantle plumes fueled by recycled subducted material. The close coinci- dence between LIPs and mass extinction has been scrutinized in many studies. Wignall (2001) addressed the correspondence between the timing of mass extinctions with the formation age of LIPs as has been attributed in the case of at least four consecutive mid-Phanerozoic examples. These examples are the end-Guada- lupian extinction/Emeishan flood basalts, the end-Permian Figure 7 Cartoon illustrating the role of mantle dynamics and extinction/Siberian Traps, the end-Triassic extinction/central supercontinents in life history and surface environment. The figure Atlantic volcanism and the early Toarcian extinction/Karoo Traps. shows the relationship between superplume, supercontinent breakup However, Wignall (2001) did not endorse a direct link, noting that and mass extinction (after Isozaki, 2009) during the Late Guadalupian the onset of eruptions in most cases slightly post-dates the main (Middle Permian; ca. 265 Ma) event, an episode termed the Illawarra phase of extinctions in these examples. However, he also noted Reversal. The model envisages two stages: (1) a strong retardation in that many of these episodes coincide with global warming and the geomagnetic field led to a break in the geomagnetic barrier marine anoxia/dysoxia, a relationship that emphasizes an impor- allowing galactic cosmic rays to infiltrate. This resulted in the tant effect on global climate. Nevertheless, there is a general formation of thick cloud layers which ultimately stopped the “oxygen consensus that the LIP events correlate closely with major changes pump”; the second stage is marked by the birth of a superplume and in oceanic and atmospheric chemistry, and, thus, could trigger its upwelling, followed by voluminous volcanic eruption leading to global mass extinctions (Saunders, 2005). plume winter. Isozaki (2009) recently evaluated the relationship between superplume, supercontinent breakup and mass extinction. The Permian magnetostratigraphic record demonstrates a significant 5. Epilogue change in geomagnetism in the Late Guadalupian (Middle Permian; ca. 265 Ma), an episode termed the Illawarra Reversal. The history of the Earth records repeated amalgamation and The Illawarra Reversal, a change in the geodynamo of the Earth’s dispersal of supercontinents starting from the first coherent core, is speculated to have resulted from the appearance of assembly Columbia during the Paleoproterozoic. Many of the a thermal instability at the 2900 km-deep coreemantle boundary configurations still remain hypothetical, and although there is that was related to mantle superplume activity. One of the major increasing evidence that the supercontinent cycles have influ- global environmental changes in the Phanerozoic occurred almost enced Solid Earth processes and biogeochemical cycle, some of simultaneously in the latest Guadalupian, as recorded in 1) mass the inferences drawn are based largely on speculative models and extinction, 2) ocean redox change, 3) sharp isotopic excursions require further quantification. The rapid erosion of super- (C and Sr), 4) sea-level drop, and 5) plume-related volcanism. mountains built up during continental amalgamation is thought to Isozaki (2009) suggested that the Illawarra Reversal, the Kamura cause enormous flux of nutrients essential for triggering the cooling event, and other unique geologic phenomena in the Late explosion of life, and switching on the Planet’s oxygen pump. Guadalupian are all consequences of the superplume activity that The assembly of supercontinents witnessed the subduction of initially triggered the breakup of Pangea. According to this model large volumes of oceanic lithosphere which move down into the (Fig. 7), a superplume rising from the deep mantle represents large deep mantle and from there to the core-mantle boundary accu- scale flow of material and energy within the planet. The upwelling mulating as slab graveyards. Their recycling is speculated to plume paired with a downwelling cold subducted slab generated provide the fuel for generating superplumes which rise up and a large scale whole-mantle convection cell. In the beginning, the eventually break apart supercontinents. Models linking super- rise of the superplume caused the Illawarra Reversal in the core plume and supercontinent disruption predict a catastrophic effect geodynamo and a cooling event on the surface. When the plume from related processes on the Earth’s surface environment head impinged the base of the Pangea supercontinent, the resultant leading to life extinction. The link between biological innovation continental rifting and formation of LIPs led to the initiation of and geological processes during Earth history offers a chal- a plume winter. Subsequent volcanic emissions and its cata- lenging topic for multidisciplinary research, not only for recon- strophic effect on the atmosphere led to mass extinction and long structing the past history, but also for predicting the future of the term oceanic anoxia (superanoxia). Two main stages have been globe. identified as follows: (1) loss of geomagnetism leading to a break in the geomagnetic barrier when galactic cosmic rays triggered extensive cloud formation and Earth’s ‘oxygen pump’ was halted; Acknowledgments (2) the second stage when the birth of the superplume and volcanic eruption lead to ‘plume winter’. Both scenarios are considered to I thank the Editor of Geoscience Frontiers for inviting this have contributed to mass extinction. contribution and Prof. Shihong Zhang for an insightful review. Supercontinent tectonics and biogeochemical cycle: A matter of ‘life and death’ 29

References Hoffman, P.F., Kaufman, A.J., Halverson, G.P., Schrag, D.P., 1998. A Neoproterozoic snowball Earth. Science 281, 1342e1346. Holland, H.D., 2006. The oxygenation of the atmosphere and oceans. Phil- Anderson, D.L., 1982. Hotspots, polar wander, Mesozoic convection and osophical Transactions of the Royal Society, Series B 361, 903e915. the geoid. Nature 297, 391e393. Hyde, W.T., Crowley, T.J., Baum, S.K., Peltier, W.R., 2000. Neo- Bradshaw, C.J.A., Brook, B.W., 2009. The Cronus Hypothesis: extinction proterozoic ‘snowball earth’ simulations with a coupled climate/ice- as a necessary and dynamic balance to evolutionary diversification. sheet model. Nature 405, 425e429. Journal of Cosmology 2, 221e229. Isozaki, Y., 2009. Illawarra Reversal: the fingerprint of a superplume that Brasier, M.D., Lindsay, J.F., 2001. Did supercontinental amalgamation triggered Pangean breakup and the end-Guadalupian (Permian) mass trigger the “Cambrian Explosion”. In: Zhuralev, A.Y., Riding, R. extinction. Gondwana Research 15, 421e432. (Eds.), The Ecology of the Cambrian Radiation. Columbia University Joseph, R., 2009. Extinction, metamorphosis, evolutionary apoptosis, and Press, New York, pp. 69e89. genetically programmed species mass death. Journal of Cosmology 2, Brocks, J.J., Love, G.D., Summons, R.E., Knoll, A.H., Logan, G.A., 235e255. Bowden, S.A., 2005. Biomarker evidence for green and purple sulphur Komabayashi, T., Maruyama, S., Rino, S., 2009. A speculation on the bacteria in a stratified Palaeoproterozoic sea. Nature 437, 866e870. structure of the D" layer: the growth of anti-crust at the core-mantle Butterfield, N.J., 1995. Secular distribution of Burgess-Shaleetype pres- boundary through the subduction history of the Earth. Gondwana ervation. Lethaia 28, 1e13. Research 15, 342e353. Butterfield, N.J., 2000. Bangiomorpha pubescens n. gen., n. sp.: implica- Komiya, T., Hayashi, M., Maruyama, S., Yurimoto, H., 2002. Intermediate tions for the evolution of sex, multicellularity, and the Mesoproter- P/T type Archean metamorphism of the Isua supracrustal belt: impli- ozoic/Neoproterozoic radiation of eukaryotes. Paleobiology 26, cations for secular change of geothermal gradients at subduction zones 386e404. and for Archean plate tectonics. American Journal of Science 302, Butterfield, N.J., 2005. Reconstructing a complex early Neoproterozoic 806e826. eukaryote, Wynniatt formation, arctic Canada. Lethaia 38, 155e169. Konhauser, K., 2009. Biogeochemistry: deepening the early oxygen Campbell, I.H., Squire, R.J., 2010. The mountains that triggered the Late debate. Nature Geoscience 2, 241e242. Neoproterozoic increase in oxygen: the Second Great Oxidation Event. Larson, R.L., 1991. Latest pulse of Earth: evidence for a mid Cretaceous Geochimica et Cosmochimica Acta 74, 4187e4206. superplume. Geology 19, 547e550. Canfield, D.E., 2005. The early history of atmospheric oxygen. Annual Li, Z.X., Li, X.H., Kinny, P.D., Wang, J., 1999. The breakup of Rodinia: Reviews of Earth and Planetary Sciences 33, 1e36. did it start with a mantle plume beneath South China? Earth and Coltice, N., Phillips, B.R., Bertrand, H., Ricard, Y., Rey, P., 2007. Global Planetary Science Letters 173, 171e181. warming of the mantle at the origin of flood basalts over superconti- Lowman, J.P., Jarvis, G.T., 1993. Mantle convection flow reversals due to nents. Geology 35, 391e394. continental collisions. Geophysical Research Letters 20, 2087e2090. Condie, K.C., 2001. Global change related to Rodinia and Gondwana. Maruyama, S., Hasegawa, A., Santosh, M., Kogiso, T., Omori, S., Gondwana Research 4, 598e599. Nakamura, H., Kawai, K., Zhao, D.P., 2009. The dynamics of big Condie, K.C., Des Marais, D.J., Abbott, D., 2001. Precambrian super- mantle wedge, magma factory, and metamorphic-metasomatic factory plumes and supercontinents: a record in black shales, carbon isotopes, in subduction zones. Gondwana Research 16, 414e430. and paleoclimates. Precambrian Research 106, 239e260. Maruyama, S., Santosh, M., 2008. Models on Snowball Earth and Conway Morris, S., 2000. The Cambrian “explosion”: slow-fuse or Cambrian explosion: a synopsis. Gondwana Research 14, 22e32. megatonnage? Proceedings of the National Academy of Science of the Maruyama, S., Santosh, M., Zhao, D.P., 2007. Superplume, supercontinent, United State of America 97, 4426e4429. and post-perovskite: mantle dynamics and antieplate tectonics on the Elewa, A.M.T., Joseph, R.W., 2009. The history, origins and causes of core-mantle boundary. Gondwana Research 11, 7e37. mass extinctions. Journal of Cosmology 2, 201e220. Meert, J.G., Liemberman, B.S., 2008. The Neoproterozoic assembly of Eyles, N., Eyles, C.H., 1989. Glacially-influenced deep-marine sedimen- Gondwana and its relationship to the Ediacaran-Cambrian radiation. tation of the Late Precambrian Gaskiers Formation, Newfoundland, Gondwana Research 14, 5e21. Canada. Sedimentology 36, 601e620. Miyata, T., Suga, H., 2001. Divergence pattern of animal gene families and Fanning, C.M., Link, P.K., 2004. U-Pb SHRIMP ages of Neoproterozoic relationship with the Cambrian explosion. BioEssays 23, 1018e1027. (Sturtian) glaciogenic Pocatello Formation, southeastern Idaho. Geology Murphy, J.B., Nance, R.D., 2003. Do supercontinents introvert or extro- 32, 881e884. vert? Sm-Nd isotope evidence. Geology 31, 873e876. Fukao, Y., Obayashi, M., Inoue, H., Nenbai, M., 1992. Subducting slabs Murphy, J.B., Nance, R.D., 2005. Do supercontinents turn inside-in or stagnate in the mantle transition zone. Journal of Geophysical Research inside-out? International Geology Review 47, 591e619. 97 (B4), 4809e4822. Murphy, J.B., Nance, R.D., Cawood, P.A., 2009. Contrasting modes of Fukao, Y., Obayashi, M., Nakakuki, T., Deep Slab Project Group, 2009. supercontinent formation and the conundrum of Pangea. Gondwana Stagnant slab: a review. Annual Reviews of Earth and Planetary Research 15, 408e420. Sciences 37, 19e46. Nisbett, E.G., Nisbett, R.E., 2008. Methane, oxygen, photosynthesis, Fukao, Y., Widiyantoro, S., Obayashi, M., 2001. Stagnant slabs in the rubisco and the regulation of the air through time. Philosophical upper and lower mantle transition region. Reviews in Geophysics 39, Transactions of the Royal Society of London B, Biological Sciences 291e323. 363, 2745e2754. Grand, S.P., 2002. Mantle shear-wave tomography and the fate of sub- Parsons, B., Sclater, J.G., 1977. An analysis of the variation of ocean floor ducted slabs. Philosophical Transactions of the Royal Society of bathymetry and heat flow with age. Journal of Geophysical Research London (Series A) 360, 2475e2491. 82, 802e827. Gurnis, M., 1988. Large-scale mantle convection and the aggregation and Peterson, K.J., Butterfield, N.J., 2005. Origin of the eumetazoa: testing dispersal of supercontinents. Nature 332, 695e699. ecological predictions of molecular clocks against the Proterozoic Harland, W.B., 2007. Origins and assessment of snowball Earth hypoth- fossil record. Proceedings of the National Academy of Science of the eses. Geological Magazine 144, 633e642. United State of America 102, 9547e9552. Hoffmann, K.H., Condon, D.J., Bowring, S.A., Crowley, J.L., 2004. U-Pb zircon Phillips, B.R., Bunge, H.P., 2007. Supercontinent cycles disrupted by date from the Neoproterozoic Ghaub Formation, Namibia: constraints on strong mantle plumes. Geology 35 (9), 847e850. Marinoan glaciation. Geology 32, 817e820. Raup, D.M., 1992. Bad Genes or Bad Luck. Norton, New York. Hoffman, P.F., 1991. Did the breakout of Laurentia turn Gondwanaland Rino, S., Kon, Y., Sato, W., Maruyama, S., Santosh, M., Zhao, D.P., 2008. inside out? Science 252, 1409e1412. The Grenvillian and Pan-Arican orogens: world’s largest orogenies 30 M. Santosh

through geologic time, and their implications on the origin of super- Vaughan, A.P.M., Storey, B.C., 2007. A new supercontinent self-destruc- plume. Gondwana Research 14, 51e72. tion mechanism: evidence from Late Triassic-Early Jurassic. Journal of Rogers, J.J.W., Santosh, M., 2002. Conﬁguration of Columbia, a Meso- the Geological Society 164, 383e392. proterozoic supercontinent. Gondwana Research 5, 5e22. Wignall, P.B., 2001. Large igneous provinces and mass extinctions. Earth Rogers, J.J.W., Santosh, M., 2004. Continents and Supercontinents. Oxford Science Reviews 53, 1e33. University Press, New York, 289 pp. Worsley, T.R., Nance, R.D., 1989. Carbon redox and climate control Santosh, M., 2010. A synopsis of recent conceptual models on supercon- through earth history: a speculative reconstruction. Paleogeography tinent tectonics in relation to mantle dynamics, life evolution and Paleoclimatology Paleoecology 75, 259e282. surface environment. Journal of Geodynamics 50, 116e133. Worsley, T.R., Nance, R.D., Moody, J.B., 1986. Tectonic cycles and the Santosh, M., Maruyama, S., Yamamoto, S., 2009. The making and history of the Earth’s biogeochemical and paleoceanographic record. breaking of supercontinents: some speculations based on superplumes, Paleoceanography 1, 233e263. superdownwelling and the role of tectosphere. Gondwana Research 15, Xiao, S.H., Knoll, A.H., 1999. Fossil preservation in the Neoproterozoic 324e341. Doushantuo phosphorite Lagerstatte, South China. Lethaia 32, Saunders, A.D., 2005. Large Igneous Provinces: origin and environmental 219e240. consequences. Elements 1, 259e263. Xiao, S.H., Laﬂamme, M., 2009. On the eve of animal radiation: Senshu, H., Maruyama, S., Rino, S., Santosh, M., 2009. Role of tona- phylogeny, ecology and evolution of the Ediacara biota. Trend in lite-trondhjemite-granite (TTG) crust subduction on the mechanism Ecology & Evolution 24 (1), 31e40. doi:10.1016/j.tree.2008.7.15. of supercontinent breakup. Gondwana Research 15, 433e442. Yale, L.B., Carpenter, S.J., 1998. Large igneous provinces and giant dyke Sheehan, P.W., 2001. The late Ordovician mass extinction. Annual swarms: proxies for supercontinent cyclicity and mantle convection. Reviews of Earth and Planetary Sciences 29, 331e364. Earth and Planetary Science Letters 163, 109e122. Shu, D., 2008. Cambrian explosion: birth of tree of animals. Gondwana Yoshida, M., Iwase, Y., Honda, S., 1999. Generation of plumes under Research 15, 219e240. a localized high viscosity lid on 3-D spherical shell convection. Silver, P., Behn, M., 2008. Intermittent plate tectonics? Science 319, Geophysical Research Letters 26 (7), 947e950. 85e88. Zhao, D.P., 2004. Global tomographic images of mantle plumes and Squire, R.J., Campbell, I.H., Allen, C.M., Wilson, C.J.L., 2006. Did the subducting slabs: insight into deep earth dynamics. Physics of the Transgondwanan Supermountain trigger the explosive radiation of Earth and Planetary Interiors 146, 3e34. animals on Earth? Earth and Planetary Science Letters 250, 116e133. Zhao, D.P., 2009. Multiscale seismic tomography and mantle dynamics. Tanner, L.H., Lucas, S.G., Chapman, M.G., 2004. Assessing the record and Gondwana Research 15, 297e323. causes of Late Triassic extinctions. Earth Science Reviews 65, 103e139. Zhang, N., Zong, S., McNarmara, A.K., 2009. Supercontinent formation van der Hilst, R.D., Widiyantoro, S., Engdahl, E.R., 1997. Evidence for deep from stochastic collision and mantle convection models. Gondwana mantle circulation from global tomography. Nature 386, 578e584. Research 15, 267e275.