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From http://www.sciencedirect.com/science/article/pii/S167498711000006X
4. Supercontinent cycle and mass extinction
4.1. Mass extinction events in earth history
Extinction is considered as an integral and essential feature of life on Earth (Bradshaw and Brook, 2009). The Archean Earth was a hot greenhouse with high level of methane and carbon dioxide marking a prolonged 600 million year global warming event. This was followed by a global freezing episode at ca. 2.3 Ga, and another warming event at ca. 1.8 Ga ( [Brocks et al., 2005], [Canfield, 2005] and [Nisbett and Nisbet, 2008]). Such extreme climatic fluctuations are considered to have driven extinction events in the early Earth. Joseph (2009) and Elewa and Joseph (2009) provide recent concise reviews on the mass extinction events in the history of our planet. The first snowball Earth event that froze the entire planet at ca. 2.3 Ga must have seriously damaged the prokaryote and eukaryote habitats from the low temperatures, and reduced levels of methane and enhanced oxygen concentration. Complex multi-cellular eukaryotes emerged and proliferated by around 1.6 to 1.2 Ga ( [Xiao and Knoll, 1999] and [Butterfield, 2000]). Acritarchs, as well as plankton, coccoid and filamentous cyanobacteria, protozoa, fungi, amoebozoans, cercozoans, and eukaryotic and marine algae had proliferated throughout the oceans and inland seas by 800 Ma (Butterfield, 2005). The process of photosynthesis started the oxygen pump leading to a drastic reduction in the concentration of CO2 (Holland, 2006). The breakup of the Neoproterozoic supercontinent Rodinia led to enhanced silicate weathering rates, which, coupled with increase in the levels of oxygen impacted the climate significantly. Methane and CO2 levels dropped, taking the Earth into the major Sturtian global ice age ( [Hoffman et al., 1998] and [Harland, 2007]). During this period, which lasted up to ca. 670 Ma, a vast number of species became extinct (Fanning and Link, 2004). Between 640 and 580 Ma, the planet witnessed yet another snowball event, the Marinoan glaciation ( [Hoffman et al., 1998], [Hoffmann et al., 2004] and [Hyde et al., 2000]) followed by a less extreme period of cooling referred to as Gaskiers, which came to a close around 580 Ma (Eyles and Eyles, 1989). The Marinoan glaciation probably witnessed the extinction of several early microscopic life forms. Following the termination of the Marinoan/Gaskiers glaciation and the warming of the
Earth, an explosion of life ensued (Peterson and Butterfield, 2005) including the evolution of megascopic Ediacarans. However, by 540 Ma, the Ediacaran fauna vanished, probably serving as an evolutionary bridge to the Cambrian explosion that followed. Beginning around 540 Ma, there was an explosive evolution of life within a period of less than 10 million years, and characterized by the body plans of modern animals ( [Conway Morris, 2000], [Peterson and Butterfield, 2005] and [Meert and Liemberman, 2008]). The Cambrian era witnessed four major extinctions during the interval 540510 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 (410360 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 (290248 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. 250200 Ma witnessed the evolution of the mammal-like therapsids, the first flying vertebrates, and the pterosaurs. This period witnessed the close-packing of the worlds land masses into one supercontinent Pangea, located in the temperate and tropical regions of Earth (Rogers and Santosh, 2004). The closing of the Triassic was marked by another mass extinction which terminated nearly half of the marine fauna and most marine reptiles (Tanner et al., 2004). The dinosaurs were the only survivors, but were also eliminated during the Cretaceous period which began at 135 Ma and came to a sudden and catastrophic end at 65 Ma. Almost 85% of all species on the globe was eliminated during the KT (Cretaceous/Tertiary) extinction event (Raup, 1992).
Among the geological reasons attributed for such mass extinction events are major climatic fluctuations including global cooling and warming events, major glaciations, fluctuations in sea level, global anoxia, volcanic eruptions, asteroid impacts and gamma rays (see Elewa and Joseph, 2009 and references therein). Obviously, many of these attributes are related to the history of continents, supercontinents and oceans on the globe. Mass extinction events are generally unknown for the Precambrian, except some speculative correlations, whereas these were common in the Phanerozoic. The assembly and dispersal of continents have influenced some of these events, but whether the individual stages of organic evolution and extinction on the planet are closely linked to Solid Earth processes remains to be investigated.
4.2. Relationship to supercontinent breakup
Yale and Carpenter (1998) synthesized information on the global distribution of large igneous provinces (LIPs) and giant dyke swarms (GDS) which suggest that both occur periodically and might be related to the mantle insulation following supercontinent assembly. The periodicity of these events (ca. 300 to 500 Ma) also broadly coincides with the supercontinent cycles. A 475 Myr gap in the LIP records between 725 and 250 Ma was explained to be related to a relatively dispersed state of continents on the globe. This period is argued to coincide with a period of Earth history characterized by oxygen, carbon, strontium and sulfur isotope ratios indicative of relatively small mantle fluxes.
Fig. 6 shows the cumulative percentage of the global occurrence of GDS, carbonatites and kimberlites, together with the model for periodic production of GDS as proposed by Yale and Carpenter (1998). The approximate time spans of the assembly and dispersal of the major supercontinents on the globe are also superimposed. Although Yale and Carpenter (1998) originally defined the step-wise steep patterns in their model GDS production curve to correlate 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 coincidence 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-Guadalupian extinction/Emeishan flood basalts, the end-Permian extinction/Siberian Traps, the end-Triassic extinction/central Atlantic volcanism and the early Toarcian extinction/Karoo Traps. However, Wignall (2001) did not endorse a direct link, noting that the onset of eruptions in most cases slightly post-dates the main phase of extinctions in these examples. However, he also noted that many of these episodes coincide with global warming and marine anoxia/dysoxia, a relationship that emphasizes an important effect on global climate. Nevertheless, there is a general consensus that the LIP events correlate closely with major changes in oceanic and atmospheric chemistry, and, thus, could trigger global mass extinctions (Saunders, 2005).
Isozaki (2009) recently evaluated the relationship between superplume, supercontinent breakup and mass extinction. The Permian magnetostratigraphic record demonstrates a significant change in geomagnetism in the Late Guadalupian (Middle Permian; ca. 265 Ma), an episode termed the Illawarra Reversal. The Illawarra Reversal, a change in the geodynamo of the Earths core, is speculated to have resulted from the appearance of a thermal instability at the 2900 km-deep coremantle boundary that was related to mantle superplume activity. One of the major global environmental changes in the Phanerozoic occurred almost simultaneously in the latest Guadalupian, as recorded in 1) mass extinction, 2) ocean redox change, 3) sharp isotopic excursions (C and Sr), 4) sea-level drop, and 5) plume-related volcanism. Isozaki (2009) suggested that the Illawarra Reversal, the Kamura cooling event, and other unique geologic phenomena in the Late Guadalupian are all consequences of the superplume activity that initially triggered the breakup of Pangea. According to this model (Fig. 7), a superplume rising from the deep mantle represents large scale flow of material and energy within the planet. The upwelling plume paired with a downwelling cold subducted slab generated a large scale whole-mantle convection cell. In the beginning, the rise of the superplume caused the Illawarra Reversal in the core geodynamo and a cooling event on the surface. When the plume head impinged the base of the Pangea supercontinent, the resultant continental rifting and formation of LIPs led to the initiation of a plume winter. Subsequent volcanic emissions and its catastrophic effect on the atmosphere led to mass extinction and long term oceanic anoxia (superanoxia). Two main stages have been identified as follows: (1) loss of geomagnetism leading to a break in the geomagnetic barrier when galactic cosmic rays triggered extensive cloud formation and Earths oxygen pump was halted; (2) the second stage when the birth of the superplume and volcanic eruption lead to plume winter. Both scenarios are considered to have contributed to mass extinction.
. Epilogue
The history of the Earth records repeated amalgamation and dispersal of supercontinents starting from the first coherent assembly Columbia during the Paleoproterozoic. Many of the configurations still remain hypothetical, and although there is increasing evidence that the supercontinent cycles have influenced Solid Earth processes and biogeochemical cycle, some of the inferences drawn are based largely on speculative models and require further quantification. The rapid erosion of supermountains built up during continental amalgamation is thought to cause enormous flux of nutrients essential for triggering the explosion of life, and switching on the Planets oxygen pump. The assembly of supercontinents witnessed the subduction of large volumes of oceanic lithosphere which move down into the deep mantle and from there to the core-mantle boundary accumulating as slab graveyards. Their recycling is speculated to provide the fuel for generating superplumes which rise up and eventually break apart supercontinents. Models linking superplume and supercontinent disruption predict a catastrophic effect from related processes on the Earths surface environment leading to life extinction. The link between biological innovation and geological processes during Earth history offers a challenging topic for multidisciplinary research, not only for reconstructing the past history, but also for predicting the future of the globe.