Geoscience Frontiers (2010) 1,21e30 available at www.sciencedirect.com China University of Geosciences (Beijing) Geoscience Frontiers journal homepage: www.elsevier.com/locate/gsf 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 hydro- thermal system in rifts with granitic basement create anomalous chemical environments enriched in nutri- ents, 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 E-mail address: [email protected]. 1674-9871 ª 2010, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. Peer-review under responsibility of China University of Geosciences (Beijing). doi:10.1016/j.gsf.2010.07.001 Production and hosting by Elsevier 22 M. Santosh 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 frag- mentation 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,