 | Supercontinent assembly and life evolution |
Supercontinent tectonics and biogeochemical cycle: A matter of life and death http://plate-tectonic.narod.ru/plum1photoalbum.html
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from http://www.sciencedirect.com/science/article/pii/S167498711000006X
. Supercontinent assembly and life evolution
The Ediacara fauna documents an important evolutionary episode at the dawn of Cambrian, prior to the so-called Cambrian explosion, and holds critical information about the early evolution of macroscopic and complex multi-cellular life (Xiao and Laflamme, 2009). The Cambrian radiation is a key episode in the history of life when a large number of animal phyla appeared in the fossil record over a geologically short period of time. The formation of the Gondwana supercontinent in the Late Neoproterozoic-Cambrian marked the final phase of a series of severe glaciations during the Neoproterozoic, culminating in the appearance of the Ediacara fauna which served as a bridge for the subsequent Cambrian radiation (Meert and Lieberman, 2008). It has been proposed that the polyphase assembly of Gondwana during the East Africa, Braziliano, Kuungan and Damaran orogenies resulted in an extensive mountain chain (Squire et al., 2006), the weathering of which delivered nutrients into a shifting oceanic realm. In addition to the fundamental biological changes that promoted the appearance and proliferation of modern life forms on Earth, it is believed that the geochemical and tectonic changes which occurred during the Ediacaran-Cambrian time enhanced the complexity of the ecosystems. Whether the Cambrian explosion was an extremely rapid event which occurred within short, and unprecedented, time duration, or a more gradual natural biological response to changing geological environment, is equivocal (Meert and Lieberman, 2008). Nevertheless, the role played by the Cambrian Gondwana assembly is emphasized in most of the models, including the formation of the 8000 km-long Transgondwanan Supermountains (Squire et al., 2006; Campell and Squire, 2010) that might have provided an effective source of rich nutrients including Fe and P to the equatorial waters, thus aiding the rapid increase in biodiversity, particularly algae and cyanobacteria, which in turn produced a marked increase in the production rate of photosynthetic oxygen. Enhanced sedimentation during these periods promoted the burial of a high fraction of organic carbon and pyrite, thus preventing their reaction with free oxygen, and leading to sustained increases in atmospheric oxygen (Campbell and Squire, 2010). Primitive bacteria which represent forerunners of modern cyanobacteria, developed the 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 innovations in the history of life on Earth which set the stage for profound changes in the redox state of the oceans and atmosphere (Konhauser, 2009).
Brasier and Lindsay (2001) evaluated the effect of the assembly of supercontinents on the radiation of life, with specific reference to the Cambrian explosion following the amalgamation of Gondwana during the Ediacaran-early Cambrian period and the accompanying widespread development of foreland basins. A rise in the rate of sediment accumulation between ca. 550 and 530 Ma suggests that rapid subsidence took place in cratonic margins and interior basins around the globe. From a synthesis of sediment patterns and accumulation rates, and carbon, strontium, and neodymium isotopes, Brasier and Lindsay (2001) suggested that rates of subsidence and uplift accompanied the dramatic radiation of animal life through the Neoproterozoic- Cambrian interval (ca. 600 to 500 Ma). Supercontinental assembly creates large marginal oceans with old, oxygen-depleted and nutrient-enriched bottom waters. As the shelf barriers begin to submerge, the nutrient-enriched bottom waters begin to invade the shelves. This leads to the flourishing of eutrophic planktons and suspension feeders and the phosphatisation preserves small shelly fossils. As the restricted carbonate platforms are drowned, pandemic early fauna flourish. With the increasing rate of space creation, condensation, winnowing and phosphogenesis, sediment ingesters flourish and invertebrates burrow deeper to retrieve buried organic matter. The high rate of space 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 nutrients (particularly derived from a tonalite-trondhjemite-granodioriteTTGcontinental crust; Maruyama and Santosh, 2008) in isolated rift basins developed through continental fragmentation provides added impetus. In addition, biomineralization aids doesnt only enhance the size of the animal phyla, but also promotes fundamental changes such as the transformation from exoskeletons 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 separated 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 NS oriented sea way along which nutrient-enriched upwelling brought about a habitable geochemical environment. The origin of metazoans, the ancestors of vertebrates is probably related to the large flux of Ca, Fe2+, HCO3, P, Na +, K +, V, Mo and other elements which aided to build their hard parts (Maruyama and Santosh, 2008). These elements are predominantly found in rocks of granitic composition and the hydrothermal system in the rifts with granitic basement created anomalous chemical environment enriched in the above nutrients. These were the primary building blocks of the skeleton and bone of the first modern animals. The content of enriched Ca2+ aided in the amalgamation of microorganisms and their transformation to large multi-cellular animals. Following the rifting of Rodinia, the sea level had dropped since 750 Ma and led to the development of a wide shallow marine platform environment, with a photic zone that offered an environment for life diversification. 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 adequate oxygen levels in the shallow marine environment and atmosphere for life to thrive, which is when ultimately the Ediacara fauna evolved. Thereafter, the mass extinctions and biomineralization coded by genome created almost all the body plans of animals by the end of Cambrian. Thus, the model of Maruyama and Santosh (2008) traces the geological processes leading to the generation of the life soup for biological evolution back to the history of fragmentation of the Rodinia supercontinent