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Five Global Mass Extinctions in the Geological History and Their Traces

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Five Global Mass Extinctions in the Geological History and Their Traces Geological Society of Hong Kong Bulletin No. 13, Issue No.1 GEOLOGICAL SOCIETY OF HONG KONG Bulletin No.13, Issue No.1 Five Global Mass Extinctions in The Geological History and Their Traces in the Hong Kong Region January, 2018 the specific geological and palaeontological evidence observed in the ‘Greater Hong Kong The readers who are interested in discussing the content of the article please email to the Geological Society of Region’ to the overall perspective of the global Hong Kong at mass extinctions, in order to better understand the [email protected] geological impacts and imprints associated with the Editors: Ir. Raymond S. M. Chan and Dr. George S. K. Ma global mass extinctions in this part of the world. Five Global Mass Extinctions in The Geological History and Their Traces in The Big Bang theory has been used to explain the the Hong Kong Region origin of the Universe (e.g. Peebles et al, 2009). It By Foo Wah Yan suggests that the Universe began to form about 13.8 billion years ago (Ga) (ESA, 2013) and led to the (M.Sc. Petroleum Geology, D.I.C., Imperial College) birth of the Solar System, in which the Earth formed General Manager, Energy Business, Polytec Resources Ltd., as an integral part, about 4.6 Ga. The first live forms Hong Kong on Earth started shortly after its formation as basic single cells bacteria about 4.3 to 3.8 billion years ago, based on the oldest fossils (single cells bacteria) Introduction found recently in Quebec, Canada, as reported in Dodd et al (2017). They are the World's oldest This paper integrates geological and environmental fossils discovered to date which provide the ‘direct sciences in probing five of the Earth’s mass evidence’ of life on Earth when our planet was still extinctions and some of their traces left in the Hong at its infancy. According to a summary by BBC Kong region. The implications from these global (2014; Fig. 1), the single-celled marine live forms mass events on the future survival of humanity are multiplied rapidly, initiated photosynthesis and also discussed in the concluding remarks. evolved rapidly into multi-cellular microorganisms, then evolved into land plants and animals, flourished and prospered. For the past 450 million These global mass extinctions events have been years, these organisms, long before the birth of linked to the earth’s extreme environmental humanity, had gone through many cycles of mass changes. The causes and effects of these extinctions and recoveries as dictated by the catastrophic events provide valuable ancient changes in the Earth’s natural environments, analogues to the modern research in climate change cosmic and solar systems’ natural processes. These and global warming, and they have significant organisms evolved according to the prevailing implications to the future survival of humanity. In environment to better adapt to, survive, reproduce addition, this paper also attempts to relate some of and propagate in the natural environment. Through 1 Geological Society of Hong Kong Bulletin No. 13, Issue No.1 natural selection, the stronger, well-adapted species acidity and salinity, worldwide sea levels, global survived the calamities and thrive while the weaker, temperatures, atmospheric concentrations, such as ill-adapted ones perished naturally. These are the carbon dioxide (CO2), hydrogen sulphide (H2S) and basic concepts of Darwin’s theory of evolution with methane (CH4), and precipitation of acid rain. They a famous motto of ‘survival for the fittest’. contributed to our present understanding of consequences of adverse environmental changes, including the trend of global warming due to In this paper, the ‘Greater Hong Kong Region’ massive greenhouse gases release related to broadly refers to the Hong Kong Special industrialization and excessive burning of fossil Administrative Region and its adjacent Southeast fuels. China areas, unless otherwise stated. The Earth is a dynamic planet constantly changing Five Global Mass Extinctions in the Geological and evolving due to internal and external factors. History These changes could take place from millions of years to instantaneous, triggered by any one or a Geological and palaeontological evidence suggests combination of such processes as plate tectonics that mass extinction has occurred at least five times and cosmic processes (e.g. asteroid and minor over the past 450 million years (Ma) (Fig. 2), each planet impacts). of which has eliminated more than half of living organisms on Earth. These catastrophic events coincide with the various ‘reef gaps’, extended The following paragraphs introduce in periods (>4 million years) of missing coral reefs chronological order the five global mass extinctions records (Veron, 2008). Coral reefs have been used (Fig. 2), discuss their causes, occurrences and as an indicator of environmental changes due to impacts, and examine their geological implications their high environmental sensitivity; their missing and imprints to the Greater Hong Kong Region (Fig. records in the geological history could be linked to 3). global environmental disasters or changes associated with events that caused global mass extinction. 1. 1 The End Ordovician Mass Extinction The first event happened at ca 440 Ma at the end of Mass extinctions occurred as a result of drastic or the Ordovician Period. The tectonic drift of the sudden environmental changes that upset the supercontinent ‘Gondwana’ to the South Pole dynamic balance of the Earth’s ecosystems. Those caused as much as 85% of sea-life forms to species that could not adapt to the new exterminate globally (Veron, 2008). Subsequently environments and ecosystems became in the Silurian, extensive areas of sea formed, exterminated in massive quantity as shown in fossil massive reef began to build up in shallow seas, first records today. These catastrophes of global scale fish evolved and plants began to colonize land. In were associated with changes in oceanic chemistry, the Greater Hong Kong areas, no rock older than 2 Geological Society of Hong Kong Bulletin No. 13, Issue No.1 400 Ma was found but from regional geological Chau Formation comprises mainly metamorphosed evidence this region was probably covered under sedimentary rocks deposited on a tidal flat or as a deeper sea conditions (Sewell et al, 2009). deltaic fan. Later in the Permian, the region became a deep continental sea in rising sea levels due to global warming as the early ‘Pangaea’ continental 2 The Late Devonian Mass Extinction fragments began moving northward through different climatic zones and merging together. The second event happened at ca 360 Ma in the late Devonian Period. Around 75% of life forms were exterminated and all (100%) living reefs were 3 3 The End Permian Mass Extinction wiped out globally (Veron, 2008). The oldest rocks found in Hong Kong, the conglomerates of the The third event happened at ca 250 Ma at the end Devonian Bluff Head Formation, were deposited at of the Permian Period. All continents moving ca 400 Ma when a fluvial deltaic environment northward finally merged into one giant prevailed in this region. These Devonian rocks crop supercontinent ‘Pangaea’, ended with the Earth’s out at two main locations in Hong Kong: on the biggest Mass Extinction also known as ‘The Great northern shore of Tolo Channel and at Ma On Shan Dying’ (Veron, 2008). The calamity caused a (Sewell et al, 2009). As described in Sewell et al staggering 96% of all life forms on earth to (2009), fragments of fossil fish placoderm, the exterminate. Numerous theories have been oldest life form, dated at 370 Ma, ever found in proposed as the cause(s) for this catastrophic event: Hong Kong, were discovered in 1980 in the asteroid impacts, flood basalt eruptions, sedimentary rock layers in Harbour Island, Plover catastrophic methane release, oxygen level drop, Cover by a fossil expert, Mr Lee Cho-min of the sea level fluctuations or some combination of these Hong Kong Polytechnic (now the Polytechnic catastrophes. University of Hong Kong). Placodermi is an extinct class of armoured prehistoric fish, whose earliest In Hong Kong, sedimentary rocks of Permian age known fossils were found in China. It evolved and are found in Tolo Harbour, mainly in Ma Shi Chau. later became extinct during the late Devonian mass Several marine fossils have been identified in the extinction at ca 360 Ma. The areas that cover the Permian siltstones and mudstones that formed in a north and south shores of the Tolo Channel, near-shore tidal flat environment. The marine life including the oldest rocks in the Bluff Head forms in the Greater Hong Kong Region suffered Formation, are now part of The Hong Kong almost total extermination during this global UNESCO Global Geopark of international catastrophe. The Great Dying of marine species in significance. the Hong Kong Region could be inferred from Veron (2008) on global scale and Sewell et al (2009) Following the Late Devonian mass extinction, at regional levels. These species extinction included Hong Kong area was submerged beneath a warm the fusilinid identified at Tung Chung limestone, shallow sea during the Carboniferous Period in brachiopods, most of the bivalves bryozoa, crinoid which calcareous muds were deposited and and 100% of the rugose corals preserved as marble in the Yuen Long Formation (Sewell et al, 2009). The Carboniferous Lok Ma 3 Geological Society of Hong Kong Bulletin No. 13, Issue No.1 Prior to the end Permian Mass Extinction, there was The fifth event happened at ca 65 Ma at a major tectonic event in China accompanied by the Cretaceous/Tertiary Boundary, also known as the intrusion of granitic magmas. Marble and skarn ‘K/T’ boundary. This event, known to have rocks, now strongly deformed (Sewell et al, 2009), associated with the extinction of dinosaurs, was were formed in contact metamorphic rocks adjacent attributed to a huge asteroid impact that struck the to the igneous intrusions.
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