Wednesday, 16 December 2015

Evidence for a Middle Permian extinction event from Spitzbergen Island.

In recent years considerable evidence has been exposed in South China for a major extinction event in the Middle Permian. This event, known as the Capitanian Extinction, could potentially have been on a scale similar to the 'Big Five' events of geological history, wiping out much of the Earth's marine life and leaving an impoverished fauna ahead of the more famous End Permian Extinction, which could help to explain the severity of that event. On the other hand it could have been a strictly local event, triggered by volcanism in southwest China and affecting only tropical fauna in a portion of the Tethyan Ocean, with no wider implications.

In a paper published in the Geological Society of America Bulletin on 14 April 2015, David Bond of the Department of Geography, Environment and Earth Sciences at the University of Hull, Paul Wignall of the School of Earth and Environment at the University of Leeds, Michael. Joachimski of the GeoZentrum Nordbayen at the Universität Erlangen-Nürnberg, Yadong Sun, also of the Geozentrum Nordbayen at the Universität Erlangen-Nürnberg, and of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, Ivan Savov, also of the School of Earth and Environment at the University of Leeds, Stephen Grasby of the Geological Survey of Canada and the Department of Geoscience at the University of Calgary, Benoit Beauchamp, also of the Department of Geoscience at the University of Calgary and Dierk Blomeier of the Norwegian Polar Institute, describe evidence from the Kapp Starostin Formation of Spitsbergen Island that suggests the Capitanian Extinction also affected the Middle Permain Boreal Ocean.

The majority of the Kapp Starostin Formation comprises Brachiopod and Bryozoan dominated limestone beds, however this breaks down in the upper third of the formation, with a series of events removing much o the fauna. The initial event causes the loss of 32 of the 40 Brachiopod species present, along with 8 of 15 Bivalves. This is followed by a period of recovery, with the replacement of the Brachiopod-dominated fauna with one dominated by Molluscs, reminiscent of the faunas that came to dominate Mesozoic seas as they recovered from the End Palaeozoic Extinction. This fauna persists for much of the remainder of the Kapp Starostin Formation, but then disappears abruptly and does not reappear. In a final stage all siliceous Sponges disappear, along with a soft-bodied fauna which left numerous trace fossils.

Brachiopods from the Kapp Starostin Formation at Kapp Starostin. (A) Brachythyrina sp.; (B) Cancrinella spitsbergiana; (C, D) indet. Echinochoncids; (E, F) Haydenella sp.; (G) Lissochonetes superba; (H) Lissochonetes superba showing papillose areas either side of the centrum; (I, J) Stenocisma sp. 2; (K, L, M) Waagenoconcha sp. Scale bar is 1 cm. Bond et al. (2015).

Dating in the Kapp Starostin Formation is imprecise, however the final extinction phase, in which Sponges and soft-bodied tracemakers are lost, coincides with a major carbon isotope excursion and is thought to represent the End Permian Extinction. Bond et al. examined oxygen and strontium isotope ratios and magnitostratigaphy across the Kapp Starostin Formation in order to try to match events to those in South China. 

Oxygen and strontium isotopes ratios reflect conditions such as temperature other environmental conditions, and generally reflect local rather than global conditions, however while ratios vary from area to area overall trends are more likely to be global, i.e. a sharp increase in the proportion of oxygen-18 in South China is also likely to be seen in Spitzbergen, and by looking at ratios of two elements enough similarities should be seen to make a comparison. 

Magnitostratgraphy uses traces of ancient magnetic fields preserved in iron minerals in rocks to trace ancient pole reversals; the poles only have two possible orientations (north pole in the north/south pole in the south or south pole in the north/north pole in the south) and these occasionally flip, with the poles exchanging positions. Pole reversals happen more-or-less at random, with periods between reversals occurring at intervals ranching from tens of thousands to millions of years, and reflected across the globe. This creates a pattern of magnetic reversals in sedimentary rocks that can be matched in different rocks across the globe.

Using these methods Bond et al. were able to establish a close match between the Spitzbergen and South China deposits, strongly suggesting that the Brachiopod extinction event seen in the Kapp Starostin Formation is the Capitanian Extinction seen in South China. They further note that a similar extinction event can be seen in the Wegener Halvø Formation of Greenland and Sverdrup Basin of Arctic Canada, suggesting that the Capitanian Extinction can be traced across the Boreal Ocean.

Bivalves from the Kapp Starostin Formation at Kapp Starostin. (A) Indet. Buchid; (B) Etheripecten keyslingiformis; (C) Grammatodon (Cosmetodon)? suzuki; (D) Palaeolima sp.; (E) Palaeoneilo sp.; (F) Retroceramus sp.; (G) Streblopteria winsnesi; (H) Streblopteria winsnesi overlain by a fragment of Cassianoides; (I, J) Vorkutopecten svalbardensis; (K) Vorkutopecten sp. Scale bar is 1 cm. Bond et al. (2015).

The cause of this extinction is harder to determine. The loss of tropical fauna in South China during the Capitanian Extinction has led to the suggestion that this could be a cooling event, preferentially affecting warm water organisms unable to cope with an abrupt drop in temperature. Such an event would be expected to affect cold-water Boreal Ocean species far less severely, as such organisms would presumably be pre-adapted to deal with cooler conditions, indeed they might be expected to expand into new areas as the cooling waters create new opportunities by wiping out warm water species. However the event appears to have affected cold water species in the Boreal Ocean just as much as warm water species in the Tethys, suggesting another cause may be responsible.

Bond et al. were able to find pyrite framboids (raspberry-shaped iron pyrite crystals) in parts of the Kapp Starostin Formation thought to have been laid down in deeper water. Such framboids only form in the absence of oxygen, raising the possibility that the Capitanian Extinction may have been linked to an anoxic ocean event. Such events have been seen elsewhere in the fossil record and are thought to be linked to break-downs in ocean circulation, which inhibit the ability of oxygen to reach deeper water. However the extinction appears to have affected shallow water species as severely as deep water species. There is no evidence of framboids in shallower water, and anoxia is a much less likely explanation for shallow water extinctions, as even when ocean circulation breaks down oxygen can be replaced in shallow water by free exchange with the atmosphere, suggesting that the cause(s) of the Capitanian Extinction are likely to be more complex.

See also... new species of Proetoid Trilobites from northeastern Oman.                                                 Trilobites dominated the seas of the early Palaeozoic, but suffered a major loss of diversity in the Devonian extinction and became extinct... fate of soil microbes during the End Permian Extinction.                                                    The End Permian Extinction is the most severe extinction event recorded in the fossil record, with the loss of around 96% of all known species, and many dominant Palaeozoic groups of organisms. The event led to the effective resetting of the Earth’s biosphere, enabling the emergence of a radically different Mesozoic Biota... cause of the end-Permian extinction.    The Earth has been dominated by multi-cellular life forms (animals, plants etc.) for a little over half a billion years, but life has not had it easy throughout all of...
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