Saturday 16 April 2016

Using mercury to assess the role of Central Atlantic Magmatic Province volcanism in the End Triassic Extinction.

The End Triassic Extinction event is considered to be the fourth of the Big Five extinction events recorded in the fossil record of the Phanerozoic Eon. On land it wiped out many large Vertebrate groups, paving the way for the Dinosaur-dominated ecosystems of the Jurassic and Cretaceous, but its effects were more severely felt in the oceans, with the total collapse of almost all carbonate reef ecosystems and the extinction of the Conodonts, a jawless Vertebrate group that first appeared in the Cambrian. The extinction is associated with a sharp rise in carbon dioxide levels, thought to have caused significant ocean acidification. This has in turn been linked to the eruption of the Central Atlantic Magmatic Province, the volcanic event that began the break-up of the supercontinent of Pangea and the formation of the Atlantic Ocean, which occurred in three or four phases over a period of about 700 000 years. However all known rocks directly associated with the Central Atlantic Magmatic Province were laid down in either terrestrial or deep marine environments, making it hard to make a direct link between the timing of these volcanic deposits and the collapse of shallow marine reef ecosystems.

In a paper published in the journal Nature Comunications on 6 April 2016, Alyson Thibodeau of the Department of Earth Sciences at the University of Toronto and the Department of Earth Sciences at Dickinson College, Kathleen Ritterbush of the Department of Geology and Geophysics at the University of Utah, Joyce Yager and Joshua West of the Department of Earth Sciences at the University of Southern California, Yadira Ibarra of the Department of Earth System Science at Stanford University, David Bottjer and William Berelson, also of the Department of Earth Sciences at the University of Southern California, Bridget Bergquist, also of the Department of Earth Sciences at the University of Toronto and Frank Corsetti, again of the Department of Earth Sciences at the University of Southern California, examine mercury concentrations across the Triassic-Jurassic boundary in shallow marine deposits at Muller Canyon in Nevada as a proxy for volcanism.

Mercury reaches the Earth's surface almost exclusively through volcanism, with major eruptions leading to significant amounts of the volatile metal entering the atmosphere, and mercury in rocks being found predominantly in volcanic deposits, and fluvial and oceanic sediments derived from such rocks. Atmospheric mercury tends to bind strongly to organic material and clay particles, tending to find its way into marine sediments within at most a few million years of erupting.

The Muller Canyon rock sequence comprises a Late Triassic Bivalve-dominated carbonate reef, which is succeeded by an Early Jurassic sedimentary sequence dominated by siltstone, with rare microscopic Gastropods and Sponge spicules. Importantly the last occurrence of the Ammonite Choristoceras crickmayi, which is used to mark the end of the Triassic, occurs seven meters below the first occurrence of the Ammonite Psiloceras spelae, which is used to mark the beginning of the Jurassic, suggesting that a good sequence across the Triassic-Jurassic boundary has been preserved.

Thibodeau et al. found that mercury levels rose sharply with the onset of the extinction event, and while they drop back after this they remain significantly higher than pre-extinction levels throughout the strata with an impoverished fauna, with several smaller peaks in mercury concentration during this time. Only when mercury levels drop back to pre-extinction levels does the biota begin to recover, and a significant Jurassic ecosystem starts to develop.

Summary of key features of the Triassic-Jurassic interval, Muller Canyon, Nevada. Panels compare (a) Mercury chemostratigraphy, (b) Ammonite species diversity; (c) benthic palaeoecology and microfacies; and (d) ecosystem state for Muller Canyon, Nevada, in association with lithology and key dates. These comparisons show that significant biotic recovery follows the mercury anomalies and provide evidence that biotic recovery began after the cessation of Central Atlantic Magmatic Province magmatism. Thibodeau et al. (2016).

This strongly suggests that the End Triassic Extinction was associated with the onset of volcanism in the Central Atlantic Magmatic Province, and that the fauna did not begin to recover until after volcanism ceased. This contrasts with previous studies, which have shown the End Triassic extinction as an abrupt event, hard to reconcile with the more drawn-out Central Atlantic Magmatism. Furthermore, while the initial extinction event was accompanied by a spike in carbon dioxide levels, likely to have caused a significant ocean acidification event, the ocean fauna remained impoverished significantly after carbon dioxide levels had returned to normal, not recovering for around two million years after the initial event, or a million years after the end of volcanism, compared to a predicted recovery timescale of 10-100 000 years for an ocean acidification event.

This suggests that while ocean acidification may have played a role in the initial extinction event, it was not the only factor involved. Thibodeau et al. note that at their highest mercury levels in the Muller Canyon deposits reach 600 parts per billion relative to organic carbon. This compares to levels of around 200 parts per billion in sediments in parts of the San Fransisco Bay area considered to be heavily polluted today as a result of the use of mercury in gold mining in California in the nineteenth century. Mercury is a powerful neurotoxin, and known to be harmful to a wide range of organisms, being highly deleterious to modern ecosystems at high levels. However Thibodeau et al. do not go as far as to claim that mercury was directly responsible for the extinction event rather than an indicator of a volcanically induced deteriorating ecosystem.

See also... 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... of an Ice Age at the start of the Middle Jurassic.                                           The Jurassic lasted approximately 65.3 million years, from about 201.3 million years ago till about 145 million years ago, and is generally considered to have been... reaction of marine invertebrates to global warming during the Early Jurassic Toarcian Extinction Event.                                        About 183 million years ago a major eruptive episode in the Karoo-Ferrar Large Igneous Province of South Africa lead to an abrupt rise in global atmospheric and...

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