The Late Ordovician Mass Extinction

Abstract

▪ Abstract Near the end of the Late Ordovician, in the first of five mass extinctions in the Phanerozoic, about 85% of marine species died. The cause was a brief glacial interval that produced two pulses of extinction. The first pulse was at the beginning of the glaciation, when sea-level decline drained epicontinental seaways, produced a harsh climate in low and mid-latitudes, and initiated active, deep-oceanic currents that aerated the deep oceans and brought nutrients and possibly toxic material up from oceanic depths. Following that initial pulse of extinction, surviving faunas adapted to the new ecologic setting. The glaciation ended suddenly, and as sea level rose, the climate moderated, and oceanic circulation stagnated, another pulse of extinction occurred. The second extinction marked the end of a long interval of ecologic stasis (an Ecologic-Evolutionary Unit). Recovery from the event took several million years, but the resulting fauna had ecologic patterns similar to the fauna that had become extinct. Other extinction events that eliminated similar or even smaller percentages of species had greater long-term ecologic effects.

Did a gamma-ray burst initiate the late Ordovician mass extinction?

“…The late Ordovician mass extinction has been related to a period of rapid cooling followed by a period of rapid global warming (Orth et al 1986;Brenchley et al 1994Brenchley et al , 1995Brenchley et al , 2003. Brenchley (1984), Brenchley et al (1991Brenchley et al ( , 1994Brenchley et al ( , 1995Brenchley et al ( , 2003, Briggs et al (1988), Owen et al (1991), Fortey & Cocks (2003), Sheehan (2001) and others have convincingly demonstrated how global cooling and warming events could have led to a mass extinction, and we do not dispute the contribution of global cooling to this extinction.…”

“…Prior to the late Ordovician cooling, temperatures were relatively warm and it is the suddenness of the climate changes and the elimination of habitats due to sea-level fall that are believed to have precipitated the extinctions (Brenchley et al 1991(Brenchley et al , 1994(Brenchley et al ,1995(Brenchley et al , 2003Melchin & Mitchell 1991;Owen et al 1991;Sheehan 2001). We emphasize that there is evidence for a possible link between GRBs and global cooling.…”

“…GRBs produce opaque nitrogen dioxide in the Earth's atmosphere, providing a mechanism for global cooling. Berner (1994), Brenchley et al (1994) and Sheehan (2001) remarked that it is a puzzle that a glaciation developed in the late Ordovician given warm climatic conditions in the middle Ordovician, although Brenchley et al (1995Brenchley et al ( , 2003 did provide an intriguing explanation based on an increase in the rate of organic carbon burial. It is noteworthy that palaeoclimate models (Herrmann & Patzkowsky 2002;Herrmann et al 2003) have shown that late Ordovician glaciation could not have occurred without some external forcing mechanism; one such impulse could be reduced solar insolation precipitated by a GRB.…”

When did the Anthropocene begin? A mid-twentieth century boundary level is stratigraphically optimal

“…For example, the beginning of the Cambrian Period (and simultaneously of the Palaeozoic Era and the Phanerozoic Eon, or base of the Cambrian System) has been placed at the appearance of a distinctive trace fossil assemblage that reflects a change in behaviour associated with the earliest burrowing bilaterian animals (emerging during the 'Cambrian explosion' of metazoan animals), and so the beginning of an abundant fossil record within strata (Landing, 1994). To take another example, the boundary between the Ordovician and Silurian periods reflects a brief, intense glacial phase that triggered one of the 'Big Five' mass extinction events, and hence profoundly altered the biota (and fossil record) of the Earth (Sheehan, 2001). Likewise, each of the Cenozoic epochs is characterized by distinctive assemblages of fossil life, with the biotic changes that help define their boundaries being rapid relative to the constancy of the biota that lasts through each epoch .…”

Organic geochemical characteristics of black shales across the Ordovician–Silurian boundary in the Holy Cross Mountains, central Poland

“…1 (Berry, 2010;Leggett, 1980;Thickpenny and Leggett, 1987;Trela and Podhalanska, 2010;Wilde, 1987) including Poland (Melchin et al, 2013;Page et al, 2007). During the Ordovician-Silurian transition, there were considerable changes in the atmosphere, ocean, biosphere and, hence, in the general marine depositional conditions (Underwood et al, 1997) inextricably linked with the Late Ordovician (End Hirnantian) glacial event (Melchin et al, 2013;Sheehan, 2001). Glaciation events Mustafa, K.A., Sephton M.A., Spathopoulos, F., Watson, J.S., Krzywiec, P. Organic geochemical characteristics of black shales across the Ordovician-Silurian boundary in the Holy Cross Mountains, Central Poland, Marine andPetroleum Geology, doi:10.1016/j.marpetgeo.2015.08.018. 3 have major influence on the productivity and preservation of organic matter due to the changes in oceanic ventilation and circulation that cause oxygenation of the seawater (Berry and Wilde, 1978;Berry, 2010;Wilde, 1987).…”