Ordovician-Silurian Mass Extinction Event
440-450 Ma at the Ordovician-Silurian transition two events occurred, and together are
ranked by many scientists as the second largest of the five major extinctions in Earth's
history in terms of percentage of genera that went extinct. 27% of all families and 57% of
all genera became extinct.
Possible causes
These extinctions are currently being intensively studied; the most commonly accepted
theory is that they were triggered by the onset of a long ice age, perhaps the most severe
glacial age of the Phanerozoic, in the Hirnantian faunal stage that ended the long, stable
greenhouse conditions typical of the Ordovician. The event was preceded by a fall in
atmospheric CO2, which selectively affected the shallow seas where most organisms
lived. As the southern supercontinent Gondwana drifted over the South Pole, ice caps
formed on it. The strata have been detected in late Ordovician rock strata of North Africa
and then-adjacent northeastern South America, which were south-polar locations at the
time. Glaciation locks up water from the world-ocean, and the interglacials free it,
causing sea levels repeatedly to drop and rise; the vast shallow intra-continental
Ordovician seas withdrew, which eliminated many ecological niches, then returned,
carrying diminished founder populations lacking many whole families of organisms.
Then they withdrew again with the next pulse of glaciation, eliminating biological
diversity at each change (Emiliani 1992 p. 491). In the North African strata, Julien
Moreau reported five pulses of glaciation from seismic sections ([1][dead link] IGCP
meeting September 2004 reports pp 26f).
This incurred a shift in the location of bottom-water formation, shifting from low
latitudes, characteristic of greenhouse conditions, to high latitudes, characteristic of
icehouse conditions, which was accompanied by increased deep-ocean currents and
oxygenation of the bottom-water. An opportunistic fauna briefly thrived there, before
anoxic conditions returned. The breakdown in the oceanic circulation patterns brought up
nutrients from the abyssal waters. Surviving species were those that coped with the
changed conditions and filled the ecological niches left by the extinctions.
Gamma ray burst hypothesis
Scientists from the University of Kansas and NASA have suggested that the initial
extinctions could have been caused by a gamma ray burst originating from an exploding
star within 6,000 light years of Earth (within a nearby arm of the Milky Way Galaxy). A
ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost
immediately, causing surface-dwelling organisms, including those responsible for
planetary photosynthesis, to be exposed to high levels of ultraviolet radiation. This would
have killed many species and caused a drop in temperatures.[7][8] While plausible, there
is no unambiguous evidence that such a nearby gamma ray burst has ever actually
occurred.
Adrian L. Melott et al estimated that gamma ray bursts from "dangerously close"
supernova explosions occur two or more times per thousand million years, and this has
been proposed as the cause of the end-Ordovician extinction.[9]
1. ^ History Channel's Mega Disasters program, "Gamma Ray Burst", 2007, rebroadcast:
2008-11-13. The program attributes the "Ordovician extinction" (sic) explicitly as the
second most grievously large extinction event after the Permian extinction.
2. ^ a b Sole, R. V., and Newman, M., 2002. "Extinctions and Biodiversity in the Fossil
Record - Volume Two, The earth system: biological and ecological dimensions of global
environment change" pp. 297-391, Encyclopedia of Global Environmental Change John
Wilely & Sons.
3. ^ a b extinction
4. ^ Causes of the Ordovician Extinction
5. ^ Rohde & Muller (2005). "Cycles in Fossil Diversity". Nature 434 (7030): 208–210.
doi:10.1038/nature03339.
6. ^ Bambach, R.K.; Knoll, A.H.; Wang, S.C. (December 2004), "Origination,
extinction, and mass depletions of marine diversity", Paleobiology 30 (4): 522–542,
doi:10.1666/0094-8373(2004)030<0522:OEAMDO>2.0.CO;2,
http://www.bioone.org/perlserv/?request=get-document&issn=00948373&volume=30&page=522
7. ^ Wanjek, Christopher (April 6, 2005). "Explosions in Space May Have Initiated
Ancient Extinction on Earth". NASA.
http://www.nasa.gov/vision/universe/starsgalaxies/gammaray_extinction.html. Retrieved
on 2008-04-30.
8. ^ "Ray burst is extinction suspect". BBC. April 6, 2005.
http://news.bbc.co.uk/1/hi/sci/tech/4433963.stm. Retrieved on 2008-04-30.
9. ^ Melott, A. et al (2004). "Did a gamma-ray burst initiate the late Ordovician mass
extinction?". International Journal of Astrobiology 3 (2): 55–61.
doi:10.1017/S1473550404001910. http://arxiv.org/abs/astro-ph/0309415. Retrieved on
2007-02-01.
* Emiliani, Cesare. (1992). Planet Earth : Cosmology, Geology, & the Evolution of
Life & the Environment. Cambridge University Press. (Paperback Edition ISBN 0-52140949-7)