Geology Reference
In-Depth Information
the rock record is spotty and incomplete, like an encyclopedia with many of its pages ran-
domly ripped out, with a few entire volumes lost. It's also often difficult to obtain exact
ages of strata and to match up the timings of formations on opposite sides of the globe.
So the disappearance of any group of animals might simply reflect a longish gap in the re-
cord. Nevertheless, as fossil databases grow and paleontologists around the world compare
notes, the largest extinction events tend to stand out against the more normal background
of life and death.
The end of the Paleozoic Era, 251 million years ago, witnessed the greatest mass extinc-
tion of all. An estimated 70 percent of land species and a whopping 96 percent of marine
species vanished—a disastrous global event called the Great Dying. Never before or since
in Earth history have so many creatures (including all the trilobites) disappeared forever.
Scientistsaren'tyetagreedonwhatcausedtheGreatDying.Itcertainlywasn'tasimple,
single cause like a giant asteroid impact; nor did it occur all at once. Indeed, multiple rein-
forcingstressfactorsmighthavecomeintoplay.Foronething,oxygenlevelshadbegunto
drop rapidly from their Carboniferous highs of 35 percent; by 251 million years ago, they
werebacktoroughly20percent.That'senoughoxygentosupportcomplexanimallife,but
the drop perhaps added stress to animals that had adapted to more profligate, demanding
high-oxygen metabolisms. The end of the Paleozoic also saw an episode of global cooling
andamodesticeage,withthickicecoveringthesouthpolarportionsofPangaea.Aconse-
quent large drop in ocean levels would have provided additional stresses by exposing most
oftheworld'scontinentalshelves.Continentalshelvesaretheocean'smostproductivebio-
sphere, so the loss of a large fraction of those shallow coastal zones would have restricted
the growth of coral reefs and other diverse shallow water ecosystems, constricting the en-
tire ocean food web.
Large-scale volcanism at the end of the Paleozoic Era, almost exactly coincident with
the mass extinction 251 million years ago, represents yet another major disruption of
Earth's biosphere—another influence of the geosphere on the biosphere. That protracted
megaeruption of as much as a million cubic miles of basalt in Siberia, one of the largest
volcanic events in Earth history, must have severely compromised Earth's environment.
For hundreds of thousands of years, pulses of volcanic ash and dust would have reduced
the Sun's input and exacerbated any ice age. The release of huge quantities of toxic sulfur
compounds would have led to acid rain and further environmental deterioration.
Asifalltheseenvironmentalinsultsweren'tenough,somescientistspointtothecollapse
of the ozone layer as yet another possible stress factor in Earth's greatest mass extinction.
Mutantfossilsporesfromend-Paleozoicrocksaroundtheworld,fromAntarcticatoGreen-
land, provide intriguing evidence, if not a smoking gun. Perhaps volcanic emissions from
Siberia triggered chemical reactions high in the atmosphere that depleted the ozone layer,
opening the window for mutagenic ultraviolet radiation.
