Given the incidence of gamma-ray bursts in the cosmos, astrophysicists estimate that at least once in the last billion years, a burst has occurred within about 6,500 light years of Earth. Now, astrophysicists at the University of Kansas and NASA Goddard Space Flight Center report that if directed at Earth, such a blast could have prompted the ice age and resulting mass extinction during the late Ordovician, 443 million years ago.
The second largest extinction event in Earth’s history, the Ordovician event killed off 60 percent of all marine invertebrates and is believed to have been caused by the sudden onset of an ice age. However, paleo-climatologists remain uncertain about what triggered the ice age during a warm period with high carbon dioxide levels.
Gamma-ray bursts break down nitrogen, which reacts with oxygen to form nitric oxide and destroys ozone. The reaction also produces nitrogen dioxide, a major component of smog, which again reacts with oxygen to form nitric oxide. This cycle perpetuates ozone destruction and produces more smog, blocking sunlight and triggering an ice age, the researchers say.
The team calculated that a 50 percent decrease in ozone would allow up to three times as much solar ultraviolet B (UVB) radiation to reach the surface. Additionally, Thomas says, “the initial flash would deliver the equivalent of five times the UVB flux of a normal, sunny summer day.”
By extrapolating from data on modern Antarctic phytoplankton exposed to increased UVB by ozone loss over the South Pole, the group concluded that a burst of this size and proximity could likely cause widespread extinctions by affecting the base of the marine food chain.
So, being good scientists, how do we test the idea?
"...searching for evidence of a gamma-ray burst in the geologic record would be quite difficult. Unlike asteroids or supernovae, the bursts are not expected to leave behind any direct evidence. So, if a chemical signature of a cosmic event were to be found at the end of the Ordovician, it would rule out the gamma-ray burst idea — and could point to another cause for the extinction."
Interesting stuff!
