July 17, 2008

Underwater Volcanoes Blamed for Mass Extinctions

More than 93 million years ago, researchers say undersea volcanic activity caused mass extinction in the worlds' oceans.

During the late Cretaceous Period, the supposed "anoxic event" starved oxygen from the ocean depths and wiped out millions of marine organisms.

Researchers from the University of Alberta, Canada, published their findings in the journal Nature. They found a telltale signature of underwater volcanism in rocks dating to the period.

Researchers say during the undersea volcanic activity, the average temperature was nearly twice that of today.

Snowy, frosty Alaska grew palm trees while large reptiles roamed northern Canada. Scientists believe the Arctic Ocean was ice-free and believe it had a lukewarm temperature.

The study found the ocean water was also devastated by a mass extinction which wiped out a type of large clam common at the time, as well as tiny ocean creatures that live on the sea floor, known as foraminifera.

Alberta researchers say, due to a sudden shift in ocean circulation, the remains of these minuscule organisms littered the seabed in thick layers, and over geological time became transformed into oil.

In the time immediately following the mass extinction, levels of greenhouse gases in the atmosphere dropped and Earth lurched into a sudden, but short-lived, period of cooling.

For years, geologists have pondered the cause of this extraordinary event.

Steve Turgeon and Robert Creaser from Alberta's department of Earth and atmospheric sciences said the cause of the devastation lies in volcanic eruptions that took place on the ocean floor, and altered the chemistry of the sea.

The evidence can be found in levels of two forms, or isotopes, of osmium found in black shale rocks.

They analyzed sedimentary rocks from cores drilled from the seabed off the coast of South America, and from mountains in Italy.

Researchers say the bed of the present-day Caribbean was formed by the huge lava flows, however researchers say the flows would have preceded the extinction by up to 23,000 years.

Tim Bralower, a geologist at Pennsylvania State University, reviewed the paper and said two theories - not mutually exclusive - have emerged to explain the chemistry of what happened next.

He says one theory is that the volcanoes erupted out metal-rich fluids that seeded the upper level of the ocean with micronutrients. Tiny life forms on the sea surface, called phytoplankton, then gorged on the food, and stored up carbon as they grew. They then sank to the sea floor and decayed, stripping the ocean of oxygen.

The other hypothesis is that the volcanoes released clouds of CO2 to the atmosphere, which then significantly warmed the climate to the extent that Earth's ocean circulation system ground to a near-halt. That meant below the surface layers, water was no longer turned over and a lack of oxygen was the result.

Dr. Bralower said scientists wrestling with some of the unknowns of climate change today, could be guided by the post-volcanism scenario.

Unanswered questions include the impact of higher temperatures on marine circulation, and whether highly debated plans to fill the ocean floor with iron filings to spur phytoplankton growth and thus soak up CO2 from the atmosphere, would ease global warming or cause oxygen starvation in the ocean depths.


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