Friday, June 22, 2012

Not with a Bang, but a Whimper?

If we could somehow disassociate ourselves from ourselves and all we are, and stand well back away from this planet earth, and hit the rewind button on our remote, taking the planet back through time (whatever that is) to its inception, and then fast forward to the present, we'd be struck by what a dynamic planet the earth is, changing constantly in fits and starts, or more accurately, eruptions, explosions, and earthquakes, in fire and in ice.

In earlier posts I've written about the earth's five mass extinction events, and the hypothesis of many scientists today that we are in the midst of a sixth mass extinction event. In examining this hypothesis, scientists have to ask themselves, what caused the first five extinctions? And then, are we seeing similar conditions now? As you might have guessed, there are no easy answers.

It's important to understand what a "mass extinction" means, and how an "event" is defined. Because we ordinary humans have no understanding of time in a geologic sense (unlike rocks), we need to know that an event in paleo terms can occur over millions of years -- there's no agreement on how many millions of years.

A mass extinction is a loss of biodiversity -- plants and animals -- that exceeds what is considered "normal," that is, things becoming extinct as an expected result of evolution, otherwise known as "background" extinction. Thus, an extinction event is a dramatic loss of plant and animal life that occurs rapidly in geologic terms.
There have been a variety of singular events hypothesized as the cause of one or another mass extinction event. Perhaps the best known extinction event and its hypothesized cause is the so-called K-T extinction, named for the timing of the event, the boundary between the end of the Mesozoic -- the Cretaceous era -- and the beginning of the Tertiary period, 65 million years ago. This boundary event marked the end of the dinosaurs, and since most of us at one time or another have been fascinated by these gigantic, strange and fearsome looking creatures, it's no wonder this particular extinction event has generated so much popular interest.
But not just dinosaurs died off in the K-T extinction. Almost all the large vertebrates on Earth, on land, at sea, and in the air became extinct by the end of the Cretaceous Period, and most plankton and many tropical invertebrates, especially reef-dwellers, became extinct. Many land plants were severely affected. Thus, the earth in all its manifestations was a very different place after K-T. Why? What happened?

Many scientists -- paleontologists, biologists, geologists, etc. -- came to believe that a large asteroid impacted the earth at that time, causing widespread devastation and triggering catastrophic global climate change. In fact, a crater has been discovered; an egg-shaped geological structure called Chicxulub, deeply buried under the sediments of the  of Mexico (North America looked like this  65 million years ago). Some scientists insist the asteroid creating the crater impacted earth exactly 65 million years ago.

Other scientists argue the Chicxulub impact predated the K-T boundary by some 300,000 years, and propose that volcanism actually caused the mass extinction. The volcanism theory centers on the so-called Deccan Traps, flood basalts similar to the Columbia River basalts of the northwestern United States. This is one of the largest volcanic provinces in the world. It consists of more than 2,000 meters of flat-lying basalt lava flows and covers an area of nearly 500,000 square km (roughly the size of the states of Washington and Oregon combined) in west-central India. Estimates of the original area covered by the lava flows are as high as 1.5 million square km. The volume of basalt is estimated to be 512,000 cubic km (the 1980 eruption of Mount St. Helens produced just 1 km3 of volcanic material).

More recently, some scientists have come to the conclusion that it was a combination of meteor strikes and massive volcanic activity that brought about the K-T mass extinction event. What seems more likely to me at least, is that over several million years, the world, rife with erupting volcanoes, became less and less hospitable for its terrestrial and marine guests. Vast amounts of dust, carbon dioxide, and sulfur dioxide poured into the atmosphere, resulting in heavy acid rain (akin to battery acid), acidic oceans, and global temperatures that initially turned frigid (possibly lasting for years) due to dust and debris, and then intolerably hot, due to the longer-term green house effects of carbon dioxide. Violent tornadoes and hurricanes ravaged the earth, the seas fell and then rose, inundating coastal areas, normal food supplies dwindled and what was left was gobbled up by the more adaptable life forms that managed to survive the initial onslaught, such as the plankton species Guembelitria cretacea, a disaster opportunist that flourished in devastated environments when few other species survived.

Chicxulub may have looked like this. This is
actually 951 Gaspra, a much larger asteroid.
And then Chicxulub made its abrupt and violent (100 million megatons equivalent) arrival on the Yucatán peninsula creating a tipping point for the vulnerable planet and spelling an end to life as it existed 65 million years ago.

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