Asteroid and comet impacts seem to be obvious culprits in mass extinctions on Earth. The heavily cratered Moon reminds us how severe earlier bombardments have been, and it’s an easy segue to note that 23 extinction events are now thought to have occurred since the beginnings of life on our planet. In the past 540 million years (the period during which abundant animal life has existed), we can identify five mass extinctions, with huge losses in particular to marine plants and animals.
The Chicxulub crater in the Yucatan is a striking piece of evidence for this scenario, but massive volcanic activity may well have played a role, and perhaps a major one. And what of the other extinctions? A new theory published in Nature seems to put a damper on the easy correlation of extinctions with impacts. Indeed, Shanan Peters (University of Wisconsin-Madison) argues that the largest factor may have been changes in ocean environments related to sea level. Says Rich Lane (National Science Foundation): “Impacts, for the most part, aren’t associated with most extinctions. There have also been studies of volcanism, and some eruptions correspond to extinction, but many do not.”
What Peters argues is that the expansion and contraction of the world’s oceans, caused by the shifting of tectonic plates and changes in climate, caused massive marine extinctions as sea levels declined. A case in point is the shallow sea that covered much of North America during the era of the dinosaurs. As it drained, animals like mosasaurs and giant sharks went extinct while life on the marine shelves changed irrevocably. Peters calls sea level changes a ‘forcing mechanism,’ one that correlates with many — not all — mass extinction events. “These results,” says the scientist, “argue for a substantial fraction of change in extinction rates being controlled by just one environmental parameter.”
All of which adds a cautionary note as we discuss the still vital need to catalog and perhaps one day intercept Earth-crossing objects. The danger they pose is real and one that should be a driver for new space technologies in support of a defensive mission we should hope we never have to fly. But in presenting that case, we should also be aware that extinction events have a varied and still unfolding set of causes, one in which impacts from the skies may play a smaller role than we had previously suspected. Good science demands that we get the facts right as we work to place impact events in a sound historical context.
The paper is Peters, “Environmental determinants of extinction selectivity in the fossil record,” published online in Nature (15 June 2008). Abstract available.
There are a lot of catastrophic things that can and will kill us, and it’s frightening to think that asteroids and comets may actually be the most preventable.
Two of my favorites (if you can call them that!) are caldera volcanoes and underwater landslides. The most obvious example of caldera volcanoes is Yellowstone which, should it erupt any time soon, would likely mean the end of America as the world’s superpower. When over half your nation is several feet deep in ash, it’s hard to keep the economy going. The good news is that it’s not due to go off for a while — at least a few thousand years, and perhaps by then we’ll be able to find a way to let Yellowstone vent (pun intended) without blowing its top.
As for underwater landslides (seaslides?) well, look no further than the “Great Crack” on the south side of the big island of Hawaii, which many scientists believe is the result of the south side of the island beginning to split away from the rest. Eventually, it will go and cause a tsunami of epic proportions all around the Pacific Rim. There is much evidence of these tsunami happening in the past with fossilized remains of sea creatures thrown hundreds of feet above the shoreline as far away as New Zealand. Suddenly Pacific shoreline real estate doesn’t look quite as attractivw, does it. Again, it’s not likely to happen any time soon, but it will happen eventually as it has happened every 10,000 years or so in the past. And those of you on the East Coast should not be too complacent. The volcanoes of the Canary Islands shed their flanks every now and then too!
We will, of course, survive as a species, should one or both of these events occur, so its still wise to focus in on the threat from above since only from the skies are we likely to find a planet-killing danger in the foreseeable future.
One catastrophe that did affect the path of Europe was the explosion of the Thera (Santorini) volcano, which destroyed the Minoan civilization. The event and its aftermath were so momentous that they survive as the myth of Atlantis.
People like ascribing momentous happenings, such as extinction events, to single causes. It appeals to our sense of economy and our love of patterns (and it may help our career!). As both Paul and tacitus point out, phenomena are often connected and result in multiple aftereffects.
The extinctions resulting from changes in sea levels are pertinent to changes brought about by global warming. And both gradual and abrupt catastrophes will fit the bill in terms of destabilizing societies on a global scale.
The complexity that Athena points out makes our study of mass extinctions wide-ranging indeed, especially in terms of the sea level changes that may be the most immediate issue for us. I’m in full agreement with tacitus that the asteroid/comet threat is preventable through sufficiently advanced technology, so the push to catalog Earth-crossing objects must continue even as we work out the interleaved details of past catastrophes and their causes. Planetary security and species survival are powerful driving forces for new technology.
Yep, Santorini is another caldera volcano and certainly went off with a bang.
In essence, it’s a race against time. If we can avoid a catastrophic natural disaster in the next few hundred years we may get to a point where we can either defend ourselves from too much harm (perhaps by predicting it well enough in advance to get most people out of the way) or even find a way to prevent the catastrophe from happening altogether. In fact, given enough warning we may already there for asteroid strikes. If we detected a monster today that will be coming our way in, say, 20 years time, we have a pretty good shot of diverting it before it get strikes. With caldera volcanoes and seaslides, etc. it may be a while yet before we can avert disaster.
Hi Folks;
At the following URL is an interesting article about the finding of water molecules in pebbles brought back from the Apollo Lunar landings. The article states that it is now believed that the Earth’s moon definitely has water, perhaps even large quantities below its surface.
http://abcnews.go.com/Technology/story?id=5342399&page=1
The ABC NEWS website is very good about providing fine articles regarding near term manned and robotic space exploration. Their SCI-TECH menu option usually has new space articles posted daily. I use this resource as my main source for current or near future manned and robotic space missions. It seem that interest in space is starting to pick up again. In some other countries, interest in manned space exploration is a real hot item in the publics mindset. I read an article recently where is was suggested by some researcher, policy maker, or manager within the U.S. that China is likely to land humans on the Moon before we go back in 2020.
Thanks;
Jim
T. rex and the Crater of Doom
Walter Alvarez
To read the entire book description or a sample chapter, please visit:
http://press.princeton.edu/titles/8650.html
Sixty-five million years ago, a comet or asteroid larger than Mt. Everest slammed into the Earth, causing an explosion equivalent to the detonation of a hundred million hydrogen bombs. Vaporized impactor and debris from the impact site were blasted out through the atmosphere, falling back to Earth all around the globe. Terrible environmental disasters ensued, including a giant tsunami, continent-scale wildfires, darkness, and cold, followed by sweltering greenhouse heat. When conditions returned to normal, half the genera of plants and animals on Earth had perished.
This horrific story is now widely accepted as the solution to a great scientific murder mystery what caused the extinction of the dinosaurs? In T. rex and the Crater of Doom, the story of the scientific detective work that went into solving the mystery is told by geologist Walter Alvarez, one of the four Berkeley scientists who discovered the first evidence for the giant impact.
The linkage of massive sea-level changes and mass extinction is well documented. Anthony Hallam, an expert in this area, has written extensively about this phenomenon.
There is a new theory:
The Gravity Theory of Mass Extinction
This theory explains the linkage between the rapid sea level changes (known as regressive-transgressive couplets) and periods of extinction in a very logical fashion. The basis of the theory is that the Earth’s surface gravity is lowered when continental tectonic plates coalesce, as they did when Pangea was formed, resulting in a shift of the Earth’s iron core away from the supercontinent.
The rapid changes in sea-level occurred when the continental tectonic plates separated and moved apart causing perturbations in surface gravity, and by extension, perturbations in sea level. I believe this new theory will eventually be accepted as the explanation for Phanerozoic mass extinctions
Luck Gave Dinosaurs an Edge
http://www.astrobio.net/news/article2872.html
A new study shows that early dinosaurs survived two mass
extinctions before they became dominant on Earth. Interestingly,
it appears that their survival may have been based on luck alone.
Cold Storage for Alien Organisms
http://www.astrobio.net/news/article2919.html
Some lunar craters may be perfect for preserving samples of life
from Earth, and possibly even from Mars or other planets. Ancient
organic remnants could have been delivered to the Moon as
debris that was thrown into space after asteroids impacted rocky
worlds in our solar system.