Location: Chemistry/Physics Building, MUN
Time: 8:00 pm
"Was the Cambrian (Ediacaran) Faunal Diversification Triggered by a 90-degree Shift of Earth's Rotation Axis?"
Speaker: Dr. Joseph Hodych, Dept. of Earth Sciences.
This is a theory brought about by an article in Science dated from 8 years ago by Dr. J.L. Kirschvink (CIT). Dr. Kirschvink also worked on the Martian meteorite ALH84001, which may have contained bacteria, and a resurrected form of the "Snowball Earth" theory. The axial shift is supposed to be evidenced by paleomagnetic data.
Dr. Hodych gave us an overview of the time frame in which life on Earth was supposed to have developed. From 3.5 to 1.8 billion years ago, the most complex organisms were bacteria. Finally, oxygen appears on earth, leaving its tell-tale trace of various oxides, etc. It is now thought that multi-cellular organisms first appeared about 600 million years ago in what is now termed the Ediacaran. The more familiar Cambrian era began about 545 million years ago. During the Cambrian Faunal Explosion, we first saw all the major body plans that now exist in organisms on the earth.
Ediacaran fauna were all soft-bodied creatures, and their fossils are rare compared to organisms from the Cambrian. Ediacaran organisms did not seem to be successful, and are not present from the Cambrian period or later. The hypothesis is that a 90-degree tilt of the earth could have caused the Ediacaran Diversification. It is important to realise this proposed tilt is an actual physical tilt, and not caused by a reversal of the earth's magnetic field, or some kind of shift in its direction.
Normally, during a shift of the earth's magnetic field (not a reversal), the core's axis will remain the same. The mantle, crust, or continents will shift. During such a shift climactic change will occur in a particular area, including the release of methane hydrates from formerly northern areas. This would cause a "methane burp", which could lead to warming, and in addition possibly trigger faunal diversification.
Dr. Kirschvink proposes a new, much quicker mechanism: inertial interchange through polar wandering. If the earth had a shape not unlike a football, spinning around the short symmetrical axis (picture two bulges at opposite sides of the equator on the current earth), then the activity of a mantle plume near a polar region could destabilise the orientation of the surface (the polar spin axis remains the same). There would be a 90-degree rotation about the axis defined by the "bulges", while the bulges and the rest of the earth continue the normal spin about the polar axis. [You really need a picture!]
There are models in the literature that indicate this is a theoretical possibility, given enough mass (ie. a sizable mantle plume), and a suitable football shape for the earth. We can pass judgement on this by using paleomagnetism, which is usually best found through magnetite in the rock. This "locks in" the magnetic field direction at the time the rock was formed.
At Skinner Cove in Western Newfoundland, there seems to be paleomagnetic evidence for no shift from the South Pole to the equator over the time period from the Ediacaran to the Cambrian. The paleomagnetism appears to be horizontal in a well-dated and non-disturbed lava flow (nearby conglomerate suggests this).
At the time the Skinner Cove results were first published, there still was a small chance the rocks could have been transported from somewhere else, even upwards of 1000 km. However, later results studied Zirconium Silicate (good dating due to Zirconium-Lead-Uranium relationship) in the lava flow. Indications of what rock the lava flowed up through give the same profile as the Long Range Mountains. So again, we seem to have put the nail in the coffin of a Cambrian flip at around 550 million years ago.
The Avalon Peninsula turns out to be one of the best areas in the world to study Ediacaran Fauna (about 575 million years ago). These were mostly stationary, bottom-attached animals (there were some worms). Because of their completely soft bodies, there are few places that have any fossil record of these creatures. Here Dr. Hodych described how the fossils were created by "injection molding" of pre-deposited mud into an overlying layer of volcanic ash.
Dr. Hodych considers he has three sets of data from the Ediacaran that are reliable. None of these indicate a 90-degree shift. The Callander complex is an example of an additional dataset, that would indicate 2 shifts, if any occurred at all, within the bounds of uncertainty. However, the geology does not rule out possible remagnetisation of the magnetite as well as with the cases of the three good sets of data. All in all, he believes there is no evidence for any inertial interchange type of pole shift.
It was noted that Star-B-Que will be at Butterpot Park this year on August 12-14. We are looking for volunteers to help out. If any member is interested they can contact an executive member, or the observing chair. An email will be sent around. In addition, members are welcome to attend the planning meeting in July.
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