Tuesday, June 03, 2008

The Carolina Bays and the Younger Dryas impact event - II

One real problem with this whole subject is the complete lack of a huge crater that was traditionally expected to be associated with energetic ET impact events. The 1908 Tunguska impact event helped turn our thinking around on this issue. In Siberia an estimated 80 million trees were blown down over an area of 830 square miles. The blast is estimated to have been the equivalent of a 10-15 megaton nuclear explosion. The bolide is thought to have been something like 100 feet in diameter, and may have been a piece of periodic comet Encke.

An article by Bob Kobres, Comet Phaethon’s Ride, has the following information about the Tunguska event.

In 1927 Leonard Kulik, a Russian scientist, was the first to locate the area devastated by the Tunguska comet. ... Kulik was astonished to find no crater even though over two thousand square kilometers of dense Siberian forest were scorched and flattened.

Astronomer C.P. Olivier, writing of Kulik's discovery for Scientific American, stated in the July 1928 issue, "In looking over this account, one has to admit that many accounts of events in old chronicles that have been laughed at as fabrications are far less miraculous than this one, of which we seem to have undoubted confirmation."

At the time of the proposed Little Dryas impact event, the earth was coming out of the last ice age, and the Laurentide ice sheet was still more than a mile thick over Canada and the northern United States. A comet that disintegrated several miles above ground, like the Tunguska comet, might have blasted away huge portions of the ice - because the explosion was several orders of magnitude greater - but left no crater.

Dr. Richard Firestone, et al, in Micrometeorite Impacts in Beringian Mammoth Tusks and a Bison Skull, a paper presented at the December 2007 AGU conference, claims to have discovered "what appear to be micrometeorites imbedded in seven Alaskan Mammoth tusks and a Siberian bison skull." These do not date to the Little Dryas impact event, however, but are a good example of something to look for in the future.

Is there any evidence other than the "black mat" for the Younger Dryas impact event?

Along the Atlantic seaboard within coastal Delaware, Maryland, New Jersey, North Carolina, South Carolina, Virginia, Georgia, and northcentral Florida, and on the Gulf of Mexico coastal plain within Texas and southwest Louisiana, are between half a million and two million elliptical depressions, ranging from one to several thousand acres in size. They are commonly known as the Carolina Bays, but have other names in other parts of the southeastern United States. Generally the southeastern end has a higher rim composed of white sand. They are named for the Bay trees that characteristically grow in many of them, not because of the frequent ponding of water. George Howard's website has a more comprehensive list of their typical features.

In 1933, the photo above kicked off the Carolina Bay debate. Edna Muldrow wrote The Comet that Struck the Carolinas in Harpers Monthly that year.

"The comet plunged down with a hiss that shook the mountains, with a crackle that opened the sky. Beneath the down plunging piston of star, compressed air gathered. Its might equaled and then exceeded that of the great star itself. It burst the comet nucleus. It pushed outward a scorching wind that must have shoved the waters upon the European shores, and on land leveled three hundred foot pines, spreading them radially outward like matches in a box. The comet struck, sending debris skyward, curtaining the east, darkening the west. Writhing clouds of steam swirled with writhing clouds of earth. For ten minutes there was a continuous bombardment, and the earth heaved and shook. For 500 miles around the focal spot of 190,000 square miles, the furnace snuffed out every form of life."

To see where these features are actually located, see George Howard's website for a wonderful Google Earth KMZ file. You'll notice that many of the "bays" he has marked don't seem to have any visible presence. The reason for this might be that since the Americas were "discovered" by the Europeans, many of the "bays" have been drained, filled in, plowed under, and paved over. I'd give Mr. Howard the benefit of the doubt.

The Eastern coast of the United States is not the only place with "Carolina Bays". Zanner C. William, in Nebraska’s Carolina Bays, a paper presented at the November 2001 GSA annual meeting, calls attention to similar landforms found in Nebraska, where they are called "Rainwater Basins". I spent about an hour using Google and Google Earth to try to find a landform in Nebraska that resembled, even slightly, a Carolina Bay. I was not successful. I'll leave this in here anyway.

The Carolina Bays of the Atlantic Coastal Plain are one of the more enigmatic geomorphological features in North America. These elliptical depressions occur in a variety of sizes, are oriented NW-SE, and have rims most visible on the southeast edge. They have often inspired flights of fancy as scientists and the public have sought ways to explain them. Part of the fascination stems from the perception that they are unique to the Coastal Plain. South of the Platte River in east central Nebraska, USGS DOQs reveal a multitude of oval shaped features that share SW-NE orientation. They occur in a variety of sizes and have rims on the southeast edge. Locally they are called Rainwater Basins. Bays occur in Coastal Plain sandy sediments; however, the Basins of Nebraska occur in a loess-mantled landscape. Orientation of Bays and Basins is opposite, but both are perpendicular to regional prevailing wind directions. Prior work of Kuzila suggested that the Nebraska Basins exist where a loess dated at ~27000 radiocarbon years before present provides a pre-existing topography. Soil survey maps of the area show that some rims of the Basins are sandy. Cores from the area indicate that a sandy landscape was buried by loess. Upper parts of these sandy deposits are well sorted; fluvial sands and gravels occur below these sorted sands. Using coring and OSL dating we are currently documenting the age of this sandy surface. Our hypothesis is that the Basins on the current land surface originally formed as blowouts or low spots in abandoned Platte River fluvial sands and gravels. The ~27000 radiocarbon years and later loess actually draped a pre-existing topography formed in these sands. We also offer that these features would be recognized as an analog of the Carolina Bays if not for their loess cover. This suggests that Carolina Bays are not unique features, and any explanation for their existence should also help explain Nebraska's Rainwater Basins.

Elliptical depressions that do look like Carolina Bays, sometimes called "oriented lakes", are found in other parts of the world, such as Perth Australia, South Africa, and Argentina.

The long axes of many of the Carolina Bays roughly converge on an area near the Great Lakes. There are several different groupings, however, with different orientations, suggesting the bays might not all be the result of a single impact event. Conversely, the variability in the orientation of the Carolina Bays could be explained as follows. If the comet struck the atmosphere at a steep angle, only those bays directly downrange would point consistently back towards the epicenter. Ones not directly in the line of fire would have a variety of orientations because the material (large blocks of ice?) blown out into space would come back down unevenly. If the comet broke up due to tidal forces and friction with the atmosphere, there could have been significant scatter in the trajectories and arrival times of the incoming pieces.

To be continued...


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