What caused the extinction of Mammoths and other megafauna12,000 years ago? How is the position of the earth relative to the sun related to the glacial-interglacial cycles of our current ice age? These questions are explored by two research papers, one from the University of Arizona, the other from the University of California – Santa Barbara.
First up is the story from Santa Barbara.
The paper: “Links between eccentricity forcing and the 100,000-year glacial cycle” by geologist Lorraine E. Lisiecki examines the timing of glacial cycles relative to the position of the earth and the sun, published April 4 in Nature, Geoscience online (available only with a subscription). (Note: the press release from UCSB had some rather questionable statements, so I emailed Ms. Lisiecki and she kindly sent me the entire paper.)
Some background: The earth’s orbit around the sun varies from nearly circular to moderately elliptical on a cycle of 100,000 years. The tilt of the earth’s axis relative to the plane of the solar system (obliquity) varies between 22.1 and 24.5 degrees and back again on a cycle to about 41,000 years. This tilt produces the variation in seasons. (Currently the Earth is tilted at 23.44 degrees from its orbital plane.) The third major variation, precession, is the direction of the axis of rotation relative to the “fixed” stars. This is like the wobble of a gyroscope. This cycle is 21,000 to 26,000 years. All of these cycles impact the amount of solar radiation impinging on the planet.
The important point about Ms. Lisiecki’s research is that she found hard evidence linking these cycles to global temperature. Lisiecki examined sea sediment cores, representing a time frame of 5 million years, from 57 locations throughout the world. The temperature proxy used was the variation of the oxygen-18 isotope gleaned from calcite in foraminifera fossils. This is a common proxy used in investigation of past temperature. (Foraminifera are rice-grain-sized, one-celled animals that have a calcium carbonate shell. They were first recognized from Cambrian-aged rocks, 550 million year ago, and many species still exist today.) In the first part of our current ice age, the glacial-interglacial cycle was 41,000 years. But 800,000 years ago that cycle changed to 100,000 years. Lisiecki found statistically significant correlations between the proxy temperature data and the timing of eccentricity and obliquity cycles. The question remains about why and how the longer cycle overcame the shorter cycle and it points out that there is a natural instability in climate during ice ages. Lisiecki proposes that “internally driven climate feedbacks” are the source of the change to a 100,000-year glacial. She does not expand on what those feedbacks are.
Now, on to the mammoths.
At the end of the last glacial epoch about 14,000 years ago, North America, and other places sported megafauna. In Arizona there were three species of mammoths, mastodons, camels, horses, large bears, large saber-toothed cats, and a critter called the glyptodont, essentially a volkwagen-sized armadillo. There were also people hunting all these animals, people of the Clovis culture. And they all disappeared between 12,000 and 10,000 years ago. Many theories abound as to why. It is speculated that the Clovis people hunted mammoths to extinction, or they brought disease which decimated the populations. Perhaps it was the climate change of the new interglacial period than changed things too much.
Following the end of glacial conditions, the climate warmed to about what it is now. But there was a cold snap, a period called the Younger Dryas. About 12,900 years ago temperatures plunged within a few decades to near glacial conditions again. That cold snap lasted about 1,000 years, followed by abrupt warming. The beginning of the Younger Dryas is marked, in some places, by a thin layer of soot. There are remains of megafauna and Clovis people below that layer, but none above it. It is speculated that a comet exploded over North America and caused continent-wide fires. The resulting climate change caused the demise of the megafauna.
Research from the University takes exception to the comet theory. The paper: “The Murray Springs Clovis site, Pleistocene extinction, and the question of extraterrestrial impact” was published in the Proceedings of the National Academy of Sciences, March 2, vol. 107 no. 9.
“Some of the evidence for the recent hypothesis of an extraterrestrial impact that caused late Pleistocene megafaunal extinctions was based upon samples collected at Murray Springs, a Clovis archaeological site in southeastern Arizona.” The researchers examined the black sooty mat and also some magnetic microspherules which can have their origin on earth as well as in comets. The researchers did not find any iridium or radiation anomalies that are commonly associated with comets (but iridium can originate from volcanoes as well).
The UofA researchers say the black mat is the result of an algal bloom from moist soil, not soot from a fire. They explain the magnetic spherules as originating “in exhaust from vehicles and power plants” that were washed into the Murray Springs site. They found sooty material only at the campsites and say it was the result of camp fires. Nanodiamonds are found in the sooty layer also. The UofA researchers say, “A common ingredient of cosmic dust, nanodiamonds are constantly raining down onto the earth’s surface, rendering them unsuitable as unequivocal evidence of an extraterrestrial impact.”
In the end, however, the UofA researchers cannot rule out a cosmic event, but they say it is very unlikely. You can read the press release here: http://uanews.org/node/31096
Other research shows that decline of megafauna began before the Younger Dryas. So, what killed the mammoths? There is much speculation, but we still don’t know the reason for sure.
The mammoth mystery continues to intrigue.