Glaciers reached Cape Cod, Massachusetts, in the most recent ice age about 20,000 years ago. But much harsher ice ages hit the Earth in an ancient geological interval known as “the Cryogenian Period” between 750 and 600 million years ago.
A team of geologists from China and the United States, including two from Washington University in St. Louis, now report evidence of at least three ice ages during that ancient time.
“The Cryogenian Period is characterized by some of the most severe glaciations in earth history. But the available age constraints are so few that geoscientists don’t even know how many glaciations occurred in the Cryogenian. Now, we believe we have evidence that there were at least three Cryogenian glaciations, and there may have been more,” says Shuhai Xiao, Ph.D., of Virginia Tech’s Department of Geosciences.
Chuanming Zhou, Ph.D., a Virginia Tech geosciences post-doctoral associate, along with Xiao, Robert Tucker, Ph.D., associate professor of earth and planetary sciences in Arts & Sciences at Washington University in St. Louis, and Zhanxiong Peng, mass spectrometer specialist in the Washington University earth and planetary sciences department , and Xunlai Yuan and Zhe Chen of Nanjing Institute of Geology and Paleontology, are co-authors of “New constraints on the ages of Neoproterozoic glaciations in South China,” published in the May 2004 issue of Geology, the journal of the Geological Society of America.
The research was supported by grants from the Chinese Academy of Sciences, China Ministry of Science and Technology, and the U.S. National Science Foundation.
Geologists have studied glacial deposits from the Cryogenian Period for many years. In recent years, it has been hypothesized that the earth was covered with ice and the oceans frozen to a depth of one or two kilometers during Cryogenian glaciations–a scenario known as snowball Earth.
“The implications are profound,” says Xiao. “There would have been a shut down of the hydrologic cycle – essentially no exchange between the atmosphere and ocean. The deep ocean would have quickly become anoxic because light would not be able to penetrate the ice to fuel algae. Above the ice, there would have been little rain or snow because there would have been little evaporation. Many organisms that lived in milder conditions would have suffered mass extinction.”
Did this happen? If so, how often? And what were the relationships between climate changes and biological evolution?
Zhou and Xiaoâ’s field investigation shows that there are at least three levels of glacial deposits in Cryogenian successions of southern China. Chinese geologists named these glacial deposits the Changan, Tiesiao, and Nantuo formations in geochronological order. In 2002, Zhou, then at Nanjing Institute of Geology and Paleontology, made a field trip to Guizhou Province in southern China and discovered a thin layer of volcanic ashes deposited between the Tiesiao and Nantuo glacial rocks.
“This was a very important discovery, because we were able to date the volcanic ashes, thereby constraining the age of the glacial deposits,” Xiao says.
Xiao and Zhou then isolated the mineral zircon (ZrSiO4) from the ash layer and sent it to Tucker and Peng at the Geochronology Laboratory at Washington University in St. Louis.
“Zircon is a common mineral that is ideally suited for U-Pb geochronology” said Tucker. “When it forms during igneous crystallization its lattice incorporates a trace amount of U and essentially no Pb. Over the course of time, two uranium isotopes (235U and 238U) decay at different rates to two different isotopes of Pb (207Pb and 206Pb). Hence, the present-day isotopic abundance of both U and Pb is a measure of the age of the mineral, in this case the age of the volcanic eruption.”
Isotopic measurements indicate that the eruptive ash layer, located between the two glacial horizons, is precisely 663 million years old.
“This is a key date. The Tiesiao glaciation must have ended before 663 million years ago, and the Nantuo glaciation began after 663 million years ago” said Xiao.
In South Australia and many other places, there is evidence for only two Cryogenian glaciations. The older one, known as Sturtian, occurred about 720 million years ago, and the younger, known as Marinoan, occurred about 630 million years ago. The new radiometric date from South China suggests that the Nantuo glaciation in South China must be equivalent to the Marinoan glaciation in South Australia.
This is also confirmed by similar stable carbon isotopes, mineral deposits and other unusual sedimentary structures.
But the exact age of the Changan and Tiesiao glacial deposits is still up in the air.
“There are two possibilities,” Xiao says. “Either they represent two pulses within a long Sturtian glaciation, or they record two discrete ice ages in South China, only one of which is recorded in South Australia by the Sturtian glacial deposit.”