New evidence supports three major glaciation events in the distant past

04/19/04



Shown is a glacial cobble from the Nantuo Formation, Guizhou Province, South China. Grooved scratch marks on this cobble were formed when it was frozen in a moving glacier and ground against the underlying pavement of the glacial valley.

Click here for a high-resolution photograph.


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 now report evidence of at least three ice ages during that ancient time.

"The Cryogenian Period is characterized by some of the worst 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 of Virginia Tech's Department of Geosciences.

Chuanming Zhou, a Virginia Tech geosciences post-doctoral associate, along with Xiao, Robert Tucker and Zhanxiong Peng of the Department of Earth and Planetary Sciences at Washington University of St. Louis, 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," the cover story in the May 2004 issue Geology, the journal of the Geological Society of America.

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 be no communication between the atmosphere and the ocean. The deep ocean would quickly become free of oxygen because light would not be able to penetrate the ice to fuel algae. Above the ice, there would be little rain or snow because there would be little evaporation. Many organisms that lived in milder conditions would become extinct."

Did this happen? If so, how often? And what were the relationships between climate changes and biological evolution?

Zhou and Xiao'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 and get some idea when the underlying and overlying glacial deposits might have formed," Xiao says.

They then isolated zircon minerals from the ash sample. To geologists, this mineral in this place was more valuable than it could ever be when used for jewelry. "Zircon is very stable," Xiao explains. "Once it crystallizes, it is a time capsule from the moment of its formation."

The zircon minerals were analyzed by Robert Tucker at Washington University in St. Louis. Radioisotope measurements suggest that the zircons were crystallized, erupted with volcanic ashes, and deposited about 663 million years ago. "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" says 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 (including sedimentary barite, which is featured on the cover of the May 2004 issue of Geology), 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."

But there is an additional twist in this business of counting and matching glacial formations from different continents. "In Newfoundland and Massachusetts, there is evidence for a much younger glaciation, known as the Gaskiers glaciation. Thus the combination of evidence requires at least three Cryogenian ice ages, the Sturtian, Marinoan, and Gaskiers," says Xiao.

It is important to sort out the sequence of events in order to figure out the causal relationship between climate change and biological evolution. Several years ago, Xiao and his graduate advisor Andrew H. Knoll at Harvard University discovered some of the earliest known animal fossils from rocks of the Doushantuo Formation in Guizhou Province. The Doushantuo Formation lies just above, and is thus younger than, the Nantuo glacial deposit. The Doushantuo Formation is dated at 599 million years old, older than the Gaskiers glaciation. The implication, Xiao explains, is that "animals evolved before the Gaskiers glaciation and they must have survived it if animals did not evolve twice."

"So now we can put the glaciation events in broad temporal relationship with evolutionary events, and our contribution can help to clarify the global picture of geological and biological evolution in the ancient past," says Xiao, "but a lot of uncertainty remains."

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