Geologists have found evidence that sea ice extended to the equator
716.5 million years ago, bringing new precision to a "snowball Earth"
event long suspected to have taken place around that time.
Led by scientists at Harvard University, the team reports on its work
in the journal Science. The new findings — based on an
analysis of ancient tropical rocks that are now found in remote
northwestern Canada — bolster the theory that our planet has, at times
in the past, been ice-covered at all latitudes.
"This is the first time that the Sturtian glaciation has been shown
to have occurred at tropical latitudes, providing direct evidence that
this particular glaciation was a ‘snowball Earth’ event," says lead
author Francis A. Macdonald, an assistant professor in the Department of
Earth and Planetary Sciences at Harvard. "Our data also suggests that
the Sturtian glaciation lasted a minimum of 5 million years."
The survival of eukaryotic life throughout this period indicates
sunlight and surface water remained available somewhere on the surface
of Earth. The earliest animals arose at roughly the same time, following
a major proliferation of eukaryotes.
Even in a snowball Earth, Macdonald says, there would be temperature
gradients on Earth and it is likely that ice would be dynamic: flowing,
thinning, and forming local patches of open water, providing refuge for
"The fossil record suggests that all of the major eukaryotic groups,
with the possible exception of animals, existed before the Sturtian
glaciation," Macdonald says. "The questions that arise from this are: If
a snowball Earth existed, how did these eukaryotes survive? Moreover,
did the Sturtian snowball Earth stimulate evolution and the origin of
"From an evolutionary perspective," he adds, "it’s not always a bad
thing for life on Earth to face severe stress."
The rocks Macdonald and his colleagues analyzed in Canada’s Yukon
Territory showed glacial deposits and other signs of glaciation, such as
striated clasts, ice rafted debris, and deformation of soft sediments.
The scientists were able to determine, based on the magnetism and
composition of these rocks, that 716.5 million years ago they were
located at sea level in the tropics, at about 10 degrees latitude.
"Because of the high albedo of ice, climate modeling has long
predicted that if sea ice were ever to develop within 30 degrees
latitude of the equator, the whole ocean would rapidly freeze over,"
Macdonald says. "So our result implies quite strongly that ice would
have been found at all latitudes during the Sturtian glaciation."
Scientists don’t know exactly what caused this glaciation or what
ended it, but Macdonald says its age of 716.5 million years closely
matches the age of a large igneous province stretching more than 1,500
kilometers (932 miles) from Alaska to Ellesmere Island in far
northeastern Canada. This coincidence could mean the glaciation was
either precipitated or terminated by volcanic activity.
Macdonald’s co-authors on the Science paper are Phoebe A.
Cohen, David T. Johnston, and Daniel P. Schrag at Harvard; Mark D.
Schmitz and James L. Crowley of Boise State University; Charles F. Roots
of the Geological Survey of Canada; David S. Jones of Washington
University in St. Louis; Adam C. Maloof of Princeton University; and
Justin V. Strauss.
This work was supported by the Polar Continental Shelf Project and
the National Science Foundation’s Geobiology and Environmental
Source : Harvard University