Scientists find signs of ‘snowball Earth’

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 of
occurring around that time.

Led
by scientists at Harvard, the team reports on its work in the latest edition of the
journal Science . The new findings — based on an analysis of
ancient tropical rocks in remote northwestern Canada — bolster the theory that
the planet has, at times in the past, been covered with ice at all latitudes.

“This
is the first time that the Sturtian glaciation [the name for that ice age] has
been shown to have occurred at tropical latitudes, providing direct evidence
that this particular glaciation was a ‘snowball Earth’ event,” said lead author
Francis A. Macdonald, an assistant professor in Harvard’s Department of Earth and
Planetary Sciences. “Our data also suggests that the Sturtian glaciation
lasted a minimum of 5 million years.”

The
survival of eukaryotic life ­­­— organisms composed of one or more cells, each with a nucleus
enclosed by a membrane — throughout this
period indicates that 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
on a snowball Earth, Macdonald said, there would be temperature gradients, and
it is likely that ice would be dynamic: flowing, thinning, and forming local
patches of open water, providing refuge for life.

“The
fossil record suggests that all of the major eukaryotic groups, with the
possible exception of animals, existed before the Sturtian glaciation,”
Macdonald said. “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 animals?”

“From
an evolutionary perspective,” he added, “it’s not always a bad thing for life
on Earth to face severe stress.”

The
rocks that 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 [light reflection] 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 said. “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 930 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 research assistant Phoebe A. Cohen; David
T. Johnston, assistant professor of earth and planetary sciences; and Daniel P.
Schrag, Sturgis
Hooper Professor of Geology and Professor of Environmental Science and
Engineering, all of Harvard. Other
co-authors are 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.

The
work was supported by the Polar Continental Shelf Project and the National Science
Foundation’s Geobiology and Environmental Geochemistry Program.