Two earth scientists say they have disproven evidence central to the highly controversial theory that much of the vast South polar ice cap has melted and refrozen in the past, and could again. The finding reduces concern that a drastic global sea-level rise could occur in the future, inundating populous coastal regions.
Scientists have been divided in a fierce debate on the stability of the East Antarctic ice sheet since 1984, when fossilized microscopic oceanic plants, called diatoms, were discovered in rocks deposited by glaciers in the Transantarctic Mountains. The 3- to 5-million-year-old fossils were seen as crucial evidence that much of the ice sheet had melted in the past, creating inland seas rich in diatoms that were subsequently "plowed" onto the Transantarctic Mountains by re-expanding glaciers.
In the March issue of the journal Geology, however, Lloyd Burckle of Lamont-Doherty and Noel Potter Jr. of Dickinson College report that they found similar diatom fossils in all sorts of Antarctic rocks--including some formed long before diatoms first appeared on earth and some clearly not deposited by glaciers. The scientists concluded that the prevalence of diatom fossils indicates that they most likely were blown by winds into the continent's interior. Their presence provides no evidence of advancing glaciers, they said.
The East Antarctic ice sheet is larger than the continental United States and 2 miles thick in places, and it locks up the water equivalent of more than a 180-foot rise in global sea-level. Its history has major implications for understanding the evolution of earth's climate and for projecting future ice sheet changes and sea-level rises.
The idea that the ice sheet could largely disappear if global temperatures rose by a few degrees emerged in the mid-1980's. David Harwood of the University of Nebraska in Lincoln and Peter Webb of Ohio State University said they found evidence that the ice sheet melted during the Pliocene, between 5 million and 3 million years ago, when temperatures were warmer than today. If the ice sheet melted then, it could be sensitive to future global warming, they reasoned.
Their evidence was the presence of Pliocene-Pleistocene-age diatoms in the Sirius Group, a collection of rock formations deposited by glaciers along the slopes of the Transantarctic Mountains, which separates east and west Antarctica. Harwood and Webb say the diatoms indicate that much of the East Antarctic ice sheet melted during the Pliocene when temperatures were warmer, exposing low-lying parts of East Antarctica to ocean flooding. According to the theory, diatoms thrived in these shallow, inland seas, and their fossils were incorporated into sediments that were pushed onto the mountains by glaciers that advanced again when the climate turned colder.
Opponents of this view say the ice sheet has been stable for 14 million years and cite geologic studies documenting the steady, incremental growth of a large-scale ice sheet, as well as ocean sediment cores around Antarctica indicating no changes caused by periodically melting ice sheets.
For opponents of the unstable ice sheet theory, the presence of Pliocene-Pleistocene-age diatoms in the Sirius Group remained a nagging problem that required an alternative explanation. In recent years, they have argued that the diatoms were introduced into the Sirius Group rocks after being blown by winds; or transported in precipitation or melting snow.
To determine whether such processes could transport oceanic diatoms so high and far from its sources, Burckle and Potter analyzed rocks from three separate regions of Antarctica. These rocks ranged in age from approximately 120 million to 400 million years old--times when scientists know that diatoms did not exist. Several sediment samples were even recovered from cracks in igneous rocks which form from the cooling of lava. The scientists found Pliocene-Pleistocene-aged diatoms in all samples, and concluded "that diatoms were introduced into them by atmospheric processes; the same may be said for diatoms found in Sirius sedimentary rocks."
The scientists said their finding rules out "the use of diatoms in the Sirius as evidence that the East Antarctic ice sheet had collapsed or even that it had been highly dynamic as late as the Pliocene."
The research was supported by the National Science Foundation.
Columbia University Record -- April 5, 1996 -- Vol. 21, No. 22