University of Buffalo geologists had to develop a new technique to calculate the size of the shelf, as other techniques only calculated when ice sheets were larger and not the opposite.
"Traditional approaches have a difficult time identifying when ice sheets were smaller," said Jason Briner, who led the study. "The outcome of our work is that we now have a tool that allows us to see how the ice sheet responded to past times that were as warm or warmer than present -- times analogous to today and the near future."
Growing ice sheets push out rocks, boulders and other debris into heaps of rubble called moraines. Because this happens only when shelves grow, fossils and rocks found in these moraines must have been picked up when the shelf was smaller and older.
Researchers looked at 250 clams they found in moraines and they were found to be 3,000 to 5,000 years old.
"Because we see the most shells dating to the 5-3000-year period, we think that this is when the most land was ice-free, when large layers of mud and fossils were allowed to accumulate before the glacier came and bulldozed them up," Briner said.
Radiocarbon dating is a very expensive procedure, so the geologists looked at the structure of amino acids found in the clams. While this is not a new process, it is the first time it is being used to age fossils used to extrapolate the age of a glacial shelf.
Their findings are published in the journal Geology.