To elaborate: Growing ice sheets are like bulldozers, pushing rocks, boulders and other detritus into heaps of rubble called moraines.
Because glaciers only do this plowing when they're getting bigger, logic dictates that rocks or fossils found in a moraine must have been scooped up at a time when the associated glacier was older and smaller.
So if a moraine contains fossils from 3,000 years ago, that means the glacier was growing and smaller than it is today 3,000 years ago.
This is exactly what the scientists saw in Greenland: They looked at 250 ancient clams from moraines in three western regions, and discovered that most of the fossils were between 3-5,000 years old.
The finding suggests that this was the period when the ice sheet's western extent was at its smallest in recent history, Briner said.
"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," he said.
Because radiocarbon dating is expensive, Briner and his colleagues found another way to trace the age of their fossils.
Their solution was to look at the structure of amino acids the building blocks of proteins in the fossils of ancient clams. Amino acids come in two orientations that are mirror images of each other, known as D and L, and living organisms generally keep their amino acids in an L configuration.
When organisms die, however, the amino acids begin to flip. In dead clams, for example, D forms of aspartic acid start turning to L's.
Because this shift takes place slowly over time, the ratio of D's to L's in a fossil is a giveaway of its age.
Knowing this, Briner's research team matched D and L ratios in 20 Arctic
|Contact: Charlotte Hsu|
University at Buffalo