Arctic Fossil Record Traces Greenland Ice Sheet's Past
November 23, 2013

Greenland Ice Sheet Was Smaller 3,000-5,000 Years Ago

April Flowers for - Your Universe Online

There have been many studies telling us how small the Greenland ice sheet is today. A new study, published in the journal Geology, reveals that the ice sheet was actually smaller between 3,000 and 5,000 years ago. Surprisingly, the sheet was as small during this period as it has ever been in recent history.

The international research team used a new technique they developed for interpreting the Arctic fossil record.

“What’s really interesting about this is that on land, the atmosphere was warmest between 9,000 and 5,000 years ago, maybe as late as 4,000 years ago. The oceans, on the other hand, were warmest between 5-3,000 years ago,” said Jason Briner, PhD, University at Buffalo associate professor of geology.

“What it tells us is that the ice sheets might really respond to ocean temperatures,” he said. “It’s a clue to what might happen in the future as the Earth continues to warm.”

The findings illuminate the history of Greenland's ice sheet, as well as provide geologists with an important new tool: a method of using Arctic fossils to deduce when glaciers were smaller than they are today.

Many techniques have been developed for figuring out when ice sheets were larger, but very few for the opposite scenario.

“Traditional approaches have a difficult time identifying when ice sheets were smaller," Briner said. "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."

The new technique involves dating fossils in piles of debris found at the edges of glaciers. Think of growing ice sheets like bulldozers. They push rocks, boulders and other detritus into heaps of rubble called moraines.

Glaciers only do this plowing when they are growing, so it stands to reason that rocks or fossils found in a moraine must have been scooped up at a time when the glacier was older and smaller.

This means that if a moraine contains fossils from 3,000 years ago, the associated glacier was growing, and smaller than today, 3,000 years ago. The fossil record in Greenland confirmed this theory.

The team examined 250 ancient clams from moraines in three western regions. They found that most of the fossils were between 3,000 and 5,000 years old, suggesting that this was the period when the ice sheet's western extent was at its smallest in recent history.

“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 prohibitively expensive, so Briner and his team found another way to trace the age of their fossils.

The researchers looked at the structure of amino acids in the fossils of ancient clams. Amino acids, which are the building blocks of proteins, 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.

Amino acids begin to flip, however, when organisms die. For example, in dead clams D forms of aspartic acid start turning to L forms.

This shift takes place slowly over time. This means the ratio of D forms to L forms in a fossil is an indicator of its age.

The researchers matched D and L ratios in 20 Arctic clamshells to their radiocarbon-dated ages. This allowed them to generate a scale showing which ratios corresponded with which ages.

They then examined the D and L ratios of aspartic acid in the 250 Greenland clamshells to come up with the fossils' ages.

Although using amino acid ratios for dating is not a new concept, applying it to the study of glaciers could help scientists better understand the history of ice — and climate change — on Earth.