Scientists picture the landforms that time forgot

A Cambridge-led team of scientists have for the first time used images of sediments beneath the sea floor to identify ancient glacial landforms – offering a new approach to tracking the development of prehistoric ice sheets.

Seismic data and detailed images providing a three-dimensional view of the ocean floor, gathered from around the mid-Norwegian continental shelf, allowed the researchers to identify features carved out by drifting icebergs and fast-flowing ice streams up to 2.5 million years ago.

It is the first time scientists have used this modern seismic technology to produce complete 3-dimensional images of glacial landforms buried beneath the modern sea floor. The approach could offer a diagnostic solution to identifying the presence and flow patterns of ancient ice sheets.

Understanding when and why ice ages took place contributes to our understanding of global climate change by showing how the frozen portion of the earth (the cryosphere) evolved, and by how much and at what rate it grew or shrank over timescales of hundreds of thousands to millions of years.

Traditionally, ancient glacial activity is identified by looking at the debris that was transported in flowing ice, and was then left behind as it melted. Much of this debris accumulates and is preserved on continental shelves beyond ice-covered coasts. But this sedimentary evidence is far from perfect - much of the debris can resemble material deposited by other processes left behind for entirely different reasons.

In a report published in the April issue of Geology, a collaboration of scientists from Cambridge, Norway and Italy describes how they used 3-dimensional seismic imaging to view complete glacial landforms in the Naust Formation, part of the mid-Norwegian shelf. The buried landforms they found are very similar to those that exist at the margins of modern ice sheets in both the Arctic and Antarctic, but were formed during the late Cenozoic period, up to 2.5 million years ago.

In particular, the group found three features that offer clear evidence of early ice sheets:

• Elongated and streamlined ridges of sediment, produced by fast-flowing ice streams. These are known as mega-scale glacial lineations and are formed when soft sediment is deposited by ice streams that drain vast former ice sheets. They provide evidence of where the ice streams existed and therefore represent the direction of past ice flow.

• Ploughmarks carved out by the keels of icebergs as the bergs ground along the seafloor. This allows scientists to track changes in the direction of iceberg drift and, hence, past ocean currents.

• Ridges and moraines formed transverse to ice flow as the ice retreated. These are often found beyond the margins of retreating glaciers in both terrestrial and marine environments and appear in clusters rather than as isolated individual features.

Professor Julian Dowdeswell, Director of the Scott Polar Research Institute at the University of Cambridge, who led the research, said: “Submarine landforms like these are found at the marine margins of almost all modern ice masses, so it seems reasonable to assume they were also features of ancient ice sheets.

“What no-one has done until now, however, is search for signs of these glacial landforms from millions of years ago. New developments in geophysics are giving us the ability to produce detailed images from hundreds of metres beneath the modern seafloor, and seismic data that enable us to produce 3-dimensional pictures of what the surface of the earth was like when it was being shaped by glaciers.

“That opens up new, exciting possibilities in identifying past ice ages which we intend to take further.”

The team are now using the same techniques to analyse buried glacial landforms from Northern Africa that could offer a glimpse of glacial activity from much earlier in earth's history. The landforms being investigated have been preserved from the Late Ordovician period, and could therefore provide information about the presence and movement of ice sheets 450 million years ago, when Africa was in a polar position and was largely covered by an ice sheet about the size of modern Antarctica.