Antarctica is covered by a vast ice sheet (Picture 1). The colours of the ice sheet indicate thickness, with blues showing thinner ice and reds, thicker ice. The image is from http://www.esa.int/spaceinimages/Images/2011/06/Antarctic_ice_sheet. Beneath the ice sheet, hidden from view (Picture 2), lies a landscape similar to that of the other continents on Earth, with mountains, hills, valleys and plains. The ice sheet colours in The thickness and extent of the ice sheet changes with changes in climate. Cooling generally leads to an overall increase in the size of the ice sheet, while warming tends to reduce the size of the ice sheet. As the ice sheet thins, the landscape under the ice progressively becomes exposed. The first to show are the tops of the highest mountains (Picture 3). When mountains extend above the ice they are known as nunataks. These nunataks act like dipsticks of the ice sheet surface because the rocks they are made of record the magnitude and time of changes in the ice sheet surface.
The Earth is continually being bombarded by high energy cosmic rays. The ice shields the rocks from these cosmic rays. When the ice sheet thins the rocks at the top of nunataks become exposed to the cosmic rays (Red A; Picture 3). Rocks are made of minerals, and some of the cosmic rays interact with atoms in the mineral quartz, changing them to what we call in-situ produced cosmogenic nuclides. Effectively a clock starts ticking once rocks are exposed to cosmic rays. As the ice sheet lowers (Picture 4 and 5) more and more of the nunataks become exposed (Blue B, Green C). The concentration of cosmogenic nuclides in the rocks reflects the duration of exposure to cosmic rays as is shown in the graph. In the example shown the top of the nunatak at Red A is exposed 30,000 years ago, Blue B is exposed later as the ice sheet thinned to below that site, about 20,000 years ago, and Green C is exposed later again, about 14000 years ago. Because Red A was exposed first, it has built up the greatest amount of cosmogenic nuclides (about 120,000 atoms/gram), while Blue B and Green C have progressively less. We sample glacially modified or deposited (erratics/till) rocks at time zero (Picture 5). We can measure the concentration of cosmogenic nuclides in minerals (subject of a future blog) to determine how long the rocks have been exposed. From that we can reconstruct the timing and rate of ice surface lowering. In other words, we can use the nunataks as dipsticks of ice surface elevation.
You may wonder what happens to cosmogenic nuclide concentrations when the rocks we sample have been eroding, or when the ice sheet thickens and buries the sample sites again. I will explain this in a future blog.