Cosmogenic isotope surface exposure dating
Surface exposure dating involves collecting such rocks and measuring the abundance of an isotope concentrated within their upper surfaces, which acts as a chemical signal for the length of time since the rock was last covered by ice.
The ANi SEED project will collect this information from a range of heights above today’s ice surface in the western Amundsen Sea Embayment of Antarctica.
These reactions are more frequent at high altitudes, where surface cosmic ray fluxes are higher.
This technique involves measuring the abundance of isotopes that are produced within rock surfaces when they are exposed to cosmic radiation.Consequently, the practical limit for the use of cosmogenic surface dating seems to be about 10 million years; after that, one old rock looks much like another.The lower limit for application of the method seems to be about ten years, because of practical limits on the accuracy with which we can measure the quantities of the relevant isotopes.Ice cover blocks the penetration of cosmic radiation, halting production of such isotopes; their abundance in a rock therefore gives a measure of the time since ice retreated: the ‘exposure age’.This diagram, showing thinning of an ice sheet from the Last Glacial Maximum (LGM) to present day, helps to visualise how this works: When an ice sheet thins, rocks transported within it (erratics – shown here as red circles) are left perched on mountainsides – known as ‘nunataks’ when surrounded by ice – at a range of heights above the modern ice sheet surface.