Surface Exposure Dating

Why use cosmogenic nuclide exposure dating?


Radiocarbon dating is abundantly used and offers very high precision dates, but we often want to date an event that is either too far in the past, or without the right type of organic matter, to be dated by 14C. While there is a slew of other dating techniques to choose from, cosmogenic nuclide exposure dating is useful for relatively young (~100 to 10 million years old) samples. If we are particularly interested in the timing of the uncovering of a surface—say, bedrock that had been covered by ice, or sediments that had been revealed by the incision of a stream—we can employ cosmogenic nuclide surface exposure dating to study that uncovering process. This is different from techniques (like 39Ar/40Ar, or U/Th) that date the formation of a rock itself.


How does 10Be exposure dating work?


Super high energy particles—mostly protons— are produced by our Sun, supernovae, and probably other extraterrestrial sources. These particles continuously enter the Earth system at incredible rates and are often, but misleadingly, called cosmic rays. When a high-energy proton collides with an atom in the Earth’s atmosphere, it can break apart that atom to produce (still high-energy) secondary radiation in the form of neutrons, protons, and other subatomic particles. Those particles continue traveling toward the Earth’s surface, likely colliding with additional atoms on the way.

Upon entering the Earth’s surface, these high-energy particles quickly collide with the relatively dense assortment of atoms around, breaking apart more atoms in the upper ~2 meters of a rock (or other) surface but failing to reach much deeper than that.  Depending on the elements that a particle collides with, it produces different end products. Collisions with Si and O, greatly abundant in the Earth’s crust and readily concentrated in quartz, produce the isotope 10Be. Because 10Be is only produced by interaction with cosmogenic radiation, measuring the concentration of 10Be in a sample can allow you to determine how long it has been exposed to cosmic radiation.

10Be nuclide production in rock surfaces not covered by ice