He's referring to beryllium-10, which is mostly generated by cosmic ray interactions with nitrogen and oxygen in the atmosphere. The Be-10 subsequently washes o

He's referring to beryllium-10, which is mostly generated by cosmic ray interactions with nitrogen and oxygen in the atmosphere. The Be-10 subsequently washes out of the atmosphere with precipitation and tends to bind in surface soils. It has a half life of 1.39 million years so it is useful for dating various geological phenomena. https://en.wikipedia.org/wiki/Beryllium-10 https://en.wikipedia.org/wiki/Beryllium-10 You can see how this beryllium isotope was used in the original paper "Climate controls on erosion in tectonically active landscapes" (linked by sradman in comment https://news.ycombinator.com/item?id=24812181 https://news.ycombinator.com/item?id=24812181) within the Materials and Methods section of the paper.