Lecture 6 - Constraining timing in the landscape: cosmogenic radionuclides

Question 1

How are cosmogenic radionuclides produced?

Answer 1

Cosmic rays isotropically bombard the solar system. When they impact with the surface of the Earth they produce in situ cosmogenic nuclides.

Question 2

What form do can cosmogenic radionuclides take?

Answer 2

• Stable isotopes: continue to accumulate with exposure (²¹Ne, ³He)
• Radioisotopes: they decay as they accumulate (²⁶Al, ¹⁰Be, ³⁶Cl)

Question 3

What is the cosmogenic isotope production in a rock the function of?

Answer 3

• latitude
• elevation
• cosmic ray flux (fluctuations in the Earth;s magnetic field)

Question 4

Production rates in cosmoradiogenic isotopes are very low. What is the production rate and the associated mineral that they would be found in in the five aforementioned isotopes?

Answer 4

¹⁰Be: 5.8 atoms g^-1 yr^-1 (from ¹⁸O) - Quartz

²⁶Al: 35 atoms g^-1 yr^-1 (from ³²Si) - Quartz

³⁶Cl 4-9 atoms g^-1 yr^-1 - Carbonates

²¹Ne 243 atoms g^-1 yr^-1 - Quartz

³He 115 atoms g^-1 yr^-1 - Olivine and Pyroxene

Question 5

What can in situ cosmogenic radionuclides be used for?

Answer 5

Determining the exposure age of a surface and determining the erosion rate.

Cannot be used for both as an exposure age can only be determined if erosion is zero.

Question 6

How is the rate of channel incision measured from a strath terrace?

Answer 6

Height of abandoned bed above water/age of strath (obtained from isotope)