A glacial moraine boulder has a 10Be concentration indicating an exposure age of 18,000 years. What geological event does this date?
AThe age of the bedrock from which the boulder was derived
BThe time when the glacier retreated and exposed the boulder to cosmic rays at the surface, recording the timing of deglaciation
CThe time when the boulder was first deposited by a river
DThe age of the cosmic rays that produced the 10Be
While buried under ice, the boulder was shielded from cosmic rays and accumulated negligible 10Be. When the glacier retreated and deposited the moraine, the boulder was exposed to cosmic radiation and 10Be began accumulating. The 18,000-year age dates the exposure event -- glacier retreat. This assumes the boulder had negligible pre-exposure (inheritance) and has not been eroded, buried by sediment, or toppled since deposition.
Question 2 True / False
Cosmogenic nuclide production rates are uniform across Earth's surface regardless of latitude or altitude.
TTrue
FFalse
Answer: False
Production rates vary significantly with latitude and altitude. Earth's magnetic field deflects low-energy cosmic rays, reducing production at low latitudes compared to high latitudes (where field lines are nearly vertical and provide less shielding). Production increases exponentially with altitude because there is less atmosphere to attenuate cosmic rays -- rates at 3000 m elevation are roughly 3-4 times higher than at sea level. Accurate age/erosion rate calculations require site-specific production rates scaled for latitude, altitude, and topographic shielding.
Question 3 Short Answer
Explain how the 26Al/10Be ratio in a quartz sample can reveal a history of burial.
Think about your answer, then reveal below.
Model answer: Both 26Al and 10Be are produced in quartz by cosmic rays with a known production ratio of ~6.75. During surface exposure, both accumulate and their ratio remains near this production ratio. If the sample is then buried (by sediment, ice, or landslide), cosmic ray production ceases and both nuclides decay radioactively. Because 26Al has a shorter half-life (0.71 Myr) than 10Be (1.39 Myr), the 26Al/10Be ratio decreases during burial. A ratio significantly below the production ratio indicates a period of shielding from cosmic rays between periods of surface exposure, and the magnitude of the depression constrains the burial duration.
The differential decay clock works because both nuclides start at a known ratio, then diverge during burial when production stops but decay continues at different rates.