Oxygen-18 is a heavier-than-normal isotope

Answer choices

1. A

   Opposite11% picked this

   Rain clouds above tropical forests are poorer in oxygen-18 than rain clouds

   We're looking for an answer that puts oxy-18 back in to the cloud at a higher rate than normal oxygen, to make up for the fact that oxy-18 drains out of the cloud more quickly than normal oxygen. If the Amazon is a place where it rains a lot (oxy-18 dumps out) and a place that's poor in oxy-18 to begin with, how can we explain how the oxy-18 level of the clouds is holding steady?

   11%
2. B

   Correct52% picked this

   Like the oceans, tropical rain forests can create or replenish rain clouds in the

   Why this is right

   Since the oxy-18 is dumping out faster than normal oxygen, any time it rains, we have to figure out how the cloud's tank is getting refilled. This answer helps give us a mechanism by which to explain that "even though it's raining an oxy-18 is leaking out, an equivalent amount is being replenished by the evaporation rising from the tropical rain forest. It's a frustratingly weak correct answer. It doesn't tell us that oxy-18 replenishes at a faster rate than normal oxygen, so it definitely doesn't settle the mystery of how the oxy-18 levels of clouds has remained constant. But of the available answers, it does the most to help give us a way to explain why.

   Skill tested: Paradox · how this choice captures the argument's function is the move to repeat next time.

   52%
3. C

   No Impact22% picked this

   The amount of rainfall over the Amazon rain forests is exactly the same as the amount of rain originally collected in the

   This answer in a sense re-affirms the passage's causal mystery. The paragraph is trying to set up a paradox where clouds form above the Atlantic Ocean, sucking up x gallons of moisture. They travel to the Amazon and dump out rain every day. This answer adds "they dump the full tank of rain that the Atlantic gave them. Every drop sucked up from the Atlantic has now fallen to the Amazon". We still get to the same paradox: given that the tank started out 20% oxy-18, and given that it's been raining a bunch, (so the oxy-18 has had lots of opportunity to leak out faster than normal oxygen), how is this band of clouds still at 20% oxy-18? We need a "putting fuel back in the tank" story to explain how the cloud's are holding steady at their level of oxy-18. This answer is purely a dumping fuel out of the tank story.

   22%
4. D

   Unrelated to Goal13% picked this

   The amount of rain recycled back into the atmosphere from the leaves of forest vegetation is exactly the same as the amount of rain

   This answer isn't talking about oxygen at all. How could this give us a story for putting oxygen-18 back into the cloud's tank?

   13%
5. E

   No Impact2% picked this

   Oxygen-18 is not a good indicator of the effect of tropical rain forests on the

   This is irrelevant. We don't care whether Oxy-18 is a good indicator of how rain forests affect the atmosphere. We just care about figuring out how clouds could dump so much Oxy-18 on the Amazon and still have a consistent level of Oxy-18.

   2%