Logical Reasoning

PT18 · S2 · Q19 Oxygen-18 is a heavier-than-normal isotope

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Oxygen-18 is a heavier-than-normal isotope of oxygen.

Scientists' Expectation

Oxy-18 is the weirder version of oxygen. In a rain cloud, there's more normal oxygen than oxy-18, but during a rainstorm, the oxy-18 dumps out at faster rate than the normal oxygen does.

If before the rainstorm, the cloud was like 80% normal oxygen and 20% oxy-18, then after the rainstorm it would be more like 87% normal, 13% oxy-18.

Since the oxy-18 drains out of the raincloud at a faster rate, we'd expect its proportional share of a cloud to be ever-shrinking.

Observed Measurements

This cloud that went from the Atlantic Ocean to the Amazon, where it rains every day, has had basically the same proportion of oxy-18 the whole time.

Goal

Given that we think the oxy-18 is draining faster than the normal oxygen, how is it holding steady to its share of the cloud's composition?

19.

Which one of the following statements, if true, best helps to resolve the conflict between scientists’ expectations, based on the known behavior of oxygen-18, and the result of their measurements of the rain clouds’ oxygen-18 content?

  1. Rain clouds above tropical forests

    Opposite

    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% picked this

  2. Correct

    Like the oceans, tropical rain

    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% picked this

  3. The amount of rainfall over

    No Impact

    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% picked this

  4. The amount of rain recycled

    Unrelated to Goal

    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?

    12% picked this

  5. Oxygen-18 is not a good

    No Impact

    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% picked this

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