Logical Reasoning

PT1 · S3 · Q11 Water vapor evaporated from the

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Water vapor evaporated from the ocean contains a greater proportion of oxygen-16 and a smaller proportion of the heavier oxygen-18 than does seawater.

Facts

Mathy Comparisons

proportion of Oxy 16 Evaporated ocean water > Ocean water

proportion of Oxy 18 Evaporated ocean water < Ocean water

Pivot (however) + Causal (because)

normally: sea water returns to ocean through precipitation >> ocean's composition is unchanged.

ice age: precipitation gets trapped on ice caps >> ? ? ?

Evaluate

It looks like the last two sentences are setting us up for an Implied Difference.

What keeps the ocean's composition unchanged is precipitation returning to the ocean. So if an ice age prevents that precipitation from returning to the ocean, then presumably the ocean's composition will change.

How will it change? The math-y comparisons at the start help us know how.

Evaporation is basically "stealing extra 16's" from the ocean. The water vapor that travels up into the sky, from the surface of the ocean, has a different 16/18 mix than the ocean water it came from. It's got more 16 and less 18 than the ocean (the heavier 18's don't wanna float up into the sky, I guess).

So when water evaporates, it's always taking . Over time, that would mean less oxy-16 in the ocean, and so the proportion of oxy-16 in the ocean would go down, and the proportion of oxy-18 would go up.

Goal

The question stem specifically asks about what is true during a typical ice age.

While we might be able to figure out how the composition of the ocean changes during an ice age, if we can make sense of the first sentence, the most important thing is looking for an answer that rewards us for figuring out that the ocean composition does change during an ice age.

That seems to be the implied difference they are setting up with the Pivot / Causal Diff-Maker in the final two sentences.

11.

Which one of the following conclusions about a typical ice age is most strongly supported by the statements above?

  1. The proportions of oxygen-16 and

    Contradicted

    The stimulus explicitly says vapor has more oxygen-16 and less oxygen-18 than seawater. The proportions in vapor and seawater are different, always. Normally and during an ice age.

    6% picked this

  2. Correct

    The concentration of oxygen-18 in

    Why this is right

    This and (E) are the only two answers saying that composition of the ocean changes, so even if we only had a fuzzy understanding of the math, we should be considering this one. The oxygen content of evaporated ocean water is higher in 16 and lower in 18 than the ocean water itself, meaning that vapor disproportionately pulls out 16 and leave behind 18. Normally that's a wash because as new vapor is leaving the ocean, old vapor (in the form of precipitation) is returning to the ocean. But in a typical ice age, the precipitation gets locked up in ice caps. So the ocean is losing oxygen-16 with no replacement, meaning the oxygen-18 share of what remains in the ocean rises.

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

    57% picked this

  3. Rain and snow contain relatively

    Contradicted

    An "interglacial" period presumably means "in between ice ages", when glaciers haven't extended well beyond the polar ice caps. We don't have any reason to think that the composition of precipitation changes during an ice age. We only think the final destination of the precipitation changes. During normal times, it returns to the ocean. During ice ages, it gets trapped on ice caps.

    12% picked this

  4. During the ice age, more

    Unsupported

    The stimulus says a large amount of precipitation falls on ice caps. Ice caps are ice/snow that is frozen on top of land. If rain falls on an ice cap, it fell on ice that's on top of land. We definitely can't say that it fell over the ocean. We never hear that where the rain falls actually shifts. It's just then when rain falls on frozen land, it gets trapped as ice. Whereas in normal terms, it would fall on land and flow back to sea.

    12% picked this

  5. The composition of seawater changes

    Unsupported

    The composition of seawater doesn't change during normal, interglacial periods. But it does change during ice ages.

    14% picked this

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