PT1.S3.Q11 - Water vapor evaporated from the ocean

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• 1 Like

  castrone Member

  March 2021 210 karma

  Yea, so its saying that the vapor created through evaporation of sea water has a higher concentration OXY- 16 and a lower concentration of OXY-18. This means that the balance of OXY-16 and OXY-18 would be disrupted in the sea itself because a lower percentage of OXY-18 is being taken out. But this usually doesn't matter bc it comes back as rain later and everything evens out. But during an ice age it doesn't return to the sea, so the sea would have higher concentrations of OXY-18.
• davejonesy Member

  March 2021 edited March 2021 50 karma

  I remember this one messed with me the first time I saw it, too.

  I think the tricky thing with this question is getting caught up in trying to full think through the ratio / proportion of O16 and O18 in the ocean, and in the rain.

  The stim tells us 1) that vapor contains more O16 than O18, 2) normally this doesn't matter because precipitation restores this balance and 3) that during ice age a larger-than normal amount of precipitation doesn't return to the ocean.

  The inference here is that point 2 & 3 taken together mean the balance of O16 and O18 in the ocean is going to be off.

  ACs

  A) this can't be right because the passage tells us different

  C) We have no reason to expect this is the case -- why would snow / rain during the ice age contain more O16 than normal?

  D) We don't know anything about land - maybe ice caps are considered land, maybe they're not, but we don't know

  E) Again we know nothing about overall change in the composition of seawater, presumably there are other components aside from O16 and O18 (like hydrogen, I'd imagine) and maybe they impact composition more.

  B This is the right answer, if only because the other answers aren't strongly supported by the text. However, it's really clearly the right answer because of what we're told at the end of the passage: that lots of ocean-evaporated precipitation doesn't fall back on the ocean, it gets caught in ice caps. What does that imply? That that balance isn't being restored. What does that imply? That we're left with more O18 in seawater because less of it evaporates, and the O16 that does evaporate gets stuck in ice.

  This is my first crack at an extended explanation...hope this helps!
• Sailor Moon LSAT Member ⭐

  March 2021 200 karma

  Water evaporated has a higher percentage of Oxygen 16 vs Oxygen 18. During the Ice age this water is trapped into ice caps and not returned to the sea.

  a. We do not know anything about a comparative proportion of oxygen in vapor to the sea water
  b. We can conclude from this that the composition of the ocean changes because more oxygen 16 leaves than is returned. So if there is less oxygen 16 in the water because a higher percentage left and was trapped by the ice, there is more oxygen 18 in the sea water. This is why answer choice B is correct.
  c. I misread this and was trying to go to fast. This is not a most strongly supported because we do not know about rain/snow having more oxygen-16 in glacial and interglacial periods.
  d. No one talks about how much of earth is covered by land vs water, and where precipitation falls.
  e. We know nothing about the rate of change of seawater.
• KI_Lawnupe9 Member

  March 2022 38 karma

  Got caught up in ocean vs seawater which are the same.