I got this wrong - I thought the seawater was a different source (body of water) than the ocean water.
VictoriaApril 28, 2019
Hi @Logan-Frye,
I agree that this passage is worded confusingly. The question is comparing water vapor from the ocean to seawater. It would make so much more sense if it was called "water vapor evaporated from the sea" or "ocean water," rather than using the words ocean and sea interchangeably as they are technically two different types of bodies of water with different characteristics. However, the only type of liquid water discussed in the passage is called "seawater," therefore, we must assume that the author is referring to the water in the ocean.
Therefore, this passage is comparing water vapor to liquid water. As water vapor contains more oxygen 16 and less oxygen 18 than liquid water, the evaporation of water would decrease the ocean's overall concentrations of oxygen 16 and increase the overall concentration of oxygen 18. Normally, this does not impact the overall concentrations of the ocean as the water vapor returns to the ocean as liquid water through precipitation; however, during an ice age, a large amount of this precipitation falls onto ice caps, where it is trapped as ice. As the water does not return to the ocean, the remaining liquid water (or seawater) has a decreased concentration of oxygen 16 and an increased concentration of oxygen 18.
Hope this is helpful! Let us know if you have any further questions or need more clarification!
Laura_21September 12, 2021
I understand that the only type of liquid water discussed in the passage is sea water, but I think it's a pretty big jump to say that seawater and ocean water are all just "water." In working through other questions it seems that such a big jump, or the need to make such a large assumption, nearly always leads to selecting an incorrect answer.
Could you please elaborate in more detail on what led you to determine that, for this particular question, seawater and ocean water should be considered collectively as "water?" Thank you!
RaviFebruary 6, 2022
@Laura_21, if you just focus on the oxygen 16 vs. 18, it helps a lot in understanding this argument. Evaporated water has more oxygen 16 than oxygen 18. This means that the ocean water left behind has a higher than normal proportion of oxygen 18. This isn't typically an issue, as all of the oxygen 16 comes back when it rains over the sea. But, the key here is that during an ice age, the rain gets trapped in the ice caps. As a result, it doesn't come back to the ocean in an ice age. This means that during an ice age when rain gets trapped in the ice, there is a higher proportion of oxygen 18 in the ocean. This gets us to B.
