#11 - Water vapor evaporated from the ocean contains a

[Luke Haqq]

Hi ashpine17!

To your first question, I don't see material from the stimulus that would make (A) true in a non-ice age. To the contrary, the stimulus indicates the proportions in the vapor and seawater are different.

To your second, yes, from the material in the stimulus, we don't know the specific proportion of oxygen-16 and oxygen-18 in seawater, though we know that there is a greater proportion of oxygen-18 in the seawater in comparison with vapor.

[lsatchallenger]

Hello!

I was initially inclined to choose B but then chose C which I realize is wrong because presumably the rain (precipitation) contains more oxygen-18 than oxygen-16 and we know the ice caps (aka snow) contains a lot of precipitation (oxygen-18) so I know this is wrong. During blind review, I chose D because my line of reasoning was the following: Water vapor oceanwater --> contains more oxygen-16 less oxygen=18 and it says compared to seawater. From this I thought that water vapor from SEAWATER --> contains more oxygen-18 less oxygen-16.

Seas and oceans are different from one another in real life -- but is my mistake that the author is using these terms synonymously? And therefore we are just differentiating between sea/ocean water and sea/ocean vapor rather than what I thought above ^^?

Hope this made some sense

Please let me know where I went wrong! Thank you!!

[Jeff Wren]

Hi challenger,

So the word "sea" has several definitions and one of them does in fact mean ocean. The first definition that comes up in the dictionary for the word "sea" is: "the expanse of salt water that covers most of the earth's surface and surrounds its landmasses." That definition is describing oceans, which cover most of the earth's surface.

It's true that the word "sea" can also refer to other bodies of water that are not oceans, such as a large lake, so it can be confusing.

In this stimulus though, seawater is being used as a synonym for ocean water. One way to spot this is the way that the word "seawater" is used in the stimulus. The stimulus is comparing the water vapor evaporated from the ocean water/seawater to the ocean water/seawater itself, basically saying that more of the oxygen-16 gets evaporated out of the ocean water/seawater than oxygen-18. (The flip side of this is that the remaining ocean water/seawater has more oxygen-18 than oxygen-16 proportionally.)

The reason that this doesn't usually matter is that the water vapor normally returns back to the ocean/sea when it rains, which would rebalance the oxygen-16/oxygen-18 proportions in the ocean/sea back to normal. During ice ages, however, not all of the rain makes it back to the ocean/sea, so that ocean water/seawater would not get rebalanced and would therefore have a higher proportion of oxygen-18 than it normally does during non-ice ages, which is described in Answer B.

You definitely weren't the only one who didn't realize this, as you can see from some of the earlier posts on this forum thread.