- Fri Oct 16, 2015 8:56 pm
#20235
Hi Sarah,
The stimulus here really only gives us information about water vapor evaporated from the ocean and its return to the ocean through precipitation. We know that the evaporated water has more O-16 and less O-18 than regular seawater, so presumably something about the evaporation process must be causing the vapor to pull out more O-16 and to leave behind more O-18 than we would ordinarily find in nonevaporated seawater. The ordinary composition of seawater (i.e., the ordinary balance between O-16 and O-18) is normally restored when the vapor is returned to the ocean through precipitation, except in ice ages when the precipitation gets trapped as ice.
Notice that we are given a reason why the composition of the ocean is ordinarily not affected by the higher ratio of O-16 to O-18 contained in the evaporated vapor: it is because the vapor returns to the ocean in form of precipitation. Thus we can infer that, without that return via precipitation, the ocean composition would be changed by the evaporation of the water vapor. If the vapor has more O-16 and less O-18 than the ocean from which it came, this ought to mean that the ocean water would be left with less O-16 and more O-18 (unless the balance was restored by the return of the vapor in the form of precipitation).
And we also know that an ice ages can prevent precipitation from returning the vapor to the ocean. Thus, we are able to conclude, as answer choice (B) does, that during an ice age, the seawater would be stuck with a higher concentration of O-18, because at least some of the seawater that had evaporated with lower-than-normal levels of O-18 would get trapped as ice and wouldn't be able to return to balance out the O-18 levels. We can safely infer something about the composition of seawater during an ice age, based on the facts we've been given.
Answer choice (C), by contrast, goes beyond the facts we've been given to draw a conclusion about rain and snow in general. All we really know is a little bit about the compositions of evaporated seawater and actual seawater, and the role played by precipitation in moderating their relationship, and one impact an ice age can have on precipitation. But we are told nothing about the composition of precipitation as a general category, and nothing specifically about the composition of rain or snow, nor about how that composition would vary between ice ages and nonglacial periods. Thus, (C) states something that we couldn't possibly know on the basis of what we are told in the stimulus.
I hope this clarifies things!
Laura
