#11 - Water vapor evaporated from the ocean contains a

[Robert Carroll]

relona,

The problem with your reasoning is that the evaporated seawater is not all returning to the ocean. Some of it is trapped as ice on ice caps. So the evaporated seawater contains a disproportionate amount of oxygen 16, and that water doesn't all return to the ocean, since some is trapped as ice. So the ocean has had its oxygen 16 depleted a bit and doesn't get it all back, since some is trapped in ice. So the ocean is being relatively depleted of oxygen 16 without replenishment; relative depletion of oxygen 16 means a greater share of oxygen 18 than before, which is what answer choice (B) says.

Robert Carroll

[cgleeson]

Hi,

Ok, so the Oxygen -16 is not making it all back to the ocean, it's trapped in ice caps. The same could be said about Oxygen -18 which is in a smaller amount. I don't see how the O2-16 can be depleted but not the O2-18 especially considering that the O2 -18 is in a smaller proportion.
I went right to answer choice D with this one.
Another question:
When a stimulus or question stem mentions "proportion" then states "many" or "some" are we looking at a percent vs amount fallacy?

Thank you kindly.

relona,

The problem with your reasoning is that the evaporated seawater is not all returning to the ocean. Some of it is trapped as ice on ice caps. So the evaporated seawater contains a disproportionate amount of oxygen 16, and that water doesn't all return to the ocean, since some is trapped as ice. So the ocean has had its oxygen 16 depleted a bit and doesn't get it all back, since some is trapped in ice. So the ocean is being relatively depleted of oxygen 16 without replenishment; relative depletion of oxygen 16 means a greater share of oxygen 18 than before, which is what answer choice (B) says.

Robert Carroll

[Adam Tyson]

What your analysis is missing, cgleeson, is the idea of concentration and proportion. If the proportion of O-16 is greater in the evaporated water than in the seawater, that means that relatively more O-16 is being removed from the ocean when compared to O-18. Thus, as the water evaporates, the relationship between 16 and 18 is changing, with 18 making up a larger percentage over time even as some of it being depleted.

I'll use my breakfast as an illustration. I typically have two eggs for breakfast and one English muffin. If this morning when I woke up I had a dozen eggs and a dozen English muffins in the fridge, then the proportion of eggs to muffins was 1:1. But my breakfast had a higher proportion of eggs than my fridge; breakfast had a 2:1 ratio. So after breakfast, I had 10 eggs in the fridge but 11 English muffins. I have fewer English muffins than before, but they now make up a greater proportion of the food in my fridge than they did. The concentration of English muffins in my fridge has increased. Not the amount, but the concentration, aka proportion relative to other things.

The problem with answer D is that "a large amount" does not prove "more" or "most." It could be that during an ice age, most of the precipitation falls over the ocean, even though "a large amount" falls on ice caps (which may not even qualify as "land" - they may still be considered part of the ocean. They are frozen ocean.)

[cgleeson]

OMG Adam! You rock ! I totally get it now...this is great. Lesson learned my friend, hope there are more like this one.

What your analysis is missing, cgleeson, is the idea of concentration and proportion. If the proportion of O-16 is greater in the evaporated water than in the seawater, that means that relatively more O-16 is being removed from the ocean when compared to O-18. Thus, as the water evaporates, the relationship between 16 and 18 is changing, with 18 making up a larger percentage over time even as some of it being depleted.

I'll use my breakfast as an illustration. I typically have two eggs for breakfast and one English muffin. If this morning when I woke up I had a dozen eggs and a dozen English muffins in the fridge, then the proportion of eggs to muffins was 1:1. But my breakfast had a higher proportion of eggs than my fridge; breakfast had a 2:1 ratio. So after breakfast, I had 10 eggs in the fridge but 11 English muffins. I have fewer English muffins than before, but they now make up a greater proportion of the food in my fridge than they did. The concentration of English muffins in my fridge has increased. Not the amount, but the concentration, aka proportion relative to other things.

The problem with answer D is that "a large amount" does not prove "more" or "most." It could be that during an ice age, most of the precipitation falls over the ocean, even though "a large amount" falls on ice caps (which may not even qualify as "land" - they may still be considered part of the ocean. They are frozen ocean.)

[Henry Z]

Hi,

Can you talk about why E is wrong?

The stimulus says the overall composition of the ocean is stable because “evaporated seawater returns to the ocean through precipitation.” But now in an ice age, when a large amount of precipitation is trapped, the evaporated seawater can’t return as fast as before, doesn’t that mean the ocean change more slowly as well?

I suppose it takes longer for the water cycle (ocean-evaporation-precipitation-ocean) to complete, so I chose E. What’s wrong?

[Adam Tyson]

The problem, Henry, is that between ice ages the composition of the ocean doesn't change at all! The stimulus tells us that "Normally, this phenomenon has no effect on the overall composition of the ocean." That means that any change during an ice age is faster than any change not during an ice age, because some change has to be faster than no change!

[Henry Z]

Hi Adam,

I guess there lies the rub. In my reading, seawater does change in interglacial periods. The stimulus says the composition of the ocean is stable only because the evaporation can return to the ocean. So before it returns, the composition is not the same. The change (that is, evaporation and precipitation) happens, just fast enough to make the ocean stable.

I know there's little use laboring over the semantics of a question as ancient as the glacial period in LSAT years, but I just find the assumption that "it's not change if it changes constantly" a big gap.

[Adam Tyson]

You're overlooking an important part of the stimulus, Henry,. The author said explicitly that "Normally, this phenomenon has no effect on the overall composition of the ocean." We should not challenge that claim! We are supposed to accept it as fact, and then draw inferences based upon it. Your analysis has you saying "but actually it DOES change," and that's the wrong way to approach the facts given to you in a Must Be True question, even if their facts conflict with what you believe to be true.

Besides, who's to say that precipitation and evaporation aren't happening simultaneously and in equal amounts, so that there literally is zero change during those periods? Sounds unlikely to me, but that doesn't mean I can just assume it isn't true!

Accept the facts and draw inferences from them. Challenging the facts goes against the task assigned to us!

[ashpine17]

wouldn't A be true normally? like in non ice age periods. it did say the rain normally falsl back and does not affect overall composition of the ocean

[ashpine17]

actually, do we NOT know the original proportion of O-16 and O-18 in the ocean before the water vapor?