Q11 - Water vapor evaporated from

[anjelica.grace]

I lucked out and guessed the correct answer, but i'm having trouble even understanding the relative proportions described in the first sentence.

help! and does increased concentration imply higher percentage?

[ohthatpatrick]

So let's say that normal seawater is 50% oxygen-16 and 50% oxygen-18.

The water vapor that evaporates up from the seawater contains a higher percentage of 16 (we can picture the heavier 18's having a harder time evaporating up).

So let's say water vapor is 70%/30% in terms of its 16/18 breakdown.

As a disproportionately larger amount of 16 gets sucked away, we should see the remaining ocean water develop a higher percentage of 18.

Remember, percentage change can occur two ways:
If I have a room full of boys and girls, and the percentage of girls suddenly goes up, that means that either:
1 - more girls came into the room
or
2 - some of the boys left the room
(or both)

So as more 16's leave the ocean, the % of 18's will go up.

However, the information goes on to say, normally there is a constant cycle of our 70/30 vapor evaporating out of the ocean while 70/30 precipitation falls back into the ocean.

In this way, the composition of the ocean stays the same.

During an ice age, the evaporation leaves the ocean, and there's isn't any precipitation returning to the ocean.

So during an ice age, as all those 16's evaporate away and get locked in an ice cap, we'll see the relative proportion of oxygen-18 in seawater go up.

And, as you suspected, 'concentration' of oxygen-18 is a paraphrase for the proportion/percentage of seawater that is oxygen-18.

So we've established why (B) is the right answer. Let's knock out the other ones:

A) This is contradicted by the first sentence.

C) "Interglacial periods" is a surprising phrase. I assume that it means "non-ice age periods". Either way, though, we don't have any information to compare the amount of oxygen-16 rain and snow have during an ice age versus in between ice ages. The rain and snow are precipitation formed from water vapor. During an ice age, the water vapor itself doesn't change. The thing that changes is that the water vapor gets trapped as ice on ice caps rather than falling back to the ocean in the form of rain or snow.

D) Another "fake comparison". We have no information to compare precipitation on land vs. precipitation over the ocean.

E) This is contradicted, if anything. We know that "normally" (i.e. during interglacial periods) the composition of seawater is not changed by the vapor/precipitation cycle. During ice ages, the composition IS changed. So if anything, we'd be able to support that the composition changes more QUICKLY than during interglacial periods. Again, though, be on the lookout for these "fake comparisons". Were we ever discussing the rate of change during an ice age vs. the rate of change in between ice ages?

Hope this helps.

[ericha3535]

Wow this question was confusing...
But let me write my thoughts out just for the hack of it.

Ok, So basically Ocean contains O16>O18 and seawater (which we don't care about its oxygen compositions but know that it has less O16 but more O18 than the ocean). Normally, ocean's composition isn't affected. Yet during the ice age, it does.

Ok, also so as far as I have understood it, this argument is saying that the vapors from sea water (through precipitation) is what causes the ocean to maintain its oxygen composition, which is O16>O18. I guess the biggest assumption one needs to make is whatever sea water vapor does to the ocean's composition, it is indeed contributing more "O16" and "give a little or take away O18" to maintain its oxygen composition, right?

Thus as a result, when such sea water vapors are trapped (more O16), then of course it's like taking away O16 from ocean's composition, causing its O18 concentration to go up.

Well here is my another 2 cents...

I don't like that assumption. The assumption again is since the sea water vapors is the factor that causes the ocean to maintain O16>O18 composition, without such factor, the proportion of Ocean's O16 will inevitably decrease during the ice age or even normal precipitation process, the sea water vapors is what gives out more O16.

Now, the question is, why can't we say the other way? Let's say during the normal days, the composition of ocean is 70% to 30%, O 16 and O18 respectively. The only reason that the ocean is maintaining such composition is because the sea water vapors take away 20% of O16 and add 20% of O18; so without sea water precipitation, the ocean is consisted of 90% to 10%. So when precipitation doesn't occur during the ice age, instead of 70 to 30, it's 90 to 10, meaning more O16...

I feel like I am rambling but... Do I make any sense?

[ohthatpatrick]

I think you're getting tripped up by the ambiguity of "seawater". "Seawater" can, and I think in this case does, mean ocean water.

There's no difference between seawater and ocean water.

All seas are connected to the oceans and drain into them (otherwise, I think we would call that body of water a 'lake').

But if we look at the math, I think your alternative scenario wouldn't work anyway.

Let's say the ocean is 70/30.

If we took "the seas" to mean separate bodies of water, then the 1st sentence dictates that their proportion has less 16 and more 18.

So let's says "the seas" are 60/40.

If the ocean starts out as 70%/30%, then as water evaporates, the percentage of 16 will go down and the percentage of 18 will go up.

Suppose after a day of evaporation, the ocean ends up being 65/35.

There's no mathematical way that 60/40 seawater could bring 65/35 ocean water back up to 70/30.

60/40 seawater combined with 65/35 ocean water would bring the 65 closer to 60 ... it would bring the 35 closer to 40.

---- man these numbers make my brain want to explode ---

The idea they're trying to get across, I believe, is this:

We start off with 70/30 ocean water.

The water that evaporates from the ocean water is higher in 16 and lower in 18 ... let's say that the vapor is 80/20.

As water evaporates from the ocean, the ocean lowers to 65/35.

But when that 80/20 vapor cycles back around, it can bring the 65 back up to 70, and it can bring the 35 back down to 30.

But, of course, during an ice age, the 80/20 vapor gets locked into ice caps, so the ocean stays at 65/35 ... justifying (B).

Hope this helps.

[ericha3535]

Oh wow...
so basically...
ocean is seawater and vice versa... I did not know they were the same things...

Basically argument is saying that the VAPOR (the gas form of seawater) of ocean has more O16 and less O18 than the ocean itself, or SEAWATER.

Oh wow...
I feel so dumb right now...

THANK YOU!!

[zip]

[quote="ohthatpatrick"]So let's say that normal seawater is 50% oxygen-16 and 50% oxygen-18.

The water vapor that evaporates up from the seawater contains a higher percentage of 16 (we can picture the heavier 18's having a harder time evaporating up).

So let's say water vapor is 70%/30% in terms of its 16/18 breakdown.

As a disproportionately larger amount of 16 gets sucked away, we should see the remaining ocean water develop a higher percentage of 18.

Remember, percentage change can occur two ways:
If I have a room full of boys and girls, and the percentage of girls suddenly goes up, that means that either:
1 - more girls came into the room
or
2 - some of the boys left the room
(or both)

So as more 16's leave the ocean, the % of 18's will go up.

Pat, this your example and last statement fail to capture the difference between relative frequency of 16 versus 18 and concentration. That is, if all that happened were that 16 were removed, without removing any water or affecting 18 levels, then only relative percentages of 16 and 18 would be affected. It's the fact that it's taking water as well that results in the correct conclusion. For example, if I took two ounces of water with a total of 5 units each of 16 and 18 and evaporated 1 ounce which removed 2 units of 16 and one unit of 18, I would be left one ounce with 3 units of 16 and 4 of 18. The relative concentration of 18 would have jumped from 2.5 units per ounce to 4 ( also 16 would have jumped from 2.5 to 3). So the concentration would go up. If I took those two ounces of water and just extracted all the 16 , but left the water and 18 intact, the concentration of 18 would remain 2.5 an ounce.

Since the problem incorporates the loss of water in conjunction with the loss of 18 and 16 in it's formulation, and assuming ( this is a most question) that the 18's can only be taken from the water evaporated, and the number will be less than all, the remaining 18 will be part of the rest of the seawater which will then have a greater concentration of 18 ( even allowing the possibility of a greater concentration of 16 if all of the 16 is not taken per unit of evaporation).

[CraigO192]

Little late to the party here, just want to confirm that I understand this passage correctly:

Water Vapor evaporating from the Ocean (O) = Higher proportion of Oxygen-16 than Seawater, a Lower proportion of Oxygen-18 than Seawater (S) ... we can say (as OhThatPatrick claimed above) 70/30 split, for example.

This is usually fine, as 70/30 evaporates from the O, it is REPLACED by the 60/40 (?) split from S through precipitation.

During the Ice Age, a large amount of precipitation doesn't make its way from S to O.

So, during an ice-age, we are expected to create a timeline as such:

O: 70/30 split evaporates. In this evaporation, O loses a 70/30 split of oxygen-16 and oxygen-18 ... theoretically bringing its Oxygen content closer to 50/50, as it is losing more Oxygen-16 than Oxygen 18... let's say 65/35 after evaporation.

S: 60/40 split evaporates. Again it is not replaced through precipitation...so the overall concentration drops to 55/45 (?)...

Therefore B is correct because the concentration of Oxygen-18 increases from 40% -> 45% (?) because, due to the ice-age, O and S are not complimenting their respective Oxygen-16/18 imbalances through evaporation and eventual precipitation.

...

Good God. I normally do alright on LR, even with tougher ones, but this one gave me a headache and threw me off the whole section -- was also having a bad day -- who knows?

Could someone double-check my reasoning or try and disprove my thinking here? I feel like I've got it down, but it seems like a much harder question than you normally see, especially for one in the early-mid section of an LR set.

[MoiseiD954]

I agree with CraigO192. Comparing to the newer PTs, this whole LR section is bizarre and question #11 is bizarre in particular. I think it should have been omitted from grading.

I think this question makes two major problematic assumptions:

1) Ocean = seawater.
Ok, fine. Maybe for a native English speaker it would not have been a problem. But for me, and maybe for other ESLs, it's nearly impossible to make this leap in the heat of the exam.

2) Ocean is made up of only oxygen-16 and oxygen 18.
I am not a chemist by any means, but I think it is safe to assume that there are more than those two elements in an ocean (it H2O, right? so maybe there is also hydrogen).
If there is more than just o-16 and o-18, then during an ice age, the % of o-18 will not increase. It will decrease because per the stimulus, the vapor takes away some o-18.