<p>As far as I know scientists use oxygen isotope 16 to 18 ratio in air trapped in glaciers (or in old foraminiftera shells) as proxy for temperature in the past. I know that $\ce{^18O}$ is heavier, and it precipitates faster, and $\ce{^16O}$ is lighter, and it evaporates faster.</p>

<p>I understand that when it's warmer, there is more evaporation and more heavy isotope left in water. Vapor moves toward poles, where it falls as rain or snow and goes back to water (this doesn't count) or stays in glacier for hundreds of thousands of years.</p>

<p>So when it's warm, we should have more $\ce{^18O}$ in ocean sediments, and less in arctic glaciers, right? So more $\ce{^16O}$ (less $\ce{^18O}$) in glaciers -> warm period.</p>

<p>On the other hand, when it's cold, precipitation occurs earlier while air moves to the poles, and heavy isotope falls down earlier, on bigger latitudes. In the air that goes to the poles there is only little heavy isotope left. So more $\ce{^16O}$ (and less $\ce{^18O}$) in glaciers -> cold period. This is contrary to the above. So how to figure out temperature from ice and sediments cores?</p>

CC BY-SA 3.0
2015-01-23T21:35:44.233Z
When you say "more 18O" do you really mean to say "higher d18O values"?  As temperature increases, doesn't mass dependent fractionation become less and less important?  At a certain point kinetic energy overwhelms the system.  For example, in a water-rock system, high dO suggests lower temperatures.  Also, when you say "ocean sediments" this is not the same as saying "in the ocean" right?  I'm not clear on what the oxygen atom is in the sediments, water?  SiO?
equant
CC BY-SA 3.0
2015-01-23T23:21:16.400Z
From http://en.wikipedia.org/wiki/Proxy_%28climate%29 "The calibration was initially done on the basis of spatial variations in temperature and it was assumed that this corresponded to temporal variations. More recently, borehole thermometry has shown that for glacial-interglacial variations ... temperature changes were twice as large as previously believed." and "To produce the most precise results, systematic cross-verification between proxy indicators (e.g. tree rings, pollen, etc.) is necessary for accuracy in readings and record-keeping."
f.thorpe
CC BY-SA 3.0
2015-01-24T20:23:49.180Z
@equant When temperature increases, so does evaporation, so I think fractionation too.

By ocean sediments I understand forams shells. It's built from CaCO3 and this shells are used as temperatures proxy.
amorfis
CC BY-SA 3.0
2015-01-24T20:27:34.707Z
@farrenthorpe Thank you for this information, but this doesn't answer my question. I want to understand the mechanism that makes isotopes proxy for temperatures in the past, and this doesn't help.
amorfis
CC BY-SA 3.0
2015-01-24T22:33:39.990Z
@amorfis you explained the mechanism in your question... I was offering a comment not an answer.
f.thorpe
CC BY-SA 3.0
2015-01-25T15:44:52.560Z
@farrenthorpe Well, still I don't know if less O16 means warmer or colder climate. So surely I don't fully understand the mechanism :) Thank you anyway.
amorfis
<paleoclimatology><sedimentology><glaciology>
How do we use oxygen isotopes as temperature proxy?
amorfis
# How do we use oxygen isotopes as temperature proxy?

(Asked by: username_0 on 2015-01-23)

As far as I know scientists use oxygen isotope 16 to 18 ratio in air trapped in glaciers (or in old foraminiftera shells) as proxy for temperature in the past. I know that $\ce{^18O}$ is heavier, and it precipitates faster, and $\ce{^16O}$ is lighter, and it evaporates faster.

I understand that when it's warmer, there is more evaporation and more heavy isotope left in water. Vapor moves toward poles, where it falls as rain or snow and goes back to water (this doesn't count) or stays in glacier for hundreds of thousands of years.

So when it's warm, we should have more $\ce{^18O}$ in ocean sediments, and less in arctic glaciers, right? So more $\ce{^16O}$ (less $\ce{^18O}$) in glaciers -> warm period.

On the other hand, when it's cold, precipitation occurs earlier while air moves to the poles, and heavy isotope falls down earlier, on bigger latitudes. In the air that goes to the poles there is only little heavy isotope left. So more $\ce{^16O}$ (and less $\ce{^18O}$) in glaciers -> cold period. This is contrary to the above. So how to figure out temperature from ice and sediments cores?

**Tags:** paleoclimatology, sedimentology, glaciology

> **Comments:**
> **username_1**: When you say "more 18O" do you really mean to say "higher d18O values"?  As temperature increases, doesn't mass dependent fractionation become less and less important?  At a certain point kinetic energy overwhelms the system.  For example, in a water-rock system, high dO suggests lower temperatures.  Also, when you say "ocean sediments" this is not the same as saying "in the ocean" right?  I'm not clear on what the oxygen atom is in the sediments, water?  SiO?  
> **username_2**: From http://en.wikipedia.org/wiki/Proxy_%28climate%29 "The calibration was initially done on the basis of spatial variations in temperature and it was assumed that this corresponded to temporal variations. More recently, borehole thermometry has shown that for glacial-interglacial variations ... temperature changes were twice as large as previously believed." and "To produce the most precise results, systematic cross-verification between proxy indicators (e.g. tree rings, pollen, etc.) is necessary for accuracy in readings and record-keeping."  
> **username_0**: @username_1 When temperature increases, so does evaporation, so I think fractionation too.  By ocean sediments I understand forams shells. It's built from CaCO3 and this shells are used as temperatures proxy.  
> **username_0**: @farrenthorpe Thank you for this information, but this doesn't answer my question. I want to understand the mechanism that makes isotopes proxy for temperatures in the past, and this doesn't help.  
> **username_2**: @username_0 you explained the mechanism in your question... I was offering a comment not an answer.  
> **username_0**: @farrenthorpe Well, still I don't know if less O16 means warmer or colder climate. So surely I don't fully understand the mechanism :) Thank you anyway.  

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## 0 answers

