Oxygen Isotopic Ratios

[coykiesaol]

Just when I thought that I understood the basics of how Oxygen Isotopes are used to determine past temperatures, I read something different!

I thought:

Water molecules with O18 do not evaporate as easily as water molecules with O16 since O18 is heavier than O16. Thus, during warm periods, one would expect the ratio of O18/O16 in marine organisms to be higher than during cold periods.

I just read:

A record of the climate for the past 5 million years is indicated by the oxygen content of cores from the Equatorial Pacific Ocean. These cores reveal numerous cycles consisting of warm (low O18/O16) and cold (high O18/O16) periods.

Which is correct; my understanding or what I just read?

Thanks

[TomB]

My understanding of the mechanism for isotope ratios in ice cores is identical to yours. I am not an expert but when I got interested in GW I researched everything and found several academic sources that that provided equivalent explanations. But I was not aware that ice core analysis could go back much more than 5000 years.

Your reference is going back 5 million years based upon equatorial conditions so it is not about ice cores. From the text you provided I guess that the proxy is microscopic animal/plant residues from sunlight dependent organisms that lived very near the surface. That hypothesis leads me to think that the surface water isotope ratios are a compliment of the ice core ratios. In ice cores we see a difference because warm conditions evaporate more O18 from the water so in the water there will be less O18.

What surprised me it that they can actually detect a difference. I would have expected dilution and mixing within the ocean surface layer to quickly erase the difference. However what they are seeing is backed by ice core data so it has merit. I sure would like to understand the statistical methods they are using to extract signal from what must be very noisy samplings.

Tom

[harryn]

Hi, I might be completely wrong here, but I think that the vapor pressure of water (how rapidly it evaporates at a given temperature) is not very sensitive to the difference of the O isotope.

IIRC, the vapor pressure is primarily due to the H bonding energy between water molecules, and this is dramatically larger than the mass difference.

As an example, butane has a much larger atomic mass than water (around 4x) but still has a much higher vapor pressure under any condition I can think of off hand.

[TomB]

Hi, I might be completely wrong here, but I think that the vapor pressure of water (how rapidly it evaporates at a given temperature) is not very sensitive to the difference of the O isotope.

IIRC, the vapor pressure is primarily due to the H bonding energy between water molecules, and this is dramatically larger than the mass difference.

As an example, butane has a much larger atomic mass than water (around 4x) but still has a much higher vapor pressure under any condition I can think of off hand.

I agree that the evaporation rate of H2O in general is fully related to the vapor pressure. It has been several years since I found the research papers on ice core date and carefully read them but they were on the web and were not really difficult reading when compared to most peer reviewed science papers. Your welcome to check on my recollection but this is what I remember.

When the researchers use ice cores as a temperature proxy they considered not the rate of evaporation but the relative percentage of O16 and O18 in the evaporated and condensed to snow H20. (While using tree rings for a temperature proxy the thickness of the rings is important but with ice cores the layer thickness in not relevant.) In ocean water O16 makes up 99.76% of the isotopes, and O18 is .002% of the isotopes. Because of the mass difference the average thermal velocity of H2O with an O18 isotope would be about 95% of the H20 with O16 an isotope. But the surface tension of water is not similarly proportional. So as the water temperature varied the relative percentage of O16 and O18 that could pass through the surface tension would vary. That is the temperature proxy derived from ice cores. (Although I don’t remember reading anything with regard to the condensation process I expect there is a compliment of the evaporation process and if so that would further amplify the O18/O16 ratio in ice cores.)

So although the evaporation rate of H20 in general would be driven by the vapor pressure and completely dependent on average nuclear mass, the comparative rates of different isotopes are a different story. I would not even begin to feel qualified to validate what the experts wrote and I read and recalled. But I do understand that if some scientist proposes a temperature proxy intended to cover 5000 years I should be able to look at the results and see the various up and down average temperature conditions that were reported in history. I see the patterns more clearly in the ice core data than I do in the tree ring data. That is not to say some graduate student didn’t spend his/her thesis year in the pub and then make up results to satisfy history, but I also have faith in the thesis committee and the peer review process to detect total fraud.

[TomB]

Hi, oxygen isotope ratio cycles are cyclical variations in the ratio of the abundance of oxygen with an atomic mass of 18 to the abundance of oxygen with an atomic mass of 16 present in some substance, such as polar ice or calcite in ocean core samples. The ratio is linked to water temperature of ancient oceans, which in turn reflects ancient climates.

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Your description is spot on but I would quibble with two words, 'cyclical' and 'ancient'.

The temperature patterns derived do seem to have repetitive characteristics, implying that the water temperature variations are also repetitive but the period and length of the cycles is not constant. The intervals between warm periods of the past 5000 years (from ice core data) were 1200, 1050 and 1050 years. The length of the warm periods was 160, 170, 200 and if the current ones just ended 161 years. To me cyclic implies a constant period and segment length from repetition to repetition, but maybe I'm just thinking like a mechanic and the variations are just noise. I suspect that explaining the variations might do more to explain climate variability that all the debates about AGW have done.

Although the ratio of O18/O16 serves as a proxy for ancient climate temperatures I see no reason why it is not a proxy for 'old' or even fairly 'recent' global temperature. There is nothing about the underlying physics that has changed so this proxy seems to be timeless.

But that said I congratulate you for saying it all in such a short paragraph. When I write something it is always too long.

Tom