3 ms·

> The highly accurate dating and long time span of those fossils have allowed scientists to estimate fluctuations in sea level, and thus ice content, by extrapo

by yread 13y ago

> The highly accurate dating and long time span of those fossils have allowed scientists to estimate fluctuations in sea level, and thus ice content, by extrapolating from variations in the ratio of oxygen isotopes How do they estimate sea level from the ratio of oxygen isotopes? I've found just this http://en.wikipedia.org/wiki/Δ18O http://en.wikipedia.org/wiki/Δ18O but it talks only about temperature, salinity and amount of evaporation

ovis 13y ago
Lighter isotopes preferentially get deposited on and form the ice sheets, so increases in ocean d18O correlate with glaciations.
kaybe 13y ago
The Rayleigh process describes the continous depletion of a reservoir through a fractionationing process - evaporation. It comes from the fact that heavier isotopes need slightly more energy for a phase change and thus get enriched in the fluid phase. In the case of water, it is mainly the stable O18 (as opposed to the abundant O16). - Lighter isotopes evaporate easier and precipitate down on land. If they don't return to the ocean for a long time due to freezing and glaciation, the ocean has a higher ratio of heavy to light isotopes. More ice means lower ocean level, and since the ratio in the ocean is dependent on the missing water mass, calculations directly give the sea level. Now how do we know what the isotope ratio was in the past? In the oceans, there are those small animals (foraminifera) that build the oxigen from water into their shells. The Wikipedia article you found discusses this. (Another fractionation process, add more calculations here. Additional temperature dependance.) When they die, they float to the ocean floor and, in time, form sediments and eventually lime rock, which can be drilled for probes. Furthermore, different kinds live in different depths, giving us even more information. This can be used for the past 70 million years, since that's the age of the oldest ocean floors. Almost (!) everything older has been subducted again. However, it is not easy to disentangle the temperature effect from the ice volume effect. You can check those out, if you're interested, they explain more thoroughly than the wikipedia article: shorter lecture script: http://www.iup.uni-heidelberg.de/institut/studium/lehre/AquaPhys/docMVEnv3_13/AqSysSkript_part2_chap1.pdf http://www.iup.uni-heidelberg.de/institut/studium/lehre/Aqua... long looooong version (read this and become a hydroscientist long): http://www-naweb.iaea.org/napc/ih/IHS_resources_publication_hydroCycle_en.html http://www-naweb.iaea.org/napc/ih/IHS_resources_publication_...
- yread 13y ago
  Thanks for this comprehensive response (I didn't expect any less from HN ;) I understood the part about O18 ending up less in the glaciers but I couldn't really grasp that if the water is in glaciers it isn't in the sea. I also thought it would be impossible to get rid of influence of the temperature (gradient with depth, latitude, ocean currents, ...) but it's probably not impossible, just difficult. Also for anybody who doesn't click in the "loooooong" version has 6 volumes...

by yread 13y ago

ovis 13y ago

Lighter isotopes preferentially get deposited on and form the ice sheets, so increases in ocean d18O correlate with glaciations.

kaybe 13y ago

The Rayleigh process describes the continous depletion of a reservoir through a fractionationing process - evaporation. It comes from the fact that heavier isotopes need slightly more energy for a phase change and thus get enriched in the fluid phase. In the case of water, it is mainly the stable O18 (as opposed to the abundant O16). - Lighter isotopes evaporate easier and precipitate down on land. If they don't return to the ocean for a long time due to freezing and glaciation, the ocean has a higher ratio of heavy to light isotopes. More ice means lower ocean level, and since the ratio in the ocean is dependent on the missing water mass, calculations directly give the sea level. Now how do we know what the isotope ratio was in the past? In the oceans, there are those small animals (foraminifera) that build the oxigen from water into their shells. The Wikipedia article you found discusses this. (Another fractionation process, add more calculations here. Additional temperature dependance.) When they die, they float to the ocean floor and, in time, form sediments and eventually lime rock, which can be drilled for probes. Furthermore, different kinds live in different depths, giving us even more information. This can be used for the past 70 million years, since that's the age of the oldest ocean floors. Almost (!) everything older has been subducted again. However, it is not easy to disentangle the temperature effect from the ice volume effect. You can check those out, if you're interested, they explain more thoroughly than the wikipedia article: shorter lecture script: http://www.iup.uni-heidelberg.de/institut/studium/lehre/AquaPhys/docMVEnv3_13/AqSysSkript_part2_chap1.pdf http://www.iup.uni-heidelberg.de/institut/studium/lehre/Aqua... long looooong version (read this and become a hydroscientist long): http://www-naweb.iaea.org/napc/ih/IHS_resources_publication_hydroCycle_en.html http://www-naweb.iaea.org/napc/ih/IHS_resources_publication_...

yread 13y ago
Thanks for this comprehensive response (I didn't expect any less from HN ;) I understood the part about O18 ending up less in the glaciers but I couldn't really grasp that if the water is in glaciers it isn't in the sea. I also thought it would be impossible to get rid of influence of the temperature (gradient with depth, latitude, ocean currents, ...) but it's probably not impossible, just difficult. Also for anybody who doesn't click in the "loooooong" version has 6 volumes...