chemistry-oxygen isotope and paleoclimatology

Oxygen ¹⁶O and paleoclimatology Solutions
How do we know what the Earth's temperature was millions of year ago?
The isotopic composition of minerals formed in the past and of water trapped in glaciers gives us an indication of the average temperature during which the ice was deposited or minerals were formed. In fact, it is the oxygen isotope composition of ice and mineral deposit that is of immense interest. Oxygen consists of three isotopes: ¹⁶O, ¹⁷O, and ¹⁸O. A large percentage ((99.762%) of the Earth's oxygen is ¹⁶O and the rest we can say is ¹⁸O. A tiny, negligible, amount is ¹⁷O but we will ignore this for simplicity. Although the percentage of ¹⁸O is small we can still measure the ratio of ¹⁸O to ¹⁶O. This means that any mineral containing oxygen, or ice, will have a measurable ratio of ¹⁸O to ¹⁶O atoms. This ratio of the ¹⁸O to ¹⁶O is temperature dependent so we can determine the temperature at the time by simply measuring the ratio of these two isotopes.
The ¹⁶O isotope is lighter than the ¹⁸O and so water molecules with the ¹⁶O isotope travel slightly faster at the same temperature than the water molecules with the heavier isotope. Because of its slightly greater speed it is easier to evaporate the lighter water molecule and so we find that a greater proportion of the lighter molecules will vaporise compared to the heavier molecules.
When water vapour condenses to form rain it is the heavier, less mobile water molecules that form rain. This leaves a greater percentage of the lighter water molecule in the air as water vapour.
The animation below shows how glacial ice forms with a relatively higher percentage of ¹⁶O than ¹⁸O. As water evaporates from the oceans ¹⁶O is concentrated in the atmosphere. As water vapour condensed to form rain the heavier isotope is removed from the atmosphere as rain leaving an even greater percentage of ¹⁶O in the air. As the atmospheric water vapour remaining travels over the poles it freezes to form glaciers.

So, as water vapour travels toward the poles, it is increasingly enriched in ¹⁶O and depleted of ¹⁸O, the extent to which this happens depends on the temperature over the poles. In this case, colder temperatures lead to a greater ¹⁸O depletion.
1) What is an isotope? Isotopes are atoms of the same element that differ in the number of neutrons found in their nucleus. 2) What do isotopes have in common? Isotopes of the same element all have the same number of protons in their nuclei. 3) Two water molecules are at the same temperature, they have the same average kinetic energy. One has an ¹⁶O isotope while the other has an ¹⁸O isotope. Which one travels with greater speed? Explain. The formula for kinetic energy is 1/2mv² If two molecules have the same kinetic energy but one is lighter it must have a greater speed (v) to make up for the lower mass. 4) In order to evaporate water molecules must break forces of attraction that act between water molecules in the liquid state. Fast moving molecules break their bonds easier than slow moving molecules. Explain why clouds are enriched with ¹⁶O isotope. Because lighter molecules travel faster they break the bonds that exist between water molecules, in the liquid state, more readily and so escape as gas. 5) Condensation occurs when water molecules slow down so that forces of attraction take hold and bind water molecules together in the liquid state. Explain why rain is enriched with ¹⁸O isotope. Because the heavier molecules are slower they are able to be held together in the liquid state by the attractive forces between water molecules. 6) Explain why glaciers have a greater percentage of ¹⁶O than sea water found in the equator. As water evaporates from the warm oceans near the equator it drifts towards the poles where it condenses and falls as rain. Rain contains a higher percentage of ¹⁸O than the water vapour it condensed from. This leaves an ¹⁶O enriched water vapour to flow to colder parts where it freezes to form glaciers. 7) How can the temperature of a region that prevailed millions of years ago be estimated using oxygen isotopes? The ratio of ¹⁶O to ¹⁸O depends on temperature. The concentration of ¹⁸O in rain water decreases as temperature also decrease. From this we can calculate with accuracy the temperature at the time glacial ice was deposited.
8) What impact would a colder world temperature have on the percentage of ¹⁸O in fossil deposits? Fossil deposits would be ¹⁸O enriched. As the world cools more glaciers form which are ¹⁶O enriched thus the ocean waters would be ¹⁸O and hence we would expect a greater proportion of ¹⁸O in mineral samples of fossils.
9) Explain why glaciers formed from water vapour in the polar regions become ¹⁸O depleted as the climate cools while fossils become ¹⁸O enriched. As water vapour moves towards the polar regions it becomes ¹⁶O enriched. This is because condensation slightly favours the ¹⁸O isotope, for the reasons mentioned above. Glaciers that form from the water vapour in the air are also ¹⁶O enriched. The water that remains in the oceans is ¹⁸O and so minerals forming in the ocean would have a higher proportion of ¹⁸O. .