Sediments, Oxygen Isotopes, and Ancient Temperatures

Water vapor enriched in oxygen-16 is transported by wind in the atmosphere from the sea to land. When the water vapor condenses and precipitates as rain, snow, or hail, the water becomes rich in oxygen-16. Eventually the oxygen-16 rich water is incorporated into rivers, lakes, glaciers, and polar ice, which are, therefore, also rich in oxygen-16. Thus the isotopic composition of groundwater and the water of rivers, lakes, and glaciers is not the same as in seas and oceans. 

Whenever atmospheric carbon dioxide dissolves in groundwater or in the water of rivers, lakes, and glaciers, it also reacts with the water to form mild carbonic acid, which renders the water slightly acidic  

The carbonic acid thus formed is rich in oxygen-16. The mildly acid ground-water as well as the water of rivers and lakes, which is, therefore, also enriched In oxygen-16, dissolves limestone from surrounding rocks, to form calcium bicarbonate, which is soluble in water  

Whenever natural water is heated and evaporates, some of the calcium bicarbonate reconverts to calcium carbonate and reprecipitates as limestone  

It follows that, depending on the temperature, whenever limestone precipitates, it includes the oxygen isotope in which the water is rich in sea waters, for example, the limestone is enriched in oxygen-18  

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Determining the temperature at which ancient sediments were formed entails two successive experimental stages (1) extracting the carbon dioxide from the sediments and (2) determining the relative amounts of the oxygen isotopes in the extracted carbon dioxide. Treating a sediment with a standard acid, such as hydrochloric or sulfuric acid, dissolves the calcium and magnesium carbonates in the sediments and... 

Figure 7 Natural variations in oxygen isotope abundances. The values in ancient precipitation (Greenland Ice, Antarctic Ice, etc.) and in forameniferal carbonate from sediments provide an excellertt record of the temperatures of the past.

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