Geochemical temperature scales

In 1947 Harold Urey, the 1934 Nobel Laureate, recognized that the temperature dependence of the isotope exchange equilibrium between water and calcite (the principal mineral in marine limestones) could be employed as a paleo-thermometer. At 298.15 K the fractionation factor for calcite-water is 1.0286, 

One of the more important applications of the carbonate paleo-temperature scale was to aid our understanding of the climate changes Earth has experienced during the last 3 million years, a period of intermittent glaciation. 8lsO data from a Caribbean sedimentary core corresponding to the most recent 700,000 years are shown in Fig. 9.2. The repeated variations in 8lsO of amplitude 1.6%o and 

Selected values of (1000 A)1/2 for quartz = mineral lsO, and pyrite = mineral 34S equilibria are found in Table 9.2. In other cases, especially at lower temperature, the temperature dependence of IE s on mineral formation may be more complicated and require empirical fits of the form, in a = Ci/T + C2/T2 or in a = Co + Ci/T + C2/T2, the C s are empirical least squares fitting parameters. 

9 Isotope Effects in Nature Geochemical and Environmental Studies 

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Catagenesis Geochemical reactions that occur in the sediments following burial on time scales greater than 1000 years and at temperatures of 50 to 150°C. 

The main trend of the geochemical reactions is toward reduction (de functionalization) that is, they follow a path in a reductive a- xis. Their time scale is millions of years and most of these reactions occur in solid phase. The reactions take place, generally, at high pressures, although temperatures very seldom surpass 1509C. As a consequence of this set of conditions (and the proper "reagents") the main (final) product of these reactions are hydrocarbons. By the same reasons, the universe of laws that rule geochemistry is quite different from the ones that rule conventional chem istry. We can think, indeed, that it is, as far from conventionaT chemistry as this is, e.g., from excited state chemistry. 

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