Читать книгу Earth Materials - John O'Brien - Страница 74

Three isotopes of oxygen occur in Earth materials (Chapter 2): oxygen‐18 (¹⁸O), oxygen‐17 (¹⁷O), and oxygen‐16 (¹⁶O). Each oxygen isotope contains eight protons in its nucleus; the remaining mass results from the number of neutrons (10, 9, or 8 respectively) in the nucleus.¹⁶O constitutes >99.7% of the oxygen on Earth,¹⁸O constitutes ~0.2%, and¹⁷O is relatively rare. The ratio¹⁸O/¹⁶O is widely used to infer important information concerning Earth history.

During evaporation, water with lighter¹⁶O is preferentially evaporated relative to water with heavier¹⁸O. During the evaporation of ocean water, water vapor in the atmosphere is enriched in¹⁶O relative to¹⁸O (lower¹⁸O/¹⁶O) while the remaining ocean water is preferentially enriched in¹⁸O relative to¹⁶O (higher¹⁸O/¹⁶O). Initially (Epstein and Mayeda 1953), these ratios were related to temperature because evaporation rates are proportional to temperature. It was proposed that higher¹⁸O/¹⁶O ratios in ocean water record higher temperatures, which cause increased evaporation and preferential removal of lighter¹⁶O. It was quickly understood that organisms using oxygen to make calcium carbonate (CaCO₃) shells could preserve this information as carbonate sediments accumulated on the sea floor over time. Such sediments would have the potential to record changes in water temperature over time; especially when the changes are large and the signal is clear (see Box 3.1).

However, it was soon realized that small, short‐term temperature signals could be largely obliterated by a second set of processes. These involve changes in global ice volumes associated with the expansion and contraction of continental glaciers, e.g., during ice ages. Glaciers expand when more snow accumulates each year than is ablated (Chapter 12). This produces a net growth in glacial ice volume. Because atmospheric water vapor largely originates by evaporation, the snow (eventually converted to ice) is enriched in¹⁶O and has a low¹⁸O/¹⁶O ratio. As glaciers expand, they store huge volumes of water with low¹⁸O/¹⁶O ratios, causing the¹⁸O/¹⁶O ratio in ocean water to progressively increase. As a result, periods of maximum glacial ice volume correlate with global periods of maximum¹⁸O/¹⁶O in marine sediments. Prior to the use of oxygen isotopes, the record of Pleistocene glaciation was known largely from glacial till deposits on the continent, and only four periods of major Pleistocene glacial expansion had been established. Subsequently, the use of oxygen isotope records from marine sediments and ice (H₂O) cores in Greenland and Antarctica has established a detailed record that involves dozens of glacial ice volume expansions and contractions during the Pliocene and Pleistocene.

¹⁸O/¹⁶O ratios are generally expressed with respect to a standard in terms of δ¹⁸O. One standard is the¹⁸O/¹⁶O ratio in a belemnite from the Cretaceous Pee Dee Formation of South Carolina, called PDB. δ¹⁸O is usually expressed in parts per thousand (mils) and calculated from: