Earth Materials. John O'Brien

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      3.3.1 Stable isotopes

      Stable isotopes contain nuclei that do not tend to change spontaneously. Instead, their nuclear configurations (number of protons and neutrons) remain constant over time. Many elements occur in the form of multiple stable isotopes with different atomic mass numbers. In many cases, these isotopes, because of their different mass, exhibit subtly different behaviors in Earth environments. These differences in behavior are recorded as differences in the ratios between isotopes that can be used to infer the conditions under which the isotopes were selectively incorporated into Earth materials. We will use oxygen and carbon isotopes to illustrate the uses of stable isotope ratios to increase our understanding of Earth materials and processes. Other stable isotopes that are commonly utilized in such studies include those of sulfur, nitrogen, and helium (Chapter 13, Box 13.2).

       Oxygen isotopes

      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:

Box 3.1 The Paleocene–Eocene thermal maximum

      In the mid‐nineteenth century, scientists recognized a rapid change in mammalian fossils that occurred early in the Tertiary era. The earliest Tertiary epoch, named the Paleocene (early life), was dominated by archaic groups of mammals that had mostly been present during the preceding Mesozoic Era. The succeeding period, marked by the emergence and rapid radiation of modern mammalian groups, was called the Eocene (dawn of life). The age of the Paleocene–Eocene boundary is currently judged to be 55.8 Ma. Later workers noted that the boundary between the two epochs was also marked by the widespread extinction of major marine groups, most prominently deep‐sea benthic foraminifera (Pinkster 2002; Ivany et al. 2018). The cause of these sudden biotic changes initially remained unknown. Oxygen and carbon isotope studies have given us some answers.

      Kennett and Stott (1991) reported a rapid rise in δ¹⁸O at the end of the Paleocene, which they interpreted as resulting from a rapid rise in temperature, since they believed that no prominent ice sheets existed at this time. Subsequent work (e.g., Zachos et al. 1993; Rohl et al. 2000; Gehler et al. 2016; Ivany et.al. 2018) has confirmed that temperatures rose ~6–8 °C at high latitudes and ~3–5 °C at low latitudes over a time interval not longer than 10 000 years. Rapid global warming, in this case the Paleocene–Eocene thermal maximum (PETM), has apparently occurred in the past, with significant implications for life on Earth. Researchers have also shown that the higher temperatures lasted for approximately 100 Ka (Pinkster 2002; Ivany et al. 2018). How long will the current period of global warming last?

      What caused the rapid global warming? Researchers studying carbon isotopes have shown that the sudden increase in temperature implied by rising δ¹⁸O values was accompanied by sudden decreases in δ¹³C. Several hypotheses have been suggested for this, most of which involve warming and the release of large quantities of¹²C from organic carbon reservoirs. Two rapid spikes in negative δ¹³C, each occurring over time periods of less than 1000 years, suggest that some releases were extremely rapid. The currently favored hypothesis involves the melting of frozen clathrates in buried ocean floor sediments. Clathrates consist of frozen water in which methane, methanol, and other organic carbon molecules are trapped. The hypothesis is that small amounts of warming caused clathrates to melt, releasing large volumes of methane to the atmosphere in sudden bursts. This would account for the sudden negative δ¹³C spikes. Because methane (CH₄) is a very effective greenhouse gas (10–20 times more effective than CO₂), this theory also accounts for the sudden warming of Earth's surface and the extinction and mammalian radiation events that mark the Paleocene–Eocene boundary. Of course scientists wonder if the current episode of global warming that already involves an order of magnitude larger release of CO₂ (Ivany et al. 2018) might be accelerated by triggering a sudden release of clathrates, and how long the effects of such releases might linger.

      Because the Cretaceous was an unusually warm period in Earth's history, with СКАЧАТЬ