Читать книгу Earth Materials - John O'Brien - Страница 75

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 high evaporation rates, PDB has an unusually high¹⁸O/¹⁶O ratio. As a result, most Pliocene–Pleistocene samples have a negative δ¹⁸O, with small negative numbers recording maximum glacial ice volumes and larger negative numbers recording minimum glacial ice volumes. Because different organisms selectively fractionate¹⁸O and¹⁶O, a range of organisms must be analyzed and the results averaged when determining global changes in¹⁸O/¹⁶O. Nothing is ever as easy as it first seems.

It should be noted that many δ¹⁸O analyses have used a different standard. This standard is the average¹⁸O/¹⁶ ratio in ocean water known as standard mean ocean water (SMOW). Of course, that has been complicated by global warming, that generally increases evaporation rates, changing the ratios in natural waters.

Because the original SMOW and PDB standards have been used up in comparative analyses, yet another standard, Vienna standard mean ocean water (VSMOW), is also used. This name is misleading as the Vienna standard is actually a pure water sample with no dissolved solids. There is currently much discussion concerning the notion of which standards are most appropriate and how δ¹⁸O and other isotope values should be reported.

Oxygen isotope analyses, as well as carbon isotopes discussed below, contribute essential data regarding Earth's paleoclimate as well as dynamic climate changes currently affecting Earth.