National Academies Press: OpenBook

Effects of Past Global Change on Life (1995)

Chapter: The Terminal Ordovician Transition

« Previous: The Eocene-Oligocene Transition
Suggested Citation:"The Terminal Ordovician Transition." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 7

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

OVERVIEW AND RECOMMENDATIONS 7 of the Early Eocene. If mean global temperature was well above that of the modern world, greenhouse warming is perhaps the most likely cause. If warmer high latitudes were accompanied by cooler equatorial conditions, then heat transport may have been the dominant control on the latitudinal temperature gradient. More generally, one can ask how the secular changes in the greenhouse capacity of the atmosphere have interacted with increasing solar luminosity, continental geographies, and orogenic uplift to produce the significant climatic oscillations recorded throughout geologic time. The Younger Dryas Cooling Events in the North Atlantic during the earliest stages of deglaciation between 14,000 and 11,400 years ago represent another example of change in thermohaline circulation. The North Atlantic is part of a circuit that extends to the Pacific—the so-called great conveyor belt (Broecker and Denton, 1989). North Atlantic deep water flows southward and is entrained in the Antarctic Current. It then passes into and through the Indian Ocean and the South Pacific to the North Pacific, where it upwells and returns by surface currents to the North Atlantic. There it loses heat that warms the climate of northern Europe and sinks again. During the emergence from the most recent glacial maximum, there was a brief expansion of glaciers between 11,400 and 10,200 years ago known as the Younger Dryas event. The cause of this reversal was in part a change in the flow of meltwater from North America to the ocean: Surface salinities declined in the North Atlantic because meltwater was diverted from the Mississippi drainage eastward to the St. Lawrence. Mixture with these buoyant waters reduced the density of the waters flowing northward into the North Atlantic and interrupted the formation at the surface of relatively dense NADW until the meltwater pulse had largely ended. Shifts in the isotopic composition of fossil planktonic foraminifera document this sequence of events, especially in the Gulf of Mexico region (see Flower and Kennett, Chapter 12). Plankton assemblages responded to changes in the salinity of surface waters. The Younger Dryas event illustrates how even a relatively small-scale perturbation of thermohaline circulation can have global oceanographic and climatic consequences. The Terminal Ordovician Transition Studies of events at the close of the Ordovician Period, about 440 m.y. ago, illustrate how it is also possible to interpret general causes and consequences of major climatic fluctuations for pre-Cenozoic intervals (see Berry et al., Chapter 2). These events are associated with one of the largest mass extinctions of all time. The stratigraphic record offers evidence that continental glaciation began at this time in the supercontinent of Gondwanaland and that the buildup of glaciers lowered sea- level by at least 50 m. Paleomagnetic data and environmental reconstructions reveal that Gondwanaland was moving over the South Pole. Marine fossils indicate a transition to a hothouse state: Hirnantian fauna, which had previously been restricted to cool water masses of the deep-sea and to high latitudes, expanded over broad regions of the ocean, replacing warm-adapted taxa that became extinct. Shifts to Hothouse Intervals A shift in the opposite direction, from the icehouse to the hothouse state, is associated with the development of warmer polar regions and deep ocean waters that are warm, sluggish, and dysaerobic to anoxic. Such a shift will automatically eliminate much life in the deep-sea. During the mid-Cretaceous highstand of sea-level, anoxic conditions extended upward into the deep portions of epicontinental seas, and it was during this interval that environmental perturbations produced pulses of biotic destruction that constitute the Cenomanian-Turonian mass extinction (see Kauffman,

Effects of Past Global Change on Life Get This Book
Buy Hardback | $65.00 Buy Ebook | $49.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

What can we expect as global change progresses? Will there be thresholds that trigger sudden shifts in environmental conditions—or that cause catastrophic destruction of life?

Effects of Past Global Change on Life explores what earth scientists are learning about the impact of large-scale environmental changes on ancient life—and how these findings may help us resolve today's environmental controversies.

Leading authorities discuss historical climate trends and what can be learned from the mass extinctions and other critical periods about the rise and fall of plant and animal species in response to global change. The volume develops a picture of how environmental change has closed some evolutionary doors while opening others—including profound effects on the early members of the human family.

An expert panel offers specific recommendations on expanding research and improving investigative tools—and targets historical periods and geological and biological patterns with the most promise of shedding light on future developments.

This readable and informative book will be of special interest to professionals in the earth sciences and the environmental community as well as concerned policymakers.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook,'s online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!