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NOVA scienceNOW: Mass Extinction

Viewing Ideas

Before Watching

1. Show that water can contain gas. Scientists hypothesize that, in the Permian period, the world's oceans became depleted of oxygen, setting off a chain of events leading to the Permian extinction. Students need to understand that atmospheric gases, such as oxygen and carbon dioxide, are soluble in water. How much of a particular gas dissolves in water depends on the temperature of the water and on the pressure of that gas above the water. The following demonstrations give students a direct experience with the fact that gas can dissolve in water.

   Demonstrate that water contains gas by opening a fresh bottle of carbonated water. Students will see gas bubbles form in the water the instant you release the pressure. And if you place a plastic bag over the top of the bottle, students will see that it fills with gas. This gas is carbon dioxide. The high pressure inside the bottle causes more carbon dioxide to dissolve in the water than would dissolve at typical ground-level atmospheric pressures. In a second demonstration, ask students if they think that tap water contains dissolved gas. Show them a glass that you had filled with tap water the previous day. They will see that the inside of the glass is lined with bubbles. Ask students to explain where these bubbles came from. (The bubbles form as gas comes out of solution. Tap water is pressurized to help it flow through the water pipes. Pressure influences the amount of gas dissolved in water. In the open glass, there is less pressure than in the water pipes, and some of the dissolved gases come out of solution, forming bubbles.)

2. Compare how differently hot and cold water retain dissolved gas. Temperature has a strong effect on how much gas can be dissolved in water. As water temperatures rise, the amount of gas dissolved in the water drops. The hypothesis explored in the segment is that the oceans warmed to the point that they were severely depleted of oxygen. If the amount of dissolved oxygen becomes critically low, aquatic life that requires oxygen, like fish, will die. Scientists contend that low oxygen levels in the world's oceans at the end of the Permian period set off a chain of events that led to the mass extinction of 95 percent of life on Earth.

   Use the following demonstration to show how temperature affects the amount of gas that can dissolve in water. (Consider having students do this activity in small groups.) Fill a bowl three-quarters full with hot tap water and another bowl three-quarters full with cold tap water. Fill two plastic cups halfway with any type of carbonated soda or sparkling water. Set a cup into each of the bowls and observe. (Make sure that the water in the bowls does not get into the cups and that there is not so much water in the bowls that it spills out when you put in the cups.) Ask students to compare how the water in the two cups behaves. (In the hot water, the bubbles will be bigger and rise to the surface more quickly than in the cold water.) In which cup will the soda go flat first? (They both will go flat, but the soda in the hot-water bath will go flat sooner than the soda in the cold-water bath.)

3. Find the site of the Siberian Traps volcanoes. Using atlases or Web sites, such as palaeo.gly.bris.ac.uk/Palaeofiles/Permian/SiberianTraps.html, locate the Siberian Traps and identify the geographical area covered by lava. As occurs in every volcanic eruption, the Traps' million-year continuous eruptions released carbon dioxide. Discuss the climatic effects of adding huge quantities of carbon dioxide to the atmosphere. Consider having students research recent volcanic eruptions (e.g., Mount St. Helens, Mt. Pinatubo, and Soufriere Hills in Montserrat) and find out how much carbon dioxide was released. Then ask students to estimate how much carbon dioxide a million years worth of erupting would have expelled.

After Watching

1. Develop a flowchart of the events that led to the Permian extinction. Reinforce the events described in the segment. On the board, write the phrases below in alphabetical or random order. Have students draw a flowchart, putting events in the correct sequence.

   • Anaerobic bacteria thrive in the oceans and produce hydrogen sulfide as a waste product
   • Atmospheric carbon dioxide levels increase
   • Atmosphere warms
   • Dissolved oxygen levels in the oceans drop
   • Hydrogen sulfide accumulates in the oceans and atmosphere
   • Most aquatic life that depends on oxygen dies
   • 95 percent of Earth's life is killed by hydrogen sulfide
   • Oceans warm
   • Volcanoes erupt

   (Volcanoes erupt -> atmospheric carbon dioxide levels increase -> atmosphere warms -> oceans warm -> dissolved oxygen levels in the oceans drop -> most aquatic life that depends on oxygen dies -> anaerobic bacteria thrive in the oceans and produce hydrogen sulfide as a waste product -> hydrogen sulfide accumulates in the oceans and atmosphere -> 95 percent of life on Earth is killed by hydrogen sulfide)

2. Grow hydrogen sulfide-producing bacteria. The early Earth had little free oxygen. Consequently, the first organisms were anaerobic, and sulfate-reducing bacteria are among the oldest life forms on Earth. To obtain energy, these bacteria oxidize organic matter and, in the process, produce hydrogen sulfide, the gas that gives rotten eggs their smell. Today, these bacteria are still common in low-oxygen environments, such as in swamps, marshes, standing waters, and the ocean floor. Ask students if they know the smell of mud from swamps. This mud often has a detectable sulfur odor. Hydrogen sulfide-producing bacteria also live in the human colon. The odor of flatus is largely due to trace amounts of hydrogen sulfide. Hydrogen sulfide is toxic to several systems in the body, although the nervous system is most affected. The gas stops cellular respiration by blocking oxygen from binding with mitochondrial enzymes. Share the information in the table below with students and have them speculate as to what concentration of hydrogen sulfide would be required to cause an extinction on a par with the Permian extinction.

   Parts per million	Effect of Hydrogen Sulfide on People
   0.0047	People can detect the characteristic rotten egg odor of hydrogen sulfide
   10-20	Eye irritation, sore throat, shortness of breath, and fluid in the lungs
   50-100	Eye damage
   150-250	Sense of smell deadened, making a person unaware of the danger
   320—530	Fluid fills the lungs to potentially lethal levels
   530-1,000	Strong stimulation of the central nervous system, leading to loss of breathing
   800	Lethal after 5-minute exposure
   Above 1,000	Lethal after a single breath

   Collect some mud from anaerobic environments, such as roadside ditches, ponds, swamps, marshes, or bodies of standing water. Gather the gooey sediments from a few inches beneath the surface. A sulfur odor will tell you there are sulfate-reducing bacteria. Put the mud in a clear container, such as a peanut butter jar, soda bottle, or baby food jar. Add water until there is an inch of water over the mud. Cover the container with plastic wrap and secure with a rubber band. Put the container in a sunny spot for several weeks, adding water as necessary. After three weeks or so, colorful colonies of bacteria will become visible at different levels in the mud. The purple-colored ones toward the bottom are purple sulfur bacteria and the green-colored ones below them are green sulfur bacteria. Both produce hydrogen sulfide as a waste product.

3. Examine historic levels of carbon dioxide. Scientists in the segment suggest that high levels of carbon dioxide caused major global warming that led to the Permian extinction. Copy the following tables for students. Have them make a line graph of the 1960-2004 data. Discuss the trend, comparing the modern levels with the historic carbon dioxide levels measured in gas bubbles trapped in Antarctic ice. (It is assumed that the gas in these bubbles reflects the composition of the atmosphere that existed when the ice formed.) Use the Antarctic ice data to calculate the average level for the past 400,000 years. (About 222 parts per million) Discuss how levels today compare with historic levels. (Today's are much higher) Over the past 400,000 years, what is the range of variation in atmospheric carbon dioxide levels? (Atmospheric carbon dioxide levels have ranged from 189.2-278 parts per million)

   Table 1: Carbon dioxide measurements from the Mauna Loa observatory

   Year	CO2 (ppm)		Year	CO2 (ppm)		Year	CO2 (ppm)		Year	CO2 (ppm)		Year	CO2 (ppm)
   1960	316.9		1970	325.7		1980	338.7		1990	354.2		2000	369.5
   1961	317.6		1971	326.3		1981	339.9		1991	355.6		2001	371.0
   1962	318.5		1972	327.5		1982	341.1		1992	356.4		2002	373.1
   1963	319.0		1973	329.6		1983	342.8		1993	357.1		2003	375.6
   1964	319.5		1974	330.3		1984	344.4		1994	358.9		2004	377.4
   1965	320.1		1975	331.2		1985	345.9		1995	360.9
   1966	321.3		1976	332.2		1986	347.1		1996	362.6
   1967	322.1		1977	333.9		1987	349.0		1997	363.8
   1968	323.1		1978	335.5		1988	351.4		1998	366.6
   1969	324.6		1979	336.9		1989	352.9		1999	368.3

   http://cdiac.ornl.gov/trends/co2/sio-mlo.htm

   
   

   Table 2: Carbon dioxide levels found in gas bubbles trapped in Antarctic ice

   Years before present	Research team led by Petit in 1999	Research team led by Barnola in 1998
   10,000	261.6	261.6
   20,000	191.6	189.2
   30,000	205.4	205.4
   40,000	209.1	209.1
   50,000	215.7	215.7
   60,000	210.4	210.4
   70,000	227.4	227.4
   80,000	221.8	221.8
   90,000	208	208
   100,000	225.9	225.9
   150,000	191.9	191.9
   200,000	242.6	242.6
   250,000	203.9	203.9
   300,000	241.9	251.7
   350,000	193	193
   400,000	278	278

   http://www.ncdc.noaa.gov/paleo/icecore/antarctica/vostok/vostok_co2.html

Web Sites

Evolution Library: Permian-Triassic Extinction
www.pbs.org/wgbh/evolution/library/03/2/l_032_02.html
Presents a short video segment in which rock layers laid down during the Permian and Triassic periods are analyzed.

Mass Extinctions of the Phanerozoic Menu
hannover.park.org/Canada/Museum/extinction/extincmenu.html
Describes mass extinctions and considers the causes of each one.

The Discovery of Global Warming
www.aip.org/history/climate/index.html#contents
Provides links on many climate change topics, including the carbon dioxide greenhouse effect.

The Permo-Triassic Extinction
palaeo.gly.bris.ac.uk/Palaeofiles/Permian/intro.html
Outlines the causes of Earth's major extinctions.

Books

The Visual Dictionary of Earth
by Geoffrey Stalker (Project Editor). Dorling Kindersley, 1993.
Includes information on Earth's geology, oceans, volcanoes, and atmosphere.

Volcanoes and Earthquakes
by Susanna Van Rose. Dorling Kindersley, 2004.
Explains how volcanoes and earthquakes occur and contains several diagrams and photographs.

Volcanoes
by Robert and Barbara Decker. W.H. Freeman and Company, 1997.
Provides detailed information about the geology of volcanoes and includes an appendix on the most notorious volcanoes.