Lecture 19

Hothouse Conditions of the Early Cenozoic: The PETM

1.  The early Cenozoic contains an abrupt climate change event that is important as a potential modern analog of greenhouse warming.  It is known as the Paleocene-Eocene Thermal Maximum -PETM-and begins about 56 million years ago. 

a. Abrupt climate changes are changes that have a sudden start, although they may continue for thousands and tens of thousands of years-the sudden change is what makes it "abrupt".

b.  The PETM is the warmest episode in the Cenozoic, and one of the warmest episodes ever in the Phanerozoic Eon.

c.  It involved global warming to such an extent that the planet became ice free, no ice at the poles, and tropical to subtropical conditions prevailed from equator to pole. 

d.  We will consider today what caused this event, how long did it last, and what caused cooling to recommence and when did sea ice return?

2.  The time scale of the Cenozoic is shown here, with the Epochs marked along the horizontal axis.

a.  The average air temperature for the Cenozoic shows that the early Cenozoic is much warmer than today.

b.  The PETM is marked by a sudden spike, followed by the general warmth of the early Eocene, until about 50 million years ago (mya).

c.  10-15 degrees centigrade warmer conditions prevailed, with tropical fossils found from equator to pole.  Sedimentary rocks associated with the warm/rainy conditions of the tropics are found at high latitudes, such as coals, bauxite (source of aluminum ore-heavily weathered silicates), and evaporites.

3.  The Eocene Epoch begins warm, and ends cool. 

a.  By about 50 million years ago, the Eocene warmth has started to fade, and cooler conditions have begun.

b.  By 40 to 45 million years ago, the Arctic Sea ice has returned to the Arctic Ocean, and cooling is well underway.

4.  We are going to look at three Eocene sites at different latitudes-low, mid, and high latitudes, and consider the kind of environment that is represented.

a.  Bighorn Basin, Wyoming

b.  Ellesmere Island, NWT, Canada, in the high Arctic

c.  Florida-the carbonate platform that is shown to be submerged in shallow marine conditions in the Eocene

5.  We begin with the Bighorn Basin, and the study of the fossil leaves.

a.  leaf size and shape is indicative of average temperatures.  Leaves of tropical plants are larger, have smooth edges, and a drip point to focus rain runoff.

b.  Leaves of mid to higher latitudes are smaller (often needles, as in the case of conifers) and don't have smooth edges.

c.  The difference is so marked that it is possible to use fossil leaves as a temperature estimator for past climate change episodes, like the PETM.

6.  Watch the YouTube interview with Scott Wing, a paleobotanist at the Smithsonian and see his fieldwork locality in the Bighorn Basin.

http://www.youtube.com/watch?NR=1&v=C9bfVKkqIoA

7.  Now let's travel up to Ellesmere Island in the high Arctic, and learn what the PETM and early Eocene environment was like through the study of the Canadian Museum of Nature

http://www.youtube.com/watch?v=AW8PQOw3jNg

8.  Finally, let's head to Florida and learn what the PETM and early Eocene environment was like through the study of the Florida Museum of Natural History

http://www.youtube.com/watch?v=pYSC6623K7I&feature=related

9.  CO2 in the atmosphere was extremely high during the PETM and early Eocene, but then dropped around 50 mya.  Shortly after that, sea ice returned to the Arctic.  The high and mid latitudes started to cool down, never to reach the high temperatures and high CO2 values of the PETM and early Eocene again. 

a.  in 2004, an Ocean Drilling Expedition to the Arctic Ocean made an important discovery about how that CO2 drop around 50 mya occurred.

b.  They discovered that in sediments of PETM and early Eocene age in the Arctic Ocean, enormous amounts of a freshwater, warm climate aquatic fern were present.

c.  The aquatic fern is Azolla, and is common today in warm, rainy environments in freshwater.  Yet here it was in huge amounts in the early Eocene in the Arctic!

d.  Azolla in such large amounts has an important effect on the environment-it draws CO2 from the atmosphere to conduct photosynthesis, but then, the plant dies and sinks

to the bottom, taking its Carbon with it.  Thus, it becomes a one-way sink for carbon dioxide, removing it from the atmosphere and burying it in the ocean sediments.

10.  Working hypothesis for why the Azolla event occurred at all: 

a.  The warm, ice free world of the early Eocene had a big effect on the small basin of the Arctic Ocean.  It is a relatively small, enclosed basin, with only one big exit (into the Atlantic), and alot of rivers that bring freshwater in. 

b.  warm, wet, early Eocene supplied freshwater and nutrients by way of the rivers to the Arctic

c.  Warm freshwater lay as a lens on the surface of the more dense ocean water, and was the perfect environment for Azolla.

d.  When temperatures cooled, enough, Azolla died off, sea ice formed, and the long term cooling got underway.

11.  Lets see this interview with Jan Beckman, one of the scientists on the IODP expedition to the Arctic that discovered the Azolla Event

http://www.youtube.com/watch?v=lH-Pn40N84k

12.  Proposed Causes of the PETM-what could have abruptly put so much carbon into the atmosphere, so rapidly? 

a.  Three hypotheses have been proposed:  volcanism and flood basalts (e.g., think Deccan Traps and Siberian Traps, for example), gas hydrates, and meteorite impacts.

b.  There are several meteorite impacts in the Eocene, but they are all in the late Eocene, AFTER the PETM and the early warm Eocene, so meteorites have been dropped from

consideration at the moment.

c.  The prime suspects are volcanism and flood basalts, and/or gas hydrates as the suppliers of this huge amount of CO2.

13.  What are gas hydrates?

a.  Gas hydrates are deposits of methane gas (CH4)  made by bacterial decay that are trapped in a "frozen" state in coastal shelf sediments along the edges of continents. 

b.  They are not stable-the gas is "frozen" in a cage of water molecules that is maintained by cold temperatures and overlying pressure of the ocean water.

c.  When temperature rises, or pressure is released, or both, the gas is freed and is released into the surrounding water column.

d.  CH4 oxidizes rapidly to CO2 and water when it meets any oxygen, either in the ocean or in the atmosphere.  Both methane and CO2 are greenhouse gasses, so this is a source of that sudden release of carbon.  Good possibilities for destabilizing these deposits and causing abrupt releases include

a.  warming deep ocean

b.  seismic activity-earthquakes, underwater landslides

14.  Gas hydrates are also important for another reason-they are huge untapped sources of clean energy-very pure methane gas that, if safely tapped, could provide significant sources of clean energy.  There is far more gas hydrate potential than all the fossil fuels we have used so far put together, so it is of great interest to the U.S. and the world to work on development of these deposits. 

15.  So the onset of the PETM was abrupt, and the hypotheses about the source of this abrupt warm event include Carbon loading from CO2 and/or methane, from volcanism and/or gas hydrates.  The early Eocene remained warm until about 50 mya, when cooling began, and by about 45 mya, sea ice returned to the Arctic.  The trigger for this cooling is hypothesized to be the impact of the Azolla Event (and probably similar events at this time).

16.  Next Lecture:  The Miocene Epoch:  The turning point-

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