Lecture 12  Permian - Triassic Extinction Event and Recovery

Focus Question:  What were the major causes of the Permian-Triassic extinction event?

1.        The extinction event that marked the boundary between the Paleozoic and the Mesozoic Eras ( the end of the Permian Period and the beginning of the Triassic Period) was the biggest extinction event in Earth’s history.  Although there is some variability in the way species extinctions are totaled, the general agreement among paleontologists is that the Permo-Triassic extinction event caused about 90% of marine taxa and about 70% of terrestrial taxa to become extinct.

a.         What are current hypotheses regarding the cause(s) of this event?

b.      What evidence is still available that we can use to test these hypotheses?

c.       Does any of this evidence support these hypotheses?

2.        Setting the Stage:  The Permian Period had certain characteristics we have already seen:

a.        The supercontinent Pangaea, extending from pole to pole, with quite a lot of continent in the equatorial and subequatorial region

b.      A steeply declining atmospheric oxygen percentage, dropping from the all time high in the Pennsylvanian.

c.       A rising atmospheric carbon dioxide percentage, increasing from a low in the Pennsylvanian.

d.      Extensive deserts  in continental interiors

3.        What kinds of processes might produce a global extinction event ?  Processes that can be global, too.

a.        Something that could affect the atmosphere:  temperature, composition

b.      Something that could affect the ocean:  temperature, circulation

c.       Something that could affect both atmosphere and ocean.

4.        What kind of evidence?

a.        Oxygen drop and carbon dioxide rise – see previous lectures on late Paleozoic.

b.      Loss of forests-the “fungal spike” – a steep rise in the abundance of fungal spores indicating a lot of decay of plants, especially trees.

c.        Stagnation of the deep ocean-a drop in dissolved oxygen in the deep ocean, marked by a change from red sediments (oxygen reached those sediments) in early Permian to grey-black (no oxygen) in late Permian, and by mid-Triassic, red sediments again.  (Remember we have seen this kind of pattern before in the BIFs)

5.        What big event at about 250 million years ago could potentially affect both atmosphere and ocean?  We need to consider the effect of a big flood basalt called the Siberian Traps (trap is from the Swedish word for step-the lava flowed out in layer upon layer, like steps).  During Permian-early Triassic  time, Siberia was almost where it is today-but much of it was a big coal swamp.  An enormous volcanic flood basalt formed there, as lava erupted from a “hotspot” plume.  The lava poured out across the region, creating the enormous flat-topped mountain range known as the Siberian Traps.  You can see the images I showed in class on the Siberia Blog posted  on the NASA website here  http://earthobservatory.nasa.gov/Features/SiberiaBlog2008/page9.php .

6.        What happens when a regional coal swamp fills with lava?  The organics “cook” and the carbon is vaporized.  Can we trace this somehow?  We can.  Remember that carbon has two stable isotopes, carbon-12 and carbon-13.  Plants prefer the lighter of the two isotopes (carbon-12), so most of the carbon in those coal swamps was carbon-12.  When that vaporized, carbon-12 in large quantities entered the atmosphere and ultimately the ocean. 

a.        The ratio of carbon-13/carbon-12 can be used to trace the carbon through time.

b.      The more negative (light, low) the ratio is, the more carbon-12 is present

c.       The more positive (heavy, high) the ratio is, the more carbon-13 is present

d.      A negative (light, low) trend in this ratio occurred at the boundary, indicating a big release of carbon-12.

7.        What about meteorites?  Meteorite impacts have long been discussed as a potential cause of the P-Tr extinction event.  There are craters that bracket the Permo-Triassic boundary, but to date, nothing on the boundary itself.  Several craters are from  210 million years and 290 million years, but none at about 250 million years.  Until a crater, or stratigraphic evidence of

8.        So, based on the evidence available, the hypothesis supported currently to explain the Permo-Triassic extinction event is the eruption of the Siberian Traps.  Here’s the way it would work:

Eruption produces carbon dioxide, sulfides, methane, which all affect the atmosphere:

a.       Long term warming, acid rain (bad news for plankton in surface ocean-base of food chain)

b.       Loss of ice sheets, warming of poles, slowing of ocean circulation

c.       Stagnation develops in deep ocean

d.      Plants on land, plankton in ocean suffer, break down of part of food chain on land and sea

The Mesozoic  begins...

9.        The Mesozoic Era is divided into 3 geologic periods:  The Triassic, Jurassic, and Cretaceous.  In the Triassic, the supercontinent Pangaea began to break up.  This break-up would take all of Mesozoic time to complete, yet the process began in mid-Triassic time.  Today, we are going to look at three areas in North America to see how this process proceeded.

a.        Triassic Rift Basins along eastern North America

b.      Shallow seaway in Gulf of Mexico area

c.       Sonoma Orogeny and others like it along the west coast of North America

10.        First, let’s look at the Triassic Rift Basins along eastern North America

a.        These basins are “graben basins”.  They formed as spreading began, and the pull-apart motion broke the crust into blocks, moving along normal faults.  Some blocks moved up relative to others (Horsts), and others down relative to others (Grabens).

b.      The grabens are low areas that fill up with sediments (and water).  These are the graben basins.

c.       As rifting occurred, some of the rifts started, then failed.  One, however, continued to the present day, forming the Atlantic Ocean, spreading from the mid-Atlantic rift.

d.      Look at the map of North America during the mid-Triassic (Ron Blakey’s map )here: http://jan.ucc.nau.edu/~rcb7/namTr230.jpg

Note the strange pattern along the eastern edge of North America.  Those long, narrow

Basins that appear as “rips” are graben basins, and they are the locations of rifting.  Most of the rifts failed, but the sediments and volcanics they contained are still visible.

a.        Road Trip to Patterson, NJ and surrounding area-including NYC.

1.        Pallisades Sill near NYC-Triassic volcanics formed during rifting

2.       Pillow basalts – formed undersea in Triassic rifting, Newark Supergroup

b.      Other graben basins-from Nova Scotia to Virginia, all showing similar features.  Triassic sediments and volcanics

11.        What kind of place on Earth today looks like those Triassic Rift Basins along eastern North America?

a.        East African rift valley lakes, especially the Afar Triangle Region in East Africa.

1.        Narrow  graben basins, filled with sediment and volcanic basalt

2.       Forming as rifting occurs

12.Basins filling with sediment in the Gulf of Mexico area-let’s go south to what is now Arizona

                a.  During Triassic time, Arizona and the 4 corners area (AZ,NM,UT and CO) were receiving a lot of sediment from the west and southwest, but had not yet dried out, as they would do in Jurassic time.

                Look at the 4 corners area map made by Ron Blakey showing the drainage and “green” landscape during this time.  http://jan.ucc.nau.edu/~rcb7/ColoPlatTriChinlePEFO.jpg   Note how the drainage is headed northwest, as opposed to today’s southwest drainage direction.  The sands and silts deposited during this time are part of the Chinle Formation.

a.        Petrified Forest National Park, Arizona-Chinle Formation sands and silts partially cover a huge

Permineralized forest of Triassic Age.

b.       Chinle Formation is exposed in the “painted desert” of this region. 

13.Western North America-a series of mountain building events (orogenies)

                a. The western edge of North America became an “active margin” (it still is).  Mountain building events occurred as slices of small continental lithospheric plate (microcontinents) collided with North America.  Several orogenic events occurred.  An example of these during Triassic-Jurassic time is the Sonoman Orogeny (named for Sonoma, California).

                1.  subduction zone formed off coast of North America, with a slab of ocean lithospheric plate subducted beneath “Sonomia”, a microcontinent.  Sediments scraped off of the descending slab formed an accretionary wedge of sediments called the “Golconda Terrane”. 

2.  Both Sonomia and the Golconda Terrane collided with North America, forming a typical mountain building event of that time.  Numerous others occurred as well.

Next Lecture:  Mesozoic Ecology on Land and Sea:  Marine Reptiles and Dinosaurs