Wednesday, March 3, 2010

Mountain building and climate change

I was talking to some students recently about a large mass extinction event which occurred at the end of a period of time that geologists call the Ordovician Period (around 444 million years ago).  The extinction event correlates fairly well to evidence we have of an ice age occurring on a southern supercontinent called Gondwana around that time.

(What most people think of as the "Ice Age" occurred between 2 million years ago and 10 thousand years ago and is only one of several ice ages the Earth has had in its long history).

Continental glaciation lowers sea levels globally (water evaporates from oceans, falls as snow, and doesn't flow back into the oceans).  During the Ordovician, much of North America was actually under water (epicontinental seas).  The cooling Earth had colder oceans (stressing marine life) and dropping sea levels exposed shallow sea floors killing off the animals living there.

North America during the Late Ordovician Period (450 Ma)

North America during the Early Silurian Period (430 Ma)
Dr. Ron Blakey (http://jan.ucc.nau.edu/~rcb7/nam.html)

One of the interesting questions one can ask about glaciation is "Why then?"  What was significant about this period of time such that global temperatures dropped?  Well there is some good evidence that global carbon dioxide (CO2) levels dropped significantly before this glaciation event (this was a time when carbon dioxide levels in the atmosphere may have been 15 times what they are today but that's a different story).

Anyway, this drop in carbon dioxide levels seems to correlate with an event occuring right here in New York at the time - the Taconic Orogeny (from the Greek "oros" meaning mountain and "genesis" for birth or origin).  During the latter part of the Ordovician Period, a chain of volcanic islands collided with the edge of the proto-North American continent due to plate tectonic convergence.  This collision thrust up Rocky Mountain sized peaks just east of where I'm sitting right now in the mid-Hudson Valley.  These were the beginning of the Appalachian Mountains.

Before the collision, there were volcanoes with massive eruptions spewing huge amounts of carbon dioxide into the Earth's atmosphere (several of these events spewed ash layers we can trace over much of eastern North America!).  After the collision, those volcanoes shut down.  What we're left with are high barren mountains (there was still essentially no significant amounts of life on land at this time - maybe a few small plants and arthropods near the shorelines).

Feldspars are minerals common in igneous and metamorphic rocks found in mountains belts such as the ancient Taconics.  Three common types of feldspar are potassium feldspar (orthoclase or microcline - KAlSi3O8), sodium plagioclase feldspar (albite - NaAlSi3O8), and calcium plagioclase feldspar (anorthite - CaAl2Si2O8).

Here's a picture of a typical igneous granite, for example:


This pink mineral grains are potassium feldspars and the white mineral grains are the sodium plagioclase feldspars (there are other minerals in this rock as well such as glassy quartz and black amphiboles).  When feldspars are exposed to water (H2O) and carbon dioxide (CO2), they chemically weather into a type of clay called kaolinite (Al2Si2O5(OH)4) as well as releasing various cations (e.g. Ca+2, K+, or Na+), silicic acid (H4SiO4), and the bicarbonate anion (HCO3-).  Here are the equations.

CaAl2Si2O8 + 2CO2 + 3H2O = Al2Si2O5(OH)4 + Ca+2 + 2HCO2-

2KAlSi3O8 + 2CO2 + 11H2O = Al2Si2O5(OH)4 + 4H4SiO4 + 2K+ + 2HCO3-

2NaAlSi3O8 + 2CO2 + 11H2O = Al2Si2O5(OH)4 + 4H4SiO4 + 2Na+ + 2HCO3-

So we have high mountains and the mountains are exposed to the elements.  Minerals in the rocks chemically weather into clays and free ions - a process which uses up carbon dioxide from the atmosphere.  So much carbon dioxide gets used up in the weathering away of these mountains that global temperatures start to fall.  This triggers the Late Ordovician ice age which causes a mass extinction of marine life.

A neat story.  The lithosphere (solid Earth), hydrosphere (water), atmosphere, and biosphere (life) are all interrelated in the Earth system.

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