global elemental cycles

elemental cycles in the environment are an interesting part of history.  for instance, the earth's atmosphere has not always been 21% oxygen, 78% nitrogen, and so on.  how do we know?  there are fossils of dragonflies that had 3 foot wingspans.  how is this related?  well...dragonflies don't have lungs.  they breath through their skin; diffusion i think.  for a dragonfly with a 3 foot wingspan to survive, a quick calculation would signify that the atmosphere would have to have greater than 30% oxygen (it's only a quick calculation if you're not calculating - i didn't calculate it, it's a fact from one of my classes a few years ago, i think it was 35%, but I would rather err on the low side).  but the accumulation of oxygen in the atmosphere is the point of interest here.  it is dependent on a mass balance of oxygen, that is, sources of atmospheric oxygen and sinks.  the source of interest is plant respiration.

the beginning of plant life probably made 35% oxygen possible.

SO - the percentage of oxygen in the atmosphere required to support the life of a 3 foot wide dragonfly means that plant life was in full force (or that humans and other animals weren't around to consume the oxygen).  i'm guessing there was some pretty intimidating plant life here (ever been to the redwood forests?)  either way, it provides some insight into the distribution of oxygen in the environment throughout earth's history.  (good back of the envelope calculation:  how much oxygen is exchanged between human lungs and the atmosphere...there are some major assumptions, but it might be interesting to find out how much or how little of an affect our breathing has on the 21% oxygen today).

so:  with that single fossil of a dragonfly, we get a lot of information.  maybe what they say is true:  archaeologists do serve a purpose.

Food for thought:  if the atmosphere was rich in oxygen, oxygen may have partitioned into oceans (which cover most of the earth), and provided the appropriate conditions (dissolved oxygen) for the evolution of respiration in the most biodiverse arena on the planet:  the ocean that is. 

cool stuff.

crunchy/environmental/sidenote:  the nitrogen cycle is the most perturbed cycle on the planet due to anthropogenic processes (my car:  it's a hyundai!)

sort of related trivia:  pangaea was the giant continent.  what was the giant ocean called? click here to find out:  http://en.wikipedia.org/wiki/Pangaea