Lecture 3 - Part I: Early Life and the slow rise of oxygen in the Earth's atmosphere

Lecture 3 - Part II: Biogeochemical Cycles - Definitions (html),  The Global Oxygen Cycle (pdf-file)

The article reporting new research questioning the validity of the still-held assumption that cyano-bacteria are responsible for most of the photosynthetic oxygen production in the Precambrian is here.


Homework assignments:
a) Required Readings for September 13, 2005:
    Catling & Zahnle, 2003: Evolution of atmospheric oxygen
    Catling et al., Science 293, 2001: Biogenic Methane, Hydrogen Escape, and the Irreversible Oxidation of Early Earth
    Wiechert, Science 298, 2003: Earth’s Early Atmosphere

b) Tasks (to be handed in Tuesday, 13^th Sept.):

• Explain why the complete burning of the biosphere does not significantly diminish atmospheric oxygen. Where does most oxygen on earth reside, i.e. which is the biggest reservoir of oxygen on earth?    2 pts
  
• Today, oxygen in the atmosphere is essentially at steady state. A disturbance from this SS will be followed by a negative feedback loop, keeping the system in SS. The sedimentation of organic matter in the oceans is part of such an important loop: Oxygen decreases in the atmosphere -->  less oxygen dissolves in the oceans --> the average depth at which there will be not enough oxygen to oxidize all sinking organic detrital matter increases --> .... complete this cycle AND compose the opposite cycle starting with "Oxygen increases in the atmosphere --> ..."     2 pts
  
• Summarize the theory that the rise of oxygen in the atmosphere can be explained by methane photolysis in the upper atmosphere.   You can find a discussion about methane photolysis at wavelengths <150 nm (hard UV light emitted from hydrogen in the sun) in this paper.    3 pts
• Consider a simple Geochemical Box Model:
  Consider an element X exchanging between two geochemical reservoirs A and B. Let M_a and M_b be the masses of X in reservoirs A and B, respectively; let τ_a and τ_b be the residence times of X in reservoirs A and B, respectively. Further let M = M_a + M_b be the total mass of X in the two reservoirs combined.

1. Show that at steady state,  M_a = M / (1 + {τ_b/τ_a})
2. In the limit τ_a >> τ_b what controls the value of M_a?
3. Consider a situation where M is injected into reservoir A at time t=0, with no further injection at later times; further assume that τ_a >> τ_b. Give an expression for M_b as a function of M, τ_a, τ_b and t. What is the characteristic time for M_b to approach steady state? What is the characteristic time for M_a to approach steady state?
4. Regarding the oxygen cycle: Which reservoir(s) resemble M_a, which one(s) M_b ?

5 pts