On the following page is a diagram (box model) of the geochemical cycle of oxyge

Question

On the following page is a diagram (box model) of the geochemical cycle of oxygen and organic carbon, from Walker (1977). Units for the reservoirs are in moles of C or O2 because the oxygen cycle complements the cycle for organic carbon. The two cycles are related via the reactions for photosynthesis and respiration/decay:

CO2 + H2O = “CH2O” + O2 (to right is photosynthesis; to left is respiration/decay).

1) What is the residence time of oxygen in the atmosphere with respect to:

a) photolysis of water and the accompanying loss of hydrogen to outer space?

b) production via photosynthesis?

c) consumption via respiration and decay of organic matter?

d) weathering?

2) About how long would Earth’s entire reservoir of fossil fuels (as estimated in the diagram) have lasted at the rates of production and consumption that prevailed in 1977?

3) If we were to burn Earth’s entire reservoir of fossil fuels (as estimated in the diagram) instantaneously, could we significantly deplete Earth’s atmosphere of oxygen? Why or why not?

4) If sunlight were somehow permanently blocked from reaching Earth’s surface, causing photolysis and photosynthesis to cease, and we make the unrealistic assumption that the other rates in the diagram would remain constant until the relevant supply reservoir became exhausted, how long would it take before oxygen disappeared from Earth’s atmosphere? What process shown in the diagram would be mainly responsible?

H to space The Organic Carbon (and oxygen) Cycles Photolysis Atmosphere of water 3.8 x 1019 moles O2 7 x 03 moles yr of 0, *7.3 x 1016 moles CO2 Energy production Respiration Photosynthesis Weathering and decay 104 moles yr 1016 moles yr 1013 moles yr 1015 moles yr Surface organic matter 2 x 1017 moles C Burial Reservoirs 1013 moles yr O Processes Sedimentary rocks Feedbacks 10:1 moles C org Fossil fuels O CO org 6 x 017 moles C CO2 H2O "CH2O" O Fig. 31. Processes (circles) and reservoirs (boxes) affecting the oxygen content of the atmosphere. (From Walker, 1974. Copyright 1974 by the American Journal of Science. New Haven. Conn. Used by permission of the publisher.) *at 400 ppmv

Explanation / Answer

photolysis of water and the accompanying loss of hydrogen to outer space with respect to residence time to be the first step of the cycle.

production via photosynthesis? as we know that already it could be generated by photosynthesis. this is the ultimate for the food cycle.

consumption via respiration and decay of organic matter? it is also play the vital role.

weathering? it should be main theme for the cycle. it may also play the vital role of the photosynthesis.

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