Jim-

Exactly how much oxygen is generated by the land vs. the ocean is definitely a matter of debate. The oxygen in the atmosphere is definitely a product of photosynthesis – by phytoplankton, land plants and other photosynthetic microbes. The latter were more important in early earth history (when all life, and thus all O2 production was in the ocean) but modern production is definitely now due to land plants and phytoplankton. Photosynthesis in the ocean works the same way as photosynthesis on land (water is split to generate O2). This oxygen then moves out of the cells where photosynthesis occurs and into the water/air.

To check your numbers, I’m using global estimates of Primary Production (the net amount of carbon fixed into sugars during photosynthesis). If we assume that the stoichiometry of carbon fixation by plants is:

 

CO2 + H2O --> CH2O(plant carbon) + O2

 

then for every mole of carbon produced, there should be a mole of oxygen produced as well. Thus any productivity measurement can be converted to moles O2 by dividing by 12 (the atomic weight of carbon). Since you are only interested in proportions, I haven’t bothered to convert the numbers. Measuring productivity is tricky and scaling local measurements up to the global scale is definitely difficult so any numbers should only be viewed as rough estimates. With that in mind, most estimates seem to suggest that production is roughly evenly divided. Taking my numbers from Schlesinger (1997) I can find estimates using two methods (Pg= Petagram =1*10^15 grams)

 

By scaling measurements from local to global:

Terrestrial 60 Pg C/year Houghton and Skole (1990)

Marine 51 Pg C/year Knauer (1993)

 

Inferring based on the isotopic composition of oxygen:

Terrestrial >90 Pg C/year Bender et al. (1994)

Marine 70 Pg C/year Bender et al. (1994)

 

As you can see by just looking at these two estimates, there can be fairly large differences depending on how you estimate. Still, they do seem to agree that terrestrial and marine production are roughly equal.

Estimates I found for the amount of terrestrial production that is due to tropical forests range from 8.3 Pg C/year (Houghton and Skole(1998) for wet and moist tropical forests) to 13.7 Pg C/year (which includes dry tropical forests from Amthor (1998)). Your high estimate may come from Leith’s 1975 estimate of 45 Pg C/year (out of 100 Pg C for total Terrestrial), which is regarded as somewhat dated and almost definitely overestimates production by tropical forests.

Pulling these numbers together I would estimate that roughly 55% of global oxygen is generated on land. Of this amount, perhaps 20% is due to tropical forests of some sort. I hope that helps!

 

Alex

 

 

References.

 

Amthor, J.S. and members of the Ecosystems Working Group (1998) Terrestrial Ecosystem Responses to Global Change: a research strategy. ORNL Technical Memorandum 1998/27, Oak Ridge National Laboratory, Oak Ridge, Tennessee. 37 pp.

 

Bender, M., T. Sowers, and L. Labeyrie. 1994. The Dole effect and its variations during the last 130,000 years as measured in the Vostok ice core. Global Biogeochemical Cycles 8: 363-376

 

Houghton, R.A. and D.L. Skole. 1990. Carbon. Pp. 393-408. In B.L. Turner, W.C. Clark, R.W. Kates, J.F. Richards, J.T Mathews, and W.B Meyer (eds.), The as Earth Transformed by Human Action. Cambridge University Press, Cambridge.

 

Knauer, G.A. 1993. Producitivity and new production of the oceanic systems. Pp. 211-231. In R. Wollast, F.T. MacKenzie, and L. Chou (eds), Interactions of C,N,P and S Biogeochemical Cycles and Global Change. Springer-Verlag, New York.

 

Lieth, H.F.H. (1975) Primary production of the major vegetation units of the world. In: Primary Productivity of the Biosphere (H. Lieth, and R.H. Whittaker, eds.). Ecological Studies 14. Springer-Verlag, New York and Berlin. pp. 203-215.

 

Schlesinger W.S. 1997. Biogeochemistry. Academic Press, New York.