336 CARBON CYCLE Figure The natural global carbon cycle Reprinted from Davidson JP, Reed WE, and Davis PM (1997) Exploring Earth: An Introduction to Physical Geography Upper Saddle River, NJ: Prentice Hall Figure The short term and long term organic carbon cycles of productivity, sedimentation, burial, and weathering Numbers refer to carbon reservoirs (in gigatons) and carbon fluxes (in gigatons year 1) Reprinted from Kump LR, Kasting JF, and Crane RG (1999) The Earth System Upper Saddle River, NJ: Prentice Hall Photosynthesis does not occur spontaneously but requires the input of solar energy Consequently, photosynthesis can take place only where sunlight can freely penetrate, on the land or within the uppermost 100 m or so of oceans and lakes On land, photosynthesis is carried out mostly by plants, whereas in the sea it is almost exclusively the domain of algae in the form of minute, free-floating silica-shelled diatoms