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Unformatted text preview: EAS 1600 Introduction to Environmental Sciences ____________________________ Class 26 - The Carbon Cycle ________________________________ Throughout this past weeks we have used the term: “biogeochemical cycle” We know by now that this term refers to the sum total of all biological, tectonic, chemical, and physical processes that recycle elements through the Earth System. This recycling includes both physical transport and chemical transformation. In the next two lectures we take a look at one of these cycles (namely, the global C cycle ) in more detail. GLOBAL BIOGEOCHEMICAL CYCLES Biogeochemical cycles relate to the transformations of nutrient elements on the earth. ¾ Why nutrient element? ¾ We are interested in elements that play a role – constructive or destructive – in one or more metabolic process. ¾ Hence the prefix “ bio ” ¾ A global biogeochemical cycle relates to the total global flow of the element, and as such it is a description of a materially closed system (and thus falls within the domain of biospherics instead of ecology ) . Biogeochemical cycle involving N ¾ photosynthesis: 5 CO 2 + 5H 2 O + 5h ν Æ 5“CH 2 O” + 5O 2 ¾ denitrification: 4HNO 3 + 5"CH 2 O” Æ 2N 2 + 5CO 2 + 7H 2 O ¾ nitrogen fixation: 2N 2 + 6H 2 O Æ 4NH 3 + 3O 2 ¾ nitrification 4NH 3 + 8O 2 Æ 4HNO 3 + 4H 2 O Biogeochemical cycle involving C ¾ photosynthesis: CO 2 + H 2 O + h ν Æ “CH 2 O” + O 2 ¾ respiration “CH 2 O” + O 2 Æ CO 2 + H 2 O Biogeochemical cycle involving S (simple) ¾ photosynthesis: 2CO 2 + H 2 S + 2H 2 O+ h ν Æ 2"CH 2 O” + H 2 SO 4 ¾ sulfate reduction H 2 SO 4 + 2"CH 2 O” Æ 2CO 2 + H 2 S + 2H 2 O Biogeochemical cycle involving S (not simple) ¾ photosynthesis: 2CO 2 + 2H 2 O + h ν Æ 2“CH 2 O” + 2O 2 ¾ sulfate reduction H 2 SO 4 + 2"CH 2 O” Æ 2CO 2 + H 2 S + 2H 2 O ¾ H 2 S oxidation in atmosphere Can you write down the reaction that is needed to balance the cycle? We will focus here on the C cycle: Of course one of prime interests in the C cycle relates to our concern over the increasing concentrations of atmospheric CO 2 and its possible climatic impact ... But before we do this, let’s introduce some basic concepts about biogeochemical cycles and how we describe them. From Chameides, W.L. and E.M. Perdue, Biogeochemical Cycles: A Computer- Interactive Course in Earth System Science and Global Change, Oxford University Press, New York, NY, 224 pp, 1997 Here is an example from your textbook related to the C Cycle (with a little added twist...) Dependence of photosynthesis on CO 2 Dependence of CO 2 on photosynthesis ¾ Also a negative feedback loop. ¾ In fact all global biogeochemical cycles, by their very nature, must have at least one negative feedback loop. Do you know why that is?...
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