FEEDBACK AND PHANEROZOIC ATMOSPHERIC CO2 AND O2

Use of systems analysis-style feedback diagrams are very useful for studying the long term carbon and sulfur cycles over millions of years involving the slow exchange of carbon and sulfur between rocks and the surficial system consisting of the combined ocean, atmosphere, biota, and soils. Tracing paths on these diagrams allows the delineation of negative and positive feedback loops as they impact the levels of both atmospheric CO2 and O2.

Examples of feedback loops affecting CO2 are: (1) the negative greenhouse-temperature+rainfall feedback as it affects the rate of uptake of CO2 by Ca-Mg silicate rock weathering; (2) the negative feedback due to CO2 fertilization of plant growth with consequent increased uptake of CO2 via silicate weathering; and (3) the positive feedback accompanying the breakdown of methane hydrates or the degassing of CH4 from swamps, oxidation of CH4 to CO2 in the atmosphere, warming due to the CO2 greenhouse effect and the thermally induced further breakdown of methane hydrates and/or degassing of swamps. This positive feedback may help explain a number of sudden warmings and positive excursions in CO2 at various times during the Phanerozoic; the loop itself can be shown to lead ultimately to either stabilization or destabilization.

Examples of feedback loops affecting O2 are: (1) the controversial simple feedbacks due to the control by O2 of weathering and burial of organic matter and pyrite; (2) negative feedback on nutrient-controlled organic matter burial due to O2-dependent Fe-P vs org-P burial or to O2-dependent nitrogen fixation; (3) the negative feedback against organic matter burial due to greater global fires and increased photorespiration accompanying higher O2; (4) the positive feedback favoring organic matter burial due to the production of microbially resistant charcoal and increased erosion plus sedimentation as a result of greater fires.