FEEDBACKS AND THE LONG TERM CARBON CYCLE

The long term carbon cycle is the cycle that operates over millions of years and that involves the slow exchange of carbon between rocks and the surficial system consisting of the ocean, atmosphere, biota, and soils. It is distinguished from the short term carbon cycle where carbon is rapidly exchanged only within the surficial system. Processes within the long term cycle are usually treated in terms of box models. A complementary representation, similar to that used in systems analysis and control theory, has been developed that illustrates cause-effect relations and negative and positive feedbacks as they impact the levels of both atmospheric CO2 and O2. The diagram also includes the cycle of phosphorus as it affects the burial of organic matter in sediments. By following paths leading from causes to effects one can trace out complex loops that demonstrate positive and negative feedback, and this allows discovery of new sub-cycles that deserve further study.

Examples of some negative feedback loops illustrated are: the CO2 induced greenhouse-temperature+rainfall feedback as it affects the rate of uptake of CO2 by Ca-Mg silicate rock weathering; the feedback due to CO2 fertilzation of plant growth with consequent increased uptake of CO2 via silicate weathering; the Lenton-Watson plant-mediated phosphate weathering feedback as a nutrient control of organic matter burial and O2 production; the feedback due to inhibited plant productivity and organic C burial as a result of extra O2 production from increased marine org-C burial; and the O2-dependent Fe-P vs org-P sediment burial feedback on O2 production by nutrient-limited sedimentary organic matter burial. Some positive feedbacks are: greenhouse warming by CO2--greater CO2 production by the weathering of ancient org-C in shales and consequent further warming; greenhouse warming by CO2--greater decomposition of methane hydrates--more CH4 and CO2 production and consequent further warming. The latter positive feedback may help explain a number of sudden warmings and positive excursions in CO2 over geologic time.