ORI GIN AL PA PER An Earth Systems Diagram for the Global Cycles of Carbon and Phosphorus and Their Effects on Atmospheric CO2 and O2 Robert A. Berner Received: 30 January 2013 / Accepted: 13 July 2013 / Published online: 24 July 2013 Springer Science+Business Media Dordrecht 2013 Abstract A complex cause–effect type earth systems diagram is presented that represents the interrelation of the global carbon and phosphorus cycles over geological time. It demonstrates how a lot of information can be represented in an extremely compact manner and how relatively unrecognized positive and negative feedbacks are revealed by tracing paths on the diagram. Emphasis is on how the C and P cycles affect the levels of atmo- spheric CO2 and O2, often via rather indirect paths. Keywords Systems analysis Carbon Phosphorus Oxygen Carbon dioxide 1 Introduction Fred Mackenzie has admirably shown (e.g., Mackenzie et al.1998, Mackenzie 2011) that box models can be used to represent complex multi-element interactions by means of fluxes (arrows) between reservoirs (boxes or circles). Compared to box models, the use of cause–effect type systems diagrams in the earth sciences has been rather limited. The few examples are Garrels et al. (1976), Saltzman and Moritz (1980, 1991), Kump (1988), Lenton and Betts (1998), Berner (1999), Lenton and Watson (2000), Berner et al. (2003), and Bergman et al. (2004). The purpose of this short note is to show how one can add a cause–effect type diagram, also using boxes and arrows, to the study of multi-element cycles, in this case the combined cycles of carbon and phosphorus. In cause–effect dia- grams, arrows DO NOT refer to simple transfers of mass, as in box models, so that no conservation of mass is implied. Clarification of what I mean by cause–effect can be illustrated by reference to Fig. 1. Starting at an arbitrary place, let’s say box A, if the value of A (in terms of temperature, R. A. Berner (&) Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109, USA e-mail: robert.berner@yale.edu 123 Aquat Geochem (2013) 19:565–568 DOI 10.1007/s10498-013-9200-0 mass, rate of a process, etc.) increases, then via a plain arrow, the temperature, mass, rate of a process, etc., for B increases. This is a direct cause–effect. An inverse cause–effect is shown by an arrow with an attached bullseye, for example, between B and C. If B increases, C decreases. If one traces the path from A to B to C and back to A, the overall effect of this loop is that a rise in A results eventually in a drop in A. This is negative feedback which leads to stabilization of the environment. Any loop that includes an odd number of arrows with bullseyes represents negative feedback. Conversely any combi- nation of arrows with no bullseyes or an even number of bullseyes represents positive feedback, or in other words, enhancement of the original fluctuation and destabilization. The loop A to B to C to D to A, with two bullseyes, thus represents positive feedback. In other words, an increase in A results in an increase in B, a decrease in C, a decrease in D, and an increase in A. 2 The Carbon–Phosphorus–CO2–O2 Systems Diagram Figure 2 shows the author’s conception of the global carbon and phosphorus cycles as they occur over long geological time scales. The effect of these cycles on atmospheric O2 and CO2 is complex, as can be seen by the many arrows leading to and away from the blue circles representing these gases. Assume that, due to a variety of processes, the level of atmospheric O2 increases which would lead to more oceanic O2. Higher oceanic O2 should favor the burial of more oxidized iron oxides (arrow A), which adsorb phosphorus from seawater (FeP). Burial of phosphorus as FeP robs the ocean of nutrient dissolved phosphate (arrow H), which in turn leads to less primary production of marine plankton (arrow Q). Less plankton growth leads to less organic burial (arrow V) and, therefore, less O2 production with a lowering of O2 (arrow C). This loop (A–H–Q–V–C) has one arrow with a bullseye resulting in overall negative feedback that would balance the initial rise in O2. Another negative feedback affecting O2 is shown by the much more direct loop D–E–C. Higher O2 should lead to more fires and the loss of land plants (arrow D). Fewer plants lead to less organic burial (arrow E) and, therefore, less O2 production and lower O2 (arrow C). A well-known example of negative feedback is the stabilization of atmospheric CO2 by silicate weathering and the atmospheric greenhouse effect (Walker et al. 1981; Berner et al. 1983) which is represented in Fig. 2 by the cycle B–L–G. Suppose the level of CO2 rose due to increased volcanic degassing. Then global temperature and rainfall would increase due to the greenhouse effect (arrow B). Higher temperatures and greater rainfall would A B D C Fig. 1 Simple example of a cause–effect system diagram. Arrows with bullseyes represent inverse responses; those without bullseyes represent direct responses (see text) 566 Aquat Geochem (2013) 19:565–568 123 cause an increase in silicate weathering (arrow L) which would in turn bring about a stabilizing drop in CO2 (arrow G). If weathering enhancement by land plants is considered, a rise in CO2 should also lead to negative feedback. Higher CO2 should lead to fertilization of land plants, increasing their biomass (arrow N) which, because of the efficacy of land plants in increasing the rate of chemical weathering, should result in increased silicate weathering (arrow S). The increased weathering should then result in a stabilizing drop in CO2 (arrow G). The enhancement of the weathering of phosphate in rocks can lead to negative feedback affecting CO2. Consider the loop B–J–P–Q–V–R, a rise in CO2 should lead to a warmer and wetter climate (arrow B). A warmer and wetter climate should lead to enhanced weathering of phosphates in rocks (arrow J). This leads to an increased flux of phosphate to the oceans, resulting in an increase in the level of nutrient aqueous P (arrow P). Higher aqueous P should lead to increased plankton productivity (arrow Q) and increased organic burial (arrow V) which in turn leads to a drop in CO2 (arrow R). This loop has one bullseye and, therefore, represents negative feedback. Methane hydrates have been added to the cycle diagram to represent positive feedback. If the climate warms, it increases the probability of the release of CH4 to the atmosphere from the decomposition of methane hydrates (arrow T). Because the diagram is intended for long geologic time, the greenhouse effect of methane is neglected because it oxidizes rapidly to CO2 in the atmosphere (arrow U). The increased CO2 should then lead to further global warming via the atmospheric greenhouse effect (arrow B). This is a process of present day concern as the climate warms. Continental Relief And Position Climate (T + pptn) Weathering Ca-Mg Silicates Volc/Met/Diag Degassing Weathering Org C Total P CO2 Ocean Circulation Land Plants Nutrient Aqueous P Organic C sed. Burial FeP Burial O2 P Q K G L E R J N D C A M F H S B CH4 Hydrate release U T Marine plankton V Fig. 2 System diagram for the long term cycles of carbon and phosphorus. (Modified from Berner 1999) Aquat Geochem (2013) 19:565–568 567 123 Figure 2 shows some other arrows and boxes (circles) that are not involved in carbon and phosphorus cycle feedbacks. The relations between ocean circulation and climate (yellow circles) are greatly simplified via small, dotted direct arrows. Boxes in orange- brown color represent one-way influences of geological processes that are assumed to be negligibly affected by feedbacks from the rest of the system. Also, there are certainly other processes involving carbon and phosphorus, such as the sedimentary burial of carbonates and diagenetic calcium phosphates, but for the purpose of simplification, and the avoidance of an incredibly complex diagram, they are not considered here. Acknowledgments I wish to complement Fred Mackenzie for his long record of excellent productive research. I especially appreciate the many informal discussions on global cycles and the environment that I have had with Fred in Hawaii over the past 22 years. References Bergman NM, Lenton TM, Watson AJ (2004) COPSE: a new model of biogeochemical cycling over Phanerozoic time. Am J Sci 304:397–437 Berner RA (1999) A new look at the long term carbon cycle. GSA Today 9:1–6 Berner RA, Lasaga AC, Garrels RM (1983) The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide and climate. Am J Sci 283:641–683 Berner RA, Beerling DJ, Dudley R, Robinson JM, Wildman RA (2003) Phanerozoic atmospheric oxygen. Annu Rev Earth Planet Sci 31:105–134 Garrels RM, Lerman A, Mackenzie FT (1976) Controls of atmospheric O2 and CO2: past, present, and future. Am Sci 63:306–315 Kump LR (1988) Terrestrial feedback in atmospheric oxygen regulation by fire and phosphorus. Nature 335:152–154 Lenton TM, Watson AJ (2000) Redfield revisited: II. What regulates the oxygen content of the atmosphere Global Biogeochem Cycles 14:249–268 Mackenzie FT (2011) Our changing planet: an introduction to earth system science and global environ- mental change, 4th edn. Prentice Hall/Pearson, Upper Saddle River, p 579 Mackenzie FT, Ver LM, Lerman A (1998) Coupled biogeochemical cycles of carbon, nitrogen, phosphorus, and sulfur in the land-ocean-atmosphere system. In: Galloway JN, Melillo JM (eds) Asian change in the context of global change. Cambridge University Press, Cambridge, pp 42–100 Saltzman B, Maasch KA (1991) A first-order model of late Cenozoic climate change. Clim Dyn 5:201–210 Saltzman B, Moritz RE (1980) A time-dependent climatic feedback system involving sea-ice extent, ocean temperature, and CO2. Tellus 32:93–118 Tim Lenton, Betts RA (1998) From daisyworld to GCMs: using models to understand the regulation of climate. In: Boutron C (ed) ERCA—Volume 3—from urban air pollution to extra-solar planets. Les Ulis, EDP Sciences, France, pp 145–167 Walker JCG, Hays PB, Kasting JF (1981) A negative feedback mechanism for the long term stabilization of Earth’s surface temperature. 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