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)
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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
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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.
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