Figures Summarizing the Global Cycles of Biogeochemically Important Elements
Author(s): William S. Reeburgh
Reviewed work(s):
Source: Bulletin of the Ecological Society of America, Vol. 78, No. 4 (Oct., 1997), pp. 260-267
Published by: Ecological Society of America
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Berkowitz, A. 1997. A simple frame
the benefits
work for considering
partnerships.
of student-scientist
In TERC and Concord Consor
report.
tium (1997). Conference
National Conference on Student &
Scientist
Partnerships.
Education
Resource
Technical
of
in
be
teachers and scien
Annual Meeting,
Chi
USA.
R. F., and J. Northfield.
The second article
is by William
S.
Reeburgh, Earth System Science,
Dr.
University
of California-Irvine.
summaries
Reeburgh
has developed
and graphical
illustrations of global
impor
cycling of several extremely
tant chemicals. These summaries and
figures will be especially
anyone teaching
graduate ecology
pertinent
undergraduate
courses.
to
and
cycles
of biologically
are an important part
of basic and advanced Earth Science,
active elements
and
Ecology,
courses.
An
Biogeochemistry
understanding
of
bio
and anthropo
cycles
genic impacts on them is also funda
in studies of global climate
mental
geochemical
260
Bulletin
of
sci
teachers. National Academy
Press, Washington,
D.C., USA.
to live
Norris, S. P. 1995. Learning
with scientific expertise: toward a
theory of intellectual communal
ism for guiding
science
the Ecological
Associates,
Hillsdale,
Jersey, USA.
and
Shulman, L. 1987. Knowledge
teaching: foundations of the new
reform. Harvard Educational Re
teaching.
S.A., and A.T. Jones. 1992.
Open work in science: a review of
Simon,
existing
cational
practice.
Resource
Massachusetts,
on Student
'TechnicalEducation
Resource Center
&
2067 Massachusetts
Partnerships: Conference
Technical
Education Re
Report.
source Center, Cambridge, Massa
chusetts, USA.
and
Ruopp, R., S. Gal, B. Drayton,
M. Pfister.
1993. Labnet:
community
of practice.
toward a
Lawrance
change. Unfortunately,
most presen
tations of biogeochemical
cycles oc
cupy one of two extremes:
they are
either presented so simply that they
contain
information
on pathways
only, or in such detail that they defy
and are useful only to
comprehension
specialists.
Further, most
in facets
specialized
workers
of
indi
and broad perspectives
and an understanding of interactions
cycles is lacking.
between
The figures presented
here are an
Cambridge,
USA.
Brian Drayton ",2 and Joni Falk'
Scientist
have
Center,
(1997). Na
Consortium
for Edu
Conference on Student & Scientist
Partnerships. Technical Education
C. 1997. Why do scien
tists want teachers and students to
do real research? In TERC and
Concord
Center
Studies, King's College,
University of London, London, UK.
TERC
and Concord
Consortium.
1997. Conference
report. National
ScienceEducation79(2):201-217.
vidual cycles,
FIGURES SUMMARIZING
THE GLOBAL CYCLES OF
BIOGEOCHEMICALLY
IMPORTANTELEMENTS
The global
of
ence
tional Conference
Illinois
cago,
(March
1997).
D.C., USA.
AERA, Washington,
Fullan, M. 1991. The new meaning of
educational change. Teachers Col
lege Press, New York, New York,
Gunstone,
development
Erlbaum
New
view 57(1):1-22.
Jersey, USA.
Pennypacker,
tists in an innovative mentorship
to
collaboration.
Paper presented
Educational
Research
American
Association
New
professional
USA.
education
the relationships
tween science
lex, Norwood,
(National Research
Council).
1996. The role of scientists in the
in
the United States. Pages 45-59
M. Hale, editor. Ecology in educa
tion. Cambridge University Press,
Cambridge, UK.
1997. Dy
Falk, J., and B. Drayton.
namics
ScienceEducation16(5):523-537.
Lemke, J. 1993. Talking science: lan
guage, learning, and values. Ab
NRC
Center, Cam
bridge, Massachusetts,
. 1993. Ecology
1994. Metacognition
and learning
to teach. International Journal of
Avenue
Cambridge, MA 02140
'Department of Biology
Boston University
Boston, MA 02215
E-mail: [email protected]
[email protected]
and use of color to distinguish pools,
fluxes,
and turnover times makes
comprehension
of individual element
cycles and comparison between ele
ment cycles straightforward.
The
figures
were
developed
as
in a graduate-level
class assignments
course in Earth Systems at the Uni
Irvine and have
versity of California,
been used in graduate as well as un
sur
dergraduate courses. Discussion
rounding
these
figures
from "What do we
do we
has evolved
include and which
to fill the middle ground and
between element
comparisons
cycles. Figures based on current lit
the global
erature values
showing
numbers
allow
being prepared, to a current "Where
did these numbers come from?" To
with
the
permit easy comparison
of C, 0, N, S, P, and Si, as
as
well
H20 and CH4, are presented in
a uniform format that shows pool or
original literature, we have used the
fac
unlits as published. Conversion
tors are supplied in the captions. The
cycles
reservoir sizes, significant
natural and
transfers or fluxes be
anthropogenic
tween pools, and residence times esti
mated from the ratio of pool size and
the major fluxes. The uniform format
Society
of America
arrows associated
note the direction
Black
use?"
as they were
attempt
with
the fluxes de
of the flux.
and white
as well
as color
versions of these figures will soon be
at the Uni
available for downloading
Table
1. Reservoir
sizes and turnover times of biologically
Turnover time (yr)
Quantity
Element
active elements.
Irvine Depart
versity of California,
ment of Earth System Science Web
I .ps.uci.edu_
site: <http://:ess
reeburgh/figures.html>. Questions,
corrections, updates, and suggestions
should be addressed by e-mail to the
Carbon (10's g C)
Sediments, rocks
Deep ocean (DIC)
Soils
Surface ocean
Atmosphere
Deep ocean (DOC)
Terrestrial biomass
Surface sediments
Marine biomass
>> 1 06
2000
<10-105
decades
5
5000
50
0.1-1000
0.1-1
77 x 106
38000
1500
1000
750
700
550-680
150
2
Oxygen (10'I mol 02)
Sedimentary rocks
Atmosphere
Long-lived biota
author.
Acknowledgments
These figures summarize the ef
forts of several groups. The carbon
cycle figure resulted from discussions
at a 1993 Dahlem
by participants
on "The Role of Non
Conference
Living Organic Matter
in the Global
Cycle"
(Zepp and Sonntag
1995). The carbon figure was used as
the level
a template and established
Carbon
37000
180
106
3 x 106
1000
219
500
11
6
50
22 days
Ocean
Biota
Surface ocean
Nitrogen (1012 g N)
Atmosphere (N2)
Sediments
Ocean (dissolved N2)
Ocean (inorganic)
Soil
Terrestrial biomass
Atmosphere (N20)
Marine biomass
Sulfur (1012 g S)
Lithosphere
Ocean
Sediments
Soils
Lakes
Marine biota
Atmosphere
4x
5x
2.2 x
6x
3x
1.3 x
1.4 x
4.7 x
109
108
107
105
105
104
104
102
2x
3x
3x
3x
1010
109
101
105
300
30
4.8
107
107
1000
2000
50
102
of detail used for the other figures.
Review articles and tables from text
books provided the information used
in the methane and water figures. The
other figures were produced and re
fined from 1993 through 1997 by
graduate students in a core course of
fered by the newly formed Depart
ment of Earth System Science at the
Irvine. Data
of California,
University
for
as
the figures was assembled
the figures were drafted
discus
after a thorough classroom
homework;
108
106
106
103
3
1
8-25 days
sion.
The
graduate students who con
tributed to this effort are: Huisheng
Bian, Paul Burke, Julia Gaudinski,
Bryan Hannegan, Adam Hirsch, Jen
nifer King, Caroline Masiello, Karena
Seth
Olsen,
Tibisay
Joe Selzler,
Perez, Shannon Regli,
and Chris Walker.
David Valentine,
W. H. Schlesinger and M. 0. Andreae
also provided helpful comments.
McKinney,
Phosphorus (1012 g P)
Sediments
Land
Deep ocean
Terrestrial biota
Surface ocean
Atmosphere
4 x 109
2 x 105
8.7 x 104
3000
2700
2 x 108
2000
1500
-50
2.6
0.028 days
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Water
Water
Vol.
Oceans
Polar ice, glacier
Groundwater
(actively exchanged)
Freshwater lakes
Saline lakes
Soil moisture
Rivers
Atmospheric water vapor
(103 km3)
1,370,000
29,000
4000
125
104
67
1.32
14
Percentage
97.61
2.08
0.29
0.009
0.008
0.005
0.005
0.0009
Turnover time (yr)
37,000
16,000
300
10-100
10-10,000
280 days
12-20 days
9 days
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of California
Irvine
Irvine,CA 92697-3100
E-mail:
nia, USA.
reeburgh@ uci.edu
Pools and fluxes inGt C and Gt C y', Gt = 10'5 g;
*= living pools; (turnover times)
Terrestrial
Atmosphere
Marine
NPP =50(
1
Deforesta
=41.4 y-'t
Combustion
(80's) 5.4 yr-t
750 (3 y)
Ann. increment = 3.2 y-1
(- +1.5 ppmv C02 Y1)
NPP = 50 y
New production
Rivers
Plants*
550-680
peat 360
(50 y)
o0.2 y-1
pDoOC:.
POC: 0.2
y-1
Coastal Ocean
20% of NPP
(> 1000 y)
microbial* 15-30 (<10 y)
POC 250-500
(<100 y)
remainder 600-800
(102-1 05 y)
Ocean CO Exchange
90anget80%9ofyNPP
Open Ocean
80% of NPP
POC 5, Living 2*
m
100
= 10 y
_____POC,l
Surface Sediments
150 (0.1 -1000 y)
(-1m)
\
Deep
8 km
7 y
POC 70-0
References:
Sediments
Hedges, 1992; Eswaran eta!., 1993;
Siegenthaler & Sarmiento, 1993;
Schimel et al., 1994
Respiration
kerogen 15x106 (>>1 my) b
methane
clathrates
11 x103
limestone 60x106
(0.1 -1 y)
(50100
y)
Sedimentation
0.
Y
_ NPP
Fig. 1. Global carbon reservoirs, fluxes, and turnover times. Major reservoirs are underlined, pool sizes and fluxes are
given in Gt (101' g) C and Gt C/yr. Turnover times (reservoir divided by largest flux to or from reservoir ) are in parenthe
ses. To convert Gt C tomoles of C, multiply by 8.3 x 1010.
Sedimentary carbonates and kerogen are the the largest carbon reservoirs, followed by marine dissolved inorganic car
bon (DIC), soils, surface sediments, and the atmosphere. The living biomass reservoir is somewhat smaller than the atmo
and respiration.
spheric carbon reservoir and actively exchanges with the atmospheric reservoir through photosynthesis
Global estimates of important fluxes or transfers between reservoirs are shown. Net primary production (NPP = gross
is approximately equal in terrestrial and marine environments. NPP
photosynthesis-respiration)
respiration. New Produc
tion = particulate organic carbon (POC) and dissolved organic carbon (DOC) exported from surface waters. Approxi
mately 20% of the ocean NPP occurs in the coastal ocean; 80% of this is deposited in surface sediments.
Turnover or residence times for the reservoirs range from >>106 yr for kerogen in the sediment reservoir to 103 -105
years for peats and soil carbon, to about 3 years for atmospheric CO2 and <1 year for ocean biomass. Because of its small
size and relatively slow equilibration with the ocean reservoir, the atmospheric carbon reservoir is presently out of balance.
The difference between atmospheric sources (deforestation and combustion) and sinks (annual atmospheric increment and
the difference
between ocean influx and efflux) is the "missing sink" of 1.8 Gt C/yr.
The major long-term sink for carbon is burial in deep sea sediments. This removal of a small portion (0.1%) of annual
NPP is responsible for oxygen in the Earth's atmosphere. Protection of photosynthetically
fixed organic carbon from oxi
dation by photosynthetic
oxygen (respiration) has permitted accumulation of oxygen in the atmosphere and ocean over
geologic time. The carbon cycle is completed by weathering of uplifted marine shales or by combustion of fossil fuels.
October 1997
263
Pools in 1015moles O2 ,Fluxes in 1015moles 02 y',
Organic pools as 02 equivalent, (turnover time)
Atmosphere
37,000 (3.7My)
Terrestrial
Gross PP 9.2 y-1
Autotroph
respiration
4.6 y-1'
Ocean
O
NPP (OC)4.6 y-1
Short-lived
biota (OC) 11 (50 y) +
Long-livedbiota
leri
soil, peat
Exchange
(OC) 180 (1000 y);
140 y
140 Y'
Surface Ocean
6 (22 d)
a 0.4
0C)_
Fires, heterotrophic
respiration 4.6 y-1;
and volcanism
-0.01 y'
Weathering
Fossil fuel combustion
0.58 y1
Sedimentary
PP 4.3 y-1
3.9 y-I
Respiration
Deep
Ocean
(OC>) -0.4 y-I
219 (-500
y)
rocks (OC) 106
Fossil fuel reserves 760
Reference:
Gross
Sedimentation (OC)
Keeling, Najjar, Bender & Tans, 1993
-0.01
'
Fig. 2. Global oxygen reservoirs, fluxes, and turnover times. Major reservoirs are underlined, pool sizes and fluxes are
given in 10'5moles 02 and 10t moles 02 per year. Turnover times (reservoir divided by largest flux to or from reservoir)
are in parentheses. To convert moles 02 to Tg 02, multiply by 3.2 x 10 1.
The atmosphere is the largest oxygen reservoir and has the longest turnover time. The atmospheric oxygen reservoir is
approximately 200-fold
larger and the turnover time is >106 times longer than the next largest reservoirs, the ocean-dis
solved oxygen reservoir and long-lived plants. The major source of oxygen is photosynthesis,
but this is almost exactly
balanced by respiration. Note that ocean sediments are shown as an oxygen source, because the long-term storage of or
ganic carbon in ocean sediments prevents oxidation and allows accumulation of oxygen in the atmosphere.
Pools and fluxes inTg N and Tg N y', Tg
= 1012 g; (turnover times)
Fixation
Natural terrestrial 190 y-'
Natural oceanic 40 y-'
crops 40 y-1
Leguminous
Chemical
fertilizer 80 y-'
Combustion
20 y-'
Terrestrial Biomass
3.5x104 (50y)
Soil
9.5 x 104 (-2000
Atmosphere
N2: 3.9-4.0x109
(107 y)
Fixed N: 1.3-1.4 x103 (-5 wk)
N 20: 1.4x103 (1_y)
River runoff
36 y
Marine Biomass
Plants: 3x102
y)
Animals:
References:
Burns & Hardy, 1975; Jaffe, 1992; McElroy et
al., 1976; Schlesinger & Hartley, 1992;
Stedman & Shetter, 1983; Soderlund &
Svensson, 1976; Galloway et al., 1995
Denitrification
Natural terrestrial 147 y1
110 y-'
Natural ocean
Biomass
burning 12 y-1
Industrial combustion
20 y
Sediments
4.0x108 (107 y)
Weathering
5 y
1.7x102
Ocean
N : 2.2x107
N20: 2.0x104
I
norglanic: 6x1 05
Organic: 2x105
Sedimentation
(burial) 14 y-1
(
Fig. 3. Global nitrogen reservoirs, fluxes, and turnover times. Major reservoirs are underlined, pool sizes and fluxes are
given in Tg (10t2 g) N and Tg N/yr. Turnover times (reservoir divided by largest flux to or from reservoir) are in parenthe
ses. To convert Tg N tomoles of N, multiply by 7.1 x 10?.
As with oxygen, the atmosphere, which is 78% N2, is the largest nitrogen reservoir. Other gaseous nitrogen species im
portant in ozone chemistry have short lifetimes and are of local importance. Nitrous oxide, an important long-lived green
house gas, is photolyzed in the stratosphere.The thermodynamicallystable formof nitrogen in thepresence of oxygen is
not N2, butNO3-.However, conversion of relatively inertN2 toother forms is limitedby themicrobially mediated nitrogen
fixation rate,and fixed nitrogen is rapidly incorporatedinto living tissue.Note that theanthropogenicnitrogen fixation rate
is about60% thatof natural fixation.Microbially mediated denitrificationcompletes thenitrogen cycle. The nitrogen bio
mass reservoirsare based on carbon reservoirsand theC:N ratio.
264
Bulletin of theEcological Society of America
Pools and fluxes Tg P, Tg = 1012g;
*
livingpools; (turnover times)
Atmosphere
0.028 (0.006 y, 53 h)
Land Biota
*3000 (47.2 y)
Jahnke,
Rivers
Reactive 1.7 - 2.5 y-1
Total 20 y-(
1992;
Berner &5 (2a00
10cean
System
9x104 (50,000 y)
Surface
1994
(0-3 km)Ocean
WY-1
P
Mineable
10,000
Fertilizer
m.7
xm04
12 y-'
42 y-'
el) Ocean
(3.0 km)
(1500 y)
\
t
biota 140 (48 d)
1
58
Sediments
4xt09 (2xlhry)
Sedimentation
Reference:
Jahnke, 1992; Berner & Rao, 1994
Fig. 4. Global
in Tg
given
ses.
To
Note
phosphorus
(1012 g) P and
convert
that
Tg
P
reservoirs, fluxes, and turnover times. Major
Tg
to moles
the phosphorus
of
times
Turnover
P/yr.
P, multiply
cycle
has
no
by
divided
(reservoir
3.2
x
largest
flux
to or
from
and
is restricted
to solid
sediments. Marine phosphorite deposits are mined and reintroduced
biomass reservoirs are derived from the carbon cycle and C:P ratios.
Atmosphere
Continental
Terrestrial dust 20 y-1
Biogenic 2.5 y-1f
Volcanoes
10 y-'
f
43 y-1
Deposition
Soils & Land Biota
3 x 105 (8.6x103 y)
1.6
) are
liquid
phases.
and
sink is burial in marine
ties. The phosphorus
pool sizes and fluxes are
reservoir
in parenthe
1010.
component,
atmospheric
reservoirs are underlined,
by
13 y-1 _
(8d)
major
activi
Pools and fluxes inTg S and Tg S y', Tg = 1012g;
(turnover times)
Marine
24 y-1
Lakes & Rivers
300 (3y)
River runoff 104 y-'
\
The
to the cycle by man's
3.2
(10 d)
COS
(5-1 Oy)
Seasalt particles 140 y-1t
I
Biogenic 15-30 y-1
159 y-'
Deposition
Open Ocean
~~Ocean
1.3 x 109 (6.8Xl106 y)
*Marine biota 30 (0.1 -1 y)
Ocean Sediments\
3 x 108 (4xl106 y)
Sedimentation
135
Lithosphere
Reference:
Andreae, 1990; Bates et al., 1992;
Charlson, Anderson &McDuff, 1992
2.4x1010
(burial)
y-1
(1.8x108 y)
Fig. 5. Natural (preindustrial) global sulfur reservoirs, fluxes, and turnover times. Major reservoirs are underlined, pool
sizes and fluxes are given in Tg (1012 g) S and Tg S/yr. Turnover times (reservoir divided by largest flux to or from reser
voir ) are in parentheses. To convert Tg S tomoles of S, multiply by 3.1 x 1010.
The lithosphere is the largest reservoir, with turnover times of approximately
major consituent, and ocean sediments, where sulfide and sulfate are the major
turnover times of >106 yr.
Carbonyl
sulfide (COS) is the longest lived gaseous
sulfur compound.
109 yr. Ocean waters, where sulfate is a
forms, have similar reservoir sizes and
Large particulate
fluxes of sea salt and terrestrial
dust are added to theatmosphere,but theirabundance is restrictedto altitudesof <1 km. The particleshave residence times
of days, close to rainout times, reflecting theircontrol by precipitation. Some volcanic emissions are injected into the
stratosphere,where theyhavemuch longer residence times.
Note that thenet preindustrialflux of sulfur (as sulfate aerosol) is from theocean to land.
October 1997
265
Pools and fluxes inTg S and Tg S yr-, Tg
(turnover times)
Atmosphere
Continental
Terrestrial dust 20 y- t
Biogenic 2.5 yr-'
emission 93 y-1t
Anthropogenic
10 y-1 t
Volcanoes
65 y-1
Deposition
1.6
Marine
81 y-' 20 y-'
(8d)
1012g;
3.2 (10 d)
COS (5-1 0y)
Seasalt particles 140 y-' t
Biogenic 15-30 y-1I
231 y- f
Deposition
Lakes & Rivers
Soils & Land Biota
3 x 105 (8.6x103 y)
300 (3y)
River runoff 213 y-'
Ocean
x 109 (6.8xl106 y)
~~~~~1.3
Lithosphere\
2.4 x 10'? (1 .8xl108 y)\
from lithosphere
Consumption
72 y-1
Weathering
Open Ocean
*Marine biota 30 (1 y)
150 y-1
Sedimentation
(burial)
135Y-1
Ocean Sediments
3x 108(4x106y)
References:
Andreae, 1990; Bates et al., 1992;
Charlson, Anderson &McDuff, 1992
Fig. 6. Global sulfur reservoirs, fluxes, and turnover times (mid-1980s). Major reservoirs are underlined, pool sizes and
fluxes are given in Tg (1012 g) S and Tg S/yr. Turnover times (reservoir divided by largest flux to or from reservoir ) are in
parentheses. To convert Tg S tomoles S, multiply by 3.1 x 10t?.
rates. Most sulfur is
additions at mid-1980s
This figure is the same as Fig. 5, but includes estimated anthropogenic
added to the atmosphere as SO2 by combustion of sulfur-rich fossil fuels (coal, oil). The SO2 is subsequently oxidized and
becomes sulfate aerosol, which is removed by precipitation.
sulfate additions is reversal of the net flux of sulfur from land to sea.
Note that one effect of anthropogenic
fluxes
Pool inTeramoles
(1012 moles),
moles y1 (turnover times)
Rivers
(150
Estuarine
Sedimentation
(gross)
5.6y-
9 x 104
surface <2 AM
| 70 AM av
deep 10-180gM
o
la(Biological 400 y)
Ocean
Atmosphere
0 ? (days)
iM av)
80% tropical
20% temperate
EoI -
5 y:
ynet
kBiological
0.6 y-1
in 1012
240 y-1
(days)
^\Production
120 y-1
\114.5
y-1
Deep
0.4 y1
Margins
(months)
Rain Rate
Weathering
2
(106-109 y)
Hydrothermal
References:
Tr6guer et al. 1995
Nelson et al. 1995
yssal
Sedimentation
29.1 y
Bhi
Deposition
120 y-1
Dissolution
p1Dsoltio
90.De0
83%Abyssal
17% Continental
Dissolution
1Export
pwelling
T<,5
Sedimentation
-
(net) 6.1 y.1
C
Dissolution
(months-centuries)
Fig. 7. Global silica reservoirs, fluxes, and turnover times. Major reservoirs are underlined, pool sizes and fluxes are given
in Tmol (1012moles) Si and Tmol Si/yr. Turnover times (reservoir divided by largest flux to or from reservoir ) are in pa
rentheses. To convert Tmol Si to Tg Si, multiply by 3.6 x 102.
im
The silica cycle is restricted to solid and aqueous phases, as there are no gaseous silica compounds of geochemical
portance.The silica cycle is dominatedby ocean processes. Silica releasedby continentalweathering is transportedin dis
solved and colloidal form by rivers to the ocean, where themajor sink is euphotic zone uptake by planktonic diatoms and
radiolariansat high latitudesand in the tropics, respectively.
266
Bulletin of theEcological Society of America
Pools in 103 km3; fluxes inkm3 y
(turnover times)
Atmosphere
13 (0.0009%)
1O1,000 y-1
Polar Ice, Glaciers
29,000 (2.08%) (16,000
lakes
Freshwater
(9 d)
0
y)
40,000
y-l
125(0.009%) (1-100y) 71,000 y-' Rivers
Saline lakes
104 (0.008%)
1.2 (0.00009%)
(10-1 000 yl
40,000
y-I
(12-20 d)
425,000
\
Soil moisture
67 (0.005%)
y-I
Ocean
~~~~~~1.37xl106
(97.61%)
385,000
(37000
y-I
y)
e
(280 d)
Groundwater
(active)
4000 (0.29%) (300 y)
References:
Schlesinger, 1993; Murray, 1992
Fig. 8. Global water
reservoirs, fluxes, and turnover times. Major reservoirs are underlined, pool sizes are given in 103
km3, and fluxes are given in km3/yr. Pool sizes, expressed as percentages of the total water budget, are given in parenthe
ses.
Water is present on Earth in three phases-solid,
all of these phases are present at some place in
liquid, and gas-and
the Earth's major water reservoirs. The ocean is the largest water reservoir, with over 97% of the Earth's water, followed
by the glacial reservoir with 2%. Both have turnover times of approximately
10,000 years. Although only slightly larger
than the river reservoir, water in the atmosphere, which accounts for only 0.0009% of the total budget, plays a very impor
tant role inmaintaining Earth's habitability. Water vapor is the most important greenhouse gas, and is responsible for some
30?C of greenhouse warming. In addition, water' s unusually high heat capacity and latent heats of vaporization and fusion
play an important role in heat storage and transport. All phases of atmospheric water play important roles in the Earth' s ra
diation budget. Atmospheric
processes
are responsible
Enteric fermentation 80 y'
Biomass burning 55 yt
Termites 20 y-t
Landfills 40 y'
Coal production 35 yt
Gas production 40 yt
Wetlands I115y'
boreal 35 y'
tropical 80 y'
Rice Production 100 yt
for transporting water from the ocean
to land.
Pools and fluxes inTg CH and Tg
yr', Tg = 10129;
COH
(turnover time)
Atmosphere
4800 (1.7 ppm, + 1% y-')
(9.6 y)
Photochemical oxidation 450 y'
Oceans and Lakes 10 y1
t
Soi consumption 40 y1
Hydrael____________
_107
5? y-I
? production 500 y-I - consumption 460 y- = 40 y' annual atmospheric increase
Fossil sources 20%
Modern biogenic sources 70-90%
References:
Cicerone &Oremland, 1988
Fung etal., 1991
Reeburgh, Whalen & Alperin, 1993
Fig. 9. Global methane reservoirs, fluxes, and turnover times. Major reservoirs are underlined, pool sizes and fluxes are
given in Tg (1012 g) CH4 and Tg CH4/yr. Turnover times (reservoir divided by largest flux to or from reservoir ) are in pa
rentheses. To convert Tg CH4 tomoles of C, multiply by 6.25 x 1010.
The methane budget is <1% of the Earth's carbon budget. Methane
is present in quantity in only three reservoirs on
Earth: as natural gas associated with fossil fuel reservoirs, as hydrates or clathrates (a cage-like structure of water ice that
containsmethane), and in the atmosphere,which is the smallest reservoir.Methane in the atmosphere is photochemically
oxidized, and the recentlyobserved increase in atmosphericconcentrationsis a resultof an imbalancebetween sources and
themajor sink, photochemicaloxidation.Research onmethane, an importantgreenhousegas, has focused on fluxes influ
encing theatmosphere.
October 1997
267