Botkin Environmental Science Earth as Living Planet 8th txtbk

118 CHAPTER 6 The Biogeochemical Cycles
the atmosphere through the respiration of living things,
through fires that burn organic compounds, and by diffusion
from the ocean. It is removed from the atmosphere
by photosynthesis of green plants, algae, and photosynthetic
bacteria and enters the ocean from the atmosphere
by the simple diffusion of carbon dioxide. The carbon
dioxide then dissolves, some of it remaining in that state
and the rest converting to carbonate (CO 3 ) and bicarbonate
(HCO 3 ). Marine algae and photosynthetic bacteria
obtain the carbon dioxide they use from the water in
one of these three forms.
Carbon is transferred from the land to the ocean in
rivers and streams as dissolved carbon, including organic
compounds, and as organic particulates (fine particles of
organic matter) and seashells and other forms of calcium
carbonate (CaCO 3 ). Winds, too, transport small organic
particulates from the land to the ocean. Rivers and streams
transfer a relatively small fraction of the total global carbon
flux to the oceans. However, on the local and regional
scale, input of carbon from rivers to nearshore areas, such
as deltas and salt marshes, which are often highly biologically
productive, is important.
Carbon enters the biota—the term for all life in a
region—through photosynthesis and is returned to the atmosphere
or waters by respiration or by wildfire. When
an organism dies, most of its organic material decomposes
into inorganic compounds, including carbon dioxide.
Some carbon may be buried where there is not sufficient
Flux to atmosphere (762 + 4.1/yr mostly due to burning fossil fuels) *
Volcanoes (0.1/yr)
Burning fossil
fuels (6.5/yr)
0.5
Land-use
change (1.6/yr)
Land
photosynthesis and
respiration (120/yr)
Oceanic photosynthesis
and respiration (90/yr)
Soil storage
(1580) *
Coal
Storage in land
plants (700)
*
Oil
?
Weathering and
erosion (0.4/yr)
(a)
(b)
*
Fossil fuel
storage (4000) *
Storage units in billions of metric tons of carbon
(GtC) and fluxes are GtC/yr. (uncertainties are ±20%)
Indicates direction of carbon transfer in GtC/yr.
Carbon stored in the
land biota, rocks, soil,
and fossil fuels
Carbon stored in
the atmosphere
?
Carbon stored in the
ocean biota, water,
and sediment
Storage in
ocean waters
38,000 *
Storage in marine sediments and
sedimentary rocks (100,000,000) *
FIGURE 6.16 The carbon
cycle. (a) Generalized global
carbon cycle. (b) Parts of the
carbon cycle simplified to illustrate
the cyclic nature of the movement
of carbon. (Source: Modified from
G. Lambert, La Recherche 18
[1987]:782–83, with some data
from R. Houghton, Bulletin of the
Ecological Society of America
74, no. 4 [1993]: 355–356, and
R. Houghton, Tellus 55B, no. 2
[2003]: 378–390), and IPCC, The
Physical Science Basis: Working
Group I. Contribution to the Fourth
Assessment Report [New York:
Cambridge University Press, 2007].)