Botkin Environmental Science Earth as Living Planet 8th txtbk

6.5 Some Major Global Biogeochemical Cycles 117
6.5 Some Major Global
Biogeochemical Cycles
With Figure 6.14’s basic diagram in mind, we can now consider
complete chemical cycles, though still quite simplified.
Each chemical element has its own specific cycle, but all the
cycles have certain features in common (Figure 6.15).
The Carbon Cycle
Carbon is the basic building block of life and the element
that anchors all organic substances, from coal and
oil to DNA (deoxyribonucleic acid), the compound that
carries genetic information. Although of central importance
to life, carbon is not one of the most abundant elements
in Earth’s crust. It contributes only 0.032% of the
weight of the crust, ranking far behind oxygen (45.2%),
silicon (29.5%), aluminum (8.0%), iron (5.8%), calcium
10, 11
(5.1%), and magnesium (2.8%).
The major pathways and storage reservoirs of the carbon
cycle are shown in Figure 6.16. This diagram is simplified
to show the big picture of the carbon cycle. Details
Oceans and land ecosystems: In the past half-century,
ocean and land ecosystems have removed about 3.1 0.5
GtC/yr, which is approximately 45% of the carbon emitted
from burning fossil fuels during that period.
Land-use change: Deforestation and decomposition of
what is cut and left, as well as burning of forests to make
room for agriculture in the tropics, are the main reasons
2.2 0.8 GtC/yr is added to the atmosphere. A small
flux of carbon (0.2 0.5 GtC/yr) from hot tropical
areas is pulled from the atmosphere by growing forests.
In other words, when considering land-use change, deforestation
is by far the dominant process.
Residual land sink: The observed net uptake of
CO 2 from the atmosphere (see Figure 6.16) by land
ecosystems suggests there must be a sink for carbon in
land ecosystems that has not been adequately identified.
The sink is large, at 2 to 3 GtC/yr, with large uncertainty
( 1.7GtC/yr). Thus, our understanding of the
carbon cycle is not yet complete.
Carbon has a gaseous phase as part of its cycle, occurring
in the atmosphere as carbon dioxide (CO 2 ) and
methane (CH 4 ), both greenhouse gases. Carbon enters
are much more complex. 12 Soil and rock minerals (slow turnover) Water
Atmosphere
Dust and
inorganic gases
Biological uptake
Biological release
Rain
and
dust
Organic material
From the
biosphere
Mineralization
Biological uptake
To the
biosphere
Soil (available)
Mineral formation
Weathering
flow
FIGURE 6.15 Basic biogeochemical cycle.