HUMAN BIOLOGY

466 Chapter 24

carbon cycle Movement
of carbon molecules from
organisms and the Earth’s
crust to the atmosphere
and oceans, then back into
organisms.

bicarbonate

and carbonate

dissolved in

ocean water

marine food webs

producers, consumers,

decomposers

marine sediments, including

formations with fossil fuels

photosynthesis aerobic

respiration

death,

sedimentation

incorporation

into sediments

diffusion between

atmosphere and ocean

burning of fossil fuels

uplifting over

geologic time

sedimentation

land food webs

producers, consumers,

decomposers

terrestrial

rocks

peat,

fossil fuels

volcanic action

burning

of fossil

fuels

deforestation

death, burial, compaction over geologic time

weathering

leaching,

runoff

soil water

(dissolved carbon)

atmosphere

(mainly carbon dioxide)

photosynthesis aerobic

respiration

combustion

of wood (for clearing

land; or for fuel)

A B

Atlantic oceans to the Atlantic and Antarctic seafloors.

There, its CO 2 moves into deep “storage” before bottom

water loops up again (Figure 24.11). This looping movement

is a factor in carbon’s distribution in the biosphere and the

global carbon “budget.”

Photosynthesizers capture billions of metric tons of

carbon atoms in organic compounds every year. How ever,

the average length of time that a carbon atom is held in any

given ecosystem varies quite a bit. For example, organic

wastes and remains decompose rapidly in tropical rain

forests, so not much carbon accumulates at the surface of

soils. In marshes, bogs, and other places where there is little

or no oxygen, decomposers cannot break down organic

compounds completely, so carbon gradually builds up in

peat and other forms of compressed organic matter. Also,

in ancient aquatic ecosystems, carbon was incorporated

in shells and other hard parts. The shelled organisms

died and sank, then were buried in sediments. The same

things are happening today. Carbon remains buried for

the Carbon Cycle

Figure 24.10 Animated! Carbon cycles

through the oceans, atmosphere, and

the bodies of organisms. A shows the

cycle through typical marine ecosystems.

B shows how carbon cycles through land

ecosystems. Yellow boxes indicate the

main carbon reservoirs. The vast majority

of carbon atoms are in sediments and

rocks, followed by ever lesser amounts

in ocean water, soil, the atmosphere, and

biomass. Here are typical annual fluxes

in the global distribution of carbon, in

gigatons:

From atmosphere to plants

by carbon fixation 120

From atmosphere to ocean 107

To atmosphere from ocean 105

To atmosphere from plants 60

To atmosphere from soil 60

To atmosphere from

fossil fuel burning 5

To atmosphere from net

destruction of plants 2

To ocean from runoff 0.4

Burial in ocean sediments 0.1

n    Carbon moves through the atmosphere and food webs on

its way to and from the oceans, sediments, and rocks.

Figure 24.10 sketches the global

carbon cycle. Sediments and rocks

hold most of the carbon, followed

by the ocean, soil, atmo sphere, and

land biomass. Carbon enters the

atmo sphere as cells engage in aero-

bic respiration, as fossil fuels burn,

and when volcanoes erupt and release it from rocks in

Earth’s crust. Most atmospheric carbon occurs as carbon

dioxide (CO 2 ). Most carbon dissolved in the ocean is in the

forms of bicarbonate and carbonate.

You’ve likely seen bubbles of CO 2 rising to the surface of

a glass of carbonated soda. Why doesn’t the CO 2 in warm

ocean surface waters escape to the atmosphere? Driven

by winds and regional differences in water density, water

makes a gigantic loop from the surface of the Pacific and

24.5

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