Environmental Engineering Reference
In-Depth Information
rests on the lithosphere as glaciers and icecaps or as icebergs floating on oceans. Water in
gaseous state extends into the lower layers of the atmosphere, the troposphere.
The
atmosphere
is the gaseous layer that surrounds the earth and extends to about 6,214
miles (10,000 km) from the surface. The lower layer, the troposphere, provides oxygen for
life support and is where the weather patterns take place. The uppermost layer of the
atmosphere offers protection against ultraviolet (UV) radiation that otherwise would affect
life negatively.
●
The
biosphere
includes all the living organisms as well as the biomass from those
organisms that have died and have not been decomposed yet. Living organisms do not
develop in isolation, but instead in communities called
ecosystems
. As a subsystem of the
biosphere, sometimes humans are placed in another sphere, the anthrosphere, which
includes humans as well as their structures (buildings, dams, bridges, roads, and so on).
●
The four spheres have defined boundaries, and interactions among them occur as the
interchange of mass and energy through the boundaries. This takes place by means of biogeo-
chemical cycles that promote the circulation of nutrients and other chemicals in a dynamic
process and the exchange of energy.
BIOGEOCHEMICAL CYCLES
The carbon cycle
The carbon cycle is extraordinarily complex and this section presents only an overview. The
carbon cycle can be divided into a long-term cycle that operates over spans encompassing
millions of years and a short-term cycle that takes place on a human time scale. It is important
to also make the distinction between preindustrial and modern carbon cycles.
The preindustrial cycle
The long-term carbon cycle has implications on deep carbon burial, formation of fossil fuels
from buried organic matter and carbonates, and the evolution of the atmospheric composition
including atmospheric carbon dioxide, oxygen, and climate. The long-term carbon cycle can
be represented by the following two chemical reactions (Berner, 2003):
CO
+
CaSiO
↔+
CaCO
SiO
[2.1]
2
3
3
2
(
)
n CO
+
n H O
↔
CH O
+
n O
[2.2]
2
2
2
2
n
As represented by Equation 2.1, carbon dioxide (CO
2
) is captured by reacting with calcium
or magnesium silicates in a process called
weathering
. The resulting calcium or magnesium
carbonates are then eroded from rocks and delivered to oceans where they precipitate into
sediments. Eventually movement of tectonic plates buries the carbon deeper into the Earth's
mantle in a process called
subduction
. High temperatures in the mantle can reverse the reaction
in Equation 2.1 and release the carbon back to the atmosphere and the oceans (Berner, 2003).
Equation 2.2 represents the capture of carbon dioxide from the atmosphere by photosynthesis.
In a long-term carbon cycle, biomass ( (CH
2
O)
n
) is deposited in marine sediments and
consequently buried deeper by subduction. High pressures and temperatures then transform
the organic matter into kerogen, oil, gas, and coal (Berner, 2003).
