The Geochemistry of the Earth - Thanatia: The Destiny of the Earth's Mineral Resources - page 108

of carbon dioxide, methane and fluorocarbons. All are believed to contribute to
global warming. Another consequence of mankind, due to the widespread use of
chlorofluorocarbons, is the depletion of the stratospheric ozone which lowers the
effectiveness of the atmosphere to protect biota against UV radiation.
5.3.1 The chemical composition of the atmosphere
In terms of its constituent gases, the atmosphere shows a notably uniform chemical
composition to heights of about 100 km (Fyfe, 1974), except for water which varies
with location and season as well as elevation. Above this altitude, the atmosphere
becomes layered and non-uniform. The atmosphere is roughly composed by (volume
content) 78% of usually inert nitrogen 1 , around 21% of oxygen, 0.93% argon, 380
ppm of carbon dioxide, a variable amount of water vapour (average 1%) and trace
amounts of other gases.
This mixture of gases is commonly known as air. The latest atmospheric geo-
chemical advances are compiled in Keeling (2004). The main components of the
troposphere and their origin are explained below based on the information provided
by Turekian (1969).
Nitrogen in the presence of oxygen at the surface of the oceans combines to form
nitrate in solution as the stable form. Both nitrogen and oxygen are maintained
at their current levels by biological processes. Oxygen is more biologically
controlled than nitrogen but both are dependent on the chemical actions of life.
The argon in the atmosphere is believed to have been produced by the radioac-
tive decay of potassium-40 and released to the atmosphere via degassing.
Methane and carbon dioxide are closely tied to biological activity. Their pres-
ence is directly sustained by production from bacteria and animals.
Manmade impurities such as sulphur dioxide and carbon monoxide, which are
responsible for the physical discomforts of smog, are sometimes found in high
concentrations in urban areas.
A summary of the composition of the atmosphere at the start of the 21st century
undertaken by Prinn (2004), from Brasseur et al. (1999) and Prinn et al. (2000) is
given in Table 5.2.
5.4 The hydrosphere
The hydrosphere is the liquid water component of the Earth. It includes oceans,
seas, lakes, rivers, rain, underground water, ice and the atmospheric water vapour
contained in clouds. It covers about 70% of the planetary surface and is home to
many plants and animals. The h ydrosphere is in continuous motion through the
1 The few exceptions are upon electrolysis by lightning and in certain biochemical processes of
nitrogen fixation.