Basics of Environmental Science

Basics of Environmental Science

(14N) by cosmic radiation, but it is unstable and decays to the commoner 12C at a steady rate. While

water is exposed to the air, both 12C and 14C dissolve into it, but once isolated from the air the

decay of 14C means that the ratio of the two changes, 12C increasing at the expense of 14C. It is

assumed that 14C forms in the air at a constant rate, so the ratio of 12C to 14C is always the same and

certain assumptions are made about the rate at which atmospheric carbon dioxide dissolves into

sea water and the rate at which water rising from the depths mixes with surface water. Whether or

not the initial assumptions are true, the older water is the less 14C it will contain, and if the

assumptions are true the age of the water can be calculated from its 14C content in much the same

way as organic materials are 14C-dated.

Carbon, oxygen, and sulphur are among the elements living organisms use and they are being cycled

constantly through air, water, and living cells. The other elements required as nutrients are also

engaged in similar biogeochemical cycles. Taken together, all these cycles can be regarded as

components of a very complex system functioning on a global scale. Used in this sense, the concept

of a ‘system’ is derived from information theory and describes a set of components which interact to

form a coherent, and often self-regulating, whole. Your body can be considered as a system in which

each organ performs a particular function and the operation of all the organs is coordinated so that

you exist as an individual who is more than the sum of the organs from which your body is made.

Biochemical cycles

The surface of the Earth can be considered as four distinct regions and because

the planet is spherical each of them is also a sphere. The rocks forming the

solid surface comprise the lithosphere, the oceans, lakes, rivers, and icecaps

form the hydrosphere, the air constitutes the atmosphere, and the biosphere

contains the entire community of living organisms.

Materials move cyclically among these spheres. They originate in the rocks

(lithosphere) and are released by weathering or by volcanism. They enter

water (hydrosphere) from where those serving as nutrients are taken up

by plants and from there enter animals and other organisms (biosphere).

From living organisms they may enter the air (atmosphere) or water

(hydrosphere). Eventually they enter the oceans (hydrosphere), where

they are taken up by marine organisms (biosphere). These return them to

the air (atmosphere), from where they are washed to the ground by rain,

thus returning to the land.

The idea that biogeochemical cycles are components of an overall system raises an obvious question:

what drives this system? It used to be thought that the global system is purely mechanical, driven by

physical forces, and, indeed, this is the way it can seem. Volcanoes, from which atmospheric gases

and igneous rocks erupt, are purely physical phenomena. The movement of crustal plates, weathering

of rocks, condensation of water vapour in cooling air to form clouds leading to precipitation—all

these can be explained in purely physical terms and they carry with them the substances needed to

sustain life. Organisms simply grab what they need as it passes, modifying their requirements and

strategies for satisfying them as best they can when conditions change

Yet this picture is not entirely satisfactory. Consider, for example, the way limestone and chalk rocks

form. Carbon dioxide dissolves into raindrops, so rain is very weakly acid. As the rain water washes

across rocks it reacts with calcium and silicon in them to form silicic acid and calcium bicarbonate,

as separate calcium and bicarbonate ions.