Geoscience Reference
In-Depth Information
C
OMMENTS ON
F
IGURE
4.2.- After mitosis and a period without
division during which the cell grows (G
0
and G
1
), the cell can divide
again, forming a cycle.
As far as biogeochemical cycles are concerned, all chemical
elements naturally present on Earth and many compounds of these
elements gradually pass from one reservoir to another and ultimately
return to the initial reservoir. Depending on the chemical element or
compound considered, the completion of a biogeochemical cycle may
take years, decades, centuries, millennia, millions or hundreds of
millions of years. The pathways of elements or compounds between
natural reservoirs depends on both physical and chemical interactions
between the reservoirs and within them the activity of living organisms
and ecosystems. This creates a very close interdependence, on the one
hand, between the physical and chemical conditions that occur on the
Earth's surface and, on the other hand, the activity of living systems.
This interdependence, which is a fundamental characteristic of the
Earth's system, started when life appeared on the planet, approximately
four billion years ago, the Earth having been formed half a billion years
earlier.
Earth System Science
, which was founded in the 1990s,
advocates the comprehensive study of the Earth as a set of integrated
and dynamic systems, using Earth sciences, life sciences, applied
sciences and social sciences.
It is not possible to describe all biogeochemical cycles associated
with marine ecosystems in a few pages. In this chapter, we will explore
the biogeochemical cycles of
four rather important chemical elements
for marine ecosystems: carbon, oxygen, nitrogen and phosphorus.
Table 4.3 shows the distribution of these four elements in different
reservoirs of the outer envelopes of the Earth. Other elements strongly
involved in marine ecosystems are silicon, sulfur and iron. The
elements involved are partly different from those in a text about
terrestrial ecosystems where there may be shortages of some resources
that are not lacking in the marine environment, e.g. H
2
O or potassium
(K). As with any rule, there are exceptions, e.g. the desiccation
experienced by marine ecosystems of the Mediterranean Sea when that
basin almost completely dried up for 1-2 million years when the
precursor of the present Strait of Gibraltar closed up in the Messinian
