Geoscience Reference
In-Depth Information
In the lithosphere, there is carbon in sedimentary rocks, which
result from the consolidation of marine sediments, in metamorphic
rocks and in some magmatic rocks. Marine sediments largely come
from organic and inorganic material of biological origin that
accumulates on the ocean floor. Over millions of years, sediments are
transformed into various rocks including limestone (CaCO
3
) or into
fossil organic matter, including oil and natural gas (whose principal
constituent is CH
4
). Within the sediments, CH
4
is also present in the
form of methane clathrate
(also called fire ice), which is a substance
where the molecules of CH
4
are trapped in thin cages of ice,
the general name of such a chemical structure being clathrate. Most of
the methane clathrate is found in deep oceanic sediments, where CH
4
is released through local bacterial transformation of organic sediments
(equation [4.14]). A small portion of methane clathrate is found on the
continental shelf at depths of hundreds of meters, originating from the
migration of CH
4
formed deeper through the thermal decomposition of
organic matter. Methane clathrate is younger than hydrocarbons, but
the quantity of carbon it represents is similar (see values in Table 4.4).
The largest reservoir of carbon on Earth consists of sedimentary rocks.
In the hydrosphere, carbon may be in dissolved or particulate
inorganic forms. Dissolved inorganic carbon (DIC) includes
the different chemical species of dissolved CO
2
(section 4.3.2). The
concentrations of DIC per unit volume of seawater are higher in deep
oceanic waters than close to the surface (see Chapter 2 of [MON 14b],
section 2.4.1). Particulate inorganic carbon (PIC) is essentially
composed of calcium carbonate (CaCO
3
). It includes calcareous
skeletons (also known as “tests”) of planktonic organisms (the tests are
external structures of organisms that include coccolithophores,
foraminifera and pteropods) and external structures of benthic
organisms (e.g. corals and shells).
The biosphere includes living organisms, for which part of the
biomass is active and part is inert (e.g. the wood of trees). On land,
most of the vegetal biomass is inert whereas, in the marine
environment, almost all the biomass is active. Hence the terrestrial
vegetal biomass has a turnover, on average, of ten years, whereas
marine vegetal biomass (phytoplankton) has a turnover, on average, of
