deposited in the ocean abysses, forming sediment in which
carbon is trapped for millions of years.
Biosphere of the Past and Paleoclimates
For a long time, geologists have known that the land-based
and marine biospheres varied greatly in the past in response
to the slow process of evolution (on the scale of millions of
years, Chap. 27, Volume 2) but also to changes in the global
climate and in the general circulation of the atmosphere and
oceans, particularly at the pace dictated by the astronomical
paleoclimate theory (Chaps. 28, 30, Volume 2).
During their reproduction cycle, most plants produce tiny
grains with a shape and decoration characteristic of their
species. These are the pollen (the male fertile element of the
flower) and spores (vegetative structure for propagation or
reproducing) which have a very hard outer shell. They are
transported by wind or streams and stored well if they land in
a low-oxygen environment. They have been found in lake
sediments and bogs, where they provide a reasonably accurate picture of the vegetation once covering the vicinity of the
lake or bog. In this way, it has been discovered that, twenty
thousand years ago, the vegetation of France was that of a
polar steppe, at the peak of the last glaciation.
Land sediments contain other fossils which also provide
information on the local climate: diatoms, mollusks, ostracods
(tiny crustaceans with a calcareous shell) living in freshwater,
larger remains of plants (stem and leaf), charcoal remains from
large natural fires, fossil remains of larger animals, such as
bones of mammals found in archaeological sites.
The link between the climate and plant fossils is so close
that statistical methods have been developed to quantify this
link, based on current observations. The resulting relationships, called ‘transfer functions’ are used to estimate past
climate conditions if the same plant associations are found in
ancient sediments as those known today (see Chap. 12,
Volume 1).
Leaving aside the abundant fossils present in coastal areas
and in coral reefs, the marine biosphere leaves many traces
of its diversity in marine sediments: fish otoliths and teeth,
aragonitic shells of pteropods (marine snails), calcareous
shells of planktonic and benthic foraminifera, coccoliths
(calcareous sheets secreted by microscopic algae, called
coccolithophorids and living in warm or temperate waters),
siliceous skeletons of marine diatoms (algae living in cold
waters rich in silica) and radiolarians (microscopic animals
living in deep water). The relationship between the temperature of the sea water and abundance of various species
of foraminifera fossils and diatoms is close, which helps to
establish transfer functions to estimate marine paleotemperatures with an accuracy close to 1–2 °C (Chap. 21,
Volume 1).
It is important to be aware that transfer functions are only
valid insofar as the species discovered in ancient sediments,
both land and marine, are the same species as those that are
present today. Even if only a rough estimate of the conditions that prevailed in the geological past is required, it is
important to remember that species evolve and are likely to
adapt to very different environments over millions of years.
Thus, in the Jurassic era, 150–200 million years ago,
well-developed coral reefs in warm waters harbored a variety
of mollusks such as Pholadomya, Tridacna and Astarte.
Their distant descendants can be found today in very different environments: Pholadomya buried in the mud in
warm coastal waters, in a reef environment for Tridacna and
in polar waters for Astarte. This shows how the reconstruction of paleoenvironments requires a cautious approach
and the comparison of the various clues found in fossils.
The Cryosphere
Water in the form of ice or snow is the cryosphere. Glaciers
and ice caps cover approximately 11% of the surface of the
Earth. Spread out over the oceans, the water they contain
would increase the sea level by about 77 m. Sea ice, which is
formed by freezing seawater, covers approximately 7% of
the oceans, but is only a few meters thick. As it floats, its
melting does not cause the sea level to rise. On land, snow
cover varies greatly with seasons.
The Role of the Cryosphere
The main property of the cryosphere is its albedo. It can
reach 80–90% for fresh snow and barely drops below 50%
when surface ice melts or when the snow covers trees. This
property introduces the second major positive feedback loop
of the climate system. Where snow or ice melts out, the
ground absorbs a larger fraction of the incident solar energy.
Containing more energy, it heats up, thus facilitating the
melting of the remaining snow and ice. The process thus
becomes amplified. In the Milankovitch theory of ice ages
(see Chap. 28, Volume 2), the decrease in incoming solar
radiation, due to the slow variations of the orbit of the Earth
around the Sun, triggers the formation of ice caps thanks to
this positive feedback. Indeed, following the decrease of
sunshine, snow accumulated in the winter does not melt
completely, in turn less energy is absorbed and the snow
starts accumulating until ice caps are formed. As we shall
see later (Chap. 25, Volume 2) the mechanism is actually
more complex, and involves feedback from the ocean and
boreal biosphere in response to changes in insolation, even if
this positive feedback does constitute the factor triggering
the start of an ice age.
18
S. Joussaume and J.-C. Duplessy
carbon is trapped for millions of years.
Biosphere of the Past and Paleoclimates
For a long time, geologists have known that the land-based
and marine biospheres varied greatly in the past in response
to the slow process of evolution (on the scale of millions of
years, Chap. 27, Volume 2) but also to changes in the global
climate and in the general circulation of the atmosphere and
oceans, particularly at the pace dictated by the astronomical
paleoclimate theory (Chaps. 28, 30, Volume 2).
During their reproduction cycle, most plants produce tiny
grains with a shape and decoration characteristic of their
species. These are the pollen (the male fertile element of the
flower) and spores (vegetative structure for propagation or
reproducing) which have a very hard outer shell. They are
transported by wind or streams and stored well if they land in
a low-oxygen environment. They have been found in lake
sediments and bogs, where they provide a reasonably accurate picture of the vegetation once covering the vicinity of the
lake or bog. In this way, it has been discovered that, twenty
thousand years ago, the vegetation of France was that of a
polar steppe, at the peak of the last glaciation.
Land sediments contain other fossils which also provide
information on the local climate: diatoms, mollusks, ostracods
(tiny crustaceans with a calcareous shell) living in freshwater,
larger remains of plants (stem and leaf), charcoal remains from
large natural fires, fossil remains of larger animals, such as
bones of mammals found in archaeological sites.
The link between the climate and plant fossils is so close
that statistical methods have been developed to quantify this
link, based on current observations. The resulting relationships, called ‘transfer functions’ are used to estimate past
climate conditions if the same plant associations are found in
ancient sediments as those known today (see Chap. 12,
Volume 1).
Leaving aside the abundant fossils present in coastal areas
and in coral reefs, the marine biosphere leaves many traces
of its diversity in marine sediments: fish otoliths and teeth,
aragonitic shells of pteropods (marine snails), calcareous
shells of planktonic and benthic foraminifera, coccoliths
(calcareous sheets secreted by microscopic algae, called
coccolithophorids and living in warm or temperate waters),
siliceous skeletons of marine diatoms (algae living in cold
waters rich in silica) and radiolarians (microscopic animals
living in deep water). The relationship between the temperature of the sea water and abundance of various species
of foraminifera fossils and diatoms is close, which helps to
establish transfer functions to estimate marine paleotemperatures with an accuracy close to 1–2 °C (Chap. 21,
Volume 1).
It is important to be aware that transfer functions are only
valid insofar as the species discovered in ancient sediments,
both land and marine, are the same species as those that are
present today. Even if only a rough estimate of the conditions that prevailed in the geological past is required, it is
important to remember that species evolve and are likely to
adapt to very different environments over millions of years.
Thus, in the Jurassic era, 150–200 million years ago,
well-developed coral reefs in warm waters harbored a variety
of mollusks such as Pholadomya, Tridacna and Astarte.
Their distant descendants can be found today in very different environments: Pholadomya buried in the mud in
warm coastal waters, in a reef environment for Tridacna and
in polar waters for Astarte. This shows how the reconstruction of paleoenvironments requires a cautious approach
and the comparison of the various clues found in fossils.
The Cryosphere
Water in the form of ice or snow is the cryosphere. Glaciers
and ice caps cover approximately 11% of the surface of the
Earth. Spread out over the oceans, the water they contain
would increase the sea level by about 77 m. Sea ice, which is
formed by freezing seawater, covers approximately 7% of
the oceans, but is only a few meters thick. As it floats, its
melting does not cause the sea level to rise. On land, snow
cover varies greatly with seasons.
The Role of the Cryosphere
The main property of the cryosphere is its albedo. It can
reach 80–90% for fresh snow and barely drops below 50%
when surface ice melts or when the snow covers trees. This
property introduces the second major positive feedback loop
of the climate system. Where snow or ice melts out, the
ground absorbs a larger fraction of the incident solar energy.
Containing more energy, it heats up, thus facilitating the
melting of the remaining snow and ice. The process thus
becomes amplified. In the Milankovitch theory of ice ages
(see Chap. 28, Volume 2), the decrease in incoming solar
radiation, due to the slow variations of the orbit of the Earth
around the Sun, triggers the formation of ice caps thanks to
this positive feedback. Indeed, following the decrease of
sunshine, snow accumulated in the winter does not melt
completely, in turn less energy is absorbed and the snow
starts accumulating until ice caps are formed. As we shall
see later (Chap. 25, Volume 2) the mechanism is actually
more complex, and involves feedback from the ocean and
boreal biosphere in response to changes in insolation, even if
this positive feedback does constitute the factor triggering
the start of an ice age.
18
S. Joussaume and J.-C. Duplessy