incorporated into aggregates of particulate fecal
material may drag down extra organic carbon into
the ocean interior).
Microscopic examination of seafloor sediments
(if shallow enough that the CaCO 3 does not dissolve)
and of material caught in sediment traps has revealed
that much of the calcium carbonate in the samples
consists of coccoliths. The flux of coccoliths
probably accounts for c. 50% of the total vertical
CaCO 3 flux in open ocean waters (in other words,
about 50% of the inorganic carbon pump),
with foraminifera shells responsible for most of the
rest. It is usually not the most numerous species
(E. huxleyi) but rather larger species (e.g., Calcidiscus quadriperforatus and Coccolithus pelagicus)
that make the greatest contributions to the total
coccolith flux.
Coccolithophores also impact on climate in other
ways, ones that are unconnected with carbon. Coccolithophores are intense producers of a chemical
called dimethylsulfoniopropionate (DMSP). The
production of DMSP leads eventually (via several
chemical transformations) to additional cloud condensation nuclei in the atmosphere and thereby to
increased cloud cover.
Coccoliths also scatter light, polarizing it in the
process. They do not reflect or block light (this
would clearly be disadvantageous for the photosynthetic cell underneath), but the difference between the refractive indices of water and of calcium
carbonate means that the trajectories of photons are
deflected by encounters with coccoliths. A small
proportion of the scattering (deflection) events are
through angles greater than 901, leading to photons
being deflected into upward directions and eventually passing back out through the sea surface. Because of this light-scattering property of coccoliths,
their bulk effect is to make the global oceans slightly
brighter than they would otherwise be. It has been
calculated that the Earth would become slightly
dimmer (the albedo of the Earth would decrease by
about 0.1% from its average global value of about
30%) were coccolithophores to disappear from the
oceans. The effect of coccoliths in enhancing water
brightness is seen in its most extreme form during
coccolithophore blooms (Figure 3).
The Past
Coccolithophores are currently the dominant type
of calcifying phytoplankton, but further back in
time there were other abundant calcifiying phytoplankton, for instance the nannoconids, which may
or may not have been coccolithophores. The fossil
calcifying phytoplankton are referred to collectively
as calcareous nannoplankton.
The first calcareous nannoplankton are seen in the
fossil record c. 225 Ma, in the late Triassic period.
Abundance and biodiversity increased slowly over
time, although they were at first restricted to shallow
seas. During the early Cretaceous (145–100 Ma),
calcareous nannoplankton also colonized the open
ocean. They reached their peak, both in terms of
abundance and number of different species (different
morphotypes) in the late Cretaceous (100–65 Mya).
‘The Chalk’ was formed at this time, consisting
of thick beds of calcium carbonate, predominantly coccoliths. Thick deposits of chalk are most
noticeable in various striking sea cliffs, including the
white cliffs of Dover in the United Kingdom, and the
Isle of Rugen in the Baltic Sea. The chalk deposits
were laid down in the shallow seas that were widespread and extensive at that time, because of a high
sea level.
Calcareous nannoplankton, along with other
biological groups, underwent long intervals of slowly
but gradually increasing species richness interspersed
with occasional extinction events. Their heyday
in the late Cretaceous was brought to an abrupt end
by the largest extinction event of all at the
K/T boundary (65 Ma), at which point B93% of
all species (B85% of genera) suddenly went extinct.
Although biodiversity recovered rapidly in the
early Cenozoic, calcareous nannoplankton have
probably never since re-attained their late Cretaceous levels.
Because the chemical and isotopic composition of
coccoliths is influenced by the chemistry of the seawater that they are synthesized from, coccoliths from
ancient sediments have the potential to record details
of past environments. Coccoliths are therefore a
widely used tool by paleooceanographers attempting
to reconstruct the nature of ancient oceans. Some of
the various ways in which coccoliths are put to use in
interpreting past conditions are as follows: (1)
elemental ratios such as Sr/Ca and Mg/Ca are used to
infer past seawater chemistry, ocean productivity,
and temperatures; (2) the isotopic composition
(d
13
C, d
18 O) of the calcium carbonate is used to infer
past carbon cycling, temperatures, and ice volumes;
(3) the species assemblage of coccoliths (some species
assemblages are characteristic of eutrophic conditions, some of oligotrophic conditions) is used to
infer trophic status and productivity. Some of the
organic constituents of coccolithophores are also
used for paleoenvironmental reconstructions. In
particular, there is a distinctive group of ketones,
termed long-chain alkenones, which are specific to
one family of coccolithophores and closely related
408 COCCOLITHOPHORES
material may drag down extra organic carbon into
the ocean interior).
Microscopic examination of seafloor sediments
(if shallow enough that the CaCO 3 does not dissolve)
and of material caught in sediment traps has revealed
that much of the calcium carbonate in the samples
consists of coccoliths. The flux of coccoliths
probably accounts for c. 50% of the total vertical
CaCO 3 flux in open ocean waters (in other words,
about 50% of the inorganic carbon pump),
with foraminifera shells responsible for most of the
rest. It is usually not the most numerous species
(E. huxleyi) but rather larger species (e.g., Calcidiscus quadriperforatus and Coccolithus pelagicus)
that make the greatest contributions to the total
coccolith flux.
Coccolithophores also impact on climate in other
ways, ones that are unconnected with carbon. Coccolithophores are intense producers of a chemical
called dimethylsulfoniopropionate (DMSP). The
production of DMSP leads eventually (via several
chemical transformations) to additional cloud condensation nuclei in the atmosphere and thereby to
increased cloud cover.
Coccoliths also scatter light, polarizing it in the
process. They do not reflect or block light (this
would clearly be disadvantageous for the photosynthetic cell underneath), but the difference between the refractive indices of water and of calcium
carbonate means that the trajectories of photons are
deflected by encounters with coccoliths. A small
proportion of the scattering (deflection) events are
through angles greater than 901, leading to photons
being deflected into upward directions and eventually passing back out through the sea surface. Because of this light-scattering property of coccoliths,
their bulk effect is to make the global oceans slightly
brighter than they would otherwise be. It has been
calculated that the Earth would become slightly
dimmer (the albedo of the Earth would decrease by
about 0.1% from its average global value of about
30%) were coccolithophores to disappear from the
oceans. The effect of coccoliths in enhancing water
brightness is seen in its most extreme form during
coccolithophore blooms (Figure 3).
The Past
Coccolithophores are currently the dominant type
of calcifying phytoplankton, but further back in
time there were other abundant calcifiying phytoplankton, for instance the nannoconids, which may
or may not have been coccolithophores. The fossil
calcifying phytoplankton are referred to collectively
as calcareous nannoplankton.
The first calcareous nannoplankton are seen in the
fossil record c. 225 Ma, in the late Triassic period.
Abundance and biodiversity increased slowly over
time, although they were at first restricted to shallow
seas. During the early Cretaceous (145–100 Ma),
calcareous nannoplankton also colonized the open
ocean. They reached their peak, both in terms of
abundance and number of different species (different
morphotypes) in the late Cretaceous (100–65 Mya).
‘The Chalk’ was formed at this time, consisting
of thick beds of calcium carbonate, predominantly coccoliths. Thick deposits of chalk are most
noticeable in various striking sea cliffs, including the
white cliffs of Dover in the United Kingdom, and the
Isle of Rugen in the Baltic Sea. The chalk deposits
were laid down in the shallow seas that were widespread and extensive at that time, because of a high
sea level.
Calcareous nannoplankton, along with other
biological groups, underwent long intervals of slowly
but gradually increasing species richness interspersed
with occasional extinction events. Their heyday
in the late Cretaceous was brought to an abrupt end
by the largest extinction event of all at the
K/T boundary (65 Ma), at which point B93% of
all species (B85% of genera) suddenly went extinct.
Although biodiversity recovered rapidly in the
early Cenozoic, calcareous nannoplankton have
probably never since re-attained their late Cretaceous levels.
Because the chemical and isotopic composition of
coccoliths is influenced by the chemistry of the seawater that they are synthesized from, coccoliths from
ancient sediments have the potential to record details
of past environments. Coccoliths are therefore a
widely used tool by paleooceanographers attempting
to reconstruct the nature of ancient oceans. Some of
the various ways in which coccoliths are put to use in
interpreting past conditions are as follows: (1)
elemental ratios such as Sr/Ca and Mg/Ca are used to
infer past seawater chemistry, ocean productivity,
and temperatures; (2) the isotopic composition
(d
13
C, d
18 O) of the calcium carbonate is used to infer
past carbon cycling, temperatures, and ice volumes;
(3) the species assemblage of coccoliths (some species
assemblages are characteristic of eutrophic conditions, some of oligotrophic conditions) is used to
infer trophic status and productivity. Some of the
organic constituents of coccolithophores are also
used for paleoenvironmental reconstructions. In
particular, there is a distinctive group of ketones,
termed long-chain alkenones, which are specific to
one family of coccolithophores and closely related
408 COCCOLITHOPHORES