The White Cliffs of Dover are an Example of Natural Carbon Sequestration
The purpose of this study is to review carbon sequestration by coccolithophores and consider it as a controllable technology. Recent research suggesting runaway global warming has inspired some scientists to propose methods to intervene in global geochemical processes and prevent future catastrophes. These include schemes to sequester carbon dioxide under oceans, use of orbiting mirrors to reflect energy into space before it can heat the atmosphere or seeding the atmosphere with sulfur compounds which precipitate clouds and reflect radiant energy. However, diverse scientific literature shows there is no agreement on the best methods to attempt and very little empirical experimentation to suggest which are practical. Additionally, there are some unexpected consequences suggested by critics which show many proposals for global geochemical engineering are fraught with danger and that they effectively replace one problem with another, rather than improve matters. However, geochemical systems are in stable equilibrium with each other and for that reason doing nothing may be the best tactic for global geochemical engineering. Geologic history has shown that natural processes give negative feedback to perturbations in the atmosphere, attenuating runaway global warming without human intervention and the White Cliffs of Dover are a great example of this. Control of calcite formation from coccoliths is a complex problem beyond our present technology.
Received: April 29, 2010;
Accepted: June 06, 2010;
Published: April 29, 2011
During a scientific meeting at Monmouth University in April 2010 the subject
of Global Warming (Ackerman and Knox, 2006) and its attenuation
by carbon sequestration was considered. In particular the role of algae was
discussed because it is strange but true that small creatures can have very
large effects on ecosystems. One group is the coccolithophores (Green
and Leadbeater, 1994) which are beyond clear observation by light microscopy
but whose effects are very large because of their cell wall chemistry (Mukkamala
et al., 2006). Coccolithophores are unicellular algae, common in
oceanic ecosystems (Reid et al., 2009), which
have a distinctive feature: cell walls composed of calcium carbonate plates
(coccoliths, Fig. 1). Late in growth, blooms of these algae
shed large amounts of their coccoliths. These turn water a milky turquoise color
(Shutler et al., 2010) similar to the rock flour
of glacial lakes (personal observation). This material is easily detected in
satellite images because of its ability to backscatter visible light (Merico
et al., 2003; Smyth et al., 2004)
against a background of more transparent sea water. Their process of cell wall
formation involves catalyzing the simple reaction of carbon dioxide (carbonic
acid) to carbonate. This is a spontaneous reaction familiar to any scientist
that has prepared a solution of sodium hydroxide then stored the solution in
a bottle with a loose cap; a fine white precipitate of sodium carbonate is formed
by atmospheric CO2 reacting with the NaOH.
||Coccolithus pelagicus (Lampitt,
2008), showing button shaped cell wall plates composed of calcium carbonate.
This species is common in cool North Atlantic waters (Cachao
and Moita, 2000)
As with most life processes coccolith formation is an enzymic process and coccolithophores
do not cause the reaction, but they do greatly increase the rate of reaction.
Much was learned about coccolith sedimentation and metamorphosis to calcite
during the boom in North Sea Oil exploration of the 1960's and 1970's. Coccoliths
are not significantly larger than the fine carbonate powders produced in chemical
reactions and sediment at about 13.8 cm day-1 (Steinmetz,
1994), so slowly that they may drift to unsaturated zones of the ocean which
allow carbonate dissolution (Honjo, 1975) before fine
carbonates can settle to the sea floor. However, because coccolithophore algae
are consumed by animals such as copepods and aggregated into fecal pellets (Ziveria
et al., 2007) they become sufficiently large to sediment rapidly
(200 m day-1, Steinmetz, 1994), accumulate
and metamorphose to the mineral chalk, a form of cemented calcite particles
(Neugebauer, 1974). This displaces the sea surface chemical
equilibrium, drawing more CO2 into reaction to carbonate. Rapid coccolith
aggregate sedimentation is a process that has sequestered large amounts of CO2
in the geological past (Gregory, 2002) and is a factor
for enormous and rapid reduction of atmospheric CO2 and therefore
global warming (Fig. 2, Salter et al.,
2007). Even so, coccoliths may suffer partial or complete dissolution after
reaching ocean sediments (Roth and Berger, 1975) because
deep sea water may not be saturated with carbonate.
These microscopic biological activities might be ignored by most people if
it were not for their large scale effects that are visible to the unaided eye:
for example, the White Cliffs of Dover (Fig. 2, Table
1). The cliffs are composed of fossilized coccolith aggregates (Saruwatari
et al., 2008) that fell to the bottom of shallow seas and built up
deposits which were mineralized to the chalky form of calcium carbonate (Mukkamala
et al., 2006). This was an important natural process in warm shallow
seas (Buitenhuis, 2000), when thick layers of calcite
formed on European continental shelves when their margins were flooded (Katz
et al., 2004; Kennedy and Garrison, 1975)
during the cretaceous epoch (Tyson and Funnell, 1987).
The White Cliffs are prominent, but they are just the tip of an iceburg of calcite
deposits that extend over large areas of North Western Europe (Fig.
3) (Hakansson et al. (1991), Buitenhuis
(2000) and citations within); naturally sequestered carbon, removed
from the carbon dioxide rich atmosphere of the cretaceous epoch.
||The White Cliffs of Dover near the South Foreland Lighthouse,
seen from St. Margaret's Bay, Dover, 51°8'6" N, 1°22'30" E (Jouan,
2006). The Lighthouse is 21 m high and the cliffs are 101 m high at
that point (Clegg, personal communication)
GLOBAL ENGINEERING AND UNINTENDED CONSEQUENCES
Coccolithophores have long been recognized as unique and interesting algae
(Siesser, 1994), but recently they have attracted interest
from global engineering theorists. Ecologists with ambitions for global environmental
engineering have looked at the White Cliffs of Dover and been inspired by the
effects of coccolithophores (Woodward et al., 2009).
Their ambition is to achieve similar large scale Carbon Sequestration and control
atmosphere composition (Wilson and Gerard, 2007; De
Baar et al., 2005). These scientists see the contribution by fossil
fuels of 5.4x1012 tonnes of carbon per year to the atmosphere as
a problem that could be solved if the converse process by coccolithophores was
induced i.e., consignment of 3.2x1012 tonnes carbon per year as carbonate
to sediments (Raven and Falkowski, 1999). These are
comparable figures and might be used like tunable factors to balance one another
and control atmospheric CO2 content. The most commonly suggested
method for achieving this process is stimulating blooms of coccolithophores
by ocean fertilization with inorganic iron (De Baar et
al., 2008; Jin et al., 2008) because
iron is the limiting nutrient in oceanic ecosystems (Pomar
and Hallock, 2008) i.e., creating Artificial White Cliffs of Dover by stimulating
||Western Europe, showing the approximate extent of calcite
deposits (Downing et al., 1993) that formed
as continental shelves were inundated (Tyson and Funnell,
1987; Hakansson et al., 1991). The White
Cliffs of Dover are indicated by "*". These deposits formed during the cretaceous
epoch, a name derived from the Latin for chalk; creta
However several nutritional and other factors determine coccolithophore blooming
(Tyrrell and Merico, 2004) and other algae may also form
blooms near the coccolith growth optima (Margalef, 1978;
Balch, 2004), making the control of coccolithophore bloom
processes tenuous. Observing field work is also limited by detecting coccolithophore
bloom as scattered light: it is not only coccoliths that create significant
back scatter (Tyrrell and Merico, 2004). Diatoms are
significantly stimulated by iron fertilization (De Baar
et al., 2005) and have highly refractive silica cell walls. Therefore
measurements that rely on satellites only must be treated with caution.
While the sight of the White Cliffs is inspiring it may be salutary to remember
how common the unintended consequences of our actions are (Hu
et al., 2003). Algal blooms, including blooms of coccolithophores
are common, but how algae thrive or die and how they bloom is a generally unknown
process and not easily manipulated by us in a controlled way (Marsh,
2003). Many of the methods of geochemical engineering have strong critics
and increasing numbers of objections have been made (Robock,
2008; Robock et al., 2009; Hegerl
and Solomon, 2009). This may be because of the very complex nature of both
atmospheric and biological systems. For example, Raven and
Falkowski (1999) list twenty two processes or factors involved in the movement
of carbon from the atmosphere to oceans and sediments i.e., these are difficult
systems to understand even if all relevant factors are independent and linear
(simple to extrapolate), or even known. Some particular unintended consequences
of coccolithophore blooms are already known (Denman, 2008;
Malin and Steinke, 2004) and include increased but unquantified
denitrification and production of the long lived greenhouse gas N2O
(Law, 2008) i.e., plans to reduce carbon dioxide using
coccolithophores would result in increases in other greenhouse gases. This is
a paradox that has been seen before, for example carbon sequestration by increased
forestation may, in some ecosystems, result in increased methane emission (Megonigal
and Guenther, 2008), another greenhouse gas.
NATURAL PROCESSES ARE IN STABLE EQUILIBRIA
Part of the problem of global environmental engineering is an assumption that
the climate is characterized by unstable equilibria which are capable of runaway
changes to new stable states. However, if this were true the earth's climate
could not recover from the many great knocks it has experienced in the past.
Examples include volcanic eruptions which have covered millions of square kilometers
in molten basalt (Pande, 2002) or darkened the atmosphere
with large volumes of particulate matter (Schroder, 2002),
which are later washed out by rain (Park et al.,
2005). When the earth has been violently struck by comets there was an initial
perturbation (Kring, 2007) then a resumption of our
present desirable climate equilibrium, not a runaway and permanent disaster.
In fact, asteroid impacts may be a regular occurrence in the history of the
earth due to the passage of the solar system through galactic debris belts and
life has always recovered (Bailer-Jones, 2009). These
violent events indicate a stable rather than unstable equilibrium between factors
controlling the climate; like a pendulum displaced from equilibrium, the atmosphere
naturally swings back from extremes. The atmosphere is not like a tall vase
which may teeter when knocked, then fall and occupy a second horizontal state
of equilibrium, far from the first. If there's many a slip twixt cup and lip
it is better to rely on natural processes, such as those that formed the White
Cliffs of Dover and the stable equilibrium of geological processes we have all
benefitted from for so long, than the uncertain outcomes of global engineering
plans which have not been adequately formulated or tested.
John Tiedemann and Dennis Rhoads inspired this study. Fang Xie of the Guggenheim library at Monmouth University was very helpful in preparation of this study.
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