Elsevier Encyclopedia of Geology

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Table 6 Sequential environmental permitting planning for recovery of building stone

Step Considerations Geologic detail

Topographic layout Defines where the recovery is to take place Scale of 1:1000–1:2000

Contour interval of one meter

Enlarged ordnance map or plane-tabled topographic

map of deposit bounds

Property bounds to 100 m lateral buffer space

Present hydrological features

Present cultural features

Exposed geological

detail

Contact of soil with bedrock, if exposed or as

predicted

Include notion of geological structure

Proposed product

stone

Dimensions and character

How stored

1. Note possible variations in the character of stone

across the breadth and depth of the planned

quarry

Frequency and method of removal from the site 2. Subdivide the quarry into separate ‘‘domains’’ if

differences exceed the flexibility of marketing

Mining concept Explains how the deposit will be opened up and how

the stone will be accessed

1. Cutting bounds, crown of cut and toe of slopes

2. Sequential advance of the cutting with final

proposed lateral bounds

3. Nature of machines to be employed

4. System of parting the stone from the outcrop or

rock mass

5. Use of explosives

Management of

runoff

Describe how precipitation and snowmelt will be

managed

Indicate presence of drainage gradient and

channelization to be placed to remove runoff to

suitable discharge

Management of

spoil

Describe dust, fine particles and sediment likely to

be generated

Rejection policy/procedure for discrimination of

stone vs. spoil

Show locations for interim spoil placement

Determine general

nature of stone

desired

Nature of stone determines the likely source

geological formation(s) to be prospected

Reduces the exploration task to Identification of

suitable terrain in lands available for recovery

Determine the

economic radius

for stone recovery

This defines the area of exploration Acquire medium-scale geologic maps of the area of

economic consideration

Access and haul

roads

Within about 8 km of centre of area of interest Examine road cuts and outcrops to ascertain the

general presence of suitable stone

Product storage

area

Placement of accumulated stone produce between

haul loads

Most stone today is palletised for unit-transport

packages of one cubic meter; avoids labour costs

in distribution to building sites

Spoil storage area Resting place for spoil accumulated from stone

removal and selection for transport

Ideally removed periodically to the final disposal

site, in accordance with quarry closure plans or

for sale as off-site embankment or fill material

Sequential quarry

reclamation

Ongoing plan of converting incremental phase-areas

of the overall stone removal site to suit the

environmental permitting permit

Normally comes as a consequence of working within

site ownership bounds and in consideration of

stability of the highwall resulting from removal of

stone from the bedrock outcrop

Management of

incidental waters

1. Runoff from rainfall and snowmelt Requires temporary ditching and/or area sumps and

pump-withdrawal to authorised discharge point2. Accommodation of seepage from the highwall

Management of

sediment

Dust and small particles subject to wind and runoff

transport from the quarry to the environment

Generally managed by placement of sedimentation

sumps and bales of cut vegetation such as ‘hay’

Requires ultimate on-site manipulation as part of the

closure plan

Closure plan Presumes that the quarry will be provided with an

acceptable after-use

Should be worked into the long-term operational

plan and be in conformance with the required

end-use of the operating permit

332 BUILDING STONE

related to lithology, conditions of origin, and geological

history (Table 3) and are reflected in mineralogy and

petrography, and include fabric and texture. Large

expenditure on the acquisition of building stone should

include a petrographical analysis for the latter features.

Locating Sources of

Building Stone

Building stone has a relatively low economic value in

the outcrop, its true value accruing from its placement

work by stone mason tradesmen. Its recovery is ex-

pensive in terms of specialized skills and by the judi-

cious use of explosives and of construction machinery

for its recovery.

A number of factors related to prospecting for

building stone deposits are well suited to engineering

geological training (Table 4).

Environmental Planning for Recovery

of Building Stone

As with most earth materials, there are two levels of

activity that apply to recovery of building stone, in-

formal and formal. For all but the weekend mason, all

other recovery activities quickly will be recognized by

environmental regulatory authorities as a form of

‘quarrying’, for which even a small stone recovery

enterprise will require submittal of a Permit Applica-

tion and some scheme of site development and inte-

grated measures for protection of the environment

(Tables 4 and 6).

Stone Masonry

Modern stone masonry nearly always serves the aes-

thetic purpose of the architect or landscape designer.

No longer are we concerned with use of stone for its

original structural role, but the manner in which the

masonry is placed provides ample opportunity for

meaningful artistic statement (Table 5).

Petrography

Petrographical examination of thin-sectioned hand

specimens recovered from prospecting will be helpful

in some instances in which the shape, colour, and/or

texture meets the client’s approval, but the geologist

remains wary of the ability of the stone to meet ex-

pectations when placed in exterior locations exposed

to the elements. As with basic identification of the

stone, such manuals as listed in the Further Reading

Section below provide an excellent desk reference.

Summary

Building stone provides a durable, aesthetically pleas-

ing construction material where wear-resistant and

climate-resistant natural stone can be used to enhance

the qualities of modern construction. Significant costs

must be paid for accessing and recovery machinery,

palleting, and transportation to the vendor’s yard.

Application of geological knowledge is essential to

providing appropriate attractive and durable stone

at an economic production cost.

Further Reading

Bates RL (1987) Stone, Clay, Glass; How Building Mater-

ials are Found and Used: Enslow.

Bell FG (1993) Engineering Geology: Oxford: Blackwell

Scientific Publications.

Blyth FGH and de Freitas MH (1984) A Geology for

Engineers, 7th edn. New York: Elsevier.

Geological Society of London (1999) Stone; Building Stone,

Rock Fill and Amourstone in Construction. Engineering

Geology Special Publications 16: 480.

Parsons and David (eds.) (1990) Stone; Quarrying and

Building in England. London: Phillmore, in Association

with The Royal Architectural Institute.

Shadman A (1996) Stone; An Introduction. London: Inter-

mediate Technical Publications.

Smith MR and Collis L (eds.) (1993) Aggregates; Sand,

Gravel and Crushed Rock Aggregates for Construction

Purposes, 2nd edn. London: Geological Society Special

Publication no. 9, p. 339.

BUILDING STONE 333

CALEDONIDE OROGENY

See EUROPE: Caledonides of Britain and Ireland; Scandinavian Caledonides (with Greenland)

a0005 CARBON CYCLE

G A Shields, James Cook University, Townsville, QLD,

Australia

Introduction

The recycling of elements between Earth’s interior, its

sedimentary sheath, the oceans, and the atmosphere

is vital to the continued functioning of Earth as a

living planet. Of all the biogeochemical cycles oper-

ating on Earth, the carbon cycle (Figure 1) is probably

the most fundamental process, without which life

could not exist. Carbon, in the form of carbon diox-

ide (CO

), represents the starting component in

almost all food chains, while at the same time per-

forming a complementary role as Earth’s thermostat,

providing an equable climate, suitable for the reten-

tion of liquid water on the surface of the planet.

Carbon in the form of bicarbonate and carbonate

ions helps to regulate ocean acidity and, in combin-

ation with calcium, provides a hard skeleton for many

marine organisms. Without the biological sequester-

ing of carbon and its eventual storage in the crust

through geological processes over millions of years,

there would be little or no free oxygen on Earth’s

surface. Similarly, without the deposition of organic

carbon and calcium carbonate shells on the seafloor,

Earth would inevitably become too hot for even the

most radical of known extremophile metabolisms. All

these important roles reflect different aspects of the

global carbon cycle, which is in reality a hierarchy of

subcycles, all operating on different time-scales.

From the moment a molecule of CO

first enters

the surface environment, whether from a volcano or

from the chimney stack of a power station, it enters

a dynamically changing realm of continual recycling.

Within a decade, the CO

molecule will very likely be

consumed by photosynthesis and converted into or-

ganic carbohydrate, either as part of a land plant or in

one of the trillions of single-celled algae in the world’s

oceans. In either case, death and decay will eventually

release the CO

back into the surface environment,

thus closing the cycle. This ‘short-term’ carbon cycle

may replay hundreds of times before burial finally

results in the addition of this carbon to the sediment-

ary pile, where some of it will remain for millions

of years as part of the ‘long-term’ carbon cycle of

sedimentation, tectonics, and weathering. On such

huge time-scales, the sequestering of atmospheric

during chemical weathering and the eventual

redeposition as carbonate rock are generally balanced

by the release of CO

from volcanoes and through

metamorphism. Although negative feedbacks tend to

regulate CO

levels over the long term, shorter term

perturbations to the carbon cycle may occur due to

pulses of massive volcanism, carbon burial, or me-

thane clathrate dissociation, all of which may have

caused considerable climatic perturbation at different

times in Earth history. The carbon cycle not only acts

as a trigger for climate change but can react to

such change, as happened during glacial–interglacial

cycles, when regional warming trends appear to have

preceded CO

increases. Interest in the short-term

carbon cycle has increased greatly of late because of

the observed elevation in atmospheric CO

concen-

trations due to fossil fuel combustion, cement manu-

facture, and land use changes. Fears that elevated

concentrations will change the future world cli-

mate have inspired research into the effects of an en-

hanced greenhouse effect on climate, sea-level, and

biodiversity. This global research effort necessitates a

good understanding of how Earth regulates climate

through the carbon cycle, including the development

of increasingly sophisticated models to describe the

‘Earth system’.

Short-Term Carbon Cycle

The short-term carbon cycle refers to the natural

cyclical processes of organic matter production and

organic decay (Figure 2). The key step in this cycle is

photosynthesis, whereby CO

is consumed by reac-

tion with water to form organic carbohydrates,

releasing oxygen in the process (eqn [1]):

þ H

O ¼ CH

O þ O

½1

Photosynthesis ! Respiration

CARBON CYCLE 335

Photosynthesis does not occur spontaneously but re-

quires the input of solar energy. Consequently, photo-

synthesis can take place only where sunlight can freely

penetrate, on the land or within the uppermost 100 m

or so of oceans and lakes. On land, photosynthesis

is carried out mostly by plants, whereas in the sea

it is almost exclusively the domain of algae in the

form of minute, free-floating silica-shelled diatoms

Figure 2 The short-term and long-term organic carbon cycles of productivity, sedimentation, burial, and weathering. Numbers refer

to carbon reservoirs (in gigatons) and carbon fluxes (in gigatons year

1

). Reprinted from Kump LR, Kasting JF, and Crane RG (1999)

The Earth System. Upper Saddle River, NJ: Prentice Hall.

Figure 1 The natural global carbon cycle. Reprinted from Davidson JP, Reed WE, and Davis PM (1997) Exploring Earth: An Introduction

to Physical Geography.

Upper Saddle River, NJ: Prentice Hall.

336 CARBON CYCLE

and calcite-shelled coccoliths. Just as photosynthesis

always consumes CO

, the reverse process, organic

respiration or decay, returns that CO

after death of

a plant or animal; this process is catalysed by the

enzymatic action of chiefly anaerobic microbes that

can be found in the digestive tracts of animals, in the

soil, or beneath the seafloor. Because organic decay is

essentially an oxidation process, oxygen is consumed

instead of being released, which is why oceanic

regions of high productivity are commonly underlain

by ‘oxygen minimum zones’, i.e., zones of low oxygen

concentration caused by the decomposition of or-

ganic matter in the water column. Organic raindown

not only transfers CO

to the deep ocean but also acts

as a biological pump for nutrients that, once stored in

the deep ocean ‘conveyor belt’, generally reemerge

only in regions of upwelling and high productivity.

Reoxidation of organic matter that has reached the

sediment may take place by a variety of mechanisms

(oxic respiration, denitrification, or sulphate reduc-

tion). However, in addition, in the deepest, most re-

ducing environments, methane may be produced by

the actions of methanogenic bacteria using two main

pathways (eqns [2a] and [2b]):

COOH ! CO

þ CH

½2a

þ CO

! CH

þ 2H

O ½2b

Methane produced in this fashion may seep back into

seawater to be reoxidized to CO

or may be stored

temporarily for thousands to millions of years as the

volatile methane clathrate.

Under normal circumstances, any particular eco-

system will be experiencing both photosynthesis and

respiration, but in different proportions. If photosyn-

thesis outweighs respiration, then the ecosystem will

act as a sink for CO

, whereas in the reverse case it

will act as a source. This ‘breathing’ of the biosphere

can best be seen in the annual fluctuations of atmos-

pheric CO

in the northern hemisphere (Figure 3).

During the northern hemisphere summer, when ter-

restrial photosynthesis and leaf growth surpass respir-

ation and decomposition, CO

levels decrease by as

much as 15 ppm in the boreal forest zone (55–65



N),

a decrease almost completely balanced by the winter

increase in CO

caused by the inevitable decompos-

ition of fallen leaves. In the southern hemisphere, the

cycle is reversed but the effect never attains more than

1 ppm. This hemisphere inequity confirms that such

seasonal CO

fluctuations are driven by the terrestrial

carbon cycle, rather than by the oceanic carbon cycle,

there being far more terrestrial biomass in the north-

ern than in the southern hemisphere. Because the

fluxes involved in the short-term carbon cycle are so

large, persistent imbalances would lead to intolerable

fluctuations in atmospheric CO

. Therefore, on time-

scales longer than about a century, this cycle of prod-

uctivity and decay must be perfectly in balance.

Nevertheless, not all CO

is eventually returned to

the atmosphere. A small proportion of the carbon

locked up in soils as organic matter may be washed

away to be buried indefinitely in coastal and marine

sedimentary basins. Similarly, a small proportion of

marine organic matter will also escape the processes

of decay by being swiftly buried in areas of high

sedimentation rate. Organic burial constitutes a leak

in the short-term carbon cycle, whereby a relatively

small proportion of CO

is continually removed from

the surface environment to be stored within Earth’s

crust. Some of this leaked CO

may eventually be

Figure 3 A three-dimensional perspective of terrestrial biosphere breathing. Reprinted from Volk T (1998) Gaia’s Body: Toward a

Physiology of the Earth

. New York: Springer-Verlag.

CARBON CYCLE 337

returned to the surface environment by tectonic pro-

cesses at convergent margins (deep-sea trenches), but

not for many millions of years, as part of the ‘long-

term’ carbon cycle of carbon burial, subduction,

uplift, weathering, and carbonate deposition.

The Long-Term Carbon Cycle

Although some carbon reaches Earth from space in

the form of impacting comets, most carbon first enters

the surface environment as volcanic CO

, originating

from deep within Earth’s interior at mid-ocean ridges,

convergent margins, and the Great Rift Valley and

other terrestrial volcanic provinces (e.g., Kam-

chatka). Some of this carbon is entirely new to the

surface environment, being derived directly from the

mantle, but some will be ‘old’ carbon, recycled from

the sediment pile by tectonic processes such as meta-

morphism. At convergent plate margins, carbonate

sediments can be carried to great depths, up to hun-

dreds of kilometres, riding on subducting ocean litho-

sphere. The high pressures and temperatures at great

depths encourage biogenic calcite (calcium carbon-

ate) to react with biogenic and detrital silica (silicon

dioxide) to form metamorphic calcium silicate min-

erals. This process releases CO

(eqn [3]) that may

eventually find its way into the atmosphere via hot

springs and seeps (Figure 4).

CaCO

þ SiO

! CaSiO

þ CO

½3

Much study is devoted to estimating the proportion

of genuinely new CO

from the mantle relative to

recycled CO

at convergent margins, but there is

currently no consensus. If volcanic outgassing were

allowed to proceed unchecked, then atmospheric par-

tial pressure of CO

(pCO

) would rise until an equi-

librium concentration was reached, balanced only by

escape of CO

into space. Exactly how hostile such an

Earth would be can be appreciated by comparison

with the furnace-like Venus, Earth’s planetary neigh-

bour and twin. Although Venus and Earth are of a

similar size and are a similar distance from the sun,

Venus has a much thicker atmosphere, made up almost

entirely of CO

, over 200 000 times more concen-

trated than in Earth’s atmosphere. There is no reason

to suspect that Venus overall is any richer in carbon

than Earth is, so Earth must possess powerful mech-

anisms of CO

sequestration, unique in the solar

system.

The first step in the permanent removal of CO

from

the atmosphere is chemical weathering (Figure 5),

whereby rainwater, made acidic by the dissolution

of aqueous CO

, corrodes rock, forming minerals

such as silicates and carbonates. In the presence of

soil, this process is accelerated by plants, which help

to concentrate CO

in the soil to levels generally 10 to

100 times higher than in the atmosphere. CO

used

up in this way may then be transported via rivers and

groundwater to the oceans, chiefly as bicarbonate

anions (HCO



), along with the weathered-out

major cations such as Si, Al, Ca, Mg, K, Na, and

Fe. Once in the ocean, some CO

may be removed

permanently from the exogenic system by chemical

precipitation as calcium carbonate, mostly in the

form of shells and skeletal elements that rain down

to the seafloor. Because CaCO

precipitation also

releases CO

back into the water column (eqn [4]),

taken up by the weathering of carbonate rocks

does not result in any long-lasting net change to at-

mospheric CO

levels. In other words, the CO

taken

up during carbonate dissolution is simply returned

during carbonate precipitation in the ocean:

CaCO

þ CO

þ H

O ¼ 2HCO



þ Ca

2þ

½4

Carbonate weathering ! Carbonate precipitation

Similarly, CO

consumed during the weathering of

Na and K silicates does not result in any net change

in atmospheric CO

because Na and K carbonate

minerals are very soluble and do not readily precipi-

tate from seawater. Equation [5] shows the net result

of Ca–Mg silicate weathering, which is the most im-

portant mechanism of permanently removing CO

from the atmosphere:

ðCaMgÞSiO

þ CO

!ðCaMgÞCO

þ SiO

½5

Much research effort has been expended to better

understand the controls on chemical weathering rates

Figure 4 The long-term carbon cycle of sedimentation, subduc-

tion, and outgassing.

338 CARBON CYCLE

and therefore the effects of these controls on the long-

term global carbon cycle. There is no consensus on

these issues because several unrelated factors are in-

volved, such as climate (both rainfall and tempera-

ture), denudation rates, weathering history, and rock

type. Through a combination of these factors, certain

tropical, humid areas of the world with freshly ex-

posed volcanic rock can exert an outsized influence

over the global carbon cycle. This is because fresh

basalts are particularlysusceptible to chemical weather-

ing and are made up predominantly of Ca–Mg

silicates. The weathering of basalts today may con-

tribute as much as a third of all the silicate-weathering

flux. Thus volcanism acts not only as a source

for atmospheric CO

, but also as one of the major

long-term sinks.

The products of chemical weathering, including

Ca, Mg, and bicarbonate ions, arrive in the oceans

in solution; concentrations build up in the

oceans until an equilibrium state is reached between

input and carbonate precipitation. Bicarbonate

(HCO



) and carbonate (CO

2

) ions, which together

make up 99% of all dissolved carbon in seawater,

help to regulate seawater pH by transferring

protons (or hydrogen ions) between carbonate

species (eqn [6]) according to the rules of carbonate

equilibria. In effect, the carbonate system in seawater

is capable of neutralizing a large portion of any

extra CO

added from, say, fossil fuel burning or

volcanism.

þ H

O ¼ H

¼ H

þ HCO



¼ 2H

þ CO

2

½6

Carbon dioxide$Carbonic acid$Bicarbonate

$Carbonate

Figure 5 The global inorganic carbon cycle of weathering, runoff, carbonate precipitation, and burial. Numbers refer to carbon

reservoirs (R; in gigatons) and carbon fluxes (F; in gigatons year

1

). Reprinted from Kump LR, Kasting JF, and Crane RG (1999) The

Earth System

. Upper Saddle River, NJ: Prentice Hall.

CARBON CYCLE 339

Many organisms catalyze the precipitation of

CaCO

from seawater through active biomineraliza-

tion and shell formation; for example, foraminifera,

coccoliths, corals, and shellfish contribute to the per-

manent removal of atmospheric CO

. Marine organ-

isms generally form their shells at relatively shallow

depths and sink to the shelf floor or abyssal depths after

death. In regions where the water depth is no greater

than about 1 km, the shells reach the seafloor more or

less intact because these waters are saturated with re-

spect to calcite and aragonite. However, at greater

depths, where pressures are greater, temperatures are

lower, and CO

concentrations are elevated, seawater

is far more corrosive to CaCO

. The level at which the

rate of carbonate dissolution balances the downward

flux of carbonate settling to the seafloor is called the

carbonate compensation depth (CCD). This depth

varies from ocean to ocean and has varied over time

in response to carbonate productivity, temperature,

atmospheric CO

, and ocean circulation changes.

Geological Evolution of the Global

Carbon Cycle

Many scientists suspect that the greenhouse effect

and, in particular, the level of CO

in the atmosphere

were different in the geological past. One major

reason for this suspicion derives from the ‘Faint

Young Sun Paradox’. Because the sun is probably

heating up over time – owing to the increasingly

exothermic nuclear fusion of lighter elements, form-

ing heavier elements in the sun’s central core – it is

probable that the input of solar energy to Earth was

significantly lower in the past, by as much as 1% for

every 100 million years, according to some estimates.

However, Earth’s sedimentary record provides evi-

dence for the existence of liquid water at least 4 bil-

lion years ago, indicating that surface temperatures

were not much lower then than they are now. There-

fore, it appears that, over time, Earth’s atmosphere

has become increasingly less efficient in retaining

solar energy. Model calculations indicate that, were

the only relevant greenhouse gas, CO

levels

must have been as much as 10 000 times higher

during the Early Precambrian. However, because me-

thane is likely to have been a major greenhouse gas

during early Earth history, before the increase of at-

mospheric oxygen levels, such estimates are likely to

represent maximum CO

concentrations only.

p0060 On geological time-scales, atmospheric CO

levels

are thought to be regulated by negative feedbacks

between climate and silicate weathering rates.

First, increased temperatures due to higher CO

levels

would accelerate chemical weathering rates, thus

effectively slowing any CO

increase. Second,

weathering requires rainfall, which would most likely

increase as temperatures rise, due to the acceleration

of the hydrological cycle. Third, enhanced CO

fertil-

ization of plant growth would also help to increase

weathering rates by stabilizing soils and encour-

aging chemical weathering over physical weathering.

This does not mean, however, that CO

levels have

remained constant over geological time. Chemical

weathering rates are imperfectly tied to CO

levels,

being related also to additional parameters, includ-

ing tectonics, vegetation cover, and palaeogeography,

whereas atmospheric CO

may be influenced by

independent changes in the input and output of

. In the geological past, higher CO

levels could

be sustained because chemical weathering rates on

the continents were lower in the absence of deep

soils, before the introduction of vascular plants in

the Devonian period. A higher PCO

was also made

possible by higher volcanic outgassing rates of mantle

and the abiotic nature of much carbonate depos-

ition. Before the introduction of pelagic carbonate

producers, such as planktic foraminifera by the Juras-

sic, carbonate deposition would have been restricted

to shallow shelf environments, thus increasing calcite

saturation levels and rendering deposition rates

vulnerable to changing sea-level (palaeogeography).

Although the introduction of land plants in the

Devonian period does not appear to have had an

immediate effect on climate (by sequestering more

atmospheric CO

), the deposition of massive peat

deposits in shallow coastal environments during the

Carboniferous and Permian periods almost certainly

did. Long-term (10

years) increases in organic carbon

burial during this interval are thought to have re-

duced CO

levels by as much as 70% (Figure 6),

causing a series of glaciations, while allowing oxygen

to build up in the atmosphere. One consequence

of higher O

is to increase air pressure; it seems

plausible that the well-recorded insect gigantism

during this interval was related to the added buoy-

ancy provided by greater air pressures. Such long-

term, but ultimately unsustainable, imbalances in

the proportion of carbon buried as organic carbon

relative to carbonate carbon may have caused changes

to both the CO

and the O

atmospheric budgets at

other times, too, but the carbon isotopic record sug-

gests that this proportion has been more or less con-

stant at 1:4, respectively, on >10

year time-scales

since 3 Ga. Geochemical models show that CO

levels

during the Phanerozoic were also higher than at

present (Figure 6), and reached peaks during the

Early Phanerozoic and more recently during the Cret-

aceous period when high PCO

is likely to have con-

tributed to the equable and balmy ‘Greenhouse’

climates typical of that interval in Earth history.

Despite the existence of regulatory feedbacks on

atmospheric CO

, the Earth system is still susceptible

340 CARBON CYCLE

Figure 6 Comparison of model predictions of atmospheric CO

over the Phanerozoic, and plant stomata-based data (black boxes).

Reprinted from Royer DL, Berner RA, and Beerling DJ (2001) Phanerozoic atmospheric CO

change: evaluating geochemical and

palaeobiological approaches. Earth Science Reviews 54: 349–392.

to shorter term (<10

years) perturbations caused by

abrupt changes to the input or output fluxes of CO

Because volcanism is insensitive to surface negative

feedbacks, short-term fluctuations in volcanic activity

may have considerable, but short-lived, consequences

for atmospheric composition and global climate due

to the release of massive amounts of greenhouse CO

Such volcanic episodes, termed ‘flood basalt events’,

are rare, but they have occurred several times over the

course of the Phanerozoic; they resulted in Large

Igneous Provinces, producing more than 1 million km

of volcanic rock, such as the Deccan and Siberian traps,

in less than 1 million years (see Large Igneous Prov-

inces). Correlations between the ages of rare igneous

events and equally rare mass extinctions suggest that

the resultant global warming may have caused mass

extinctions, such as at the Permian–Triassic boundary

(Siberian traps) (see Palaeozoic: End Permian Extinc-

tions). Rapid episodes of global warming may also

cause the sudden decomposition of volatile methane

clathrate, which constitutes a massive reservoir of

carbon, semi-permanently stored within sediments at

high pressures and low temperatures, both on land and

at sea. The abrupt release of methane (a greenhouse gas

that quickly reverts to CO

, another greenhouse gas)

into the atmosphere would accelerate global warming.

Precisely this may have happened at the Paleocene–

Eocene boundary (Figure 7) as well as at other times

in Earth history.

Glacial–Interglacial Cycles

and CH

concentrations can be reconstructed

back more than 200 000 years using air bubbles

locked up in the Antarctic ice-cap; past concentrations

clearly parallel changes in temperature during the

more recent glacial–interglacial cycles (Figure 8),

although they appear to lag consistently behind

temperature by as much as 1000 years. This time lag

suggests that whatever mechanisms are involved, a

finite amount of warming is required before CO

and CH

outgassing becomes significant. Because tem-

perature changes in Antarctica generally preceded

temperature changes in the northern hemisphere, it

is also possible that CO

changes may have been

caused initially by changes in climate, but that in-

creased CO

and other trace gases acted to amplify

those climatic changes. The ‘coral reef hypothesis’

was originally proposed to explain the observed

increase in atmospheric CO

recorded in glacial ice

during times of deglaciation. This hypothesis con-

siders that shallow shelf (or neritic) carbonate de-

position is significantly lower when shelf space is

decreased during glacial sea-level lowstands, whereas

carbonate deposition would increase during sea-level

transgressions. Because carbonate precipitation

returns CO

back into the atmosphere, an increase

in shelf space during deglaciation would serve to

increase atmospheric pCO

and decrease the alkalin-

ity of surface seawater, at least in the short term

(Figure 9). Calculations suggest that sea-level can

account for at least half of the observed increase in

levels. On longer time-scales, the effects of sea-

level on carbonate deposition would be offset by

changes to the deep-sea preservation of carbonate

shells, but the efficacy of this feedback depends on

the link between the shallow environment and the

deep, which is limited by the sluggish pace of ocean

CARBON CYCLE 341

Figure 8 Atmospheric CO

, temperature, and CH

records determined from the Vostok ice-cores in Antarctica. Reprinted from

Wigley TML and Schimel DS (eds.) (2000) The Carbon Cycle. Cambridge: Cambridge University Press.

Figure 7 Schematic diagram showing the Latest Paleocene Thermal Maximum (LPTM) dissociation hypothesis. Massive quantities

C-rich CH

are released into the water column via sediment failure, causing short-lived negative excursions in the marine

carbonate d

C record and increases in atmospheric CO

. PAL, Paleocene. Reprinted from Katz ME, Pak DK, Dickens GR, and Miller

KG (1999) The source and fate of massive carbon input during the Latest Paleocene Thermal Maximum.

Science 286: 1531–1533.

342 CARBON CYCLE

Elsevier Encyclopedia of Geology - vol I A-E

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