Contents
Carbon Cycle Heavy Metals Nitrogen Cycle Sulfur Cycle
Carbon Cycle
E G Nisbet, Royal Holloway University of London, Egham, Surrey, UK
Copyright 2003 Elsevier Science Ltd. All Rights Reserved.
Geological Sources of Carbon
Historical Background
Carbon has probably been present on the Earth’s surface from the earliest accretion of the planet.
During the Hadean Eon (4.56 to about 4.0 Ga ago, where 1 Ga is 109years), the atmosphere was probably rich in carbon gases. Frequent major impacts would have occurred, the largest of which would have removed any existing atmosphere, to be replaced by fresh infall and degassing from the interior. Volcanoes in the early ocean would have hosted vigorous hydrothermal systems, and basalt ejecta after each impact would have reacted with sea water, with precipitation of carbonate. At the close of the Hadean, the atmosphere was probably mainly carbon gases and nitrogen, over a deep ocean.
On Venus, the loss of hydrogen atoms from the top of the atmosphere removed nearly all hydrogen from the water that once covered the surface of the planet, dehydrating it. The atmosphere became dominantly carbon dioxide, and is now in effective equilibrium with the surface rocks, such that carbon dioxide and rock silicate react to give carbonate and quartz, at roughly 5001C and 90 MPa. There is now very little hydrogen left in the planet, except in sulfuric acid in clouds. The interior is probably consequently oxidized by residual oxygen.
On Earth, planetary evolution proceeded differently and the surface has probably sustained liquid water for over 4 Ga, with occasional excursions into somewhat more extreme states, including possible snowball states (but always with some liquid water).
Through-out the Archean Eon (roughly 4.0–2.5 Ga ago) the air was probably mainly carbon dioxide and nitrogen, over deep oceans, possibly with methane-rich epi-sodes. During the Archean, life appeared, and has dramatically modified the composition of the atmos-phere, such that the air has probably been a biological construction for most of the past 3.5 Ga.
The first possible evidence for carbon-managing life on Earth is in43.7-Ga-old rocks in Greenland. By around 3.5 Ga ago, certainly by 3.0 Ga ago, oxygenic photosynthesis was the dominant control on the carbon gas content of the atmosphere. Since then, O2
and CO2 have been obverse and reverse sides of the same coin. The chief mediators of the interexchange of carbon and oxygen between organic matter and air were first the cyanobacteria and purple bacteria.
Today, descended from them, are the chloroplasts (from cyanobacteria) and mitochondria (from purple bacteria) in plants and mitochondria in animals. The enzyme rubisco, which is of great antiquity, is the principal means of capture of carbon from the air, but rubisco in effect works in reverse if concentrations change. In the Proterozoic Eon (2.5–0.56 Ga ago) the air changed greatly. Most opinion is that initially in the Archean the air may have been dominantly carbon dioxide and nitrogen, but possibly around 2.3–2.0 Ga ago, there may have been a major event when oxygen increased markedly in the air, and the atmos-pheric burden of its reverse, carbon dioxide, dropped.
Since then, the atmosphere has been made of nitro-gen and oxynitro-gen with other minor gases, and water vapor. Only the argon in the air is not biologically managed.
In the Phanerozoic Eon (the last 0.56 Ga of Earth history), the development of animal life with hard parts, such as shells, precipitated carbonate widely (although it should be noted that microbial precipita-tion had occurred extensively in the Precambrian).
The colonization of the land, and the development of dense forests from the Carboniferous onwards, has allowed the development of a subtle and complex 196 BIOGEOCHEMICAL CYCLES/Carbon Cycle
atmospheric management regime, with short-term regulation of the carbon dioxide/oxygen balance by processes such as fire and seasonal growth/decay cycles. Although Precambrian oil is known, the deposition of oil and coal has particularly occurred in the Phanerozoic, especially from the start of the Carboniferous. This has had substantial impact on the global carbon budget.
Nevertheless, the basis of the biogeochemical reg-ulation of the air remains the same – chloroplasts and mitochondria, descended from Archean bacteria, and the inorganic chemical constraints of weathering and precipitation.
Geological Stores of Carbon
Over time, large geological stores of carbon have developed (Figure 1). Though there are estimates of the size of these stores, they are very poorly con-strained. Isotopic evidence suggests that for every four carbon atoms degassed from the interior of the Earth, three have been sequestered into carbonate, and one captured by rubisco and incorporated into organic matter.
The highest-level geological store of carbon is organic matter in soft sediment and in piles of sediment only affected by burial metamorphism.
This matter includes buried organic material that is slowly being processed by microbial recyclers and by the inorganic effects on buried organic matter of the burial pressure and temperature. In addition, much charcoal is stored in sediment, deposited from erosion after forest fires. Methanogens in sediment, and also inorganic metamorphism of deeper carbon-rich sedi-ment, produce methane. In the Arctic at a few hundred meters depth, and elsewhere in thick piles of sediment on continental margins worldwide under 500–1000 m water, very large quantities of methane are stored as methane hydrate, an icelike methane/water com-pound. Estimates of the total methane store are around 1019g, an order of magnitude more carbon than there is in the biosphere.
Sedimentary rocks include stored carbon in oil, coal, and carbonate. Oil, gas, and coal are metamor-phic products. Low-grade thermal maturation of organic matter produces oil and gas; coal comes especially from buried plant matter such as Carbon-iferous trees and bog deposits. Particular geological periods have, for various reasons, produced remark-able amounts of carbon-rich deposits. Cretaceous rocks host a disproportionate proportion of the world’s oil, possibly reflecting the organic blooms after huge carbon gas emissions in the Cretaceous from mantle–plume volcanism. Carboniferous rocks, as their name implies, are rich in coal across Europe
and North America, formed in ancient equatorial rainforest settings.
Carbonate sedimentary rocks are very widespread, particularly in Mesozoic sequences. Today, carbonate reefs still occur (e.g., the Australian Great Barrier Reef), but some Mesozoic reefs may have been immense. The Earth’s crust also contains a wide assemblage of crustal metamorphic rocks that include carbon (e.g., diamond) that has at one time been cycled through the atmosphere.
Geological Cycling
Geological processes cycle carbon continually from the interior to the atmosphere and then back to the interior. Carbon is degassed from volcanoes as carbon dioxide, carbon monoxide, and methane. Some vol-canoes also emit carbonate lavas (e.g., in the East African rift region), and rare kimberlite eruptions bring up carbon directly as diamonds.
The volcanic carbon gases are either (75–80%) rained out and, via bicarbonate, eventually deposited as carbonate, when combined with calcium, magne-sium, strontium, etc., weathered from rocks, or (20–
25%) captured by organic matter and eventually deposited as such (e.g., kerogen).
Carbon is returned through the interior of the planet in several ways. Some sediment, especially carbonate, coal, and oil, is laid down directly on continents and eventually metamorphosed or deformed in mountain building events (with loss of CO2 back to the air).
Other sediment is subducted and carbon dioxide is either driven off into the overlying lithosphere at a few tens of kilometers depth, or driven off at deeper level, eventually to ascend and return to the air via volca-noes. A small amount of carbon is not returned to the air but continues down into the deeper mantle and, in turn, a small amount is emitted from deep mantle via deep-sourced mantle–plume volcanoes such as Hawaii.
Reservoirs and Fluxes of Carbon in the Ocean–Atmosphere–Biosphere
System
Figure 1summarizes the main reservoirs of carbon in the atmosphere–ocean system, and the main fluxes between them. Biological stores are small but the annual fluxes are large. Geological stores are vast, many orders of magnitude larger than the biological stores (though too poorly quantified for numbers to be given here), but annual fluxes are relatively small.
Geological stores include carbon in carbonate (about three-quarters of Earth’s near-surface store of carbon) and also once-organic carbon (about one-quarter), BIOGEOCHEMICAL CYCLES/Carbon Cycle 197
such as coal, oil, and gas. Hydrate reserves of carbon (mainly methane in offshore sediments) are probably around 1019g.
The atmosphere contains about 730 Pg carbon, and the ocean about 38 000 Pg where 1 petagram is 1015g, or a billion tons, using the US version of billion). Soil carbon is of the order of 1500 Pg, and the total plant carbon is around 500 Pg. Estimates of reservoirs and fluxes given here are from the Intergovernmental Panel on Climate Change (IPCC), the best recent overall assessment.
Global Carbon Cycles
The global carbon system can be seen as a set of nested cycles (Figure 1). The inmost cycles are the atmos-pheric cycles. Carbon dioxide has an effective lifetime in the air of several centuries. Once emitted, an atom of carbon in a molecule of carbon dioxide is likely to survive in the air over this period before the carbon is
returned to the water or ground, although it may be processed in the meanwhile. The first cycle is the diurnal photosynthesis/respiration cycle, within the seasonal cycle of growth and decay. Around this is wrapped the longer-term cycling of carbon between air and the Earth’s surface (mostly ocean) over millennia (see exchanges inFigure 1).
On an even longer time scale the geological cycles operate. Carbon dioxide is taken up as carbonate in sediments and altered seafloor rock. The carbonate is either carried down subduction zones by the plate system, and there driven off to emerge as carbon dioxide at volcanoes, or is accreted into continents, where carbon dioxide is driven off by metamorphic processes. Calcium, strontium, etc., erode from sili-cate rock, then combine with the carbon dioxide to precipitate carbonate and return the cycle. In addition, some carbon is accreted to the bases of continents as diamond. On a longer time scale yet, carbon is returned from the deep mantle to the midocean ridges, and thence back to the surface.
light
Soil 1500 Fossil organic
Carbonate rock CH4
hydrate CO2
CO from
subduction CO2 Mantle
CO2 Metamorphic
CO2
Diamonds
Oceans 38000 Atmosphere
730
>0.1 120
Reduced organic
debris Plankton, marine animals
Carbonate
CarbonateThermocline
Organic-rich sediment
Lysocline Photosynthesis
90 90
Plume volcano (eg Hawaii)
Detritus
DIC DOC
Organic-rich sediment
Detritus 360 Inert
150
Soil carbon 1080 60 Respiration
55 Animal respiration 120 Photo-synthesis
60 Net pp 4
Fire
Land plants and land animals Oceans Plate cycle
light Air
88 103 90 45
11 42 0.1 58
33 0.01
Plankton DIC
DOC
Transport Thermocline
Sediment
Sediment CaCO3dark Lysocline 0.4
100 m
3500 m
Volcano
Plume
Midocean ridge
Ca++ Dic+Doc
120
Plants 500 Carbonatereefs
Carbonatein basalt
Figure 1 Main diagram: components of the global carbon cycle. Boxes show reservoirs in PgC, during the 1980s. Arrows show fluxes per year in PgC. Numerical values from IPCC. Inset circles show details of specific parts of the cycle. Left: land biosphere; center: oceanic exchanges; right: longer-term geological cycling.
198 BIOGEOCHEMICAL CYCLES/Carbon Cycle
Geological sources, in addition to the volcanic carbon gases already discussed, include metamorphic degassing, through carbonated water in hot springs and other emissions. The total flux from geological sources is minuscule compared to annual fluxes within the biosphere, but over hundreds of millions of years the geological cycling exchanges surface carbon effectively with the interior. On land and in the oceans, the principal intermediary between atmospheric car-bon and captured carcar-bon is now, as in the past, the enzyme rubisco. This preferentially selects isotopically light 12C, and consequentially inorganic carbonate made from the residual carbon is preferentially enriched in the heavier 13C. By means of rubisco, plants capture carbon from the air. A substantial amount of the carbon is respired back by the plants, and most of the rest by animals, bacteria, and archaea.
Carbon Cycling on Land
Most carbon capture on land is in the growth of plants.
All residents of temperate latitudes are familiar with the seasonal cycle of growth and decay. About 120 petagrams of C per year is captured by photosynthesis in plants (gross primary production: GPP), of which half (60 PgC year!1) is very rapidly respired. Thus net primary production (NPP) of the land is about 60 PgC year!1 – contrast this with the total carbon burden of the atmosphere of 730 Pg. Most of this NPP is fairly rapidly returned to the air. About 4 PgC year!1 is returned via fires, mostly as CO2but with significant CO and CH4. Fires produce significant charcoal, which is very resistant and can survive for hundreds of millions of years abstracted from the air; thus, to this extent, fire is a long-term remover of carbon from the air. Animals and microbially mediated decay return 55 PgC year!1via heterotrophic respiration. Much of this occurs via detritus (e.g., leaf fall) – some of the detritus decays quickly, while a part enters the soil carbon reservoir and returns to the air over a much longer time period. The flux of dissolved carbon captured via plants to inert soil carbon is about 0.4 PgC year!1, eventually carried by rivers as dis-solved organic carbon (DOC). Added to the 0.4 PgC year!1 flux of dissolved inorganic carbon (DIC) derived from CaCo3; this gives a total river flux of about 0.8 PgC year!1.
Carbon Cycling in the Oceans
Air exchanges gas with water, for example in the bubbles of wave crests under the winds. Carbon dioxide dissolved in the ocean (DIC) occurs as CO2, HCO!3, and CO2!3 . The DIC input to the oceans is about 90 PgC year!1, and return about 88 PgC year!1.
The near-surface waters of the oceans host plankton photosynthesizers, including both abundant cyano-bacterial picoplankton and a wide variety of euka-ryote plankton. Both cyanobacteria and the genetical-ly related chloroplasts of eukaryote phytoplankton use rubisco. Feeding on these are zooplankton and the complex chain of marine life. The GPP of the oceans is about 103 PgC year!1, of which 58 PgC year!1 are recycled by autotrophic respiration, leaving an NPP of 45 PgC year!1(compare with the 60 PgC year!1 land production – see below). In total for land and sea, the annual NPP is over 100 PgC, and gross annual drawdown from the air isB200 PgC, contrasted to the atmospheric reservoir of 730 PgC. Much of this is rapidly recycled back to the air.
However, some material remains abstracted from the air. The detritus from particulate organic matter (POC) in the oceans (everything from dead bacteria to dead whales), plus the sinking of DOC, exports carbon into the deeper water below the thermocline that marks the lower boundary of the biologically produc-tive upper waters. Physical exchange transports 33 PgC year!1 downwards as DIC. In this deeper water below the thermocline, heterotrophic respira-tion by marine animals occurs, and 42 PgC year!1 is recycled upwards as DIC. Some material descends to the sea bottom. Also, at the seafloor, both deposition (in shallow water) and dissolution of carbonate occur (in the deepest water).
Geological Controls
Volcanism emits variable quantities of carbon gases to the air. In many years, few eruptions may occur; then many in one year; rarely, cataclysmic eruptions occur.
Metamorphism also causes emissions of carbon gases, principally carbon dioxide via hydrothermal waters (e.g., in hot springs). Total geological emissions are B0.02–0.05 PgC year!1, but highly variable.
Chemical weathering is the long-term remover of carbon from the ocean–atmosphere system. Rain-water and carbon dioxide form carbonic acid. This then reacts with rock silicate to extract the calcium and precipitate with it as carbonate:
H2OþCO2!H2CO3
CaSiO3þH2CO3!CaCO3þSiO2þH2O The carbonate and quartz are eventually returned either to the mantle by subduction or to the deep crust by tectonic processes, and cycled back again to the surface either via magma or as metamorphic volatiles.
Dissolution of carbonate rock also occurs. Carbonic acid in soils attacks limestones, and the net result is transport of carbonate to the seafloor and return of BIOGEOCHEMICAL CYCLES/Carbon Cycle 199
carbon dioxide to the air. As the carbonic acid is derived from the atmosphere, this dissolution/precip-itation cycle has no net impact on the carbon dioxide burden of the air, but is important in the fluxing of dissolved inorganic carbon.
CaCO3ðlimestonesÞ þH2CO3
!CaCO3ðshells;etc:Þ þCO2þH2O
Global Distribution of Carbon Dioxide
The NOAA-CMDL ‘flying carpet’, diagram (more properly the zonally-averaged surface of mole frac-tion), which incorporates measurements from a worldwide net of monitoring time series, illustrates carbon dioxide distribution in the world atmosphere (Figure 2). The figure is built from long-term time series of many stations, using intercalibrated data sets.
Several features of the diagram are notable. First, let us consider the interhemispheric gradient. There is far more land in the Northern Hemisphere, and the dominance of the north reflects the high biological productivity of the northern land biomes. The marked seasonality is caused by the seasonal variation – in northern spring, massive uptake of carbon dioxide occurs as leaf growth takes carbon dioxide out of the air. In return, the rise in northern autumn, leaf fall, and microbial oxidation return the carbon dioxide to the air (note that the word ‘autumn’ is used here to avoid commenting on the ‘fall rise’). The diagram is shown on a multiyear scale, but local continuous records show further diurnal detail, as carbon dioxide is taken up in the warm afternoon, and respired back in the cooler evening.
The Southern Hemisphere shows reverse seasona-lity, which records both local southern effects, and also the windblown transport from the north.
Overriding the year-on-year cycles, the steady anthropogenic growth is well illustrated in the dia-gram, especially in the South Pole record.
Methane and Carbon Monoxide
Methane
The major sources of methane are methanogenic archaea and combustion, both natural and human-induced. Somewhat over 500 Tg (1 Tg is 1012g or a million tons) of methane are emitted each year, of which roughly 300 Tg are in some way linked to human activity and 200 Tg are from broadly natural sources. The major natural sources are wetlands, including both tropical wetlands such as African papyrus swamps and South American swamps, and boreal (northern) wetlands and peat bogs (especially in Siberia and Canada). Natural grass and forest fires, set by lightning, are also major sources. Human-induced emissions are from leaks from the gas industry, from coal mines, from cows and other domestic ruminants, from rice fields, and from human-lit fires, including tropical wildfires. Note that in some cases defining the natural/human division is not simple: human-planted rice fields emit, but may replace natural wetlands that also emitted methane.
Methane has a lifetime in the air of about a decade in the modern atmosphere. The principal sink of meth-ane is OH, from water vapor in the atmosphere. OH is most abundant in the tropical troposphere, and OH abundance follows the sun in its progress north and south. Thus this is the main locus of methane removal.
The second, much more minor sink of methane is soil microbial oxidation by methanotrophic bacteria.
The methane flying carpet diagram (Figure 3) appears similar to the CO2 diagram, but is subtly very different. The major sources are seasonal – wetland emissions vary logarithmically with temper-ature and are thus much greater in hot mid-to-late summer. In contrast, fossil fuel emissions are larger in winter, especially gas leaks and emissions from coal pulverization. In any place, the chief sink, OH, is sharply seasonal, moving across the (mainly tropical) latitudes with the sun.
The diagram illustrates the dominance of the Northern Hemisphere, especially in human activity.
The marked seasonality in the north records both production and the southward transport of methane as low-methane summer winds enter the cool north and displace the high-methane northern air south-wards. In the Southern Hemisphere, fires contribute, as does the arrival of high-methane northern air.
The South Pole record illustrates the overall growth well.
380 370 360 350 340 CO2 (µmol mol−1)
60° N 30° N
300°° S 60° S
90° S Latitude
01 92 93 94 95 96 97 98 99 00
Year
Figure 2 Carbon dioxide time series record by latitude. (>http://
www.cmdl.noaa.gov/ccgg/gallery/index.php?pageType=folder
&currDir=./Data_Figures) (Courtesy of NOAA-CMDL with thanks.) 200 BIOGEOCHEMICAL CYCLES/Carbon Cycle