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ScienceWeek
CARBON CYCLE, FOSSIL FUELS, AND THE ATMOSPHERE
The following points are made by Robert A. Berner (Nature 2003 426:323):
1) The carbon cycle is a variety of processes that take place over timescales ranging from hours to millions of years. Processes occurring over the shorter periods include photosynthesis, respiration, air-sea exchange of carbon dioxide and humus accumulation in soils. However, it is the long-term carbon cycle, occurring over millions of years, that is of interest when considering the origin of fossil fuels.
2) The long-term cycle is distinguished by the exchange of carbon between rocks and the surficial system, which consists of the ocean, atmosphere, biosphere, and soils. The long-term carbon cycle is the main controller of the concentration of atmospheric carbon dioxide and (along with the sulphur cycle) atmospheric oxygen over a geological timescale(1,2). It can be represented succinctly by the generalized reactions(3,4):
(1) CO2 = CaSiO3 CaCO3 + SiO2
(2) CO2 + H2O CH2O + O2
3) These reactions summarize many intermediate steps. Equation (1), going from left to right, represents the uptake of atmospheric carbon dioxide during the weathering on land of calcium (and magnesium) silicates, with the dissolved weathering products (Ca2+, Mg2+ and HCO3-) delivered to the ocean and precipitated there as calcium and magnesium carbonates in sediments. Going from right to left, equation (1) represents the deep burial and thermal decomposition of carbonates with the liberated carbon dioxide released into the atmosphere and oceans.
4) Equation (2) is of particular interest. Going from left to right, it represents net global photosynthesis (photosynthesis minus respiration), as manifested by the burial of organic matter (CH2O) in sediments. The buried organic matter is eventually transformed, mostly to kerogen, but some becomes oil, gas and coal. Equation (2), going from right to left, represents the oxidative weathering of organic matter exposed by erosion on the continents (and to a lesser extent the combination of thermal decomposition of organic matter to reduced gases with consequent oxidation of the gases in the atmosphere). The burning of fossil fuels by humans has caused a large increase in the rate of organic matter oxidation compared with that of the natural weathering process. This increase is apparently an acceleration by a factor of about 100. Thus, humans have greatly perturbed the long-term carbon cycle.
5) In summary: The long-term carbon cycle operates over millions of years and involves the exchange of carbon between rocks and the Earth's surface. There are many complex feedback pathways between carbon burial, nutrient cycling, atmospheric carbon dioxide and oxygen, and climate. New calculations of carbon fluxes during the Phanerozoic eon (the past 550 million years) illustrate how the long-term carbon cycle has affected the burial of organic matter and fossil-fuel formation, as well as the evolution of atmospheric composition.(5)
References (abridged):
1. Garrels, R. M., Lerman, A. & Mackenzie, F. T. Controls of atmospheric O2 and CO2 -- past, present, and future. Am. Sci. 64, 306-315 (1976)
2. Holland, H. D. The Chemistry of the Atmosphere and Oceans (Wiley, New York, 1978)
3. Ebelmen J. J. Sur les produits de la decomposition des especes minerales de la famille des silicates. Annu. Rev. Mines 12, 627-654 (1845)
4. Urey, H. C. The Planets: their Origin and Development (Yale Univ., New Haven, 1952)
5. IPCC (Intergovernmental Panel on Climate Change) Climate Change 2001: Synthesis Report (IPCC, Geneva, 2001)
Nature http://www.nature.com/nature
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IMPACT OF A PERMO-CARBONIFEROUS HIGH O2 EVENT ON THE TERRESTRIAL CARBON CYCLE.
The following points are made by D.J. Beerling and R.A. Berner (Proc. Nat. Acad. Sci. 2000 97:12428) :
1) Independent models predicting the Phanerozoic (past 600 million years) history of atmospheric O(2) partial pressure (pO(2)) indicate a marked rise to approximately 35% in the Permo-Carboniferous, around 300 million years before present, with the strong potential for altering the biogeochemical cycling of carbon by terrestrial ecosystems. This potential, however, would have been modified by the prevailing atmospheric pCO(2) value.
2) The authors use a process-based terrestrial carbon cycle model forced with a late Carboniferous paleoclimate simulation to evaluate the effects of a rise from 21 to 35% pO(2) on terrestrial biosphere productivity and assess how this response is modified by current uncertainties in the prevailing pCO(2) value.
3) The authors report that the results indicate that a rise in pO(2) from 21 to 35% during the Carboniferous reduced global terrestrial primary productivity by 20% and led to a 216-Gt (1 Gt = 10^(12) kg) C reduction in the vegetation and soil carbon storage, in an atmosphere with pCO(2) = 0.03%. However, in an atmosphere with pCO(2) = 0.06%, the CO(2) fertilization effect is larger than the cost of photorespiration, and ecosystem productivity increases leading to the net sequestration of 117 Gt C into the vegetation and soil carbon reservoirs. In both cases, the effects result from the strong interaction between pO(2), pCO(2), and climate in the tropics.
4) From this analysis, the authors deduce that a Permo-Carboniferous rise in pO(2) was unlikely to have exerted catastrophic effects on ecosystem productivity (with pCO(2) = 0.03%), and if pCO(2) levels at this time were >0.04%, the water-use efficiency of land plants may even have improved.
Proc. Nat. Acad. Sci. http://www.pnas.org
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GEOCHEMISTRY: ON RIVER CARBON AND THE CARBON CYCLE
In its most general outline, the term "carbon cycle", in geochemistry and Earth science, refers to the movement of carbon from an atmospheric inorganic state to a biospheric organic state and then back to an atmospheric inorganic state. In detail, there are several pathways from biospheric organic carbon to atmospheric inorganic carbon, one of which, of great importance, is the movement of organic carbon into the hydrosphere, principally via rivers that empty into oceans, with oceanic dissolved organic carbon a reservoir for movement of organic carbon to an atmospheric inorganic state. Concerning the transfer of dissolved organic carbon from rivers to oceans, there are puzzles that have not yet been solved.
The following points are made by Wolfgang Ludwig (Nature 2001 409:466):
1) The author presents a commentary on new data concerning river carbon (P.A. Raymond and J.E. Bauer: Nature 25 Jan 01 409:497). The author (Ludwig) points out that dissolved organic carbon in the oceans is one of the largest reservoirs in the global carbon cycle, this reservoir comparable in size to all of the carbon in terrestrial plants, or to all of the carbon in form of carbon dioxide in the atmosphere. The input of terrestrial organic carbon from rivers, the main source of most constituents of sea water, could fill the oceanic reservoir in only a few thousand years, which (according to radiocarbon dating) is apparently the average age of oceanic organic carbon. But although there ought to be a great amount of terrestrial-derived organic carbon in the oceans, geochemical studies indicate there is apparently very little, and the fate of river-transported carbon ("riverine carbon") once it enters the oceans is unclear.\
2) Almost all of the organic carbon on Earth is created via photosynthesis, whether on land or in water, but on land the process produces characteristic markers, so that terrestrial carbon should be traceable after it has entered the oceans. For example, many land plants synthesize certain compounds, such as *lignin or *tannin, which are absent in marine *phytoplankton. In principle, therefore, detecting these biomarkers in the oceans can reveal if carbon had a terrestrial origin. The other widely used method involves measuring the ratio between the two stable carbon isotopes, C-13 and C-12, in the bulk organic matter. Most land plants produce carbon that is more depleted in C-13 than carbon produced by marine phytoplankton, which results in higher isotopic ratios in marine than in terrestrial carbon.
3) Raymond and Bauer (2001) now present an analysis of organic materials in four rivers (Amazon [BR], Hudson [New York, US], (York [Virginia, US], Parker [Massachusetts, US] by radiocarbon dating (carbon-14, carbon-13 measurements), and they report the organic carbon in these rivers is up to several thousand years old [*Note #1]. This is in sharp contrast with the general belief that most of the organic carbon in rivers should be relatively "fresh". The particulate organic carbon (i.e., the fraction retained on a filter) was especially old (C-14-depleted). From these results, and laboratory evidence that suggests selective degradation of young (C-14-rich) dissolved organic carbon over the residence times of river and coastal waters, Raymond and Bauer conclude that pre-aging and degradation may alter significantly the structure, distribution, and quantities of terrestrial organic matter before its delivery to the oceans. The implication is that the absence of riverine carbon in the oceans is only apparent and due to the fact that we have not been able to distinguish riverine carbon from marine-generated carbon.
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Notes:
Note #1: Carbon-14 dating depends on the decay of carbon-14 to nitrogen. Carbon-14 is continually formed in nature by the interaction of neutrons with nitrogen-14 in the Earth's atmosphere, the required neutrons produced by cosmic rays interacting with the atmosphere. The carbon-14 from this reaction is converted to carbon dioxide by reaction with atmospheric oxygen and mixed and uniformly distributed with the atmospheric carbon dioxide containing stable carbon-12. Since living organisms use atmospheric carbon dioxide either directly or indirectly, their systems contain the constant ratio of carbon-12 to carbon-14 that exists in the atmosphere. Death of an organism terminates the equilibrium process: no fresh carbon dioxide is added to the dead substance, and the carbon-14 present in the dead substance decays with a half-life of 5730 years, while carbon-12 in the dead substance remains what it was at death. Measurement of the carbon-14 activity at a given time thus allows calculation of the time elapsed after the death of the organism.
lignin: A complex organic polymer and major component of wood.
tannin: A complex astringent substance occurring widely in plants, particularly in leaves, unripe fruits, and tree bark.
phytoplankton: Small, usually microscopic, aquatic plants capable of photosynthesis; e.g., unicellular algae. Phytoplankton and plankton are not equivalent. The term "plankton" is a general designation for various drifting microscopic aquatic organisms in the upper regions of the oceans, both photosynthetic and non-photosynthetic.
Nature http://www.nature.com/nature
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