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ScienceWeek
EARTH SCIENCE: ON FOSSIL ORGANIC CARBON IN MARINE SEDIMENTS
The following points are made by A.F. Dickens et al (Nature 2004 427:336):
1) Marine sediments act as the ultimate sink for organic carbon, sequestering otherwise rapidly cycling carbon for geologic timescales(1,2). Sedimentary organic carbon burial appears to be controlled by oxygen exposure time in situ(3,4), and much research has focused on understanding the mechanisms of preservation of organic carbon(5).
2) In this context, combustion-derived black carbon has received attention as a form of refractory organic carbon that may be preferentially preserved in soils and sediments. However, little is understood about the environmental roles, transport and distribution of black carbon.
3) Black carbon (BC) is the heterogeneous, aromatic and carbon-rich residue of biomass burning and fossil-fuel combustion. It is formed on land, and may be eroded from soils and carried by rivers or via aerosol transport to the ocean. BC in open-ocean sediments is from 2400 to 13,900 yr older than the co-deposited bulk organic carbon (OC)12. It is possible that BC is small and light enough to be stored in the oceanic dissolved organic carbon pool before being deposited in marine sediments, accounting for the older age of the BC. Alternatively, BC may age on land before transport to the ocean, either in the soil carbon pool or as fossil BC from ancient forest fires stored in rocks. Existing data cannot distinguish between these storage and transport mechanisms.
4) In summary: The authors apply isotopic analyses to graphitic black carbon samples isolated from pre-industrial marine and terrestrial sediments. The authors find that this material is terrestrially derived and almost entirely depleted of radiocarbon, suggesting that it is graphite weathered from rocks, rather than a combustion product. The widespread presence of fossil graphitic black carbon in sediments has therefore probably led to significant overestimates of burial of combustion-derived black carbon in marine sediments. It could be responsible for biasing radiocarbon dating of sedimentary organic carbon, and also reveals a closed loop in the carbon cycle. Depending on its susceptibility to oxidation, this recycled carbon may be locked away from the biologically mediated carbon cycle for many geologic cycles.
References (abridged):
1. Hedges, J. I. & Keil, R. G. Sedimentary organic matter preservation: an assessment and speculative synthesis. Mar. Chem. 49, 81–115 (1995)
2. Berner, R. A. Biogeochemical cycles of carbon and sulfur and their effect on atmospheric oxygen over Phanerozoic time. Palaeogeogr. Palaeoclimatol. Palaeoecol. 75, 97–122 (1989)
3. Hartnett, H. E., Keil, R. G., Hedges, J. I. & Devol, A. H. Influence of oxygen exposure time on organic carbon preservation in continental margin sediments. Nature 391, 572–574 (1998)
4. Hedges, J. I. et al. Sedimentary organic matter preservation: a test for selective degradation under oxic conditions. Am. J. Sci. 299, 529–555 (1999)
5. Hedges, J. I. et al. The molecularly-uncharacterized component of nonliving organic matter in natural environments. Org. Geochem. 31, 945–958 (2000)
Nature http://www.nature.com/nature
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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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