Carbonate cycles ancient

Dickens (1999) cautions that in modem times a similar reoccurrence is prevented by deeper oceans than in ancient times. However, the importance of the LPTM perturbation is that it is the only analog available in the geological record for understanding how the global carbon cycle and other systems is related to a rapid, massive input of fossil fuel such as that which may be occurring in modern industrial times. 

Veizer J. (1985) Carbonates and ancient oceans Isotopic and chemical record on time scales of 107 - 109 years. In The Carbon Cycle and Atmospheric CO2 Natural Variations, Archean to Present (eds. E.T. Sundquist and W.S. Broecker), pp. 595-601. Geophys. Monographs, Washington, D.C. 

Radiocarbon dating has attracted considerable attention. Carbon-14 is produced in the upper atmosphere by cosmic-ray bombardment of nitrogen-14. It is oxidised to carbon dioxide and eventually absorbed and incorporated in the tissues of plants and animals. The time taken for a carbon atom to complete such a carbon-cycle and return to the upper atmosphere is, on average, about 500 years. As the half-life of is 5568 years, the specific activity of carbon in the carbon cycle is roughly constant. But carbon removed from this life-embracing cycle by conversion to, and retention in, a solid such as wood, bone or shell loses activity at a rate determined by the decay constant for Thus the specific activity of carbon in a rock, a fossil plant or bone, or ancient artifact gives its age (Libby, 1951). Measurements are not easy because of the low specific activities but are of considerable and improving accuracy. 

Figure 26-18 shows the carbon cycle. Carbon dioxide in the atmosphere is in equilibrium with an enormous quantity that is dissolved in oceans, lakes, and streams. Some of this dissolved CO2 was once in the form of calcium carbonate (CaC03), the main component of the shells of ancient marine animals. The shells were eventually converted into limestone, which represents a large store of carbon on Earth. When the limestone was exposed to the atmosphere by receding seas, it weathered under the action of rain and surface water, producing carbon dioxide. Some of this CO2 was released into the atmosphere. This process continues today. 

Carbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 ( C) to determine the age of carbonaceous materials up to about 60 000 years. The technique was developed in 1949 [35]. Although fossil fuels have their origin in ancient biomass, they are not considered biomass because they contain carbon that has been out of the carbon cycle for a very long time. This means that the C14 content is zero compared to biomass that is by far younger. 

Freeman, K.H., and J.M. Hayes. 1992. Fractionation of carbon isotopes by phytoplankton and estimates of ancient CO2 levels. Global Biogeochemical Cycles 6 185-198. 

Freeman KH, Hayes JM (1992) Fractionation of carbon isotopes by phytoplankton and estimates of ancient CO2 levels. Global Biogeochem Cycles 6 185-198 Freeman KH, Hayes JM, Trendel JM, Albrecht P (1990) Evidence from carbon isotope measurements for diverse origins of sedimentary hydrocarbons. Nature 343 254-256 Freyer HD (1979) On the C-record in tree rings, I, C variations in northern hemisphere trees during the last 150 years, TeUus 31 124-137... 

Schmidt, M. W. I., Skjemstad, J. O., and Jager, C. (2002). Carbon isotope geochemistry and nanomorphology of soil black carbon Black chernozemic soils in central Europe originate from ancient biomass burning, Global Biogeochem. Cycles 16, Art. No. 1123. 

Sleep N. H. and Zahnle K. (2001) Carbon dioxide cycling and impheations for chmate on ancient Earth. ]. Geophys. Res. 106, 1373-1399. 

Carbon dioxide also enters the atmosphere when plants and animals decompose. Recall from Chapter 22 that the remains of ancient plants and animals were converted under pressure to fossil fuels. When fossil fuels are humed, the carbon is converted to CO2. As you ll learn, the huming of fossil fuels and other human activities may he disrupting the balance of the carhon cycle. 

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