Carbon excursions

Fig. 4.14. Tracing climate change in the Miocene. Shown here are records of ice volume and temperature (based on foraminiferal S 0) and relative organic carbon burial (based on foraminiferal S C), compared with the CO2 estimates of Pagani et al. (1999), and tectonic events that may have affected ocean heat transport. Trends in CO2 are consistent with organic carbon burial and CO2 drawdown during the Monterey Excursion, but cannot explain the Miocene Climatic Optimum (MCO) or expansion of the East Antarctic Ice Sheet (EAIS).

This rule holds reasonably well when C or t varies within a narrow range for acute exposure to a gaseous compound (Rinehart and Hatch, 1964) and for chronic exposure to an inert particle (Henderson et al., 1991). Excursion of C or / beyond these limits will cause the assumption Ct = K to be incorrect (Adams et al., 1950, 1952 Sidorenko and Pinigin, 1976 Andersen et al., 1979 Uemitsu et al., 1985). For example, an animal may be exposed to 1000 ppm of diethyl ether for 420 min or 1400 ppm for 300 min without incurring any anesthesia. However, exposure to 420,000 ppm for lmin will surely cause anesthesia or even death of the animal. Furthermore, toxicokinetic study of fiver enzymes affected by inhalation of carbon tetrachloride (Uemitsu et al., 1985), which has a saturable metabolism in rats, showed that Ct = K does not correctly reflect the toxicity value of this compound. Therefore, the limitations of Haber s rule must be recognized when it is used in interpolation or extrapolation of inhalation toxicity data. 

Figure 16. (a) Ca isotope record from marine carbonates (De La Rocha and DePaolo 2000). The variations are inferred to reflect variations in the isotopic composition of seawater (which is heavier by about 1,4%o). The small excursions of S Ca reflect changes in the global weathering cycle they are recast in (b) in terms of the ratio of the flux of calcium being delivered to the ocean by weathering (Fw) to the flux of Ca being removed from the ocean by carbonate sedimentation (c) Smoothed record of benthic foraminiferal 5 0 for the Cenozoic time period from Zachos et al. (2001). 

The use of graphitized carbons can impart significant stability to high-voltage excursions and appears suitable for automotive fuel cell use. However, a number of workers have reported fhaf the performance of Pt on these supports is poorer than that found for conventional Pt catalysts. For example. 

Figure 3.49 summarizes the oxygen isotope curve for the last 65 Ma. The most pronounced warming trend is expressed by a 1.5%o decrease in 8 0 and occurred early in the Cenozoic from 59 to 52 Ma, with a peak in Early Eocene. Coinciding with this event is a brief negative carbon isotope excursion, explained as a massive release of methane into the atmosphere (Norris and Rohl 1999). These authors used high resolution analysis of sedimentary cores to show that two thirds of the carbon shift occurred just in a few thousand years, indicating a catastrophic release of carbon from methane clathrates into the ocean and atmosphere. 

Kump LR (1989) Alternative modeling approaches to the geochemical cycles of carbon, sulfur and strontium isotopes. Am J, Sd 289 390-410 Kump LR (2005) Ironing out biosphere oxidation. Science 307 1058-1059 Kump LR, Arthur MA (1999) Interpreting carbon-isotope excursions carbonates and organic matter, Chem Geol, 161 181-198... 

The 2003 ACGIH threshold limit value-time-weighted average (TLV-TWA) for carbon dioxide is 5000 ppm (9000 mg/m ) with a short-term excursion limit of 3 0,000 ppm (54,000 mg/m ). 

On the basis of scale-up considerations from 0.4-L runs, each sample extraction was conducted at 1950 50 lb/in.2 and 37-45 °C, lasted about 110 min, and involved passing approximately 11,200 standard liters of carbon dioxide through the aqueous solution. Because pressure-flow rate excursions might occur in the large-scale apparatus and lead to the rupture of glass traps, a series of three stainless steel impingers maintained at —76 °C were used to collect the organics present in the effluent carbon dioxide stream. 

Vqq, of adsorbed carbon monoxide against electrode potential at gold electrodes, for CO-saturated (ca. ImM) solutions. Electrolytes were ( , ) 0.1 M HCIO, ( ) 0.1 M NaClO, and (A) 0.1 M KF. The arrowed dashed curves represent the sequential peak frequencies obtained upon potential excursions from 100 mV to 500 mV and return, into the region where CO electrooxidation, occurs. The solid straight line, drawn through the points obtained at potentials (< 100 mV) where adsorbed CO is stable towards electrooxidation, has a slope of about 50 cm l V. ... 

This short excursion into the diverse field of natural and artificial cyclopropane architectures highlights the fact that cyclopropanes continue to provide stimuli for and challenges to current concepts of synthesis, structure and theory. It s amazing what three carbons can do. 

For example, in the carbon cycle consider the balance between terrestrial photosynthesis and respiration-decay. If the respiration and decay flux to the atmosphere were doubled (perhaps by a temperature increase) from about 5200 x 1012 to 10,400 x 1012 moles y-l, and photosynthesis remained constant, the CO2 content of the atmosphere would be doubled in about 12 years. If the reverse occurred, and photosynthesis were doubled, while respiration and decay remained constant, the CO2 content of the atmosphere would be halved in about the same time. An effective and rapid feedback mechanism is necessary to prevent such excursions, although they have occurred in the geologic past. On a short time scale (hundreds of years or less), the feedbacks involve the ocean and terrestrial biota. As was shown in Chapter 4, an increase in atmospheric CO2 leads to an increase in the uptake of CO2 in the ocean. Also, an initial increase in atmospheric CO2 could lead to fertilization of those terrestrial plants which are not nutrient limited, provided there is sufficient water, removal of CO2, and growth of the terrestrial biosphere. Thus, both of the aforementioned processes are feedback mechanisms that can operate in a positive or negative sense. An increased rate of photosynthesis would deplete atmospheric CO2, which would in turn decrease photosynthesis and increase the oceanic evasion rate of CO2, leading to a rise in atmospheric CO2 content. More will be said later about feedback mechanisms in the carbon system. 

If a complete cell is charged to, e.g., 4.1 V, then the potential Z carbon of the fully lithiated negative electrode will be about 0.1 V vs. Li/Li+. Therefore, the potential Eoxiie of the fully charged positive electrode in this example will be 4.2 V vs. Li/Li+. Needless to say that this trivial relationship must be remembered when data for half cells (vs. metallic lithium) are compared to the data for complete cells. An important consequence of this trivial relationship is the potential excursion of the counterelectrode in the case of an anomalous behavior of the carbon electrode (and vice versa). Imagine that, in the previous example the potential of the carbon would shift to 0.3 V vs. Li/Li+ due to a malfunction of the carbon electrode. If the end-of-charge voltage of the complete cell would be the same, namely 4.1V, then the potential of the positive electrode would be 4.4 V vs. Li/Li+. In such a case, the safety of the entire cell could be compromised. 

The impelling force in these excursions of atoms up the valley sides is molecular collision. Collisions impart energy to the bromine molecules and to the molecule containing the two carbon atoms and permit them to rise to the top of the pass. In most cases the collisions are not violent enough to carry them to the top of the pass and they fall back to the valley floors. Even when a bromine atom gets to the top of the pass it can fall either way and become part of either a stable C-Br compound or a stable Br-Br molecule. At this particular position at the top of the pass... 

Berger, W.H. Carbon dioxide excursions in the deep sea record Aspects of the problem, p. 502-542, Andersen,... 

Ball, S.C. et ah. The effect of dynamic and steady state voltage excursions on the stability of carbon supported Pt and PtCo catalysts, ECS Trans., 3, 595, 2006. 

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