Carbon isotopes, photochemical

S. Opsahl, R.G. Zepp (2001). Photochemically-induced alteration of stable carbon isotope ratios (delta C) in terrigenous dissolved organic carbon. Geophys. Res. Lett., 28,2417-2420. 

If we attribute the anomalous isotope behavior of oxygen to the mechanism of photochemical self-shielding in the photolysis of CO, should we not expect similar effects in carbon isotopes The ratio in solar system... 

This new and novel method to study the photochemical degradation of Kevlar-29 fabric in air divides into four steps (1) fabric cleaning, (2) photolysis at specified temperature and time in 0.2 atm - 02, (3) preparation of the degraded (DMAc-soluble) sample surface for decarboxylation at 25° and 196°C in the concentrated sulfuric acid, and (4) the total carbon dioxide analyses by gas chromatography and the isotopic carbon dioxide ( °C02 and 48co2) ratios by GC-mass spectrometer. 

In recent years, tremendous progress has been achieved in the analysis of the isotope composition of important trace compounds in the atmosphere. The major elements - nitrogen, oxygen, carbon - continually break apart and recombine in a multitude of photochemical reactions, which have the potential to produce isotope fractionations (Kaye 1987). Isotope analysis is increasingly employed in studies of the cycles of atmospheric trace gases e.g., CH4 and N2O, which can give insights into sources and sinks and transport processes of these compounds. The rationale is that various sources have characteristic isotope ratios and that sink processes are accompanied by isotope fractionation. 

Kasting (2001) argues in support of the view of Farquhar et al (2000) (but see also Ohmoto et al, 2001) that sulfur isotope fractionation changed around 2.3 Ga. This opinion is based on the claim, from comparison of sulfur isotopes, that so-called mass independent fractionation occurred as a result of gas-phase photochemical reactions, particularly photolysis of SO2. Such fractionation would be much more likely to occur in a I0W-O2 atmosphere in which sulfur was present in a variety of oxidation states. Thus, the claim that fractionation changed around 2.3 Ga ago can be seen as supporting the notion that there was a substantial rise in O2 around this time. This, however, raises the question if cyanobacterial oxygen production had been sufficient to create the mbisco fingerprint in carbonates as early as 2.7-3.0Ga ago, why did the rise of free O2 only occur 400-700 Myr later ... 

Phosgene, containing 2-4% by volume of ac, was obtained by photochemical equilibration (using 253.7 nm mercury line) with isotope-enriched carbon monoxide (see Section 5.1.1) [2135] ... 

Laureni and co-workers have studied the photochemical decomposition of 1,2,3-selena and 1,2,3-thiadiazoles in argon or nitrogen matrix. In each case they could identify the products as the ethynylselenol or thiol (136) and the seleno-or thioketene (137). In addition, in the case of 1,2,3-selen-adiazole acetylene was also detected. Using isotopically labelled substrates, they demonstrated that a major portion of the ethynylthiol formed from 1,2,3-thiadiazoles must have undergone an equilibriation of the carbons, probably through the symmetrical thiirene intermediate (138). In the case of the selenium compound, however, their results showed that the selenirene is not on the route to the ethynylselenol (136, X = Se). 

In addition to its effects on microbial activity, solar UVR has direct effects on decomposition. A variety of recent studies have provided evidence that CDOM undergoes a complex array of other photoreactions that can involve a decrease in average molecular weight accompanied by cleavage to a variety of photoproducts [3-5,8,24,25,68,70,74,94-118], changes in isotopic content [60,119] and consumption of oxygen [59,68,106,116]. These reactions include the direct photochemical mineralization of the CDOM to carbon monoxide and dissolved inorganic carbon (DIC). Of these various direct pathways, the photoproduction of DIC is most efficient. 

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