Investigation of Mercury, Carbon and Oxygen Isotopes in the Environment

Emission of mercury from the burning of coal is considered one of the important anthropogenic sources of atmospheric mercury.Along with current measurements of the isotopic composition of atmospheric mercury being conducted in our laboratory, we have analyzed mercury emitted from a coal fired power plant.Biswas and others (2008) previously reported variations in the isotopic composition of mercury in samples of coal deposits.Since the combustion of coal is expected to release virtually all its mercury, we expect similar isotopic patterns in coal and total emitted mercury.The emitted mercury exists in various physical and chemical forms, each possessing distinct properties that affect atmospheric transport.Flue gases have been sampled in the stacks of a coal-fired electric power plant.The Ontario Hydro method was used to collect mercury of the flue gases.The method, a modification of EPA method 29 uses oxidant solutions (KCl, H 2 O 2 -HNO 3 and KMnO 4 -H 2 SO 4 ) in its sampling train.In addition gaseous reactive mercury was sampled downwind in large volume rain samples.The measurements shows a similar mass independent fractionation with coal samples.But, displays a wide range of mass dependent fractionation ( 198 Hg -1.239 to 2.294).We found that samples in Hg 2+ are light isotope enriched, while in Hg 0 these are reverse.Plant specimens collected from an altitudinal range of 3700 to 4500m above sea level in the Zhada Basin in southwesten Tibet were identified into species and analyzed for their stable carbon isotopic compositions.These plants represent 35 species and 11 families.Their stable carbon isotope ratios ( 13 C) show that although C 3 plants are the dominant vegetation in local ecosystems a few C 4 plants also exist in the study area in the warmest months.The C 4 grasses found in the Zhada Basin are Achnatherum splendens and Pennisetum flaccidum of the family Graminaea, Atriplex centralasiatica and Salsola ruthenica of the family Chenopodiaceae and Poa lahulensis of Poaceae .The 13 C data indicate that among the 35 plant species found in the Zhada basin, 30 are C 3 plants (31 samples representing 30 different species) and have 13 C values ranging from -30.3 to -21.1, with a mean of -25.1+2 (n=31).C 4 grasses (6 samples representing 5 different species) from the basin yielded 13 C values ranging from -15.6 to -13.6, with a mean of -14.3+0.7(n=5), and account for ~14% of all species collected.The discovery of C 4 species in the Zhada Basin shows that C 4 plants can exist at high elevations xi though they are few in number as cold conditions do not favor C 4 grasses.The stable carbon isotope ( 13 C) analysis of tooth enamel from modern herbivores show that C 4 grasses do not contribute significantly to the diets of modern herbivores in the Zhada Basin and thus confirm the current dominance of C 3 vegetation in the area.Stable isotope analyses of tooth enamel from both fossil and modern herbivores were used to reconstruct the modern and ancient diets and paleoenvironment of the Zhada Basin, southwestern Tibetan Plateau.The 13 C values of enamel samples from wild Tibetan ass (Equus kiang) and domestic cows (Bos primigeniuss) from the Zhada Basin are -9.4 , which indicate a diet comprising predominantly of C 3 plants and are consistent with the current dominance of C 3 vegetation in the area.However, some of the serial samples from the Bos primigenius show 13 C values higher than -8, which suggests the consumption of small amounts of C 4 grasses and/or CAM plants by the Bos primigenius.The bulk enamel- 13 C values of fossil herbivores including Hipparion, Rhinocerotidaes, Bovidaes, Elephantidaes and Cervidae are -9.2 0.1, indicating that these ancient mammals, like modern herbivores in the area, fed primarily on C 3 vegetation and lived in an environment dominated by C 3 plants.The 18 O values of serial tooth enamel samples from modern herbivores show intra-tooth variations of ~2-5.The oxygen isotope ratios ( 18 O) of fossil herbivores are more negative relative to those of modern Bos primigenius and Tibetan asses.There is no inverse correlation between 13 C and 18 O values within individual fossil teeth from 5.3 to 3.5Ma, confirming that little or no C 4 grasses were consumed in the basin from 5.3 to 3.1 Ma is consistent with the bulk enamel isotope data.The intra-tooth 18 O variations of fossil herbivores ranged from ~ 1 to ~5, similar to those observed in modern teeth.The present day climate in the Zhada Basin is cold and does not favor the growth of C 4 grasses.Although C 4 plants have been found in the Zhada Basin, they do not contribute significantly to local biomass as well as herbivores' diets as indicated by the enamel 13 C values.The bulk and serial isotope data suggest that cold and C 3 -dominated environments have been a feature of the Zhada Basin since at least 5.3 Ma.

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