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, H2O2-HNO3 and KMnO4-H2SO4) 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 (δ198Hg -1.239 to 2.294). We found that samples in Hg2+ are light isotope enriched, while in Hg0 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 (δ13C) show that although C3 plants are the dominant vegetation in local ecosystems a few C4 plants also exist in the study area in the warmest months. The C4 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 δ13C data indicate that among the 35 plant species found in the Zhada basin, 30 are C3 plants (31 samples representing 30 different species) and have δ13C values ranging from -30.3 / to -21.1 /, with a mean of -25.1+2 / (n=31). C4 grasses (6 samples representing 5 different species) from the basin yielded δ13C 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 C4 species in the Zhada Basin shows that C4 plants can exist at high elevations though they are few in number as cold conditions do not favor C4 grasses. The stable carbon isotope (δ13C) analysis of tooth enamel from modern herbivores show that C4 grasses do not contribute significantly to the diets of modern herbivores in the Zhada Basin and thus confirm the current dominance of C3 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 δ13C 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 C3 plants and are consistent with the current dominance of C3 vegetation in the area. However, some of the serial samples from the Bos primigenius show δ13C values higher than -8 /, which suggests the consumption of small amounts of C4 grasses and/or CAM plants by the Bos primigenius. The bulk enamel-δ13C 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 C3 vegetation and lived in an environment dominated by C3 plants. The δ18O values of serial tooth enamel samples from modern herbivores show intra-tooth variations of ~2-5 /. The oxygen isotope ratios (δ18O) of fossil herbivores are more negative relative to those of modern Bos primigenius and Tibetan asses. There is no inverse correlation between δ13C and δ18O values within individual fossil teeth from 5.3 to 3.5Ma, confirming that little or no C4 grasses were consumed in the basin from 5.3 to 3.1 Ma is consistent with the bulk enamel isotope data. The intra-tooth δ18O 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 C4 grasses. Although C4 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 δ13C values. The bulk and serial isotope data suggest that cold and C3-dominated environments have been a feature of the Zhada Basin since at least 5.3 Ma.

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