名古屋大学年代測定総合研究センターシンポジウム報告

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Over the past five years, gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) analyses of phospholipid fatty acids (PLFAs) has been increasingly used to link organic matter (OM) sources with sedimentary bacteria. This technique has been applied across diverse estuarine and coastal sediments, including lower Laguna Madre, TX, an oligotrophic, coastal lagoon dominated by a single OM source, seagrasses; shelf stations, a eutrophic coastal region receiving multiple sources of OM, hypoxic regions that occur seasonally and deep slope and abyssal plain sediments of the Gulf of Mexico (GOM). Previous reports using the Laguna Madre data as examples, have been used to make comparisons of PLFA 16:0 and PLFA 15:0 isotope ratios and PLFA 16:0 and total organic carbon isotope ratios. Deviations from the 1:1 line in the former indicate living or recently senescent sources of organic matter are not predominantly bacterial. Deviations from the 1:1 line in the latter indicate living or recently senescent sources of organic matter differ isotopically from detrital or older OM in sediments. Prior to the work of Goni et al. (1998), carbon isotope ratios of OM in GOM sediments were interpreted as marine in origin. Based on a series of geochemical measurements, Goni et al. suggested that GOM sediments are largely composed of terrestrial organic carbon (OCterr). Furthermore, They went on to show that shelf and slope sediments were primarily C3 and C4 respectively. I report on the preferential utilization of autochthonous OM by sedimentary bacteria at the sediment surface and the shift to recalcitrant, terrestrially derived OM with depth.

Environmental liability cases involving spilled or released petroleum products into ocean ecosystems require oil identification techniques that are unambiguous and conclusive, even in situations where oils have been subjected to secondary environmental processes, such as, evaporation and dissolution. The ability and functionality of the Compound Specific Isotope Correlation (CSIC) technique is tested to determine its reliability to characterize released petroleum using the carbon isotope ratios (13C/12C) of the individual gasoline-range compounds (C5-C9). In particular, this thesis studies the potential of CSIC as a robust diagnostic tool, to identify and correlate marine releases of oil with their sources, especially those having undergone evaporative weathering. Three crude oils (Alberta Sweet Mixed Blend, Lacula and Louisiana) added to synthetic seawater were exposed to mechanically simulated wave energy and controlled evaporative weathering at 10 oC. Time-series sampling of the gasoline-range vapour fractions from the headspace employed Solid Phase Micro Extraction (SPME). SPME-Continuous Flow-Isotope Ratio Mass Spectrometry (SPME-CF-IRMS) determined the molecular abundances and stable carbon isotope ratios (δ13C) of the gasoline-range compounds of the original and weathered oils. Evaporation rates over the maximum 20 hour period varied for the 3 study oils. Most (74%) of the individual compounds measured in the oils display a δ13C enrichment with progressive evaporation with approximately half of the compounds in all 3 oils showing fractionation of the carbon isotopes ≤ 0.5‰ within measurement precision. The magnitude of carbon isotope shift observed in compounds pre-vs. post-weathering ranges from 0 to 2.8 ±0.6‰. There is no clear relationship identified between the degree of 13C enrichment in the oils and groupings such as chemical class, structure or carbon number. The overall weighted average 13C enrichment for all compounds in the 3 oils is approximately 1‰. Toluene was the only compound consistently exhibiting comparatively high 13C enrichment (1.6‰, 1.8‰ and 2.8‰) in all 3 oils after evaporative weathering. Hierarchical Cluster Analysis (HCA) treatment of the CSIC data set can reliably discriminate between the 3 oils despite evaporative weathering and δ13C changes. HCA is also able to unambiguously relate the three weathered oils back to their respective original unweathered oil. Diagnostic shifts in δ13C of individual compounds in an oil may potentially be used to trace weathered oils back to the source, and possibly give a estimation of time since release. However the typically rapid rate of evaporation for the gasoline-range fractions limits the time that an oil can be successfully identified by CSIC. / Graduate

Climate change is expected to have significant and uncertain impacts on methane (CH4) emissions from northern peatlands. Biogeochemical models can extrapolate site-specificCH(4) measurements to larger scales and predict responses of CH4 emissions to environmental changes. However, these models include considerable uncertainties and limitations in representing CH4 production, consumption, and transport processes. To improve predictions of CH4 transformations, we incorporated acetate and stable carbon (C) isotopic dynamics associated with CH4 cycling into a biogeochemistry model, DNDC. By including these new features, DNDC explicitly simulates acetate dynamics and the relative contribution of acetotrophic and hydro-genotrophic methanogenesis (AM and HM) to CH4 production, and predicts the C isotopic signature (delta C-13) in soil C pools and emitted gases. When tested against biogeochemical and microbial community observations at two sites in a zone of thawing permafrost in a subarctic peatland in Sweden, the new formulation substantially improved agreement with CH4 production pathways and delta C-13 in emitted CH4 (delta C-13-CH4), a measure of the integrated effects of microbial production and consumption, and of physical transport. We also investigated the sensitivity of simulated delta C-13-CH4 to C isotopic composition of substrates and, to fractionation factors for CH4 production (alpha(AM) and alpha(HM)), CH4 oxidation (alpha(MO)), and plant-mediated CH4 transport (alpha(TP)). The sensitivity analysis indicated that the delta C-13-CH4 is highly sensitive to the factors associated with microbial metabolism (alpha(AM), alpha(HM), and alpha(MO)). The model framework simulating stable C isotopic dynamics provides a robust basis for better constraining and testing microbial mechanisms in predicting CH4 cycling in peatlands.

In order to achieve a more accurate input value for groundwater age determining models, δ13CVPDB values for soil gas were evaluated at 50 cm depths in locations throughout Utah in order to define correlations between δ13C and environmental parameters. 16 sites were chosen that exploited large changes in elevation and latitude which provided variations in climate, precipitation, plant community, etc. Gas samples were collected over 1-1½ years, and soil samples were collected at depth during installations. Field and laboratory studies were also used to evaluate CO2 and δ13C change with depth. It was discovered that in mountainous recharge areas, the mean δ13C value was −21.8 ± 2.8‰ (2σ). Use of this value reduced the uncertainty in groundwater age models by close to half relative to the current assumed range of -13‰ to -28‰. One arid and one semi-arid site were located at Pilot Valley (mean -11.0‰ ± 2.0, 2σ) and the mouth of Cedar Canyon (mean -17.5‰ ± 0.8, 2σ). The high values at these locations may be due to low root respiration and low microbial activity rates caused by high temperatures, low precipitation, and low vegetation density. Correlations among environmental parameters and δ13C values were discovered with ANOVA, but the differences were small enough that for practical purposes they are insignificant.

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Bark beetle outbreaks are becoming more intense and severe when coupled with the effects of climate change. Engelmann spruce (Picea engelmannii) is one such species facing large-scale, epidemic spruce beetle outbreaks. Large-scale disturbances, such as beetle outbreaks, have major consequences for the future success of the ecosystem, thus highlighting the importance of understanding what promotes amplified outbreaks as well as their effects on future seedling establishment. Our research focused on two parts of a large-scale beetle outbreak: the mortality of spruce trees and the subsequent regeneration of seedlings. Our first study examined the timing of spruce mortality during an outbreak in order to identify the extent to which drought promotes host species mortality. Trees that are drought stressed have less resources to defend themselves against beetle attacks, however, the warmer temperatures associated with droughts also promote a more rapid population expansion of spruce beetles. We were specifically interested in determining the contribution that host drought stress plays during an epidemic outbreak. Our second study analyzed the patterns of regenerating seedlings with an aim to identify changes associated with the outbreak. Specifically, we were interested in how an epidemic outbreak changes the drivers of seedling establishment.

Grass-dominated ecosystems in many regions around the world have experienced increased abundance of woody plants during the past 100 yrs. In the Rio Grande Plains of southern Texas, subtropical woodlands, dominated by C3 trees/shrubs capable of symbiotic N2-fixation, have become significant components of landscapes that were once dominated by C4 grasslands. Upland areas in this region now consist of small discrete clusters and large groves of woody vegetation embedded in a grassy matrix, while lower-lying portions of the landscape are dominated by closed-canopy woodlands. I used soil δ13C in conjunction with aerial photography and geostatistics to quantify landscape-scale vegetation dynamics in uplands of this savanna parkland. Spatial patterns of soil δ13C in grids and transects traversing woody patches indicated larger woody groves were formed from small discrete clusters of woody plants that spread laterally and eventually coalesced. Soil δ13C contour maps revealed some clusters are currently growing rapidly towards each other and might coalesce into groves in the near future, while some clusters remained relatively stable. Kriged maps of soil δ13C provided a strong spatial context for future studies aimed at understanding the functional consequences of this change in landscape structure. The dominant invading woody plant, honey mesquite (Prosopis glandulosa), was important in determining the spatial pattern of soil δ13C, supporting the hypothesis that they serve as recruitment foci and facilitate the establishment of subordinate woody species. Leaf δ15N values suggested that the N2-fixing mesquite influenced the N nutrition of nearby non-N2-fixing shrubs, thus, suggesting a mechanism by which mesquite could facilitate establishment of other woody species. In closed-canopy drainage woodlands, however, spatial patterns of soil δ13C were no longer controlled by the presence of mesquite, but by the amount of soil organic carbon and soil texture. The positive correlation between silt+clay and soil δ13C indicates that the formation of organomineral complexes and microaggregates may slow SOC turnover rates and favor the persistence of C4-derived SOC from the original grassland. This study enhances our understanding of potential patterns, causes and consequences of grassland to woodland conversions which are underway today in many grass-dominated ecosystems around the world.