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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.

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.

Microbes, including Bacteria, are globally important mediators of elemental transformations in the marine water column, but not until recently has their biomass been suggested to contribute significantly to carbon export flux. Here I characterize lipid and carbon isotopic signatures in marine particulate organic matter (POM) explicitly at microbial size scales, and I quantitatively explore how these signatures are transferred down the water column. In the North Pacific Subtropical Gyre (NPSG) an isotopically-enriched pool of submicron POM appears to dominate export to mesopelagic depths, supporting recent observations that bacterioplankton communities contribute to export flux in proportion to their biological abundance. In the Eastern Tropical North Pacific (ETNP) complex pathways emerge for the flux of POM to the deep ocean. I use the largest data set to date for natural \(^{13}C\) signatures of individual water column lipids to reveal that submicron and larger-size suspended POM size classes are isotopically distinct. Results point to de novo production of lipids above and within the oxygen minimum zone. I develop quantitative models to deconvolve the signatures of sinking and in situ sources of these lipids. Results converge on a best-fit model for downward flux in the ETNP that includes both surface-derived and sub-photic zone lipids. Overall results from the modern ocean suggest that approximately half of total suspended POM is submicron in size, much of it is bacterial in origin, and despite the small size of this material, it participates dynamically in water column export flux. These results also suggest some revised interpretations of organic matter signatures in the geologic record. I formulate a quantitative model of marine microbial production and degradation, and reproduce "inverse" isotopic signatures found in lipids and organic matter preserved in Proterozoic sedimentary rocks. Results suggest that the disappearance of this inverse \(^{13}C\) pattern was a consequence of the shift from Bacteria to Eukarya as dominant producers of marine autotrophic biomass. Together, results of this thesis reveal that heterogeneity in the isotopic signatures of marine suspended POM is associated with particle size, and by extension, must be a function of the composition of the total planktonic community. / Earth and Planetary Sciences

Stable carbon isotope analyses of the humin fraction of the soil organic matter were conducted on more than 160 soil profiles from Tikal, Guatemala. The profiles were collected from near areas associated with the earthworks of Tikal; an ancient ditch and parapet construction hypothesized to have formed ancient boundaries of the polity. In addition to the isotope analyses, the physical and chemical characteristics of the horizons were determined. Maize, a C4 plant, formed an integral part of the ancient Maya diet and is the only known C4 plant cultivated by the Maya. Prior to and subsequent to the ancient Maya occupation of Tikal, the landscape was dominated by C3 forest vegetation. Over the centuries C4 plant biomass including rhizodeposition decomposed to form soil organic matter that contains a distinct C4 signature reflecting the vegetation history of the area. Forested areas anciently cleared for agriculture were identified through interpretation of significant isotopic shifts that signaled past vegetation changes. Buried horizons were encountered in the upland depressions and bajo wetlands. The aggraded soil deposits were likely the result of increased human activity related to settlement and agriculture. The buried horizons and the overlying sediments exhibited stable carbon isotope shifts associated with forest clearance and maize agriculture. Geospatial analysis of the stable carbon isotope ratios indicated that ancient Maya agriculture was focused on deeper footslope and toeslope soils in both bajos and upland depressions. Some evidence of infield agriculture or food processing was also encountered in connection with ancient settlement at upland locations. The soil data provide insight into ancient land use and sustainability that could potentially contribute to subsistence and population reconstruction models.

The chronostratigraphic correlation of the Silurian units in Iowa is complicated by complex carbonate depositional environments and poor biostratigraphic control. In this study, we integrate conodont biostratigraphic data with carbon isotope (δ 13Ccarb) chemostratigraphic data from the Blanding, Hopkinton, Scotch Grove, and LaPorte City formations of Iowa to provide a precise chronostratigraphic framework for regional and global correlation. Conodonts from the LaPorte City Formation of eastern Iowa (East-Central Iowa Basin) in the Garrison Core indicate an early to middle Telychian age for the formation. Conodonts diagnostic of the Pterospathodus eopennatus Superzone, Pterospathodus eopennatus ssp. nov. 2 Zone, and Pterospathodus amorphognathoides angulatus Zone were recovered, allowing for the first direct comparison of the stratigraphic ranges of conodont species from the North American Midcontinent and the Baltic basin. The morphology of Pseudolonchodina fluegeli ssp. n. Männik (2007) co-occurs with Pseudolonchodina fluegeli fluegeli sensu Männik (2007a) in the LaPorte City Formation and are not stratigraphically separated in Iowa as they are in the East Baltic. Wurmiella? polinclinata polinclinata ranges much lower in the East-Central Iowa Basin (Pt. eopennatus ssp. nov. 2 Zone) than the Baltic Basin, so it cannot be used as an index fossil diagnostic of the Pt. am. amorphognathoides Zone in global correlations. Three carbon isotope excursions were recovered from the Hopkinton and Scotch Grove formations. The late Aeronian (herein renamed “Farmers Creek”) carbon isotope excursion and a heretofore unrecorded carbon isotope excursion were recorded from the Hopkinton Formation in the SS-10 Core (Jones County). The Farmers Creek Excursion can be correlated to Johnson County, where it was recorded in the Hopkinton Formation by McAdams et al. (in prep). The Valgu excursion was recovered from the uppermost Hopkinton Formation through the overlying Buck Creek Quarry Member of the Scotch Grove Formation in the Garrison Core (Benton County). This excursion can be correlated to Johnson County, where it was recorded in the same formations. The integration of conodont biostratigraphic and carbon isotope chemostratigraphic data from the Silurian of Iowa allows for the first regional chronostratigraphic correlations at a resolution finer than stage level. The oxygen and carbon isotope values from the Garrison Core and the evidence for post-diagenetic karsting and fluid movement through the units may provide further evidence that the dolomitization process of the LaPorte City Formation was halted by the influx of meteoric phreatic water. This study shows the first high-resolution chronostratigraphic correlation of Silurian units in Iowa, and highlights the utility of integrated carbon isotope chemostratigraphy as a tool for chronostratigraphic correlation in strata with poor biostratigraphic control. The conodonts recovered from the LaPorte City Formation shows the utility of the small limestone formations on the northwest flank of the East-Central Iowa Basin for refining global Silurian conodont biostratigraphic zonation.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous, toxic contaminants that are released to the environment from various petrogenic and pyrogenic sources. In an effort to more clearly identify and trace sources of PAHs in the environment, purification and compound specific isotope analysis methods were developed to accurately measure the stable carbon isotope ratio of individual PAHs. Development of the method included improving accuracy and precision of the isotopic measurement by producing highly pure extracts using various chromatographic techniques. The method was refined by improving compound separations using purification techniques and high resolution chromatographic columns. The purification method consists of alumina/silica gel column chromatography, gel permeation chromatography and thin layer chromatography. The mean recovery of PAHs after the purification procedure was approximately 80 %. Sample purities after purification were verified by GC/FID and full scan mass spectrometry. To better resolve peaks and provide more accurate stable carbon isotope measurements, various gas chromatographic conditions were evaluated. The precision of the method ranged between 0.08 and 0.43 . The analytical protocols were evaluated to confirm compositional and stable isotopic integrity during purification and stable isotopic analysis. To confirm the utility of the purification and isotope analysis methods, various environmental samples from marine, land and lacustrine environments were analyzed. The isolates were analyzed for the composition and the stable carbon isotope ratios of PAHs. The stable carbon isotope ratio was measured by GC/IRMS and the results, along with quantitative compound compositions, were used to characterize and identify the contaminant sources. The sources of the PAHs in the study areas were differentiated by PAH molecular ratios and confirmed by stable carbon isotope ratios. This study confirms that compound specific isotope analysis of pollutants by GC/IRMS can be used to identify PAH sources in environmental samples. The study also confirms that the purification and stable carbon isotope analysis methods that were developed can be used to accurately measure the stable carbon isotope ratios of PAHs in environmental samples for the purpose of source identification. GC/IRMS measurement of stable isotopic compositions can be an effective fingerprinting method when used in conjunction with traditional molecular composition methods.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous, toxic contaminants that are released to the environment from various petrogenic and pyrogenic sources. In an effort to more clearly identify and trace sources of PAHs in the environment, purification and compound specific isotope analysis methods were developed to accurately measure the stable carbon isotope ratio of individual PAHs. Development of the method included improving accuracy and precision of the isotopic measurement by producing highly pure extracts using various chromatographic techniques. The method was refined by improving compound separations using purification techniques and high resolution chromatographic columns. The purification method consists of alumina/silica gel column chromatography, gel permeation chromatography and thin layer chromatography. The mean recovery of PAHs after the purification procedure was approximately 80 %. Sample purities after purification were verified by GC/FID and full scan mass spectrometry. To better resolve peaks and provide more accurate stable carbon isotope measurements, various gas chromatographic conditions were evaluated. The precision of the method ranged between 0.08 and 0.43 . The analytical protocols were evaluated to confirm compositional and stable isotopic integrity during purification and stable isotopic analysis. To confirm the utility of the purification and isotope analysis methods, various environmental samples from marine, land and lacustrine environments were analyzed. The isolates were analyzed for the composition and the stable carbon isotope ratios of PAHs. The stable carbon isotope ratio was measured by GC/IRMS and the results, along with quantitative compound compositions, were used to characterize and identify the contaminant sources. The sources of the PAHs in the study areas were differentiated by PAH molecular ratios and confirmed by stable carbon isotope ratios. This study confirms that compound specific isotope analysis of pollutants by GC/IRMS can be used to identify PAH sources in environmental samples. The study also confirms that the purification and stable carbon isotope analysis methods that were developed can be used to accurately measure the stable carbon isotope ratios of PAHs in environmental samples for the purpose of source identification. GC/IRMS measurement of stable isotopic compositions can be an effective fingerprinting method when used in conjunction with traditional molecular composition methods.

Stable carbon isotope ratios (δ13C) in tree-ring cellulose are modified by environmental conditions occurring during carbon fixation. Researchers have however not reached a consensus as to whether extractives, lignin and/or hemicelluloses, all with specific isotopic signatures, should be removed prior to dendroisotopic analysis. The topic of the first paper dealt with the comparison of Thuja occidentalis L. wood components and their suitability for subsequent dendroisotopic analyses. It was recommended that holocellulose be isolated since an alpha-cellulose yield may be too low for subsequent mass spectrometer analysis, especially when narrow rings are encountered and multiple stable isotope analyses are to be performed per sample. The second paper investigated the associations between the ring-width and δ13C chronologies with climate variables. The δ13C chronology spanned from 1650 to 2006 A.D. and incorporated dead and living T. occidentalis trees selected from two sites in central Manitoba, Canada. Compared to the δ13C values, ring width was more often associated with climate conditions in the year prior to ring formation. However, moisture stress was limiting for both radial growth and carbon assimilation. During the year of ring-formation, ring width was associated with spring and early summer conditions whereas, δ13C was more indicative of overall summer conditions. Nonetheless, each of ring width and δ13C contained individualistic climate information which could be used in tandem for long-term climate reconstruction.

Stable carbon isotope ratios (δ13C) in tree-ring cellulose are modified by environmental conditions occurring during carbon fixation. Researchers have however not reached a consensus as to whether extractives, lignin and/or hemicelluloses, all with specific isotopic signatures, should be removed prior to dendroisotopic analysis. The topic of the first paper dealt with the comparison of Thuja occidentalis L. wood components and their suitability for subsequent dendroisotopic analyses. It was recommended that holocellulose be isolated since an alpha-cellulose yield may be too low for subsequent mass spectrometer analysis, especially when narrow rings are encountered and multiple stable isotope analyses are to be performed per sample. The second paper investigated the associations between the ring-width and δ13C chronologies with climate variables. The δ13C chronology spanned from 1650 to 2006 A.D. and incorporated dead and living T. occidentalis trees selected from two sites in central Manitoba, Canada. Compared to the δ13C values, ring width was more often associated with climate conditions in the year prior to ring formation. However, moisture stress was limiting for both radial growth and carbon assimilation. During the year of ring-formation, ring width was associated with spring and early summer conditions whereas, δ13C was more indicative of overall summer conditions. Nonetheless, each of ring width and δ13C contained individualistic climate information which could be used in tandem for long-term climate reconstruction.