Selenium (Se) is an essential trace element, which, with multiple oxidation states and six stable isotopes, has the potential to be a powerful paleo-environmental proxy. In this study, Se concentrations and isotopic compositions were analyzed in a suite of about 120 samples of fine-grained marine sedimentary rocks and sediments spanning the entire Phanerozoic. While the selenium concentrations vary greatly (0.22 to 72 ppm), the δ82/76Se values fall in a fairly narrow range from -1 to +1 , with the exception of laminated black shales from the New Albany Shale formation (Devonian), which have δ82/76Se values of up to +2.20 . Black Sea sediments (Holocene) and sedimentary rocks from the Alum Shale formation (Late Cambrian) have Se/TOC ratios and δ82/76Se values close to those found in modern marine plankton (1.72x10-6±1.55x10-7 mol/mol and 0.42±0.22 ). (Note: TOC = total organic carbon.) For the other sedimentary sequences, the Se/TOC ratios indicate enrichment in selenium relative to marine plankton. Additional input of isotopically light terrigenous Se (δ82/76Se ≈ -0.42 ) may explain the Se data measured in recent Arabian Sea sediments (Pleistocene). The very high Se concentrations in sedimentary sequences that include the Cenomanian-Turonian Ocean Anoxic Event (OAE) 2 possibly reflect a significantly enhanced input of volcanogenic Se to the oceans. As the latter has an isotopic composition (δ82/76Se ≈ 0 ) not greatly different from marine plankton, the volcanogenic source does not impart a distinct signature to the sedimentary Se isotope record. The lowest δ82/76Se values are observed in the OAE2 samples from Demerara Rise and Cape Verde Basin cores (δ82/76Se = -0.95 to 1.16 ) and are likely due to fractionation associated with microbial or chemical reduction of Se oxyanions in the euxinic water column. In contrast, a limiting availability of seawater Se during periods of increased organic matter burial is thought to be responsible for the elevated δ82/76Se values and low Se/TOC ratios in the black shales of the New Albany Shale formation. Overall, our results suggest that Se data may provide useful information on paleodepositional conditions, when included in a multi-proxy approach.

No description available.

Dans ce travail, nous avons appliqué les méthodes de calcul électronique basés sur la théorie de la fonctionnelle de la densité pour calculer les propriétés de partage isotopique à l'équilibre de l'hydrogène, de l'oxygène et du silicium entre divers matériaux d'intérêt en sciences de la Terre. La compréhension de ces mécanismes de partage est un enjeu majeur en géochimie, car le contenu isotopique des minéraux constitue un témoin des événements géochimiques. Le calcul électronique permet de déterminer les propriétés vibrationnelles complètes d'un matériau au niveau harmonique,<br />desquelles on déduit ses propriétés de partage. Notre but est de valider une approche ab initio, i.e. libre de tout paramètre issu de l'expérience, permettant la prédiction du partage entre deux matériaux. On s'est intéressé aux matériaux: quartz, vapeur d'eau, glace, kaolinite, lizardite, enstatite, forstérite, brucite et gibbsite. Certains ont fait l'objet d'études expérimentales précises de leurs différentes propriétés physiques, ce qui nous permet de valider notre approche, et tous sont de grand intérêt en sciences de la Terre, ce qui nous permet de proposer de nouvelles lois de partage en fonction de la température, et de vérifier la validité de celles déjà proposées. C'est la première fois qu'une telle étude systématique est réalisée à partir d'une approche ab initio, et sur autant de systèmes différents. On a donc procédé à une étude la plus complète possible des sources d'erreur susceptibles d'influencer le résultat final. Par ailleurs, cette approche nous permet d'analyser de façon plus détaillée les processus physiques à l'origine du partage isotopique dans ces différents matériaux.

Reproduction de : Thèse de doctorat : Science des agroressources : Toulouse, INPT : 2007. / Titre provenant de l'écran-titre. Bibliogr. 191 réf.

Thèse de doctorat : Sciences de l'Univers et de l'environnement : Paris Est : 2008. / Titre provenant de l'écran-titre.

Thesis (Ph. D.)--University of Chicago, Dept. of Anthropology, August 2001. / Includes bibliographical references. Also available on the Internet.

The rate of carbon turnover in soil is a balance between the input of carbon by plants through their roots and associated fungi and the loss of carbon due to plant and microbial respiration, oxidation and leaching. Soil carbon dynamics are notoriously difficult to measure, and being able to separate total soil respiration into its autotrophic and heterotrophic components would help understanding of carbon cycling processes. Where autotrophic respiration originates from roots and their associated mycorrhizal fungi, using newly fixed carbon, and heterotrophic respiration originates from the breakdown of older soil organic matter. By calculating the δ¹³C signature of respired CO₂ (the ratio of the abundances of C isotopes ¹²C and ¹³C) it is possible to determine whether it is of heterotrophic or autotrophic origin. In this study a 6 chamber, constant CO₂ concentration measuring apparatus was developed to determine both the rate of CO₂ efflux and to collect undisturbed CO₂ samples for isotope analysis. This apparatus was tested using live soil samples with different δ¹³C values (-22 ‰ to -27 ‰) and respiration rates (2 – 8 µmol m⁻² s⁻¹) obtained from various locations in New Zealand. Testing involved taking samples using the respiration apparatus, then incubating the same samples in a bag, and then comparing the two. There was no difference between the results from the soil respiration apparatus and the bags (R²=0.96, p=0.0002). Twelve microcosms including soil and grass were extracted from a newly converted dairy farm and placed into in growth cabinets. Diurnal courses of partitioned soil respiration were made over 24 hours with constant soil temperature to eliminate temperatures effect on soil respiration. Half were then covered with 90% shade cloth for 12 days to test if a reduction in light (and therefore newly fixed carbon) would have any effect on soil respiration. There was a significant reduction in soil respiration, yet no detectable change in the δ¹³C of soil respired CO₂ under heavily shaded treatment. There was however there was a shift towards heterotrophic dominated respiration. This shows that while L. perenne is resilient to surrounding conditions it is susceptible to change if exposed to different conditions for prolonged periods of time. The use of this new technique in the field will allow improved understanding of factors effecting soil C efflux.

Understanding the paleoclimate of a region is important, especially when trying to determine the extent of natural climate variability within the context of anthropogenic impacts. Recent anomalous periods of climate change in the Late Holocene, including the Little Ice Age and Medieval Warm Period, could possibly repeat in the future, having significant worldwide consequences. This holds especially true for tropical and subtropical karst environments, where limited paleoclimate proxies provide minimal data regarding past climate change. An investigation into past climate change in Belize using fulvic acids from cave sediments shows periods of drought during the collapse of the Maya society around 1400 years ago. Comparison of changes in the carbon isotope data from the fulvic acids agree with speleothem records, but more closely reflect changes in the vegetation above the cave, showing Maya population decline through waning agriculture. Further investigation of using fulvic and other organics acids are examined from cave sediments in Florida. The data show fulvic acid carbon isotopes are the most robust recorders of climate change, agreeing with several nearby speleothem d¹8O and d¹³C records from west-central Florida. A more detailed record of climate change in Florida through a calibration study of precipitation and cave dripwater oxygen and hydrogen isotopes revealed that the amount effect dominates rainfall in west-central Florida. Homogenization of epikarst dripwater gives average d¹8O values representative of the annual amount-weighted average of precipitation d¹8O for the area, suggesting speleothem isotope records reflect changes in rainfall amount. Examination of two speleothems from west-central Florida show complex teleconnection and solar forcing mechanisms responsible for past climate changes. A high-resolution stable isotope, trace element, and time series analysis study for the last 1500 years shows variability during the LIA and MWP, pointing to a combined influence of Pacific and Atlantic teleconnection mechanisms, especially the ITCZ, NAO and PDO, being responsible for precipitation variability. Long-term reconstruction of the mid-Holocene and Late Pleistocene from another speleothem reveals differences in temperature and precipitation between glacial and interglacial conditions in Florida. Climate proxies from the tropics and subtropics provide additional clues to global climate change crucial to understanding future water availability.

Graduate

During the Medieval period of Denmark, economic and trade relations grew inter-regionally, with culture, ideas, and products being transferred on a more regular basis through the 11th to 13th centuries. Beginning around 1050 AD and lasting until AD 1536, the country faced drastic climatic changes, shifting economic and agricultural practices, and disease outbreaks (most notably the bubonic plague). The current study seeks to investigate mobility during this period from two medieval cemeteries around Horsens, Denmark: the rural site of Sejet and the urban site of Ole Wormsgade, both used throughout the 12th to 16th centuries. A previous isotopic analysis using oxygen indicated that some movement was seen at these sites, with three individuals identified as potential migrants from other Scandinavian regions. This study compares the existing oxygen isotopic data with variations in 87Sr/86Sr ratios from these samples. Such ratios represent local bedrock baselines of strontium, which are slightly different between eastern and western Denmark. ICP-MS was used to measure 87Sr/86Sr ratios, and results are interpreted in the context of climatic changes and shifting socioeconomic practices. The results suggest that longer-distance movements into Denmark were seen during the Early Medieval Period. The potential migrant identified in this research points to movement towards the rural population at Sejet, and could possibly connect this migration with marriage rules influencing immigration at the time. This research demonstrates that movement during the medieval period of Denmark was a complex, dynamic, and multilinear process during a time of increasing urbanization. / October 2015