Cycling of two carbon substrates of contrasting lability by heterotrophic biofilms across a nutrient gradient of headwater streams

Anthropogenic impacts can significantly alter stream nutrient and dissolved organic carbon (DOC) delivery and composition. Nutrient and DOC cycling in headwater streams, however, are linked via a variety of complex feedbacks that are, in part, influenced by DOC composition emphasizing the need to investigate coupled nutrient–DOC interactions. This study assessed differential incorporation and mineralization of ¹³C labeled glucose and vanillin by heterotrophic microbes within epilithic biofilm communities in four temperate headwater streams spanning a 100-fold range in total dissolved nitrogen and soluble reactive phosphorous concentrations. The substrates were traced via ¹³C analyses of DOC, dissolved inorganic carbon, bulk biofilm, and individual biofilm phospholipid fatty acids (PLFA) to assess total incorporation of the substrates and the distribution of substrate use within the heterotrophic community. Results indicate greater nutrient uptake by high nutrient streams with glucose additions relative to vanillin additions and support the hypothesis that nutrient retention in high nutrient streams is hampered by a lack of labile C sources. Vanillin-derived C uptake was only detectable in PLFA from the highest nutrient stream and was dominated by eukaryotic organisms, likely including fungi. This suggests biofilms in high nutrient streams are better adapted to access relatively slow turnover substrates perhaps due to their composition and overall structure. PLFA-based glucose use efficiencies were greatest in the lowest nutrient stream supporting the hypothesis that labile DOC sources are used more efficiently by heterotrophs in less impacted streams, while biofilms of high nutrient streams are better adapted to utilizing a wider array of DOC sources. This adaption is likely a result of exposure to the lower quality DOC pools in high-nutrient streams resulting from high DOC uptake supported, in part, by fast turnover autochthonous sources of DOC. Nutrient retention in nutrient-rich streams, however, is still likely limited by readily bioavailable DOC leading to lower nutrient retention and downstream nutrient enrichment.     

The TRANSMED Atlas： geological-geophysical fabric of the Mediterranean region——Final report of the project

Geological research on the Mediterranean region is presently characterized by the transition from disciplinary to multidisciplinary research, as well as from national to international investigations. In order to synthesize and integrate the vast disciplinary and national datasets which are available, it is necessary to implement maximum interaction among geoscientists of different backgrounds. The creation of project-oriented task forces in universities and other research institutions, as well as the development of large international cooperation programs, is instrumental in pursuing such a multidisciplinary and supranational approach. The TRANSMED Atlas, an official publication of the 32nd International Geological Congress (Florence 2004), is the result of an international scientific cooperation program which brought together for over two years sixtythree structural geologists, geophysicists, marine geologists, petrologists, sedimentologists, stratigraphers, paleogeographers, and petroleum geologists coming from eighteen countries, and working for the petroleum industry, academia, and other institutions, both public and private. The TRANSMED Atlas provides an updated, synthetic, and coherent portrayal of the overall geological-geophysical structure of the Mediterranean domain and the surrounding areas. The initial stimulus for the Atlas came from the realization of the extremely heterogeneous nature of the existing geological-geophysical data about such domain. These data have been gathered by universities, oil companies, geological surveys and other institutions in several countries, often using different procedures and standards. In addition, much of these data are written in languages and published in outlets that are not readily accessible to the general international reader. By synthesizing and integrating a wealth of preexisting and new data derived from surficial geology, seismic sections at various scales, and mantle tomographies, the TRANSMED Atlas provides for the first time a coherent geological overview of the Mediterranean region and represents an ideal springboard for future studies.     

Seafloor spreading anomalies and crustal ages of the Clarion-Clipperton Zone

The Clarion-Clipperton Zone (CCZ) of the central Pacific is one of the few regions in the world’s oceans that are still lacking full coverage of reliable identifications of seafloor spreading anomalies. This is mainly due to the geometry of the magnetic lineations’ strike direction sub-parallel to the Earth’s magnetic field vector near the equator resulting in low amplitude magnetic anomalies, and the remoteness of the region which has hindered systematic surveying in the past. Following recently granted research licenses for manganese nodules in the CCZ by the International Seabed Authority, new magnetic data acquired with modern instrumentation became available which combined with older underway data make the identification of seafloor spreading anomalies possible for large parts of the CCZ and adjacent areas. The spreading rates deduced from the seafloor spreading patterns show a sharp increase at the end of Chron 21 (47.5 Ma) which corresponds to the age of the bend in the Hawaii-Emperor seamount chain and an associated plate tectonic reorganisation in the Central Pacific. An accurate map of crustal ages for the central-eastern Pacific based on our anomaly picks may provide a basis for improved plate tectonic reconstructions of the region.     

Halogen systematics in the Mallik 5L-38 gas hydrate production research well,Northwest Territories,Canada: Implications for the origin of gas hydrates under terrestrial permafrost conditions

The authors report here halogen concentrations in pore waters and sediments collected from the Mallik 5L-38 gas hydrate production research well, a permafrost location in the Mackenzie Delta, Northwest Territories, Canada. Iodine and Br are commonly enriched in waters associated with CH₄, reflecting the close association between these halogens and source organic materials. Pore waters collected from the Mallik well show I enrichment, by one order of magnitude above that of seawater, particularly in sandy layers below the gas hydrate stability zone (GHSZ). Although Cl and Br concentrations increase with depth similar to the I profile, they remain below seawater values. The increase in I concentrations observed below the GHSZ suggests that I-rich fluids responsible for the accumulation of CH₄ in gas hydrates are preferentially transported through the sandy permeable layers below the GHSZ. The Br and I concentrations and I/Br ratios in Mallik are considerably lower than those in marine gas hydrate locations, demonstrating a terrestrial nature for the organic materials responsible for the CH₄ at the Mallik site. Halogen systematics in Mallik suggest that they are the result of mixing between seawater, freshwater and an I-rich source fluid. The comparison between I/Br ratios in pore waters and sediments speaks against the origin of the source fluids within the host formations of gas hydrates, a finding compatible with the results from a limited set of ¹²⁹I/I ratios determined in pore waters, which gives a minimum age of 29 Ma for the source material, i.e. at the lower end of the age range of the host formations. The likely scenario for the gas hydrate formation in Mallik is the derivation of CH₄ together with I from the terrestrial source materials in formations other than the host layers through sandy permeable layers into the present gas hydrate zones.     

Proximity effects on semiconducting mineral surfaces II:: Distance dependence of indirect interactions

In a previous study, we described proximity effects on surfaces of the semiconducting minerals galena and pyrite, whereby a chemical reaction at one surface site modifies the reactivity of a remote surface site several Ångstroms or even nanometers away (Becker et al., 2001). The modification of interest does not arise because of a direct “through space” interaction between the two sites, but rather an indirect interaction via the electronic structure of the substrate. Here we investigate the distance and direction dependence of proximity effects using quantum mechanical modeling. The direct and indirect interactions between co-adsorbed oxygen atoms and between adsorbed oxygen atoms and point defects on vacuum-terminated galena (100) surfaces were modeled. Density functional theory cluster and plane wave pseudopotential calculations were used to calculate the modifications to the adsorption energy as a function of separation. Energy-distance plots indicate that the proximity effect energy can become very strong at separations decreasing below about 5 to 6 Å, and persist at increasing separations up to 12 Å in a slowly decaying form. A strong attractive indirect interaction out-competes direct electrostatic repulsion for O-vacancy interactions. An oscillatory asymptotic behavior is found for co-adsorbed O-O indirect interactions, which indicates that the proximity effect energy can vary with surface crystallographic direction. It implies the presence of a strong organizing force on like adatoms that may explain the progressive oxidation of certain sulfide minerals by patchwork growth. These findings begin to pave the way for improved adsorption isotherms and extended surface complexation models that will include the specific influence of semiconductor-type proximity effects.     

Mechanical controls on collision-related compressional intraplate deformation

Intraplate compressional features, such as inverted extensional basins, upthrust basement blocks and whole lithospheric folds, play an important role in the structural framework of many cratons. Although compressional intraplate deformation can occur in a number of dynamic settings, stresses related to collisional plate coupling appear to be responsible for the development of the most important compressional intraplate structures. These can occur at distances of up to ±1600 km from a collision front, both in the fore-arc (foreland) and back-arc (hinterland) positions with respect to the subduction system controlling the evolution of the corresponding orogen. Back-arc compression associated with island arcs and Andean-type orogens occurs during periods of increased convergence rates between the subducting and overriding plates. For the build-up of intraplate compressional stresses in fore-arc and foreland domains, four collision-related scenarios are envisaged: (1) during the initiation of a subduction zone along a passive margin or within an oceanic basin; (2) during subduction impediment caused by the arrival of more buoyant crust, such as an oceanic plateau or a microcontinent at a subduction zone; (3) during the initial collision of an orogenic wedge with a passive margin, depending on the lithospheric and crustal configuration of the latter, the presence or absence of a thick passive margin sedimentary prism, and convergence rates and directions; (4) during post-collisional over-thickening and uplift of an orogenic wedge. The build-up of collision-related compressional intraplate stresses is indicative for mechanical coupling between an orogenic wedge and its fore- and/or hinterland. Crustal-scale intraplate deformation reflects mechanical coupling at crustal levels whereas lithosphere-scale deformation indicates mechanical coupling at the level of the mantle-lithosphere, probably in response to collisional lithospheric over-thickening of the orogen, slab detachment and the development of a mantle back-stop. The intensity of collisional coupling between an orogen and its fore- and hinterland is temporally and spatially variable. This can be a function of oblique collision. However, the build-up of high pore fluid pressures in subducted sediments may also account for mechanical decoupling of an orogen and its fore- and/or hinterland. Processes governing mechanical coupling/decoupling of orogens and fore- and hinterlands are still poorly understood and require further research. Localization of collision-related compressional intraplate deformations is controlled by spatial and temporal strength variations of the lithosphere in which the thermal regime, the crustal thickness, the pattern of pre-existing crustal and mantle discontinuities, as well as sedimentary loads and their thermal blanketing effect play an important role. The stratigraphic record of collision-related intraplate compressional deformation can contribute to dating of orogenic activity affecting the respective plate margin.