Polymer–matrix composites degrade under the influence of UV radiation in the range of the 290–400 nm band. The degradation of polymer–matrix composites exposed to UV radiation is characterized by ...extensive aging of the epoxy matrix, resulting in deterioration of their mechanical properties. Glass fibers/epoxy resin composites were made by an out-of-autoclave method whereas a fiber optic sensor was placed between different layers of laminates. In our work, we used a fiber Bragg grating sensor covered with graphene oxide and embedded in a polymer matrix composite to monitor UV radiation intensity. Measurements of UV radiation may allow monitoring the aging process of individual components of the polymer composite. In order to estimate the number of microcracks of epoxy resin, microstructure observations were carried out using a scanning electron microscope.
Marine cyanobacterial mats were cultured on coastal sediments (Nivå Bay, Øresund, Denmark) for over three years in a closed system. Carbonate particles formed in two different modes in the mat: (i) ...through precipitation of submicrometer-sized grains of Mg calcite within the mucilage near the base of living cyanobacterial layers, and (ii) through precipitation of a variety of mixed Mg calcite/aragonite morphs in layers of degraded cyanobacteria dominated by purple sulfur bacteria. The d13C values were about 2‰ heavier in carbonates from the living cyanobacterial zones as compared to those generated in the purple bacterial zones. Saturation indices calculated with respect to calcite, aragonite, and dolomite inside the mats showed extremely high values across the mat profile. Such high values were caused by high pH and high carbonate alkalinity generated within the mats in conjunction with increased concentrations of calcium and magnesium that were presumably stored in sheaths and extracellular polymer substances (EPS) of the living cyanobacteria and liberated during their post-mortem degradation. The generated CaCO3 morphs were highly similar to morphs reported from heterotrophic bacterial cultures, and from bacterially decomposed cyanobacterial biomass emplaced in Ca-rich media. They are also similar to CaCO3 morphs precipitated from purely inorganic solutions. No metabolically (enzymatically) controlled formation of particular CaCO3 morphs by heterotrophic bacteria was observed in the studied mats. The apparent alternation of in vivo and post-mortem generated calcareous layers in the studied cyanobacterial mats may explain the alternation of fine-grained (micritic) and coarse-grained (sparitic) laminae observed in modern and fossil calcareous cyanobacterial microbialites as the result of a probably similar multilayered mat organization.
Two global isotopic events, the early Sheinwoodian (early Wenlock) and that at the Silurian—Devonian transition, have been comprehensively studied in representative carbonate successions at ...Kytayhorod and Dnistrove, respectively, in Podolia, Ukraine, to compare geochemistry and biotic changes related correspondingly to the Ireviken and Klonk events. These two large-scale isotope excursions reveal different regional ecosystem tendencies. The well-defined increasing trend across the Llandovery—Wenlock boundary in siliciclastic input, redox states and, supposedly, bioproductivity, was without strict correlative relations to the major 13C enrichment event. The environmental and biotic evolution was forced by eustatic sea-level fluctuations and two-step climate change toward a glaciation episode, but strongly modified by regional epeirogeny movements due to location near the mobile Teisseyre-Törnquist Fault Zone. Thus, the global early Sheinwoodian biogeochemical perturbation was of minor depositional significance in this epeiric sea, as in many other Laurussian domains. Conversely, the Podolian sedimentary record of the Klonk Event exhibits temporal links to the abrupt δ13C anomaly, overprinted by a tectonically driven deepening pulse in the crucial S–D boundary interval. This carbon cycling turnover was reflected in the regional carbonate crisis and cooling episodes, paired with a tendency towards eutrophication and recurrent oxygen deficiency, but also with major storms and possible upwelling. Faunal responses in both Podolian sections follow some characters of the Silurian pattern worldwide, as manifested by conodont changeover prior to the major early Sheinwoodian isotopic/climatic anomaly. This contrasts with the relative brachiopod and chitinozoan resistances in the course of the Ireviken Event. Also, during the Klonk Event, a moderate faunal turnover, both in benthic and pelagic groups, occurred only near the very beginning of the prolonged 13C-enriched timespan across the system boundary, possibly due to progressive dysoxia and temperature drop. The characters point to a peculiarity of the Klonk Event by comparison with the Silurian global events, and some similarity already to the succeeding Devonian transgressive/anoxic episodes.
The large-scale global biogeochemical perturbation across the Silurian–Devonian (S–D) boundary, recorded in the major positive excursion of the δ
13C time curve (Klonk Event) of an amplitude of 2.5 ...to 3.0‰ (max. 4.0‰) in Europe and up to 5.0‰ in North America, reflects a unique combination of palaeogeographic, biogeochemical and evolutionary processes in the late Caledonian geodynamic setting. The steady sulfur isotopic ratios show an overall stability of the S–D oceanic geochemical system as a whole and do not indicate any synchronous changes in anoxic deep oceanic and sediment processes. Therefore this led us to a hypothesis that the crucial changes that contributed to the recorded carbon cycling turnover are related to rapidly evolving ocean–continent–biosphere feedback. Coastal zones of the latest Silurian epicontinental seas accumulated considerable quantities of organic carbon from early vascular vegetation, which explosively expanded to inhabit vast near-coastal shallows and deltas. Large primary production of these early terrestrial plants and rapidly enhanced sedimentary burial of organic carbon were also responsible for CO
2 drawdown, which have resulted in reversed-greenhouse effect and a global climatic cooling tendency. This feedback was blocked when the sea level gradually dropped and led to shrinking of the Silurian epicontinental seas and the growing climatic deterioration during the S–D transition limited primary production. Furthermore, continued processes of regressive abrasion and erosion limited the storage of organic carbon, as well as the efficiency of the carbonate factory. During the following Early Devonian greenhouse interval, marine regression and active latest Caledonian tectonism promoted progressive weathering of the sedimentary organic matter. The oxidation of C-rich deposits caused the subsequent growth of CO
2 levels in the Early Devonian, culminating in the warming of global climate.
The variability of δ
13C and δ
18O was determined within the columnal facet of individual ossicles, within different regions of skeletons and within bulk skeletons of extant stalked crinoids. ...Isotopic compositions of individual ossicles may vary by up ~
1‰ for both isotopes, whereas isotopic variability within a skeleton may be as high as ~
2.8‰ for δ
13C and ~
1.2‰ for δ
18O. In contrast, mean isotopic compositions and variations are similar for different specimens of a single species from any particular locality. Isotopic variation was evaluated between higher taxonomic groups of crinoids, including Isocrinida, Comatulida, Bourgueticrinida and Cyrtocrinida. Skeletons of isocrinids, comatulids and bourgueticrinids are consistently more negative in δ
13C than those of cyrtocrinids. This difference may be as high as ~
10‰, and is unrelated to the place of origin. Such isotopic differences reflect distinct physiological differences between crinoid groups we studied. Overall, their δ
18O values show weak temperature dependence, which are overshadowed by the strong influence of physiological or vital effects on the isotopic composition of crinoid skeletal carbonate. Thus great caution needs to be exercised when using the stable isotope composition of crinoids as an environmental proxy.
► Crinoids exhibit a large vital effect in the fractionation of light stable isotopes. ► Large differences in isotope composition exist at higher taxonomic levels in crinoids. ► Great care should be taken in the use of crinoid skeletons as environmental proxies.
The Miocene Globigerina Limestone of the Maltese islands contains widespread omission surfaces with very different characteristics and origins.
The terminal Lower Globigerina Limestone hardground ...(TLGLHg) formed during a period of falling sea level. Coccolith assemblages suggest shallowness. Sedimentary structures and trace fossil assemblages, indicate increasing frequency of storm events and erosional episodes, towards the surface. Calcite cementation which took place around
Thalassinoides burrows and formed irregular nodules was followed by dissolution of aragonite. It is suggested that lithification was linked to microbial reactions involving organic matter.
In contrast two later surfaces, the terminal Middle Globigerina Limestone omissionground (TMGLOg), which marks the Lower to Middle Miocene boundary, and the Fomm-ir-Rih local hardground (FiRLHg) both contain early diagenetic dolomite. Lithification took place in two phases. The dolomite is interpreted to have formed beneath the sea floor; it was subsequently exhumed and partially corroded as the precipitation of calcitic and phosphatic cements took place around burrows open to the circulation of sea water.
Abstract
A rich collection of exceptionally preserved Lower Triassic fossil fish remains obtained during the Polish Spitsbergen Expedition of 2005 includes many isolated teeth be− lieved to belong to ...a saurichthyid actinopterygian. Stable isotope analysis (δ
13
C and δ
18
O) of putative Saurichthys teeth from the Hornsund area (South Spitsbergen) acting as a paleoenvironmental proxy has permitted trophic−level reconstruction and comparison with other Lower Triassic fish teeth from the same location. The broader range of δ
13
C values ob− tained for durophagous teeth of the hybodont selachian, Lissodus, probably reflects its mi− gratory behaviour and perhaps a greater feeding diversity. X−ray microcomputed tomogra− phy (XMT), a non−destructive technique, is used for the first time in order to elucidate de− tails of tooth histology, the results of which suggest that the method has considerable poten− tial as a future analytical tool.
The phenomena of (i) inverse correlation between the oceanic carbon and sulfur isotopic curves, and (ii) covariation between the oceanic carbon and oxygen isotopic curves at all their major ...excursions appear as paradoxes in the current paradigm of global biogeochemical cycles. These phenomena, however, are fully explicable by our model proposing that the ocean alternates between two general modes: stagnant, stratified, and net autotrophic (overfed) ocean, on the one hand, and vigorously mixed and net heterotrophic (hungry) ocean, on the other. This model is in fact strongly supported by the carbon isotopic evidence. The directions of change in the isotopic ratios of carbon, oxygen, and sulfur should be different in the lower, anoxic box of a stratified ocean than in the upper, oxic box; whereas ocean destratification and mixing of the two boxes should lead to coeval shifts in the oceanic isotopic curves of these elements. The model has far-reaching implications for (i) the causal explanation of both secular trends and major shifts in the oceanic isotopic curves, and (ii) for the application of oxygen isotopic data for paleotemperature and paleoenvironment determinations.
The phenomena of (i) inverse correlation between the oceanic carbon and sulfur isotopic curves, and (ii) covariation between the oceanic carbon and oxygen isotopic curves at all their major ...excursions appear as paradoxes in the current paradigm of global biogeochemical cycles. These phenomena, however, are fully explicable by our model proposing that the ocean alternates between two general modes: stagnant, stratified, and net autotrophic (overfed) ocean, on the one hand, and vigorously mixed and net heterotrophic (hungry) ocean, on the other. This model is in fact strongly supported by the carbon isotopic evidence. The directions of change in the isotopic ratios of carbon, oxygen, and sulfur should be different in the lower, anoxic box of a stratified ocean than in the upper, oxic box; whereas ocean destratification and mixing of the two boxes should lead to coeval shifts in the oceanic isotopic curves of these elements. The model has far-reaching implications for (i) the causal explanation of both secular trends and major shifts in the oceanic isotopic curves, and (ii) for the application of oxygen isotopic data for paleotemperature and paleoenvironment determinations.
PDF
