197,082 research outputs found
Re-evaluation of the age model for North Atlantic Ocean Site 982 – arguments for a return to the original chronology
No full textRecently, the veracity of the published chronology for the Pliocene section of North Atlantic Ocean Drilling Program Site 982 was called into question. Here, we examine the robustness of the original age model as well as the proposed age model revision. The proposed revision is predicated on an apparent mis-identification of the depth to the Gauss-Matuyama (G/M) polarity chronozone reversal boundary (2.581 Ma) based on preliminary shipboard paleomagnetic data and offers a new chronology which includes a hiatus between ~3.2 and 3 Ma. However, an even more accurate shore-based, u-channel-derived polarity chronozone stratigraphy for the past ~2.7 Ma supports the shipboard composite stratigraphy and demonstrates that the original estimate of the depth of the G/M reversal in the Site 982 record is correct. Thus, the main justification forwarded to support the revised chronology no longer exists. We demonstrate that the proposed revision results in a pronounced anomaly in sedimentation rates proximal to the proposed hiatus, erroneous assignment of marine-isotope stages in the Site 982 Pliocene benthic stable oxygen isotope stratigraphy, and a markedly worse correlation of proxy records between this site and other regional paleoclimate data. We conclude that the original chronology for Site 982 is a far more accurate age-model than that which arises from the published revision. We strongly recommend the use of the original chronology for all future work at Site 982
Recommended from our members
Dithiolane ligands for semiconductor quantum dots
No full textWe have investigated the potential of the dithiolane ring to anchor organic compounds on the surface of CdSe–ZnS core–shell quantum dots. In particular, we have synthesized three monomeric ligands, incorporating an azobenzene chromophore and a single dithiolane anchor in their molecular skeleton, as well as a polymeric ligand with multiple chromophoric labels and anchoring groups. All compounds co-adsorb on the surface of preformed quantum dots, together with their native tri-n-octylphosphine oxide surfactants, when chloroform dispersion of the organic ligands and the inorganic nanoparticles are heated under reflux for 24–72 h. The reaction time dictates the average number of azobenzene chromophores incorporated in the final assemblies, which can range from 6 up to 92. However, the modified quantum dots retain a substantial hydrophobic character and are not soluble in water, despite the presence of hydrophilic poly(ethylene glycol) chains in three of the four dithiolane ligands. The adsorbed azobenzene chromophores can be switched from trans to cis configurations with ultraviolet stimulations. The photochemical process is thermally reversibly and, in the case of the polymeric ligand, results in the photomodulation of the luminescence intensity of the nanostructured construc
Electron and energy transfer mechanisms for fluorescence modulation with photochromic switches
No full textThe electronic and structural changes associated with photochromic transformations can be exploited to modulate the emission intensity of fluorescent compounds on the basis of electron and energy transfer processes. Indeed, fluorescent and photochromic components can be joined covalently or noncovalently into molecular or supramolecular assemblies respectively and the emission of one component can be regulated by operating the other with optical stimulations. In fact, the photoinduced and reversible interconversion of the colorless and colored states of the photochromic component are accompanied by significant changes in absorption. These changes alter the degree overlap between the absorption band of a photochrome and the emission band of a fluorophore and, thus, activate or prevent the transfer of energy from the latter to the former. Here is presented a survey of diverse fluorophore–photochrome conjugates, macromolecular constructs, supramolecular assemblies, photoswitchable nanoparticles and multilayer arrays which operate successfully with optical inputs
World
No full textThe Mid-Pliocene Warm Period (MPWP, 2.9 to 3.3 Ma), along with older Pliocene (3.2 to 5.3 Ma) records, offers potential past analogues for our 400-ppmv world. The coastal geology of western and southern coasts of the Republic of South Africa exposes an abundance of marine deposits of Pliocene and Pleistocene age. In this study, we report differential GPS elevations, detailed stratigraphic descriptions, standardized interpretations, and dating of relative sea-level indicators measured across ~700 km from the western and southern coasts of the Cape Provinces. Wave abrasion surfaces on bedrock, intertidal sedimentary structures, and in situ marine invertebrates including oysters and barnacles provide precise indicators of past sea levels. Multiple sea-level highstands imprinted at different elevations along South African coastlines were identified. Zone I sites average +32 ± 5 m (six sites). A lower topographic Zone II of sea stands were measured at several sites around +17 ± 5 m. Middle and late Pleistocene sites are included in Zone III. Shoreline chronologies using 87Sr/86Sr ages on shells from these zones yield ages from Zone I at 4.6 and 3.0 Ma and Zone II at 1.04 Ma. Our results show that polar ice sheets during the Plio-Pleistocene were dynamic and subject to significant melting under modestly warmer global temperatures. These processes occurred during a period when CO2 concentrations were comparable to our current and rapidly rising values above 400 ppmv
Chiroptical switching based on the photoinduced proton transfer between homopolymers bearing side-chain spiropyran and azopyridine moieties
No full textA novel optically-active polymeric methacrylate, bearing the (S)-3-hydroxy pyrrolidinyl group linked through the nitrogen atom to an azopyridine chromophore, was synthesized and investigated in combination with a polymeric methacrylate incorporating the photoisomerizable spiropyran chromophore in the side-chain. In the presence of acid, this system can be operated in solution to modulate the protonation of the azopyridine moieties by photoisomerizing the spiropyran component. In addition to UV-vis spectroscopy, the proton transfer process occurring between the macromolecular components can be also followed by CD spectroscopy, thus behaving as a chiroptical switch
Pliocene shorelines and the epeirogenic motion of continental margins: a target dataset for dynamic topography models
No full textGlobal mean sea level during the mid-Pliocene epoch (∼3 Ma), when CO2 and temperatures were above present levels, was notably higher than today due to reduced global ice sheet coverage. Nevertheless, the extent to which ice sheets responded to Pliocene warmth remains in question owing to high levels of uncertainty in proxy-based sea level reconstructions as well as solid Earth dynamic models that have been used to evaluate a limited number of data constraints. Here, we present a global dataset of 10 wave-cut scarps that formed by successive Pliocene sea level oscillations and which are observed today at elevations ranging from ∼6 to 109 m above sea level. The present-day elevations of these features have been identified using a combination of high-resolution digital elevation models and field mapping. Using the MATLAB interface TerraceM, we extrapolate the cliff and platform surfaces to determine the elevation of the scarp toe, which in most settings is buried under meters of talus. We correct the scarp-toe elevations for glacial isostatic adjustment and find that this process alone cannot explain observed differences in Pliocene paleo-shoreline elevations around the globe. We next determine the signal associated with mantle dynamic topography by back-advecting the present-day three-dimensional buoyancy structure of the mantle and calculating the difference in radial surface stresses over the last 3 Myr using the convection code ASPECT. We include a wide range of present-day mantle structures (buoyancy and viscosity) constrained by seismic tomography models, geodynamic observations, and rock mechanics laboratory experiments. Finally, we identify preferred dynamic topography change predictions based on their agreement with scarp elevations and use our most confident result to estimate a Pliocene global mean sea level based on one scarp from De Hoop, South Africa. This inference (11.6 ± 5.2 m) is a downward revision and may imply that ice sheets were relatively resistant to warm Pliocene climate conditions. We also conclude, however, that more targeted model development is needed to more reliably infer mid-Pliocene global mean sea level based on all scarps mapped in this study
Recommended from our members
Photochromic Polymers Based on the Photoinduced Opening and Thermal Closing of [1,3]Oxazine Rings
No full textTwo macromolecular constructs incorporating a single polymer backbone with multiple photochromic side chains are developed. Both systems are prepared from preformed photochromic [1,3]oxazines after the ring-opening polymerization of their norbornene appendages. In solution, UV illumination of these polymers opens the [1,3]oxazine rings in their side chains in less than 6 ns and with a quantum yield of 0.09 in both instances. The photogenerated species incorporate a 4-nitrophenolate chromophore, and hence, their formation is accompanied by the appearance of an intense band in the visible region of the
absorption spectrum. The photoproducts revert spontaneously to the original state with first-order kinetics inmicroseconds. Furthermore, both photochromic polymers tolerate hundreds of switching cycles with no sign of degradation, even in the presence of molecular oxygen. Thus, this design logic and choice of
functional building blocks can translate into the realization of innovative photoresponsive materials with excellent photochromic performance
Self-Assembling Catenanes and Rotaxanes
No full textRecently, self-assembly processes have been used for the construction of a wide diversity of molecular and supramolecular architectures. We have developed an approach to self-assembling mechanically-interlocked structures such as catenanes and rotaxanes. The methodology relies upon the stereoelectronic complementarity between pi-electron deficient bipyridinium-based components and pi-electron rich hydroquinone- or 1,5-dioxynaphthalene-based components. The driving forces responsible for the self-assembly are (i) pi-pi stacking interactions between the bipyridinium units and either the hydroquinone or 1,5-dioxynaphthalene rings, together with (ii) hydrogen bonding interactions between the CH-acidic protons of the bipyridinium units and the polyether oxygen atoms incorporated within the pi-electron rich components. By employing this approach, we have been able to self-assemble a large number of [2]catenanes, [3]catenanes, [4]catenanes and, very recently, two [5]catenanes, incorporating five interlocked macrocyclic subunits. We have also achieved the template-directed synthesis of several [2]rotaxanes, [3]rotaxanes, and a dendritic [4]rotaxane. Here, we report the syntheses of several mechanically-interlocked molecular compounds, along with a description of their dynamic properties in solution and their electrochemical behaviour. Furthermore, the possibility of controlling, by external stimuli - chemical or electrochemical - the dynamic processes occurring in solution, within these molecules, with the ultimate goal of generating molecular machines, is illustrated
Chromatography of Mechanically-Interlocked Molecular Compounds
No full textCatenanes and rotaxanes are molecules composed of mechanically interlocked components which are not linked to each other by covalent bonds, These molecular assemblies behave as discrete molecules with defined properties significantly different from those of the parent ''free'' components. High-performance liquid chromatography has been employed successfully to characterize some tetracationic catenanes and rotaxanes incorporating either cyclobis(paraquat-p-phenylene) or cyclobis(paraquat-4,4'-biphenylene) as the charged components and either hydroquinone-containing macrocycles or dumbbell-shaped entities as the neutral components. in each case, significant differences in the retention times of the mechanically interlocked molecular compounds, in comparison with those of their components as their ''free'' forms, were observed
Synthesis and photoresponsive behavior of optically active methacrylic homopolymers containing side-chain spiropyran chromophores
No full textNovel optically-active methacrylic homopolymers bearing in the side chain one or more chiral groups of one single configuration (based on the L-lactic acid residue) linked to the spiropyran chromophore, have
been successfully synthesized and fully characterized.
These intrinsically chiral polymers exhibit remarkable thermal stability, with glass transition temperatures in the range 100–130 C and decomposition temperatures around 270 C. The chiroptical characterization indicates the occurrence of asymmetric induction on the electronic transitions of the side-chain chromophore related to the number of L-lactic acid residues interposed between the main
chain and the spiropyran chromophore.
In the presence of acid, these systems can be used to modulate the protonation of polymeric azopyridine moieties upon photoisomerization of the spiropyran group. In addition to UV–Vis spectroscopy, the
proton transfer process occurring between the macromolecular components can be also followed by CD spectroscopy, the system thus behaving as a chiroptical switch
- …
