67,130 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
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
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
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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
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
Mining e-mail content for author identification forensics
No full textWe describe an investigation into e-mail content mining for author identification, or authorship attribution, for the purpose of forensic investigation. We focus our discussion on the ability to discriminate between authors for the case of both aggregated e-mail topics as well as across different email topics. An extended set of e-mail document features including structural characteristics and linguistic patterns were derived and, together with a Support Vector Machine learning algorithm, were used for mining the e-mail content. Experiments using a number of e-mail documents generated by different authors on a set of topics gave promising results for both aggregated and multi-topic author categorisation
PHOTOINDUCED PROTON TRANSFER IN MACROMOLECULAR SYSTEMS
No full textPhotochromic materials are well known candidates for several technological applications such as devices for the optical storage of informations, optical switches and in general as materials exhibiting photoresponsive properties when irradiated with light of suitable frequency and intensity. In particular in the last few years a great interest grew up on using these compounds as building blocks for the construction of molecular logic gates able to perform simple logic operations (AND, NOT and OR). These molecular switches are based on the interplay between chemical, electrical or optical stimulations and absorbance, luminescence or redox outputs
In this regard, spiropyran (SP), a three state molecular switch, has been widely studied and resulted promising for obtaining a device able to elaborate information by detecting inputs of one form and transducing them into outputs of another one1. As previously reported [6-8], irradiation of this colourless compound with UV light induces heterolytic cleavage of the spiro carbon-oxygen bond, thus producing the ring opened and intensively coloured merocyanine form (ME). This process is thermically and photochemically reversible. Furthermore the addition of acid induces the formation of the protonated merocyanine MEH which, if irradiated with Visible light, releases a proton and comes back to SP. The absorption and emission properties of these three states (SP, ME and MEH) are significantly different and allow to use these compounds for repeated data recording and optical data storage
Crowdsourcing in the Quaternary sea level community: Insights from the Pliocene
No full textIn order to establish the 'fingerprint' of past sea level changes, many field measurements of paleo sea level from globally distributed locations are needed. It is because this problem requires a geographically expansive database that it becomes an ideal candidate for crowdsourcing techniques. In order to crowdsource sea level data from the Mid-Pliocene Warm Period, we developed three tools: PlioWiki, RSL calc and RSL map. PlioWiki is a web portal, open to contributions, where investigators can share knowledge on Pliocene to Quaternary relative sea levels. RSL calc is a standardized, ready-to-use tool for field geologists to log their own sea level field observations and, if they desire, submit new data to an open access database of relative sea level markers. RSL map allows one to visualize and query the database built with RSL calc on a Google Map interface. Here we describe these tools and discuss the advantages of crowdsourcing, relative to traditional approaches, for the creation of sea level databases for any time period. © 2012 Elsevier Ltd
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
The Mid-Pliocene sea-level conundrum: Glacial isostasy, eustasy and dynamic topography
No full textDetermining eustatic sea level during the Mid-Pliocene warm period (~ 3.3 to 2.9 Ma) has been a central but elusive goal in the study of past warm climates. Estimates of eustatic sea level based on geologic data span a broad range; variation that we now recognize is due in part to geographically varying post-depositional displacement caused by glacial isostatic adjustment and dynamic topography. In this study, we combine field observations and glacial isostatic adjustment modeling to estimate the dynamic topography signal in three areas that are important to paleo-sea level studies of the Mid-Pliocene warm period (South Africa, West Australia and southeastern United States). We show that dynamic topography played a significant role in the post-depositional displacement of Pliocene, and even younger Pleistocene, shorelines. In this regard, we provide a robust paleo-sea level elevation data set, corrected for glacial isostatic adjustment, that can be used to evaluate predictions from mantle flow models of dynamic topography
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