1,721,120 research outputs found
The TEX86 paleotemperature proxy
No full textThe TEX86 paleothermometer is based upon the distribution of archaeal membrane lipids (“GDGTs”) in marine sediments. GDGTs are ubiquitous, abundant and relatively resistant to degradation; as such, the TEX86 paleothermometer has been used to reconstruct sea surface temperature (SST) during the Cenozoic and early Mesozoic. In this chapter, we review the principles of the TEX86 proxy and developments made over the last two decades. We also discuss its application as a paleotemperature proxy and explore existing challenges and limitations
Temperature changes across the Paleocene-Eocene Thermal Maximum – a new high-resolution TEX86 temperature record from the Eastern North Sea Basin
No full textThe Paleocene-Eocene Thermal Maximum (PETM; ∼55.9 Ma) was a hyperthermal event associated with large carbon cycle perturbations, sustained global warming, and marine and terrestrial environmental changes. One possible trigger and/or source of the carbon release that initiated the PETM is the emplacement of the North Atlantic Igneous Province (NAIP). This study focuses on an expanded section of marine clays and diatomite on Fur Island in northern Denmark, where the entire PETM sequence has been identified by a negative ∼4.5‰ δ
13C
TOC excursion. This remarkably well-preserved section also contains >180 interbedded ash layers sourced from the NAIP, making it an ideal site for investigating the correlations between large-scale volcanism and environmental changes. This study provides a new and complete high-resolution TEX
86-derived sea-surface temperature (SST) reconstruction over the entire PETM and the post-PETM section (up to about 54.6 Ma). The palaeothermometry record indicates an apparent short-lived cooling episode in the late Paleocene, followed by a pronounced temperature response to the PETM carbon cycle perturbations with a ∼10 °C SST increase during the PETM onset (up to ∼33 °C). Extreme SSTs fall shortly after the PETM onset, and continue to decrease during the PETM body and recovery, down to anomalously cool SSTs post-PETM (∼11–23 °C). Both phases of potential cooling coincide with proxies of active NAIP volcanism, suggesting a causal connection, although several overprinting non-thermal factors complicate interpretations of the TEX
86 values. Indices of effusive and explosive NAIP volcanism are largely absent from the Danish stratigraphy during the PETM body, though a re-emergence toward the end of the PETM suggest NAIP volcanism might have played a role in the PETM termination in the North Sea. This new SST record completes the previous fragmented view of climate changes at this globally important PETM site, and indicates large temperature variations in the North Sea during the earliest Eocene that are possibly linked to NAIP volcanism.
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Dead Sea leaf wax stable carbon and hydrogen isotopes in sediment core DSDDP 5017-1-A
No full textCompound-specific hydrogen and carbon isotopes were measured on the C30 fatty acid in the Dead Sea Deep Drilling Project Core 5017-1-A
Biomarker approaches for reconstructing terrestrial environmental change
No full textThe response of the terrestrial biosphere to warming remains one of the most poorly understood and quantified aspects of the climate system. One way to test the behavior of the Earth system in warm climate states is to examine the geological record. The abundance, distribution, and/or isotopic composition of source-specific organic molecules (biomarkers) have been used to reconstruct terrestrial paleoenvironmental change over a range of geological timescales. Here, we review new or recently improved biomarker approaches for reconstructing (a) physical climate variables (land temperature, rainfall), (b) ecosystem state variables (vegetation, fire regime), and (c) biogeochemical variables (soil residence time, methane cycling). This review encompasses a range of key compound classes (e.g., lipids, lignin, and carbohydrates). In each section, we explore the concept behind key biomarker approaches and discuss their successesas paleoenvironmental indicators. We emphasize that analyzing several biomarkers in tandem can provide unique insights into the Earth system. ▪ Biomarkers can be used to reconstruct terrestrial environmental change over a range of geological timescales. ▪ A multi-proxy biomarker approach provides novel insights into climate and the environment.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
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