1,721,008 research outputs found
Oligocene climate forcing and palaeoceanography of the equatorial Pacific (paper presented at: AGU Fall Meeting, San Francisco, 2004)
No full text: A planktonic and benthic foraminiferal stable isotope stratigraphy of the Oligocene equatorial Pacific (Ocean Drilling Program, Site 1218) was generated at 6 kyr resolution between magnetochrons C9n and C11n.2n (26.4-30 Ma on a newly developed astronomically calibrated time scale). Our data allow a detailed examination of Oligocene paleoceanography, the evolution of the early cryosphere and the influence of orbital forcing on glacioeustatic sea level variations. The Oligocene climate and ice sheet dynamics were strongly influenced by orbital forcing. Spectral analysis reveals power and coherency for obliquity (40 kyr period) and eccentricity (110 kyr, 405 kyr) orbital bands, with an additional strong imprint of the eccentricity and 1.2 Myr obliquity amplitude cycle, which drove ice sheet oscillations in the southern hemisphere. Heavy oxygen isotope intervals correspond to maxima in carbon isotopes, indicating substantial changes in the carbon cycle that accompanied the glacial events. Planktonic and benthic foraminifera O are used to constrain the magnitude and timing of major fluctuations in ice volume and global sea level change. Glacial episodes, related to obliquity and eccentricity variations, occurred at 29.16, 27.91 and 26.76 Ma. These correspond to glacioeustatic sea level fluctuations of 50 to 65 meters. High amplitude cyclic variations are recorded in the carbon isotope signal of planktonic and benthic foraminifera, the water column carbon isotope gradient and estimated percent carbonate of bulk sediment. Maxima in C and the increased C values are associated with each of the glacial events. Alteration of high latitude temperatures and Antarctic ice volume thus had a significant impact on the global carbon burial and equatorial productivity. We investigate the implications of a close correspondence between oxygen and carbon isotope events and long term amplitude envelope extrema in astronomical calculations during the Oligocene, and develop a new naming scheme for stable isotope events, based on the 405 kyr eccentricity cycle count
Data report: Oligocene paleoceanography of the Equatorial Pacific Ocean: planktonic and benthic foraminifer stable isotope results from Site 1218
No full textFluctuations in the position of the Proto Gulf Stream: Evidence from high-resolution stable isotope results
No full textThe Gulf Stream is prominent western boundary current in the North Atlantic. However, little is known about the evolution of the Gulf Stream, particularly during times of elevated greenhouse gases and global temperatures. High-resolution (3 k.y.) stable isotope analyses (?18O, ?13C) were conducted on middle Eocene (ca. 40 - 37 Ma) planktonic foraminifera from the western North Atlantic (Ocean Drilling Program Site 1052). The study exposed a more complicated pattern of climate variability than was formerly anticipated, with large (>1 per mil) and rapid (>10 k.y.) variations in ?18O. The magnitude of change is greater than that seen in open-ocean Pleistocene records but could not have been caused by ice-volume and/or sea-level fluctuations. Instead, the oxygen isotope shifts resulted primarily from large oscillations in sea-surface temperatures with shifts of up to 12°C. Climatic modeling results have indicated the presence of a Gulf Stream analogue during the Eocene. The movement of the Gulf Stream would cause sea-surface temperatures to change dramatically over a small area. High frequency instability of sea-surface temperatures may have been attributable to deflections in the position of the Gulf Stream across the Blake Plateau. There is a strong 400 kyr cyclicity evident in the sea-surface temperature record, suggesting that fluctuations in the position of the Gulf Stream were caused by feedbacks within the climate system, in response to orbital changes in solar insolation
Oligocene climate dynamics
Full text linkA planktonic and benthic foraminiferal stable isotope stratigraphy of the Oligocene equatorial Pacific (Ocean Drilling Program, Site 1218) was generated at 6 kyr resolution between magnetochrons C9n and C11n.2n (~26.4–30 Ma on a newly developed astronomically calibrated timescale). Our data allow a detailed examination of Oligocene paleoceanography, the evolution of the early cryosphere, and the influence of orbital forcing on glacioeustatic sea level variations. Spectral analysis reveals power and coherency for obliquity (40 kyr period) and eccentricity (~110, 405 kyr) orbital bands, with an additional strong imprint of the eccentricity and 1.2 Myr obliquity amplitude cycle, driving ice sheet oscillations in the Southern Hemisphere. Planktonic and benthic foraminifera ?18O are used to constrain the magnitude and timing of major fluctuations in ice volume and global sea level change. Glacial episodes, related to obliquity and eccentricity variations, occurred at 29.16, 27.91, and 26.76 Ma, corresponding to glacioeustatic sea level fluctuations of 50–65 m. Alteration of high-latitude temperatures and Antarctic ice volume had a significant impact on the global carbon burial and equatorial productivity, as cyclic variations are also recorded in the carbon isotope signal of planktonic and benthic foraminifera, the water column carbon isotope gradient, and estimated percent carbonate of bulk sediment. We also investigate the implications of a close correspondence between oxygen and carbon isotope events and long-term amplitude envelope extrema in astronomical calculations during the Oligocene, and develop a new naming scheme for stable isotope events, on the basis of the 405 kyr eccentricity cycle count
Systematic taxonomy of middle Miocene Sphaeroidinellopsis (planktonic foraminifera)
Full text linkThe taxonomy and phylogeny of the Miocene to Recent genus Sphaeroidinellopsis have been documented in previous studies, but the evolution of this lineage remains unclear. Some authors have debated this genus in the past, choosing a variety of parameters to discriminate the morphospecies. Here we present new scanning electron microscope analyses of specimens from Ocean Drilling Program (ODP) Site 925 (Ceara Rise, western equatorial Atlantic) and ODP Site 959 (Deep Ivorian Basin, eastern equatorial Atlantic). Our study reveals transitional individuals Sphaeroidinellopsis disjuncta–Sphaeroidinellopsis kochi, a speciation event never described previously. These transitional specimens are characterized by extreme morphological features such as elongated and sac-like final chambers, requiring amendments to the current classification and taxonomy of these morphospecies. In this paper, an alternative hypothesis is presented and discussed, to assess these new observations within the evolutionary mosaic of Sphaeroidinellopsis
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
Pacific Oligocene reference curve (invited talk)
No full textWe present an uninterrupted chronology of climate and ocean carbon chemistry from ODP Site 1218 recovered in the equatorial Pacific, from the Eocene/Oligocene to the Oligocene/Miocene boundary, ~34 to 23 Ma. Using astronomically age calibrated data we find a strong imprint of the 405, 127 and 96-thousand-year (kyr) Earth's eccentricity as well as a dominant influence of the 1.2 million year (Myr) obliquity amplitude modulation cycles on periodically re-occurring Oligocene glacial and carbon cycle events. In combination, these astronomical modulations act as the "heartbeat" of the Oligocene climate system. The response of the climate system to intricate orbital variations is striking and suggests a fundamental role of the carbon cycle in the interaction between solar forcing and climate. Our record provides a new high-resolution view of the Oligocene climate system, prompts a re-evaluation of the previously hypothesised late Oligocene deglaciation, and sheds new light on Oligocene inter-ocean basin gradients between the Atlantic, Southern Ocean, and Pacific. Salient observations include foraminiferal benthic stable oxygen and carbon isotopes that co-vary, a phase lag of d13C w.r.t. d18O for the 405 kyr cycle, preferential filtering of longer orbital periods in d13C, presumably due to TCO2 reservoir buffering. We then use simple orbitally forced carbon cycle box models and manage to re-create the patterns observed in our data, including the overall strong amplitude of 405 kyr cycles in d13C. These models show a strong amplification of the lower astronomical frequencies, as observed in our data
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