1,721,112 research outputs found
Improved astronomically tuned timescales for the Late Neogene (abstract of paper presented at EUG X, Strasbourg, France, 28 Mar - 1 Apr 1999)
No full textRecent attempts to tune geological timescales to Milankovitch-type cycles rely on matching features in proxy records to astronomically calculated obliquity and precession cycles. They often use tuning targets such as the 65°N summer insolation curve. It has been shown (Laskar 1993) that the exact position of insolation peaks in time depends on the parameters chosen for dynamical ellipticity and tidal dissipation (the Earth model). Hence a better knowledge of these parameters is needed before precise timescales for the pre Pliocene can be developed using traditional methods (Lourens, 1996).High quality geological records that have been tuned to an astronomical target have been published from both the Pacific (especially ODP Leg 138; Shackleton et al., 1995) and the Atlantic (especially ODP Leg 154; Shackleton and Crowhurst, 1997) although two different astronomical solutions have been used for the two studies. We have re-examined these data sets to place them in a consistent time scale, and to improve signal-to-noise ratio by stacking different data sets. Our objective is to optimise the separation in the data of that variability that is independent of the chosen Earth model (the amplitude modulation of the precession and obliquity components, extracted by complex demodulation) from the components that do depend on the chosen Earth model (the mean frequencies for obliquity and climatic precession). This operation can be performed iteratively on older sequences to optimise the astronomical solution for longer time intervals
Paleoclimatologia dell'ultima deglaciazione nel Bacino di Cefalù-Mar Tirreno meridionale
No full textREND. FIS. ACC. LINCE
Plio-Pleistocene nannofossil biostratigraphy and calibration to oxygen isotope stratigraphies from Deep Sea Drilling Project Site 607 and Ocean Drilling Program Site 677
No full textEvolutionary events within early Pleistocene Gephyrocapsa assemblages provide at least four biostratigraphically useful events occurring in the Matuyama Chron after the Olduvai subchron. Four additional, late Pliocene-early Pleistocene events are also investigated. The successful astronomical calibration of oxygen isotope stratigraphies from Deep Sea Drilling Project site 607 and Ocean Drilling Program site 677 in the Matuyama Chron permits calibration of the biostratigraphic events to these uniquely resolved isotope chronologie
Orbital tuning and re-tuning: art and science (abstract of paper presented at AGU Fall Meeting, San Francisco, 6-10 Dec 2002)
No full textIt is difficult to imagine a situation wherein (a) a geological record built up at a constant rate or (b) a geological deposit built up at a rate that was independent of climate. In addition all geological proxies for climate parameters are subject to both observational noise, and variability in their relationship with climate. Hence considerable judgement is needed in first selecting geological records that will be amenable to "tuning" to a record of changing distribution of solar insolation as calculated by an astronomer, and then in carrying out the operation. Tuning a geological record to an incorrect insolation record, or to the wrong time interval in a correct insolation record, or to a simplified "caricature" of the true insolation record can all provide considerable insights in some circumstances. Nevertheless from the point of view of the geological time scale it is important to carry out the procedure correctly, with the best available astronomical data. It is also important that one can utilize the tuning in order to verify the astronomical calculations. We have recently re-tuned a number of records on the basis of the new calculations discussed by Laskar (2001). Astronomical Solutions for Paleoclimates Studies. Eos Trans. AGU, 82(47), Fall Meet. Suppl. Abstract U11A-01. In the Pliocene re-tuning is generally straightforward and uninformative because the astronomical changes are small and the tuning is so unambiguous that little judgement is required. In the Early Miocene and Oligocene the situation is more interesting. During this time available data suggests that the climatic variability is dominated by 41-ky obliquity cycles but attaining a correct tuning requires identification of the weaker precession signal and making use of the longer-term (100-ky and 400-ky) eccentricity modulation. A surprising aspect of the tuning published by us in 1999 was that between 20Ma and 24Ma there was long-term (about 1.1My) modulation of the obliquity signal that matched that predicted; this match broke down in the earlier part of the record.Re-tuning the records for this interval required a slight "stretching" to take account of the fact that 400-ky eccentricity maxima in the Oligocene are around 100ky older than in the previous astronomical solution. This obviously required the addition of a very small number of additional obliquity cycles; thus it was necessary to make a judgement anew regarding the interpretation of each cycle. This in turn enables us to assess the degree of robustness of the tuning. The fact that in the new tuning the amplitude modulation of the obliquity signal in the 20Ma to 24Ma time window still matches the calculated modulation is not surprising since in this time-window the amplitude envelope of obliquity moved back in age to the same extent as the eccentricity signal, as a result of the new calculations. However, the fact that the re-tuned data now demonstrates an amplitude modulation of the obliquity signal that remains in phase with the calculated signal back to 30Ma, despite the fact that the re-tuning did not entail significant re-interpretation of the record, strongly suggests both that the new solution represents an "improvement" and that the coherence with this long-term modulation constitutes a very strong independent validation of this approach to geological time scale development
Response of the pelagic enviroment to palaeoclimatic changes in the central Mediterranean Sea during the Late Quaternary.
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Early-Middle Pleistocene deep circulation in the western subtropical Atlantic: southern hemisphere modulation of the North Atlantic Ocean
No full textEarly-Middle Pleistocene climate and deep ocean hydrography have been reconstructed using oxygen and carbon isotope ratio measurements in planktonic and benthic foraminifera from a high-deposition-rate sedimentary succession recovered at the Bermuda Rise, in the northern Sargasso Sea (Ocean Drilling Program Leg 172, Site 1063). The site’s water depth makes it sensitive to changes in the balance between North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) in the deep North Atlantic, allowing the evaluation of perturbations in deep water production at different timescales. Millennial-scale fluctuations, superimposed on the longer period oscillations of orbital origin, occurred during all the observed climate states. The highest amplitude sub-Milankovitch fluctuations were mainly associated with interglacials, particularly after the intensification of the glacial regime at c. 900 ka. Using the benthic carbon isotope signal as a water mass tracer, and by comparing the δ13C record to a suite of drill sites in the North Atlantic and Pacific oceans, it has been possible to infer that the relative strength of AABW production varied through time, as did that of NADW. A scenario in which the two different source components of deep water can undergo dramatic changes in their circulation regime through time should be taken into consideration when evaluating the role of thermohaline circulation in global climate change
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
Punctuated evolution of global climate cooling during the late Middle to early Late Miocene: high-resolution planktonic foraminiferal and oxygen isotope records from the Mediterranean
No full textVariations 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
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