1,721,285 research outputs found
Radiocarbon calibration uncertainties for tree-ring chronologie Avigliana_A15
No full textRadiocarbon dating is the most commonly used chronological tool in archaeological and environmental sciences dealing with the past 50,000 years, making the radiocarbon calibration curve one of the most important records in paleosciences. For the past 12,560 years, the radiocarbon calibration curve is constrained by high quality tree-ring data. Prior to this, however, its uncertainties increase rapidly due to the absence of suitable tree-ring 14C data. Here, we present new high-resolution 14C measurements from 3 floating tree-ring chronologies from the last deglaciation. By using combined information from the current radiocarbon calibration curve and ice core 10Be records, we are able to absolutely date these chronologies at high confidence. We show that our data imply large 14C-age variations during the Bølling chronozone (Greenland Interstadial 1e) – a period that is currently characterized by a long 14C-age plateau in the most recent IntCal13 calibration record. We demonstrate that this lack of structure in IntCal13 may currently lead to erroneous calibrated ages by up to 500 years
Carbon cycle simulations results and estimates of the 14C production rate over the last 55,000 years
No full textThis data set contains carbon cycle simulations results and estimates of the 14C production rate Q over the last 55,000 years, as well as some not yet online available supporting time series. Some data sets cover up to the last 100,000 years. They have been compiled here in order to improve how independent approaches of Q when used in carbon cycle model frameworks improve simulations of atmospheric Δ14C in comparison to reconstructions, e.g. from IntCal20.
In detail, the following is contained:
a) three reconstructions (data-based estimates) of the 14C production rate based on (1) cosmogenic radionuclides from ice cores; (2) geomagnetic field data and (3) a 10Be stack from marine sediment records;
b) deconvolved (model-based) estimates of the 14C production rate from two different models, the carbon cycle box model BICYCLE-SE (8 scenarios) and the LSG OGCM (3 scenarios), both externally forced with atmospheric Δ14C, e.g. from IntCal20;
c) model-internal simulated atmospheric Δ14 when the data-based estimates of have been applied to force the the BICYCLE-SE model (11 scenarios)
d) simulated atmospheric CO2 from the BICYCLE-SE model (1 scenario);
e) supporting data: (1) a smoothed version from Hulu Cave Δ14 as has been used within IntCal20 as prior to calculate the marine reservoir age of contributing marine records; (2) a corrected version of the stack of cosmogenic radionuclides from ice cores underlying our ice core based version of Q
Tree-rings reveal two strong solar proton events in 7176 and 5259 BCE
No full textAbstract The Sun sporadically produces eruptive events leading to intense fluxes of solar energetic particles (SEPs) that dramatically disrupt the near-Earth radiation environment. Such events have been directly studied for the last decades but little is known about the occurrence and magnitude of rare, extreme SEP events. Presently, a few events that produced measurable signals in cosmogenic radionuclides such as 14 C, 10 Be and 36 Cl have been found. Analyzing annual 14 C concentrations in tree-rings from Switzerland, Germany, Ireland, Russia, and the USA we discovered two spikes in atmospheric 14 C occurring in 7176 and 5259 BCE. The ~2% increases of atmospheric 14 C recorded for both events exceed all previously known 14 C peaks but after correction for the geomagnetic field, they are comparable to the largest event of this type discovered so far at 775 CE. These strong events serve as accurate time markers for the synchronization with floating tree-ring and ice core records and provide critical information on the previous occurrence of extreme solar events which may threaten modern infrastructure.Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschun
14C and 10Be around 1650 cal BC
No full textThere is a debate about the date of the Minoan eruption of Santorini as reconstructed from a branch of an olive tree that was buried alive in trephra on Santorini2 and the dating of an ice core horizon attributed to this eruption.3 The olive branch was 14C-wiggle matched to the 14C calibration curve and yielded an age range of 1627-1600 bc4 while the counting of annual layers in the Greenland ice cores produced an age of 1642 (±5) bc.5 Here I will study the relative timing of the two time scales by comparing the cosmic ray signal as recorded by 14C in tree rings and 10Be in ice cores. The result reveals an intriguing age difference that is similar to the dating difference mentioned above. The origin of the difference is unclear. This analysis supports both the dating of the Santorini eruption with the olive branch and the identification of this eruption in the ice cores, but it suggests unrecognised uncertainties in the tree ring 14C data, the ice core chronology or both
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
Solar activity during the last 1000 yr inferred from radionuclide records
No full textIdentification of the causes of past climate change requires detailed knowledge of one of the most important natural factors—solar forcing. Prior to the period of direct solar observations, radionuclide abundances in natural archives provide the best-known proxies for changes in solar activity. Here we present two independent reconstructions of changes in solar activity during the last 1000 yr, which are inferred from 10Be and 14C records. We analyse the tree-ring 14C data (SHCal, IntCal04 from 1000 to 1510 AD and annual data from 1511 to 1950 AD) and four 10Be records from Greenland ice cores (Camp Century, GRIP, Milcent and Dye3) together with two 10Be records from Antarctic ice cores (Dome Concordia and South Pole). In general, the 10Be and 14C records exhibit good agreement that allows us to obtain reliable estimates of past solar magnetic modulation of the radionuclide production rates. Differences between 10Be records from Antarctica and Greenland indicate that climatic changes have influenced the deposition of 10Be during some periods of the last 1000 yr. The radionuclide-based reconstructions of past changes in solar activity do not always agree with the sunspot record, which indicates that the coupling between those proxies is not as close as has been sometimes assumed. The tree-ring 14C record and 10Be from Antarctica indicate that recent solar activity is high but not exceptional with respect to the last 1000 yr
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
Cosmogenic radionuclides reveal an extreme solar particle storm near a solar minimum 9125 years BP
Full text linkDuring solar storms, the Sun expels large amounts of energetic particles (SEP) that can react with the Earth’s atmospheric constituents and produce cosmogenic radionuclides such as 14C, 10Be and 36Cl. Here we present 10Be and 36Cl data measured in ice cores from Greenland and Antarctica. The data consistently show one of the largest 10Be and 36Cl production peaks detected so far, most likely produced by an extreme SEP event that hit Earth 9125 years BP (before present, i.e., before 1950 CE), i.e., 7176 BCE. Using the 36Cl/10Be ratio, we demonstrate that this event was characterized by a very hard energy spectrum and was possibly up to two orders of magnitude larger than any SEP event during the instrumental period. Furthermore, we provide 10Be-based evidence that, contrary to expectations, the SEP event occurred near a solar minimum
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