1,721,328 research outputs found
Three hundred eighty thousand year long stable isotope and faunal records from the Red Sea : influence of global sea level change on hydrography
Full text linkStable isotope and faunal records from the central Red Sea show high-amplitude oscillations for the past 380,000 years. Positive δ18O anomalies indicate periods of significant salt buildup during periods of lowered sea level when water mass exchange with the Arabian Sea was reduced due to a reduced geometry of the Bab el Mandeb Strait. Salinities as high as 53‰ and 55‰ are inferred from pteropod and benthic foraminifera δ18O, respectively, for the last glacial maximum. During this period all planktonic foraminifera vanished from this part of the Red Sea. Environmental conditions improved rapidly after 13 ka as salinities decreased due to rising sea level. The foraminiferal fauna started to reappear and was fully reestablished between 9 ka and 8 ka. Spectral analysis of the planktonic δ18O record documents highest variance in the orbital eccentricity, obliquity, and precession bands, indicating a dominant influence of climatically - driven sea level change on environmental conditions in the Red Sea. Variance in the precession band is enhanced compared to the global mean marine climate record (SPECMAP), suggesting an additional influence of the Indian monsoon system on Red Sea climates
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
(Table 1) Age determination of sediment core M35003-4
No full text(Table 1) Age determination of sediment core M35003-
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
Centennial to multidecadal timeseries of North Atlantic climatology during the last two glacial-interglacial transitions and marine-terrestrial teleconnections
No full textCon el fin de identificar y comprender la secuencia de eventos y los mecanismos involucrados en las transiciones glaciales-interglaciares y el establecimiento de los posteriores períodos interglaciares, es esencial mejorar nuestro entendimiento acerca de la variabilidad del clima y la sensibilidad del océano en áreas clave como el Atlántico Norte a escalas de tiempo de alta resolución. El Atlántico Norte es una región muy sensible a los cambios de las condiciones climáticas durante terminaciones glaciares debido a su proximidad a las capas de hielo del Hemisferio Norte. Por otra parte, el Atlántico Norte es una región de formación de aguas profundas, lo cual es crítico en el ajuste del modo de la circulación meridional atlántica (AMOC). Por tanto, cualquier perturbación en esta región es de importancia global. Reconstrucciones climáticas del Atlántico Norte durante el último periodo interglacial (Estadio Isotópico Marino 5e, en sus siglas en inglés, MIS 5e) revelan condiciones climáticas en común con las proyecciones que los modelos climáticos predicen para el siglo 21, bajo la continua influencia humana y el aumento de gases de efecto invernadero. Por tanto, el período MIS5e se considera un buen análogo para el futuro calentamiento climático. Esta tesis contribuye en este sentido con reconstrucciones climáticas de alta resolución que muestran la variabilidad de la climatología superficial oceánica del Atlántico Norte durante el actual (Holoceno) y el último (MIS5e) períodos interglaciares y sus respectivas previas deglaciaciones (T1, T2): las secuencias MIS6-T2-MIS5e (150-105 ky) y MIS2-T1-Holoceno (25-0 ky) secuencias. Los datos se generan a partir de muestras de sedimento marino procedentes del punto de sondeo ODP 976 en el Mar de Alborán, Mediterráneo occidental, que recibe las señales del clima del Atlántico Norte a través de la atmósfera y la advección de aguas superficiales atlánticas que entran en el mar Mediterráneo a través del estrecho de Gibraltar. Se presentan series temporales de "proxies" medidos en el foraminífero planctónico, Globigerina bulloides, relativos a temperatura superficial del océano (derivada de la proporción Mg/Ca, SSTMg/Ca) y d18O, los cuales se combinan para indicar la variabilidad salinidad de la superficie del océano (d18Osw). Las altas tasas de sedimentación en ODP 976, (del orden de 23 cm/ky; Bernasconi et al., 1999), nos permiten resolver la variabilidad a escalas de tiempo de centenal a multi-decenales. A fin de evaluar el grado en que los perfiles generados ODP976 de hecho representan la climatología marina de la zona se comparan con perfiles similares en otros puntos de referencia en el Mar de Alborán , ODP 977 ( Martrat et al., 2004) y MD95-2043 (Cacho et al., 1999, 2002); en el margen Ibérico occidental, MD95-2042 (Shackleton et al., 2000, 2002, 2004), MD95-2040 (Schönfeld et al., 2003), MD99-2339 (Voelker et al., 2006) y MD99-2334 (Skinner et al., 2003), y con registros de d18O de testigos de hielo de Groenlandia: NGRIP (NGRIP members , 2004), NEEM (NEEM community members, 2013 ) y el registro sintético de variabilidad climática (Barker et al., 2011). Durante la secuencia de MIS2-T1-Holoceno, los perfiles de datos están estrechamente correlacionados, mostrando los registros del ODP976 una mayor resolución temporal, confirmando así la calidad de estos registros para representar la evolución climática del Atlántico Norte. Las diferentes secuencias captan características transiciones en múltiples pasos relacionados con la incursión de la acelerada desintegración de las capas de hielo, aunque a diferentes magnitudes y tiempos relativos. La característica más frecuente de ambas terminaciones glaciales es el calentamiento acelerado de 5-6 °C inmediatamente después de los respectivos períodos de deshielo deglacial asociados a los eventos Heinrich H1 y H11. Durante T1 esto ocurre hace ̃15 ky, es decir, durante el episodio Bølling/Allerød, cuando on ODP 976 se registra un calentamiento de SSTMg/Ca a niveles interglaciares de 20°C. Hace 12,5 ky, las SSTMg/Ca descienden ̃4-4,5 °C durante el período "Younger Dryas" (YD) que se documenta en los registros ODP 976 como un evento en dos fases, con un período seco y frío inicial entre 12,5-12 ky seguido de un intervalo más húmedo comprendido entre 12-11,5 ky. Durante el Holoceno, las SSTMg/Ca se estabilizan alrededor de 17.5 °C, coincidiendo con mediciones presentes de SST estacional de primavera del Mar de Alborán (MEDATLAS II, Medar Group, 2002). En comparación, T2 estuvo regida casi en su totalidad, por un solo evento masivo deshielo que coincidió con H11 o que contuvo el principal evento de H11 en el mismo. Este evento se destaca claramente en el perfil de SSTMg/Ca y los datos de d18Osw que representan los cambios de salinidad. Este escenario de una oleada masiva de agua de deshielo se apoya en registros bentónicos de d13C del Atlántico Norte (MD95-2042, U1308) que revelan una baja formación de aguas profundas del Atlántico Norte (NADW) y una débil circulación de retorno durante ese período. Una vez el evento H11 llega a su fin, tiene lugar un abrupto e ininterrumpido incremento de SSTMg/Ca de ̃ 5 °C en 1 ky, alcanzando los niveles de SST interglaciares (̃ 20°C) hace 129 ky. Este incremento de SSTMg/Ca ocurre es coetáneo al rápido establecimiento de flora interglaciar, tal y como evidencian los registros de polen que reconstruyen la expansión de bosque templado y mediterráneo en ODP 976 (Combourieu-Nebout, sin publicar) y el margen ibérico (Sánchez Goñi et al., 2003, 2012; Tzedakis, comunicación personal), lo cual sugiere un inicio sincrónico del período interglacial en ambos ambientes terrestres y marinos. Con el fin de superar las dificultades para obtener datación absoluta de la secuencia MIS6-T2- MIS5e, esta tesis genera un modelo de edad alternativo para los registros del ODP976 basados en la estrecha correlación del Atlántico Norte y la SST del Mediterráneo con la variabilidad del monzón asiático y las precipitaciones del Mediterráneo. Esas variables ambientales se representan en registros de espeleotemas y, por tanto, se correlaciona el registro de SSTMg/Ca del ODP 976 con los perfiles de d18O de las espeleotemas de Antro del Corchia, en Italia (Drysdale et al., 2004, 2009), BD-La Chaise, en Francia (Couchoud et al., 2009) y la Cueva Dongge, en China (Kelly et al., 2006). La transferencia de las cronologías basadas en series de uranio de las espeleotemas a los perfiles de datos del ODP976 ofrece una nueva cronología que ayuda a una mejor correlación de los registros de los testigos continentales, marinos y de hielo. Una incertidumbre importante durante T2 se deriva de la discrepancia observada entre los registros de Corchia y los registros de ODP976 SSTMg/Ca; discrepancia que proponemos posiblemente refleja cambios en las trayectorias atmosféricas durante el evento H11 de masas de aire húmedo que llegan al Mediterráneo.In order to identify and understand the timing of events and mechanisms involved in glacialinterglacial transitions and the establishment of interglacial periods it is essential to learn about the climate variability and ocean sensitivity of key areas such as the North Atlantic at high resolution time scales. This region is highly sensitive to the changes of climatic conditions during glacial terminations because of its proximity to the Northern Hemisphere ice sheets. Moreover, the North Atlantic is a prime location where surface-to-deep-ocean coupling occurs that is critically involved with setting the mode of the Atlantic Meridional Overturning Circulation. Hence any perturbations in this region likely are of global consequence. North Atlantic climate reconstructions of the last interglacial period (Marine Isotope Stage, MIS 5e) show many features that model projections predict for the 21st century under continuing human influence and increasing greenhouse gas forcing. Hence, the MIS5e period is considered a good analogue for future climate warming. This thesis contributes to this theme with high resolution records that depict the variability of North Atlantic surface ocean climatology during the present (Holocene) and last (MIS5e) interglacial periods and the preceding deglaciations: the MIS6-T2-MIS5e (150-105 ky) and MIS2- T1-Holocene (25-0 ky) sequences. The data are generated for ODP Site 976 in the Alboran Sea, westernmost Mediterranean, which receives North Atlantic climate signals through the atmosphere and the advection of Atlantic inflow waters entering the Mediterranean Sea. Time series are presented of planktonic (Globigerina bulloides) Mg/Ca derived sea surface temperature (SSTMg/Ca) and d18O that both in combination are used to indicate sea surface salinity variability (d18Osw). High rates of sediment deposition at the site, in the order of 23 cm/ky (Bernasconi et al., 1999), allow us to resolve variability at centennial to multi-decadal timescales. In order to assess the extent to which the ODP976 data profiles indeed represent the marine climatology in the area they are compared with similar profiles from other sites in the Alboran Sea, ODP 977 (Martrat et al., 2004) and MD95-2043 (Cacho et al., 1999, 2002); at the western Iberian margin, MD95-2042 (Shackleton et al., 2000, 2004), MD95-2040 (Schönfeld et al., 2003), MD99- 2339 (Voelker et al., 2006) and MD99-2334 (Skinner et al., 2003), and with Greenland ice core d18O records NGRIP (NGRIP members, 2004), NEEM (NEEM community members, 2013) and the synthetic record of Greenland climate variability (Barker et al., 2011). During the MIS2-T1- Holocene sequence the data profiles are tightly correlated while ODP976 consistently displays highest temporal resolution hence confirming the quality of this site as representing the North Atlantic climate evolution. Multiple-step features connected with the incursion of accelerated icesheet disintegration are captured by the ODP 976 palaeo-climatic records across the MIS6-T2- MIS5e and MIS2-T1-Holocene sequences reflecting periodic ice-sheet break down albeit at different magnitudes and relative timing. The most prevalent feature of both glacial terminations is the accelerated warming by 5-6°C immediately after the major deglacial meltwater surges associated with Heinrich events H1 and H11. During T1 this occurs at ~15 ky i.e., during the Bolling/Allerod episode, when ODP976 registers a warming of SSTMg/Ca to full-interglacial levels of 20°C. At 12.5 ky, SSTMg/Ca decreases by ~4-4.5°C into the Younger Dryas cold phase that is documented in the ODP 976 records as a two-phase event with an initial dry and cold period at 12.5-12 ky followed by a more humid interval at 12-11.5 ky. Holocene SSTMg/Ca stabilizes around 17.5°C closely matching today's Alboran Sea region modern spring SST (MEDATLAS II, Medar Group, 2002). T2 by comparison was almost entirely controlled by a single massive meltwater surge that was coincident with H11 or that embedded the main H11 event in it. This event clearly stands out in the SSTMg/Ca profile and the computed d18Osw data depicting salinity changes. The scenario of a single massive meltwater surge is supported by North Atlantic benthic d13C records (MD95-2042, U1308) revealing low North Atlantic Deep Water (NADW) formation and weak overturning circulation at this time. Once H11 ends, an abrupt and uninterrupted SSTMg/Ca increase of ~5°C occurs within 1 ky, reaching full interglacial SST levels (~20°C) at 129 ky. This SSTMg/Ca shift is accompanied by the rapid establishment of interglacial flora evidenced by an abrupt expansion of temperate and Mediterranean forest pollen taxa in ODP 976 (Combourieu-Nebout, unpublished) and the Iberian margin (Sánchez Goñi et al., 2012; Shackleton et al., 2003; Tzedakis, pers. comm.), which suggests an in-phase onset of the interglacial period in the regional terrestrial and marine environments. In order to overcome the difficulties of obtaining precise dating of the MIS6-T2-MIS5e sequence this thesis attempts generating an alternative age model for ODP976 that exploits the tight correlation of North Atlantic and Mediterranean temperature with the variability of the Asian monsoon and Mediterranean rainfall. Those environmental variables are depicted in speleothem records and hence the ODP 976 SSTMg/Ca record is correlated with the speleothem profiles of Antro del Corchia in Italy (Drysdale et al., 2004, 2009), BD-La Chaise in France (Couchoud et al., 2009) and Dongge Cave in China (Kelly et al., 2006). Transferring the uranium-series dated chronologies of those speleothems to the ODP976 data profiles provides a new chronology that aids an improved correlation of continental, marine and ice core records. A major uncertainty derives from the offset structure of T2 in the Corchia and La Chaise profiles from that of the ODP976 SSTMg/Ca profile which possibly reflects shifted atmospheric trajectories during H11 of moist air masses reaching the Mediterranean
Surface and deep circulation off South Africa : Agulhas leakage influence on the meridional overturning circulation during the last 345 kyr
Full text linkDescripció del recurs: el 11 d'agost de 2010Investigaciones paleocenográficas y simulaciones numéricas están mostrando que el Hemisferio Sur desempeña un papel importante, tanto en los cambios de la circulación global como en la definición del estado climático terrestre. Una región clave para la circulación global se encuentra en la región alrededor de Sudáfrica. Aquí, aguas superficiales y de termoclina del Océano Índico son transferidas al Atlántico a través de anillos y de filamentos de la Corriente de Agulhas. En profundidad, la transferencia de agua se produce en la dirección opuesta, con Agua Profunda del Atlántico Norte (North Atlantic Deep Water, NADW), abandonando la cuenca atlántica y entrando en la índica. La entrada de aguas cálidas y saladas de Agulhas al Atlántico Sur se cree que puede influir en el Atlántico Norte a través de perturbaciones en los balances de flotabilidad, siendo crucial para el modo y estabilidad de la Circulación Meridional Atlántica (Meridional Overturning Circulation, MOC). En esta Tesis presento registros de 345.000 años de isótopos estables de foraminíferos planctónicos y bentónicos, Cd/Ca de bentónicos, Mg/Ca de planctónicos, granoclasificación de arcillas (sortable silt) y censos de foraminíferos de un registro combinado de dos testigos sedimentarios recuperados a profundidades medias (~ 2500 m) aguas adentro de Sudáfrica. Los registros permiten discernir la circulación superficial y profunda pasada y evaluar la conexión entre el flujo superficial de aguas índicas hacia el Atlántico y el flujo de aguas profundas atlánticas hacia el Índico. La circulación profunda del Atlántico se cree que era diferente durante el Último Máximo Glacial (Last Glacial Maximum, LGM), con NADW siendo sustituida por un agua intermedia (Glacial North Atlantic Intermediate Water, GNAIW) y con Aguas de Componente Sur (Southern Component Water, SCW) expandiéndose más hacia el norte e invadiendo la cuenca a menores profundidades. Hemos encontrado evidencias de que la circulación profunda durante las partes intermedias de los períodos glaciales MIS 6 y 8 (Estadio Isotópico Marino: Marine Isotope Stages, MIS) era similar a la del LGM. Asimismo, encontramos indicios de que la SCW es una mezcla de un agua superior bien ventilada (Upper Southern Component Water, USCW) e inferior mal ventilada (Lower Southern Component Water, LSCW). Durante las fases iniciales de ambos períodos glaciales, la circulación profunda probablemente no difería substancialmente de la actual. Esto, juntamente con valores de los registros de superficie similares a los presentes sugiere un modo de conexión entre la transferencia de aguas de Agulhas y la MOC similar a la actual. Desde estos valores tipo¬interglacial en los períodos glaciales tempranos, los registros superficiales muestran tendencias crecientes, alcanzando niveles máximos mucho antes de las Terminaciones y colapsando a niveles interglaciales al entrar en los períodos cálidos MIS 5e, 7e y 9c. Estudios previos y modelos numéricos muestran una presencia considerable de aguas de Agulhas en el Atlántico durante las Terminaciones y sugieren que esta entrada de aguas cálidas y saladas podría haber sido fundamental para el restablecimiento de las condiciones interglaciales. Nuestros registros demuestran este postulado ya que la máxima presencia de aguas de Agulhas ocurre en paralelo con la recuperación de niveles interglaciales en los registros profundos. Es posible que el colapso de aguas de Agulhas al comienzo de los interglaciales indique el restablecimiento de un modo moderno de conexión entre el leakage de aguas de Agulhas a través de anillos y la liberación de aguas cálidas y saladas acumuladas en el Índico. Curiosamente, durante MIS 2 y las partes intermedias de MIS 6 y 8, los indicadores de circulación superficial registran una fuerte presencia de aguas de Agulhas a la vez que la SCW dominaba la hidrografía profunda. Esto indicaría que en estos momentos, o bien la fuerte presencia de aguas de Agulhas no generaría una anomalía de flotabilidad en el Atlántico Sur de modo eficiente o bien que otros factores, por ejemplo, la propia climatología del Atlántico Norte, podrían prevalecer sobre la anomalía de salinidad impuesta por las aguas de Agulhas.Paleocenographic research and numerical model simulations increasingly are emphasizing that the Southern Hemisphere plays an important role, both in global circulation changes and in defining the state of Earth's climate. A key region for the global circulation system is found in the neighbourhood of southern Africa. Here, surface and thermocline Indian Ocean Waters are transferred to the Atlantic by rings and filaments of the Agulhas Current. At depth, the water is transferred in the opposite direction, with North Atlantic Deep Water (NADW) leaving the Atlantic and entering the Indian. The input of warm, salty Agulhas Waters to the South Atlantic is thought to influence the far distant North Atlantic by way of buoyancy perturbation, being crucial for the mode and stability of the Meridional Overturning Circulation (MOC). In this Thesis I present 345,000-year-long records of planktonic and benthic foraminifera stable isotopes, benthic Cd/Ca, planktonic Mg/Ca, sortable silt and foraminiferal census counts from a spliced record of two mid-depth (~2500 m) sediment cores from off South Africa. The records allow deciphering the past surface and deep circulation in the area and assessing the linkage between both, the surface flux of Indian waters towards the Atlantic and the deep flux of Atlantic waters towards the Indian. The Atlantic deep circulation is thought to have been different during the Last Glacial Maximum (LGM), with NADW being replaced by Glacial North Atlantic Intermediate Water (GNAIW) and Southern Component Waters (SCW) spreading further north and bathing the basin to shallower depths. We find evidence that the deep circulation during middle phases of the glacials Marine Isotope Stage (MIS) 6 and 8 was similar to that of the LGM. At the same time, we find indications that SCW was a mixture of an Upper well-ventilated component (USCW) and a Lower poorly-ventilated one (LSCW). During early glacial phases of both, MIS 6 and 8, the deep Atlantic circulation was plausibly not much different from today's. This, in conjunction with values of the surface records similar to present ones, indicates presence of Agulhas Water at our site and suggests a persistence of the modern-type linkage between Agulhas Leakage and MOC. From these interglacial-like values at the initiation of the glacial periods, the surface records display increasing trends reaching maximum values well before glacial Terminations and collapsing to interglacial levels as the warm periods MIS 5e, 7e and 9c are reached. Previous studies and numerical models show a prominent presence of Agulhas Waters in the South Atlantic during glacial Terminations and suggest that this input could have been pivotal for the reestablishment of interglacial climate conditions. Our records demostrate this contention as maximum presence of Agulhas Waters occurs in parallel with the recovery to interglacial levels of the deep circulation records. It is possible that the collapse of Agulhas Waters at the beginning of interglacial periods indicates the reestablishment of a modern-type mode of Agulhas Water Leakage through rings and the release of warm saline waters stored in the Indian Ocean. Intriguingly, during MIS 2 and mid-glacial MIS 6 and 8, the surface circulation proxies record a strong presence of Agulhas Waters while SCW was dominating the deep hydrography. This indicates that at these times either the presence of Agulhas Waters was unable to efficiently generate a buoyancy anomaly in the South Atlantic or that other factors were able to overcome the salt anomaly imposed by the Agulhas Waters, for instance North Atlantic climatology
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