1,721,230 research outputs found
A depositional model for hydromagnesite-magnesite playas near Atlin, British Columbia, Canada
No full textThis study formulates a comprehensive depositional model for hydromagnesite-magnesite playas. Mineralogical, isotopic and hydrogeochemical data are coupled with electron microscopy and field observations of the hydromagnesite-magnesite playas near Atlin, British Columbia, Canada. Four surface environments are recognized: wetlands, grasslands, localized mounds (metre-scale) and amalgamated mounds composed primarily of hydromagnesite [Mg(CO)(OH)·4HO], which are interpreted to represent stages in playa genesis. Water chemistry, precipitation kinetics and depositional environment are primary controls on sediment mineralogy. At depth (average ≈ 2 m), Ca-Mg-carbonate sediments overlay early Holocene glaciolacustrine sediments indicating deposition within a lake post-deglaciation. This mineralogical change corresponds to a shift from siliciclastic to chemical carbonate deposition as the supply of fresh surface water (for example, glacier meltwater) ceased and was replaced by alkaline groundwater. Weathering of ultramafic bedrock in the region produces Mg-HCO groundwater that concentrates by evaporation upon discharging into closed basins, occupied by the playas. An uppermost unit of Mg-carbonate sediments (hydromagnesite mounds) overlies the Ca-Mg-carbonate sediments. This second mineralogical shift corresponds to a change in the depositional environment from subaqueous to subaerial, occurring once sediments 'emerged' from the water surface. Capillary action and evaporation draw Mg-HCO water up towards the ground surface, precipitating Mg-carbonate minerals. Evaporation at the water table causes precipitation of lansfordite [MgCO·5HO] which partially cements pre-existing sediments forming a hardpan. As carbonate deposition continues, the weight of the overlying sediments causes compaction and minor lateral movement of the mounds leading to amalgamation of localized mounds. Radiocarbon dating of buried vegetation at the Ca-Mg-carbonate boundary indicates that there has been ca 8000 years of continuous Mg-carbonate deposition at a rate of 0·4 mm yr. The depositional model accounts for the many sedimentological, mineralogical and geochemical processes that occur in the four surface environments; elucidating past and present carbonate deposition
High-resolution palaeohydrological reconstruction of central Italy during the Holocene
No full textThe endorheic nature of Lake Trasimeno in combination with its position in central Italy makes it a relevant site to better constrain spatial differences in Holocene climatic variability in the Mediterranean area. Herein, we present a high-resolution ostracod record from the Holocene section of an 8.59-m-long sedimentary core, which is compared with historical data to distinguish anthropogenic and climatic signals. The occurrence, abundance and vanishing of ostracod species are directly controlled by lake-level variations, which are in turn related to global and regional climatic changes (i.e. moisture variations). The total organic carbon content as well as observed lithological changes provide additional information about Lake Trasimeno’s hydrological and trophic conditions in the past. Most important variations have been identified at ca. 10,000 cal. yr BP, when the lacustrine basin changed from a temporary to a permanent waterbody (from Sarsicypridopsis aculeata to Candona angulata association). The highest lake level and the total absence of ostracods occur at around 9000 cal. yr BP. The recorded humid phase persisted up to ca. 4200 cal yr BP since when a lake-level decreasing trend started and continued until the present day (Candona angulata, Cyprideis torosa and Darvinula stevensoni associations). The frequency of changes in the relative abundance of the main species shows centennial variations (i.e. C. angulata, C. torosa and Darvinula stevensoni). As historical evidences yield that human interventions to control the lake level remained unsuccessful in the past, Lake Trasimeno records an almost pristine climatic signal during most of the Holocene, which is quite unusual in the highly populated Mediterranean area
Geophysical evidence of multiple glacier advances in Lago Fagnano (54°S), southernmost Patagonia
Full text linkThe Island of Tierra del Fuego, at the southernmost extreme of Patagonia, is located in one of the most extensively glaciated areas of the Southern Hemisphere outside Antarctica during the late Pleistocene. The Lago Fagnano region, at ∼54°30′S and ∼68°W, has experienced several phases of glacier growth and retreat since the Last Glacial Maximum (LGM). We illustrate these phases using combined geomorphological, geophysical and coring surveys in Lago Fagnano itself, a ∼105 km-long, E–W-oriented glacio-tectonic basin. We identify and map a complex set of submerged frontal, central and lateral moraines covered by lacustrine sediments using seismic stratigraphic analysis of multi-channel profiles imaging the sub-lake floor. We then combine these geophysical data with field observations and regional maps of similar structures around the lake to reconstruct the spatial behavior of the Fagnano paleo-glacier since the LGM. We interpret the preserved frontal moraines as having formed during at least 20 re-advance stages of the glacier within a long-term deglaciation interval post-LGM. Preliminary tephrochronological dating of a ∼7.5 m long core indicates a step-wise deglaciation pattern comprising a final glacier re-advance stage at ∼11.2 ka BP.Fil: Waldmann, Nicolas. Universidad de Ginebra; SuizaFil: Ariztegui, Daniel. Universidad de Ginebra; SuizaFil: Anselmetti, Flavio S.. Swiss Federal Institute of Aquatic Science and Technology. Department of Surface Waters; SuizaFil: Coronato, Andrea Maria Josefa. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; ArgentinaFil: Austin Jr., James A.. University of Texas at Austin; Estados Unido
Arid and humid phases in central Italy during the Late Pleistocene revealed by the Lake Trasimeno ostracod record
No full textA multiproxy approach in a sediment core from Lake Trasimeno has been used to reconstruct the climate history of central Italy during the Late Pleistocene to Early Holocene period (ca. 47,000â9,000 cal yr B.P.). Ostracod assemblages and sedimentological data (lithology and carbonate content) have been used to infer past hydrological changes in the area. Ostracods were analyzed throughout the core using diversity indexes and multivariate statistic analyses (Cluster and PCA). Three main associations linked to lake level and salinity variations were recognized: 1) the C. torosa association, indicating permanent lacustrine conditions with high lake levels and low salinities; 2) the S. aculeata association, linked to very shallow/temporary waters with higher salinity conditions; and 3) the S. aculeata-E. mareotica association pointing to temporary water conditions and the highest salinities. Furthermore, the presence of C. fuscata and L. blankenbergensis during wide parts of the Late Pleistocene indicates temperatures lower than present days. Alternations of these three ostracod associations compares well with the oxygen isotope curve from Greenland (NGRIP) and are thus interpreted as climatically driven. At the Holocene transition (Termination 1), the ostracod associations indicate a delay in the increasing warming and humidity with respect to the NGRIP temperature record
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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