1,721,026 research outputs found
Statistically learning Archean carbonate diagenesis
No full textGeochemical data are often very noisy due to the large natural heterogeneity of the geological materials as well as oversimplification of rock classification. This has serious repercussions on the precision of our knowledge of the deep past as we often rely solely on geochemical proxies to investigate the geological evolution of Archean and Proterozoic environments. Here statistical learning procedures were applied to achieve unbiased classification of Neoarchean stromatolitic dolostone textures on the basis of the distribution of their trace elements and rare earth elements (REE) investigated through laser ablation induced coupled plasma - mass spectrometry (LA-ICP-MS). Multivariate statistical analyses and supervised statistical learning have revealed that different dolomite fabrics, thought as products of aggrading diagenesis and recrystallization, are in fact chemically indistinguishable. The diagenetic processes that cause the re-crystallization of dolomite and the consequent change of textures, is not affecting the distribution of major and trace elements inherited by the depositional environment or during early stages of diagenesis. At the same time the algorithm has revealed that an optically homogeneous microcrystalline dolomite sample may in fact be geochemically inhomogeneous because of processes of ripening and recrystallization occurred at an early stage of marine diagenesis and that have contributed to element mobilization. Statistical learning has succeeded in recognizing chemofacies which not always overlap with dolomite textures and fabrics highlighting the importance of crystallographic and diagenetic studies before any study of carbonates as geochemical proxies
The astrobiological potential of the Makgadikgadi Basin, Botswana: Field analogue for planetary exploration
No full textTerrestrial analogue sites have been crucial for studying Martian geology and mineralogy, integrating the direct evidence available from Mars through remote sensing and in situ measurements carried out by the instruments on board robotic missions. Studying readily available and accessible terrestrial analogues of Martian fossil or extant environments is considered the most efficient way to answer crucial scientific questions. These analogues offer opportunities to collect a range of geological and microbiological data. The Makgadikgadi Basin (MKB) in Botswana is one of such environments hosting a system of salt pans presenting striking similarities with Mars playa deposits. The MKB presents layered mounds, relict fan deltas with inverted channels, polygonal structures and evaporitic crusts harboring communities of extremophiles. The present-day MKB is predominantly fed by groundwater and local precipitations in an overall arid to semi-arid climate, characterized by high UV radiation and salinity, deposition of evaporitic minerals and authigenic clays. The shallow subsurface of the MKB pans is covered by diagenetic features (duricrusts) including silcretes and calcretes. These pans can serve as test beds for the physical and chemical characteristics of playa deposits on Mars and help improve our understanding of the conditions that might support life outside our planet
Partitioning and distribution of silver in sediment-hosted Cu-Ag deposits: Evidence from the Ghanzi-Chobe Belt portion of the Kalahari Copper Belt
No full textThe Khoemacau Mining (KCM) Cu-Ag deposits in northwestern Botswana are comprised of several mineralised zones, including from south to north: the Banana Zone (made up of the Chalcocite Zone, New Discovery, North Limb, South Limb Definition, and the North East Fold), Mango, Zone 5, Zone 5 North, Boseto, Zeta NE, and Zone 6. The KCM Cu-Ag deposits are characterised by two styles of mineralisation that include disseminated and structurally controlled styles of mineralisation, both typified by the scarcity of Ag-bearing minerals. Laser ab lation inductively coupled mass spectrometry (LA-ICP-MS) and electron probe microanalysis (EPMA) techniques were employed to (i) track the sources of Ag by assessing the Ag budget of the sulphide phases, (ii) assess the partitioning and distribution of Ag in coexisting sulphide phases, and (iii) outline the mode of occurrence of Ag in the host sulphides. Bornite, chalcocite, wittichenite, tetrahedrite-like mineral, covellite, and galena were observed to be Ag-endowed. Bornite and tetrahedrite-like mineral are the principal hosts of Ag and contain, on average, up to 2090 and 4107 ppm Ag, respectively. But when bornite coexists with chalcocite, Ag preferentially partitions into chalcocite (mean Ag concentrations range from 423 to 1050 ppm) than in bornite (mean Ag concentrations range from 201 to 309 ppm). In the chalcocite-wittichenite assemblage, Ag partitions equally in these phases with mean concentrations of 1890 and 1854 ppm, respectively. Galena from the North East Fold contains a significant amount of Ag (mean 477 ppm) than that one from Zone 5, which is Ag-poor (mean up to 87 ppm). Unlike the spatially associated sulphides, chalcopyrite, however, is a poor host for Ag (mean concentration 1.3-104 ppm). The LA-ICP-MS data demonstrated that Ag is essentially bound in the crystal lattice of bornite, chalcocite, wittichenite, tetrahedrite-like mineral, covellite, and locally galena, as shown by the homogeneous distribution of Ag in the elemental maps, small to medium standard deviations (averaging 2-44% of the mean) and relatively flat time-resolved depth profiles. However, large variations in the Ag concentration between laser spots (averaging 51-70% of the mean) from the same sample, as observed in some bornite II grains from the New Discovery and the North East Fold mineralised zones, may indicate the presence of Ag-rich micro to nanoparticles in bornite II. The initial mineralising fluid is thought to have been saturated in Ag and precipitated Ag-rich phases in the mineralised zones southwest of the KCM Property, including the Chalcocite Zone, New Discovery, North Limb, South Limb Definition, and the North East Fold. Subsequently, it became Ag-undersaturated with time as it precipitated Ag-poor phases in the mineralised zones northeast of the KCM Project (such as Zone 5). The positive correlations (R > 0.60) displayed between Ag and As, Se, Sb, Hg, Te, and Bi in the Cu-(Fe)-sulphides demonstrate that these elements could be used as pathfinder elements in sediment-hosted Cu-Ag deposit exploration
Layered mound, inverted channels and polygonal fractures from the Makgadikgadi pan (Botswana): Possible analogues for Martian aqueous morphologies
No full textLayered mounds and inverted channels with polygonal fractures from the Ntwetwe Pan in the Makgadikgadi Basin (central Botswana) have been herein investigated. These morphologies are from an evaporitic basin (the Makgadikgadi Basin) that is the remnant of an ancient Pleistocene lake and is currently part of the world's largest evaporitic system.The mounds in the Ntwetwe Pan are characterized by a layered structure and low relief (max. 5 m above the pan floor) and can be in excess of 2 km wide. The mounds consist mainly of loose (non-lithified) sand and silt with high moisture contents, even during the dry season. Geophysical investigations have shown that groundwater processes, particularly those related to the capillary fringe that rises and conveys moisture through the mounds, are factors that make mound sediments resistant to wind erosion.The inverted channels, identified in the southern part of the Ntwetwe Pan, are characterized by gentle reliefs and depressions, which depend upon the distribution of calcretes and indurated sediments. Large scale (up to 100 m wide) polygonal fractures localized at the front of the channels, disappear at the transition with the present-day pan floor.We consider that these particular mounds, within the Ntwetwe Pan, are remnants of the strandline of the paleo-Makgadikgadi Lake, and that the inverted channels represent distributary channels of a relict fan delta, formed by an ephemeral river, most likely the paleo-Boteti River, during a Lake Paleo-Makgadikgadi highstand stage. We consider that large scale (up to 100 m wide) polygonal fractures, located on the channel-mouth lobes, represent large-scale desiccation cracks formed by rapid water evaporation from delta deposits.The results of this investigation highlight the importance of the paleo-drainage system and its interactions with the water table and wind-deflation as main geomorphological factors within salt pan environments. The mounds in the Makgadikgadi pans also show strong geomorphic similarities to spring mounds on the surface of Mars, localized in equatorial layered deposits (ELDs). These ELDs mounds are considered to result from cyclical groundwater upwelling, evaporation and wind deflation. The geological processes that resulted in the formation of mounds within the Makgadikgadi pans may, therefore, help to explain how similar layered deposits formed on Mars and confirm existing theories
Variability in microplastic sediment concentrations in the Upper Okavango Delta
No full textRiver pollution with microplastic particles (MP) has become a global scale problem. Recent research has shown that MP can remain in riverbed sediment for several decades, where they interact with the sediment and biota, potentially impacting food webs, while slowly releasing chemical additives. Thus, a good understanding of the MP distribution within riverbed sediment is critical to quantify potential risks to riverine ecosystems. Here we present first results of MP sediment concentrations for the upper Okavango Delta (Panhandle) in Northern Botswana, which home to about 90,000 people.
The Panhandle comprises meandering channels, oxbow lakes and a wide alluvial plain. River flows in the Okavango Basin are dominated by a strong seasonal pulse. At the upstream end (Mohembo), discharge varies between <100 m3/s during low flow and 700 m3/s during peak flow (March-April).
Sediment samples were collected at 7 sites between Mohembo and Sepupa by scooping. A 15 cm-long push core was also collected to assess MP distribution in depth. Organic matter was digested using either Fenton’s reagent or only H2O2. Each digested sample was split; part analysed with Raman spectroscopy to identify main polymers and part stained with Nile Red for fast MP quantification using fluorescent microscopy. Sediment grain size analysis was conducted using laser diffraction.
Fluorescent microscopy showed MP sediment concentrations from around 57 MP/kg dry weight in some of the oxbows to 400 MP/kg in the main channel. About 92% of the identified MP were classified as fragments, varying in length between 70-1065 μm (lower detection limit was 64 μm). The remainder of the particles were identified as fibres. Raman spectroscopy showed PET, PP and PVC as the main polymer types. Future studies will focus on determining basin-wide MP concentrations and their relation to sediment transport patterns
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
- …
