1,721,159 research outputs found
In situ Nd isotopic analysis of geological materials by laser ablation MC-ICP-MS
No full textNeodymium (Nd) isotopes have widespread applications in the earth sciences, both in geochronology and in provenance/tracer studies. The isotopic analyses are conventionally done by chemical separation of pure Nd from the sample matrix. This process is time consuming and loses potentially vast amounts of information held at small spatial scales. Here, we describe a laser ablation protocol for the measurement of Nd isotopes in geological materials with relatively high Nd concentrations (100 ppm; e.g., apatite, titanite, and ferromanganese nodules). The procedure allows, for the first time, the exploitation of information held in Nd isotopes at scales of less than 0.1 mm. The principal analytical issues to be overcome are the interferences from isobaric Sm isotopes, the correction for which must be very accurate in order to maintain a high accuracy for the Nd isotopic ratios. We show that careful attention to mass discrimination effects along with an iterative correction procedure allows a correction that results in an accuracy on the 143Nd/144Nd isotope ratio of the order of 0.5 epsilon units (0.005%) for Sm/Nd ratios as high as 1.2. The total analysis time is short (2–5 min), enabling a large number of analyses (>30) in each analytical session. Spatial resolution is governed by the Nd concentration and the required accuracy and for apatites and titanites it is typically 90 µm. Analytical precision is degraded by around a factor of 2–3 relative to the best TIMS and solution MC-ICP-MS data. Crucially, however, the rapid sample throughput and the extremely high spatial resolution by far outweigh this reduction in precision. A high resolution study of an Atlantic ferromanganese crust illustrates the usefulness of the described approach.Neodymium (Nd) isotopes have widespread applications in the earth sciences, both in geochronology and in provenance/tracer studies. The isotopic analyses are conventionally done by chemical separation of pure Nd from the sample matrix. This process is time consuming and loses potentially vast amounts of information held at small spatial scales. Here, we describe a laser ablation protocol for the measurement of Nd isotopes in geological materials with relatively high Nd concentrations (100 ppm; e.g., apatite, titanite, and ferromanganese nodules). The procedure allows, for the first time, the exploitation of information held in Nd isotopes at scales of less than 0.1 mm. The principal analytical issues to be overcome are the interferences from isobaric Sm isotopes, the correction for which must be very accurate in order to maintain a high accuracy for the Nd isotopic ratios. We show that careful attention to mass discrimination effects along with an iterative correction procedure allows a correction that results in an accuracy on the 143Nd/144Nd isotope ratio of the order of 0.5 epsilon units (0.005%) for Sm/Nd ratios as high as 1.2. The total analysis time is short (2–5 min), enabling a large number of analyses (>30) in each analytical session. Spatial resolution is governed by the Nd concentration and the required accuracy and for apatites and titanites it is typically 90 µm. Analytical precision is degraded by around a factor of 2–3 relative to the best TIMS and solution MC-ICP-MS data. Crucially, however, the rapid sample throughput and the extremely high spatial resolution by far outweigh this reduction in precision. A high resolution study of an Atlantic ferromanganese crust illustrates the usefulness of the described approach
The oceanic biogeochemistry of nickel and its isotopes: New data from the South Atlantic and the Southern Ocean biogeochemical divide
Full text linkISSN:0012-821XISSN:1385-013XISSN:1385-013
No change in the neodymium isotope composition of deep water exported from the North Atlantic on glacial-interglacial time scales
No full textQuantifying past circulation is a vital part of testing our understanding of the modern and future climate system. The isotopic composition of neodymium (Nd) in marine precipitates has considerable promise as a recorder of past circulation patterns, but its robust application requires knowledge of the end-member compositions in order to correctly deconvolute a downstream signal. We show here, using in situ, high temporal resolution analyses of ferromanganese crusts from the North Atlantic, that the Nd isotopic composition of deep water during times of much more extensive Northern Hemisphere ice cover was no different than the modern-day interglacial value. This result is surprising, but greatly simplffies the use of Nd isotopes as tracers of the strength and patterns of circulation in the Atlantic in the past. © 2007 Geological Society of America
The significance of monazite U–Th–Pb age data in metamorphic assemblages; a combined study of monazite and garnet chronometry
No full textIn this study, a coupled U–Th–Pb isotopic and EMP chemical study was carried out in situ on monazite (micro-inclusions within garnet as well as matrix grains) from rocks recovered from two chronologically well-constrained areas of the Himalayan orogen. Monazite inclusions within garnets from three samples yield ages of 44–36 Ma, whereas matrix grains within one sample are typically younger (30–26 Ma). Y depletion of these younger matrix grains indicates that they grew after garnet had crystallised. The recognition of episodic monazite growth during regional metamorphism, first at greenschist facies (before garnet growth) and secondly at upper amphibolite facies (post-dating garnet growth), allows relative growth ages of the occluding garnet to be calculated. These are in excellent agreement with Sm–Nd garnet ages from surrounding units. This approach not only provides a ready means of obtaining porphyroblast growth ages but also allows the combination of precise U–Th–Pb data from metamorphic monazite with thermobarometric information obtained from rock-forming minerals
Authigenic Pb isotopes from the Laurentian Fan: Changes in chemical weathering and patterns of North American freshwater runoff during the last deglaciation
No full textClimate-dependent chemical weathering trends have a strong impact on the dissolved Pb isotopic composition of continental runoff during glacial terminations, so that this tracer can be used to reconstruct the impact of the North American deglaciation on aspects of freshwater runoff and ocean chemistry. Here we present authigenic Fe–Mn oxyhydroxide-derived Pb isotope records from lower Laurentian Fan sediments that trace the local deglacial continental runoff signal through the Gulf of St. Lawrence. We use these records to investigate changes in the Pb isotopic composition of the North American runoff, and their relationship to deglacial processes.The new Pb isotope records are very similar to those from the subtropical deep NW Atlantic locations, though the new data are at higher resolution and exhibit much greater amplitudes of change due to their Laurentide Ice Sheet-proximal setting. Unradiogenic compositions (206Pb/204Pb as low as 18.8 at 18.5 ka) during the latest glacial change towards highly radiogenic compositions during the early Holocene (206Pb/204Pb = 20.1 at 11.2 ka). Late Holocene 206Pb/204Pb values return to intermediate values around 206Pb/204Pb = 19.6. These new data support previous inferences that the Pb isotopic composition of the deep Atlantic is controlled during deglaciation by changes in the pattern and intensity of continental chemical weathering. In more detail, the most significant change in the entire Pb isotope record starts at 12.5 ka and lasts until about 11 ka. This observation, of strong Pb isotopic changes across a climatically cold period in the North Atlantic region, suggests that at this location Pb isotopic compositions recorded in deep marine authigenic Fe–Mn oxyhydroxides not only trace deglacial changes in chemical weathering intensity but also highly depend on major freshwater runoff routes in interior North America. Our data suggest a gradual opening of the eastward freshwater runoff route in the course of the Younger Dryas
Variable Quaternary chemical weathering fluxes and imbalances in marine geochemical budgets
No full textRivers are the dominant source of many elements and isotopes to the ocean. But this input from the continents is not balanced by the loss of the elements and isotopes through hydrothermal and sedimentary exchange with the oceanic crust, or by temporal changes in the marine inventory for elements that are demonstrably not in steady state1, 2, 3, 4. To resolve the problem of the observed imbalance in marine geochemical budgets, attention has been focused on uncertainties in the hydrothermal and sedimentary fluxes1, 2, 3, 4. In recent Earth history, temporally dynamic chemical weathering fluxes from the continents are an inevitable consequence of periodic glaciations5, 6, 7, 8, 9. Chemical weathering rates on modern Earth are likely to remain far from equilibrium owing to the physical production of finely ground material at glacial terminations10, 11, 12, 13 that acts as a fertile substrate for chemical weathering. Here we explore the implications of temporal changes in the riverine chemical weathering flux for oceanic geochemical budgets. We contend that the riverine flux obtained from observations of modern rivers is broadly accurate, but not representative of timescales appropriate for elements with oceanic residence longer than Quaternary glacial–interglacial cycles. We suggest that the pulse of rapid chemical weathering initiated at the last deglaciation has not yet decayed away and that weathering rates remain about two to three times the average for an entire late Quaternary glacial cycle. Taking into account the effect of the suggested non-steady-state process on the silicate weathering flux helps to reconcile the modelled marine strontium isotope budget with available data. Overall, we conclude that consideration of the temporal variability in riverine fluxes largely ameliorates long-standing problems with chemical and isotopic mass balances in the ocean.<br/
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
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