1,721,050 research outputs found
Controls on the solute geochemistry of subglacial discharge from the Russell Glacier, Greenland Ice Sheet determined by radiogenic and stable Sr isotope ratios
No full textWe used radiogenic and stable Sr isotope ratios (87Sr/86Sr and δ88/86Sr) to examine controls on solute acquisition in subglacial discharge from the Russell Glacier, a land-terminating lobe of the Greenland Ice Sheet (GrIS) located in western Greenland. The study focused on two melt seasons in 2014 and 2015. Subglacial discharge 87Sr/86Sr ratios are 13,000 ppm higher than those measured for bedload and suspended sediment digests, and are more similar to those of bedload sediment leachates. These results point to the preferential dissolution of minerals with high 87Sr/86Sr ratios. Analyses of mineral separates from bulk rocks demonstrate that biotite, chlorite, hornblende, and K-feldspar have relatively high 87Sr/86Sr ratios. Subglacial discharge δ88/86Sr values are ∼0.10‰ higher than those for bedload and suspended sediment digests. Isotope fractionation during secondary mineral formation and/or adsorption cannot account for the difference between subglacial discharge and bedrock δ88/86Sr values, as suspended and bedload sediment leachates and digests produced similar δ88/86Sr values and are within the range for bulk silicate Earth. Consistent with the interpretation of 87Sr/86Sr ratios, we attribute the difference to the preferential dissolution of minerals with high δ88/86Sr values. Mineral separates display a wide range of δ88/86Sr values (∼0.40‰). Those having high δ88/86Sr values include hornblende and K-feldspar, as well as apatite and titanite. Taken together, the preferential weathering of predominately silicate minerals explains the Sr isotope geochemistry of subglacial discharge.Subglacial discharge from the Russell Glacier feeds the proglacial Akuliarusiarsuup Kuua River (AKR). Along a 32 km transect of the AKR from the GrIS margin toward the coast, riverine 87Sr/86Sr ratios increase from ∼0.722 to ∼0.747 in an approximately step-wise pattern that corresponds to point-source inputs of additional subglacial discharge. Major cation concentrations and 87Sr/86Sr ratios minimally vary along lengths of the transect with no hydrological inputs. This suggests that proglacial chemical weathering is negligible and likely does not contribute significantly to GrIS solute fluxes.In general, this study supports the recent contention that silicate mineral weathering dominates the solute geochemistry of GrIS subglacial discharge in contrast to valley glaciers, which typically show substantial contributions from carbonate and sulfide weathering regardless of primary bedrock composition. Ice sheet subglacial chemical weathering may therefore have a greater impact on long-term atmospheric CO2 drawdown than previously realized
Silicate versus carbonate weathering in Iceland: New insights from Ca isotopes
No full text) to trace sources of Ca in Icelandic rivers. We report elemental and Ca isotope data for rivers, high- and low-temperature groundwater, basalt, hydrothermal calcite (including Iceland Spar), and stilbite and heulandite, which are two types of zeolites commonly formed during low-grade metamorphism of basalt. In agreement with previous research, we find that rivers have higher ?44/40Ca values than basalt, with a maximum difference of ?0.40‰. This difference may reflect isotope fractionation in the weathering zone, i.e., preferential uptake of 40Ca during clay mineral formation, adsorption, and other geochemical processes that cycle Ca. However, calcite ?44/40Ca values are also up to ?0.40‰?higher than bedrock values, and on a diagram of ?44/40Ca versus Sr/Ca, nearly all waters plot within a plausible mixing domain bounded by the measured compositions of basalt and calcite, with glacial rivers plotting closer to calcite than non-glacial rivers. Calcite and heulandite form during hydrothermal alteration of basalt in the deep lava pile and often occur together in metabasalts now exposed at the surface. Because heulandite ?44/40Ca values are ?1–2‰?lower than basalt, we suggest that 40Ca uptake by heudlandite explains the relatively high ?44/40Ca values of calcite and that calcite weathering in turn elevates riverine ?44/40Ca values. High mechanical erosion rates are known to facilitate the exposure and weathering of calcite, which explains the isotopic contrast between glacial and non-glacial watersheds. Using a mixing model, we find that calcite weathering provides ?0–65% of the Ca in non-glacial rivers and ?25–90% of the Ca in glacial rivers, with silicate weathering providing the remainder. Icelandic hydrothermal calcite contains mantle carbon. Noting that zeolite facies metamorphism and hydrothermal fluid circulation are ubiquitous characteristics of basaltic eruptions and assuming that hydrothermal calcite in other basaltic settings also contains mantle carbon, we suggest that the contribution of basalt weathering to long-term CO2 drawdown and climate regulation may be less significant than previously realized
Radiogenic and stable Sr isotope ratios (87Sr/86Sr, d88/86Sr) as tracers of riverine cation sources and biogeochemical cycling in the Milford Sound region of Fiordland, New Zealand
No full textThis study reports radiogenic Sr isotope ratios (87Sr/86Sr), stable Sr isotope ratios (?88/86Sr), and major ion concentrations for river, rock, sediment, soil, and plant samples collected from the Cleddau and Hollyford catchments in the Milford Sound region of Fiordland, New Zealand. The catchments primarily drain gabbro, but some tributaries access limestone and volcanogenic sediments. The goal of the study was to understand controls on riverine ?88/86Sr values in a landscape with multiple factors that may influence chemical weathering, including dense vegetation, high rainfall, and abundant, freshly-eroded Holocene fluvio-glacial and landslide debris.Rivers draining gabbro have higher ?88/86Sr values than bedrock, by as much as ?0.14‰, and the ?88/86Sr values strongly correlate with molar Ca/Sr ratios (R2 = 0.69). Leaching of rocks and sediment reveals no evidence for the preferential dissolution of minerals having high ?88/86Sr values and Ca/Sr ratios. In-stream Sr isotope fractionation seems unlikely because comparison against 87Sr/86Sr and Ca/Sr ratios demonstrates that riverine ?88/86Sr values conservatively trace water-mass mixing. The riverine data are best explained by the input of soil water, which is distinct from potential bedrock end-members (i.e., silicates and carbonates) based on ?88/86Sr but indistinguishable in terms of Ca/Sr and 87Sr/86Sr. While strontium isotope fractionation during secondary mineral formation and pedogenesis is possible, clay mineral formation is minor and most soils are poorly developed. Instead, soil water ?88/86Sr values more likely reflect plant uptake. Plant samples yielded a wide range of ?88/86Sr values, but on average, they are lower than those for bedrock, consistent with the expectation that plants preferentially incorporate lighter Sr isotopes. Mass-balance constraints, together with 87Sr/86Sr ratios, indicate that soil water ?88/86Sr values are ?0.30‰ higher than bedrock ?88/86Sr values, and mixing calculations show that the plant-fractionated soil water pool contributes ?27% of the riverine Sr. For tributaries accessing limestone and volcanogenic sediments, Ca/Sr and 87Sr/86Sr ratios appear consistent with two-component mixing between silicate and carbonate weathering, but ?88/86Sr values reveal a third contribution from soil water inputs, similar to gabbro catchments.The results of this study suggest that Sr isotopes behave conservatively during water mass mixing and stream transport but non-conservatively in soil, where plant uptake can elevate soil water ?88/86Sr values relative to bedrock. Plant uptake, or related biogeochemical processes, such as ion-exchange on organic matter surfaces, also appear to modify soil water Ca/Sr ratios. Many studies use 87Sr/86Sr and Ca/Sr ratios to apportion riverine solutes between silicate and carbonate weathering, but Ca/Sr ratios may be non-conservative in densely vegetated areas. The stable Sr isotope tracer shows promise for resolving riverine cation sources, as well as effects from biological cycling
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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