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Comparing machine learning isoscapes of 87Sr/86Sr ratios of plants on the island of Sardinia: Implications for the use of isoscapes for assessing the provenance of biological specimens
Get PDFStrontium isotope analysis is widely used to evaluate the provenance and mobility of biological specimens. Frequently applied in archaeology, palaeontology, ecology, forensics, and food science, strontium isotope analysis compares the 87Sr/86Sr ratio of a specimen against a strontium isoscape – a representation of expected 87Sr/86Sr ratios across a landscape – to identify areas that are more and/or less likely to be the source of the specimen. Strontium isoscapes are built using different methods, but all approaches start with empirical 87Sr/86Sr ratios sampled from areas with known coordinates and use them to assign likely 87Sr/86Sr ratios to unknown areas. Following the publication of Bataille et al., 2018 and Bataille et al., 2020, machine learning using a random forest algorithm has become a common method of producing strontium isoscapes. Despite the recognition that this method requires training with local ratios, especially in geologically complex regions, very little work has evaluated machine learning isoscapes' accuracy. This study compares and evaluates two previously published machine learning isoscapes of Sardinia against new empirical data provided by the project ZANBA. The ZANBA data is then used to create a third machine learning map of Sardinia, which is tested against previously published empirical data. The three isoscapes show different levels of predictive accuracy, with more primary data points leading to more correct predictions. However, a densely sampled landscape did not create an isoscape that gave substantially more accurate predictions than a moderately densely sampled landscape when tested against primary data from outside the original sampling areas. Areas of an isoscape with low root mean squared error (RMSE), which is often interpreted as indicating accuracy, did not necessarily give more correct predictions. Finally, a machine learning isoscape of Sardinia that incorporates both new and previously published empirical data is presented
Trace Metal Variations through the Tartan and Waipawa Formations: Implications for the Environment of Deposition
No full textAn inorganic geochemical study of the Late Paleocene organic matter-rich Waipawa and Tartan formations was undertaken in order to investigate the depositional environment. The formation varies in thickness between 2 and 50 metres and is distributed across many of New Zealand’s Cenozoic basins, where it forms an important
potential hydrocarbon source rock. This study measured major and trace elements which can be loosely grouped into redox sensitive, biologically influenced, terrestrially sourced, and rare earth elements (REE). The study focused on three sections through the Waipawa and Tartan formations: Angora Quarry in the East Coast Basin, and the Great
South Basin hydrocarbon exploration wells Kawau-1A and Pakaha-1.
At Angora Quarry, x-ray fluorescence (XRF) was used to measure the major constituents Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, SO₃, K₂O, CaO, TiO₂, MnO and Fe₂O₃. inductively-coupled plasma mass spectrometry (ICP-MS) was used to measure Li, Ca, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, Ge, As, Rb, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Ba, REE, Hf, Tl, Pb, Th and U.
For Pakaha-1 and Kawau-1A side wall core samples, ICP-MS was used to
measure Ti, V, Cr, Mn, Co, Ni, Cu, As, Se, Rb, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Hf, Ta, W, Tl, Pb, Bi, Th and U. Insufficient sample was available for XRF on these samples.
No major changes in oxygen concentration during deposition were recorded by redox-sensitive elements from Angora Quarry and Pakaha-1 sediments; however samples from Kawau-1A and from a section 1 km upstream from Angora Quarry were deposited under somewhat oxygen-depleted conditions. As the anoxic and suboxic indicators show significantly lower variations than under present day anoxic
environments, and in Angora Quarry CaO and SO₃ are significantly depleted with higher aluminosilicates a rapid deposition is required to explain the preservation of the organic matter. In the Great South Basin wells, the clay content correlates directly with increased gamma ray levels measured by well logs. Increased influx of terrestrial clays has been linked to marine transgressions in many New Zealand sediments and is been taken to mean the same for the Waipawa and Tartan formations. The oxygen depletion indicates that water depths during deposition
exceeded 50 metres. The depositional model proposed here, therefore, is that of a major marine transgression that flooded and eroded near-shore swamps, re-depositing the terrestrial organic matter offshore. The increased nutrients released by this would have stimulated bioproductivity and locally, where conditions were suitable, depleted the
oxygen content of the water column. This study also suggests ternary diagrams are valuable for calculating the enrichment of elements affected by two processes, such as Sr, which is related to both
detrital Al and related to biological Ca. Ga, Ba and Al content are also related on a ternary diagram indicating the similar terrestrial and biological relationships for Ba and Ga. W was found to behave in a similar way to Bi. Enrichment factors proved less useful than absolute enrichment for Kawau-1A, where detrital input varied greatly and
was found to be significantly different in composition to average shale as defined by Wedephol (1971)
Assembling Dacite in a Continental Subduction Zone: A Case Study of Tauhara Volcano
No full textMount Tauhara is the largest dacitic volcanic complex of onshore New Zealand and comprises seven
subaerial domes and associated lava and pyroclastic flows, with a total exposed volume of ca. 1 km3. The
dacites have a complex petrography including quartz, plagioclase, amphibole, orthopyroxene,
clinopyroxene, olivine and Fe‐Ti oxides and offer an excellent opportunity to investigate the processes
and timescales involved in assembling dacitic magma bodies in a continental subduction zone with in
situ and mineral specific analytical techniques.
Whole rock major and trace element data and Pb isotopes ratios define linear relationships indicating
that the dacites are generated by mixing of silicic and mafic magmas. Two groups of samples define
separate mixing trends between four endmembers on the basis of La/Yb ratios, 87Sr/86Sr ratios and Sr
contents. The older Western and Central Domes have low 87Sr/86Sr (0.7042‐0.7046) and high LREE/HREE
(LaN/YbN = 8.0‐11.5) and Sr (380‐650 ppm) compared to the younger Hipaua, Trig M, Breached and Main
Domes, which have higher 87Sr/86Sr (0.7047‐0.7052) and lower LREE/HREE (LaN/YbN = 6.5‐7.5) and Sr
(180‐400 ppm).
In situ mineral major and trace element chemistry of mineral phases, as well as Sr and Pb isotope ratios
of mineral separates have been used to: (i) fingerprint the origin of each crystal phase; (ii) constrain the
chemistry of the four endmembers involved in the mixing events and; (iii) estimate the timing of mixing
relative to eruption and the ascent rate of the dacitic magmas. The presence of quartz and analyses of
quartz‐hosted melt inclusions are used to fingerprint the chemistry of the silicic endmembers, which is a
rhyolitic melt with a major element chemistry similar to that of either the Whakamaru Group Ignimbrite
melts (Western, Central and Trig M Domes) or intermediate between that of the Whakamaru and the
Oruanui Ignimbrite melts (Hipaua, Breached and Main Domes). Similarly, Ba‐Sr concentrations and Sr
isotopic signatures of plagioclase show that this phenocryst phase also predominantly crystallized from
the rhyolitic melt. Variations in the Mg# and trace element chemistry of clinopyroxenes suggest they
were formed both in the mixed dacitic melts and in a mafic endmember. The chemistry of the mafic
endmembers have been traced using a combination of back‐calculated Sr melt concentrations from
clinopyroxene with the highest Mg# in each sample group, and the linear trends between whole rock
SiO2 content and most elements. These results indicate that dacites erupted from the Western and
Central Dome were generated by the mixing of a high alumina basalt and a rhyolitic melt and Trig M
Dome dacites were generated by the mixing of an andesite with a rhyolitic melt. Magmas erupted from
Hipaua, Breached and Main Domes were also produced by the mixing of an andesitic melt and a rhyolitic
body with a composition intermediate between that of the Whakamaru and the Oruanui melt bodies.
Trace element data and 87Sr/86Sr ratios of amphibole demonstrate that it crystallized from the mixed
dacitic melt. Thermobarometric conditions obtained from amphibole indicate that the magma mixing
event that produced the dacites occurred within a magma chamber located at ca. 9 km depth and ca.
900°C with the exception of Trig M Dome which occurred deeper at 13 km and 950°C. Diffusion profiles
of Ti in quartz and Fe‐Mg in clinopyroxene indicate the magma mixing events occurred < 6 months prior
to eruption. Amphibole reaction rims show the magma to have ascended over 2‐3 weeks for each dome,
with the exception of Main Dome where reaction rims were not present in the amphibole, suggesting
the ascent rate was faster than 0.2 m/s (< 6 hours)
Ultra-precise titanium stable isotope measurements by double-spike high resolution MC-ICP-MS
No full textIn this contribution, we present a new technique for the ultra-precise determination of titanium stable isotope composition (expressed as δ49Ti or deviation of the 49Ti/47Ti ratio to the reference standard) of geological samples by multi-collection plasma source mass spectrometry (MC-ICPMS) using the double spike method to correct for instrumental mass bias. Tails of polyatomic spectral interferences on 46Ti are accounted for by using sample-standard bracketing in high-resolution mode. Choice of ideal double and triple spike composition is investigated and results show that analytical error for a single measurement is optimised for a 47Ti–49Ti double spike composed of ca. 50% of each spike and mixed with ca. 52% of sample. Measurements of pure Ti solution show that internal error on single measurements of ca. 0.010‰ (95% c.i.) is attainable on δ49Ti, in agreement with the error model. Due to the lack of a widely available reference isotopic standard for titanium, all results are expressed as deviations relative to newly created reference material (OL-Ti standing for Origins Laboratory – titanium) prepared from an ultra-pure titanium metal rod. A range of analytical tests demonstrates the robustness of our method. An external reproducibility of ca. 0.020‰ (2sd) is routinely achievable for Ti stable isotopes. Data for a range of basaltic rock standards as well as a subduction zone basalt reference suite is presented and show that the Ti stable isotope compositions of terrestrial basalt show resolvable variations but are overall very close to the OL-Ti reference standard. The average Ti isotopic composition of the basalts studied here is the present best estimate of the upper mantle composition; δ49Ti = +0.004 ± 0.062‰ (2sd)
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Interactions de faibles profondeurs et géochimie des basaltes d'îles océaniques : implications sur les modes d'acquisition de la signature isotopique et sur la topogique mantellique
No full textSystematic measurements of Sr-Nd-Pb isotopic compositions in Oceanic Island Basalts (OIB) for 30 years have shown the complex nature of mantle sources. However, measured variations in these samples can be related either to variable proportions of the components present in the deep source of the basalts, or to interactions between the pristine melts and the environments encountered during their ascent to the surface during partial melting, plume-lithosphere interaction, assimilation and fractional crystallization or post-eruption seawater interaction processes. Therefore, extracting the signature of shallow-level interactions from the global geochemical signature is a key to retrieving the source information and to investigating mantle topology in greater details. Means to identify and see through these shallow-level interactions are first presented before being applied to 2 case studies done on 2 islands in different geodynamic setting (São Nicolau Island, Cape Verde archipelago, intraplate setting and old lithosphere; São Jorge Island, Azores archipelago, Plume-ridge interaction setting on a young lithosphere). Major and trace elements as well as Sr-Nd-Pb isotopes measurements show that an important part of the isotopic variations can be explained by shallow-level interactions rather than source heterogeneity. These results are extended from the island-scale to the archipelago-scale in both cases. Identification of such processes in locality playing a key role in the definition of mantle topology allows generalizing the conclusions of the 2 local studies to a global scale. On this basis, the existence and nature of a sub-primitive component common to all OIB (FOZO, PHEM, C), inferred from non-filtered OIB data compilation, is then discussed.Les compositions isotopiques de Sr, Nd et Pb des basaltes d'îles océaniques ont été mesurées de manière systématique depuis 30 ans. Les variations de ces compositions sont classiquement interprétées comme la signature du mélange entre des composants présents au sein des panaches mantelliques. Cependant elles peuvent aussi représenter des interactions entre les magmas et les réservoirs qu'ils traversent au cours de leurs remontées vers la surface. Après avoir mis en évidence les processus pouvant altérer la composition initiale d'un magma et proposer des moyens de s'en affranchir, 2 études locales sont éffectuées (îles de Sao Nicolau et de Sao Jorge, archipels du Cap Vert et des Açores). Ces deux études montrent la part importante des variations d'origine superficielle dans ces deux localités. Ces résultats sont ensuite généralisés à d'autres localités clés de la topologie mantellique et permettent de discuter d'un composant primitif ubiquiste dans les panaches mantellique
Variations on the Author
Get PDF“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
Get PDFWe 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
Get PDFWe 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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