1,721,037 research outputs found
Testing the Cretaceous greenhouse hypothesis using glassy foraminiferal calcite from the core of the Turonian tropics on Demerara Rise
No full textGlassy Turonian foraminifera preserved in clay-rich sediments from the western tropical Atlantic yield the warmest equivalent δ18O sea-surface temperatures (SSTs) yet reported for the entire Cretaceous-Cenozoic. We estimate Turonian SSTs that were at least as warm as (conservative mean ~30 °C) to significantly warmer (warm mean ~33 °C) than those in the region today. However, if independent evidence for high middle Cretaceous pCO2 is reliable and resulted in greater isotopic fractionation between seawater and calcite because of lower sea-surface pH, our conservative and warm SST estimates would be even higher (32 and 36 °C, respectively). Our new tropical SSTs help reconcile geologic data with the predictions of general circulation models that incorporate high Cretaceous pCO2 and lend support to the hypothesis of a Cretaceous greenhouse. Our data also strengthen the case for a Turonian age for the Cretaceous thermal maximum and highlight a 20–40 m.y. mismatch between peak Cretaceous-Cenozoic global warmth and peak inferred tectonic CO2 production. We infer that this mismatch is either an artifact of a hidden Turonian pulse in global ocean-crust cycling or real evidence of the influence of some other factor on atmospheric CO2 and/or SSTs. A hidden pulse in crust cycling would explain the timing of peak Cretaceous-Cenozoic sea level (also Turonian), but other factors are needed to explain high-frequency (~10–100 k.y.) instability in middle Cretaceous SSTs reported elsewhere
Diffuse hydrothermal fluids from Lucky Strike hydrothermal vent field: evidence for a shallow conductively heated system
No full textChemical analysis of 18 diffuse hydrothermal fluids collected from the Lucky Strike hydrothermal vent site reveals the presence of less than 7% high-temperature hydrothermal end-member. Temperature-Li and temperature-Mg relationships for the diffuse fluids indicated two sample groups. One group of three samples had temperatures which fell on or below the temperatures expected for conservative mixing between seawater and high-temperature vent fluids, consistent with mixing and cooling. The second grouping had diffuse fluid temperatures greater than expected from conservative mixing of high-temperature end-member fluids and seawater. This indicates that the diffuse hydrothermal effluent was heated conductively during its formation. The Lucky Strike site is characterized by a hydrothermal slab of basaltic fragments indurated by silica and barite. It is proposed that the diffuse fluids circulate within the cracks of the slab, forming a shallow short-pass hydrothermal convection system. The source of heat is likely to be high-temperature hydrothermal fluids circulating beneath the hydrothermal slab. The chemistry of the diffuse fluids shows no evidence for silica precipitation, suggesting that the slab is not actively forming from the diffuse fluids at the present. <br/
Linking basement carbonate vein compositions to porewater geochemistry across the eastern flank of the Juan de Fuca Ridge, ODP Leg 168
No full textLeg 168 of the Ocean Drilling Program (ODP) investigated the heat flow, fluid chemistry and crustal alteration associated with ridge flank hydrothermal systems. Ten sites were drilled on the eastern flank of the Juan de Fuca Ridge, along an 80 km transect, between 20 and 100 km east of the spreading centre. Recovered cores consisted of 100-500 m of sediment with shallow penetration (1.7-48.1 m) into the underlying igneous basement (0.8-3.6 Ma). Here we use the composition of calcium carbonate minerals, from veins within the upper basement, to reconstruct the evolving chemistry of hydrothermal fluids with increasing crustal age and sediment cover thickness. We show for the first time a clear link between the alteration of the basement rocks as recorded by secondary minerals, and the near-basement sedimentary pore fluids, which are often assumed to be representative of the basement fluids responsible for low temperature alteration of the upper crust. Carbonates precipitated from basement fluids that ranged in strontium isotopic composition from near-modern seawater (87Sr/86Sr[ap]0.70918) to the near-basement pore fluid values at any one site. 87Sr/86Sr ratios are independent of mineralogy with both aragonite and calcite precipitating from variably evolved fluids with the range in carbonate 87Sr/86Sr increasing with crustal age. A parallel geochemical evolution of basement fluids and sediment porewaters is shown since 87Sr/86Sr ratios of near-basement pore fluids decrease from 0.709013 to 0.707108 away from the ridge axis. A correlation exists between 87Sr/86Sr ratios and [delta]18O-calculated fluid temperatures, with more geochemically evolved carbonates having precipitated from warmer fluids. Basement fluid compositions, calculated from carbonate Sr, Mg, Fe and Mn concentrations combined with suitable partition coefficients, are also temperature-dependent. Given an observed increase in basement temperature with age, from 16[deg]C to 64[deg]C along the transect, a progressive chemical development of basement fluid is demonstrated. Carbonate veins in volcanic basement from ODP Holes 504B and 896A, on the Costa Rica Rift, record the same temperature compositional evolution of basement fluid as those from the Juan de Fuca Ridge flank. Although these locations have different thermal histories and therefore must have experienced different temporal geochemical evolution of basement fluid, basement temperature appears to be the dominant control on basement fluid composition
Hydrothermal alteration of the lower oceanic crust: Sr isotopic constraints from the CCSP CY-4 drill hole, Troodos Ophiolite, Cyprus (abstract of paper presented at AGU Fall Meeting, San Francisco, 8-12 Dec 2003)
No full textOptimization of an inductively coupled plasma-optical emission spectrometry method for the rapid determination of high-precision Mg/Ca and Sr/Ca in foraminiferal calcite
No full textThe ability to measure Mg/Ca and Sr/Ca in foraminiferal calcite has increased in importance following their identification as palaeoceanographic tracers with significant potential to constrain past changes in ocean chemistry and temperature. However, in order to tackle palaeoceanographic problems, it is essential to develop methods for the determination of these ratios that have both rapid sample throughput and high analytical precision. Here, we demonstrate the importance of simultaneous measurement for high-precision Mg/Ca and Sr/Ca analysis. Our method is readily applicable to automated analysis over a wide range of Ca concentrations and is not subject to significant matrix effects. Using a “dual view? inductively coupled plasma–optical emission spectrometer (ICP-OES), supported by a dedicated autosampler, we have investigated three nebulizers and two spray chambers and determined optimum analytical parameters. We show that there are consistent relationships between signal-to-noise ratio and gas flow rates and that the working sample dilution range has an important effect on linear response. Further, for this instrument it is important to manually constrain the timing of measurements to obtain truly simultaneous readings. Using the method developed, precisions of better than 0.21% are obtained for Mg/Ca and Sr/Ca in solutions containing between 1 and 4 ppm Ca with no significant matrix effect, using a Glass Expansion “Conikal? nebulizer coupled with a “Tracey? spray chamber, in autosampler mode
Linking basement vein compositions to porewater geochemistry across the eastern flank of the Juan de Fuca ridge, ODP Leg 168 (abstract of paper presented at AGU Fall Meeting, San Francisco, 8-12 Dec 2003)
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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
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