Search results for "502"
showing 10 items of 7391 documents
The Influence of Crystal Size Distributions on the Rheology of Magmas: New Insights From Analog Experiments
2017
This study examines the influence of particle size distributions on the rheology of particle suspensions by using analogue experiments with spherical glass beads in silicone oil as magma equivalent. The analyses of 274 individual particle-bearing suspensions of varying modality (uni-, bi- tri- and tetramodality), as well as of polymodal suspensions with specific defined skewness and variance, are the first data set of its kind and provide important insights in the relationship between the solid particles of a suspension and its rheological behaviour. Since the relationship between the rheology of particle bearing suspensions and its maximum packing fraction ϕm is well established by several…
Linking tephrochronology and soil characteristics in the Sila and Nebrodi mountains, Italy
2017
Recent studies have demonstrated that soils formed on pyroclastic ash deposits are much more common in the Mediterranean area than previously assumed. These soils are an important key to understanding past volcanic events and landscape evolution. Chronological information in soils of Quaternary volcanic events, however, remains still poorly understood in southern Italy. Using a multi-method forensic approach, we explore the origin and age of volcanic deposits (soils) in Sicily and Calabria. The geochemical signature of the soil was compared to the chemical fingerprint of the magmas of potential source areas of southern Italian volcanoes. The results indicate that the investigated soils on t…
A record of seafloor methane seepage across the last 150 million years
2020
<p>Methane seepage at the seafloor is a source of carbon in the marine environment and has long been recognized as an important window into the deep geo-, hydro-, and bio-spheres. However, the processes and temporal patterns of natural methane emission over multi-million-year time scales are still poorly understood. The microbially-mediated methane oxidation leads to the precipitation of authigenic carbonate minerals within subseafloor sediments, thus providing a potentially extensive record of past methane emission. In this study, we used data on methane-derived authigenic carbonates to build a proxy time series of seafloor methane emission over the last 150 My. We quantitat…
Interpretation of the nitrogen isotopic composition of Precambrian sedimentary rocks: Assumptions and perspectives
2016
International audience; Nitrogen isotope compositions in sedimentary rocks (d(15)N(sed)) are routinely used for reconstructing Cenozoic N-biogeochemical cycling and are also being increasingly applied to understanding the evolution of ancient environments. Here we review the existing knowledge and rationale behind the use of d(15)N(sed) as a proxy for the Precambrian N-biogeochemical cycle with the aims of (i) identifying the major uncertainties that affect analyses and interpretation of nitrogen isotopes in ancient sedimentary rocks, (ii) developing a framework for interpreting the Precambrian d(15)N(sed) record, (iii) testing this framework against a database of Precambrian d(15)N(sed) va…
Ocean Acidification and the End-Permian Mass Extinction: To What Extent does Evidence Support Hypothesis?
2012
International audience; Ocean acidification in modern oceans is linked to rapid increase in atmospheric CO 2 , raising concern about marine diversity, food security and ecosystem services. Proxy evidence for acidification during past crises may help predict future change, but three issues limit confidence of comparisons between modern and ancient ocean acidification, illustrated from the end-Permian extinction, 252 million years ago: (1) problems with evidence for ocean acidification preserved in sedimentary rocks, where proposed marine dissolution surfaces may be subaerial. Sedimentary evidence that the extinction was partly due to ocean acidification is therefore inconclusive; (2) Fossils…
Essential features of the polytypic charoite-96 structure compared to charoite-90
2011
AbstractCharoite, ideally (K,Sr,Ba,Mn)15–16(Ca,Na)32[(Si70(O,OH)180)](OH,F)4·nH20, is a rock-forming mineral from the Murun massif in Yakutia, Sakha Republic, Siberia, Russia, where it occurs in a unique alkaline intrusion. Charoite occurs as four different polytypes, which are commonly intergrown in nanocrystallme fibres. We report the structure of charoite-96(a =32.11(6),b =19.77(4),c =7.23(1) Å, β = 95.85(9)°,V =4565(24) Å3, space groupP21/m),which was solvedab initioby direct methods on the basis of 2676 unique electron diffraction reflections collected by automated diffraction tomography and refined toR1/wR2=0.34/0.37. The structure of charoite-96 is related to that of the charoite-90,…
Environmental change during the Early Cretaceous in the Purbeck-type Durlston Bay section (Dorset, Southern England): a biomarker approach.
2007
20 pages; International audience; The Purbeck-type section (Durlston Bay, Dorset, UK) exhibits littoral lagoonal to lacustrine facies. It shows a gradual climatic/environmental change from semi-arid conditions associated with evaporites at the Jurassic–Cretaceous transition, to a more humid climate at the end of the Berriasian. Though generally organic-poor (total organic carbon, TOC, <1.3%), the Durlston Bay section shows an organic-rich episode (TOC up to 8.5%) located at the transition from evaporitic to more humid facies. A biomarker study was performed in order to determine the origin of the organic matter (OM) in the section and see if changes in organic sources accompanied the genera…
Impact of basin burial and exhumation on Jurassic carbonates diagenesis on both sides of a thick clay barrier (Paris Basin, NE France).
2014
27 pages; International audience; Several diagenetic models have been proposed for Middle and Upper Jurassic carbonates of the eastern Paris Basin. The paragenetic sequences are compared in both aquifers to propose a diagenetic model for the Middle and Late Jurassic deposits as a whole. Petrographic (optical and cathodoluminescence microscopy), structural (fracture orientations) and geochemical (δ18O, δ13C, REE) studies were conducted to characterize diagenetic cements, with a focus on blocky calcite cements, and their connection with fracturation events. Four generations of blocky calcite (Cal1-Cal4) are identified. Cal1 and Cal2 are widespread in the dominantly grain-supported facies of t…
The behaviour of zirconium, hafnium and rare earth elements during the crystallisation of halite and other salt minerals
2017
Abstract Halite crystals from Messinian and Tortonian evaporites from Sicily and Spain and current precipitated halite crystals and the relative parent brines (active evaporation systems) were investigated in order to evaluate the Zr, Hf and Rare Earth Element (REE) behaviour. Halite crystallisation from evaporating brines fractionates Zr, Hf and REE through a two-step process. During the first step, dissolved complexes of studied elements are scavenged onto the surfaces of crystallising halite. During the second step, elements are co-precipitated into the crystal lattice as it grows. The first step mechanism is determined by the dissolved REE speciation. In saltworks where carbonate-REE co…
Zr/Hf ratio and REE behaviour: A coupled indication of lithogenic input in marginal basins and deep-sea brines
2019
Abstract The distribution of dissolved Zr, Hf and Rare Earth Elements (yttrium and lanthanides, hereafter referred to as REE) in the Eastern Mediterranean seawater column was measured in the Kryos basin to evaluate the lithogenic contribution from both Nile River and Sahara and Arabian desert dust. We found dissolved Zr/Hf ratios below the signature of crustal rocks and chondrites; a phenomenon likely driven by the dissolution of the Mn-rich coating of atmospheric dust particles delivered from the desert. In deeper waters, Zr/Hf ratios are clustered close to the signature of crustal rocks and chondrites according to the different Zr and Hf dissolved speciation. The Zr/Hf ratio observed in t…