401 research outputs found
Pumice from volcanoes: its possible role fĂĽr the iron and silica budget of the surface ocean and diatom growth
Photoacoustics Modelling using Amplitude Mode Expansion Method in a Multiscale T-cell Resonator
The photoacoustic (PA) effect consisting of the generation of an acoustic
signal based on the absorption of light has already demonstrated its potential
for various spectroscopic applications for both gaseous and solid samples. The
signal produced during photoacoustic spectroscopy (PAS) measurement is,
however, usually weak and needs to be amplified. This is achieved by using a
photoacoustic cell resonator where acoustic resonances are utilized to
significantly boost the signal. Therefore, a PA resonator has a significant
role in PAS measurement set-ups. When designing or optimizing a new PA
resonator, numerical methods are generally used to simulate the photoacoustic
signal generation. In this paper, the amplitude mode expansion (AME) method is
presented as a quick and accurate simulation tool. The method is used to
simulate the photoacoustic signal in a multi-scale T-cell resonator over a wide
frequency range. The AME method is based on eigenmode expansion and
introduction of losses by quality factors. The AME simulation results are
compared and analyzed against the results from the viscothermal method.
Reasonably good agreement is obtained between the two methods. However, small
frequency shifts in the resonances of the AME method are noted. The shifts are
attributed to the location of the dominant mode within the T-cell. The
viscothermal method is considered the most accurate method for simulating the
photoacoustic signal in small resonators. However, it is computationally very
demanding. The AME method provides a much faster simulation alternative. This
is particularly useful in the design and optimization of photoacoustic
resonators where numerical methods are preferred over experimental measurements
due to their speed and low cost.Comment: Comsol Conference 201
Recycling of geochemically heterogenous oceanic crust: significance for the origin of ocean island basalts
Evidence for Miocene subduction beneath the Alboran Sea (Western Mediterranean) from 40Ar/39Ar age dating and the geochemistry of volcanic rocks from holes 977A and 978A
Volcanic pebbles in gravels from Sites 977 and 978 in the Alboran Sea (western Mediterranean) were dated (using the 40Ar/
39Ar single-crystal laser technique) and analyzed for their major- and trace-element compositions (determined by X-ray fluorescence
and inductively coupled plasma-mass spectrometry). The samples range from basalts to rhyolites, and belong to the
tholeiitic, calc-alkaline, and shoshonitic series. Single-crystal and step-heating laser 40Ar/39Ar analyses of plagioclase, sanidine,
biotite, and amphibole phenocrysts from basaltic to rhyolitic samples indicate that eruptions occurred between 6.1 and 12.1 Ma.
The age data conform to the stratigraphy and agree with microfossil ages, when available. The major-element and compatible
trace-element data of samples with H2O < 4 wt% show systematic variations, consistent with fractionation of the observed phenocryst
phases (plagioclase, olivine, clinopyroxene, magnetite, hornblende, quartz, and biotite). The incompatible-element patterns
formed by normalizing to primitive mantle for all samples show spiked patterns with peaks generally at mobile elements
and troughs at immobile elements, in particular Nb and Ta. The calc-alkaline affinities and the incompatible-element systematics
are characteristic of subduction zone volcanism, which indicates that subduction occurred beneath the eastern Alboran from
6 to at least 12 Ma. We propose that the change in chemistry from calc-alkaline and potassic to sodic compositions between 5–
6 Ma reflects detachment of the subducting slab. Uplift of the Strait of Gibraltar, associated with this detachment, could have
caused the Messinian Salinity Crises
Flow of Canary mantle plume material through a subcontinental lithospheric corridor beneath Africa to the Mediterranean: REPLY
Volcanic ash as fertiliser for the surface ocean
Iron is a key limiting micro-nutrient for marine primary productivity. It can be supplied to the ocean by atmospheric dust deposition. Volcanic ash deposition into the ocean represents another external and so far largely neglected source of iron. This study demonstrates strong evidence for natural fertilisation in the iron-limited oceanic area of the NE Pacific, induced by volcanic ash from the eruption of Kasatochi volcano in August 2008. Atmospheric and oceanic conditions were favourable to generate a massive phytoplankton bloom in the NE Pacific Ocean which for the first time strongly suggests a connection between oceanic iron-fertilisation and volcanic ash supply
Density-dependent adjustment of inducible defenses
International audiencePredation is a major factor driving evolution, and organisms have evolved adaptations increasing their survival chances. However, most defenses incur trade-offs between benefits and costs. Many organisms save costs by employing inducible defenses as responses to fluctuating predation risk. The level of defense often increases with predator densities. However, individual predation risk should not only depend on predator density but also on the density of conspecifics. If the predator has a saturating functional response one would predict a negative correlation between prey density and individual predation risk and hence defense expression. Here, we tested this hypothesis using six model systems, covering a taxonomic range from protozoa to rotifers and crustaceans. In all six systems, we found that the level of defense expression increased with predator density but decreased with prey density. In one of our systems, i.e. in Daphnia, we further show that the response to prey density is triggered by a chemical cue released by conspecifics and congeners. Our results indicate that organisms adjust the degree of defense to the acute predation risk, rather than merely to predators’ densities. Our study suggests that density-dependent defense expression reflects accurate predation-risk assessment and is a general principle in many inducible-defense systems
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