345 research outputs found
Densidades, tamanho de grupo e reprodução de emas no Pantanal Sul.
Este estudo sobre a ecologia das emas no Pantanal foi uma primeira experiĂȘncia na regiĂŁo, e teve o objetivo de avaliar as possibilidades de utilização da espĂ©cie nas fazendas do Pantanal da NhecolĂąndia. A população estimada, atravĂ©s de um levantamento aĂ©reo, foi de 6.500 emas adultas, em todo o Pantanal. Na fazenda Nhumirim foram encontrados 73 grupos de emas durante o estudo, e o nĂșmero de grupos variou ao longo do ano, de 2 a 17 indivĂduos. A razĂŁo sexual foi de 1 macho para 3,6 fĂȘmeas. Os ninhos foram feitos pelos machos, em ĂĄreas abertas e em ĂĄreas fechadas. Nos 2 anos do estudo foram encontrados 26 ninhos, e o nĂșmero de ovos variou de 5 a 25. O principal predador dos ninhos foi o tatu-peba. A população de emas no Pantanal estĂĄ bem conservada e existe possibilidade do uso sustentado da espĂ©cie.bitstream/item/37302/1/BP55.pd
Impact of permeability evolution in igneous sills on hydrothermal flow and hydrocarbon transport in volcanic sedimentary basins
Sills emplaced in organic-rich sedimentary rocks trigger the generation and
migration of hydrocarbons in volcanic sedimentary basins. Based on seismic
and geological observations, numerical modeling studies of hydrothermal flow
around sills show that thermogenic methane is channeled below the intrusion
towards its tip, where hydrothermal vents nucleate and transport methane to
the surface. However, these models typically assume impermeable sills and
ignore potential effects of permeability evolution in cooling sills, e.g.,
due to fracturing. Here, we combine a geological field study of a volcanic
basin (NeuquĂ©n Basin, Argentina) with a hybrid finite-elementâfinite-volume method (FEMâFVM) of numerical modeling
of hydrothermal flow around a sill, including hydrocarbon generation and
transport. Our field observations show widespread veins within sills
composed of graphitized bitumen and cooling joints filled with solid bitumen
or fluidized shale. Raman spectroscopy indicates graphitization at
temperatures between 350 and 500ââC, suggesting fluid flow within the
intrusions during cooling. This finding motivates our modeling setup, which
investigates flow patterns around and through intrusions that become porous
and permeable upon solidification. The results show three flow phases
affecting the transport of hydrocarbons generated in the contact aureole:
(1) contact-parallel flow toward the sill tip prior to solidification, (2)
upon complete solidification, sudden vertical âflushingâ of overpressured
hydrocarbon-rich fluids from the lower contact aureole towards and into the
hot sill along its entire length, and (3) stabilization of hydrocarbon
distribution and fading hydrothermal flow. In low-permeability host rocks,
hydraulic fracturing facilitates flow and hydrocarbon migration toward the
sill by temporarily elevating porosity and permeability. Up to 7.5â% of
the generated methane is exposed to temperatures >400ââC in the simulations and may thus be permanently stored as graphite in or
near the sill. Porosity and permeability creation within cooling sills may
impact hydrothermal flow, hydrocarbon transport, and venting in volcanic
basins, as it considerably alters the fluid pressure configuration, provides
vertical flow paths, and helps to dissipate overpressure below the sills.</p
Distinct degassing pulses during magma invasion in the stratified Karoo Basin â New insights from hydrothermal fluid flow modelling
Magma emplacement in organicârich sedimentary basins is a main driver of past environmental crises. Using a 2D numerical model, we investigate the process of thermal cracking in contact aureoles of cooling sills and subsequent transport and emission of thermogenic methane by hydrothermal fluids. Our model includes a MohrâCoulomb failure criterion to initiate hydrofracturing and a dynamic porosity/permeability. We investigate the Karoo Basin, taking into account hostârock material properties from borehole data, realistic total organic carbon content, and different sill geometries. Consistent with geological observations, we find that thermal plumes quickly rise at the edges of saucerâshaped sills, guided along vertically fractured high permeability pathways. Contrastingly, less focused and slower plumes rise from the edges and the central part of flatâlying sills. Using a novel upscaling method based on sillâtoâsediment ratio we find that degassing of the Karoo Basin occurred in two distinct phases during magma invasion. Rapid degassing triggered by sills emplaced within the top 1.5 km emitted ~1.6·103 Gt of thermogenic methane, while thermal plumes originating from deeper sills, carrying a 12âtimes greater mass of methane, may not reach the surface. We suggest that these large quantities of methane could be reâmobilized by the heat provided by neighboring sills. We conclude that the Karoo LIP may have emitted as much as ~22.3·103 Gt of thermogenic methane in the half million years of magmatic activity, with emissions up to 3 Gt/year. This quantity of methane and the emission rates can explain the negative ÎŽ13C excursion of the Toarcian environmental crisis.
Key Points
Sill geometry and emplacement depth as well as intruded host rock type are the main factors controlling methane mobilization and degassing
Dehydrationârelated porosity increase and poreâpressureâinduced hydrofracturing are important mechanisms for a quick transport of methane from sill to the surface
The Karoo Basin may have degassed ~22.3·103 Gt of thermogenic methane in the half million years of magmatic activit
Arc magmas sourced from melange diapirs in subduction zones
Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 5 (2012): 862-867, doi:10.1038/ngeo1634.At subduction zones, crustal material is recycled back into the mantle. A certain proportion, however, is returned to the overriding
plate via magmatism. The magmas show a characteristic range of compositions that have been explained by three-component
mixing in their source regions: hydrous fluids derived from subducted altered oceanic crust and components derived from the thin
sedimentary veneer are added to the depleted peridotite in the mantle beneath the volcanoes. However, currently no uniformly
accepted model exists for the physical mechanism that mixes the three components and transports them from the slab to the
magma source.
Here we present an integrated physico-chemical model of subduction zones that emerges from a review of the combined findings
of petrology, modelling, geophysics, and geochemistry: Intensely mixed metamorphic rock formations, so-called mélanges, form
along the slab-mantle interface and comprise the characteristic trace-element patterns of subduction-zone magmatic rocks. We
consider mélange formation the physical mixing process that is responsible for the geochemical three-component pattern of the
magmas. Blobs of low-density mélange material, so-called diapirs, rise buoyantly from the surface of the subducting slab and
provide a means of transport for well-mixed materials into the mantle beneath the volcanoes, where they produce melt. Our model
provides a consistent framework for the interpretation of geophysical, petrological and geochemical data of subduction zones.H.M. was funded
by the J. LamarWorzel Assistant Scientist Fund and the
Penzance Endowed Fund in Support of Assistant Scientists.
Funding from NSF grant #1119403 (G. Harlow)
is acknowledged.2013-05-1
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