Geophysical Research Letters | EPFL Graph Search

Geophysical Research Letters est une revue scientifique bimensuelle de géosciences à comité de lecture publiée par l'American Geophysical Union et créée en 1974. En 2020, son rédacteur en chef est Harihar Rajaram, professeur à l'Université Johns-Hopkins. Objectifs et portée La revue vise à publier rapidement des rapports de recherche concis sur une ou plusieurs des disciplines couvertes par l'American Geophysical Union, telles que les sciences de l'atmosphère, la Terre solide, les sciences spatiales, l'océanographie, l'hydrologie, les processus de surface terrestre et la cryosphère. La revue publie également des analyses par invitation qui couvrent les avancées réalisées au cours des deux ou trois dernières années. Le lectorat cible est la communauté des sciences de la Terre, la communauté scientifique au sens large et le grand public. Indexation Cette revue est indexée dans les bases suivantes : Selon les Journal Citation Reports, la revue a un facteur d'impa

An evaluation of surface micro and meso plastic pollution in pelagic ecosystems of western Mediterranean Sea

Luiz Felippe De Alencastro, Florian Faure, Pascal Hagmann

In the 90’s, the discovery of high concentrations of plastic debris in the oceans generated interest and concern of general public as well as scientific community. Since, the accumulation of plastic debris in the North Pacific and North Atlantic gyres has been widely reported. Surprisingly, only few quantitative studies exist concerning the Mediterranean Sea. The purpose of this study was to examine the distribution, abundance and characteristics of surface micro (size 0.33 – 5.0 mm) and meso (size 5.0 – 50 mm) plastic debris, as compared to the abundance of organic material, in the western Mediterranean Sea. 41 samples have been collected during 2 cruises in September 2011 and August 2012 in regions distributed between positions 38-44°N and 001-010°E (Gulf of Lion, Eastern Spanish coast, Balearic Islands, Sardinia, Corsica). Sample collection was performed using a surface net tow of 330 μm mesh size. Samples were sieved to separate particles smaller than 5 mm from larger ones; plastic pieces were extracted from the plankton and organic matter, and sorted in 6 categories: fragments, pellets, thin films, foams, lines and others (tar, etc.). Each were counted and weighted, as was the organic matter once dried. The concentrations of micro-plastic debris were measured in term of particles number and particles mass per surface unit. The average values of 130’000 parts/km2 and 58’000 mg/km2, respectively were measured, with highest concentrations reaching values superior to 420’000 parts/km2 and 140’000 mg/km2. A qualitative analysis mainly revealed the presence of fragments (77% in mass), but also thin films (13%), foams (7%), pellets (2%) and lines (2%). The average ratio of micro-plastic mass to dry organic material mass has been measured at 1.04. A fraction of the samples was collected in low wind conditions (u10 < 4 m/s), while the other part was collected under medium wind conditions (4 ≤ u10 ≤ 11 m/s). As the vertical distribution of micro-plastic is known to be highly sensitive to wind stress, the values have been corrected and re-interpreted using the methodology proposed by Kukulka & al. 2012. The resulting corrected values suggest that the employed sampling process underestimated the surface micro-plastic concentration by an average factor of 1.55. The average concentrations of meso-plastic debris reached the number of 5’500 parts/km2 and the mass of 120’000 mg/km2, mostly fragments (57%) and thin films (34%). The results revealed that (1) plastic marine debris are widely distributed in western Mediterranean sea, (2) plastic debris concentrations in the Mediterranean Sea are of the same order of magnitude than those measured in the North Atlantic or North Pacific subtropical gyres and (3) plastic debris concentrations are about the same order of magnitude than dry organic material, arguing in favor of the hypothesis of potential misinterpretation and ingestion of these particles by plankton feeders in the sea. Kukulka T, Proskurowski G, Moret-Ferguson S, Meyer D, Lavender Law K, 2012. Geophysical Research Letters, vol 39.

2013

Transition from a radial to a 3D buoyancy-driven hydraulic fracture

Brice Tanguy Alphonse Lecampion, Andreas Möri

Magmatic dikes are a naturally occurring type of fluid-driven fractures (Rivalta et al. 2015) propagating in the lithosphere due to buoyant forces emerging from the density difference between the host material and the injected fluid (more precisely the difference between the gradients of the in-situ minimum stress and the hydrostatic pore-fluid pressure). The form of such buoyant fractures has been extensively studied in the context of magmatic dikes and shows the appearance of a head-tail structure (Rivalta et al. 2015). 2D plane-strain evaluations highlighted the fact that lubrication flow in the tail is driving the growth of the hydrostatic head (Lister & Kerr 1991; Roper & Lister 2005), which has been confirmed by Germanovich et al. (2014) for the 3D finger-like geometry. We investigate the 3D transition of a radial hydraulic fracture emerging from a point source towards a buoyancy-driven fracture, focussing on the case of a constant injection rate in homogeneous conditions (homogeneous material properties and buoyancy contrast). We use scaling arguments to characterize the transition and the late time buoyant behavior of the fracture and validate the findings with the fully coupled planar 3D hydraulic fracture growth solver PyFrac (Zia & Lecampion 2020). Our results confirm a characteristic transition time / length scale from a radial to a buoyant fracture depending on a single dimensionless parameter. The same parameter can be used to predict the fractures overrun (maximum breadth / stabilized breadth). A second, very slow transition takes place between the elongated fracture and the late time fully-developed 3D head-tail structure (similar to the solution of Germanovich et al. (2014)). Our simulations support the appearance of a 3D toughness dominated head structure. They also confirm the importance of the viscous tail as the driving mechanism for the dynamics of such a 3D Weertman’s pulse (form of the head). We quantify how and when the transition from a radial to a buoyant fracture takes place, and it’s implications on the late time behavior. REFERENCES Germanovich, L. N. & Garagash, D. I. & Murdoch, L. & Robinowitz, M. 2014: Gravity-driven hydraulic fractures, AGU Fall meeting Lister, J., R. & Kerr, R. C. 1992: Fluid-mechanical models of crack propagation and their application to magma transport in dykes, Journal of Geophysical research: Solid Earth, 96(B6), 10049-10077 Rivalta, E. & Taisne, B. & Bunger, A. P. & Katz, R. F. 2015: A review of mechanical models of dike propagation: Schools of thought, results and future directions, Tectonophysics, 638, 1-42 18th Swiss Geoscience Meeting, Zurich 2020 Roper, S. M. & Lister, J. R. 2005: Buoyancy-driven crack propagation from an over-pressured source, J. Fluid Mech., 536, 79-98 Zia, H. & Lecampion, B. 2020: PyFrac: A planar 3d hydraulic fracture simulator, Computer Physics Communications, page 107368

2020

An evaluation of surface micro and meso plastic pollution in pelagic ecosystems of western Mediterranean Sea

Luiz Felippe De Alencastro, Florian Faure, Pascal Hagmann

2013

Transition from a radial to a 3D buoyancy-driven hydraulic fracture

Brice Tanguy Alphonse Lecampion, Andreas Möri

2020