Dominant amphipods of Posidonia oceanica seagrass meadows display considerable trophic diversity          下载免费PDF全文

Loïc N. Michel  Patrick Dauby  Sylvie Gobert  Martin Graeve  Fabienne Nyssen  Nicolas Thelen  Gilles Lepoint 《Marine Ecology》2015,36(4):969-981

Gut content examination and trophic markers (fatty acids, stable isotopes of C and N) were combined to delineate the diet of the dominant species of amphipods from Mediterranean Posidonia oceanica seagrass meadows and to highlight trophic diversity among this community. Our results indicate that, although all dominant species heavily relied on macroalgal epiphytes, considerable interspecific dietary differences existed. Carbon stable isotope ratios notably showed that some of the amphipod species favored grazing on epiphytes from leaves or litter fragments (Apherusa chiereghinii, Aora spinicornis, Gammarus aequicauda), while others such as Dexamine spiniventris preferred epiphytes from rhizomes. The remaining amphipods (Caprella acanthifera, Ampithoe helleri and Gammarella fucicola) readily consumed both groups. In addition, SIAR modeling suggested that most species had a mixed diet, and relied on several food items. Fatty acid analysis and gut contents revealed that contributions of microepiphytic diatoms and of benthic and suspended particulate organic matter to the diet of amphipods were anecdotal. None of the examined species seemed to graze on their seagrass host [low 18:2(n‐6) and 18:3(n‐3) fatty acids contents], but Gammarus aequicauda partly relied on seagrass leaf detritus, as demonstrated by the lesser ¹³C‐depletion of their tissues. Overall, our findings suggest that amphipods, because of their importance in the transfer of organic matter from primary producers and detritus to higher rank consumers, are key items in P. oceanica‐associated food webs.  相似文献   

Orbital and thermal evolutions of four potential targets for a sample return space mission to a primitive near-Earth asteroid     

Patrick Michel  Marco Delbo 《Icarus》2010,209(2):520-534

In this paper, we present our study of the orbital and thermal evolutions, due to solar radiative heating, of four near-Earth asteroids (NEAs) considered as potential target candidates for sample return space missions to primitive asteroids. We used a dynamical model of the NEA population to estimate the most likely source region and orbital history of these objects. Then, for each asteroid, we integrated numerically over their entire lifetime a set of 14 initially indistinguishable orbit (clones), obtained by small variations of the nominal initial conditions. Using a thermal model, we then computed surface and sub-surface temperatures of these bodies during their dynamical history. Our aim is to determine whether these bodies are likely to have experienced high temperature level, and whether great temperature changes can be expected due to the orbital changes as well as their maximum and minimum values. Such information is important in the framework of sample return space missions whose goal is to bring back pristine materials. The knowledge of the temperature range of materials at different depth over the orbital evolution of potential targets can help defining sampling strategies that ensure the likelihood that unaltered material will be brought back. Our results suggest that for all the considered potential targets, the surface has experienced for some time temperatures greater than 400 K and at most 500 K with 50% probability. This probability drops rapidly with increasing temperature. Sub-surface materials at a depth of only 3 cm are much more protected from high temperature and generally do not reach temperatures exceeding 450 K (with 50% probability). They should thus be unaltered at this depth at least from a Sun-driven heating point of view. On the other hand, surface material for some of the considered objects can have a range of temperature which can make them less reliable as pristine materials. However, it is assumed here that the same material is constantly exposed to solar heat, while regolith turnover may occur. The latter can be caused by different processes such as seismic shaking and/or impact cratering. This would reduce the total time that materials are exposed to a certain temperature. Thus, it is very likely that a sample collected from any of the four considered targets, or any primitive NEA with similar dynamical properties, will have components that will be thermally unaltered as long as some of it comes from only 3 to 5 cm depth. Such a depth is not considered difficult to reach with some of the current designs of sampling devices.  相似文献   

Thick sections of layered ultramafic cumulates in the Oman ophiolite revealed by an airborne hyperspectral survey: Petrogenesis and relationship to mantle diapirism     

Harold Clénet  Georges Ceuleneer  Patrick Pinet  Bénédicte Abily  Yves Daydou  Esther Harris  Isma Amri  Céline Dantas 《Lithos》2010,114(3-4):265-281

Using the HyMap instrument, we have acquired visible and near infrared hyperspectral data over the Maqsad area of the Oman ophiolite (~ 15 × 60 km). This survey allowed us to identify and map the distribution of clinopyroxene-rich cumulates (inter-layered clinopyroxenites and wehrlites) whose occurrence was previously undocumented in this area. The cumulates reach several hundred meters in thickness and crop out at distances exceeding 15 km on both sides of the Maqsad former spreading centre. They occur either in mantle harzburgites, as km-sized layered intrusions surrounded by fields of pegmatitic dykes consisting of orthopyroxene-rich pyroxenite and gabbronorites, or at the base of the crustal section where they are conformably overlain by cumulate gabbros. These ultramafic cumulates crystallized from silica- and Mg-rich melts derived from a refractory mantle source (e.g. high Cr#, low [Al₂O₃], low [TiO₂]). These melts are close to high-Ca boninites, although, strictly speaking, not perfect equivalents of present-day, supra-subduction zone, boninites. Chemical stratigraphy reveals cycles of replenishment, mixing and fractional crystallization from primitive (high Mg#) melts, typical of open magma chambers and migration of inter-cumulus melts. The TiO₂ content of clinopyroxene is always low (≤ 0.2 wt.%) but quite variable compared to the associated pegmatites that are all derived from a source ultra-depleted in high field strength elements (HFSE). This variability is not caused by fractional crystallization alone, and is best explained by hybridization between the ultra-depleted melts (parent melts of the pegmatites) and the less depleted mid-ocean ridge basalts (MORB) parent of the dunitic–troctolitic–gabbroic cumulates making up the crustal section above the Maqsad diapir.We propose that, following a period of magma-starved spreading, the Maqsad mantle diapir, impregnated with tholeiitic melts of MORB affinity, reached shallow depths beneath the ocean ridge. This diapir induced melting of the formerly accreted and hydrothermally altered lithosphere. At this stage, these boninitic-like lithospheric melts crystallized as pegmatitic dykes. As the diapir continued to rise, the amount of MORB reaching shallow depths increased, together with the surrounding temperature, leading to the formation of magma chambers where the crystallization of layered cumulates became possible. These cumulates remained rich in pyroxene and devoid of plagioclase as long as the contribution of MORB-derived melts was moderate relative to the lithospheric-derived melts. As the contribution of MORB to the refilling of the magma chamber increased, gabbroic cumulates started to crystallize.  相似文献   

Alpine tectonics in the Calabrian–Peloritan belt (southern Italy): New 40Ar/39Ar data in the Aspromonte Massif area     

Thomas Heymes  Patrick Monié  Nicolas Arnaud  Arnaud Pêcher  Jean-Pierre Bouillin  Roberto Compagnoni 《Lithos》2010,114(3-4):451-472

This study provides new ⁴⁰Ar/³⁹Ar geochronological constraints on the age of the Alpine tectonics in the Aspromonte Massif (southern part of the Calabrian–Peloritan belt). This massif exposes the upper units of the Calabride Complex which originated from the European continental margin. The Calabride Complex was incorporated in the Alpine orogenic wedge and then integrated into the Apennines and Maghrebides fold-and-thrust belts. Throughout the Calabride Complex there is evidence for a two stage tectonic history, which remains however rather poorly dated: Alpine nappe stacking is followed by extensional reworking along the former thrust contacts or along new detachment surfaces. Our new ages suggest that exhumation of the uppermost units, which accompanied nappe stacking, probably started at 45 Ma and that the deepest units were almost completely exhumed at 33 Ma. This kinematics probably corresponds to syn-orogenic extension while the end of exhumation is clearly related to the extensional tectonics dated at 28.6 Ma along detachment structures.Our geochronological data reveal a very short lag time between accretional and extensional processes in this part of the Mediterranean Alpine orogenic belt. The direction of extension, when the units are restored to their initial position (i.e. before the opening of the Western Mediterranean basins and the bending of the arc) is NNE–SSW. Such a direction does not fit with the eastward slab-retreat model generally put forward to explain extension in the Western Mediterranean. In contrast, we provide evidence for roughly N–S middle Oligocene extension in the accretionary prism, not previously described in this part of the Mediterranean domain.  相似文献   

Multi-stage reaction history in different eclogite types from the Pakistan Himalaya and implications for exhumation processes     

Franziska D.H. Wilke  Patrick J. O'Brien  Uwe Altenberger  Matthias Konrad-Schmolke  M. Ahmed Khan 《Lithos》2010,114(1-2):70-85

Metabasites were sampled from rock series of the subducted margin of the Indian Plate, the so-called Higher Himalayan Crystalline, in the Upper Kaghan Valley, Pakistan. These vary from corona dolerites, cropping out around Saif-ul-Muluk in the south, to coesite–eclogite close to the suture zone against rocks of the Kohistan arc in the north. Bulk rock major- and trace- element chemistry reveals essentially a single protolith as the source for five different eclogite types, which differ in fabric, modal mineralogy as well as in mineral chemistry. The study of newly-collected samples reveals coesite (confirmed by in situ Raman spectroscopy) in both garnet and omphacite. All eclogites show growth of amphiboles during exhumation. Within some coesite-bearing eclogites the presence of glaucophane cores to barroisite is noted whereas in most samples porphyroblastic sodic–calcic amphiboles are rimmed by more aluminous calcic amphibole (pargasite, tschermakite, and edenite). Eclogite facies rutile is replaced by ilmenite which itself is commonly surrounded by titanite. In addition, some eclogite bodies show leucocratic segregations containing phengite, quartz, zoisite and/or kyanite. The important implication is that the complex exhumation path shows stages of initial cooling during decompression (formation of glaucophane) followed by reheating: a very similar situation to that reported for the coesite-bearing eclogite series of the Tso Morari massif, India, 450 km to the south-east.  相似文献   

Development of a Darcy-Brinkman model to simulate water flow and tracer transport in a heterogeneous karstic aquifer (Val d’Orléans, France)     

Ali Salim Joodi  Stanislas Sizaret  Stéphane Binet  Ary Bruand  Patrick Alberic  Michel Lepiller 《Hydrogeology Journal》2010,18(2):295-309

Darcy’s law is the equation of reference widely used to model aquifer flows. However, its use to model karstic aquifers functioning with large pores is problematic. The physics occurring within the karstic conduits requires the use of a more representative macroscopic equation. A hydrodynamic model is presented which is adapted to the karstic aquifer of the Val d’Orléans (France) using two flow equations: (1) Darcy’s law, used to describe water flow within the massive limestone, and (2) the Brinkman equation, used to model water flow within the conduits. The flow equations coupled with the transport equation allow the prediction of the karst transfer properties. The model was tested by using six dye tracer tests and compared to a model that uses Darcy’s law to describe the flow in karstic conduits. The simulations show that the conduit permeability ranges from 5?×?10^?6 to 5.5?×?10^?5?m² and the limestone permeability ranges from 8?×?10^?11 to 6?×?10^?10?m². The dispersivity coefficient ranges from 23 to 53 m in the conduits and from 1 to 5 m in the limestone. The results of the simulations carried out using Darcy’s law in the conduits show that the dispersion towards the fractures is underestimated.  相似文献   

A field study (Massachusetts, USA) of the factors controlling the depth of groundwater flow systems in crystalline fractured-rock terrain   总被引：4，自引：3，他引：1  

David F. Boutt  Patrick Diggins  Stephen Mabee 《Hydrogeology Journal》2010,18(8):1839-1854

Groundwater movement and availability in crystalline and metamorphosed rocks is dominated by the secondary porosity generated through fracturing. The distributions of fractures and fracture zones determine permeable pathways and the productivity of these rocks. Controls on how these distributions vary with depth in the shallow subsurface (<300 m) and their resulting influence on groundwater flow is not well understood. The results of a subsurface study in the Nashoba and Avalon terranes of eastern Massachusetts (USA), which is a region experiencing expanded use of the fractured bedrock as a potable-supply aquifer, are presented. The study logged the distribution of fractures in 17 boreholes, identified flowing fractures, and hydraulically characterized the rock mass intersecting the boreholes. Of all fractures encountered, 2.5% are hydraulically active. Boreholes show decreasing fracture frequency up to 300 m depth, with hydraulically active fractures showing a similar trend; this restricts topographically driven flow. Borehole temperature profiles corroborate this, with minimal hydrologically altered flow observed in the profiles below 100 m. Results from this study suggest that active flow systems in these geologic settings are shallow and that fracture permeability outside of the influence of large-scale structures will follow a decreasing trend with depth.  相似文献   

Application of drone-captured thermal imagery in aiding in the recovery of meteorites within a snow-covered strewn field     

Patrick J. A. Hill  Libby D. Tunney  Christopher D. K. Herd 《Meteoritics & planetary science》2023,58(3):421-432

The rapid recovery of meteorites mitigates the exposure of astromaterials to the terrestrial environment and subsequent contamination. Modern fireball observatories have enabled the more accurate triangulation of fireball trajectories, which has aided in the location of strewn fields, in the case of meteorite-producing events. Despite this advancement, most meteorite searches still use manual searching to locate any meteorite falls, which is often labor-intensive and has a slow coverage rate (km² day⁻¹). Recent work has begun exploring the application of drone technology to the recovery of meteorites; however, most of this work has focused on falls in arid environments. Our study examines the utilization of drones with thermal imaging technology to aid in the recovery of meteorites that have fallen on a snow-covered field. We created a simulated strewn field that included meteorite specimens as well as Earth rocks with similar properties (“meteowrongs”). Thermal imagery was utilized to determine whether the thermal contrast between meteorites and snow could aid in the identification of meteorites. We found that the thermal contrast was significant enough that meteorites were readily identifiable within thermal images; however, it was not significant enough to distinguish between the meteorites and the meteowrongs. The utilization of thermal imagery in conjunction with visible imagery has the potential to aid in the rapid recovery of meteorites in snow-covered landscapes.  相似文献   

Truncated dunes as evidence of the 2004 tsunami in North Sumatra and environmental recovery post‐tsunami     

Claire Kain  Christopher Gomez  Patrick Wassmer  Franck Lavigne  Deirdre Hart 《New Zealand geographer》2014,70(3):165-178

The 2004 tsunami transformed the coast of Indonesia. This research investigates a sand dune area in Lampuuk, Sumatra, that was scoured by tsunami flow. We assessed geomorphology one‐year post‐event and examine the timescale of vegetation recovery. Ground Penetrating Radar (GPR) evidence shows an eroded succession of thin dipping units, overlain by aeolian layers 0 to 50 cm thick. Incipient dunes were absent, indicating limited dune rebuilding at one‐year post‐tsunami, possibly resulting from channelised airflow and the absence of vegetation. Recolonisation by vegetation was initially limited but progressed rapidly between 2005 and 2011, highlighting the temporal non‐linearity of recovery processes.  相似文献   

Formation and evolution of sand deserts in Xinjiang,Northwest China：Ⅰ. Provenances of desert sands     

ZHU Bingqi  YU Jingjie  QIN Xiaoguang  Patrick RIOUAL  LIU Ziting  XIONG Heigang 《地理学报》2014,24(1):177-190

Sandy desert landscape is a geological product of arid climate and abundant sand materials supply in natural conditions, therefore the formation of sandy desert is an ideal studying object for understanding the interaction between various stratigraphic/epigenetic spheres of the earth system. However, until now, the knowledge about the provenance and formation of these deserts in Xinjiang, Central Asia is diverse and inconsistent, a systematic review is necessary. Ancient aeolian sand sediments in the internal areas of these deserts and their twin dust deposits in the Cenozoic strata surrounding the Xinjiang deserts are direct and indirect clues reflecting the provenance of sand materials and the formation and evolution of deserts. Based on the geochemical, mineralogical and isotopic evidences of desert sands and relevant deposits in the peripheral regions, this paper reviews the research progress on the development of the sandy deserts in Xinjiang, Northwest China. Many proofs proposed that desert sands in Xinjiang were mainly sourced from the ancient pluvial, alluvial and fluvial sediments and were eventually achieved because of the local sand supply. It pointed out that the settings of tectonic structure in Xinjiang had made sediments both in the Tarim Basin and the Junggar Basin being influenced greatly by regional hydrological system and aeolian processes originated from the planetary wind system of westerly, the East Asian winter monsoon and the topographical mountain-valley winds. However, the directions of transportation paths are different between the hydrological dynamics and the aeolian dynamics, which are decided by the slantwise structures of the basins tectonics and the regional atmospheric circulation routes, respectively, and have determined anisotropies of the transport pathway of these sediments delivered by different agents. This results in a geological cycle of detrital sediment transportation in the two large basins and thus largely increases the degree of sediment mixing, alternation and recycle between younger and older sediment sources, as well as the complexity and diversity of sand provenances.  相似文献