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Satellite altimetry provides an ocean mean sea surface (MSS) map with global coverage and overall excellent precision. However, in some areas, like the Vanuatu archipelago, the coverage and resolution of this tool are not sufficient to correctly map the short scale undulations of the sea surface, due to numerous islands and to strong lithospheric and mantle heterogeneities. New applications such as seafloor geodesy may require local mean surface representation with improved resolution in specific areas. We used sea surface height collected with kinematic GPS during three different cruises in 2004, 2006 and 2007 to reconstruct a homogeneous sea surface map around Santo Island, Vanuatu. We assess the accuracy of this GPS-derived local sea surface to 6–19 cm and evaluate the quality of existing altimetry and gravity-derived MSSs on the Vanuatu archipelago. Observed short scale undulations are interpreted as due to local geodynamics.  相似文献   
2.
The eruption of the Pelagatos scoria cone in the Sierra Chichinautzin monogenetic field near the southern suburbs of Mexico City occurred less than 14,000 years ago. The eruption initiated at a fissure with an effusive phase that formed a 7-km-long lava flow, and continued with a phase of alternating and/or simultaneous explosive and effusive activity that built a 50-m-high scoria cone on the western end of the fissure and formed a compound lava flow-field near the vent. The eruption ended with the emplacement of a short lava flow that breached the cone and was accompanied by weak explosions at the crater. Products consist of a microlite-rich high-Mg basaltic andesite. Samples were analyzed to determine the magma’s initial properties as well as the effects of degassing-induced crystallization on eruptive style. Although distal ash fallout deposits from this eruption are not preserved, a recent quarry exposes a large section of the scoria cone. Detailed study of exposed layers allows us to elucidate the mode of cone-building activity. Petrological and textural data, combined with models calibrated by experimental work and melt-inclusion analyses of similar magmas elsewhere, indicate that the magma was initially hot (>1,200°C), gas-rich (up to 5 wt.% H2O), crystal-poor (~10 vol.% Fo90 olivine phenocrysts) and thus poorly viscous (40–80 Pa s). During the early phase, low magma ascent velocity at the fissure vent allowed low-viscosity magma to degas and crystallize during ascent, producing lava flows with elevated crystal contents at T < 1,100°C, and blocky surfaces. Later, the closure of the fissure by cooling dikes focused the magma flow at a narrow section of the fissure. This led to an increased magma ascent velocity. Rapid and shallow degassing (<3 km deep) triggered ~40 vol.% microlite crystallization. Limited times for gas-escape and higher magma viscosity (6 × 105–4 × 106 Pa s) drove strong explosions of highly (60–80 vol.%) and finely vesicular magma. Coarse clasts broke on landing, which implies brittle behavior due to complete solidification. This requires sufficient time to cool and in turn implies ejection heights of over 1 km, which is much higher than “normal” Strombolian activity. Hence, magma viscosity significantly impacts eruption style at monogenetic volcanoes because it affects the kinetics of shallow degassing. The long-lasting eruptions of Jorullo and Paricutin, which produced similar magmas in western México, were more explosive. This can be related to higher magma fluxes and total erupted volumes. Implications of this study are important because basaltic andesites are commonly erupted to form monogenetic scoria cones of the Trans-Mexican Volcanic Belt.  相似文献   
3.
The climates of the mid-Holocene (MH), 6,000 years ago, and of the Last Glacial Maximum (LGM), 21,000 years ago, have extensively been simulated, in particular in the framework of the Palaeoclimate Modelling Intercomparion Project. These periods are well documented by paleo-records, which can be used for evaluating model results for climates different from the present one. Here, we present new simulations of the MH and the LGM climates obtained with the IPSL_CM5A model and compare them to our previous results obtained with the IPSL_CM4 model. Compared to IPSL_CM4, IPSL_CM5A includes two new features: the interactive representation of the plant phenology and marine biogeochemistry. But one of the most important differences between these models is the latitudinal resolution and vertical domain of their atmospheric component, which have been improved in IPSL_CM5A and results in a better representation of the mid-latitude jet-streams. The Asian monsoon’s representation is also substantially improved. The global average mean annual temperature simulated for the pre-industrial (PI) period is colder in IPSL_CM5A than in IPSL_CM4 but their climate sensitivity to a CO2 doubling is similar. Here we show that these differences in the simulated PI climate have an impact on the simulated MH and LGM climatic anomalies. The larger cooling response to LGM boundary conditions in IPSL_CM5A appears to be mainly due to differences between the PMIP3 and PMIP2 boundary conditions, as shown by a short wave radiative forcing/feedback analysis based on a simplified perturbation method. It is found that the sensitivity computed from the LGM climate is lower than that computed from 2 × CO2 simulations, confirming previous studies based on different models. For the MH, the Asian monsoon, stronger in the IPSL_CM5A PI simulation, is also more sensitive to the insolation changes. The African monsoon is also further amplified in IPSL_CM5A due to the impact of the interactive phenology. Finally the changes in variability for both models and for MH and LGM are presented taking the example of the El-Niño Southern Oscillation (ENSO), which is very different in the PI simulations. ENSO variability is damped in both model versions at the MH, whereas inconsistent responses are found between the two versions for the LGM. Part 2 of this paper examines whether these differences between IPSL_CM4 and IPSL_CM5A can be distinguished when comparing those results to palaeo-climatic reconstructions and investigates new approaches for model-data comparisons made possible by the inclusion of new components in IPSL_CM5A.  相似文献   
4.
The climates of the mid-Holocene (MH, 6,000 years ago) and the Last Glacial Maximum (LGM, 21,000 years ago) have been extensively documented and as such, have become targets for the evaluation of climate models for climate contexts very different from the present. In Part 1 of the present work, we have studied the MH and LGM simulations performed with the last two versions of the IPSL model: IPSL_CM4, run for the PMIP2/CMIP3 (Coupled Model Intercomparion Project) projects and IPSL_CM5A, run for the most recent PMIP3/CMIP5 projets. We have shown that not only are these models different in their simulations of the PI climate, but also in their simulations of the climatic anomalies for the MH and LGM. In the Part 2 of this paper, we first examine whether palaeo-data can help discriminate between the model performances. This is indeed the case for the African monsoon for the MH or for North America south of the Laurentide ice sheet, the South Atlantic or the southern Indian ocean for the LGM. For the LGM, off-line vegetation modelling appears to offer good opportunities to distinguish climate model results because glacial vegetation proves to be very sensitive to even small differences in LGM climate. For other cases such as the LGM North Atlantic or the LGM equatorial Pacific, the large uncertainty on the SST reconstructions, prevents model discrimination. We have examined the use of other proxy-data for model evaluation, which has become possible with the inclusion of the biogeochemistry morel PISCES in the IPSL_CM5A model. We show a broad agreement of the LGM–PI export production changes with reconstructions. These changes are related to the mixed layer depth in most regions and to sea-ice variations in the high latitudes. We have also modelled foraminifer abundances with the FORAMCLIM model and shown that the changes in foraminifer abundance in the equatorial Pacific are mainly forced by changes in SSTs, hence confirming the SST-foraminifer abundance relationship. Yet, this is not the case in all regions in the North Atlantic, where food availability can have a strong impact of foraminifer abundances. Further work will be needed to exhaustively examine the role of factors other than climate in piloting changes in palaeo-indicators.  相似文献   
5.
Ocean Dynamics - Short waves growth is characterized by nonlinear and dynamic processes that couple ocean and atmosphere. Ocean surface currents can have a strong impact on short wave steepness and...  相似文献   
6.
The 690?km2 Tacámbaro-Puruarán area located at the arc-front part of the Michoácan-Guanajuato volcanic field in the Trans-Mexican Volcanic Belt (TMVB) records a protracted history of volcanism that culminated with intense monogenetic activity in the Holocene. Geologic mapping, 40Ar/39Ar and 14C radiometric dating, and whole-rock chemical analyses of volcanic products provide insights to that history. Eocene volcanics (55–40?Ma) exposed at uplifted blocks are related to a magmatic arc that preceded the TMVB. Early TMVB products are represented by poorly exposed Pliocene silicic domes (5–2?Ma). Quaternary (<2?Ma) volcanoes (114 mapped) are mainly scoria cones with lavas (49 vol.%), viscous lava flows (22 vol.%), and lava shields (22 vol.%). Erupted products are dominantly either basaltic andesites (37 vol. %), or andesites (17 vol.%), or span across both compositions (28 vol.%). Basalts (9 vol.%), dacites (4 vol.%), shoshonites (2 vol.%), and other alkali-rich rocks (<3 vol.%) occur subordinately. Early-Pleistocene volcanism was bimodal (dacites and basalts) and voluminous while since 1?Ma small-volume eruptions of intermediate magmas have dominated. Higher rates of lithospheric extension in the Quaternary may have allowed a larger number of small, poorly evolved dikes to reach the surface during this period. Eruptive centers as old as 1.7?Ma are aligned in a NE direction parallel to both, basement faults and the direction of regional compressive stress, implying structural control on volcanic activity. Data suggest that volcanism was strongly pulsatory and fed by localized low-degree partial melting of mantle sources. In the Holocene, at least 13 eruptions occurred (average recurrence interval of 800?years). These produced ~3.8?km3 of basaltic andesitic to andesitic magma and included four eruptions dated at ~1,000; 4,000; 8,000; and 11,000?years bc (calibrated 14C ages). To date, this is one of the highest monogenetic eruption frequencies detected within such a small area in a subduction-related arc-setting. These anomalous rates of monogenetic activity in an area with thick crust (>30?km) may be related to high rates of magma production at depth and a favorable tectonic setting.  相似文献   
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8.
One of the most remarkable features of the exceptionally well preserved 26 km3 Socompa debris avalanche deposit is the evidence for topographically driven secondary flow. The avalanche formed by sector collapse of Socompa stratovolcano and spread 40 km across a pre-existing basin, forming a sheet of ∼50 m average thickness. As the avalanche impinged on the western and northern margins of the basin, it was reflected back, forming a secondary flow that continued to travel 15 km down a gentle slope at an oblique angle to the primary flow, the front of the return wave being preserved frozen on the surface of the deposit as a prominent escarpment. Satellite images, aerial photos, digital elevation models and field observations were used to reconstruct the sequence of events during avalanche emplacement, and in particular during secondary flow. The avalanche sheet was divided into distinct terrane groups, each believed to have experienced a particular strain history during emplacement. Evidence for avalanche reflection includes clearly recognizable secondary slide masses, sub-parallel sets of curvilinear shear zones, headwall scarps separating the (primary) levée from the secondary terranes, extensional jigsaw breakup of surface lithologies during return flow, and cross cutting, or deflection, of primary flow fabrics by secondary terranes. Reflection off the basin margin took place in an essentially continuous manner, most major return motions being simultaneous with, or shortly following, primary flow. The secondary flow occurred as a wave that swept obliquely across the primary avalanche direction, remobilizing the primary material, which was first compressed, then stretched, as it passed over and rearward of the wave front. As return flow occurred, surface lithologies were rifted in a brittle manner, and the slabs were sheared pervasively as they glided and rotated back into the basin; some sank into the more fluidal interior of the avalanche, which drained out into a prominent distal lobe. Extension by factors of up to 1.8 took place during return flow. Secondary flow took place on slopes of only a few degrees, and the distal lobe flowed 8 km on a slope of ∼1°. Overall the avalanche is inferred to have slid into place as a fast-moving sheet of fragmental rock debris, with a leading edge and crust with near-normal friction and an almost frictionless, fluidal interior and base. The avalanche emplacement history deduced from field evidence is consistent with the results of a previously published numerical model of the Socompa avalanche.  相似文献   
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