71 research outputs found
Lakes and subglacial hydrological networks around Dome C, East Anatrctica
No full textPrecise topography from European Remote-sensing Satellite radar altimetry and high density of airborne radio-echo sounding in the area surrounding Dome C, Antarctica, show a link between surface features and subglacial lakes. In this paper, we extend the study to fine structures by computing a curvature-based coefficient (c y) related to surface undulations. These coefficient variations reveal many surface undulations, and some elongated features of this parameter seem to link known subglacial lakes. A population of high values of this coefficient, assumed to correspond to transitions between sliding and non-sliding flow regime, strengthen the appearance of a network which would link most of the lakes in the area. The existence of such a network impacts on ice-flow dyna ics and on subglacial-lake studies
Rapid change of snow surface properties at Vostok, East Antarctica, revealed by altimetry and radiometry
No full textInternational audienceWe present results of snow surface properties using the ENVISAT dual frequency altimeter at S (3.2 GHz) and Ku (13.6 GHz) bands and the AMSR-E microwave radiometer at frequencies ranging between 6 and 36 GHz in the Vostok region, East Antarctica. The altimetric time series observed between 2002 and 2008 show variations at 3 different time scales (daily, seasonal and inter-annual), that correlate directly with variations in the snow surface properties. In this study we focus on the analysis of the rapid daily event, occurring on February 14th 2005, that created a jump of the backscatter coefficient of up to 5.3 dB at the S band and 2.5 dB at the Ku band. The ratio of V/H-polarization brightness temperature slowly decreased in December and January 2005, and suddenly increased on February 14th 2005. The origin of this rapid event is investigated using AWS data from Vostok station, altimetric and radiometric data simultaneously. Both snow surface density and roughness are found to vary during this event. This event is shown to be synchronous with strong wind occuring during a period of anomalous wind direction, and the presence of surface hoar. These particular conditions certainly modified the snow surface roughness and thus impacted the altimetric signal. We finally investigate the impact of this event on the calculation of the regional ice-sheet mass-balance using different corrections of height with echo shape variations. It is shown to be negligible only if the full echo shape correction (Legresy et al., 2006) is used
Sea ice meltwater and Circumpolar Deep Water drive contrasting productivity in three Antarctic polynyas
No full textAuthor Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 124(5), (2019): 2943-2968, doi:10.1029/2019JC015071.In the Southern Ocean, polynyas exhibit enhanced rates of primary productivity and represent large seasonal sinks for atmospheric CO2. Three contrasting east Antarctic polynyas were visited in late December to early January 2017: the Dalton, Mertz, and Ninnis polynyas. In the Mertz and Ninnis polynyas, phytoplankton biomass (average of 322 and 354 mg chlorophyll a (Chl a)/m2, respectively) and net community production (5.3 and 4.6 mol C/m2, respectively) were approximately 3 times those measured in the Dalton polynya (average of 122 mg Chl a/m2 and 1.8 mol C/m2). Phytoplankton communities also differed between the polynyas. Diatoms were thriving in the Mertz and Ninnis polynyas but not in the Dalton polynya, where Phaeocystis antarctica dominated. These strong regional differences were explored using physiological, biological, and physical parameters. The most likely drivers of the observed higher productivity in the Mertz and Ninnis were the relatively shallow inflow of iron‐rich modified Circumpolar Deep Water onto the shelf as well as a very large sea ice meltwater contribution. The productivity contrast between the three polynyas could not be explained by (1) the input of glacial meltwater, (2) the presence of Ice Shelf Water, or (3) stratification of the mixed layer. Our results show that physical drivers regulate the productivity of polynyas, suggesting that the response of biological productivity and carbon export to future change will vary among polynyas.This work was cofunded by the Australian Antarctic Division research projects AAS 4131 and 4291. This project was also supported by the Australian Government Cooperative Research Centres Programme through the Antarctic Climate & Ecosystems (ACE CRC). S. Moreau and C. Genovese were supported by the Australian Research Council's Special Research Initiative for Antarctic Gateway Partnership (project ID SR140300001). V. Puigcorbé and M. Roca‐Martí are grateful for the support from Pere Masque and Edith Cowan University. M.C. Arroyo was supported by the Dickhut Fellowship, administered by the Virginia Institute of Marine Science. The authors would like to thank the officers and crew of the R/V Aurora Australis for their logistic support, the CSIRO hydrochemists for their analyses of nutrient concentrations, and E. J. Yang for her microscope analysis of phytoplankton species. We also want to thank two anonymous reviewers for their very good comments on this study. The data presented in this paper are available on the Australian Antarctic Division (AAD) Data Centre at https://data.aad.gov.au/aadc/metadata/metadata_by_parameter.cfm.2019-09-2
EFFECTS OF COLD-ROLLING AND POST-DEFORMATION ANNEALING ON THE MARTENSITIC TRANSFORMATION OF A TiNi SHAPE MEMORY ALLOY
No full textThe thermoelastic transformation of shape-memory titanium -nickel alloys is highly influenced by the microstructural state of the alloy and particularly by the plastic deformation of the metal. In this study, the influence of cold-work and annealing on martensitic and austenitic transformations is performed with "in situ" thermoelectric power measurements during thermal cycling. In the recrystallized state the transformations are very well defined, and the thermal hysteresis is generally less than 30 degrees. After work hardening the transformation becomes "diffuse" and is no longer observed if the deformation is greater than a critical value of approximately 25%. The effects of post-deformation heat treatments were also characterized using a heavily cold- worked metal within a temperature range of 300-600°C. Thermoelectric power measurements applied to the characterization of shape-memory alloy transformation proves to be a high performance tool displaying a great sensitivity to martensitic transformation
IN-SITU TEM OF THE AMORPHIZATION REACTION IN Al-Pt MULTILAYERS
No full textLa reaction d'amorphisation d'échantillons multicouches A1-Pt est observée in-situ par microscopie électronique à transmission. Nous montrons que la réaction d'amorphisation commence a basse température (150°C à 200°C). En termes de composition, la phase amorphe est située entre les composés intermétalliques Al21Pt8 et Al3Pt2. La réction d'amorphisation dans le système A1-Pt est discutée en termes des critères de stabilité thermodynamique, cinétiques et structuraux, proposés pour les systèmes où une réaction d'amorphisation en phase solide est observée.The amorphization reaction of A1-Pt multilayer samples is monitored during in-situ transmission electron microscopy. It is shown that the amorphization reaction starts at low temperatures (150°C to 200°C). The compositional region of the amorphous phase is estimated to be situated between the Al21Pt8 and the Al3Pt2 compounds. The occurrence of a solid state amorphization reaction in the A1-Pt system is discussed in relation to the thermodynamic, kinetic and structural stability criteria proposed for systems in which solid state amorphization reactions are observed
Radar altimetry measurements over antarctic ice sheet: A focus on antenna polarization and change in backscatter problems
No full textISI Document Delivery No.: 010LG Times Cited: 1 Cited Reference Count: 25 Cited References: Arthern RJ, 2001, J GEOPHYS RES-ATMOS, V106, P33471, DOI 10.1029/2001JD000498 Brenner AC, 2007, IEEE T GEOSCI REMOTE, V45, P321, DOI [10.1109/TGRS.2006.887172, 10.1106/TGRS.2006.887172] Davis CH, 2004, IEEE T GEOSCI REMOTE, V42, P2437, DOI 10.1109/TGRS.2004.836789 DAVIS CH, 1993, J GLACIOL, V39, P687 FUNG AK, 1982, IEEE T GEOSCI REMOTE, V20, P528, DOI 10.1109/TGRS.1982.350421 Lacroix P, 2009, REMOTE SENS ENVIRON, V113, P2633, DOI 10.1016/j.rse.2009.07.019 Lacroix P, 2008, REMOTE SENS ENVIRON, V112, P1712, DOI 10.1016/j.rse.2007.08.022 Legresy B, 1999, GEOPHYS RES LETT, V26, P2231, DOI 10.1029/1999GL900531 Legresy B, 1998, J GLACIOL, V44, P197 Legresy B, 2005, REMOTE SENS ENVIRON, V95, P150, DOI 10.1016/j.rse.2004.11.018 Li J., 2002, INT S PHYS MECH PROC, P233 Li J, 2004, ANN GLACIOL, V38, P309, DOI 10.3189/172756404781814988 Matzler C., 1987, Remote Sensing Reviews, V2 Pritchard HD, 2009, NATURE, V461, P971, DOI 10.1038/nature08471 Remy F, 2009, REMOTE SENS-BASEL, V1, P1212, DOI 10.3390/rs1041212 Remy F, 2006, IEEE T GEOSCI REMOTE, V44, P3289, DOI 10.1109/TGRS.2006.878444 RIDLEY JK, 1988, INT J REMOTE SENS, V9, P601 Tran N, 2008, IEEE T GEOSCI REMOTE, V46, P3694, DOI 10.1109/TGRS.2008.2000818 Vincent P, 2006, SENSORS-BASEL, V6, P208, DOI 10.3390/s6030208 Wingham D., 2009, GEOPHYS RES LETT, V36 Wingham DJ, 2006, PHILOS T R SOC A, V364, P1627, DOI 10.1098/rsta.2006.1792 Wingham DJ, 2006, ADV SPACE RES-SERIES, V37, P841, DOI 10.1016/j.asr.2005.07.027 Wingham DJ, 1998, SCIENCE, V282, P456, DOI 10.1126/science.282.5388.456 Zwally HJ, 2005, J GLACIOL, V51, P509, DOI 10.3189/172756505781829007 Zwally HJ, 2002, J GLACIOL, V48, P199, DOI 10.3189/172756502781831403 Remy, F. Flament, T. Blarel, F. Benveniste, J. 1 ELSEVIER SCI LTD OXFORD ADV SPACE RESIn this paper, we investigate the impact of the error due to the penetration of the altimetric wave within the snowpack. The phenomenon has two different impacts. The first one, due to temporal change in snow characteristics, affects the ice sheet volume trend as derived from altimetric series. The second one, because of both the anisotropy of the ice sheet surface properties and of the linear antenna polarization, introduces a difference in measurements at crossover points. These two phenomena are the cause of what are probably the most critical limitations to the interpretation of long-term altimetric series in term of mass balance and to the comparison between or data fusion of different missions. Moreover, they will lead to the largest error when comparing data from EnviSat with data from CryoSat, because of the different orbits, or with data from AltiKa, because of the different radar frequencies. We show that waveform distortions due to snow characteristics fluctuation are complex. In the central part of the East Antarctica, the height and the leading edge width fluctuations vary together while elsewhere, height fluctuations may occur with no variations in the waveform shape, mostly during winter. As a consequence, these induced errors cannot be corrected with solely the help of the backscatter: waveform shape parameters are also needed. They are however not enough to fully correct these two errors. We propose an empirical correction for these effects. We show that crossover differences may be significantly reduced to up 22 cm. In terms of volume change, the estimation may vary up to 4 cm/yr at cross-overs depending on the correction used and is reduced in average to 2.3 cm/yr with our correction. The difference between the height trends estimated with both corrections is weak in average but may locally reach 5 cm/yr with a clear geographical pattern. (C) 2012 COSPAR. Published by Elsevier Ltd. All rights reserved
- …
