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31.
Airborne laser altimetry survey of Glaciar Tyndall, Patagonia 总被引:1,自引:1,他引:0
Kristian Keller Gino Casassa Andrs Rivera Rene Forsberg Niels Gundestrup 《Global and Planetary Change》2007,59(1-4):101
The first airborne laser altimetry measurements of a glacier in South America are presented. Data were collected in November of 2001 over Glaciar Tyndall, Torres del Paine National Park, Chilean Patagonia, onboard a Twin Otter airplane of the Chilean Air Force. A laser scanner with a rotating polygon-mirror system together with an Inertial Navigation System (INS) were fixed to the floor of the aircraft, and used in combination with two dual-frequency GPS receivers. Together, the laser–INS–GPS system had a nominal accuracy of 30 cm after data processing. On November 23rd, a total of 235 km were flown over the ablation area of Glaciar Tyndall, with 5 longitudinal tracks with a mean swath width of 300 m, which results in a point spacing of approximately 2 m both along and across track. A digital elevation model (DEM) generated using the laser altimetry data was compared with a DEM produced from a 1975 map (1:50,000 scale — Instituto Geográfico Militar (IGM), Chile). A mean thinning of − 3.1 ± 1.0 m a− 1 was calculated for the ablation area of Glaciar Tyndall, with a maximum value of − 7.7 ± 1.0 m a− 1 at the calving front at 50 m a.s.l. and minimum values of between − 1.0 and − 2.0 ± 1.0 m a− 1 at altitudes close to the equilibrium line altitude (900 m a.s.l.). The thinning rates derived from the airborne survey were similar to the results obtained by means of ground survey carried out at 600 m of altitude on Glaciar Tyndall between 1975 and 2002, yielding a mean thinning of − 3.2 m a− 1 [Raymond, C., Neumann, T.A., Rignot, E., Echelmeyer, K.A., Rivera, A., Casassa, G., 2005. Retreat of Tyndall Glacier, Patagonia, over the last half century. Journal of Glaciology 173 (51), 239–247.]. A good agreement was also found between ice elevation changes measured with laser data and previous results obtained with Shuttle Radar Topography Mission (SRTM) data. We conclude that airborne laser altimetry is an effective means for accurately detecting glacier elevation changes in Patagonia, where an ice thinning acceleration trend has been observed during recent years, presumably in response to warming and possibly also drier conditions. 相似文献
32.
Melt inclusions in high-An plagioclase from the Gorda Ridge: an example of the local diversity of MORB parent magmas 总被引:2,自引:0,他引:2
R. L. Nielsen Jennifer Crum Rene Bourgeois Kaylea Hascall Lance M. Forsythe Martin R. Fisk David M. Christie 《Contributions to Mineralogy and Petrology》1995,122(1-2):34-50
The use of ocean floor basalt chemistry as a tool to investigate mantle composition and processes requires that we work with
basalts that have been modified little since leaving the mantle. One source of such basalts is melt inclusions trapped in
primitive crystals. However, obtaining information from these melt inclusions is complicated by the fact that melt inclusions
in natural basalts are essentially always altered by post-entrapment crystallization. This requires that we develop techniques
for reconstructing the original trapped liquid compositions. We conducted a series of experiments to reverse the effects of
post-entrapment crystallization by re-heating the host crystals to their crystallization temperature. For these experiments
we used plagioclase crystals separated from a single Gorda Ridge lava. The crystallization temperature for these crystals
was determined by a set of incremental re-heating experiments to be ∼1240–1260° C. The inclusions are primitive, high Ca-Al
basaltic melts, saturated with plagioclase, olivine and Al-rich chromite at low pressure. The inclusion analyses can be linked
to the host lava composition by low pressure fractionation. The major element composition of the re-homogenized melt inclusions
within each crystal is relatively constant. However, the incompatible element analyses have extremely wide ranges. The range
of La/Sm and Ti/Zr from inclusions analyzed from a single sample from the Gorda Ridge exceeds the range reported for lavas
sampled from the entire ridge. The pyroxene compositions predicted to be in equilibrium with the melt inclusion trace element
signature cover much of the range represented by pyroxenes from abyssal peridotites. The volumetric proportions of the magmas
entering the base of the crust can be evaluated using frequency distribution of melt inclusion compositions. This distribution
suggests that the array of magmas was skewed towards the more depleted compositions, with little evidence for an enriched
component in this system. This pattern is more consistent with a dynamic flow model of the mantle or of a passive flow model
where the melts produced in the peripheral areas of the melting regime were not focused to the ridge.
Received: 5 January 1995 / Accepted: 13 June 1995 相似文献
33.
The orogenic volcanic rocks of Erromango island are divided into a Quaternary theoleiitic group and an older Pliocene one showing calc-alkalic affinity. The microprobe mineralogy of these lavas agrees with this geochemical distinction and indicates a marked iron enrichment trend in the recent tholeiitic lavas, whereas titanomagnetite occurs as early formed phenocrysts (Al, Cr, Mg-rich) only in Pliocene basalts. Some Ni and Mg-rich olivine phenocrysts (Fo88) and unusual chemically zoned clinopyroxene megacrysts are described from Pliocene basic lavas and interpreted as deep-fractionated minerals. The large compositional range of these latter (diopside core to augite rim, through salite intermediate zone) leads us to discuss the physical conditions and the corresponding cation substitutions prevailing during their formation. Diopside cores crystallized at depths of about 20 km from a relatively ‘primitive’ melt. The formation of salite is considered to be the result of concomitant fractionation of Mg-rich phases and increase of fO2. An early stage of fractionation of Cr-bearing diopside + Ni-bearing olivine would account for the typical Cr and Ni depletion of Erromango lavas. 相似文献
34.
Dr. Bruce Forsberg Dr. Jose Marcus Godoy Dr. Reynaldo Victoria Dr. Luiz Antonio Martinelli 《GeoJournal》1989,19(4):399-405
A 210Pb chronology of sedimentation in a small floodplain lake in the Jamari river basin (Rondonia, Brazil) provided insight into the relationship between development and erosion in the Brazilian Amazon. Positive correlations between increased sedimentation in the lake and levels of deforestation and mining in the Jamari basin suggested a major increase in erosion associated with these activities. This increase in erosion could diminish agricultural yields in the basin and significantly reduce the useful life of a hydro-electric plant recently built on the river. 相似文献
35.
Reynaldo L. Victoria Dr. Luiz A. Martinelli Jeffrey E. Richey Dr. Allan H. Devol Dr. Bruce R. Forsberg Dr. Maria N. G. Ribeiro 《GeoJournal》1989,19(1):45-52
Soil samples were collected on the floodplains of the Amazon River and its principal Brazilian tributaries during dry, early rising water, and early falling water periods. The concentrations of basic cations and pH in these alluvial soils were always higher than those in the more common terra firme soils while the concentrations of aluminum were generally lower. Among the alluvial soils, those from the main channel floodplain were generally higher in basic cations and pH, and lower in aluminum than those from the tribuary floodplains. The concentrations of basic cations in soils along the main channel floodplain decreased downstream. No significant difference was found in the levels of basic cations, pH, or aluminum between sampling periods. 相似文献
36.
Gravity field terrain effect computations by FFT 总被引:2,自引:2,他引:2
René Forsberg 《Journal of Geodesy》1985,59(4):342-360
The widespread availability of detailed gridded topographic and bathymetric data for many areas of the earth has resulted
in a need for efficient terrain effect computation techniques, especially for applications in gravity field modelling. Compared
to conventional integration techniques, Fourier transform methods provide extremely efficient computations due to the speed
of the Fast Fourier Transform (FFT. The Fourier techniques rely on linearization and series expansions of the basically unlinear terrain effect integrals, typically
involving transformation of the heights/depths and their squares. TheFFT methods will especially be suited for terrain reduction of land gravity data and satellite altimetry geoid data.
In the paper the basic formulas will be outlined, and special emphasis will be put on the practial implementation, where a
special coarse/detailed grid pair formulation must be used in order to minimize the unavoidable edge effects ofFFT, and the special properties ofFFT are utilized to limit the actual number of data transformations needed. Actual results are presented for gravity and geoid
terrain effects in test areas of the USA, Greenland and the North Atlantic. The results are evaluated against a conventional
integration program: thus, e.g., in an area of East Greenland (with terrain corrections up to10 mgal), the accuracy ofFFT-computed terrain corrections in actual gravity stations showed anr.m.s. error of0.25 mgal, using height data from a detailed photogrammetric digital terrain model. Similarly, isostatic ocean geoid effects in the
Faeroe Islands region were found to be computed withr.m.s. errors around0.03 m 相似文献
37.
Mireille Polve Rene C. Maury Sebastien Jego Herve Bellon Ahmed Margoum Graciano P. Yumul Jr Betchaida D. Payot Rodolfo A. Tamayo Jr Joseph Cotten 《Resource Geology》2007,57(2):197-218
Baguio, in the Central Cordillera of Northern Luzon, is a district that displays porphyry copper and epithermal gold mineralization, associated with Early Miocene–Pliocene–Quaternary calc‐alkaline and adakitic intrusions. Systematic sampling, K‐Ar dating, major and trace elements, and Sr, Nd, Pb isotopic analyses of fresh magmatic rocks indicate three magmatic pulses: an Early Miocene phase (21.2–18.7 Ma), a Middle–Late Miocene phase (15.3–8 Ma) and finally a Pliocene–Quaternary event (3–1 Ma). The first phase emplaced evolved calc‐alkaline magmas, essentially within the Agno Batholith complex, and is thought to be related to the westward‐dipping subduction of the West Philippine Basin. After a quiescence period during which the Kennon limestone was deposited, magmatic activity resumed at 15.3 Ma, in connection with the start of the subduction of the South China Sea along the Manila Trench. It emplaced first petrogenetically related and relatively unradiogenic low‐K calc‐alkaline lavas and intermediate adakites. Temporal geochemical patterns observed from 15.3 to 1 Ma include progressive enrichment in K and other large ion lithophile elements, increase in radiogenic Sr and Pb and corresponding decrease in radiogenic Nd. These features are thought to reflect the progressive addition to the Luzon arc mantle wedge of incompatible elements largely inherited from South China Sea sediments. The origin of the long quiescence period, from 8 to 3 Ma, remains problematic. It might represent a local consequence of the docking of the Zambales ophiolitic terrane to Northern Luzon. Then, magmatic activity resumed at 3 Ma, emplacing chemically diversified rocks ranging from low K to high K and including a large proportion of adakites, especially during the Quaternary (dacitic plugs). The authors tentatively relate this diversity to the development of a slab tear linked with the subduction of the fossil South China Sea ridge beneath the Baguio area. 相似文献
38.
Three different methods of handling topography in geoid determination were investigated. The first two methods employ the
residual terrain model (RTM) remove–restore technique, yielding the quasigeoid, whereas the third method uses the classical
Helmert condensation method, yielding the geoid. All three methods were used with the geopotential model Earth Gravity Model
(1996) (EGM96) as a reference, and the results were compared to precise global positioning system (GPS) levelling networks
in Scandinavia. An investigation of the Helmert method, focusing on the different types of indirect effects and their effects
on the geoid, was also carried out. The three different methods used produce almost identical results at the 5-cm level, when
compared to the GPS levelling networks. However, small systematic differences existed.
Received: 18 March 1999 / Accepted: 21 March 2000 相似文献
39.
Sabine Hahn Erika Griesshaber Wolfgang W. Schmahl Rolf D. Neuser Ann‐Christine Ritter Rene Hoffmann Dieter Buhl Andrea Niedermayr Anna Geske Adrian Immenhauser 《Sedimentology》2014,61(6):1625-1658
Throughout much of Earth's history, marine carbonates have represented one of the most important geological archives of environmental change. Several pivotal events during the Phanerozoic, such as mass extinctions or hyperthermal events have recently been associated with ocean acidification. Nevertheless, well‐defined geological proxies for past ocean acidification events are, at best, scarce. Here, experimental work explores the response of bivalve shell ultrastructure and isotope geochemistry (δ13C, δ18O and δ26Mg) to stressful environments, in particular to sea water acidification. In this study, the common blue mussel, Mytilus edulis, was cultured (from early juvenile stages to one year of age) at four pH regimes (pHNBS 7·2 to pH 8·0). Shell growth rate and ultrastructure of mainly the calcitic portion of the shells were compared between experimental treatments. Specimens exposed to low‐pH environments show patches of disordered calcitic fibre orientation in otherwise well‐structured shells. Furthermore, the electron backscattered diffraction analyses reveal that, under acidified conditions, the c‐axis of the calcite prisms exhibits a bimodal or multi‐modal distribution pattern. Similar shell disorder patterns have been reported from mytilids kept under naturally acidified sea water conditions. In contrast, this study found no evidence that different pH regimes affect shell carbon, oxygen or magnesium isotope ratios. Based on these observations, it is proposed that: (i) stressful environments, in this case low sea water pH, predictably affect bivalve biomineralization patterns; and (ii) these findings bear potential as a novel (petrographic) proxy for ancient sea water acidification. An assessment of the applicability of these data to well‐preserved fossil shell material from selected time intervals requires additional work. 相似文献
40.
Powerful computers and dedicated software allow effective data mining and scientific analyses in astronomical plate archives. We give and discuss examples of newly developed algorithms for astronomical plate analyses, e.g., searches for optical transients, as well as for major spectral and brightness changes. 相似文献