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1.
This paper is concerned with identifying the spatial and temporal patterns in the annual maximum and minimum water level in the Pearl River Delta (PRD) region. The Mann–Kendall test and Pettitt test are used to detect trends and abrupt change points, and the Trend Free Pre-Whitening (TFPW) approach then eliminates the effect of serial correlation in data series with significant autocorrelation. Approximately fifty years of the annual hydrological variables from 18 stations in the three major rivers (the West River, the North River, and the East River) are examined. The changing trends of the extremes in water level show different features in different parts of the PRD region. Generally speaking, in the upper part of the delta, the water levels show a decreasing trend while in the middle and lower part there is an increasing trend. This spatial pattern of the extreme water level variation is unlikely to be due to a long-term change in stream flow in the PRD region because the water level changes do not always coincide with the extreme stream flow variations. Sand excavation initiated in the 1980s and continuing for more than 20 years in almost all tributaries around the PRD region is one of the most serious intensive human activities affecting water levels. The result of the Pettitt test indicates that most abrupt change points occurred in 1980s–1990s, which reveals that sand excavation and channel regulation are likely to have been the most significant factors contributing to the change over this period. These anthropogenic activities modify the annual extreme water level dramatically in a way that affects the morphology of river channels and estuaries of the PRD and also the redistribution of discharge. However, there are differences in the geographic locations of significant trends for the water level investigated, which implies that these impacts are not spatially uniform.  相似文献   

2.
The sediment load delivered from the Huanghe (Yellow River) to the sea has decreased sharply to 0.15 × 109 metric tons per year (0.15 Gt/yr) between 2000 and 2005, and now represents only 14% of the widely cited estimate of 1.08 Gt/yr. The river seems to be reverting to the pristine levels characteristic of the middle Holocene, prior to human intervention. Datasets from 1950 to 2005 from four key gauging stations in the main stream reveal distinct stepwise decreases in sediment load, which are attributed to both natural and anthropogenic impacts over the past 56 yr. Completions of two reservoirs, Liujiaxia (1968) and Longyangxia (1985), in the upper reaches of the river and their joint operations have resulted in stepwise decreases in sediment load coming from the upper reaches. Effective soil conservation practices in the middle reaches since the late 1970s, combined with the operation of the Sanmenxia and Xiaolangdi reservoirs, have also caused stepwise decreases in sediment load at Huayuankou in the middle reaches, but the decrease differs from that observed in the upper reaches. Decrease in precipitation is responsible for 30% of the decrease in sediment load at Huayuankou, while the remaining 70% is ascribed to human activities in the river basin, of which soil conservation practices contribute 40% to the total decrease. Sediment retention within reservoirs accounts for 20% of the total sediment load decrease, although there was notable sediment retention within the Xiaolangdi reservoir from 2000 to 2005. The remaining 10% of the decrease in sediment load is a result of the operation of reservoirs in the upper reaches. In the lower reaches, 20% of the sediment passing Huayuankou has been lost as a result of channel deposition and water abstraction. Soil conservation practices and the operation of reservoirs have lowered the content of coarser sediment (D > 0.05 mm) at Huayuankou, and reduced channel deposition in the lower reaches. In contrast, sediment loss owing to water abstraction in the lower reaches has increased considerably as water consumption for agricultural needs has increased. Therefore, the combined effects of climate change and human activities in the upper, middle, and lower reaches have resulted in stepwise decreases in the sediment load delivered from the Huanghe to the sea. The Huanghe provides an excellent example of the altered river systems impacted by climate change and extensive human activities over the past 56 yr. Further dramatic decreases in sediment load and water discharge in the Huanghe will trigger profound geological, morphological, ecological, and biogeochemical responses in the estuary, delta, and coastal sea.  相似文献   

3.
The Magdalena, a world-class river, in the top ten in terms of sediment load ∼ 150 MT/yr, is the largest river discharging directly into the Caribbean Sea. Data on water discharge, sediment load, and dissolved load of the Magdalena River is presented as an initial interpretation of coastal ecosystems changes in relation to water discharge and sediment load from the Magdalena. During the 1972–1998 yr-period, the Magdalena River has delivered approximately 4022 MT of sediment to the Caribbean coast. The river reflects high inter-annual variability and delivers large portions of its fluvial discharge and sediment loads in short periods of time. The analysis of annual deviations from the 27-yr mean sediment load indicates that 59% of the total sediment load variability of the Magdalena at Calamar could be attributed to flashy peak events. Further analyses of sediment load anomalies suggest that there was a high discharge period in the Magdalena River between 1985 and 1995 and another one in the Canal del Dique between 1985 and 1992. These increasing trends in sediment load coincide with the overall decline of live coral cover around the Rosario Islands, a 145 km2 coral reef complex in the Caribbean Sea that constitutes a marine protected area. The comparison of live coral: algae ratios for the 1983–2004 yr-period, also indicates that there has been an associated increase in the percentage of algae cover (i.e., Grande Island 1983 = 5%, 2004 = 59%). Other analyses show that nearly 850 ha of seagrass existing in the Cartagena Bay in the 1930s, only 76 ha remained in 2001, which is less than 8% of the original cover. There has been a mix of multiple stressors (natural and anthropogenic; local, regional and global; temporal and chronic) affecting the coastal ecosystems in the area, but the effect of the Magdalena River runoff has been constant and very prolonged (several decades). The impacts of heavy sediment loads and freshwater discharges from the Canal del Dique to Cartagena Bay have greatly contributed to the partial disappearance of coral formations and also to a considerable reduction in abundance of seagrass beds in the bay and neighboring areas.  相似文献   

4.
During the last half of the 20th century, cumulative annual discharge from 137 representative rivers (watershed areas ranging from 0.3 to 6300 × 103 km2) to the global ocean remained constant, although annual discharge from about one-third of these rivers changed by more than 30%. Discharge trends for many rivers reflected mostly changes in precipitation, primarily in response to short- and longer-term atmospheric–oceanic signals; with the notable exception of the Parana, Mississippi, Niger and Cunene rivers, few of these “normal" rivers experienced significant changes in either discharge or precipitation. Cumulative discharge from many mid-latitude rivers, in contrast, decreased by 60%, reflecting in large part impacts due to damming, irrigation and interbasin water transfers. A number of high-latitude and high-altitude rivers experienced increased discharge despite generally declining precipitation. Poorly constrained meteorological and hydrological data do not seem to explain fully these “excess” rivers; changed seasonality in discharge, decreased storage and/or decreased evapotranspiration also may play important roles.  相似文献   

5.
Cryospheric change in China   总被引：16，自引：0，他引：16  
This paper provides an overview of the current status of the cryosphere in China and its changes. Up-to-date statistics of the cryosphere in China are summarized based on the latest available data. There are 46,377 glaciers in China, covering an area of 59,425 km2. The glacier ice reserve is estimated to be about 5600 km3 and the annual glacier runoff is about 61.6 × 109 m3. The continuous snow cover extent (> 60 days) in China is about 3.4 × 106 km2 and the maximum water equivalent is 95.9 × 109 m3 yr− 1. The permafrost area in China is about 1.72 × 106 km2. The total ground ice reserve on the Qinghai–Tibetan Plateau is estimated to be about 10,923 km3. Recent investigations indicated that glacier areas in China have shrunk about 2–10% over the past 45 yr. Total glacier area has receded by about 5.5%. Snow mass has increased slightly. Permafrost is clearly degrading, as indicated by shrinking areas of permafrost, increasing depth of the active layer, rising of lower limit of permafrost, and thinning of the seasonal frost depth. Some models predict that glacier area shrinkage could be as high as 26.7% in 2050, with glacier runoff increasing until its maximum in about 2030. Although snow mass shows an increasing trend in western China, in eastern China the trend is toward decreasing snow mass, with increasing interannual fluctuations. Permafrost degradation is likely to continue, with one-third to one-half of the permafrost on the Qinghai–Tibetan Plateau anticipated to degrade by 2100. Most of the high-temperature permafrost will disappear by then. The permafrost in northeastern China will retreat further northward.  相似文献   

6.
Accumulation of organic matter (OM) was studied in four ombrotrophic peat bogs in Finland: Harjavalta (vicinity of a Cu–Ni smelter), Outokumpu (near a closed Cu–Ni mine), Alkkia (Ni-treated site) and Hietajärvi (a pristine site). At each sampling site, two peat cores (15 × 15 × 100 cm) were taken. Age-dating of peat was determined using 210Pb method (CRS model). The local annual temperature sum and precipitation for the past 125 years were modeled. The objective was to compare recent net accumulation rates of heavy metal polluted ombrotrophic peat bogs with those of a pristine bog, and to study the relationship between weather and net accumulation rates. Based on 210Pb age-dating, the upper 16-cm peat layer at Harjavalta, 35 cm at Outokumpu and 25 cm at Hietajärvi represents 125 years of peat formation, yielding the following average peat accumulation rates: Harjavalta 1.3 mm year− 1, Outokumpu 2.8 mm year− 1 and Hietajärvi 2.0 mm year− 1. At the Alkkia site, the Ni treatment in 1962 had completely stopped the peat accumulation. Net accumulation rates were related to precipitation at Outokumpu, Harjavalta and Hietajärvi sites. In addition, emissions released from the nearby located Cu–Ni smelter could have affected negatively net OM accumulation rate at Harjavalta site.  相似文献   

7.
A digital 3D-reconstruction of the Baltic Ice Lake's (BIL) configuration during the termination of the Younger Dryas cold phase (ca. 11 700 cal. yr BP) was compiled using a combined bathymetric–topographic Digital Terrain Model (DTM), Scandinavian ice sheet limits, Baltic Sea Holocene bottom sediment thickness information, and a paleoshoreline database maintained at the Lund University. The bathymetric–topographic DTM, assembled from publicly available data sets, has a resolution of 500 × 500 m on Lambert Azimuthal Equal Area projection allowing area and volume calculations of the BIL to be made with an unprecedented accuracy. When the damming Scandinavian ice sheet margin eventually retreated north of Mount Billingen, the high point in terrain of Southern central Sweden bordering to lower terrain further to the north, the BIL was catastrophically drained resulting in a 25 m drop of the lake level. With our digital reconstruction, we estimate that approximately 7800 km3 of water drained during this event and that the ice dammed lake area was reduced by ca. 18%. Building on previous results suggesting drainage over 1 to 2 years, our lake volume calculations imply that the freshwater flux to the contemporaneous sea in the west was between about 0.12 and 0.25 Sv. The BIL reconstruction provides new detailed information on the paleogeography in the area of southern Scandinavia, both before and after the drainage event, with implications for interpretations of geological records concerning the post-glacial environmental development.  相似文献   

8.
Sediment has accumulated in Isfjorden, a deep fjord in Spitsbergen, at a rate of 1.7 km3/k.y. during the past 13 k.y. Between 200 ka and 13 ka the fjord was free of ice for 120 k.y. Assuming a similar sediment delivery rate during this ice-free time, 200 km3 of sediment would have accumulated in the fjord. An alternative calculation based on erosion rates suggests that 400 km3 could have been delivered to Isfjorden during this 120 k.y.Seismic studies have identified a 330 km3 package of sediment on the continental shelf and slope west of Isfjorden. This sediment is believed to have accumulated between 200 ka and 13 ka. Herein we argue that this is sediment that was originally deposited in the fjord, and that it was transferred to the shelf by glaciers in the 70 ka during which the fjord was occupied by ice. Calculations using a steady-state numerical model suggest that the sediment could have been moved in a deforming layer of subglacial till and in subglacial melt streams at rates of 7.6 × 106 m3 a−1 and 0.3 × 106 m3 a−1, respectively, resulting in a total flux of 7.9 × 106 m3 a−1. It is unlikely that much sediment was moved in a basal layer of dirty ice, as intense basal melting would have inhibited sediment entrainment.Of the time that glaciers occupied the fjord, 60% would have been required to evacuate the accumulated sediment. During the remaining time, the ice could have been deepening the fjord.  相似文献   

9.
The heat needed to melt snow over the Tien Shan mountains and Japanese Islands for 10-day period (TDP) was estimated. Melting curves and a map of snowmelt duration were obtained through the long-term data from 79 stations in the Tien Shan mountains and 20 stations in the Japanese Islands. At high elevations in the mountains, about 40% of the snow melts during penultimate 10 days of snow cover. In the Japanese Islands, about 80% of the snow melts during the last 20 days of snow cover. Over the mountains, 0.13×104 MJ m2 year−1 is needed to melt snow in the northern and western Tien Shan where maximum snow accumulation occurred. The volume of air cooled 10 °C by snowmelt amounted to 4.4×106 km3 year−1 over the Tien Shan mountains and 3×106 km3 year−1 over the Japanese Islands. The most significant impact of snowmelt on air temperature was observed at an elevation of 2500 m in the western and northern Tien Shan. Air that was cooled 10 °C could reach an elevation of 2.1 km day−1. Over the Japanese Islands, energy losses from snowmelt amounted to 0.26×1014 MJ year−1 and the maximum occurred over Honshu Island. The heat loss from snowmelt in the Tien Shan mountains and Japanese Islands amounted to about 2/3 of heat loss in the Eurasian continental plains.  相似文献   

10.
Mean-sea-level data from coastal tide gauges in the north Indian Ocean were used to show that low-frequency variability is consistent among the stations in the basin. Statistically significant trends obtained from records longer than 40 years yielded sea-level-rise estimates between 1.06–1.75 mm yr− 1, with a regional average of 1.29 mm yr− 1, when corrected for global isostatic adjustment (GIA) using model data. These estimates are consistent with the 1–2 mm yr− 1 global sea-level-rise estimates reported by the Intergovernmental Panel on Climate Change.  相似文献   

11.
A long series of lakes (~ 150) borders the Patagonian Andes (south of ~ 38°S), most of which are a geomorphologic relict of Pleistocene glaciations. Employing instrumental records, we inspected lake water level departures from seasonal variations in seven proglacial lakes: Lacar, Mascardi, Steffen, Escondido, Puelo, Vinter, and Argentino. Lakes north of ~ 42°S show maximum gage (water) level during austral winter months; lakes between ~ 42° and ~ 45°S appear transitional; the one lake south of ~ 50°S (Argentino) shows maximum water level in early autumn. Most lakes show moderate level fluctuation throughout yearly records and, in general, show heteroscedacity. Furthermore, Hurst exponents reveal persistent behavior (i.e., long-term memory effect) in all water level series. In most lakes there are no trends in deseasonalized mean and maximum water levels (Seasonal Kendall test). Lake Mascardi–Manso River system (mostly fed by melt water from the retreating Manso Glacier) is a contrasting example that shows a decreasing trend during summer months that we ascribe to the also declining ice volume. Harmonic analysis (Fourier and wavelet transform) of deseasonalized mean and maximum water level time series shows interannual and decadal periodicities that we link to the occurrence of El Niño and/or the Antarctic Oscillation. The associated phase spectrum indicates that there is a ~ 13-month lag between ENSO occurrences and its effect on anomalous lake water levels. Increased snow accumulation during austral winters usually follows summertime El Niño events, which normally result in increased melt water volume that occurs with about one-year delay during the following (austral) spring/summer.  相似文献   

12.
The possible effects of trace-gas induced climatic changes on Pyramid and Yellowstone Lakes are assessed using a model of lake temperature. The model is driven by years of hourly meteorological data obtained directly from the output of double-CO2 experiments (2 × CO2) conducted with a regional climate model nested in a general circulation model. The regional atmospheric model is the climate version of the National Center for Atmospheric Research/Pennsylvania State University mesoscale model, MM4.Average annual surface temperature of Pyramid Lake for the 2 × CO2 climate is 15.5 ± 5.4°C (±1 σ), 2.8°C higher than the control. Annual overturn of the lake ceases as a result of these higher temperatures for the 2 × CO2 climate. Evaporation increases from 1400 mm yr−1 in the control to 1595 mm yr−1 in the 2 × CO2 simulation, but net water supplied to the Pyramid Lake basin increases from −6 mm yr−1 in the control to +27 mm yr−1 in the 2 × CO2 simulation due to increased precipitation.For the open water periods, the average annual surface temperature of Yellowstone Lake is 13.2 ± 5.1°C for the 2 × CO2 climate, a temperature 1.6°C higher than the control. The annual duration of ice cover on the lake is 152 days in the 2 × CO2 simulation, a reduction of 44 days relative to the control. Warming of the lake for the 2 × CO2 climate is mostly confined to the near-surface. Simulated spring overturn for the 2 × CO2 climate occurs earlier in the year and fall overturn later than in the control. Evaporation increases from 544 mm yr−1 to 600 mm yr−1 in the 2 × CO2 simulation, but net water supplied to the Yellowstone Lake basin increases from +373 mm yr−1 in the control to +619 mm yr−1 due to increased precipitation. The effects of these climatic changes suggest possible deterioration of water quality and productivity in Pyramid Lake and possible enhancement of productivity in Yellowstone Lake.  相似文献   

13.
Elevation dependency of climate change signals has been found over major mountain ranges such as the European Alps and the Rockies, as well as over the Tibetan Plateau. In this study we examined the temporal trends in monthly mean minimum temperatures from 116 weather stations in the eastern Tibetan Plateau and its vicinity during 1961–2006. We also analyzed projected climate changes in the entire Tibetan Plateau and its surroundings from two sets of modeling experiments under future global warming conditions. These analyses included the output of the NCAR Community Climate System Model (CCSM3) with approximately 150 km horizontal resolution for the scenario of annual 1% increase in atmospheric CO2 for future 100 years and physically-based downscaling results from the NCAR CAM3/CLM3 model at 10' × 10' resolution during three 20-year mean periods (1980–1999, 2030–2049 and 2080–2099) for the IPCC mid-range emission (A1B) scenario. We divided the 116 weather stations and the regional model grids into elevation zones of 500 m interval to examine the relationship of climatic warming and elevation. With these corroborating datasets, we were able to confirm the elevation dependency in monthly mean minimum temperature in and around the Tibetan Plateau. The warming is more prominent at higher elevations than at lower elevations, especially during winter and spring seasons, and such a tendency may continue in future climate change scenarios. The elevation dependency is most likely caused by the combined effects of cloud-radiation and snow-albedo feedbacks among various influencing factors.  相似文献   

14.
Subglacial lakes and jökulhlaups in Iceland   总被引：1，自引：0，他引：1  
Active volcanoes and hydrothermal systems underlie ice caps in Iceland. Glacier–volcano interactions produce meltwater that either drains toward the glacier margin or accumulates in subglacial lakes. Accumulated meltwater drains periodically in jökulhlaups from the subglacial lakes and occasionally during volcanic eruptions. The release of meltwater from glacial lakes can take place in two different mechanisms. Drainage can begin at pressures lower than the ice overburden in conduits that expand slowly due to melting of the ice walls by frictional and sensible heat in the water. Alternatively, the lake level rises until the ice dam is lifted and water pressure in excess of the ice overburden opens the waterways; the glacier is lifted along the flowpath to make space for the water. In this case, discharge rises faster than can be accommodated by melting of the conduits. Normally jökulhlaups do not lead to glacier surges but eruptions in ice-capped stratovolcanoes have caused rapid and extensive glacier sliding. Jökulhlaups from subglacial lakes may transport on the order of 107 tons of sediment per event but during violent volcanic eruptions, the sediment load has been 108 tons.  相似文献   

15.
We present the first calculation of the kinetic Sunyaev–Zel’dovich (kSZ) effect due to the inhomogeneus reionization of the universe based on detailed large-scale radiative transfer simulations of reionization. The resulting sky power spectra peak at ℓ = 2000–8000 with maximum values of [ℓ(ℓ + 1)Cℓ/(2π)]max  4–7 × 10 −13. The scale roughly corresponds to the typical ionized bubble sizes observed in our simulations, of 5–20 Mpc. The kSZ anisotropy signal from reionization dominates the primary CMB signal above ℓ = 3000. At large-scales the patchy kSZ signal depends only on the source efficiencies. It is higher when sources are more efficient at producing ionizing photons, since such sources produce larger ionized regions, on average, than less efficient sources. The introduction of sub-grid gas clumping in the radiative transfer simulations produce significantly more power at small-scales, but has little effect at large-scales. The patchy reionization kSZ signal is dominated by the post-reionization signal from fully-ionized gas, but the two contributions are of similar order at scales ℓ  3000 − 104, indicating that the kSZ anisotropies from reionization are an important component of the total kSZ signal at these scales.  相似文献   

16.
Climatic changes of the 20th century have altered the water cycle in the Andean basins of central Argentina. The most visible change is seen in the mountain glaciers, with loss of part of their mass due to decreasing thickness and a substantial recession in the last 100 years. This paper briefly describes the results of glacier mass balance research since 1979 in the Piloto Glacier at the Cajón del Rubio, in the headwaters of Las Cuevas River, presenting new results for the period 1997–2003. Very large interannual variability of net annual specific balance is evident, due largely to variations in winter snow accumulation, with a maximum net annual value of + 151 cm w.e. and a minimum value of - 230 cm w.e. Wet El Niño years are normally associated with positive net annual balances, while dry La Niña years generally result in negative balances. Within the 24-year period, 67% of the years show negative net annual specific balances, with a cumulative mass balance loss of - 10.50 m water equivalent (w.e.). Except for exceptions normally related to El Niño events, a general decreasing trend of winter snow accumulation is evident in the record, particularly after 1992, which has a strong effect in the overall negative mass balance values. The glacier contribution to Las Cuevas River runoff is analysed based on the Punta de Vacas River gauge station for a hypothetical year without snow precipitation (YWSP), when the snowmelt component is zero. Extremely dry years similar to a YWSP have occurred in 1968–1969, 1969–1970 and 1996–1997. The Punta de Vacas gauge station is located 62 km downstream from Piloto Glacier, and the basin contains 3.0% of uncovered glacier ice and 3.7% of debris-covered ice. The total glacier contribution to Las Cuevas River discharge is calculated as 82 ± 8% during extremely dry years. If glacier wastage continues at the present trend as observed during the last 2 decades, it will severely affect the water resources in the arid central Andes of Argentina.  相似文献   

17.
We present a scientific case approached through high quality 3D NIR spectroscopy performed with CIRPASS, attached to the Gemini South telescope. A binary mass concentration at the nucleus of the galaxy M 83 was suggested by Thatte et al. [A&A 364 (2000) L47] and Mast et al. [BAAA 45 (2002) 98. Astroph#0505264] determined the possible position of the hidden secondary mass concentration with 2D H-alpha kinematics. The preliminary results of the NIR study presented here are based in almost 1500 spectra centered in the wavelength 1.3 μm, with a spectral resolving power of 3200. They allow us to unveil, with 0.36″ (6.4 pc) sampling and subarcsecond resolution, the velocity field in a region of 13″ × 9″ around the optical nucleus. We confirm that the optical nucleus is not located at the most important center of symmetry of the ionized gas velocity field. The largest black hole that could fit to the circular motion in this kinematic center should have a mass not larger than 3 × 106(sin i)−1 M solar masses.  相似文献   

18.
Multi-wavelength photometric observations of Cygnus X-3 were carried out at 18 cm through to 450 μm, complemented by X-ray (2–10 keV) observations. The system was mildly active with cm fluxes at 150–250 mJy. We find the spectrum to be flat with a spectral index of zero. Using a modified Wolf-Rayet wind model, and assuming emission is generated in synchrotron emitting jets from the source, we find an upper-limit to the magnetic field of 20 G at a distance 5×1012 cm is required.  相似文献   

19.
Climate changes and recent glacier behaviour in the Chilean Lake District   总被引：1，自引：1，他引：0  
Atmospheric temperatures measured at the Chilean Lake District (38°–42°S) showed contrasting trends during the second half of the 20th century. The surface cooling detected at several meteorological stations ranged from − 0.014 to − 0.021 °C a− 1, whilst upper troposphere (850–300 gpm) records at radiosonde of Puerto Montt (41°26′S/73°07′W) revealed warming between 0.019 and 0.031 °C a− 1. Regional rainfall data collected from 1961 to 2000 showed the overall decrease with a maximum rate of − 15 mm a− 2 at Valdivia st. (39°38′S/73°05′W). These ongoing climatic changes, especially the precipitation reduction, seem to be related to El Niño–Southern Oscillation (ENSO) phenomena which has been more frequent after 1976. Glaciers within the Chilean Lake District have significantly retreated during recent decades, in an apparent out-of-phase response to the regional surface cooling. Moreover, very little is known about upper troposphere changes and how they can enhance the glacier responses. In order to analyse their behaviour in the context of the observed climate changes, Casa Pangue glacier (41°08′S/71°52′W) has been selected and studied by comparing Digital Elevation Models (DEMs) computed at three different dates throughout the last four decades. This approach allowed the determination of ice elevation changes between 1961 and 1998, yielding a mean thinning rate of − 2.3 ± 0.6 m a− 1. Strikingly, when ice thinning is computed for the period between 1981 and 1998, the resulting rate is 50% higher (− 3.6 ± 0.6 m a− 1). This enhanced trend and the related area loss and frontal retreat suggests that Casa Pangue might currently be suffering negative mass balances in response to the upper troposphere warming and decreased precipitation of the last 25–30 yr, as well as debris cover would not prevent the glacier from a fast reaction to climate forcing. Most of recent glaciological studies regarding Andean glaciers have concentrated on low altitude changes, namely frontal variations, however, in order to better understand the regional glacier changes, new data are necessary, especially from the accumulation areas.  相似文献   

20.
It is now well accepted that surface processes provide a critical feedback on the surface tectonic deformation, whatever it is, orogenic building or basin evolution. However, the idea that the influence of these processes may go below the crustal levels, is less common. In this preliminary study, we use coupled thermo-mechanical numerical models to investigate the possible influence of surface processes on the styles of continental collision, in particular, continental subduction. For that, we further exploit the recent successful model of continental subduction of the early stages of India–Asia collision by Toussaint et al. [Toussaint G., Burov, E., and J.-P. Avouac, Tectonic evolution of a continental collision zone: a thermo-mechanical numerical model, Tectonics, 23, TC6003, doi:10.1029/2003TC001604, 2004b.]. On the example of India–Asia-like settings, we show that not only the surface topography but also the total amount of subduction may largely vary as function of denudation rate (controlled by the coefficient of erosion, k). Erosion provides a dynamic discharge of the hanging wall of the major thrust zone, whereas the sedimentation increases loading on the footwall and this helps down-thrusting of the lower plate. Both processes reduce the resistance of the major thrust and subduction channel to subduction. However, very strong or very slow erosion/sedimentation enhance the possibility of plate coupling and promote whole-scale thickening or buckling. The maximal amount of subduction is thus achieved for some intermediate values of erosion rates when the tectonic uplift rate is fine-balanced by the denudation rate. In our case the optimal balance is reached for the values of k on the order of 3000 m2/yr. We then extended our model beyond the conditions of India–Asia collision, in terms of the tested range of k and convergence rates. The experiments suggest that for provided settings, both extra slow (k < 50–100 m2/yr) and extra rapid erosion (k > 6000–8000 m2/yr) limit, by up to 50%, the total amount of subduction, if not totally prevent it. The model demonstrates the large capability of surface processes to adopt to different deformation styles: the orogenic building and subduction successfully develop (subduction number, S > 0.5) in the range of k between 500 m2/yr and 6000 m2/yr at convergence rates ranging from 1 cm/yr to 6 cm/yr. Within this range, some peculiar features of orogenic style such as the geometry of the accretion prism, amount of upper crustal subduction, horizontal progression of the mountain range/thrust fault and the amount of exhumation of metamorphic facies are sometimes quite different. We conclude that surface processes may control deep, mantle level tectonic evolution.  相似文献   

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