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1.
We investigate late glacial and Holocene climate change recorded in Lake Baikal using the oxygen isotope composition of diatom silica (δ18ODIAT). Evaporation from the lake is minor, and the temperature fractionations of δ18O are unable to explain variations in the δ18ODIAT record alone. Isotopically, low meltwater input from glaciers may have some influence on δ18ODIAT, but the assumed periods of climatic warming and wastage do not coincide with large shifts in δ18ODIAT. There is a gradual oxygen isotope lowering from 27.0‰ to 20.6‰ over the late glacial, while, during the Holocene, δ18ODIAT values return to relatively high values. Previous studies of the modern oxygen and hydrogen isotope composition of Lake Baikal's inputs reveal that fluvial input to the lake's North Basin are isotopically lower than fluvial input from South Basin rivers. This north–south gradient of river δ18O and δD is mainly due to the greater input from isotopically low winter precipitation in the north and isotopically higher summer precipitation in the south. As a result, the δ18ODIAT record from Lake Baikal can at least in part be explained by varying input from these sources related to seasonal changes in precipitation. Changes in atmospheric conditions may have a role in altering seasonality and the distribution of precipitation over Lake Baikal's catchment. A feedback mechanism is well known linking higher Eurasian spring snow cover extent (ESSC) to the development of anticyclonic conditions and low precipitation the following summer in the areas south of Lake Baikal. A simultaneous increase in the importance of depleted water (snowmelt) input from the north and decreased enriched summer precipitation in the south is needed to explain depletions in δ18O of lake water and subsequently δ18ODIAT during colder periods. The opposite of this situation is required to enrich lake water during warmer periods. The analysis of δ18O from diatom silica is a useful proxy for environmental change, especially in lakes, like Lake Baikal, where carbonates are absent or diluted. However, analysis must be based on near pure diatom samples as even trace amounts of silt can have a dominating effect on δ18ODIAT values.  相似文献   

2.
The climatological signal of δ18O variations preserved in ice cores recovered from Puruogangri ice field in the central Tibetan Plateau (TP) was calibrated with regional meteorological data for the past 50 years. For the period AD 1860–2000, 5-yearly averaged ice core δ18O and a summer temperature reconstruction derived from pollen data from the same ice core were compared. The statistical results provide compelling evidence that Puruogangri ice core δ18O variations represent summer temperature changes for the central TP, and hence regional temperature history during the past 600 years was revealed. A comparison of Puruogangri ice core δ18О with several other temperature reconstructions shows that broad-scale climate anomalies since the Little Ice Age occurred synchronously across the eastern and southern TP, and the Himalayas. Common cold periods were identified in the 15th century, 1625–1645 AD, 1660–1700 AD, 1725–1775 AD, 1795–1830 AD, 1850–1870 AD, 1890–1920 AD, 1940–1950 AD, and 1975–1985 AD. The period 1725–1775 AD was one of the most prolonged cool periods during the past 400 years and corresponded to maximum Little Ice Age glacier advance of monsoonal temperate glaciers of the TP.  相似文献   

3.
A Marine Isotope Stage (MIS) 3/early MIS 2 section from a structural high along the east coast of the North Basin of Lake Baikal was analysed for diatoms, C/N ratios, and organic carbon isotope ratios. Diatoms were present throughout MIS 3 and early MIS 2, with high concentrations of the planktonic taxa Cyclotella sp. c.f. gracilis between 54 and 51.5 kyr BP indicating relatively warm, interstadial, conditions. Following a %TOC inferred climatic cooling between 43.2 and 39.1 kyr BP, evidence of a more muted δ13C(organic) and %TOC inferred climatic warming from c. 39.1–34.7 kyr BP coincides with a period of very high diatom concentrations, indicating high aquatic productivity, at the Buguldeika Saddle in the South Basin of Lake Baikal. No evidence exists for a ‘Kuzmin’ catchment erosional event in the North Basin during MIS 3. This, however, may reflect the location of the coring site away from major riverine inputs. Abrupt climatic cooling at the culmination of both warm phases in the North Basin are associated, on the basis of the palaeomagnetic age-model and correlations to existing sites in Lake Baikal, with the initiation of Heinrich events 5 (c. 50 kyr BP) and 4 (c. 35 kyr BP), respectively, in the North Atlantic. The amount of organic material declines across the MIS 3/MIS 2 transition while constant C/N ratios suggest organic material to be predominantly derived from phytoplankton. An increase in δ13C(organic) at the MIS 3/MIS 2 transition may therefore indicate changes in aquatic productivity, pCO2 or the inorganic carbon pool.  相似文献   

4.
In this paper we report about a small region on the northern scarp of Olympus Mons showing an increase of the 3 μm hydration band in the OMEGA spectra, together with low superficial temperatures. Although water ice clouds can occurs on the flank of big martian volcanoes, radiative transfer modeling indicates that atmospheric water ice alone cannot justify the shape of the observed band. A fit of the 1.9–3 μm absorption features is obtained by hypothesizing that the study region consists of a mixture of dust and water ice covered by an optically thin (τ=0.08 at 3 μm) layer of dust. Thermal modeling also suggests that water ice in this region may be stable during most of the martian year due to the saturation of the atmosphere. If water ice is responsible for the observed spectral behavior, it might consist of a number of ice or snow patches possibly deposited in small depressions.  相似文献   

5.
Three years of regular weekly/biweekly monitoring of seasonal changes in temperature, transparency, chlorophyll a (CHL) and bacteria [erythrosine-stained microscopic counts and cultivable colony forming units (CFUs)] at the vertical profile in the South basin of Lake Baikal (51°54′195″N, 105°04′235″E, depth 800 m) were evaluated. In more detail, the structure and function of phytoplankton and the microbial loop in the euphotic layer at the same site were investigated during the late-winter–early-spring period under the ice. The depth of euphotic zone (up to 1% of surface irradiation) was 35 to 40 m. Primary production was measured three times a week with the 14C method in 2, 10, 20, 30 and 40 m. Maximum production was found in 10 m, with lower values towards the surface (light inhibition) and towards the lower layers. The total production in cells larger than 1 μm in the column (0–40 m) was 204–240 mg C d−1 m−2, 30–40% of it being in cells 1–3 μm (mostly picocyanobacteria), which represented roughly 9% of the total chlorophyll a (estimated from pigment analyses). A major part of phytoplankton biomass was formed by diatoms (Synedra acus Hust., Asterionella formosa Hass. and Stephanodiscus meyerii Genkal & Popovskaya). Total production (including extracellular, dissolved organic matter) was 235–387 mg C day−1 m−2, and the exudates were readily used by bacteria (particles 0.2–1 μm). This part amounted to 1–5% of cellular production in 2 to 20 m and 11–77% of cellular production in 20–40 m, i.e., in light-limited layers. From 0 to 30 m, chlorophyll a concentration was 0.8 to 1.3 μg l−1, wherefrom it decreased rapidly to 0.1 μg l−1 towards the depth of 40 m. Bacteria (DAPI-stained microscopic counts) reached 0.5–1.4×106 ml−1; their cell volumes measured via image analysis were small (average 0.05 μm−3), often not well countable when erythrosine stain was used. Bacterial biomasses were in the range of 6–21 μg C l−1. Numbers of colony forming units (CFUs) on nutrient fish-agar were c. 3–4 orders lower than DAPI counts. The amounts of heterotrophic protists were low, whereby flagellates reached 6 to 87 ml−1 and ciliates, 0.2–1.2 ml−1 (mostly Oligotrichida). Bacterial production was measured in the same depths as primary production using 3H-thymidine (Thy) and 14C-leucine (Leu) uptake. Consistently, bacterial abundances, biomasses, thymidine and leucine production were higher by 30–50% in layers 2, 10 and 20 m compared with that in the deeper 30 and 40 m, where cellular primary production was negligible. Leucine uptake in the deeper layers was even three times lower than in the upper ones. From the comparison of primary and bacterial production, bacteria roughly use 20–40% of primary production during 24 h in the layers 2 to 20 m.  相似文献   

6.
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   

7.
Australia, New Zealand and South America are the main sources of terrestrial climate change records for midlatitudes in the Southern Hemisphere. The advantage of studying the New Zealand record is that its vegetation has been subject to human influence for only the last thousand years. Vegetation records for Auckland are important because earlier work indicates that during the Last Glacial Maximum, the boundary between scrubland and forest lay in the Auckland region. Auckland is situated in a volcanic field and the coring site was in the crater of a small extinct volcano (Crater Hill, formed about 29 ka BP). The 4-m long core contained sediment dating from c. 5 to c. 18 ka BP. We present pollen and diatom records from this core.The pollen records from basal clays indicate southern beech forest (mainly Nothofagus menziesii) was present in the region around Crater Hill from 18 to 14.5 ka BP. At this time, there were areas of scrub in the crater surrounding a hardwater lake. The southern forest limit could well have been close to the site. Records from overlying peat indicate beech forest was replaced by Podocarp broadleaf forest as the Last Glacial ended. Metrosideros spp. (coastal forest trees) peak in the early Holocene. This coincides with an impoverished diatom flora which indicates drier conditions in the basin. When the lake reformed in the Holocene on peat its water was more acidic.  相似文献   

8.
The recovery of several thousand meteorite fragments from Antarctica has led to speculation that accumulations may occur on the Earth's other major ice caps. Meteorites falling over the past 80,000 years on the Devon Island ice cap in the Canadian Arctic may be exposed at the surface near the ice cap margin. From the terrestrial meteorite flux, ice movement rates, and fragmentation factors it is calculated that 12,560 samples of 150 g mass are potentially concentrated in a 65 km2 zone along the northwest margin. A search of this region on foot and by helicopter in July, 1981, failed to recover any specimens. Although metre-sized gneissic boulders, plucked from the underlying Precambrian basement, were concentrated in this zone it is postulated that the unseasonal 30 cm snow cover on the ice prohibited the recognition of possible meteorite specimens, which may average only 5 cm in diameter.  相似文献   

9.
Increased melting on glaciers and ice sheets and rising sea level are often mentioned as important aspects of the anticipated greenhouse warming of the earth's atmosphere. This paper deals with the sensitivity of Greenland's ice mass budget and presents a tentative projection of the Greenland component of future sea level rise for the next few hundred years. To do this, the ‘Villach II temperature scenario’ is prescribed,output from a comprehensive mass balance model is used to drive a high-resolution 3-D thermomechanic model of the ice sheet.The mass balance model consists of two parts: the accumulation part is based on presently observed values and is forced by changes in mean anr tempeerature. The ablation model is based on the degree-day method and accounts for daily and annual temperature cycle, a different degree-day factor for ice and snow melting and superimposed ice formation. Under present-day climatic conditions, the following total mass balance results (in ice equivalent per years): 599.3 × 109 m3 of accumulation, 281.7 × 109m3 of runoff assuming a balanced budget, 317.6 × 109m3 of iceberg calving. A 1K uniform warming is then calculated to increase the runoff by 119.5 × 109 m3. Since accumulation also increases by 32 × 109 m3, this leads to reduction of the total mass balance by 887.5 × 109 m3 of ice, corresponding to a sea level rise of 0.22 mm/yr. For temperature increase larger than 2.7 K, runoff, exceeds accumulation, and if ice sheet dynamics were to remain unchanged, this would add an extra amount of 0.8 mmyr to the worl's oceans.Imposing the Villach II scenario (warming up to 4.23 K) and accumulating mass balance changes forward in time (static response) would then result in a global sea level rise of 7.1 cm by 2100 AD, but this figure may go up to as much as 40 cm per century in case the warming is doubled. In a subsequent dynamic model involving the ice flow, the ice sheet is found to produce a counteracting effect by dynamically producing steeper slopes at the margin, thereby reducing the area over which runoff can take place. This effect is particularly apparent in the northeastern part of the ice sheet, and is also more pronounced for the smaller temperature perturbations. Nevertheless, all these experiments certainly highlight the vulnerability of the Greenland ice sheet with respect to a climatic warming.  相似文献   

10.
The snow cover of the Northern Patagonia Icefield (NPI) was monitored after applying the Normalized Difference Snow Index (NDSI) and the Red/NIR band ratio to 134 Moderate Resolution Imaging Spectroradiometer (MODIS) images captured between 2000 and 2006. The final results show that the snow cover extent of the NPI fluctuates a lot in winter, in addition to its seasonal behaviour. The minimum snow cover extent of the period (3600 km2) was observed in March 2000 and the maximum (11,623 km2) in August 2001. We found that temperature accounts for approximately 76% of the variation of the snow cover extent over the entire icefield. We also show two different regimes of winter snow cover fluctuations corresponding to the eastern and the western sides of the icefield. The seasonality of the snow cover on the western side was determined by temperature rather than precipitation, while on the east side the seasonality of the snow cover was influenced by the seasonal behaviour of both temperature and precipitation. This difference can be explained by the two distinct climates: coastal and continental. The fluctuations in the winter snow cover extent were more pronounced and less controlled by temperature on the western side than on the eastern side of the icefield. Snow cover extent was correlated with temperature R2 = 0.75 and R2 = 0.74 for the western and eastern sides, respectively. Since limited meteorological data are available in this region, our investigation confirmed that the change in snow cover is an interesting climatic indicator over the NPI providing important insights in mass balance comprehension. Since snow and ice were distinguished snow cover fluctuations can be associated to fluctuations in the snow accumulation area of the NPI. In addition, days with minimum snow covers of summer season can be associated to the period in which Equilibrium Line Altitude (ELA) is the highest.  相似文献   

11.
Hielo Patagónico Sur (HPS), an icefield in Southern Patagonia, is the largest temperate ice mass in the southern hemisphere. Despite continued research efforts during the last decade many glaciological variables, especially mass balance, are still poorly known. This is partly because access to the icefield is difficult due to remoteness and persistent harsh weather conditions. Therefore, remote sensing appears to be a more suitable tool for the acquisition of data. In this work we present a remote sensing study of snow zonation on HPS using Landsat 5 Thematic Mapper data acquired on 12 March 2001. By using image processing and classification techniques, proved to be useful in other glaciated regions, we map for the first time the extent and occurrence of major snow zones on the whole HPS. We separate between two classes of ice and three classes of snow. Ice facies are classified as bare or debris (i.e. dirt) covered ice, covering 2454 km2 and 777 km2 respectively, or 18.4% and 5.8% of the icefield on the day of image acquisition. Snow types are classified according to spectral differences in the images, following the glacier facies concept. Two of the three snow cover types are interpreted to represent differences in snow grain size within a fairly homogeneous snow pack whereas the third one is interpreted to represent the slush zone. A first order altitudinal control on the distribution of these snow facies is evident. In addition, our results show that snow accumulation on HPS is markedly controlled by the interaction of strong west–northwest snow-bearing winds and the rough mountainous terrain. In order of decreasing altitude we find that the two snow facies and the slush facies occupy 3819 km2, 3292 km2 and 2295 km2 respectively, or 28.6%, 24.6% and 17.2% of the icefield, on the day of image acquisition. Estimates of equilibrium line altitude using our results yield values of 800–900 m above sea level for the western side and 1500–1600 m above sea level for the eastern side, with an accumulation area ratio of 0.74.  相似文献   

12.
Snow algae in a 45.97-m-long ice core from the Tyndall Glacier (50°59′05″S, 73°31′12″W, 1756 m a.s.l.) in the Southern Patagonian Icefield were examined for potential use in ice core dating and estimation of the net accumulation rate. The core was subjected to visual stratigraphic observation and bulk density measurements in the field, and later to analyses of snow algal biomass, water isotopes (18O, D), and major dissolved ions. The ice core contained many algal cells that belonged to two species of snow algae growing in the snow near the surface: Chloromonas sp. and an unknown green algal species. Algal biomass and major dissolved ions (Na+, K+, Mg2+, Ca2+, Cl−, SO42−) exhibited rapid decreases in the upper 3 m, probably owing to melt water elution and/or decomposition of algal cells. However, seasonal cycles were still found for the snow algal biomass, 18O, D-excess, and major ions, although the amplitudes of the cycles decreased with depth. Supposing that the layers with almost no snow algae were the winter layers without the melt water essential to algal growth, we estimated that the net accumulation rate at this location was 12.9 m a− 1 from winter 1998 to winter 1999, and 5.1 m from the beginning of winter to December 1999. These estimates are similar to the values estimated from the peaks of 18O (17.8 m a− 1 from summer 1998 to summer 1999 and 11.0 m from summer to December 1999) and those of D-excess (14.7 m a− 1 from fall 1998 to fall 1999 and 8.6 m a− 1 from fall to December 1999). These values are much higher than those obtained by past ice core studies in Patagonia, but are of the same order of magnitude as those predicted from various observations at ablation areas of Patagonian glaciers.  相似文献   

13.
We present a 50-year pollen record at near-annual resolution from Gahai Lake in the Qaidam Basin on the northeastern Tibetan Plateau. Chronology of a 22-cm short core was established by 210Pb and 137Cs analysis. The pollen results at 0.5 cm intervals show large changes in Artemisia/Chenopodiaceae (A/C) ratios from < 0.2 to 0.95 in the last 50 years. High (low) A/C ratios represent increase (decrease) in steppe pollen production, which correspond to high (low) relative humidity observed at nearby Delingha weather station. On the basis of good correspondence with instrumental records and carbonate content from Gahai Lake, we conclude that A/C ratio is sensitive to moisture change and can be a very useful index in reconstructing paleoclimate of arid regions. Comparison with pollen and snow accumulation data from Dunde ice core suggests that effective moisture at low and high elevations shows the opposite relationship when mountain precipitation was extremely high, possibly due to topography-induced uplifting and subsiding air dynamics.  相似文献   

14.
A pollen record from the core sediments collected in the northern part of Lake Baikal represents the latest stage of the Taz (Saale) Glaciation, Kazantsevo (Eemian) Interglacial (namely the Last Interglacial), and the earliest stage of the Zyryanka (Weichselian) Glaciation. According to the palaeomagnetic-based age model applied to the core, the Last Interglacial in the Lake Baikal record lasted about 10.6 ky from 128 to 117.4 ky BP, being more or less synchronous with the Marine Isotope Stage 5e. The reconstructed changes in the south Siberian vegetation and climate are summarised as follows: a major spread of shrub alder (Alnus fruticosa) and shrub birches (Betula sect. Nanae/Fruticosae) in the study area was a characteristic feature during the late glacial phase of the Taz Glaciation. Boreal trees e.g. spruce (Picea obovata) and birch (Betula sect. Albae) started to play an important role in the regional vegetation with the onset of the interglacial conditions. Optimal conditions for Abies sibirica–P. obovata taiga development occurred ca. 126.3 ky BP. The maximum spread of birch forest-steppe communities took place at the low altitudes ca. 126.5–125.5 ky BP and Pinus sylvestris started to form forests in the northern Baikal area after ca. 124.4 ky BP. Re-expansion of the steppe communities, as well as shrubby alder and willow communities and the disappearance of forest vegetation occurred at about 117.4 ky BP, suggesting the end of the interglacial succession. The changes in the pollen assemblages recorded in the sediments from northern Baikal point to a certain instability of the interglacial climate. Three phases of climate deterioration have been distinguished: 126–125.5, 121.5–120, and 119.5–119 ky BP. The penultimate cooling signal may be correlated with the cool oscillation recorded in European pollen records. However, such far distant correlation requires more careful investigation.  相似文献   

15.
Glacial bedform patterns and sediments deposited by the temperate and polythermal Late Devensian ice sheet in north-central Ireland record changes in the processes, location, and magnitude of subglacial meltwater throughout the last full glacial cycle (21–14 14C kyear BP). Meltwater characteristics are related directly to basal ice thermal regime and ice dynamics, including ice velocity and shifts in the location of ice centres. Therefore, reconstructed meltwater characteristics may provide insight into wider controls on dynamic ice behaviour. A range of meltwater-related features are present across north-central Ireland. These include tunnel valleys, drumlin leeside sequences, eskers, and boulder lags. Other bedforms including Rogen moraines were modified by meltwater activity along ice streams. Meltwater was stored subglacially in two contrasting regions located beneath or near ice centres in north-central Ireland. (1) The Lough Erne Basin is developed in a lowland depression occupied partly by subglacial Rogen moraine ridges which were formed around the time of the last glacial maximum. Meltwater was stored between Rogen ridge crests and released by hydraulic jacking associated with drumlinisation (16.6 14C kyear BP) and ice streaming (13.8 14C kyear BP). (2) The Lough Neagh Basin occupies a similar lowland depression and was the location of an ice sheet centre throughout the last glacial cycle. No bedforms are present beneath or immediately surrounding Lough Neagh. A larger, more continuous meltwater lake existed in the Lough Neagh depression, probably sealed by a region of cold-based ice outside lake margins. Water escaped through regional-scale tunnel valleys, particularly the Poyntzpass channel which was active during the Carlingford ice readvance (Killard Stadial, correlated with Heinrich event 1 at 14.5 14C kyear BP). Overall, reconstructed subglacial lake characteristics and drainage mechanisms are related closely to basal ice thermal regime and substrate relief (controlling lake geometry), and provide insight into controls on overall ice sheet dynamics.  相似文献   

16.
It is investigated whether conditions for melting can be temporarily created in the upper sub-surface parts of snow/ice-packs on Mars at subzero surface temperatures by means of the solid-state greenhouse effect, as occurs in snow- and ice-covered regions on Earth. The conditions for this possible temporary melting are quantitatively described for bolometric albedo values A = 0.8 and A = 0.2, and with model parameters typical for the thermo-physical conditions at snow/ice sites on the surface of present Mars. It is demonstrated by numerical modelling that there are several sets of parameters which will lead to development of layers of liquid water just below the top surface of snow- and ice-packs on Mars. This at least partial liquefaction occurs repetitively (e.g. diurnally, seasonally), and can in some cases lead to liquid water persisting through the night-time in the summer season. This liquid water can form in sufficient amounts to be relevant for macroscopic physical (rheology, erosion), for chemical, and eventually also for biological processes. The creation of temporary pockets of sub-surface water by this effect requires pre-existing snow or ice cover, and thus is more likely to take place at high latitudes, since the present deposits of snow/ice can mainly be found there. Possible rheologic and related erosion consequences of the appearance of liquid sub-surface water in martian snow/ice-packs are discussed in view of current observations of recent rheologic processes.  相似文献   

17.
This paper is a comparison and compilation of lichenometric and geomorphic studies performed by two independent teams in the Cordillera Blanca, Peru, in 1996 and 2002 on 66 “Little Ice Age” moraines of 14 glaciers. Using eleven new control points, we recalibrated the initial rapid growth phase of the previously established Rhizocarpon subgenus Rhizocarpon growth curve. This curve was then used to estimate the age of “Little Ice Age” moraines. The time of deposition of the most prominent and numerous terminal and lateral moraines on the Pacific-facing side of the Cordillera Blanca (between AD 1590 and AD 1720) corresponds to the coldest and wettest phase in the tropical Andes as revealed by ice-core data. Less prominent advances occurred between AD 1780 and 1880.  相似文献   

18.
Debris-flow activity on the forested cone of the Ritigraben torrent (Valais, Swiss Alps) was assessed from growth disturbances in century-old trees, providing an unusually complete record of past events and deposition of material. The study of 2246 tree-ring sequences sampled from 1102 Larix decidua Mill., Picea abies (L.) Karst. and Pinus cembra ssp. sibirica trees allowed reconstruction of 123 events since AD 1566. Geomorphic mapping permitted identification of 769 features related to past debris-flow activity on the intermediate cone. The features inventoried in the study area covering 32 ha included 291 lobes, 465 levées and 13 well-developed debris-flow channels. Based on tree-ring records of disturbed trees growing in or next to the deposits, almost 86% of the lobes identified on the present-day surface could be dated. A majority of the dated material was deposited over the last century. Signs of pre-20th century events are often recognizable in the tree-ring record of survivor trees, but the material that caused the growth anomaly in trees has been completely overridden or eroded by more recent debris-flow activity.Tree-ring records suggest that cool summers with frequent snowfalls at higher elevations regularly prevented the release of debris flows between the 1570s and 1860s; the warming trend combined with greater precipitation totals in summer and autumn between 1864 and 1895 provided conditions that were increasingly favorable for releasing events from the source zone. Enhanced debris-flow activity continued well into the 20th century and reconstructions show a clustering of events in the period 1916–1935 when warm–wet conditions prevailed during summer in the Swiss Alps. In contrast, very low activity is observed for the last 10-yr period (1996–2005) with only one debris-flow event recorded on August 27, 2002. Since sediment availability is not a limiting factor, this temporal absence of debris-flow activity is due to an absence of triggering events, which not only shifted from June and July to August and September over the 20th century, but also seemed to be initiated primarily by persistent precipitation rather than summer thunderstorms. From the reconstructions, based on RCM simulations, there are indications that debris-flow frequencies might continue to decrease in the future, as precipitation events are projected to occur less frequently in summer but become more common in spring or autumn.  相似文献   

19.
Understanding the collisional behavior of ice dust aggregates at low velocity is a key to determining the formation process of small icy bodies such as icy planetesimals, comets and icy satellites, and this collisional behavior is also closely related to the energy dissipation mechanism in Saturn’s rings. We performed head-on collision experiments in air by means of free-falling centimeter-sized sintered snowballs with porosities from 44% to 80% at impact velocities from 0.44 m s?1 to 4.12 m s?1 at ?10 °C. In cases of porosity larger than 70%, impact sticking was the dominant collision outcome, while bouncing was dominant at lower porosity. Coefficients of restitution of snow in this velocity range were found to depend strongly on the porosity rather than the impact velocity and to decrease with the increase of the porosity. We successfully measured the compaction volume of snowballs after the impact, and it enabled us to estimate the dynamic compressive strength of snow with the assumption of the energy conservation between kinetic energy and work for deformation, which was found to be consistent with the upper limit of static compressive strength. The velocity dependence of coefficients of restitution of snow was analyzed using a Johnson’s model, and a diagram for collision outcomes among equal-sized sintered snowballs was successfully drawn as a function of porosity and impact velocity.  相似文献   

20.
Caleb I. Fassett 《Icarus》2007,189(1):118-135
Ceraunius Tholus, a Hesperian-aged volcano in the Tharsis region, is characterized by small radial valleys on its flanks, and several larger valleys originating near its summit caldera. All of these large valleys drain from near the lowest present portion of the caldera rim and down the flanks of the volcano. The largest valley debauches into Rahe Crater (an oblique impact crater), forming a depositional fan. Recent study of climate change on Mars suggests that many low-latitude regions (especially large volcanic edifices) were periodically the sites of snow accumulation, likely triggered by variations in spin orbital parameters. We apply a conductive heat flow model to Ceraunius Tholus that suggests that following magmatic intrusion, sufficient heating would be available to cause basal melting of any accumulated summit snowpack and produce sufficient meltwater to cause the radial valleys. The geometry of the volcano summit caldera suggests that meltwater would also accumulate in a volumetrically significant caldera lake. Analysis of the morphology and volumes of the largest valley, as well as depositional features at its base, suggest that fluvial erosion due to drainage of this summit caldera lake formed the large valleys, in a manner analogous to how valleys were formed catastrophically from a lake in Aniakchak caldera in Alaska. Moreover, the event which carved the largest valley on Ceraunius Tholus appears to have led to the formation of a temporary lake within Rahe Crater, at its base. The more abundant, small valleys on the flanks are interpreted to form by radial drainage of melted ice or snow from the outside of the caldera rim. Comparison of Ceraunius Tholus with the volcano-capping Icelandic ice sheet Myrdalsjokull provides insight into the detailed mechanisms of summit heating, ice-cap accumulation and melting, and meltwater drainage. These observations further underline the importance of a combination of circumstances (i.e., climate change to produce summit snowpack and an active period of magmatism to produce melting) to form the valley systems on some martian volcanoes and not on others.  相似文献   

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