A Late Quaternary catastrophic flood in the Lahul Himalayas

Impressive flood deposits are described resulting from a catastrophic lake outburst in the Upper Chandra valley in the Lahul Himalaya, northern India. Reconstructions of the former glacial lake, Glacial Lake Batal, and the discharges were undertaken using landforms and sediment data. The glacial dam burst released 1.496 km³ of water in 0.72 days, with peak discharges of between 21000 and 27000 m³ s⁻¹ at Batal. Dating by OSL suggests the flood occurred ca. 36.9 ± 8.4 to 43.4 ± 10.3 ka ago. This cataclysmic flood was responsible for major resedimentation and landscape modification within the Chandra valley.     

Evidence for Holocene megafloods down the Tsangpo River gorge, southeastern Tibet

Lacustrine and alluvial terraces and sediments record the extent of at least two Holocene glacially dammed lakes immediately upstream of the Tsangpo River gorge at the eastern syntaxis of the Himalaya. The larger lake covered 2835 km², with a maximum depth of 680 m and contained an estimated 832 km³ of water; the smaller lake contained an estimated 80 km³ of water. Radiocarbon dating of wood and charcoal yielded conventional radiocarbon ages of 8860 ± 40 and 9870 ± 50 ¹⁴C yr B.P. for the higher set of lake terraces, and 1220 ± 40 and 1660 ± 40 ¹⁴C yr B.P. for sediments from the lower terraces. Catastrophic failure of the glacial dams that impounded the lakes would have released outburst floods down the gorge of the Tsangpo River with estimated peak discharges of up to 1 to 5 × 10⁶ m³ s⁻¹. The erosive potential represented by the unit stream power calculated for the head of the gorge during such a catastrophic lake breakout indicates that post-glacial megafloods down the Tsangpo River were likely among the most erosive events in recent Earth history.     

Peak discharge of a Pleistocene lava-dam outburst flood in Grand Canyon, Arizona, USA

The failure of a lava dam 165,000 yr ago produced the largest known flood on the Colorado River in Grand Canyon. The Hyaloclastite Dam was up to 366 m high, and geochemical evidence linked this structure to outburst-flood deposits that occurred for 32 km downstream. Using the Hyaloclastite outburst-flood deposits as paleostage indicators, we used dam-failure and unsteady flow modeling to estimate a peak discharge and flow hydrograph. Failure of the Hyaloclastite Dam released a maximum 11 × 10⁹ m³ of water in 31 h. Peak discharges, estimated from uncertainty in channel geometry, dam height, and hydraulic characteristics, ranged from 2.3 to 5.3 × 10⁵ m³ s⁻¹ for the Hyaloclastite outburst flood. This discharge is an order of magnitude greater than the largest known discharge on the Colorado River (1.4 × 10⁴ m³ s⁻¹) and the largest peak discharge resulting from failure of a constructed dam in the USA (6.5 × 10⁴ m³ s⁻¹). Moreover, the Hyaloclastite outburst flood is the oldest documented Quaternary flood and one of the largest to have occurred in the continental USA. The peak discharge for this flood ranks in the top 30 floods (>10⁵ m³ s⁻¹) known worldwide and in the top ten largest floods in North America.     

Timing of glaciation and last glacial maximum paleoclimate estimates from the Fish Lake Plateau, Utah

The High Plateaus of Utah include seven separate mountain ranges that supported glaciers during the Pleistocene. The Fish Lake Plateau, located on the eastern edge of the High Plateaus, preserves evidence of at least two glacial advances. Four cosmogenic ³He exposure ages of boulders in an older moraine range from 79 to 159 ka with a mean age of 129 ± 39 ka and oldest ages of 152 ± 3 and 159 ± 5 ka. These ages suggest deposition during the type Bull Lake glaciation and Marine Oxygen Isotope Stage (MIS) 6. Twenty boulder exposure ages from four different younger moraines indicate a local last glacial maximum (LGM) of ~ 21.1 ka, coincident with the type Pinedale glaciation and MIS 2. Reconstructed Pinedale-age glaciers from the Fish Lake Plateau have equilibrium-line altitudes ranging from 2950 to 3190 m. LGM summer temperature depressions for the Fish Lake Plateau range from −10.7 to −8.2°C, assuming no change in precipitation. Comparison of the Fish Lake summer temperature depressions to a regional dataset suggests that the Fish Lake Plateau may have had a slight increase (~ 1.5× modern) in precipitation during the LGM. A series of submerged ridges in Fish Lake were identified during a bathymetric survey and are likely Bull Lake age moraines.     

Glacial Lake Vitim, a 3000-km outburst flood from Siberia to the Arctic Ocean

A prominent lake formed when glaciers descending from the Kodar Range blocked the River Vitim in central Transbaikalia, Siberia. Glacial Lake Vitim, evidenced by palaeoshorelines and deltas, covered 23,500 km² and held a volume of ~ 3000 km³. We infer that a large canyon in the area of the postulated ice dam served as a spillway during an outburst flood that drained through the rivers Vitim and Lena into the Arctic Ocean. The inferred outburst flood, of a magnitude comparable to the largest known floods on Earth, possibly explains a freshwater spike at ~ 13 cal ka BP inferred from Arctic Ocean sediments.     

Relationships among satellite chlorophylla, river inputs, and hypoxia on the Louisiana Continental shelf, Gulf of Mexico

SeaWiFS ocean color measurements were used to investigate interannual, monthly, and weekly variations in chlorophylla (chla) on the Louisiana shelf and to assess relationships with river discharge, nitrate load, and hypoxia. During the study period (2000–2003), interannual changes in shelf-wide chla concentrations averaged over January–July ranged from +57% to −33% of the 4-yr average, in accord with freshwater discharge changes of +20% to −29% and nitrate load changes of +20% to −35% from the Mississippi and Atchafalaya Rivers. Chla variations were largest on the shelf between the Mississippi and Atchafalaya Deltas. Within this region, which corresponds spatially to the area of most frequent hypoxia, lowest January–July mean chla concentrations (5.5 mg m⁻³ over 7,000 km²) occurred during 2000, the year of lowest freshwater discharge (16,136 m³ s⁻¹) and nitrate load (55,738 MT N d⁻¹) onto the shelf. Highest January–July mean chla concentrations (13 mg m⁻³ over 7,000 km²) were measured in 2002, when freshwater discharge (27,440 m³s⁻¹) and nitrate load (101,761 MT N d⁻¹) were highest and second highest, respectively. Positive correlations (R²=0.4–0.5) were found between chla and both fresh water and nitrate loads with 0 to 1 month lags, with the strongest relationships just west of the Mississippi Delta. In 2001, unusually clear skies allowed the identification of distinct spring and summer chla blooms west of the Mississippi Delta 4–5 wk after peaks in river discharge. East of the delta, the chla concentrations peaked in June and July, following the seasonal reversal in the coastal current. A clear linkage was not detected between satellite-measured chla and hypoxia during the 4-yr period, based on a time series of bottom oxygen concentrations at one station within the area of most frequent hypoxia. Clear relationships are confounded by the interaction of physical processes (wind stress effects) with the seasonal cycle of nutrient-enhanced productivity and are influenced by the prior year's nitrate load and carbon accumulation at the seabed.     

Hydrogeological and geochemical control of the variations of Rn concentrations in a hard rock aquifer: Insights into the possible role of fracture-matrix exchanges

To investigate the possible variations of Rn concentration in crystalline rocks as a function of flow conditions, a field study was carried out of a fractured aquifer in granite. The method is based on the in situ measurement of Rn in groundwater, aquifer tests for the determination of hydraulic characteristics of the aquifer and laboratory measurement of Rn exhalation rate from rocks. A simple crack model that simulates the Rn concentration in waters circulating in a fracture intersecting a borehole was also tested. The Rn concentrations in groundwaters from boreholes of the study site ranged from 192 to 1597 Bq L⁻¹. The Rn exhalation rates of selected samples of granite and micaschist were determined from laboratory experiments. The results yielded fluxes varying from 0.5 to 1.3 mBq m⁻² s⁻¹ in granite and from 0.5 to 0.9 mBq m⁻² s⁻¹ in micaschists. Pumping tests were performed in the studied boreholes to estimate the transmissivity and calculate the equivalent hydraulic aperture of the fractures. Transmissivities ranged from 10⁻⁵ to 10⁻³ m² s⁻¹. Using the cubic law, hydraulic equivalent fracture apertures were calculated to be in the range of 0.5–2.3 mm.     

A Late Pleistocene lacustrine transgression in the Fehmarn Belt,southwestern Baltic Sea

In order to reconstruct the late Pleistocene seismic stratigraphy, 550 km of high-density 3.5 kHz sub-bottom seismic-reflection profiles were recorded within a 70 km² area in the narrow offshore border zone between Germany and Denmark. A depositional unit 3 was analysed for its seismic facies association, and in the central study area mound, oblique, channel, reflection-free, shingled, hummocky, broken and parallel associations occurred. The rims of the association shows mound, oblique, hummocky and minor parallel seismic facies. Stratigraphic control was available from 32 coring sites. In the central study area, unit 3 represents rhythmic layers of silt with fine sand and clay in an overall fining-upward 3 to 5-m-thick sequence. At the rim of the basin, the unit comprises fine sand, silt and rhythmic layers of silt with clay and fine sand. Unit 3 is proposed as having formed during the transgression of a lake, partly in contact with the Baltic Ice Lake, some time between 10.8–10.0 ¹⁴C ka BP. After 10.0 ¹⁴C ka BP the water body stagnated when contact with the Baltic Basin was cut off. Analysis of base level of the subunits implies tectonic activity resulting in subsidence of the central study area which could possibly have influenced the transgressive development.     

GIS‐based reconstruction of Late Weichselian proglacial lakes in northwestern Russia and Belarus

Reconstructing ice‐lake histories is of considerable importance for understanding deglacial meltwater budgets and the role of meltwater reservoirs for sea‐level rise in response to climate warming. We used the latest data on chronology and ice‐sheet extents combined with an isostatically adjusted digital elevation model to reconstruct the development of proglacial lakes in the area of the Karelian ice stream complex of the Late Weichselian Scandinavian Ice Sheet on the East European Plain. We derived the deglacial ice lake development in seven time‐slices from 19 to 13.8 ka, assuming the individual ice‐marginal positions to be isochronous throughout the studied domain. Modelling is based on mapping of critical drainage thresholds and filling the depressions that are potentially able to hold meltwater. Such an approach underestimates the real dimensions of the ice lakes, because the role of erosion at the thresholds is not considered. Our modelling approach is sensitive to the (local) ice‐margin location. Our results prove the southward drainage of meltwater during the glacier extent maxima and at the beginning of deglaciation whereas rerouting to the west had taken place already around 17.5 ka, which is some 1.5 ka earlier than hitherto supposed. The total ice‐lake volume in the study area was lowest (~300 km³) during the maximum glacier extent and highest (~2000 km³) during the highstand of the Privalday Lake at c. 14.6 ka. At 14.6–14.4 ka, the Privalday Lake drained to the early Baltic Ice Lake. The released ~1500 km³ of water approximately corresponds to 20% of the early Baltic Ice Lake water volume and therefore it is unlikely that it was accommodated there. Thus, we argue that the additional meltwater drained through the Öresund threshold area between the early Baltic Ice Lake and the sea, becoming a part of the Scandinavian Ice Sheet's contribution to the Meltwater Pulse 1A event.     

Outburst floods from glacial Lake Missoula

The Pleistocene outburst floods from glacial Lake Missoula, known as the “Spokane Floods”, released as much as 2184 km³ of water and produced the greatest known floods of the geologic past. A computer simulation model for these floods that is based on physical equations governing the enlargement by water flow of the tunnel penetrating the ice dam is described. The predicted maximum flood discharge lies in the range 2.74 × 10⁶−13.7 × 10⁶ m³ sec⁻¹, lending independent glaciological support to paleohydrologic estimates of maximum discharge.     

Le cordon sableux Limani-Yagoua, extrême-nord Cameroun, et son rôle hydraulique

The Limani-Yagoua mega sand-ridge, covering an area of 330,000 km² is well know in the extreme north of Cameroun. Similar structures have also been described in the Chad Republic (Bongor-Koro-Toro), Niger (Tal) and Nigeria (Bama). Two possible origins can be considered: (i) circum-lacustrine around the Lake Chad Basin, interpreted as a Holocene or Pleistocene palæo-shoreline, or (ii) æolian accumulation as a mega sand dune in an area of basin subsidence. The sand-ridge in northern Cameroun is an extremely permeable aquifer with discharge rates of 3–6 m³ h⁻¹. High nitrate levels (27–127 mg l⁻¹) have been observed in several boreholes. Tritium at 4.1 ut indicates recent groundwater recharge, suggesting infiltration of rainfall damned by the sand-ridge.     

Holocene hydrological reconstructions from stable isotopes and paleolimnology, Cordillera Real, Bolivia

Multiproxy analyses of sediment cores from Lago Taypi Chaka Kkota (LTCK) Cordillera Real, Bolivia, provide a record of drier conditions following late Pleistocene deglaciation culminating in pronounced aridity between 6.2 and 2.3 ka B.P. Today LTCK is a glacial-fed lake that is relatively insensitive to changes in P–E because it is largely buffered from dry season draw-down through the year-round supply of glacial meltwater. This was not the case during the middle to late Holocene when glaciers were absent from the watershed. Lake-water δ¹⁸O values inferred from δ¹⁸O analysis of sediment cellulose range from −12.9 to −5.3‰ and average −8.7‰ between 6.2 and 2.3 ka B.P. Modern lake-water δ¹⁸O from LTCK averages −14.8‰ which is compatible with the δ¹⁸O_lw value of −14.3‰ for the surface sediment cellulose. Analyses of δ¹⁸O from modern surface waters in 23 lakes that span the range from glacial-fed to closed basin vary from −16.6 to −2.5‰. This approximates the magnitude of the down-core shift in δ¹⁸O_lw values in LTCK during the middle to late Holocene from −12.9 to −5.3‰. Strong paleohydrologic change during the middle Holocene is also evident in diatom assemblages that consist of shallow-water, non-glacial periphytic taxa and bulk organic δ¹³C and δ¹⁵N that show increases likely resulting from degradation of lacustrine organic matter periodically exposed to subaerial conditions. Neoglaciation began after 2.3 ka B.P. as indicated by changes in the composition of the sediments, lower δ¹⁸O values, and a return to diatom assemblages characteristic of the glacial sediments that formed during the Late Pleistocene. Collectively, these data indicate that the past 2.3 ka B.P. have been the wettest interval during the Holocene. Millennial-scale shifts in the paleohydrologic record of LTCK during the early to middle Holocene conform to other regional paleoclimatic time-series, including Lake Titicaca and Nevado Sajama, and may be driven by insolation and resultant changes in atmospheric circulation and moisture supply. In contrast, an apparent 1200-year lag in the onset of wetter conditions at LTCK (2.3 ka B.P.) compared to Lake Titicaca (3.5 ka B.P.) provides evidence for variable sub-regional hydrologic response to climate change during the middle to late Holocene.     

Holocene hydrological variability of Lake Ladoga,northwest Russia,as inferred from diatom oxygen isotopes

This article presents a new comprehensive assessment of the Holocene hydrological variability of Lake Ladoga, northwest Russia. The reconstruction is based on oxygen isotopes of lacustrine diatom silica (δ¹⁸O_diatom) preserved in sediment core Co 1309, and is complemented by a diatom assemblage analysis and a survey of modern isotope hydrology. The data indicate that Lake Ladoga has existed as a freshwater reservoir since at least 10.8 cal. ka BP. The δ¹⁸O_diatom values range from +29.8 to +35.0‰, and relatively higher δ¹⁸O_diatom values around +34.7‰ between c. 7.1 and 5.7 cal. ka BP are considered to reflect the Holocene Thermal Maximum. A continuous depletion in δ¹⁸O_diatom since c. 6.1 cal. ka BP accelerates after c. 4 cal. ka BP, indicating Middle to Late Holocene cooling that culminates during the interval 0.8–0.2 cal. ka BP, corresponding to the Little Ice Age. Lake‐level rises result in lower δ¹⁸O_diatom values, whereas lower lake levels cause higher δ¹⁸O_diatom values. The diatom isotope record gives an indication for a rather early opening of the Neva River outflow at c. 4.4–4.0 cal. ka BP. Generally, overall high δ¹⁸O_diatom values around +33.5‰ characterize a persistent evaporative lake system throughout the Holocene. As the Lake Ladoga δ¹⁸O_diatom record is roughly in line with the 60°N summer insolation, a linkage to broader‐scale climate change is likely.     

Chronology of latest Pleistocene mountain glaciation in the western Wasatch Mountains, Utah, U.S.A.

Understanding the timing of mountain glacier and paleolake expansion and retraction in the Great Basin region of the western United States has important implications for regional-scale climate change during the last Pleistocene glaciation. The relative timing of mountain glacier maxima and the well-studied Lake Bonneville highstand has been unclear, however, owing to poor chronological limits on glacial deposits. Here, this problem is addressed by applying terrestrial cosmogenic ¹⁰Be exposure dating to a classic set of terminal moraines in Little Cottonwood and American Fork Canyons in the western Wasatch Mountains. The exposure ages indicate that the main phase of deglaciation began at 15.7 ± 1.3 ka in both canyons. This update to the glacial chronology of the western Wasatch Mountains can be reconciled with previous stratigraphic observations of glacial and paleolake deposits in this area, and indicates that the start of deglaciation occurred during or at the end of the Lake Bonneville hydrologic maximum. The glacial chronology reported here is consistent with the growing body of data suggesting that mountain glaciers in the western U.S. began retreating as many as 4 ka after the start of northern hemisphere deglaciation (at ca. 19 ka).     

Late Pleistocene and Holocene glaciation of the Fish Lake valley,northeastern Alaska Range,Alaska

We reconstructed a chronology of glaciation spanning from the Late Pleistocene through the late Holocene for Fish Lake valley in the north‐eastern Alaska Range using ¹⁰Be surface exposure dating and lichenometry. After it attained its maximum late Wisconsin extent, the Fish Lake valley glacier began to retreat ca. 16.5 ka, and then experienced a readvance or standstill at 11.6 ± 0.3 ka. Evidence of the earliest Holocene glacial activity in the valley is a moraine immediately in front of Little Ice Age (LIA) moraines and is dated to 3.3–3.0 ka. A subsequent advance culminated at ca. AD 610–900 and several LIA moraine crests date to AD 1290, 1640, 1860 and 1910. Our results indicate that ¹⁰Be dating from high‐elevation sites can be used to help constrain late Holocene glacial histories in Alaska, even when other dating techniques are unavailable. Close agreement between ¹⁰Be and lichenometric ages reveal that ¹⁰Be ages on late Holocene moraines may be as accurate as other dating methods. Copyright © 2009 John Wiley & Sons, Ltd.     

Total particulate and reactive gaseous mercury in ambient air on the eastern slope of the Mt. Gongga area,China

Total particulate mercury (TPM) and reactive gaseous mercury (RGM) concentrations in ambient air on the eastern slope of the Mt. Gongga area, Sichuan Province, Southwestern China were monitored from 25 May, 2005 to 29 April, 2006. Simultaneously, Hg concentrations in rain samples were measured from January to December, 2006. The average TPM and RGM concentrations in the study site were 30.7 and 6.2 pg m⁻³, which are comparable to values observed in remote areas in Northern America and Europe, but much lower than those reported in some urban areas in China. The mean seasonal RGM concentration was slightly higher in spring (8.0 pg m⁻³) while the minimum mean concentration was observed in winter (4.0 pg m⁻³). TPM concentrations ranged across two orders of magnitude from 5.2 to 135.7 pg m⁻³ and had a clear seasonal variation: winter (74.1 pg m⁻³), autumn (22.5 pg m⁻³), spring (15.3 pg m⁻³) and summer (10.8 pg m⁻³), listed in decreasing order. The annual wet deposition was 9.1 μg m⁻² and wet deposition in the rainy season (May–October) represented over 80% of the annual total. The temporal distribution of TPM and RGM suggested distinguishable dispersion characteristics of these Hg species on a regional scale. Elevated TPM concentration in winter was probably due to regional and local enhanced coal burning and low wet deposition velocity. The RGM distribution pattern is closely related to daily variation in UV radiation observed during the winter sampling period indicating that photo-oxidation processes and diurnal changes in meteorology play an important role in RGM generation.     

Numerical simulation of the paleohydrology of glacial Lake Oshkosh, eastern Wisconsin, USA

Proglacial lakes, formed during retreat of the Laurentide ice sheet, evolved quickly as outlets became ice-free and the earth deformed through glacial isostatic adjustment. With high-resolution digital elevation models (DEMs) and GIS methods, it is possible to reconstruct the evolution of surface hydrology. When a DEM deforms through time as predicted by our model of viscoelastic earth relaxation, the entire surface hydrologic system with its lakes, outlets, shorelines and rivers also evolves without requiring assumptions of outlet position. The method is applied to proglacial Lake Oshkosh in Wisconsin (13,600 to 12,900 cal yr BP). Comparison of predicted to observed shoreline tilt indicates the ice sheet was about 400 m thick over the Great Lakes region. During ice sheet recession, each of the five outlets are predicted to uplift more than 100 m and then subside approximately 30 m. At its maximum extent, Lake Oshkosh covered 6600 km² with a volume of 111 km³. Using the Hydrologic Engineering Center-River Analysis System model, flow velocities during glacial outburst floods up to 9 m/s and peak discharge of 140,000 m³/s are predicted, which could drain 33.5 km³ of lake water in 10 days and transport boulders up to 3 m in diameter.     

Late Pleistocene and Holocene vegetation and climate records from Lake Kotokel, central Baikal region

Climatically driven Late Pleistocene and Holocene vegetation changes were reconstructed based on pollen records from the sediments of Lake Kotokel and Cheremushka Bog, located on the eastern shore of Lake Baikal. The described paleoenvironmental record has higher resolution than records collected from Lake Baikal and unites individual events identified in prior studies of bottom and onshore cores. Remarkable shifts in landscapes and expansions of index plants are as follows. Forest tundra and/or forest steppe landscape with birch, spruce, Artemisia, and Poaceae prevailed at ca. 50–25 ¹⁴C kyr BP. Tundra and/or steppe vegetation dominated by Artemisia and Poaceae was typical for the Last Glacial Maximum. The expansion of shrub birch and willow occurred at ca. 15.5 ¹⁴C kyr BP. Two peaks of spruce expansion at ca. 47.5–42.4 ¹⁴C kyr BP (Karginian time) and at ca. 14.5–13 ka (Bølling-Allerød warm intervals) suggest that the condition were more humid than today. A slight increase in Artemisia at ca. 11–10.5 ¹⁴C kyr BP (13–12 ka) was indicative of the Younger Dryas event. An expansion of birch forests with fir at ca. 12–6.4 ka suggests higher humidity. The currently dominant Scots and Siberian pine forests with birch expanded since 6.4 ka.     

Ground ice recharge via brine transport in frozen soils of Victoria Valley, Antarctica: Insights from modeling δO and δD profiles

Soils in the McMurdo Dry Valleys, Antarctica contain ice and considerable amounts of salt. Ice often occurs at shallow depth throughout the Dry Valleys and other areas of hyperarid permafrost, notably on Mars. This common occurrence of shallow ice is enigmatic; however, since according to published sublimation models it should disappear relatively quickly (at rates of order 0.1 mm a⁻¹) due to vapor loss to the atmosphere. This loss may be offset by recharge from snowmelt infiltrating and freezing in the soil. Herein, we present a first quantitative estimate of this recharge based on measured vertical profiles of δD and δ¹⁸O that reveal considerable detail about the sources and sinks of ice. We model these profiles, taking into account the salt content and a soil temperature record along a 1.6 m depth profile of ∼10 ka old ice-cemented soils in Victoria Valley, Antarctica. The stable isotopes of ice are enriched in heavy isotopes at the top of the ice cement (20 cm depth); both δD and δ¹⁸O values plotted against depth exhibit a concave upward curve. At depth, the isotope composition is similar to that of Lake Victoria and modern meteoric water. The concave shape of the isotope profile is suggestive of downward advection-dispersion of snowmelt water enriched in heavy isotopes into the ice cement. Our advection-dispersion model, coupled with field data, enables us to quantify the advective flux and dispersion of melt water into the ice. The advective velocity and dispersion coefficient depend on the time since advection began and the ice-to-brine ratio; they are, respectively, of the order of 10⁻¹¹-10⁻¹⁰ m s⁻¹ and 10⁻¹²-10⁻¹¹ m² s⁻¹. These values suggest that over the ∼10 ka time period, a total of 190 mm water infiltrated into the ice-cemented ground. The isotope composition and deuterium excess values of the uppermost ice cement can be modeled from snowmelt water enriched in salts using open system-Rayleigh fractionation. To develop the isotopic signature of the upper ice cement requires evaporation of ∼95% of the snowmelt water. Based on 190 mm brine infiltrating into the soil requires an initial total of ∼4 m of snowmelt water. This corresponds to ∼0.4 mm a⁻¹ suggesting that, under the current climate condition, water from snowmelt is sufficient to compensate modeled sublimation rates, and therefore conserve ground ice in Victoria Valley.     

A 90,000-year tephrostratigraphic framework of Aso Volcano, Japan

A detailed 90,000-year tephrostratigraphic framework of Aso Volcano, southwestern Japan, has been constructed to understand the post-caldera eruptive history of the volcano. Post-caldera central cones were initiated soon after the last caldera-forming pyroclastic-flow eruption (90 ka), and have produced voluminous tephra and lava flows. The tephrostratigraphic sequence preserved above the caldera-forming stage deposits reaches a total thickness of 100 m near the eastern caldera rim. The sequence is composed mainly of mafic scoria-fall and ash-fall deposits but 36 silicic pumice-fall deposits are very useful key beds for correlation of the stratigraphic sequence. Explosive, silicic pumice-fall deposits that fell far beyond the caldera have occurred at intervals of about 2500 years in the post-caldera activity. Three pumice-fall deposits could be correlated with lava flows or an edifice in the western part of the central cones, although the other silicic tephra beds were erupted at unknown vents, which are probably buried by the younger products from the present central cones. Most of silicic eruptions produced deposits smaller than 0.1 km³, but bulk volumes of two silicic eruptions producing the Nojiri pumice (84 ka) and Kusasenrigahama pumice (Kpfa; 30 ka) were on the order of 1 km³ (VEI 5). The largest pyroclastic eruption occurred at the Kusasenrigahama crater about 30 ka. This catastrophic eruption began with a dacitic lava flow and thereafter produced Kpfa (2.2 km³). Total tephra volume in the past 90,000 years is estimated at about 18.1 km³ (dense rock equivalent: DRE), whereas total volume for edifices of the post-caldera central cones is calculated at about 112 km³, which is six times greater than the former. Therefore, the average magma discharge rate during the post-caldera stage of Aso Volcano is estimated at about 1.5 km³/ky, which is similar to the rates of other Quaternary volcanoes in Japan.