molar gas ratios of air entrapped in ice a new tool to determine the origin of relict massive ground ice bodies in permafrost abstract the molar ratios of atmospheric gases change during dissolution in water due to differences in their relative solubilities we exploited this characteristic to develop a tool to clarify the origin of ice formations in permafrost regions extracted from ice molar gas ratios can distinguish buried glacier ice from intrasedimental ground ice formed by freezing groundwaters an extraction line was built to isolate gases from ice by melting and trapping with liquid he followed by analysis of n2 o2 ar 18oo2 and 15nn2 by continuous flow mass spectrometry the method was tested using glacier ice aufeis ice river icing and intrasedimental ground ice from sites in the canadian arctic o2 ar and n2 ar ratios clearly distinguish between atmospheric gas in glacial ice and gases from intrasedimental ground ice which are exsolved from freezing water 15nn2 and 18oo2 in glacier ice aufeis ice and intrasedimental ground ice do not show clear distinguishing trends as they are affected by various physical processes during formation such as gravitational settling excess air addition mixing with snow pack and respiration

basal ice facies and supraglacial melt out till of the laurentide ice sheet tuktoyaktuk coastlands western arctic canada glacially deformed massive ice and icy sediments mi is in the eskimo lakes fingerlands and summer island area of the tuktoyaktuk coastlands western arctic canada show in the same stratigraphic sequences features characteristic of both basal glacier ice and intrasedimental ice basal ice features comprise 1 ice facies and facies groupings similar to those from the basal ice layers of contemporary glaciers and ice sheets in alaska greenland and iceland 2 ice crystal fabrics similar to those from basal ice in antarctica and ice cored moraines on axel heiberg island canada and 3 a thaw or erosional unconformity along the top of the mi is buried by glacigenic or aeolian sediments intrasedimental ice consists of pore ice and segregated ice formed within pleistocene sands deposited before glacial overriding the co existence of basal and intrasedimental ice within the mi is records their occurrence within the basal ice layer of the laurentide ice sheet stagnation of the ice sheet and melt out of till from the ice surface allowed burial and preservation of the basal ice layer on a regional scale the widespread occurrence of supraglacial melt out till with clast fabrics similar to those in the underlying ice suggests that such till can be well preserved during partial thaw of a continental ice sheet in lowlands underlain by continuous permafrost

thermokarst lake basin sediments tuktoyaktuk coastlands western arctic canada three stages of deposition are distinguished in thermokarst lake basin sequences in ice rich permafrost of the tuktoyaktuk coastlands western arctic canada 1 widespread retrogressive thaw slumping around lake margins that rapidly transports upland sediments into thermokarst lakes forming a distinctive basal unit of impure sand and or diamicton 2 a reduction or cessation of slumping because of the pinching out of adjacent ground ice slump stabilization or climatic cooling that reduces the input of clastic sediment permitting reworking of sediment around lake margins and suspension settling principally in basin centres 3 lakes drain and deposition may continue by gelifluction and accumulation of in situ peat or aeolian sand radiocarbon dating of detrital peat and wood from a progradational sequence basal unit defines a lateral younging trend in the direction of progradation a progradation rate is calculated to be 4 cm yr 1 consistent with rapid deposition during stage 1 above the nonuniform nature of the trend is attributed to episodic influxes of old organic material by slumping and reworking by waves and currents in comparison with thermokarst lake basin sequences previously described in alaska canada and siberia the middle unit of those in the tuktoyaktuk coastlands is similar whereas the basal unit is generally thicker and by contrast often contains diamicton these differences are attributed respectively to larger scale resedimentation of upland sediments by retrogressive thaw slumping and debris flow deposition in thermokarst lakes in the tuktoyaktuk coastlands compared with the sediments within supraglacial lakes in areas of moderate to high relief the middle unit of thermokarst lake basin sequences in the tuktoyaktuk coastlands lacks clastic varves and the basal unit is much thinner and texturally less variable these differences are attributed to higher relief and larger volumes of meltwater and glacigenic sediment in supraglacial lakes which promote more suspension settling and resedimentation of glacigenic sediment than in thermokarst lakes in the tuktoyaktuk coastlands it may be impossible to distinguish glacial and periglacial thermokarst lake basin sediments in permafrost areas of incomplete deglaciation not only is it often difficult to distinguish intrasedimental and buried glacier ice but the depositional processes associated with thaw of both ice types are presumably the same and the host sediments very similar

factors controlling riffle scale hyporheic exchange flows and their seasonal changes in a gaining stream a three dimensional groundwater flow model 1 subsurface flow within a single riffle of a low gradient gravel bed stream was modeled in three dimensions using modflow a finite difference groundwater flow model model simulations showed that exchange flows can only occur in this low gradient gaining stream because of a zone of alluvial sediment around the stream that has much higher permeability than the surrounding catchment k 10 4 m s 1 compared with k 10 6 to 10 8 m s 1 the key factors controlling exchange flow within the alluvial zone were identified as the hydraulic conductivity of the alluvium the hydraulic gradient between upstream and downstream ends of the riffle and the flux of groundwater entering the alluvium from the sides and beneath in the study riffle each of these factors changes with season causing a reversal of flow paths in the alluvium and a reduction in exchange flows from about 0 2 0 5 m 3 d per meter stream length in summer to about 0 008 0 04 m 3 d 1 per meter stream length during fall to spring the model also revealed that exchange flows are up to twice as strong but more variable at the sides of the stream than near the center and that vertical flow paths beneath the channel are more persistent under the range of conditions modeled than lateral flow paths into the banks

ice core evidence for extensive melting of the greenland ice sheet in the last interglacial evidence from ice at the bottom of ice cores from the canadian arctic islands and camp century and dye 3 in greenland suggests that the greenland ice sheet melted extensively or completely during the last interglacial period more than 100 ka thousand years ago in contrast to earlier interpretations the presence of dirt particles in the basal ice has previously been thought to indicate that the base of the ice sheets had melted and that the evidence for the time of original growth of these ice masses had been destroyed however the particles most likely blew onto the ice when the dimensions of the ice caps and ice sheets were much smaller ice texture gas content and other evidence also suggest that the basal ice at each drill site is superimposed ice a type of ice typical of the early growth stages of an ice cap or ice sheet if the present day ice masses began their growth during the last interglacial the ice sheet from the earlier illinoian glacial period must have competely or largely melted during the early part of the same interglacial period if such melting did occur the 6 meter higher than present sea level during the sangamon cannot be attributed to disintegration of the west antarctic ice sheet as has been suggested

variations in permafrost thickness in response to changes in paleoclimate solutions to the stefan problem for the motion of the base of ice bearing permafrost in response to changes in paleoclimate were obtained using perturbation finite difference and finite element methods paleotemperature models were used to investigate the thickness response to compare solution methods to determine the current state of the permafrost and to determine constraints on the models the perturbation and finite difference methods used the approximation of linear temperature profiles while the finite element method did not there was a transient thickness response of about 41 kyr implying that paleotemperature records of greater length are needed for models and that the permafrost loses its memory of past conditions for much longer times faster thawing rates slower freezing rates and greater variations in thickness were found for the perturbation and finite difference solutions compared to the finite element solution these results appear to be caused by the simplifying assumptions in the former solutions a lag about 20 kyr exists between changes in surface temperature and thickness response and a small thermal offset is apparent in the finite element solution small asymmetries exist in the freezing and thawing rates and the thickness response paleotemperature models based on ice cores predict current permafrost thicknesses that are too large models with the long term mean surface temperature of permafrost within a few degrees of the current value of 11 c and with the last glacial period temperatures no more than 6 8 c lower are compatible with current prudhoe bay conditions these include models developed for east siberia from isotopic profiles in deep sea sediments and for barrow alaska modified for prudhoe bay these models predict that the permafrost thickness at prudhoe bay varied by about 10 or less 60 m over the last glacial cycle freezing and thawing rates were less than 6 mm yr 1 at present this permafrost should be near its long term equilibrium thickness and thawing at 1 mm yr 1

lake stratigraphy implies an 80 000 yr delayed melting of buried dead ice in northern russia sediment cores from lakes kormovoye and oshkoty in the glaciated region of the pechora lowland northern russia reveal sediment gravity flow deposits overlain by lacustrine mud and gyttja the sediments were deposited mainly during melting of buried glacier ice beneath the lakes in lake kormovoye differential melting of dead ice caused the lake bottom to subside at different places at different times resulting in sedimentation and erosion occurring only some few metres apart and at shifting locations as further melting caused inversion of the lake bottom basal radiocarbon dates from the two lakes ranging between 13 and 9 ka match with basal dates from other lakes in the pechora lowland as well as melting of ice wedges this indicates that buried glacier ice has survived for ca 80 000 years from the last glaciation of this area at 90 ka until about 13 ka when a warmer climate caused melting of permafrost and buried glacier ice forming numerous lakes and a fresh looking glacial landscape copyright 2003 john wiley sons ltd

segregated intrusive ice of subglacial meltwater origin in retrogressive thaw flow headwalls richardson mountains nwt canada abstract extensive beds of icy sediments are exposed in the headwalls of active retrogressive thaw flows which developed on the aklavik plateau richardson mountains nwt sedimentary and isotopic characteristics defined two units debris rich ice overlain by icy diamicton these units originated from the same glacial sediment deposited during the late wisconsin westward expansion of the laurentide ice sheet the icy diamicton is a result of the development of thaw flows and mass movement processes during the early holocene warm interval which reworked the sediments the ice in the diamicton is characterized by average 18o and d values of 22 6 1 8 and 181 15 respectively which are similar to the modern isotropic composition of precipitation for that area the debris rich ice is more depleted in 18o and d with 18o ranging from 30 to 27 and d ranging from 230 to 210 air bubbles enclosed in the ice have 13c values ranging between 24 and 19 and a co2 concentration nine times higher than the expected concentration of air entrapped in modern glacier ice but similar to that of soil co2 both physical and isotopic properties of the debris rich ice suggest that it is segregated intrusive ice that formed after the retreat of the laurentide ice sheet during permafrost aggradation as sediment laden subglacial meltwater moved through a proglacial talik this article discusses the mechanism involved in the formation of massive segregated intrusive ice and how it differs from both buried glacier ice and holocene massive intrasedimental ice

high resolution stable water isotopes as tracers of thaw unconformities in permafrost a case study from western arctic canada abstract the knowledge of past permafrost conditions is of importance to assess the potential magnitude of changes that periglacial environments may experience as a result of climate warming or disturbance to assess if past thaw unconformities may be preserved from isotopic and geochemical discontinuities within permafrost this study investigates the distribution of ground ice stable water isotopes and major cations in two permafrost cores collected in a hummocky terrain site near inuvik northwest territories canada a site where the evolution of the active layer during a recent period of permafrost degradation and subsequent aggradation was documented based on the high resolution isotope geochemistry profiles closed system rayleigh type ionic segregation and isotope fractionation occurred during thermally induced water migration into shallow permafrost and its freezing along a negative soil temperature gradient due to thermally induced water migration into permafrost 18 o may not always be able to identify thaw unconformities however the calculation of the 18 o enrichment factors between ice and water e 18 o i w may be used to determine position of thaw unconformities in permafrost if thaw events are followed by permafrost aggradation the approach of using e 18 o i w provides additional information regarding past permafrost conditions that can complement change in cryostructures observed along natural exposures

pathways of arsenic from sediments to groundwater in the hyporheic zone evidence from an iron isotope study summary ssulfide fe content and heavy fe isotopic signatures of the bulk core sediments all indicate anoxic and sulfidic conditions in the hyporheic zone the relationship between ssulfide and fe contents suggests that fe iii oxides hydroxides are transferred between non sulfidic fe ii minerals and fe ii sulfides under anoxic and sulfidic conditions respectively the fe isotope composition provides further evidence that microbial dissimilatory reduction of fe iii and the formation of fe ii sulfides and non sulfidic fe ii minerals are the dominant fe geochemical pathways and take place at different depths in the hyporheic zone in the upper sections of the core a and b with depth less than 10 m microbial fe iii reduction and non sulfidic fe ii minerals formation govern the fe cycling and the fe isotope composition in hyporheic water and bulk sediments microbial fe iii and so 4 2 reduction and interaction between produced fe ii aq and fe ii sulfides precipitate control 56fe values of sediments and water sample in the midsections 13 19 m of the core a conversely abiotic fe iii reduction by hs determines the bulk 56fe values of core sediments and water in the midsections 13 19 m of the core b microbial so 4 2 reduction is limited and microbial fe iii reduction controls the 56fe values of water and sediments at the bottom of both cores the variation of 56fe values and the as concentration in hyporheic water are similar at each depth indicating that as enrichment in the water is strongly associated with the microbial reduction of fe iii oxides hydroxides and the formation of fe ii sulfides and non sulfidic fe ii minerals the enriched 56fe values of high as water concentrations suggest that microbial reduction of fe iii oxides hydroxides is the dominant process that promotes as mobility in the hyporheic zones

oriented lake development in the context of late quaternary landscape evolution mckinley bay coastal plain western arctic canada abstract oriented lakes characterized by elongate forms central basins and shallow littoral shelves are common features of circum arctic coastal lowlands the environmental conditions geological processes and chronology associated with the development of oriented lakes however are little known but essential for understanding how such arctic lowlands evolve using combined techniques of field and drill log stratigraphy and sedimentology luminescence and radiocarbon dating methods and geomorphic mapping we reconstruct the landscape evolution leading toward oriented lake formation on the mckinley bay coastal plain of western arctic canada a region with over 900 oriented lakes most lakes with deep central basins are inherited from a preglacial braidplain ca 73 27 ka and alluvial braided channel network that extended beyond the glacial limit ca 18 6 14 3 ka eolian erosion active during the lateglacial and postglacial period ca 12 8 1 9 ka reworked fluvial deposits eolian processes modified existing basins and created other shallow deflationary basins as small barchanoid dunes migrated under cold dry paraglacial conditions between about 12 8 and 10 7 ka vegetation cover developed at the onset of the early holocene climatic optimum ca 10 7 ka and parabolic dunes were active between 9 6 and 4 6 ka thus oriented lakes developed in basins conditioned by fluvial and eolian processes in the absence of much near surface ground ice lateral expansion of deep basin lakes and shallow stabilized deflationary basins predominated during the late holocene through wind induced wave and current processes overall this sequence of oriented lake formation does not support a thaw lake cycle but rather small basin evolution of a periglacial landscape

mathematical models of compaction consolidation and regional groundwater flow summary a 3 d theory of groundwater flow and consolidation that allows for compressibility of the soil or sediment matrix is that due to biot 1941 however this theory is inapplicable to virgin consolidation tests where the deformation of the soil along the critical state surface is accomplished by elastic plastic flow in fact biot s model cannot even be applied to overconsolidated soils where the deformation is elastic as it does not conserve the mass of the solid in this paper we provide a modified version of the model and show how it can be extended to allow for elastic plastic deformation and large amplitude deformations in elastic plastic compaction a particular example of the potential application of this theory is to the groundwater flow induced by the compaction of sediments by an overriding ice sheet and we show how the commonly used dupuit approximation can be employed to simplify the theory

hydraulic and thermal response of groundwater surface water exchange to flooding in an experimental aquifer summary the effects of flooding on interactions between streams and shallow aquifers are poorly studied due to the unpredictable nature of flooding events this study examines groundwater surface water interactions in a constructed experimental meander over several simulated flood events using hydrologic and thermal monitoring detailed steady state hydraulic head measurements were taken of the stream surface and in 59 piezometer nests during baseflow bankfull flow and overbank flooding conditions allowing for three dimensional hydrologic characterization additionally a lithium tracer test was conducted and pressure transducers and thermistors were deployed to capture transient behavior results demonstrate extensive coupling between the stream and adjacent alluvial aquifer under all discharge conditions water table elevation responds rapidly to changes in stream stage and re equilibrates to order of magnitude increases in discharge within about 1 h though flooding elevates the water table steady state hydraulic gradients within the meander are independent of stream stage this results from the flow boundary condition imposed by the stream which deepens in response to flooding but has a slope that is essentially independent of discharge in addition to the stream boundary flow within the meander is also controlled by loss of stream water to the subsurface which directs hydraulic gradients towards the base of the meander finally the temperature distribution within the meander during normal and bankfull flow mimics the water table distribution and results from advected warm stream water being progressively cooled by vertical conduction in the direction of groundwater flow these findings suggest that areas of high reaction rates within meanders hot spots will be sensitive to whether a stream reach is gaining or losing water to the subsurface further the location of these hot spots of is likely independent of stream stage though other controls on reaction rates may be affected

hyporheic exchange due to channel spanning logs 1 the flow of river water around large woody debris lwd creates pressure gradients along the riverbed that drive a large zone of river groundwater mixing or hyporheic exchange flume experiments and numerical simulations show that river water downwells into the riverbed upstream of a channel spanning log and upwells downstream exchange rates are greatest near the log and decay exponentially with distance upstream and downstream we developed equations for bed pressure profiles and hyporheic exchange rates in the vicinity of a channel spanning log that can be used to evaluate the impact of lwd removal or reintroduction on hyporheic mixing the magnitude of pressure disturbance along the bed and thus hyporheic exchange increases with the fraction of channel depth blocked by the log and channel froude number exchange rates are relatively insensitive to relative depth of the log gap ratio at natural densities lwd in lowland streams drives reach averaged hyporheic exchange rates similar to a ripple covered bed however the length scales and residence times of hyporheic exchange due to lwd are greater by removing lwd from streams humans have altered patterns and rates of hyporheic exchange which influence habitat distribution and quality for invertebrates and fish

major end moraines of younger dryas age on wollaston peninsula victoria island canadian arctic implications for paleoclimate and for formation of hummocky moraine discussion discussion in referring to several large moraine belts n arctic canada dyke and savelle 2000 state a consensus is emerging that these moraines are cored by glacier ice p 615 they further state that ice cored morainal belts in arctic canada would resemble southern areas of hummocky moraine were they to lose their ice cores p 617 more investigation and research is required before these statements can be accepted in lieu of theories proposed by other investigators for the origin of ice within hummocky arctic topography and for the evolution of this topography the claimed consensus is that of dyke et al 1992 sharpe 1992 and st onge and mcmartin 1995 dyke et al 1992 stated that many of the moraines present on prince of wales island consist of buried glacier ice sharpe 1992 reported exposures of relatively debris free ice containing a few debris rich bands underlying up to 2m of diamicton within areas of hummocky moraine he noted the contact between the diamicton and ice as abrupt st onge and mcmartin 1995 reported icy sediments overlain by bouldery diamicton with a sharp and subhorizontal contact reticulate ice was noted in the diamicton near the contact all contacts between till and ice appear characteristic of segregation ice developing in the base of and under a finetextured soil mackay and dallimore 1992 in addition the above sequences are the reverse of the normal continental glacier sequence where relatively clean glacier ice overlies debris rich glacier ice st onge and mcmartin believed i the hydrogeochemistry and isotopic values of reticulate ice ice wedge ice and the observed icy sediments and ii the slope of the co isotope plot from their icy sediment samples support a glacial origin for the icy sediments the latter is the strongest argument for a glacial origin of the icy sediments as the slope is similar to that for meteoric water lorrain and demeur 1985 however differentiation of segregation ice and glacial ice by co isotope relation has since been seriously questioned french and harry 1990 mackay and dallimore 1992 the hydrogeochemistry and isotope values reported by st onge and mcmartin 1995 for the different ice types simply confirm different origins of the ice types and do not necessarily indicate a glacial origin for the icy sediments dyke and savelle s 2000 analysis of ice cored topography excluded the mainland west of the amundson gulf where a hummocky belt of topography on the tuktoyaktuk peninsula and adjacent areas fig 1 was studied in detail by mackay cf 1963 1971 1973 1983 and his colleagues mackay and stager 1966 rampton and mackay 1971 rampton 1974 rampton and bouchard 1975 gell 1976 rampton 1988a 1988b dallimore and wolfe 1988 mackay and dallimore 1992 they concluded that the ice cores found within hummocky morainal topography along the tuktoyaktuk peninsula and yukon coastal plain rampton 1982 pollard and dallimore 1988 harry et al 1988 are intrasedimental ice the conclusion is based on evidence summarized in rampton 1988a and mackay and dallimore 1992 namely similar geochemistry and isotope values for massive ice and ice within the underlying sandy sediments in numerous profiles indicating a common water source and downward freezing isotope plots having linear trends through ice and underlying interbedded sand and ice and co isotope plots showing a linear trend down through till ice and sand both trends suggesting downward freezing petrographic data from massive ice indicative of a segregation origin the common presence of reticulate ice within the till immediately over the massive ice reticulate ice develops in fine grained sediments as permafrost aggrades the asso

stream aquifer interactions and hyporheic exchange in gaining and losing sinuous streams 1 the irregular planform morphology of rivers leads to formation of hyporheic zones along its banks this study investigates how hyporheic exchange in stream banks whose planform is idealized as sinusoidal is affected by net gains from and net losses of water to the adjacent aquifer these effects are studied via numerical modeling of groundwater flow adjacent to sinuous channels across a broad range of sinuosity and gain loss magnitude hyporheic zone areas and fluxes both decrease exponentially with increasing magnitude of net gain or loss relative to the case where the stream has no net flux of water neutral residence time through the hyporheic zone also decreases with gain loss magnitude the hyporheic zones become constrained near the apex of bends indicating that these areas could be hot spots for mixing and biogeochemical processing hyporheic zones in channels with smaller sinuosity are more prone to hyporheic flux and area reduction while very sinuous channels are able to maintain a hyporheic zone even under largely losing or gaining conditions equations fitted to the suite of simulation results allow for prediction of hyporheic flux area and residence time on the basis of aquifer hydraulic conductivity channel sinuosity and the ratio of along valley and across valley mean head gradients

nature and origin of ground ice sandhills moraine southwest banks island western canadian arctic the sandhills moraine is a late wisconsinan lateral moraine complex on southwest banks island the occurrence of ice ablation landforms ground ice slumps kettle lakes and catastrophic lake drainage in winter suggests the presence of substantial bodies of massive ground ice the distinctive hummocky topography of the sandhills moraine is thought to reflect partial melt out of this ice stratigraphic observations indicate that the ice is overlain irregularly and unconformably by glacigenic sediments notably pebbly clay till and or sandy gravels outwash while the ice itself possesses numerous and variable mineral inclusions faults and foliations petrofabric analyses indicate a strongly preferred orientation to the ice crystals it is suggested that these characteristics are best explained if the ground ice is interpreted as relict glacier ice

active mud volcanoes on the continental slope of the canadian beaufort sea the major geochemical characteristics of red sea brine are summarized for 11 brine filled deeps located along the central graben axis between 19 n and 27 n the major element composition of the different brine pools is mainly controlled by variable mixing situations of halite saturated solution evaporite dissolution with red sea deep water the brine chemistry is also influenced by hydrothermal water rock interaction whereas magmatic and sedimentary rock reactions can be distinguished by boron lithium and magnesium calcium chemistry moreover hydrocarbon chemistry concentrations and 13 c data of brine indicates variable injection of light hydrocarbons from organic source rocks and strong secondary bacterial or thermogenic degradation processes a simple statistical cluster analysis approach was selected to look for similarities in brine chemistry and to classify the various brine pools as the measured chemical brine compositions show remarkably strong concentration variations for some elements the cluster analysis indicates two main classes of brine type i brine chemistry oceanographer and kebrit deeps is controlled by evaporite dissolution and contributions from sediment alteration the type ii brine suakin port sudan erba albatross discovery atlantis ii nereus shaban and conrad deeps is influenced by variable contributions from volcanic magmatic rock alteration the chemical brine classification can be correlated with the sedimentary and tectonic setting of the related depressions type i brine filled deeps are located slightly off axis from the central red sea graben a typical collapse structure formation which has been defined for the kebrit deep by evaluating seismic and geomorphological data probably corresponds to our type i brine type ii brine located in depressions in the northern red sea i e conrad and shaban deeps could be correlated to volcanic intrusion extrusion related deep formation the chemical indications for hydrothermal influence on conrad and shaban deep brine can be related to brines from the multi deeps region in the central red sea where volcanic magmatic fluid rock interaction is most obvious the strongest hydrothermal influence is observed in atlantis ii brine central multi deeps region which is also the hottest red sea brine body in 2011 68 2 c

defining the hyporheic zone in a large tidally influenced river summary an investigation was conducted to characterize the spatial and temporal distribution of the hyporheic zone of a large tidally influenced river the field site is located on the fraser river in british columbia canada approximately 30 km from its outlet to the ocean the physical attributes of the riverbed were mapped using geophysical techniques coupled with sediment sampling the spatial and temporal distribution of groundwater composition beneath the riverbed was determined through detailed profiling contaminated fresh groundwater discharges through a narrow band of the riverbed at a distance approximately 88 105 m from the shoreline coinciding with the termination of a massive silty unit saline groundwater as part of a regional flow system dominates the riverbed sediments from 105 m beyond the shoreline towards the centre of channel three water types occur within the upper 2 m of the riverbed sediments a result of both mixing of river water contaminated fresh groundwater and saline groundwater and modification by cation exchange reactions the interaction of these waters produced distinct zones of ca hco 3 na cl and ca cl type waters the distribution of groundwater solutes indicates that during a single tidal cycle river water penetrates the riverbed to a depth of approximately 15 cm but the long term effects of tidal pumping of river water into the riverbed is observed to a depth of approximately 1 m below the river bed

cryostratigraphy of the klondike muck deposits west central yukon territory four late quaternary cryostratigraphic units are recognized in the unconsolidated valley bottom deposits of the klondike area yukon territory three of the units in ice rich loessal sediments of pre wisconsinan or wisconsinan age collectively compose the king solomon formation they are overlain by a holocene organic unit the units are distinguished by their cryostratigraphic characteristics and oxygen isotope ratios of included ground ice the basal unit is the last chance creek member a pre late wisconsinan deposit containing preserved ice wedges 18o 28 to 26 d 225 to 209 the overlying quartz creek member a late wisconsinan unit is dominated by organic rich loess massive ice is noticeably absent although the sediments are ice rich the isotopic composition of ice in this unit is characteristic of full glacial conditions 18o 32 to 29 d 234 to 257 an abrupt change to warmer and wetter conditions at the end of glaciation prior to the holocene is recorded by the i

using noble gas ratios to determine the origin of ground ice abstract argon krypton and xenon have different solubilities in water meaning their ratios in water are different from those in atmospheric air this characteristic is used in a novel method to distinguish between ice bodies which originate from the compaction of snow i e buried snow banks glacial ice vs ice which forms from the freezing of groundwater i e pingo ice ice which forms from the compaction of snow has gas ratios similar to atmospheric air while ice which forms from the freezing of liquid water is expected to have gas ratios similar to air equilibrated water this analysis has been conducted using a spike dilution noble gas line with gas extraction conducted on line samples were mixed with an aliquot of rare noble gases while being melted then extracted gases are purified and cryogenically separated samples have been analysed from glacial ice buried snow bank ice intrusive ice wedge ice cave ice and two unknown ice bodies ice bodies which have formed from different processes have different gas ratios relative to their formation processes

effect of experimental wood addition on hyporheic exchange and thermal dynamics in a losing meadow stream 1 stream restoration structures such as large wood can enhance shallow river groundwater exchange or hyporheic exchange and alter temperature dynamics in restored reaches we added and then removed channel spanning logs in a second order mountain meadow stream to test short term impacts on hyporheic exchange streambed temperatures and surface water temperatures based on vertical seepage measurements and numerical simulations of hyporheic fluid and heat flow large wood addition increased hyporheic exchange and altered streambed temperatures in this losing stream meter scale hyporheic exchange cells formed beneath large wood upwelling pore water downstream of logs stabilized diel temperature cycles across 8 of the streambed creating localized but potentially valuable thermal refuge exchange rates were 0 1 of channel discharge too small to impact the range of diel temperature signals in surface water however the lag between downstream and upstream diel temperature signals was slightly greater with large wood which may indicate that surface storage zones rather than hyporheic storage zones increased thermal retardation losing conditions limited the spatial extent and rates of hyporheic exchange near large wood impacts of large wood reintroduction on hyporheic exchange depend on ambient groundwater discharge or recharge streambed permeability channel froude number large wood blockage ratio and large wood spacing in many streams large wood reintroduction may increase hyporheic habitat volume and complexity but may not increase exchange rates enough to alter surface water temperature or chemistry surface storage zones such as eddies and pools can still influence heat and solute retention in the channel

a full scale field experiment 1978 1995 on the growth of permafrost by means of lake drainage western arctic coast a discussion of the method and some results on 13 august 1978 a lake on the western arctic coast was artificially drained in a multidisciplinary experiment on the growth of permafrost on the unfrozen bottom of the drained lake a bowl shaped talik unfrozen basin with a maximum depth of about 32 m underlay the lake bottom prior to drainage in the first winter after drainage downward freezing started on the exposed lake bottom and upward freezing from permafrost beneath the talik after drainage the soft lake bottom sediments hardened from water loss and freeze thaw consolidation gradual thinning of the active layer at many sites was accompanied by ground uplift and the growth of aggradational ice downward and upward freezing has resulted in solute rejection freezing point depressions pore water expulsion from the freezing of the saturated lake bottom sands and convective heat transfer from groundwater flow in an open hydrologie system the increasingly saline intrapermafrost groundwater flowing at an increasingly negative temperature be

nature and origin of a pleistocene age massive ground ice body exposed in the chapman lake moraine complex central yukon territory canada abstract a massive ground ice body was found exposed in the headwall of a thaw flow developed within the chapman lake terminal moraine complex on the blackstone plateau ogilvie mountains central yukon territory which is contemporaneous to the reid glaciation based on visible cryostructures in the 4 m high headwall two units were identified massive ground ice overlain sharply by 2 m of icy diamicton the nature and origin of the chapman lake massive ground ice was determined using cryostratigraphy petrography stable o h isotopes and the molar concentration of occluded gases co2 o2 n2 and ar entrapped in the ice a new technique in the field of periglacial geomorphology that allows to distinguish between glacial and non glacial intrasedimental ice collectively the results indicate that the chapman lake massive ground ice formed by firn densification with limited melting refreezing and underwent deformation near its margin given that the massive ground ice body consists of relict glacier ice it suggests that permafrost persisted at least locally on plateau areas in the central yukon territory since the middle pleistocene in addition the d value of chapman lake relict glacier ice suggests that the ice covering the area during the reid glaciation originated from a local alpine glaciation in the ogilvie mountains

calculation of strong ground motion due to an extended earthquake source in a laterally varying structure abstract lateral heterogeneity of the crust combined with the complexity of an extended earthquake source may account for observations of strong ground motion not easily explained by simplified source models in a laterally homogeneous medium both the effects of an extended source and lateral heterogeneity can be examined using a forward modeling technique that combines distributed nonuniform slip on a fault plane with dynamic ray tracing in a three dimensional medium using this technique velocity seismograms are calculated at ranges less than 10 km for a finite source which occurs in a wedge shaped zone of low velocity this structure is chosen to approximate the structure of fault zones found in reflection and refraction surveys across active faults in california iterative paraxial ray tracing is used to solve the two point ray tracing problem the low velocity zone surrounding the fault causes arrivals in the laterally varying structure to be delayed and rays to leave the fault more closely to the fault normal at most locations examined the effect of the three dimensional structure on isochrone velocity is small compared to the effects on geometric spreading and radiation pattern because rays are bent substantially in the near source region the seismograms calculated for the laterally varying structure vary substantially in amplitude and polarity from seismograms calculated for the laterally homogeneous structure

mixing of surface and groundwater induced by riverbed dunes implications for hyporheic zone definitions and pollutant reactions 1 the hyporheic zone is often defined as where mixing of surface water and groundwater occurs in shallow sediments beneath and adjacent to rivers this mixing is credited with creating unique biogeochemical conditions that can attenuate contaminants from either upstream surface water or groundwater under gaining conditions however reactions of contaminants upwelling from groundwater may be more dependent on such mixing than contaminants from surface water we numerically modeled mixing between hyporheic flow paths induced by riverbed dunes and flow paths of adjacent upwelling of deeper groundwater results show that only 12 7 or less tracer mass upwelling from deeper groundwater dispersed across into hyporheic flow paths originating in surface water the spatial extent of a mixing defined hyporheic zone was smaller than a hyporheic zone defined as hydrologic flow paths leaving and returning to surface water mixing dependent reactions will therefore be localized within a thin mixing zone yet vary considerably with site conditions for example mixing in homogeneous sediments was controlled most by variation in hydraulic conductivity and upwelling flow rate which primarily affected mixing zone length by contrast introduction of heterogeneity increased mixing primarily by increasing mixing zone thickness consistent with studies of flow focusing in aquifers finally dispersivity is a critical parameter for which data are needed for shallow sediments our results help clarify hyporheic zone definitions and potential for mixing dependent reactions in particular the biogeochemically reactive portion of riverbed sediments from the perspective of upwelling contaminants does not necessarily spatially coincide with more traditional hydrologic conceptions of the hyporheic zone

surficial materials and related ground ice conditions slave province n w t canada surficial mapping and geologic information on the nature and evolution of surficial materials in the slave geologic province indicate that the geotechnical properties and potential ground ice contents associated with these materials depend largely upon their provenance depositional conditions and the postglacial climatic history this information may be used to provide a regional scale view of the distribution of ground ice conditions and terrain sensitivities associated with various surficial materials in till veneers and blankets ground ice content is generally low as suggested by lack of thermokarst and other permafrost features however distinctive surface relief in hummocky till including kettle depressions rim ridges and shallow thaw flowslides may be attributed to massive ice resulting in sensitive till terrain although many outwash sediments have low ice contents near the surface massive ice ranging from 5 to 10 m thick is present in some eskers and ice contact outwash sediments these are associated with thermokarst slope movement and collapse features indicative of meltout or creep of large bodies of massive ice the terrain sensitivity associated with these deposits is typically low to moderate due to the coarse grained nature of the sediments in contrast terrain sensitivity is high and active layer detachment slides are common along the coronation gulf coast where frozen silty clay marine sediments contain a wide range of ice contents results from this study may be applied to a much more extensive area of the glaciated western arctic mainland and adjacent arctic coastal plain in which materials with a similar glacial history are found resume la cartographie et l evolution de depots de surface dans la province des esclaves demontrent que les proprietes geotechniques et le potentiel pour la presence de glace de sol associes aux depots dependent largement de l origine des depots l histoire glaciaire et une connaissance de l histoire climatique postglaciaire cette information peut servir a fournir une vue regionale de la distribution de glace de sol et la sensibilite de terrain associee aux divers depots de surface la quantite de glace de sol dans les couvertures et placages de till peut etre minime telle que suggeree par l absence de thermokarst ou des formes periglaciaires par contre dans le till bossele des structures de contact glaciaire et des glissements retrogressifs par liquefaction resultent en formes de relief distinctives qui peuvent etre attribuees a la glace de sol enfouie meme si certains eskers ont l apparence d avoir de la glace de sol en faible teneur la glace massive d une epaisseur d e5a1 0 mpeut se retrouver ou les eskers demontrent les effets de thermokarst de reptation et des structures d effondrement la sensibilite de terrain associee a ces depots est generalement faible a moyenne a cause d une granulometrie grossiere le long des rivieres ou des sables grossiers littoraux recouvrent les silts argilleux marins geles contenant la glace de sol d une epaisseur variable la sensibilite de terrain est elevee et des glissements par liquefaction sont communs les resultats de cette etude peuvent etre appliques dans l arctique central et de l ouest ou les depots de surface ayant une origine glaciaire semblable sont presents

permafrost on the tibet plateau china abstract the tibet plateau represents a unique permafrost environment being a result of high elevation caused by land uplift in the order of 3000 m over the last 2 million years under present climatic conditions the lower altitudinal limit of permafrost in the north is at 4200 m a s l which means that if the plateau were 1000 m lower than present there would be hardly any permafrost the permafrost is ice poor because during the quaternary the area was unglaciated and little water was available for the formation of massive ground ice furthermore today s climate is arid and little moisture is available for segregated and pore ice formation as a result mid portion desiccation of the active layer does not occur to the same extent as in high latitudes permafrost temperature on the plateau depends mainly upon altitude and varies irregularly with latitude permafrost is warm and thin compared with high latitude polar permafrost in both north america and russia and thus more sensitive to changes in climate and surface conditions a recent warming trend in ground temperature has been monitored and if maintained permafrost will become relict within the next ca 150 years permafrost features are few due to the arid environment many of the pingos on the plateau are genetically associated with faults and therefore of the open system type faults are the most powerful factor disturbing the thermal regime of the plateau permafrost

massive ice of the tuktoyaktuk area western arctic coast canada the extensive coastal exposure of massive underground ice at peninsula point southwest of tuktoyaktuk northwest territories is believed to be intrasedimental ice the ice grew beneath a frozen d

thermal regime of dune covered sediments under gaining and losing water bodies 1 we investigate the effects of current bed form induced flow and heat transport through permeable bottom sediments overlain by a marine or terrestrial water column that is gaining or losing deep groundwater heat transport is forced by the diel variation of temperature in the water column the investigation utilizes sequentially coupled simulations of turbulent flow in the water column and darcy flow and heat transport in the sediments the simulations address the question when where and by how much are diel water column temperature variations transmitted into sediments subjected to ambient groundwater discharge this is crucial information for detecting observing and predicting temperature sensitive biogeochemical and ecological processes in the bottom sediments when the groundwater gain or loss is small it has no appreciable effect on temperatures in the sediments which are controlled by heat conduction and current bed form induced heat advection as losing discharge increases the temperature signal from the water column penetrates deeper into the sediments with the largest temperature variations found under a downwelling zone along the stoss side of the bed form and damped temperature variations found near a narrow upwelling zone below the crest similar patterns are observed under gaining conditions but with temperature variations penetrating to shallower depths the interfacial exchange zone is diminished by upward movement of deep groundwater large gains or losses of deep groundwater prevent the formation of an interfacial exchange zone making heat transport almost vertically one dimensional the sensitivity of the sediment thermal regime to hydrodynamic conditions increases with increasing water column current reynolds number and with sediment permeability

cryostratigraphy paleogeography and climate change during the early holocene warm interval western arctic coast canada botanical and cryostratigraphic records from northwest canada indicate that the climate of the early holocene was considerably warmer than today tree line was over 100 km farther north and a thaw unconformity dating from 8000 14c years bp formed at the base of an active layer 2 5 times thicker than at present numerous thermokarst lake basins formed in the preceding millennia both the botanical and cryostratigraphic indices described are products of summer conditions previous reconstructions of early holocene climate have not assessed the significance of paleocoastal location on the seasonality and extent of apparent climate warming at present there is a steep gradient in growing season conditions between cooler sites on the beaufort sea coast and warmer inland locations winter conditions are more uniform because both sea and land are snow covered coastal retreat in the region has been rapid due to sea level rising over a gently sloping shelf containing readily erodible sediments the coastlin

linearized inversion for fault rupture behavior application to the 1984 morgan hill california earthquake we present a technique to infer the rupture history of an earthquake from near source records of ground motion unlike most previous studies each point on the fault is assumed to slip only once when the rupture front passes with a spatially variable slip intensity in this parameterization the data are linearly related to slip intensity but nonlinearly related to rupture time we perform a linearized inversion for slip intensity and rupture time by iteratively perturbing an assumed starting model the inverse problem for the model perturbation is solved using a tomographic back projection technique smoothing and inequality constraints are applied to ensure that the resulting solution is stable and feasible asymptotic ray theory is used to calculate theoretical seismograms and partial derivatives with respect to model parameters in test cases with noisy synthetic data sets we found that it is possible to distinguish between rupture models having variable slip amplitude and models having variable rupture velocity provided the station coverage is adequate we apply the technique to recordings of the april 24 1984 morgan hill earthquake the results indicate that slip amplitude on the fault plane was extremely variable and that the rupture front did not propagate uniformly away from the hypocenter in particular rupture was delayed on a 12 km2 section of the fault approximately 14 km to the southeast of the hypocenter the rupture front surrounded the region which subsequently failed with a component of rupture propagation back toward the hypocenter similar behavior has been observed in dynamic rupture models with stress or strength inhomogeneities this segment of the fault ruptured with a large slip amplitude releasing 12 of the total seismic moment from 4 of the total area of the aftershock zone the surface trace of this section of the fault is characterized by a complex left step that could act to increase the normal stress acting across the fault however the distribution of aftershocks suggests that the fault at depth is simpler and that it may bend to the right in either case our rupture model suggests that this segment of the fault represents an asperity which initially resisted rupture but eventually ruptured massively we estimate the shear fracture energy for this earthquake to be 2 106 j m2

on the isotopic composition in δd and δ18o of water and ice during freezing experiments on progressive unidirectional freezing are coi1 ducted to determine rhe evol ution in 00 and 51 bo of successive water samples and ice layers ta ken during the course of freezing r es ults indicate t hal lhi s evoluti on takes pl ace in a bd bibo diagram a long a strai ght line wi th a characteri sti c slope this slope different from that due to the precip itation effec t gives a finger print of the occurrence of a freezing or of a melt ing refreez ing process in the stud ied reservoir residual water was sampled at the beginning of the experiment after 40 80 85 90 and 95 of the material had frozen when the freezing process was complete the ice core was recovered and sectioned the samples covered the freezing ranges from 0 to 10 10 to 20 20 to 50 50 to 60 60 to 70 70 to 80

in stream geomorphic structures as drivers of hyporheic exchange 1 common in stream geomorphic structures such as debris dams and steps can drive hyporheic exchange in streams exchange is important for ecological stream function and restoring function is a goal of many stream restoration projects yet the connection between in stream geomorphic form hydrogeologic setting and hyporheic exchange remains inadequately characterized we used the models hec ras modflow and modpath to simulate coupled surface and subsurface hydraulics in a gaining stream containing a single in stream geomorphic structure and to systematically evaluate the impact of fundamental characteristics of the structure and its hydrogeologic setting on induced exchange we also conducted a field study to support model results model results indicated that structure size background groundwater discharge rate and sediment hydraulic conductivity are the most important factors determining the magnitude of induced hyporheic exchange followed by geomorphic structure type depth to bedrock and channel slope model results indicated channel spanning structures were more effective at driving hyporheic flow than were partially spanning structures and weirs were more effective than were steps across most structure types downwelling flux rate increased linearly with structure size yet hyporheic residence time exhibited nonlinear behavior increasing quickly with size at low structure sizes and declining thereafter important trends in model results were observed at the field site and also interpreted using simple hydraulic theory thereby supporting the modeling approach and clarifying underlying processes

last glacial maximum age for the northwest laurentide maximum from the eagle river spillway and delta complex northern yukon abstract the eagle river spillway and braid delta complex provide a record of the maximum extent of the northwest laurentide ice sheet and diversion of meltwater from bonnet plume basin into the interior basins of non glaciated northern yukon development of the spillway can be characterized in three distinct zones based on the distribution of erosion and deposition along each reach erosion dominated channel initiation and incision followed by channelization and coarse clastic deposition along channel margins and into tributary valleys and lastly fine grained deltaic and lacustrine sedimentation in the lower channel reach deltaic sedimentation within the spillway is crudely coarsening upward from alternating beds of massive clay and silt to ripple cross bedded sand all sediments occur in rapidly aggrading forms with no evidence for a significant hiatus in deposition radiocarbon ages on woody plant macrofossils and spruce needles are non finite while radiocarbon ages on macrofossils from herbaceous plant taxa and insects with steppe tundra ecological affinity from the upper part of the delta range from 15 840 90 to 21 600 1300 14c yr bp these ages coupled with the rapidly aggrading nature of the delta sediments and landform suggest an age of ca 15 16 000 14c yr bp non finite and mixed ages underscore the significant problem of reworked well preserved macrofossils in arctic environments and the need for careful selection of both fragile and ecologically representative macrofossils to establish reliable chronologies

seasonal hyporheic dynamics control coupled microbiology and geochemistry in colorado river sediments 2016 american geophysical union all rights reserved riverbed microbial communities play an oversized role in many watershed ecosystem functions including the processing of organic carbon cycling of nitrogen and alterations to metal mobility the structure and activity of microbial assemblages depend in part on geochemical conditions set by river groundwater exchange or hyporheic exchange to assess how seasonal changes in river groundwater mixing affect these populations in a snowmelt dominated fluvial system vertical sediment and pore water profiles were sampled at three time points at one location in the hyporheic zone of the colorado river and analyzed by using geochemical measurements 16s rrna gene sequencing and ecological modeling oxic river water penetrated deepest into the subsurface during peak river discharge while under base flow conditions anoxic groundwater dominated shallower depths over a 70 cm thick interval riverbed sediments were therefore exposed to seasonally fluctuating redox conditions and hosted microbial populations statistically different from those at both shallower and deeper locations additionally microbial populations within this zone were shown to be the most dynamic across sampling time points underlining the critical role that hyporheic mixing plays in constraining microbial abundances given such mixing effects we anticipate that future changes in river discharge in mountainous semiarid western u s watersheds may affect microbial community structure and function in riverbed environments with potential implications for biogeochemical processes in riparian regions

basal glacier ice and massive ground ice different scientists same science whilst glaciologists and permafrost researchers investigate ice bodies using similar techniques there has been surprisingly little collaboration between the two communities this paper examines the potential benefits of interdisciplinary research into the formation of basal ice beneath glaciers and the origin of massive ice in glaciated permafrost regions active collaboration in these areas has already improved our understanding of the formation of basal ice beneath cold based glaciers the critical role played by basal freezing in controlling the dynamic behaviour of stagnating ice streams and the significance of glacier permafrost interactions at the margins of pleistocene ice sheets however in order to promote future collaboration certain obstacles need to be overcome the contrasting ice classification schemes employed by glaciologists and permafrost scientists for example need to be unified in order to allow detailed comparisons of ice rich sequences in both environments this could in turn enable exciting research advances most notably by facilitating the identification of preserved remnants of pleistocene ice sheets within permafrost regions that provide a potentially invaluable and currently largely untapped source of palaeoglaciological information

convective transport within stable river sediments local pressure variations of the order of 100 1000 n m2 can be observed between the upstream and downstream faces of the typically triangular shaped dunelike sediment structures that form at the sediment water interface of rivers laboratory experiments were conducted examining the influence of this localized pressure variation on contaminant transport processes within the sediment numerical modeling of the in bed flow via boundary element methods was also undertaken in order to predict convective transport under typical field conditions the laboratory experiments and numerical simulation of the in bed flow in several rivers verified that the pressure distribution observed on the sediment surface and the resulting interstitial fluid convection can control transport of chemically inert nonsorbed contaminants in stable sediments in bed peclet numbers were of the order of 100 1000 indicating the negligible influence of diffusion under the conditions examined

permafrost response to surface temperature change and its implications for the 40 000 year surface temperature history at prudhoe bay alaska the geothermal data of lachenbruch et al 1982 in ice bearing permafrost at prudhoe bay are interpreted in terms of the temperature history on a time scale of 40 000 years or longer to this end two theories are developed a forward theory to calculate the response of ice bearing permafrost to a surface temperature disturbance and an inverse theory to calculate parameters characterizing the surface temperature history from suitable measurements in this permafrost the inverse theory shows that two pieces of information characterizing the surface temperature history on a long time scale and the equilibrium geothermal heat flux can usually be determined but the results depend upon the quality of the data the structure of the surface temperature disturbance and the timing of the measurements the memory of the permafrost may range from near zero to hundreds of thousands of years the geothermal data indicate that the prudhoe bay ground surface temperature has a 40 000 year average of minus 10 9 plus minus 3 0c and a value at glacial maximum of minus 10 9 plus minus 4 6c the limits correspond to the estimated uncertainties in the data and they depend somewhat on the structure of the surface temperature disturbance over the last 40 000 years or so the minus 10 9c temperaturemore is the same as that which prevailed before the 1 8 k warming of the past century the limits placed on the variation of the ground surface temperature by this interpretation are not in accord with limits placed by some existing interpretations of other paleoclimatic indicators it is concluded that a consistent understanding of climate history and geothermal regime in northern alaska does not yet exist less

stratigraphy and glaciotectonic structures of permafrost deformed beneath the northwest margin of the laurentide ice sheet tuktoyaktuk coastlands canada the upper 5 20 m of ice rich permafrost at three sites overridden by the northwest margin of the laurentide ice sheet in the tuktoyaktuk coastlands western arctic canada comprise massive ice beneath ice rich diamicton or sandy silt the diamicton and silt contain 1 truncated ice blocks up to 15 m long 2 sand lenses and layers 3 ice veins dipping at 20 30 4 ice lenses adjacent and parallel to sedimentary contacts and 5 ice wedges the massive ice is interpreted as intrasedimental or buried basal glacier ice and the diamicton and silt as glacitectonite that has never thawed deformation of frozen ground was mainly ductile in character deformation was accompanied by sub marginal erosion of permafrost which formed an angular unconformity along the top of the massive ice and supplied ice clasts and sand bodies to the overlying glacitectonite after deformation and erosion ceased postglacial segregated ice and ice wedge ice developed within the deformed permafrost

reduction of the hyporheic zone volume due to the stream aquifer interaction 1 pore water in stream sediments is continuously exchanged with the surface water from the overlying stream this exchange of water and solutes that occurs across the stream sediment interface plays an important role for fluvial ecology because of the unique biochemical conditions rich biodiversity and high rates of metabolism while many studies have observed the extent of the hyporheic zone to be modified by changes in the level of the groundwater table the actual importance of this interaction is still difficult to quantify here we focus on the case of bedform induced hyporheic exchange to show how the the volume of hyporheic sediments that receive water from the stream is significantly reduced by the upwelling of subsurface water a simple scaling relationship for the assessment of maximum depth of the hyporheic zone is proposed by relating hyporheic flow to the groundwater discharge in an aquifer with given hydraulic properties and head difference between the stream and the aquifer

late wisconsinan erosion and eolian deposition summer island area pleistocene mackenzie delta northwest territories optical dating and implications for glacial chronology the summer island area of the pleistocene mackenzie delta western canadian arctic is thought to have been last glaciated during the early wisconsinan 35 ka with a suggested late wisconsinan 13 ka glacial limit located 80 100 km to the south cryostratigraphic observations in the summer island area identify eolian and fluvial sediments overlying an erosion surface on summer island eolian sand occurs within sand wedges the tops of which are contiguous with an eolian sand sheet deposited on a formerly bare ice surface this paleo ice surface is attributed to fluvial erosion and that in turn is attributed to proglacial meltwater activity the age of erosion is inferred to be shortly before 14 ka determined by optical dating using infrared stimulation of luminescence from feldspar of the eolian sand wedges this 14 ka meltwater erosion is significantly north of the generally accepted limit of late wisconsinan glaciation in this coastal area suggesting that ice last covered the whole of rich

luminescence dating of mid to late wisconsinan aeolian sand as a constraint on the last advance of the laurentide ice sheet across the tuktoyaktuk coastlands western arctic canada luminescence dating of pre glacial sand in the tuktoyaktuk coastlands northwest territories discounts an early wisconsinan age for the last laurentide glaciation to cross the arctic coastal plain and supports a late wisconsinan age aeolian dune sand from the kittigazuit formation near cliff point on the southern shore of liverpool bay predates till deposited during the toker point stade potassium rich feldspar from three stratigraphic sections 35 km up ice from the toker point glacial limit provides thermoluminescence age estimates that range from 18 7 2 0 to 9 1 1 0 ka and infrared stimulated luminescence age estimates of 23 8 5 1 to 11 0 2 1 ka quartz from four of the same samples provides optically stimulated luminescence age estimates of 16 5 1 0 to 13 7 0 9 ka collectively these estimates reject the early wisconsinan age for the toker point stade glaciation inferred hitherto from radiocarbon dating a review of pre glacial and post glacial age estimates from the region indicates

the permafrost regime in the mackenzie delta beaufort sea region n w t and its significance to the reconstruction of the palaeoclimatic history observed depths to the base of the ice bearing permafrost bpf as determined from geophysical well log and survey information in the mackenzie delta and offshore beaufort sea continental shelf show significant variation depths range from over 700m beneath the northeastern portion of richards island and the adjacent offshore region to the north to less than 100m in the modern mackenzie delta and offshore mackenzie bay regions along the tuktoyaktuk peninsula permafrost thicknesses exceed 600m but tend to thin to less than 100m in a southeasterly direction the wide variation in the thickness and distribution of permafrost is due in part to differences in the subsurface deltaic lithologies and to the complexity of the past surface temperature history exhibited for example by fluctuations of the mean annual ground temperature geological cross sections showing variation in the tertiary and upper cretaceous deltaic sequences indicate a good correlation with the permafrost distribution in particular where the sand and gravel units of the iperk and kugmallit sequences predominate permafrost is thick owing to the high thermal conductivities of the ice rich sediments conversely where the low thermal conductivity shales and mudstones of the richards sequence are present permafrost is accordingly thinner numerical models of permafrost aggradation in the mackenzie delta region suggest that early and late wisconsinan surface temperatures were of the order 18 c a mid wisconsinan rise in temperature to approximately 8 c similar to mean surface temperature today is also indicated this proposed palaeoclimate is sufficient to account for the observed maximum permafrost thicknesses in the region

influence of ground ice and permafrost on coastal evolution richards island beaufort sea coast n w t a long term sediment budget 1947 1985 for northern richards island shows that when ground ice and offshore erosion are accounted for there is a near balance between headland erosion and coastal deposition excess ice constitutes about 20 of the total volume of eroded material from the headlands with massive ground ice contributing nearly 9 and segregated ice lenses and ice wedges making up the remainder coastal response to major storms in 1987 and 1993 suggests that erosion is episodic with short periods of intense disruption followed by readjustment of cliff profiles processes characteristic of this environment include mechanical erosion of ice bonded sediments creating unstable erosional niches mechanical failure of niches along ice wedge planes and longer term thermal erosion of ice bonded sediments where ice contents are high localized thaw slumps initiated by coastal erosion may retreat at rates substantially higher than those observed at other sections of the coast cliff top retreat r

comments on debris covered glaciers in the sierra nevada california and their implications for snowline reconstruction by d h clark m m clark and a r gillespie

geophysical mapping of ground ice using a combination of capacitive coupled resistivity and ground penetrating radar northwest territories canada 1 the nature and distribution of ground ice are two of the most unpredictable geological variables in near surface deposits characterized by continuous permafrost subsurface information about ground ice distribution and structure can be obtained either by invasive and environmentally destructive techniques like drilling and excavation or by noninvasive low impact geophysical methods in this study coordinated measurements by two complementary geophysical tools capacitive coupled resistivity ccr and ground penetrating radar gpr were used to map ground ice in a variety of locations in the mackenzie delta region of the western canadian arctic both ccr and gpr systems are highly portable especially on snow covered surfaces and very effective in collecting data under winter conditions when cold ground temperatures ensure that nearly all liquid water is frozen and signal penetration is enhanced ccr and gpr readily detect stratigraphic differences including the contacts between massive ice deposits and enclosing sediments gpr is widely used in permafrost research but ccr has been used in only a few studies this is the first study to combine results from both systems by collecting complementary data sets along coincident transects we demonstrate that when combined these data increase the quality and interpretation of subsurface information beyond what could be determined by either of the instruments alone the complementary nature of these two geophysical tools facilitated the detection and mapping of massive ground ice ice rich sediments ice wedges thermokarst and basic stratigraphic relationships this study breaks new ground by documenting the benefits of using these techniques together in permafrost investigations

eastern beringia and beyond late wisconsinan and holocene landscape dynamics along the yukon coastal plain canada terrestrial permafrost archives along the yukon coastal plain northwest canada have recorded landscape development and environmental change since the late wisconsinan at the interface of unglaciated beringia i e komakuk beach and the northwestern limit of the laurentide ice sheet i e herschel island the objective of this paper is to compare the late glacial and holocene landscape development on both sides of the former ice margin based on permafrost sequences and ground ice analyses at these sites involved a multi proxy approach including sedimentology cryostratigraphy palaeoecology of ostracods stable water isotopes in ground ice hydrochemistry and ams radiocarbon and infrared stimulated luminescence irsl dating ams and irsl age determinations yielded full glacial ages at komakuk beach that is the northeastern limit of ice free beringia herschel island to the east marks the late wisconsinan limit of the northwest laurentide ice sheet and is composed of ice thrust sediments containing plant detritus as young as 16 2 cal ka bp that might provide a maximum age on ice arrival late wisconsinan ice wedges with sediment rich fillings on herschel island are depleted in heavy oxygen isotopes mean delta o 18 of 29 1 parts per thousand this together with low d excess values indicates colder than modern winter temperatures and probably reduced snow depths grain size distribution and fossil ostracod assemblages indicate that deglaciation of the herschel island ice thrust moraine was accompanied by alluvial proluvial and eolian sedimentation on the adjacent unglaciated yukon coastal plain until similar to 11 cal ka bp during a period of low glacio eustatic sea level the late glacial holocene transition was marked by higher than modern summer temperatures leading to permafrost degradation that began no later than 11 2 cal ka bp and caused a regional thaw unconformity cryostructures and ice wedges were truncated while organic matter was incorporated and soluble ions were leached in the thaw zone thermokarst activity led to the formation of ice wedge casts and deposition of thermokarst lake sediments these were subsequently covered by rapidly accumulating peat during the early holocene thermal maximum a rising permafrost table reduced peat accumulation and extensive ice wedge growth resulted from climate cooling starting in the middle holocene until the late 20th century the reconstruction of palaeolandscape dynamics on the yukon coastal plain and the eastern beringian edge contributes to unraveling the linkages between ice sheet ocean and permafrost that have existed since the late wisconsinan

the 1984 morgan hill california earthquake the morgan hill california earthquake magnitude 6 1 of 24 april 1984 ruptured a 30 kilometer long segment of the calaveras fault zone to the east of san jose although it was recognized in 1980 that an earthquake of magnitude 6 occurred on this segment in 1911 and that a repeat of this event might reasonably be expected no short term precursors were noted and so the time of the 1984 earthquake was not predicted unilateral rupture propagation toward the south southeast and an energetic late source of seismic radiation located near the southeast end of the rupture zone contributed to the highly focused pattern of strong motion including an exceptionally large horizontal acceleration of 1 29g at a site on a dam abutment near the southeast end of the rupture zone

hyporheic exchange with heterogeneous streambeds laboratory experiments and modeling 1 hyporheic exchange is generally analyzed with the assumption of a homogeneous hyporheic zone in reality streambed sediments have a heterogeneous structure and this natural heterogeneity produces spatially variable interfacial fluxes and complex hyporheic exchange patterns to assess the basic effects of sediment structure on hyporheic exchange we performed salt and dye injection experiments in a recirculating laboratory flume with two heterogeneous sediment beds characterized by negative exponential correlated random hydraulic conductivity fields dye injections showed that the hyporheic flow structure was controlled by the spatial relationship of bed forms to high and low permeability regions of the streambed as no existing model could represent these effects we developed a new finite element model to calculate the pore water flow field resulting from the interaction of the bed form induced boundary head distribution and the heterogeneous sediment structure a numerical particle tracking approach was then used to assess the resulting hyporheic exchange the combined flow transport model did an excellent job of predicting the complex hyporheic flow pathways in the heterogeneous bed and the net hyporheic exchange up to t 30 hours the heterogeneous hydraulic conductivity field caused both greater spatial variability in the water flux through the bed surface and a greater average interfacial flux than would have occurred with a homogeneous bed the layered correlation structure of the streambed produced an effective anisotropy that favored longitudinal pore water flow and caused a relatively rapid decrease of the mean pore water velocity with depth as a result solute penetration into the bed was confined to a more shallow region than would have occurred with a homogeneous bed the combination of faster near surface transport and shallower solute penetration produced a shorter mean hyporheic residence time on the basis of the combination of experimental results and model simulations we conclude that the structural heterogeneity of streambed sediments produces more spatially limited hyporheic exchange that occurs with greater spatial variability and at a higher overall rate

cryostructures in permafrost tuktoyaktuk coastlands western arctic canada the shape and distribution of ice and sediment within frozen ground constitute its cryostructure observations on perennially frozen sediments in the tuktoyaktuk coastlands enable six cryostructures to be identified 1 structureless 2 lenticular 3 layered 4 reticulate 5 crustal and 6 suspended these cryostructures can be transitional or composite five cryofacies types can also be identified according to volumetric ice content and grain size they can be grouped into cryofacies assemblages the utility of the descriptive classification of ice rich sediments is illustrated from a palaeothaw layer at crumbling point

burial and preservation of a 30 000 year old perennial snowbank in red creek valley ogilvie mountains central yukon canada this study describes the origin and age of a body of massive ground ice exposed in the headwall of a thaw slump in the red creek valley central yukon canada the site is located beyond the limits of pleistocene glaciation in central yukon and within the southern limit of the modern continuous permafrost zone the origin of the massive ground ice which is preserved under a fine grained diamicton containing thin layers of tephra was determined through ice petrography stable o h isotope composition of the ice and gas composition of occluded air entrapped in the ice the age of the massive ground ice was established by identifying the overlying tephra and radiocarbon dating of a muck deposit preserved within the ice collectively the results indicate that the massive ground ice formed by snow densification with limited melting refreezing and is interpreted as being a buried perennial snowbank the muck deposit within the ice which yielded an age of 30 720 340 14 c a bp and the dawson tephra 25 300 14 c a bp overlying the perennial snowbank indicates that the snowbank accumulated at roughly the transition between marine isotope stages 3 and 2 dry climatic conditions at this time and possibly high winds enabled the snowbank to accumulate in the absence of extensive local valley glaciation as occurred in the mountains to the south in addition to documenting the persistence of relict permafrost and ground ice to warming climate in regions where they are predicted to disappear by numerical models this study presents evidence of an isotopic biosignature preserved in a body of massive ground ice

distribution and activity of ice wedges across the forest tundra transition western arctic canada remote sensing regional ground temperature and ground ice observations and numerical simulation were used to investigate the size distribution and activity of ice wedges in fine grained mineral and organic soils across the forest tundra transition in uplands east of the mackenzie delta in the northernmost dwarf shrub tundra ice wedge polygons cover up to 40 of the ground surface with the wedges commonly exceeding 3 m in width the largest ice wedges are in peatlands where thermal contraction cracking occurs more frequently than in nearby hummocky terrain with fine grained soils there are fewer ice wedges rarely exceeding 2 m in width in uplands to the south and none have been found in mineral soils of the tall shrub tundra although active ice wedges are found there throughout peatlands in the spruce forest zone small relict ice wedges are restricted to peatlands at tundra sites winter temperatures at the top of permafrost are lower in organic than mineral soils because of the shallow permafrost table occurrence of phase change at 0 c and the relatively high thermal conductivity of icy peat due to these factors and the high coefficient of thermal contraction of frozen saturated peat ice wedge cracking and growth is more common in peatlands than in mineral soil however the high latent heat content of saturated organic active layer soils may inhibit freezeback particularly where thick snow accumulates making the permafrost and the ice wedges in spruce forest polygonal peatlands susceptible to degradation following alteration of drainage or climate warming

the chronostratigraphy of late pleistocene glacial and periglacial aeolian activity in the tuktoyaktuk coastlands nwt canada aeolian periglacial sand deposits are common in the tuktoyaktuk coastlands of western arctic canada regionally extensive and thick aeolian sand sheet deposits have been observed in two major stratigraphic settings within a sand unit characterized by large aeolian dune deposits and interbedded with glaciofluvial outwash from the laurentide ice sheet lis small localized sand sheets have also been observed along the tops of sandy bluffs within sequences of drained thermokarst lakes deposits and as an involuted veneer above buried basal ice of the lis on the basis of radiocarbon and optically stimulated luminescence osl dates from preserved periglacial aeolian sand sheets and dunes a regional chronostratigraphy is presented which indicates that both extensive dunes and sand sheets accumulated mainly between ca 30 and 13 ka a switch to dominantly sand sheet aggradation at ca 14 13 ka with sand sheets forming widely until ca 8 ka is attributed to a surface armouring by glacial deposits associated with the advance of the lis and b amelioration of the climate from cold aridity an absence of osl dates between ca 8 and 1 ka suggests that sand sheets stabilized during much of the holocene local sand sheet aggradation during recent centuries has occurred near sandy bluffs and on the floors of drained thermokarst lakes the osl dates constrain the maximum extent of the lis in the tuktoyaktuk coastlands to marine isotope stage 2

the principles of cryostratigraphy abstract cryostratigraphy adopts concepts from both russian geocryology and modern sedimentology structures formed by the amount and distribution of ice within sediment and rock are termed cryostructures typically layered cryostructures are indicative of syngenetic permafrost while reticulate and irregular cryostructures are indicative of epigenetic permafrost cryofacies can be defined according to patterns of sediment characterized by distinct ice lenses and layers volumetric ice content and ice crystal size cryofacies can be subdivided according to cryostructure where a number of cryofacies form a distinctive cryostratigraphic unit these are termed a cryofacies assemblage the recognition if present of i thaw unconformities ii other ice bodies such as vein ice ice wedges aggradational ice and thermokarst cave pool ice and iii ice sand and gravelly pseudomorphs is also important in determining the nature of the freezing process the conditions under which frozen sediment accumulates and the history of permafrost

an assessment of evidence for pingos on mars using hirise pingos are small hills with cores of ice formed by injection and freezing of pressurized water the possibility of pingos on mars is of particular interest because of the associated implications for liquid water we have systematically searched for candidate pingos using images from the high resolution imaging science experiment hirise camera since pingos are expected to develop surface fractures due to extension of the frozen ground over the ice core we have searched for fractured features and identified a variety of mounds these features are confined to the martian mid latitudes in the bands where gullies are also most common the observed fractured mounds have a variety of morphologies and are likely of multiple origins isolated fractured mounds found on the floors of gullied craters in the southern hemisphere match the general morphologic characteristics of terrestrial pingos and are the best candidates for martian pingos but there is currently no direct evidence for presence of ice cores and it is difficult to produce the necessary water volumes so these features should still be interpreted with caution other fractured mounds appear more likely to be erosional remnants of an unusual mantling layer or possibly thermokarst structures flat topped mounds in utopia have some characteristics fracture pattern and latitudinal distribution consistent with pingos but differ in other aspects such as shape and setting while we do not rule out a pingo origin we prefer an erosional model for these enigmatic features

strong motion seismology strong motion seismology is concerned with earthquake ground motion in the amplitude range that poses the threat of human injury or property damage because the occurrence of strong motion cannot be predicted in advance and because the time between significant events at any one place may range from tens to thousands of years strong motion data is recorded on triggered instruments that typically must be maintained in readiness for years between periods of significant recording as a consequence the strong motion data set is sparse because of the engineering need for estimates of future ground motion the scope of the field of strong motion seismology encompasses seismic source theory and any other aspect of seismology that can provide insights helpful in making the best possible estimates given the limited data this relationship with the rest of seismology is a reciprocal one in that strong motion data gives perhaps the best information available on earthquake source processes

periglacial landscape dynamics in the western canadian arctic results from a thermokarst lake record on a push moraine herschel island yukon territory abstract ice rich permafrost landscapes are sensitive to climate and environmental change due to the melt out of ground ice during thermokarst development thermokarst processes in the northern yukon territory are currently not well documented lake sediments from herschel island 69 36 n 139 04 w in the western canadian arctic provide a record of thermokarst lake development since the early holocene a 727 cm long lake sediment core was analyzed for radiographic images magnetic susceptibility granulometry and biogeochemical parameters organic carbon nitrogen and stable carbon isotopes based on eight calibrated ams radiocarbon dates the sediment record covers the last 11 500 years and was divided into four lithostratigraphic units a to d reflecting different thermokarst stages thermokarst initiation at the study area began 11 5 cal ka bp from 11 5 to 10 0 cal ka bp lake sediments of unit a started to accumulate in an initial lake basin created by melt out of massive ground ice and thaw subsidence between 10 0 and 7 0 cal ka bp unit b the lake basin expanded in size and depth attributed to talik formation during the holocene thermal maximum higher than modern summer air temperatures led to increased lake productivity and widespread terrain disturbances in the lake s catchment thermokarst lake development between 7 0 and 1 8 cal ka bp unit c was characterized by a dynamic equilibrium where lake basin and talik steadily expanded into ambient ice rich terrain through shoreline erosion once lakes become deeper than the maximum winter lake ice thickness thermokarst lake sediments show a great preservation potential however site specific geomorphic factors such as episodic bank shore erosion or sudden drainage through thermo erosional valleys or coastal erosion breaching lake basins can disrupt continuous deposition a hiatus in the record from 1 8 to 0 9 cal ka bp in lake herschel likely resulted from lake drainage or allochthonous slumping due to collapsing shore lines before continuous sedimentation of unit d recommenced during the last 900 years

seasonal growth bands in pingo ice field studies of pingo ice show that alternating clear and bubble bands may grow when water freezes at the top of a subpingo water lens a study of the banding at one excellent exposure of pingo ic

arsenic mobilization in the hyporheic zone of a contaminated stream arsenic behavior was examined in a contaminated stream by sampling the dissolved 0 45 m arsenic and metals in surface water shallow hyporheic zone water and adjacent ground water surface water was oxic and slightly basic and ground water was anoxic and acidic hyporheic zone water had ph values of 6 7 dissolved oxygen concentrations mostly between 0 and 3 mg l 1 and mean concentrations of most metals inbetween surface and ground water sample concentrations however arsenic and iron were enriched in the hyporheic zone most of the hyporheic zone dissolved arsenic was in the form of as iii which is considered to be more toxic to some organisms than as v in the oxic surface water 20 of the total dissolved as was found to occur in its reduced form we hypothesize that upon burial and reduction of fe oxyhydroxides in the streambed sediment bound arsenic is transferred into the dissolved phase as as iii and it is subsequently released into the surface water where it does not immediately reoxidize a continual flux of reduced as to the surface water maintains as iii concentrations above that expected in oxygenated surface waters

hyporheic temperature dynamics and heat exchange near channel spanning logs 1 the flow of river water around large woody debris lwd creates pressure gradients along the riverbed that drive river groundwater mixing or hyporheic exchange and heat transport within the hyporheic zone we quantify hyporheic fluid and heat exchange induced by current interaction with channel spanning logs using two approaches laboratory flume experiments and numerical simulations that link turbulent open channel fluid flow porous fluid flow and heat transport flume and numerical experiments show that logs produce a characteristic diel temperature pattern within sediment that shifts with log blockage ratio a fraction of the channel depth blocked by the log channel froude number and sediment permeability upstream of a log downwelling water transports the river s diel thermal signal deep into the sediment downstream upwelling water from shallow flow paths has a thermal signal similar to the surface water while upwelling water from deep flow paths forms a wedge of buffered low amplitude temperatures since most hyporheic water emerges from shallow flow paths upwelling water has limited potential to buffer surface water temperature because of historic channel clearing practices modern rivers have unnaturally low densities of lwd a key implication is that lwd removal has contributed to thermal homogenization and potential degradation of hyporheic habitats lwd reintroduction is a promising strategy to improve vertical connectivity in rivers and increase thermal patchiness within the hyporheic zone however hyporheic exchange near lwd may not impact diel surface water temperatures at the reach scale

thawing of massive ground ice in mega slumps drives increases in stream sediment and solute flux across a range of watershed scales 1 ice cored permafrost landscapes are highly sensitive to disturbance and have the potential to undergo dramatic geomorphic transformations in response to climate change the acceleration of thermokarst activity in the lower mackenzie and peel river watersheds of northwestern canada has led to the development of large permafrost thaw slumps and caused major impacts to fluvial systems individual mega slumps have thawed up to 106 m3of ice rich permafrost the widespread development of these large thaw slumps up to 40 ha area with headwalls of up to 25 m height and associated debris flows drive distinct patterns of stream sediment and solute flux that are evident across a range of watershed scales suspended sediment and solute concentrations in impacted streams were several orders of magnitude greater than in unaffected streams in summer slump impacted streams displayed diurnal fluctuations in water levels and solute and sediment flux driven entirely by ground ice thaw turbidity in these streams varied diurnally by up to an order of magnitude and followed the patterns of net radiation and ground ice ablation in mega slumps these diurnal patterns were discernible at the 103 km2 catchment scale and regional disturbance inventories indicate that hundreds of watersheds are already influenced by slumping the broad scale impacts of accelerated slumping are indicated by a significant increase in solute concentrations in the peel river 70 000 km2 these observations illustrate the nature and magnitude of hydrogeomorphic changes that can be expected as glaciogenic landscapes underlain by massive ice adjust to a rapidly changing climate

combined role of advective pumping and mechanical dispersion on time scales of bed form induced hyporheic exchange this study analyzes the effect of advective pumping and pore scale dispersion on bed form induced hyporheic exchange advection and dispersion play a competitive role in the exchange dynamics betwe

macropores as preferential flow paths in meander bends macropores are subsurface connected void spaces caused by processes such as fracture of soils micro erosion and fauna burrows they are common near streams e g hyporheic and riparian zones and may act as preferential flow paths between surface and groundwaters affecting hydrologic and biogeochemical processes we tested the hydrologic function of macropores by constructing an artificial macropore within the saturated zone of a meander bend open macropore om and later filling its upstream end partially filled macropore pfm for each treatment we injected saline tracer at an upgradient monitoring well within the meander and monitored downgradient hydraulics and tracer transport pressure transducers in monitoring wells indicated hydraulic gradients within the meander were 32 higher perpendicular to and 6 higher parallel to the macropore for the om than for the pfm additionally hydraulic conductivities measured via falling head tests were 29 to 550 times higher along the macropore than in nearby sediment we used electrical conductivity probes in wells and electrical resistivity imaging to track solute transport transport velocities through the meander were on average 9 and 21 higher per temporal moment analysis and observed tracer peak respectively for the om than for the pfm furthermore temporal moments of tracer breakthrough analysis indicated downgradient longitudinal dispersion and breakthrough tracer curve tailing were on average 234 and 182 higher for the om respectively this suggests the om enabled solute transport at overall shorter timescales than the matrix but also increased tailing our results demonstrate the importance of macropores to meander bend hydrology and solute transport copyright 2012 john wiley sons ltd

thermokarst processes and the origin of crater rim gullies in utopia and western elysium planitia abstract we have identified a number of gullies that could be aqueous in origin near or at the rim of several impact craters in utopia planitia and western elysium planitia 30 0 59 0 n 241 0 291 0 w based on the sharpness of their incisions and the general absence of superposed craters we ascribe a relatively recent origin to the gullies scalloped depressions are commonplace throughout the region as well as on the crater walls rims and floors near the areas of gully issuance occasionally the depressions cross cut the gully debris aprons suggesting that the formation of some depressions is even more recent than that of the gullies previous research has proposed that the depressions are collapse basins formed by thermokarst processes on earth thermokarst landforms occur in areas of low gradient topography where the permanently frozen ground permafrost is ice rich and has undergone a change in thermal equilibrium this change can be triggered by long term or episodic cyclic climate change and accompanying rises in mean temperatures towards 0 c as well as by rises in seasonally sustained summer temperatures well above 0 c in order to explain the origin of the rim or near rim gullies we invoke high obliquity and the possibility that this region of mars experienced obliquity driven rises in temperature atmospheric pressure and humidity sufficient to keep surface water and near surface ground ice stable for extended periods of time we propose that gully formation is closely related to local freeze thaw processes that in turn generate a thermokarst landscape of which the gullies are a part this geological and climatological scenario comprises the following steps 1 an inundation of meltwater at high obliquity due to the thawing of an atmospherically deposited snowpack or ice sheet and the subsequent saturation of the underlying regolith to tens of metres of depth 2 loss of water on the surface perhaps as obliquity decreases slightly followed by the progressive freezing of the saturated regolith this creates an aggrading mass of ice rich regolith 3 obliquity induced temperature rises that engender the thaw drainage and partial evaporation of the near surface ice rich regolith 4 localised formation of thermokarst collapse basins alases as water is evacuated from these basins 5 formation of gullies near or at some impact crater rims as the result of meltwater migration from nearby alases through the thawed regolith to the areas of gully issuance although the plains materials in this region are in part very old possibly hesperian or even noachian the mantling deposits and their deformation by thermokarst processes appears to be relatively young this suggests that recent climatic conditions could have been episodically warmer and wetter than had been previously thought

water table dynamics and groundwater surface water interaction during filling and draining of a large fluvial island due to dam induced river stage fluctuations 1 dam controlled river stage fluctuations alter groundwater surface water interaction between persistent bars and islands and the rivers bounding them by rapidly changing hydraulic gradients and expanding hyporheic zones a 300 m long and 80 m wide sand gravel island with established vegetation located on the colorado river austin texas usa is subjected to 1 m daily river stage variations due to upstream dam operations piezometer nests with probes monitored the evolution of the water table and groundwater flow paths through several cycles of dam induced stage fluctuations results show that hydraulic head and the water table within the island closely track the river stage associated with dam release water table mounds and depressions which overlap in time were mapped through the course of one storage release cycle over which 4 000 m 3 of water moved in and out of the island dam operations have drastically altered groundwater surface water connectivity between the colorado river and the fluvial island aquifer by pumping substantial amounts of water in and out of the aquifer during dam release and storage cycles