climate instability during the last interglacial period recorded in the grip ice core isotope and chemical analyses of the grip ice core from summit central greenland reveal that climate in greenland during the last interglacial period was characterized by a series of severe cold periods which began extremely rapidly and lasted from decades to centuries as the last interglacial seems to have been slightly warmer than the present one its unstable climate raises questions about the effects of future global warming

evidence for stratigraphic distortion in the greenland ice core project grip ice core during event 5e1 120 kyr bp from gas isotopes 1 the disturbed stratigraphy of the ice in the lowest 10 of the greenland grip ice core has been previously demonstrated using gas measurements 18o of o2 and ch4 on a few meters depth scale however rapid ice isotopic variations on the scale of 20 cm are experienced in the bottom of the grip ice core with complex chemical signatures that make them difficult to reconcile with a disturbed stratigraphy of the ice this is the case for event 5e1 first described as a dramatic cooling 120 kyr bp we analyzed at a 5 cm resolution the isotopic composition of the air from 2 m of the grip bottom ice core covering event 5e1 the 15n measurements combined with a basic firn modeling lead to the solid conclusion that the rapid event 5e1 is not a climatic event rapid variations of 18o of o2 18oatm are in agreement with a disturbed ice stratigraphy however the double peak shape of the 18oatm recalling chemical data at the same depth requires processes of diffusion after the mixing or even postcoring placing limits to the interpretation of some classical paleoclimatic proxies in small scale mixed ice 1 m

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

effects of large floods on sediment transport and reach morphology in the cobble bed sainte marguerite river abstract sediment transport rates were estimated for two flood events on the cobble bed sainte marguerite river in the saguenay region canada morphologic methods were used to derive one set of estimates and a combination of the meyer peter and muller equation with a dimensionless sediment transport ratio after dietrich et al nature 340 1989 215 was used to derive another set of estimates both sets of estimates give consistent results for the first event which had a decade scale return period and for the second event which was the largest flood on record and had a century scale return period the transport occurring during the second event was an order of magnitude greater than that occurring during the first event despite this disparity in the transport intensity of the two events the channel morphology remained qualitatively similar the observed degree of channel stability is attributed to a change of channel pattern and the initiation of bed degradation following channel rectification in the 1960s

northern westerlies during the last glacial maximum results from cmip5 simulations abstractbased upon simulations from phase 5 of the coupled model intercomparison project cmip5 the vertical and regional characteristics of the northern westerlies during the last glacial maximum lgm are investigated in this study at the northern hemispheric scale all nine available models simulate a poleward shift of the 200 hpa jet and eight models simulate an equatorward shift of the 850 hpa jet compared to the preindustrial period these shifts are of approximately 2 3 latitude for the arithmetic multimodel mean the upper tropospheric cooling in the tropics possibly due to reduced latent heat release is expected to account for the poleward shift of the 200 hpa jet through the thermal wind relationship changes in the midlatitude baroclinic instability in response to the amplified polar cooling are associated with the jet stream in the lower troposphere through anomalous eddy activity in particular the types of predominant baroclinic eddies are regionally dependent the behavior of the 8

riverine dunes on the coastal plain of georgia usa abstract relict eolian dunes on the georgia coastal plain line the north and east sides of many se trending streams and contain important paleoenvironmental information a detailed analysis of four dune fields characterizes dune geomorphology stratigraphy sedimentology and soils georgia s inland dunes range from parabolic to infilled parabolic to irregular in form typically merge to form long chains that parallel source streams and can occur as distinct bands typically the dunes are composed of 95 quartz sand grains with more than half the sand falling in the medium sand fraction dunes overlie flood plain and pointbar sands backswamp and peat deposits fluvial terraces and other dunes in places organic deposits occur on top of dunes cross bedding is observable in dunes more than about 4 m thick and indicates eastward dune migration paleosols found within some dunes indicate holocene reworking conditions favoring dune formation likely included at least seasonal aridity postdepositional modification of the dunes includes a smoothing of the dune surface a clay increase in the c horizon spodic horizon development bioturbation removal of weatherable minerals and fluvial incision

glaciochemistry of polar ice cores a review human activities have already modified the chemical composition of the natural atmosphere even in very remote regions of the world the study of chemical parameters stored in solid precipitation and accumulated on polar ice sheets over the last several hundred thousand years provides a unique tool for obtaining information on the composition of the preindustrial atmosphere and its natural variability over the past this paper deals with the chemistry of polar ice focused on the soluble mineral na nh4 k ca mg h f cl no3 so4 and h2o2 and organic methanesulfonate ch3so3 formate hcoo acetate ch3coo and formaldehyde hcho species and their interpretation in terms of past atmospheric composition aerosols and water soluble gaseous species we discuss ice core dating the difficulties connected with trace measurements and the significance of the ionic composition of snow we examine temporal from the last decades back to the last climatic cycle and spatial including examples from coastal as well as central areas of greenland and antarctica variations in the ionic budget of the precipitation and evaluate ice core studies in terms of the chemical composition of our past atmosphere we review 1 how greenland and antarctic ice cores that span the last few centuries have provided information on the impact of human activities and 2 how the chemistry of deep ice cores provides information on various past natural phenomena such as climatic variations glacial interglacial changes el nino volcanic eruptions and large boreal forest fires

glacial flour dust storms in the gulf of alaska hydrologic and meteorological controls and their importance as a source of bioavailable iron 1 iron is an essential micronutrient that limits primary productivity in much of the ocean including the gulf of alaska goa however the processes that transport iron to the ocean surface are poorly quantified we combine satellite and meteorological data to provide the first description of widespread dust transport from coastal alaska into the goa dust is frequently transported from glacially derived sediment at the mouths of several rivers the most prominent of which is the copper river these dust events occur most frequently in autumn when coastal river levels are low and riverbed sediments are exposed the dust plumes are transported several hundred kilometers beyond the continental shelf into iron limited waters we estimate the mass of dust transported from the copper river valley during one 2006 dust event to be between 25 80 ktons based on conservative estimates this equates to a soluble iron loading of 30 200 tons we suggest the soluble fe flux from dust originating in glaciofluvial sediment deposits from the entire goa coastline is two to three times larger and is comparable to the annual fe flux to goa surface waters from eddies of coastal origin given that glaciers are retreating in the coastal goa region and in other locations it is important to examine whether fluxes of dust are increasing from glacierized landscapes to the ocean and to assess the impact of associated fe on marine ecosystems

chronology and provenance of last glacial peoria loess in western iowa and paleoclimatic implications geologic archives show that the earth was dustier during the last glacial period one model suggests that increased gustiness stronger more frequent winds enhanced dustiness we tested this at loveland iowa one of the thickest deposits of last glacial age peoria loess in the world based on k rb and ba rb loess was derived not only from glaciogenic sources of the missouri river but also distal loess from non glacial sources in nebraska optically stimulated luminescence osl ages provide the first detailed chronology of peoria loess at loveland deposition began after 27 ka and continued until 17 ka osl ages also indicate that mass accumulation rates mars of loess were not constant mars were highest and grain size was coarsest during the time of middle peoria loess accretion 23 ka when 10 m of loess accumulated in no more than 2000 yr and possibly much less the timing of coarsest grain size and highest mar indicating strongest winds coincides with a summer insolation minimum at high latitudes in north america and the maximum southward extent of the laurentide ice sheet these observations suggest that increased dustiness during the last glacial period was driven largely by enhanced gustiness forced by a steepened meridional temperature gradient

wildfire morphologic change and bed material transport at fishtrap creek british columbia abstract in 2003 a wildfire burned nearly two thirds of the fishtrap creek watershed including the forested floodplain annually repeated surveys of the stream channel combined with bed material tracer studies in 2006 and 2007 indicate that the channel has become unstable and that the bed material transport dynamics have changed since the fire using these data the morphologic changes have been quantified and event scale bed material transport rates have been estimated for four post fire freshets the immediate response of the channel during the 2005 and 2006 freshets was a progressive shift in channel morphology from a relatively featureless plane bed morphology devoid of significant sediment accumulations to a riffle pool morphology in which bars cover a significant proportion of the bed this transformation occurred in association with moderate bed material sediment transport rates and only minor bank erosion within the study reach extensive bank erosion occurred during the 2007 and 2008 freshets producing a dramatic change in channel morphology and increasing the local bed material transport rates by an order of magnitude post fire monitoring of streamflow and suspended sediment concentrations a surrogate for upstream sediment supply indicates that post fire increases are not detectable presented in eaton et al in press and our analysis demonstrates that major morphologic changes can occur following wildfire even when the common external drivers of stream discharge and sediment supply remain unchanged

morphological controls on the downstream passage of a sediment wave in a gravel bed stream sediment waves in river systems have been widely reported although few studies have examined the interaction between these waves and the morphology of the reaches through which they pass this interaction determines how waves are modified as they propagate downstream this study documents the origin and downstream passage of an avulsion generated sediment wave through a 374 m study reach of the allt dubhaig scotland a nested survey framework was adopted with volumes calculated from cross sections spaced between 10 and 40 m apart documenting the origin and downstream passage of the wave the wave moved through an intensively c 1 m cross section spacing monitored 120 m stretch reach a within the study reach allowing assessment of sediment exchanges between the incoming wave and the local morphology successive surveys show the movement of the wave through and out of the reach and also that areas where wave sediment was deposited did not always correspond with areas of subsequent erosion reach a was divided into three morphologically distinct sub reaches 1a 2a and 3a within which sediment fluxes and the three dimensional distribution of erosion deposition were estimated sediment wave input into 1a and 2a relatively stable sub reaches caused forced bar aggradation and erosion of sediment from elsewhere within the reach which then became part of the wave the downstream transfer of this sediment into unstable 3a caused aggradation and in response widespread erosion which increased the magnitude of the sediment wave as it exited reach a sediment exchange between the recipient reach and the wave depends upon local morphological stability and is a crucial process affecting wave magnitude and attenuation the macroscale sediment wave interacted with rather than overwhelmed the recipient morphology 1998 john wiley sons ltd

north atlantic eddy driven jet in interglacial and glacial winter climates the atmospheric westerly flow in the north atlantic na sector is dominated by atmospheric waves or eddies generating via momentum flux convergence the so called eddy driven jet the position of this jet is variable and shows for the present day winter climate three preferred latitudinal states a northern central andsouthernpositioninthena here theauthorsanalyzethebehavioroftheeddy drivenjetunderdifferent glacial and interglacial boundary conditions using atmosphere land only simulations with the ccsm4 climate model as state of the art climate models tend to underestimate the trimodality of the jet latitude the authors apply a bias correction and successfully extract the trimodal behavior of the jet within ccsm4 the analysis shows that during interglacial times i e the early holocene and the eemian the preferred jet positions are rather stable and the observed multimodality is the typical interglacial character of the jet during glacial times the jet is strongly enhanced its position is shifted southward and the trimodal behavior vanishes this is mainly due to the presence of the laurentide ice sheet lis the lis enhances stationary waves downstream thereby accelerating and displacing the na eddy driven jet by anomalous stationary momentum flux convergence additionally changes in the transient eddy activity caused by topography changes as well as other glacial boundary conditions lead to an acceleration of the westerly winds over the southernnaattheexpenseofmorenorthernareas consequently bothstationaryandtransienteddiesfoster the southward shift of the na eddy driven jet during glacial winter times

response and recovery of a subalpine stream following a catastrophic flood the july 15 1982 lawn lake flood in rocky mountain national park colorado was caused by the failure of a 79 yr old earthen dam peak discharges of the flood far exceeded naturally occurring flows and it caused severe channel disturbance along most of roaring river and some parts of fall river this study documents the geomorphic response of a 5 km reach of fall river in the 5 yr following the flood in 1983 the first year after the lawn lake flood snowmelt flows were well above average these high flows together with very high sediment yields from roaring river resulted in significant geomorphic changes on reaches of fall river downstream during the 1983 snowmelt runoff 15 5 x 106 kg of bed load sediment was eroded from the upper part of the study area these loads were at least 1 000 times higher than before the lawn lake flood most of this sediment was then deposited in a highly sinuous reach of fall river in the lower part of the study area this reach had not been much affected by the lawn lake flood but sedimentation during the period of high flow in 1983 completely filled in the channel resulting in the formation of a continuous 2 3 km long depositional zone in 1984 sediment yield from roaring river declined dramatically and this trend continued for the next 3 yr by 1987 the bed load sediment yield in the upper reaches of fall river was only about 0 4 x 106 kg yr the decline in sediment loads resulted in progressive erosion and recovery of the original channel of fall river in the depositional zone reach recovery in the upstream part of the depositional zone was complete by 1985 recovery in the downstream part of the depositional zone took longer because of the continued supply of sediment and because the sediment was mobile less of the time as of 1987 about 80 of the material initially stored in the sedimentation zone had been eroded bed load sediment yields at a sampling site 1 km downstream of the terminus of the depositional zone ranged from 5 x 106 to 11 x 106 kg yr but showed no significant decline over the 5 yr study period the average rate of bed load transport through this reach was at least 100 times greater than before the lawn lake flood but few discernible channel changes resulted from the higher loads

gravel transport and morphological change in braided sunwapta river alberta canada the relation between morphological change and patterns of variation in bedload transport rate in braided streams was observed by repeated daily topographic surveys over a 25 day study period in a 60 m reach of the proglacial sunwapta river alberta canada there are two major periods of morphological change each lasting several days and each involving the complete destruction and reconstruction of bar complexes bar complex destruction was caused by redirection of the flow and by downstream extension of the confluence scour zone upstream reconstruction involved accretion of unit bars on bar head flank and tail and in one case was initiated by disection of a large lobate unit bar high rates of sediment movement measured from net scour and fill of the cross sections coincided with these morphological changes sediment was supplied from both bed and bank erosion and patterns and distances of transfer were highly variable rates of transport estimated by matching upstream erosional volumes with downstream deposition were much greater than those estimated from either a step length approach or a sediment budget measurements of scour and fill and observations of morphological change indicate that step lengths virtual transport distances were typically 40 100m during a diurnal discharge cycle shorter step lengths occurred when transfer was confined to a single anabranch and longer steps involved channel changes at the scale of the entire reach sediment budgeting was used to describe the spatial patterns of sediment transport associated with the morphological changes and to estimate minimum daily reach averaged transport rates mean bedload transport rates correlate with discharge but with considerable scatter the largest deviations from the mean relation can be tied to phases of channel incision bank erosion scour hole migration bar deposition and channel filling apparently controlled by changes and fluctuations in sediment supply from upstream independent of discharge these are interpreted as field evidence of autopulses or macropulses in bedload transport previously observed only in laboratory models of braided streams

deglaciation of a soft bedded laurentide ice sheet we present a series of numerical reconstructions of the laurentide ice sheet during the last deglaciation 18 7 14c ka that evaluates the sensitivity of ice sheet geometry to subglacial sediment deformation these reconstructions assume that the laurentide ice sheet flowed over extensive areas of water saturated deforming sediment soft beds corresponding to the st lawrence lowland the great lakes region the western prairies of the u s and canada and the hudson bay and hudson strait regions sediment rheology is based on a constitutive law that incorporates experimental results from late wisconsin till deposited by the laurentide ice sheet which suggest only mildly nonlinear viscoplastic behavior by varying the effective viscosity of till we produced four reconstructions for the ice sheet during the last glacial maximum 18 14c ka and two reconstructions each of the ice sheet at 14 13 12 11 and 10 14c ka we also produced one reconstruction for 9 8 4 8 and 7 14c ka reconstructions that assume a low effective viscosity for all areas of deforming sediment show a multidomed ice sheet with a large bowl shaped depression over hudson bay and thin ice 1000 m above modern sea level over the western and southern margins those reconstructions that assume a higher effective viscosity of till in the hudson bay region than for the western and southern margins also show a multidomed ice sheet but with considerably thicker ice over hudson bay and a more northerly position of the central ice divide these two different geometries may represent ice sheet orographic changes associated with a heinrich event further increases in effective viscosity of till approaching the effective viscosity of ice would result in a high monolithic ice dome centered over hudson bay reinforcing the notion that a multidomed ice sheet reflects the distribution of substrate geology modeled ice surface geometry at the last glacial maximum shows many of the same general features as previous reconstructions that incorporate deformable beds our reconstructions with higher effective till viscosities in hudson bay also agree with the ice 4g reconstructions peltier 1994 which are based on inversion of relative sea level data for the early part of the last deglaciation 18 13 14c ka but then depart significantly from ice 4g beginning at about 12 14c ka due to differing assumptions of the history of deglaciation modeled ice volume for the last glacial maximum suggests a glacioeustatic change of 50 55 m by a soft bedded laurentide ice sheet this would increase as the effective viscosity of till increases subsequent ice volume changes through the last deglaciation generally parallel the trend of eustatic rise recorded at barbados new guinea and tahiti but suggest that the laurentide ice sheet was not the source of meltwater pulse 1a

transport direction of peoria loess in nebraska and implications for loess sources on the central great plains abstract in the midwestern united states large rivers draining the laurentide ice sheet lis were the most important sources of peoria loess deposited during the last glaciation loess deposition near those rivers may have responded primarily to ice sheet dynamics rather than direct effects of climatic change in contrast it has been proposed that thick peoria loess on the central great plains was derived mainly from unglaciated landscapes northwest of the main loess deposits in this study transport directions inferred from more than 600 measurements of peoria loess thickness in nebraska are used to test the hypothesis that much of the peoria loess on the great plains is nonglaciogenic a strong northwest to southeast thickness trend indicates that most peoria loess in nebraska was transported from one or more unglaciated northwestern source areas rather than from glacially influenced river floodplains the missouri river draining the lis the platte river draining alpine glaciers and the elkhorn river unglaciated basin were secondary sources their contribution is not detectable beyond a distance of 40 60 km peoria loess deposition on the central great plains was largely a direct response to climatic change in the unglaciated source region

planform cyclicity in an unstable reach complex fluvial response to environmental change long and short term channel changes are documented and analysed for a historically unstable reach of the river severn at llandinam mid wales long term changes the last 150 years reconstructed from 10 archival sources are characterized by channel planform switching between meandering 1836 1840 and 1948 1963 and braided 1884 1903 and 1975 present phases short term changes monitored by detailed planform surveys over a 2 5 year period showed smaller scale channel adjustments involving channel switching bar accretion and channel expansion phases of braiding at llandinam have been triggered by extrinsic controls primarily flooding but intrinsic controls floodplain sediments planform evolution and channel gradient have been influential in priming the reach prior to destabilization flow regulation on the river severn since 1968 has partly frozen the planform of the contemporary braid zone management of channel planform adjustments where environmental change is phased in over time must be informed by a knowledge of the potential for triggered planform switches in addition the effects of environmental change on fluvial systems are often historically contingent upon the state of the channel at the time of impact 1998 john wiley sons ltd

orbital tuning chronology for the vostok climate record supported by trapped gas composition abstract we present data on the o2 n2 ratios of trapped gas samples for the entire length of the vostok climate record as in other cores o2 n2 ratios in these samples are less than the atmospheric ratio by a small and variable amount because o2 is selectively excluded during the gas trapping process and because o2 is also preferentially lost in poorly preserved core samples samples younger than 150 ka have large and variable o2 depletions samples older than 200 ka have o2 n2 ratios that replicate well and vary smoothly with depth we plot o2 n2 ratios of well replicated samples older than 160 ka using a chronology derived by matching the 18o of paleoatmospheric o2 18oatm to northern hemisphere june insolation on this timescale o2 n2 varies coherently with local 78 s summertime insolation based on time series analysis of the o2 n2 record and the dynamics of snow metamorphism at the surface we conclude that summertime insolation influences physical properties of ice grains that control the degree of o2 exclusion during bubble closeoff o2 n2 in vostok is thus arguably a property that records local summertime insolation and can be used to test independent chronologies for the core we show that the 18oatm chronology supported by the coincidence of o2 n2 ratios with insolation is also compatible with recent radiometric dating of corals from high sea stands we further successfully test the 18oatm tuning chronology by showing that it predicts a chronology for the gisp2 core which is essentially indistinguishable from the standard gisp2 chronology and therefore in excellent agreement with the radiometric chronology of hulu cave china an accurate chronology for the vostok ice core is now in place

geochemical variations in peoria loess of western iowa indicate paleowinds of midcontinental north america during last glaciation peoria loess deposited in western iowa during the last glacial maximum lgm shows distinct geochemical and particle size variations as a function of both depth and distance east of the missouri river geochemical and particle size data indicate that peoria loess in western iowa probably had two sources the missouri river valley and a source that lay to the west of the missouri river both sources indicate that lgm paleowinds in western iowa had a strong westerly component similar to interpretations of previous workers a compilation of loess studies in iowa and elsewhere indicates that westerly winds were dominant during loess transport over much of the midcontinent south of the laurentide ice sheet which is not in agreement with paleowinds simulated by atmospheric general circulation models agcms agcms consistently generate a glacial anticyclone with easterly or northeasterly winds over the laurentide ice sheet and the area to the south of it loess deposition in the midcontinent during the lgm may be a function of infrequent northwesterly winds that were unrelated to the presence of the glacial anticyclone

size and distance of travel of unconstrained clasts on a streambed we have examined the relation between the mean distance of movement during individual flood events and the mean particle size of unconstrained clasts in gravel bed rivers distance of movement was scaled using the mean distance of movement of the median size group of the surface material size was scaled using the median b axis diameter of the subsurface material the scaled data are fitted with a nonlinear function clasts larger than the median of surface material exhibit a steep decline of mobility with an upper limit for mobility near di d50sub 10 smaller materials exhibit a less sensitive relation we suppose that trapping of small clasts affects their travel distance these results indicate that the movement of free surface stones is size preferred a subordinate analysis of free constrained and buried particles reveals that bed surface structure acts to reduce differential movement

what have we learnt from palaeoclimate simulations there has been a gradual evolution in the way that palaeoclimate modelling and palaeoenvironmental data are used together to understand how the earth system works from an initial and largely descriptive phase through explicit hypothesis testing to diagnosis of underlying mechanisms analyses of past climate states are now regarded as integral to the evaluation of climate models and have become part of the toolkit used to assess the likely realism of future projections palaeoclimate assessment has demonstrated that changes in large scale features of climate that are governed by the energy and water balance show consistent responses to changes in forcing in different climate states and these consistent responses are reproduced by climate models however state of the art models are still largely unable to reproduce observed changes in climate at a regional scale reliably while palaeoclimate analyses of state of the art climate models suggest an urgent need for model improvement much work is also needed on extending and improving palaeoclimate reconstructions and quantifying and reducing both numerical and interpretative uncertainties

surface winds across eastern and midcontinental north america during the last glacial maximum a new data model assessment abstract last glacial maximum lgm proxy evidence of surface wind direction across eastern and midcontinental north america comes primarily from loess and dune deposits and overwhelmingly suggests surface winds had a strong westerly component however the season of sediment deposition and the temporal scale of wind information preserved in these deposits remains uncertain furthermore paleoclimate model simulations over the last several decades have indicated a predominance of easterly winds across this region due to the presence of an anticyclone over the laurentide ice sheet as well as katabatic winds flowing off the ice sheet and over the adjacent land surface here we reassess model data near surface wind direction agreement using nine general circulation models participating in the lgm experiment of the third paleoclimate model intercomparison project pmip3 and a compilation of previously published paleowind directions from loess and dune deposits dating to the lgm we find the highest overall model proxy data agreement in winter december february indicating predominantly westerly winds across the region in the lgm model simulations we also find high zonal and meridional wind direction agreement in spring march may and fall september november in many models winter spring and fall also have faster mean daily near surface wind speeds in the lgm simulations relative to pre industrial control simulations thus this model data assessment suggests lgm aeolian deposition in the study region likely occurred dominantly in these three seasons at times when local conditions favorable for aeolian deflation coincided with high wind speed events models that agree best with the proxy data have strengthened aleutian and icelandic low pressure systems and a weakened laurentide high pressure system which constrains the spatial footprint of the laurentide high to the ice sheet reducing northeasterly winds near the ice sheet margin a weaker laurentide high in turn coincides with warmer surface temperatures over the ice sheet and the north atlantic the strength and location of semi permanent pressure systems were thus key controls on surface wind direction across midcontinental and eastern north america during the lgm

stratigraphy and chronology of mississippi valley loess in western tennessee the stratigraphy and chronology of loess deposits in the mississippi valley provide an important proxy record of the number and timing of glacial advances into the upper mississippi drainage basin the stratigraphy of loess and paleosols from two exposures in northwestern tennessee reveals three major loess units that are correlated with the loveland loess roxana silt and peoria loess from oldest to youngest detailed measurements of clay and secondary pedogenic iron contents suggest that the loveland loess may consist of multiple loess units which have been pedogenically welded to one another to form a composite sangamon soil the mineralogy of the coarse silt fraction from one exposure reveals both primary mineralogical differences between the peoria loess and the roxana silt and pedogenic overprinting by the sangamon soil grain size and iron data suggest that the degree of pedogenic alteration increases from west to east away from the loess source area which probably reflects the role of sedimentation rate on the degree of pedogenic alteration of loess however only the peoria loess shows the expected exponential eastward decrease in thickness the thickness of older loess units appears to be controlled more by the occurrence of localized erosion than by distance from source area most samples are resistant to solar resetting of the thermoluminescence signal which if unrecognized could yield thermoluminescence ages that significantly underestimate the true age of the deposits the best match between thermoluminescence age using the total bleach method and radiocarbon age of the peoria loess and roxana silt is achieved when either long durations of optical bleaching by sunlight 32 hr or artificial ultraviolet uv light are applied to define the residual thermoluminescence level partial bleach analysis indicates that long duration optical bleaching by sunlight or uv lamp bleaching does not systematically overbleach samples of peoria loess and roxana silt three thermoluminescence age estimates between 38 4 1 and 53 5 ka from the basal half of the roxana silt and six age estimates between 18 2 and 25 3 ka from the basal half of the peoria loess are consistent with numerous finite radiocarbon dates from around the central mississippi valley thermoluminescence age estimates suggest that the oldest of the intra loveland loess units is 127 12 ka one age and may have been deposited during marine oxygen isotope stage 6 the younger two of these loess units were deposited between 113 10 and 70 6 ka three ages this latter age range for loess that underlies the top of the presumed sangamon soil is problematic because it requires that pedogenesis may have begun as late as approximately 70 ka moreover loess deposited between about 120 and 70 ka suggests that the southern margin of the laurentide ice sheet was within some part of the upper mississippi valley during this interval this conflicts with some glacial geologic evidence but it is consistent with global sea level and 18o records of ice volume which indicate several intervals of moderate glaciation during this time

runoff and erosion processes after a forest fire in mount carmel a mediterranean area in the mediterranean forest area of israel fires increase runoff and sediment yield rates relative to undisturbed forested land the september 1989 fire covered an area of 4 km2 in the main recreation area of mount carmel a typical mediterranean forest area the lithology is chalk and limestone and about 40 of the burnt area has steep slopes exceeding 30 three study plots were established in burnt and unburnt areas plots were located on 100 300 m2 areas with different slope exposures runoff and sediments were collected after each storm by a collector system a hydrometric station was established draining an area of 1 km2 in the burnt zone rainfall was measured by two recorders and several rain gauges at the experimental sites in the first rainfall season after the fire runoff and sediment yield were 500 and 100 000 times higher respectively in the burnt areas rainfall intensity is a dominant factor in runoff and sediment yield rates in the measured basin total runoff in the first year was 1 6 revegetation recovery of the area was rapid as shown by the results from the second season runoff decreased by one order of magnitude from an average of 10 mm to 1 5 mm sediment yield decreased by two orders of magnitude from 1200 g m 2 to 10 g m 2 the third season 1991 1992 was an exceptionally rainy year and therefore runoff and sediment yield increased but to less than in the first year runoff and sediment yield are related to vegetation cover rainfall intensity soil properties slope steepness and exposure and fire intensity logging activities after fire increase sediment yields through its effects on vegetation cover and soil fire severity increases the potential for erosion

complex channel responses to changes in stream flow and sediment supply on the lower duchesne river utah channel responses to flow depletions in the lower duchesne river over the past 100 years have been highly complex and variable in space and time in general sand bed reaches adjusted to all perturbations with bed level changes whereas the gravel bed reaches adjusted primarily through width changes gravel bed reaches aggraded only when gravel was supplied to the channel through local bank erosion and degraded only during extreme flood events a 50 reduction in stream flow and an increase in fine sediment supply to the study area occurred in the first third of the 20th century the gravel bed reach responded primarily with channel narrowing whereas bed aggradation and four large scale avulsions occurred in the sand bed reaches these avulsions almost completely replaced a section of sinuous channel about 14 km long with a straighter section about 7 km long the most upstream avulsion located near a break in valley slope and the transition from a gravel bed upstream and a sand bed downstream transformed a sinuous sand bed reach into a braided gravelbed reach and eventually into a meandering gravel bed reach over a 30 year period later an increase in flood magnitudes and durations caused widening and secondary bed aggradation in the gravel bed reaches whereas the sand bed reaches incised and narrowed water diversions since the 1950s have progressively eliminated moderate flood events whereas larger floods have been less affected the loss of frequent flooding has increased the duration and severity of drought periods during which riparian vegetation can establish along the channel margins as a result the channel has gradually narrowed throughout the study area since the late 1960s despite the occasional occurrence of large floods no tributaries enter the duchesne river within the study area so all reaches have experienced identical changes in stream flow and upstream sediment supply d 2004 elsevier b v all rights reserved

the basic ingredients of the north atlantic storm track part i land sea contrast and orography understanding and predicting changes in storm tracks over longer time scales is a challenging problem particularly in the north atlantic this is due in part to the complex range of forcings land sea contrast orography sea surface temperatures etc that combine to produce the structure of the storm track the impact of land sea contrast and midlatitude orography on the north atlantic storm track is investigated through a hierarchy of gcm simulations using idealized and semirealistic boundary conditions in a highresolution version of the hadley centre atmosphere model hadam3 this framework captures the largescale essence of features such as the north and south american continents eurasia and the rocky mountains enabling the results to be applied more directly to realistic modeling situations than was possible with previous idealized studies the physical processes by which the forcing mechanisms impact the large scale flow and the midlatitude storm tracks are discussed the characteristics of the north american continent are found to be very important in generating the structure of the north atlantic storm track in particular the southwest northeast tilt in the upper tropospheric jet produced by southward deflection of the westerly flow incident on the rocky mountains leads to enhanced storm development along an axis close to that of the continent s eastern coastline the approximately triangular shape of north america also enables a cold pool of air to develop in the northeast intensifying the surface temperature contrast across the eastern coastline consistent with further enhancements of baroclinicity and storm growth along the same axis

delta15n measurements as a calibration tool for the paleothermometer and gas ice age differences a case study for the 8200 b p event on grip ice isotopic composition measurement on atmospheric nitrogen 15n trapped in ice cores is established as a gravitational enrichment indicator recently it was discovered that thermal diffusion has a significant influence on this parameter during periods of large temperature fluctuations severinghaus et al 1998 here we present results for the 8200 b p event the largest temperature excursion during the holocene which underpin this view and extend it to medium range temperature changes the thermal diffusion component is about one third of the gravitational enrichment for this event the shortness of this cooling event does not significantly affect the gravitational enrichment as documented by rather constant close off depths therefore the 15n residuals can be directly used to estimate temperature changes hence calibrating the paleothermometer since our 15n record can resolve temperature variations as short as a few decades it complements the calibration technique of borehole temperature measurements for which the resolution decreases rapidly with increasing age we obtained a relation 18oice temperature of 0 30 0 11 k which agrees nicely with the temporal dependence for cold periods about 0 30 k and is significantly lower than today s greenland spatial relation 0 67 k this corresponds to a range of the surface temperature shift for the 8200 b p event in central greenland between 5 4 and 11 7 k with a best estimate of 7 4 k by matching the 15n variations with 18oice variations we estimate the mean gas ice age difference to be 209 15 years which is in good agreement with model estimates

chronology of inland eolian dunes on the coastal plain of georgia usa most inland eolian dunes associated with rivers on the georgia coastal plain probably date to glacial periods direct dating of the dune sand by optically stimulated luminescence dating combined with limiting ages from radiocarbon dating shows that dunes formed during isotope stage 2 stage 3 the transition between stages 4 and 5 and isotope stage 6 or earlier most of the dates indicate dune activity between 30 000 and 15 000 years ago holocene activity included limited and local reworking of the crests of some thick dunes along streams that flow from the piedmont

transport direction of wisconsinan loess in southeastern minnesota a study of wisconsinan loess in part of southeastern minnesota confirms earlier suggestions that much of the loess in this region was not derived flora the floodplain of the mississippi river two wisconsinan loess units the peoria loess and roxana silt occur in the study area peoria loess 1 8 m thick fines systematically eastward from an abrupt western border toward the mississippi there are no apparent grain size trends away from other adjacent rivers peoria loess thickness generally decreases eastward but is highly variable probably because of differential erosion potential sources for this unit are pre wisconsinan sediments on the iowan erosion surface immediately west of the border of thick loess and wisconsinan glacial sources tens or hundreds of kilometers to the west the underlying roxana silt up to 1 m thick occurs only near the mississippi and fines away from that river the roxana silt deposit could reflect aggradation of the mississippi floodplain because of glacial activity upstream and does not have clear implications for upland vegetation near the study area by contrast transport of the peoria loess from distant glacial sources or wind erosion of upland surfaces closer to the loess deposit both imply extensive very sparsely vegetated surfaces west of the study area when the peoria loess accumulated

channel adjustment and a test of rational regime theory in a proglacial braided stream the upstream reach of the sunwapta river alberta provides a useful quasi experimental field case of channel adjustment in a proglacial stream historically the formation of a proglacial lake deprived the river of its coarse sediment supply for several decades and lead to a dramatic decrease in braiding intensity close to the lake while braiding intensity increased further downstream this response to the reduction of gravel input is consistent with previous experimental results subsequent construction activity and channelization close to the lake have contributed to the continuation of these temporal and spatial trends in channel pattern the current state of adjustment of the river morphology can be explained in the context of these historical changes using rational regime equations the study reach has no tributaries and bed material size decreases twofold along the reach while width and braiding intensity increase yet channel slope decreases by only 10 the absence of any significant change in discharge downstream along the reach allows testing of regime equations under conditions in which discharge is held constant the current downstream trends in slope and fluctuations in width are predicted reliably from rational regime equations but not by the existing empirical hydraulic geometry relations the rational equations incorporate the effect of grain size and slope on channel width and the effect of width and grain size on channel slope the regime equations are successful even though they were devised for single channel gravel streams the small 10 decrease in slope along the reach despite a halving of median grain size is attributed to the counteracting positive effect on slope of the downstream increase in braiding intensity and width the downstream increase in braiding intensity must be largely the result of decreasing grain size this confirms the influence of grain size on channel pattern thresholds and demonstrates using spatial transitions in channel pattern that channel pattern predictions based on stream power alone are inadequate

virtual rate and mean distance of travel of individual clasts in gravel bed channels travel distances in gravel bed rivers of tagged particles of various sizes are related to excess stream power estimated for peak discharge mean distance of movement irrespective of grain size is weakly correlated with stream power especially near the threshold of movement there may be several reasons for the weak correlation including variable effects of bed structure varying magnitudes of sediment mobilizing events and sampling problems grain size itself is of marginal significance the virtual rate of travel calculated using total time for which the flow is larger than that needed to initiate clast movement also bears a weak relation to the excess stream power over the period better results are obtained by relating the virtual rate of travel to the first peak of the flow event only this implies that the initial seeding of the tagged particles dominates the observations nonetheless an underlying general relation is indicated by the data which are derived from a wide range of flow regime types

channel reach morphology in mountain drainage basins a classification of channel reach morphology in mountain drainage basins synthesizes stream morphologies into seven distinct reach types colluvial bedrock and five alluvial channel types cascade step pool plane bed pool rime and dune ripple coupling reach level channel processes with the spatial arrangement of reach morphologies their links to hillslope processes and external forcing by confinement ripar ian vegetation and woody debris defines a process based framework within which to assess channel condition and response potential in mountain drainage basins field investigations demonstrate character istic slope grain size shear stress and roughness ranges for different reach types observations consistent with our hypothesis that alluvial channel morphologies reflect specific roughness configurations ad justed to the relative magnitudes of sediment supply and transport ca pacity steep alluvial channels cascade and step pool have high ratios of transport capacity to sediment supply and are resilient to changes in discharge and sediment supply whereas low gradient alluvial channels pool rime and dune ripple have lower transport capacity to supply ra tios and thus exhibit significant and prolonged response to changes in sediment supply and discharge general differences in the ratio of transport capacity to supply between channel types allow aggregation of reaches into source transport and response segments the spatial distribution of which provides a watershed level conceptual model linking reach morphology and channel processes these two scales of channel network classification define a framework within which to in vestigate spatial and temporal patterns of channel response in moun tain drainage basins

fire related sedimentation events on alluvial fans yellowstone national park u s a abstract we document initiation and flow processes deposit facies and geomorphic effects of forest fire related sedimentation on small alluvial fans in yellowstone national park brief intense convective storm precipitation on steep basins burned in the 1988 fires produced sedimentation events involving a variety of depositional processes on fans over the course of all documented events flows on fans progressed from higher to lower sediment concentration events were often dominated by either debris flows or relatively sediment poor streamflow processes in some events however flows ranging a over wide spectrum of sediment concentration produced significant fan deposits debris flows were generated by progressive sediment bulking involving pervasive surface runoff and rill erosion on st ep upper basin slopes followed by deep incision as flows progressed down channels debris flow deposits show a marked decline in thickness and coarse gravel content downfan often with extensive distal gravel poor facies we recognized a relatively minor percentage of noncohesive debris flow and hyperconcentrated flow facies with sorting and stratification intermediate between muddy debris flow and streamflow facies these were deposited where dilute flows bulked with coarse sediment by eroding channel alluvium or earlier deposits of the event below incised fan channels streamflows expanded as sheetfloods which prograded lower fans with distally fining deposits basins 3 km2 typically produced streamflow events on fans but sediment texture and availability on slopes and in channels are primary factors determining flow processes on fans of smaller basins burned soil surfaces provided abundant silt and clay for debris flow generation but because soil surface sediment was stripped and or compacted over time the lack of available fines resulted in dominance of streamflow processes in later events

channel morphology gradient profiles and bed stresses during flood in a step pool channel abstract the stability of a step pool channel is investigated on the basis of measurements during a median annual flood the study reaches range in gradient from 5 to 10 and the flow was 3 m3 s 1 in an 8 m wide channel the estimated shear stress based on the channel gradient was 102 pa which implies that the channel ought to move half meter class boulders calculations of the shear stress using flow resistance equations yield values of about 30 pa these figures imply that the largest boulders in the channel could be moved in fact the channel is stable the effective stress remains well below the threshold to move the large structure forming boulders the steps are stable structures that result from interlocking of the larger stones they are semi randomly located depending on the location of keystones that are deposited in the channel by debris flows or fall out of the banks the high variation in step pool wavelength step height residual pool depth and pool length is indicative of the random nature of step location and structure along a reach there is no convincing evidence that special conditions govern their formation

channel and woody debris characteristics in adjacent burned and unburned watersheds a decade after wildfire park county wyoming abstract large variability in responses of stream sediment and large woody debris lwd to severe fire has limited the accurate prediction of the magnitude and duration of fire effects on streams conditions in one absaroka range stream that was severely burned in 1988 were compared to those in an adjacent undisturbed stream to improve understanding of fire effects on channel and lwd characteristics beyond the first few years ten reaches of each stream were sampled during summer 1999 average bankfull channel width was greater in burned jones creek than in unburned crow creek lwd frequency and overall frequency of lwd accumulations were greater in crow creek than jones creek debris jam frequency was greater in jones creek after accounting for differences in the frequency of pieces with length greater than channel width larger piece size and better anchoring contributed to more frequent small accumulations of lwd in crow creek differences between streams in lwd frequency are consistent with greater mobility of debris in burned jones creek lwd associated fine sediment deposits were thicker but less frequent along jones creek than crow creek

steering of westerly storms over western north america at the last glacial maximum the last glacial maximum hydroclimate over western north america differed from the modern climate a proxy model comparison suggests that the glacial storm track was squeezed and steered by atmospheric high pressure systems

components and mechanisms of hydrologic cycle changes over north america at the last glacial maximum abstractthis study examines the differences in the moisture budget over north america between the last glacial maximum lgm and modern climate as simulated by nine models from paleoclimate modell

modeling the densification of polar firn including heat diffusion application to close off characteristics and gas isotopic fractionation for antarctica and greenland sites 1 modeling the densification of polar firn is essential to estimate variations of close off characteristics density close off depth delta age in relation with past climate parameters temperature and accumulation rates furthermore the air composition in the firn is modified by gravitational and thermal fractionation and stable isotope measurements of permanent gases like nitrogen or argon can provide information on the amplitude of these fractionations in this work we propose a new model coupling firn densification and heat diffusion in addition to the determination of the firn thickness and gas ice age differences the model allows a reconstruction of the time evolution of the temperature for different sites in antarctica vostok and greenland gisp2 and therefore the evolution of gravitational and thermal isotopic fractionations in firn under present day conditions the modeled profiles are in good agreement with the available temperature measurements in firn for sites with low accumulation rates such as vostok the results show the existence of temperature gradients in the firn column even when no rapid climatic changes occur the comparison of the modeled 15n results to measurements allows to better constrain the 18o temperature relationship used to infer the surface temperature history and for gisp2 the model validates the long term borehole based temperature

the missing dimension effects of lateral variation on 1 d calculations of fluvial bedload transport most calculations of bedload transport in rivers including those in numerical models of aggradation and degradation are 1 d all hydraulic and transport rate calculations are averaged over the channel width because bedload transport laws are nonlinear width averaged calculations will underestimate the true bedload flux if there is any local spatial variation in either the bed or the flow this paper analyses the effects on bedload transport capacity of spatial variation in applied and critical c shear stress separately and in combination a simple but versatile statistical model is used to represent variability in with allowance for differences between sand and gravel bed rivers and for below bankfull flow bedload flux is shown to increase greatly with the variance of especially in gravel bed rivers variability in c through bed patchiness may increase reduce or make little difference to bedload flux depending on the correlation between and c simple width averaging leads to severe underestimation of bedload transport in most conditions some alternatives are considered the findings have implications for sediment routing models srms but further research is needed to explore the issue fully

assessment of channel changes due to long term bedload supply decrease roubion river france this paper presents the observations made in the watershed of the upper roubion river which drains a submediterranean mountainous environment and in two of its main tributaries the soubrion and bine creeks our aim was to analyze contemporary morphological and sedimentary changes in response to a progressive decrease in bedload supply this decrease is linked to an increase of forest cover in the watershed during the 20th century following rural depopulation and to a correlated decrease in peak flows several variables were systematically measured width slope bed level change sediment size along 23 km of river whereas dendrochronological data floodplain cross sections and estimated recurrence intervals of sediment entrainment based on critical shear stress calculations were also produced for specific sites to complete our analysis evidence of three major phenomena was obtained on both the roubion and its tributaries i bed degradation ii development of stable pavement in incised reaches and iii channel narrowing associated with the development of alluvial forest on recently abandoned portions of the pre existing active channel forest development began in the 1950s these three phenomena show positively covariant relationships that lead us to propose a descriptive model of mountain fluvial system adjustments to progressive decrease of bedload

fluvial response a decade after wildfire in the northern yellowstone ecosystem a spatially explicit analysis forest fire is a vital ecological process capable of inducing complex fluvial response but the integration of these effects across entire watersheds remains poorly understood we collected downstream cross sectional and geomorphic data acquired geographic information on land cover and forest fire and performed spatially explicit statistical analyses to examine fire related impacts in catchments burned to varying degrees generalized least squares gls regression models suggested that channels with a greater percentage of burned drainage area were associated with markedly higher cross sectional stream power relatively smaller width depth ratios and lower bank failure rates 12 to 13 years after the fires these results implied that streams became more powerful in the aftermath of forest fire and that net incision had been the primary response in second to fourth order channels since the 1988 yellowstone fires the extensive geographic coverage of our data spanning multiple basins with measurements spaced every 100 m allowed us to hypothesize a process response model based on these results we suggest that a wave of fire related sediment propagates through burned catchments high runoff events or even moderate flows provide sufficient energy to evacuate the finer grained material delivered from burned hillslopes to the channel network over a period of 5 10 years the combination of elevated post fire discharges and decreased sediment supply then induces an episode of incision site specific channel changes are highly variable because streams can accommodate post fire increases in energy and sediment supply through multiple modes of adjustment characterizing the spatial distribution of stream power would provide a valuable management tool because this variable is strongly associated with percent burned drainage area and integrates several elements of complex fluvial response future research focused on the channel substrate and its evolution through time is needed but our results indicate a fundamental linkage between fire and fluvial processes d 2002 elsevier science b v all rights reserved

geomorphic implications of glacier outburst flooding noeick river valley british columbia an outburst flood from ape lake in october 1984 severely affected channel and floodplain morphology of noeick river this event estimated to have exceeded 103 m3 s 1 was as much as 2 4 times greater than the probable maximum meteorological flood storage of water in ponds lakes and floodplain margins resulted in attenuation of the flood peak as it progressed downstream as a result flood effects were most evident in the alluvial reaches of upper noeick river below ape lake between km 10 and km 23 here the channel zone widened from a preflood average of 75 m to almost 200 m the area of eroded valley bottom increased by about 2 5 times and a cumulative length of 2 5 km of valley side colluvial fans was eroded transported sediment stored in the floodplain and along valley margins resulted in aggradation of up to 0 75 m except in restricted reaches where limited sediment supply and increased stream power caused degradation of up to 3 m a second and fractionally larger outburst flood in august 1

response and recovery of the eel river california and its tributaries to floods in 1955 1964 and 1997 abstract northwestern california is prone to regional high magnitude winter rainstorms which repeatedly produce catastrophic floods in the basins of the northern coast ranges major floods on the eel river in 1955 and 1964 resulted in substantial geomorphic changes to the channel adjacent terraces and tributaries this study evaluated the changes and the effects of a moderate flood in 1997 through field observations and examination of aerial photographs that spanned from 1954 to 1996 the purpose was to document the nature and magnitude of geomorphic responses to these three floods and assess the rates and controls on the recovery of the eel river and its tributaries channel widening from extensive bank erosion was the dominant geomorphic change along the lower eel river during major floods as a result of the 1964 flood the largest amount of widening was 195 m and represented an 80 change in channel width channel narrowing characterized the periods after the 1955 and 1964 floods more than 30 years after the 1964 flood however the river had not returned to pre flood width which suggests that channel recovery required decades to complete a long recovery time is unusual given that the eel river is located in an area with a superhumid climate and has an exceptionally high sediment yield this long recovery time may reflect highly seasonal precipitation and runoff which are concentrated in 3 5 months each winter in contrast to the main stem of the eel river the dominant effects of floods on the tributaries of the eel river were rapid aggradation of channel bed and valley floor followed by immediate downcutting dendrogeomorphic data aerial photographs and field observations indicate that thick wedges of gravel derived largely from hillslope failures in upper reaches of the tributaries are deposited at and immediately upstream of the mouths of tributaries as the stage of the eel river exceeded that of the tributaries during major floods in the waning stages of the flood the tributaries cut through the gravel at a rate equal to the lowering of the eel and generated unpaired terraces and nickpoints the complete process of deposition and incision can occur within a few days of peak discharge although reworking of some sediment on the valley floor may continue for years after large floods channel morphology in the tributaries appears to be a product of infrequent high magnitude events the morphology of the tributary channel also appears to be greatly influenced by the frequency and magnitude of mass wasting in headwater areas of small basins

thermal fractionation of air in polar firn by seasonal temperature gradients 1 air withdrawn from the top 5 15 m of the polar snowpack firn shows anomalous enrichment of heavy gases during summer including inert gases following earlier work we ascribe this to thermal diffusion the tendency of a gas mixture to separate in a temperature gradient with heavier molecules migrating toward colder regions summer warmth creates a temperature gradient in the top few meters of the firn due to the thermal inertia of the underlying firn and causes gas fractionation by thermal diffusion here we explore and quantify this process further in order to 1 correct for bias caused by thermal diffusion in firn air and ice core air isotope records 2 help calibrate a new technique for measuring temperature change in ice core gas records based on thermal diffusion severinghaus et al 1998 and 3 address whether air in polar snow convects during winter and if so whether it creates a rectification of seasonality that could bias the ice core record we sampled air at 2 m depth intervals from the top 15 m of the firn at two antarctic sites siple dome and south pole including a winter sampling at the pole we analyzed 15n 14n 40ar 36ar 40ar 38ar 18o 16o of o2 o2 n2 84kr 36ar and 132xe 36ar the results show the expected pattern of fractionation and match a gas diffusion model based on first principles to within 30 although absolute values of thermal diffusion sensitivities cannot be determined from the data with precision relative values of different gas pairs may at siple dome 40ar 4 is 66 2 as sensitive to thermal diffusion as 15n in agreement with laboratory calibration 18o 2 is 83 3 and 84kr 48 is 33 3 as sensitive as 15n the corresponding figures for summer south pole are 64 2 81 3 and 34 3 accounting for atmospheric change the figure for o2 n2 4 is 90 3 at siple dome winter south pole shows a strong depletion of heavy gases as expected however the data do not fit the model well in the deeper part of the profile and yield a systematic drift with depth in relative thermal diffusion sensitivities except for kr constant at 34 4 suggesting the action of some other process that is not currently understood no evidence for wintertime convection or a rectifier effect is seen

a model for fire induced sediment yield by dry ravel in steep landscapes sediment flux from hillslopes to channels commonly increases following wildfires with implications for the carbon cycle river habitats and debris flow hazards although much of this material is transported via dry ravel existing ravel models are not applicable to hillslopes with gradients greater than the angle of repose which can constitute the majority of mountainous terrain to fill this knowledge gap we develop a continuity model for sediment storage by vegetation dams on steep hillslopes to predict sediment yields following wildfire the maximum volume of sediment stored prior to wildfire is set to be a function of vegetation density the capacity of plants to impound sediment and the contributing hillslope area time is required after fire to establish vegetation and replenish hillslope sediment storage which introduces vegetation regrowth rate soil production rate and fire recurrence interval as important variables that affect ravel yield model results for the san gabriel mountains california predict that sediment yield can increase by several orders of magnitude following fire these results are consistent with field data of ravel yield 30 mm per contributing area of hillslope in 5 months we collected following the 2009 station fire as well as postfire sediment flux recorded by 93 debris basins in contrast to previous work our model shows that heightened postfire sediment yields can be explained by a change in hillslope sediment storage independent of major changes in the soil production rate and landscape form over geomorphic timescales

episodic deposition of illinois valley peoria silt in association with lake michigan lobe fluctuations during the last glacial maximum the chronology and cause of millennial depositional oscillations within last glacial loess of the central lowlands of the united states are uncertain here we present a new age model that indicates the peoria silt along the illinois river valley accumulated episodically from 28 500 to 16 000 cal yr bp as the lake michigan lobe margin fluctuated within northeastern illinois the age model indicates accelerated loess deposition coincident with regional glacial advances during the local last glacial maximum a weakly developed paleosol the jules geosol represents a period of significantly slower deposition from 23 700 to 22 000 cal yr bp a gastropod assemblage based reconstruction of mean july temperature shows temperatures 6 10 c cooler than modern during peoria silt deposition stable oxygen and carbon isotope values 18 o and 13 c of gastropod carbonate do not vary significantly across the pedostratigraphic boundary of the jules geosol suggesting slower loess accumulation was a result of reduced glacial sediment supply rather than direct climatic factors however a decrease in 18 o values occurred between 26 000 and 24 000 cal yr bp synchronous with the lake michigan lobe s southernmost advance this 18 o decrease suggests a coupling of regional summer hydroclimate and ice lobe position during the late glacial period

assessing the effect of vegetation related bank strength on channel morphology and stability in gravel bed streams using numerical models bank strength due to vegetation dominates the geometry of small stream channels but has virtually no effect on the geometry of larger ones the dependence of bank strength on channel scale affects the form of downstream hydraulic geometry relations and the meandering braiding threshold it is also associated with a lateral migration threshold discharge below which channels do not migrate appreciably across their floodplains a rational regime model is used to explore these scale effects it parameterizes vegetation related bank strength using a dimensionless effective cohesion cr the scale effects are explored primarily using an alluvial state space defined by the dimensionless formative discharge q and channel slope s which is analogous to the q s diagrams originally used to explore meandering braiding thresholds the analyses show that the effect of vegetation on both downstream hydraulic geometry and the meandering braiding threshold is strongest for the smallest streams in a watershed but that the effect disappears for q 106 the analysis of the migration threshold suggests that the critical discharge ranges from about 5 m3 s to 50 m3 s depending on the characteristic rooting depth for the vegetation the analysis also suggests that where fires frequently affect riparian forests channels may alternate between laterally stable gravel plane bed channels and laterally active riffle pool channels these channels likely do not exhibit the classic dynamic equilibrium associated with alluvial streams but instead exhibit a cyclical morphologic evolution oscillating between laterally stable and laterally unstable end members with a frequency determined by the forest fire recurrence interval copyright 2008 john wiley sons ltd

loess of the upper mississippi valley driftless area abstract loess of the driftless area includes four distinct late quaternary lithostratigraphic units the wyalusing new loveland roxana and peoria formations erosion has removed parts or all of the pre late wisconsian loess at many sites these formations consist largely of loess and retransported loess that typically occurs on uplands terraces and valley margins in the region the oldest widespread formation probably 125 000 yr b p is here formally named the wyalusing formation the three younger formations correlate with the loveland probably 125 000 yr b p roxana 55 000 25 000 yr b p and peoria 25 000 12 000 yr b p loesses recognized elsewhere in the midcontinental united states the soil stratigraphic morphology of the wyalusing loveland and roxana peoria couplets appears to represent two distinct loess sedimentation sequences related to major expansion and contraction of the laurentide ice sheet during oxygen isotope stages 6 and 3 2 respectively the occurrence frequency of loess units is inversely related to age illustrating the erosional nature of the driftless area landscape the occurrence of four loess units at some sites in driftless area stratigraphic sections corroborates loess stratigraphy along the length of the mississippi valley where typically not more than four or five loess units are found and they represent only the late and middle pleistocene

estimation of temperature change and of gas age ice age difference 108 kyr b p at vostok antarctica air trapped in ice core bubbles provides our primary source of information about past atmospheres air isotopic composition 15n 14n and 40ar 36ar permits an estimate of the temperature shifts associated with abrupt climate changes because of isotope fractionation occurring in response to temperature gradients in the snow layer on top of polar ice sheets a rapid surface temperature change modifies temporarily the firn temperature gradient which causes a detectable anomaly in the isotopic composition of nitrogen and argon the location of this anomaly in depth characterizes the gas age ice age difference age during an abrupt event by correlation with the d or 18o anomaly in the ice we focus this study on the marine isotope stage 5d 5c transition 108 kyr b p a climate warming which was one of the most abrupt events in the vostok antarctica ice isotopic record petit et al 1999 a steplike decrease in 15n and 40ar 4 from 0 49 to 0 47 possibly a gravitational signal due to a change in firn thickness is preceded by a small but detectable 15n peak possibly a thermal diffusion signal we obtain an estimate of 5350 300 yr for age close to the model estimate of 5000 years obtained using the vostok glaciological timescale our results also suggest that the use of the present day spatial isotope temperature relationship slightly underestimates but by no more than 20 15 the vostok temperature change from present day at that time which is in contrast to the temperature estimate based on borehole temperature measurements in vostok which suggests that antarctic temperature changes are underestimated by up to 50 salamatin et al 1998

last glacial loess in the conterminous usa the conterminous united states contains an extensive and generally well studied record of last glacial loess the loess occurs in diverse physiographic provinces and under a wide range of climatic and ecological conditions both glacial and non glacial loess sources are present and many properties of the loess vary systematically with distance from loess sources united states midcontinent last glacial loess is probably the thickest in the world and our calculated mass accumulation rates mars are as high as 17 500 g m 2 yr at the bignell hill locality in nebraska and many near source localities have mars greater than 1500 g m 2 yr these mars are high relative to rates calculated in other loess provinces around the world recent models of last glacial dust sources fail to predict the extent and magnitude of dust flux from the mid continent of the united states a better understanding of linkages between climate ice sheet behaviour routing of glacial meltwater land surface processes beyond the ice margin and vegetation is needed to improve the predictive capabilities of models simulating dust flux from this region r 2003 elsevier ltd all rights reserved

the long term control of vegetation and woody debris on channel and flood plain evolution insights from a paired catchment study in southeastern australia abstract numerous case studies have demonstrated that alluvial and semi alluvial rivers in se australia have undergone dramatic metamorphosis in historical times however very few studies place these changes within a long term evolutionary context as a consequence the magnitude of and ultimate controls on the changes to river form and processes are not fully appreciated in this study a paired catchment analysis is undertaken between two moderate sized sand bed rivers in east gippsland australia from the thurra river direct insight is gained into the predisturbance control exerted by riparian vegetation and wood in a lowland alluvial river this river is effectively in the same condition today as it was at the time of the arrival of europeans in australia in contrast the adjacent cann river which has been settled by europeans for 150 years but was previously very similar to the thurra river exhibits stark differences today channel morphodynamics observed within the thurra river when coupled with historical and geomorphic evidence for the former condition of the cann river provide a detailed reference by which the recent changes to the cann river are measured chronostratigraphic evidence from both flood plains places recent channel behavior within an evolutionary context extending well into the pleistocene since european settlement the study reach of the cann river has experienced a 360 increase in channel depth a 240 increase in channel slope a 700 increase in channel capacity and up to a 150 fold increase in the rate of lateral channel migration the contemporary condition of the cann river channel differs profoundly to that which has prevailed over at least the previous 27 ka the first order control on the historical channel metamorphosis is the removal of riparian vegetation and woody debris wd numerous thresholds have been crossed as a result of historical channel changes particularly the relationship between average length of woody debris pieces and channel width consequently channel recovery will not simply be achieved by reintroducing the pre existing riparian vegetation and woody debris load devising sound management strategies and realistic management targets for the cann river or any similarly altered river requires an understanding of the long term channel and flood plain evolutionary history as well as the underlying controls on historical changes

modelling geomorphic response to environmental change in an upland catchment in the uk s upland catchments river terraces and alluvial features indicate a history of periodic aggradation and degradation linked to holocene changes in land use primarily deforestation and climate change altering flood frequency and magnitude although both factors are important calculating their individual effects is complicated by the likelihood of their concurrent alteration to investigate the relative impacts of land use and climate change a cellular model is applied to the upland catchments of cam gill beck above starbotton north yorkshire this is divided into 1 million 2 m by 2 m grid cells to which a range of process laws are applied these include approximate expressions for mass movement rates soil creep the influence of vegetation and hillslope hydrology as well as fluvial erosion and deposition in ten grain size fractions this provides a good representation of valley floor geometry while retaining a fully dynamic interaction with the surrounding valley sides previous applications of this model have shown the detailed formation of bars and berms as well as examples of braiding avulsion and channel range running on a silicon graphics origin 2000 computer an ensemble of simulations were completed bracketing a wide range of environmental scenarios involving changes in flood frequency magnitude and vegetation cover over time scales ranging from 10 to 100 years these showed that decreasing tree cover and increasing rainfall magnitude individually produced similar 25 to 100 increases in sediment discharge whereas in combination they generated a 1300 rise furthermore channels formed by the model in response to increased rainfall magnitudes are located where relic channels are found in cam gill beck implying that these are the products of previous periods of high rainfall magnitudes copyright 2000 john wiley sons ltd

on the interpretation of chinese loess as a paleoclimate indicator the records of wind blown dust i e loess in china are some of the most important terrestrial records of past climate changes stretching back over the last 10 ma in the paleoclimate literature intervals of increased dust generation have been almost always interpreted as being associated with more intense or prolonged wintertime conditions here it is shown that in accordance with modern observations dust outbreaks in asia are predominantly springtime phenomena during spring frequent cyclogenesis in the lee of the mongolian altai and the passage of strong cold fronts produce the intense windstorms that loft and entrain dust into the air the meteorology governing such outbreaks is likely robust in past climates contrary to the common paleoclimate presumption it is actually the breakdown of the siberian high that permits the dust producing windstorms to occur the importance of cold fronts in generating such windstorms suggests that cooling of high latitude climate during the miocene or during glacial intervals might play a significant role in the signal recorded in the loess deposits the unique springtime factors that generate dust storms are an example of why the common partitioning of asian climate into a winter and summer monsoon is oversimplified and can be misleading

post wildfire erosion response in two geologic terrains in the western usa abstract volumes of eroded sediment after wildfires vary substantially throughout different geologic terrains across the western united states these volumes are difficult to compare because they represent the response to rainstorms and runoff with different characteristics however by measuring the erosion response as the erodibility efficiency of water to detach and transport sediment on hillslopes and in channels the erosion response from different geologic terrains can be compared specifically the erodibility efficiency is the percentage of the total available stream power expended to detach remobilize or transport a mass of sediment erodibility efficiencies were calculated for the i initial detachment and for the ii remobilization and transport of sediment on the hillslopes and in the channels after wildfire in two different geological terrains the initial detachment efficiencies for the main channel and tributary channel in the granitic terrain were 10 9 and 5 4 and were similar to those for the volcanic terrain which were 5 5 and 1 1 no initial detachment efficiency could be measured for the hillslopes in the granitic terrain because hillslope measurements were started after the first major rainstorm the initial detachment efficiency in the volcanic terrain was 1 3 0 41 the average remobilization and transport efficiencies associated with flash floods in the channels also were similar in the granitic 0 18 0 57 and volcanic 0 11 0 41 terrains on the hillslope the remobilization and transport efficiency was greater in the volcanic terrain 2 4 than in the granitic terrain 0 65 however this may reflect the reduced sediment availability after the first major rainstorm 30 min maximum rainfall intensity 90 mm h 1 in the granitic terrain while easily erodible fine colluvium remained on the hillslope after the first rainstorm 30 min maximum rainfall intensity 7 2 mm h 1 in the volcanic terrain the erosion response in channels and on hillslopes of the granitic and volcanic terrains was similar when compared using erodibility efficiencies

mechanisms of dust emission from pleistocene loess deposits nebraska usa 1 saltation bombardment is commonly believed to be the primary mechanism by which large quantities of dust can be generated direct entrainment of silt on the other hand is thought to generate minimal quantities of dust if so loess landscapes should rarely be major sources of dust unless disturbed by human activity to test the mechanisms by which loess can be eroded by the wind we used the portable in situ wind erosion lab pi swerl pi swerl tests were conducted on benches that were carved into loess at field locations coarse to fine loess was also collected and tested in a controlled setting using 1 dry unconsolidated loess and 2 crusted loess results indicate that in most cases the threshold friction velocity for silt is less than that for sand and that most dust generation occurs by direct entrainment of dust saltation even in coarse loess was nonexistent to intermittent in a few cases saltation of sand or soil aggregates preceded dust generation or enhanced emissions after direct entrainment began emission fluxes of loess are potentially high in both proximal and distal settings suggesting that loess is easily entrainable if vegetation density is low we hypothesize that an arid and windy climate of the late pleistocene paired with lower vegetation density facilitated large scale erosion of peoria loess in nebraska to generate wind aligned ridges and troughs more broadly our results indicate that loess landscapes have the potential to be major sources as well as sinks of dust

modeling channel morphodynamic response to variations in large wood implications for stream rehabilitation in degraded watersheds abstract anthropogenic modification of forests has often decoupled streams from riparian ecosystems and altered natural wood recruitment processes extensive research has shown that large wood significantly impacts channel dynamics especially in small and intermediate sized forested streams where wood pieces are similar in length to channel width and many stream rehabilitation efforts now involve the addition of large wood to streams the primary objective of this research is to investigate the relation between large wood and reach scale channel morphology and hydraulics using a physical model in order to better inform stream rehabilitation programs and future modeling efforts four experiments each comprising numerous five hour runs were conducted using a froude scaled stream table with wood loads scaled to 0 m3 m2 0 011 m3 m2 0 016 m3 m2 and 0 022 m3 m2 the addition of large wood significantly decreased the reach averaged velocity in all experiments and was associated with decreased sediment transport and increased sediment storage in the reach increases in bed and water surface slope compensated for the loss of energy available to transport sediment and enabled the system to reach a new steady state within the equivalent of 6 to 9 years adding wood increased pool frequency as well as the variability in cross sectional depth while causing the reach to undergo a transition from a plane bed to a riffle pool morphology retention of fine sediment increased the availability of fish spawning substrate while increased water stage improved connectivity between the channel and the floodplain the changes in habitat complexity were generally related to the wood load added to the reach but were also dependent on the orientation and arrangement of the pieces these results demonstrate that wood may exert a primary control on channel morphodynamics and the availability of aquatic habitat in intermediate sized streams and suggest that the benefits from stream rehabilitation efforts are highly dependent on project scale the relatively long time needed to realize habitat benefits demonstrates that long term monitoring of rehabilitation projects is necessary

late pleistocene dune activity in the central great plains usa abstract stabilized dunes of the central great plains especially the megabarchans and large barchanoid ridges of the nebraska sand hills provide dramatic evidence of late quaternary environmental change episodic holocene dune activity in this region is now well documented but late pleistocene dune mobility has remained poorly documented despite early interpretations of the sand hills dunes as pleistocene relicts new optically stimulated luminescence osl ages from drill cores and outcrops provide evidence of late pleistocene dune activity at sites distributed across the central great plains in addition late pleistocene eolian sands deposited at 20 25 ka are interbedded with loess south of the sand hills several of the large dunes sampled in the sand hills clearly contain a substantial core of late pleistocene sand thus they had developed by the late pleistocene and were fully mobile at that time although substantial sand deposition and extensive longitudinal dune construction occurred during the holocene many of the late pleistocene osl ages fall between 17 and 14 ka but it is likely that these ages represent only the later part of a longer period of dune construction and migration at several sites significant late pleistocene or holocene large dune migration also probably occurred after the time represented by the pleistocene osl ages sedimentary structures in late pleistocene eolian sand and the forms of large dunes potentially constructed in the late pleistocene both indicate sand transport dominated by northerly to westerly winds consistent with late pleistocene loess transport directions numerical modeling of the climate of the last glacial maximum has often yielded mean monthly surface winds southwest of the laurentide ice sheet that are consistent with this geologic evidence despite strengthened anticyclonic circulation over the ice sheet mobility of large dunes during the late pleistocene on the central great plains may have been the result of cold short growing seasons with relatively low precipitation and low atmospheric co 2 that increased plant moisture stress limiting the ability of vegetation to stabilize active dune sand the apparent coexistence of large mobile dunes with boreal forest taxa suggests a late pleistocene environment with few modern analogs

late holocene channel changes of the middle trent channel response to a thousand year flood record this paper presents recent work on the floodplain sedimentology of the middle trent using data from gravel pits archaeological sites and documentary sources the middle trent has been unusually active during the holocene in comparison with other large lowland rivers in the british isles the holocene floodplain fill is dominated by sands and gravels with abundant structural evidence of changes in channel pattern and channel type a thousand year record of channel change has been reconstructed from palaeochannels and gravel units with radiocarbon dating of brushwood palaeomagnetic dating of fine channel fills dendrochronological dating of timber structures and dating via archaeological typologies the trent also has a reasonably well recorded flood history at least since the 11th century ad a comparison of the flood record and channel change indicates that the same degree of morphological and sedimentary response is not necessarily associated with floods of similar magnitudes i e there is no constant relationship between event magnitude and landform change instead the response seems dependent on the existing state of the channel and medium term trajectory of channel change there is evidence at both the hemington and colwick reaches of a cycle of channel change involving a change in channel typology and dating evidence that this may have migrated downstream the results of this work provide a medium term perspective on channel change which may be more appropriate for large british rivers than short term monitoring for both model validation and planning purposes

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

interactions between stationary waves and ice sheets linear versus nonlinear atmospheric response this study examines the mutual interaction between topographically forced atmospheric stationary waves and continental scale ice sheets using a thermomechanical ice sheet model coupled to a linear

prospects for interregional correlations using wisconsin and holocene aridity episodes northern gulf of mexico coastal plain abstract luminescence dating of extensive dune fields and associated eolian sandsheets provided a chronology of recently recognized pleistocene and early holocene dry climate episodes in the currently humid warm temperate northern northeastern gulf of mexico region scattered parabolic dunes and clusters of intersecting parabolic dunes along with elongated shore transverse and shore parallel dunes developed these landforms occur in a 390 km long and 2 to 3 km wide semicontinuous belt in southeast alabama and northwestern florida dune elevations reach 22 m sangamon coastal barrier sectors were the primary source of the eolian sand deflation was coeval with early wisconsin to mid holocene marine low sea levels and associated distant shorelines early holocene dune dates were synchronous with indications of a hypsithermal dry interval in southeast louisiana the yucatan and the south atlantic seaboard overlapping with dry episodes in yucatan and the high plains texas dunes and louisiana and texas prairie mounds especially in the southwest texas coast still dominated by dry climate suggests intervals of early to late holocene drought the dates provide the basis for identifying and correlating wisconsin early and late holocene climate phases between currently semiarid and humid coastal and interior areas they contribute to future studies including interregional paleoclimate modeling although pleistocene coastal eolian deposition coincided with glaciation in the northern interior and with cooler temperatures of a reduced gulf of mexico holocene aridity phases may have been related to major variations in the position of high pressure cells storm tracks and branches of the jet stream and even to prolonged la nina conditions

late quaternary loess in northeastern colorado part ii pb isotopic evidence for the variability of loess sources a new application of the pb isotopic tracer technique has been used to determine the relative importance of different silt sources for late wisconsin loess in the central great plains of eastern colorado samples of the peoria loess collected throughout the study area contain k feldspar derived from two isotopically and genetically distinct sources 1 glaciogenic material from early and middle proterozoic crystalline rocks of the colorado province and 2 volcaniclastic material from the tertiary white river group exposed on the northern great plains pb isotopic compositions of k feldspar in loess from two dated vertical sections at beecher island and last chance colorado vary systematically implying climatic control of source availability we propose a model whereby relatively cold conditions promoted the advance of front range valley glaciers discharging relatively little glaciogenic silt but strong winds caused eolian erosion of white river group silt due to a decrease in vegetation cover during warmer periods valley glaciers receded and discharged abundant glaciogenic silt while surfaces underlain by the white river group were stabilized by vegetation isotopic data from eastern colorado loess sections record two warm cold warm cycles during late wisconsin time between about 21 000 and 11 000 radiocarbon yr b p similar to results from other studies in the united states and greenland

effects of aggradation and degradation on riffle pool morphology in natural gravel channels northwestern california after the flood of december 1964 12 gaging sections in northern california widened as much as 100 and aggraded as much as 4 m and then degraded to stable levels during a period of 5 years or more as channels aggraded bed material became finer and low to moderate flow through gaging sections in pools became shallower faster and steeper comparisons of longitudinal profiles also show the diminishment of pools as well as a decrease in bar relief accompanying the excessive sediment load as gaging sections degraded hydraulic geometries recovered to a limited degree full recovery probably depends on channel narrowing and further depletion of sediment supply the hydraulic changes with aggradation indicate an increase in the effectiveness of moderate discharges less than i to 2 year recurrence interval annual flood series to transport bed load and shape the bed bars become smaller pools preferentially fill and rimes armored with relatively small gravel tend to erode headward during falling stages and form a gentler gradient excess sediment can thus be more readily transported out of channels when additional contributions from watersheds are usuall slight

conditions for generation of fire related debris flows capulin canyon new mexico comparisonoftheresponsesofthreedrainagebasinsburnedbythedomefireof1996innewmexicoisusedtoidentifythe hillslope channelandfirecharacteristicsthatindicateasusceptibilityspecificallytowildfire relateddebrisflow summer thunderstorms generated three distinct erosive responses from each of three basins the capulin canyon basin showed widespreaderosivesheetwashandrillingfromhillslopes andseverefloodingoccurredinthechannel thenorthtributary basin exhibited extensive erosion of the mineral soil to a depth of 5 cm and downslope movement of up to boulder sized material andatleastonedebrisflowoccurredinthechannel negligiblesurfacerunoffwasobservedinthesouthtributary basin thenegligiblesurfacerunoffobservedinthesouthtributarybasinisattributedtothelimitedextent andseverityof the fire in that basin the factors that best distinguish between debris flow producing and flood producing drainages are drainage basin morphology and lithology a rugged drainage basin morphology an average 12 per cent channel gradient andsteep roughhillslopescoupledwithcolluviumandsoilweatheredfromvolcaniclasticandvolcanicrockspromotedthe generation of debris flows a less rugged basin morphology an average gradient of 5 per cent and long smooth slopes mantled with pumice promoted flooding flood and debris flow responses were produced without the presence of waterrepellentsoils thecontinuityandseverityoftheburnmosaic theconditionoftheriparianvegetation theconditionofthe fibrous root mat accumulations of dry ravel and colluvial material in the channel and on hillslopes and past debris flow activity appearedtohavelittlebearingonthedistinctiveresponsesofthebasins publishedin2000byjohnwiley sons ltd

late quaternary loess in northeastern colorado part i age and paleoclimatic significance loess in eastern colorado covers an estimated 14 000 km 2 and is the westernmost part of the north american midcontinent loess province stratigraphic studies indicate there were two periods of loess deposition in eastern colorado during late quaternary time the first period spanned ca 20 000 to 12 000 14 c yr b p ca 20 14 ka and correlates reasonably well with the culmination and retreat of pinedale glaciers in the colorado front range during the last glacial maximum the second period of loess deposition occurred between ca 11000 and 9000 14 c yr b p this interval may be holocene or may correlate with a hypothesized younger dryas glacial advance in the colorado front range sedimentologic mineralogic and geochemical data indicate that as many as three sources could have supplied loess in eastern colorado these sources include glaciogenic silt derived from the colorado front range and two bedrock sources volcaniclastic silt from the white river group and clays from the pierre shale the sediment sources imply a generally westerly paleowind during the last glacial maximum new carbon isotope data combined with published faunal data indicate that the loess was probably deposited on a cool steppe implying a last glacial maximum july temperature depression relative to the present of at least 5 6 c overall loess deposition in eastern colorado occurred mostly toward the end of the last glacial maximum under cooler and drier conditions with generally westerly winds from more than one source

lgm summer climate on the southern margin of the laurentide ice sheet wet or dry abstract regional climate simulations are conducted using the polar fifth generation pennsylvania state university psu ncar mesoscale model mm5 with a 60 km horizontal resolution domain over north america to explore the summer climate of the last glacial maximum lgm 21 000 calendar years ago when much of the continent was covered by the laurentide ice sheet lis output from a tailored ncar community climate model version 3 ccm3 simulation of the lgm climate is used to provide the initial and lateral boundary conditions for polar mm5 lgm boundary conditions include continental ice sheets appropriate orbital forcing reduced co2 concentration paleovegetation modified sea surface temperatures and lowered sea level the simulated lgm summer climate is characterized by a pronounced low level thermal gradient along the southern margin of the lis resulting from the juxtaposition of the cold ice sheet and adjacent warm ice free land surface this sharp thermal gradient anchors the midtropospheric

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

late pleistocene paleowind directions atlantic coastal plain u s a abstract parabolic dunes transverse dunes and fields of irregular dunes are common along the left banks the north or east sides of streams of the atlantic coastal plain from georgia to delaware and probably beyond similar dunes occur in the appalachian piedmont but are less common the features have been eroded to some extent and are normally recognizable on topographic maps only as areas of irregular topography with wet and dry depressions the larger parabolic dunes can be recognized on aerial photographs and satellite images because the associated soils are poor and the vegetative cover relatively sparse the dunes are of eolian origin and late pleistocene in age it is thought that most of the dunes are late wisconsin in age and contemporary with the carolina bays a small sample of paleowind directions based on average directions of parabolic dune axes indicates that late wisconsin winds in georgia were westerly and grain sizes suggest that the winds were stronger than present day coastal winds of the area paleowind directions shift progressively toward the south across the carolinas and are southwesterly in north carolina they change abruptly to the northwest in the delmarva peninsula following the theory that carolina bays are former lakes that are elongated perpendicular to the effective wind direction the fact that paleowind directions derived from carolina bay orientations are virtually identical to those from parabolic dune orientations strongly supports the wind orientation theory of carolina bay origin and confirms the validity of the small sample of directions obtained from parabolic dunes by analogy with dunes of the eastern shore of lake michigan and the atlantic and pacific coasts the atlantic coast plain dunes do not necessarily indicate arid conditions the sediment load of some streams may have been sandier than at present and streams with sandy braided channels may have been common

comparison of pi swerl with dust emission measurements from a straight line field wind tunnel 1 the portable in situ wind erosion lab pi swerl was developed to measure dust emissions from soil surfaces this small portable unit can test the emissivity of soils in areas that are difficult to access with a field wind tunnel and can complete a larger number of tests in less time the pi swerl consists of a cylindrical enclosure containing an annular flat blade that rotates at different speeds which generates shear stress upon the surface the shear stress generated by pi swerl results in the entrainment of particles including dust pi swerl was developed to provide an index of dust emission potential comparable to the field wind tunnel the pi swerl dust emission results were compared against those obtained from a 12 m long 1 m wide 0 75 m high straight line suction type portable field wind tunnel by conducting collocated tests at 32 distinct field settings and soil conditions in the mojave desert of southern california clay to sand rich soils that displayed a range of crusting gravel cover and disturbance were tested the correspondence between dust emissions mg m 2 s 1 for the two instruments is nearly 1 1 on most surfaces deviation between the two instruments was noted for densely packed gravel surfaces for rough surfaces a correction can be applied to the pi swerl that results in comparable dust emission data to the wind tunnel pi swerl can be used to complement research efforts in aeolian geomorphology aimed to quantify spatial and temporal patterns of dust emissions as well as air quality research related to dust emissions