source and origin of roxana silt and middle wisconsinan midcontinent glacial activity abstract thick roxana silt middle wisconsinan in central and southwestern illinois traditionally has been interpreted as loess derived from valley train deposits in the ancient mississippi river valley winters et al h a winters j j alford and r l rieck quaternary research 29 25 35 1988 recently suggested that the roxana was not directly related to glacial activity but was derived from sediment produced by increased shoreline and spillway erosion associated with a fluctuating ancestral lake michigan because 1 paleoenvironmental and paleohydrologic conditions inferred in the hypothesis are unlikely for a loess depositional system and 2 loess did not accumulate during late wisconsinan deglaciation under conditions similar to those hypothesized we suggest the hypothesis should be rejected roxana distribution suggests the major source was drainage from the upper mississippi river valley and variations in loess thickness in illinois can be explained by consideration of valley width depth orientation and postdepositional erosion tills in the headwaters region of the ancient mississippi drainage system in minnesota and wisconsin occur in the appropriate stratigraphic position and have colors and mineralogic compositions that suggest they could be the parent till of the roxana we believe a valley train source for thick roxana is most probable and urge continued consideration of middle wisconsinan glaciation in the upper great lakes area

chronology of late wisconsinan glaciation in middle north america abstract we propose a chronology of late wisconsinan glacial fluctuations in middle north america from alberta to wisconsin based on radiocarbon dates derived solely from wood previous chronologies of the southwestern margin of the north american continental ice sheet have depended to a considerable degree on radiocarbon dates from fine grained organic sediment this material is commonly contaminated with older carbon resulting in chronologic confusion by using only dates from wood much of the confusion disappears however because of the scarcity of wood dates only four of the sixteen identified fluctuations are accurately dated an advance into iowa about 14 000 to 13 500 bp an advance into south dakota and iowa about 12 300 bp an advance into the lake michigan basin about 11 700 bp and an advance into the lake superior basin about 9900 bp in addition the beginning of late wisconsinan glaciation before 20 000 bp is fairly well documented none of the fluctuations in the western part of the region are accurately dated

ice sheet sources of sea level rise and freshwater discharge during the last deglaciation we review and synthesize the geologic record that constrains the sources of sea level rise and freshwater discharge to the global oceans associated with retreat of ice sheets during the last deglaciation the last glacial maximum 26 19 ka was terminated by a rapid 5 10 m sea level rise at 19 0 19 5 ka sourced largely from northern hemisphere ice sheet retreat in response to high northern latitude insolation forcing sea level rise of 8 20 m from 19 to 14 5 ka can be attributed to continued retreat of the laurentide and eurasian ice sheets with an additional freshwater forcing of uncertain amount delivered by heinrich event 1 the source of the abrupt acceleration in sea level rise at 14 6 ka meltwater pulse 1a 14 15 m includes contributions of 6 5 10 m from northern hemisphere ice sheets of which 2 7 m represents an excess contribution above that derived from ongoing ice sheet retreat widespread retreat of antarctic ice sheets began at 14 0 15 0 ka which together with geophysical modeling of far field sea level records suggests an antarctic contribution to this meltwater pulse as well the cause of the subsequent younger dryas cold event can be attributed to eastward freshwater runoff from the lake agassiz basin to the st lawrence estuary that agrees with existing lake agassiz outlet radiocarbon dates much of the early holocene sea level rise can be explained by laurentide and scandinavian ice sheet retreat with collapse of laurentide ice over hudson bay and drainage of lake agassiz basin runoff at 8 4 8 2 ka to the labrador sea causing the 8 2 ka event

laurentide cordilleran and montane glaciation in the western peace river grande prairie region alberta and british columbia canada abstract geological investigations over the last decade in the western peace river grande prairie district of northwestern alberta and northeastern british columbia have provided important chronostratigraphic information regarding laurentide cordilleran and montane quaternary glacial events numerous 14 c age determinations as well as stratigraphic and sedimentological analyses of numerous exposures indicate that this area was first glaciated by cordilleran ice sometime before the late wisconsinan during the late wisconsinan ca 22 000 14 000 bp laurentide glacier ice covered much of the area between approximately 15 000 and 10 000 bp montane ice affected only the western part of the peace region there is no evidence for synchroneity of the ice advances and therefore no evidence for ice coalescence in this region the resultant ice free corridor was most likely a laterally fluctuating zone which existed during a several thousand year period in the late pleistocene climatic conditions along the corridor during this period were probably unfavorable for human migration but adequate for some plants and small animals numerous isolated areas along the western foothills of the northern rocky mountains west of the laurentide glaciation limit were never glaciated during the late wisconsinan

history of late glacial runoff along the southwestern margin of the laurentide ice sheet abstract the routing of runoff from the retreating laurentide ice sheet was controlled by a combination of ice marginal position and topography isostatic depression also played an important role in the evolution of the ice marginal landscape a major redirection of drainage occurred after ice retreated north of the missouri escarpment about 12 ka bp runoff that had previously flowed south to the missouri river could then be routed eastward to lake agassiz which in turn drained to the upper mississippi river valley changes in drainage routing in the region west of glacial lake agassiz typically were abrupt as new and lower paths along the retreating ice margin became available each new basin was larger than its predecessor and its headwaters extended farther west four distinct sequential phases of ice marginal drainage occurred between 12 and 10 7 ka bp the james sheyenne souris pembina and qu appelle assiniboine starting with the qu appelle assiniboine phase about 11 2 ka bp lake agassiz collected runoff from as far west as the rocky mountains by about 10 7 ka bp ice had retreated north of the lower reaches of the saskatchewan river allowing meltwater to by pass the qu appelle assiniboine basin at about the same time the eastern outlets of lake agassiz opened initiating drainage to the north atlantic ocean from this vast region drainage routes evolved by a common sequence of events beginning with the impoundment of proglacial lakes along the ice margin most of these lakes drained catastrophically resulting in the formation of spillways with distincitve geomorphic features that include broad scoured and streamlined subupland channels along with subsequently incised narrow deep inner channels characteristic deposits of these outbursts include boulder mantles on subupland channel floors boulder gravel bars within the inner channels and large primarily subaqueous fans in lake basins that received the flood bursts these outbursts progressed from basin to basin causing the drainage of other proglacial lakes until they reached lake agassiz spillways associated with the next younger drainage phase were commonly incised across the abandoned lake floors

late quaternary events in the lethbridge area alberta new observations in the lethbridge area permit a more complete reconstruction of the landscape history in late wisconsinan and holocene time the plain of glacial lake macleod the lethbridge moraine and the plain of glacial lake lethbridge became exposed in that order and in quick succession almost immediately thereafter a discontinuous mantle of loess began to accumulate on those surfaces the presence of glacier peak layer g or manyberries tephra near the base of the loess indicates these events occurred just before 11 200 bp the similarity of this chronology to that established for the older green lake end moraine in the cypress hills region and the fact that the younger buffalo lake moraine was formed before 11 000 bp indicate that deglaciation of southern alberta proceeded very rapidly the oldest buried paleosol observed near lethbridge began to form shortly after 11 200 bp subsequently and throughout the holocene intervals of loess deposition alternated with soil forming intervals at least

a reconstruction of moorhead and emerson phase environments along the eastern margin of glacial lake agassiz rainy river basin northwestern ontario mapping of quaternary geology in the rainy river lowland northwestern ontario resulted in discovery of several fossil bearing localities organic remains are associated with both the moorhead and

a high resolution record of climate variability and landscape response from kettle lake northern great plains north america abstract a decadal scale multiproxy record of minerals pollen and charcoal from kettle lake north dakota provides a high resolution record of climate and vegetation change spanning the entire holocene from the northern great plains ngp in north america the chronology is established by over 50 ams radiocarbon dates this record exhibits millennial scale trends evident in other lower resolution studies but with much more detail on short term climate variability and on the rapidity and timing of major climatic shifts as a proxy for precipitation we utilize the rate of endogenic carbonate sedimentation which depends on groundwater inflow which in turn depends on precipitation independent cluster analyses of mineral and pollen data reveal major holocene mode shifts at 10 73 ka ka cal yr bp 9 25 ka and 4 44 ka the early holocene 11 7 9 25 ka was generally wet with perhaps a trend to higher evaporation associated with warming temperatures a switch from calcite to aragonite deposition associated with a severe but brief drought occurred at 10 73 ka from 10 73 ka to 9 25 ka climate was generally humid but punctuated at 100 300 yr intervals by brief droughts including the most severe drought of the entire holocene at 9 25 ka this event was coeval with the 9 3 9 2 ka event in the greenland ice cores and observed at a number of sites worldwide in contrast the prominent 8 2 ka event in greenland is not remarkable at kettle lake the prominence of the 9 25 event locally in the ngp may be due to a major drawdown and northward retreat of lake agassiz at this time reducing its mesoclimatic effect on the ngp and thrusting the region into an insolation controlled regime the mid holocene 9 25 4 44 ka was characterized by great variability in moisture on a multi decadal scale with severe droughts alternating with more humid periods the high abundance of the weedy but drought intolerant ambrosia generally during the mid holocene and specifically during the multi decadal drought periods is seemingly paradoxical but can be explained by high interannual variability of moisture overlaid on the multi decadal variability the late holocene 4 44 ka present was also characterized by multi decadal variability in moisture but was generally wetter than the mid holocene and the magnitude of variability was less the trends in wet dry mineral pollen and charcoal proxies were similar to the mid holocene but late holocene mineral pollen assemblages are distinct from mid holocene the shift to wetter climate in the late holocene was more gradual than the abrupt shift to arid conditions 9 25 ka which may explain the asymmetric retreat and readvance of forest along the eastern margin of the ngp precipitation variations in the ngp have been linked with pacific and atlantic sea surface temperatures and mid holocene drought in the ngp has been linked with sustained la nina like conditions in the pacific these linkages may explain the decadal and millennial scale variations seen in the ngp but cause of the prominent century scale variations remains elusive

response of the irish ice sheet to abrupt climate change during the last deglaciation we summarize 121 14c and in situ cosmogenic 10be and 36cl ages that constrain fluctuations of the irish ice sheet iis since the last glacial maximum lgm that can be linked to abrupt climate changes in the north atlantic region these data provide a robust means to date ice sheet fluctuations because similar age events can be identified from widely spaced sites they are constrained by stratigraphy and they can be related to large changes in the configuration of a dynamic ice sheet the following events are recognized 1 ams 14c ages and 10be and 36cl ages as well as offshore data suggest that the last maximum advance of the iis occurred between 27 and 23 cal ka 2 deglaciation began on the western continental shelf and in the irish sea basin at 23 cal ka dated sites from around ireland constrain subsequent widespread retreat of ice sheet margins from the shelf onto irish coastlines during the cooley point interstadial 20 cal ka ams 14c ages suggest that the cooley point interstadial continued until 18 2 cal ka b p during this interstadial retreat of the ice sheet margin into the northern irish sea basin indicates that the iis may have lost up to two thirds of its mass 3 dated sites from northwestern and eastern ireland indicate that the cooley point interstadial was terminated by ice readvance during the clogher head stadial at 18 2 cal ka 4 the linns interstadial was a brief interval 17 0 16 5 cal ka of ice recession following the clogher head stadial that is identified from raised marine sediments in eastern and northern ireland and cosmogenic ages elsewhere in ireland 5 the iis subsequently readvanced during the killard point stadial reaching its maximum extent 16 5 cal ka indicating that the readvance began sometime earlier 6 widespread retreat of the iis began by 15 5 cal ka during the rough island interstadial 6 readvance of cirque glaciers in western ireland occurred during the younger dryas equivalent nahanagan stadial

air photo based change in channel width in the minnesota river basin modes of adjustment and implications for sediment budget abstract the minnesota river and major tributaries have experienced large increases in discharge over the past century aerial photograph based measurements of channel width were made for the 1938 2015 period at 16 multibend subreaches by digitizing the area between vegetation lines and dividing by centerline length results show considerable increases in width for the main stem 0 62 0 10 y and major tributaries 0 31 0 08 y but are inconclusive for smaller channels width

small profile concavity of a fine bed alluvial channel although most natural streams have concave upward longitudinal profiles there are conflicting perspectives about whether an alluvial channel is less concave than a bedrock channel alluvial channels can be classified as coarse bed and fine bed channels depending on the bed grain size both are transport limited but the threshold of motion differs greatly whereas a coarse bed alluvial channel can be claimed to be as concave as a bedrock channel we claim that a fine bed channel is distinguishably less concave we derive the concavity index of a fine bed alluvial channel using the power law relationships emergent at a steady state river network for known ranges of the scaling parameters our formulation informs a range of concavity index as 0 07 0 09 for a fine bed alluvial channel our analyses of previous laboratory experiments and real fine bed alluvial channels in the midwestern u s and northern europe also support our conclusions i e small profile concavity of steady state fine bed alluvial channels

deglacial flood origin of the charleston alluvial fan lower mississippi alluvial valley abstract large magnitude flooding of the mississippi river from proglacial lakes agassiz and superior most likely occurred between 11 300 and 10 900 and 9900 and 9500 yr b p the charleston alluvial fan a depositional remnant of one of these floods is located at the head of a wide alluvial plain near charleston missouri the fan is an elongate convex up sand body 16 24 km composed of medium and fine grained sand at least 8 m thick this sand contrasts with the older coarse grained sand of the braided stream surface to the west and south and younger silty clay of the meandering stream level to the north and east a weakly developed soil separates the underlying braided steam deposits from the alluvial fan a bulk soil radiocarbon date of 10 590 200 yr b p from the contact between the fan and clays of the meandering stream system indicates that the charleston fan was deposited near the end of the early interval of flooding from lake agassiz about 10 900 yr b p if the charleston fan is the last remnant of deglacial flooding in the lower mississippi valley then deposition of significant quantities of sediment from largemagnitude floods between 10 000 and 9500 yr b p did not extend into the lower mississippi valley through thebes gap

deglaciation basin formation and post glacial climate change from a regional network of sediment core sites in ohio and eastern indiana abstract many paleoclimate and landscape change studies in the american midwest have focused on the late glacial and early holocene time periods 16 11 ka but little work has addressed landscape change in this area between the last glacial maximum and the late glacial 22 16 ka sediment cores were collected from 29 new lake and bog sites in ohio and indiana to address this gap the basal radiocarbon dates from these cores show that initial ice retreat from the maximal last glacial ice extent occurred by 22 ka and numerous sites that are 100 km inside this limit were exposed by 18 9 ka post glacial environmental changes were identified as stratigraphic or biologic changes in select cores the strongest signal occurs between 18 5 and 14 6 ka these midwestern events correspond with evidence to the northeast suggesting that initial deglaciation of the ice sheet and ensuing environmental changes were episodic and rapid significantly these changes predate the onset of the bolling postglacial warming 14 8 ka as recorded by the greenland ice cores thus deglaciation and landscape change around the southern margins of the laurentide ice sheet happened 7 ka before postglacial changes were felt in central greenland

flow mechanism of the des moines lobe of the laurentide ice sheet rapid flow of the des moines lobe of the laurentide ice sheet may have been related to its unlithified substrate new reconstructions of the lobe based on moraine elevations sediment subsidence during moraine deposition and flow direction indicators indicate that the lobe may have been 3 times thicker than in previous reconstructions nevertheless implied basal shear stresses are 15 kpa so internal ice deformation was not significant instead the lobe likely moved by a combination of sliding plowing of particles through the bed surface and bed shear consolidation tests on basal till yield preconsolidation stresses of 125 300 kpa so effective normal stresses on the bed were small a model of sliding and plowing indicates that at such stresses most particles gripped by the ice may have plowed easily through the till bed resulting in too small a shear traction on the bed to shear it at depth consistent with this prediction measurements of orientations of clasts in basal till yield a weak fabric implying pervasive bed shear strain less than 2 although some stronger fabrics have been reported by others we infer tentatively that movement was principally at the bed surface by plowing

archaeological constraints on some geological interpretations of a late wisconsinan site at varsity estates calgary alberta abstract late glacial history in western alberta has been a subject of debate as this area lies in the zone of assumed contact of the cordilleran glaciers and laurentide ice shield the geographic limits and timing of the coalescence of the two ice masses however are not clear except for the west central part of the province with the documented merger in the athabasca valley recently arguments for a complete late wisconsinan coalescence in the prairies all along the rocky mountain foothills have been presented based on 14c dates 21 ka and cosmogenic 36cl dates

the application of radiocarbon age estimates to the dating of glacial sequences an example from the miami sublobe ohio u s a abstract despite the importance of dating glacial sequences little evidence has been available to highlight the stratigraphic issues associated with the interpretation of age analysis a compilation of radiocarbon data from the miami sublobe of the laurentide ice sheet provides a large enough data set more than 133 dates to identify at least two sources of error sample and unit variability and transportation problems analysis of different samples from the same stratigraphic level commonly differ by more than the reported one sigma error and 2 000 year differences do occur glacial entrainment and transportation emplace older organic material over younger in situ materials and this effect may also give age errors in the data set of up to 2 000 years at a minimum multiple radiocarbon ages from a sequence of units are necessary to assign a reliable age to events within a glacial sequence however representing multiple ages poses its own problems a graphical representation that combines the probability of each sample appears to be one solution

geologic and topographic controls on fast flow in the laurentide and cordilleran ice sheets ice streams are fast flowing arteries which play a vital role in the dynamics and mass balance of present day ice sheets although not fully understood fast flow dynamics are intimately coupled with geologic topographic thermal and hydrologic conditions of the underlying bed these are difficult observables beneath contemporary ice sheets hindering elucidation of the processes which govern ice streaxn behavior for past ice sheets the problem is antithetic geologic evidence of former ice streams exists but spatim and temporal histories are uncertain however detailed knowledge of bed geology and topography is available in many places we take advantage of this information to compile terrain characteristics relevant to fast flow dynamics in the laurentide and cordilleran ice sheets using seed points where fast flowing wisconsinan ice has been geologically inferred discriminant analysis of a suite of north american geologic and topographic properties yields a concise measure of ice bed coupling strength our analysis suggests that the interior plains and continental shelf regions of north america have low basal coupling relative to areas of variable relief or exposed bedrock in the cordillera and on the canadian shield we conclude that the interior plains and continental shelves are both topographically and geologically predisposed to large scale basal flows i e ice streaxns or surge lobes this result holds independent of whether the mechanism of fast flow is sediment deformation or decoupled sliding over the bed

glacial lake saskatchewan and lake agassiz deltas in east central saskatchewan with special emphasis on the nipawin delta the nipawin delta extends eastward from the town of nipawin in east central saskatchewan the prince albert and fort a la corne deltas formed in lake saskatchewan at the mouths of the south and north saskatchewan rivers whereas the nipawin and mossy vale deltas formed in lake agassiz at the mouth of the saskatchewan river these deltas became lower in elevation and younger eastward clays beneath the nipawin and mossyvale deltas can be traced beneath the prince albert and fort a la corne deltas with little change in thickness suggesting that most of the clays beneath the nipawin and mossyvale deltas were deposited during the formation of the prince albert and fort a la corne deltas the nipawin delta formed between 10 200 and 9500 bp the campbell strandline emerson phase was the shoreline of the nipawin delta the mossyvale delta formed about 9500 bp the surface of the mossyvale delta mccauleyville level is about 40 m below the surface of the nipawin delta campbell level

geochemical evolution of lacustrine brines from variable scale groundwater circulation abstract evaporative groundwater fed lakes in the glaciated north american great plains vary widely in chemistry a contributing cause is chemical variability of source groundwater intercepted by specific lakes caused in part by differing depths of groundwater circulation aqueous chemical characteristics of 61 lakes and 160 groundwater samples were compared for an area where such lakes are common in eastern montana western north dakota results indicate that groundwater chemistry varies according to depth in a similar fashion within different aquifers lake water evaporation from initial groundwater solutions typical of three depths was geochemically modeled using phrqpitz based on a pitzer treatment of activities and equilibria results show that chemistry of most lake waters in the study area may correspond to that predicted from evaporation of shallow and intermediate depth groundwater but not of deep groundwater as postulated in some previous investigations lakes in shallow surface depressions receive water primarily from shallow local groundwater flow lakes located in deep or broad topographic depressions may additionally receive groundwater from deeper circulation in the field area studied relative dominance of anions sulfate vs carbonate in brines is a signature for inferred depth of source also diagnostic is the suite of brine salts formed na so4 mg salts for shallow flow these plus na co3 salts for intermediate depth flow such source signatures will vary from area to area according to depth variations in groundwater chemistry and in stratigraphy chemical evolution of lake water is a two stage process with a groundwater path influenced by residence time depth of circulation aquifer mineralogy and related factors and a surface path influenced by evaporation rates lake aquifer hydraulics and lake geochemical reactions groundwater flow patterns may affect the former set of factors thereby indirectly controlling lake water chemistry

late pleistocene depositional systems of metropolitan toronto and their engineering and glacial geological significance the municipality of metropolitan toronto area 480 km2 population 2 15 million is centrally located on the late pleistocene sedimentary infill of the laurentian channel a broad bedrock low up to 115 km wide connecting the huron and ontario basins this channel forms part of a relict late tertiary drainage network the laurentian river modified by pleistocene glacial erosion and infilled by over 100 m of glacial and interglacial sediments the subsurface stratigraphy of the channel fill below metropolitan toronto has been established from many different data sources and is depicted in this paper as a series of cross sections with a total length of nearly 105 km the subsurface stratigraphy has been divided provisionally into five depositional complexes which have been mapped in the subsurface along several transects these are 1 a glacial complex of illinoian age 2 a lacustrine complex of sangamon interglacial and earliest wisconsinan sediments 120 000 75 000 bp 3 a glaciolacust

glacial meltwater cooling of the gulf of mexico gcm implications for holocene and present day climates an atmospheric general circulation model the ncar ccm has been used to investigate the possible effects that reduced gulf of mexico sea surface temperatures sst could have on regional and hemispheric climates 18o records and terrestrial evidence indicate at least two major glacial meltwater discharges into the gulf of mexico subsequent to the last glacial maximum it is probable that these discharges reduced gulf of mexico sst we have conducted three numerical experiments with imposed gulf wide sst coolings of 3 c 6 c and 12 c and find in all three experiments significant reductions in the north atlantic storm track intensity along with a strong decrease in transient eddy water vapor transport out of the gulf of mexico surface pressures are higher over the north atlantic indicating a reduction of the climatological icelandic low the region is generally cooler and drier with a reduction in precipitation that agrees well with evidence from greenland ice cores other statistically significant changes occur across the northern hemisphere but vary between the three experiments in particular warmer wetter conditions are found over europe for both the 6 c and 12 c sst reductions but cooler conditions are found for the 3 c reduction this indicates a dependence in both the sign and magnitude of the model response on the magnitude of the imposed sst anomaly the results suggest that the present day north atlantic storm track is dependent on warm gulf of mexico sst for much of its intensity they also suggest that meltwater induced coolings may help account in part for some of the climatic oscillations that occurred during the last glacial interglacial transition

pattern of southern laurentide ice sheet margin position changes during heinrich stadials 2 and 1 abstract with concerns over rising global temperatures ice sheet stability and implications for sea level rise the responses of former ice sheets to past climate change provide useful insights into linkages between the atmosphere cryosphere and oceans one example is the behavior of the laurentide ice sheet lis during heinrich stadials times of cooler sea surface temperature sst in the north atlantic using existing cosmogenic surface exposure ages varve chronologies radiocarbon ages and optically stimulated luminescence ages from the southern lis margin this report tests the hypothesis that the southern side of the lis advanced during times of surface cooling of the north atlantic this test reconstructs the timing of local glacial maxima and the times immediately pro and preceding them from multiple areas and compares them to the timing of heinrich stadial 2 hs2 26 6 23 6 ka and to the timing of heinrich stadial 1 hs1 19 3 15 3 ka during hs2 southern sector lobes new england region huron erie lobe lake michigan lobe green bay lobe and the chippewa lobe advanced towards their maximum position except for the des moines lobe for which there is no applicable data during hs1 these same lobes experienced substantial retreat but the des moines lobe advanced thus the southern laurentide margin behaved differently during the two heinrich stadials this pattern may be attributed to differing east west temperature gradients across north america and associated changes in atmospheric circulation

till stratigraphy and late glacial events in the lake michigan lobe of eastern wisconsin four late wisconsinan reddish brown tills are present along the lake michigan shoreline in eastern wisconsin these tills are distinguished by differences in grain size distribution clay mineral content and stratigraphic position the ozaukee and haven tills underlie the two creeks forest bed the younger valders till although not present at a forest bed locality is also pre twocreekan age based on several arguments including correlation with tills of the adjoining green bay lobe one greatlakean age till the two rivers till has been recognized and this clearly overlies the two creeks forest bed as previously reported we conclude that the valders till is most likely of pre twocreekan age and that valderan is an inappropriate term as a substage name for post twocreekan time and that the use of the term greatlakean is justified by stratigraphic studies

coarse textured basal zones in thin loess deposits products of sediment mixing and or paleoenvironmental change abstract the purpose of this research was to characterize and interpret the coarse basal zones that are common in thin loess deposits that overlie coarser textured sediment to that end we sampled nine pedons in northeastern wisconsin and the upper peninsula of michigan each of which had formed in thin 55 cm loess over sandy glacial sediment at most of these sites the loess became noticeably coarser near the lithologic discontinuity the loess has a primary particle size mode in the coarse silt or fine very fine sand fraction 30 65 m and a secondary mode in the fine or medium sand fraction 200 400 m we attribute the secondary mode to mixing of underlying sands into the loess either during loess deposition or by post depositional pedoturbation in thin loess pedoturbation processes can penetrate into the underlying sediment facilitating mixing upward as far as 50 cm into the loess silt from the loess has also been mixed into the underlying sandy sediment alternatively in some pedons the loess itself coarsens with depth the particle size mode for the loess becomes increasingly coarser with depth this coarsening suggests that during loess deposition one or more of the following was occurring 1 wind velocities were decreasing over time 2 the textural character of the loess source area s were changing decreasing or 3 additional source areas with finer textured sediment became more dominant over time our research demonstrates the utility of detailed particle size data for detecting and interpreting the paleoenvironmental history of loess we also document the extent to which pedoturbation can impact the original textural characteristics of loess or any sediment that occurs as a thin surficial mantle

age verification of the lake gribben forest bed and the younger dryas advance of the laurentide ice sheet analysis of nine wood samples from the lake gribben forest bed near lake gribben michigan yielded a combined age of 10 025 100 14c years bp which confirms and refines prior age estimates for the bed the stratigraphic position of these samples below a prograding ice contact fan indicates the time that a glacial margin reached the southern edge of the lake superior basin geomorphic tracing and correlation of associated deposits indicate that a contemporaneous margin extended almost 1000 km from duluth minnesota across the lake superior basin to north bay ontario along the southern shore of lake superior ice margin expansion began during and ended at the close of the younger dryas a surging glacier system would not produce a nearly linear moraine system across both a major basin lake superior and a major upland abitibi upland therefore we attribute this advance of the laurentide ice sheet to climatic forcing of the younger dryas event

paleomagnetic dating of late glacial and postglacial sediments in lake superior the oldest unit of the late quaternary sedimentary sequence of lake superior consists of glacial till that was deposited about 9900 years bp during the last glacial readvance into the lake superior basin the till deposits are unconformably overlain by a thin sequence of bedded sands and silts which are overlain by a thick sequence of varved sediments 10 m in thickness which in turn are overlain by postglacial silty clays unfortunately 14c has not been found to be a useful tool in dating the late glacial and postglacial sediments of lake superior however paleodeclination and paleoinclination logs provide a useful method of correlating and indirectly dating cores taken of the late glacial and postglacial sediments it is concluded from paleomagnetic dating that rhythmic sedimentation ceased before 9200 years bp in the southeastern lake proper and at about 8700 years bp in the southeastern bay areas however rhythmic sedimentation continued until about 8200 years bp in the northern part of lake su

quaternary geology and geomorphology of the dinosaur provincial park area and surrounding plains alberta canada the identification of former glacial lobes drainage diversions and meltwater flood tracks abstract the quaternary geology and geomorphology of the lower red deer river drainage basin centred on the dinosaur provincial park badlands provides information on pre late wisconsinan drainage patterns and the dynamics of former lobes of the laurentide ice sheet in western canada patterns of deglaciation proglacial lake evolution and spillway incision are also reconstructed based upon the distribution of surface materials and glacial glaciofluvial landforms the empress group fluvial and glaciolacustrine sediments which could be as young as 27 ka bp infill the precursor red deer river and its tributaries and document the initial advance of glacier ice into southern alberta glaciotectonic disturbance of older sediments and bedrock the production of deformation tills and the construction of a megafluting complex and cupola hills record the advances of a glacier lobe centred over the study area stratified inter and intra till beds record pulses of subglacial meltwater between phases of subsole deformation the thickening of tills towards the margin of the lobe represents a till wedge an expected product of sediment advection by glaciers moving over deformable beds the eastern margin of the glacier lobe is demarcated by the interlobate suffield hummocky moraine belt which contains overprinted thrust ridges which record diachronous oscillations of neighbouring lobes within the ice sheet proglacial and glaciofluvial sediments were deposited in the area in association with proglacial lake bassano patricia which drained eastwards when the suffield moraine was dissected by spillways changes in the size of glacial lake bassano patricia are clearly documented by a sequence of spillway incisions which culminated in the erosion of scabland topography and the initiation of a new course for the red deer river a 15 km southward diversion of the main channel in distinct contrast to the documented incision histories of other small rivers in alberta one tree creek and little sandhill creek did not start major incisions of the quaternary sediments over buried valley positions until the late holocene when environmental conditions were characterized by higher precipitation

late wisconsinan and holocene history of southwestern saskatchewan the late wisconsinan laurentide ice sheet reached the limit of glaciation along the north facing slope of the unglaciated part of the cypress hills nunatak but elsewhere around the cypress hills and wood mountain uplands of southwestern saskatchewan it did not reach the limit of glaciation an interval of deglaciation was followed by a readvance of the ice sheet about 15 ka to a position near the late wisconsinan limit and was marked by strong flow of keewatin ice from the north and weaker flow of hudson ice from the northeast final deglaciation resulted in the formation of glacial lakes around the cypress hills nunatak a network of ice marginal and subglacial trenches presently occupied by the frenchman valley and its tributaries was formed when these lakes drained along over and under ice towards the western part of the wood mountain upland to the east the main trench joined an ancestral part of frenchman valley along the ice free southern slopes of wood mountain upland meltwater from the ic

optical ages indicate the southwestern margin of the green bay lobe in wisconsin usa was at its maximum extent until about 18 500 years ago abstract samples for optical dating were collected to estimate the time of sediment deposition in small ice marginal lakes in the baraboo hills of wisconsin these lakes formed high in the baraboo hills when drainage was blocked by the green bay lobe when it was at or very near its maximum extent therefore these optical ages provide control for the timing of the thinning and recession of the green bay lobe from its maximum position sediment that accumulated in four small ice marginal lakes was sampled and dated difficulties with field sampling and estimating dose rates made the interpretation of optical ages derived from samples from two of the lake basins problematic samples from the other two lake basins south bluff and feltz basins responded well during laboratory analysis and showed reasonably good agreement between the multiple ages produced at each site these ages averaged 18 2 ka n 6 and 18 6 ka n 6 respectively the optical ages from these two lake basins where we could carefully select sediment samples provide firm evidence that the green bay lobe stood at or very near its maximum extent until about 18 5 ka the persistence of ice marginal lakes in these basins high in the baraboo hills indicates that the ice of the green bay lobe had not experienced significant thinning near its margin prior to about 18 5 ka these ages are the first to directly constrain the timing of the maximum extent of the green bay lobe and the onset of deglaciation in the area for which the wisconsin glaciation was named

the timing and nature of late quaternary vegetation changes in the northern great plains usa and canada a re assessment of the spruce phase abstract this paper revises the chronology for the northward migration of picea glauca white spruce across the northern great plains following the recession of the laurentide ice sheet and reinterprets the species composition and structure of the late glacial vegetation on the basis of pollen and plant macrofossil analysis the timing of spruce migration is based on 26 14 c ages obtained from picea macrofossils the date for the appearance of white spruce in southern south dakota usa remains unchanged 12 600 14 c yr bp ca 15 000 cal yr bp but its arrival in southern saskatchewan canada by 10 300 14 c yr bp ca 12 100 cal yr bp is about 1500 years later than previously estimated based on an organic sediment date picea glauca thus migrated northwards at an average rate of 0 38 km 14 c year 0 30 km calendar year significantly slower than the previously published rate of 2 km 14 c year white spruce trees probably inhabited lake shorelines whereas prairie parkland and boreal plants occupied both lowlands and uplands forming an open white spruce parkland this interpretation differs from a previous reconstruction of a boreal type spruce forest and thus offers another paleoclimatic interpretation precipitation was probably low and summer temperatures relatively mild averaging about 19 c

ice streams in the laurentide ice sheet identification characteristics and comparison to modern ice sheets this paper presents a comprehensive review and synthesis of ice streams in the laurentide ice sheet lis based on a new mapping inventory that includes previously hypothesised ice streams and includes a concerted effort to search for others from across the entire ice sheet bed the inventory includes 117 ice streams which have been identified based on a variety of evidence including their bedform imprint large scale geomorphology topography till properties and ice rafted debris in ocean sediment records despite uncertainty in identifying ice streams in hard bedrock areas it is unlikely that any major ice streams have been missed during the last glacial maximum laurentide ice streams formed a drainage pattern that bears close resemblance to the present day velocity patterns in modern ice sheets large ice streams had extensive onset zones and were fed by multiple tributaries and where ice drained through regions of high relief the spacing of ice streams shows a degree of spatial self organisation which has hitherto not been recognised topography exerted a primary control on the location of ice streams but there were large areas along the western and southern margin of the ice sheet where the bed was composed of weaker sedimentary bedrock and where networks of ice streams switched direction repeatedly and probably over short time scales as the ice sheet retreated onto its low relief interior several ice streams show no correspondence with topography or underlying geology perhaps facilitated by localised build up of pressurised subglacial meltwater they differed from most other ice stream tracks in having much lower length to width ratios and have no modern analogues there have been very few attempts to date the initiation and cessation of ice streams but it is clear that ice streams switched on and off during deglaciation rather than maintaining the same trajectory as the ice margin retreated we provide a first order estimate of changes in ice stream activity during deglaciation and show that around 30 of the margin was drained by ice streams at the lgm similar to that for present day antarctic ice sheets but this decreases to 15 and 12 at 12 cal ka bp and 10 cal ka bp respectively the extent to which these changes in the ice stream drainage network represent a simple and predictable readjustment to a changing mass balance driven by climate or internal ice dynamical feedbacks unrelated to climate or both is largely unknown and represents a key area for future work to address

meltwater and precipitation runoff to the north atlantic arctic and gulf of mexico from the laurentide ice sheet and adjacent regions during the younger dryas runoff from north america may have played a significant role in ocean circulation and climate change during the last deglaciation because the driving force behind such changes may have been related to salinity of the north flowing atlantic ocean conveyor circulation it is critical to know the volume timing and location of fresh water entering the north atlantic from the melting laurentide ice sheet during the younger dryas cold episode 11 000 10 000 years b p there was a two fold increase in the volume of meltwater plus precipitation runoff to more than 1700 km yr 1 flowing through the st lawrence valley to the north atlantic mainly because retreating ice allowed the glacial lake agassiz basin to drain eastward into the great lakes at this time there was a corresponding decline in discharge from lake agassiz through the mississippi river to the gulf of mexico runoff to the arctic ocean also increased at about the beginning of the younger dryas from 740 to 900 km yr 1 because of the capture of what is now the headwater region of the mackenzie river watershed this in combination with rising sea level and warming climate may have increased the amount of pack ice reaching the north atlantic through the norwegian sea from the arctic ocean at 10 000 years b p eastward overflow from the western interior of north america was blocked by advancing ice again forcing overflow to the gulf of mexico and possibly to the northwest into the arctic ocean although total runoff to the oceans from all regions draining from the laurentide ice sheet did not vary substantially between 12 000 and 9000 years b p if discharge to the gulf of mexico is excluded fresh water reaching the north atlantic averaged 4000 km yr 1 during the younger dryas in contrast to 2870 km yr 1 just before this cold episode and 3440 km yr 1 just after it

modeling the subglacial hydrology of the james lobe of the laurentide ice sheet to investigate the drainage conditions that might be expected to develop beneath soft bedded ice sheets we modeled the subglacial hydrology of the james lobe of the laurentide ice sheet from hudson bay to the missouri river simulations suggest the james lobe had little effect on regional groundwater flow because the poorly conductive upper cretaceous shale that occupies the upper layer of the bedrock would have functioned as a regional aquitard this implies that general northward groundwater flow out of the williston basin has likely persisted throughout the quaternary moreover the simulations indicate that the regional aquifer system could not have drained even the minimum amount of basal meltwater that might have been produced from at the glacier bed therefore excess drainage must have occurred by some sort of channelized drainage network at the ice till interface using a regional groundwater model to determine the hydraulic conductivity for an equivalent porous medium in a 1 m thick zone between the ice and underlying sediment and assuming conduit dimensions from previous theoretical work we use a theoretical karst aquifer analog as a heuristic approach to estimate the spacing of subglacial conduits that would have been required at the ice till interface to evacuate the minimum water flux results suggest that for conduits assumed to be on the order of a tenth of a meter deep and up to a meter wide inter conduit spacing must be on the order of tens hundreds of meters apart to maintain basal water pressures below the ice overburden pressure while evacuating the hypothesized minimum meltwater flux r 2007 elsevier ltd all rights reserved

anomalous radiocarbon ages from a holocene detrital organic lens in alaska and their implications for radiocarbon dating and paleoenvironmental reconstructions in the arctic abstract eleven radiocarbon age determinations clearly show that a lens of holocene fluvial organic debris on the alaskan arctic coastal plain contains mostly pre holocene organic material radio carbon ages of identified plant macrofossils indicate the material was deposited about 9000 to 9500 yr b p radiocarbon analyses of bulk samples from this deposit however range from 13 300 to 30 300 yr b p most of the old organic matter seems to be in the smaller size fractions in the deposit particularly in the fraction between 0 25 and 0 5 mm but all size fractions are contaminated particular caution must be exercised in submitting bulk samples for radiocarbon dating from areas where conditions favor redeposition of isotopically dead carbon

width change of a planar magma path implication for the evolution and style of volcanic eruptions abstract the width change of a planar path filled with magma is analyzed assuming that the process is controlled by laminar flow of an incompressible viscous magma between movable parallel walls of country rock the analysis shows that the width of a planar path changes at a rate is proportional to the effective opening pressure which is defined as the pressure of the penetrating magma plus the tensile stress component stretching the fluid interface when the opening pressure is a positive constant the fissure widens at an accelerating rate and finally diverges at a critical time constant which depends on the initial size of the fissure and the viscosity of magma as well as on the opening pressure in contrast the fissure gradually narrows under a negative opening pressure this result is applicable to a variety of magmatic processes first the orientation of a volcanic fissure or a dike is such that the tensile stress component is maximized because a fissure with this orientation becomes wider than those at other orientations as it has the minimum value of critical time constant second local irregularities in the geometrical and mechanical conditions of the magma sheet lead to significant variations in fissure width in the widening process this nonuniform increase in width is further exaggerated by mechanical and thermal erosion of the wall rock which makes the irregular surfaces irrecoverable in this manner a planar magma path tends to be split into isolated cylindrical conduits when the magma pressure is reduced and the fissure closes as a whole some actual fissure eruptions such as those at kilauea and izu oshima show evolutionary processes consistent with this prediction another application of the analysis is to explain how the style of a volcanic eruption is determined by the tectonics specifically extensional tectonics which gives a positive opening pressure for any magma pressure allows magma to ascend dispersively along various paths and produces a group of small monogenetic volcanoes in contrast contractional tectonics in which new fissures can be made only for a very high magma pressure concentrates magma into a restricted number of vents and leads to the growth of large polygenetic volcanoes

postglacial history and paleoecology of wampum manitoba a former lagoon in the lake agassiz basin a small paleolagoon along the margin of the glacial lake agassiz basin near wampum manitoba 130 km southeast of winnipeg contains an unusually thick and continuous sequence of late glacial and holocene organic sediment cores at 17 sites through this sequence provide insight into the fluctuations of the level of lake agassiz the timing of the construction of the lake9s most extensive strandline the upper campbell beach and the paleoecology of this area during the past 10 000 14 c yr the history of this area is established from 26 new ams radiocarbon dates siliceous microfossils plant macrofossils mineralogy sediment characteristics and stratigraphic relationships following deglaciation at wampum about 11 14 c ka deposition of largely unfossiliferous silty clay with occasional ice rafted clasts represents relatively deep water conditions in lake agassiz overlying this is a spatially and temporally variable mixture of silt sand peat gyttja and marl containing an allochthonous assemblage of siliceous microfossils and plant macrofossils with a wide range of radiocarbon dates these sediments were deposited in an embayment of lake agassiz mainly in shallow quiet waters as the lake transgressed about 10 ka over the moorhead low water phase surface between about 9 7 and 9 3 ka the wampum embayment was progressively isolated from lake agassiz by growth of the upper campbell spit resulting in the formation of a lagoon organic rich deposits on the lakeward side of the lagoon were buried 9 3 9 4 ka by sand and gravel of the spit and a thin overwash of sand marks this event throughout the lagoon at about this time lake agassiz fell below the upper campbell beach level subsequent accumulation at wampum consists mainly of algal gyttja during the early holocene and marl during the middle holocene only minor fluctuations in hydrological conditions occurred except for slightly warmer and drier conditions between 6 2 and 4 0 14 c ka these stable shallow water conditions may be due partly to persistent groundwater contributions to the site progressive encroachment of rooted aquatics led to present day fen conditions peat accumulation across the basin began by about 4 0 ka

deglaciation of fennoscandia abstract to provide a new reconstruction of the deglaciation of the fennoscandian ice sheet in the form of calendar year time slices which are particularly useful for ice sheet modelling we have compiled and synthesized published geomorphological data for eskers ice marginal formations lineations marginal meltwater channels striae ice dammed lakes and geochronological data from radiocarbon varve optically stimulated luminescence and cosmogenic nuclide dating this is summarized as a deglaciation map of the fennoscandian ice sheet with isochrons marking every 1000 years between 22 and 13 cal kyr bp and every hundred years between 11 6 and final ice decay after 9 7 cal kyr bp deglaciation patterns vary across the fennoscandian ice sheet domain reflecting differences in climatic and geomorphic settings as well as ice sheet basal thermal conditions and terrestrial versus marine margins for example the ice sheet margin in the high precipitation coastal setting of the western sector responded sensitively to climatic variations leaving a detailed record of prominent moraines and other ice marginal deposits in many fjords and coastal valleys retreat rates across the southern sector differed between slow retreat of the terrestrial margin in western and southern sweden and rapid retreat of the calving ice margin in the baltic basin our reconstruction is consistent with much of the published research however the synthesis of a large amount of existing and new data support refined reconstructions in some areas for example the lgm extent of the ice sheet in northwestern russia was located far east and it occurred at a later time than the rest of the ice sheet at around 17 15 cal kyr bp we also propose a slightly different chronology of moraine formation over southern sweden based on improved correlations of moraine segments using new lidar data and tying the timing of moraine formation to greenland ice core cold stages retreat rates vary by as much as an order of magnitude in different sectors of the ice sheet with the lowest rates on the high elevation and maritime norwegian margin retreat rates compared to the climatic information provided by the greenland ice core record show a general correspondence between retreat rate and climatic forcing although a close match between retreat rate and climate is unlikely because of other controls such as topography and marine versus terrestrial margins overall the time slice reconstructions of fennoscandian ice sheet deglaciation from 22 to 9 7 cal kyr bp provide an important dataset for understanding the contexts that underpin spatial and temporal patterns in retreat of the fennoscandian ice sheet and are an important resource for testing and refining ice sheet models

an updated radiocarbon based ice margin chronology for the last deglaciation of the north american ice sheet complex abstract the north american ice sheet complex naisc consisting of the laurentide cordilleran and innuitian ice sheets was the largest ice mass to repeatedly grow and decay in the northern hemisphere during the quaternary understanding its pattern of retreat following the last glacial maximum is critical for studying many facets of the late quaternary including ice sheet behaviour the evolution of holocene landscapes sea level atmospheric circulation and the peopling of the americas currently the most up to date and authoritative margin chronology for the entire ice sheet complex is featured in two publications geological survey of canada open file 1574 dyke et al 2003 quaternary glaciations extent and chronology part ii dyke 2004 these often cited datasets track ice margin recession in 36 time slices spanning 18 ka to 1 ka all ages in uncalibrated radiocarbon years using a combination of geomorphology stratigraphy and radiocarbon dating however by virtue of being over 15 years old the ice margin chronology requires updating to reflect new work and important revisions this paper updates the aforementioned 36 ice margin maps to reflect new data from regional studies we also update the original radiocarbon dataset from the 2003 2004 papers with 1541 new ages to reflect work up to and including 2018 a major revision is made to the 18 ka ice margin where banks and eglinton islands once considered to be glacial refugia are now shown to be fully glaciated our updated 18 ka ice sheet increased in areal extent from 17 81 to 18 37 million km2 which is an increase of 3 1 in spatial coverage of the naisc at that time elsewhere we also summarize region by region significant changes to the deglaciation sequence this paper integrates new information provided by regional experts and radiocarbon data into the deglaciation sequence while maintaining consistency with the original ice margin positions of dyke et al 2003 and dyke 2004 where new information is lacking this is a pragmatic solution to satisfy the needs of a quaternary research community that requires up to date knowledge of the pattern of ice margin recession of what was once the world s largest ice mass the 36 updated isochrones are available in pdf and shapefile format together with a spreadsheet of the expanded radiocarbon dataset n 5195 ages and estimates of uncertainty for each interval

reconstruction of past lake levels and their relation to groundwater hydrology in the parkers prairie sandplain west central minnesota abstract the parkers prairie sandplain lies along the prairie forest border in west central minnesota an area with ample palynological evidence of significant environmental change during the late pleistocene and holocene past lake levels provide independent evidence of environmental change over the same temporal and spatial scales as pollen analysis we used the method established by digerfeldt 1986 1988 to determine the lowest past levels of three closed basin lakes in the sandplain the method uses a combination of evidence from macrofossils coarse minerogenic matter and organic matter from a transect of cores taken in the near shore region to reconstruct past lake levels lower than present carbonate content magnetic susceptibility and pollen content in the cores provided supplementary evidence and aided cross correlation among the cores each of the three lakes had its lowest level between about 7200 and 6700 yr b p lake levels ranged from 2 8 m to 6 2 m lower than modern levels the greatest lake level lowering occurred for the lake farthest from the river that drains the sandplain the least lake level lowering occurred for the lakes close to the river this spatial pattern of lake level lowering implies that a lowering of the regional water table caused by a reduction in groundwater recharge was the ultimate cause of the lower lake levels the results demonstrate that surface water hydrology is not the sole determinant of lake level and that there are possible dangers in paleoclimatic interpretations of past lake levels based on the assumption of hydrologic isolation of basins

assessment of river bank erosion in southern minnesota rivers post european settlement article i nfo river bank erosion is one of the major sources of sediment for many rivers around the world with the cur rent emphasis on developing total maximum daily loads tmdls for impaired waters in the united states there is heightened interest in quantifying background sediment levels in rivers in this study we assessed variations in river bank erosion over time using a combination of 1855 public land survey system plats aerial photographs from 1938 to 2009 and light detection and ranging lidar data from 2005 to 2009 for sediment impaired rivers in southern minnesota results showed that bank erosion was episodic making comparisons of erosion rates from dissimilar time intervals unreliable for comparable time intervals average river bank retreat rates 0 51 m yr 1 from 1855 to 1938 vs 0 37 m yr 1 from 1938 to 2009 were statistically similar t 2 13 p 0 14 suggesting that bank erosion rates have remained stable since european settlement comparisons over shorter time intervals of 1938 1971 and 1971 2009 also showed similar statistical trends t 0 76 p 0 45 with average river bank retreat rates of 0 57 and 0 50 m yr 1 respectively however additional 145 observations of bank retreat were found in the period 1971 2009 relative to 1938 1971 in dicating that the number of actively eroding river banks may have increased over time contrary to assump tions made in the literature bank erosion measurements using lidar data showed a poor relationship r 2

twentieth century warming of the tropical atlantic captured by sr u paleothermometry coral skeletons are valuable archives of past ocean conditions however interpretation of coral paleotemperature records is confounded by uncertainties associated with single element ratio thermometers including sr ca a new approach sr u uses u ca to constrain the influence of rayleigh fractionation on sr ca decarlo et al 2016 here we build on the initial pacific porites sr u calibration to include multiple atlantic and pacific coral genera from multiple coral reef locations spanning a temperature range of 23 15 30 12 c accounting for the wintertime growth cessation of one bermuda coral we show that sr u is strongly correlated with the average water temperature at each location r2 0 91 p 0 001 n 19 we applied the multi species spatial calibration between sr u and temperature to reconstruct a 96 year long temperature record at mona island puerto rico using a coral not included in the calibration average sr u derived temperature for the period 1900 1996 is within 0 12 c of the average instrumental temperature at this site and captures the 20th century warming trend of 0 06 c per decade sr u also captures the timing of multi year variability but with higher amplitude than implied by the instrumental data mean sr u temperatures and patterns of multi year variability were replicated in a second coral in the same grid box conversely sr ca records from the same two corals were inconsistent with each other and failed to capture absolute sea temperatures timing of multi year variability or the 20th century warming trend our results suggest that coral sr u paleothermometry is a promising new tool for reconstruction of past ocean temperatures

reply to comments by carlson et al 2009 on radiocarbon deglaciation chronology of the thunder bay ontario area and implications for ice sheet retreat patterns

evidence of cordilleran late wisconsinan glaciers in the ice free corridor abstract glaciers flowing down the athabasca river valley during the last glacial period were fed both by local montane glaciers and by cordilleran ice moving easterly across the continental divide from british columbia resulting in a major eastward surge into the ice free corridor the timing of this event has been disputed for many years and has important implications for the study of paleoenvironmental conditions and the north south migration of biota in the corridor chronologic and lithostratigraphic data from the jasper region suggest that this event took place in the late wisconsinan subtill radiocarbon dates on wood in the athabasca river valley indicate that non glacial alluvial sediments began accumulating there sometime before 48 ka a finite date of 29 100 560 bp gsc 3792 on wood under till near jasper provides a maximum age for the onset of the last glaciation in the region although ice from west of the rocky mountain trench probably did not cross the continental divide until after 21 5 ka the occurrence of supraglacial metamorphic erratics as well as the distribution and nature of mapped glacial deposits and the trend of subglacial paleoflow indicators in the region east of the rockies suggests that late wisconsinan glaciers flowing from the mountains coalesced with laurentide glaciers and were deflected southeasterly along the mountain front an even more extensive glacial event may have effected the region in pre late wisconsinan times the model of late wisconsinan glaciation presented here is in agreement with all known radiocarbon data from the area and is especially attractive when supratill dates on non woody lacustrine organics are viewed in light of possible contamination by old carbon

correlation of glacial deposits of the huron lake michigan and green bay lobes in michigan and wisconsin michigan and eastern wisconsin were glaciated several times during the pleistocene ice erosion overdeepened river valleys in the weaker units of the sequence of sedimentary rocks now exposed as a result of structural basining glacial erosion led to development of the three relatively deep bedrock basins now occupied by lakes huron michigan and superior as a result whenever an ice margin crossed the area it had a lobate form with ice tongues extending relatively far south in the bedrock basins while much of the area between the lobes was ice free lobe margins within the basins terminated in proglacial lakes the ice lobes of importance in the area are a the huron lobe and its saginaw sublobe b the lake michigan lobe and c the green bay lobe fig 1 the granulometry and mineralogy of the drift related to each lobe reflect the nature of both the bedrock and older surficial units upice in much of the area the glacially related deposits are over 30 m thick and in some places the thickness is more than 300 m the surface deposits nearly everywhere are late wisconsin in age and relatively little is known of the glacial history of the area prior to that time formal names have not been established for lithostratigraphic units in michigan and most units in wisconsin have only recently been formally defined mickelson et al 1984

ostracod faunas in a large late pleistocene ice dammed lake southern ontario canada abstract in the lake ontario basin of mid continent north america near toronto canada a low diversity micro crustacean ostracod community dominated by candona caudata and candona subtriangulata occurs in several poorly sorted pebbly mud diamicts units sunnybrook seminary and meadowcliffe tills the diamicts were deposited in an ancestral ice dammed lake ontario with a level at least 90 m above the modern lake during the early and middle phases of the wisconsin glaciation 25 000 75 000 yr b p poorly sorted sediments accumulated by ice rafting and mud deposition from meltwater plumes and are separated by shallower water deltaic facies comparison of the modern habitat ranges of ostracods specifically their water depth temperature and dissolved oxygen preferences provides important palaeolimnological data regarding late pleistocene ice dammed lakes

stratigraphy and correlation of the glacial deposits of the des moines and james lobes and adjacent areas in north dakota south dakota minnesota and iowa the glacial deposits of the des moines and james lobes record the extension of late wisconsin laurentide ice from canada into minnesota and iowa and into north dakota and south dakota respectively fig 1 chart 1 d e in this review we also discuss wisconsin age deposits of adjacent parts of north dakota and pre wisconsin deposits in north dakota south dakota and southwestern minnesota chart 1 b c e

age of the frenchman valley and associated drift south of the cypress hills saskatchewan canada frenchman valley extends about 340 km from the cypress hills saskatchewan to its confluence with the milk river in montana in one area the valley is 3 km wide and it was 180 m deep prior to deposition of as much as 80 m of landslide debris channel sand and gravel and alluvial and colluvial deposits six radiocarbon ages ranging from 3440 to 11 460 bp suggest the frenchman valley is wisconsinan the frenchman valley could not have formed in its side hill position if late wisconsinan ice had not been present south at the cypress hills

late wisconsinan deglaciation of alberta processes and paleogeography abstract the scarcity of lake basins in alberta dating earlier than 11 000 bp has been interpreted as indicating the continued presence of active glacial ice because of the related implication that the presence of ice precludes the existence of an ice free corridor it is useful to examine this issue more closely due to the effects of deglaciation many areas of alberta were dominated by chaotic ice stagnation conditions with continual reversal of topography and rapid transformation of the surface the dynamic nature of the environment affects arguments regarding the existence of an ice free corridor as well as having implications for archeological site formation preservation and discovery deglaciation models utilizing paradigms of active ice retreat may result in dichotomous depictions of the land surface as either ice covered or deglaciated due to the insulating properties of supraglacial debris stagnant ice results in a situation that is simultaneously neither and both of the above conditions while the landscape is ice covered and thus not ice free an accessible landscape characterized by glacial karst topography exists on top of the stagnating ice surface general reconstructions of the impact of stagnant ice and glacial karst topography on the paleogeography of alberta are presented for discussion

holocene eolian activity in the minot dune field north dakota stabilized eolian sand is common over much of the great plains region of the united states and canada including a subhumid area of 1500 km2 near minot north dakota eolian landforms consist of sand sheets and northwest trending parabolic dunes dunes and sand sheets in the minot field are presently stabilized by a cover of prairie grasses or oak woodland stratigraphic studies and accelerator mass spectrometry radiocarbon dating of paleosols indicate at least two periods of eolian sand movement in the late holocene pedologic data suggest that all of the dune field has experienced late holocene dune activity though not all parts of the dune field may have been active simultaneously similar immobile element ti zr la ce concentrations support the interpretation that eolian sands are derived from local glaciofluvial and glaciolacustrine sediments however glaciolacustrine and glaciofluvial source sediments have high ca concentrations from carbonate minerals whereas dune sands are depleted in ca

application of till pebble lithology to the differentiation of glacial lobes in southern alberta variations in the pebble composition of surface till in southern alberta indicate the presence of three contemporaneous lobes in the last continental glacier the ice lobe domains have similar pebble associations and differ only in the frequency of occurrence of rock constituents a large part of the percentage variability within each domain is caused by irregular entrainment of locally derived stones which dilutes pebble contents and results in a lithology substantially different from the provenance composition the study has shown that the regional trends are preserved and can be recognized in the locally modified lithologies but the latter have to be isolated and evaluated differently from the regular provenance lithologies the common diluting agent in the area is fluvial gravel that is widespread on erosional bedrock surfaces and in preglacial valleys because the gravel is generally coarse pebble fractions of larger size are more affected by local dilution than are successively finer fractions q

unstable behavior of the laurentide ice sheet over deforming sediment and its implications for climate change geologic records of fluctuations of the laurentide ice sheet margin following the most recent glacial maximum ca 20 000 14c yr b p identify fundamental differences in ice sheet behavior depending on subglacial bed conditions rapid and irregular icemargin fluctuations occurred only over areas of deforming sediment indicating nonclimatic forcing controlled by the inherent instability of coupled ice sheet deforming sediment dynamics in contrast largely uninterrupted ice margin retreat with no evidence of significant readvance occurred over rigid bed areas indicating stable behavior unstable ice sheet behavior was most pronounced from 15 000 until 10 000 14c yr b p by which time most of the ice margin had retreated onto a rigid bed unstable ice sheet behavior would have been an integral component in controlling variable fluxes of icebergs and meltwater as well as meltwater routing to the north atlantic thus affecting thermohaline circulation the abrupt climate oscillations in the north atlantic region that ended at 10 000 14c yr b p may thus have their origin in the inherently unstable behavior of the laurentide ice sheet overriding deforming sediment

origin of the first global meltwater pulse following the last glacial maximum well dated sea level records show that the glacioeustatic rise following the last glacial maximum was characterized by two or possibly three brief intervals of rapid sea level rise separating periods with much lower rates these very high rates of sea level rise indicate periods of exceptionally rapid deglaciation of remaining ice sheets the laurentide ice sheet is commonly targeted as the source of the first and largest of the meltwater pulses mwp ia between 14 200 12 200 14c years b p and 13 700 years ago 11 700 14c years b p in all oceanic records of deglaciation of the former northern hemisphere ice sheets that we review only those from the gulf of mexico and the bermuda rise show evidence of low 18o values at the time of mwp ia identifying the southern laurentide ice sheet as a potential source for mwp ia we question this source for mwp ia however because 1 ice sheet models suggest that this sector of the ice sheet contributed only a fraction 10 of the sea level needed for mwp ia 2 melting this sector of the ice sheet at the necessary rate to explain mwp ia is physically implausible and 3 ocean models predict a much stronger thermohaline response to the inferred freshwater pulse out of the mississippi river into the north atlantic than is recorded this leaves the antarctic ice sheet as the only other ice sheet capable of delivering enough sea level to explain mwp ia but there are currently no well dated high resolution records to document this hypothesis these conclusions suggest that reconstructions of the laurentide ice sheet in the ice 4g model which are constrained to match the sea level record may be too low for time periods younger than 15 000 years ago furthermore 18o records from the gulf of mexico show variable fluxes of meltwater from the southern margin of the laurentide ice sheet which can be traced to the opening and closing of eastward draining glacial lake outlets associated with surging ice sheet behavior these variable fluxes through eastern outlets were apparently sufficient to affect formation of north atlantic deep water thus underscoring the sensitivity of this process to changes in freshwater forcing

comments on palaeoecology and palaeogeography near cochrane alberta canada just after the last major high stand of glacial lake calgary

landscape evolution valley excavation and terrace development following abrupt postglacial base level fall many high latitude fluvial systems are adjusting to base level changes since the last glaciation channels that experienced base level fall may still be incising often through glacial diamictons tills these tills can be quite competent behaving more like weak bedrock than unconsolidated sediment and erode at a fast pace thus providing a unique opportunity to test models of channel incision and knickpoint migration in transient systems here we integrate light detection and ranging lidar topography strath terrace chronology and numerical modeling to determine knickpoint migration and incision history of the le sueur river in central minnesota usa results indicate that the le sueur river is best modeled as a detachment limited channel with downstream coarsening related to lag clasts from tills playing a critical factor in longitudinal profile development the le sueur river meanders as it incises so we coupled the best fit incision model to a meander model to determine valley excavation history the excavation history was used to determine a natural background erosion rate prior to land use changes associated with settlement and agricultural expansion in the mid 1800s we compared background fine sediment silt and clay erosion rates with historic decadal average annual suspended loads results show that modern fine sediment contributions from sources associated with valley excavation are three times higher than modeled presettlement loads recent changes in hydrology associated with land use and climate change have increased flows in rivers leading to higher sediment loads not just from field erosion but from increased bank and bluff erosion in the deeply incised valleys

landform signature of the laurentide and cordilleran ice sheets across alberta during the last glaciation government geological survey maps and research publications have portrayed the distribution of glacial landforms associated with the advance and retreat of the laurentide and cordilleran ice sheets across alberta at a local regional and continental scale to date this information has not been systematically synthesized into a single compilation at a consistent scale although this original work provided valuable information to constrain reconstructions of former ice sheet extent configuration and flow geometry its derivation primarily from the interpretation of aerial photographs and the shuttle radar topography mission 90 m digital elevation model dem may result in methodological inconsistencies and spatial biases these biases together with challenges associated with geomorphic mapping in densely forested areas of western and northern alberta limit the usefulness of previous mapping when applied to inversion based ice sheet reconstructions which have specific input data demands recently ligh

multi phase evolution of gnammas weathering pits in a holocene deglacial granite landscape minnesota usa the morphometry of 85 gnammas weathering pits from big stone county in western minnesota allows the assessment of the relative ages of the gnamma population the ratio between maximum and minimum depths is independent of the initial size of the cavity and only depends on the weathering evolution therefore the distribution of depth ratios can be used to assess the gnamma population age and the history of weathering the asymmetrical distribution of depth ratios measured in big stone county forms three distinct populations when these sets are analyzed independently the correlation r2 between maximum and minimum depths is greater than 0 95 each single population has a normal distribution of depth ratios and the average depth ratios value for each population are 1 1 60 0 05 2 2 09 0 04 and 3 2 42 0 08 the initiation of gnamma formation followed the exhumation of the granite in the region this granite was till and saprolite covered upon retreat of the ice from the last glacial maximum nearby outcrops are striated but the study site remained buried until it was exhumed by paleofloods issuing from a proglacial lake these holocene aged gnammas in western minnesota were compared with gnammas of other ages from around the world our new results are in accordance with the hypothesis that values represent the evolution of gnammas with time under temperate to cold climate dynamics phases of the formation of new gnammas may result from changes in weathering processes related to climate changes copyright 2007 john wiley sons ltd

deglaciation chronology and revegetation in northwestern ontario along a 420 km transect in northwestern ontario canada sediments from four lakes were analyzed with respect to lithology pollen and macrofossils radiocarbon dates show that the region was deglaciated between ca 11 500 and 8000 years bp and periods of both rapid ice retreat and readvance influenced the history of glacial lake agassiz in the south the ice sheet was succeeded by a lengthy interval of park tundra with stands of spruce ash and elm the ash and elm seem to have disappeared during a suggested cool period 11 100 10 200 years bp farther north the park tundra phase lasted not more than 50 100 years after ca 10 200 years bp before boreal trees dominated the climatic change around 10 200 years bp permitted the very rapid migration of spruce larch birch and jack or red pine into northwestern ontario from northern minnesota the migration routes for pinus strobus white pine alnus rugosa and a crispa were divided however one from the south south of lake superior and one from t

the use of paleomagnetic curves for chrono stratigraphic correlation of late quaternary fluvial estuarine and lacustrine stratigraphic sequences in nipigon bay lake superior ontario canada abstract a study was carried out in nipigon bay lake superior to evaluate the use of paleomagnetic curves in correlating late quaternary stratigraphic sequences from a variety of sedimentary environments within a limited geographic area oscillations of the paleodeclination and paleoinclination curves result from secular variations of the earth s magnetic field with time so any correlation horizons picked on the character of either of these curves could be used for chrono stratigraphic correlation from one core to another within nipigon bay high resolution paleodeclination and paleoinclination curves were compiled for cores taken from six fluvial estuarine and lacustrine sites both the paleodeclination and the paleoinclination curves show marginal correlation for cores taken from within a specific sedimentary environment and reasonable correlation for cores taken from different sedimentary environments paleomagnetic curves can be used for chrono stratigraphic correlation and in addition show considerable promise as a method for absolute age dating of late quaternary stratigraphic sequences

modeling the complexity of different recently deglaciated soil landscapes as a function of map scale the scale at which a soil landscape soilscape is viewed has a significant impact on soil pattern and interpretations made from those patterns recently deglaciated soilscapes are particularly spatially complex in order to understand how scale impacts pattern on complex soilscapes we used a gis to examine soil maps for 13 counties in the northern united states all affected by late wisconsinan glaciation we used an arck macro language script to change the map scale and when the change was to a smaller scale group dissolve soil map units based on similarities to a prescribed list of neighboring map unit characteristics similarity criteria included drainage class taxonomic great group parent material and slope soilscape complexity was measured at nine different scales and is based on various pattern metrics number of punctate soil units km 2 map unit polygons km 2 map unit boundary length km 2 and boundary length polygon 1 km 2 soilscape complexity as a function of scale was then examined by regressing pattern metric data against the size of the minimum map unit for each of the nine scales extrapolation of the regression lines to 1 10 000 a scale larger than is typically mapped illustrated how much additional information might accrue if these counties were to be mapped at that larger scale in most cases 2 10 times more map units would have been recognized and delineated at the two times larger map scale but map unit boundary lengths would have increased by only about 1 5 times whether this additional information is of such a magnitude that it could justify remapping some of these complex landscapes at larger scales is an economic decision our study provides much needed data on the magnitude of information gained by mapping soilscapes at larger scales d 2004 elsevier b v all rights reserved

late wisconsin landform distribution and glacier bed conditions in wisconsin abstract the late wisconsin laurentide ice sheet advanced across permafrost and reached its maximum extent in wisconsin between about 18 000 and 15 000 years ago deep permafrost persisted in southern wisconsin until about 14 000 years ago and in northern wisconsin until about 13 000 years ago we suggest that during maximum glaciation a zone about 5 km wide in the south and 20 km wide in the north along the margin of the late wisconsin glacier was frozen to its bed meltwater from farther behind the margin where the bed was at least locally thawed cut a series of closely spaced tunnel channels through the frozen bed zone these channels most likely formed episodically and they were the source for much of the meltwater stream sediment deposited in broad outwash plains beyond the ice margin frozen bed conditions near the margin also likely contributed to increased upward shearing of sediment and the accumulation of thicl supraglacial sediment in northern areas up ice from the frozen bed zone the glacier bed was at least locally thawed in a zone about 75 km wide extensive drumlin fields formed in the area of the bed that was thawed by about 13 000 years ago permafrost melted in northern wisconsin and thawed bed conditions probably extended to the ice margin throughout wisconsin and adjacent areas after about 13 000 years ago in northern wisconsin the glacier was sliding on its bed and forming drumlins out to the ice margin and thick supraglacial sediment no longer accumulated

interplay between spatially explicit sediment sourcing hierarchical river network structure and in channel bed material sediment transport and storage dynamics understanding how sediment moves along source to sink pathways through watersheds from hillslopes to channels and in and out of floodplains is a fundamental problem in geomorphology we contribute to advancing this understanding by modeling the transport and in channel storage dynamics of bed material sediment on a river network over a 600 year time period specifically we present spatiotemporal changes in bed sediment thickness along an entire river network to elucidate how river networks organize and process sediment supply we apply our model to sand transport in the agricultural greater blue earth river basin in minnesota by casting the arrival of sediment to links of the network as a poisson process we derive analytically under supply limited conditions the time averaged probability distribution function of bed sediment thickness for each link of the river network for any spatial distribution of inputs under transport limited conditions the analytical assumptions of the poisson arrival process are violated due to in channel storage dynamics where we find large fluctuations and periodicity in the time series of bed sediment thickness the time series of bed sediment thickness is the result of dynamics on a network in propagating altering and amalgamating sediment inputs in sometimes unexpected ways one key insight gleaned from the model is that there can be a small fraction of reaches with relatively low transport capacity within a nonequilibrium river network acting as bottlenecks that control sediment to downstream reaches whereby fluctuations in bed elevation can dissociate from signals in sediment supply

near channel versus watershed controls on sediment rating curves predicting riverine suspended sediment flux is a fundamental problem in geomorphology with important implications for water quality land and water resource management and aquatic ecosystem health to advance understanding we evaluated environmental and landscape factors that influence sediment rating curves srcs we generated srcs with recent total suspended solids tsss and discharge data from 45 gages on 36 rivers throughout the state of minnesota usa watersheds range from 32 to 14 600 km2 and represent distinct settings regarding topography land cover and geologic history rivers exhibited three distinct src shapes simple power functions threshold power functions and peaked or negative slope functions we computed src exponents and coefficients describing the steepness of the relation and the tss concentration at median flows respectively in addition to quantifying watershed topography climate hydrology geology soil type and land cover we used lidar topography to characterize the near channel environment upstream of gages we used random forest models to analyze relations between src parameters and attributes of the watershed and the near channel environment the models correctly classify 78 of src shapes and explain 37 60 of variance in src parameters we find that src steepness exponent is strongly related to near channel morphological characteristics including near channel relief channel gradient and presence of lakes along the local channel network but not to land use in contrast land use influences tss concentrations at moderate and low flow these findings suggest that the near channel environment controls changes in tss as flows increase whereas land use drives median and low flow tss conditions

a mammoth mammuthus primigenius tooth from late wisconsin deposits near embden north dakota and comments on the distribution of woolly mammoths south of the wisconsin ice sheets a well preserved third molar of a woolly mammoth mammuthus primigenius was recovered from sand and gravel forming the highest herman prominent strandline of lake agassiz near embden in western cass county north dakota the herman strandline is estimated to have formed about 11 500 years bp and presumably the tooth is of similar age perhaps the animal lived in a tundra like area near the lake agassiz shoreline additional evidence suggests that woolly mammoths occupied a tundra like range south of the wisconsin ice sheets extending from southern british columbia to the atlantic continental shelf off virginia

the impact of glacial lake runoff on the goldthwait and champlain seas the relationship between glacial lake agassiz runoff and the younger dryas abstract the freshwater flux to the north atlantic ocean during the last deglaciation included runoff meltwater plus precipitation from glacial lake st lawrence ca 11 6 ka bp from glacial lake agassiz 10 9 9 9 and 9 5 8 0 ka bp and from glacial lake barlow ojibway 9 5 8 0 ka bp runoff from the glacial lakes does not appear to have mixed with the deep water of the goldthwait sea in the gulf of st lawrence and was part of the surface outflow to the north atlantic ocean radiocarbon dated invertebrate faunal assemblages show that the major impact of lake agassiz runoff to the north atlantic ocean during the 10 9 to 9 9 ka bp interval occurred after 10 5 ka bp resulting in the freshening of the champlain sea the younger dryas cold episode on the other hand began about 11 0 ka bp the discordance between the major impact of lake agassiz runoff on the champlain sea and the beginning of the cold episode indicates that lake agassiz runoff did not trigger the younger dryas cooling however the runoff from glacial lake agassiz may have sustained the cold climate during the latter part of the younger dryas cold episode