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

widespread middle holocene dune formation in the eastern upper peninsula of michigan and the relationship to climate and outlet controlled lake level dating of five widely spaced noncoastal dunes in eastern upper michigan by optically stimulated luminescence of quartz establishes that eolian sand was last mobilized between ca 7 and 5 5 ka in the region although this interval corresponds to the drier altithermal hypsithermal period climate alone may not have been sufficiently arid to cause dune formation in this area instead it appears that dune formation may also be linked to depressed water tables associated with outlet controlled low levels in lakes michigan and superior following dune stabilization peat began to accumulate in interdune areas ca 4 4 ka

evaluation of simple geochemical indicators of aeolian sand provenance late quaternary dune fields of north america revisited abstract dune fields of quaternary age occupy large areas of the world s arid and semiarid regions despite this there has been surprisingly little work done on understanding dune sediment provenance in part because many techniques are time consuming prone to operator error experimental highly specialized expensive or require sophisticated instrumentation provenance of dune sand using k rb and k ba values in k feldspar in aeolian sands of the arid and semiarid regions of north america is tested here results indicate that k rb and k ba can distinguish different river sands that are sediment sources for dunes and dune fields themselves have distinctive k rb and k ba compositions over the basin and range and great plains regions of north america the hypothesized sediment sources of dune fields are reviewed and assessed using k rb and k ba values in dune sands and in hypothesized source sediments in some cases the origins of dunes assessed in this manner are consistent with previous studies and in others dune fields are found to have a more complex origin than previously thought use of k rb and k ba for provenance studies is a robust method that is inexpensive rapid and highly reproducible it exploits one of the most common minerals found in dune sand k feldspar the method avoids the problem of using simple concentrations of key elements that may be subject to interpretative bias due to changes in mineralogical maturity of quaternary dune fields that occur over time

asexual reproduction of tubular fossils in the terminal neoproterozoic dengying formation south china abstract tubular fossils are an abundant and diverse component of late ediacaran benthic communities however with rare exceptions their development and modes of asexualreproduction remain poorly resolved with the deeper research and more specimens recovered the types of asexual reproduction of tubular fossils are clarified three dimensionally phosphatized tubes from the ediacaran dengying formation in lijiagou southern shaanxi province china provide a window into the reproduction of late ediacaran tubular fossils in this paper we document two different styles of asexual reproduction associated with two morphotypes of the biomineralized tubular fossil and propose upper ediacaran index fossil cloudina type a with bud from the aperture of parent tube and type b with new bud from the lateral of parent tube type c with two similar diameter individual tubes and the new genus multiconotubus type e and type f with individual tubes but living in the parent tube what is the most important is that based on the internal structure of dichotomous multiconotubus and observing means of x ray micro tomography we can see clearly the internal structure and we propose a new growth strategy for this organism in this paper we emphasize the asexual budding reproduction and the multiple common wall reproduction

little ice age aridity in the north american great plains a high resolution reconstruction of salinity fluctuations from devils lake north dakota usa the little ice age was an interval between about ad 1500 and 1850 characterized by advancing glaciers in mountainous regions of europe and western north america however it is unclear whether this cool moist period was truly global in extent or how it was manifested in other regions with different climatic controls a high resolution reconstruction of salinity fluctuations in devils lake north dakota based on fossil diatoms ostracode shell geochemistry and bulk carbonate geochemistry indicates that saline conditions prevailed throughout much of the recent past these results suggest an arid climate in the northern great plains throughout the little ice age and that during this interval climatic gradients between the great plains and regions to both the east and west may have been quite steep

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

predatorial borings in late precambrian mineralized exoskeletons the late precambrian tube forming cloudina the earliest known animal to produce a mineralized exoskeleton shows evidence of having been attacked by shell boring organisms of more than 500 tubes from shaanxi province china 2 7 have rounded holes 40 to 400 micrometers in diameter the relation between the size of the holes and the width of the bored tubes suggests that the attacking organism was a predator selecting its prey for size if true this would be the oldest case of predation in the fossil record and would support the hypothesis that selection pressures from predation was a significant factor in the evolution of animal skeletons around the precambrian cambrian boundary

radiocarbon dating of soil organic matter radiocarbon ages of soil organic matter are evaluated with a model which incorporates the dynamics of the14c content of soil organic matter measured14c ages of soil organic matter or its fractions are always younger than the true ages of soils due to continuous input of organic matter into soils differences in soil c dynamics due to climate or soil depth will result in significantly different14c signatures of soil organic matter for soils of the same age as a result the deviation of the measured14c age from the true age of soil formation could differ significantly among different soils or soil horizons our model calculations also suggest that14c ages of soil organic matter will eventually reach a steady state provided that no climatic or ecological perturbations occur once a soil or a soil horizon has reached a steady state 14c dating of soil organic matter will provide no useful information regarding the age of the soil however for soils in which steady state has not been reached it is possible to estimate the age of soil formation by modeling the measured14c contents of soil organic matter radiocarbon dating of buried soils could in general overestimate the true age of the burial by as much as the steady state age of the soil or soil horizon

late holocene eolian activity in the mineralogically mature nebraska sand hills abstract the age of sand dunes in the nebraska sand hills has been controversial with some investigators suggesting a full glacial age and others suggesting that they were last active in the late holocene new accelerator mass spectrometry radiocarbon ages of unaltered bison bones and organic rich sediments suggest that eolian sand deposition occurred at least twice in the past 300014c yr b p in three widely separated localities and as many as three times in the past 80014c yr at three other localities these late holocene episodes of eolian activity are probably the result of droughts more intense than the 1930s dust bowl period based on independent great plains climate records from lake sediments and tree rings however new geochemical data indicate that the nebraska sand hills are mineralogically mature eolian sands in nebraska have lower k feldspar and k2o rb and ba contents than most possible source sediments and lower k feldspar contents than dunes of similar age in colorado the most likely explanation for mineralogical maturity is reduction of sand sized k feldspar to silt sized particles via ballistic impacts due to strong winds over many cycles of eolian activity therefore dunes of the nebraska sand hills must have had a long history probably extending over more than one glacial interglacial cycle and the potential for reactivation is high with or without a future greenhouse warming

spatial and temporal patterns of late quaternary eolian deposition eastern colorado u s a abstract eolian sediment covers about 60 of colorado east of the rocky mountains about 30 of the sediment is sand and 70 is loess initially flood plains were the principal sources of eolian sediment but during the holocene dunes formed from older eolian sand and alluvium on uplands since latest pleistocene time dominant dune forming winds have been northwesterly in the northern part of the region and southwesterly in the southern part at present sand sheets and dunes mainly parabolic types are stable and covered with vegetation in dunes sand is commonly 20 30 m thick but elsewhere averages

regional aridity in north america during the middle holocene increased aridity throughout the great plains and rocky mountain region during the middle holo cene has been documented from pollen records aeolian proxy variables in lake cores and active sand dune migration varve calibration provided by a continuously varved record of the holocene from a core from elk lake northwestern minnesota shows that the influx of aeolian clastic material increased beginning about 8 ka and ended about 3 8 ka with peak aeolian activity at about 6 ka if aeolian influx to elk lake corresponds in time to aeolian influx in other lakes and to maximum dune activity in minnesota dune fields then the varve calibration in elk lake provides precise time calibration of periods of peak aeolian activity in minnesota palaeowind studies from the minnesota dune fields show that the dominant wind direction when the dunes were active was from the northwest the same as the dominant wind direction in dune fields throughout the great plains and rocky mountains if the mid holocene aeolian act

aeolian sand depositional records from western nebraska landscape response to droughts in the past 1500 years the great plains is dominated by presently stabilized dune fields that are indicators of extreme drought in the late holocene this study focused on deciphering the timing of reactivation of dunes in western nebraska stratigraphy adjacent to dune dammed lakes reveals aeolian sand separated by palaeosols indicating mobilization of aeolian sand followed by landscape stability the chronology of aeolian sand depositional events is constrained using the luminescence based single aliquot regeneration method providing resolution to relate dune movement to tree ring and palaeolimnologic records of drought there are at least a six aeolian depositional events in the past 1500 years with apparent mean ages of 1390 130 670 70 470 40 240 40 140 20 and 70 10 yr all study sites show evidence for aeolian accumulation in the twentieth century potentially reflecting the 1930s drought significant aeolian activity is coincident with the tree ring identified sixteenth century megadrought indicating widespread landscape impacts

dune dammed paleovalleys of the nebraska sand hills intrinsic versus climatic controls on the accumulation of lake and marsh sediments although running water is the dominant geomorphic agent on earth eolian processes can gain ascendancy in regions where the climate is arid vegetation is sparse and abundant sand is available for transport with climate change the boundaries between fluvial dominated and eolian dominated areas may shift although there have been few reports in the north american literature of river systems blocked by dune sand our work in the nebraska sand hills provides evidence of multiple episodes of such blockage events during prolonged arid intervals in latest pleistocene and middle holocene time eolian dune sand blocked two large valley systems in western nebraska these blockages raised the water table of the high plains aquifer as much as 25 m over an area of 7000 km2 and created over one thousand lakes wetlands far removed from the discharge points of the buried paleovalley system are strongly alkaline exceeding 250 000 mg l total dissolved solids tds relatively fresh 280 mg l tds flow through lakes are present at the distal end of the system where the gradient of the water table is steep and the cross section of the buried valley is large anomalously thick marsh and lake sediments accumulated in deep paleovalleys upstream of dune dams near the southern margin of the sand hills our cores and radiocarbon dates from blue and crescent lakes reveal their histories to be quite distinct from adjacent swan lake these differences are best explained by multiple blockage events our work explains why lakes are most abundant in the driest part of the sand hills it also provides another line of evidence for major dune activity in the sand hills region during holocene time and shows that factors other than regional climate specifically location height and hydraulic conductivity of dune dams can control the rise and fall of the ground water table and the chemistry of lakes

stratigraphy and geochronology of upper quaternary eolian sand on the southern high plains of texas and new mexico united states eolian sand in dune fields and sand sheets cover 10 000 km 2 10 of the southern high plains of northwestern texas and eastern new mexico these deposits are concentrated in three west east trending belts of dunes the muleshoe lea yoakum and andrews dune fields from north to south that appear to be eastern extensions of the mescalero monahans dune system in the pecos river valley and in the seminole sand sheet a discontinuous accumulation of sheet sands between the lea yoakum and monahans andrews dunes the most common landforms are parabolic dunes associated with blowouts coppice dunes and sand sheets all typical of sandy vegetated semiarid landscapes barchan dunes in keeping with a relatively limited sand supply and an underlying surface that is relatively hard blackwater draw formation pleistocene and fence row dunes historic dunes formed along field boundaries these eolian deposits accumulated episodically in the late pleistocene and holocene and provide clues to the history of regional drought and aridity the earliest phase of sedimentation occurred when sheet sands were deposited between 11 000 and 8000 14 c yr b p probably in several phases based on archaeological data as a result of episodic drought beginning between 11 000 and 10 000 yr b p eolian deposits dating between 8000 and 3000 yr b p are rare although eolian sediment 8000 4500 14 c yr b p is ubiquitous in the draws that cross the region and paleoenvironmental indicators show that the region was subjected to aridity throughout middle holocene time the middle holocene deposits most likely were remobilized in late holocene time most of the deposits and landforms of the dune fields and sand sheets are late holocene dating before 4000 14 c yr b p and mostly before 1500 14 c yr b p the muleshoe and lea yoakum dunes and the seminole sand sheet underwent substantial eastward expansion in late holocene time buried soils and radiocarbon ages show that the eolian sand accumulated in several stages probably in response to cyclic drought the muleshoe dunes accumulated after ca 1300 cal yr b p ca 1400 14 c yr b p after ca 750 670 cal yr b p ca 850 750 14 c yr b p just after ca 500 cal yr b p ca 450 14 c yr b p and during the nineteenth and twentieth centuries the lea yoakum dunes were active after ca 3400 cal yr b p ca 3200 14 c yr b p and historically the seminole sand sheet was active ca 430 330 cal yr b p ca 360 14 c yr b p and in the twentieth century the andrews dunes were subjected to at least two phases of eolian sedimentation after ca 2320 cal yr b p ca 2320 14 c yr b p comparisons with eolian chronologies from other regions on the great plains suggest that dune mobilization was a regional phenomenon after ca 2300 cal yr b p ca 2300 14 c yr b p after ca 1500 1400 cal yr b p ca 1650 1550 14 c yr b p after ca 700 cal yr b p ca 800 14 c yr b p between 500 and 300 cal yr b p ca 450 300 14 c yr b p and in the nineteenth century the climatic fluctuations responsible for mobilizing the dunes probably were relatively minor yet the landscape impacts were substantial resulting in widespread wind erosion and dune construction

late 18th century drought induced sand dune activity great sand hills saskatchewan geomorphic evidence and optical ages from seven locations indicate that widespread dune activity occurred within the last 200 years in the great sand hills region of southwestern saskatchewan optical ages n 36 define an interval of dune activity bracketed by the earliest age of back ridges in the seward sand hills 185 8 years and the average age of stabilized dune heads about 105 years during this interval parabolic dunes were active in all areas studied these ages indicate that the most recent interval of activity was initiated about ad 1800 and continued at a level higher than present for approximately 80 years the most likely cause of dune activation was lower than average precipitation relative to 1960 1991 values through the 1700s culminating in drought in the late 1700s as evidenced in dendroclimatic records from the cypress hills and from the rocky mountain foothills dunes affected by such climatically induced regional activity require many decades to restabilize historical ob

modern and late holocene wind regimes over the great plains central u s a the great plains of the central united states contain numerous presently stabilized dune fields that were active during the late holocene the most recent period of reactivation occurred about 700 1000 years ago during the medieval climatic anomaly a period of frequent severe drought recognized throughout the north american west dune orientations and internal bedding preserve information about paleowinds from this time period we compared paleowind information with modern winds and potential sand transport analyzed from meteorological data while both reconstructed paleowinds and modern winds include a strong northwesterly component over the northern to central portions of the great plains a significant difference exists to the south during the late holocene southwesterly winds had a strong influence on dune forms over the central and southern great plains while modern potential sand transport is dominated by southerly to southeasterly winds that bring moisture to the region from the gulf of mexico hypothetical dune types forming under modern winds would be different and have different crestal orientations than those preserved from the late holocene in order for such a shift in southerly winds to take place it is likely that 1 the typical position of the bermuda high in summer was shifted either to the east or to the south resulting in weaker low level winds from the south and southeast and greater extent of southwesterly winds on the great plains and or 2 drought likely enhanced by evapotranspiration and soil moisture feedbacks brought the normally higher level southwesterly flow down to the land surface given our understanding of megadroughts over the great plains as well as recent climate predictions for increased warmth and drought over this area future reactivation of the dunes seems likely

geochemical evidence for an eolian sand dam across the north and south platte rivers in nebraska abstract geochemical and geomorphic data from dune fields in southwestern nebraska provide new evidence that the nebraska sand hills once migrated across the north and south platte rivers and dammed the largest tributary system to the missouri river the lincoln county and imperial dune fields which lie downwind of the south platte river have compositions intermediate between the nebraska sand hills quartz rich and northeastern colorado dunes k feldspar rich the most likely explanation for the intermediate composition is that the lincoln county and imperial dunes are derived in part from the nebraska sand hills and in part from the south platte river the only mechanism by which the nebraska sand hills could have migrated this far south is by complete infilling of what were probably perennially dry north platte and south platte river valleys such a series of events would have required an extended drought both for activation of eolian sand and decreased discharges in the platte river system a nearby major tributary of the north platte river is postulated to have been blocked by eolian sand about 12 000 14c yr b p we propose that an eolian sand dam across the plattes was constructed at about this same time

holocene development of parabolic dunes in the central st lawrence lowland quebec abstract stabilized parabolic dunes in the central st lawrence lowland are oriented ne sw in the postulated direction of dune building winds coming from anticyclonic air circulation induced by the retreating laurentide ice sheet about 10 000 yr ago the eolian chronology reconstructed from several sections in mixed dune peatland environments indicates that postglacial plant colonization characterized by a fortuitous assemblage of arctic subarctic and boreal elements preceded dune formation during champlain sea regression around 10 000 yr b p confined peatlands and small forests were buried by eolian sands between 10 000 and 7500 yr b p under dry and temperate conditions this eolian episode lasted about 2500 14 c yr and ended when cyclonic air circulation similar to the present humid climatic regime was established following the breakup and disappearance of the laurentide ice sheet over hudson bay and peripheral areas dune stabilization through paludification of well drained eolian sands about 7500 yr b p suggests a major shift in climate toward wetter conditions that have been characteristic during most of the holocene in eastern north america minor eolian erosion induced by wildfire was recorded during late holocene time about 1250 yr b p anthropogenic perturbation logging and agriculture practice was also responsible for recent very local eolian activity

eolian sand transport pathways in the southwestern united states importance of the colorado river and local sources geomorphologists have long recognized that eolian sand transport pathways extend over long distances in desert regions along such pathways sediment transport by wind can surmount topographic obstacles and cross major drainages recent studies have suggested that three distinct eolian sand transport pathways exist or once existed in the mojave and sonoran desert regions of the southwestern united states one hypothesized pathway is eolian sand transport from the eastern mojave desert of california into western arizona near parker and would require sand movement across what must have been at least a seasonally dry colorado river valley we tested this hypothesis by mineralogical geochemical and magnetic analyses of eolian sands on both sides of the colorado river as well as sediment from the river itself results indicate that dunes on opposite sides of the colorado river are mineralogically distinct eastern california dunes are feldspar rich whereas western arizona dunes are quartz rich derived from quartz rich colorado river sediments because of historic vegetation changes little new sediment from the colorado river is presently available to supply the parker dunes based on this study and previous work the colorado river is now known to be the source of sand for at least three of the major dune fields of the sonoran desert of western arizona and northern mexico on the other hand locally derived alluvium appears to be a more important source of dune fields in the mojave desert of california although many geomorphologists have stressed the importance of large fluvial systems in the origin of desert dune fields few empirical data actually exist to support this theory the results presented here demonstrate that a major river system in the southwestern united states is a barrier to the migration of some dune fields but essential to the origin of others published by elsevier science ltd

chronology and geochemistry of late holocene eolian deposits in the brandon sand hills manitoba canada abstract accelerator mass spectrometry and conventional radiocarbon age determinations of organic matter from paleosols indicate that the brandon sand hills area of southern manitoba has been subjected to recurrent intervals of eolian activity in the past 5000 years although precise regional correlations are precluded by dating uncertainties periods of most notable paleosol development occurred around 2300 to 2000 1400 to 1000 and 600 to 500 cal yr bp with eolian activity occurring before and after each of these periods episodes of eolian activity may correspond to periods of regional drought whereas paleosols mark periods of increased moisture availability and stabilization by vegetation the geochemistry of the eolian sands paleosols and source sediments indicates that partial leaching of carbonates occurs from pedogenesis during humid climatic phases and that this is probably the primary mechanism of carbonate depletion of eolian sands in this area recent trends in sand dune activity from historic aerial photography and early explorers accounts indicate that the few active dunes that presently exist have stabilized at a rate of 10 20 per decade despite several severe droughts in the 20th century this may be attributed to pre settlement droughts that were more severe than those in historic times although regional dune stabilization may also be related in part to the spread of forest cover in the past few hundred years

shell structure and distribution of cloudina a potential index fossil for the terminal proterozoic cloudina bearing biosparites and biomicrites in the lower part of the nama group namibia contain a wide morphological diversity of shell fragments that can all be attributed to the two named species c hartmannae and c riemkeae the curved to sinuous tubular shells of cloudina were multi layered each shell layer was 8 to 50 micrometers thick and in the form of a slightly flaring tube with one end open and the other closed growth appears to have been periodic with successive shell layers forming within older layers each added layer was slightly elevated from the previous layer at the proximal end and was asymmetrically placed within the older layer so that only a portion of the new shell layer was fused to the previous layer this type of growth left a relatively large unminerialized area between the shell layers which was often partially or fully occluded by early marine cements the thin shell layers exhibit both plastic and brittle deformation and were likely formed of a rigid caco3 impregnated organic rich material often the shell layers are preferentially dolomitized suggesting an original mineralogy of high magnesian calcite both species in the nama group formed thickets or perhaps bioherms and this sedentary and gregarious habit suggests that cloudina was probably a filter feeding metazoan of at least a cnidarian grade of organization the unusual shell structure of cloudina gives rise to a characteristic suite of taphonomic and diagenetic features that can be used to identify cloudina bearing deposits within the nama group and in other terminal proterozoic deposits around the world species of cloudina occur in limestones from brazil spain china and oman in sequences consistent with a latest proterozoic age assignment in addition supposed lower cambrian pre trilobitic shelly fossils from northwest mexico and the white inyo mountains in california and nevada including sinotubulites nevadatubulus and wyattia are all either closely related to or con generic with cloudina hence it is probable that these outcrops are latest proterozoic in age and that cloudina or cloudina like organisms were widely distributed at that time it is possible moreover to suggest that metazoan biomineralization occurred on a global scale by the latest proterozoic at the same time that evidence for complex multicellularity and locomotion in animals appears in siliciclastic ediacaran rocks in the form of body and trace fossils

simulation of the climate of 18 000 years bp results for the north american north atlantic european sector and comparison with the geologic record of north america abstract the large ice sheets in north america and europe and the extensive sea ice cover in the north atlantic at the time of the last glacial maximum must have greatly modified the surface temperature patterns and in turn the location and intensity of the surface winds and jet streams a general circulation model was used to simulate the january and july patterns of temperature precipitation and wind for 18 ka bp boundary conditions for the model consisting of ice sheet location and height sea ice location and sea surface temperature were prescribed from climap 1981 the model results are illustrated and described for the north american north atlantic european sector the jet stream splits around the north american ice sheet and the southern branch strengthens considerably compared to present over the southern portion of the united states the sea ice margin of the north atlantic and the southern edge of the european ice sheet geologic evidence principally from north america of wind temperature and moisture conditions is assessed from sand dune and loess records estimates of snowline depression pollen records and lake level studies the geologic evidence is generally compatible with the model simulation

stratigraphic evidence of desertification in the west central great plains within the past 1000 yr stratigraphic and geomorphic relations archeological data and eight radiocarbon ages at five widely scattered localities in northeastern colorado indicate that eolian sand was mobilized over broad areas within the past 1000 yr the mobilization began after 1 ka was episodic and ended at some as yet undetermined time prior to the latter part of the nineteenth century given that climate model simulations suggest only slight variation in average surface temperature and annual precipitation in this region during the past 1000 yr this part of the great plains evidently is near the threshold of widespread eolian sand transport under the present climate

stabilized dune ridges in northern saskatchewan stabilized dune ridges occurring in northern saskatchewan have previously been identified as variedly as ice crack moraines and longitudinal dunes investigations of their morphological structural and sedimentary attributes reveal that they are indeed of eolian origin but they form a particular group within the parabolic dune association namely the cree lake type dune ridges the ridges occur in association with other types of parabolic dunes and other eolian features such as loess and wind abraded glacial blocks and bedrock outcrops the dunes and the associated eolian features were all formed by southeasterly paleowinds of uniform direction the dune ridges developed from primary parabolic dunes of simple and composite types through the process of dune elongation at the same time exposed rock surfaces were abraded by the wind and loess was deposited downwind from the developing dune fields the southeasterly direction of the paleowinds which is almost directly opposite to the direction of

holocene dune activity and environmental change in the prairie parki and and boreal forest central saskatchewan canada past aeolian activity was reconstructed at four dunefields in the prairie parkland and boreal forest of central saskatchewan to elucidate landscape response to environmental change optical ages from stabilized dunes in the boreal transition ecoregion indicate two episodes of activity the first at about 11 ka corresponds to a period of early holocene parkland and grassland cover following deglaciation and drainage after about 13 0 ka and brief establishment of boreal forest the second between about 7 5 and 5 ka corresponds to a period of mid holocene parkland grassland cover optical ages from dunefields in the prairie parkland primarily record mid holocene activity between about 7 5 and 4 7 ka corresponding to a period of grassland cover with some reworking continuing into the late holocene although this area was deglaciated by about 13 5 ka there is no evidence of early holocene dune activity suggesting that mid holocene activity may have reworked earlier deposits here consequently much of the morphology and stratigraphy observed in these dunefields are associated with mid holocene activity likely associated with increased aridity and reduced vegetation cover at that time this study provides the most northerly evidence of mid holocene dune reactivation on the great plains lending support to the assertion that aeolian activity was widespread at that time

late quaternary stratigraphy and geochronology of the western killpecker dunes wyoming usa abstract new stratigraphic and geochronologic data from the killpecker dunes in southwestern wyoming facilitate a more precise understanding of the dune field s history prior investigations suggested that evidence for late pleistocene eolian activity in the dune field was lacking however luminescence ages from eolian sand of 15 000 yr as well as folsom 12 950 11 950 cal yr b p and agate basin 12 600 10 700 cal yr artifacts overlying eolian sand indicate the dune field existed at least during the latest pleistocene with initial eolian sedimentation probably occurring under a dry periglacial climate the period between 13 000 and 8900 cal yr b p was characterized by relatively slow eolian sedimentation concomitant with soil formation erosion occurred between 8182 and 6600 cal yr b p on the upwind region of the dune field followed by relative stability and soil formation between 5900 and 2700 cal yr b p the first of at least two latest holocene episodes of eolian sedimentation occurred between 2000 and 1500 yr followed by a brief 500 yr episode of soil formation a second episode of sedimentation occurring by at least 700 yr may coincide with a hypothesized medieval warm period recent stabilization of the western killpecker dunes likely occurred during the little ice age 350 100 yr b p the eolian chronology of the western killpecker dunes correlates reasonably well with those of other major dune fields in the wyoming basin suggesting that dune field reactivation resulted primarily due to departures toward aridity during the late quaternary similar to dune fields on the central great plains dune fields in the wyoming basin have been active under a periglacial climate during the late pleistocene as well as under near modern conditions during the latest holocene

late quaternary eolian sand depositional record for southwestern kansas landscape sensitivity to droughts abstract eolian deposits and landforms are ubiquitous in western kansas particularly south of the arkansas river with the presence of a stabilized dune field stratigraphic studies and associated optical dating reveal a complex depositional history for this dune field spanning the late quaternary the oldest eolian deposits from ca 16 to 12 ka completely or partially bury high terraces with ages of ca 16 and 33 ka it is unknown whether these eolian deposits reflect regional aridity and or a change in sediment availability with the transition of the arkansas river from a single channel to a braided system there is pervasive evidence for episodic eolian erosion and deposition ca 9 8 to 6 3 ka generally coincident with loess deposition on upland surfaces olson c g nettleton w d porter d a brasher b r 1997 middle holocene aeolian activity on the high plains of west central kansas holocene 7 3 255 261 and thus it is inferred to reflect regional aridity sites within the dune field show a sequence of eolian sands and weak buried soils reflecting either dune migration or sand sheet accretion at ca 1490 430 380 320 180 and 70 yr ago which correspond well with continental scale droughts in the tree ring record eolian sand in the arkansas river dune field may be derived principally with variability in fluvial activity and in climate during the late pleistocene to the middle holocene with substantial reactivation of eolian systems during decadal scale drought variability in the past 2000 yr

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

sediment magnetic signature of climate in modern loessic soils from the great plains our magnetic analysis of 40 modern loessic soil profiles from the midwestern united states shows that the upper soil horizons of all sites are characterized by an increase in the concentration of fine grained do0 1mm ferrimagnetic minerals magnetite and or maghemite our sites were selected from stable upland positions along a precipitation gradient extending from sw nebraska mean ann precip o500 mm yr to central missouri mean ann precip 41000 mm yr changes in magnetic remanence parameters irm arm and magnetic susceptibility w between soil and parent material vary systematically along the transect and are likely to reflect changes in source area and transport distance magnetic enhancement is calculated for several magnetic parameters by taking the ratio menhanced mparent material where m stands for either arm irm or w our analyses show that magnetic enhancement of modern soils based on irm and w correlates with changes in mean annual precipitation the best correlation however is observed when arm is used as a proxy of magnetic enhancement which might reflect the bias of arm towards small single domain grains of likely pedogenic origin our study shows that magnetic soil properties in combination with non magnetic proxies such as solum depth b horizon color depth to carbonates or other weathering indices have the potential to yield reliable reconstructions of paleoclimate in parts of the great plains where well preserved loessic paleosols exist

timing and origin for sand dunes in the green river lowland of illinois upper mississippi river valley usa abstract the recent increase in dune studies in north america has been heavily focused in the great plains while less attention has historically been given to the dune fields east of the mississippi river here we report ages and suggest a potential sediment source for sand dunes in the green river lowland illinois which may provide a better understanding of the dynamic interactions between eolian glacial lacustrine and fluvial processes that shaped the landscapes of the upper midwest seven coherent optically stimulated luminescence ages osl or optical ages obtained from four sites suggest that major dune construction in the green river lowland occurred within a narrow time window around 17 500 ago this implies either an enhanced aridity or an episodic increase of sediment supply at 17 500 years ago or combination of the both contrary to previous assertions that dune sand was sourced from the deflation of the underlying outwash sand deposited when the lake michigan lobe retreated from the area we propose that green river lowland dunes sand originated from the green bay lobe through the rock river specifically sediment supply increased in the rock river valley during drainage of glacial lake scuppernong which formed between 18 000 and 17 000 years ago when the green bay lobe retreated from its terminal moraine the lake drained catastrophically through the rock river valley providing glacial sediment and water to erode the preexisting sandy sediments throughout the remainder of the late pleistocene the laurentide ice sheet drained into larger more northerly glacial lakes that in turn drained through other river valleys therefore the dunes in the green river lowland formed only during the catastrophic drainage of glacial lake scuppernong but were stabilized through the remainder of the pleistocene this scenario explains the abrupt dune construction around 17 500 years ago and explains the lack of later dune activity up to the pleistocene holocene transition osl and radiocarbon ages also indicate that dunes were reactivated during the early middle and late holocene some eolian activation occurred within well defined dry intervals in the upper midwest suggesting that increased aridity may have been the primary driver in mobilizing sand however many ages do not correspond to drier periods in contrast to the relative coherency of the pleistocene osl ages from multiple study sites the holocene osl ages do not overlap from one site to another suggesting that increased aridity alone cannot explain the multiple phases of dune reactivation in the holocene therefore we conclude that the combined effect of localized disturbances and greater aridity acted in concert to increase eolian sand activity in the holocene the multiple periods of eolian activity during the holocene suggest a high potential for future sand activation in the region and these results are informative for environmental prediction and potential future mitigation

variation in radiocarbon ages of soil organic matter fractions from late quaternary buried soils radiocarbon dating of three organic matter fractions total humic acid and residue isolated from late quaternary buried soils of the central great plains reveals that there often are considerable differences among but no consistent order to the ages of fractions for late holocene soils the residue fraction or the total fraction generally produces the oldest age for late pleistocene soils however no fraction was consistently the oldest the absence of a consistent sequence of fraction ages is attributed to postburial contamination of soils when bulk samples from the same soil were split and sent to two laboratories different radiocarbon ages were usually obtained the variability in radiocarbon ages of soil organic matter confirms that caution should be taken when using radiocarbon ages obtained from different laboratories to make regional stratigraphic correlations

radiocarbon geochemistry of modern and ancient arctic lake systems baffin island canada the accuracy of arctic lake chronologies has been assessed by measuring the14c activities of modern carbon sources and applying these isotopic mass balances to dating fossil lake materials small 1 km2 shallow 25 m arctic lakes with watersheds 12 km2have soil and peat stratigraphic sections with14c activities ranging from 98 to 51 modern the14c activity of particulate organic carbon dissolved organic carbon and dissolved inorganic carbon from lake and stream waters ranges from 121 to 95 modern the sediment water interface of the studied lakes shows consistent14c ages of 100014c yr although the14c activity of living aquatic vegetation is 115 modern radiocarbon measurements of components of the lacustrine carbon pool imply that the 100014c yr age of the sediment water interface results from deposition of14c depleted organic matter derived from the watershed

lungfish burrows in the upper triassic chinle and dolores formations colorado plateau abstract vertical to inclined cylindrical trace fossils that occur in the upper triassic chinle and dolores formations on the colorado plateau are interpreted to be the casts of lungfish burrows the casts which are as much as 11 cm in diameter and as much as 1 6 m long were formed by passive silicilastic and carbonate sedimentation into apparently abandoned lungfish burrows locally the burrow fillings are overwhelmingly abundant and many intersect and have destroyed former burrow fillings superposition of bioturbation episodes has obliterated most primary sedimentary structures this bioturbation has contributed to the mottled coloration and the knobby weathering texture of the rocks the burrow fillings occur ubiquitously in three lithofacies comprising 1 purple and white mottled ilicified sandstone and siltstone 2 red and brown siltstone and mudstone and 3 pink and green limestone these strata were deposited in a continental environment that included fluvial channels and floodplains sand sheets and playa mudflats and lacustrine basins marshes and deltas the identification of the trace fossils as the positive casts of lungfish burrows is based on their morphologic similarity to previously identified lungfish burrows and to available hand specimens the widespread occurrence of the lungfish burrows in the chinle and dolores formations attests to the extensive habitat that supported lungfish in the late triassic and to conditions favorable for burrow preservation analogy with the environments that support modern lungfish populations suggests that the late triassic climate in the study area provided sufficient moisture to support large populations of lungfish and that this climate was probably punctuated by seasonally dry periods

evidence of active dune sand on the great plains in the 19th century from accounts of early explorers eolian sand is extensive over the great plains of north america but is at present mostly stabilized by vegetation accounts published by early explorers however indicate that at least parts of dune fields in nebraska colorado kansas new mexico and texas were active in the 19th century based on an index of dune mobility and a regional tree ring record the probable causes for these periods of greater eolian activity are droughts accompanied by higher temperatures which greatly lowered the precipitation to evapotranspiration ratio and diminished the cover of stabilizing vegetation in addition observations by several explorers and previous historical studies indicate that rivers upwind of great plains dune fields had shallow braided sandy channels as well as intermittent flow in the 19th century wide braided sandy rivers that were frequently dry would have increased sand supplies to active dune fields we conclude that dune fields in the great plains are extremely sensitive to climate change and that the potential for reactivation of stabilized dunes in the future is high with or without greenhouse warming

formation of linear and parabolic dunes on the eastern snake river plain idaho in the nineteenth century abstract geomorphic and stratigraphic evidence show periods of distinct hydrologic excess and deficit sometime in the past 500 years on the eastern snake river plain natural exposures at the current shore of mud lake contain a lacustrine silty sand beneath eolian sands the latter deposit is expressed at the surface as hairpin parabolic and linear dunes these dunes can be traced into and across lakes a clear sign of recent aridity two shallow water gastropods and two wood fragments from the lacustrine silty sand yield the respective ams 14 c ages of ca 705 and 988 year bp and 195 and 305 year bp the offset in ages indicates a lake hard water effect of aquatic gastropods with the wood ages providing the closest age constraint optically stimulated luminescence single aliquot regeneration ages of 150 30 and 120 20 year bp on quartz grains from the overlying eolian sands further constrains a drop in lake level and subsequent aridity to the 19th century this lacustrine dune sequence is probably the geomorphic expression of a distinct wet period followed by a severe drought between ad 1845 and 1856 which has been documented in the dendroclimatic records for the rocky mountains and the western great plains lake fluctuations and dune formation is associated with greater subdecadal variability in spring precipitation than recorded in the past 100 years

origin of the late quaternary dune fields of northeastern colorado stabilized eolian deposits mostly parabolic dunes and sand sheets cover much of the landscape of northeastern colorado and adjacent parts of southwestern nebraska in four geographically distinct dune fields stratigraphic and soil geomorphic relations and accelerator radiocarbon dating indicate that at least three episodes of eolian sand movement occurred between 27 ka and 11 ka possibly between 11 ka and 4 ka and within the past 1 5 ka thus eolian sand deposition took place under both glacial and interglacial climatic conditions in the youngest episodes of eolian sand movement holocene parabolic dunes partially buried pleistocene sand sheet deposits late holocene sands in the fort morgan and wray dune fields to the south of the south platte river have trace element ratios that are indistinguishable from modern south platte river sands but different from ogallala formation bedrock which has previously been cited as the main source of dune sand on the great plains sands in the greeley dune field to the north of the south platte river have trace element concentrations that indicate a probable laramie formation source measurements of parabolic dunes indicate paleowinds from the northwest in all dune fields in good agreement with resultant drift directions calculated for nearby weather stations thus paleowinds were probably not significantly different from present day winds and are consistent with a south platte river source for the fort morgan and wray dune fields and a laramie formation source for the greeley dune field sand accumulated downwind of the south platte river to form the fort morgan duen field in addition sand was also transported farther downwind over the upland formed by the calcrete caprock of the ogallala formation and deposited in the lee of the upland on the southeast side because of high wind energy the upland itself served as a zone of sand transport but little or no sand accumulation took place on this surface these studies which demonstrate the importance of fluvial source sediments for dune fields in colorado may be applicable to other dune fields in north america because modern drift potentials in northeastern colorado are among the highest in the world the present stability of dunes in the region may be in part a function of the dunes being supply limited rather than solely transport limited extensive 7700 km2 late holocene dunes document that eolian sand in northeastern colorado is very sensitive to small changes in climate or fluvial source conditions

middle holocene mobilization of aeolian sand in western upper michigan and the potential relationship with climate and fire a forested dunefield covers the baraga plains in western upper michigan the ages of five dunes from across the dunefield were determined by optically stimulated luminescence osl employing the single aliquot regenerative dose sar method on quartz grains the results indicate that sand mobilization was restricted to the middle holocene at around 7 ka four environmental variables rapid climatic oscillations fire strong winds and a dry climate are invoked as a combined cause of dune formation this study provides further evidence for the wide extent of aeolian activity in the upper midwest of north america during the middle holocene

origin of late quaternary dune fields on the southern high plains of texas and new mexico mostly stabilized late holocene eolian sands on the southern high plains of the united states were studied to determine their origins and to assess whether present dune stability depends more strongly on sediment supply sediment availability or transport limitations geomorphic sedimentological and geochemical trends indicate that late holocene dunes formed under westerly paleowinds broadly similar to those of today mineralogical and geochemical data indicate that the most likely source for the sands is not the pecos river valley but the pleistocene blackwater draw formation an older extensive eolian deposit in the region these observations suggest that new sand is supplied whenever vegetation cover is diminished to the extent that the blackwater draw formation can be eroded in agreement with modern observations of wind erosion in the region we conclude therefore that southern high plains dunes are stabilized primarily due to a vegetation cover the dunes are thus sediment availability limited this conclusion is consistent with the observation that in the warmest driest part of the region where vegetation cover is minimal dunes are currently active over a large area geochemical data indicate that southern high plains dunes are the most mineralogically mature quartz rich sands yet studied in the great plains which suggests a long history of eolian activity either in the dune fields or during deposition of the blackwater draw formation

geochemical and mineralogical evidence from eolian sediments for northwesterly mid holocene paleowinds central kansas usa a prominent 4500 km2 dune eld in the great plains is the great bend sand prairie of south central kansas dunes here overlie late quaternary alluvium and were reactivated extensively in the late holocene geomorphic and soil evidence suggests that the most likely eolian sand source is the arkansas river valley to the northwest nevertheless orientations of stabilized dunes indicate that the most recent dune forming winds came from the south or southwest in agreement with modern wind data mineralogy and trace element concentrations in eolian sands of the great bend sand prairie are similar to those of the arkansas river which permits the arkansas river as a sediment source ca and sr abundances which re ect small amounts of carbonate minerals are higher in arkansas river sand compared to eolian sands and show a systematic depletion away from the arkansas river to the southeast these trends are likely due to carbonate mineral depletion downwind from abrasion and size reduction thus paleowinds probably were northwesterly during initial deposition northwesterly winds occur today when dry paci c derived air is dominant we hypothesize that the residence time of this air mass was much greater while dunes initially formed possibly during a warmer and drier mid holocene period 2000 elsevier science ltd and inqua all rights reserved