borehole effects on downhole seismic measurements there are increasing interests in acquiring crosshole and vsp surveys at frequency up to 1 khz or more with the intention to resolve the fine details of structures and lithology between wells bregman et al 1989 harris 1988 tura 1991 however at frequencies on the order 1 khz the existence of the fluid filled borehole has a strong influence on the downhole measurements depending on the frequency and angle of incidence as well as the formation properties the measured displacements on the borehole wall or pressure at the center of borehole may be significantly different from those of the incident wave without taking proper care of this effect any imaging or inversion algorithm which utilizes both the amplitude and phase information of recorded energy will produce erroneous results

macrofossil and tree ring evidence for a long term forest succession and mid holocene hemlock decline abstract paludified dune fields in southern quebec provided a unique opportunity to analyze a mid holocene forest succession ranging from xero mesophilous forests to treed peatlands which registered the hemlock decline a major biostratigraphic event in eastern north america our fossil tree chronology established from 66 radiocarbon dates of tree stems indicates that white pine and hemlock were present at the study site about 6000 5700 yr b p hemlock reached a maximum 4900 4800 yr b p and then declined between 4600 and 3800 yr b p the species recovered around 3700 yr b p and then decreased when white pine and eastern latch dominated the site after 3000 yr b p and between 2600 and 2200 yr b p respectively tree ring signatures of fossil hemlocks suggest the continuous influence of biotic factors possibly insect defoliators throughout the record as shown by recurrent suppressed growth pattern microscopic examination of the hemlock fossil stems disclosed no evidence of fungal disease the hemlock decline was probably caused by regional posthypsithermal climatic changes toward moister conditions

climate change in southern illinois usa based on the age and δ13c of organic matter in cave sediments abstract matrix supported diamicton and uniform to laminated silty fine grained sediment deposited from about 42 500 to 27 600 cal yr b p under slackwater conditions nearly filled two caves in southwestern illinois at some point most of the sediment was flushed from the caves and from about 22 700 to 4000 cal yr b p floods deposited a drape of sandy and silty sediment on remnant slackwater successions cobbly alluvium and bedrock especially from 7700 to 4000 cal yr b p clay mineral analyses of the slackwater cave sediment reveal a provenance of chiefly petersburg silt a smectite and illite rich proglacial lacustrine unit present in the overlying illinois episode glacial succession today remnants of the ancient subterranean slackwater deposits nearly fill several secondary passages and in at least two locations cover a cobble mantled strath terrace 1 3 to 1 5 m above active stream channels slumping and sinkhole formation appear to have been important mechanisms for deposition of the ancient subterranean deposits slumping of these surficial deposits and associated vegetation can occur along the flanks of sinkholes in addition to sinkhole formation and enter caves however the finer organics some of them comminuted during transport into the caves become part of the cave alluvium this finer organic fraction is the modern analog of the humified organic matter disseminated in slackwater sediment dated in this investigation by radiocarbon methods twenty four 14c ages on humified organic matter provide chronologic control the 13c values of the organic matter reflect the proportion of c4 type to c3 type vegetation growing in and around swallets and sinkholes at the time of redeposition drought tolerant c4 type vegetation was more prevalent relative to c3 type vegetation from 42 500 to 31 200 cal yr b p compared to conditions from 28 800 cal yr b p to the present the 13c values are consistent with the results from other investigations of speleothems and organic matter from loessial paleosols

the magnitude of error in conventional bulk sediment radiocarbon dates from central north america abstract although the carbon reservoir problem with bulk sediment radiocarbon dates from lakes has long been recognized many synoptic studies continue to use chronologies derived from such dates for four sites in central north america we evaluate chronologies based on conventional radiocarbon dates from bulk sediment versus chronologies based on accelerator mass spectrometry ams radiocarbon dates from terrestrial plant macrofossils the carbon reservoir error varies among sites and temporally at individual sites from 0 to 8000 yr an error of 500 2000 yr is common this error has important implications for the resolution of precise event chronologies

buried soils in a perched dunefield as indicators of late holocene lake level change in the lake superior basin abstract a stratigraphic analysis of buried soils within the grand sable dunes a dune field perched 90 m above the southern shore of lake superior reveals a history of eolian activity apparently linked with lake level fluctuations over the last 5500 yr a relative rise in the water plane of the nipissing great lakes initially destabilized the lakeward bluff face of the grand sable plateau between 5400 and 4600 14 c yr b p this led to the burial of the sable creek soil by eolian sediments derived from the bluff face subsequent episodes of eolian activity appear to be tied to similar destabilizing events high lake levels may have initiated at least four and perhaps eleven episodes of dune building as expressed by soil burials within the dunes intervening low lake levels probably correlate with soil profile development which varies from the well developed sable creek spodosol catena to thin organic layers containing in place stumps and tree trunks paleoecological reconstructions available for the area do not imply enough climatic change to account for the episodic dune activity burial of soils by fine fraction sediments links dune building episodes with destabilization of the lower lake facing bluff which is rich in fines

middle holocene dry period in the northern midwestern united states lake levels and pollen stratigraphy abstract four cores from lake mendota in the yahara river valley of south central wisconsin provide pollen charcoal and sediment stratigraphic evidence of a middle holocene dry period in the northern midwest the lake level was lower between about 6500 and 3500 yr b p indicating that runoff from the upper yahara river basin may have ceased during the drier seasons of the year derived estimates of precipitation between 6500 and 3500 yr b p indicate a decrease of about 10 down to about 700 mm from the present precipitation value of 800 mm a warming between 6500 and 3500 yr b p is also interpreted based on a change of vegetation from rich mesophytic forest before 6500 yr b p to quercus savanna with increased charcoal abundances after that time after 3500 yr b p a closed quercus forest and decreased charcoal suggest a cooler and wetter climate the qualitative paleoecological interpretations quantitative precipitation reconstructions based on pollen from the region including the lake mendota data and estimates of hydrologic budget based on lake level changes all show indications of a dry and warm middle holocene evidence from a large regional array of sites also supports these interpretations

holocene paleoenvironments in southeastern minnesota chasing the prairie forest ecotone this paper uses a multi proxy approach involving pollen plant macrofossils speleothem isotopes and alluvial history of streams to reconstruct the history of prairie expansion and contraction along the prairie forest border of southeastern minnesota usa early holocene forests were replaced by prairie along this border but eastward expansion of prairie stalled for 2000 yr when the prairie forest ecotone stabilized prairie invaded the area from the west 8000 9000 yr b p but mesic forest remained less than 100 km to the east until about 6000 yr b p changes in n 13 c values in speleothem calcite that reflect the rise of c4 grasses correlate well with the presence of c4 grass species identified in the plant macrofossil record periods of large floods correlate with speleothem evidence of dry summers increased cool season precipitation both resulting in less plant cover to absorb moisture and change to prairie vegetation fl 2002 elsevier science b v all rights reserved

holocene climatic change in the northern midwest pollen derived estimates mapping of holocene pollen data in the midwestern united states has revealed several broadscale vegetational changes that can be interpreted in climatic terms these changes include 1 the early holocene northward movement of the spruce dominated forest and its later southward movement after 3000 yr b p and 2 the eastward movement of the prairie forest border into southwestern wisconsin by 8000 yr b p and its subsequent westward retreat after 6000 yr b p when certain basic assumptions are met multiple regression models can be derived from modern pollen and climate data and used to transform the pollen record of these vegetational changes into quantitative estimates of temperature or precipitation to maximize the reliability of the regression equations we followed a sequence of procedures that minimize violations of the assumptions that underlie regression analysis reconstructions of precipitation during the holocene indicated that from 9000 to 6000 yr b p precipitation decreased by 10 to 25 over much of the midwest while mean july temperature increased by 0 5 to 2 0 c at 6000 yr b p precipitation was less than 80 of its modern values over parts of wisconsin and minnesota after 6000 yr b p precipitation generally increased while mean july temperature decreased in the north and increased in the south the time of the maximum temperature varies within the midwest and is earlier in the north and later in the south

vertical distribution of total carbon nitrogen and phosphorus in riparian soils of walnut creek southern iowa abstract subsurface lithology plays an important role in many riparian zone processes but few studies have examined how sediment nutrient concentrations vary with depth in this study we evaluated concentrations of nutrients n c and p with depth in a riparian zone of the glaciated midwest a total of 146 sediment samples were collected from 24 cores that extended to a maximum depth of 3 6 m at eight sites in the riparian zone of walnut creek subsurface deposits were predominantly silt loam becoming coarser and more variable with depth nitrogen and carbon content ranged from

effects of carbon dioxide on mineral weathering rates at earth surface conditions the weathering of silicate minerals is an important long term control on the global carbon budget while the rate of mineral weathering is influenced by the atmospheric variations in atmospheric carbon dioxide the only measurements of those effects have occurred during dissolution experiments at temperatures much higher than earth surface conditions thus any climate models that include such a relationship may not be able to fully couple variations in atmospheric carbon dioxide with the lithospheric sinks our study presents a relationship for the dependence of plagioclase dissolution rates on pco2 based on field data from a site in the southeastern sierra nevada drainages a series of canyons that have similar drainages show wide variability in water chemistry that is attributed to variations in pco2 from geothermal sources this setting allowed us to isolate the effect of pco2 on weathering rates in conditions relevant to climate models the results show that mineral dissolution rates are proportional to pco2 0 45 when the observed variations are attributed solely to variations in pco2 this relationship is likely to be more applicable to climate models than prior laboratory derived data 2007 elsevier b v all rights reserved

responses of floods to holocene climatic change in the upper mississippi valley abstract dimensions of holocene relict channels and sedimentological characteristics of point bars associated with these relict channels were used to reconstruct a holocene history of long term changes in magnitudes of 1 58 yr floods in upper mississippi valley watersheds of southwestern wisconsin the reconstructed record of floods shows relatively large and persistent nonrandom departures from contemporary long term average flood magnitudes the flood history indicates climatic changes that are broadly similar to climatic changes indicated from fossil pollen in the same region the holocene floods ranged from about 10 15 larger to 20 30 smaller than contemporary floods of the same recurrence frequency large floods were characteristic between about 6000 4500 and 3000 2000 yr b p and during a brief interval after 1200 yr b p small floods were common between about 8000 6500 4500 3000 and 2000 1200 yr b p these fluvial responses were found to be closely associated with a long term episodic mobility and storage of sediments in the wisconsin watersheds during periods of relatively large floods relatively rapid lateral channel migration either reworked or removed extensive tracts of valley bottom alluvium in contrast during periods of relatively small floods relatively slow lateral channel migration is apparent and the channel and floodplain system appear to have been relatively stable

pollen plant macrofossils and microvertebrates from mid holocene alluvium in east central iowa usa comparative taphonomy and paleoecology abstract pollen plant macrofossils and micromammals are commonly used in paleoecological interpretations but rarely are found associated because of distinctly separate taphonomic pathways into the fossil record the lilienthal local biota dating between 5920 60 and 6300 80 14 c b p is a rare exception as the site contains all three proxies juxtaposed in alluvium impounded upstream from a fossil log near the headwaters of mud creek east central iowa u s a a variety of taphonomic processes can strongly influence the composition of fossil assemblages this paper compares the taphonomic framework for each fossil group interprets the paleoecologic signal from each and then compares the three interpretations pollen and plant macrofossils are found in organic silts indicating deposition in a quiet water environment microvertebrates ordinarily occur in sand sized sediments and show evidence of fluvial transport despite these different pathways all lilienthal assemblages indicate that a largely closed mesic deciduous forest grew along mud creek in mid holocene time therefore a narrow north south ecotone must have existed between the forest and tall grass prairie biomes in eastern iowa this robust data set provides a detailed picture of the paleoecology and paleogeography of this past environment

late holocene development of a floodplain along a small meandering stream northern quebec canada this paper describes the activity of a small meandering stream and the development of its floodplain during the last 4600 years calendar years bp in the northern boreal zone of quebec three trenches were excavated across the floodplain s full width and permitted the interpretation of morphosedimentary units in relation to modern analogs chronological controls within trenches was provided by the dendrochronological and radiocarbon dating of buried tree trunks from 4600 to 2900 cal bp and from 1000 to 120 cal bp the channel migrated and constructed its floodplain at very slow rates mostly because of low flow velocities vegetated streambanks and the cohesive texture of marine sediments reworked by the channel vertical accretion rates were extremely variable on the floodplain with high rates proximal to the modern channel and low rates over distal also older portions of the floodplain following a major channel shift meander cut off or avulsion around 2900 cal bp channel migration appears to have been constrained to a narrow zone adjacent to the modern channel within this constrained zone the migrating channel has reworked its own sediments leading to a marked unconformity between 2900 1000 cal bp it is thought that underlying marine sediments protrusions and perhaps the forested banks protected older alluvial sediments from being eroded during the last three millenniums our study shows that small boreal floodplains may contain in a very small area abundant and diversified archives of their evolution

isotopic evidence for younger dryas aridity in the north american midcontinent determining the impact of the younger dryas yd climate event on the unglaciated north american midcontinent has proved difficult due to a scarcity of suitable paleoclimate proxies here we present a well dated carbon isotope 13c record from a buried soil sequence in southwestern missouri which reveals a large isotopic excursion during the yd chronozone in this region of the modern prairie forest border the 13c signature of soil organic matter is a reliable indicator of past climatic change because 13c values are controlled primarily by the relative abundance of c3 and c4 plants which is tied to the environmental setting between ca 13 200 and 11 900 yr ago the abundance of c4 grasses increased by upwards of 50 of the total biomass indicating expansion of grassland most likely driven by increased aridity during this period environmental gradients in the midcontinent must have been very steep because at the same time that a c4 rich prairie existed in southwestern missouri spruce forests grew in iowa illinois and ohio

terrestrial sensitivity to abrupt cooling recorded by aeolian activity in northwest ohio usa article i nfo article history received 7 may 2010 available online 21 february 2011 optically stimulated luminescence dated sand dunes and pleistocene beach ridges in northwest ohio are used to reconstruct landscape modification more than 5000 yr after deglaciation four of the osl ages 13 3 11 1 ka cluster around the younger dryas cold event five ages 10 8 8 2 ka cluster around the preboreal one young age 0 9 0 7 ka records more recent aeolian activity and one age of 15 1 13 1 ka dates a barrier spit in lake warren in northwest ohio both landscape instability recorded by aeolian activity and a vegetation response recorded by pollen are coeval with the younger dryas however the climate conditions during the preboreal resulting in aeolian activity are not recorded in the available pollen records from this we conclude that aeolian dunes and surfaces susceptible to deflation are sensitive to cooler drier episodes of climate and can complement pollen data younger dryas and preboreal aged aeolian activity in northwestern ohio coincides with aeolian records elsewhere in the great lakes region east of the prairie forest ecotone

intensity gradients and reversals in late glacial environmental change in east central north america abstract radiocarbon dated pollen sites from the allegheny plateau east and the till plains west document significant vegetational and climatic change between 15 000 and 9000 14 c bp south of the great lakes although limited by the problems of no analog vegetation pollen transfer functions tf and response surfaces rs quantify the climatic changes including a cold oscillation chronologically equivalent to the european younger dryas phase i before ca 13 000 14 c bp conditions across the region were similar january 20 to 16 c july 12 c rs or 19 c tf mean annual precipitation 80 to 90 cm year 1 phase iia between ca 13 000 and 11 000 14 c bp n s and e w climate gradients were initiated till plains temperatures increased by ca 8 c in january and by 8 c rs or 2 3 c tf in july plateau increases for january were 3 6 c and for july were 8 c rs or 1 2 c tf precipitation generally increased somewhat phase iib ca 11 000 to 10 000 14 c bp gradients became less pronounced the western sites show a climate reversal january drop of ca 5 to 2 c july drop to ca 1 2 c becoming more like the plateau sites which show a brief small fluctuation this is followed by an abrupt 10 to 20 cm year 1 reduction in precipitation january warming of ca 3 c and continued cooler julys phase iii after 10 000 14 c bp gradients similar to those of today developed the changes in phases iib and iii are probably linked to the drainage history of the great lakes and via that history to the changes on the european continent

a 130 000 year long pollen record from pittsburg basin illinois abstract pittsburg basin in south central illinois contains a sediment record extending from the present back to the end of the late illinoian glaciation when central illinois was covered with picea pinus forest during the last interglaciation a temperate deciduous forest more diverse than holocene quercus carya forest replaced the illinoian late glacial boreal forest prairie pollen types and the charcoal pollen ratio indicating fire frequency temporarily increased then forest with high juniperus percentages became dominant once more as the charcoal pollen ratio dropped after the last interglaciation the charcoal pollen ratio increased again and prairie and wetland surrounded pittsburg basin through the entire wisconsinan glacial age the area was still prairie in late wisconsinan time but with some picea and pinus during the holocene the region has been a mixture of prairie and quercus carya forest during the last interglaciation pittsburg basin was surrounded by vegetation different from that surrounding it during the present interglaciation rather than indicating substantial differences in climate between analogous phases of different glacial interglacial cycles this variation may be due to changes in fire frequency which could be caused by small changes in climate human activity or differences in soil

the timing of regional lateglacial events and post glacial sedimentation rates from lake superior abstract we analyze both new and previously published paleomagnetic records of secular variation psv from lake superior sediment cores and compare these records to correlated rhythmite varve thickness records to determine post glacial sedimentation rates and to reassess the termination of glaciolacustrine varves in the basin the results suggest that offshore sedimentation rates have exhibited considerable spatial variation over the past 8000 years particularly during the mid holocene we attribute offshore mid holocene sedimentation changes to alterations in whole basin circulation perhaps precipitated by a greater dominance of the gulf of mexico air mass during the summer season nearshore bays are characterized by high sedimentation rates for at least 1000 years after varve cessation and during a period between around 4500 and 2000 cal bp after 2000 cal bp sedimentation rates subsided to earlier rates the increases between 4500 and 2000 cal bp are probably due to lake level fall after the nipissing ii highstand the older glaciolacustrine varve thickness records suggest that the influx of glacially derived sediment ended abruptly everywhere in the lake except near the lake nipigon inlets multiple sediment cores reveal 36 anomalously thick varves previously ascribed to the formation of the nakina moraine which were deposited just prior to varve cessation in the open lake the psv records support the observation that the cessation of these thick varves is a temporally correlative event occurring at 9035 170 cal bp calibrated years before 1950 ca 7950 8250 14 c bp this date would correlate to the eastern diversion of lake agassiz and glacial meltwater into lake ojibway

paleoenvironmental records of water level and climatic changes from the middle to late holocene at a lake erie coastal wetland ontario canada pollen and diatom assemblages and peat stratigraphies from a coastal wetland on the northern shore of lake erie were used to analyze water level and climatic changes since the middle holocene and their effects on wetland plant communities peat deposition began 4700 cal yr b p during the nipissing ii transgression which was driven by isostatic rebound at that time a diatom rich wild rice marsh existed at the site water level dropped at the end of the nipissing rise at least 2 m within 200 yr leading to the development of shallower water plant communities and an environment too dry for most diatoms to persist the sharp decline in water level was probably driven primarily by outlet incision but climate likely played some role the paleoecological records provide evidence for post nipissing century scale transgressions occurring around 2300 1160 700 and 450 cal yr b p the chronology for these transgressions correlates with other studies from the region and implies climatic forcing peat inception in shallow sloughs across part of the study area around 700 cal yr b p coincides with the little ice age these records considered alongside others from the region suggest that the little ice age may have resulted in a wetter climate across the eastern great lakes region

the anatomy of a climatic oscillation vegetation change in eastern north america during the younger dryas chronozone century scale climate changes reshaped circulation patterns over the north atlantic and adjacent regions during the last glacialto interglacial transition here we show that vegetation across eastern north america shifted dramatically at the beginning and end of the younger dryas chronozone ydc 12 900 11 600 cal yr b p when changes in ocean circulation rapidly cooled and then warmed the north atlantic sea surface on both the site specific scale and the continental scale vegetation changed only gradually during the millennia before 15 000 13 000 cal yr b p and after 11 000 9000 cal yr b p the ydc but climate changes ca 12 900 and 11 600 cal yr b p altered the vegetation on both spatial scales within centuries plant associations changed and some taxa rapidly migrated hundreds of kilometers 300 km within b100 yr in limited regions near the north atlantic coast abrupt cooling ca 12 900 cal yr b p resulted in a return to earlier vegetation types elsewhere however the vegetation patterns during the ydc were distinct from those of both earlier and later intervals they indicate abrupt non reversing seasonal temperature changes that were probably related to atmospheric circulation changes during the ydc rather than to the direct influence of north atlantic sea surface temperatures atmospheric circulation patterns during the ydc were unique within the last 21 000 yr because of a unique combination of climate controls insolation ice sheet extent and atmospheric composition were significantly different from their full glacial states even when the north atlantic returned to near full glacial conditions the ydc vegetation patterns demonstrate 1 rapid ecological responsiveness to abrupt climate change and 2 spatially varied patterns of ydc climate change r 2002 elsevier science ltd all rights reserved

mapped plant macrofossil and pollen records of late quaternary vegetation change in eastern north america abstract macrofossil presence absence maps and isopoll maps in 3000 year intervals show how ranges and abundance maxima for 12 eastern north american tree and shrub taxa picea p glauca p mariana larix laricina abies balsamea tsuga canadensis pinus strobus p resinosa p banksiana betula papyrifera b alleghaniensis b series humiles have changed from the last glacial maximum to the present the macrofossil maps corroborate patterns shown by the isopoll maps and provide spatial detail and taxonomic resolution the macrofossils confirm the inference from pollen data that unglaciated southeastern north america was cooler during the last glacial maximum 18 and 15 ka than simulated by the cohmap experiments with the ncar ccm0 general circulation model the geographic distribution of macrofossil occurrences during the late glacial 12 and 9 ka indicate that migration lag of boreal species did not occur at regional to subcontinental scales and that pollen assemblages lacking modern analogs resulted from climate gradients lacking modern analogs early holocene 9 and 6 ka macrofossil maps show rapid northward expansion of tree species ranges into deglaciated regions the data also show modest contractions of northern range limits of temperate species and expansions of southern range limits of boreal species in response to cooling trends during the late holocene 3 and 0 ka comparison of modern macrofossil maps of nine of these taxa with corresponding range maps confirm that the macrofossils record the geographic ranges accurately comparison of the modern macrofossil maps with maps of tree growing stock volume shows that for some taxa abies tsuga macrofossil occurrences were most frequent in regions of maximum tree abundance comparison of modern isopoll maps with the modern range and growing stock volume maps indicate that in contrast to the macrofossil data the pollen data provide poorer resolution of range limits for most taxa but better indications of abundance maxima and minima within the ranges

reconstruction of holocene lake level from diatoms chrysophytes and organic matter in a drainage lake from the experimental lakes area northwestern ontario canada abstract a paleolimnological study of lake level changes in lake 239 rawson lake a headwater lake in the experimental lakes area in northwestern ontario indicates large fluctuations have occurred over the holocene analyses are based on diatoms the proportion of chrysophyte scales to diatoms and organic matter content from near shore sediment cores quantitative estimates of lake level are based on a diatom inferred depth model that was developed from surficial sediments collected along several transects in lake 239 declines of 1 3 m occurred during the late holocene whereas declines of at least 8 m occurred during the more arid mid holocene these results provide the first substantive evidence of large declines in lake level in northwestern ontario during the mid holocene conditions during the mid holocene may provide a partial view of future conditions under increasing global temperatures

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

up in the refrigerator geomorphic response to periglacial environments in the upper mississippi river basin usa abstract james c knox was best known for his work on holocene and historical changes in fluvial systems but he also had a long standing interest in the effects of late pleistocene periglacial environments on landscape evolution in parts of the upper mississippi river basin that were just outside the laurentide ice sheet margin or as knox put it up in the refrigerator knox and others in the quaternary community of the midwestern u s often suggested that hillslope erosion was accelerated under periglacial conditions so that glacial periods have had a dominant effect on the landscape we see today this paper reviews the evidence and reasoning supporting that view in a study area of the upper mississippi basin bordered on three sides by ice margins of the last glaciation including the wisconsin driftless area and adjacent landscapes sparse but compelling paleoecological data and relict ice or sand wedge polygons provide clear evidence for a cold climate and widespread permafrost around the peak of the last glaciation in highly dissected relatively high relief parts of the study area the loess and soil stratigraphy on ridgetops and the colluvial mantles on steeper slopes are best explained by highly effective hillslope erosion including solifluction during and just after the last glacial maximum knox used the post depositional truncation of a loess unit to quantify contrasting late pleistocene and holocene sediment yields from a small driftless area watershed while the late pleistocene yield indicates accelerated erosion it is still lower than modern sediment yields in many tectonically active or semiarid landscapes and it may reflect deposition of highly erodible loess as well as effects of periglacial conditions the views of knox and other midwestern geomorphologists on landscape evolution through glacial interglacial cycles were highly influenced by the work of robert v ruhe ruhe proposed that an episode of widespread erosion during and just after the last glacial maximum can explain enigmatic aspects of quaternary stratigraphy and the soil landscape on the iowan erosion surface a very low relief landscape of the study area ruhe s key evidence is still valid though it needs to be separated from an implausible model of landscape evolution interpretation of the iowan erosion surface and other low relief landscapes just outside the ice margin also requires recognition of the profound effect of eolian processes on those landscapes under periglacial conditions many new insights on landscape evolution in the study area could result from wider application of cosmogenic nuclide based methods to assess glacial interglacial changes in basinwide rates of erosion and residence time of soils just as important a need exists for much more field based characterization of hillslope fluvial and eolian sediments for comparison with those of modern permafrost regions and past periglacial environments in europe

patterns of holocene environmental change in the midwestern united states abstract four pollen sequences along a transect from north central iowa to southeast wisconsin reveal the distribution of prairie and forest during the holocene and test the use of pollen isopolls in locating the holocene prairie forest border prairie was dominant in central iowa and climate was drier than present from about 8000 to 3000 yr b p during the driest part of this period in central iowa 6500 5500 yr b p mesic forest prevailed in eastern iowa and wisconsin suggesting conditions wetter than at present prairie replaced the mesic forest about 5400 yr b p in eastern iowa but did not extend much farther east mesic forests were replaced in southern wisconsin and northern illinois about 5400 yr b p by xeric oak forests this change from mesic to xeric conditions at 5400 yr b p was widespread and suggests that the intrusion of drier pacific air was blocked by maritime tropical air from the gulf of mexico until the late holocene in this area

unprecedented last glacial mass accumulation rates determined by luminescence dating of loess from western nebraska abstract a high resolution chronology for peoria last glacial period loess from three sites in nebraska midcontinental north america is determined by applying optically stimulated luminescence osl dating to 35 50 m quartz at bignell hill nebraska an osl age of 25 000 yr near the contact of peoria loess with the underlying gilman canyon formation shows that dust accumulation occurred early during the last glacial maximum lgm whereas at devil s den and eustis nebraska basal osl ages are significantly younger 18 000 and 21 000 yr respectively at all three localities dust accumulation ended at some time after 14 000 yr ago mass accumulation rates mars for western nebraska calculated using the osl ages are extremely high from 18 000 to 14 000 yr much higher than those calculated for any other pre holocene location worldwide these unprecedented mars coincide with the timing of a mismatch between paleoenvironmental evidence from central north america and the paleoclimate simulations from atmospheric global circulation models agcms we infer that the high atmospheric dust loading implied by these mars may have played an important role through radiative forcing in maintaining a colder than present climate over central north america for several thousand years after summer insolation exceeded present day values