modification and preservation of environmental signals in speleothems speleothems are primarily studied in order to generate archives of climatic change and results have led to significant advances in identifying and dating major shifts in the climate system however the climatological meaning of many speleothem records cannot be interpreted unequivocally this is particularly so for more subtle shifts and shorter time periods but the use of multiple proxies and improving understanding of formation mechanisms offers a clear way forward

first results from the radio plasma imager on image the radio plasma imager rpi is a 3 khz to 3 mhz radio sounder incorporating modern digital processing techniques and long electronically tuned antennas that is flown to large radial distances into the high latitude magnetosphere on the imager for magnetopause to aurora global exploration image satellite clear echoes similar to those observed by ionospheric topside sounders are routinely observed from the polar cap ionosphere by rpi even when image is located at geocentric distances up to approximately 5 earth radii using an inversion technique these echoes have been used to determine electron density distributions from the polar cap ionosphere to the location of the image satellite typical echoes from the plasmapause boundary observed from outside the plasmasphere are of a diffuse nature indicating persistently irregular structure echoes attributed to the cusp and the magnetopause have also been identified those from the cusp have been identified more often and with greater confidence

multi proxy speleothem record of climate instability during the early last interglacial in southern turkey abstract a stalagmite from dim cave in southern turkey contains a climate record documenting rapid and significant changes in amounts of precipitation between 132 ka and 128 ka during the penultimate glacial interglacial transition some u th dates have been compromised by carbonate dissolution but rigorous selection and tuning to 18o records from other speleothems has generated a robust age model growth rate was initially very slow but a rapid increase at 129 ka was accompanied by strong negative trends in 18o and 13c a combination implying the onset of much wetter conditions isotopic values at 129 ka suggest that groundwater recharge rates and biogenic activity in the soil zone exceeded those of the early holocene a significant isotopic enrichment event at 128 ka during which there was alternating aragonite and calcite deposition documents a strong drying event with a duration of 200 years a concurrent decrease in 87sr 86sr ratios indicates increased groundwater residence times and the cumulative evidence suggests amounts of rainfall fell from well above to slightly below present day levels similar 18o enrichment events are present in coeval speleothem records from southwest france and the northern alps and these together with pollen evidence from italy greece and the iberian margin of drier conditions at this time imply that a climate anomaly extended across the northern mediterranean borderlands the timing duration and structure of this episode are consistent with marine evidence of strong north atlantic cooling early in the last interglacial and there is a resemblance to the holocene 8 2 ka event recorded globally in many proxy climate archives

physical conditions at the base of a fast moving antarctic ice stream boreholes drilled to the bottom of ice stream b in the west antarctic ice sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1 6 bars of the ice overburden pressure these conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick the mechanics of ice streaming plays a role in the response of the ice sheet to climatic change

a high resolution record of holocene climate change in speleothem calcite from cold water cave northeast iowa high precision uranium thorium mass spectrometric chronology and 18o 13c isotopic analysis of speleothem calcite from cold water cave in northeast iowa have been used to chart mid holocene climate change significant shifts in 18o and 13c isotopic values coincide with well documented holocene vegetation changes temperature estimates based on 18o 16o ratios suggest that the climate warmed rapidly by about 3 c at 5900 years before present and then cooled by 4 c at 3600 years before present initiation of a gradual increase in 13c at 5900 years before present suggests that turnover of the forest soil biomass was slow and that equilibrium with prairie vegetation was not attained by 3600 years before present

asymmetric vegetation responses to mid holocene aridity at the prairie forest ecotone in south central minnesota abstract the mid holocene ca 8000 4000 cal yr bp was a time of marked aridity throughout much of minnesota and the changes due to mid holocene aridity are seen as an analog for future responses to global warming in this study we compare the transition into ca 9000 7000 yr ago and out of ca 5000 2500 yr ago the mid holocene mh period at kimble pond and sharkey lake located along the prairie forest ecotone in south central minnesota using high resolution 5 36 yr sampling of pollen charcoal sediment magnetic and loss on ignition properties changes in vegetation were asymmetrical with increasing aridity being marked by a pronounced shift from woodland forest dominated landscape to a more open mix of grassland and woodland savanna in contrast at the end of the mh grassland remained an important component of the landscape despite increasing effective moisture and high charcoal influxes median 2 7 4 0 vs 0 6 1 7 mm 2 cm 2 yr 1 at start of mh suggest the role of fire in limiting woodland expansion asymmetric vegetation responses variation among and within proxies and the near absence of fire today suggest caution in using changes associated with mid holocene aridity at the prairie forest boundary as an analog for future responses to global warming

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

complex response of a midcontinent north america drainage system to late wisconsinan sedimentation abstract the geomorphic evolution of mud creek basin in eastern iowa u s a serves to illustrate how geomorphic influences such as sediment supply valley gradient climate and vegetation are recorded in the alluvial stratigraphic record sediment supply to the fluvial system increased significantly during the late wisconsinan through a combination of periglacial erosion and loess accumulation subsequent evolution of the holocene alluvial stratigraphic record reflects long term routing of the late wisconsinan sediment through the drainage basin in a series of cut and fill cycles whose timing was influenced by hydrologic response to change in climate and vegetation when viewed in a regional context the alluvial stratigraphic record appears to reflect a long term complex response of the flu ial system to increased sediment supply during the late wisconsinan hydrologic and sediment supply changes accompanying the spread of euroamerican agriculture to the basin in the 1800s dramatically upset trends in sedimentation and channel behavior established during the holocene

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

landscape evolution alluvial architecture environmental history and the archaeological record of the upper mississippi river valley abstract the alluvial fill in the upper mississippi river valley umv is a palimpsest of past landscapes environments and physical evidence of human life ways the valley has undergone significant changes in fluvial style during the time humans have occupied its landscapes including changes in channel pattern location of depocenters and sediment lithology remnants of late glacial braidplains that predate human presence in the upper midwest occur as sandy terraces and terrace fills a subsequent major change in alluvial architecture resulted from fundamental changes in seasonal water and sediment input that marked the end of glacial meltwater input into the valley about 12 4 ka 10 500 14c yr b p a shift to net transport of sandy bedload and storage of fine grained overbank sediment on the floodplain accompanied the change to an island braided channel pattern at that time the holocene channel belt has been significantly narrower and the zone of sediment storage is reduced relative to that of the late glacial river major holocene depocenters include alluvial fans and colluvial slopes floodbasins natural levees and fluvial fans at the junction of large tributaries climate models and paleobotanical and isotopic studies indicate that shifts in large scale patterns of atmospheric circulation and moisture transport into the mid continent of north america induced hydrologic and vegetation changes that strongly influenced flood frequency and magnitude the delivery of sediment from tributary basins and the evolution of the umv landscape understanding the alluvial architecture of the valley and the temporal spatial distribution of biotic environments and processes that have buried mixed altered or destroyed archaeological deposits is essential to develop strategies for sampling the valley for evidence of past human activity and for properly interpreting the archaeological record

speleothem evidence for holocene fluctuations of the prairie forest ecotone north central usa carbon and oxygen isotopic trends from seven midwestern speleothems record significant offsets in the timing of middle holocene vegetation change interactions of dry pacific and moist gulf of mexico air masses maintained a sharp moisture gradient across iowa minnesota and wisconsin such that the arrival of prairie was offset by 2000 years between caves and pollen sites located only 50 km apart oxygen isotopes shift concomitantly with carbon in most cases although these changes are believed to represent increased evaporative enrichment of 18o prior to infiltration during the prairie period

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

role of the plasmapause in dictating the ground accessibility of elf vlf chorus this study explores the manner in which the plasmapause is responsible for dictating which magnetospheric source regions of elf vlf chorus are able to propagate to and be received by midlatitude stations on the ground first we explore the effects of plasmapause extent on ground based observations of chorus via a 3 month study of ground based measurements of chorus at palmer station antarctica l 2 4 50 s geomagnetic latitude and data on the plasmapause extent from the image euv instrument it is found that chorus normalized occurrence peaks when the plasmapause is at l 2 6 somewhat higher than palmer s l shell and that this occurrence peak persists across a range of observed chorus frequencies next reverse ray tracing is employed to evaluate the portion of the equatorial chorus source region distributed in radial distance and wave normal from which chorus is able to reach palmer station via propagation in a nonducted mode the results of ray tracing are similar to those of observations with a peak of expected occurrence when the plasmapause is at l 3 the exact location of the peak is frequency dependent this supports the conclusion that the ability of chorus to propagate to low altitudes and the ground is a strong function of instantaneous plasmapause extent and that peak occurrence of chorus at a given ground station may occur when the l shell of the plasmapause is somewhat beyond that of the observing station these results also suggest that chorus observed on the ground at midlatitude stations propagates predominantly in the nonducted mode

z mode sounding within propagation cavities and other inner magnetospheric regions by the rpi instrument on the image satellite 1 when the radio plasma imager rpi on the image satellite operates in the inner plasmasphere and at moderate to low altitudes over the poles pulses emitted at the low end of the rpi 3 khz to 3 mhz sounding frequency range can propagate in the z mode as well as the whistler mode at medium altitudes within the plasmasphere discrete z mode echoes with turning points earthward of image are often observed analogous in form to the regular and oblique z mode echoes found on topside sounder records in the polar regions where fpe fce 1 usually obtains the z mode echoes tend to be diffuse and to exhibit properties such as an intensity decrease at the local value of fce where there is a significant change with frequency in the topology of the z mode refractive index surface at altitudes up to a few thousand km in polar or near polar regions z mode soundings regularly provide information on the local value of fpe whereas passive recordings that depend upon indications of the upper hybrid resonance frequency fuh usually fail to do so within the transition region from the auroral zone to the plasmasphere and within the plasmasphere itself at altitudes from 2000 km upward we find evidence of the z mode trapping phenomenon noted previously in connection with natural wave emissions detected at polar latitudes this phenomenon involves echoes returning both from turning points in denser regions earthward of the sounder and from more tenuous regions above within z mode cavities discrete z mode echoes are found to propagate back and forth along geomagnetic field aligned paths between upper and lower altitude reflection points the echoes present unique forms depending upon whether image is located above or below a minimum in the altitude profile of the z mode cutoff frequency through an inversion process such echoes make possible determination of the electron density profile along geomagnetic field lines to distances of thousands of km above the location of image all on the basis of echo delay information within frequency bands only 20 to 30 khz wide the forms of such density profiles are in general agreement with results reported from conventional rpi x mode sounding from higher altitudes as well as with predictions based upon a diffusive equilibrium model of density along plasmasphere field lines

a stalagmite record of changes in atmospheric circulation and soil processes in the brazilian subtropics during the late pleistocene we present a high resolution 116 000 year carbon stable isotope record from a stalagmite from southern brazil which has been precisely dated using the u series method evaluation of carbon and oxygen isotope ratios together with the speleothem growth history suggest that the carbon isotopic composition of the speleothem is primarily controlled by biogenic co2 supply from the soil which is in turn affected by temperature and secondarily rainfall amount thus the speleothem provides evidence of paleoenvironmental change in southern brazil during the last glacial period predominantly high d 13 c values and low stalagmite growth rates reflect persistent cool conditions during most of the glacial period in subtropical brazil this cooling is probably related to an intensified extratropical circulation with more frequent and intense cold surges reaching a maximum at approximately 19 ky b p this cooling tendency is interrupted during periods of high obliquity values within the full glacial period at 93 85 and 47 40 ky b p and after 19 ky b p when a dramatic decrease in d 13 c marks the deglaciation time in the continent unlike d 18 o the d 13 c record does not exhibit a strong response to precessional forcing instead it shows a strong 40 ky obliquity signal here we propose that local temperature and thus the biological processes in the soil are primarily steered by the gradients of temperature between low and mid high latitudes which influence the meridional heat transport these gradient changes in turn are paced by obliquity r 2006 published by elsevier ltd

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

evidence for increased cool season moisture during the middle holocene isotopic analyses of three stalagmites from cold water cave northeast iowa united states reveal nearly identical 13 c trends from ca 7 to 2 ka however 18 o patterns differ by as much as 3 from 5 7 to 3 2 ka these disparate 18 o values reflect evaporative 18 o enrichment in meteoric water prior to infiltration suggesting that previously calculated temperatures based on a single cold water cave stalagmite overestimated middle holocene warming the coincidence of elevated middle holocene growth rates in the stalagmites with the lowest oxygen isotopic compositions indicates that the middle holocene was marked by a predominance of coolweather precipitation during a period of increased overall aridity

observations of structured optical emissions and particle precipitation equatorward of the traditional auroral oval 1 high sensitivity all sky images from south central alaska show the common occurrence of slow moving faint optical emissions at 557 7 and 427 8 nm well equatorward of the discrete aurora these emissions appear over a continuum of forms ranging from homogenous longitudinal bands to bands with irregular structure on their poleward edge to widely distributed arrays of vortex like curls and widely spaced spots on the order of 10 km in diameter these forms appear to correspond to various stages in the temporal evolution of nearly corotating precipitation regions populated by particles injected from more distant areas of the magnetosphere and may exhibit morphological control by an instability operating on cold plasma near the plasmapause although these phenomena are most common in the evening hours and typically persist for hours at a time one case demonstrates that the features can remain throughout the night on occasion these faint optical features appear to be colocated with regions of enhanced background counts in dmsp particle measurements and to be associated with the ring current outer radiation belt region equatorward of subauroral polarization streams saps the features can occur in conjunction with hf and mf radio frequency absorption at levels which are often too low to show up on routine absorption instruments such as the riometer this has practical implications on the operation and interpretation of ionospheric interaction experiments carried out in the subauroral region

response of bankfull flood magnitudes to holocene climate change uinta mountains northeastern utah long term variations in holocene fl ood magnitude were quantifi ed from the bankfull dimensions of abandoned channels preserved on flsurfaces in the northern uinta mountains of northeastern utah cross sectional areas of abandoned channels were reconstructed and relationships derived from the modern gage records were used to estimate bankfull discharges from bankfull cross section areas the results indicate systematic nonrandom variations of bankfull fl oods in the northern uinta mountains large fl oods as much as 10 15 greater than modern dominated from 8500 to 5000 calendar yr b p and again from 2800 to 1000 cal yr b p small fl oods as much as 15 20 less than modern characterize the periods from 5000 to 2800 cal yr b p and from 1000 cal yr b p to near present the middle and late holocene record of bankfull fl ood magnitude compares well with independent evidence for climatic variation in the area the early holocene record indicates that larger than modern bankfull fl oods coincide with warmer than modern mean annual temperature we hypothesize that an increased range of magnitude for seasonal solar radiation during the early holocene favored the accumulation and rapid melting of deep snowpacks in the high uinta mountains thus producing large fl oods despite warmer mean annual temperatures the episode of smaller than modern bankfull fl oods between 5000 and 2800 cal yr b p coincides with records of increased forest fi re frequency in the northern uintas larger than modern fl oods from 2800 to 1000 cal yr b p coincide with a local decrease in forest fi re frequency and evidence for minor local glacial readvances the decrease in fl magnitudes following 1000 cal yr b p corresponds to numerous local and regional records of warming during the medieval climatic anomaly

floodplain sedimentation in the upper mississippi valley natural versus human accelerated abstract understanding the time scales and pathways for response and recovery of rivers and floodplains to episodic changes in erosion and sedimentation has been a long standing issue in fluvial geomorphology floodplains are an important component of watershed systems because they affect downstream storage and delivery of overbank flood waters and they also serve as sources and temporary sinks for sediments and toxic substances delivered by river systems here 14c and 137cs isotopic dating methods are used along with ages of culturally related phenomena associated with mining and agriculture to determine rates of sedimentation and morphologic change for a reach of the upper mississippi river and adjacent tributaries in southwestern wisconsin and northwestern illinois the most important environmental change that influenced fluvial activity in this region during last 10 000 years involved the conversion of a late holocene mosaic of prairie and forest to a landscape dominated by cropland and pastureland associated with euro american settlement results presented herein for the upper mississippi valley umv show that the shift from pre agriculture natural land cover to landscape dominance by agricultural land use of the last 175 200 years typically increased rates and magnitudes of floodplain sedimentation by at least an order of magnitude accelerated overbank flooding led to increased bank heights on tributary streams and in turn contributed to more frequent deep flows of high energy these high energy flows subsequently promoted bank erosion and lateral channel migration and the formation of a historical meander belt whose alluvial surface constitutes a new historical floodplain inset against the earlier historical floodplain the new historical floodplain serves as a flume like channel that provides efficient downstream transport of water and sediment associated with moderate and large magnitude floods floodplains on lower tributaries however continue to experience rates of overbank sedimentation that are of anomalously high magnitude given improved land cover and land conservation since about 1950 this lower valley anomaly is explained by minimal development of historical agriculture period meander belts because of relatively low stream power in these channel and floodplain reaches of relatively low gradient in general long term pre agriculture rates of vertical accretion between about 10 000 and 200 years ago averaged about 0 2 mm yr 1 in tributary watersheds smaller than about 700 km2 and about 0 9 mm yr 1 on the floodplain of the upper mississippi river where the contributing watershed area increases to about 170 000 km2 on the other hand rates of historical vertical accretion during the period of agricultural dominance of the last 200 years average between 2 and 20 mm yr 1 with short episodes of even higher rates during times of particularly poor land conservation practices significant hydrologic effects of mining and agricultural started by the 1820s and became widespread in the study region by the mid 19th century the hydrologic and geomorphic influences of mining were relatively minor compared to those related to agriculture high resolution dating of floodplain vertical accretion deposits shows that large floods have frequently provided major increments of sedimentation on floodplains of tributaries and the main valley upper mississippi river the relative importance of large floods as contributors to floodplain vertical accretion is noteworthy because global atmospheric circulation models indicate that the main channel upper mississippi river should experience increased frequencies of extreme hydrologic events including large floods with anticipated continued global warming instrumental and stratigraphic records show that coincident with global warming a shift to more frequent large floods occurred since 1950 on the upper mississippi river and these floods generally contributed high magnitudes of floodplain sedimentation

force mass and energy budgets of the crary ice rise complex antarctica the stress mass and energy dissipation budgets of crary ice rise are analyzed using field data collected during the 1983 1985 austral summers and in previous field programs in addition the net back pressure and ice discharge rate along the grounding lines of ice streams are calculated to assess the effect of the ice rise on the surrounding flow comparison of the ice rise budgets with the analysis of grounding line data confirms the influence of the ice rise on ice sheet stability and suggests that crary ice rise may have formed recently in response to an acceleration of one of the ice streams it is concluded that feedback between ice stream acceleration and ice rise formation may control the future evolution of the above ice stream and promote long term grounding line stability in the face of strong natural fluctuations

small scale field aligned plasmaspheric density structures inferred from the radio plasma imager on image 1 among the objectives of the radio plasma imager rpi on image is the observation of the earth s plasmasphere from the satellite s polar orbit with apogee 8 re geocentric distance and perigee near 1200 km altitude this objective is here pursued by 1 remote sounding from high altitude regions outside the main plasmasphere 2 sounding within the plasmasphere and 3 in situ passive measurements of natural wave activity during sounding of the plasmasphere from both outside and inside the plasmapause rpi echoes that follow non field aligned ray paths are usually not the discrete traces on range versus frequency records plasmagrams that are predicted by ray tracing simulations in smooth magnetospheric density models instead such rpi echoes exhibit various amounts of spreading from 0 5 re to 2 re in virtual range range assuming free space speed of light propagation the range spreading is attributed to scattering from partial reflection from and propagation along geomagnetic field aligned electron density irregularities there exists a substantial body of theoretical work on mechanisms that might explain the appearance of such irregularities both within and beyond the plasmasphere that the spread producing irregularities are field aligned is suggested by the efficiency with which rpi excites discrete echoes that propagate along the geomagnetic field sometimes into both hemispheres the spatial distributions and scale sizes of the spread producing irregularities remain to be investigated the rpi echo data however coupled with earlier evidence from topside sounders and whistler mode instruments suggest that they can have cross field scale sizes within a range from 200 m to over 10 km and electron densities within 10 of background rpi is found capable of detecting plasmapause locations from distances of 3 re or more when minimal signal integration is used the location and range of density values of a steep plasmapause can be determined from distances of order 1 re and echoes can at times be returned from points extending inward from the plasmapause to locations where the electron density reaches 3000 cm 3 which is usually at l 3

a large drop in atmospheric 14c 12c and reduced melting in the younger dryas documented with 230th ages of corals paired carbon 14 14 c and thorium 230 230 th ages were determined on fossil corals from the huon peninsula papua new guinea the ages were used to calibrate part of the 14 c time scale and to estimate rates of sea level rise during the last deglaciation an abrupt offset between the 14 c and 230 th ages suggests that the atmospheric 14 c 12 c ratio dropped by 15 percent during the latter part of and after the younger dryas yd this prominent drop coincides with greatly reduced rates of sea level rise reduction of melting because of cooler conditions during the yd may have caused an increase in the rate of ocean ventilation which caused the atmospheric 14 c 12 c ratio to fall the record of sea level rise also shows that globally averaged rates of melting were relatively high at the beginning of the yd thus these measurements satisfy one of the conditions required by the hypothesis that the diversion of meltwater from the mississippi to the st lawrence river triggered the yd event

parent material influence on soil response to vegetation change southeastern minnesota u s a abstract soil morphology changes dramatically across the former transition from forest to grassland in the midwestern u s a that vegetation boundary shifted as a result of holocene climatic change and fire suppression following euroamerican settlement but the timescale of soil response to those vegetation changes and the factors that influence it are poorly known on steep colluvial slopes of southeastern minnesota mollisols with thick dark a horizons typically associated with grassland are found today under deciduous forest soils with much thinner and or lighter colored a horizons occur immediately up and downslope of the forest covered mollisols most of the soils with thick a horizons are not in topographic settings found to favor organic matter accumulation in other landscapes principal component analysis highlights important axes of textural and mineralogical variation among horizons of these soils often related to parent material properties soils with and without thick a horizons are separated along one principal component heavily loaded by high ph reflecting the presence of dolomite fragments in the hillslope sediment that soils with thick dark a horizons formed in stable c isotope analysis reveals that the mollisols with thick dark a horizons under forest had c input from vegetation with abundant c4 grasses in the past public land survey data also indicate replacement of grassland or savanna by forest since 1854 on colluvial slopes where forest covered mollisols occur today we propose that the soils with thick dark a horizons under forest today reflect a lagged response to vegetation change from grassland or savanna to closed forest their thick a horizons and some c4 derived organic matter may be preserved because dolomite weathering releases abundant ca and mg that favor aggregation and organic matter stabilization soils with thin a horizons just up or downslope may also have formed partly under grasses but have responded more quickly and or more substantially to vegetation change

late quaternary landscape evolution of the upper dnister valley western ukraine abstract the broad and flat valley of the upper dnister in western ukraine is characterized by a complex setting of ecotopes and a relatively natural state of floodplain dynamics excellent geo archives extended peat bogs and postglacial river terraces document late quaternary landscape evolution with special regard to changes in fluvial morphodynamics vegetation and human impact the terraces were studied by detailed geomorphological mapping as well as sedimentological and pedological analyses in connection with information from historical maps vegetation history was studied by palynological methods in combination with radiocarbon dating two late pleistocene nt1 and nt2 and seven holocene h1 h7 river terraces are evident the ecological transformation during the period of climatic warming at the end of the last glacial maximum lgm caused the most significant change in runoff and sediment load as well as in the vegetation cover the dnister changed from a braided to a meandering river system it was soon after the spreading of postglacial forests with an increasing dominance of broadleaved trees that the pollen diagrams reflected the first signs of anthropogene influence intensified agricultural land use since the iron age is mirrored in the increasing amount of herbaceous pollen as well as the rise of fluvial sediment redeposition modern hydraulic engineering on the dnister has caused visible transformations because of these impacts the stability of the upper dnister ecosystem has decreased significantly and is now highly sensitive to environmental change

holocene environments reconstructed from plant macrofossils in stream deposits from southeastern nebraska usa plant macrofossils from sediments exposed in cutbanks of small streams on the great plains yield details of the history of the vegetation and climate and potentially of the hydrologic behaviour of the streams during the past 9000 radiocarbon years five zones are recognized by visual inspection macrofossils in zone 1 c 9000 to c 8500 bp indicate that upland deciduous forest was sparsely present but that prairie weedy and wetland habitats were also well established zone 2 c 8500 to c 5800 bp is sparsely represented but upland trees disappeared riparian trees and wetland plants were sparse and weeds and prairie taxa fluctuated from low to high values this interval was probably the warmest driest part of the holocene zone 3 c 5800 to c 3100 bp had a rich riparian forest along with well developed prairies disturbed habitats and wetlands this period was more moist but had intermittent droughts zone 4 c 3100 to c 2700 bp has peaks in many prairie and weedy elements whereas riparia

geology of the pre illinoian sediment in the bridgeport terrace lower wisconsin river valley wisconsin the bridgeport terrace in the lower wisconsin valley is characterized by pre iuinoian till like sediment at the west end of the terrace that grades eastward into fluvial sediment with eastward dipping foreset beds that underlie an eastward sloping depositional surface the bridgeport sediment overlies a rock bench that protected it from erosion stratigraphic relationships indicate the valley was already incised below the rock bench by the time the bridgeport sediment was deposited a pre iuinoian age for the bridgeport terrace is indicated by deeply weathered sediment and by the reversed remanent magnetism of fine grained units in the terrace gravel furthermore a well developed probable sangamon age paleosol is developed underneath two loess units that overlie the fluvial sediment the clay mineralogy of till like sediment at the west end of the terrace is similar to pre illinoian till units in iowa and west central wisconsin

recent changes in the dynamic condition of the ross ice shelf antarctica variations in the amplitude of radar echoes from the bottom of the grid western half of the ross ice shelf have been analyzed contrary to the results of a similar analysis performed for the grid eastern sector of the ice shelf bands of low signal strength downstream from both crary ice rise and the siple coast do not correlate with modern flow lines the difference in direction between the radar bands downstream of crary ice rise and the present velocity vectors and the absence of a comparable trend farther east suggest to us that the grounding line around crary ice rise retreated within the last 1000 years this hypothesis is reinforced by the observation of several domes and hollows in ice thickness downstream of crary ice rise which are similar to a hollow now located in the wake of the ice rise and a dome on its eastern flank we interprete this as evidence for a rapid increase in flow around the ice rise which carried downstream the ice topography formed around the ice rise similar but less detailed evidence found downstream of the siple coast suggests that there was a regional retreat of the west antarctic grounding line

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

basin scale reconstruction of the geological context of human settlement an example from the lower mississippi valley usa abstract large river valleys integrate hydroclimatic change differently than smaller ones whereas sedimentary deposits in smaller valleys serve to record temporally short lived and or low amplitude hydrological and geomorphic responses larger valleys are relatively insensitive to changes at these scales however because of their size larger valleys archive sedimentary records of longer term geographically extensive flooding and climate related landscape evolution recent work in the upper tensas basin of the lower mississippi river valley usa demonstrates the utility of analyzing sediments from a large geomorphically and fluvially complex river basin as a proxy for significant climatic and landscape changes and their relation to human history a 6000 year sedimentary archive is used to explore the interplay between climate change landscape evolution and human responses to changing environmental parameters once the mississippi river basin stabilized following glacial retreat and deceleration of sea level rise the history of the river and its inhabitants was dominated by long periods of landscape stability punctuated by episodes of significant landform alteration modifications of the landscape are correlated with intervals of rapid climate change recorded in globally distributed multiproxy data sets periods of significant landscape instability and fluvial re adjustment are recorded for the periods ca 4800 3800 3000 2500 and 1000 800 cal bp these punctuations in the history of the lower mississippi valley are related to periods of major cultural transformation and suggest climate change plays a significant role in the long term history of human occupation in this river valley this research demonstrates how geoarcheology can provide information on human environmental relationships and long term landscape histories at temporal and spatial scales relevant to understanding cultural responses to climate change

spread f and the structure of equatorial ionization depletions in the southern anomaly region combined optical and radio sensors provide a unique characterization of the structure of equatorial emission depletion regions connected to rising bubbles over the magnetic equator in chile as part of the miseta campaign in fall 1994 a ccd enhanced all sky imaging photometer provided optical images of the postsunset appearance and motions of the depletion bands at a magnetic dip latitude of 11 s concurrently a digisonde collocated with the photometer monitored the appearance of spread f in between the ionograms the sounder operated as a doppler interferometer identifying the locations of f layer irregularities associated with the spread f they were found to lie inside the emission depletion regions the hf sounder requiring orthogonality with the field aligned f layer irregularities to generate the spread structure tracked these irregularities inside the emission depletion bands as they drifted eastward ray tracing simulations show that the radio waves become trapped within the depletion regions when the depletions are within 300 km of the sounder site model calculations indicate that the sounder rays encounter orthogonality with the earth s magnetic field within the depletion bubble southward from the site consistent with the local dip angle the combination of optical images with hf radio sounding demonstrated that radio imaging in the equatorial ionosphere can be done with a digital ionosonde that operates as a doppler interferometer the digisonde measurements and ray tracing show for the first time that the spread f signatures on ionograms are the result of coherent scatter from irregularities primarily within the walls of the depletion

impact of euro american settlement on a riparian landscape in northeast iowa midwestem usa an integrated approach based on historical evidence floodplain sediments fossil pollen plant macrofossils and insects european settlement and attendant forest clearance and agricultural activities in northeastern iowa caused changes in the landscape vegetation insect fauna and water quality unequalled in rate and magnitude since the melting of wisconsinan glaciers historical documents show that the upper part of the roberts creek drainage basin was settled between ad 1840 and 1856 and the area was under intensive cultivation by 1880 extensive soil erosion beginning at this time resulted in increased runoff and more frequent flooding aggradation rates increased by one to two orders of magnitude over those in presettlement times and the entire floodplain was covered with up to 1 m of sediment channel widening between about 1880 and 1930 allowed the stream to accommodate greater floods overbank deposition decreased and further deposits were restricted mostly to the channel belt the presettlement vegetation was a stable mix of wet meadows and riparian shrubs on the floodplain a rich aquatic community in the stream

comparison of multiple proxy records of holocene environments in the midwestern united states we compare four emerging approaches to reconstructing holocene vegetation and climate from south of the glacial border in northeastern iowa united states pollen plant macrofossils carbon isotopic 13 c values from alluvial organic matter and carbon isotopic values in stalagmites from a nearby cave all show similar paleovegetational and paleoclimatic trends during the holocene pollen and plant macrofossils show a rapid change from forest to prairie about 6000 cal yr b p followed by a return of oaks to a presumably savanna like community about 3500 cal yr b p the 13 c values in alluvial organic matter and the percentage of c 4 plants both increase ca 6300 cal yr b p and then decrease in the last 3500 years in the cave 13 c values rise beginning at 6000 cal yr b p to a broad peak ca 4500 to 3000 cal yr b p and decrease thereafter pollen and plant macrofossils record the composition of the vegetation that produced the isotopic signals and verify c 3 c 4 interpretations based on the isotopic records we demonstrate that these methods are complementary but that any single method will provide an accurate reconstruction of past environments