new absolute time scale for the quaternary climate in the chinese loess region by grain size analysis continuous loess sequences in china provide not only information for paleoclimatic reconstructions but also permit the establishment of a reliable absolute time scale for the quaternary an empirical relationship is developed between the grain size of the loess and the sedimentation rate in a short well dated reference section step 1 this relationship is extrapolated to the complete quaternary section step 2 after derivation of the sedimentation rate by measuring the grain size in all the layers of the sequence the duration of deposition of each layer is obtained by dividing thickness by sedimentation rate and subsequently the age of each point in the sequence is calculated by adding together all the duration values of the overlying layers the chronology of the extrapolated part of the section obtained in this way is independent of dates provided by other methods and may also be checked by them

an 15 000 year record of el nino driven alluviation in southwestern ecuador debris flows have deposited inorganic laminae in an alpine lake that is 75 kilometers east of the pacific ocean in ecuador these storm induced events were dated by radiocarbon and the age of laminae that are less than 200 years old matches the historic record of el nino events from about 15 000 to about 7000 calendar years before the present the periodicity of clastic deposition is greater than or equal to 15 years thereafter there is a progressive increase in frequency to periodicities of 2 to 8 5 years this is the modern el nino periodicity which was established about 5000 calendar years before the present this may reflect the onset of a steeper zonal sea surface temperature gradient which was driven by enhanced trade winds

holocene loess in illinois revealed by osl dating implications for stratigraphy and geoarchaeology of the midwest united states abstract most uplands in illinois are blanketed by late wisconsinan peoria loess and the conventional wisdom is that little or no dust accumulation occurred during the holocene 11 7 ka to present in a recent effort to investigate if illinois holocene loess was deposited and preserved we applied optically stimulated luminescence osl dating to seven locations where loess is thick these include four sites along the mississippi river in westernmost illinois and three loess bluffs south of the illinois river in central illinois our results suggest that holocene aged loess is preserved along the mississippi river in the state of illinois in contrast no holocene loess was found south of the illinois river in the central portion of the state in addition to its spatially limited distribution holocene loess thicknesses range only up to 1 m nevertheless loess stratigraphy and osl dating results provide the first holocene loess reported east of the missouri river our new finding also resolves the long time archeological puzzle that a number of artifacts of several thousand years old were buried by upland loess along the mississippi river

late quaternary chronostratigraphic framework of terraces and alluvium along the lower ohio river southwestern indiana and western kentucky usa abstract the lower ohio river valley is a terraced fluvial landscape that has been profoundly influenced by quaternary climate change and glaciation a modern quaternary chronostratigraphic framework was developed for the lower ohio river valley using optically stimulated luminescence osl dating and allostratigraphic mapping to gain insights into the nature of fluvial responses to glacial interglacial stadial interstadial transitions and holocene climate change river deposits t0 youngest to t7 oldest were mapped along a 75 km reach of the lower ohio river and were dated using 46 osl and 5 radiocarbon samples the examination of cores combined with osl and radiocarbon dating shows that fluvial sediments older than marine oxygen isotope stage mis 2 are present only in the subsurface aggradation during mis 6 illinoian glaciation filled the valley to within 7 m of the modern floodplain and by 114 ka mis 5e sangamon interglacial the ohio river had scoured the mis 6 sediments to 22 m below the modern floodplain surface there were no fluvial sediments in the valley with ages between mis 5e and the middle of mis 3 the mis 3 ages 39 ka and stratigraphic position of t5 deposits suggest the ohio river aggraded 8 14 m during mis 4 or mis 3 near the end of mis 3 the ohio river incised the mid last glacial mid wisconsinan deposits 10 m and began aggrading again by 30 ka aggradation continued into mis 2 with maximum mis 2 aggradation occurring before 21 ka which is coincident with the global last glacial maximum lgm as the ohio river adjusted to changing fluxes in sediment load and discharge following the lgm it formed a sequence of fill cut terraces in the mis 2 outwash that get progressively younger with decreasing elevation ranging in age from 21 ka to 13 ka from 14 ka to 13 ka the ohio river rapidly incised 3 m to form a new terrace and by 12 ka at the onset of the holocene the ohio river established a meandering channel pattern the river formed a broad floodplain surface from 12 ka to 6 ka and then incised 1 m and formed a fill cut terrace from 6 ka to 5 ka after 5 ka likely in response to mid holocene drought in north america the ohio river incised 5 m and by 4 ka the river began aggrading again the ohio river has aggraded 4 m since aggradation began at 4 ka the chronostratigraphic framework and reconstructed history developed here suggest that the lower ohio river is highly sensitive to glacial interglacial transitions and abrupt holocene climate change and responds rapidly to these allogenic forcings

eolian processes and heterogeneous dust emissivity during the 1930s dust bowl drought and implications for projected 21st century megadroughts the 1930s dust bowl drought on the us great plains was an environmental crisis with failure of agricultural systems landscape denudation and elevated atmospheric dust loads poor agricultural practices were implicated for triggering widespread eolian erosion and heightened dust emissions but this assumption is called into question this study classified land surface changes in southwest kansas from aerial images taken in 1936 and 1939 to infer surficial processes dust sources and associated emissivity in total seven distinctive surface classes were identified from an arcgis analysis of spectral reflectance values connected to surface vegetation cover and eolian activity demonstrating a strikingly heterogeneous landscape response to the drought stratigraphic studies indicate accumulation of up to 4 m of eolian sand in places with erosion of a subjacent silty pre 1930s soil surface potential dust emissivity estimates for particulate matter were derived from the distribution of classified land surf

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

east asia winter monsoon variations on a millennial time scale before the last glacial interglacial cycle grain size and magnetic susceptibility measurements on samples from a typical loess palaeosol sequence on the central chinese loess plateau are used to reconstruct the pleistocene east asian monsoon climate the coarse grained fraction i e the weight percentage 30 m of the bulk grain size distribution is used as a sensitive proxy index of the east asia winter monsoon strength on the basis of an absolute time scale time series variations of this proxy show that winter monsoon strengths varied on millennial time scales during the periods 145 165 240 280 320 350 390 440 600 640 860 890 900 930 and 1330 1400 kyr bp the wavelength of these climatic oscillations varied between 1 89 and 4 0 kyr as is shown by spectral analysis using the multitaper method although numerical simulation experiments show that high frequencies also can arise from measurement errors in the grain size analysis the frequencies prove to be sufficiently stable when the spectral analysis is repeated with a different number of tapers for the time being we do not correlate these climatic oscillations with palaeoclimatic records in the north atlantic deep sea sediments because both time scales need to be further improved our data however certainly demonstrate that millennial scale east asian winter monsoon variations in the last 1 4 million years can be detected from terrestrial loess records copyright 1999 john wiley sons ltd

a 250 000 year record of lunette dune accumulation on the southern high plains usa and implications for past climates abstract a concentration of lake playa basins occurs on the southern high plains shp of northwest texas and eastern new mexico associated with these lake playas are lee side lunettes positioned on their southeast margins ranging in height from 1 5 to 10 m an osl dating program was applied to 30 samples from lunettes associated with large lakes and small playa basins samples were extracted from trenched dune sections or from deep cores earlier shp lunette investigations show depositional ages primarily in the late wisconsin and holocene this research extends the timing of lunette accretion to the middle pleistocene the earliest recorded deposition for these features the expanded chronology permitted investigation into dune morphology on nested lunettes built on contracting lake margins outer lunettes formed prior to inner dunes but simultaneous deposition occurred on downwind ridges as younger lee side dunes were constructed large lake lunettes were inactive during discreet shp pluvial episodes from early wisconsin to lgm conversely these lunettes accreted when climatic conditions promoted basin desiccation and aeolian deflation this suggests their mode of formation contrasts with lunette models recognized for other regions from post lgm to earliest holocene active lunette accretion occurred from 16 to 11 ka followed by a significant period of lunette construction during the mid holocene late holocene lunette deposition was interrupted by intervals of landscape stability lunette deposition between 1300s and 1700s corresponds with drought intervals recognized in tree ring records from adjacent regions recent lunette activity on the plateau is contemporary with the 1930 s dust bowl further insight into shp response to changing climatic conditions was given by comparing lunette depositional events with previous investigations on sedimentation intervals for draws lake playa basins and sand sheets

signature of magnetic enhancement in a loessic soil in nebraska united states of america our study of a loessic soil profile from east central nebraska shows that the a horizons of the modern soil are characterized by higher concentrations of fine grained b0 1 am magnetic minerals this pedogenic magnetic component leads to higher values of concentration dependent parameters such as magnetic susceptibility v isothermal remanent magnetization irm and anhysteretic remanent magnetization arm combined with increases in frequency dependent susceptibility vfd and arm irm ratios hysteresis properties are relatively insensitive towards the presence of this pedogenic magnetic component the magnetic properties of the soil profile are dominated by ferrimagnetic magnetite or maghemite analyses of bsoftq irm sirm and bhardq irm hirm however do show that approximately 80 90 of the remanence carrying magnetic component exists in the form of hematite or goethite and that the magnetically enhanced horizons are enriched in both ferri and antiferromagnetic minerals the pedogenic magnetic component is most likely caused by the conversion of paramagnetic iron bearing minerals to ferriand antiferromagnetic minerals soil compaction lessivage or decalcification cannot explain the observed magnetic soil properties magnetic analyses of loess paleosol sequences from the midwestern united states may yield valuable information about regional variability of paleoclimate based on the fine grained nature of the pedogenic magnetic component we expect grain size dependent proxies arm arm irm vfd to yield better paleoclimatic information than low field magnetic susceptibility used in previous analyses d 2004 elsevier b v all rights reserved

correlation of climate cycles in middle mississippi valley loess and greenland ice two complete late wisconsin loess successions in the middle mississippi river valley reveal 39 and 41 alternating paleosol a and c horizons striking changes in soil color iron content and carbonate content define four major and two minor paleosol a horizon complexes which were interpreted to represent wisconsin interstadials 1 2 3 4 and semiinterstadials 1 5 and 2 5 respectively the timing of wisconsin interstadials matches that of corresponding greenland interstadials midcontinent loess and greenland ice records as well as rates of atmospheric 14c production have periodicities in common suggesting a solar influence only a persistent heat and moisture supply could produce prominent paleosol complexes near the continental ice margin this record suggests that el nino southern oscillation variability has amplified solar forcing and resultant tropical heat and moisture transport played a significant role in millennial and centennial scale climate cycles during the late wisconsin glaciation over the northern hemisphere

relationships between soil organic carbon and precipitation along a climosequence in loess derived soils of the central great plains usa abstract understanding the effects of varying average annual precipitation on organic carbon in loess derived soils can assist the reconstruction of paleoclimates from buried paleosol sequences as well as facilitate the prediction of soil organic carbon soc change in response to future climate forcings the goal of this study was to examine trends in soc and its relationships to particle size distribution psd and soil aggregation in surface loess along a precipitation gradient in the central great plains of the united states soil cores were collected from undisturbed portions of seven pioneer cemeteries to a depth of 50 cm across a transect spanning northwest kansas into western missouri pioneer cemeteries were selected to minimize any effect from land use and disturbance soil cores were cut into 2 5 cm segments which were prepared and analyzed for bulk density soc and aggregated and disaggregated psd from laser diffraction annual precipitation depth and psd were used in the multivariate analyses to explain the distribution of soc and complexed organic carbon coc an indicator of soil physical quality as well as bulk density precipitation was the variable that most explained soc coc and bulk density a proxy for microaggregation geometric mean shift gms was developed for this study and defined as the difference between the geometric mean of the aggregated and disaggregated psds microaggregation occurred below an oc clay ratio of 0 163 in the upper 50 cm of the loess derived soils analyzed among the variables considered coc had the highest coefficient of determination r 2 0 871 our findings indicate that future climate forcings resulting in precipitation changes may have an effect on coc and thus soil physical quality of loess derived soils

a new method to constrain soil development time using both osl and radiocarbon dating abstract the time over which a soil has developed since the parent materials were deposited and subaerially exposed referred to here as soil development time is of considerable interest to pedologists geomorphologists geologists archeologist and paleoclimatologists soil development time has been estimated both indirectly based on the degree of soil development and directly by radiocarbon dating of soil organic matter optically stimulated luminescence osl dating of parent material deposition and through exposure dating using cosmogenic nuclides here we propose a new model for estimating the soil development time by subtracting the low molecular weight lmw or pyrolysis volatile 14c age of the uppermost a horizon of a soil from the osl age of the c horizon of the parent material soil time osl age parent c horizon 14c age lmw fraction of uppermost a horizon the lmw organic compound fraction represents the most mobile carbon compounds in the soil organic carbon pool and is least resistant to physical chemical and biodegradation in the soil environment potentially yielding the youngest 14c age osl dating of the c horizon provides an estimate of the time since deposition of unconsolidated parent material thus the difference in the two ages can be used to estimate the time that has elapsed for pedogenic alteration from deposition of the parent material to the most recent additions of soil organic carbon we applied this new approach in four scenarios 1 in a modern soil that developed downward in loess 2 in a buried soil developed in aggrading loess a cumulative soil which grows upward 3 in a buried soil formed in dune sand and 4 in a counterexample showing that simple subtraction is not always appropriate because of changes in soil s parent materials from loess to sand these case studies reveal that this approach constrains soil development time more reasonably than can be commonly done with either osl or radiocarbon dating alone the difference in 14c ages between the uppermost and lowermost sola of a soil profile almost certainly underestimates the time of soil development and the difference in osl ages taken from sediment units overlying and underlying a buried soil most likely overestimates it a combination of osl and radiocarbon dating better constrains soil development time broadening the applications of these two dating methods beyond their use for cross checking the accuracy of the other dating method

sediment accumulation and sedimentation rates in playas on the high plains of western kansas usa abstract grassland ecosystems are some of the most modified ecosystems in the world largely due to conversion to cropland and the high plains of the central united states is no exception playa wetlands are ubiquitous features of the high plains and have experienced increased sediment accumulation which is the primary cause of reduced playa ecosystem function seven playas on the high plains of western kansas five with cropland dominated watersheds and two with grassland dominated watersheds were examined to quantify the amount of recent past 100 175 yr sediment accumulated within playas and to establish chronologies of playa filling thicknesses of recent sediment were determined in the field and soil cores were extracted from study playas four playa cores were analyzed for 210pb to establish the chronology of recent sediment accumulation and sedimentation rates chronologies of pre agricultural sediments and sedimentation rates were calculated by 14c dating of buried soils preserved within playa cores radiocarbon ages indicate playa stratigraphic records span the holocene and that these playas have persisted on the landscape since at least the late pleistocene long term natural sedimentation rates were highest during the pleistocene holocene transition and early holocene at 2 4 cm century during the middle and late holocene sedimentation rates averaged 1 cm century cropland playas accumulated 10 15 cm of recent sediment at average rates of 6 5 10 cm century while grassland playas accumulated only 2 cm of recent sediment conversion of watersheds to cropland has greatly accelerated sediment accumulation within playas which is generally resulting in a decline in critical playa ecosystem functions

el nino variability off peru during the last 20 000 years here we present a high resolution marine sediment record from the el nino region off the coast of peru spanning the last 20 000 years sea surface temperature photosynthetic pigments and a lithic proxy for el nino flood events on the continent are used as paleo el nino southern oscillation proxy data the onset of stronger el nino activity in peru started around 17 000 calibrated years before the present which is later than modeling experiments show but contemporaneous with the heinrich event 1 maximum el nino activity occurred during the early and late holocene especially during the second and third millennium b p the recurrence period of very strong el nino events is 60 80 years el nino events were weak before and during the beginning of the younger dryas during the middle of the holocene and during medieval times the strength of el nino flood events during the last millennium has positive and negative relationships to global and northern hemisphere temperature reconstructions

introducing an improved multi proxy approach for paleoenvironmental reconstruction of loess paleosol archives applied on the late pleistocene nussloch sequence sw germany to study paleoenvironmental and paleoclimatic changes in loess paleosol sequences lps most proxies except for screening parameters have often been applied at low sampling density typically 1 2 per stratigraphic unit this might lead to results that overlook paleovegetation effects that were not only restricted to paleosols but also affected underlying loess material here a new profile was investigated in the loess paleosol sequence lps at nussloch sw germany using a high resolution multi proxy approach to improve our understanding of the late pleistocene record which serves as a standard sequence in central europe this approach included several geophysical and geochemical parameters as well as frequencies of root related features grain size distribution and osl dating enabled correlation of the new section with the nearby standard profile p4 revealing considerable variations of the thickness of stratigraphic units due to local small scale variations in depositional conditions and paleogeomorphology of longitudinal greda structures magnetic susceptibility revealed weathering of the entire lps under more humid conditions than those described for se european and chinese lps it further enabled a rough differentiation between an interval that was influenced by pedogenic processes lower pleniglacial to lower part of the upper pleniglacial and loess that was not markedly affected by soil formation color measurements confirmed these results and enabled the identification of several paleosols in the upper pleniglacial for the uppermost part of the lps low magnetic susceptibility and inorganic geochemical parameters such as low fe ti ratios indicated only weak or no pedogenic influence in contrast considerable variations e g in carbonatic carbon contents and organic carbon nitrogen c n ratio might be related to the high frequency of calcified root remains up to 200 rhizoliths m 2 and up to 12 500 microrhizoliths m 2 in certain depth intervals of the lps entailing incorporation and degradation of organic matter frequencies of biopores which are suggested to derive largely from former root growth and to a lesser extent from insect and earthworm bioturbation were related to paleosols and thus confirmed the pedostratigraphy an improved assessment of terrestrial sedimentary archives can be achieved by high resolution multi proxy analysis and three dimensional investigation of terrestrial sediments that are prone to bioturbation c 2014 elsevier b v all rights reserved

carbon cycle sequestration in buried soils rapid deposition of wind borne silt after the end of the last glacial period buried a large reservoir of organic carbon in the deep soil geochemical analyses suggest that this sequestered soil carbon could be released to the atmosphere if exposed to decomposition

geodetic imaging mega thrust coupling beneath the himalaya abstract the 2015 gorkha earthquake highlights the urgent need for understanding seismicity in the himalaya which is envisaged to be governed by stick slip behavior on the main himalayan thrust mht beneath the southern edge of the tibetan plateau we integrate gps data in southern tibet with those public available for geodetic imaging of the frictional properties of the seismogenic fault aiming at a better understanding of the strain buildup on the largest continental mega thrust and its relation to great earthquakes the postseismic effects due to viscoelastic relaxation from three large earthquakes 1934 bihar mw 8 4 1950 assam mw 8 5 and 2005 kashmir mw 7 6 are corrected in the coupling modeling the new model constrained by all gps velocities spirit leveling rates and insar interferograms spread between the eastern and western himalayan syntaxes confirms previous findings that the mht is fully locked in the uppermost 15 20 km of crust below which an abrupt decrease in coupling occurs with coefficients reduced from 0 8 to 0 2 within a narrow transition zone of 30 50 km in width generally the coupling pattern is modified little after removing the postseismic contributions of the 1934 bihar and 2005 kashmir earthquakes from gps velocities meanwhile the high coupling region around the eastern himalayan syntaxis is widened from 100 km to 140 km after removing the perturbation induced by the 1950 assam earthquake the spatial variation of fault coupling coincides well with the epicentral areas of historical earthquakes in the sikkim and bhutan himalaya the locking zone is wider than elsewhere along the himalaya the strain buildup for the past 300 years under the bhutan himalaya would be sufficient to power a maximum earthquake of mw 8 6

a 10 000 year record of dune activity dust storms and severe drought in the central great plains dune fi elds and loess deposits of the great plains of north america contain stratigraphic records of eolian activity that can be used to extend the short observational record of drought we present a 10 000 yr reconstruction of dune activity and dust production in the central great plains region based on 95 optically stimulated luminescence ages the integration of data from both eolian sand and loess is an important new aspect of this record clusters of ages defi ne episodes of extensive eolian activity which we interpret as a response to frequent severe drought at 1 0 0 7 ka and 2 3 4 5 ka with peaks centered on 2 5 and 3 8 ka sustained eolian activity occurred from 9 6 to 6 5 ka parts of this record may be consistent with hypotheses linking holocene drought to sea surface temperature anomalies in the pacifi c or atlantic oceans or to the el nino southern oscillation phenomenon but the record as a whole is diffi cult to reconcile with any of these hypotheses

el nino like climate teleconnections in new england during the late pleistocene a glacial varve chronology from new england spanning the 4000 year period from 17 500 to 13 500 calendar years before the present was analyzed for evidence of climate variability during the late pleistocene the chronology shows a distinct interannual 3 to 5 years band of enhanced variability suggestive of el nino southern oscillation enso teleconnections into north america during the late pleistocene when the laurentide ice sheet was near its maximum extent and climatic boundary conditions were different than those of today this interannual variability largely disappears by the young end of the 4000 year chronology with only the highest frequency components roughly 3 year period persisting this record provides evidence of enso like climate variability during near peak glacial conditions

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

reconstruction of sea surface conditions at middle to high latitudes of the northern hemisphere during the last glacial maximum lgm based on dinoflagellate cyst assemblages abstract a new calibration database of census counts of organic walled dinoflagellate cyst dinocyst assemblages has been developed from the analyses of surface sediment samples collected at middle to high latitudes of the northern hemisphere after standardisation of taxonomy and laboratory procedures the database comprises 940 reference data points from the north atlantic arctic and north pacific oceans and their adjacent seas including the mediterranean sea as well as epicontinental environments such as the estuary and gulf of st lawrence the bering sea and the hudson bay the relative abundance of taxa was analysed to describe the distribution of assemblages the best analogue technique was used for the reconstruction of last glacial maximum lgm sea surface temperature and salinity during summer and winter in addition to sea ice cover extent at sites from the north atlantic n 63 mediterranean sea n 1 and eastern north pacific n 1 three of the north atlantic cores from the continental margin of eastern canada revealed a barren lgm interval probably because of quasi permanent sea ice six other cores from the greenland and norwegian seas were excluded from the compilation because of too sparse assemblages and poor analogue situation at the remaining sites n 54 relatively close modern analogues were found for most lgm samples which allowed reconstructions the new lgm results are consistent with previous reconstructions based on dinocyst data which show much cooler conditions than at present along the continental margins of canada and europe but sharp gradients of increasing temperature offshore the results also suggest low salinity and larger than present contrasts in seasonal temperatures with colder winters and more extensive sea ice cover whereas relatively warm conditions may have prevailed offshore in summer from these data we hypothesise low thermal inertia in a shallow and low density surface water layer

spatial pattern of grain size in the late pliocene red clay deposits north china indicates transport by low level northerly winds abstract the confirmation of an eolian origin of the tertiary red clay from north china would make it a valuable terrestrial paleoclimatic record however it is still controversial as to which wind primarily transported the dust forming the red clay onto the loess plateau careful consideration of this question is a prerequisite for better understanding the history of atmospheric circulation in central and east asia during the miocene and pliocene epochs in this paper the grain size distributions of bulk samples and chemically isolated quartz grains of the late tertiary red clay were investigated to explore the potential transportation mechanisms of the dust as well as the atmospheric circulation at the time of deposition the samples were taken from six localities spanning across 550 km on the loess plateau the results show that the mean grain size of both bulk and quartz samples decreases southward as does the coarse fraction content 42 m these data not only give new evidence for the eolian origin of the red clay but also indicate that the red clay was probably transported mainly by northerly low level winds or the winds at least had a strong northerly component therefore the spatial distribution of grain size of the red clay is comparable to that of the overlying quaternary eolian loess which demonstrates a remarkable northwest southeast decrease in grain size sedimentation rate and thickness during the last glacial coinciding with the dominant wind direction of the northwesterly east asian winter monsoon however the grain size of the red clay is generally finer than the corresponding loess this indicates that the red clay may have been transported by a weaker winter monsoon associated with stronger pedogenic modification and or the dust source area was not extensive compared to that during the quaternary moreover the high content of grains coarser than 19 m transported mainly by energetic low level winds over short distances suggests that the dusts were not transported by upper level westerly

isotopic and rock magnetic expression of environmental change at the pleistocene holocene transition in the central great plains abstract in a region too dry for the effective use of the traditional palynological approach loess deposits of the central great plains offer a source of proxy information for the reconstruction of past climates the brady soil a ubiquitous feature developed within the uppermost late pleistocene peoria loess recorded the character of vegetation change associated with climate shifts during the pleistocene holocene transition stable carbon isotope ratio analysis and two room temperature magnetic parameters were used to examine the nature of the change during this period 13 c values derived from soil organic matter indicate that in the late pleistocene minor populations of drought tolerant c4 plants were interspersed among the dominant cool season grasses c3 in the west while c3 grasses were accompanied by trees in the east by the early holocene c4 grasses were the dominant vegetation in the west and contributed substantially to the 13 c signal in the east magnitude of the 13 c shift suggests an increase in july mean daily temperature of 8 10 c according to existing models magnetic susceptibility and frequency dependence signal the cessation of loess deposition and development of the brady soil and indicate a spatial pattern in the intensity of weathering across the region

physical processes involved in the 1988 drought and 1993 floods in north america abstract an analysis of the spring summer 1988 drought and 1993 floods over north america reveals a reversal in the sign of anomalies in several fields large sea surface temperature anomalies of opposite signs existed in the tropical pacific with strong la nina conditions in 1988 and a mature el nino in 1993 the distribution of tropical convection in the convergence zones and associated latent heating of the atmosphere were correspondingly altered implying a large scale switch in the anomalous tropical heating and forcing of extratropical quasi stationary waves in the atmosphere influencing the subtropical jet stream over the north pacific and across north america in 1988 the jet stream and the closely related storm track of high frequency disturbances in the upper troposphere were displaced into canada well north of the normal location the farthest north of any year from 1979 to 1993 in 1993 a broader jet stream and the storm track were displaced well south of normal to a more springlike location a

magnetic susceptibility and mineral zonations controlled by provenance in loess along the illinois and central mississippi river valleys abstract magnetic susceptibility ms patterns have proven useful for regional stratigraphic correlations of zones within thick oxidized peoria and roxana silts along the illinois and central mississippi river valleys for more than 350 km variations in ms of c horizon loess are controlled by silt sized magnetite content and are interpreted to reflect changes in sediment provenance due to fluctuations of the superior and lake michigan glacier lobes and the diversion of the mississippi river to its present course grain size distributions and scanning electron microscopic observations indicate that stratigraphic changes in ms are not significantly influenced by eolian sorting or diagenetic dissolution respectively three compositional zones lower middle and upper are delineated within peoria silt which usually can be traced in the field by ms the occurrence of clay beds interstadial soils and or subtle color changes these zones can be correlated with but are generally of more practical use than previously studied dolomite zones mckay 1977 or clay mineral zones frye et al 1968 however mineralogical analyses can help to substantiate zone boundaries when in question ms and compositional zones may indirectly record a climatic signal primarily through the effect that global cooling has had on ice lobe fluctuations in the upper mississippi drainage basin

isotopic and geomorphic evidence for holocene climate southwestern kansas abstract the cimarron bend of southwestern kansas is characterized by loess dunes and sand sheets that reflect a complex eolian alluvial history stratigraphic evidence indicates that at least two major dune and sand sheet reactivation episodes occurred these follow intervals of alluviation stability and soil development in the holocene radiocarbon ages from paleosol horizons bracket these periods and their 13 c values provide evidence of a shift from a cooler wetter climate in the late pleistocene and early holocene dominated by c 3 plants to a warmer drier middle holocene period characterized by an increase in c 4 plants stable isotopic results for late holocene soils may reflect local rather than regional climatic conditions our results compare favorably with studies in the central and southern great plains together these results imply that a regional change toward a warmer drier climate occurred from the early to the middle holocene on the southern great plains

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

loess record of dry climate and aeolian activity in the early to mid holocene central great plains north america thick holocene bignell loess sections provide new evidence for the timing of early to middle holocene aridity in the central great plains the immediate source of loess in these sections was dune fields just upwind based on thickness trends and grain size data thus periods of rapid loess accumulation indicate episodes of extensive dune activity under drier than present climate in typical bignell loess sections a thick zone of coarse textured loess with minimal pedogenic alteration is interpreted as a record of the most rapid sustained holocene loess accumulation optical ages indicate that loess in this zone was deposited beginning about 9000 10000 years ago at five study sites accumulation ended shortly after 6500 years ago at three sites but possibly earlier at the other two in two well dated sections the average rate of loess accumulation between 9000 10000 and 6500 years ago was about 1 6 3 5 times greater than in the late holocene thus we infer that the most extensive sustained holocene

mineralogic and climatic interpretations of the luochuan loess section china based on diffuse reflectance spectrophotometry we examined the top 135 m that is the entire pleistocene of the classic luochuan section on the chinese loess plateau with a diffuse reflectance spectrophotometer from the near ultraviolet through the visible and into the near infrared from the reflectance data we calculated sample brightness which with some caveats is a reasonable proxy for magnetic susceptibility mineralogic changes were identified by factor analyzing the first derivative of the percent reflectance data and examining samples with high factor scores two factors which explain about 96 of the cumulative variance are distinguished by the relative proportion of hematite and goethite the minerals that are responsible for the color changes in the loess sequence both hematite and goethite are present in both loess and paleosol but goethite dominates in loess whereas hematite dominates in paleosol the goethite factor exhibits an inverse correlation with magnetic susceptibility the hematite factor exhibits a weak positive correlation with susceptibility paleoclimatic interpretations are drawn from comparison of susceptibility to the concentration of spectrally identified hematite based on this comparison paleosols in the early pleistocene wucheng formation are characterized by conditions that are drier than today a dry summer monsoon whereas later pleistocene paleosols are characterized by a wet summer monsoon

pleistocene climates in china dated by magnetic susceptibility the magnetic susceptibility of loess and interbedded soils in central china varies with the degree of pedogenesis and serves as a proxy measure of climate the concentration of magnetic minerals in the sediment is inversely proportional to sedimentation rates throughout the brunhes epoch susceptibility measurements combined with the reversal stratigraphy provide a time scale that is independent of astronomic chronology on this scale the susceptibility record closely parallels the oxygen isotope fluctuations in deep sea sediments suggesting a close interdependence of the chinese dust falls the volume of land based ice and global climate

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

evidence for a warm last glacial maximum in the nordic seas or an example of shortcomings in uk 37 and uk 37 to estimate low sea surface temperature the occurrence of ice free conditions in the nordic seas during the last glacial maximum has been demonstrated using microfossil and biomarker approaches an accurate picture of the prevailing oceanographic conditions may be more difficult to ascertain thus uncertainties for most proxies increase at the cold extreme of temperature calibrations and glacial sediments contain small amounts of microfossils and biomarkers here we discuss sea surface temperature estimates derived from alkenone indices for the last glacial maximum in the nordic seas that provide a surprising scenario where surface temperatures were warmer than at present we conclude that the uk37 and uk37 estimates may not be interpreted in terms of sea surface temperature as several factors may have conspired to bias the molecular signal however using an alternative index based on the abundance of the c37 4 alkenone we estimate a maximum temperature of 6 c

isotope evidence of paleo el nino southern oscillation cycles in loess paleosol record in the central united states abstract the 13 c of soil carbonate in rhizoconcretions collected from aloess paleosol sequence in the central united states indicates that grow ing season c 3 c 4 plant ratio oscillated by 35 on a 900 200 yr timescale during the late wisconsinan glaciation the pattern appears inphase with advance and retreat of the southern margin of the lauren tide ice sheet suggesting influence by paleo el nino southern oscilla tion cycles the 13 c of soil organic matter indicates that the annual av erage c 3 c 4 plant ratio oscillated only by 18 with a periodicity of450 100 yr and closely matched the cyclic pattern of loess paleosollayers it suggests a periodic enhancement of the penetration of thegulf of mexico air over the region during this time keywords carbon isotope loess paleosol glacial phase paleo enso cycle introduction the el nino southern oscillation enso causes global scale climateanomalies the el nino and la nina are two counterparts of enso whichare associated with warming and cooling in surface water in the eastern trop ical pacific on the basis of modeling studies cane 1998 cane andclement 1999 and clement and cane 1999 proposed that frequency in tensity and duration of long term el nino or la nina intervals have oscil lated on a millennial time scale during the past 150 k y they proposed thatduring glacial periods these long term enso cycles might alter the globalclimate via atmospheric teleconnections in which el nino dominant inter vals may cause retreat and la nina dominant intervals may cause advanceof the southern margin of the laurentide ice sheet the enso sensitive re gions currently in north america partially overlap the area that was coveredby the laurentide ice sheet during the last glaciation fig 1 modernla nina conditions cause heavy snowfall whereas el nino conditions re duce winter precipitation in these regions http www ogp noaa gov enso it is reasonable to assume that la nina or el nino dominant intervals mighthave controlled advance and retreat of the southern margin of the laurentideice sheet if these intervals oscillate on a long term time scale in this paper we use stable isotope ratios from a loess paleosol exposure in the centralunited states to seek evidence for millennial scale paleo enso cycles

evidence supporting holocene loess in illinois with new proxy data and historical regional perspectives on loess deposition in the upper mississippi river valley abstract we thank grimley et al 2019 for their constructive comments on our publication on holocene loess in illinois revealed by optically stimulated luminescence osl dating miao et al 2018 in the journal quaternary science reviews their skepticism on the preservation of holocene loess in illinois provides us with a good opportunity to clarify several key issues pedoturbation including bioturbation the methodology of soil research and our osl dating results in addition we will discuss the more fundamental issue behind this debate what controls loess deposition in the midwestern usa direct glacial sediment supply or broader scale climate change finally we also report new high resolution grain size magnetic susceptibility and color data from one of our holocene loess sites

non magnetic indicators of pedogenesis related to loess magnetic enhancement and depletion examples from the czech republic and southern siberia abstract we carried out a multiparameter investigation for non magnetic proxies of the intensity of pedogenesis in loess paleosol sections from two different climate settings dolni v stonice central europe and krasnogorskoye southern siberia the work was performed in order to clarify the problem of two opposite scenarios of magnetic susceptibility ms patterns in these two profiles the chinese model with magnetically enhanced paleosols and the siberian alaskan model with magnetically depleted paleosols the age constraints on the krasnogorskoye section were inferred from an independent correlation with the well dated dolni v stonice section based on ms and sediment color cie l and with the ms log from lake baikal the pedologic description of the loess paleosol sections and their correlation was considerably improved by the use of vis spectral proxies cie l reflectance in the red band their weathering intensity can be effectively characterized by diffuse reflectance spectroscopy drs and geochemical cec and rb sr proxies whereas the ms is strongly dependent on the type of soil chernozems vs brown soils the geochemical proxies in particular cec proved to be relatively independent of the soil type these parameters are less affected by region specific conditions than the ms signal the non magnetic proxies indicate relatively dissimilar climatic trends at the two sites during the last glacial interglacial cycle a long distance correlation based solely on the ms signal can be adversely affected by switching between the chinese and the siberian alaskan magnetic modes this having been observed at the krasnogorskoye section in general no single universal proxy of weathering or pedogenesis intensity would be applicable across broad regions or considerable climatic gradients paleoclimatologic interpretations of large regions should always be based on a carefully designed multi proxy approach

magnetic properties of loess deposits in australia and their environmental significance abstract there are vast dry areas in australia but few loess sections have been reported mackenzie s waterholes creek mwc profile in new south wales was the deepest and first dated loess profile in this paper various measurements of magnetic parameters from mwc were carried out and comparison was made with those of luochuan profile which lies in the central chinese loess plateau clp the types of magnetic minerals are similar to the clp magnetite maghemite limonite and hematite however the magnetic properties of the australian aeolian sediment are quite different frequency dependent magnetic susceptibility fd is rather high while values of magnetic susceptibility are low the relative content of hard paramagnetic magnetic components is much higher than clp whereas the content of maghemite and hematite is much less furthermore hard magnetic minerals increase with depth while soft maghemite decrease with depth maghemite is more sensitive than magnetite under pedogenic condition of waterlogged environments it increases along with enhancement of magnetic susceptibility in luochuan section however decreases in mwc profile this very likely indicates that the pedogenesic processes in mwc took place under an environment in which maghemite magnetite were unstable the modern annual precipitation in the mwc area is 924 mm which is about 300 mm higher than that in luochuan that extra moisture may lead mwc to exceed the stable range for maghemite magnetite those minerals therefore were gradually converted to more stable goethite causing magnetic susceptibility to decrease fe and mn nodules are abundant in the lower part of section implying mildly reducing conditions that allowed fe and mn to be mobilised and enriched in near the oxidation front indicating that under waterlogged and reducing conditions wind blown maghemite magnetite were transformed to iron hydroxides such as goethite limonite or other paramagnetic minerals resulting in reduced magnetic susceptibility the susceptibility from most of the section shows a generally positive relationship to the loess particle size and seems to fit the alaskan wind vigor model however fe chemical weathering and movement in mwc suggests that the precondition for the wind vigor model cannot be satisfied and that the susceptibility variation can only be explained by pedogenesis

high resolution paleoclimatic information from chinese eolian sediments based on grayscale intensity profiles abstract grayscale intensity profiles from photographic images offer a rapid means of obtaining paleoclimate proxy records from chinese loess dune sand and paleosols although the data can be obtained using conventional 35 mm film images a digital camera and laptop computer will produce a high resolution grayscale profile at a field site within minutes comparison of grayscale profiles with profiles of magnetic susceptibility measured down loess and dune sand sections at sites on the loess plateau and tibetan plateau in a range of altitudes and climatic regimes shows that the two parameters are highly correlated therefore grayscale intensity is a convenient alternative to magnetic susceptibility for generating paleoclimatic data in the loess and desert margin regions of china the resolution of both grayscale and susceptibility profiles ultimately is limited by bioturbation which is most pronounced in paleosols

optical and radiocarbon ages of stacked paleosols and dune sands in the nebraska sand hills usa abstract optical ages for eolian sands from the nebraska sand hills indicate periods of extensive eolian activity at ca 115 25 840 70 2300 240 and 3560 340 a activity was also noted at single sampling locations at ca 6180 370 8430 510 and 13110 800 a many of these ages are similar to those noted by earlier authors optical ages from samples collected within paleosols indicate shorter and possibly less extensive periods of eolian activity at approximately 1220 150 1590 110 and possibly 1950 150 a during which the paleosol sands accumulated what was originally interpreted as a single 1 2 m thick paleosol is shown by optical dating to consist of three or more welded soils developed within eolian sands with optical ages of ca 3800 240 2740 240 1560 110 and possibly 1930 140 a each of which match eolian pulses recognized elsewhere scatter in some optical ages is attributable to intersection of sand filled rodent burrows extending in outcrop 1 5 m below the contact between paleosol and overlying topset beds a 5310 360 a optical age for one probable intersected burrow provides evidence for upward or lateral transport of older sands

mid holocene climates of the americas a dynamical response to changed seasonality simulations of the climatic response to mid holocene 6 ka bp orbital forcing with two coupled ocean atmosphere models foam and csm show enhancement of monsoonal precipitation in parts of the american southwest central america and northernmost south america during northern hemisphere summer the enhanced onshore flow that brings precipitation into central america is caused by a northward displacement of the inter tropical convergence zone driven by cooling of the equatorial and warming of the northern subtropical and mid latitude ocean ocean feedbacks also enhance precipitation over the american southwest although the increase in monsoon precipitation there is largely driven by increases in land surface temperature the northward shift in the equatorial precipitation band that causes enhanced precipitation in central america and the american southwest has a negative feedback effect on monsoonal precipitation in northern south america the simulations demonstrate that mid holocene aridity in the mid continent of north america is dynamically linked to the orbitally induced enhancement of the summer monsoon in the american southwest with a spatial structure wet in the southwest and dry in the mid continent similar to that found in strong monsoon years today changes in winter precipitation along the west coast of north america in central america and along the gulf coast caused by southward displacement of the westerly storm tracks indicate that changes in the northern hemisphere winter monsoon also play a role in regional climate changes during the mid holocene although the simulations with foam and csm differ in detail the general mechanisms and patterns are common to both the model results thus provide a coherent dynamical explanation for regional patterns of increased or decreased aridity shown by vegetation lake status and aeolian data from the americas

vegetation dynamics during the pleistocene holocene transition in the central great plains usa the holocene pleistocene transition in the upland loess mantled regions of the central great plains is punctuated by the brady soil which separates the late pleistocene peoria loess and the holocene bignell loess previous research on the brady soil at the old wauneta roadcut site in southwestern nebraska has produced paleoenvironmental information based on well constrained luminescence and radiocarbon ages stable carbon isotope data and chemical and physical data while the research indicated high effective moisture during formation of the brady soil and a shift to warm season c4 vegetation from the cool season c3 dominated vegetation of the peoria loess those data do not provide any detail as to plant community composition and significant underlying climatic inferences assemblages of phytoliths and other biosilicates extracted from the brady soil provide specific information on vegetation communities and indicate shifts of plant taxa comprising these assemblages short cell phytolith count data rev

apparent long term cooling of the sea surface in the northeast atlantic and mediterranean during the holocene reconstructions of upper ocean temperature t during the holocene 10 0 ka b p were established using the alkenone method from seven high accumulation sediment cores raised from the northeast atlantic and the mediterranean sea 361n 751n all these paleo t records document an apparent long term cooling during the last 10 kyr in records with indication of a constant trend the apparent cooling ranges from 0 27 to 0 151c kyr 1 records with indication of time variable trend show peak to peak amplitudes in apparent temperatures of 1 2 2 91c a principal component analysis shows that there is one factor which accounts for a very large fraction 67 of the total variance in the biomarker paleo t records and which dominates these records over other potential secondary influences two possible contributions are 1 a widespread surface cooling which may be associated with the transition fromthe hypsithermal interval b9 5 7 ka b p to the neoglaciation b5 7 0 ka b p and 2 a change in the seasonal timing and or duration of the growth period of alkenone producers prymnesiophyte algae the first contribution is consistent with many climate proxy records from the northeast atlantic area and with climate model simulations including milankovitch forcing the second contribution is consistent with the divergence between biomarker and summer faunal paleo t fromearly to late holocene observed in two cores further work is necessary and in particular the apparent discordance between biomarker and faunal t records for the relative stable holocene period must be understood to better constrain the climatic and ecological contributions to the apparent cooling observed in the former records r 2002 elsevier science ltd all rights reserved

possible solar forcing of century scale drought frequency in the northern great plains a 2100 yr decadal resolution salinity and aridity proxy record of lacustrine ostracode shell mg ca ratios from a closed basin lake in the northern great plains shows statistically significant periodicities of 400 200 130 and 100 yr these periodicities are similar to the three principal solar oscillation periods 420 218 143 yr as inferred from the atmospheric radiocarbon record suggesting strong solar forcing of century scale drought frequency with a fundamental 400 yr period and its 2nd 3rd and 4th harmonics our proxy record correlates visually and statistically cross spectral analysis with the atmospheric 14 c record solar proxy and the gisp2 greenland ice sheet project two 18 o record climate proxy showing that solar minima are in phase with drought periods in the northern great plains and cold periods in greenland this spectral similarity together with phase correlation indicates a possible teleconnection of century scale global climate fluctuations through common solar forcing

overview of glacial atlantic ocean mapping glamap 2000 glamap 2000 presents new reconstructions of the atlantic s sea surface temperatures sst at the last glacial maximum lgm defined at both 21 500 18 000 years b p last isotope maximum and 23 000 19 000 years b p maximum glacial sea level low stand and orbital minimum of solar insolation epilog working group see mix et al 2001 these reconstructions use 275 sediment cores between the north pole and 60 s with carefully defined chronostratigraphies four categories of core quality are distinguished more than 100 core sections provide a glacial record with subcentennial to multicentennial scale resolution sst estimates are based on a new set of almost 1000 reference samples of modern planktic foraminifera and on improved transfer function techniques to deduce sst from census counts of microfossils including radiolarians and diatoms new proxies also serve to deduce sea ice boundaries the glamap 2000 sst patterns differ significantly in crucial regions from the climap 1981 reconstruction and thus are important in providing updated boundary conditions to initiate and validate computational models for climate prediction

loess record of the pleistocene holocene transition on the northern and central great plains usa various lines of evidence support conflicting interpretations of the timing abruptness and nature of climate change in the great plains during the pleistocene holocene transition loess deposits and paleosols on both the central and northern great plains provide a valuable record that can help address these issues a synthesis of new and previously reported optical and radiocarbon ages indicates that the brady soil which marks the boundary between late pleistocene peoria loess and holocene bignell loess began forming after a reduction in the rate of peoria loess accumulation that most likely occurred between 13 5 and 15 cal ka brady soil formation spanned all or part of the bolling allerod episode approximately 14 7 12 9 cal ka and all of the younger dryas episode 12 9 11 5 cal ka and extended at least 1000 years beyond the end of the younger dryas the brady soil was buried by bignell loess sedimentation beginning around 10 5 9 cal ka and continuing episodically through the holocene evidence for a brief increase in loess influx during the younger dryas is noteworthy but very limited most late quaternary loess accumulation in the central great plains was nonglacigenic and was under relatively direct climatic control thus brady soil formation records climatic conditions that minimized eolian activity and allowed effective pedogenesis probably through relatively high effective moisture optical dating of loess in north dakota supports correlation of the leonard paleosol on the northern great plains with the brady soil thick loess in north dakota was primarily derived from the missouri river floodplain thus its stratig raphy may in part reflect glacial influence on the missouri river nonetheless the persistence of minimal loess accumu lation and soil formation until 10 cal ka at our north dakota study site is best explained by a prolonged interval of high effective moisture correlative with the conditions that favored brady soil formation burial of both the brady soil and the leonard paleosol by renewed loess influx probably represents eolian system response that occurred when gradual change toward a drier climate eventually crossed the threshold for eolian activity overall the loess paleosol sequences of the central and northern great plains record a broad peak of high effective moisture across the late pleistocene to holocene boundary rather than well defined climatic episodes corresponding to the bolling allerod and younger dryas episodes in the north atlantic region

sources and paleoclimatic significance of holocene bignell loess central great plains usa abstract this paper reexamines the stratigraphy sources and paleoclimatic significance of holocene bignell loess in the central great plains a broadly similar sequence of loess depositional units and paleosols was observed in thick bignell loess sections up to 300 km apart suggesting that these sections record major regional changes in the balance between dust deposition and pedogenesis new optical ages together with previously reported radiocarbon ages indicate bignell loess deposition began 9000 11 000 yr ago and continued into the late holocene some bignell loess is

response of planktic foraminiferal size to late quaternary climate change 1 geographical size distribution within entire holocene foraminiferal assemblages is related to global environmental gradients such as temperature primary productivity and environmental variability this study demonstrates that these correlations are also recognizable in late quaternary assemblages from three locations in the south atlantic on temporal and latitudinal scales the size response to temporal paleoenvironmental changes during glacial interglacial cycles mimics the geographic holocene size variability the amplitude of size variability is directly related to the amplitude of the climatic fluctuations as shown by the stable size temperature relationship over time the documented changes in the assemblage size are caused by species replacement and intraspecific size variability the relative importance of these processes depends on the environmental setting species have been shown to reach their maximum size and abundance under certain optimum conditions and decrease in size if environmental conditions differ from these optima we confirm that late quaternary species sizes were largest at paleotemperatures identical to holocene ones

biopores and root features as new tools for improving paleoecological understanding of terrestrial sediment paleosol sequences abstract the important role of roots and rhizosphere processes is accepted for the top soil but still under debate for the deep subsoil including soil parent material especially for terrestrial sediments like loess and dune sands roots and root traces are mostly recognized in profile descriptions but not interpreted in the paleoenvironmental context further synchrony of sediment deposition and root trace formation is commonly assumed this is challenged by partially large maximum rooting depths of plants exceeding the soil depth and by frequent occurrence of secondary carbonates and biopores of potential root origin below recent soil and paleosols to improve understanding of paleoenvironmental records in terrestrial sediment paleosol sequences recent roots and root traces including calcified roots and root derived biopores were investigated in six soil loess and dune sand profiles across central and se europe visualization of small carbonate accumulations diameter 1 mm frequently called pseudomycelia by x ray microtomographic scanning and morphologic comparison with rhizoliths calcified roots diameter mostly 3 20 mm up to 100 mm possible indicate root origin of the former therefore requiring renaming to microrhizoliths quantification of roots biopores rhizoliths and microrhizoliths on horizontal levels yielded maximum frequencies of 2100 m 2 4100 m 2 196 m 2 and 12 800 m 2 respectively considering the pore volume remaining from former root growth this indicates their significant contribution to structural properties of the sediments and paleosols depth distribution of roots and root traces was frequently related to soil and paleosols respectively and mostly showed maximum frequencies within or immediately below these units root traces are therefore not necessarily of similar age like the surrounding sediment but are typically of younger age the time lag between root traces and the surrounding stratigraphic unit can vary between small time periods likely decades to centuries in case of microrhizoliths and several millenia in case of larger rhizoliths penetrating several stratigraphic units with assumed radii of former rhizosphere extension of 5 mm for microrhizoliths a frequency of 12 500 m 2 corresponded to 100 rhizosphere area in the respective depth interval these findings emphasize the meaning of root traces in sediment paleosol sequences potential temporal and spatial inhomogeneity of root growth on the one hand especially for shrub and tree vegetation and occurrence of root remains of different age and origin in identical depth intervals on the other hand hamper the assessment of the chronologic context of these with the surrounding sediment or paleosol nevertheless root traces in terrestrial archives provide valuable information with respect to paleovegetation and paleoenvironmental conditions if their chronological context is known

middle and late holocene dune reactivation in the nebraska sand hills usa a combination of optical dating of aeolian sand and radiocarbon dating of palaeosols and interdune sediments has allowed some refinements to the chronostratigraphy of the nebraska sand hills our results directly confirm previous inferences that multiple phases of aeolian deposition including megabarchan dune reactivation occurred within the sand hills during the middle and late holocene optical dates on stratified aeolian barchan dune sediments from two localities provide ages ranging from c 6 ka to 300 years ago weakly developed palaeosols intercalated within the dune sediments and other forms of post depositional modification indicate periods of relative stability and reduced dune mobilization optical dates on the upper 12 15 m of two dune exposures indicate repeated possibly regionally correlative reactivations which occurred between c 400 500 and 200 300 years ago comparison of late holocene aeolian activity with local and regional palaeoclimatic proxy records suggests that dune reacti

thermoluminescence ages of loess and associated sediments in central nebraska usa abstract thick deposits of quaternary loess maximum 60 m more typically 20 30 m occur in the loess hills and south of the platte river in central nebraska a programme of field sampling and thermoluminescence dating has provided new age estimates for the younger aeolian stratigraphic units in the upper part of the sequence dates from aeolian sands below the gilman canyon formation at two sites indicated ages ranging from 64 7 to 48 5 ka providing a maximum age for this unit a date of 35 3 ka was obtained from near the top of a truncated gilman canyon sequence in one section but radiocarbon evidence from other sites suggests that slow accumulation of dust accompanied by pedogenesis and syn depositional reworking continued in some areas until at least 23 ka ago the gilman canyon formation is overlain by up to 25 m of grey peoria loess which accumulated between c 22 ka and 10 5 ka ago between c 10 5 ka and 9 ka ago the rate of dust accumulation decreased apparently due to a reduction in sediment supply from the great plains river valleys thereby allowing the widespread development of a soil the brady soil after 9 ka ago the rate of dust deposition again accelerated resulting in the accumulation of up to 5 m of bignell loess the main source of bignell loess was probably also the valleys of ephemeral great plains rivers during periods when the region experienced a drier climate than present dating evidence suggests that some time after 3 2 ka ago the rate of dust accumulation again slowed allowing development of the modern soil it is suggested that this change reflected a reduction in dust availability related to a change towards slightly wetter climate and associated changes in river regime

stable isotope composition of soil organic matter and phytoliths as paleoenvironmental indicators abstract the stable c isotope composition of organic matter and opal phytoliths in diverse ecosystems demonstrate that soils carry a c isotopic signature that reflects long term inputs of above and below ground c 3 or c 4 biomass the utility of these isotopic characterization data for paleoenvironmental study is based on a knowledge of dominant soil forming processes as well as geomorphic and climatic conditions this paper reports on both the theory and applicability of isotopic characterization of soil organic c and opal phytoliths the theoretical perspectives are discussed in light of broad environmental applications details of the sample collection and preparation are provided with insights into quantities and pretreatment required for stable c isotopic characterization two case studies from the central great plains region are presented paleoclimatic interpretations are discussed for a portion of the central great plains these interpretations are based on the stable c isotope data recovered from holocene paleosols these data indicate higher proportions of c 3 vegetation persisted during the early holocene the concordance in the c isotopic signatures of soil organic matter and phytoliths provide strong biological evidence of regionally cooler conditions this c isotopic concordance also appears during the mid holocene c isotope values indicate an increase in the proportion of c 4 vegetation which reflects regionally warmer climatic conditions than present isotopic discordance in soil organic c and phytoliths can indicate the degree of diagenesis resulting from pedological alteration geomorphic and pedologic evidence indicate that the c isotope discordance between soil organic matter and opal phytoliths is the result of local topographic variations and spatial heterogeneity associated with plant distribution

late quaternary environmental reconstructions of playa lunette system evolution on the central high plains of kansas united states playas are small ephemeral wetlands found throughout semiarid and arid regions around the world lunettes are dunes that form downwind of some large playas two playa lunette systems on the high plains were investigated to reconstruct paleoenvironments during playa lunette system evolution both playa lunette systems are composed of sediment spanning 40 k y with earliest deposits accumulating during at least marine isotope stage mis 3 and possibly starting as early as mis 5 the gilman canyon formation pedocomplex dating between 42 and 28 ka dominates this period and is ubiquitous throughout the playa lunette systems climate during mis 3 was similar to modern with warm temperatures low effective moisture and playa floors exposed long enough to allow pedogenesis during mis 2 climate was relatively cool with higher effective precipitation and playas were inundated for longer periods the regionally expressed brady soil of the pleistocene holocene transition caps these deposits and dates between 11 8 and 9 4 ka during the pleistocene holocene transition climate warmed yet moisture availability remained relatively high several distinct shifts in 13 c identify rapid climate changes associated with the bolling allerod younger dryas chronosequence warming continued into the holocene though moisture availability was highly variable holocene soils are common and date between 8 3 and 3 6 ka playa lunette system stratigraphy represents a continuum of the upland loess sequence though deposits are altered by playa hydrology geomorphic processes alternated between dissolution and fluvial eolian driven processes as climate changed and playas developed due to the aggradational environment and sensitivity of these systems high resolution records of environmental change throughout their evolution are preserved

palaeoclimatic signals recognized by chemometric treatment of molecular stratigraphic data abstract the high resolution stratigraphy of various marker compounds has been studied using gc hplc and gc ms in a 13 m gravity core recovered from the kane gap region eastern equatorial atlantic which provides a record of the glacial interglacial episodes over the last million years downhole variations in many presumed source specific components are observed e g in n alkanes from terrigenous land plants and dinosterol from dinoflagellates which may be due to perturbations or cyclicities resulting from climatic change fluctuations in the unsaturation of alkenones attributable to variations in water temperatures show correlations with the glacial interglacial cycles recorded in the 18 o values for planktonic foraminifera thereby providing a potential organic geochemical measure of past climates these molecular abundance data can be linked to the palaeotemperature record following computer treatments using principal component and spectral analyses molecular stratigraphy shows promise as a new chemostratigraphical tool where other means of stratigraphy fail for example through calcium carbonate dissolution

factors affecting the magnetic susceptibility of a loess soil sequence barton county kansas usa abstract to interpret climatic signatures from magnetic susceptibility ms in the barton county loess soil sequence central kansas we tested the influence of particle size and carbonate concentration on magnetic susceptibility first sand silt and clay were separated and ms was then measured on each of the three size fractions to assess the influence of chemical treatments and carbonate on ms signals the chemical agents used to remove carbonate organic matter and free iron oxides were applied to another set of samples and ms was measured before and after each of the treatments the results show that the bulk ms differentiates among stratigraphic units but the environmental interpretation is far from straightforward soils have stronger ms signals than the loess units in the upper portion while the carbonate rich soils have weaker ms signals in the lower portion due to the ms dilution effect of carbonate sand ms peaks in bignell loess peoria loess and ll1 may suggest that in loess units fewer sand sized magnetic minerals were changed by weathering processes into clay sized magnetic minerals than in soils a low bulk ms at the bottom of the reddish pedocomplex to the bottom of ls 1 is attributable to a combination of a high sand content and a high carbonate concentration the silt ms peak in the barton sand might have been derived from the parent materials comparing with mineralogic and chemical indices clay ms might be closer to being a proxy of weathering intensity although clay ms dominates the bulk ms signals silt ms and sand ms affect the bulk ms signals in two ways 1 changes in the percentages of silt and sand and 2 mineralogical variations in silt ms and sand ms from one stratigraphic unit to another this study demonstrates that 40 of the variation in the bulk ms is accounted for by the clay ms and 56 of the variation by the silt ms and clay ms together after adding the sand ms 66 of the variation is accounted for if diamagnetic carbonate is taken into consideration 76 of the variation in bulk ms is accounted for by the ms of the three size fractions and the ms dilute effect of carbonate

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 records of environmental change in high plains playa wetlands kansas us holocene environmental change is poorly characterized throughout much of the high plains playa wetlands are sites with appreciable potential for reconstructing environmental change because of their environmental sensitivity radiocarbon litho and magneto stratigraphic and stable carbon isotope 13c data for four playas on the kansas high plains indicate significant change occurred throughout the holocene which had profound impacts on playa ecosystem functions e g groundwater recharge surface water storage and habitat minimum 13c values and buried soils observed during the pleistocene holocene transition suggest sufficient moisture to support vegetative cover and promote pedogenesis low magnetic susceptibility rapid increase in 13c and weak pedogenic alteration in overlying deposits suggest early holocene aridity with increased landscape instability and sediment deposition early to middle holocene stratigraphy transitioned from loess with little pedogenic modification into overlying buri

isolation of the syndepositional magnetic susceptibility signals from loessic paleosols of china abstract bulk magnetic susceptibility ms has been widely used to differentiate the loess and soil units and acts as a useful proxy of the intensity of summer monsoon to reconstruct the paleoclimate on the chinese loess plateau since soil development occurs not only at the actual ground surface but also to a greater depth bulk ms is overprinted by the post depositional pedogenesis and can only be regarded as representative of a time period rather than a specific point in time therefore the bulk ms record as a naturally smoothed curve can illustrate the low frequency milankovitch cycles but may be unable to adequately resolve high frequency sub orbital climatic changes based on the pedosedimentary processes of soil formation the magnetic susceptibility variation msv model is proposed to isolate the syndepositional enhancements of ms that may be directly linked to the climatic conditions of dust deposition after an indirect test of the concept underlying the msv model using an atmospheric methane data set from taylor dome antarctica we further verify the model directly from a loessic paleosol by the tentative correlation between the msv record of chinese loess at luochuan profile and 18o of the ngrip ice core within the last interglacial period our results show that msv model may improve our understanding of widely used ms proxy itself and climate variation within the interglacial time periods

latest pleistocene to holocene loess in the central great plains optically stimulated luminescence dating and multi proxy analysis of the enders loess section nebraska usa abstract loess deposits of the central great plains usa and their intercalated soils provide a detailed record of climatically driven changes within the aeolian system during the pleistocene holocene transition and the holocene here we present a detailed optically stimulated luminescence osl chronology as well as multi proxy analysis obtained for the first time on the enders section located in southwestern nebraska central great plains the section records multiple episodes of rapid loess deposition alternating with soil formation rapid accumulation of late pleistocene peoria loess was replaced around 13 14 ka by formation of the brady soil until 9 5 0 6 ka the holocene bignell loess then buried the brady soil and accumulated episodically throughout the holocene the loess paleosol stratigraphy since the late pleistocene at the enders site is very similar to that at other sites in western nebraska and the newly developed osl chronology based on three grain size classes adds new confidence to earlier dating the high resolution grain size profile from enders shares many features with similar data from the previously studied wauneta site including three peaks of fine grained material just above and within the brady soil likely representing response to millennial scale climatic changes during the pleistocene holocene transition this study demonstrates the potential for developing high resolution well dated paleoclimatic records from the loess of the central great plains contrasts between great plains and eurasian loess records reflect differences in the late pleistocene to holocene climatic evolution and other factors influencing the loess system

the effect of sea surface properties on shell morphology and size of the planktonic foraminifer neogloboquadrina pachyderma in the north atlantic abstract the variability in size and shape of shells of the polar planktonic foraminifer neogloboquadrina pachyderma have been quantified in 33 recent surface sediment samples throughout the northern atlantic ocean and correlated with the properties of the ambient surface waters the aim of the study was to determine whether any of the morphological features could be used to reconstruct sea surface properties in the polar realm of the north atlantic where most paleotemperature proxies appear to fail the analyses revealed that shell morphology is only weakly controlled by habitat properties whereas shell size showed a strong correlation with sea surface temperature the regression of mean shell size on sea surface temperature revealed the presence of two trends among the sinistrally coiled shells a continuous increase in shell size with decreasing sst in sediments deposited under polar water masses and a continuous increase in shell size with increasing sst in samples from transitional waters the second trend mirrors the trend observed for dextrally coiled shells which are frequent in the same samples and signal the presence of n incompta the identical mean shell size trends among the sinistral and dextral specimens in the temperate samples confirms the results of earlier genetic studies which indicated the existence of a small but distinct proportion of opposite coiling in n incompta to which the sinistral shells in the temperate samples could be attributed the linear correlation between mean shell size and sea surface temperature in the polar domain summer sst

a multiproxy reconstruction of the evolution of deep and surface waters in the subarctic nordic seas over the last 30 000yr on the basis of various lithological mircopaleontological and isotopic proxy records covering the last 30 000 calendar years cal kyr the paleoenvironmental evolution of the deep and surface water circulation in the subarctic nordic seas was reconstructed for a climate interval characterized by intensive ice sheet growth and subsequent decay on the surrounding land masses the data reveal considerable temporal changes in the type of thermohaline circulation open water convection prevailed in the early record providing moisture for the fennoscandian barents ice sheets to grow until they reached the shelf break at 26 cal kyr and started to deliver high amounts of ice rafted debris ird into the ocean via melting icebergs low epibenthic 18o values and small sized subpolar foraminifera observed after 26 cal kyr may implicate that advection of atlantic water into the nordic seas occurred at the subsurface until 15 cal kyr although modern like surface and deep water conditions first developed at 13 5 cal kyr thermohaline circulation remained unstable switching between a subsurface and surface advection of atlantic water until 10 cal kyr when ird deposition and major input of meltwater ceased during this time two depletions in epibenthic 13c are recognized just before and after the younger dryas indicating a notable reduction in convectional processes despite an intermittent cooling at 8 cal kyr warmest surface conditions existed in the central nordic seas between 10 and 6 cal kyr however already after 7 cal kyr the present day situation gradually evolved verified by a strong water mass exchange with the arctic ocean and an intensifying deep convection as well as surface temperature decrease in the central nordic seas this process led to the development of the modern distribution of water masses and associated oceanographic fronts after 5 cal kyr and eventually to today s steep east west surface temperature gradient the time discrepancy between intensive vertical convection after 5 cal kyr but warmest surface temperatures already between 10 and 6 cal kyr strongly implicates that widespread postglacial surface warming in the nordic seas was not directly linked to the rates in deep water formation

causes and consequences of a middle pleistocene origin of the modern planktonic foraminifer neogloboquadrina pachyderma sinistral switches in coiling direction of shells of the planktonic foraminifer neogloboquadrina pachyderma have been extensively used as a paleotemperature proxy from the late miocene through the quaternary exploiting the specific modern polar affinity of the sinistrally coiled form here we have reexamined the origin of the modern sinistral form in pliocene and pleistocene sediments from ocean drilling program site 1014 in the eastern north pacific by using morphometric and stable isotope data we show that pliocene and pleistocene sinistral forms of n pachyderma are morphologically and ecologically different and that modern type sinistral form first appeared ca 1 ma these findings suggest that n pachyderma sinistral should not be used for calibrated paleoceanographic reconstructions prior to the middle pleistocene and reinforce the need to assess the origin of specific ecological affinities of other modern planktonic foraminifer species the first appearance of the modern form and the subsequent evolution of its mean shell size appear to have been synchronous in the north pacific and the north atlantic such hemispheric evolutionary synchronicity implies existence of trans arctic gene flow the modern type shell morphology and the specific cold water affinity of n pachyderma sinistral may have evolved in response to the onset of the 100 k y climate regime in the middle pleistocene

high resolution proxy record of holocene climate from a loess section in southwestern nebraska usa abstract multi proxy analysis was used to produce a high resolution paleoclimatic record from an exceptionally thick section of the holocene bignell loess near wauneta southwestern nebraska in the central great plains the wauneta section has excellent age control based on optically stimulated luminescence osl and radiocarbon dating and records multiple episodes of rapid loess deposition alternating with slower deposition and soil formation the lowermost and uppermost osl ages obtained from the bignell loess are 10 250 610 years 5 9 m depth and 100 10 years 0 1 m depth respectively as a result the holocene has been temporally confined stratigraphically the bignell loess overlies the late pleistocene peoria loess deposited 21 14 ka and the two units are separated by the brady soil which is distinguished by its color and other pedogenic features l a b color parameters and organic carbon content of bignell loess are sensitive proxies to differentiate drought induced aeolian sediment layers from the intercalated soil horizons soil organic carbon derived 13 c data suggest that the c 3 dominated floral environment during peoria loess deposition shifted dramatically to a c 4 dominated environment during brady soil formation in response to a warming trend even greater c 4 abundance characterized the late holocene high resolution 13 c data support the contention that c 3 vs c 4 vegetation change in the holocene reflects ecosystem response to frequent vegetation disturbance under arid conditions time series analysis reveals that 13 c and color parameters display high frequency variation with periodicities of 103 118 years and 103 years respectively similar periodicities were also reported in studies of north dakota lakes though the physical mechanism responsible is uncertain comparison of bignell loess color and tropical pacific sea surface temperatures ssts allows evaluation of a proposed teleconnection between drought in the great plains and la nina like conditions in the tropical pacific the loess color index and eastern tropical pacific sst display broad similarities through the late pleistocene and holocene that are consistent with this teleconnection on the other hand drought centered at 3800 years ago is not consistent with the teleconnection and the end of early holocene aridity at the wauneta section around 6500 years ago is much earlier than the corresponding rise in sst and increase in el nino frequency in the eastern tropical pacific

climatic interpretation of the luochuan and lingtai loess sections china based on changing iron oxide mineralogy and magnetic susceptibility abstract hematite and goethite estimated from diffuse reflectance spectra and magnetic susceptibility ms have been determined from both the luochuan and lingtai loess sections of the loess plateau of central china changing concentrations of these iron oxides and ms were used to determine paleoclimatic change based on a model that relates pedogenic production of hematite and magnetite to precipitation we propose two precipitation phases on the loess plateau during the last 2 6 ma from the wucheng red clay boundary up to paleosol s5 mis 13 precipitation on the loess plateau was moderate probably not frequently exceeding 450 mm yr and highly seasonal from s5 to the present precipitation during interglacial times has gradually increased to the present levels of 600 650 mm yr for the localities examined throughout the pleistocene spanning both glacial and interglacial conditions hematite to goethite ratios display a consistent decrease this decrease suggests that precipitation on the loess plateau has been getting less seasonal and or has been occurring during times when temperature was lower in addition to these long term changes it also appears that iron oxide records from the loess plateau exhibit 400 500 ka cycles from the wucheng red clay boundary to s5 these 400 500 ka cycles may be analogous to similar length cycles recorded in 13 c marine records in the south china sea and may be linked through weathering and silica production as a result of the asian monsoon

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

color characteristics of chinese loess and its paleoclimatic significance during the last glacial interglacial cycle abstract the soil color is widely used in paleoclimate and paleoenvironment reconstructions in the chinese loess plateau to better understand the color spatial changes during the glacial interglacial cycle the soil color lightness l characteristic spectra magnetic susceptibilities and mean grain sizes of three loess paleosol sequences were compared results showed that high l and low hematite to goethite ratios hm gt appeared in loess units and low l and high hm gt ratios accompanied paleosol layers indicating glacial interglacial hydrothermal oscillation l in the yulin section was higher than in the chaona and lihuacun sections indicating that different precipitations have great effect on l furthermore hm gt magnetic susceptibility lf and mean grain size are correlated closely with l l and hm gt not only document climatic variations in the glacial interglacial cycle vis a vis loess paleosol sequences but also can identify heinrich cold events and millennial scale dansgaard oeschger d o warm events it suggests that soil color responds sensitively to global climate change driven by ice volumes l and hm gt curves exhibit higher frequencies and larger amplitudes than magnetic susceptibility lf curves indicating that l and hm gt can be regarded as sensitive and reliable proxies for characterizing high resolution climate change during the last glacial interglacial cycle

el nino like pattern in ice age tropical pacific sea surface temperature sea surface temperatures ssts in the cold tongue of the eastern equatorial pacific exert powerful controls on global atmospheric circulation patterns we examined climate variability in this region from the last glacial maximum lgm to the present using a sst record reconstructed from magnesium calcium ratios in foraminifera from sea floor sediments near the galapagos islands cold tongue sst varied coherently with precession induced changes in seasonality during the past 30 000 years observed lgm cooling of just 1 2 c implies a relaxation of tropical temperature gradients weakened hadley and walker circulation southward shift of the intertropical convergence zone and a persistent el nino like pattern in the tropical pacific this is contrasted with mid holocene cooling suggestive of a la nina like pattern with enhanced sst gradients and strengthened trade winds our results support a potent role for altered tropical pacific sst gradients in global climate variations

episodic holocene loess deposition in central nebraska abstract the mirdan canal section in central nebraska usa provides evidence for multiple episodes of holocene loess deposition in the central great plains this irrigation canal cut exposes more than 4 m of holocene sediment that overlies the late wisconsinan peoria loess the oldest holocene sediment is local slopewash that fills gullies but overlying beds are interpreted as holocene loess bignell loess three buried soil profiles occur within the holocene sediment and have yielded uncalibrated 14 c ages of 3010 yr bp uppermost buried soil and 8790 yr bp third buried soil from top a fourth buried soil yielded an age of 9330 yr bp and is correlated with the brady soil the bignell loess has a darker color slightly more organic matter more clay more coarse silt and sand less smectite and less volcanic glass than the underlying peoria loess the bignell loess is most plausibly interpreted as dust from local flood plains containing a mixture of reworked peoria loess and older silts with contrasting mineralogy episodes of loess deposition occurred in the early middle and late holocene probably during periods of dry climate but possibly also as a consequence of large erosive floods that made wind erodible sediment available on nearby flood plains

palaeoclimatic and sedimentary history from magnetic susceptibility of loess in china the magnetostratigraphic age of the oldest loess sediments in the central chinese loess plateau investigated so far has recently been placed at ca 2 4 million years b p thus glaciations in central asia which are believed to provide the detritus for the formation of chinese loess commenced at the same time as the first ice rafted debris sank into the ocean basins of the northern hemisphere shackleton et al 1984 lithologic palaeontologic geochemical and especially rock magnetic variations throughout the loess column are indicative of palaeoclimatic fluctuations during loess deposition the variations of magnetic low field susceptibility in loess closely resemble those of the marine oxygen isotope record shackleton and opdyke 1976 and establish for the first time a continuous record of the pleistocene glacial and interglacial stages in a continental section spectral analysis of the rock magnetic time series indicates slow apparent sedimentation during interglacial periods which are characterized mainly by calcium dissolution and related compaction of the soils variable aeolic sediment influx as well as erosion and or polygenetic soil formation may also strongly influence the sedimentary history

sedimentary aggregates in the peoria loess of nebraska usa loess grain size data used to infer transport direction or wind strength are generally derived from vigorously disaggregated samples however these data may not adequately represent the effective particle size distribution during loess transport if the transported dust contained aggregates of finegrained material thin sections of minimally altered c and bc horizons in the late pleistocene peoria loess of nebraska usa indicate the presence of aggregates with diameters of 30 1000 am the larger aggregates 250 am are unlikely to have been transported and are interpreted as the result of soil faunal activity and other pedogenic processes after deposition aggregates smaller than 250 am could have a similar origin but laser diffraction particle size analysis suggests that many are sedimentary particles comparison of minimally and fully dispersed particle size distributions from each sampling site was used to estimate the modal diameter of aggregates the aggregate modal diameter becomes finer with decreasing loess thickness representing increasing distance from the source a similar trend was observed in the modal diameter of fully dispersed particle size distributions which represents the mode of sand and silt transported as individual grains we interpret both trends as the result of sorting during transport supporting the interpretation that many of the aggregates were transported rather than formed in place aggregate content appears to increase with distance from the source explaining a much more rapid downwind increase in clay content than would be expected if clay were transported as particles smaller than 2 am diameter although the peoria loess of nebraska contains sedimentary aggregates many of the coarse silt and sand grains in

stratigraphy of the younger dryas chronozone and paleoenvironmental implications central and southern great plains abstract the great plains of the united states was the setting for some of the earliest research in north america into patterns and changes in the character of late pleistocene environments and their effects on contemporary human populations many localities in the region have well stratified records of terminal pleistocene and early holocene human paleoindian activity and past environments these have proven important in debates over the character of the younger dryas chronozone ydc 11 000 10 000 14c bp 12 900 11 700 cal bp in the continental interior this paper reviews the lithostratigraphic record of the ydc on the central and southern great plains and summarizes paleobiological records largely isotopic the goal is to determine if there is any uniformity in the timing character direction and or magnitude of changes in depositional environments or broader geomorphic systems before during or after the ydc in order to address the question of the character of environments through this time the stratigraphic records of the late pleistocene to early holocene transition and in particular the stratigraphic records of the ydc vary through time and space the data clearly show that a host of geomorphic processes produced the terminal pleistocene and early holocene stratigraphic records of the great plains moreover the ydc is not necessarily manifest as a distinct lithostratigraphic or biostratigraphic entity in these different types of deposits and soils the various geomorphic systems of the great plains did not behave synchronously in response to any common climate driver these stratigraphic records reflect local environmental conditions and probably a complex response to the reorganization of mid latitude climates in the terminal pleistocene and early holocene

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

arctic ocean during the last glacial maximum atlantic and polar domains of surface water mass distribution and ice cover on the basis of 52 sediment cores analyzed and dated at high resolution the paleoceanography and climate of the last glacial maximum lgm were reconstructed in detail for the fram strait and the eastern and central arctic ocean sediment composition and stable isotope data suggest three distinct paleoenvironments 1 a productive region in the eastern to central fram strait and along the northern barents sea continental margin characterized by atlantic water advection frequent open water conditions and occasional local meltwater supply and iceberg calving from the barents sea ice sheet 2 an intermediate region in the southwestern eurasian basin up to 84 85 n and the western fram strait characterized by subsurface atlantic water advection and recirculation a moderately high planktic productivity and a perennial ice cover that breaks up only occasionally and 3 a central arctic region north of 85 n in the eurasian basin characterized by a low salinity surface water layer and a thick ice cover that strongly reduces bioproduction and bulk sedimentation rates although the total inflow of atlantic water into the arctic ocean may have been reduced during the lgm its impact on ice coverage and halocline structure in the fram strait and southwestern eurasian basin was strong

lateral and depth variation of loess organic matter overprint related to rhizoliths revealed by lipid molecular proxies and x ray tomography abstract terrestrial sediments like loess are well known for their paleoenvironmental significance although organic carbon contents are commonly very low loess and organic matter lom thereof is regarded as important terrestrial archive for vegetation and climate during deposition however the lom signal is prone to contamination by om of other age and origin than the synsedimentary vegetation e g by postsedimentary deep rooting plants we hypothesized that the influence of rhizosphere effects related to deep rooting plants varies with depth in quality quantity and distance to the former root the 13 m thick late pleistocene loess paleosol sequence at nussloch sw germany contains rhizoliths calcified roots of holocene age with the carbonatic encrustation leading to preservation of former root deposits rhizoliths allow for assessment of rhizosphere processes that occurred during the root s lifetime several horizontal transects comprising rhizoliths surrounding loess rhizoloess up to a distance of 11 cm from rhizoliths and root free reference loess from different depth intervals were analyzed for their carbon c alkane and fatty acid fa composition alkane proxies like carbon preference index cpi average chain length acl as well as n c27 n c31 and n c29 n c31 ratios indicated grass vegetation as origin of lom while rhizoliths derived from woody vegetation several lipid molecular proxies e g short chain long chain alkanes long chain alkane composition and long chain very long chain fa indicated the incorporation of considerable amounts of root and rhizomicrobial om the rhizosphere effect i e postsedimentary overprint of initial lom in the vicinity of roots was not restricted to few mm around the former root but notable to distances of at least 5 cm and possibly more this was confirmed by x ray tomography analyses enabling identification of small calcified root remains and non calcified root channels in the former rhizosphere which could not be identified under field conditions in depth intervals with high rhizolith frequency this entailed lateral overprint of large parts of loess which in depths of 1 95 and 3 2 m below present surface can cover the whole loess due to extension of the former rhizosphere of 5 cm regarding the high rhizolith frequency maximizing at 200 m 2 at 2 6 m depth at nussloch these findings raise the importance of deep rooting plants as potential source for significant postsedimentary overprint of loess paleosol sequences