lake devlin and pinedale glacial history front range colorado abstract glacial lake devlin was an ice margin lake that formed during pinedale time when the north boulder creek valley glacier blocked the mouth of a tributary near the downvalley limit of glaciation the lake was about 2 km long and as much as 60 m deep and was fed by meltwater from small valley glaciers it accumulated sediment apparently without interruption for about 10 000 yr the inception of lake devlin between 23 500 and 21 000 yr b p provides a limiting date for the maximum expansion of pinedale ice because the lake could not form until the north boulder creek glacier had advanced to within 2 km of its downvalley limit the maximum lowering of timberline which occurred about 19 000 yr b p is assumed to coincide with the pinedale glacial maximum apparent timberline elevations determined from picea pinus pollen ratios suggest that ice front positions during the time between 23 000 and 19 000 yr b p may have been only slightly less extensive than at the pinedale maximum lake devlin drained catastrophically about 13 000 yr b p which was after ice had receded from the outlet area pinedale glaciation in the front range began about or after 30 000 yr b p and final deglaciation occurred between 15 000 and 12 000 yr b p

the use of the common pb isotope composition of detrital k feldspar grains as a provenance tool and its application to upper carboniferous paleodrainage northern england k feldspar is a common detrital mineral in sandstones it contains little u or th and hence the common pb isotope composition of unaltered k feldspar is that of the source rock pb isotope variations in igneous and metamorphic crustal rocks define broad spatial patterns that make the pb signature of detrital k feldspar grains a useful provenance tool however it is unclear how robust this signal is and to what extent it can be modified by weathering transport burial diagenesis and or exposure to hydrothermal fluids this study reports an evaluation of the technique using two granite arkose pairs from helmsdale northern scotland and shap northwest england where the composition of the detrital grains can be compared to feldspars in the granite from which they were derived the results of this test have informed a pilot study using the pb isotope composition of detrital k feldspar in upper carboniferous fluvial sedimentary rocks of northern england to constrain the sediment routing pb isotope compositions have been measured using an in situ high resolution single grain technique that can distinguish original composition heterogeneity and subsequent alteration within individual sand grains the results for the granite arkose pairs show that the common pb isotope signature of the detrital k feldspar grains matches the in situ feldspars in the source granites the signal is independent of grain size medium grained sand to granules but alteration along a paleo regolith at helmsdale has resulted in perturbations in pb composition and an increase in the errors on the analyses the application of the technique to arkosic carboniferous serpukhovian bashkirian millstone grit group sandstones in the pennine basin reveals two discrete populations of k feldspars that cannot be distinguished petrographically matching of the compositions to potential sourcelands to the north and northeast of the basin suggest a far traveled 500 km source of k feldspar from the archean paleoproterozoic lewisian rocks of northwest scotland or their extension to the north but also an equally important proximal source emanating from the southern uplands belt of scotland in what would have been the lower part of the hinterland drainage basin of the millstone grit

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

evidence of cyclic dust deposition in the us great plains during the last deglaciation from the high resolution analysis of the peoria loess in the eustis sequence nebraska usa the peoria loess unit is a well defined stratigraphical unit in the upper pleistocene of the north american quaternary deposited between 30 25 ka and about 12 ka ago it has been indicated that this unit shows the highest known worldwide depositional rate for eolian deposits as its thickness varies near the source area between 19 m and 46 m extreme values that are not even recorded in the chinese sequences the results of our present investigation indicate that this particular unit is not homogenous its shows different subunits where lithological variation can be observed through the occurrence of embryonic gley horizons alternating with laminated loess furthermore the grain size analysis shows cycles corresponding to variations in the eolian dynamics responsible for dust transportation and deposition a grain size index interpreted as characterizing the eolian dynamics higher values corresponding to stronger wind conditions shows higher values than those observed in europe a comparison of this index is proposed with the greenland dust and 18 o records it shows that the main climatic history as corresponding to events occurring mainly in north atlantic domain is recorded in the peoria loess deposits however the variation in the magnitude of the eolian events indicates differences from the european loess sequences the strong north atlantic coolings expressed by heinrich events as recorded in europe by coarser deposits are not differentiated in the studied sequence hence they follow more closely observations obtained off california for the north east pacific domain 2007 elsevier b v all rights reserved

loess deposits from the tajik depression central asia magnetic properties and paleoclimate abstract continuous accumulation of dust during the quaternary has formed widespread loess deposits in southern tajikistan the magnetic susceptibility variations of these aeolian sediments have recorded the paleoclimatic fluctuations in a similar direct and complete way to the well studied sediments from the central loess plateau in china 11 susceptibility measurements were carried out in situ at intervals of 10 cm along seven partly overlapping sections in the loess paleosol sequence of karamaidan ca 100 km east of dushanbe nearly 250 oriented samples were taken for rock magnetic investigations careful field observation revealed a total actual loess thickness of ca 100 m for a composite profile with thirteen intervening pedocomplexes pc standard demagnetization techniques yielded a characteristic component of natural remanent magnetization nrm which was used to obtain a well defined magnetostratigraphy the section covers a total timespan of approximately 1 m y including the brunhes and the youngest part of the matuyama polarity chron the susceptibility variations correlate excellently with those of the chinese loess record at xifeng 22 although minor differences in the relative amplitude of the susceptibility signal occur close correlation with the astronomically tuned oxygen isotope record 26 enables refined dating of the loess sequence at karamaidan and demonstrates the global significance of the recorded paleoclimatic variations especially with respect to paleoprecipitation on land during the quaternary

a loess paleosol record of climate and glacial history over the past two glacial interglacial cycles 150 ka southern jackson hole wyoming abstract loess accumulated on a bull lake outwash terrace of marine oxygen isotope stage 6 mis 6 age in southern jackson hole wyoming the 9 m section displays eight intervals of loess deposition loess 1 to loess 8 oldest each followed by soil development our age depth model is constrained by thermoluminescence meteoric 10 be accumulation in soils and cosmogenic 10 be surface exposure ages we use particle size geochemical mineral magnetic and clay mineralogical data to interpret loess sources and pedogenesis deposition of mis 6 loess was followed by a tripartite soil thin loess complex soils 8 7 and 6 apparently reflecting the large climatic oscillations of mis 5 soil 8 mis 5e shows the strongest development loess 5 accumulated during a glacial interval 76 69 ka mis 4 followed by soil development under conditions wetter and probably colder than present deposition of thick loess 3 43 51 ka mis 3 was followed by soil development comparable with that observed in soil 1 loess 1 mis 2 accumulated during the pinedale glaciation and was followed by development of soil 1 under a semiarid climate this record of alternating loess deposition and soil development is compatible with the history of yellowstone vegetation and the glacial flour record from the sierra nevada

middle to late pleistocene loess record in eastern nebraska usa and implications for the unique nature of oxygen isotope stage 2 abstract new subsurface data reveal a nearly continuous stratigraphic record of middle to late pleistocene loess sedimentation preserved beneath upland summits in eastern nebraska usa thickness and grain size trends as well as pedologic evidence indicate significant changes in loess sources accumulation rates and depositional environments the newly defined kennard formation accumulated in the middle pleistocene and may represent multiple thin increments of distal loess from nonglacial sources on the great plains the overlying loveland loess up to 18 m thick and deposited during oxygen isotope stage 6 ois 6 illinoian glaciation probably records the emergence of the missouri river valley as a major glaciogenic loess source the prominent sangamon geosol formed through long term pedogenic alteration of the upper loveland loess during ois 5 and 4 thin loess of the gilman canyon formation records slow loess accumulation and pedogenic alteration in ois 3 the peoria loess ois 2 is similar in thickness to loveland loess but may have accumulated more rapidly in an environment less favorable to bioturbation more importantly comparison of peoria and loveland loess thickness trends indicates much greater influx of nonglaciogenic loess from the great plains during ois 2 than in ois 6 suggesting colder and or drier conditions in the midcontinent during ois 2 than in earlier glacial stages

origin of the rajmahal traps and the 85 e ridge preliminary reconstructions of the trace of the crozet hotspot the 85 e ridge is a buried aseismic ridge approximately parallel to and west of the ninetyeast ridge in the northeastern indian ocean it was previously shown to be of probable volcanic origin emplaced on very young oceanic crust but no satisfactory model of emplacement of the rocks was offered we propose a model of origin of the rajmahal traps of northeastern india the 85 e ridge and afanasy nikitin seamount as the trace of the hotspot that now lies beneath the crozet islands in the southern indian ocean this reconstruction places the kerguelen hotspot which formed the ninetyeast ridge at the triple junction between greater india australia and antarctica before the breakup of eastern gondwana

late pleistocene and holocene seasonal temperatures reconstructed from fossil beetle assemblages in the rocky mountains abstract mutual climatic range mcr analysis was applied to 20 fossil beetle assemblages from 11 sites dating from 14 500 to 400 yr b p the fossil sites represent a transect of the rocky mountain region from northern montana to central colorado the analyses yielded estimates of mean july and mean january temperatures the oldest assemblage 14 500 yr b p yielded mean july values of 10 11 c colder than present and mean january values 26 30 c colder than present postglacial summer warming was rapid as indicated by an assemblage dating 13 200 yr b p with mean july values only 3 4 c cooler than modern by 10 000 yr b p several assemblages indicate warmer than modern mean summer and winter values by 9000 yr b p mcr reconstructions indicate that both summer and winter temperatures were already declining from an early holocene peak mean july values remained above modern levels and mean january values remained below modern levels until 3000 yr b p a series of small scale oscillations followed

stratigraphy and palaeoclimatic significance of late quaternary loess palaeosol sequences of the last interglacial glacial cycle in central alaska abstract loess is one of the most widespread subaerial deposits in alaska and adjacent yukon territory and may have a history that goes back 3 ma based on mineralogy and major and trace element chemistry central alaskan loess has a composition that is distinctive from other loess bodies of the world although it is quartz dominated central alaskan loess was probably derived from a variety of rock types including granites metabasalts and schists detailed stratigraphic data and pedologic criteria indicate that contrary to early studies many palaeosols are present in central alaskan loess sections the buried soils indicate that loess sedimentation was episodic or at least rates of deposition decreased to the point where pedogenesis could keep ahead of aeolian input as in china loess deposition and pedogenesis are likely competing processes and neither stops completely during either phase of the loess soil formation cycle loess deposition in central alaska took place before and probably during the last interglacial period during stadials of the mid wisconsin period during the last glacial period and during the holocene an unexpected result of our geochronological studies is that only moderate loess deposition took place during the last glacial period our studies lead us to conclude that vegetation plays a key role in loess accumulation in alaska factors favouring loess production are enhanced during glacial periods but factors that favour loess accumulation are diminished during glacial periods the most important of these is vegetation boreal forest serves as an effective loess trap but sparsely distributed herb tundra does not thus thick accumulations of loess should not be expected where tundra vegetation was dominant and this is borne out by modern studies near the treeline in central alaska much of the stratigraphic diversity of north american loess including that found in the central lowlands the great plains and alaska is explained by a new model that emphasizes the relative importance of loess production factors versus loess accumulation factors

towards identifying the origin of metamorphic components in austrian loess insights from detrital rutile chemistry thermometry and u pb geochronology abstract trace element cr nb zr geochemistry and u pb geochronology of detrital rutile grains provide constraints on the nature of potential metamorphic sources of loess in austria cr and nb compositions reveal rutile sources with a predominance of metapelitic lithologies for all three loess samples by contrast zr in rutile geothermometry indicates that formation temperatures for loess rutiles around krems at the bohemian massif bm proved to be ca 100 125 c higher t median 717 and 737 c for two samples from stratzing and krems than those at wels close to the eastern alps ea t median 612 c implying that rutiles in loess at wels were derived from alpine lower amphibolite grade rocks while those in loess around krems from rocks formed under upper amphibolite to granulite grade conditions metamorphic temperatures recorded in rutiles from loess at the bm closely match the two reaction thermobarometry estimates of the last high t overprint 7 11 kbar and 700 800 c of rocks making up the varied series and gfohl units around krems this event dated at 340 to 350 ma is believed to be captured by the detrital rutile u pb data as well e g 349 21 and 352 32 ma all these data demonstrate the significance of proximal bm sources varied series and gfohl unit drained by the danube in loess formation around krems fluvial transport is believed to have played a substantial role in entraining and accumulating rutiles for subsequent aeolian deflation for loess near wels while the ultimate sources of these grains are likely to be in the eastern alps the recycling of these rutiles from the flysch zone of the ea cannot be ruled out despite the uncertainties of cr nb based source discriminations in some cases and the potential decoupling of zr and u pb systematics recognized in rutiles this study demonstrates that rutiles may provide unique details on the nature and metamorphic history of their parent rocks if used with caution and by knowing its limitations under certain conditions it is anticipated that beyond zircon rutile geochemistry will become a powerful provenance tracer in loess studies too this seems to be particularly important to gain a better understanding on the erosion and rutile fertility of different metamorphic terrains and their contributions to loess material issues that have not been adequately addressed thus far

lead isotopic evidence for the origin of paleo and mesoproterozoic rocks of the colorado province u s a abstract lead isotopic ratios of k feldspars and whole rocks from 1 7 and 1 4 ga plutonic rocks of the colorado province are relatively non radiogenic for 207 pb 204 pb plotting below the average crust model curve of stacey and kramers 1975 indicating that the terrane was derived primarily from juvenile mantle material slightly more radiogenic ratios in the northern part of the terrane near the archean wyoming province suggest minor inclusion of an older component the data from 1 7 ga plutons plot in a broad field suggesting two episodes of re equilibration with whole rock pb probably related to heating events in the mesoproterozoic 1 4 ga and cretaceous 70 ma possible differences in calculated whole rock th u coupled with slight pb isotopic variations along the north south transect suggest either a terrane boundary through central colorado near salida and gunnison or fundamental differences in source rocks metasedimentary vs metavolcanic u pb analyses of multigrain splits of detrital zircons from quartzites throughout the colorado province have failed to identify archean detritus the oldest 207 pb 206 pb ages found in two samples of quartzite from northern colorado are about 2 0 ga perhaps derived from rocks of the trans hudson orogen in contrast to 2 75 ga detrital zircon in a paleoproterozoic quartzite from the southern part of the wyoming province while we are not yet able to discern if these ages are true provenance ages or mixtures of archean and paleoproterozoic components the absence of easily recognizable archean zircons supports other isotopic data and a conclusion that most of the paleoproterozoic crust of the colorado province was ultimately derived from a juvenile at 1 8 ga mantle reservoir

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

the geochemistry of loess asian and north american deposits compared abstract loess is widely distributed over asia and north america and constitutes one of the most important surficial deposits that serve as terrestrial records of the quaternary the oldest pleistocene loess in china is likely 2 6 ma thus spanning much or all of the pleistocene in north america most loess is no older than the penultimate glacial period with the exception of alaska where the record may go back to 3 0 ma on both continents loess deposits date primarily to glacial periods and interglacial or interstadial periods are represented by paleosols both glacial and non glacial sources of silts that comprise the bulk of loess deposits are found on both continents although loess has been considered to be representative of the average upper continental crust there are regionally distinctive compositions of loess in both asia and north america loess deposits in asia from yakutia tajikistan and china have compositionally distinct major element compositions due to varying abundances of silicate minerals carbonate minerals and clay minerals in north america loess in the mississippi river valley the great plains and alaska are also distinguishable with regard to major element composition that reflects highly diverse source sediments trace element geochemistry sc th zr and the rare earth elements also shows regional diversity of loess bodies in both asia and north america on both continents most loess bodies show significant contributions from later cycle altered sedimentary rocks as opposed to direct derivation from igneous rocks further some loess bodies have detectable contributions from mafic igneous rocks as well as major contributions from average upper crustal felsic rocks intercalated paleosols in loess sections show geochemical compositions that differ significantly from the underlying loess parent materials ratios of soluble to insoluble elements show depletions in paleosols due to chemical weathering losses of calcite dolomite plagioclase mica apatite and smectite in asia and north america the last interglacial paleosol is more weathered than equivalent modern soils which could be due either to a climate that was warmer and more humid a longer period of pedogenesis or both in asia early pleistocene loess and paleosols are both more weathered than those from the middle and late pleistocene forming prior to a mid pleistocene aridification of asia from uplift of the tibetan plateau understanding the geochemistry of loess and paleosols can tell us much about past atmospheric circulation past temperature and moisture regimes and even tectonic processes

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

surface winds across eastern and midcontinental north america during the last glacial maximum a new data model assessment abstract last glacial maximum lgm proxy evidence of surface wind direction across eastern and midcontinental north america comes primarily from loess and dune deposits and overwhelmingly suggests surface winds had a strong westerly component however the season of sediment deposition and the temporal scale of wind information preserved in these deposits remains uncertain furthermore paleoclimate model simulations over the last several decades have indicated a predominance of easterly winds across this region due to the presence of an anticyclone over the laurentide ice sheet as well as katabatic winds flowing off the ice sheet and over the adjacent land surface here we reassess model data near surface wind direction agreement using nine general circulation models participating in the lgm experiment of the third paleoclimate model intercomparison project pmip3 and a compilation of previously published paleowind directions from loess and dune deposits dating to the lgm we find the highest overall model proxy data agreement in winter december february indicating predominantly westerly winds across the region in the lgm model simulations we also find high zonal and meridional wind direction agreement in spring march may and fall september november in many models winter spring and fall also have faster mean daily near surface wind speeds in the lgm simulations relative to pre industrial control simulations thus this model data assessment suggests lgm aeolian deposition in the study region likely occurred dominantly in these three seasons at times when local conditions favorable for aeolian deflation coincided with high wind speed events models that agree best with the proxy data have strengthened aleutian and icelandic low pressure systems and a weakened laurentide high pressure system which constrains the spatial footprint of the laurentide high to the ice sheet reducing northeasterly winds near the ice sheet margin a weaker laurentide high in turn coincides with warmer surface temperatures over the ice sheet and the north atlantic the strength and location of semi permanent pressure systems were thus key controls on surface wind direction across midcontinental and eastern north america during the lgm

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

geochemical evidence of saharan dust parent material for soils developed on quaternary limestones of caribbean and western atlantic islands abstract most previous workers have regarded the insoluble residues of high purity quaternary limestones coral reefs and oolites as the most important parent material for well developed clay rich soils on caribbean and western atlantic islands but this genetic mechanism requires unreasonable amounts of limestone solution in quaternary time other possible parent materials from external sources are volcanic ash from the lesser antilles island arc and saharan dust carried across the atlantic ocean on the northeast trade winds soils on quaternary coral terraces and carbonate eolianites on barbados jamaica the florida keys united states and new providence island bahamas were studied to determine which if either external source was important caribbean volcanic ashes and saharan dust can be clearly distinguished using ratios of relatively immobile elements al 2 o 3 tio 2 ti y ti zr and ti th comparison of these ratios in 25 soils where estimated ages range from 125 000 to about 870 000 yr shows that saharan dust is the most important parent material for soils on all islands these results indicate that the northeast trade winds have been an important component of the regional climatology for much of the quaterary saharan dust may also be an important parent material for caribbean island bauxites of much greater age

mineralogical and geochemical trends across the loess plateau north china abstract the mineralogy and major element geochemistry of malan loess the uppermost loess layer were studied along two transects from the baiyu mountains in the north to the wei river lowlands 350 km to the south the mineralogy of the coarse silt fine silt and clay fractions is generally uniform across the loess plateau however in the coarse silt fraction there is a decrease in the weight percentage of heavy minerals and in the hornblende epidote ratio southwards also in the clay fraction mica contents increase slightly southwards and smectite increases northwards along one transect the major element geochemistry of malan loess shows several clear trends silicon sodium and magnesium generally decrease from north to south while aluminium iron potassium titanium and manganese generally increase the observed mineralogical and chemical patterns appear to be mainly related to the changing particle size from sandy loess in the north to clayey loess in the south slightly higher potassium and phosphorus contents in the clayey loess suggest that this loess has a greater natural fertility overall the results confirm particle size evidence that malan loess was derived from a desert source area to the north and northwest of the loess plateau

a direct link between the china loess and marine δ 18 o records aeolian flux to the north pacific most studies of quaternary climates conducted during the past 150 years have focused on glacial sequences and for the past 25 years marine sediments have provided significant further understanding of glacial and interglacial palaeoclimates an important goal of palaeoclimatology however to link the well known detailed continental records to marine records of similar resolution has yet to be realized the classic loess sequences of china provide an apparently continuous record of continental climate over the past 2 4 million years1 2 the oxygen isotope 18o timescale developed from the marine record3 gives us a global chrono stratigraphy of late pleistocene climates previously it has been suggested in studies of the aeolian component in pelagic sediments that it may be possible to link these two records directly by finding a suitable core from the north west pacific downwind from china4 5 here we report the results of such an effort for core v21 146 fig 1 and tie the last 500 000 years of the chinese loess soil sequence directly to the 18o timescale and in so doing refine the dating of regional loess and soil horizons

origin and paleoclimatic significance of late quaternary loess in nebraska evidence from stratigraphy chronology sedimentology and geochemistry muhs d r bettis iii e a aleinikoff j n mcgeehin j p beann j skipp g marshall b d roberts h m johnson w c benton r 2008 origin and paleoclimatic significance of late quaternary loess in nebraska evidence from stratigraphy chronology sedimentology and geochemistry gsa bulletin 120 11 12 1378 1407

towards an orbital time scale for chinese loess deposits abstract in this study we analyzed the grain size of the baoji loess soil sequence at 10 cm intervals results show that grain size variations are very sensitive to loess soil alternations with loess units being much coarser than soils such a change in loess soil grains is interpreted as the result of the glacial interglacial cyclic variations in the intensity of the winter monsoon winds out of siberia the grain size record is thus employed as a proxy indicator of the winter monsoon circulation and tuned to the orbital records calculated recently by berger and loutre 1991 under the control of major magnetic reversals the tuning is independent of any correlation with 18o signals in the deep sea sediments the resulting grain size time scale is tightly constrained as suggested by the following facts 1 the filtered obliquity and precession components from the grain size data on the orbital time scale closely match the theoretical orbital records 2 ages of the major magnetic reversals estimated from the grain size time scale are in good agreement with the k ar dated ages 3 there is close coherence between the baoji grain size time series and the orbital variations at the orbital frequency bands over the entire 0 2 5 ma period and 4 the grain size record on the orbital time scale shows a close similarity to the orbitally tuned dsdp site 607 and odp site 677 18o records examination of time dependent characteristics of the grain size record suggests that there are two major shifts of dominant periodicities in the long term monsoonal cycles one occurring at about 0 8 ma bp and the other around 1 7 ma bp

geochemistry and mineralogy of late quaternary loess in the upper mississippi river valley usa provenance and correlation with laurentide ice sheet history abstract the midcontinent of north america contains some of the thickest and most extensive last glacial loess deposits in the world known as peoria loess peoria loess of the upper mississippi river valley region is thought to have had temporally varying glaciogenic sources resulting from inputs of sediment to the mississippi river from different lobes of the laurentide ice sheet here we explore a new method of determining loess provenance using k rb and k ba values in k feldspars and micas in loess from a number of different regions in north america results indicate that k rb and k ba values can distinguish loess originating from diverse geologic terrains in north america further different loess bodies that are known to have had the same source sediments using other criteria have similar k rb and k ba values we also studied three thick loess sections in the upper mississippi river valley region at each site the primary composition of the loess changed over the course of the last glacial period and k rb and k ba values parallel changes in carbonate mineral content and clay mineralogy we thus confirm conclusions of earlier investigators that loess composition changed as a result of the shifting dominance of different lobes of the laurentide ice sheet and the changing course of the mississippi river we conclude that k rb and k ba values are effective robust and rapid indicators of loess provenance that can be applied to many regions of the world

a late quaternary record of eolian silt deposition in a maar lake st michael island western alaska abstract recent stratigraphic studies in central alaska have yielded the unexpected finding that there is little evidence for full glacial late wisconsin loess deposition because the loess record of western alaska is poorly exposed and not well known we analyzed a core from zagoskin lake a maar lake on st michael island to determine if a full glacial eolian record could be found in that region particle size and geochemical data indicate that the mineral fraction of the lake sediments is not derived from the local basalt and is probably eolian silt deposition took place from at least the latter part of the mid wisconsin interstadial period through the holocene based on radiocarbon dating based on the locations of likely loess sources eolian silt in western alaska was probably deflated by northeasterly winds from glaciofluvial sediments if last glacial winds that deposited loess were indeed from the northeast this reconstruction is in conflict with a model derived reconstruction of paleowinds in alaska mass accumulation rates in zagoskin lake were higher during the pleistocene than during the holocene in addition more eolian sediment is recorded in the lake sediments than as loess on the adjacent landscape the thinner loess record on land may be due to the sparse herb tundra vegetation that dominated the landscape in full glacial time herb tundra would have been an inefficient loess trap compared to forest or even shrub tundra due to its low roughness height the lack of abundant full glacial eolian silt deposition in the loess stratigraphic record of central alaska may be due therefore to a mimimal ability of the landscape to trap loess rather than a lack of available eolian sediment

geochemical characterization of the luochuan loess paleosol sequence china and paleoclimatic implications abstract chemical major and trace element and isotopic sr nd analyses of the luochuan loess paleosol sequence in china were performed in order to examine the following problems 1 potential source heterogeneity 2 element behavior during pedogenesis 3 paleoclimatic implications and 4 the use of loess data to determine the average composition of the upper continental crust new results clearly indicate that rb sr u and ce abundances are severely depleted in paleosols relative to their parental loess the behavior of these elements is strongly controlled by the breakdown of primary minerals such as carbonate sr or by alteration processes in highly oxidizing environments ce and u the pattern of element variations through the upper section of the luochuan sequence mimics the magnetic susceptibility signal both in position and in intensity thus reinforcing the pedogenesic origin of the enhanced magnetic susceptibility in paleosols all loess samples display highly uniform ree patterns except for ce characterized by the upper continental crust ucc ratios la yb n 10 and eu eu 0 66 the paleosol ree patterns are similar to those of the loess with the same lree hree fractionation but they also show distinct and variable negative ce anomalies nd and sr isotopic compositions are rather uniform in both loess and paleosols and do not vary with stratigraphic position nd 10 5 to 9 2 87 sr 86 sr 0 715 to 0 719 the isotopic homogeneity in the luochuan sequence strongly suggests a uniform source region during the entire period of deposition since 800 ka ago to the present several conclusions may be drawn from the present study 1 1 no geochemical distinction can be made between loess layers l1 to l7 the sources and the dust storm trajectories must have been essendally the same for the last 800 ka 2 2 some elements e g ca rb sr u ce are strongly fractionated by pedogenesis between loess and paleosols ce mobility in soils is clearly demonstrated but the process responsible for this leaching is still poorly understood 3 3 systematic variations of element abundances and ratios between loess and paleosols can be used as chemical indicators for pedogenesic intensity and so for paleoclimatic change these chemical indicators serve a function similar to that of oxygen isotopes in deep sea sediments pelagic foraminifera or that of magnetic susceptibility in loess sequences they are different recorders of paleoclimatic change 4 4 the striking uniformity of ree patterns and la th ratios in the luochuan loess and paleosols as well as in loess worldwide is an excellent starting point for estimating the average composition of the upper continental crust

geochemical evidence for the origin of late quaternary loess in central alaska loess is extensive in central alaska but there are uncertainties about its source and the direction of paleo winds that deposited it both northerly and southerly winds have been inferred the most likely sources of loess are the tanana river south the nenana river southeast and the yukon river north late quaternary loess in central alaska has immobile trace element compositions cr sc th ta th sc th u eu eu gdn ybn that indicate derivation mostly from the tanana river however other ratios as sb zr hf lan ybn and quantitative modeling indicate that the yukon river was also a source during the last glacial period there may have been a longer residence time of the siberian and canadian high pressure cells along with a strengthened aleutian low pressure cell this would have generated regional scale northeasterly winds and explains derivation of loess from the yukon river however superim posed upon this synoptic scale circulation there may have been strong southerly katabatic winds from expanded glaciers on the northern flank of the alaska range these winds could have provided eolian silt from the tanana river yukon river and tanana river sediments are highly calcareous whereas fairbanks area loess is not this suggests that carbonate leaching in loess kept ahead of sedimentation and that late quaternary loess in central alaska was deposited relatively

on the formation of desert loess sequences of quartz rich coarse 20 63 m silt occur in many low and midlatitude unglaciated arid and semiarid areas and have been termed desert loess the processes by which these deposits are generated have been debated for decades all hypotheses to explain their origin seek to provide mechanisms for the generation of silt sized material without glacial grinding which is the main process involved in the production of coarse silt at high latitudes possible mechanisms for the formation of coarse silt in arid regions include derivation from preexisting siltstones mechanical weathering of silicate rocks and abrasion of sand grains in active dune environments during intense transport events examination of the characteristics of desert loess and field and laboratory experiments to assess the role of dune areas as a source of coarse silt by abrasion and or resuspension of residual fines suggests that many loess sequences are dominated by locally derived coarse silt improvements in the characterization of desert loess particle size mineralogy and geochemistry are needed however to identify sources and sinks of coarse silt especially when combined with climatic back trajectory analysis properly scaled experiments and modeling of particle collisions will also help to better quantify the effectiveness of abrasion in the generation of coarse silt in support of field observations

grain size of quartz as an indicator of winter monsoon strength on the loess plateau of central china during the last 130 000 yr the chinese loess paleosol sequence constitutes an important record of variations in asian monsoon climate over the past 2 4 myr magnetic susceptibility of loess and paleosols has been used as a proxy for summer monsoon intensity while median grain size has been regarded as a measure of the strength of winter monsoon winds that were responsible for most of the dust transport however median grain size is only an approximate index of winter monsoon strength because both paleosols and loess have been modified to various degrees by weathering processes that have produced pedogenic clay the quartz component of loess and paleosols is largely unaffected by weathering processes and therefore constitutes a more reliable proxy index of monsoon wind strength median grain size qmd and maximum grain size qmax values of monomineralic quartz isolated from the loess paleosol section at luochuan in the central loess plateau are characterized by two main intervals during the last ca 130 000 yr when these parameters were significantly greater than 9 and 85 m respectively and three main intervals when they were lower the data imply that the winter monsoon weakened during the intervals with low qmd and qmax values which coincide with marine oxygen isotope stages 5 3 and 1 and was strongest ca 67 000 and 20 000 yr ago during isotope stages 4 and 2 however both quartz grainsize records display second order high frequency high amplitude variations which are lacking in the magnetic susceptibility record that imply rapid and significant changes in atmospheric conditions that affect dust transport and deposition

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

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

change in atmospheric mineral aerosols in response to climate last glacial period preindustrial modern and doubled carbon dioxide climates desert dust simulations generated by the national center for atmospheric research s community climate system model for the current climate are shown to be consistent with present day satellite and deposition data the response of the dust cycle to last glacial maximum preindustrial modern and doubled carbon dioxide climates is analyzed only natural non land use related dust sources are included in this simulation similar to some previous studies dust production mainly responds to changes in the source areas from vegetation changes not from winds or soil moisture changes alone this model simulates a 92 33 and 60 change in dust loading for the last glacial maximum preindustrial and doubled carbon dioxide climate respectively when impacts of carbon dioxide fertilization on vegetation are included in the model terrestrial sediment records from the last glacial maximum compiled here indicate a large underestimate of deposition in continental regions probably due to the lack of simulation of glaciogenic dust sources in order to include the glaciogenic dust sources as a first approximation we designate the location of these sources and infer the size of the sources using an inversion method that best matches the available data the inclusion of these inferred glaciogenic dust sources increases our dust flux in the last glacial maximum from 2 1 to 3 3 times current deposition

lead isotopic fingerprinting of tectono stratigraphic terranes east central alaska common lead isotopic compositions have been determined on feldspars from meta igneous rocks from nine tectono stratigraphic terranes or subterranes in east central alaska most of the terranes have distinct and well defined signatures in terms of isotopic composition thus most can be distinguished on conventional lead isotopic diagrams lead isotopic ratios provide evidence for 1 possible sources for the igneous rocks 2 time of metamorphism 3 correlation of terrane fragments and 4 delineation of juxtaposed terranes determination of lead isotopic ratios from igneous rocks can be useful in characterizing tectono stratigraphic terranes as to mantle or crustal origin and in correlation particularly where terrane relationships are enigmatic

last glacial loess in the conterminous usa the conterminous united states contains an extensive and generally well studied record of last glacial loess the loess occurs in diverse physiographic provinces and under a wide range of climatic and ecological conditions both glacial and non glacial loess sources are present and many properties of the loess vary systematically with distance from loess sources united states midcontinent last glacial loess is probably the thickest in the world and our calculated mass accumulation rates mars are as high as 17 500 g m 2 yr at the bignell hill locality in nebraska and many near source localities have mars greater than 1500 g m 2 yr these mars are high relative to rates calculated in other loess provinces around the world recent models of last glacial dust sources fail to predict the extent and magnitude of dust flux from the mid continent of the united states a better understanding of linkages between climate ice sheet behaviour routing of glacial meltwater land surface processes beyond the ice margin and vegetation is needed to improve the predictive capabilities of models simulating dust flux from this region r 2003 elsevier ltd all rights reserved

a new indicator of glacial dispersal lead isotopes abstract lead isotope ratios from glacigenic sediments have been successfully used as glacial indicators to define dispersal patterns in mineralized areas of the canadian shield and the appalachian orogen sediments adjacent to and down ice from volcanogenic massive sulphide vms deposits have pb isotope ratios that reflect an input of pb from the deposits these ratios are distinctly different from those of the surrounding country rocks vertical sections overlying one of the deposits show that the pb isotope signature is preserved in much of the succession although incursions of pb from other sources are also identified by pb isotope ratios that are much higher than those of the deposit sections away from known deposits have isotopic ratios characteristic of the country rocks pb from some vms deposits can form an ideal glacial indicator the reasons for this are i the deposits can be considered point sources ii the pb in old vms deposits is isotopically different from the country rocks iii the pb abundance in the deposits is significantly enriched relative to average crustal abundances iv the pb isotope ratios are independent of the pb concentration in samples containing low to moderate amounts of pb v isotope ratio measurements can be made on small quantities of material and vi the isotope ratios of till enriched in vms derived pb are not measurably affected by weathering

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

using pca to characterize and differentiate loess deposits in wisconsin and upper michigan usa abstract data on loess thickness as well as a variety of particle size fractions were determined for 875 samples taken from several loess sheets across wisconsin and upper michigan because the particle size characteristics of these samples varied widely within and between the loess sheets we used principal components analysis pca as a way of isolating the major textural components or signals that exist across the various loess sheets the purpose of this research was to examine and interpret these principal components or particle size signatures common to the different loesses in order to better understand the loess sheets character formation mechanisms and likely sources our initial assumption that many of these loesses varied markedly from the classical silty loess was supported by the particle size data and the pca although component 1 was interpreted as thick silty loess dominated by medium silts component 2 was mainly composed of very fine sand and coarse silt and is better viewed as coarse loess components 3 and 4 were less texturally homogeneous and may reflect mixing between the loess and the underlying sediment especially where the loess is

chronologic evidence for multiple periods of loess deposition during the late pleistocene in the missouri and mississippi river valley united states implications for the activity of the laurentide ice sheet abstract the loess stratigraphy of the mid continental u s is an important proxy record for the activity of the laurentide ice sheet in north america one of the most outstanding problems is deciphering the age of loess deposits in this area during the late pleistocene radiocarbon dating of snails and thermoluminescence dating of the fine silt fraction 4 11 m from loess at the loveland loess type section loveland iowa and a recent excavation at the pleasant grove school section madison county illinois provide new chronologic control on loess deposition in the mississippi missouri river valley chronology indicates that the loveland loess is illinoian in age 135 20 ka but is not correlative with the teneriffe silt which is dated to 77 8 ka concordant radiocarbon and thermoluminescence age estimates demonstrate that the roxana silt and a correlative loess in iowa the pisgah formation is probably 40 30 ka old these age estimates in conjunction with previous results indicate that there were four periods of loess deposition during the last 150 ka at 25 12 ka 45 30 ka 85 70 ka and at ca 135 20 ka this chronology of loess deposition supports the presence of both a late illinoian and early wisconsinan loess and associated soils thus there may be more than one soil in the loess stratigraphy of the mid continental u s with morphologies similar to the sangamon soil the last three periods of loess deposition may be correlative with periods of elevated dust concentrations recorded in the dye 3 ice core from southern greenland this is particularly significant because both areas possibly had the same source for eolian particles reconstructions of atmospheric circulation for glacial periods show a southerly deflected jet stream that could have transported dust from the mid continental usa to southern greenland lastly the inferred record of loess deposition is parallel to a chronology for deglaciation of the laurentide ice sheet deciphered from chronologic and stratigraphic studies of raised glacial and marine sediments in the hudson bay lowlands canada these chronologies indicate that the laurentide ice sheet was quite dynamic during the late pleistocene advancing and retreating across north america at least four times during the last 150 ka

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

late quaternary loess in northeastern colorado part ii pb isotopic evidence for the variability of loess sources a new application of the pb isotopic tracer technique has been used to determine the relative importance of different silt sources for late wisconsin loess in the central great plains of eastern colorado samples of the peoria loess collected throughout the study area contain k feldspar derived from two isotopically and genetically distinct sources 1 glaciogenic material from early and middle proterozoic crystalline rocks of the colorado province and 2 volcaniclastic material from the tertiary white river group exposed on the northern great plains pb isotopic compositions of k feldspar in loess from two dated vertical sections at beecher island and last chance colorado vary systematically implying climatic control of source availability we propose a model whereby relatively cold conditions promoted the advance of front range valley glaciers discharging relatively little glaciogenic silt but strong winds caused eolian erosion of white river group silt due to a decrease in vegetation cover during warmer periods valley glaciers receded and discharged abundant glaciogenic silt while surfaces underlain by the white river group were stabilized by vegetation isotopic data from eastern colorado loess sections record two warm cold warm cycles during late wisconsin time between about 21 000 and 11 000 radiocarbon yr b p similar to results from other studies in the united states and greenland

a geochemical study of loess and desert sand in northern china implications for continental crust weathering and composition abstract chemical compositional variations of desert sands and loess loess like deposits river deposits and saline lacustrine deposits collected mostly from the southern margin of the taklimakan desert china provide strong constraints on the chemical weathering processes of continental crust and also on the provenance compositions of these quaternary deposits constant but lower concentrations of al relative to the average upper continental crust ucc in these quaternary deposits could be attributed to chemical weathering of al rich minerals such as feldspar and mica and removal of clay minerals by wind and water large variations of mobile na ca mg and sr compared to relatively immobile al k and ba resulted from the chemical weathering of plagioclase and carbonate minerals salinization and carbonation of the arid lands most trace elements are not significantly fractionated their variations in absolute concentration are caused by the dilution of quartz evaporates and carbonate minerals variations in sn concentrations probably depend on relative contents of some heavy minerals high sr ba low th u k cs ratios of the saline lake deposits suggest differences in aqueous chemistry of these elements the occurrence of the m type tetrad effect a ree pattern with four convex curves of ree compositions found mainly in the river deposits is considered the result of water particle interaction during chemical weathering of crustal materials y ho ratios of these samples are significantly higher than those of chondrite and the ucc this suggests different chemical behavior of these two elements and possibly the enrichment of carbonate materials the low lree hree feature of the saline lake deposits can be interpreted as due to the different aqueous chemical behavior of the ree s however it can also be reasonably considered as the result of inheritance from their provenance since lree and lree hree variations may be related to the geological setting of the source according to some elements that are not significantly fractionated the average provenance composition of these quaternary deposits are probably characterized by enrichments of ni cr and v over fe higher la th lower la sc and th sc and lower ta w and nb w ratios compared with the ucc composition

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

late quaternary loess in northeastern colorado part i age and paleoclimatic significance loess in eastern colorado covers an estimated 14 000 km 2 and is the westernmost part of the north american midcontinent loess province stratigraphic studies indicate there were two periods of loess deposition in eastern colorado during late quaternary time the first period spanned ca 20 000 to 12 000 14 c yr b p ca 20 14 ka and correlates reasonably well with the culmination and retreat of pinedale glaciers in the colorado front range during the last glacial maximum the second period of loess deposition occurred between ca 11000 and 9000 14 c yr b p this interval may be holocene or may correlate with a hypothesized younger dryas glacial advance in the colorado front range sedimentologic mineralogic and geochemical data indicate that as many as three sources could have supplied loess in eastern colorado these sources include glaciogenic silt derived from the colorado front range and two bedrock sources volcaniclastic silt from the white river group and clays from the pierre shale the sediment sources imply a generally westerly paleowind during the last glacial maximum new carbon isotope data combined with published faunal data indicate that the loess was probably deposited on a cool steppe implying a last glacial maximum july temperature depression relative to the present of at least 5 6 c overall loess deposition in eastern colorado occurred mostly toward the end of the last glacial maximum under cooler and drier conditions with generally westerly winds from more than one source

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

provenance of loess in the central great plains u s a based on nd sr isotopic composition and paleoenvironmental implications abstract loess of the central great plains u s a records intervals of quaternary aeolian dust accumulation at rates among the highest known worldwide this study used nd sr isotopic analysis to investigate the provenance of middle pleistocene to holocene loess in western and central nebraska u s a essential information for understanding the paleoenvironmental conditions that allowed such rapid accumulation to occur at some times and not others nd sr isotopic data suggest that dust from unglaciated great plains landscapes has been the primary component of loess accumulated at both very high and low rates since the middle pleistocene however loess isotopic compositions all require an additional minor source with higher 87sr 86sr and lower end most likely debris from precambrian rocks in the rocky mountains carried to the great plains by the platte river system the contribution from this secondary source probably including glacially eroded sediment was greater in marine isotope stage mis 3 and early mis 2 but decreased somewhat just after the last glacial maximum lgm at the bignell hill section earlier research showed that the highest accumulation rates in that section also occurred after the lgm thus our results likely indicate increased dust emission from unglaciated landscapes at that time greater plant moisture stress in a warming climate with still low co2 could have played a role in that increased dust production along with lagged vegetation response however a strengthened westerly component of surface winds is the most straightforward explanation

dirtmap the geological record of dust atmospheric dust is an important feedback in the climate system potentially affecting the radiative balance and chemical composition of the atmosphere and providing nutrients to terrestrial and marine ecosystems yet the potential impact of dust on the climate system both in the anthropogenically disturbed future and the naturally varying past remains to be quantified the geologic record of dust provides the opportunity to test earth system models designed to simulate dust records of dust can be obtained from ice cores marine sediments and terrestrial loess deposits although rarely unequivocal these records document a variety of processes source transport and deposition in the dust cycle stored in each archive as changes in clay mineralogy isotopes grain size and concentration of terrigenous materials although the extraction of information from each type of archive is slightly different the basic controls on these dust indicators are the same changes in the dust flux and particle size might be controlled by a combination of a source area extent b dust emission efficiency wind speed and atmospheric transport c atmospheric residence time of dust and or d relative contributions of dry settling and rainout of dust similarly changes in mineralogy reflect a source area mineralogy and weathering and b shifts in atmospheric transport the combination of these geological data with process based forward modelling schemes in global earth system models provides an excellent means of achieving a comprehensive picture of the global pattern of dust accumulation rates their controlling mechanisms and how those mechanisms may vary regionally the dust indicators and records of terrestrial and marine palaeoenvironments dirtmap data base has been established to provide a global palaeoenvironmental data set that can be used to validate earth system model simulations of the dust cycle over the past 150 000 years

radiocarbon ages of soils and charcoal in late wisconsinan loess south central nebraska abstract the farmdale soil occurs below late wisconsinan loess throughout the u s midwest at the la sena site in the central great plains humates in the farmdale interstadial soil have a corrected age of 21 000 yr b p humates in a buried bt horizon and a bulk sample of overlying loess 2 5 m above the farmdale interstadial soil have ages of 17 000 and 19 000 yr b p respectively in the republican river valley picea spruce charcoal is common in the lower meter of peoria loess near bloomington nebraska humates from burned organic matter only 60 cm above the base of peoria loess have a corrected age of ca 19 000 yr b p

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