sedimentology detrital zircon geochronology and stable isotope geochemistry of the lower eocene strata in the wind river basin central wyoming shallow subduction of the farallon plate beneath the western united states has been commonly accepted as the tectonic cause for the laramide deformation during late cretaceous through eocene time however it remains unclear how shallow subduction would produce the individual laramide structures critical information about the timing of individual laramide uplifts their paleoelevations at the time of uplift and the temporal relationships among laramide uplifts have yet to be documented at regional scale to address the question and evaluate competing tectonic models the wind river basin in central wyoming is fi lled with sedimentary strata that record changes of paleogeog raphy and paleoelevation during laramide deformation we conducted a multidisciplinary study of the sedimentology detrital zircon geochronology and stable isotopic geochemistry of the lower eocene indian meadows and wind river formations in the northwestern corner of the wind river basin in order to improve understanding of the timing and process of basin evolution source terrane unroofi ng and changes in paleoelevation and paleoclimate depositional environments changed from alluvial fans during deposition of the indian meadows formation to low sinuosity braided river systems during deposition of the wind river formation paleocurrent directions changed from southwestward to mainly eastward through time conglomerate and sandstone compositions suggest that the washakie and or western owl creek ranges to the north of the basin experienced rapid unroofi ng ca 55 5 54 5 ma producing a trend of predominantly mesozoic clasts giving way to precambrian basement clasts upsection rapid source terrane unroofi ng is also suggested by the upsection changes in detrital zircon u pb ages detrital zircons in the upper wind river formation show age distributions similar to those of modern sands derived from the wind river range with up to 20 of zircons derived from the archean basement rocks in the wind river range indicating that the range was largely exhumed by ca 53 51 ma the rise of these ranges by 51 ma formed a confi ned valley in the northwestern part of the basin and promoted development of a meandering fl uvial system in the center of the basin the modern paleodrainage confi guration was essentially established by early eocene time carbon isotope data from paleosols and modern soil carbonate show that the soil co 2 respiration rate during the early eocene was higher than at present from which a more humid eocene paleoclimate is inferred atmosphere pco 2 estimated from paleosol carbon isotope values decreased from 2050 450 ppmv to 900 450 ppmv in the early eocene consistent with results from previous studies oxygen isotope data from paleosol and fl uvial cement carbonates show that the paleoelevation of the wind river basin was comparable to that of the modern great plains 500 m above sea level and that local relief between the washakie and wind river ranges and the basin fl oor was 2 3 0 8 km up to 1 km of post laramide regional net uplift is required to form the present landscape in central wyoming

isotopic composition of old ground water from lake agassiz implications for late pleistocene climate a uniform oxygen isotope value of 25 per mil was obtained from old ground water at depths of 20 to 30 meters in a thick deposit of clay in the southern part of the glacial lake agassiz basin the lake occupied parts of north dakota and southern manitoba at the end of the last glacial maximum and received water from the ice margin and the interior plains region of canada ground water from thick late pleistocene age clay deposits elsewhere a till in southern saskatchewan and a glaciolacustrine deposit in northern ontario show the same value at similar depths these sites are at about 50 n latitude span a distance of 2000 kilometers and like the lake agassiz sites have a ground water velocity of less than a few millimeters per year the value of 25 per mil is characteristic of meltwater impounded in the southern basin of lake agassiz this value corresponds to an estimated air temperature of 16 c compared with the modern temperature of 0 c for this area

bicadinanes and related compounds as maturity indicators for oils and sediments abstract biomarker maturity indices have proven useful in petroleum exploration to assess the thermal maturity of oils and sediments however their application to deltaic petroleum systems is often hindered by source and diagenetic interferences recently we have described several new maturity indices based on the catagenetic products of the plant biopolymer polycadinene these indices should be largely immune to biochemical effects during diagenesis since little microbial activity will remain at temperatures sufficient to dissociate the polymer here we report changes in five polycadinene related maturity indices through a marine fluvio deltaic depositional sequence in the south sumatra basin indonesia the results are compared with the maturity required for oil generation as estimated by kinetic modelling and with conventional maturity indicators such as sterane epimerisation and the methyl phenanthrene index the data confirm that the polycadinene indices are less subject to source and diagenetic interferences than those based on the sterane and aromatic hydrocarbon distributions values for several of the indices have also been measured for 17 tertiary oils from southeast asia papua new guinea new zealand and australia contrary to an earlier report the bicadinane maturity indicator bmi 1 continues to change into the oil window and hence may be useful in ranking the relative maturity of oils as well as sediments the bmi 1 values for two oils having unusually low sterane maturities are normal leading us to question whether the low sterane maturity of many tertiary age oils is due to their source rocks having been heated more rapidly than those of older oils

oxygen and carbon isotopes in modern fresh water ostracod valves assessing vital offsets and autecological effects of interest for palaeoclimate studies abstract the isotopic composition of modern benthic ostracod and mollusc carbonate the oxygen isotope composition of lake water 18ow the carbon isotope composition of dissolved inorganic carbon 13cdic and water temperatures were monitored during a year at different water depths in two lakes ammersee and starnberger see southern germany the aim was to test whether the valves are built in isotopic equilibrium and how seasonal water temperatures are recorded in the shells we find systematic offsets between 18o of valve carbonate compared to 18o expected for a theoretical calcite formed in isotopic equilibrium offsets are higher than 2 for different species of the candona group 1 5 for cytherissa lacustris and around 0 8 for limnocythere inopinata and darwinula stevensoni they seem to be constant for all instars of a species and temperature independent mean water temperatures of the calcification period are readily reflected in the valve 18o if corrected for the offsets 13c of valve carbonate is mainly controlled by 13c of the dissolved carbonate in ambient lake water but seems to be biased by micro habitat effects and vital offsets the study provides information on the seasonality of valve formation thus potentially allowing the quantification of temporal and spatial water temperature gradients from specific 18o records

brunei darussalam characteristics of selected petroleums and source rocks abstract the development of three tertiary deltaic complexes has resulted in the deposition of up to 10 km of sandstones and shales comprising the sources and reservoirs for crude oils that occur onshore near offshore and with future exploration efforts those likely to be encountered in deepwater reservoirs north of the brunei coastline we examined a series of offshore oils and onshore rock samples in brunei darussalam a to delineate oil family groups and their source rock characteristics and b to assess the source potential of the sedimentary sequence with respect to lithology and depositional setting twelve offshore oils and 53 shales coaly shales and coals were examined the oils contain indicators of allochthonous e g bicadinanes oleananes and autochthonous e g cholestanes and methylcholestanes components in the source organic matter predictable geographic variations of this mixed input are clearly evident in the sample set e g allochthonous input appears to increase in offshore brunei to the northeast although this molecular source signature is relatively clear migration of these oils from deep and unidentified source rocks has resulted in extensive migration contamination with respect to the tetracyclic and pentacyclic hydrocarbons this contamination has resulted in strong correlations between certain molecular maturity indicators and the present day temperature of the reservoirs liquid hydrocarbon source rock potential is present in the tidal and coastal embayment facies and is greatest in the miocene coals neither the shales nor coaly shales contain significant oil generative potential the thermal immaturity of the sample set precludes valid oil source rock correlations without conducting artificial maturation experiments on the coals

a full glacial biota from southeastern iowa usa wisconsinan full glacial silts filling a swale exposed in conklin quarry johnson co iowa contain a large and diverse biota that includes pollen bryophytes vascular plant macrofossils small mammals molluscs and insects radiocarbon dates on wood from the top middle and bottom of the swale fill respectively were 16710 270 17 170 205 and 18090 190 yr bp the pollen diagram is dominated by picea spruce pinus pine and cyperaceae sedge and it records low pollen accumulation rates plant macrofossils include a number of tundra species along with picea and larix larch needles and small pieces of wood the insect fauna contains many species now confined to the forest tundra transition zone of northwestern yukon and alaska small mammals include the tundra indicators dicrostonyx collared lemming and probably microtus miurus singing vole together with boreal forest taxa the molluscs include extinct and relict species and show the widest range in present geographic distribution but rocky mountain and especially northern elements predominate in the swale fill all these lines of evidence lead to consistent palaeoclimato logical interpretation and palaeoecological reconstruction the dominant habitats represented by the biota and sedimentary environment collectively included open calcareous silty to sandy or gravelly upland sites minerotrophic fens wetlands pond or stream side clayey to sandy shores and shallow possibly ephemeral cold clear water ponds mean july temperatures were probably 11 to 13 c cooler than at present the biota indicates that a picea larix krummholz with extensive tundra openings was present in southeastern iowa between 18090 and 16710 yr bp

atmospheric oxygenation caused by a change in volcanic degassing pressure around two and a half billion years ago following the end of the archaean eon the atmosphere turned from anoxic to weakly oxic in what is known as the great oxidation event using a model of volcanic degassing gaillard et al demonstrate that a preceding period of continental crust formation may have been the trigger they propose that as continents emerged and volcanoes became increasingly subaerial rather than submarine magmatic volatiles were degassed at lower pressures leading to a progressive oxidation of the gases released this shift to a release of sulphur as sulphur dioxide rather than as hydrogen sulphide could then have fed marine sulphate reduction and the eventual oxygenation of earth s atmosphere the precambrian history of our planet is marked by two major events a pulse of continental crust formation at the end of the archaean eon and a weak oxygenation of the atmosphere the great oxidation event that followed at 2 45 billion years ago this oxygenation has been linked to the emergence of oxygenic cyanobacteria1 2 and to changes in the compositions of volcanic gases3 4 but not to the composition of erupting lavas geochemical constraints indicate that the oxidation state of basalts and their mantle sources has remained constant since 3 5 billion years ago5 6 here we propose that a decrease in the average pressure of volcanic degassing changed the oxidation state of sulphur in volcanic gases initiating the modern biogeochemical sulphur cycle and triggering atmospheric oxygenation using thermodynamic calculations simulating gas melt equilibria in erupting magmas we suggest that mostly submarine archaean volcanoes produced gases with so2 h2s 1 and low sulphur content emergence of the continents due to a global decrease in sea level and growth of the continental crust in the late archaean then led to widespread subaerial volcanism which in turn yielded gases much richer in sulphur and dominated by so2 dissolution of sulphur in sea water and the onset of sulphate reduction processes could then oxidize the atmosphere

interpreting basal sediments and plant fossils in kettle lakes insights from silver lake michigan usa we report on pollen plant macrofossils and associated lithostratigraphy of a sediment core extracted from the base of silver lake a kettle lake in northern lower michigan usa which reveal a co

migration of contaminants in groundwater at a landfill a case study 3 tritium as an indicator of dispersion and recharge abstract since the beginning of the era of major atmospheric tests of nuclear weapons in 1953 rain and snow has contained concentrations of 3 h that have been above the natural levels that existed prior to 1953 as a consequence 3 h now serves as an identifier or tracer of post 1952 water in groundwater flow systems in this study the distribution of 3 h in and near the contaminant plume at the borden landfill was determined by analysis of samples from multilevel point samplers and from bundle piezometers the contaminant plume which was delineated using cl and so 2 4 patterns was found to contain a large zone of tritiated water and a smaller zone with no detectable 3 h much of the zone of untritiated water exists in the frontal segment of the plume this is consistent with the fact that landfilling operations at the site began in 1940 and that as the plume developed in the period from 1940 to 1952 any natural 3 h that may have existed in the plume during this period is now below the detection level an estimate of the 3 h concentrations in water that has recharged the aquifer beneath the landfill was developed using the nearly continuous record for 3 h in precipitation at ottawa ontario with comparisons to less frequent data from other sampling stations the ottawa record indicates that during the years in the 1960 s in which peak 3 h concentrations occurred values of thousands of tu in precipitation were common and levels of many hundreds of tu or higher existed almost continuously in the zone of tritiated water in the plume and in the tritiated zone in the natural groundwater up and downgradient of the plume all of the sample points except three in the plume had concentrations below 150 tu two of these three have concentrations below 290 tu the general occurrence in the aquifer of 3 h levels that are much below the high levels that existed in precipitation prior to the 1970 s is attributed to radioactive decay and to the effect of longitudinal hydrodynamic dispersion that has caused mixing of the tritiated groundwater with older untritiated water an analytical solution to the one dimensional form of the advection dispersion equation was used with the method of superposition to simulate the distribution of 3 h along the central flow path in the contaminant plume simulated 3 h distributions using longitudinal dispersivity values in the range of 30 60 m gave 3 h distributions similar to the field values although the longitudinal dispersivity range of 30 60 m is far above the range reported from laboratory experiments on sand and although it is much above the range obtained in a small scale tracer test at the borden site it is within the range reported in the literature based on the calibration of models to regional contaminant migration patterns in glaciofluvial or alluvial aquifers the strong longitudinal dispersion is believed to be caused by mixing between the ubiquitous layers of fine and medium grained sand and silty sand that comprise the aquifer an estimate of the average annual rate of recharge that has contributed to the groundwater zone beneath the landfill during the bomb tritium period was obtained by comparing the input of 3 h to an area segment of the landfill with the 3 h contained within a groundwater flow tube leading from the specified area along the centerline of the contaminant plume this approach yielded a minimum average annual recharge rate of 50 with the main cause of uncertainty being the extent of the area in which recharge occurs

spatial and temporal variation in precipitation isotopes in the sydney basin australia summary the 2h and 18o composition of 816 precipitation samples collected between february 2005 and october 2009 from four sites across the sydney basin australia were analysed representing the first published precipitation isotope dataset for the region monthly values weighted averages and local meteoric water lines are presented for use in hydrological applications the 4 year precipitation weighted average values ranged from 2h 20 16 18o 4 50 and d 15 88 for lucas heights in the coastal region to 2h 29 48 18o 5 97 and d 18 33 for mt werong in the highlands west of sydney for the sydney basin as a whole we recommend an overall local meteoric water line of 2h 8 01 0 05 18o 16 8 0 3 precipitation in the sydney basin is isotopically different from the nearest gnip sites with a significantly higher d excess high d excess values may be the result of low humidity at the sea surface during evaporation at mid latitude sources however recycling of moisture either locally or along a moisture transport trajectory may also be an important factor a clear coastal effect was observed but generally temperature and amount effects were found to be weak

applications and limitations of mango s light hydrocarbon parameters in petroleum correlation studies mango 1987 reported on the remarkable invariance of four isoheptanes in petroleums regardless of the absolute concentration the ratio of sums of concentration of 2 methylhexane 2 3 dimethylpentane 3 methylhexane 2 4 dimethylpentane remains constant the k1 ratio value being around unity gas chromatographic analyses of an additional 500 oils contradict this invariance as a suite of oils from the cretaceous northwest basin argentina deviate significantly by having a ratio of 1 67 still oils derived from the same structural kerogen have invariant ratios that can vary from a lowest value of 0 79 found for the midland oil set mango 1990 up to a highest value of 1 67 found for the cretaceous northwest basin mango 1990 postulated so called parent daughter schemes having an invariance in the ratio of isoheptanes and dimethylcyclopentanes an invariance unique for a homologous set of oils but possibly distinctly different from another suite of homologous oils it will be shown here that this difference in invariance can be used in oil oil correlation studies and that this technique is especially powerful for oil condensate correlation condensates in which the concentration of biological markers is quite often below the limit of detection further the concentration of light hydrocarbons needed for this approach can be determined from whole oil gas chromatography without any pre treatment thus providing a rapid and cheap tool for correlation studies

oxygen isotopic composition of ferric oxides from recent soil hydrologic and marine environments low temperature synthesis experiments on ferric oxide water systems have resulted in disparate oxygen isotope fractionation temperature t curves in this study recent ferric oxides mostly goethites of holocene age were collected and analyzed from a variety of modern soil stream and marine environments where formation temperature and the oxygen isotopic composition 18o of the water from which ferric oxides precipitated can be independently measured or estimated this allows comparison of experimental t relationships with data from natural systems selective dissolution methods were refined for the pretreatment of fine grained minerals in order to obtain reliable 18o values for pure and crystalline ferric oxides the difference 18o between the 18o value of goethite and that of local mean meteoric water ranges from 1 5 to 6 3 for soil goethites from new jersey indiana michigan iowa south dakota and taiwan we argue that these variations are largely the result of differences between the 18o of formation water and that of local mean meteoric water induced probably by 18o enrichment of soil waters by evaporation or other processes in soil horizons where ferric oxides are forming a marine goethite sample from scotland and a subaqueous bog iron sample from new jersey which can not be biased by evaporative processes provide crucial natural evidence that the difference in 18o between goethite and formation water is 1 5 at 10 c this result is consistent with our prior laboratory synthesis results bao and koch 1999 but in conflict with other experimental calibrations given the highly variable 18o value of soil or other surface water as well as the potential of initially formed ferric oxides for reequilibration with subsurface burial fluids during maturation to crystalline phases an understanding of formational and diagenetic conditions is absolutely essential when attempting to use the oxygen isotope composition of ferric oxides as a paleoclimatic proxy

absence of altonian glaciation in illinois abstract illinois was free of glacier ice from about 130 000 to 25 000 yr b p deposits of this time interval in northeastern illinois represented by core samples from test hole isgs s 30 are composed of leached nonglacigenic stratified or pedoturbated silty sediments that typically are organic rich and pedogenically modified these deposits overlie the sangamon soil which is identified by its stratigraphic position soil morphology and distinct alteration products in the clay mineral fraction including a variably swelling vermiculite like phase and a randomly interstratified kaolinite 10 a phase the regional extent of these deposits suggests that if altonian ice existed in the lake michigan basin it did not extend westward beyond the cuesta of silurian dolomite that rims the southwestern shore of lake michigan

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

episodes of late holocene aridity recorded by stalagmites from devil s icebox cave central missouri usa two stalagmites from devil s icebox cave central missouri display similar 13 c and 18 o values and trends during the late holocene positive 13 c excursions at 3 5 2 6 ka and 1 2 0 9 ka are interpreted to reflect drier conditions these elevated stalagmite 13 c values could have plausibly been driven by increasing c4 plant abundances over the cave or an increased contribution of bedrock carbon both of which could reflect decreased effective moisture a lack of corresponding oxygen isotopic anomalies during these intervals suggests that neither mean annual temperature nor the seasonality of precipitation changed concomitantly with dryness both of the 13 c excursions identified in our stalagmite record are roughly

what caused the rise of atmospheric o2 abstract oxygenic photosynthesis appears to have evolved well before o 2 levels increased in the atmosphere at around 2 4 ga this has led to numerous suggestions as to what may have kept o 2 suppressed and then eventually allowed it to rise these suggestions include changes in the recycling of carbon and sulfur relative to water or hydrogen a switch from dominantly submarine to dominantly subaerial volcanism gradual oxidation of the continents and a concomitant decrease in reduced metamorphic gases a decline in deposition of banded iron formations a decline in nickel availability and various proposals to increase the efficiency of photosynthesis several of these different mechanisms could have contributed to the rise of o 2 although not all of them are equally effective to be considered successful any proposed mechanism must make predictions that are consistent with the carbon isotope record in marine carbonates which shows relatively little change with time apart from transient but occasionally spectacular excursions the reasons for this constancy are explored here but are not fully resolved in the process of making these comparisons a self consistent redox balance framework is developed which will hopefully prove useful to others who may work on this problem and to astronomers who may one day try to decipher spectral signatures of oxygen on earth like exoplanets

glacial meltwater in lake huron during early postglacial time as inferred from single valve analysis of oxygen isotopes in ostracodes abstract 18o measurements of benthic ostracodes are used to reconstruct the 18o history of lake huron and georgian bay water between 10 600 and 7600 14c yr b p this 18o record was calibrated using a comparison of the 18o values of modern ostracodes and lake huron water where a fractionation of 1 0358 was measured between the oxygen isotope ratios of the most isotopically positive ostracode candona subtriangulata and lake water the most positive shell 18o value was used because it is precipitated in the cold 0 to 2 c water common to both deep and shallow environments the 18o of lake huron water reflects a large glacial meltwater component for much of its history before 7600 14c yr b p times of low lake level correlate with the highest ratio of glacial meltwater to local precipitation in the basin most negative 18o values georgian bay water was more negative in 18o than lake huron water of the same age this reflects a higher proportion of glacial meltwater in georgian bay and its separation from lake huron during times of low lake level

pattern of southern laurentide ice sheet margin position changes during heinrich stadials 2 and 1 abstract with concerns over rising global temperatures ice sheet stability and implications for sea level rise the responses of former ice sheets to past climate change provide useful insights into linkages between the atmosphere cryosphere and oceans one example is the behavior of the laurentide ice sheet lis during heinrich stadials times of cooler sea surface temperature sst in the north atlantic using existing cosmogenic surface exposure ages varve chronologies radiocarbon ages and optically stimulated luminescence ages from the southern lis margin this report tests the hypothesis that the southern side of the lis advanced during times of surface cooling of the north atlantic this test reconstructs the timing of local glacial maxima and the times immediately pro and preceding them from multiple areas and compares them to the timing of heinrich stadial 2 hs2 26 6 23 6 ka and to the timing of heinrich stadial 1 hs1 19 3 15 3 ka during hs2 southern sector lobes new england region huron erie lobe lake michigan lobe green bay lobe and the chippewa lobe advanced towards their maximum position except for the des moines lobe for which there is no applicable data during hs1 these same lobes experienced substantial retreat but the des moines lobe advanced thus the southern laurentide margin behaved differently during the two heinrich stadials this pattern may be attributed to differing east west temperature gradients across north america and associated changes in atmospheric circulation

deglacial hydroclimate of midcontinental north america abstract during the last deglaciation temperatures over midcontinental north america warmed dramatically through the bolling allerod underwent a cool period associated with the younger dryas and then reverted to warmer near modern temperatures during the early holocene however paleo proxy records of the hydroclimate of this period have presented divergent evidence we reconstruct summer relative humidity rh across the last deglacial period using a mechanistic model of cellulose and leaf water 18o and d combined with a pollen based temperature proxy to interpret stable isotopes of sub fossil wood midcontinental rh was similar to modern conditions during the last glacial maximum progressively increased during the bolling allerod peaked during the younger dryas and declined sharply during the early holocene this rh record suggests deglacial summers were cooler and characterized by greater advection of moisture laden air masses from the gulf of mexico and subsequent entrainment over the mid continent by a high pressure system over the laurentide ice sheet these patterns help explain the formation of dark colored cumulic horizons in many great plains paleosol sequences and the development of no analog vegetation types common to the midwest during the last deglacial period likewise reduced early holocene rh and precipitation correspond with a diminished glacial high pressure system during the latter stages of ice sheet collapse

oxygen and hydrogen isotopes of rainfall and dripwater at desoto caverns alabama usa key to understanding past variability of moisture transport from the gulf of mexico abstract the southeast and the us gulf coast in particular are notably lacking isotope data in the water cycle despite the fact that moisture transport from the gulf of mexico gom has a considerable influence on both regional and continental rainfall patterns this study reports time series of oxygen and hydrogen isotopes acquired over a 3 year period 2005 2008 from gom derived rainfall cave dripwater and shallow groundwaters and offers valuable insights on the links between factors controlling regional rainfall and the ubiquitous karst hydrology amount weighted mean monthly rainwater 18o and d values in tuscaloosa alabama range from 1 5 to 8 3 and 1 2 to 49 5 respectively and show mean seasonal amplitudes of 4 18o and 25 d in comparison d excess values display large seasonal amplitudes of 10 20 resulting from differences in the degree of evaporation from falling raindrops between summer and winter months and correlate well with the coeval air temperature r2 0 59 p dripwater samples from nearby desoto caverns show weak 18o and d seasonal variations and record only 20 and 51 of the 18o and 2h enrichments respectively discerned in the rainwater 3 year time trends the seasonal and interannual amplitude attenuations in the dripwaters are attributed to a relatively thick overlying bedrock 30 40 m and a relatively large well mixed epikarst storage reservoir residence time of water in the cave s epikarst is estimated to be 1 3 months based on high resolution flow rate data our investigation suggests that global atmospheric circulation patterns enso and bermuda high likely govern the interannual 18o and d isotope trends discerned in the water cycle compartments but much longer time series are required to confirm our conjectures the results of this study form a solid basis for future acquisition and interpretation of climate proxy records from regional speleothems

lakewater paleothermometry from deep lake minnesota during the deglacial holocene transition from combined δ18o analyses of authigenic carbonate and aquatic cellulose abstract carbonate cellulose and bulk organic isotope data from varved sediments of deep lake minnesota provide a record of paleohydrologic and paleoclimatic change during the deglacial holocene transition from 12 000 to 7500 cal bp deep lake became enriched in 18 o in response to increased influence of maritime tropical moisture from the gulf of mexico between 12 000 and 9500 cal bp culminating in regional expansion of pinus and quercus and then again as a result of increased aridity with the establishment of more zonal circulation and prairie vegetation by 8500 cal bp summer lake water temperatures simultaneously increased during the deglacial holocene transition as suggested by decoupling of carbonate 18 o and cellulose inferred lake water 18 o values between 11 000 and 9500 cal bp and then declined briefly around 8600 cal bp possibly in response to the cooling event associated with the final drainage of glacial lakes agassiz and ojibway into the labrador sea although the carbonate cellulose paleothermometer generated unrealistically high values these can readily be explained by imposing kinetic effects that occur during rapid carbonate precipitation and conservative estimates of a seasonal lag effect in the synthesis of cellulose with respect to carbonate these new results suggest that climatic variations during the deglacial holocene transition in northwestern minnesota were driven mainly by changes in large scale atmospheric circulation and that lake agassiz played a comparatively minor role except during its final drainage

a high resolution speleothem record of climatic variability at the allerod younger dryas transition in missouri central united states stable isotopic analysis and 234 u 230 th mass spectrometry dating of a single calcite stalagmite from onondaga cave east central missouri reveal climatic variability between 13 200 and 12 400 yr bp coincident with the aller d younger dryas transition in the gisp2 greenland ice cores n 18 o values of speleothem calcite mark decreasing temperatures between 13 200 and 13 000 yr bp while more rapidly decreasing n 13 c values record a sharp shift toward c3 vegetation at approximately 13 100 yr bp because of uncertainties in the chronological control of both this speleothem and the gisp2 record and the brevity of this speleothem record v800 years direct comparison of the two sequences is difficult the interval preserved by this speleothem may span the inter aller d cold period iacp alone both the iacp and the start of the younger dryas or only the first half of the younger dryas in any case these data demonstrate that the climate of the north american interior deteriorated significantly and rapidly during the interval associated with the aller d younger dryas transition fl 2001 elsevier science b v all rights reserved

mid wisconsinan stratigraphy and paleoenvironments at the st charles site in south central iowa a sequence of loess mantled sediments exposed along clanton creek provides the first evidence of fluvial pedologic and biotic environments before the last glacial maximum in south central iowa two fining upward fluvial sequences one inset into the other are exposed radiocarbon ages indicate that the alluvial fills are about 34 000 yr old basal gravel in the oldest fill contains well preserved mammoth mammuthus bones sparse seeds from this horizon suggest weedy flood plain conditions the younger alluvial fill contains well preserved pollen plant macrofossils and insects the pollen is dominated by nonarboreal taxa and pinus suggesting a prairie border or savanna environment vascular plant and bryophyte macofossils indicate a variety of aquatic and marsh environments on the flood plain insects are mostly sympatric in mixed conifer and hardwood forest that extends along latitude 47 to 49 between eastern north dakota and new england but forest beetles are rare and the fauna is dominated by openground forms the site is interpreted as an open flood plain dotted with marshes and oxbow lakes it was surrounded by open woodland or savanna similar to that in north eastern north dakota at present july temperatures at the st charles site were probably 3 to 5 c cooler than those at present the pre loess stratigraphy of the site differs markedly from that of upland sites and demonstrates that correlation between upland and lowland sequences cannot be done without adequate dating

hydrogeology and redox geochemistry of ch4 in a late wisconsinan till and loess sequence in central iowa the hydrogeology and redox geochemistry of ch4 in two hydrogeological settings within a late wisconsinan till and loess sequence in central iowa were investigated using ch4 eh dissolved o2 no3 n nh4 n dissolved fe so4 h2s dissolved organic carbon doc particulate organic carbon poc 13c ch4 2h ch4 13c of dissolved inorganic carbon dic and 14c ch4 depleted 13c ch4 and 2h ch4 values enriched 13c dic values and the lack of higher hydrocarbons suggest that the ch4 is microbial and is formed by co2 reduction carbon 14 dates of ch4 indicate that 14 000 to 17 000 year old poc and doc in the 1oess and till compose the substrate for methanogenesis production of ch4 probably began after deglaciation at about 12 000 years b p and has continued to the present groundwater recharge particularly in the upland topographic position at the site has decreased ch4 concentrations in the upper part of the till

delayed deglaciation or extreme arctic conditions 21 16 cal kyr at southeastern laurentide ice sheet margin 1 the conventionally accepted ages of the last glacial maximum lgm retreat of the southeastern laurentide ice sheet lis are 26 21 cal kyr derived from bulk sediment radiocarbon ages and 28 23 cal kyr varve estimates utilizing accelerator mass spectrometry ams 14c dating of earliest macrofossils in 13 lake bog inorganic clays we find that vegetation first appeared on the landscape at 16 15 cal kyr suggesting ice had not retreated until that time the gap between previous age estimates and ours is significant and has large implications for our understanding of ocean atmosphere linkages older ages imply extreme arctic conditions for 9 5 cal kyr a landscape with no ice yet no deposition in lakes our new ams chronology of lis retreat is consistent with marine evidence of deglaciation from the n atlantic showing significant freshwater input and sea level rise only after 19 cal kyr with a cold meltwater lid perhaps delaying ice melt

evidence for dilution of deep confined ground water by vertical recharge of isotopically heavy pleistocene water new analyses of the isotopic composition of water sup 14 c dating of dissolved inorganic carbon and order of magnitude darcy calculations suggest that a dilute body of water trending north south in the cambrian ordovician aquifer of iowa was emplaced as vertical recharge of pleistocene age water from the base of the des moines lobe of late wisconsin time the recharge occurred through more than 300 m of overlaying silurian to mississippian age rocks the delta sup 18 o values range from minus 10 per thousand to minus 9 per thousand for the dilute water body and are consistent with a mixture of des moines lobe meltwater and precipitation found today in the north central us these results suggest that 1 the climate at the end of the last glaciation was mild and 2 a ground water stable isotope signature similar to that of modern precipitation in an aquifers recharge area is not a priori evidence for relatively recent recharge

classification and thermal history of petroleum based on light hydrocarbons abstract classifications of oils and kerogens are described two indices are employed termed the heptane and isoheptane values based on analyses of gasoline range hydrocarbons the indices assess degree of paraffinicity and allow the definition of four types of oil normal mature supermature and biodegraded the values of these indices measured in sediment extracts are a function of maximum attained temperature and of kerogen type aliphatic and aromatic kerogens are definable only the extracts of sediments bearing aliphatic kerogens having a specific thermal history are identical to the normal oils which form the largest group 41 in the sample set this group was evidently generated at subsurface temperatures of the order of 138 149 c 280 300 f defined under specific conditions of burial history it is suggested that all other petroleums are transformation products of normal oils

atmospheric oxygenation and volcanism arising from f gaillard b scaillet n t arndt 229 232 2011 10 1038 nature10460 around 2 5 billion years ago earth s atmosphere turned from anoxic to oxic in what is known as the great oxidation event gaillard et al 1 suggest that this oxygenation was caused by the emergence of the continents and a shift in volcanism from predominantly submarine to primarily subaerial conditions because the ratio of volcanic so2 to h2s in their model increases with this shift they argue that the atmosphere became more oxidized but their model also predicts that outgassing of co2 decreases at the lower pressure of continental volcanism and that this should act against atmospheric oxygenation because co2 is the substrate for the production of oxygen from photosynthesis hence their mechanism may not trigger a rise in atmospheric o2

an attempt to understand the behavior of dissolved organic carbon in coastal aquifers of pondicherry region south india groundwaters of the coastal region are significant and serve as a fragile ecosystem in the sedimentary terrain the pondicherry region is characterized by different geological formations of distinct geological epochs cretaceous tertiary and quaternary ninety three samples were collected from specific aquifers with respect to spatial distribution alluvium 31 upper cuddalore 13 lower cuddalore 13 other tertiary 7 cretaceous 24 and mixed multiple completion aquifer 5 the collected samples were analyzed for major ions ca2 mg2 na k hco3 cl po42 so42 f and doc stables isotopes of 18o d and 13c were also analyzed along with heavy metals like al ba fe sr cd and zn the 18o ranges between 6 85 and 2 15 and d ranges between 43 94 and 19 41 the distribution of these environmental isotopes in groundwater of this layered aquifer sequence was also attempted a comparison of the isotopic data with the rainfall local meteoric water line with a equation of d 7 398 18o 5 067 indian meteoric water line and global meteoric water line was performed the 13c values for groundwater ranges from 5 3 to 18 1 the result indicates that the groundwater in the study area is mainly meteoric in origin and few samples show evidence of evaporation the dissolved organic carbon doc is a very important component in biogeochemical cycling of elements characterized by high susceptibility to leaching the range of the doc in the study area is 0 16 mg l factor analysis was applied to classify the groundwater samples and to identify geochemical processes controlling groundwater geochemistry the alluvium aquifers show a complex hydrogeochemistry than the older aquifers the major factor influencing the hydrogeochemistry of the region is rock water interaction and anthropogenic processes hence metal mobility of the groundwater shows the correlation of metal bonding and doc the relation between the doc and heavy metal concentration in the aquifers was also identified

higher plant biomarkers reflect palaeovegetation changes during jurassic times abstract the relative abundances of three higher plant derived biomarkers retene cadalene and ip ihmn have been measured in marine sedimentary rocks from the northwest margin of australia it is thought that each biomarker represents input from a different plant type the distributions of these three compounds form a fingerprint representing higher plant input hpf variations in hpf in oxfordian sediments were nearly identical in all three locations with retene becoming very abundant relative to the other two compounds with decreasing age of the sediment this finding strongly suggests that the composition of terrestrial input during deposition largely determines hpf and that the possible effects of diagenesis and catagenesis on the distribution of the three biomarkers are relatively unimportant the marked increase in the abundance of retene relative to that of cadalene during the oxfordian is interpreted to reflect an increase in the contribution of plants that produced precursors for retene i e conifers brought about by a significant change in climate this was exemplified by measuring the distributions of retene and cadalene expressed in the higher plant parameter hpp for a suite of sediments from the carnarvon basin western australia covering the complete jurassic period the hpp profile displays three major cycles each covering a period of at least 10 million years this profile not only compared well with published palaeoclimate data but also showed a remarkable similarity with second order cycles in the global sea level curve thus strongly supporting the proposal that variations in hpf and hpp are indications of changes in palaeoclimate the relation with global sea level further suggests that global factors e g the atmospheric carbon dioxide concentration may play a major role in determining the observed variations in the distributions of these higher plant derived biomarkers

minimum limiting deglacial ages for the out of phase saginaw lobe of the laurentide ice sheet using optically stimulated luminescence osl and radiocarbon methods

paleoclimatic investigations using isotopic signatures of the late pleistocene holocene groundwater of the stratified aquifers in kuwait abstract the study attempts to determine paleo recharge sources for two major aquifer units of kuwait namely kuwait group aquifer kga and dammam formation dfm were collected and analyzed for major ions and isotopes the obtained results of 14c ages indicated that the groundwater samples ranged from 31 9 ky to 3 9 ky in dfm and 23 3 ky to 0 8 ky in kga the variations in 18o signatures with respect to age reflected that there were two major sources of rainwater such as enriched southern indian ocean monsoon and the depleted northwesterly mediterranean vapor source the samples found to show variations during the three marine isotopic stages mis 1 3 heinrich events bolling allerod and dryas time periods in both aquifers a shift in the intertropical convergence zone itcz was witnessed in the groundwater of both aquifers from 7 8 ky to 4 9 ky the variation in the holocene climate is mainly due to the influence of tropospheric easterly jet governed by the swing in itcz as a result of holocene volcanism the mean 18o values of dfm samples 3 81 reflect depletion than kga samples 2 31 and the d excess values were 0 95 and 2 05 respectively thus it could be inferred that the dfm was predominantly recharged during late pleistocene period and it was in a relatively cooler climate than recharge environment of kga the study infers that kga was recharged between 26on and 26 8on latitudes and that of the dfm was noted to be recharged south west of kuwait in two different patches between 25 8on and 26 2on and 26 7on 27 4on latitudes during late pleistocene the kga was concluded to be recharged between 25 5on and 27 5on latitudes south of kuwait during holocene

groundwater flow velocity and age in a thick fine grained till unit in southeastern wisconsin abstract piezometer nests were installed at study sites in each of five north south trending end moraines of the late pleistocene oak creek formation in southeastern wisconsin the formation is composed primarily of a fine grained glacial diamicton till and laterally continuous and discontinuous coarse grained lake and meltwater stream sediment it overlies the silurian dolomite aquifer which is a source of drinking water to rural areas the average vertical linear velocity and age of ground water in the oak creek formation were estimated by three methods darcy s law environmental isotopes including 3 h 2 h 18 o and 14 c dissolved inorganic carbon and solute transport modeling of 18 o the f 1 and metro sites in the tinley moraine showed excellent agreement among the three estimates of vertical velocity and showed the lowest velocity values 0 3 0 5 cm year 1 downward which suggests that diffusion controls vertical mass transport at these sites although the extrapolated maximum age of ground water is 35 000 years ground water below 75 m at these sites is probably not older than 15 000 years which is the maximum age of the formation estimates of velocity showed less agreement at study sites in the lake border moraine system to the east and ranged from about 0 2 to 20 7 cm year 1 maximum groundwater age could range from 213 to 6000 years higher and more variable velocities perhaps owing to thinner and more heterogeneous sediment in these areas suggest that diffusion may not dominate vertical mass transport heterogeneity and fractures may also promote the development of groundwater flow systems dominated by lateral flow because of the uncertainty about the nature of groundwater flow velocity and age in the formation east of the tinley moraine future waste disposal activity in southeastern wisconsin should be confined to the thickest parts of the tinley moraine near the present f 1 and metro sites

rapid least squares inversion of apparent resistivity pseudosections by a quasi newton method1 a fast inversion technique for the interpretation of data from resistivity tomography surveys has been developed for operation on a microcomputer this technique is based on the smoothness constrained least squares method and it produces a two dimensional subsurface model from the apparent resistivity pseudosection in the first iteration a homogeneous earth model is used as the starting model for which the apparent resistivity partial derivative values can be calculated analytically for subsequent iterations a quasi newton method is used to estimate the partial derivatives which reduces the computer time and memory space required by about eight and twelve times respectively compared to the conventional least squares method tests with a variety of computer models and data from field surveys show that this technique is insensitive to random noise and converges rapidly this technique takes about one minute to invert a single data set on an 80486dx microcomputer

gaillard et al reply replying to j f kasting d c catling k zahnle 10 1038 nature11274 2012 kasting 1 question the model of ref 2 in which we suggest that the oxygenation of the atmosphere around 2 45 gyr ago was promoted by the emergence of subaerial volcanism producing volcanic gases with much more elevated so2 h2s ratios than submarine volcanism

isotopic composition of precipitation in central iowa abstract the isotopes of hydrogen and oxygen have been used extensively as indicators of groundwater age 3h and paleotemperature of groundwater recharge 18o 16o 2h 1h however comparisons of values in groundwater with those in recent precipitation are few this study describes and interprets the isotopic composition of precipitation in central iowa for the year 1992 an unusually mild winter driven by south westerly flow of the jet stream was followed by an unusually cool and predominantly dry summer that was caused by a persistent northwesterly flow in the upper atmosphere temperature was below normal 9 0 c precipitation was above normal 868 4 mm and was mostly derived from a gulf of mexico moisture source fifty one precipitation samples were collected from 37 events and analyzed for hydrogen and oxygen isotopes tritium activities ranged from 0 to 44 tritium units tu and the weighted mean 3h activity was 11 02 tu a value consistent with international atomic energy agency iaea data anomalously high 3h activities suggested input of technogenic 3h weighted mean values of 18o and 2h were 8 02 and 53 62 respectively and the equation of the local meteoric water line was 2h 9 26 18o 4 65 weighted mean 18o and 2h values from the gulf pacific moisture source 10 80 74 82 were more depleted than those ascribed solely to a gulf of mexico moisture source 7 24 47 39 seasonal differences in temperature primarily controlled these differences in isotopic composition

increased subaerial volcanism and the rise of atmospheric oxygen 2 5 billion years ago the hypothesis that the establishment of a permanently oxygenated atmosphere at the archaean proterozoic transition approximately 2 5 billion years ago occurred when oxygen producing cyanobacteria evolved is contradicted by biomarker evidence for their presence in rocks 200 million years older to sustain vanishingly low oxygen levels despite near modern rates of oxygen production from approximately 2 7 2 5 billion years ago thus requires that oxygen sinks must have been much larger than they are now here we propose that the rise of atmospheric oxygen occurred because the predominant sink for oxygen in the archaean era enhanced submarine volcanism was abruptly and permanently diminished during the archaean proterozoic transition observations are consistent with the corollary that subaerial volcanism only became widespread after a major tectonic episode of continental stabilization at the beginning of the proterozoic submarine volcanoes are more reducing than subaerial volcanoes so a shift from predominantly submarine to a mix of subaerial and submarine volcanism more similar to that observed today would have reduced the overall sink for oxygen and led to the rise of atmospheric oxygen

abrupt changes in north american climate during early holocene times recent studies of the greenland ice cores have offered many insights into holocene climatic dynamics at decadal to century timescales1 2 3 despite the abundance of continental records of holocene climate few have sufficient chronological control and sampling resolution to compare with the greenland findings4 butannually laminated sediments varves from lakes can provide high resolution continental palaeoclimate data with secure chronologies here we present analyses of varved sediments from deep lake in minnesota usa trends in the stable oxygen isotope composition of the sedimentary carbonate indicate a pronounced climate cooling from 8 9 to 8 3 kyr before present probably characterized by increased outbreaks of polar air decreased precipitation temperatures and a higher fraction of the annual precipitation falling as snow the abrupt onset of this climate reversal over several decades was probably caused by a reorganization of atmospheric circulation and cooling of the arctic airmass in summer that resulted from the final collapse of the laurentide ice near hudson bay and the discharge of icebergs from the quebec and keewatin centres into the tyrell sea the timing and duration of this climate reversal suggest that it is distinct from the prominent widespread cold snap that occurred 8 200 years ago in greenland and other regions1 5 6 no shifts in the oxygen isotope composition of sediment carbonate occurred at 8 2 kyr before present at deep lake but varve thickness increased dramatically probably as a result of increased deposition of aeolian dust taken together our data suggest that two separate regional scale climate reversals occurred between 9 000 and 8 000 years ago and that they were driven by different mechanisms

biomarker and n alkane isotope profiles for tertiary oils relationship to source rock depositional setting abstract biomarker and n alkane isotope profiles have been measured for 29 late cretaceous tertiary oils from se asia china papua new guinea new zealand and the u s a the results are interpreted with respect to six kinds of source rock depositional setting fluvio deltaic fd freshwater transitional tr lacustrine lac saline lacustrine sal marine deltaic md and marine carbonate mc a comparison of biomarker and n alkane isotope results suggests that parameters such as the oleanane hopane ratio may overestimate the higher plant contribution to marine oils the abundance of bicadinanes is much more variable than that of oleanane probably because of a specific association with the highly resinous dipterocarp hardwoods of se asia however traces do occur in an oil from outside the paleogeographic range of the dipterocarp family confirming that there is also a more general angiosperm source the bicadinane isomeric distribution is shown to depend on maturity leading to new maturity parameters which are resistant to even very heavy biodegradation retention indices based on the regular hopane series are provided for the main bicadinane and methyl bicadinane isomers and for a group of oleanane related triterpanes of unknown structure the presence of the latter compounds may indicate locally deposited as opposed to transported higher plant matter in the source source rock depositional setting is the primary control on the shape of the n alkane isotope profile with negatively sloping curves being characteristic of fd and tr oils and flat or positively sloping curves typical of marine oils the difference is probably related to the bacterial reworking of higher plant matter in the fd environment a slight isotopic anomaly at n c 17 correlates with the abundance of algal derived steranes and may indicate a minor marine contribution to the source of an fd oil

the dekalb mounds of northeastern illinois as archives of deglacial history and postglacial environments article i nfo article history received 28 august 2009 available online 12 june 2010 the type dekalb mounds of northeastern illinois usa 42 0 n 88 7 w are formed of basal sand and gravel overlain by rhythmically bedded fines and weathered sand and gravel generally from 2 to 7 m thick the fines include abundant fossils of ostracodes and uncommon leaves and stems of tundra plants rare

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

paleobotanical and environmental implications of a buried forest bed in northern lower michigan usa sediments and stratigraphy at the tower buried forest site provide a glimpse into the immediate post glacial environment of northern lower michigan usa at this site sandy glacial outwash is overlain by 1 37 cm of peat associated with a wetland above which are 2 flat lying spruce and larch logs and branches that date between 10 910 and 10 340 cal years bp above the woody materials are 55 75 cm of sand interbedded with organic muck laminae which we interpret as local alluvium this stratigraphic sequence is overlain by muck soil materials associated with the modern wetland pollen and plant macrofossil analysis of the lower peat indicate that a tundra boreal parkland had been established here shortly after final deglaciation later picea glauca white spruce picea mariana black spruce larix laricina larch and abies balsamea balsam fir became abundant in an open boreal forest spaghnum peatland subsequent increases in pinus pine and typha cattail indicate drying and possibly warming conditions at ca 10 340 cal years bp or a few centuries prior nearly two millennia after final deglaciation water flowed across the site possibly knocking over the trees interbedded sand and muck deposits above the flat lying logs are interpreted as alluvial deposits from this event after which the wetland became quiescent again

composition and origin of fuel from the hut of explorer robert falcon scott cape evans antarctica geochemical analyses of the fuel used for the motor driven sledges used by the explorer robert falcon scott for his 1911 1912 quest to the south pole indicate that it is a straight run gasoline the presence of bicadinanes oleanane and other oleanoid angiosperm markers indicate that the feedstock oil was likely to have been sourced from terrestrial source rocks of tertiary age in the south east asian region the overall chemical composition of the fuel in its present state indicates that it may have been too heavy for usage in polar regions

a late pleistocene holocene noble gas paleotemperature record in southern michigan 1 noble gas temperatures ngts and 14c derived ages in groundwaters of the michigan basin reveal a ground temperature of 1 c toward the end of the last glacial maximum lgm suggesting that groundwater recharge occurred under the laurentide ice sheet lis cover in addition to the general warming observed since the lgm the ngt record indicates an abrupt warming event between 12 8 and 11 1kyrs bp correlative to the bolling allerod boa warm phases ice sheet linked changes in freshwater delivery to the north atlantic together with changes in the north atlantic deep water nadw circulation are possible causes of such abrupt climate shifts in northeastern us pleistocene waters yielding the lowest ngts have the highest 18o and d values suggesting an atmospheric circulation pattern distinct from today with a stronger moisture component from the gulf of mexico possibly due to the presence of the lis which weakened the pacific westerly flow

the hydrogen and oxygen isotopic composition of precipitation evaporated mine water and river water in montana usa summary the isotopic composition of 42 samples of rain and snow collected in 2004 were used to construct a local meteoric water line lmwl for butte montana the derived equation d 7 31 18 o 7 5 r 2 0 987 represents one of the first published lmwls based on direct precipitation for any location in the northern rocky mountains samples of underground and surface mine waters in butte including the berkeley pit lake and a nearby tailings pond define a linear trend with a much lower slope and intercept than the lmwl d 5 00 18 o 49 5 r 2 0 991 consistent with non equilibrium evaporation at an average relative humidity of roughly 65 detailed evaporation calculations are presented which indicate that the shallow berkeley pit lake was approximately 25 evaporated in october 2003 whereas the surface of the tailings pond was at least 50 evaporated the intersection of the lmwl and mine water evaporation trend was used to calculate the average composition of recharge water to the flooded mine complex d 139 18 o 18 0 these values are considerably lighter than the weighted total of precipitation for the 2004 calendar year d 118 18 o 15 3 which is partly explained by the unusually low snowfall that montana experienced in 2004 based on this study the lmwl recently proposed by kendall and coplen 2001 kendall c coplen t b 2001 distribution of oxygen 18 and deuterium in river waters across the united states hydrological processes 15 1363 1393 from regression of isotopic data from a number of montana rivers is more accurately interpreted as an evaporation line isotopic trends based on river data should be treated with caution particularly in a semi arid region such as montana where rivers are often influenced by dams and irrigation withdrawals

atmospheric changes in north america during the last deglaciation from dune wetland records in the midwestern united states reconstructing moisture conditions during the early retreat of the laurentide ice sheet lis may help define the phase relationship of deglacial conditions and atmospheric changes over north america here we report two sediment successions near the concurrent ice margins in the midwest of the u s a to show that dune formation was active from 17 7 to 14 7 and from 12 8 to 11 8 thousand years ago ka and lowland soils formed at 14 7 and wetland sediments developed from 13 5 to 12 8 ka our results suggest that moisture conditions in the midwestern u s a were inversely phased with those in the southwestern u s a during the heinrich event 1 h1 and younger dryas yd stadials the northern area became drier and southern area became wetter whereas during the bolling allerod b a interstadials the northern area became wetter and southern area became drier this pattern is also observed on phase change levels within the h1 interval in detail from 17 7 to 16 5 ka during the h1b phase major aridity in the midwest corresponded to wet conditions in the southwest u s a from 16 5 to 15 0 ka during the h1a phase climate reversed from dry to wet in the midwest whereas conditions went from wet to dry in the southwest from 15 0 to 14 7 ka during the transition to the bolling allerod t b a phase a return of aridity in the midwest corresponded to a return of humidity in the southwest

chronological framework for the deglaciation of the lake michigan lobe of the laurentide ice sheet from ice walled lake deposits a revised chronological framework for the deglaciation of the lake michigan lobe of the south central laurentide ice sheet is presented based on radiocarbon ages of plant macrofossils archived in the sediments of low relief ice walled lakes we analyze the precision and accuracy of 15 ams 14 c ages of plant macrofossils obtained from a single ice walled lake deposit the semi circular basin is about 0 72 km wide and formed of a 4 to 16 m thick succession of loess and lacustrine sediment inset into till the assayed material was leaves buds and stems of salix herbacea snowbed willow the pooled mean of three ages from the basal lag facies was 18 270 50 14 c a bp 21 810 cal a bp an age that approximates the switch from active ice to stagnating conditions the pooled mean of four ages for the youngest fossil bearing horizon was 17 770 40 14 c a bp 21 180 cal a bp material yielding the oldest and youngest ages may be obtained from sediment cores located at any place within the landform based on the estimated settling times of overlying barren rhythmically bedded sand and silt the lacustrine environment persisted for about 50 more years at a 67 confidence level the dated part of the ice walled lake succession persisted for between 210 and 860 cal a modal value 610 cal a the deglacial age of five moraines or morainal complexes formed by the fluctuating margin of the lake michigan lobe have been assessed using this method there is no overlap of time intervals documenting when ice walled lakes persisted on these landforms the rapid readvances of the lobe during deglaciation after the last glacial maximum probably occurred at some point between the periods of ice walled lake sedimentation copyright 2011 john wiley sons ltd

hydrothermal fe fluxes during the precambrian effect of low oceanic sulfate concentrations and low hydrostatic pressure on the composition of black smokers abstract modern mid ocean ridge hydrothermal systems typically release vent fluids to the ocean with dissolved h2s in excess of fe these fluids are the consequence of high temperature interactions between sulfate rich seawater and mid ocean ridge basalt at conditions near the critical point for seawater the precipitation of fes and fes2 during venting titrates most of the fe from the fluid significantly reducing the net flux of fe to the open ocean here we suggest that hydrothermal fluids emanating from precambrian seafloor systems older than 1 8 ga had fe h2s ratios 1 and with fh2 higher than today because seawater lacked its primary oxidant dissolved sulfate ion this predominance of fe over h2s would have promoted the establishment of an iron rich deep ocean and the deposition of banded iron formations bif accordingly the end of bif deposition at 1 8 ga was the result of the buildup of sulfate in seawater from oxidative weathering and its return at 750 ma the result of reductions in seawater sulfate concentrations during snowball earth episodes enhanced by elevated fe concentrations during depressurization of hydrothermal systems by large eustatic sea level falls moreover precambrian chemosynthetic vent communities may have been based on h2 synthesis rather than on h2s oxidation as they largely are today

episodic deposition of illinois valley peoria silt in association with lake michigan lobe fluctuations during the last glacial maximum the chronology and cause of millennial depositional oscillations within last glacial loess of the central lowlands of the united states are uncertain here we present a new age model that indicates the peoria silt along the illinois river valley accumulated episodically from 28 500 to 16 000 cal yr bp as the lake michigan lobe margin fluctuated within northeastern illinois the age model indicates accelerated loess deposition coincident with regional glacial advances during the local last glacial maximum a weakly developed paleosol the jules geosol represents a period of significantly slower deposition from 23 700 to 22 000 cal yr bp a gastropod assemblage based reconstruction of mean july temperature shows temperatures 6 10 c cooler than modern during peoria silt deposition stable oxygen and carbon isotope values 18 o and 13 c of gastropod carbonate do not vary significantly across the pedostratigraphic boundary of the jules geosol suggesting slower loess accumulation was a result of reduced glacial sediment supply rather than direct climatic factors however a decrease in 18 o values occurred between 26 000 and 24 000 cal yr bp synchronous with the lake michigan lobe s southernmost advance this 18 o decrease suggests a coupling of regional summer hydroclimate and ice lobe position during the late glacial period

morphometric analysis of ice walled lake plains in northern illinois implications of lake elongation by wind induced dual cycle currents abstract ice walled lake plains iwlps are rounded flat topped mounds that formed in stagnant ice environments along the margins of the laurentide ice sheet we conducted detailed morphometric and statistical analyses of the shape size and orientation of more than 400 iwlps identified from aerial photos aided with lidar data in dekalb county illinois usa lake elongation theories include extraterrestrial impact e g the carolina bays ice flow dynamics and crevasses and wind induced currents that preferentially erode the shorelines perpendicular to the dominant wind direction the results indicate that elliptical iwlps with a perimeter greater than 3050 m have preferred orientations roughly normal to the paleo wind direction as indicated by contemporaneous parabolic dunes located 50 km to the west the orientations of the iwlps with a perimeter less than 1220 m are scattered and show no apparent trend the iwlp orientation is not related to ice flow dynamics or glacial crevasses because no statistically significant relationship exists with regard to the ice flow as proxied by the moraine direction the orientation of large iwlps in dekalb county are consistent with wind induced lake elongation observed in modern permafrost thaw lakes suggesting that the prevailing wind also played an important role in controlling the orientation of iwlps during the last glacial period and led to the preferred orientation we see today

controlled moraines origins characteristics and palaeoglaciological implications abstract controlled moraines are supraglacial debris concentrations that become hummocky moraine upon de icing and possess clear linearity due to the inheritance of the former pattern of debris rich folia in the parent ice linearity is most striking wherever glacier ice cores still exist but it increasingly deteriorates with progressive melt out as a result moraine linearity has a low preservation potential in deglaciated terrains but hummocky moraine tracts previously interpreted as evidence of areal stagnation may instead record receding polythermal glacier margins in which debris rich ice was concentrated in frozen toe zones recent applications of modern glaciological analogues to palaeoglaciological reconstructions have implied that a controlled moraine development can be ascribed to a specific process e g englacial thrusting or supercooling and b controlled moraine preservation potential is good enough to imply the occurrence of the specific process in former glacier snouts e g ancient polythermal or supercooled snouts these assumptions are tested using case studies of controlled moraine construction in which a wide range of debris entrainment and debris rich ice thickening mechanisms are seen to produce the same geomorphic features polythermal conditions are crucial to the concentration of supraglacial debris and controlled moraines in glacier snouts via processes that are most effective at the glacier permafrost interface end moraines lie on a process form continuum constrained by basal thermal regime the morphological expression of englacial structures in controlled moraine ridges is most striking while the moraines retain ice cores but the final deposits landforms tend to consist of discontinuous transverse ridges with intervening hummocks preserving only a weak impression of the former englacial structure these are arranged in arcuate zones of hummocky moraine up to 2 km wide containing ice walled lake plains and lying down flow of streamlined landforms produced by warm based ice a variety of debris entrainment mechanisms can produce the same geomorphic signature spatial and temporal variability in process form relationships will lead to the sequential development of different types of end moraines during the recession of a glacier or ice sheet margin

groundwater flow and velocity in a 500 ka pre illinoian till eastern iowa few hydrology studies have investigated glacial till older than illinoian time 300 000 bp despite these older tills overlying a large portion of north america an 8 and 6 well monitoring well nest installed into a 31 m thick pre illinoian till sequence near cedar rapids iowa was characterized using traditional hydrologic methods and chemical tracers the aquitard system consists of about 9 m of fine grained oxidized pre illinoian till overlying 22 m of unoxidized till and devonian dolomite bedrock hydraulic conductivity ranged from 10 7 m s in oxidized till and 10 10 m s in unoxidized till hydraulic head relations indicated downward groundwater flow through the till profile with hydraulic gradients steepest near the unoxidized till bedrock interface tritium and nitrate concentrations indicated recent 50 years old recharge to a depth of 9 12 m below land surface 18o and 2h results ranged between 6 2 to 7 9 and 38 0 to 50 9 respectively and plotted near the local meteoric water line a 1 per mil shift toward less negative 18o values with depth may suggest a climate change signal contained in the till water but more data are needed to verify this trend vertical groundwater velocity through the unoxidized till was estimated to range from 0 4 to 5 7 cm year the thickness of unoxidized pre illinoian till in linn county was estimated from available records and contoured against vertical travel times to evaluate the effectiveness of pre illinoian till in preventing nitrate migration to underlying bedrock aquifers

geotechnical model for great plains hummocky moraine formed by till deformation below stagnant ice abstract hummocky or disintegration moraine is a widespread late wisconsin glacial terrain type across the glaciated great plains of mid continent north america this terrain consists of chaotic hummocks and enclosed kettle hollows that are widely underlain by clay rich till the prevailing interpretation of this terrain is one of supraglacial deposition from debris covered stagnant ice and is based largely on geomorphologic comparisons with modern valley glacier margins recently in contrast it has been argued that hummocky moraine can form subglacially by the pressing of fine grained deformable till under the overburden pressure of stagnant ice in this paper we test the postulated subglacial origin of hummocky moraine using a geotechnical model of a deforming soil till the model confirms that hummocky topography can be the result of failure and deformation of fine grained till by differential loading below stagnant ice differential pressures at the ice bed contact arise from the complete or partial perforation of thin stagnant ice sheet lobes by numerous water filled doline like depressions the growth and decay of ponds as ice down wastes result in repeated cycles of subglacial till failure and landform development resulting in the observed random linear circular elongate and other patterns of hummocks ridges and hollows formerly streamlined preferentially oriented subglacial landforms such as drumlins and rogen moraines can also be overprinted forming washboard moraine

depositional history of the north taymyr ice marginal zone siberia a landsystem approach the sediment landform associations of the northern taymyr peninsula in arctic siberia tell a tale of ice sheets advancing from the kara sea shelf and inundating the peninsula probably three times during the weichselian in each case the ice sheet had a margin frozen to its bed and an interior moving over a deforming bed the north taymyr ice marginal zone ntz comprises ice marginal and supraglacial landsystems dominated by thrust block moraines 2 3 km wide and large scale deformation of sediments and ice large areas are still underlain by remnant glacier ice and a supraglacial landscape with numerous ice walled lakes and kames is forming even today the proglacial landsystem is characterised by subaqueous e g deltas or terrestrial e g sandar environments depending on location altitude and time of formation dating results osl c 14 indicate that the ntz was initiated ca 80 kyr bp during the retreat of the early weichselian ice sheet and that it records the maximum limit of a middle weichselian glaciation ca 65 kyr bp during both these events proglacial lakes were dammed by the ice sheets part of the ntz was occupied by a thin late weichselian ice sheet 20 12 kyr bp resulting in subaerial proglacial drainage copyright c 2002 john wiley sons ltd

the glacial origins of relict pingos wales uk ramparted depressions doughnut shaped debris cored ridges with peat and or sediment filled central basins are commonly perceived to represent the relict collapsed forms of permafrost ground ice mounds i e pingos or lithalsas in wales uk ramparted depressions of late pleistocene age have been widely attributed to permafrost related processes however a variety of alternative glacial origins for these enigmatic landforms are also consistent with the available geological and geomorphological evidence although previous studies have barely considered such alternative processes of formation from detailed geophysical sedimentological and remote sensing studies at two field sites we demonstrate that i the wastage of stagnating glacier ice is a viable alternative explanation for the formation of ramparted depressions in wales ii the glacial geomorphology and geology of these landforms is analogous to supraglacial and subglacial landforms from the last laurentide and fennoscandian ice sheets iii these landforms have significant potential for characterising the nature of deglaciation around the margins of the irish sea during the last glacial cycle and may record evidence for the overextension and stagnation of the south eastern margin of the irish sea ice stream and iv investigations of ramparted depressions within formerly glaciated terrains must consider both glacial and periglacial mechanisms of formation

holocene environmental and climatic change in the northern great plains as recorded in the geochemistry of sediments in pickerel lake south dakota the sediments in pickerel lake northeastern south dakota provide a continuous record of climatic and environmental change for the last 12000 yr sediments deposited between 12 and 6 ka radiocarbon show extreme variations in composition oxygen and carbon isotopic composition of bulk carbonate carbon isotopic composition of organic matter and magnetic susceptibility these variations re ect changes in sources of moisture regional vegetation types precipitation evaporation balance ground and surface water in ux water residence time erosion lake productivity water level and water temperature the total carbonate content of late pleistocene sediments steadily increased from 20 at the base of the core to as much as 80 in sediments deposited between 11 and 9 ka by about 8 ka the total carbonate content of the sediments had declined to about 40 where it remained with little variation for the past 8 kyr suggesting relatively stable conditions there are marked increases in values of d13c and d18 oi n bulk carbonate and d13c of organic matter in sediments deposited between 10 and 6 ka as evaporation increased and the vegetation in the watershed changed from forest to prairie this shift toward more 18o enriched carbonate may also re ect a change in source or seasonality of precipitation during this early holocene interval the organic carbon oc content of the sediments remained relatively low 2 3 but then increased rapidly to 4 5 between 7 and 6 ka re ecting the rapid transition to a prairie lake the oc content uctuates slightly between 4 and 6 in sediments deposited over the past 6 kyr like oc and total carbonate most variables measured show little variation in the 13 m of sediment deposited over the past 6 kyr particularly when compared with early holocene variations although the magnetic susceptibility of this upper 13 m of sediment is generally low 10 si units the upper six meters of the section is marked by striking 1 m cycles ca 400 500 yr periodicity in susceptibility these cycles are interpreted as being due to variations in the in ux of eolian detrital clastic material century scale cyclic variations in di erent proxy variables for aridity and eolian activity from sediments deposited over the past 2000 yr in other lakes in the northern great plains as well as in sand dune activity suggest that aridity cycles were the dominant feature of late holocene climate of the northern great plains 2000 elsevier

stable isotopic composition of precipitation in the semi arid north central portion of the us great plains abstract where data are available hydrologic studies may use stable oxygen and hydrogen isotopes to investigate groundwater surface water interaction groundwater recharge and advective diffusive transport to estimate groundwater ages or to unravel paleohydrology such studies require that the isotopic composition of precipitation be known as precipitation is a major input to groundwater and surface water systems oxygen 18 18 o and deuterium 2 h data for precipitation are lacking for the semi arid portion of the north central great plains of the us and thus there is need to establish additional meteoric water lines as isotope input functions across the region as well as to develop better understanding of the isotopic climate linkages that control oxygen and hydrogen isotope ratios in precipitation this study examines the 18 o and 2 h composition of precipitation for a representative site near north platte nebraska in the semi arid north central great plains during the years 1989 through 1994 oxygen 18 values range from 30 5 to 1 7 deuterium values range from 228 to 11 yearly arithmetic mean values for the north platte station are 9 8 and 71 respectively weighted yearly means for 18 o and 2 h over the 6 year period were 9 6 and 69 respectively north platte values show a strong isotopic enrichment between winter and summer precipitation and a strong 18 o t correlation r 2 0 845 for mean monthly values of 0 54 per degree celsius the local meteoric water line for the site is 2 h 7 66 18 o 4 96

a four component mixing model for water in a karst terrain in south central indiana usa using solute concentration and stable isotopes as tracers the study area lies in a highly karstified carbonate terrain in south central indiana sinkholes conduits and caves form large secondary pathways for the subsurface flow as a result the discharge from a main emergence point for the subsurface flow system the orangeville rise quickly responds to the storm events and shows wide variations in flow rate water chemistry and stable isotopic compositions these responses are attributed to the mixing of water in secondary pathways in the study area recharge occurs through the thick mantled karst plain and the sinkhole plains and the role of soil layer and epikarst in the recharge process is of great importance rain dic 2 hco3 mg l 13cdic 7 soil water dic 544 hco3 mg l 13cdic 14 7 epikarstic water dic 224 hco3 mg l 13cdic 13 6 and phreatic diffuse flow water dic 299 hco3 mg l 13cdic 11 8 generally showed unique and constant dissolved inorganic carbon dic and 13cdic values over time using dic and 13cdic as tracers a four component mixing model was established for the karstic flow system by constructing the discharge hydrograph separation curves the mixing ratio of each component rain 10 6 soil 3 1 epikarstic 52 3 and phreatic 34 0 water was determined for the orangeville rise discharge over the testing period of 104 h after the storm event of 10 4 90 vadose water occupied 55 4 of spring discharge and this demonstrates the importance of the unsaturated zone especially the epikarst in the karstic flow systems

trimethylnaphthalenes in crude oils and sediments effects of source and maturity abstract three sedimentary sequences and a number of crude oils have been analysed for trimethylnaphthalenes tmns using gas chromatography the sediments were found to contain enhanced relative abundances of 1 2 5 tmn and 1 2 7 tmn in samples of cretaceous age and younger this was especially pronounced in lower maturity sediments containing type iii organic matter older samples containing type iii organic matter were found to contain enhanced relative abundances of 1 2 5 tmn and lower relative abundances of 1 2 7 tmn 1 2 7 tmn appears to be derived directly from the structural degradation of oleanane type triterpenoids present in angiosperms and as such is proposed as a marker for this class of plant and indirectly therefore also for samples deposited since the early cretaceous the crude oils have been classified according to their trimethylnaphthalene compositions this classification enables crudes of higher plant origin to be identified and to be further classified according to whether the organic matter pre dated or post dated the evolution of angiosperms

hydrogen and oxygen isotopic records in monthly scales variations of hydrological characteristics in the different landscape zones of alpine cold regions monthly scale variations in stable isotopic composition in precipitation and various water bodies were analyzed in this study using data collected from a field station experimental observation was performed to analyze processes in the glacier snow zone alpine cold desert zone marsh meadow zone alpine shrub zone and mountain grassland zone based on the isotopic and hydrochemical analyses of glacier snow frozen soil groundwater and other water sources in the headwater catchment of the alpine cold region delta o 18 and delta d values of precipitation and various water bodies in mafengou river basin exhibited strong seasonal variations precipitation and various water bodies were rich in delta o 18 and delta d during the wet season and relatively negative in delta o 18 and delta d during the dry season the isotopic content of various water bodies in mafengou river basin was dependent on temperature elevation of the recharge area and amount of precipitation lower temperatures were typically associated with lower delta o 18 and delta d values and are believed to be mainly responsible for the seasonal patterns observed in the isotopic values of various water bodies in mafengou river basin in the alpine cold region seasonal variations were larger in streams where recent precipitation is the main source of the flow and smaller in streams where groundwater is the dominant source of water c 2013 published by elsevier b v

northern hemisphere ice sheet responses to past climate warming during periods of glaciation the northern hemisphere was swathed by large ice sheets a review of ice sheet retreat during the last two deglaciations shows that land based ice sheets responded rapidly to rising summer insolation whereas marine based ice sheets underwent a delayed but more abrupt response

atmospheric sulfur rearrangement 2 7 billion years ago evidence for oxygenic photosynthesis mass independently fractionated sulfur isotopes mif s provide strong evidence for an anoxic atmosphere during the archean moreover the temporal evolution of mif s shows increasing magnitudes between 2 7 and 2 5 ga until the start of the great oxidation event g o e at around 2 4 ga the conclusion of a completely anoxic atmosphere up to the g o e is in contrast to recent studies on redox sensitive elements which suggest slightly oxidizing conditions during continental weathering already several hundred million years prior to the g o e in order to investigate this apparent inconsistency we present multiple sulfur isotopes for 2 71 ga pyritic black shales derived from the kidd creek area ontario canada these samples display high positive 33s values up to 3 8 and the typical late archean slope in 36s 33s of 0 9 in contrast the time period before 3 2 2 73 ga is characterized by greatly attenuated mif s magnitudes and a slope in 36s 33s of 1 5 we attribute the increase in 33s magnitude as well as the contemporaneous change in the slope of 36s 33s to changes in the relative reaction rate of different mif s source reactions and changes in atmospheric sulfur exit channels both of these are dependent on atmospheric ch4 co2 and o2 mixing ratios we propose a distinct change in atmospheric composition at 2 7 ga resulting from increased fluxes of oxygen and methane as the best explanation for the observed neoarchean mif s record our data and modeling results suggest that oxygenic photosynthesis was a major contributor to primary productivity 2 7 billion years ago

application of the tritium interface method for determining recharge rates to unconfined drift aquifers ii non homogeneous case the depth of penetration of bomb tritium into the saturated zone was used to calculate the rate of groundwater recharge to an unconfined homogeneous drift aquifer in cape cod massachusetts u s a the aquifer is of glaciofluvial origin and is composed of well sorted sand and gravel tritium concentration in groundwater ranged from 0 to 85 t u bomb tritium 5 t u was found in 45 groundwater samples collected from 1 5 to 27 1 m below the water table prebomb tritium concentrations 5 t u were identified in 4 samples collected from 25 6 to 41 1 m below the water table assuming piston type flow beneath groundwater divides the rate of recharge to the aquifer was found to range from 33 7 to 40 6 cm yr 1 this range agrees favorably with recharge estimates previously determined from water budget analyses

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

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

ice walled lake plains implications for the origin of hummocky glacial topography in middle north america abstract ice walled lake plains are prominent in many areas of hummocky till topography left behind as the laurentide ice sheet melted from middle north america the formation of the hummocky till topography has been explained by 1 erosion by subglacial floods 2 squeezing of subglacial till up into holes in stagnant glacial ice or 3 slumping of supraglacial till the geomorphology and stratigraphy of ice walled lake plains provide evidence that neither the lake plains nor the adjacent hummocks are of subglacial origin these flat lake plains up to a few kilometers in diameter are perched as much as a few tens of meters above surrounding depressions they typically are underlain by laminated fine grained suspended load lake sediment many ice walled lake plains are surrounded by a low rim ridge of coarser grained shore sediment or by a steeper rim ridge of debris that slumped off the surrounding ice slopes the ice walled lakes persisted for hundreds to thousands of years following glacial stagnation shells of aquatic molluscs from several deposits of ice walled lake sediment in south central north dakota have been dated from about 13 500 to 10 500 b p calibrated radiocarbon ages indicating a climate only slightly cooler than present this is confirmed by recent palaeoecological studies in nearby non glacial sites to survive so long the stagnant glacial ice had to be well insulated by a thick cover of supraglacial sediment and the associated till hummocks must be composed primarily of collapsed supraglacial till

the kankakee torrent and other large meltwater flooding events during the last deglaciation illinois usa abstract evidence of the kankakee torrent ekblaw and athy 1925 includes boulder bars formed on a scoured bedrock surface west of kankakee illinois and overflow channels that connect several moraine dammed basins wauponsee watseka and pontiac willman and payne 1942 geomorphic evidence of a large scale flood event in the illinois valley includes features such as erosional residuals hajic 1990 the age of the kankakee torrent is about 19 000 cal yr bp based on the pooled mean of four radiocarbon ages of tundra plant stems and leaves from the oswego channel complex median probability 18 930 cal yr bp 1 range 18 870 18 970 cal yr bp analysis of recently obtained sediment cores from the middle illinois river valley near havana illinois has revealed the bedrock surface is defended by a mantle of bouldery debris buried by 15 m of mostly slackwater lake sediment radiocarbon ages of needles archived in the lake sediment reveal evidence for an early lake phase that post dates the kankakee torrent 18 030 17 530 cal yr bp and a later lake phase 15 690 13 040 cal yr bp the radiocarbon ages indicate that the deeply buried bouldery rubble was deposited by the kankakee torrent consideration of isostasy indicates that the earlier lake phase at havana may have been associated with downward flexure of land surface in response to glacier loading the younger lake phase was caused in part by deposition of a sediment dam the savanna deer plain terrace at the mouth of the illinois river the lake shoaled due to passing of the isostatic forebulge across the area

holocene precipitation seasonality captured by a dual hydrogen and oxygen isotope approach at steel lake minnesota abstract middle holocene 8 to 4 ka bp warmth and aridity are well recorded in sediment archives from midcontinental north america however neither the climatic driver nor the seasonal character of precipitation during this period is well understood because of the limitations of available proxy indicators for example an important challenge is to distinguish among the interacting effects of evaporation temperature or precipitation seasonality in existing 18 o records from the region here we combine hydrogen isotopes of palmitic acid and oxygen isotopes of carbonate to derive lake water isotopic values during the holocene at steel lake in north central minnesota in combination these data enable us to separate variations in evaporation from variations in the isotopic composition of input waters to lake variations in evaporation are used as a proxy for aridity and lake water input isotopic values are used as a proxy for the isotopic values of meteoric precipitation our results suggest that lake water input isotopic values were more negative during the middle holocene than at present to test whether these more negative values are related to temperature or precipitation seasonality we compare pollen inferred temperatures and the expected isotopic value of precipitation resulting from these temperatures to the reconstructed precipitation isotopic values results suggest that middle holocene warmth and aridity were associated with increased evaporation rates and decreased summer precipitation these inferences are consistent with climate simulations that highlight the role of seasonal insolation and sea surface temperatures in driving variations in precipitation seasonality during the holocene results also suggest that changes in holocene precipitation seasonality may have influenced the expansion of the prairie forest border in minnesota as well as regional variations in grassland community composition this study demonstrates the efficacy of the dual hydrogen and oxygen isotope approach for distinguishing changes in evaporation and precipitation seasonality in the paleolimnological record