holocene paleoenvironmental changes inferred from diatom assemblages in sediments of kusawa lake yukon territory canada abstract the southwest yukon territory canada is an important region for recovering sensitive records of holocene paleoclimatic change more information is needed however to constrain the timing of the major holocene climatic transitions and to understand associated impacts on different ecosystems for example paleolimnological studies have focused on small lakes and ponds but the history of large lakes has received little study we analyzed diatom assemblages species richness valve concentrations and biogenic silica in the sediments of kusawa lake 60 16 5 n 136 10 9 w 671 m a s l to reconstruct the responses of this large surface area 142 km 2 deep z max 135 m freshwater ecosystem to holocene climatic transitions diatoms colonized the lake soon after ice retreat around 11 000 cal yr bp assemblages throughout the record were dominated by planktonic types diatom concentrations and biogenic silica were high during the holocene thermal maximum between 10 700 and 7300 cal yr bp then began to decrease in response to cooling associated with orbitally driven reductions in insolation diatom assemblages shifted towards taxa with lower surface water temperature optima after 8300 cal yr bp perhaps in response to abrupt and progressive cooling our study confirms that diatom assemblages in large lakes are sensitive to regional scale paleoclimatic changes

a regional scale climate reconstruction of the last 4000 years from lakes in the nebraska sand hills usa abstract high resolution paleohydrological reconstructions were carried out in five shallow lakes in the nebraska sand hills across an east west transect in order to 1 determine whether long term droughts of the past 4000 years were spatially and temporally coherent across the region 2 distinguish local variation in climate or hydrology from regional patterns of change and 3 compare the paleolimnological results with the existing dune inferred drought records diatom inferred lake level was reconstructed for all sites and compared with other regional records alterations between high and low lake levels were frequent during the past 4000 years which suggests that shifts between dry and wet periods were prevalent across the sand hills extended multi decadal to centennial scale droughts were more common prior to 2000 years bp while the last two millennia were hydrologically more variable and climate conditions alternated on shorter timescales despite some discrepancies among the five records the paleohydrological reconstructions refine the holocene drought history of the nebraska sand hills particularly between 2200 and 4000 cal a bp many of the observed drought events are contemporaneous with severe droughts documented at sites in the northern great plains and rocky mountains lending support for the severity and regional significance of these events in western north america

holocene lake level trends in the rocky mountains u s a abstract the availability of water shapes life in the western united states and much of the water in the region originates in the rocky mountains few studies however have explicitly examined the history of water levels in the rocky mountains during the holocene here we examine the past levels of three lakes near the continental divide in montana and colorado to reconstruct holocene moisture trends using transects of sediment cores and sub surface geophysical profiles from each lake we find that mid holocene shorelines in the small lakes 4 110 ha were as much as 10 m below the modern lake surfaces our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000 2000 years ago we also discuss evidence for millennial scale moisture variation including an abruptly initiated and terminated wet period in colorado from 4400 to 3700 cal yr bp and find only limited evidence for low lake stands during the past millennium the extent of low water levels during the mid holocene which were most severe and widespread ca 7000 4500 cal yr bp is consistent with the extent of insolation induced aridity in previously published regional climate model simulations like the simulations the lake data provide no evidence for enhanced zonal flow during the mid holocene which has been invoked to explain enhanced mid continent aridity at the time the data including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium confirm the ability of large boundary condition changes to push western water supplies beyond the range of recent natural variability

synchronous climatic change inferred from diatom records in four western montana lakes in the u s rocky mountains late holocene environmental and climatic conditions were reconstructed from diatom assemblages in sediment cores from four western montana lakes crevice lake foy lake morrison lake and reservoir lake the lakes show synchroneity in timing of shifts in diatom community structure but the nature of these changes differs among the lakes two of the sites provide highly resolved records of hydrologic balance while the other two stratigraphic sequences primarily record temperature impact on lake thermal structure all four lakes show significant change in five discrete intervals 2200 2100 1700 1600 1350 1200 800 600 and 250 cal yr bp the similarities in the timing of change suggest overlying regional climatic influences on lake dynamics the 800 600 cal yr bp shift is evident in other paleorecords throughout the great plains and western us associated with the transition from the medieval climate anomaly to the little ice age large scale climatic mechanisms that influence these lake environments may result from atmospheric circulation patterns that are driven by interactions between pacific and atlantic sea surface temperatures which are then locally modified by topography

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

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

a diatom record of late holocene climate variation in the northern range of yellowstone national park usa the changes in diatom species composition in a sediment core from crevice lake yellowstone national park spanning the past 2550 yr were used to reconstruct long term limnological and ecological conditions that may be related to late holocene climate variability planktic forms dominate the fossil diatom assemblages throughout this record but changes in species dominance indicate varying nutrient levels over time particularly phosphorus the changes in the nutrient concentrations in the lake were probably driven by changes in temperature and wind strength that affected the duration of water column mixing and thus the extent of nutrient recycling from deep waters prior to 2100 cal before present bp stephanodiscus minutulus and synedra tenera dominated suggesting long cool springs with extensive regeneration of phosphorus from the hypolimnion that resulted from isothermal mixing from 2100 to 800 cal bp these species were replaced by cyclotella michiganiana and cyclotella bodanica these species are characteristic of lower nutrient concentrations and are interpreted here to reflect warm summers with long periods of thermal stratification from 800 to 50 cal bp s minutulus dominated the diatom assemblage suggesting a return to lengthy mixing during spring the most dramatic late holocene changes in the fossil diatom assemblages occurred during the transition from the medieval period to the little ice age approximately 800 cal bp

three dimensional modeling of lacustrine diatom habitat areas improving paleolimnological interpretation of planktic benthic ratios three dimensional modeling of lake morphometry enables the calculation of lake volume planar surface area and basin surface area which are critical components of a conceptual model of how planktic and benthic habitat areas change with changing lake level we have applied three dimensional modeling to foy lake montana and compared model results of changing ratios of planktic and benthic habitat areas at all possible lake levels to sediment cores recovered from the lake the model allows us to produce semiquantitative depth reconstructions and greatly improves lake level reconstruction of this morphometrically complex basin the conceptual model also was modified to examine the influence of resuspension versus gravity redistribution of diatomaceous material on the sedimentary record comparison of the model results with both long and short sedimentary records from foy lake montana suggests that planktic benthic ratios may be a better predictor of mean lake depth than maximum lake depth the model developed here is simple enough to be easily implemented has few requirements can be adapted to suit any lake basin and can be augmented to examine a variety of different lake processes that affect habitat availability

annual precipitation in the yellowstone national park region since ad 1173 cores and cross sections from 133 limber pine pinus flexilis james and douglas fir pseudotsuga menziesii mirbel franco at four sites were used to estimate annual july to june precipitation in the yellowstone national park region for the period from ad 1173 to 1998 examination of the long term record shows that the early 20th century was markedly wet compared to the previous 700 yr extreme wet and dry years within the instrumental period fall within the range of past variability and the magnitude of the worst case droughts of the 20th century ad 1930s and 1950s was likely equaled or exceeded on numerous occasions before ad 1900 spectral analysis showed significant decadal to multidecadal precipitation variability at times this lower frequency variability produces strong regime like behavior in regional precipitation with the potential for rapid high amplitude switching between predominately wet and predominately dry conditions over multiple time scales strong yellowstone region precipitation anomalies were almost always associated with spatially extensive events spanning various combinations of the central and southern u s rockies the northern u s southern canadian rockies and the pacific northwest 2007 university of washington all rights reserved

holocene seasonal variability inferred from multiple proxy records from crevice lake yellowstone national park usa article i nfo a 9400 yr old record from crevice lake a semi closed alkaline lake in northern yellowstone national park was analyzed for pollen charcoal geochemistry mineralogy diatoms and stable isotopes to develop a nu anced understanding of holocene environmental history in a region of northern rocky mountains that re ceives both summer and winter precipitation the limited surface area conical bathymetry and deep water 31 m of crevice lake create oxygen deficient conditions in the hypolimnion and preserve annually laminated sediment varves for much of the record pollen data indicate that the watershed supported a closed pinus dominated forest and low fire frequency prior to 8200 cal yr bp followed by open parkland until 2600 cal yr bp and open mixed conifer forest thereafter fire activity shifted from infrequent stand replacing fires initially to frequent surface fires in the middle holocene and stand replacing events in recent centuries low values of 18 o suggest high winter precipitation in the early holocene followed by steadily drier conditions after 8500 cal yr bp carbonate rich sediments before 5000 cal yr bp imply warmer summer conditions than after 5000 cal yr bp high values of molybdenum mo uranium u and sulfur s indicate anoxic bottom waters before 8000 cal yr bp between 4400 and 3900 cal yr bp and after 2400 cal yr bp the diatom record indicates extensive water column mixing in spring and early summer through much of the holocene but a period between 2200 and 800 cal yr bp had strong summer stratification phosphate limita tion and oxygen deficient bottom waters together the proxy data suggest wet winters protracted springs and warm effectively wet summers in the early holocene and less snowpack cool springs warm dry sum mers in the middle holocene in the late holocene the region and lake experienced extreme changes in win ter spring and summer conditions with particularly short springs and dry summers and winters during the roman warm period 2000 cal yr bp and medieval climate anomaly 1200 800 cal yr bp long springs and mild summers occurred during the little ice age and these conditions persist to the present although the proxy data indicate effectively wet summer conditions in the early holocene and drier conditions in the middle and late holocene none point specifically to changes in summer precipitation as the cause in stead summer conditions were governed by multi seasonal controls on effective moisture that operated over multiple time scales

a 2200 yr record of hydrologic variability from foy lake montana usa inferred from diatom and geochemical data abstract a 2200 yr long high resolution 5 yr record of drought variability in northwest montana is inferred from diatoms and 18o values of bio induced carbonate preserved in a varved lacustrine core from foy lake a previously developed model of the diatom response to lake level fluctuations is used to constrain estimates of paleolake levels derived from the diatom data high frequency decadal fluctuations in the de trended 18o record mirror variations in wet dry cycles inferred from banff tree rings demonstrating the sensitivity of the oxygen isotope values to changes in regional moisture balance low frequency multi centennial isotopic changes may be associated with shifts in the seasonal distribution of precipitation from 200 b c to a d 800 both diatom and isotope records indicate that climate was dry and lake level low with poor diatom preservation and high organic carbon nitrogen ratios subsequently lake level rose slightly although the climate was drier and more stable than modern conditions at a d 1200 lake level increased to approximately 6 m below present elevation after which the lake fluctuated between this elevation and full stage with particularly cool and or wetter conditions after 1700 the hydrologic balance of the lake shifted abruptly at 1894 because of the establishment of a lumber mill at the lake s outlet spectral analysis of the 18o data indicates that severe droughts occurred with multi decadal 50 to 70 yr frequency

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

centennial eolian cyclicity in the great plains usa a dominant climate pattern of wind transport over the past 4000 years proxy evidence at decadal resolution from late holocene sediments from pickerel lake northeastern south dakota shows distinct centennial cycles 400e700 years in magnetic susceptibility contents of carbonate organic carbon and major elements abundance in ostracodes and d 18 o and d 13 c values in calcite proxies indicate cyclic changes in eolian input productivity and temperature maxima in magnetic susceptibility are accompanied by maxima in aluminum and iron mass accumulation rates mars and in abundances of the ostracode fabaeformiscandona rawsoni this indicates variable windy and dry conditions with westerly wind dominance including during the medieval climate anomaly maxima in carbonates organic carbon phosphorous and high d 13 c values of endogenic calcite indicate moister and less windy periods with increased lake productivity including during the little ice age and alternate with maxima of eolian transport times of the maunder sporer and wolf sunspot minima are characterized by maxima in d 18 o values and aluminum mars and minima in d 13 c values and organic carbon content we interpret these lake conditions during sunspot minima to indicate decreases in lake surface water temperatures of up to 4e5 c associated with decreases in epilimnetic productivity during

lake sediments record large scale shifts in moisture regimes across the northern prairies of north america during the past two millennia six high resolution climatic reconstructions based on diatom analyses from lake sediment cores from the northern prairies of north america show that shifts in drought conditions on decadal through multicentennial scales have prevailed in this region for at least the last two millennia the predominant broad scale pattern seen at all sites is a major shift in moisture regimes from wet to dry or vice versa depending on location that occurred after a period of relative stability these large scale shifts at the different sites exhibit spatial coherence at regional scales the three canadian sites record this abrupt shift between anno domini 500 and 800 and subsequently conditions become increasingly variable all three u s sites underwent a pronounced change but the timing of this change is between anno domini 1000 and 1300 thus later than in all of the canadian sites the mechanisms behind these patterns are poorly understood but they are likely related to changes in the shape and location of the jet stream and associated storm tracks if the patterns seen at these sites are representative of the region this observed pattern can have huge implications for future water availability in this region

multidecadal drought and holocene climate instability in the rocky mountains time series analysis of a diatom inferred drought record suggests that holocene hydroclimate of the northern rocky mountains has been characterized by oscillation between two mean climate states the dominant climate state was initiated at the onset of the holocene ca 11 ka under this climate state drought was strongly cyclic recurring at frequencies that are similar to twentieth century multidecadal phase changes of the pacific decadal oscillation this pattern remained consistent throughout much of the midholocene continuing until ca 4 5 ka after this time the mean climate state changed and drought recurrence became unstable periods of cyclic drought alternated with periods of less predictable drought the timing of this shift in climate was coincident with widespread severe drought in the mid continent of north america overall the strongest periodicity in severe drought occurred during the mid holocene when temperatures in the northern rocky mountains were warmer than today

a new chronology for the end triassic mass extinction article i nfo the transition from the triassic to jurassic period initiating the age of the dinosaurs approximately 200 ma is marked by a profound mass extinction with more than 50 genus loss in both marine and continental realms this event closely coincides with a period of extensive volcanism in the central atlantic magmatic province camp associated with the initial break up of pangaea but a causal relationship is still debated the triassic jurassic t j boundary is recently proposed in the marine record at the first occurrence datum of jurassic ammonites post dating the extinction interval that concurs with two distinct perturbations in the carbon isotope record the continental record shows a major palynological turnover together with a prominent change in tetrapod taxa but a direct link to the marine events is still equivocal here we develop an accurate chronostratigraphic framework for the t j boundary interval and establish detailed trans atlantic and marine continental correlations by integrating astrochronology paleomagnetism basalt geochemistry and geobiology we show that the oldest camp basalts are diachronous by 20 kyr across the atlantic ocean and that these two volcanic pulses coincide with the end triassic extinction interval in the marine realm our results support the hypotheses of phanerozoic mass extinctions resulting from emplacement of large igneous provinces lips and provide crucial time constraints for numerical modelling of triassic jurassic climate change and global carbon cycle perturbations

a tree ring based reconstruction of the atlantic multidecadal oscillation since 1567 a d 1 we present a tree ring based reconstruction of the atlantic multidecadal oscillation amo which demonstrates that strong low frequency 60 100 yr variability in basin wide 0 70 n sea surface temperatures ssts has been a consistent feature of north atlantic climate for the past five centuries intervention analysis of reconstructed amo indicates that 20th century modes were similar to those in the preceding 350 yr and wavelet spectra show robust multidecadal oscillations throughout the reconstruction though the exact relationships between low frequency sst modes higher frequency 7 25 yr atmospheric modes e g north atlantic oscillation arctic oscillation and terrestrial climates must still be resolved our results confirm that the amo should be considered in assessments of past and future northern hemisphere climates

triassic jurassic organic carbon isotope stratigraphy of key sections in the western tethys realm austria the late triassic period is recognized as one of the five major mass extinctions in the fossil record all these important intervals in earth history are associated with excursions in c isotope records thought to have been caused by perturbations in the global carbon cycle the nature and causes of c isotopic events across the triassic jurassic t j transition however are poorly understood we present several new high resolution organic c isotope records from the eiberg basin austria including the proposed global boundary stratotype section and point gssp for the base of the jurassic the triassic jurassic boundary interval in these records is characterized by the initial and main negative organic carbon isotope excursions cie of up to 8 the initial and main cies are biostratigraphically constrained by first and last occurrences of boundary defining macro and microfossils e g ammonites high resolution c isotope records appear to be an excellent correlation proxy for this period in the eiberg basin pyrolysis analysis demonstrates increased hydrogen index hi values for organic matter coinciding with the initial cie terrestrial organic matter influx and mass occurrences of green algae remains may have influenced the c isotope composition of the sedimentary organic matter this may have contributed to the extreme amplitude of the initial cie in the eiberg basin

the quest for chron e23r at partridge island bay of fundy canada camp emplacement postdates the end triassic extinction event at the north american craton the partridge island stratigraphic section at the bay of fundy maritime canada reveals a continental sedimen tary succession with the end triassic mass extinction level closely followed by basalts of the central atlantic magmatic province camp new paleomagnetic data show that a short reverse magnetic polarity chron correlative to e23r of the newark geomagnetic polarity time scale gpts is present below the extinction event organic carbon isotope data and basalt geochemistry further indicate that the onset of camp emplacement in the bay of fundy was roughly synchronous with emplacement in the newark basin but slightly postdates the oldest camp volcanism in morocco by 20 ka these re sults confirm the potential for long distance camp correlations based on geochemical trace elements indicate substantiate provincialism of latest triassic palynoflora and suggest a very concise period 100 ka of camp emplacement in the northern atlantic region resume la section stratigraphique de l ile partridge dans la baie de fundy maritimes canada revele une succession se dimentaire continentale dans laquelle le niveau d extinction massive de la fin du trias est suivi de pres par des basaltes de la province magmatique centre atlantique camp de nouvelles donnees paleomagnetiques montrent qu un chrone de breve inversion de polarite magnetique correle a e23r de l echelle de temps geomagnetique de polarite de newark est present sous l evenement d extinction des donnees d isotopes du carbone organique et la geochimie des basaltes indiquent de plus que le debut de la mise en place de la camp dans la baie de fundy a ete passablement contemporain avec la mise en place du bassin newark mais cette mise en place a eu lieu peu de temps apres le plus ancien volcanisme camp au maroc soit environ 20 ka ces resultats confirment le potentiel de correler la camp sur de longues distances en se basant sur des ele ments traces geochimiques ils indiquent et valident le provincialisme de la plus recente palynoflore du trias et suggerent une tres courte periode 100 ka pour la mise en place de la camp dans la region de l atlantique nord traduit par la redaction

regional atmospheric circulation change in the north pacific during the holocene inferred from lacustrine carbonate oxygen isotopes yukon territory canada analyses of sediment cores from jellybean lake a small evaporation insensitive groundwater fed lake provide a record of changes in north pacific atmospheric circulation for the last 7500 yr at 5 to 30 yr resolution isotope hydrology data from the southern yukon indicate that the oxygen isotope composition of water from jellybean lake reflects the composition of mean annual precipitation 18op recent changes in the 18o of jellybean sedimentary calcite 18oca correspond to changes in the north pacific index npi a measure of the intensity and position of the aleutian low al pressure system this suggests that 18op variability was related to the degree of fractionation during moisture transport from the gulf of alaska across the st elias mountains and that holocene shifts were controlled by the intensity and position of the al following this model between 7500 and 4500 cal yr b p long term trends suggest a predominantly weaker and or westward al between 4500 and 3000 cal yr b p the al shifted eastward or intensified before shifting westward or weakening between 3000 and 2000 cal yr b p rapid shifts eastward and or intensification occurred 1200 and 300 cal yr b p holocene changes in north pacific atmospheric circulation inferred from jellybean lake oxygen isotopes correspond with late holocene glacial advances in the st elias mountains changes in north pacific salmon abundance and shifts in atmospheric circulation over the beaufort sea

century scale paleoclimatic reconstruction from moon lake a closed basin lake in the northern great plains estimates of past lake water salinity from fossil diatom assemblages were used to infer past climatic conditions at moon lake a climatically sensitive site in the northern great plains a good correspondence between diatom inferred salinity and historical records of mean annual precipitation minus evapotranspiration p et strongly suggests that the sedimentary record from moon lake can be used to reconstruct past climatic conditions century scale analysis of the holocene diatom record indicates four major hydrological periods an early holocene transition from an open freshwater system to a closed saline system by 7300 b p which corresponds with a transition from spruce forest to deciduous parkland to prairie and indicates a major shift from wet to dry climate a mid holocene period of high salinity from 7300 to 4700 b p indicating low effective moisture p et a transitional period of high salinity from 4700 to 2200 b p characterized by poor diatom preservation and a late holocene period of variable lower salinity during the past 2 200 yr indicating fluctuations in effective moisture

fire and vegetation history during the last 3800 years in northwestern montana abstract foy lake in northwestern montana provides a record of annual to decadal scale landscape change sedimentary charcoal and pollen analyses were used to document fire and vegetation changes over the last 3800 years which were then compared to similar records from ad 1880 to 2000 the long term record at foy lake suggests shifts between forest and steppe as well as changes in fire regime that are likely the result of climate change fire activity inferred from the frequency of charcoal peaks averaged 18 fire episodes 1000 years from 3800 to 2125 cal year bp and increased from 16 fire episodes 1000 years at 2125 cal year bp to 22 episodes 1000 years at 750 cal year bp a period when the pollen data suggest that steppe vegetation yielded to increasing patches of forest cover between 2125 and 750 cal year bp increased forest cover produced more background charcoal than before and after this period when vegetation was dominated by steppe between 750 and 75 cal year bp steppe has expanded and fire episode frequency averaged 33 episodes 1000 years increasing to a maximum of 40 episodes 1000 years at ca 300 cal year bp and then decreasing to present levels since ad 1880 the pollen record indicates an increase in shrubs and grasses from ad 1895 to 1960 as a result of vegetation changes associated with timber harvesting and livestock grazing no fires have been documented in the foy lake watershed since ad 1880 charcoal from the extralocal fires of ad 1910 burning over 4 111 249 ha in idaho montana and wyoming however is present in foy lake between ad 1970 and 2000 increased arboreal pollen in the record is consistent with observations that the forest has become more closed the activities of euro americans have led to a decline in forest cover between ad 1880 and 1970 followed by a recent increase as trees are now growing in areas previously occupied by steppe euro americans are likely the cause of a reduction in fire activity in watershed since ad 1880

microstructural investigations on strongly stained olivines of the chassignite nwa 2737 and implications for its shock history abstract olivines from the meteorite northwest africa 2737 nwa 2737 show unique characteristics such as strong brownish staining the occurrence of perpendicular sets of bright lamellae and the exsolution of metallic nanoparticles these features were investigated by transmission electron microscopy and spectroscopic techniques to understand their formation and consequent implications for the shock history of nwa 2737 areas showing optically an intense brown staining are characterised by the occurrence of finely dispersed metallic nanoparticles an extreme high density 10 16 m 2 of polygonised dislocations with burgers vector 001 and a high abundance of trivalent iron 16 as determined by electron energy loss spectroscopy in contrast the bright lamellae contain individual dislocation lines with burgers vector 001 of a slightly lower density 10 14 m 2 almost no metallic nanoparticles and significantly lower trivalent iron content model mass balance calculations suggest that the high content of trivalent iron can be incorporated as a laihunite component into the cores of polygonised dislocations this is charge balanced by the formation of metallic nanoparticles trivalent iron is likely responsible for the intense brown colour whereas nanoparticles darken these areas considerably and cause an intense red slope in the infrared range the proposed diffusion of trivalent iron into the cores of polygonised dislocations and the associated exsolution of metallic iron nanoparticles are likely fundamental processes for intense olivine staining during meteoritic impacts in general detailed analyses of variously stained regions suggest that the bright lamellae have experienced slightly higher shock induced strain causing a recrystallisation of olivine which occurred simultaneously with the polygonisation and exsolution processes resulting in the olivine staining of adjacent areas the existence of individual dislocations can be interpreted as the result of a two stage shock history for nwa 2737 after a first impact the material of nwa 2737 was likely embedded in a hot ejecta blanket this would ensure sufficient time at elevated temperatures for the polygonisation of dislocations and the recrystallisation of bright lamellae a second impact resulted in the emission of individual dislocations into recrystallised olivine and the ejection of that meteorite from mars

influence of late holocene climate on northern rocky mountain mammals an exceptionally rich paleontological site containing thousands of mammalian fossils and well dated with 18 radiocarbon samples provides evidence of late holocene ecological response to climatic change in northern yellowstone national park wyoming the mammalian fauna composed of 10 597 identified specimens shows surprising affinity to the local habitat with little evidence of long distance transport of faunal elements thus revealing the faithfulness of a fossil site to the community from which it is derived the mammals illustrate ecological sensitivity to a series of mesic to xeric climatic excursions in the sagebrush grassland ecotone during the past 3200 yr from 3200 cal yr b p to a maximum of 1100 cal yr b p the species composition of mammals indicates wetter conditions than today beginning about 1200 cal yr b p the fauna becomes more representative of xeric conditions with maxima in xeric indicator taxa and minima in mesic indicator taxa concordant with the medieval warm period circa 1000 to 650 yr b p cooler wetter conditions which prevailed for most of the little ice age 700 to 100 yr b p in general correspond to a return to a more mesic mammalian fauna a warm period within the little ice age is documented by a xeric fauna these data show that mammalian ecological sensitivity to climatic change over this intermediate time scale holds promise for predictions about the impacts of future global warming

geochemical evidence for hydroclimatic variability over the last 2460 years from crevice lake in yellowstone national park usa abstract a 2460 year long hydroclimatic record for crevice lake yellowstone national park montana was constructed from the 18 o values of endogenic carbonates the 18 o record is compared to the palmer hydrologic drought index phdi and pacific decadal oscillation pdo indices as well as inferred discharge of the yellowstone river during the last century high 18 o values coincide with drought conditions and the warm phase of the pdo index low 18 o values coincide with wet years and a negative pdo index comparison of tree ring inferred discharge of the yellowstone river with the 18 o record over the last 300 years indicates that periods of high discharge i e wet winters with significant snow pack correspond with low 18 o values extrapolating this relationship we infer wet winters and high river discharge for the periods of 1090 1030 970 870 670 620 and 500 430 cal years bp the wet intervals at 670 and 500 cal bp are synchronous with similar events in banff canada and walker lake nevada the wet intervals at 970 and 670 cal bp overlap with wet intervals at walker lake and major drought events identified in the western great basin these results suggest that the northern border of yellowstone national park straddles the boundary between northern rocky mountains and great basin climate regimes

an extended organic carbon isotope record across the triassic jurassic boundary in the queen charlotte islands british columbia canada abstract new lithologic and organic carbon isotope data are presented for the triassic jurassic boundary section at kennecott point queen charlotte islands british columbia canada the previously reported late norian to earliest hettangian record is extended by over 130 m and three new isotopic features are revealed the record now shows a negative offset in baseline carbon isotope values from approximately 29 in the late norian to 31 in the hettangian this offset is accompanied by the previously reported 2 negative excursion at the triassic jurassic boundary and a 5 positive excursion in the early hettangian there is a significant long term negative isotopic trend in the hettangian interval of the section which may be due to camp volcanism the positive excursion is attributed to a decline in bio calcification as well as changes in microbial ecology both related to the mass extinction at the triassic jurassic boundary

climate and tectonic effects on holocene development of an alpine lake muge co se margin of tibet south west china is an important area for determining variability of the sw asian monsoon it is however also tectonically very active palaeolimnological studies in this region generally focus on climate as a direct driver of ecological change rarely considering the effects of tectonic activity and catchment ontogeny we present diatom and organic matter data from a holocene sediment record from muge co an alpine lake located on an active fault zone in the southeast margin of tibet and explore the possible drivers of ecological change a major change occurred at 9000 cal bp when the diatom assemblage switched from benthic to predominantly planktonic forms and organic matter content decreased rapidly comparison of the holocene diatom record with a regional data set of lake surface sediments using detrended canonical correspondence analysis dcca suggests that this change in diatom development was mainly associated with changing water depth most likely linked to a seismically triggered landslide

widespread drought episodes in the western great lakes region during the past 2000 years geographic extent and potential mechanisms we compared high resolution reconstructions of peatland water table depth at two raised bogs in the western great lakes region separated by 1000 km the sites included minden bog in southeastern michigan and hole in the bog in north central minnesota our objectives were to 1 determine whether large decadal to multidecadal droughts of the past 2000 yrs were spatially and temporally coherent across the region and 2 assess the underlying mechanisms of widespread droughts in the region we found a strong correlation between bog inferred records of moisture variability in the two regions r 0 53 between 2100 and 600 bp extreme drought events centered on 1850 1800 1650 1000 800 and 700 bp are recorded in both regions many of these drought events were contemporaneous with large droughts already documented at sites far to the west in the great plains and rocky mountains to identify potential modern climate analogues we used empirical orthogonal function eof analysis to reveal spatial modes of variability in drought and precipitation records of north america for the past century the first eofs of annual precipitation and palmer drought severity index pdsi were similar to each other and consistent with a pattern of widespread continental drought correlations between these eofs and annual sea surface temperatures ssts indicate strong relationships to ssts in both the atlantic and pacific basins we hypothesize that widespread droughts between 1000 and 700 bp were related to amplification of a spatial mode of moisture variability associated with an anomalously warm north atlantic a warm mid latitude north pacific and an anomalously cold tropical pacific a more extensive network of hydroclimate records developed using a consistent set of methods and proxies could be used in conjunction with high resolution records of past ssts to test this hypothesis

development of an eemian mis 5e interglacial palaeolake at sokli n finland inferred using multiple proxies a 12 m long lacustrine record from sokli n finland was analyzed for diatoms non pollen palynomorphs macrofossils pollen and geochemistry in order to reconstruct the development of a high latit

reconstruction of holocene environmental changes in the kiso ibi nagara compound river delta nobi plain central japan by diatom analyses of drilling cores abstract holocene environmental changes were reconstructed by analyzing diatom assemblages from sediment cores ym kzn nk and mc from the nobi plain central japan five diatom assemblage zones were identified 1 at the beginning of the holocene freshwater species were dominant 2 then marine and brackish marine species increased indicating transgression 3 in the middle holocene proportions of marine and brackish marine species became almost constant with marine species dominant 4 marine species began to be replaced by freshwater species indicating marine regression as a result of delta progradation and 5 freshwater species again became dominant these diatom assemblages correlate with previously defined lithological units zones 1 and 2 with unit b fluvial to coastal plain zone 3 with unit c inner bay or prodelta zone 4 mainly with unit d1 delta front slope and zone 5 with units d2 delta front platform and e delta plain and flood plain although the shoreline migrated landward transgression faster than it migrated seaward regression transgressive diatom assemblage changes decrease in marine brackish water species took up to 1000 years whereas regressive changes required only a few hundred years diatom analysis is useful for reconstructing not only holocene sea level changes and sedimentary environments but also local geographic effects

the little ice age in the canadian rockies abstract this paper reviews the evidence and history of glacier fluctuations during the little ice age lia in the canadian rockies episodes of synchronous glacier advance occurred in the 12th 13th early 18th and throughout the 19th centuries regional ice cover was probably greatest in the mid 19th century although in places the early 18th century advance was more extensive glaciers have lost over 25 of their area in the 20th century selective preservation of the glacier record furnishes an incomplete chronology of events through the 14th 17th centuries in contrast varve sequences provide continuous annually resolved records of sediments for at least the last millennium in some highly glacierized catchments such records have been used to infer glacier fluctuations evaluation of recent proxy climate reconstructions derived from tree rings provides independent evidence of climate fluctuations over the last millennium most regional glacier advances follow periods of reduced summer temperatures reconstructed from tree rings particularly ca 1190 1250 1280 1340 1690s and the 1800s reconstructed periods of higher precipitation at banff alberta since 1500 are 1515 1550 1585 1610 1660 1680 and the 1880s glacier advances in the early 1700s the late 1800s and in places the 1950 1970s reflect both increased precipitation and reduced summer temperatures negative glacier mass balances from 1976 to 1995 were caused by decreased winter balances the glacier fluctuation record does not contain a simple climate signal it is a complex response to several interacting factors that operate at different timescales evaluation of climate proxies over the last millennium indicates continuous variability at several superimposed timescales dominated by decade century patterns only the 19th century shows a long interval of sustained cold summers this suggests that simplistic concepts of climate over this period should be abandoned and replaced with more realistic records based on continuous proxy data series the use of the term lia should be restricted to describing a period of extended glacier cover rather than being used to define a period with specific climate conditions

direct versus indirect climate controls on holocene diatom assemblages in a sub tropical deep alpine lake lugu hu yunnan sw china the reconstruction of holocene environmental changes in lakes on the plateau region of southwest china provides an understanding of how these ecosystems may respond to climate change fossil diatom assemblages were investigated from an 11 000 year lake sediment core from a deep alpine lake lugu hu in southwest china an area strongly influenced by the southwest or the indian summer monsoon changes in diatom assemblage composition notably the abundance of the two dominant planktonic species cyclotella rhomboideo elliptica and cyclostephanos dubius reflect the effects of climate variability on nutrient dynamics mediated via thermal stratification internal nutrient cycling and catchment vegetation processes statistical analyses of the climate diatom interactions highlight the strong effect of changing orbitally induced solar radiation during the holocene presumably via its effect on the lake s thermal budget in a partial redundancy analysis climate solar insolation and proxies reflecting catchment process pollen percentages c n ratio were the most important drivers of diatom ecological change showing the strong effects of climate catchment vegetation interactions on lake functioning this diatom record reflects long term ontogeny of the lake catchment ecosystem and suggests that climatic changes both temperature and precipitation impact lake ecology indirectly through shifts in thermal stratification and catchment nutrient exports

diatom response to climate forcing of a deep alpine lake lugu hu yunnan sw china during the last glacial maximum and its implications for understanding regional monsoon variability abstract the yunnan plateau of southwest china is strongly influenced by the asian monsoon and is a critical area in terms of determining its development and variability since the last glacial maximum high resolution diatom analysis of a 14c dated sediment core retrieved from lugu lake yunnan provides a detailed history of palaeoenvironment changes from 30 000 to 10 000 cal yr bp a model of climate diatom interactions linking thermal stratification ice cover and catchment derived nutrients is proposed to explain the observed changes prior to 24 500 cal yr bp the dominance of the planktonic cyclotella rhomboideo elliptica indicates a period of stable stratification and low nutrient levels with a moderate change of temperature and precipitation in the southwest monsoon region during the lgm 24 500 14 500 cal yr bp the loss of the planktonic diatoms and a switch to small fragilaria spp suggest an altered thermal regime in the lake i e weaker stratification and possibly ice cover from 18 000 cal yr bp the expansion of planktonic taxa stephanodiscus and the asterionella formosa and increased toc are indicative of increased nutrient availability reflecting altered vegetation and soil processes due to rising regional temperatures after 14 500 cal yr bp a rapid reversal to dominance by planktonic diatoms indicates a further increase in temperature and continued development of catchment vegetation and soils the abrupt changes in diatom species composition around 24 500 and 14 500 cal yr bp suggest that the ecological status of the lake exceeded limnological thresholds interestingly however the younger dryas event is not clearly recognizable in either the diatom or pollen records in the lugu lake sediment record in a partial redundancy analysis 44 2 of the variability in the total diatom assemblages in the period 30 000 10 000 cal yr bp was accounted for by changes in summer solar radiation the relationships between the diatom assemblages and the c n and pollen data are further evidence of climate catchment vegetation interactions and their impact on lake functioning the composition of diatom assemblages in the lugu lake sediment core reflects direct and indirect i e via catchment processes climate forcing and in particular the strength of the southwest monsoon which in turn corresponds to orbitally induced variability in northern hemisphere summer solar insolation from 30 000 cal yr bp to 10 000 cal yr bp