climate instability during the last interglacial period recorded in the grip ice core isotope and chemical analyses of the grip ice core from summit central greenland reveal that climate in greenland during the last interglacial period was characterized by a series of severe cold periods which began extremely rapidly and lasted from decades to centuries as the last interglacial seems to have been slightly warmer than the present one its unstable climate raises questions about the effects of future global warming

potential analogues for paleoclimatic variations in eastern interior alaska during the past 14 000 yr atmospheric circulation controls of regional temperature and moisture responses the paleoclimatic history of a region can be viewed as a series of surface temperature and moisture anomalies through time the effects of changes in large scale climatic controls e g insolation major circulation controls can be mediated by the influence of smaller scale controls e g topographic barriers coastlines this may result in heterogenous surface climatic responses at the regional and sub regional scale divergent paleoclimatic trajectories between regions may be explainable in terms of such meso scale patterns using modern analogues for paleoclimate we examine how the sequence of climatic variations in eastern interior alaska during the interval 12 000 0 14c yr bp could have been generated by specific atmospheric circulation patterns fossil pollen and lake level records document the long term trends in temperature and effective moisture for the region water balance modelling provides additional estimates of paleoprecipitation synoptic climatological patterns are described using the modern instrumental record of upper level and sea level pressure surface temperature and precipitation at 12 000 14c yr bp eastern interior alaska was cooler and drier than present a situation generated today by a southward displacement of the jet stream conditions warmer and drier than present at 9000 14c yr bp may have been generated by increased ridging north of alaska and a weakened westerly circulation warmer wetter conditions than present possibly prevailed in the late middle holocene these might reflect ridging over alaska and troughing further west cool wet conditions feature enhanced westerly flow into alaska through an eastward shift in the east asian trough and positive pressure anomalies in the north pacific they may be analogous to cold periods of the little ice age the analogues demonstrate how surface conditions in other parts of beringia may sometimes be similar to while at other times different from those in the eastern interior these broader spatial patterns provide hypotheses about past climates that can be tested with paleoclimatological data for example the widespread positive temperature anomalies associated with the warm dry 9000 14c yr bp analogue fit with the expansion northward of the eastern siberian treeline the anomalously cool conditions in northeast siberia associated with the warm wet analogue may explain the continued late middle holocene treeline advance in alaska while there was retreat in siberia

the bonneville estates rockshelter rodent fauna and changes in late pleistocene middle holocene climates and biogeography in the northern bonneville basin usa abstract excavations at bonneville estates rockshelter nevada recovered rodent remains from stratified deposits spanning the past ca 12 500 14c yr bp 14 800 cal yr bp specimens from horizons dating to the late pleistocene and early holocene include species adapted to montane and moist and cool habitats including yellow bellied marmot marmota flaviventris and bushy tailed woodrat neotoma cinerea shortly after 9000 14c bp 10 200 cal yr bp these mammals became locally extinct or nearly so taxonomic diversity declined and the region became dominated by desert woodrats neotoma lepida and other species well adapted to xeric low elevation settings the timing and nature of changes in the bonneville estates rodent fauna are similar to records reported from nearby homestead and camels back caves and provide corroborative data on terminal pleistocene early holocene environments and mammalian responses to middle holocene desertification moreover the presence of northern pocket gopher thomomys talpoides at bonneville estates adds to a sparse regional record for that species and similar to homestead cave it appears that the ca 9500 14c yr bp 10 800 cal yr bp replacement of the northern pocket gopher by botta s pocket gopher in the great salt lake desert vicinity was also in response to climate change

isotopic and geochemical evidence of palaeoclimate changes in salton basin california during the past 20 kyr 1 δ18o and δ13c records in lake tufa deposits abstract investigation of 18 o and salt content of surface waters in the salton basin california shows that there is a strongly positive correlation between 18 o values and salt contents and that the 18 o of salton sea is much heavier than that of its input waters due to strong evaporation using a steady state 18 o model we have discussed the 18 o of lake cahuilla an ancient lake in the basin under through flowed closed and overflowed situations an ams 14 c dated stable isotope record of a 60 cm lake tufa slice lc 1 from the ancient shoreline of lake cahuilla indicates that the lake experienced closed and or overflowed conditions between 1 300 and 20 500 yr bp the variations in 18 o and 13 c values of lc 1 ranging from 8 52 to 2 50 pdb and from 0 8 to 3 76 respectively reflect changes in the relative humidity in salton basin and variations in the colorado river inflow and hence climatic conditions in the colorado river drainage basin the tufa record exhibits palaeoclimates of salton basin as follows 1 relatively dry climate between 20 and 18 kyr bp was followed by a wet climate between 18 and 17 kyr bp 2 decreasing effective moisture precipitation evaporation from 17 to 16 kyr bp corresponded to heinrich 1 event whereas effective moisture increased at ca 15 5 kyr bp during the trans us wet period contrasting patterns between the lc 1 record and the greenland ice core and chinese speleothem records during the bolling allerod interstadial from 15 2 to 11 7 kyr bp may be caused by age uncertainty of the lc 1 record further study of this interval is needed 3 wet warm early holocene corresponding to the maximum effective moisture period perhaps brought about by the strengthening of north american monsoon 4 toward 2 kyr bp the climate became dryer apparently resulting from weakening of the monsoons especially around 6 2 and 2 5 kyr bp 5 a wet regime occurred in the basin during the medieval warm period

late quaternary environmental change in the bonneville basin western usa abstract excavation and analyses of small animal remains from stratified raptor deposits spanning the last 11 5 ka together with collection and analysis of over 60 dated fossil woodrat midden samples spanning the last 50 ka provide a detailed record of changing climate in the eastern great basin during the late pleistocene and holocene sagebrush steppe dominated the northern bonneville basin during the full glacial suggesting that conditions were cold and relatively dry in contrast to the southern basin which was also cold but moister limber pine woodlands dominated 13 11 5 ka indicating increased dryness and summer temperatures 6 7 c cooler than present this drying trend accelerated after 11 5 ka causing lake bonneville to drop rapidly eliminating 11 species of fish from the lake from 11 5 8 2 ka xerophytic sagebrush and shadscale scrub replaced more mesophilic shrubs in a step wise fashion a variety of small mammals and plants indicate the early holocene was 3 c cooler and moister than at present not warmer as suggested by a number of climatic models the diversity of plants and animals changed dramatically after 8 2 ka as many species disappeared from the record some of the upland species returned after 4 ka and great salt lake became fresh enough at 3 4 and 1 2 ka to support populations of utah chub

estimating superparamagnetism in marine sediments with the time dependency of coercivity of remanence superparamagnetic grains ultrafine 50 nm particles that do not retain a magnetic remanence at room temperature can form through several biogeochemical pathways in marine sediments knowing the distribution of such particles therefore provides a means of better understanding the past diagenetic history of sediments and evaluating their potential record of environmental change herein we employ magnetic hysteresis measurements in a new way to determine the abundance of superparamagnetic sp grains we start with a phenomenological model that describes a dependency of coercivity of remanence hcr with measurement time as a function of superparamagnetic grain content calculated time dependencies of hcr agree with experimental data suggesting that the model can be used to derive quantitative sp estimates when applied to pelagic sediments of the western equatorial pacific ocean ocean drilling program site 805c the time dependencies of hcr reveal a 25 30 sp increase starting just above the modern iron redox boundary irb next we compare our approach with a previous method that detects sp grains through the thermal demagnetization of saturation remanence acquired at very low temperature 20 k we find this method fails to record the sp changes detected using the time dependency of hcr and suggest that the cause is a sensitivity of the low temperature data to small differences in magnetic mineralogy that accompany diagenesis specifically differential maghemitization causes varying suppression of the remanence changes associated with the verwey transition at 120 k the removal of maghemite shells on magnetic grains below the irb results in greater remanence changes at the verwey transition and a corresponding sp underestimate using the low temperature method in addition some iron oxyhydroxides paramagnetic at room temperature can become ferromagnetic at very low temperatures resulting in an sp overestimate in oxic sediments above the irb the increase of sp content in the zone of magnetic mineral reduction observed with the time dependency of hcr may be related to the breakdown of larger magnetic grains and the activity of dissimilatory iron reducing bacteria superparamagnetism measured by the time dependency of hcr may be useful for tracking the history of these biogeochemical processes in marine sediments

a modern analogue of western united states summer palaeoclimate at 6000 years before present a modern summer climate analogue of western united states 6 ka palaeoclimate based on broad scale patterns from proxy data and general circulation model simulations provides details concerning its sum mer synoptic climatology august 1955 serves as a close analogue featuring an amplified subtropical ridge aloft and anticyclonic flow near the surface over the central united states and stronger westerly flow along the canadian northwest united states border the august 1955 summer temperature pattern similar to those from general circulation model simulations exhibits positive anomalies throughout most of the continental interior the august 1955 precipitation pattern corresponds very closely with patterns of effective moisture from proxy data showing widespread aridity and some stronger monsoonal activity in the southwest the analogue suggests that the amplified subtropical ridge is a very important feature of 6 ka palaeoclimate with the circulation pattern differing from that in the early holocene implying interactions between both milankov itch and sub milankovitch forcing factors

late pleistocene and early holocene rivers and wetlands in the bonneville basin of western north america abstract field investigations at dugway proving ground in western utah have produced new data on the chronology and human occupation of late pleistocene and early holocene lakes rivers and wetlands in the lake bonneville basin we have classified paleo river channels of these ages as gravel channels and sand channels gravel channels are straight to curved digitate and have abrupt bulbous ends they are composed of fine gravel and coarse sand and are topographically inverted i e they stand higher than the surrounding mudflats sand channels are younger and sand filled with well developed meander scroll morphology that is truncated by deflated mudflat surfaces gravel channels were formed by a river that originated as overflow from the sevier basin along the old river bed during the late regressive phases of lake bonneville after 12 500 and prior to 11 000 14c yr b p dated samples from sand channels and associated fluvial overbank and wetland deposits range in age from 11 000 to 8800 14c yr b p and are probably related to continued sevier basin overflow and to groundwater discharge paleoarchaic foragers occupied numerous sites on gravel channel landforms and adjacent to sand channels in the extensive early holocene wetland habitats reworking of tools and limited toolstone diversity is consistent with theoretical models suggesting paleoarchaic foragers in the old river bed delta were less mobile than elsewhere in the great basin

separation of local signals from the regional paleomonsoon record of the chinese loess plateau a rock magnetic approach we propose a method based on thermal unblocking of low temperature saturation remanent magnetization for a quantitative estimation of the superparamagnetic cullity 1972 fraction size d 30 nm of magnetite produced by pedogenesis in the chinese loess plateau liu 1988 we applied this method to the proxy climatic records of the last 130 ka from two sites 250 km apart but separated by the mountain range liupan shan xifeng to the east 35 7 n 107 6 e and baicaoyuan to the west 36 2 n 105 0 e currently have humid and arid microclimates respectively as expected the superparamagnetic fraction increases during known warm temperature intervals at each site furthermore the more humid site clearly has higher overall superparamagnetic fractions during most of the last 130 ka however during the period 5 to 10 ka ago the relative humidity at both sites was the same within experimental errors bulk grain size evidence confirms the magnetic data and we suggest that the present easterly summer monsoon in china came from a more southerly direction during this time to flow parallel to liupan shan resulting in very similar summer humidity at xifeng and baicaoyuan

link between benthic oxygen isotopes and magnetic susceptibility in the red clay sequence on the chinese loess plateau 1 recent rock magnetic work on the red clay sequence on the chinese loess plateau clp convincingly demonstrates that the enhancement mechanisms of low frequency magnetic susceptibility i e measured at 470 hz lf in the red clay sequence are similar to those in the loess paleosol sequence therefore lf in the red clay sequence should indicate precipitation intensity received by the clp as is the case in the overlying loess paleosol sequence based on this result we compared lf in the red clay sequence with benthic oxygen isotope records we infer that the primary precipitation source on the clp varies over time in three phases during 8 1 4 5 ma the east asian summer monsoon dominates during 4 5 4 ma the polar westerlies dominate during 4 0 ma the east asian summer monsoon dominates we attribute these precipitation source shifts on the clp to the closure of the panama seaway around 4 5 ma and the tibetan uplift during the interval 4 2 6 ma

grain sizes of susceptibility and anhysteretic remanent magnetization carriers in chinese loess paleosol sequences detailed rock magnetic studies show that susceptibility mass specific and anhysteretic remanent magnetization arm of the chinese loess paleosol sequences are carried by almost identical magnetic carriers therefore the ratio arm or equivalently arm where is defined as 0 and 0 is the intercept susceptibility of the plot of versus arm and arm is field normalized arm can be used to quantify the grain size of and arm carriers by determining this ratio for three chinese loess paleosol profiles jiuzhoutai yuanbao and yichuan characterized by different degrees of environmentally controlled pedogenesis and sedimentation rates we show that the lower grain size limit of aeolian magnetic particles in the less pedogenically altered loess units is about 100 300 nm in the finer grained pseudosingle domain psd grain size range in contrast the grain sizes of pedogenically produced magnetic particles for mature paleosols dominantly cover both the superparamagnetic sp and single domain sd ranges on the basis of plots of arm against samples can be divided into four regions i ii iii and iv region i corresponds to the least pedogenically altered primary loess samples with arm of 0 165 0 24 samples in region ii a transition zone between the least altered loess and the onset of development of paleosols have values identical to those in region i but have lower arm of 0 09 0 165 with increasing susceptibility in zone iii arm is positively correlated with indicating the gradually increasing influence of sp particles finally in zone iv with higher than 6 5 10 7 m3 kg 1 arm is independent of the variations in suggesting that arm is totally controlled by the pedogenic finest grained particles and the size distribution of these particles remains almost constant the development of soils in the chinese loess revealed by these three profiles from three sites can be clearly explained by a continuous process of pedogenesis increasing from zone i to zone iv the definition of the pedogenic zones can help to improve our understanding of the underlying mechanisms and variability of pedogenesis and thus could enable more successful and accurate separation of the authentic pedogenic signals from the background signal of the aeolian inputs at different loess sites worldwide

rock magnetic proxies of climate change in the loess palaeosol sequences of the western loess plateau of china summary results of the first detailed study of the climate proxy record in the loess palaeosol sequence at xining one of the few palaeoclimate sites in the currently arid western loess plateau of china illustrate the importance of making many types of rockmagnetic measurements other than susceptibility a multiparameter approach yielded confirmation that here as elsewhere in the loess plateau the susceptibility enhancement in palaeosols was caused primarily by ultrafine magnetite and maghaemite nevertheless magnetic enhancement was caused not exclusively by changes in relative grain size but also by variations in concentration and mineralogy of the magnetic fraction the effects of concentration variations were removed through normalization of susceptibility and anhysteretic remanence with saturation magnetization and saturation remanence respectively the resulting signal was ascribed more confidently to variation in magnetic grain size which in turn was interpreted as a better proxy of pedogenesis than simple susceptibility variations in magnetic mineralogy were also determined to constrain interpretations further the data were then used to discuss climate history at xining finally results from xining were compared with other western sites and contrasted with eastern sites in summary 1 data is presented from a new loess plateau site which also appears to yield a global climate signal 2 a demonstration is made of a more rock magnetically robust way to separate concentration composition and grain size controls on susceptibility and other magnetic parameters and 3 models are provided for inter regional comparisons of palaeoclimate proxy records

hydrologic variability and the onset of modern el nino southern oscillation a 19 250 year record from lake elsinore southern california there are very few terrestrial palaeoclimate archives spanning the last glacial maximum through the holocene from coastal southern california yet knowledge of past climate dynamics is critical for assessing present and future constraints on the region s dwindling freshwater resources we present initial results from two drill cores extracted from the present day edge of lake elsinore southern california s largest natural lake using a multiproxy approach including lithologic description mass magnetic susceptibility loi 550 c total organic matter and loi 950 c total carbonate we infer first order long term climate change over the past 19 250 calendar years furthermore we suggest possible first order forcing mechanisms that drive this change which include presence absence of continental ice sheets insolation and complex ocean atmosphere interactions our results indicate four distinct millennial scale climate modes over the past 19 250 calendar years these modes include a wet last glacial maximum 19 250 17 120 cal yr bp a relatively dry late glacial holocene transition 17 120 9450 cal yr bp a wet early holocene 9450 7670 cal yr bp and a highly variable mid to late holocene climate i e alternating wet dry cycles 7670 cal yr bp present we attribute the mid to late holocene climate interval to the onset of el nino southern oscillation ca 7000 cal yr bp and a more vigorous hydrologic system these results are supported by a variety of regional terrestrial and marine palaeoclimate archives copyright 2005 john wiley sons ltd

oxygen isotope records of goethite from ferricrete deposits indicate regionally varying holocene climate change in the rocky mountain region u s a oxygen isotopes of goethite from ferricrete deposits were measured from both northern and southern rocky mountain localities to assess geographic variability in regional holocene paleoclimate change a f3 7xincrease in oxygen isotope values of 14 c dated goethites in the northern rocky mountains suggests a regional scale relative increase in amounts of isotopically heavy summer precipitation since the early holocene in contrast oxygen isotope values from the southern rocky mountains increase abruptly f2 1xat f6200 14 c yr b p then decrease f2 4xbetween f2000 14 c yr b p and the present we interpret this period of relatively high y 18 o values as evidence for a middle holocene warm period combined with a relatively strong summer monsoon these variable climate records suggest that the rocky mountains of the western united states have had a spatially heterogeneous holocene climate history d 2003 university of washington all rights reserved

patterns and causes of millennial scale climate change in the pacific northwest during marine isotope stages 2 and 3 abstract a high resolution pollen record from fargher lake washington shows a series of millennial scale changes in vegetation and climate during marine isotope stages mis 3 and 2 a comparison of fargher lake with pollen records from little lake oregon and carp lake washington reveals regional patterns of change during the early part of mis 3 ca 60 000 45 000 cal yr bp fargher and carp lakes indicate transitions between open and closed vegetation and cold dry and warm wet conditions between 45 000 and 27 500 cal yr bp all three records show shifts between low and high elevation taxa that indicate a series of temperature changes these temperature changes are similar to those in the santa barbara basin and may be explained by changes in the strength of the aleutian low during mis 2 27 500 14 000 cal yr bp shifts between wet and dry adapted taxa were most prominent at little lake there is also evidence for two minor warmings mis 2 climate variations do not correlate well with those in the santa barbara basin however precipitation changes in the pacific northwest during mis 2 correlate with evidence of ice sheet surges into the north atlantic and were probably related to variations in the strength and position of the glacial anticyclone and the jet stream

understanding geomorphic responses to environmental change a 19 000 year case study from semi arid central texas usa understanding the impact of past climatic changes on landscape stability is crucial in order to predict and mitigate the effects of future changes however in arid and semi arid environments reconstructions are often hampered by a poor understanding of the relationship between sediment deposition and climate we present here data from central texas a region that is sensitive to environmental change but has received relatively little attention the study integrates a chronology of 29 optically stimulated luminescence osl ages from six sedimentary sites in a range of depositional contexts with a 19 000 a climate record derived from pollen extracted from the adjacent boriack bog by comparing the two records we aimed to assess the relationship between climate change and geomorphic activity data show that extensive aeolian and colluvial deposition occurred during the mid to late holocene with sedimentation generally increasing during more arid phases however a number of depositional events on slopes were associated with moister episodes and sediment was also deposited in a summit setting immediately after phases of increased precipitation linkages between climate change and geomorphic response were therefore complex being controlled by both sediment supply and transport energy the climatic fluctuations identified in the boriack bog record highlight the sensitivity of central texas to environmental change while disparities in conditions recorded between it and other palaeoenvironmental sites in the southern usa emphasise the need for further work in order to enhance understanding of landscape sedimentary response to climate copyright c 2010 john wiley sons ltd

the effects of late quaternary climate and pco2 change on c4 plant abundance in the south central united states the late quaternary was a time of substantial environmental change with the past 70 000 years exhibiting global changes in climate atmospheric composition and terrestrial floral and faunal assemblages we use isotopic data and couple climate and vegetation models to assess the balance between c3 and c4 vegetation in texas during this period the carbon isotope composition of fossil bison mammoth and horse tooth enamel is used as a proxy for c3 versus c4 plant consumption and indicates that c4 plant biomass remained above 55 through most of texas from prior to the last glacial maximum lgm into the holocene these data also reveal that horses did not feed exclusively on herbaceous plants consequently isotopic data from horses are not reliable indicators of the c3 c4 balance in grassland biomes estimates of c4 percentages from coupled climate vegetation models illuminate the relative roles of climate and atmospheric carbon dioxide co2 concentrations in shaping the regional c4 signal c4 percentages estimated using observed modern climate vegetation relationships and late quaternary climate variables simulated by a global climate model are much lower than those indicated by carbon isotope values from fossils when the effect of atmospheric co2 concentration on the competitive balance between c3 and c4 plants is included in the numerical experiment however estimated c4 percentages show better agreement with isotopic estimates from late quaternary mammals and soils this result suggests that low atmospheric co2 levels played a role in the observed persistence of c4 plants throughout the late quaternary

loess record of climatic changes during mis5 in the hexi corridor northwest china abstract the hexi corridor is adjacent to deserts that serve as one of the dust sources of chinese loess the rate of aeolian dust accumulation is higher in the corridor than those in central and western part of chinese loess plateau so the loess might have recorded more details of climate changes the approximately 12 m thick s1 paleosol sequence at the shagou loess section comprises five soils and four intervening loess horizons color index carbonate content and grain size variation within the loess paleosol sequence are used as proxies of the east asian monsoon and movement of the desert margin the eemian mis 5e equivalent lacks obvious variations in the shagou record a transition layer in this section indicates the complexity of the penultimate deglaciation during substage 5d 5a sub orbital fluctuations of monsoon and desert variations are recorded both in the proxies and stratrigraphy being coincident with cold events and warm interstadials in north atlantic and greenland our records suggest that the east asian monsoon was affected by polar and high latitude climate variations even in the last interglacial

magnetic enhancement and palaeoclimate study of a loess palaeosol couplet across the loess plateau of china summary the origin of the milankovitch palaeoclimatic signal so dramatically recorded in the chinese loess remains uncertain at least five hypotheses have been put forward to explain the well documented enhancement of magnetic susceptibility in palaeosols which formed when conditions were warmer and wetter we report here a magnetic study of a prominent loess soil couplet l4 s3 at three widely separated sites where present day mean annual rainfall and temperature range from 350mm to 700mm and from 6 c to 13 c the properties determined are laboratory remanences isothermal irm anhysteretic arm viscous vrm including viscous decay room temperature susceptibility and thermomagnetic behaviour at fields beyond 100 mt the irm spectra are all essentially identical both in shape and in absolute magnitude suggesting a uniform magnetic component widespread across the plateau subtracting the signal of this component isolates a second component the amount of which varies by an order of magnitude between sites the magnetic properties of this latter material are remarkably similar to those reported for bacterial magnetite in geological sediments and modern soils viscous decay of vrm takes place in two distinct stages one of which falls to zero in 10min we attribute magnetic enhancement which is the source of the palaeoclimatic signal to in situ pedogenetic formation of magnetite which we characterize in our interpretation as two separate forms of bacterial magnetofossils the most magnetic sample requires a mass fraction of about half of one per cent of this biogenic material

an organic geochemical record of sierra nevada climate since the lgm from swamp lake yosemite abstract sediment records from swamp lake sl in the central sierra nevada california provide evidence of climatic change on millennial and centennial timescales over the last 20 000 years total organic carbon toc abundance varied in concert with elemental and isotopic tracers of organic matter c n 13 c org 15 n biogenic silica content total magnetic susceptibility and sediment lithology we interpret the down core proxy records as representing the response of the lake environment in terms of temperature seasonal ice cover mixing regimes runoff and in situ om and nutrient cycling to shifting climate states these environmental factors in turn drove changes in algal productivity om sources microbial om regeneration and secondary production and detrital input the late pleistocene 19 7 10 8 cal kyr bp was dominated by fluctuations between relatively warm dry intervals with high toc 17 4 16 5 15 8 15 0 13 9 13 2 11 4 11 0 cal kyr bp and cold wet intervals 16 5 15 8 14 8 13 9 13 1 11 6 11 0 10 7 cal kyr bp characterized by low toc and high detrital input the holocene 10 7 cal kyr bp present was characterized by three abrupt increases in toc after 10 8 8 0 and 3 0 cal kyr bp and numerous century scale fluctuations toc increases reflected enhanced lake productivity and om recycling and reduced detrital input in response to changing winter temperature and hydrologic regimes inferred environmental changes at sl correlate with other sierra nevada paleorecords and with reconstructed sea surface temperatures along the california margin parallel changes in the sl and sst records over the past 20 000 years provide new evidence that continental climate in the sierra nevada and the california current system have responded on multiple timescales to common drivers in north pacific ocean atmospheric circulation

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

climate variations in europe over the past 140 kyr deduced from rock magnetism rapid shifts in climate during the last glacial are now well documented particularly from the oxygen isotope records of the two greenland ice cores grip1 2 and gisp23 in the grip record1 2 these climate events are also seen during the preceding eemian interglacial which may be an analogue for the future climate warmed by the greenhouse effect but these shifts are not found in the eemian section of the gisp2 core3 casting doubt on whether the rapid shifts in the grip oxygen isotope record really do represent a climate signal here we present magnetic susceptibility pollen and organic carbon records from maar lake deposits in the massif central france these data provide an independent record of past climate and we find that they correlate well with the ice core records during the last glacial during the eemian two rapid cooling events seen in our record also correlate with those seen in the grip ice core supporting the idea that rapid climate change did occur in the eemian interglacial and demonstrating that it extended to continental europe

magnetism of chinese loess deposits summary the age of chinese loess deposits has long been disputed biostratigraphical and earlier magnetostratigraphical investigations placed the entire loess formation within the pleistocene and ascertained a maximum loess age of about 1 2 myr a new collection of nearly 500 samples from a natural outcrop and a borehole section near lochuan lat 35 8 n long 109 2 e shaanxi province has been dated by magnetic stratigraphy thermal cleaning of the natural remanent magnetization nrm removes a strong secondary component of viscous origin along the present geomagnetic field which resides largely in magnetite the characteristic nrm component is due to haematite which is thought to be of chemical origin rhythmical intensity variations of nrm and initial susceptibility depend on the loess lithology and may reflect climatic changes during loess deposition the palaeomagnetic results are consistent between the two sections and yield a clearly defined magnetic polarity zonation the brunhes matuyama boundary and the jaramillo subchron have been positively identified in both outcrops at exactly the same stratigraphic level the olduvai subchron has been found in the borehole section which records the entire loess sequence most probably the formation of chinese loess began shortly after the matuyama gauss polarity transition therefore a late pliocene age of about 2 4 myr is assigned to the oldest loess sediments measured

comparison of oxygen isotope records from the gisp2 and grip greenland ice cores recent results1 2 from the greenland ice core project grip summit ice core suggest that the climate in greenland has been remarkably stable during the holocene but was extremely unstable for the time period represented by the rest of the core spanning the last two glaciations and the intervening eemian inter glacial here we present the complete oxygen isotope record for the greenland ice sheet project 2 gisp2 core drilled 28 km west of the grip core we observe large rapid climate fluctuations throughout the last glacial period which closely match those reported for the grip core however in the bottom 10 of the cores spanning the eemian interglacial and the previous glacia tion there are significant differences between the two records it is possible that ice flow may have altered the chronological sequences of the stratigraphy for the bottom part of one or both of the cores considerable further work will be necessary to evaluate the likelihood of this and the extent to which it will still be possible to extract meaningful climate information from the lowest sections of the cores

paleowind directions from the magnetic fabric of loess profiles in central alaska abstract two parallel profiles of alternating loess and paleosol horizons from the halfway house site west of fairbanks in central alaska were evaluated in terms of their potential for preserving paleowind directions combined the profiles span specmap marine oxygen isotope stages 1 6 approximately the last 150 ka our analytical tools consisted of measuring the sample s anisotropy of magnetic susceptibility and characterizing the magnetic mineralogy by multiple approaches both primary eolian and secondary reworked magnetic fabrics are recognized along certain intervals of both profiles primary eolian fabrics display a horizontal magnetic foliation and a well defined magnetic lineation corresponding to the paleodirection of sediment transport and deposition a systematic change in transport direction is observed where long term prevailing winds appear to shift from a nw se to n s direction during glacial and interglacial periods respectively

a quantitative model of magnetic enhancement in loessic soils we present a quantitative model for the climatic dependence of magnetic enhancement in loessic soils the model is based on the widely accepted hypothesis that ultrafine magnetite precipitates during alternating wetting and drying cycles in the soil micropores the rate at which this occurs depends on the frequency of drying wetting cycles and on the average moisture of the soil both parameters are estimated using a statistical model for the soil water balance that depends on frequency and intensity of rainfall events and on water loss by evapotranspiration monthly climatic tables are used to calculate the average soil moisture and the rate of pedogenic magnetite production which is proportional to a new parameter called magnetite enhancement proxy mep our model is tested by comparing mep calculated for known present day climates with the magnetic enhancement of modern soils the magnetic enhancement factor defined as the ratio between a given magnetic enhancement parameter and mep is expected to be a site independent constant we show that magnetic enhancement differences between soils from the chinese loess plateau and from midwestern u s are explained by our model which yields similar magnetic enhancement factors for the two regions our model is also successful in predicting the mean annual rainfall threshold above which magnetic enhancement declines in a given type of climate

timing and extent of late quaternary paleolakes in the trans pecos closed basin west texas and south central new mexico abstract the trans pecos closed basin is a hydrographically closed region covering 20 000 km 2 centered on salt basin 160 km east of el paso texas geomorphic and limnetic evidence have been used to identify four major highstands for lake king during the last glacial maximum lgm additional geomorphic features from a second recently identified paleolake lake sacramento have been found in the beargrass subbasin a nested subbasin approximately 75 km northwest of salt basin radiocarbon ages of the organic material in lake king sediments date four abrupt climate changes and rapid lacustrine transgressions during the lgm with a quasi periodicity of 2000 yr geomorphic evidence in the beargrass subbasin identifies lake cycles contemporaneous with those in lake king the dates for these transgressions correlate with the dates of freshening events identified by researchers in paleolake basins elsewhere in new mexico the quasi periodicity of the events approximates that of dansgaard oeschger events identified from greenland ice cores the contemporaneity of the trans pecos transgressions with transgressive events in other basins in the region suggests that paleolakes in the region were in phase with respect to abrupt climate changes during the latter stages of the lgm

sp grain production during thermal demagnetization of some chinese loess palaeosol summary progressive thermal demagnetization of natural samples to above 300 c may cause significant alterations of magnetic mineral phases such changes are commonly monitored by magnetic susceptibility ms determinations at every demagnetization step in an attempt to characterize in more detail alteration products created during thermal demagnetization of some chinese loess palaeosols l1 s1 ms was measured at both room temperature ms293k and at liquid nitrogen temperature ms77k between each demagnetization step we have used the difference between ms77k and ms293k temperature dependent susceptibility ktd ms293k ms77k to monitor production of superparamagnetic sp magnetic grains ktd is a function of 1 paramagnetic susceptibility according to the curie law k 1 t that predicts a theoretical 3 83 times increase in ms at 77k 2 sp grains at 293 k that passes into the single domain sd state on cooling to 77 k causing a dramatic reduction in ms77k 3 magnetite grains exhibiting magnetic changes when passing through the verwey transition at 120 k will also reduce ms77k we observe a linear relationship between frequency dependent ms kfd and temperature dependent susceptibility ktd indicating that sp sd blocking of magnetite may be the dominant factor of ms change during cooling high values of ktd indicates high sp concentrations and an increase of ktd with increasing demagnetization temperature is mainly attributed to the production of sp grains since the amount of magnetite exhibiting changes in ms at the verwey transition is unlikely to increase during heating of loess palaeosol to 600 c in air this interpretation is also based on results from zero field heating irm77k 1t zero field cooling of irm77k 1t and short time 100 s viscous decay of irm500mt our findings suggest that sp grains are produced continuously in both loess and palaeosol with thermal treatment and that sp production in palaeosol apparently commence at the surprisingly low temperature of around 200 c

paleoclimate simulations for north america over the past 21 000 years features of the simulated climate and comparisons with paleoenvironmental data maps of upper level and surface winds and of surface temperature and precipitation illustrate the results of a sequence of global paleoclimatic simulations spanning the past 21 000 years for north america we review a the large scale features of circulation temperature and precipitation that appear in the simulations from the ncar community climate model version 1 ccm 1 b the implications of the simulated climate for the past continental scale distributions of three plant taxa picea spp pseudotsuga menziesii and artemisia tridentata which are broadly representative of the vegetation across the continent and c the potential explanations in terms of atmospheric circulation or surface energy and water balance processes for mismatches between the simulations and observations most of the broad scale features of previous paleoclimatic simulations with the ncar ccm 0 for north america are present in the current simulations many of the elements of a conceptual model based on previous climate simulations that describes the controls of paleoclimatic variations across north america during the past 21 000 years are found in simulations reviewed here these include 1 displacement of the jet stream by the laurentide ice sheet to the south of its present position in both winter and summer 2 generation of a glacial anticyclone over the ice sheet at the lgm and the consequent induction of large scale sinking motions induced over eastern north america 3 changes in the strength of surface atmospheric circulation features through time including weakening of the aleutian low in winter and strengthening of the eastern pacific and bermuda high pressure systems in summer as the ice sheet decreased in size 4 development of a heat low at the surface and a strengthened ridge in the upper atmosphere over the continent at the time of the maximum summer insolation anomaly 5 increases in summer temperature earlier in regions remote from the ice sheet these increases appear earlier in the present ccm 1 simulations than in the previous ccm 0 ones however and 6 continuation of negative winter temperature anomalies into the middle holocene in general simulated surface conditions that are discordant with paleoenvironmental observations can be attributed to the simulation of particular atmospheric circulation patterns e g those that suppress precipitation or advect warm air into a region with these mismatches amplified in beringia and the southeastern united states by surface energy and water balance processes

how long and how stable was the last interglacial abstract surface ocean indicators in the north atlantic during marine isotope stage mis 5 correlate closely with the vegetational succession in northeastern france the melisey i silty layer which marks the end of the last interglacial biozone in la grande pile pollen record appears coeval with the polar front advance c24 registered in the core v29 191 by a sharply increased presence of ice rafted detritus and the cold water foraminifer neogloboquadrina pachyderma sinistral since this event is younger than the peak of mis 5d the last interglacial as recognized in northern france correlates not only with the mis 5e but also with a substantial part of mis 5d the last interglacial in la grande pile was twice as long as the holocene and the climate in its first half was apparently not less stable than during the current interglacial if the future natural climates were to develop as analogs of the past then the onset of the next glacial environments on land would be still many millennia ahead

differences between east asian and indian monsoon climate records during mis3 attributed to differences in their driving mechanisms evidence from the loess record in the sichuan basin southwestern china and other continental and marine climate records abstract asian monsoons vary with global climatic change as is well recorded by chinese loess paleosol sequences a radiocarbon and osl dated grain size and magnetic susceptibility record of the loess paleosol sequence from the chengdu basin southwestern china contains a greatly amplified mis 3 indian monsoon climate record comparison of this record with other continental and marine climate records in the temporal and frequency domains indicates that the high amplitude mis3 feature dominates the indian monsoon region and weakens northward but is not observed in east asian monsoon climate records these observed differences in the amplitude of mis 3 climate variations between the two monsoonal regions may be caused by the different effects of the precession insolation forcing on the very different monsoonal driving mechanisms the indian monsoon which originates from the subtropical mascarene high in the southern hemisphere carries large precession signals from the equatorial and tropic zones leading to the overall intensification of mis 3 climate signals the northward decrease of the indian monsoon strength reduces precessional climate signals carried by the monsoon on the other hand the east asian monsoon is largely generated from the western pacific subtropical high in the northern subtropical zone where the impact of precessional insolation forcing and heat vapor transport are much weaker thus exerting a more limited spatial and temporal influence on climate records

age and palaeoclimatic significance of the loess of lanzhou north china extending in a broad arc that abuts the sandy non gobi deserts the loess plateau of northern china1 3 is one of the most massive accumulations of loess in the world the loess sequence is typically characterized by an alternation of silty or sandy loess with more clay rich palaeosols these alternations in conjunction with their enclosed faunas and distinctive mineralogies have been interpreted as reflecting pleistocene glacial interglacial cycles3 8 because it holds implications for the climatic and anthropological history of china9 10 the definition of a reliable chronological framework for loess deposition is of great interest recent palaeomagnetic studies4 5 have indicated that loess deposition in shaanxi province commenced 2 4 myr ago to assess the synchrony of loess accumulation across the loess plateau we have dated a 330 m thick loess sequence near lanzhou gansu province the magnetostrati graphical results reported here indicate that the base of this loess succession dates from 1 3 my r ago this young age in comparison to the shaanxi sequence is attributed to uplift along the northern fringe of the tibetan plateau11 that precluded early pleistocene loess preservation in this mountainous region palaeosols in the basal loess occur on average once every 25 kyr suggesting that climates conducive to soil forming events may have been modulated by orbital precession in the early pleistocene

quantitative estimates of palaeoprecipitation at xifeng in the loess plateau of china abstract using10be recent studies heller et al 1993 beer et al 1993 have tried to reconstruct palaeoclimatic records by separating the secondary pedogenic and primary aeolian detrital contributions to the magnetic susceptibility of chinese loess here we use a technique which monitors the temperature dependence of magnetic remanence to estimate the magnetisation and relative contribution of the superparamagnetic sp single domain sd and multidomain md ferrimagnetic components in the xifeng loess section on the assumption that the sp ferrimagnetic component is dominantly of pedogenic origin the magnetisation of this component has also been determined for samples taken from the topmost palaeosol s0 at six sites over the loess plateau and directly correlated with present day precipitation at these sites this correlation is then used to estimate quantitatively palaeoprecipitation for the topmost 16 m of the xifeng section which covers about the last 140 000 years the data indicate a considerably reduced precipitation during the glacial loess forming periods and a significantly higher level of precipitation than today during the last interglacial period

holocene fire and vegetation along environmental gradients in the northern rocky mountains abstract holocene records of fire vegetation and climate were reconstructed from four sites in the bitterroot range region of the northern rocky mountains in order to examine the vegetation and fire histories and evaluate the hypothesis proposed by whitlock and bartlein 1993 regarding the effects of increased summer insolation on precipitation patterns vegetation history in the series of sites was broadly similar in the late glacial period the pollen data suggest open parkland dominated by picea or alpine meadow which reflect conditions cooler and drier than present these open forests were replaced in the early to middle holocene by forests composed mainly of pinus and pseudotsuga which suggest conditions warmer than present modern forest compositions were in place by ca 3000 cal yr bp and small variations in the timing of the vegetation shifts reflect local differences among sites the long term trends in fire occurrence support the hypothesis proposed by whitlock and bartlein 1993 that precipitation regimes were sharpened during the early holocene summer insolation maximum but their location has remained unchanged as a result of topographic constraints sites located in areas currently summer dry were drier than present during the early holocene and fires were more frequent conversely sites located in the areas that are summer wet at present were wetter than present in the early holocene and fires were less frequent on millennial time scales it appears that the climate boundary is controlled by topography and does not shift

late quaternary paleoclimate in semiarid northeastern brazil from u series dating of travertine and water table speleothems abstract fossil travertine and subaqueous speleothems in presently dry caves suggest periods of enhanced ground water recharge during the quaternary period in semiarid northeastern brazil travertine deposits and water table speleothems were dated by the 230 th 234 u technique yielding new evidence for the timing of past periods of increased rainfall the travertine was deposited in two distinct phases extensive in situ deposits during marine isotope stage 2 and fragmentary indurated deposits dating to ca 400 000 yr b p possibly marine isotope stage 10 or 12 dating of water table speleothems showed that the regional water table was 13 1 m above the present level at the last glacial maximum a second site yielded ages suggesting a still higher water table during marine isotope stage 6 these new data demonstrate that northeastern brazil did not follow the general pattern of a dry last glacial maximum as suggested by other continental records in amazonia and southeastern brazil and as depicted in recent general circulation model gcm simulations our results thus indicate that significant variability in regional rainfall patterns occurred in the area during glaciations and caution against overgeneralization of paleoclimate patterns

paleoclimatic implications of magnetic susceptibility in late pliocene quaternary sediments from deep drilling core sg 1 in the western qaidam basin ne tibetan plateau 1 lake sediments are important archives of paleoclimate change and erosion history a 938 5 m long core sg 1 of lacustrine sediments dated at 2 77 ma to 0 1 ma was obtained from the western qaidam basin in the northeastern tibetan plateau consisting of dark grayish mudstone and grayish siltstone intercalated with salts and fine sandstones in the upper part magnetic susceptibility data combined with detailed rock magnetic properties were analyzed for revealing the significance of ferro i magnetic concentration for past changes of climate and erosion mass specific susceptibility shows a striking cyclic and long term variation samples with high values are dominated by magnetite and maghemite with pseudo single domain properties in contrast samples with low values contain maghemite from single domain to multidomain and additionally a significant fraction of hematite the driving mechanism of variation can be explained by three alternative models 1 different source regions with alternations of wind and cryoclastic erosion in a wider hinterland dry cold climate and surface runoff erosion from a narrower area more humid climate and 2 and 3 low temperature oxidation occurring either in the lake sediments dry climate or in the catchment area during weathering more humid climate trends of match with changes in sedimention rates and are roughly synchronous with the deep sea 18o record on a glacial interglacial timescale therefore the concentration of magnetic minerals in the western qaidam basin sediments is likely controlled by both tectonic influence and paleoenvironmental changes but can be best interpreted by alternations and trends of dry cold and more humid periods due to asian drying and global cooling

authigenic calcium carbonate flux in groundwater controlled lakes implications for lacustrine paleoclimate records groundwater dominated lakes are an important feature of many landscapes their sediments are a particularly valuable source of paleoenvironmental information in semiarid regions where perennial lakes may otherwise be scarce where groundwater and lake composition are favorable carbonate mineral precipitation evaporative concentration of lake water and microbial processes can combine to strongly deplete dissolved ca relative to influent groundwaters the authigenic carbonate flux acf can then become limited by water column cation availability and thereby be coupled to groundwater inflow rates and aquifer recharge here we analyze sedimentary records from two marl producing groundwater controlled lakes and demonstrate a link between one dimensional acf and the palmer drought severity index pdsi a measure of land surface wetness in a restricted outflow lake with high carbonate alkalinity acf is enhanced during historically wet climatic periods in response to increased aquifer recharge rates acf in this lake declines during droughts a neighboring dilute lake with a high rate of groundwater outflow shows comparatively weak coupling between acf and pdsi history ionic chemistry carbonate mineral equilibria and 13 c patterns of dissolved inorganic carbon show that the sensitivity of the acf signal depends on the degree of evaporative evolution of lake water and the mineral saturation state of the water column under conditions of stratification and ice cover copyright 2005 elsevier ltd

radiocarbon chronology andδ13c analysis of mid to late holocene aeolian environments guadalupe mountains national park texas usa the red dunes of guadalupe mountains national park are quartzose sand sheets and dunes stabilized by sparse plant cover stratigraphy radiocarbon dating and 13c analysis were examined in two of the larger arroyos for correlating evidence of aeolian deposition which in this basin is taken as an indication of arid conditions four quartz sand lithostratigraphic units representing intervals of aeolian activity are identified radiocarbon dates provide constraint on the timing of the three youngest quartz sand units a mid holocene aeolian interval represented by a single radiocarbon date of 6350 70 bp is interpreted as transitional to an altithermal regime a neopluvial period dated at 3370 70 bp separates mid and late holocene aeolian activity late holocene time is characterized by generally drier conditions than present and almost continuous aeolian activity from 1690 80 bp to 350 0 bp stable carbon isotope analysis suggests an abrupt shift to generally cooler summer temperatures during thi

quantifying ultrafine pedogenic magnetic particles in chinese loess by monitoring viscous decay of superparamagnetism we propose a new approach for quantifying pedogenically produced ultrafine nanometer scale magnetic particles in chinese loess we have recorded the viscous decay t 100 s of imposed isothermal remanent magnetization irm assumed to be selectively carried by superparamagnetic particles jv application of viscosity measurements relaxometry to loess paleosol sequences from the se extremity of the chinese loess plateau reveals that jv aptly reflects contributions of viscous superparamagnetic sp particles to the total remanence carried by an assemblage of magnetic particles time decay of jv can be determined with a higher resolution than the conventionally used frequency dependent susceptibility fd for characterizing the relative abundance of sp particles the nearly logarithmic time decay of irm implies a rather broad grain size distribution of pedogenic particles in chinese loess that may range from a lower size limit of 10 nm to an upper limit of 40 nm with a prevalent grain size maximum of 21 25 nm numerical calculations of the time dependence of irm decay demonstrate that the actual contribution of fine grained pedogenic magnetite maghemite to the total remanence is at least twice as large as that estimated by both fd and our proposed jv parameter we therefore propose that determination of superparamagnetic contributions by the time dependence of irm viscosity is a useful technique for tracking pedogenic processes and hence paleoclimatic conditions of chinese loess

a record of the blake event during the last interglacial paleosol in the western loess plateau of china abstract a high resolution record of the blake event has been obtained from a 40 m well dug in a loess section at jiuzhoutai in the city of lanzhou in the western loess plateau of china the paleomagnetic signal is principally carried by magnetite both in loess and paleosols the blake event is located at the boundary between paleosol s1 c of the last interglacial equivalent to mis 5e in deep sea records and loess l2 2 mis 5d and is characterized by a triple feature consisting of two short reversed intervals separated by a short normal interval the directional changes are abrupt as previously revealed in other records and suggest geocentric dipole field behavior before after and during the event but not necessarily during n r or r n transitions the age and duration of the event are estimated as between 119 97 and 114 47 kyr bp and 5 5 kyr respectively from thermoluminescence dating and astronomically tuned climate stratigraphy based on high resolution magnetic susceptibility

quaternary highstands in bear lake valley utah and idaho surficial deposits in bear lake valley utah and idaho demonstrate that bear lake and its surrounding wetlands expanded and contracted several times during the quaternary period deposits in 40 outcrops and the geomorphology of the valley were studied to reconstruct the water level history temporal control was provided by amino acid geochronology on 29 samples of aquatic mollusks calibrated by 14 c dating and tephrochronology samples of three molluscan genera were heated and analyzed to determine intergeneric differences in aspartic asp and glutamic glu acid racemization the covariance of asp and glu racemization in these genera was used to identify aberrant amino acid analyses the rate of glu racemization was calibrated by using a parabolic model that was used to estimate sample ages although the paucity of exposures uncertainty of the depositional setting and postdepositional uplift of deposits hamper an accurate reconstruction of water level in bear lake valley several conclusions can be drawn during the middle pleistocene lacustrine or wetland environments in northern bear lake valley may have extended up to 1830 masl meters above sea level 6000 ft twice between ca 1000 100 ka bear hollow phase during the late pleistocene the shoreline of bear lake transgressed at least three times 1 to 1817 masl 5960 ft at 47 39 cal calendar year corrected ka jensen spring phase 2 to 1814 masl 5950 ft at ca 16 15 cal ka raspberry square phase and 3 to 1814 masl at ca 9 cal ka willis ranch phase bear river currently bypasses bear lake but flowed into the lake during highstands and allowed the water level to rise to the valley threshold at nounan narrows water levels have lowered to 1806 masl 5923 ft since the middle pleistocene likely because of downcutting by bear river at nounan narrows because downcutting at nounan narrows controls the maximum attainable water level in bear lake valley effects of climate change on lake level are limited and bear lake may actually have been smaller than today when lake bonneville transgressed to its maximum shoreline during the last glacial maximum

inter profile correlation of the chinese loess paleosol sequences during marine oxygen isotope stage 5 and indications of pedogenesis abstract low field magnetic susceptibility has been widely used to determine the pedostratigraphy of the chinese loess paleosol sequences however uncertainties remain in correlating between the loess magnetic susceptibility and the marine oxygen isotope records because susceptibility variations are affected by both global and local paleoclimatic changes to provide a more sound paleoclimatic interpretation of magnetic susceptibility variations age models across marine oxygen isotope stage mis 5 for the jiuzhoutai jzt and yuanbao yb sections western chinese loess plateau were constructed through an integrated approach by linking the major pedostratigraphic boundaries of the loess profiles to the specmap oxygen isotope curve and by correlating relative magnetic paleointensity records with both the sint800 global paleointensity stack from marine sediments and 36cl records from the grip ice core results indicate good correlation of sirm60 mt a residual remanence of saturation isothermal remanent magnetization after a 60 mt alternating field demagnetization variations between these two sites which agree well with fluctuations in subtropical atlantic sea surface temperatures all cooling events recorded by ice core and atlantic marine sediments within mis5 have counterparts in sirm60 mt sirm60 mt is partially controlled by the degree of low temperature oxidation which is strongly temperature dependent however strong pedogenesis can decrease sirm60 mt due to further oxidation of partially oxidized magnetites above some critical points therefore we propose that sirm60 mt is best suited to record paleotemperature changes in loess profiles from the western chinese loess plateau where pedogenesis is the weakest furthermore by inter profile correlation between the yb and jzt sections we note that the seemingly uniform sub paleosol unit with a broad susceptibility peak previously assigned to mis5c between 34 4 and 37 4 m in the yb profile actually consists of two independent units lower part of s1l1 mis5b and s1s2 mis5c this indicates that susceptibility values can be strongly affected by local factors e g mainly precipitation therefore beside the simplistic traditional paleoclimatic interpretation of variations in loess susceptibility involving only cold dry and warm humid scenarios cold humid and warm dry scenarios should also be considered

a postglacial palaeoecological record from the san juan mountains of colorado usa fire climate and vegetation history continuous sediment charcoal and pollen records were developed from a 4 5 m sediment core from little molas lake lml 3370 m elevation san juan county co lml formed by 11 200 cal bp subsequent to glacial retreat turbated clay and gyttja was derived from in lake productivity and outwash sediments from the drainage basin from 1 1 200 cal bp until 10 200 cal bp cessation of glacial input and replacement of tundra with picea forest correlates with the termination of the younger dryas and indicates warming an increase in diploxylon pollen cf p ponderosa probably from lower elevations reflects the influence of the southwestern monsoon c 10 160 cal bp pollen ratios indicate that picea and other conifers persisted near the lake for the remainer of the holocene the driest holocene period occurs c 6200 to 5900 cal bp when lake levels were the lowest as indicated by all the proxy records wetter conditions during the last c 2600 cal bp favoured the expansion of p edulis and p ponderosa lat

rock magnetic constraints for the mid miocene climatic optimum from a high resolution sedimentary sequence of the northwestern qaidam basin ne tibetan plateau abstract the thick sequence of cenozoic deposits in the qaidam basin of the ne tibetan plateau has great potential for understanding the tectonic and climatic evolution of the region in this study we performed detailed rock magnetic analyses of a well dated 20 7 11 2 ma upper cenozoic sedimentary sequence from the huatugou htg section in the nw qaidam basin the magnetic susceptibility of the sequence is relatively low before 17 4 ma high during 17 4 14 5 ma and lower after 14 5 ma again this pattern is consistent with the occurrence of the mid miocene climatic optimum mmco during 17 4 14 5 ma with global climatic cooling thereafter measurements of multiple rock magnetic parameters indicate a peak in the concentration of fine grained magnetic minerals during 17 4 14 5 ma this feature demonstrates that the enhancement of the magnetic susceptibility is due to the pedogenic production of superparamagnetic sp and single domain sd magnetite suggesting the occurrence of relatively high effective precipitation and high temperature in the qaidam basin at this time comparing with regional and global climatic records the warm and humid climatic conditions coincide with the mmco implying that global climate changes were responsible for the climatic evolution in the qaidam basin throughout the mmco however after 14 5 ma the rapid uplift of the northern tibetan plateau caused it to reach a critical elevation which blocked the moisture supply to the qaidam basin and may also have contributed to the observed climatic changes

small mammal data on early and middle holocene climates and biotic communities in the bonneville basin usa abstract archaeological investigations in camels back cave western utah recovered a series of small mammal bone assemblages from stratified deposits dating between ca 12 000 and 500 14c yr b p the cave s early holocene fauna includes a number of species adapted to montane or mesic habitats containing grasses and or sagebrush e g lepus townsendii marmota flaviventris reithrodontomys megalotis and brachylagus idahoensis which suggest that the region was relatively cool and moist until after 8800 14c yr b p between ca 8600 and 8100 14c yr b p these mammals became locally extinct taxonomic diversity declined and there was an increase in species well adapted to xeric low elevation habitats including ground squirrels lepus californicus and neotoma lepida the early small mammal record from camels back cave is similar to the 11 300 6000 14c yr b p mammalian sequence from homestead cave northwestern utah and provides corroborative data on bonneville basin paleoenvironments and mammalian responses to middle holocene desertification

the younger dryas phase of great salt lake utah usa abstract field investigations at the public shooting grounds a wildlife management area on the northeastern shore of great salt lake and radiocarbon dating show that the great salt lake rose to the gilbert shoreline sometime between 12 9 and 11 2 cal ka we interpret a ripple laminated sand unit exposed at the public shooting grounds and dated to this time interval as the nearshore sediments of great salt lake deposited during the formation of the gilbert shoreline the ripple laminated sand is overlain by channel fill deposits that overlap in age 11 9 11 2 cal ka with the sand and by wetland deposits 11 1 to 10 5 cal ka consistent accelerator mass spectrometry radiocarbon ages were obtained from samples of plant fragments including those of emergent aquatic plants but mollusk shells from spring and marsh deposits yielded anomalously old ages probably because of a variable radiocarbon reservoir effect the bonneville basin was effectively wet during at least part of the younger dryas global cooling interval however conflicting results from some great basin locations and proxy records indicate that the regional effects of younger dryas cooling are still not well understood

millennial scale climatic oscillations during the last interglaciation in central china repeated southward excursions of north atlantic polar water during the last interglaciation 18 o stage 5 130 74 ka are recorded by planktonic foraminifera and ice rafted detritus in north atlantic sediment cores and greenland ice sheet cores display quasi synchronous fluctuations comparable high frequency variations in the east asian winter monsoon climate are discernible in three loess paleosol profiles in central china that span the last interglaciation peak values of the 40 m quartz fraction and bulk sediment samples from the s1 last interglacial accretionary paleosol complex reflect major dust flux events when winter monsoon winds strengthened frequent oscillations of the dust flux and nine significant dust events are recorded six events falling between ca 110 and 70 ka are correlated with cold peaks c19 24 identified in north atlantic cores two comparable dust peaks occur within paleosol s1ss3 substage 5e the older of these dating to ca 121 ka may correlate with a brief cold event recently recognized in high resolution marine and terrestrial climate proxy records

the role of maghemite in the enhancement of magnetic signal in the chinese loess paleosol sequence an extensive rock magnetic study combined with citrate bicarbonate dithionite treatment we have conducted a rock magnetic study of selected chinese loess samples before and after citrate bicarbonate dithionite cbd treatment our study is more extensive than previous ones and reveals new details of the magnetic carriers which are related to paleoclimate change three pairs of paleosol s1 the top paleosol layer in the lishi formation and loess l2 underlying s1 from three localities xining xifeng and jixian were investigated by using a variety of magnetic methods after the paleosols were treated with cbd which removes all maghemite grains and the finest grains of magnetite magnetic susceptibility decreased from 77 10 8 to 20 10 8 m3 kg in xining from 165 10 8 to 30 10 8 m3 kg in xifeng and from 180 10 8 to 25 10 8 m3 kg in jixian anhysteretic remanent magnetizations arm were reduced to less than one tenth of the original values in the three sites and saturation magnetization js decreased by about 60 in xining and 70 in xifeng and jixian these reduced values in the paleosols after cbd treatment are similar to those of the corresponding underlying loess samples which however did not show significant decrease after cbd treatment room temperature frequency dependence of susceptibility high temperature dependence of susceptibility and low temperature sirm demagnetization of the paleosol and loess samples before and after cbd treatment confirm that in addition to superparamagnetic sp particles magnetite and or maghemite the cbd removable magnetic carriers also include maghemite of grain sizes larger than sp

rapid changes in ocean circulation and heat flux in the nordic seas during the last interglacial period the apparent similarity of climate variability in the north atlantic region in the last interglacial period1 5 and the present interglacial holocene has recently been challenged by the rapid oscillations in climate conditions indicated by some marine6 7 and terrestrial climate records8 10 for the last interglacial ocean circulation in the northern north atlantic seems to be intimately coupled to the processes of climate change on various time scales11 12 so that climate variability and the associated mechanisms of change should be well recorded by sediments from these high latitudes previous studies in this region5 13 have indeed indicated an apparently less stable last interglacial climate than at middle latitudes of the north atlantic ocean4 here we present detailed records from marine sediments in the nordic seas of oceanographic conditions during the last interglacial the records show three large sea surface temperature fluctuations a weakening of the east west sea surface temperature gradient with time and changes in deep water properties in contrast similar analyses of a core from the same region indicate that sea surface temperature during the holocene has been relatively stable our data along with those from the labrador sea7 indicate rapid changes in ocean circulation and oceanic heat fluxes at high northern latitudes during the last interglacial which may have been associated with marked temperature changes on adjacent continents

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

sediment and rock strength controls on river incision into bedrock recent theoretical investigations suggest that the rate of river incision into bedrock depends nonlinearly on sediment supply challenging the common assumption that incision rate is simply proportional to stream power our measurements from laboratory abrasion mills support the hypothesis that sediment promotes erosion at low supply rates by providing tools for abrasion but inhibits erosion at high supply rates by burying underlying bedrock beneath transient deposits maximum erosion rates occur at a critical level of coarse grained sediment supply where the bedrock is only partially exposed fine grained sediments provide poor abrasive tools for lowering bedrock river beds because they tend to travel in suspension experiments also reveal that rock resistance to fluvial erosion scales with the square of rock tensile strength our results suggest that spatial and temporal variations in the extent of bedrock exposure provide incising rivers with a previously unrecognized degree of freedom in adjusting to changes in rock uplift rate and climate furthermore we conclude that the grain size distribution of sediment supplied by hillslopes to the channel network is a fundamental control on bedrock channel gradients and topographic relief

millennial scale climatic change during the last interglacial period superparamagnetic sediment proxy from paleosol s1 western chinese loess plateau detailed magnetic analyses of samples from the jiuzhoutai well section on the western chinese loess plateau reveal high resolution signals of summer monsoon change the last interglacial period is represented here by the 8 meter thick s1 paleosol complex we have used low temperature remanence studies to analyze variations in the concentration of superparamagnetic sp grains which previous studies have shown to be largely of pedogenic origin the sp concentration interpreted as a proxy for the extent of pedogenesis shows millennial scale variations within s1 we conclude that the last interglacial period in asia was characterized by rapid climate fluctuations with at least one brief return to near glacial conditions in the middle of oxygen isotope substage 5e

an enhanced role for the tropical pacific on the humid pleistocene holocene transition in southwestern north america abstract climate effects on landscape evolution during the late pleistocene holocene transition 14 6 8 ka in southwestern north america traditionally are linked to the activity of the north american monsoon and to vegetation change related to a decrease in winter precipitation acting in response to orbital cyclicity we performed an integrated analysis of regional alluvial fan lacustrine and paleobotanical records for the area comparing them with hemispheric and regional paleoclimate proxies our focus was on the potential role the tropical pacific has as a synoptic pattern modulator and moisture source for hydrogeomorphic activity in the region our analysis indicates that the onset of alluvial fan aggradation in most of the region at 13 5 ka could have been a response to semi permanent el nino like conditions in the tropical pacific which enhanced the frequency of winter frontal storms as well as increased penetration of tropical cyclones in the region the north american monsoon was restricted in extent and intensity until 7 ka and probably was not a major factor in alluvial fan aggradation a second stage of alluvial fan aggradation from 11 5 to 9 ka was dominated by hyper concentrated flows and sheet flood sedimentation along with deposition in fluvial settings storms were probably were linked to landfall of enhanced water vapor bands in the leading edge of winter extra tropical cyclones with moisture advected directly from the tropical pacific at 8 ka favorable conditions for the occurrence of these storms waned and storm tracks shifted northward analysis of modern analogs for storm types described above as prevalent during this period indicates that changes in circulation patterns across the tropical pacific can affect storm properties enough to explain the observed geomorphic effects regardless of other factors traditionally considered of large impact like vegetation change our results suggest that the tropical pacific plays a larger role than currently thought in landscape evolution of the region

a re evaluation and spatial analysis of evidence for a younger dryas climatic reversal in beringia abstract an objective classification of paleoclimatic proxies from 75 lake and peat records is used to re evaluate data quality and inferred climatic patterns in beringia during the younger dryas mapped data reveal coherent but spatially complex regional patterns suggesting the younger dryas was characterized by 1 cooling in southern alaska eastern siberia and portions of northeastern siberia and 2 uniform to warmer than present conditions through most of central alaska northeastern siberia and possibly the russian far east and northern alaska the beringian patterns correspond to distinctive large scale climatic forcings although in some locations further modified by more local influences such as topography general circulation models and modern synoptic climatology provide a conceptual framework for exploring possible mechanisms responsible for the observed changes forcings and associated climatic responses consistent with the proxy data include 1 lowered sea surface temperatures in the north atlantic and pacific reducing temperature and precipitation in eastern siberia 2 intensified aleutian low and lowered sea surface temperatures causing cooler summers and higher winter precipitation in southern alaska 3 stronger pacific subtropical high and an eastward shift of the east asian trough reducing summer temperatures in southern alaska and causing relative warmth in northeastern siberia and 4 strong high pressure system producing warm dry conditions in interior alaska

isothermal remanence experiments at room and at liquid nitrogen temperature application to soil studies a simple magnetic mineral identification technique is proposed which measures isothermal remanent magnetisation irm acquired in different field strengths at 293 k and 77 k magnetic parameters are introduced that use these irms in order to distinguish concentration and type of ferromagnetic minerals in different magnetic states the method determines easily contributions of superparamagnetic ferrimagnetic minerals in environmental magnetism studies in addition high coercivity goethite and possibly superparamagnetic hematite are identified the analysis of the magnetic properties of a soil profile on a late pleistocene terrace from the jarama river in central spain is given as an example

spatial variability of late quaternary paleoclimates in the western united states abstract paleoclimatic interpretation of proxy data is complicated sometimes by the appearance of heterogeneous patterns of climatic responses across networks of sites modern climate analogues for the western united states similar to those patterns of atmospheric circulation of 18 000 and 9000 yr b p were examined in order to explain such patterns of spatial heterogeneity modern analogues were defined by comparing modern atmospheric circulation patterns with those simulated by general circulation models maps of temperature and precipitation anomalies of the modern analogues reveal patterns of spatial heterogeneity which resemble the patterns of effective moisture compiled from paleoclimatic data january 1957 was found to be a reasonable 18 000 yr b p analogue and it features isolated areas of increased wetness in the northern great basin and increased dryness in the northwest interior analogues for 9000 yr b p from a composite of 11 augusts display patterns of spatial heterogeneity of effective moisture over most of the mountainous areas the analogues suggest that spatial heterogeneity of climate is the rule rather than the exception over much of the western united states with the climatic anomalies at any particular time representing the outcome of the mediation of large scale atmospheric circulation controls by smaller scale topographic features

compound soil horizons with mixed calcic and argillic properties examples from the northern great basin usa abstract palaeosols between deposits of the last two deep lake cycles in the northern great basin have multiple carbonate engulfed horizons and compound calcic and argillic properties at depth they are difficult to explain pedogenetically because they bifurcate in places especially in basin centres where they are separated by sediments of a minor lake expansion of probable early wisconsinan age one sequence of processes can account for all phenomena two cycles of colluviation and synchronous admixture of loess particles were each succeeded by the formation of an argillic over a calcic horizon carbonate enrichment resulting from the later period of soil formation affected the buried older soil this led to welding of the two soils and the formation of a compound horizon carrying the argillic properties of the first and the calcic properties of the second pedogenic phase steady aeolian influx to the same soil surface the commonly invoked mechanism cannot explain the bifurcated palaeosols and is not needed to explain the welded ones

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr