variability of the north atlantic thermohaline circulation during the last interglacial period studies of natural climate variability are essential for evaluating its future evolution greenland ice cores suggest that the modern warm period the holocene has been relatively stable for the past 9 000 years much less is known about other warm interglacial periods which comprise less than 10 of the climate record during the past 2 5 million years here we present high resolution ocean sediment records of surface and deep water variables from the bermuda rise spanning the last interglacial period about 118 000 127 000 years ago in general deep water chemical changes are coincident with transitions in surface climate at this site the records do not show any substantial fluctuations relative to the much higher variability observed during the preceding and subsequent cool climates the relatively stable interglacial period begins and ends with abrupt changes in deep water flow we estimate using 230 th measurements to constrain the chronology that transitions occur in less than 400 years

eemian cooling in the norwegian sea and north atlantic ocean preceding continental ice sheet growth changing conditions in the north atlantic region may drive global climate changes1 2 according to previous reconstructions of the last interglacial the eemian north atlantic sea surface tempera tures ssts were similar to present day values3 in the norwegian sea even warmer conditions appeared as a single pulse of short duration4 5 whereas the greenland ice record suggests that the warm interglacial air temperatures were interrupted by several cold periods6 here we use faunal and stable isotope analyses of foraminifera in two sediment cores from the north atlantic ocean and norwegian sea to reconstruct high resolution records of sst and sea surface salinity sss during the eemian interglacial our results which differ significantly from the greenland record6 show a sharp decrease in sst and sss of the norwegian sea associated with a more moderate cooling and freshening of the north atlantic at the middle of isotope substage 5e several millennia before the beginning of continental ice sheet growth changes in the norweg ian sea surface conditions appear to have represented an important climate change affecting global atmospheric and thermohaline circulations

micromorphlogy of recent and buried soils in a semi arid region of northwestern india abstract profiles of four recent soils and of eleven soils buried under archaeological sites of known ages were investigated in an area between the ghaggar valley and the northern piedmont of the arawalli hills in northwestern india the principal approach was through study of micromorphology data were also obtained on mineralogies of several size fractions characteristics shared by the recent and buried soils indicate that essentially the same processes operated in their formation the oldest soils are buried under protohistoric sites and thus were formed at least 5000 years ago they are developed on sediments rich in sand sized mica flakes which indicate an alluvial origin others are buried beneath historic sites with ages ranging from 500 b c to 1500 a d these soils and the recent ones have a silt fraction low in mice major features of both buried and recent soils are attributed to biological activity reflected in extensive channeling and additions of excrements throughout profiles local distribution of clay silt and carbonates indicates translocation of these components furthermore the forms of calcitic features indicate modifications of various kinds evidence of biological activity as expressed in characteristics of the soils becomes progressively weaker from older to younger profiles soil development seems to have weakened over that period as well these changes are correlated with the decline with time in the proportions of mica flakes in coarse fractions the decline is attributed to lack of alluvial sedimentation since the protohistoric period and to greater transport and winnowing of sediments by wind we believe this has reduced fertility of the soils formed during the last 4000 years

abrupt changes in deep atlantic circulation during the transition to full glacial conditions 1 six ocean drilling program odp sites in the northwest atlantic have been used to investigate kinematic and chemical changes in the western boundary undercurrent wbuc during the development of full glacial conditions across the marine isotope stage 5a 4 boundary 70 000 years ago sortable silt mean grain size ss measurements are employed to examine changes in near bottom flow speeds together with carbon isotopes measured in benthic foraminifera and planktic foraminiferal fragmentation as proxies for changes in water mass chemistry a depth transect of cores spanning 1 8 4 6 km depth allows changes in both the strength and depth of the wbuc to be constrained across millennial scale events ss measurements reveal that the flow speed structure of the wbuc during warm intervals interstadials was comparable to modern holocene conditions however significant differences are observed during cold intervals with higher relative flow speeds inferred for the shallow component of the wbuc 2 km depth during all cold stadial intervals including heinrich stadial 6 and a substantial weakening of the deep component 3 4 km during full glacial conditions our results therefore reveal that the onset of full glacial conditions was associated with a regime shift to a shallower mode of circulation involving glacial north atlantic intermediate water that was quantitatively distinct from preceding cold stadial events furthermore our chemical proxy data show that the physical response of the wbuc during the last glacial inception was probably coupled to basin wide changes in the water mass composition of the deep northwest atlantic

milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr bp an atmospheric general circulation model has been used to simulate the climates of january and july at 3 000 yr intervals from 15 kyr bp onwards the model simulates a strengthened monsoon circulation and increased precipitation in the northern hemisphere tropics culminating at 9 6 kyr bp the computed hydrological budgets were used to estimate the area of closed lakes between 8 9 and 26 6 n giving with minor exceptions results in good agreement with geological evidence

relaxation oscillators in concert a framework for climate change at millennial timescales during the late pleistocene 1 using a box model of the north atlantic ocean and a parameterization of heinrich events we suggest that self sustained oscillations of the large scale oceanic circulation provide a framework to accommodate crucial elements of late pleistocene climate variability i dansgaard oeschger style oscillations with varying interstadial length ii synchronization between dansgaard oeschger stadials and heinrich events and iii younger dryas type events the latter result from the restart of the oscillations after a glacial maximum and can be regarded as dansgaard oeschger stadials overprinted by rapidly changing boundary conditions

the impact of environmental change and human land use on alluvial valleys in the loess plateau of china during the middle holocene abstract in recent years a growing number of studies have reported proxy evidence for holocene climatic and environmental change on the loess plateau of north central china pollen from lake cores and sediment analyses from loess sections firmly point to a warm moist mid holocene climatic optimum archaeological evidence indicates that at the same time human agricultural settlements expanded into this area during the middle neolithic period a geoarchaeological study of sediment sections from several valleys within the catchment of the yiluo river henan province indicates a sequence beginning with a stable landscape soil development and high water tables in the early mid holocene which was exploited by early neolithic millet farmers in the peiligang period subsequently there was a rapid shift to episodes of hillslope erosion and increased valley alluviation in the later mid holocene the resulting alluvial sediments dating from 5300 5010 cal bc through 2130 1870 cal bc represent clay rich water logged deposits that contain sediment and phytolith evidence for the first rice paddy farming in this region this coincides with the first migration of neolithic rice farmers to the area in the yangshao period ca 5500 cal bc these fills temporally and stratigraphically linked to archaeological sites and human land use practices suggest that they originated as a direct result of human impact on the landscape after ca 2000 cal bc holocene valley incision in this catchment led to narrowing floodplains the disappearance of the valley wetland deposits and a reduction in land available for paddy farming

rising social complexity agricultural intensification and the earliest rice paddies on the loess plateau of northern china geoarchaeological studies of landscapes immediately adjacent to archaeological sites can contribute information on the direct impact of small scale societies on their associated landscapes this direct connection allows us to understand aspects of the motivations economic decision making and agricultural strategies and how they affected local site catchments the origin and spreading of farming communities onto the loess plateau of northern china provides a good example of this we recorded sediment profiles that were immediately adjacent to the site of huizui in the yiluo river basin here we identified evidence for human land use beginning with the early mid holocene deposits which are consistent with stable hillslope soils indicating that the first mixed forager millet farmers of the peiligang neolithic had a very light ecological footprint on the landscape this is in contrast to the later middle neolithic yangshao period farmers sediments from the yangshao period revealed paleolandscape and phytolith evidence for the earliest neolithic paddy farming well outside of the natural habitat of wild rice in addition to evidence for massive deforestation and soil erosion a 15 m deep sediment sequence containing sets of gravels beginning ca 7200 cal bp and gleyed soils dating from ca 6600 cal bp contained rice phytoliths archaeological waste suggesting manuring and micromorphological data indicating trampling these signs of intensive landscape management go hand in hand with rapidly increasing social complexity

changes in meridional temperature and salinity gradients in the north atlantic ocean 30 72 n during the last interglacial period eight deep sea sediment cores from the north atlantic ocean ranging from 31 to 72 n are studied to reconstruct the meridional gradients in surface hydrographic conditions during the interval of minimum ice volume within the last interglacial period using benthic foraminiferal 18o measurements and estimates of sea surface temperature sst and sea surface salinity sss we show that summer ssts and ssss decreased gradually during the interval of minimum ice volume at high latitude sites 52 72 n whereas they were stable or increased during the same time period at low latitude sites 31 41 n this increase in meridional gradients of ssts and ssss may have been due to changes in the latitudinal distribution of summer and annual average insolation and associated oceanic and atmospheric feedbacks these trends documented for the eemian ice volume minimum period are similar to corresponding changes observed during the holocene and may have had a similar origin

weathering and clay mineral formation in two holocene soils and in buried paleosols in tadjikistan towards a quaternary paleoclimatic record in central asia abstract the upper part of the karamaydan section tadjikistan shows the most detailed loess paleosol sequence yet known for the brunhes chron and the central and lower parts of the chashmanigar section provide similar detail for most of the matuyama chron to enable paleoclimates to be deduced the primary and secondary minerals in the silt and clay fractions must be determined separately to evaluate the type and intensity of mineral weathering and clay mineral formation to distinguish between inherited and pedogenetically formed clay minerals the original petrographic homogeneity of the parent material from which a soil developed must be established the main sources of pedogenic clay minerals are phyllosilicates in the silt fractions illites and vermiculites are the dominant pedogenetically formed clay minerals in the b or bt horizons of the holocene climaphytomorphic soils and in all paleosols s and pedocomplexes pk in karamaydan and chashmanigar except s xvii in which large amounts of smectites were formed there is little difference in the type and amount of pedogenic clay mineral formation between the holocene soils and the paleosols in the brunhes epoch at karamaydan as well as during most of the matuyama epoch at chashmanigar these results indicate that the climates of the interglacials represented by the b or bt horizons of the buried paleosols of young mid and old pleistocene age were similar to that of the holocene

deepwater source variations during the last climatic cycle and their impact on the global deepwater circulation the degree of similarity of the 13c records of the planktonic foraminiferal species n pachyderma and of the benthic foraminiferal genus cibicides in the high latitude basins of the world ocean is used as an indicator of the presence of deepwater sources during the last climatic cycle whereas continuous formation of deep water is recognized in the southern ocean the norwegian sea stopped acting as a sink for surface water during isotope stage 4 and the remainder of the last glaciation however deep water formed in the north atlantic south of the norwegian sea during the last climatic cycle as early as isotope substage 5d and this area was also the only active northern source during stages 4 2 a detailed reconstruction of the geographic distribution of 13c in benthic foraminifera in the atlantic ocean during the last glacial maximum shows that the most important deepwater mass originated from the southern ocean whereas the glacial north atlantic deep water cannot be traced south of 40 n at shallower depth an oxygenated 13c rich intermediate water mass extended from 45 n to 15 s in the pacific ocean a ventilation higher than the modern one was also found in open ocean in the depth range 700 2600 m and is best explained by stronger formation of intermediate water in high northern latitudes

sensitivity studies of northern hemisphere glaciation using an atmospheric general circulation model abstract the u k university global atmospheric modeling programme gcm is used to investigate whether the growth of northern hemisphere ice sheets could have been initiated by changes of orbital parameters and sea surface temperature two different orbital configurations corresponding to the present day and 115 kyr bp are used the reduced summer solar insulation in the northern hemisphere results in a decrease of the surface temperature by 4 to 10 c in the northern continents and to perennial snow in some high latitude regions therefore the model results support the hypothesis that a deficit of summer insulation can create conditions favorable for initiation of ice sheet growth in the northern hemisphere a decreased sea surface temperature northward of 65 n during the northern hemisphere summer may contribute to the maintenance of ice sheets a simple mixed layer ocean model coupled to the gcm indicates that the changes of sea surface temperature and extension of sea ice due to insulation changes pl

sea ice induced changes in ocean circulation during the eemian we argue that arctic sea ice played an important role during early stages of the last glacial inception two simulations of the institut pierre simon laplace coupled model 4 are analyzed one for the time of maximum high latitude summer insolation during the last interglacial the eemian and a second one for the subsequent summer insolation minimum at the last glacial inception during the inception increased arctic freshwater export by sea ice shuts down labrador sea convection and weakens overturning circulation and oceanic heat transport by 27 and 15 respectively a positive feedback of the atlantic subpolar gyre enhances the initial freshening by sea ice the reorganization of the subpolar surface circulation however makes the atlantic inflow more saline and thereby maintains deep convection in the nordic seas these results highlight the importance of an accurate representation of dynamic sea ice for the study of past and future climate changes

the climate in europe during the eemian a multi method approach using pollen data the last interglacial period the eemian offers a testbed for comparing climate evolution throughout an interglacial with the current warm period we present here results from climatic reconstructions from 17 sites distributed across the european continent allowing an assessment of trends and regional averages of climate changes during this period we use a multi method approach to allow for an improved assessment of the uncertainties involved in the reconstruction in addition the method takes into account the errors associated with the age model the resulting uncertainties are large but allow a more robust assessment of the reconstructed climatic variations than in previous studies the results show a traditional three part eemian with an early optimum followed by slight cooling and eventually a sharp drop in both temperatures and precipitation this sequence is however restricted to the north as this latter change is not observed in the south where temperatures remain stable for longer these variations led to marked variation in the latitudinal temperature gradient during the eemian the difference between the two regions is also noticeable in the magnitude of changes with greater variations in the north than the south some evidence is found for changes in lapse rates however a greater number of sites is needed to confirm this

can milankovitch orbital variations initiate the growth of ice sheets in a general circulation model the goddard institute for space studies climate model is used to investigate whether the growth of ice sheets could have been initiated by solar insolation variations three different orbital configurations are used corresponding to 116 and 106 kyr b p and a modified insolation field with greater reductions in summer insolation at high northern latitudes the time periods chosen are those in which geophysical evidence suggests ice sheets may have been growing rapidly the reduced summer insolation field characteristic of all the experiments is thought to be a necessary condition for allowing snow to last through the summer and ice sheets to build the results show that the model fails to maintain snow cover through the summer at locations of suspected initiation of the major ice sheets despite the reduced summer and fall insolation when 10 m thick ice was inserted in all locations where continental ice sheets existed during the last glacial maximum the model failed to maintain it as well producing energy and mass imbalances which would remove the ice within 5 years only when the ocean surface temperatures were adjusted to their peak ice age values was the model able to keep any of the additional ice and then only in a very restricted region of northern baffin island the experiments indicate there is a wide discrepancy between the model s response to milankovitch perturbations and the geophysical evidence of ice sheet initiation as the model failed to grow or sustain low altitude ice during the time of high latitude maximum solar radiation reduction 120 110 kyr b p it is unlikely it could have done so at any other time within the last several hundred thousand years if the model results are correct it indicates that the growth of ice occurred in an extremely ablative environment and thus demanded some complicated strategy or else some other climate forcing occurred in addition to the orbital variation influence and co2 reduction which would imply we do not really understand the cause of the ice ages and the milankovitch connection if the model is not nearly sensitive enough to climate forcing it could have implications for projections of future climate change

progress in paleoclimate modeling this paper briefly surveys areas of paleoclimate modeling notable for recent progress new ideas including hypotheses giving a pivotal role to sea ice have revitalized the low order models used to simulate the time evolution of glacial cycles through the pleistocene a prohibitive length of time for comprehensive general circulation models gcms in a recent breakthrough however gcms have succeeded in simulating the onset of glaciations this occurs at times most recently 115 kyr b p when high northern latitudes are cold enough to maintain a snow cover and tropical latitudes are warm enhancing the moisture source more generally the improvement in models has allowed simulations of key periods such as the last glacial maximum and the mid holocene that compare more favorably and in more detail with paleoproxy data these models now simulate enso cycles and some of them have been shown to reproduce the reduction of enso activity observed in the early to middle holocene modeling studies have demonstrated that the reduction is a response to the altered orbital configuration at that time an urgent challenge for paleoclimate modeling is to explain and to simulate the abrupt changes observed during glacial epochs i e dansgaard oescher cycles heinrich events and the younger dryas efforts have begun to simulate the last millennium over this time the forcing due to orbital variations is less important than the radiance changes due to volcanic eruptions and variations in solar output simulations of these natural variations test the models relied on for future climate change projections they provide better estimates of the internal and naturally forced variability at centennial time scales elucidating how unusual the recent global temperature trends are

distribution and genesis of calcitic pedofeatures within a rapidly aggrading loess paleosol sequence in china abstract the distribution of calcitic pedofeatures within part of a buried paleosol complex on gaolanshan mountain near lanzhou in the western semi arid part of the loess plateau of china is interpreted in terms of a pedosedimentary model controlled by changes in dominance of regional climatic monsoonal forces during periods of moderate loess accumulation rates and moisture availability accretionary pedogenesis was characterised by localised dissolution and redistribution of calcite and development of compound calcitic depletion concentration pedofeatures these took one of two forms i equigranular hypidiotopic calcitic coatings or infillings 100 350 m and calcitic depletion hypocoatings the reconstruction provides the basis for explaining the ubiquitous occurrence of high concentrations of sand size grains and nodules of calcite within weakly leached horizons of other paleosols in the western part of the loess plateau of china their presence reflects not only the influence of semi arid conditions prevailing during the main stable soil forming intervals but also preceding drier periods of accretionary pedogenesis associated with transitional changes in relative dominance of dry northwesterly winter monsoons responsible for the transport of the dust to that of moist southwesterly and southeasterly monsoons the study emphasises the general need for a pedosedimentary approach when attempting to interpret loess paleosol sequences

on the structure and origin of major glaciation cycles 1 linear responses to milankovitch forcing time series of ocean properties provide a measure of global ice volume and monitor key features of the wind driven and density driven circulations over the past 400 000 years cycles with periods near 23 000 41 000 and 100 000 years dominate this climatic narrative when the narrative is examined in a geographic array of time series the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles we argue that the 23 000 and 41 000 year cycles of glaciation are continuous linear responses to orbitally driven changes in the arctic radiation budget and we use the phase progression in each climatic cycle to identify the main pathways along which the initial local responses to radiation are propagated by the atmosphere and ocean early in this progression deep waters of the southern ocean appear to act as a carbon trap to stimulate new observations and modeling efforts we offer a process model that gives a synoptic view of climate at the four end member states needed to describe the system s evolution and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems solutions to problems involving systems of such complexity are not born full grown like athena from the head of zeus rather they evolve slowly in stages each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step victor p starr

a 420 000 year deuterium excess record from east antarctica information on past changes in the origin of precipitation at vostok here we present and analyze the complete vostok ice cores deuterium excess record which spans the last four climatic cycles back to 420 000 years b p to extract paleoclimate information from this record we use isotopic modeling showing that changes in deuterium excess d d 8 18o of vostok precipitation reflect changes in the average temperature of oceanic moisture sources over the last 250 000 years the deuterium excess is strongly anticorrelated with changes in earth s obliquity this reflects changes in the relative contribution of low and high latitudes to the vostok precipitation resulting from changes in the latitudinal annual mean insolation gradient governed by the obliquity however this modulation by obliquity is not observed prior to 250 000 years b p we attribute this difference to the ice flow the deeper ice under vostok station having accumulated in a location upstream vostok receiving precipitation from different oceanic origins despite this difference between the earlier and the later portions of the record the deuterium excess changes during all the glacial inceptions are similar in amplitude and timing relative to the deuterium decrease the glacial inceptions are characterized by high deuterium excess indicating the significant role of the tropics in supplying moisture to the already cold poles

the tibetan plateau as amplifier of orbital scale variability of the east asian monsoon 1 asian monsoon climate variability at geological time scales is modulated by both the earth s orbital changes and tectonic uplift of the tibetan plateau tp here using bandpass filtered versions of previously published high resolution geological records from chinese loess we show that the orbital scale variability of the east asian summer monsoon easm has increased dramatically since the late pliocene climate model simulations indicate that the increase in the variability of the northern easm at both precession and tilt periods may be at least partially attributed to the uplift of the tp

late eemian warming in the nordic seas as seen in proxy data and climate models 1 we analyze a transient simulation of the last glacial inception in a climate model of intermediate complexity focusing on sea ice ocean circulation dynamics in the north atlantic and nordic seas as northern high latitude summer insolation decreases toward the end of the eemian interglacial arctic sea ice export to the north atlantic increases this surface fresh water transport weakens deep water formation in the north atlantic and the near surface circulation of the subpolar gyre as a consequence the relative contribution of subpolar gyre waters to the atlantic inflow into the nordic seas is reduced giving way to more warm and saline subtropical waters from the north atlantic current we thus find an episode of relatively high heat and salt transport into the nordic seas during the last glacial inception between 119 000 and 115 000 years before present this stabilizes deep ocean convection in the region and warms scandinavia during a phase of low insolation these findings are in good agreement with proxy data from the nordic seas and north atlantic at the end of the warm interval sea surface temperature drops by about 3 c marking the onset of large scale glacier growth over scandinavia

response of global deep water circulation to earth s climatic change 135 000 107 000 years ago the response of deep ocean circulation to major climatic events of the past 135 000 years is inferred using carbon 13 analyses of benthic foraminifera from the major ocean basins the results demonstrate that the circulation of the world ocean deep water is very sensitive to climate and that it changes drastically during climatic transitions

loess and early land use geoarchaeological investigation at the early neolithic site of guobei southern chinese loess plateau abstract prehistoric land use at the guobei site and its relationship with the local environment are examined by applying osl dating micromorphological examination and geo physical analysis the majority of the osl dates are of early to middle holocene ages and are thus comparable to many osl dates derived from other studies in the same region according to the particle size analysis silt sized particles 2 60 m were predominant throughout the profiles examined however there are spatial and temporal variations of different size groups of particles throughout the profiles which provide complementary information for the micromorphological interpretation the total organic component of the samples examined through loi is relatively high all 2 with those of the overlying holocene deposits higher than those of the underlying malan loess by about 0 2 moreover in all three profiles the highest organic contents appear in the palaeosols confirming that there was greater organic accumulation during soil formation periods the groundmass of most slides collected from the early to middle holocene horizons displays a very homogeneous pattern while the abundance and distribution of different kinds of pedo features mainly including clay textural calcitic iron mn and crustal features vary greatly temporally and spatially these different lines of information demonstrate diversified pedo sedimentary processes due to variations in micro environmental conditions and cultural activities we discuss the importance of a palaeo ecological perspective allowed by the geoarchaeological study to an improved understanding of the relationship between loess changing hydrology prehistoric farming practice and land use and long term landscape change in the chinese loess area this will thus contribute to a comparison on the dynamic relationship between loess and prehistoric farming in other regions of the world such as europe and north america

orbital forcing of arctic climate mechanisms of climate response and implications for continental glaciation progress in understanding how terrestrial ice volume is linked to earths orbital configuration has been impeded by the cost of simulating climate system processes relevant to glaciation over orbital time scales 10 3 10 5 years a compromise is usually made to rep resent the climate system by models that are averaged over one or more spatial dimensions or by three dimensional models that are limited to simulating par ticular snapshots in time we take advantage of the short equilibration time 310 years of a climate model consisting of a three dimensional atmosphere coupled to a simple slab ocean to derive the equilibrium climate response to accelerated variations in earths orbital configuration over the past 165 000 years prominent decreases in ice melt and increases in snowfall are sim ulated during three time intervals near 26 73 and 117 thousand years ago ka when aphelion was in late spring and obliquity was low there were also significant decreases in ice melt and increases in snowfall near 97 and 142 ka when eccentricity was relatively large aph elion was in late spring and obliquity was high or near its long term mean these glaciation friendly time intervals correspond to prominent and secondary phases of terrestrial ice growth seen within the marine d 18 o record both dynamical and thermal effects contribute to the increases in snowfall during these periods through increases in storm activity and the fraction of precipi tation falling as snow the majority of the mid to high latitude response to orbital forcing is organized by the properties of sea ice through its influence on radiative feedbacks that nearly double the size of the orbital forcing as well as its influence on the seasonal evolution of the latitudinal temperature gradient

the demise of the last interglacial recorded in isotopically dated speleothems from the alps abstract the transition from interglacial to glacial climate has attracted much attention in paleoclimatic research partly because orbital forcing dictates that such a return to a new ice age is inescapable albeit in the distant future only the detailed series of steps that leads to glacial inceptions remains poorly understood however and studies of past glacial inceptions are often compromised by incomplete records imprecise chronological control and or uncertainties associated with proxy transfer functions here we present results from u th dated speleothems from a cave in the austrian alps entrische kirche cave which grew continuously between ca 127 and 114 ka and intermittently also during subsequent stadials and interstadials the alpine setting of this cave is prone to periglacial and glacial processes as well as temperature and precipitation changes which are recorded in these cave deposits we use high resolution stable isotope data to constrain paleoenvironmental changes associated with the demise of the last interglacial unprecedented in detail for central europe peak interglacial conditions are characterized by high 18 o values and high growth rates from ca 127 to 124 ka a major drop in 18 o by 3 occurred at ca 118 ka which coincided with cold event c26 in the north atlantic this isotopic shift which occurred at or close to isotopic equilibrium cannot be explained by simple uniform cooling modelling suggests that enhanced seasonality dominated by significant changes in winter precipitation are required to reconcile the isotope data with the fact that calcite deposition continued across the mis 5e d transition i e no cave freezing this period of enhanced seasonality lasted for several millennia during which the catchment area of the cave remained geomorphologically stable and ice free eventually speleothem growth terminated at ca 114 ka which is in line with widespread ice rafting in the north atlantic cold event c24 and the onset of fully stadial conditions during mis 5d

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

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

is high obliquity a plausible cause for neoproterozoic glaciations 1 the main features of the low latitude neoproterozoic glaciations remain the subject of controversial debates concerning possible triggers here we use an agcm coupled with a slab ocean to test one of the earliest and simplest explanation for tropical glaciations a higher obliquity of the earth s rotation axis we show that high obliquity may result in an extensive glaciation during the sturtian episode 750 ma due to the location of continental masses in the tropical areas but cannot produce a large glaciation in the case of mid to high latitude paleogeographies such as those thought to typify the varangian vendian episodes 620 580 ma

a late eemian aridity pulse in central europe during the last glacial inception how do ice ages begin it s an obvious question to ask as we enjoy the relative luxury of an interglacial but a hard one to answer a look at past transitions may give some clues as to how this period will one day come to an end a climate reconstruction based on sediments found beneath a lake in the eifel mountains in germany provides evidence of an extreme climate event lasting 468 years right at the end of the last interglacial dust storms aridity bushfires and the loss of trees associated with a warm climate coincided with a southward shift of the warm waters of the north atlantic drift in terms of insolation the rate of delivery of the sun s radiation to earth conditions then were similar to those we have now investigating the processes that led to the end of the last interglacial period is relevant for understanding how our ongoing interglacial will end which has been a matter of much debate see for example refs 1 2 a recent ice core from greenland demonstrates climate cooling from 122 000 years ago3 driven by orbitally controlled insolation with glacial inception at 118 000 years ago4 5 6 7 8 here we present an annually resolved layer counted record of varve thickness quartz grain size and pollen assemblages from a maar lake in the eifel germany which documents a late eemian aridity pulse lasting 468 years with dust storms aridity bushfire and a decline of thermophilous trees at the time of glacial inception we interpret the decrease in both precipitation and temperature as an indication of a close link of this extreme climate event to a sudden southward shift of the position of the north atlantic drift the ocean current that brings warm surface waters to the northern european region the late eemian aridity pulse occurred at a 65 n july insolation of 416 w m 2 close to today s value of 428 w m 2 ref 9 and may therefore be relevant for the interpretation of present day climate variability

possible role of atmosphere biosphere interactions in triggering the last glaciation we coupled a global biome model iteratively with an atmospheric general circulation model to study the possible role of vegetation in the climate system at the time of glacial inception 115 000 years ago orbital forcing alone was not sufficient to initiate glaciation when other components of the climate system were kept as present atmospheric composition oceans biosphere and cryosphere summers were however cold enough to induce major vegetation shifts in high northern latitudes southward migration of the boreal forest tundra limit helped to create favourable conditions for continental ice sheet growth with increasing snow depth and duration in labrador arctic canada and northern western fennoscandia these results support a role for biogeophysical feedback in initiating glaciations

variability of monsoon climate in east asia at the end of the last glaciation abstract high resolution paleomonsoon proxy records from peat and eolian sand paleosol sequences at the desert loess transition zone in china denote a rapid oscillation from cold dry conditions 11 200 10 60014c yr b p to cool humid conditions 10 600 10 20014c yr b p followed by a return to cold dry climate 10 200 10 00014c yr b p variations in precipitation proxies suggest that significant climatic variability occurred in monsoonal eastern asia during the younger dryas interval late glacial climate in the chinese desert loess belt that lies downwind from europe was strongly influenced by cold air from high latitudes and from the north atlantic via the westerlies the inferred precipitation variations were likely caused by variations in the strength of the siberian high which influenced the pressure gradient between land and ocean and therefore influenced the position of the east asian monsoon front

bio climatic imprints on a holocene loess palaeosol from china micromorphology heavy minerals pollen and soil properties were examined in a typical holocene loess profile in the north of the loess plateau consisting of a palaeosol locally known as the ansai palaeosol underlain by the malan loess and overlain by modern loess the palaeosol consists of an upper humus rich ab horizon over a clay rich bt horizon the humus rich horizon is intensely weathered contains precipitated calcitic material derived from the overlying modern loess and has both high pollen content and diversity clay coatings in the clay rich horizon indicate formation by eluviation illuviation of clay pedogenetic characteristics and pollen analysis imply that the vegetation during the time of most intensive soil development is likely to have been a warm temperate forest a complex interpretation of the loess palaeosol sequence recognizes several development stages the phase of soil development maximum and minimum limiting ages for which are ca 8800 and 4400 c 14 years bp involved continuing but reduced loess deposition and successive holocene bio climatic environment overprinted new features on the palaeosol c 2003 elsevier ltd all rights reserved

increasing vegetation and climate gradient in western europe over the last glacial inception 122 110 ka data model comparison high resolution terrestrial pollen and marine planktic and benthic isotopes coarse fraction and n pachyderma s analyses have been performed in the marine isotope stage mis 5 interval of images core md99 2331 retrieved in the northwestern iberian margin this study shows the occurrence of a zeifen interstadial stadial succession on land and in the ocean during the first part of mis 5e in northwestern iberia the eemian is marked from 126 to 122 ka by the development of deciduous quercus forest at the same time as mediterranean forest colonised southern iberia and deciduous quercus corylus forest occupied northernmost european regions from 121 to 115 ka carpinus betulus forest developed in nw iberia indicating a winter cooling by 2degreesc on land and an increase in annual precipitation by 100 200 min along with a sea surface temperature sst decreasing trend off iberia a similar cooling has been documented at the same time in northern germany 52degreesn by the replacement of deciduous forest by coniferous abies picea formations implying a southward displacement of the deciduous tree line between similar to60degreesn and 50degreesn as early as 120 ka the southward migration of the tree line between 72degreesn and 58degreesn simulated by the earth model of intermediate complexity mobidic from 122 and 120 ka and considered as a major process to initiate the last glaciation is therefore compatible with data between 115 and 110 ka the substantial ice accumulation in northern high latitudes mis 5e 5d transition was synchronous with successive drops c26 and c25 in northeastern atlantic sst in northwestern iberia abies pinus trees developed at the expense of quercus carpinus forest a tundra like environment occupied northern germany marking the end of the interglacial in northwestern europe at 115 ka and boreal forest likely colonised northeastern france the first displacement of the vegetation belts at 121 ka was enhanced at 115 ka indicating an amplification of the vegetation and climate gradients in northeastern atlantic and european borderlands probably related with the well developed ice caps at that time the comparison between the general trend in the estimated and simulated mobidic winter and summer temperatures for latitudes between 35 and 45degreesn shows that both follow quite straightforwardly the precession signal although the simulated and reconstructed temperatures agree better in the south than north of 40degreesn annual precipitation is exhibiting opposite trend in the data and in the model this contradiction is likely the fact that the zonal climate simulated by the model may not accurately represent the regional climate features as reconstructed from the pollen c 2004 elsevier b v all rights reserved

sensitivity of glaciation to initial snow cover co2 snow albedo and oceanic roughness in the ncar ccm we present results from numerical experiments made with a gcm the ncar ccm1 that were designed to estimate the annual balance between snow fall accumulation and ablation for geographically important land regions for a variety of conditions we also attempt to assess the reliability of these results by investigating model sensitivity to changes in prescribed physical parameters experiments were run with an initial imposition of 1 m of midwinter snowcover over all northern hemisphere land points over alaska western canada siberia and the tibetan plateau the model tended to retain this snow cover through the summer and in some cases increase its depth as well we define these regions as glaciation sensitive and note some correspondence between them and source regions for the pleistocene ice sheets an experiment with greatly reduced co2 100 ppm showed a tendency towards spontaneous glaciation i e the model remained snow covered throughout the summer over the same geographic regions noted above with 200 ppm co2 roughly equal to values at the last glacial maximum snow cover over these regions did not quite survive the summer on a consistent basis combining 200 ppm co2 and 1 m of initial northern hemisphere snow cover yielded glaciation sensitive conditions agreeing remarkably well with locations undergoing glaciation during the pleistocene to assess the reliability of these results we have determined minimal model uncertainty by varying two of the empirical coefficients in the model within physically plausible ranges in one case surface roughness of all ocean gridpoints was reduced by an order of magnitude leading to local 10 reductions in precipitation snowfall a change hard to distinguish from inherent model variability in the other case the fraction of a land grid square assumed to be occupied by snow cover for albedo purposes was varied from one half to unity large changes occurred in the degree of summer melting and in some cases the sign of the net balance changed as fractional snow cover was changed we conclude that the model may be able to reveal regions sensitive to glaciation but that it cannot yield a reliable quantitative computation of the magnitude of the net snow accumulation that can be implicitly or explicitly integrated through time

true to milankovitch glacial inception in the new community climate system model the equilibrium solution of a fully coupled general circulation model with present day orbital forcing is compared to the solution of the same model with the orbital forcing from 115 000 years ago the difference in snow accumulation between these two simulations has a pattern and a magnitude comparable to the ones inferred from reconstructions for the last glacial inception this is a major improvement over previous similar studies and the increased realism is attributed to the higher spatial resolution in the atmospheric model which allows for a more accurate representation of the orography of northern canada and siberia the analysis of the atmospheric heat budget reveals that as postulated by milankovitch s hypothesis the only necessary positive feedback is the snow albedo feedback which is initiated by reduced melting of snow and sea ice in the summer however this positive feedback is almost fully compensated by an increased meridionalheat transportin theatmosphereandareducedconcentrationoflow arcticclouds in contrasttosimilar previous studies the ocean heat transport remains largely unchanged this stability of the northern north atlanticcirculation is explainedby the regulatingeffect of thefreshwaterimportthroughthe naresstrait and northwest passage and the spiciness import by the north atlantic current

the long term paleomonsoon variation recorded by the loess paleosol sequence in central china abstract the loess paleosol sequence in central china during the last 2 5 ma is a good record of the asian monsoon variation which could be considered as the concrete realization of global paleoclimatic cycles the geological and biological evidence of the loess paleosol sequence indicates that the loess was mainly the dust deposits transported by the northerly wind of winter monsoons and also suffered to some extent from pedogenesis caused by weakened summer monsoons and that the paleosol accreted by the rather slow dust accumulation was closely related to strengthened summer monsoons an alternating occurrence of loess and paleosol in the luochuan section implies the variation history of warm humid climate with a summer monsoon dominance and cold dry climate with a winter monsoon dominance the magnetic susceptibility of loess and paleosol can be regarded as a proxy index of asian monsoon variation an analysis of the susceptibility curve and structure characters of luochuan section shows that the asian monsoon circulation was strengthened from early pleistocene to late pleistocene and the 500 ka bp point and 1200 ka bp point seem to be a threshold and a turning point respectively in terms of the long term paleomonsoon variation in asia

heavy mineral record of the holocene environment on the loess plateau in china and its pedogenetic significance abstract three sections of holocene loess on the loess plateau were studied to document environmental and pedogenetic changes in heavy minerals in the fine sand to fine silt fraction 0 25 0 01 mm the sections were in a north south transect and ranged in age from 10 000 200 year b p to the present the heavy minerals consist mainly of homblende epidote and opaque minerals however there are differences in the content of unstable minerals and in the surface characteristics of grains indices based on mineral ratios and grain surface characteristics suggest that the so called heilutu soil which has been recognized as a modern soil was subjected to variations in natural weathering conditions aeolian loess deposition and human impacts its profile indicates that it is a polygenetic soil

late quaternary dust flow on the chinese loess plateau abstract the dust flux of over 25 g cm2 103yr was reached in the central part of the chinese loess plateau during the last glacial maximum this is more than three times higher than during the last interglacial and early glacial time the grain size of the pleniglacial dust reached a peak shortly after 20 000 years ago and testifies to the occurrence of strong winds and frequent dust storms

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

archaeology of the anthropocene in the yellow river region china 8000 2000 cal bp although archaeological analysis emphasizes the importance of climatic events as a driver of historical processes we use a variety of environmental and archaeological data to show that human modif

high resolution palynological record off the iberian margin direct land sea correlation for the last interglacial complex abstract we present high resolution pollen dinocyst and isotopic data for the last interglacial complex from marine core md952042 southwestern margin of the iberian peninsula 37 48 n 10 10 w 3148 m direct land sea correlation from this core indicates that during this period north atlantic sea surface temperatures were in phase with iberian climate our palynological analysis suggests a younger dryas like event at the marine isotope stage mis 6 5 transition the analysis also indicates that the eemian spans from the lightest isotopic values of mis 5e ca 126 ky bp to the heavier isotopic values towards the mis 5e 5d transition therefore the eemian is not entirely equivalent to mis 5e pollen analysis identifies four climatic phases of low amplitude during the eemian a mediterranean climate in southwestern europe is gradually replaced by oceanic conditions the middle of the eemian is characterized by an increase in precipitation on the land and ocean associated with a slight cooling this seems to be the result of a displacement of the polar front as far south as southern europe during this period after the eemian three relatively short climatic phases on land melisey i st germain ia and montaigu cold event occurred contemporaneously with three shifts of sea surface temperatures the montaigu event first identified in terrestrial pollen sequences is therefore also recorded in core md952042 on the basis of pollen dinocyst and planktonic isotopic data our results also show that the warm periods of mis 5 are not characterized by similar climatic conditions on land

present day kinematics of asia derived from geologic fault rates active fault geometry available quaternary rates on major faults and the far field plate motion are used to determine crustal kinematics in the collision zone between india and asia using a finite element formalism to construct a spherical shell model of the asian continent with embedded faults we seek a velocity solution approaching the motion of rigid blocks by minimizing the elastic strain energy in the fault bounded blocks by doing so we test the assumption that long term deformation within continents is mostly localized into major faults in the solution fault motion accounts for more than 80 of the deformation allowing us to describe our velocity model in terms of quasi rigid block rotations on the sphere south china is rotating clockwise about a pole located southwest of borneo implying an e ese velocity vector of 11 mm yr for a point at shanghai in agreement with the velocity vector determined by very long baseline interferometry heki et al 1995 the eastward movement of south china is accommodated by oblique extension along the red river fault at a rate of 10 5 mm yr in the south and by the combination of left lateral strike slip motion on the qinling dabie shan fault and the counterclockwise rotation of the ordos and adjacent blocks in the north the tarim rotates clockwise with respect to dzungaria about a pole located at 44 n 92 e consistent with increasing crustal shortening toward the west throughout the tien shan assuming incompressibility a crustal volume budget over a domain encompassing the tertiary mountain ranges in asia shows that over the last 10 000 years 73 4 of the north south shortening between india and asia has been absorbed by thickening of the lithosphere and 27 has been accommodated by lateral extrusion of continental blocks the present day predominance of thickening in asia results from the relatively slow eastward motion of south china controlled by strike slip faulting in the qinling shan and oblique extension in northeastern china

ice sheet inception subgrid hypsometric parameterization of mass balance in an ice sheet model studies of ice sheet inception and mountain glaciation are limited by model resolution in both atmospheric and glaciologic models the accumulation and ablation of snow and ice are controlled by local climate which is sensitive to topographic features that are subgrid in continent scale investigations climate patterns are complex in areas of high relief but the first order influence of elevation on temperature and atmospheric humidity is quantifiable and is central to glacier mass balance we make substantial improvements to the descriptions of accumulation and ablation in a large scale ice sheet model by incorporating subgrid topographic information the distribution of relief in the large scale model is estimated through a hypsometric parameterization based on the minimum maximum and median topographic elevation this permits accounting of high elevation accumulation and melting in deep valleys we combine this with a parameterization of downslope subgrid ice flux simulating but not explicitly describing the role of alpine glaciers the detailed budgeting of mass balance greatly improves our skill in modelling present day ice distribution in north america we anticipate similar improvements in our ability to model nucleation centres of the quaternary ice sheets this approach may also prove useful in projections of glacier and ice field response to climate change

enhanced ice sheet growth in eurasia owing to adjacent ice dammed lakes large proglacial lakes cool regional summer climate because of their large heat capacity and have been shown to modify precipitation through mesoscale atmospheric feedbacks as in the case of lake agassiz 1 several large ice dammed lakes with a combined area twice that of the caspian sea were formed in northern eurasia about 90 000 years ago during the last glacial period when an ice sheet centred over the barents and kara seas 2 blocked the large northbound russian rivers 3 here we present high resolution simulations with an atmospheric general circulation model that explicitly simulates the surface mass balance of the ice sheet we show that the main influence of the eurasian proglacial lakes was a significant reduction of ice sheet melting at the southern margin of the barents kara ice sheet through strong regional summer cooling over large parts of russia in our simulations the summer melt reduction clearly outweighs lake induced decreases in moisture and hence snowfall such as has been reported earlier for lake agassiz1 we conclude that the summer cooling mechanism from proglacial lakes accelerated ice sheet growth and delayed ice sheet decay in eurasia and probably also in north america

the impact of precession changes on the arctic climate during the last interglacial glacial transition three sensitivity experiments using an ocean atmosphere general circulation model oagcm are conducted to simulate the climate impact of precession the relative contributions of components of the hydrological cycle including the albedo of arctic sea ice advection of atmospheric water vapor and sea surface temperature to the summer arctic melt process are evaluated timing of the perihelion is varied in each experiment with meteorological spring sp winter wp and autumn ap perihelion corresponding to conditions at 110 115 and 120 ky bp respectively obliquity is unchanged at the 115 ky level which is lower than today the experiments are assessed relative to the present day control which has been shown to simulate current conditions based on observations in the sp experiment top of the atmosphere toa insolation is weaker than today between the summer solstice and autumnal equinox in the ap case representing the interglacial it is less intense between vernal equinox and summer solstice but stronger during the remainder of the year although the incident solar radiation is reduced in summer in the sp experiment increased melting of snow is found primarily as a result of feedbacks from the delayed seasonal cycle of hydrologic components this is in contrast to both the wp and ap cases in which the perennial snow cover is simulated at the time of the last glacial inception 115 ky bp the wp experiment shows lower insolation to the high northern latitudes in late spring and summer mainly as a result of lower obliquity than today dynamical ocean atmosphere interactions in response to precession maintain the reduced sea ice melting in late spring strengthen the annual equator to pole sea surface temperature sst gradient and increase atmospheric moisture convergence in glaciation sensitive regions in both the wp and

sea surface temperatures of southern midlatitudes 0 160 kyr b p 1 late quaternary climate fluctuated between extremes of glaciations lasting 90 kyr on average and interglacial episodes lasting 10 kyr still largely unknown are the spatial and temporal evolution of these global climate states with vigorous debate still underway on the mechanisms responsible for glacial inceptions and terminations though it is widely believed that the southern hemisphere oceans play a central role in global climate changes few paleoclimate records exist from there here we present three new alkenone derived sst records from the midlatitude southern hemisphere spanning the last 160 kyr a full glacial interglacial cycle our subtropical sst records from the last glacial period are characterized by 1 warming 47 23 kyr b p when high latitudes in both hemispheres cooled and 2 milder temperatures during the penultimate glacial period than during the last glacial interval these sst features are found to be of tropical to subtropical wide extent implying increased thermal gradients at times of high latitude ice sheet growth this has implications for the vigor of atmospheric and upper ocean circulation and the transport of heat and moisture to the poles that may have been instrumental in the growth and maintenance of polar ice sheets during glacial periods

micromorphology of loess paleosol sequences a record of paleoenvironmental change abstract the record of paleoenvironmental change retained within thin sections from loess paleosol sequences is discussed with reference to case studies from western europe china and northwestern usa the paleoenvironmental significance of individual micromorphological features such as calcitic concentrations clay coatings and cryogenic structures are first evaluated and the value of recognising and interpreting microstratigraphic relationships of features in terms of paleoenvironmental change is illustrated recent developments have seen a greater emphasis on using micromorphology within a pedosedimentary approach whereby both pedogenic and sedimentary processes are taken into account when interpreting loess paleosol sequences this has led not only to the acquisition of important information on conditions during phases of enhanced loess accretion and periods of climatic transition but also to an increased awareness of spatial changes in processes and environmental controls during particular time periods micromorphologically based reconstructions have enabled the isolation of key stages of pedosedimentary development although absolute dating of stages is often limited by lack of age controls a novel approach aimed at addressing this limitation afforded by presence of datable tephra in microstratigraphic association with key micromorphological features is discussed

the reunion subchron vegetation and climate history of the northeastern russian arctic inferred from the lake el gygytgyn pollen record abstract the 318 m thick sediment record from lake el gygytgyn provides unique opportunities for a detailed examination of environmental changes during the reunion subchron polarity reversal event 2 1384 2 1216 myr bp in the northeastern russian arctic the paper describes vegetation and climate fluctuations between 2 15 and 2 10 myr bp as inferred from palynological data biome reconstructions indicate that throughout this interval the tundra tund biome generally has higher affinity scores as compared to cold steppe step or cold deciduous forest clde an exception is the climatic optimum between 2 139 and 2 131 myr bp coinciding with marine isotope stage 81 approximately the reunion subchron when the clde biome has the highest scores landscape openness indices suggest that more closed vegetation characterized most of the interval between 2 146 and 2 127 myr bp when deciduous forest and shrubs expanded in the regional vegetation and climate was relatively warm and wet peaks in green algal colonies botryococcus and zygnema type spores 2 150 2 146 2 131 2 123 and 2 112 2 102 myr bp indicate expansions of shallow water habitats and lowered lake levels comparisons with biome reconstructions from other interglacial intervals at lake el gygytgyn suggest that precession related summer insolation intensity and obliquity related duration of summer daylight are major controls on the onset of interglaciations whereas obliquity probably plays a more significant role on vegetation succession at northern high latitudes during the pleistocene

toward the snowball earth deglaciation the current state of knowledge suggests that the neoproterozoic snowball earth is far from deglaciation even at 0 2 bars of co2 since understanding the termination of the fully ice covered state is essential to sustain or not the snowball earth theory we used an atmospheric general climate model agcm to explore some key factors which could induce deglaciation after testing the models sensitivity to their parameterizations of clouds co2 and snow we investigated the warming effect caused by a dusty surface associated with ash release during a mega volcanic eruption we found that the snow aging process its dirtiness and the ash deposition on the snow free ice are key factors for deglaciation our modelling study suggests that under a co2 enriched atmosphere a dusty snowball earth could reach the deglaciation threshold

simulating the amplification of orbital forcing by ocean feedbacks in the last glaciation according to milankovitch theory the lower summer insolation at high latitudes about 115 000 years ago allowed winter snow to persist throughout summer leading to ice sheet build up and glaciation1 but attempts to simulate the last glaciation using global atmospheric models have failed to produce this outcome when forced by insolation changes only2 3 4 5 these results point towards the importance of feedback effects for example through changes in vegetation or the ocean circulation for the amplification of solar forcing6 7 8 9 here we present a fully coupled ocean atmosphere model of the last glaciation that produces a build up of perennial snow cover at known locations of ice sheets during this period we show that ocean feedbacks lead to a cooling of the high northern latitudes along with an increase in atmospheric moisture transport from the equator to the poles these changes agree with available geological data10 11 12 13 14 15 and together they lead to an increased delivery of snow to high northern latitudes the mechanism we present explains the onset of glaciation which would be amplified by changes in vegetation in response to weak orbital forcing

role of orbitally induced changes in tundra area in the onset of glaciation the link between glacial interglacial cycles and changes in insolation due to variations in the earth s orbital parameters is well established1 4 but of the attempts to simulate incipient glaciation using three dimensional general circulation models gcms driven by orbital forcing alone5 10 only one10 has been successful gcm experiments7 11 show that reduced summer insolation 115 000 years ago during an interglacial to glacial climate shift produces sufficient high latitude cooling to cause expansion of tundra at the expense of boreal forest11 which in turn can induce more cooling11 14 here we show using a global climate model that the increase in surface albedo under snow covered conditions that results from a biome model estimate11 of tundra expansion 115 000 years ago is sufficient to induce glaciation over extreme northeastern canada if the additional cooling from this estimated tundra expansion induces further expansion then widespread glaciation occurs at latitudes above 65 n these results suggest that the climate feedback from high latitude tundra expansion might have contributed to the onset of the most recent glaciation

response of air sea carbon fluxes and climate to orbital forcing changes in the community climate system model 1 a global general circulation model coupled to an ocean ecosystem model is used to quantify the response of carbon fluxes and climate to changes in orbital forcing compared to the present day simulation the simulation with the earth s orbital parameters from 115 000 years ago features significantly cooler northern high latitudes but only moderately cooler southern high latitudes this asymmetry is explained by a 30 reduction of the strength of the atlantic meridional overturning circulation that is caused by an increased arctic sea ice export and a resulting freshening of the north atlantic the strong northern high latitude cooling and the direct insolation induced tropical warming lead to global shifts in precipitation and winds to the order of 10 20 these climate shifts lead to regional differences in air sea carbon fluxes of the same order however the differences in global net air sea carbon fluxes are small which is due to several effects two of which stand out first colder sea surface temperature leads to a more effective solubility pump but also to increased sea ice concentration which blocks air sea exchange and second the weakening of southern ocean winds that is predicted by some idealized studies occurs only in part of the basin and is compensated by stronger winds in other parts

a movable trigger fossil fuel co2 and the onset of the next glaciation 1 the initiation of northern hemisphere ice sheets in the last 800 kyr appears to be closely controlled by minima in summer insolation forcing at 65 n beginning from an initial typical interglacial pco2 of 280 ppm the climber 2 model initiates an ice sheet in the northern hemisphere when insolation drops 0 7 standard deviation or 15 w m2 below the mean this same value is required to explain the history of climate using an orbitally driven conceptual model based on insolation and ice volume thresholds paillard 1998 when the initial baseline pco2 is raised in climber 2 a deeper minimum in summertime insolation is required to nucleate an ice sheet carbon cycle models indicate that 25 of co2 from fossil fuel combustion will remain in the atmosphere for thousands of years and 7 will remain beyond one hundred thousand years archer 2005 we predict that a carbon release from fossil fuels or methane hydrate deposits of 5000 gton c could prevent glaciation for the next 500 000 years until after not one but two 400 kyr cycle eccentricity minima the duration and intensity of the projected interglacial period are longer than have been seen in the last 2 6 million years

report of 1st discussion group the last interglacial in high latitudes of the northern hemisphere terrestrial and marine evidence abstract the paleoclimatic informations preserved at some selected sites from high northern latitudes correlative with the last interglacial maximum of isotopic substage 5e have been compiled on a hemisphere scale the mapped compilation reveals much warmer climate conditions on average than present in both terrestrial and marine environments and indicates a northward shift of subpolar and boreal bioclimatic zones the data also suggest that temperature and precipitation gradients were significantly different than those of the holocene with particularly efficient latitudinal and eastward transport of warm air masses the paleoenvironmental and paleoclimate trends through the last interglacial sensu lato isotopic stage 5 are more difficult to assess because the chronostratigraphical framework is poorly constrained nevertheless the data suggest important regionalism in the climatostratigraphical trends in particular the large amplitude climatic cyclicity that characterizes the last interglacial of europe is barely observed elsewhere moreover by the end of the last interglacial isotopic substage 5a conditions slightly cooler than present are recorded in western europe while relatively warm conditions similar to today apparently prevailed in circumpolar regions adjacent to the north atlantic despite the onset of ice growth the last interglacial provides paleosynoptic situations unlike those of the present that may contribute to understanding interactions within the environmental system on a hemispheric scale there is however an imperative need for uniformization of data sets and for more accurate chronological control on regional scales

comparison of milankovitch periods between continental loess and deep sea records over the last 2 5 ma abstract a high resolution east asian winter monsoon proxy record reconstructed from the baoji loess section in china shows two major shifts in climate modes over the past 2 5 ma one occurring at about 1 7 1 6 ma bp and the other at about 0 8 0 5 ma bp the 1 7 1 6 ma shift is characterized by a rather abrupt transition of winter monsoon variability from various periodicities to dominant 41 ka cycles and accompanied by a substantial increase in intensity of winter monsoon winds as manifested by an increase in average loess grain size the 0 8 0 5 ma event shows a relatively gradual transition from constant 41 ka cycles to predominant 100 ka climatic oscillations with a significant increase in amplitude the 0 8 0 5 ma shift matches that registered in deep sea 18 o records whereas the 1 7 1 6 ma shift is absent in global ice volume changes this comparison suggests that at about 1 6 ma bp the ice sheets in the northern hemisphere may have reached a critical size sufficient to modulate changes in the global climate system the discrepancy of climate cyclicity between loess and deep sea records over the 2 5 1 6 ma interval suggests that the older matuyama climate evolution cannot be understood simply by a regular 41 ka cycle model on a global scale more long proxy records derived from continental deposits are needed

on the structure and origin of major glaciation cycles 2 the 100 000 year cycle climate over the past million years has been dominated by glaciation cycles with periods near 23 000 41 000 and 100 000 years in a linear version of the milankovitch theory the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity but the 100 000 year radiation cycle arising from eccentricity variation is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing we present phase observations showing that the geographic progression of local responses over the 100 000 year cycle is similar to the progression in the other two cycles implying that a similar set of internal climatic mechanisms operates in all three but the phase sequence in the 100 000 year cycle requires a source of climatic inertia having a time constant similar to 15 000 years much larger than the other cycles similar to 5 000 years our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source when these ice sheets forced by precession and obliquity exceed a critical size they cease responding as linear milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses in our model the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity driven modulations in the 23 000 year sea level cycle during an interval when sea level is forced upward from a major low stand by a milankovitch response acting either alone or in combination with an internally driven higher frequency process ice sheets grounded on continental shelves become unstable mass wasting accelerates and the resulting deglaciation sets the phase of one wave in the train of 100 000 year oscillations whether a glacier or ice sheet influences the climate depends very much on the scale the interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger thereby gradually increasing its radius of influence

palaeosol development in the chinese loess plateau as an indicator of the strength of the east asian summer monsoon evidence for a mid holocene maximum the interpretation of the stalagmite delta o 18 record from southern china as an indicator of east asian summer monsoon easm rainfall amount or intensity is strongly debated soil development in the chinese loess plateau clp situated in the modern summer monsoon domain has long been treated as a direct and reliable easm proxy in this study we compiled 81 loess and 229 palaeosol dates from 77 profiles across the clp and synthesized the data statistically in order to investigate the easm history during the holocene the resulting history of holocene loess deposition and palaeosol development demonstrates that 1 loess deposition decreased and palaeosol formation increased across the clp after 11 5 ka 1 ka 1000 cal yr bp revealing a gradually intensified easm during the early holocene 2 numerous palaeosols developed in the clp between 8 8 ka and 3 4 ka indicating a generally strong easm during the mid holocene 3 the number of palaeosol dates decreased rapidly at around 3 ka suggesting a sudden retreat of the easm in the late holocene these trends of easm evolution are broadly consistent with variations in aeolian sand activity in the four main sandlands the mu us otindag horqin and hulun buir sandlands along the modern monsoon fringe in northern china and are supported by a wide range of independent evidence in addition we also found that high loess deposition and stronger aeolian activity occurred at similar to 5 ka representing a weak easm event within the context of an overall strong mid holocene easm the strong mid holocene easm revealed by palaeosol development in the clp and aeolian sand activity in the sandlands in northern china contrasts with the early holocene easm maximum suggested by stalagmite delta o 18 data from southern china our results are compatible with the proposal that the stalagmite delta o 18 variations in china do not result from changes in easm rainfall amount or intensity but instead reflect changes in moisture source driven by variations in atmospheric oceanic circulation c 2014 elsevier ltd and inqua all rights reserved

correlation between climate events in the north atlantic and china during the last glaciation episodes of massive iceberg release heinrich events 1 3 into the north atlantic ocean during the last glaciation were associated with recurring episodes of unusually cold north atlantic surface water bond cycles 4 and cold air temperatures over greenland dansgaard oeschger events 5 6 four of the youngest of these cold events have also been reported in climate records from sites outside the north atlantic region7 but until now the entire suite has been identified only in north atlantic marine sediments greenland ice core records and tentatively in french lake sediments8 here we examine grain size data from chinese loess and intercalated accretionary palaeosols of last glacial age for evidence of similar climate signals remote from the north atlantic region we see grain size maxima with ages that match those of the last six heinrich events which we interpret as an indication of the changing strength of the east asian winter monsoon which largely controls the transport and deposition of central asian aeolian dust thus it seems that these heinrich events have left their signature in the chinese loess record this is consistent with simulations of the glacial climate9 which imply that the climates of the north atlantic and china were linked by the effect of westerly winds

orbital forcing of the inception of the laurentide ice sheet according to the astronomical theory of palaeoclimates the glacial interglacial oscillations of the pleistocene have their origin in the insolation forcing caused by the orbital variations of the earth1 the potential response of the climatic system to the resulting insolation perturbations has been investigated mainly using zonally averaged energy balance models ebm 2 6 atmospheric general circulation models gcm include many essential processes such as the hydrological cycle neglected in ebms but because of their high computational cost can be used only for snapshot reconstructions of climate at certain times in the past we show here that the climatic transition at the end of the last interglacial is a particularly relevant period for gcm studies of the sensitivity of climate to the astronomical variations of insolation we have used a low resolution spectral gcm to simulate two annual cycles with the insolation conditions of 125 000 yr bp and 115 000 yr bp for the 115 kyr simulation we find an annual mean cooling over canada of more than 2 k and increased precipitation we suggest that such a pattern of response to the insolation changes would favour the extension of permanent snow cover over the labrador area and could have triggered formation of the laurentide ice sheet

micromorphology and genesis of paleosols in the luochuan loess section china pedostratigraphic and environmental implications the loess paleosol sequence in the well known potou section near luochuan covers the entire pleistocene back to 2 5 ma in a much more complete sequence than any known from europe or north america to deduce a record of the pleistocene climatic history an understanding of the genesis of the paleosols is important main diagnostic features normally observable in surface soils have disappeared in the case of fossil soils therefore micromorphology is of outstanding importance because it enables the reconstruction of original properties and polygenetic soil development especially in pedocomplexes the upper part of the loess paleosol sequence can well be compared with the 18o record shown in the specmap curve of the brunhes epoch although the sequence of paleosols shows partly an even more detailed record of cold and warm stages and allows more specific informations regarding the paleoclimate of the interglacial periods cf fig 2 the last four paleosols represented by the s1 pedocomplex the upper part of the s2 and the s3 are genetically well comparable with the natural holocene soil indicating a similar climate with a tall grass steppe vegetation the s6 is a steppe soil also whereas the five soils of the s4 the s5 complex and the s7 are forest soils indicating more moist and partly warmer interglaciations than the holocene the loess paleosol sequence of the matuyama epoch seems to give more paleoclimatic informations than the deep sea cores known so far the lishi loess below the b m boundary down to 84 m in the section contains two more soils s11 and s13 which are developed under steppe vegetation in interglaciations similar to the present one and four forest soils the s8 complex the upper soil of the s9 and the s10 indicating again more moist interglaciations than the holocene most of the numerous soils in the wusheng loess are probably formed synsedimentarily as alternation of soil formation and addition of loess the red clay at the basis consists of four intensively developed but truncated paleosols the parent material of at least the lower part must be regarded as pliocene loess