a penultimate glacial monsoon record from hulu cave and two phase glacial terminations oxygen isotope records of three stalagmites from hulu cave china extend the previous high resolution absolute dated hulu asian monsoon record from the last to the penultimate glacial and deglacial periods the penultimate glacial monsoon broadly follows orbitally induced insolation variations and is punctuated by at least 16 millennial scale events we confirm a weak monsoon interval between 135 5 1 0 and 129 0 1 0 ka prior to the abrupt increase in monsoon intensity at asian monsoon termination ii based on correlations with both marine ice rafted debris and atmospheric ch 4 records we demonstrate that most of marine termination ii the full rise in antarctic temperature and atmospheric co 2 and much of the rise in ch 4 occurred within the weak monsoon interval when the high northern latitudes were probably cold from these relationships and similar relationships observed for termination i we identify a two phase glacial termination process that was probably driven by orbital forcing in both hemispheres affecting the atmospheric hydrological cycle and combined with ice sheet dynamics

a stable isotope study of cave seepage waters an isotope study 18o 2 h of 62 samples of waters from both the interior of the nerja cave malaga southern spain and the exterior sinkholes and springs at the site was carried out rainfall water in the study area was also studied the mean value of seepage waters can be considered as the mean isotopic value of the precipitation waters of the area the origin and aquifers of these different types of waters has also been studied obtaining a clear meteoric origin both for the seepage waters and the waters from the wells and springs but the aquifer of the latter is different showing more negative isotopic values this suggests that they could have mixed with waters from other aquifers which are either deeper or originate at other altitudes

melt extraction from partially molten regions beneath mid ocean ridges abstract a complete set of two dimensional equations for conservation of mass momentum and energy of a two phase system of melt in a deformable matrix is used to derive analytically a model for the vertical distribution of the melt fraction also a set of asymptotic relations for parameters in the velocity field of the matrix and the melt in the neighbourhood of the symmetry axis of sea floor spreading is obtained these relations have been used to derive solutions appropriate to the melt extraction from the partially molten regions beneath mid ocean ridges and the conditions for their existence the results show the convergence of the melt streamlines towards the spreading center it is caused by 1 a horizontal gradient of the piezometric pressure associated with the deformations of the matrix and 2 an upward increase of the porosity due to melting for accepted values of the relevant parameters a focusing effect of melt towards spreading centers exists thereby providing an explanation for the mid ocean paradox of a narrow region of ridge axis volcanism overlying a broad melt production zone

major features and forcing of high latitude northern hemisphere atmospheric circulation using a 110 000 year long glaciochemical series the greenland ice sheet project 2 glaciochemical series sodium potassium ammonium calcium magnesium sulfate nitrate and chloride provides a unique view of the chemistry of the atmosphere and the history of atmospheric circulation over both the high latitudes and mid low latitudes of the northern hemisphere interpretation of this record reveals a diverse array of environmental signatures that include the documentation of anthropogenically derived pollutants volcanic and biomass burning events storminess over marine surfaces continental aridity and biogenic source strength plus information related to the controls on both high and low frequency climate events of the last 110 000 years climate forcings investigated include changes in insolation of the order of the major orbital cycles that control the long term behavior of atmospheric circulation patterns through changes in ice volume sea level events such as the heinrich events massive discharges of icebergs first identified in the marine record that are found to operate on a 6100 year cycle due largely to the lagged response of ice sheets to changes in insolation and consequent glacier dynamics and rapid climate change events massive reorganizations of atmospheric circulation that are demonstrated to operate on 1450 year cycles changes in insolation and associated positive feedbacks related to ice sheets may assist in explaining favorable time periods and controls on the amplitude of massive rapid climate change events explanation for the exact timing and global synchroneity of these events is however more complicated preliminary evidence points to possible solar variability climate associations for these events and perhaps others that are embedded in our ice core derived atmospheric circulation records

putting the younger dryas cold event into context abstract the younger dryas event is by far the best studied of the millennial scale cold snaps of glacial time yet its origin remains a subject of debate the long held scenario that the younger dryas was a one time outlier triggered by a flood of water stored in proglacial lake agassiz has fallen from favor due to lack of a clear geomorphic signature at the correct time and place on the landscape the recent suggestion that the younger dryas was triggered by the impact of a comet has not gained traction instead evidence from chinese stalagmites suggests that rather than being a freak occurrence the younger dryas is an integral part of the deglacial sequence of events that produced the last termination on a global scale

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

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

instability of glacial climate in a model of the ocean atmosphere cryosphere system in contrast to the relatively stable climate of the past 10 000 years during glacial times the north atlantic region experienced large amplitude transitions between cold stadial and warm interstadial states in this modeling study we demonstrate that hydrological interactions between the atlantic thermohaline circulation thc and adjacent continental ice sheets can trigger abrupt warming events and also limit the lifetime of the interstadial circulation mode these interactions have the potential to destabilize the thc which is already more sensitive for glacial conditions than for the present day climate thus providing an explanation for the increased variability of glacial climate

strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation the climate of the last glacial period was extremely variable characterized by abrupt warming events in the northern hemisphere accompanied by slower temperature changes in antarctica and variations of global sea level it is generally accepted that this millennial scale climate variability was caused by abrupt changes in the ocean thermohaline circulation here we use a coupled ocean atmosphere sea ice model to show that freshwater discharge into the north atlantic ocean in addition to a reduction of the thermohaline circulation has a direct effect on southern ocean temperature the related anomalous oceanic southward heat transport arises from a zonal density gradient in the subtropical north atlantic caused by a fast wave adjustment process we present an extended and quantitative bipolar seesaw concept that explains the timing and amplitude of greenland and antarctic temperature changes the slow changes in antarctic temperature and its similarity to sea level as well as a possible time lag of sea level with respect to antarctic temperature during marine isotope stage 3

lessons learned from oxygen isotopes in modern precipitation applied to interpretation of speleothem records of paleoclimate from eastern asia abstract variability in oxygen isotope ratios collected from speleothems in chinese caves is often interpreted as a proxy for variability of precipitation summer precipitation seasonality of precipitation and or the proportion of 18o to 16o of annual total rainfall that is related to a strengthening or weakening of the east asian monsoon and in some cases to the indian monsoon we use modern reanalysis and station data to test whether precipitation and temperature variability over china can be related to changes in climate in these distant locales we find that annual and rainy season precipitation totals in each of central china south china and east india have correlation length scales of 500 km shorter than the distance between many speleothem records that share similar long term time variations in 18o values thus the short distances of correlation do not support though by themselves cannot refute the idea that apparently synchronous variations in 18o values at widely spaced 500 km caves in china are due to variations in annual precipitation amounts we also evaluate connections between climate variables and 18o values using available instrumental measurements of 18o values in precipitation these data from stations in the global network of isotopes in precipitation gnip show that monthly 18o values generally do not correlate well with either local precipitation amount or local temperature and the degree to which monthly 18o values do correlate with them varies from station to station for the few locations that do show significant correlations between 18o values and precipitation amount we estimate the differences in precipitation amount that would be required to account for peak to peak differences in 18o values in the speleothems from hulu and dongge caves assuming that 18o scales with the monthly amount of precipitation or with seasonal differences in precipitation insofar as the present day relationship between 18o values and monthly precipitation amounts can be applied to past conditions differences of at least 50 in mean annual precipitation would be required to explain the 18o variations on orbital time scales which are implausibly large and inconsistent with published gcm results similarly plausible amplitudes of seasonal cycles in amounts or in seasonal variations in 18o values can account for less than half of the 4 5 difference between glacial and interglacial 18o values from speleothems in china if seasonal cycles in precipitation account for the amplitudes of 18o values on paleoclimate timescales they might do so by extending or contracting the durations of seasons a frequency modulation of the annual cycle but not by simply varying the amplitudes of the monthly rainfall amounts or monthly average 18o values amplitude modulation allowing that several processes can affect seasonal variability in isotopic content we explore the possibility that one or more of the following processes contribute to variations in 18o values in chinese cave speleothems different source regions of the precipitation which bring different values of 18o in vapor different pathways between the moisture source and the paleorecord site along which exchange of 18o between vapor surface water and condensate might differ a different mix of processes involving condensation and evaporation within the atmosphere or different types of precipitation each may account for part of the range of 18o values revealed by speleothems and each might contribute to seasonal differences between past and present that do not scale with monthly or even seasonal precipitation amounts

ice age terminations 230th dated oxygen isotope records of stalagmites from sanbao cave china characterize asian monsoon am precipitation through the ends of the third and fourthmost recent ice ages as a result am records for the past four glacial terminations can now be precisely correlated with those from ice cores and marine sediments establishing the timing and sequence of major events in all four cases observations are consistent with a classic northern hemisphere summer insolation intensity trigger for an initial retreat of northern ice sheets meltwater and icebergs entering the north atlantic alter oceanic and atmospheric circulation and associated fluxes of heat and carbon causing increases in atmospheric co2 and antarctic temperatures that drive the termination in the southern hemisphere increasing co2 and summer insolation drive recession of northern ice sheets with probable positive feedbacks between sea level and co2

possible solar origin of the 1 470 year glacial climate cycle demonstrated in a coupled model dansgaard oeschger events are rapid climate fluctuations that occurred during and at the end of the last ice age with remarkable regularity there were 23 such events between 110 000 and 23 000 years bp with a periodicity of 1 470 years identify the source of this cycle and it should be possible to say what triggered these events the sun had been excluded as a possible cause because of the lack of a 1 470 year spectral contribution in records of solar variability despite this braun et al present a new hypothesis that convincingly explains the 1 470 year period as the net result of two well known solar cycles the devries and gleissberg cycles with periods of 210 years and 87 years many palaeoclimate records from the north atlantic region show a pattern of rapid climate oscillations the so called dansgaard oeschger events with a quasi periodicity of 1 470 years for the late glacial period1 2 3 4 5 6 various hypotheses have been suggested to explain these rapid temperature shifts including internal oscillations in the climate system and external forcing possibly from the sun7 but whereas pronounced solar cycles of 87 and 210 years are well known8 9 10 11 12 a 1 470 year solar cycle has not been detected8 here we show that an intermediate complexity climate model with glacial climate conditions simulates rapid climate shifts similar to the dansgaard oeschger events with a spacing of 1 470 years when forced by periodic freshwater input into the north atlantic ocean in cycles of 87 and 210 years we attribute the robust 1 470 year response time to the superposition of the two shorter cycles together with strongly nonlinear dynamics and the long characteristic timescale of the thermohaline circulation for holocene conditions similar events do not occur we conclude that the glacial 1 470 year climate cycles could have been triggered by solar forcing despite the absence of a 1 470 year solar cycle

interpretation of speleothem calcite δ13c variations evidence from monitoring soil co2 drip water and modern speleothem calcite in central texas we studied the sources and transport of carbon in two active karst systems in central texas inner space cavern is and natural bridge north and south caverns nb to provide new insights into the interpretation of speleothem cave calcite deposit carbon isotope compositions we have determined the 13c values of soil co2 13cs in grassland and savanna above these caves with 13c values of cave drip water 13chco3 and modern speleothem calcite grown on artificial substrates 13ccc we compare 13chco3 values from direct drip sites where water was sampled immediately upon discharging from the cave ceiling to values from indirect sites where water was sampled after flowing along a prolonged path within the cave that allowed for longer co2 degassing and have found that direct drip sites yield consistently lower 13chco3 values the 13chco3 values of direct drip water below savanna 10 6 0 5 and 12 6 0 2 in nb and is respectively are indistinguishable from is or similar to nb calculated 13chco3 values in equilibrium with measured soil co2 beneath trees 13 5 to 11 3 for juniper trees above nb and 13 6 to 12 6 for mixed oak and elm trees above is respectively at is the 13chco3 values of direct drip water are higher below grassland 9 7 0 3 than below savanna 12 6 0 2 these results suggest that the 13chco3 values of drip waters that initially enter the caves are controlled by deep rooted plants where present and are minimally influenced by host rock dissolution and or prior calcite precipitation pcp the 13chco3 values of indirect drip water vary seasonally with relatively low values during the summer 10 8 0 8 and 9 2 0 4 under juniper savanna at nb and under grassland at is respectively that are similar to the direct drip 13chco3 values 10 6 0 5 and 9 7 0 3 under savanna at nb and under grassland at is respectively the relatively high 13chco3 values of indirect drip sites during the winter 13chco3 8 6 0 8 at nb and 8 0 0 1 at is result from co2 degassing of water along in cave flow paths we also present decade long records of modern calcite 13c values from direct and indirect drip sites at is the 13ccc values vary seasonally with lower values during the summer and higher values during the winter and with smaller amplitude variations at the direct drip site such seasonal variations can be used as a geochronological tool in some speleothems that do not contain visible lamina the summer 13ccc values of direct drip calcite are similar to 13ccc values predicted from soil co2 collected beneath trees above that drip site the occurrence of highest 13ccc values during the winter when cave co2 concentrations are low highlights the significance of ventilation driven changes in cave air pco2 modern calcite 13c values are also negatively correlated with drip rate which suggests that 13ccc variations are controlled by kinetic effects during degassing and calcite precipitation associated with the drip water exposure time to a low pco2 environment in all at the caves we investigated variability in speleothem 13ccc values primarily reflect presence absence of deep rooted vegetation and kinetic isotope effects we therefore infer that increased aridity may result in higher 13c values of vegetation lower drip rates and more drip water degassing and thus higher 13ccc values of speleothem calcite

hydraulic interpretation of cross stream variations in bed load transport the hydraulic control of bed load transport rates in nahal yatir and nahal eshtemoa two coarse grained ephemeral channels in the semiarid northern negev israel provides a rare opportunity to infer the spanwise variation in bed shear stress from an analysis of cross stream variations in bed load transport rate automatic sediment transport monitoring stations were used to obtain synchronous measurements of bed load discharge at a number of locations across the widths of two straight channel reaches in both streams channel average bed load fluxes demonstrated a common and well defined response to changing channel average shear stress and approximated the transporting capacity of the flow over much of the range of monitored discharges however transport rates measured at the channel margins are only half those at the channel centerline and at high discharges a marked asymmetry in the pattern of bed load transport develops across the central section of the widest channel this variation in bed load discharge over the two channel cross sections is thought to reflect lateral variations in shear stress induced by sidewall drag and more tentatively the generation and disposition of cellular secondary currents however no systematic relation is found for the ratios of sediment fluxes at off center sampling locations and those recorded at the channel center even though the off center locations are thought to move into and out of the region affected by sidewall drag as aspect ratio of the flow decreases and increases with changing water stage the results suggest that it is difficult to generalize about the changing influence of the sidewall on local shear and bed load as aspect ratio changes during the course of a flood

a minimum thermodynamic model for the bipolar seesaw 1 the simplest possible model is proposed to explain a large fraction of the millennial climate variability measured in the isotopic composition of antarctic ice cores the model results from the classic bipolar seesaw by coupling it to a heat reservoir in this thermal bipolar seesaw the heat reservoir convolves northern time signals with a characteristic timescale applying the model to the data of grip and byrd we demonstrate that maximum correlation can be obtained using a timescale of about 1000 1500 years higher correlations are obtained by first filtering out the long term variability which is due to astronomical and greenhouse gas forcing and not part of the thermal bipolar seesaw the model resolves the apparent confusion whether northern and southern climate records are in or out of phase synchronous or time lagged

a 3000 yr annually laminated stalagmite record of the last glacial maximum from hulu cave china abstract a high resolution annual layer counted and 230 th dated multi proxy record is constructed from a stalagmite in hulu cave china these proxies including 18 o annual layer thickness alt gray level gl and sr ca cover a time span of 3000 yr from 21 to 24 ka the physical proxies alt and gl and the geochemical index sr ca all primarily reflecting karst hydrological processes vary in concert and their coherence is supported by wavelet analyses variations in the 18 o data agree with fluctuations in the alt and sr ca records on multi decadal to centennial scales suggesting that the hulu 18 o signal is strongly associated with varying local rainfall amounts on short timescales a monsoon failure event at 22 2 ka correlates with a decrease in tropical rainfall a reduction in global ch 4 and an ice rafted event in the north atlantic this correlation highlights roles of the asian monsoon and tropical hydrological cycle in modulating global ch 4 because the high latitude emission was inhibited during the last glacial maximum lgm spectral analysis of the 18 o record displays peaks at periodicities of 139 59 53 43 30 23 and 19 15 yr the absence of typical centennial solar cycles may be related to muted changes in ocean circulation during the lgm

stochastic resonance in the north atlantic large abrupt changes between warm and cold modes of north atlantic climate exhibit spectral power at 1500 years yet some climatic changes were linked to outburst floods and other events that are unlikely to have been truly periodic we hypothesize that a weak periodic forcing has combined with noise from ice sheet related events to cause the observed mode switches this stochastic resonance hypothesis predicts a recurrence pattern between warmings that is distinct from the predictions of simple periodic and stochastic models and at least some other models the ice isotopic data from central greenland ice cores are consistent with the stochastic resonance hypothesis but not with other models we have tested we thus support arguments for the existence of a periodicity of 1500 years in the north atlantic climate system and for the importance of ice sheet events in forcing north atlantic changes

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

the eastern mediterranean paleoclimate as a reflection of regional events soreq cave israel the climate of the eastern mediterranean region of the last 60 ky was determined by a high resolution study of the oxygen and carbon isotopic composition 1500 measurement pairs of speleothems from the soreq cave israel with chronology provided by 53 precise 230 th 234 u tims ages the high precision of the speleothem tims ages permits us to determine the timing of regional climatic events in the eastern mediterranean region and to see if they correlate with global events during the period from 60 to 17 ky the 18 o and 13 c values were generally 2 2 5 higher than during the period from 17 ky to present this is consistent with the climatic transition from glacial to interglacial within the 60 to 17 ky period the soreq cave stable isotope profile includes four cold peaks at 46 35 25 and 19 ky and 2 warm peaks at 54 and 36 ky in addition the period 17 ky has two more cold peaks at 16 5 and from 13 2 to 11 4 ky the ages of four of the six cold peaks correlate well with the ages of three heinrich events h1 h2 h5 and with the age of the younger dryas however the other two heinrich events are not reflected in the soreq cave record several other isotope peaks which appear during the last 7 ky are contemporaneous with regional climatic events in the middle east and north africa in addition to the drop in 18 o and 13 c observed between the last glacial and the holocene sharp simultaneous drops in 234 u 238 u 0 ratios sr concentrations and in 87 sr 86 sr are also observed suggesting that the latter are climate related these variations are interpreted in terms of major changes in the temperature the mean annual rainfall and its isotopic composition the isotopic composition of the mediterranean vapor source the soil moisture conditions and in the mixing proportions of sources with different 87 sr 86 sr ratios sea spray dust particles and dolomitic host rock

millennial scale asian monsoon variability during the late marine isotope stage 6 from hulu cave china a precisely 230 th dated stalagmite 13 c profile from hulu cave china is presented to characterize the frequency and pattern of millennial scale asian monsoon am variability from 160 6 to 132 5 ka evidence for an antiphased relationship of the 13 c and 18 o on the millennial scale suggests that the 13 c is indicative of the local hydrological cycle associated with changes in am strength owing to the 13 c responding to am changes more sensitively than the 18 o we could identify 15 strong am events that correlate to cold intervals recorded in antarctic ice cores within 230 th dating uncertainty this result supports a dynamic link of am strength and southern hemispheric climates via the cross equatorial airflows power spectrum analysis shows a predominant periodicity of 1 5 2 5 ka for the 13 c profile similar to the dansgaard oeschger frequency during the last glacial period moreover the am events are characterized by rapid transitions at the onset suggesting that the observed millennial scale am variability is likely forced by northern high latitude climates via north south shifts of the intertropical convergence zone associated with the bipolar seesaw mechanism as evidence for a common mechanism for ice age terminations a strong am event 134 ka surrounding termination ii is analogous to the bolling allerod warming interval

a high resolution monsoon record of millennial scale oscillations during late mis 3 from wulu cave south west china a speleothem record from south western china characterizes in detail the millennial scale changes in asian monsoon am intensity from 39 3 to 28 7 ka the calcite 18o profile with an average resolution of 8 years shows several strong monsoon events concurrent with greenland interstadials gis 8 4 to gain a systematic perspective of am millennial scale variability the new and previously reported data from the same cave are combined showing that am variation exhibits a broad similarity with greenland ice 18o records and with antarctica but in an opposite sense for the interval that encompasses gis 5 and gis 4 1 however our stalagmite 18o record depicts a sustained strong monsoon with no distinctive oscillation between these interstadials another prominent characteristic in our record is a gradual transition into chinese interstadial cis 8 which is well constrained by an annually laminated sequence we find that an initial rise in monsoon intensity lasting a few centuries significantly precedes the abrupt onset of cis 8 in the am realm this suggests that atmospheric moisture and heat transport are probably capable of inducing abrupt climate change when a rapid reorganization of ocean atmosphere circulations passes a tipping point

precise dating of dansgaard oeschger climate oscillations in western europe from stalagmite data the signature of dansgaard oeschger events millennial scale abrupt climate oscillations during the last glacial period is well established in ice cores and marine records but the effects of such events in continental settings are not as clear and their absolute chronology is uncertain beyond the limit of 14 c dating and annual layer counting for marine records and ice cores respectively here we present carbon and oxygen isotope records from a stalagmite collected in southwest france which have been precisely dated using 234u 230th ratios we find rapid climate oscillations coincident with the established dansgaard oeschger events between 83 000 and 32 000 years ago in both isotope records the oxygen isotope signature is similar to a record from soreq cave israel and deep sea records indicating the large spatial scale of the climate oscillations the signal in the carbon isotopes gives evidence of drastic and rapid vegetation changes in western europe an important site in human cultural evolution we also find evidence for a long phase of extremely cold climate in southwest france between 61 2 0 6 and 67 4 0 9 kyr ago