millennial scale climate variability during the last glacial period in the tropical andes millennial scale climate variation during the last glacial period is evident in many locations worldwide but it is unclear if such variation occurred in the interior of tropical south america and if so how the low latitude variation was related to its high latitude counterpart a high resolution record derived from the deep drilling of sediments on the floor of lake titicaca in the southern tropical andes is presented that shows clear evidence of millennial scale climate variation between 60 and 20 ka bp this variation is manifested by alternations of two interbedded sedimentary units the two units have distinctive sedimentary geochemical and paleobiotic properties that are controlled by the relative abundance of terrigenous or nearshore components versus pelagic components the sediments of more terrigenous or nearshore nature likely were deposited during regionally wetter climates when river transport of water and sediment was higher whereas the sediments of more pelagic character were deposited during somewhat drier climates regionally the majority of the wet periods inferred from the lake titicaca sediment record are correlated with the cold events in the greenland ice cores and north atlantic sediment cores indicating that increased intensity of the south american summer monsoon was part of near global scale climate excursions

decadal time scale variability recorded in the quelccaya summit ice core delta o 18 isotopic ratio series and its relation with the sea surface temperature the spectral characteristics of the delta sup 18 o isotopic ratio time series of the quelccaya ice cap summit core are investigated with the multi taper method mtm the singular spectrum analysis ssa and the wavelet transform wt techniques for the 500 y long 1485 1984 period the most significant at the 99 8 level cycle according to the mtm f test has a period centered at 14 4 y while the largest variance explaining oscillation according to the ssa technique has a period centered at 12 9 y the stability over time of these periods is investigated by performing evolutive mtm and ssa on the 500 y long delta sup 18 o series with a 100 y wide moving window it is shown that the cycles with largest amplitude and that the oscillations with largest extracting variance have corresponding periods aggregated around 13 5 y that are very stable over the period between 1485 and 1984 the wt of the same isotopic time series reveals the existence of a main oscillation around 12 y which are also very stable in time the relation between the isotopic data at quelccaya and the annual sea surface temperature sst field anomalies is then evaluated for the overlapping 1919 1984 period significant global correlation and significant coherency at 12 1 y are found between the isotopic series and the annual global sea surface temperature gsst series moreover the correlation between the low over 8 y frequency component of the isotopic time series and the annual sst field point out significant values in the tropical north atlantic this region is characterized by a main sst variability at 12 8 y

present day south american climate abstract this paper documents the main features of the climate and climate variability over south america on the basis of instrumental observations gathered during the 20th century it should provide a modern reference framework for paleoclimate research in south america targeting high resolution proxies over the past few centuries several datasets suitable for present day climate research are first described highlighting their advantages as well as their limitations we then provide a basic physical understanding of the mean annual cycle of the precipitation and atmospheric circulation over the continent and the adjacent oceans in particular the diversity of precipitation temperature and wind patterns is interpreted in terms of the long meridional extent of the continent and the disruption of the large scale circulation caused by the andes cordillera the contrasting oceanic boundary conditions and the landmass distribution similarly the intensity and timing of the interannual and interdecadal climatic fluctuations exhibit considerable geographical dependence as some regions are more influenced by large scale phenomena rooted in the tropical oceans while others are more influenced by high latitude phenomena the impact of these large scale phenomena over south america is documented by a regression analysis between selected atmospheric indices and the precipitation and temperature fields we have included a discussion on the seasonality and long term stability of such impacts and complemented our general description by an updated review of the literature on climate variability over specific regions

millennial scale northwest african droughts related to heinrich events and dansgaard oeschger cycles evidence in marine sediments from offshore senegal 1 we present a suite of new high resolution records 0 135 ka representing pulses of aeolian fluvial and biogenic sedimentation along the senegalese continental margin a multiproxy approach based on rock magnetic element and color data was applied on three cores enclosing the present day northern limit of the itcz a strong episodic aeolian contribution driven by stronger winds and dry conditions and characterized by high hematite and goethite input was revealed north of 13 n these millennial scale dust fluxes are synchronous with north atlantic heinrich stadials fluvial clay input driven by the west african monsoon predominates at 12 n and varies at dansgaard oeschger time scales while marine productivity is strongly enhanced during the african humid periods and marine isotope stage 5 from latitudinal signal variations we deduce that the last glacial itcz summer position was located between core positions at 12 26 and 13 40 n furthermore this work also shows that submillennial periods of aridity over northwest africa occurred more frequently and farther south than previously thought

variability of southwest indian summer monsoon precipitation during the bolling allerod we have generated a high resolution 20 yr 2 3 0 th dated stalagmite oxygen isotope record from timta cave in the western himalaya in india that documents southwest indian summer monsoon ism precipitation variations during the bolling allerod interstadial from 15 2 to 11 7 ka compared with the glacial and younger dryas ism precipitation was enhanced during the bolling allerod ism precipitation was apparently coupled to variations in the east asian monsoon and north atlantic climate on millennial and multicentennial time scales during the deglaciation analyses of a high growth rate interval 2 5 yr resolution encompassing the late bolling early alterod suggest that multidecadal monsoon variability was an important aspect of ism behavior at that time the frequency spectrum of ism precipitation during this time interval is similar to that of the 1 4 c record and other ism precipitation records during the latest holocene this raises the hypothesis that multidecadal climate dynamics during the late boiling early allerod may have been similar to those that operated during the last several millennia even though the boundary conditions of these two time intervals were very different

interhemispheric space time attributes of the dansgaard oeschger oscillations between 100 and 0 ka the persistence stability and interhemispheric phasing of the dansgaard oeschger do climate oscillation over the last glacial period 0 100 ka has been evaluated in oxygen isotope records of three polar ice cores grip gisp2 and byrd station and a midlatitude deep sea core from the north atlantic ocean md95 2042 the results show that do oscillations in atmospheric conditions occurred in both northern and southern polar ice records although in the southern records the oscillations had at most only ca one tenth the power of those in the north the do oscillations first appeared duringmarine isotope stage mis 4 and the average spectral power of the northern hemisphere do oscillations increased markedly during mis 3 30 38 ka the do mode in the gisp2 record is confined to the frequency band 1 1 59 kyr to 1 1 37 kyr but in the grip record the mode exhibits strong frequency splittingover a band that is wider by ca 50 time frequency analysis shows that in grip the do mode underg oes a frequency modulation that is phase locked with the earth s obliquity cycle this modulation does not appear in nearby gisp2 in the north atlantic marine record do oscillations behaved somewhat differently appearingsporadically duringmis 5 and 4 the planktonic do oscillations increased in spectral power duringmis 3 leadingpeak power in the gisp2 record by ca 3 kyr do oscillations were relatively stable in all five records duringmis 3 they could not be detected unequivocally in any of the records duringthe holocene 0 11 ka at other times the do mode in all of the records was amplitude modulated by earth s orbital parameters finally interhemispheric phasingof do oscillations over 10 90 ka was assessed between the byrd station and gisp2 records and between the benthic and planktonic records of core md95 2042 coherency studies reveal an apparent time lead of byrd do oscillations over gisp2 by 384770 yr 2s level and of the north atlantic benthic over planktonic do oscillations by 208733 yr this apparent time lead of the southern over the northern temperature proxies is likely the consequence of the distinctive harmonic shapes affecting the northern rectangular vs southern triangular do oscillations the actual northern southern relationship as suggested by modelling and other recent data studies is most probably in simple antiphase cross phase of 1801 r 2002 published by elsevier science ltd

correlated millennial scale changes in surface hydrography and terrigenous sediment yield inferred from last glacial marine deposits off northeastern brazil abstract the stable isotope composition of planktonic foraminifera correlates with evidence for pulses of terrigenous sediment in a sediment core from the upper continental slope off northeastern brazil stable oxygen isotope records of the planktonic foraminiferal speciesglobigerinoides sacculiferandglobigerinoides ruber pink reveal sub milankovitch changes in sea surface hydrography during the last 85 000 yr warming of the surface water coincided with terrigenous sedimentation pulses that are inferred from high xrf intensities of ti and fe and which suggest humid conditions in northeast brazil these tropical signals correlate with climatic oscillations recorded in greenland ice cores dansgaard oeschger cycles and in sediment cores from the north atlantic heinrich events trade winds may have caused changes in the north brazil current that altered heat and salt flux into the north atlantic thus affecting the growth and decay of the large glacial ice sheets

quaternary glaciation and hydrologic variation in the south american tropics as reconstructed from the lake titicaca drilling project a 136 m long drill core of sediments was recovered from tropical high altitude lake titicaca bolivia peru enabling a reconstruction of past climate that spans four cycles of regional glacial advance and retreat and that is estimated to extend continuously over the last 370 000 yr within the errors of the age model the periods of regional glacial advance and retreat are concordant respectively with global glacial and interglacial stages periods of ice advance in the southern tropical andes generally were periods of positive water balance as evidenced by deeper and fresher conditions in lake titicaca conversely reduced glaciation occurred during periods of negative water balance and shallow closed basin conditions in the lake the apparent coincidence of positive water balance of lake titicaca and glacial growth in the adjacent andes with northern hemisphere ice sheet expansion implies that regional water balance and glacial mass balance are strongly influenced by global scale temperature changes as well as by precessional forcing of the south american summer monsoon

four climate cycles of recurring deep and surface water destabilizations on the iberian margin centennial climate variability over the last ice age exhibits clear bipolar behavior high resolution analyses of marine sediment cores from the iberian margin trace a number of associated changes simultaneously proxies of sea surface temperature and water mass distribution as well as relative biomarker content demonstrate that this typical north south coupling was pervasive for the cold phases of climate during the past 420 000 years cold episodes after relatively warm and largely ice free periods occurred when the predominance of deep water formation changed from northern to southern sources these results reinforce the connection between rapid climate changes at mediterranean latitudes and century to millennial variability in northern and southern polar regions

north atlantic forcing of amazonian precipitation during the last ice age the last glacial period was marked by rapid reorganizations of oceanic and atmospheric circulation speleothem records from the amazon basin suggest that precipitation variability was linked to these events the last glacial period was marked by multiple abrupt reorganizations of ocean and atmosphere circulation1 on thousand year timescales slowing of the atlantic meridional overturning circulation was associated with cooling in the high northern latitudes whereas strengthened circulation was linked to northern warming1 2 in the tropics these millennial scale events were primarily reflected in altered patterns of precipitation3 these hydrologic fluctuations induced ecological changes in the atlantic seaboard and the high andes2 but less is known about the amazon basin here we reconstruct precipitation over amazonian ecuador over the past 94 000 years using a 18o record from speleothems collected in santiago cave in western amazonia we interpret the variability of the 18o record as changes in the source and amount of precipitation with the exception of the period between 40 000 and 17 000 years ago abrupt high frequency changes coincide with shifts in north atlantic circulation indicating a high latitude influence on amazonian precipitation over millennial timescales on longer timescales the record shows a relationship to precessional changes in the earth s orbit in light of the lack of extreme aridity in our records we conclude that ecosystems in western amazonia have not experienced prolonged drying over the past 94 000 years

100 kyr fluvial cut and fill terrace cycles since the middle pleistocene in the southern central andes nw argentina abstract fluvial fill terraces in intermontane basins are valuable geomorphic archives that can record tectonically and or climatically driven changes of the earth surface process system however often the preservation of fill terrace sequences is incomplete and or they may form far away from their source areas complicating the identification of causal links between forcing mechanisms and landscape response especially over multi millennial timescales the intermontane toro basin in the southern central andes exhibits at least five generations of fluvial terraces that have been sculpted into several hundred meter thick quaternary valley fill conglomerates new surface exposure dating using nine cosmogenic 10be depth profiles reveals the successive abandonment of these terraces with a 100 kyr cyclicity between 75 7 and 487 34 ka depositional ages of the conglomerates determined by four 26al 10be burial samples and u pb zircon ages of three intercalated volcanic ash beds range from 18 141 to 936 170 ka indicating that there were multiple cut and fill episodes although the initial onset of aggradation at 1 ma and the overall net incision since ca 500 ka can be linked to tectonic processes at the narrow basin outlet the superimposed 100 kyr cycles of aggradation and incision are best explained by eccentricity driven climate change within these cycles the onset of river incision can be correlated with global cold periods and enhanced humid phases recorded in paleoclimate archives on the adjacent bolivian altiplano whereas deposition occurred mainly during more arid phases on the altiplano and global interglacial periods we suggest that enhanced runoff during global cold phases due to increased regional precipitation rates reduced evapotranspiration or both resulted in an increased sediment transport capacity in the toro basin which outweighed any possible increases in upstream sediment supply and thus triggered incision compared with two nearby basins that record precessional 21 kyr and long eccentricity 400 kyr forcing within sedimentary and geomorphic archives the recorded cyclicity scales with the square of the drainage basin length

climate in the western cordillera of the central andes over the last 4300 years abstract the distichia peat core obtained in the carhuasanta valley near nevado mismi cordillera chila provides information on climatic and environmental conditions over the last 4300 years the relative changes in the stable carbon isotope composition of plant remains preserved in the core reflect major temperature fluctuations in the western cordillera of the southern peruvian andes these temperature variations can be additionally linked with the changes in precipitation patterns by analysing c and c n ratio in the core relatively warm and moist conditions prevailed from 4280 to 3040 cal yrs bp bc 2330 1090 with a short colder dry episode around 3850 cal yrs bp bc 1900 the most prominent climate changes recorded in the peat occurred between 3040 and 2750 cal yrs bp bc 1090 800 when the initial warming turned to a rapid cooling to temperatures at least 2 c lower than the mean for the late holocene initially drier conditions within this event turned to a short wet phase after 2780 cal yrs bp bc 830 when the temperature increased again this event coincides with significant changes in peat and ice core records in the central andes matching the timing of the global climate event around 2 8 cal ka bp climatic conditions in the study area became relatively dry and stable after the event for about 800 years highly variable temperatures and humidity prevailed during the last 2000 years when an extended warm and relatively humid period occurred between 640 and 155 cal yrs bp ad 1310 1795 followed by predominantly colder and drier conditions the established 13 c peat record represents the first continuous proxy for the temperature in the southern peruvian andes dated by the ams 14 c distichia peat is wide spread in the andes and the proposed approach can be applied elsewhere in high altitudes where no other traditional climate proxies are available

the role of north brazil current transport in the paleoclimate of the brazilian nordeste margin and paleoceanography of the western tropical atlantic during the late quaternary abstract reconstructions of surface paleoceanographic conditions of the western equatorial atlantic and past climates of the adjacent northeast brazilian the nordeste continental margin were undertaken by analyzing sediments from a piston core and associated gravity and box cores recovered from 3107 meter water depth at 0 20 n on the equatorial brazilian continental slope the record is dated by radiocarbon analysis and oxygen isotopic stratigraphy of planktonic foraminifers and spans from near modern to approximately 110 ka high resolution xrf analysis provides insight into the paleoclimate history of the nordeste during the last glacial interval several large amplitude and abrupt peaks are observed in the time series of ti ca and are usually accompanied by peaks of fe k together these record periods of increased precipitation and intense weathering on the adjacent continent and increased terrestrial sediment discharge from nordeste rivers into the atlantic within the limits of dating accuracy most ti ca peaks correlate with heinrich events in the north atlantic this record thus corroborates and extends back in time the previous record of arz et al 1998 determined on sediment cores from farther southeast along the nordeste margin stable oxygen isotopic analysis and mg ca paleothermometry on the near surface dwelling planktonic foraminiferal species globierinoides ruber find that mean sea surface temperature sst during glacial time 20 55 ka n 97 was 23 89 0 79 c and the mean sst during the late holocene 0 5 ka n 14 was 26 89 0 33 c ssts were 0 5 2 c higher and inferred sea surface salinities were lower during most of the periods of elevated ti ca thus as observed in previous studies the western equatorial atlantic was warm at least locally and the adjacent southern tropical continent was wet at the same time that the high latitude north atlantic was cold using the syntrace ccsm3 fully coupled climate model with transient forcing for the period 22 ka to present we find that decreased transport of the north brazil current co occurs with reduced atlantic meridional overturning circulation and colder than normal ssts in the north atlantic region these simulated conditions are invariably associated with significantly increased precipitation in the nordeste region

late pleistocene climate conditions in the north chilean andes drawn from a climate glacier model a climate glacier model was used to reconstruct late glacial climate conditions from two case study glaciers at 18 and 22 s in the arid sub tropical western andes of northern chile the model uses i the geometry of the late glacial maximum glaciation ii modern diurnal and annual cycles amplitudes and lapse rates of the climate iii empirical statistical sublimation melt and accumulation models developed for this area and iv dynamic ice flow through two known cross sections for steady state conditions the model is validated with modern conditions and compares favorably with the glaciological features of today the mass balance model calculates the modern equilibrium line altitude at 18 s as high as 5850 m field data 5800 m whereas no glaciers exist in the fully arid southern area at 22 s despite altitudes above 6000 m and continuous permafrost for late glacial times the model results suggest a substantial increase in tropical summer precipitation ap 840 50 10 mm a 1 for the northern test area 1000 10 30 mm a 1 for the southern test area and a moderate temperature depression t 4 4 0 1 0 2 c at 18 s 3 2 0 1 c at 22 s extratropical frontal winter precipitation june september was 15 of the total annual precipitation a scenario with higher winter precipitation from the westerlies circulation belt does not yield a numerical solution which matches the observed geometry of the glaciers therefore we conclude that an equatorward displacement of the westerlies must be discarded as a possible explanation for the late pleistocene glaciation in the andes of northern chile

millennial scale precipitation changes in southern brazil over the past 90 000 years 1 a u th dated 90 000 year long speleothem oxygen isotope record from southern brazil anti correlates remarkably with the cave calcite records from eastern china but positively correlates with the speleothem record from northeastern brazil suggesting an interhemispheric anti phasing of rainfall on both millennial and orbital timescales likely related to displacement in the mean position of the intertropical convergence zone and associated asymmetry in hadley circulation the phase relationships among these records are consistent with the hypothesis that abrupt climate events during the last glacial period are triggered by oceanic circulation changes in the high latitudes and enhanced by tropical feedbacks

interhemispheric anti phasing of rainfall during the last glacial period abstract we have obtained a high resolution oxygen isotopic record of cave calcite from caverna botuvera 27 13 s 49 09 w southern brazil which covers most of the last 36 thousand years ka with an average resolution of a few to several decades the chronology was determined with 46 u th ages from two stalagmites tests for equilibrium conditions show that oxygen isotopic variations are primarily caused by climate change we interpret our record in terms of meteoric precipitation changes hence the variability of south american monsoon sam intensity the oxygen isotopic profile broadly follows local insolation changes and shows clear millennial scale variations during the last glacial period with amplitudes as large as 3 but with smaller centennial scale shifts 1 during the holocene the overall record is strikingly similar to but strongly anti correlated with a number of records from the northern hemisphere we compared our record to other precisely dated contemporaneous records from hulu cave eastern china minima in 18o wet periods intense sam at our site are synchronous with maxima in 18o dry periods weak east asian monsoon eam in eastern china within precise dating errors and vice versa this anti phased precipitation relationship between two low latitude locations may be interhemispheric in extent based on comparison with records from other sites precipitation anti phasing may be related to north south shifts in the mean position of the intertropical convergence zone itcz and asymmetry in hadley circulation in two hemispheres associated not with seasonal changes as observed today but with millennial scale climate shifts the millennial scale atmospheric see saw patterns that we observe could have important controls and feedbacks on climate within hemispheres because of water vapor s greenhouse properties

does a monsoon climate exist over south america the climatology and the basic state of the summertime circulation and rainfall over south america are studied using assimilation products from the data assimilation system of goddard earth observing system 1 geos1 and satellite derived rainfall results indicate the existence of a regional summer monsoon circulation regime induced by strong diabatic heating over the subtropical south american highland centered at the altiplano plateau sensitivity of the results to the assimilation scheme is tested by comparing that with the national centers for environmental prediction ncep reanalysis and with satellite rainfall estimates results show general agreement between the model produced rainfall anomaly and the satellite estimates as well as consistency between the basic circulation features in the geos 1 and the ncep reanalyses a case study of 1989 90 south american summer monsoon sasm reveals the following characteristics 1 in late spring the onset of sasm is signaled by an abrupt merging of the upper tropospheric double westerly jets one in the subtropics and the other in the subpolar region into a single jet in the midlatitudes this is followed by the establishment of a vortex to the southeast of altiplano and occurrence of heavy precipitation over subtropical eastern brazil 2 during the mature phase of sasm the heavy rainfall zone moves over the altiplano plateau and the southernmost brazilian highland the fully established sasm features are the following a an enhancement of equatorial north atlantic trade wind which emanates from the sahara high and crosses the equator over the south american continent b a buildup of strong northwesterlies along the eastern side of the tropical andes and c development of the south atlantic convergence zone in the southernmost position with strong convective activity meanwhile the upper tropospheric return flow emerges from an anticyclone formed over the altiplano plateau crosses the equator and sinks over northwestern africa 3 the withdrawal of sasm in late summer is signaled by the resplitting of the midlatitude westerly jet at the same time the low level northwest monsoon flow diminishes reducing the moisture supply and leading to the termination of heavy precipitation over the subtropical highland results also show that the above mentioned characteristics of sasm are clearly linked to the tropospheric temperature changes over the central south american highland sensible versus latent heating over the highland are bound to play an important role in the evolution of sasm to provide further support of presence of a monsoon climate over south america sasm is compared and contrasted to the classic east asian summer monsoon easm many similar features including evolution characteristics between the two systems have been identified contrasting aspects of the sasm from the easm are also discussed it is pointed out that a number of monsoonal characteristics of the climate of south america such as the seasonal reversal of the low level wind become apparent only when the strong annual mean wind is removed based on the characteristic features and their evolution the authors conclude that a monsoon climate does exist over south america

nature and causes of quaternary climate variation of tropical south america abstract this selective review of the quaternary paleoclimate of the south american summer monsoon sasm domain presents viewpoints regarding a range of key issues in the field many of which are unresolved and some of which are controversial 1 el nino southern oscillation variability while the most important global scale mode of interannual climate variation is insufficient to explain most of the variation of tropical south american climate observed in both the instrumental and the paleoclimate records 2 significant climate variation in tropical south america occurs on seasonal to orbital i e multi millennial time scales as a result of sea surface temperature sst variation and ocean atmosphere interactions of the tropical atlantic 3 decadal scale climate variability linked with this tropical atlantic variability has been a persistent characteristic of climate in tropical south america for at least the past half millennium and likely far beyond 4 centennial to millennial climate events in tropical south america were of longer duration and perhaps larger amplitude than any observed in the instrumental period which is little more than a century long in tropical south america these were superimposed upon both precession paced insolation changes that caused significant variation in sasm precipitation and eccentricity paced global glacial boundary conditions that caused significant changes in the tropical south american moisture balance as a result river sediment and water discharge increased and decreased across tropical south america lake levels rose and fell paleolakes arose and disappeared on the altiplano glaciers waxed and waned in the tropical andes and the tropical rainforest underwent significant changes in composition and extent to further evaluate climate forcing over the last glacial cycle 125 ka we developed a climate forcing model that combines summer insolation forcing and a proxy for north atlantic sst forcing to reconstruct long term precipitation variation in the sasm domain the success of this model reinforces our confidence in assigning causation to observed reconstructions of precipitation in addition we propose a critical correction for speleothem stable oxygen isotopic ratios which are among the most significant of paleoclimate proxies in tropical south america for reconstruction of variation of paleo precipitation or sasm intensity however it is already well known that any particular 18o value observed in speleothem carbonate is affected by two processes that have nothing to do with changes in precipitation amount the influence of temperature on carbonate water isotopic fractionation in the cave and the influence of changing 18o of seawater quantitatively accounting for both artifacts can significantly alter the interpretations of speleothem records in tropical south america both adjustments act in the same direction and have the tendency to increase the true amplitude of the paleo hydrologic signal but by different amounts in glacial and inter glacial stages these corrections have even graver implications for the interpretation of tropical northern hemisphere speleothem records e g chinese speleothems where the combined adjustments tend to decrease or even eliminate the true signal amplitude

155 000 years of west african monsoon and ocean thermal evolution a detailed reconstruction of west african monsoon hydrology over the past 155 000 years suggests a close linkage to northern high latitude climate oscillations ba ca ratio and oxygen isotope composition of planktonic foraminifera in a marine sediment core from the gulf of guinea in the eastern equatorial atlantic eea reveal centennial scale variations of riverine freshwater input that are synchronous with northern high latitude stadials and interstadials of the penultimate interglacial and the last deglaciation eea mg ca based sea surface temperatures ssts were decoupled from northern high latitude millennial scale fluctuation and primarily responded to changes in atmospheric greenhouse gases and low latitude solar insolation the onset of enhanced monsoon precipitation lags behind the changes in eea ssts by up to 7000 years during glacial interglacial transitions this study demonstrates that the stadial interstadial and deglacial climate instability of the northern high latitudes exerts dominant control on the west african monsoon dynamics through an atmospheric linkage

mechanisms of abrupt climate change of the last glacial period 1 more than a decade ago ice core records from greenland revealed that the last glacial period was characterized by abrupt climate changes that recurred on millennial time scales since their discovery there has been a large effort to determine whether these climate events were a global phenomenon or were just confined to the north atlantic region and also to reveal the mechanisms that were responsible for them in this paper we review the available paleoclimate observations of abrupt change during the last glacial period in order to place constraints on possible mechanisms three different mechanisms are then reviewed ocean thermohaline circulation sea ice feedbacks and tropical processes each mechanism is tested for its ability to explain the key features of the observations particularly with regard to the abruptness millennial recurrence and geographical extent of the observed changes it is found that each of these mechanisms has explanatory strengths and weaknesses and key areas in which progress could be made in improving the understanding of their long term behavior both from observational and modeling approaches are suggested finally it is proposed that a complete understanding of the mechanisms of abrupt change requires inclusion of processes at both low and high latitudes as well as the potential for feedbacks between them some suggestions for experimental approaches to test for such feedbacks with coupled climate models are given

the timing and magnitude of mountain glaciation in the tropical andes the andes of ecuador peru and bolivia host the majority of the world s tropical glaciers in the tropical andes glaciers accumulate during the wet season austral summer and ablate year round precipitation is delivered mainly by easterlies and decreases both n s and e w chronological control for the timing of glacial advances in the tropical andes varies in ecuador six to seven advances have been identified dating is based on radiocarbon ages timing of the local last glacial maximum lgm and the existence of younger dryas advances remain controversial in peru local variability in glaciation patterns is apparent surface exposure dating in the cordillera blanca and junin plain suggests that the local lgm may have been early 30 ka although uncertainties in age calculations remain the local lgm was followed by a lateglacial readvance stillstand and preceded by larger glaciations in contrast preliminary data from an intervening massif indicate that the largest moraines are lateglacial chronologies from bolivia also suggest local variability in leeward milluni and san francisco valleys local lgm moraines descend to 4300 m above sea level a s l whereas in windward zongo valley lateglacial moraines reach 3400 m a s l atlantic and pacific sea surface temperatures el nino southern oscillation and insolation changes all likely play roles in mediating tropical andean glacial cycles copyright 2008 john wiley sons ltd additional supporting information may be found in the online version of this article

transient simulation of last deglaciation with a new mechanism for bolling allerod warming we conducted the first synchronously coupled atmosphere ocean general circulation model simulation from the last glacial maximum to the bolling allerod ba warming our model reproduces several major features of the deglacial climate evolution suggesting a good agreement in climate sensitivity between the model and observations in particular our model simulates the abrupt ba warming as a transient response of the atlantic meridional overturning circulation amoc to a sudden termination of freshwater discharge to the north atlantic before the ba in contrast to previous mechanisms that invoke amoc multiple equilibrium and southern hemisphere climate forcing we propose that the ba transition is caused by the superposition of climatic responses to the transient co 2 forcing the amoc recovery from heinrich event 1 and an amoc overshoot

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

a 45 kyr palaeoclimate record from the lowland interior of tropical south america we present a well dated high resolution 45 kyr lake sediment record reflecting regional temperature and precipitation change in the continental interior of the southern hemisphere sh tropics of south america the study site is laguna la gaiba llg a large lake 95 km2 hydrologically linked to the pantanal an immense seasonally flooded basin and the world s largest tropical wetland 135 000 km2 lake level changes at llg are therefore reflective of regional precipitation we infer past fluctuations in precipitation at this site through changes in i pollen inferred extent of flood tolerant forest ii relative abundance of terra firme humid tropical forest versus seasonally dry tropical forest pollen types and iii proportions of deep versus shallow water diatoms a probabilistic model based on plant family and genus climatic optima was used to generate quantitative estimates of past temperature from the fossil pollen data our temperature reconstruction demonstrates rising temperature by 4 c at 19 5 kyr bp synchronous with the onset of deglacial warming in the central andes strengthening the evidence that climatic warming in the sh tropics preceded deglacial warming in the northern hemisphere nh by at least 5 kyr we provide unequivocal evidence that the climate at llg was markedly drier during the last glacial period 45 0 12 2 kyr bp than during the holocene contrasting with sh tropical andean and atlantic records that demonstrate a strengthening of the south american summer monsoon during the global last glacial maximum 21 kyr bp in tune with the 20 kyr precession orbital cycle holocene climate conditions occurred as early as 12 8 12 2 kyr bp when increased precipitation in the pantanal catchment caused heightened flooding and rising lake levels in llg in contrast to this strong geographic variation in lgm precipitation across the continent expansion of tropical dry forest between 10 and 3 kyr bp at llg strengthens the body of evidence for widespread early mid holocene drought across tropical south america

the history of south american tropical precipitation for the past 25 000 years long sediment cores recovered from the deep portions of lake titicaca are used to reconstruct the precipitation history of tropical south america for the past 25 000 years lake titicaca was a deep fresh and continuously overflowing lake during the last glacial stage from before 25 000 to 15 000 calibrated years before the present cal yr b p signifying that during the last glacial maximum lgm the altiplano of bolivia and peru and much of the amazon basin were wetter than today the lgm in this part of the andes is dated at 21 000 cal yr b p approximately coincident with the global lgm maximum aridity and lowest lake level occurred in the early and middle holocene 8000 to 5500 cal yr b p during a time of low summer insolation today rising levels of lake titicaca and wet conditions in amazonia are correlated with anomalously cold sea surface temperatures in the northern equatorial atlantic likewise during the deglacial and holocene periods there were several millennial scale wet phases on the altiplano and in amazonia that coincided with anomalously cold periods in the equatorial and high latitude north atlantic such as the younger dryas

distichia peat a new stable isotope paleoclimate proxy for the andes article i nfo global climate variability is a well documented fact however the human contribution to climate change is now being vigorously debated therefore a better understanding of past natural climate variability may help to establish the actual anthropogenic contribution to the observed climatic trend a variety of high resolution proxies now exist for documenting climate variability that has occurred in the northern hemisphere over the last 10 ka in contrast high resolution paleoclimate records are more limited for regions such as high altitudes in the andes south america however many regions of the andes contain a rich but as yet overlooked paleoclimate archive in the form of thick peat deposited in situ by the distichia plant in our study based on altitudinal transect from the peruvian andes we found a statistically significant and strong relationship between the stable carbon isotope composition of distichia and air temperature r 0 92 pb0 01 we also confirmed good preservation of relative differences in the original stable carbon isotope composition in peat derived from this plant our calibration showed that a decrease of 0 97 0 23 in the stable carbon isotope composition of distichia peat reflects a 1 c increase in mean air temperature of the growing seasons this relationship can be used as a new high resolution proxy for reconstruction of paleotemperature variations over the past several thousand years in the andes mountains based on distichia peat cores

a 60 000 year record of hydrologic variability in the central andes from the hydrogen isotopic composition of leaf waxes in lake titicaca sediments abstract a record of the hydrogen isotopic composition of terrestrial leaf waxes d wax in sediment cores from lake titicaca provides new insight into the precipitation history of the central andes and controls of south american summer monsoon sasm variability since the last glacial period comparison of the d wax record with a 19 kyr d record from the nearby illimani ice core supports the interpretation that precipitation d is the primary control on d wax with a lesser but significant role for local evapotranspiration and other secondary influences on d wax the titicaca d wax record confirms overall wetter conditions in the central andes during the last glacial period relative to a drier holocene during the last deglaciation abrupt d wax shifts correspond to millennial scale events observed in the high latitude north atlantic with dry conditions corresponding to the bolling allerod and early holocene periods and wetter conditions during late glacial and younger dryas intervals we observe a trend of increasing monsoonal precipitation from the early to the late holocene consistent with summer insolation forcing of the sasm but similar hydrologic variability on precessional timescales is not apparent during the last glacial period overall this study demonstrates the relative importance of high latitude versus tropical forcing as a dominant control on glacial sasm precipitation variability

a semi analytical model for multi stage fractured horizontal wells summary multi stage fractured horizontal wells are widely applied when developing tight reservoirs and shale gas reservoirs as such testing and evaluating fractured horizontal wells productivity become necessary for further improving the wells performance analyzing post fracturing transient pressure data provides estimation of some key parameters that affect the productivity including effective fracture lengths fracture conductivities fracture skin factors and average formation permeability this paper presents a semi analytical model based on green s functions and the source sink method to facilitate the transient pressure analysis for a multi stage fractured horizontal well in a closed box shaped reservoir four kinds of fluid flow in the multi stage fractured horizontal well system including fluid from the reservoir to the fractures and from the reservoir to the horizontal wellbore fluid inside the fractures as well as fluid flow inside the horizontal wellbore are all taken into consideration compared with previous models this study considers the pressure drops caused by pipe flow inside the wellbore for the model s accuracy and robustness the fractures and the horizontal wellbore are discretized into vertical plane segments and horizontal line segments respectively correspondingly the fluid flow from the reservoir to the fracture as well as the fluid flow directly from the reservoir to the horizontal wellbore at each segment are modeled by analytical solutions of vertical plane source and horizontal line source respectively in addition the fluid flow inside the fractures is modeled as 1 d linear flow the fluid flow inside the horizontal wellbore is described with penmatcha and aziz s model 1999 finally interface flux and pressure continuity conditions are used to couple the equations of aforementioned four kinds of fluid flow the effects of fluid flow directly from the reservoir to the horizontal wellbore fracture spacing fracture lengths fracture conductivities and fracture skin factors on the transient pressure behavior are studied and type curves are generated the results suggest that in a tight or shale gas reservoir fracture stage fracture lengths conductivities and skin factors have significant influence on the transient pressure behavior during a testing period while the fluid flow directly from the reservoir into the horizontal wellbore reduces the pressure drops slightly then a field case is analyzed and reliable results are obtained this model can be further applied to optimize the fracture spacing and fracture lengths for a multi stage fractured horizontal well

low latitude ice cores record pacific sea surface temperatures matic records suggested that a more conventional temperature interpretation might be more appropriate with lower isotopic values during the presumed to be cooler little ice age and increasingly enriched precipitation during the 20th century warming phase thompson 2001 3 both of these conflicting interpretations suffer from inadequate calibration of the observed isotopic record in relation to climatic conditions during periods of snow accumulation on the ice caps here we use the results of on site meteorological measurements to determine the seasonality of the ice core record and then assess how the isotopic records relate to temperature precipitation and the larger scale circulation associated with snowfall at these high elevation sites we then extend the analysis to ice core records from the asian monsoon region this empirical approach to the interpretation of the isotopic record in low latitude ice cores complements theoretical studies by broecker 1997 and pierrehumbert 1999

indian ocean climate and an absolute chronology over dansgaard oeschger events 9 to 13 oxygen isotope ratios of a stalagmite from socotra island in the indian ocean provide a record of changes in monsoon precipitation and climate for the time period from 42 to 55 thousand years before the present the pattern of precipitation bears a striking resemblance to the oxygen isotope record from greenland ice cores with increased tropical precipitation associated with warm periods in the high northern latitudes the largest change at the onset of interstadial 12 occurred very rapidly in about 25 years the chronology of the events found in our record requires a reevaluation of previously published time scales for climate events during this period

sensitivity of the atlantic intertropical convergence zone to last glacial maximum boundary conditions 1 recent paleoproxy records suggest that the mean latitude of the atlantic intertropical convergence zone itcz varied synchronously with north atlantic climate over a range of timescales throughout the holocene and last glacial maximum we show that the present day meridional mode of atmosphere ocean variability in the tropical atlantic is a potentially useful model for understanding these paleoclimate changes the tropical atlantic in a coupled atmospheric general circulation and slab ocean model responds to last glacial maximum conditions with a southward displacement of the itcz this response arises primarily through the land ice sheet that forces increased north atlantic trades analogous to the forcing on the present day meridional mode changes to sea ice coverage and to ocean heat transport associated with a weakened atlantic thermohaline circulation also cause a meridional mode response though through different mechanisms our results highlight the potential for tropical atlantic paleoclimate to be driven from the high latitude influences in particular land ice on glacial interglacial timescales

timing of the late quaternary glaciation in the andes from 15 to 40 s the central andes play a pivotal role for glacier and climate reconstruction along the american pole equator pole transect nevertheless knowledge about late quaternary palaeoenvir onmental changes in this region is extremely limited with the advent and application of surface exposure dating during the last few years the establishment of more detailed glacial chronologies could provide important insights into forcings and mechanisms of glaciation and climate change this paper reviews previously published exposure ages and compares them with independent age control on glacial chronologies and with information about the palaeohydrological conditions although available data are still very limited and there are remaining systematic uncertainties related to surface exposure dating the following simplified palaeoglacial palaeoclimate model is presented to serve as a testable hypothesis for future studies i glaciers in the humid tropical andes were mainly temperature sensitive and therefore advanced during temperature minima advances are dated to 20 25 ka 15 ka and 11 13 ka i e synchronous to the global last glacial maximum lgm the heinrich event 1 and the younger dryas antarctic cold reversal ii further south and west precipitation decreases and glaciers therefore become more precipitation sensitive maximum or at least prominent glacial advances as far south as 308 s are dated into the lateglacial i e synchronous with lake transgression phases tauca 14 18 ka and coipasa 11 13 ka iii between 30 and 408 s glaciers reached their maximum extent much earlier during the pre lgm 35 40 ka this is attributed to a northward shift and or intensification of the westerlies whereas conditions during the global lgm were too dry to allow for significant glacial advances iv south of 408 glaciers become temperature sensitive again and reached their maximum accordingly synchronous to the global lgm copyright 2008 john wiley sons ltd

a 370 000 year record of vegetation and fire history around lake titicaca bolivia peru fossil pollen and charcoal analyses of sediments from lake titicaca peru bolivia provide a record of palaeoclimatic variation spanning four full glacial cycles pollen aquatic microfossils and charcoal as well as previously published data including diatom assemblages carbonate content and stable carbon isotopic ratios of organic carbon indicate that interglacials were warm and dry whereas the peaks of glacials were cold and wet each of the interglacials documented in the record are somewhat different with those of mis 5e and mis 9 inducing lower lake levels and a drier vegetation signature than those of mis 7 and 1 the presence of charcoal particles in sediments deposited during previous interglacials provides evidence of the long term role of fire in shaping andean ecosystems

early warming of tropical south america at the last glacial interglacial transition glaciation in the humid tropical andes is a sensitive indicator of mean annual temperature here we present sedimentological data from lakes beyond the glacial limit in the tropical andes indicating that deglaciation from the last glacial maximum led substantial warming at high northern latitudes deglaciation from glacial maximum positions at lake titicaca peru bolivia 16 s and lake junin peru 11 s occurred 22 000 to 19 500 calendar years before the present several thousand years before the bolling allerod warming of the northern hemisphere and deglaciation of the sierra nevada united states 36 5 to 38 n the tropical andes deglaciated while climatic conditions remained regionally wet which reflects the dominant control of mean annual temperature on tropical glaciation

a late quaternary diatom record of tropical climatic history from lake titicaca peru and bolivia abstract a composite high resolution diatom stratigraphy from three piston cores and one box core in the deep sub basin of lake titicaca reveals large moisture variations during the past 30 kyr in the altiplano region diatom sequences indicate orbital and millennial scale variability in water level and salinity the pelagic freshwater diatom species cyclotella andina and cyclotella stelligera dominate glacial age sediments suggesting that the lake was above its present outlet generally wet conditions continued until 11 000 cal yr bp as indicated by high percentages of freshwater planktonic diatoms large pulses of benthic diatom species between about 11 000 and 10 000 cal yr bp suggest brief intervals of large amplitude declines in lake level during the early holocene 10 000 8500 cal yr bp a freshwater diatom assemblage suggests overflowing conditions pelagic freshwater diatoms are replaced ca 8500 cal yr bp by the salinity indifferent species cyclotella meneghiniana and by benthic taxa indicating the beginning of lake regression during the mid holocene 6000 3500 cal yr bp the abundance of the saline taxon chaetoceros muelleri coupled with high abundances of epiphytic and epipelic diatoms indicates maximum salinity and lowest lake levels in the entire 30 000 year record lake transgression began ca 4000 cal yr bp and the lake achieved modern levels by about 1500 cal yr bp these water level changes imply changes in effective moisture most likely resulting from large precipitation changes precipitation was high throughout the last glacial maximum 21 000 18 000 cal yr bp likely due to an enhanced south american summer monsoon during peak summer insolation in the southern hemisphere in contrast the mid holocene transition was dryer than today in association with an austral summer insolation minimum and the subsequent weakening of the summer monsoon

episodic eolian deposition in the past ca 50 000years in the alto ilo dune field southern peru abstract the alto ilo dune field in the coastal desert of southern peru is located at the edge of the atacama desert and is one of the driest landscapes on earth a stratigraphic sequence of eolian sands paleosols and debris flow deposits are identified in this dune field spanning the past 55 ka we infer that eolian deposition is preceded by an increase in sediment supply from local alluvial fans that are upwind of the alto ilo dune field and thus reflects periods of relative hydrologic excess eolian sediment supply may increase also with the concomitant fall in sea level providing an additional foreland source area for eolian particles the presence of paleosols indicate millennial scale periods of locally wetter conditions possibly associated with spread of lomas vegetation fed from coastal fog sources debris flows reflect significant rainfall events possibly associated with enso variability the chronology is provided by optically stimulated luminescence using multiple aliquot regeneration protocols which reveal four major eolian depositional episodes at ca 55 to 45 ka 38 to 27 ka 22 to 16 ka and 12 ka eolian deposition in the late pleistocene dominates the record and is generally coincident with metrics of hydrologic excess from the peruvian andes and the altiplano which appears to translate into enhanced sediment availability on the coast a significant period of soil development between 18 and 13 ka is consistent with timing of central andean pluvial event and indicates widespread wet conditions the holocene record of eolian deposition is sparse reflecting limited alluvial fan sources and decreasing particle availability upwind with the rise in postglacial sea level

uplift rate transients at subduction margins due to earthquake clustering coastal uplift is common in continental fore arc systems with elevated paleoshorelines indicating that uplift rates can vary dramatically over time on individual margins the origins of these changes in uplift rates are examined using a global data set of paleoshorelines together with 2 d numerical models of subduction systems empirical paleoshoreline data n 282 from eight subduction margins indicate that uplift rates are generally not steady state and varied by up to a factor of 20 during the late quaternary 125 ka on many subduction margins uplift rates increase to the present day a finding which we attribute to sampling bias toward those locations where holocene uplift rates have been highest with respect to other global margins which have undergone fast subsidence or no vertical motion e g a property akin to the so called sadler effect paleoshorelines and 2 d models suggest that transient uplift rates at subduction margins are mainly a short term 20 ka phenomenon that cannot be accounted for by plate boundary scale processes such as changes in the rates of plate convergence sediment underplating or isostatic unloading instead time variable uplift rates are ascribed to temporal clustering of large magnitude earthquakes on upper plate faults and to a lesser extent the subduction thrust the potential for future damaging earthquakes and tsunamis may have been underestimated at active subduction margins with no measureable holocene uplift and in such cases late quaternary paleoshorelines could provide an important constraint for hazard analysis

a new estimate of the holocene lowstand level of lake titicaca central andes and implications for tropical palaeohydrology new evidence from piston cores and high resolution seismic reflection data shows that water levels in lake titicaca were as much as 100 m below the present level during the early to mid holocene between 6 and 3 8 14c kyr bp climatological and modelling studies indicate that lake titicaca rainfall depends on convective activity in upwind amazonia the lake level data therefore suggest a drier amazon basin during this time this view is bolstered by an excellent match between the titicaca lake level curve and decreased methane concentrations in greenland ice previously ascribed to drying of low latitude wetlands blunier et al 1995 the postglacial history of lake titicaca fits a global pattern of lake level change in the tropics characterized by opposite phasing between the southern and northern hemispheres this pattern is most likely the result of orbital controls over the intensity of summer insolation

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

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

abrupt change in tropical african climate linked to the bipolar seesaw over the past 55 000 years 1 the tropics play a major role in global climate dynamics and are vulnerable to future climate change we present a record of east african climate since 55 ka preserved in lake malawi sediments that indicates rapid shifts between discrete climate modes related to abrupt warming d o events observed in greenland although the timing of the malawi events cannot be determined exactly our age model implies that they occur prior to their greenland counterparts consistent with southward excursions of the intertropical convergence zone during greenland stadials the magnitude of each of the events recorded in malawi sediments corresponds to the scale of the subsequent greenland warming this suggests that a tropical component of climate sets a template for abrupt high northern latitude climate fluctuations associated with the bipolar seesaw

holocene hydrologic variation at lake titicaca bolivia peru and its relationship to north atlantic climate variation a growing number of sites in the northern hemisphere show centennial to millennial scale climate variation that has been correlated with change in solar variability or with change in north atlantic circulation however it is unclear how or whether these oscillations in the climate system are manifest in the southern hemisphere because of a lack of sites with suitably high sampling resolution in this paper we reconstruct the lake level history of lake titicaca using the carbon isotopic content of sedimentary organic matter to evaluate centennial to millennial scale precipitation variation and its phasing relative to sites in the northern hemisphere the pattern and timing of lake level change in lake titicaca is similar to the ice rafted debris record of holocene bond events demonstrating a possible coupling between precipitation variation on the altiplano and north atlantic sea surface temperatures ssts the cold periods of the holocene bond events correspond with periods of increased precipitation on the altiplano holocene precipitation variability on the altiplano is anti phased with respect to precipitation in the northern hemisphere monsoon region more generally the tropical andes underwent large changes in precipitation on centennial to millennial timescales during the holocene copyright 2005 john wiley sons ltd

late quaternary glaciation of the cordillera real bolivia glacial geological studies in tropical mountain areas of the southern hemisphere can be used to address two issues of late pleistocene climate change the global synchroneity of deglaciation and the magnitude of temperature reduction in the tropics radiocarbon dates from the cordillera real and from other areas in peru and bolivia suggest that late pleistocene glaciation culminated between 14 000 and 12 000 yr bp followed by rapid deglaciation because deglaciation was apparently synchronous with that in northern hemisphere regions insolation change at high latitudes may not have been the only factor that produced global deglaciation at this time late pleistocene glaciation in the cordillera real culminated when precipitation was 200 mm yr 1 higher and temperatures were 3 5 1 6 c lower than today this produced an equilibrium line altitude depression of about 300 100 m on the western side of the cordillera prior to this conditions were drier and probably at least as cold however the lack of moraines in the cordillera real dated to the last glacial maximum ca 18000 yr bp precludes using the equilibrium line altitude method to quantitatively evaluate the discrepancy between warm sea surface temperatures and cold terrestrial conditions reconstructed with other proxies for this time period

different precipitation patterns across tropical south america during heinrich and dansgaard oeschger stadials abstract detailed knowledge about tropical south american precipitation patterns during heinrich h and dansgaard oeschger do stadials provides relevant insights into the possible evolution of amazonian hydroclimate under future climate change sediment core geob16224 1 ca 7 n raised from a site in the continental slope off french guiana in western equatorial atlantic under the influence of the amazon river discharge documents the impacts of h and do stadials on both inorganic i e fe ca record and organic i e alkenone c37 concentration and c37 c38 ratio geochemistry between 41 and 13 ka bp our results show millennial scale covariations of increased fe ca values with decreased c37 concentration and c37 c38 ratios during h and do stadials comparing our high temporal resolution data with previously published records from ca 17 n to 4 s we are able to differentiate the influence of h and do stadials upon tropical south american precipitation we find that records under the influence of the south american summer monsoon e g western amazon and the northern edge of the intertropical convergence zone itcz e g northernmost south america exhibit strong climate shifts during both h and do stadials but regions under the influence of the southern edge of the itcz e g northeastern brazil experience a weaker reaction during do stadials than during h stadials

millennial scale sequence stratigraphy numerical simulation with dionisos abstract we investigate whether depositional sequences can form on 1000 y or millennial scale and what stratal architecture can develop as the result of these short term variations abrupt climate changes are caused by a complex interplay between atmospheric oceanic and cryospheric processes dansgaard oeschger d o cycles of 1000 y and bond cycles of 7000 y have been identified in climate studies since the early 1990s a 3d forward stratigraphic model dionisos was used in this study to analyze the possible stratigraphic architecture that may evolve in response to the millennial scale climatic cycles according to current knowledge no detectable eustatic changes occur in a d o cycle but sea level may change slightly through several d o cycles an abrupt 20 m fall and subsequent rise characterize the heinrich events between bond cycles our modeling included three experiments i stable sea level ii slightly rising sea level and iii slightly falling sea level between heinrich events the applied fluvial water discharge and sediment supply varied according to the millennial climatic variations in each experiment the modeling experiments lead to the formulation of a conceptual model for millennial scale stratigraphy relevant to glacial periods the millennial scale sequences belong to a two fold hierarchy defined by a series of short d o cycles nested within longer bond cycles which in turn are separated by the sharp heinrich events the stacking patterns predicted between heinrich events include i alternating thicker and thinner bedsets of normal regressive highstand progradation hst on d o scale if sea level is stable ii highstand systems tract transgressive systems tract hst tst sequences on d o scale if the sea level is rising and iii thickening and thinning forced regressive bedsets on d o scale if the sea level is falling in case iii the bond scale falling stage systems tract fst has two distinct parts a proximal slightly and gradually downstepping unit followed by a strongly offlapping unit deposited offshore the intra fst surface that separates the two units corresponds to the heinrich sea level drop and is referred to in this paper as the heinrich discontinuity this type of sequence consists of fst lst tst and no hst may form

evolution of the lake titicaca basin and its diatom flora over the last 370 000 years abstract in recent years deep drilling undertaken as part of the international continental drilling program has generated multiple long lacustrine sedimentary records to reconstruct continental paleoclimate in many cases the tectonic and geomorphic history of these basins is under constrained and poorly known which affects the interpretation of climate history from geophysical geochemical and paleobiotic proxies in the sedimentary record in addition non analog biotic assemblages that reflect evolutionary processes may constrain the reconstruction of past environments in the drill core record of lake titicaca spanning the last 370 ka the diatom stratigraphy reflects both the influence of climate and the long term evolution of the lake basin and its biota in the upper part of the drill core sequence glacial intervals were deep and dominated by freshwater planktic taxa and peak interglacial intervals were shallow and dominated by benthic species some with saline affinities in the basal sections of the drill core record benthic diatoms are dominant in both glacial and interglacial units with freshwater taxa dominating the glacial strata this suggests that the ancient lake basin was shallower during intervals of both wet and dry climate and that the modern deep lake may result from a progressive subsidence and deepening of the basin over time in addition morphological evolution in one of the major lineages of planktic diatoms cyclostephanos indicates substantial change in the limnological environment that affected species morphology and may have driven speciation

timing of millennial scale climate change in antarctica and greenland during the last glacial period a precise relative chronology for greenland and west antarctic paleotemperature is extended to 90 000 years ago based on correlation of atmospheric methane records from the greenland ice sheet project 2 and byrd ice cores over this period the onset of seven major millennial scale warmings in antarctica preceded the onset of greenland warmings by 1500 to 3000 years in general antarctic temperatures increased gradually while greenland temperatures were decreasing or constant and the termination of antarctic warming was apparently coincident with the onset of rapid warming in greenland this pattern provides further evidence for the operation of a bipolar see saw in air temperatures and an oceanic teleconnection between the hemispheres on millennial time scales

lake level variability in salar de coipasa bolivia during the past 40 000 yr various paleoclimatic records have been used to reconstruct the hydrologic history of the altiplano relating this history to past variability of the south american summer monsoon prior studies of the southern altiplano the location of the world s largest salt flat the salar de uyuni and its neighbor the salar de coipasa generally agree in their reconstructions of the climate history of the past 24 ka some studies however have highly divergent climatic records and interpretations of earlier periods in this study lake level variation was reconstructed from a 14 m long sediment core from the salar de coipasa these sediments span the last 40 ka lacustrine sediment accumulation was apparently continuous in the basin from 40 to 6 ka with dry or very shallow conditions afterward the fossil diatom stratigraphy and geochemical data 13 c 15 n ca c n indicate fluctuations in lake level from shallow to moderately deep with the deepest conditions correlative with the heinrich 1 and younger dryas events the stratigraphy shows a continuous lake of variable depth and salinity during the last glacial maximum and latter stages of marine oxygen isotope stage 3 and is consistent with environmental inferences and the original chronology of a drill core from salar de uyuni