oxygen and carbon isotopes in modern fresh water ostracod valves assessing vital offsets and autecological effects of interest for palaeoclimate studies abstract the isotopic composition of modern benthic ostracod and mollusc carbonate the oxygen isotope composition of lake water 18ow the carbon isotope composition of dissolved inorganic carbon 13cdic and water temperatures were monitored during a year at different water depths in two lakes ammersee and starnberger see southern germany the aim was to test whether the valves are built in isotopic equilibrium and how seasonal water temperatures are recorded in the shells we find systematic offsets between 18o of valve carbonate compared to 18o expected for a theoretical calcite formed in isotopic equilibrium offsets are higher than 2 for different species of the candona group 1 5 for cytherissa lacustris and around 0 8 for limnocythere inopinata and darwinula stevensoni they seem to be constant for all instars of a species and temperature independent mean water temperatures of the calcification period are readily reflected in the valve 18o if corrected for the offsets 13c of valve carbonate is mainly controlled by 13c of the dissolved carbonate in ambient lake water but seems to be biased by micro habitat effects and vital offsets the study provides information on the seasonality of valve formation thus potentially allowing the quantification of temporal and spatial water temperature gradients from specific 18o records

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

extensive carbonate algal bioherms in upper pleistocene saline lakes of the central altiplano of bolivia during the upper pleistocene the central altiplano of bolivia was repeatedly flooded by deep and extensive saline lakes in response to climatic fluctuations development of carbonate algal bioherms took place during at least three major periods of lacustrine highstands discontinuously covering the 300 km long and 100 km wide lacustrine slopes and terraces up to an elevation of 100 m above the surface of the modern halite crust of uyuni distribution size and shape of the bioherms are diverse due to various factors e g the nature and morphology of the substrate and the hydrodynamic conditions that prevailed during growth on larger palaeoterraces the build ups coalesced to form platform like carbonate accumulations although the morphologies closely resemble those induced by cyanobacteria they were predominantly constructed by other plant communities probably dominated by filamentous green algae cyanobacterial communities flourished in association with these plants but they did not contribute significantly to the architecture of the bioherms they participated to encrust the plant stems and algal bushes or to form thin laminated layers covering the build ups a prominent feature of some bioherms is their composite structure due to repeated algal growth during successive lacustrine episodes that were separated by subaerial exposures with moderate erosional effects the build ups located between 3660 and 3680 m elevation display up to three major parts 1 a massive inner core formed during an early minchin highstand before 40 ka 2 a large peripheral envelope deposited at about 40 ka late minchin and 3 a thinner outermost crust formed during a late glacial event lake level dropped during interlacustrine stages sometimes leading to desiccation and deposition of salt layers in the deepest parts of the system i e the present day salar of uyuni

modern and last local glacial maximum snowlines in the central andes of peru bolivia and northern chile late pleistocene snowlines in the central andes were 500 1200 m lower than at present radiocarbon dates imply that the late pleistocene glacial maximum in the region occurred prior to 20 14c ka but lack of maximum limiting ages adds considerable uncertainty to the exact timing snowline modeling demonstrates that snowlines in the eastern and western cordilleras of the central andes respond differently to temperature and precipitation changes in the eastern cordillera the snowline is near the level of the annual 0 c isotherm and melting occurs throughout the year here snowlines are sensitive to temperature changes but relatively insensitive to accumulation changes in the western cordillera the snowline rises 1000 higher owing to increasing aridity and the snowline exhibits a much stronger sensitivity to accumulation changes the consistent 1200 m snowline depression along the eastern cordilleras of the central andes can be modeled by a mean annual cooling of 5 9 c this is inconsistent with the 2 c cooling in tropical sea surface temperatures suggested by climap reconstructions the 800 1000 m snowline depression in the western cordillera cannot be accounted for solely by cooling but also requires an increase in precipitation

isotopic tracers of paleohydrologic change in large lakes of the bolivian altiplano we have developed an 87sr 86sr 234u 238u and 18o data set from carbonates associated with late quaternary paleolake cycles on the southern bolivian altiplano as a tool for tracking and understanding the causes of lake level fluctuations distinctive groupings of 87sr 86sr ratios are observed ratios are highest for the ouki lake cycle 120 95 ka at 0 70932 lowest for coipasa lake cycle 12 8 11 4 ka at 0 70853 and intermediate at 0 70881 to 0 70884 for the salinas 95 80 ka inca huasi 45 ka sajsi 24 20 5 ka and tauca 18 1 14 1 ka lake cycles these sr ratios reflect variable contributions from the eastern and western cordilleras the laca hydrologic divide exerts a primary influence on modern and paleolake 87sr 86sr ratios waters show higher 87sr 86sr ratios north of this divide most lake cycles were sustained by slightly more rainfall north of this divide but with minimal input from lake titicaca the coipasa lake cycle appears to have been sustained mainly by rainfall south of this divide in contrast the ouki lake cycle was an expansive lake deepest in the northern poopo basin and spilling southward these results indicate that regional variability in central andean wet events can be reconstructed using geochemical patterns from this lake system

recent advances in sar interferometry time series analysis for measuring crustal deformation abstract synthetic aperture radar sar interferometry is a technique that permits remote detection of deformation at the earth s surface and has been used extensively to measure displacements associated with earthquakes volcanic activity and many other crustal deformation phenomena analysis of a time series of sar images extends the area where interferometry can be successfully applied and also allows detection of smaller displacements through the reduction of error sources here we review recent advances in time series sar interferometry methods that further improve accuracy this is particularly important when constraining displacements due to processes with low strain rates such as interseismic deformation we include examples of improved algorithms applied to image deformation associated with the 2010 eruption of eyjafjallajokull volcano in iceland slow slip on the guerrero subduction zone in mexico and tectonic deformation in western anatolia turkey

holocene environments from stable isotope stratigraphy of ostracods and authigenic carbonate in chilean altiplano lakes abstract sediment cores from three shallow 4000 m a s l paleoclimatic profiles show coherency within tropical subtropical circulation patterns climatic signals however are partially masked by local environmental systems results suggest similar arid mid holocene to less arid late holocene trends in the three lakes although each lake shows distinct response characteristics linked to differences in regional moisture balance isotopic signatures for each lake group in discrete 18o vs 13c populations high isotopic variability results from episodic flooding and brine evolution in the shallow lakes laguna seca 18 11 s 69 14 w 4500 m a s l is characterized by a diverse ostracod assemblage of at least 8 species indicating highest moisture levels among the three sites high 13c values are the result of continuous evasion of co2 derived from volcanically charged groundwaters and springs plus the formation of travertine and photosynthetic activity in the lake a 3 decrease in 18o about 4 7 m suggests an abrupt shift to wetter conditions at the transition from mid to late holocene lagunas miscanti 23 44 s 67 46 w 4140 m a s l and del negro francisco 27 28 s 69 14 w 4125 m a s l each contain only one ostracod species limnocythere sappaensis attesting to higher salinities although sediment facies and ostracod continuity and abundances imply a history of higher effective moisture at laguna del negro francisco sediment cores from each lake document more arid conditions in the lower sections with a sharp shift to wetter conditions in the upper sections highly variable isotope values at the transition from mid to late holocene suggest rapid short term climate shifts during this transition this overall trend can be correlated from laguna seca laguna miscanti and laguna del negro francisco to lago titicaca implying climatic coherency along the altiplano isotopic profiles show second order fluctuations and drought events that suggest short term variability along the transect in order to explain differences in the effective moisture among the three sites we postulate interactions of regional air masses in the north convective tropical precipitation dominates laguna seca as for lakes in the titicaca region effective moisture for lagunas del negro francisco and miscanti follows a south to north gradient generated by cold air masses from cut offs of pacific westerlies colliding with moist tropical air masses coherent shifts in the past therefore imply changes in available tropical air mass moisture

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

influence of climate change and uplift on colorado plateau paleotemperatures from carbonate clumped isotope thermometry the elevation history of earth s surface is key to understanding the geodynamic processes responsible for the rise of plateaus we investigate the timing of colorado plateau uplift by estimating depositional temperatures of tertiary lake sediments that blanket the plateau interior and adjacent lowlands using carbonate clumped isotope paleothermometry a measure of the temperature dependent enrichment of 13 c 18 o bonds in carbonates comparison of modern and ancient samples deposited near sea level provides an opportunity to quantify the influence of climate and therefore assess the contribution of changes in elevation to the variations of surface temperature on the plateau analysis of modern lake calcite from 350 to 3300 m elevation in the southwestern united states reveals a lake water carbonate temperature lct lapse rate of 4 2 0 6 c km analysis of miocene deposits from 88 to 1900 m elevation in the colorado river drainage suggests that the ancient lct lapse rate was 4 1 0 7 c km and temperatures were 7 7 2 0 c warmer at any one elevation than predicted by the modern trend the inferred cooling is plausible in light of pliocene temperature estimates off the coast of california and the consistency of lapse rates through time supports the interpretation that there has been little or no elevation change for any of the samples since 6 ma together with previous paleorelief estimates from apatite u th he data from the grand canyon our results suggest most or all of the plateau s lithospheric buoyancy was acquired 80 60 ma and do not support explanations that ascribe most plateau uplift to oligocene or younger disposal of either the farallon or north american mantle lithosphere

13c 18o isotope signatures and clumped isotope thermometry in foraminifera and coccoliths accurate constraints on past ocean temperatures and compositions are critical for documenting climate change and resolving its causes most proxies for temperature are not thermodynamically based appear to be subject to biological processes require regional calibrations and or are influenced by fluid composition as a result their interpretation becomes uncertain when they are applied in settings not necessarily resembling those in which they were empirically calibrated independent proxies for past temperature could provide an important means of testing and or expanding on existing reconstructions here we report measurements of abundances of stable isotopologues of calcitic and aragonitic benthic and planktic foraminifera and coccoliths relate those abundances to independently estimated growth temperatures and discuss the possible scope of equilibrium and kinetic isotope effects the proportions of 13c 18o bonds in these samples exhibits a temperature dependence that is generally similar to that previously been reported for inorganic calcite and other biologically precipitated carbonate containing minerals apatite from fish reptile and mammal teeth calcitic brachiopods and molluscs aragonitic coral and mollusks most species that exhibit non equilibrium 18o 16o 18o and 13c 12c 13c ratios are characterized by 13c 18o bond abundances that are similar to inorganic calcite and are generally indistinguishable from apparent equilibrium with possible exceptions among benthic foraminiferal samples from the arctic ocean where temperatures are near freezing observed isotope ratios in biogenic carbonates can be explained if carbonate minerals generally preserve a state of ordering that reflects the extent of isotopic equilibration of the dissolved inorganic carbon species

on the fidelity of shell derived δ18oseawater estimates article i nfo 18 oseawater a robustproxyforsurfacesalinity we assessthe fidelity of shell derived 18 oseawater estimates for the surface dwelling foraminifer globigerinoides ruber white using an atlantic meridional coretop transect spanning basin scale temperature and salinity gradients shell derived and observed 18 oseawater values are well correlated r 2 0 77 but a large systematic bias is observed shell mg ca ratios are significantly elevated above values expected from observed and isotopic calcification temperatures in the saline subtropical gyres of both hemispheres this temperature independent shell mg ca ratio variability termed excess mg ca i s highly correlated with surface salinity r 2 0 77 and the observed salinity dependence 27 4 is much higher than indicated by culture studies 6 2 our coretop data are used to develop new atlantic basin temperature and salinity calibration equations that are accurate 1 1 c and 0 20 respectively precise r 2 0 82 and 0 81 respectively and verifiable using previously published data these results are valid for the relatively high salinities of the subtropical atlantic 35 5 37 3 we discover that inclusion of other published data from lower salinity regions b35 indicates little or no excess mg ca taken together these results point to a strongly non linear positive salinity effect on shell mg ca ratios that significantly affects the accuracy of sst and 18

late pleistocene climate and water budget of the south american altiplano abstract geomorphic evidence from the peruvian bolivian altiplano indicates that during episodes prior to 28 000 and around 12 500 11 000 yr b p lakes covered an area about six and four times as large as at present respectively within the constraints of the heat and water budget model calculations are used to estimate the precipitation rate that would allow hydrologic equilibrium on this basis it is suggested that rainfall on the altiplano during the episodes of enlarged lakes was respectively some 300 and 200 mm annum 1 larger than at present representing increases of about 75 and 50 respectively field evidence suggests that the episodes of enlarged lakes on the altiplano may have preceded or coincided with periods of maximum glaciation in the neigh boring andes in this region with high elevation of the ice equilibrium line increased precipitation is particularly conducive to glaciation

lake highstands on the altiplano tropical andes contemporaneous with heinrich 1 and the younger dryas new insights from 14 c u th dating and δ 18 o of carbonates this study provides new geochronological and stable isotope constraints on late pleistocene fluctuations in lake level that occurred in the closed watershed of the central altiplano between 25 and 12 ka u series isochrons and 14 c ages from carbonates are used to confirm and refine the previous chronology published placzek et al 2006b our new data support three successive lake highstands during the late pleistocene i the lake sajsi cycle from 25 to 19 ka that culminated at 3670 m at about 22 ka almost synchronously with the global last glacial maximum ii the lake tauca cycle that lasted from 18 to 14 5 ka and was characterized by the highest water level reached at least 3770 m from 16 5 to 15 ka iii the lake coipasa cycle from 12 5 to 11 9 ka that reached an elevation of 3700 m 42 m above the elevation of the salar de uyuni 3658 m these high amplitude lake level fluctuations are in phase with the cold warm oscillations that occurred in the north atlantic and greenland during the late pleistocene heinrich 1 bolling allerod younger dryas such temporal coincidence supports the hypothesis that wet events recorded in the central altiplano are controlled by the northesouth displacement of the inter tropical convergence zone resulting from changes in the meridional temperature gradient finally the oxygen isotope ratios measured in these lacustrine carbonates allows for calculation of the 18 o value of paleolake waters estimates of water 18 o v smow are 2 8 0 7 for lake tauca and 1 6 0 9 for lake coipasa these data are used to constrain changes in lake hydrology and can be interpreted to indicate that the proportion of precipitation arising from local water recycling was less than 50

lake level chronology on the southern bolivian altiplano 18 23 s during late glacial time and the early holocene stratigraphic analyses of outcrops shorelines and diatoms from the southern bolivian altiplano uyuni coipasa basin reveal two major lacustrine phases during the late glacial period and the early holocene based on a chronology established by radiocarbon and u th control a comparison of14c and230th 234u ages shows that during times of high lake level radiocarbon ages are valid however during low water periods 14c ages must be corrected for a reservoir effect the lacustrine tauca phase started a little before 16 00014c yr b p and the lake level reached its maximum between 13 000 and 12 00014c yr b p a dry event ticana occurred after ca 12 000 and before 950014c yr b p a moderate lacustrine oscillation coipasa event occurred between ca 9500 and 850014c yr b p using a reservoir corrected conventional14c chronology comparisons between the lake level chronology in the uyuni coipasa basin and data from other southern tropical areas of south america suggest that the lacustrine evolution may reflect large scale climatic changes

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

u series dating of impure carbonates an isochron technique using total sample dissolution abstract u series dating is a well established technique for age determination of late quaternary carbonates materials of sufficient purity for nominal dating however are not as common as materials with mechanically inseparable aluminosilicate detritus detritus contaminates the sample with extraneous th we propose that correction for contamination is best accomplished with the isochron technique using total sample dissolution tsd experiments were conducted on artificial mixtures of natural detritus and carbonate and on an impure carbonate of known age results show that significant and unpredictable transfer of radionuclides occur from the detritus to the leachate in commonly used selective leaching procedures the effects of correcting via leachate residue pairs and isochron plots were assessed isochrons using tsd gave best results followed by isochron plots of leachates only

initiation and development of the laurentide and cordilleran ice sheets following the last interglaciation abstract fossil records from sites overridden by or adjacent to the laurentide ice sheet indicate that the climate of the last interglaciation oxygen isotope substage 5e ca 130 116 ka was warmer than today following the last interglaciation the laurentide ice sheet first developed during stage 5 over keewatin quebec and baffin island along its northern margin the ice sheet reached its maximum extent of the last glaciation during stage 5 the ice sheet advanced across baffin island onto the continental shelf early during stage 5 5d whereas the advance into the western canadian arctic occurred late during stage 5 5b the ice sheet also may have advanced into the st lawrence lowland during substage 5b although this event may be younger stage 4 the hudson bay lowland became ice free during substage 5a retreat of the ice sheet on baffin island occurred during late stage 5 probably substage 5a the exact timing of retreat from the western canadian arctic is unknown but it occurred before 48 ka the southern sector including the st lawrence lowland was ice free during late stage 5 the hudson bay lowland may have remained ice free through stage 4 and much of stage 3 because of conflicting chronologies however it is more likely that this area was glaciated throughout stage 3 and perhaps stage 4 nevertheless the data demonstrate that the lowland was ice free during part of the last glaciation an independent ice cap developed over the appalachian uplands and advanced across nova scotia during stage 4 perhaps as far as the edge of the continental shelf the ice cap remained active over nova scotia as a setellite to the laurentide ice sheet throughout the remainder of the last glaciation the ice sheet advanced into the st lawrence lowland during stage 4 and subsequently overwhelmed the local ice cap in the appalachian uplands advancing perhaps into northern new england but not farther south the lowland remained covered by the ice sheet until late stage 2 the ice sheet may also have advanced into the lake ontario basin during stage 4 the position of the northern margin of the laurentide ice sheet during stage 4 is not known but it remained an unknown distance behind its maximum position reached during stage 5 cores from baffin bay indicate a substantial decrease in high latitude glaciation during stage 4 following retreat the keewatin sector of the ice sheet may have remained over much of northwestern canada as a quasi stable ice mass until it readvanced during stage 2 similarly the baffin island sector of the ice sheet may have remained largely intact the southern margin of the ice sheet may have advanced into the lake ontario basin and upper mississippi valley during the middle of stage 3 ca 50 ka reaching its maximum extent of the last glaciation during stage 2 ca 18 21 ka advance of the northern margin was younger ca 8 13 ka than that of the southern margin this advance was less extensive than the penultimate advance stage 5 paleoenvironmental records indicate that the last interglaciation in areas covered by and near the cordilleran ice sheet was as warm as or warmer than present the cordilleran ice sheet appears to have developed during stage 5 or 4 at that time it advanced over southern british columbia and into the northern puget lowland there is no record of this event in northern areas that were later covered by the ice sheet the ice sheet disappeared before 59 ka at the beginning of a lengthy nonglacial interval paleoenvironmental records indicate that climate was similar to the present during part of this interval the ice sheet was absent and glaciers probably were confined to mountain areas throughout stage 3 climatic deterioration marking the end of this nonglacial interval may have begun as early as 29 ka by 14 15 ka the ice sheet had achieved its maximum extent of the last glaciation because there are few suitable dating methods capable of resolving events beyond the radiocarbon limit and because sites that preserve a record of events from the last glaciation are spatially restricted we consider this synthesis as tentative and subject to significant revision as dating methods improve nevertheless this perspective of the north american ice sheets through the last glaciation demonstrates their complex and dynamic behavior and attendant rapid fluctuations in ice volume

a new greenland ice core chronology for the last glacial termination 1 we present a new common stratigraphic timescale for the north greenland ice core project ngrip and grip ice cores the timescale covers the period 7 9 14 8 kyr before present and includes the bolling allerod younger dryas and early holocene periods we use a combination of new and previously published data the most prominent being new high resolution continuous flow analysis cfa impurity records from the ngrip ice core several investigators have identified and counted annual layers using a multiparameter approach and the maximum counting error is estimated to be up to 2 in the holocene part and about 3 for the older parts these counting error estimates reflect the number of annual layers that were hard to interpret but not a possible bias in the set of rules used for annual layer identification as the grip and ngrip ice cores are not optimal for annual layer counting in the middle and late holocene the timescale is tied to a prominent volcanic event inside the 8 2 kyr cold event recently dated in the dye 3 ice core to 8236 years before a d 2000 b2k with a maximum counting error of 47 years the new timescale dates the younger dryas preboreal transition to 11 703 b2k which is 100 150 years older than according to the present grip and ngrip timescales the age of the transition matches the gisp2 timescale within a few years but viewed over the entire 7 9 14 8 kyr section there are significant differences between the new timescale and the gisp2 timescale the transition from the glacial into the bolling interstadial is dated to 14 692 b2k the presented timescale is a part of a new greenland ice core chronology common to the dye 3 grip and ngrip ice cores named the greenland ice core chronology 2005 gicc05 the annual layer thicknesses are observed to be log normally distributed with good approximation and compared to the early holocene the mean accumulation rates in the younger dryas and bolling periods are found to be 47 2 and 88 2 respectively

the greenland ice core chronology 2005 15 42 ka part 2 comparison to other records a new greenland ice core chronology gicc05 based on multi parameter counting of annual layers has been obtained for the last 42 ka here we compare the glacial part of the new time scale which is based entirely on records from the northgrip ice core to existing time scales and reference horizons covering the same period these include the grip and northgrip modelled time scales the meese sowers gisp2 counted time scale the shackleton fairbanks grip time scale sfcp04 based on 14c calibration of a marine core the hulu cave record three volcanic reference horizons and the laschamp geomagnetic excursion event occurring around greenland interstadial 10 gicc05 is generally in good long term agreement with the existing greenland ice core chronologies and with the hulu cave record but on shorter time scales there are significant discrepancies around the last glacial maximum there is a more than 1 ka age difference between gicc05 and sfcp04 and a more than 0 5 ka discrepancy in the same direction between gicc05 and the age of a recently identified tephra layer in the northgrip ice core both sfcp04 and the tephra age are based on 14c dated marine cores and fixed marine reservoir ages for the laschamp event gicc05 agrees with a recent independent dating within the uncertainties

late local glacial maximum in the central altiplano triggered by cold and locally wet conditions during the paleolake tauca episode 17 15 ka heinrich 1 the timing and causes of the last deglaciation in the southern tropical andes is poorly known in the central altiplano recent studies have focused on whether this tropical highland was deglaciated before synchronously or after the global last glacial maximum 21 ka bp in this study we present a new chronology based on cosmogenic 3he 3he c dating of moraines on cerro tunupa a volcano that is located in the centre of the now vanished lake tauca 19 9 s 67 6 w these new 3he c ages suggest that the tunupa glaciers remained close to their maximum extent until 15 ka bp synchronous with the lake tauca highstand 17 15 ka bp glacial retreat and the demise of lake tauca seem to have occurred rapidly and synchronously within dating uncertainties at 15 ka bp we took advantage of the synchronism of these events to combine a glacier model with a lake model in order to reconstruct precipitation and temperature during the lake tauca highstand this new approach indicates that during the tauca highstand 17 15 ka bp the centre of the altiplano was characterized by temperature 6 5 c cooler and average precipitation higher by a factor ranging between 1 6 and 3 compared to the present cold and wet conditions thus persisted in a significant part of the southern tropical andes during the heinrich 1 event 17 15 ka bp this study also demonstrates the extent to which the snowline of glaciers can be affected by local climatic conditions and emphasizes that efforts to draw global climate inferences from glacial extents must also consider local moisture conditions

abrupt temperature changes in the western mediterranean over the past 250 000 years a continuous high resolution western mediterranean sea surface temperature sst alkenone record spanning the past 250 000 years shows that abrupt changes were more common at warming than at cooling during marine isotope stage mis 6 sst oscillated following a stadial interstadial pattern but at lower intensities and rates of change than in the dansgaard oeschger events of mis 3 some of the most prominent events occurred over miss 5 and 7 after prolonged warm periods of high stability climate during the whole period was predominantly maintained in interglacial interstadial conditions whereas the duration of stadials was much shorter

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

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

stable isotope paleoaltimetry a critical review stable isotope paleoaltimetry makes use of systematic trends in the distribution and isotopic composition of modern precipitation with climate and topography and of the potential to estimate the isotopic composition of paleoprecipitation from authigenic in situ formed minerals to illustrate the usefulness as well as potential limitations of this method we review 1 processes controlling the isotopic composition of modern precipitation 2 stable isotope data from modern precipitation across regions of high topography and 3 stable isotope data from authigenic minerals that have been used to infer paleotopography from this we conclude that stable isotope studies of authigenic minerals can permit useful inferences on paleotopography with uncertainties that critically depend on a detailed understanding of local to global scale paleoclimate during the time interval of interest

a synchronized dating of three greenland ice cores throughout the holocene as part of the effort to create the new greenland ice core chronology 2005 gicc05 a synchronized stratigraphical timescale for the holocene parts of the dye 3 grip and ngrip ice cores is made by using volcanic reference horizons in electri cal conductivity measurements to match the cores the main annual layer counting is carried out on the most suited records only exploit ing that the three ice cores have been drilled at locations with different climatic con ditions and differences in ice flow however supplemental counting on data from all cores has been performed between each set of reference horizons in order to verify the valid ity of the match after the verification the main dating is transferred to all records us ing the volcanic reference horizons as tie points an assessment of the mean annual layer thickness in each core section confirms that the new synchronized dating is consistent for all three cores the data used for the main annual layer counting of the past 7900 years are the dye 3 grip and ngrip stable isotope records as the high accumulation rate at the dye 3 drill site makes the seasonal cycle in the dye 3 stable isotopes very resistant to firn diffusion an effort has been made to extend the dye 3 holocene record the new syn chronized dating relies heavily on this record of 75 000 stable isotope samples the dat ing of the early holocene consists of an already established part of gicc05 for grip and ngrip which has now been transferred to the dye 3 core gicc05 dates the younger dryas termination as defined from deuterium excess to 11 703 b2k 130 years earlier than the previous grip dating

tropical climate changes at millennial and orbital timescales on the bolivian altiplano tropical south america is one of the three main centres of the global zonal overturning circulation of the equatorial atmosphere generally termed the walker circulation1 although this area plays a key role in global climate cycles little is known about south american climate history here we describe sediment cores and down hole logging results of deep drilling in the salar de uyuni on the bolivian altiplano located in the tropical andes we demonstrate that during the past 50 000 years the altiplano underwent important changes in effective moisture at both orbital 20 000 year and millennial timescales long duration wet periods such as the last glacial maximum marked in the drill core by continuous deposition of lacustrine sediments appear to have occurred in phase with summer insolation maxima produced by the earth s precessional cycle short duration millennial events correlate well with north atlantic cold events including heinrich events 1 and 2 as well as the younger dryas episode at both millennial and orbital timescales cold sea surface temperatures in the high latitude north atlantic were coeval with wet conditions in tropical south america suggesting a common forcing

u th dating of lake lisan late pleistocene dead sea aragonite and implications for glacial east mediterranean climate change u series dating can be an effective means to obtain accurate and precise ages on quaternary carbonates however most samples require a correction for u and th in admixed detritus this complication is often addressed through generation of u th isochrons requiring analyses of several coeval samples in addition presence of water derived hydrogenous th in the carbonate can cause inaccuracies in isochron ages this study reports a high resolution u series chronology of sediments deposited by lake lisan the last glacial precursor of the dead sea the strategy employed combines multiple measurements from a few stratigraphic heights and fewer analyses from many heights in a single described and measured section the resulting chronology is based on ages at 22 heights in a 40 m thick section covering the interval of 70 14 calendar ka bp the effects of admixed detritus are evaluated using trace elements nearly pure aragonite samples indicated by very low abundances of insoluble elements such as nb and zr were found to contain hydrogenous th which causes the uncorrected u 230th age of a modern sample to be 2 5 ka nevertheless accurate ages have been obtained by correcting for the detrital and aqueous interferences the resulting ages are in stratigraphic order and their accuracy is evidenced by consistency of lisan formation u series and 14c ages with the coral based calendar radiocarbon age calibration the u th ages provide a context to unravel the limnological history of lake lisan boundaries between the lower middle and upper stratigraphic units correspond to the mis 4 3 and 3 2 transitions respectively during mis 2 and 4 the lake generally showed a stable two layer configuration and a positive fresh water balance reflected by deposition of laminated aragonite detritus dry intervals during mis 2 and 4 are indicated by thick gypsum layers and an inferred depositional hiatus which are temporally associated with heinrich events h1 at 17 ka and h6 at 65 ka respectively during mis 3 the lake level was unstable with intermittent dry periods indicated by abundant clastic layers and a significant hiatus between 43 49 ka clastic layers are associated with dansgaard oeschger events during mis 3 and indicate lake level declines during abrupt northern hemisphere warmings overall the climate of the eastern mediterranean region shows a strong linkage to the northern hemisphere climate with increasing lake size and stability during cold periods and fluctuations and dessication during warmings and heinrich events

u th dating of carbonate platform and slope sediments absolute chronology of marine sediment beyond the 14c age range provides a test for models of climate change and has many other applications u th techniques have been used for such chronology by dating corals but extending these techniques to marine sediment is complicated by the presence of significant initial 230th both in detrital material and scavenged from seawater in this study we investigate four methods of solving the initial 230th problem for a particular type of marine sediment the aragonite rich sediments of carbonate platforms and slopes bulk sediment u th analyses can be corrected for initial th to yield ages with 2 to 3 kyr precision for highstand periods when sediment aragonite contents are particularly high uncertainty on the corrections causes inadequate precision for sediment from other periods however removal of scavenged th before analysis would enable a dramatic increase in this precision but has not proved successful despite a range of chemical leach approaches using heavy liquids to separate the various carbonate minerals found in bahamas sediment enables an isochron approach to correct for initial th but the presence of initial th from two sources requires correction or removal of one source of initial th before the other is deconvolved by the isochron quantitative removal of detrital material before isochron analysis proves a successful approach such isochron data demonstrate that although sediment remains closed to u th on a centimetre scale nuclides are moved from grain to grain by recoil such intergrain exchange is expected to be observed in all sediments containing mineral grains with different u concentrations measured 234u 238u allows the recoil movement to be corrected and results in isochron ages with precision sometimes as low as 3 kyr the accuracy of this approach has been proved by dating samples within the 14c age range sediments spanning the penultimate deglaciation have also been dated after a small correction for bioturbation the age for this event is found to be 135 2 3 5 ka this date is 8 kyr before the peak in northern hemisphere insolation and suggests that deglaciation is initiated by a mechanism in the southern hemisphere or tropics this isochron approach shows considerable promise for dating of sediments older than this event which will provide further information about the timing and mechanisms of global climate change

strategies for successful u th dating of paleolake carbonates an example from the bolivian altiplano we report over 90 u th dates from carbonates deposited around paleolakes on the bolivian altiplano petrographic and chemical data for tufas and the siliciclastic detritus contained within them allow 1 assessment of possible diagenetic effects 2 a development of a strategy for selection of carbonate samples with low initial th contents and 3 assessment of the uncertainty due to initial th this strategy allows us to produce precise u th dates from lacustrine carbonates the principal consideration in dating such carbonates is the composition and quantity of initial th incorporated into the carbonate and additional uncertainty is introduced because this initial th may have two sources in lacustrine deposits isochron plots measured 230th 232th and x ray diffraction and trace element chemistry of silicic residues all favor regional soil and dust as the sole source of initial th in carbonates from the bolivian altiplano a situation that simplifies single sample dating of altiplano carbonates

δ18o and δ13c values of modern brachiopod shells abstract researchers have not rigorously tested the hypothesis that calcite from modern brachiopod shells is precipitated in oxygen isotope equilibrium with ambient seawater isotopic variability at the intraspecimen and intertaxon levels has not been examined without such data for modern brachiopods similar data from ancient brachiopods cannot be accurately interpreted in this study a survey is made of 18 o and 13 c values of terebratulid rhynchonellid thecideidine and craniacean brachiopods from antarctica the bay of fundy curacao japan new zealand norway puget sound palau sicily and south africa this suite of samples provides a wide range of taxonomic levels temperatures salinities and depositional environments for evaluating the degree of isotopic equilibrium attained during precipitation of brachiopod calcite new data indicate that modem brachiopod calcite is not always precipitated in oxygen and carbon isotope equilibrium with ambient seawater calcite from the primary layer and specialized shell structures hinge brachidium foramen interarea muscle scars are depleted in both 18 o and 13 c a characteristic of biological fractionation or vital effects often found in other calcerous marine organisms our findings suggest that these portions of the brachiopod shell should be avoided during sampling of ancient brachiopods secondary layer calcite the material most often analyzed in ancient brachiopods has higher 18 o and 13 c values which approach and sometimes correspond with predicted equilibrium values therefore secondary layer calcite is the most suitable portion of the brachiopod shell for use as an ancient seawater proxy although near equilibrium precipitation in secondary layer calcite is encouraging to those studying the isotopic composition of ancient oceans these data come with caveats large intraspecimen variability in the 18 o values of secondary layer calcite 1 in some samples limits the use of brachiopods as precise indicators of the 18 o value or temperature of ancient seawater the 18 o and 13 c values of secondary layer calcite deviate from predicted equilibrium values toward the lower values of the primary layer and specialized shell structures indicating a measurable vital effect in most cases measured 18 o values are lower than the calculated equilibrium values dictated by seasonal variations in the 18 o value and temperature of seawater ontogenetic variations in the 13 c values of secondary layer calcite have also been measured and must be assessed when interpreting ancient data modern brachiopods occur in a wide range of seawater salinities and temperatures therefore the relation between salinity and the 18 o value of seawater must be known to correctly calculate the temperature of calcite precipitation from secondary layer 18 o values errors of up to 15 c are found if ambient seawater is assumed to have a constant 18 o value e g 0 smow and the 18 o values of secondary layer calcite are used to calculate temperature coupled with vital effects and intraspecimen variability these errors can obscure the resolution of secular variations in the rock record