droughts in the east asian summer monsoon margin during the last 6 kyrs link to the north atlantic cooling events abstract teleconnections to the high latitudes forcing by the tropical oceans and solar variability have all been suggested as dominant factors in the sub millennial global climate changes yet there is little consensus as to the relative importance of these factors for the east asian summer monsoon easm variability this study presents the results of high resolution analyses of ca and mg concentrations mg ca ratio 18 o and 13 c values of endogenic calcites from a sediment core from dali lake in the easm margin in order to investigate the sub millennial easm variability and its possible driving forces during the last 6 kyrs increases in these chemical proxy data were interpreted as drought events in the region due to the intensive evaporation losses overwhelming the water input to the lake the chemical proxy data in this study combined with multi proxy indicators including grain size component and total organic carbon concentrations from the same sediment core imply that declines in the easm intensity may have played a dominant role in triggering the drought events during the last 6 kyrs the results indicate that the easm intensity significantly declined at the intervals of 5 8 4 75 3 2 2 8 1 65 1 15 and 0 65 0 2 kyrs bp large declines in the easm intensity during the last 6 kyrs correspond in time to occurrences of ice rafted debris in the north atlantic indicating that millennial to centennial scale changes in the easm intensity were mainly controlled by climatic processes occurring in the northern high latitudes these data imply that persistent global warming may be favorable for the strengthening of the easm circulation and for the transportation of more rainfall to the semi arid regions of northern china on sub millennial scales

migration of the siple dome ice divide west antarctica the non linearity of the ice flow law or a local accumulation low over an ice divide can cause isochrones internal layers to be shallower under the divide relative to the flanks forming a divide bump in the internal layer pattern this divide signature is analyzed using ice flow models and inverse techniques to detect and quantify motion of the siple dome ice divide west antarctica the principal feature indicating that migration has occurred is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide the calculated migration rate is 0 05 0 50 m a 1 toward ice stream d and depends slightly on whether the divide bump is caused by the non linearity of ice flow or by a local accumulation low our calculations also suggest a strong south north accumulation gradient of 5 10 x 10 6 a 1 in a narrow zone north of the divide a consequence of divide migration is that pre holocene ice is thickest about 0 5 km south of the present divide position divide motion indicates that non steady processes possibly associated with activity of the bounding ice streams are affecting the geometry of siple dome the migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths this suggests that major asynchronous changes in the elevation or position of the bounding ice streams are unlikely over at least the past 10 3 10 4 years

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

the importance of tropical sea surface temperature patterns in simulations of last glacial maximum climate abstract prescribed sst experiments are performed using the national center for atmospheric research s community climate model version 3 general circulation model to isolate the contribution of the tropical ssts reconstructed by the climate long range investigation mapping and prediction study climap to the modeled global atmospheric circulation anomalies at the last glacial maximum lgm the changes in tropical sst patterns cause changes in tropical convection that force large 300 m in 500 mb geopotential height changes in northern hemisphere wintertime circulation these midlatitude circulation changes occur despite the small 1 c change in the mean tropical sst between the present and the climap reconstruction in fact the midlatitude circulation changes due to the difference in the tropical sst pattern between the present and the climap reconstruction are greater than the circulation changes due to a uniform tropical sst cooling of 3 c or those due to the presence of the lgm ice sheets the

contribution of enso variability to the east asian summer monsoon in the late holocene abstract the asian monsoon am is an important atmosphere circulation system of the global climate system acting as a bridge across high northern latitude and tropic climates however dynamical origins of am variations during the holocene remain unclear here we present stable isotope records 18 o and 13 c of a holocene stalagmite from niu cave central china to address the nature and causes of east asian summer monsoon easm changes our 18 o record shows a gradual weakening of the easm intensity from the middle to the late holocene corresponding with a decrease in northern hemisphere summer insolation and an enhancement of enso activity the 13 c profile exhibits similar variability to 18 o record suggesting that regional hydrological changes were primarily controlled by the large scale summer monsoon circulation power spectrum analyses of 18 o and 13 c records show that the easm intensity varies at a dominant periodicity of 830 yr probably correlated to an enso like cycle furthermore millennial scale shifts in our records exhibit a strong correlation with that in enso proxies over the past 3000 years the easm enso relationship may be related to the shifts in the mean position of walker circulation and western north pacific subtropical high our finding suggests that millennial scale monsoon changes in the late holocene are primarily modulated by changes in tropical ocean atmosphere circulation rather than variations in the north atlantic thermohaline circulation

ice age ice sheet rheology constraints from the last glacial maximum form of the laurentide ice sheet state ot the art thermomechanical models of the modern greenland ice and the ancient laurenticle ice sheet that covered canada at the last glacial maximum lgm are not able to explain simultaneously the observed forms of these cryospheric structures when the same anisotropy enhanced version of the conventional glen flow law is employed to describe their rheology the lgm laurenticle ice sheet predicted to develop in response to orbital climate forcing is such that the ratio of its thickness to its horizontal extent is extremely large compared to the aspect ratio inferred on the basis of surface geomorphological and solid earth geophysical constraints we show that if the glen flow law representation of the rheology is replaced with a new rheology based upon very high quality laboratory measurements of the stress strain rate relation then the aspect ratios of both the modern greenland ice sheet and the laurenticle ice sheet that existed at the lgm are simultaneously explained with little or no retuning of the flow law

a high resolution geochemical record from lake edward uganda congo and the timing and causes of tropical african drought during the late holocene abstract high resolution analyses of the elemental composition of calcite and biogenic silica bsi content in piston cores from lake edward equatorial africa document complex interactions between climate variability and lacustrine geochemistry over the past 5400 years correlation of these records from lake edward to other climatically forced geochemical and lake level records from lakes naivasha tanganyika and turkana allows us to develop a chronology of drought events in equatorial east africa during the late holocene major drought events of at least century scale duration are recorded in lacustrine records at about 850 1500 2000 and 4100 cal year bp of these the most severe event occurred between about 2050 and 1850 cal year bp during which time lake edward stood about 15 m below its present level numerous additional droughts of less intensity and or duration are present in the lake edward record some of which may be correlated to other lacustrine climate records from equatorial east africa these events are superimposed on a long term trend of increasingly arid conditions from 5400 to about 2000 cal year bp followed by a shift toward wetter climates that may have resulted from an intensification of the winter indian monsoon although the causes of decade to century scale climate variability in the east african tropics remain obscure time series spectral analysis suggests no direct linkage between solar output and regional rainfall rather significant periods of 725 125 63 72 31 25 and 19 16 years suggest a tight linkage between the indian ocean and african rainfall and could result from coupled ocean atmosphere variability inherent to the tropical monsoon system

deformation in the vicinity of ice divides calculs numeriques aux elements finis de la variation de la vitesse horizontale avec la profondeur au voisinage d une difference de la glace symetrique isotherme et ou glissement sur un lit plat

implications of the form of the flow law for vertical velocity and age depth profiles in polar ice valeurs des exposants de la loi de fluage des glaciers devon island canada et camp century groenland

testing solar forcing of pervasive holocene climate cycles the temporal and spatial extent of holocene climate change is an area of considerable uncertainty with solar forcing recently proposed to be the origin of cycles identified in the north atlantic region to address these issues we have developed an annually resolved record of changes in irish bog tree populations over the last 7468 years which together with radiocarbon dated bog and lake edge populations extend the dataset back to 9000 yr ago the irish trees underpin the internationally accepted radiocarbon calibration curve used to derive a proxy of solar activity and allow us to test solar forcing of holocene climate change tree populations and age structures provide unambiguous evidence of major shifts in holocene surface moisture with a dominant cyclicity of 800 yr similar to marine cycles in the north atlantic indicating significant changes in the latitude and intensity of zonal atmospheric circulation across the region the cycles however are not coherent with changes in solar activity both being on the same absolute timescale indicating that holocene north atlantic climate variability at the millennial and centennial scale is not driven by a linear response to changes in solar activity copyright 2005 john wiley sons ltd

a flow law for anisotropic ice and its application to ice sheets a new formulation of the flow law of polycrystalline ice was derived for an rbitrary c axis orientation fabric pattern the rate factor of the flow law is given by a geometrical factor that can be calculated with c axis fabric data and stress condition a theoretical calculation combining this flow law with a lattice rotation model by intracrystalline slip in the constituent grains predicts the development of characteristic fabric patterns observed in many laboratory experiments and in the field according to this flow model of polycrystalline ice if significant recrystallization does not occur uniaxial compression makes the ice body harder with increasing strain whereas simple shear makes the ice body softer with strain this may significantly affect ice sheet dynamics

rate controlling processes in the creep of polar ice influence of grain boundary migration associated with recrystallization abstract information on the deformation modes and recrystallization processes in polar ice sheets was obtained thanks to the analysis of the ice structure along two deep ice cores from greenland and antarctica it is shown that the deformation of polar ice at low stresses is produced by intracrystalline slip accommodated by grain boundary migration gbm a deformation model based on an equilibrium between work hardening and recovery processes has been developed the decrease in dislocation density is due to both gbm associated with grain growth and the formation of boundaries by recrystallization the value of the dislocation density along the byrd and grip ice cores obtained thanks to the model is about 1 10 11 m 2 this value is in agreement with data from synchrotron x ray diffraction on samples taken along the core this deformation model can account for the deformation of polar ice at low stresses it is shown that the flow law with a stress exponent lower than 2 can be related to the efficiency of gbm as a recovery process

high resolution temperature and evaporation changes during the late holocene in the northeastern arabian sea in order to reconstruct the monsoonal variability during the late holocene we investigated a complete annually laminated sediment record from the oxygen minimum zone omz off pakistan for oxygen isotopes of planktic foraminifera and alkenone derived sea surface temperatures sst significant sst changes of up to 3 c which cannot be explained by changes in the alkenone producing coccolithophorid species inferred from the gephyrocapsa oceanica emiliania huxleyi ratio suggest that sst changes are driven by changes in the monsoon strength our high decadal resolution data indicate that the late holocene in the northeastern arabian sea was not characterized by a stable uniform climate as inferred from the greenland ice cores but by variations in the dominance of the sw monsoon conditions with significant effects on temperatures highest sst fluctuations of up to 3 0 c and 2 5 c were observed for the time interval from 4600 to 3300 years b p and during the past 500 years the significant short term sst changes during the past 500 years might be related to climatic instabilities known from the northern latitudes little ice age and confirm global effects surface salinity values reconstructed from 18o records after correction for temperature related oxygen isotope fractionation suggest that in general the past 5000 years were characterized by higher than recent evaporation and more intense sw monsoon conditions however between 4600 and 3700 years b p evaporation dropped sw monsoon weakened and ne monsoon conditions were comparatively enhanced for the past 1500 years we infer strongly fluctuating monsoon conditions comparisons of reconstructed salinity records with ice accumulation data from published tibetan ice core and tibetan tree ring width data reveal that during the past 2000 years enhanced evaporation in the northeastern arabian sea correlates with periods of increased ice accumulation in tibet and vice versa this suggests a strong climatic relationship between both monsoon controlled areas

triclinic transpression zones with inclined extrusion data from natural transpressional shear zones reveal a large range of lineation orientations that can spread along complete girdles one possible way to account for such a type of fabrics is variation in the extrusion direction this work presents a new theoretical model of oblique transpression with inclined extrusion the model is kinematic and the flow is considered as homogeneous and steady the angle between the simple shear direction and the extrusion direction is one of the main parameters controlling the final fabric low values of 40 can generate strain ellipsoids very close to the abscissa axis of the flinn plot depending on the vorticity of the flow the finite strain and the actual value of the angle the model predicts the generation of lineation fabrics ranging from point maxima to patterns with opposite plunge senses these patterns can display monoclinic nearly monoclinic or clearly triclinic symmetry lineation patterns spreading along complete girdles can be obtained by allowing the extrusion direction to deviate less than 30 about the vertical in either direction

paleohydrology of an upper aptian lacustrine system from northeastern brazil integration of facies and isotopic geochemistry abstract the codo formation records the initial evolutionary stages of an intracontinental rift system formed along the brazilian equatorial margin in the late aptian deposits of this unit exposed on the eastern margin of the grajau basin include gypsum bituminous black shales and limestones these lithologies were formed in a low energy well stratified anoxic and hypersaline lake system developed in a dominantly arid semi arid climate this lacustrine succession is internally organized into three categories of shallowing upward cycles with the first and second order cycles being related to seismic activity associated with fault reactivations and the third order cycles recording climatic fluctuations studies emphasizing petrography and analysis of the geochemical tracers fe mg sr mn na and ca helped to identify the sedimentary facies that kept a primary signal which were thus appropriate for isotopic investigations aiming paleoenvironmental and paleohydrologic reconstructions the results of this study revealed a wide distribution of dominantly low carbon and oxygen isotope values in carbonates ranging from 5 69 to 13 02 and from 2 71 to 10 80 respectively this paper demonstrates that at least in the particular case of oxygen the isotope ratios vary according to seismically induced shallowing upward cycles with values in general lower at their bases where central lake deposits dominate and progressively higher upward where marginal lake deposits are more widespread in addition to confirming a depositional signature for the analysed samples this behavior allowed the development of a seismic induced isotope model the lighter isotope ratios appear to be related to flooding events promoted by subsidence which resulted in the development of a perennial lake system while heavier isotope values are related to ephemeral lake phases favored by uplift and or increased stability furthermore the results show that a closed lake system dominated as indicated by the overall good positive covariance i e 0 42 to 0 43 between the carbon and oxygen isotopes though open phases are also recorded by negative covariance values of 0 36 during closed phases the 18 o displayed the highest range of variation i e 3 63 to 4 89 due to increased residence time while this variation was low i e 0 09 to 1 87 during open lake phases when there was a balance in the water isotope composition maintained by continuous basin inflow

facies architecture of flexural margin lowstand delta deposits in lake edward east african rift constraints from seismic reflection imaging abstract lake edward is single half graben rift basin unique in the east african rift system as a result of its connection to lake george via the kazinga channel a paleo river valley which is incised into the flexural margin of the lake using recently acquired normal incidence seismic reflection data and thirty three lake bottom sediment samples we examine the consequences of this hydrologic connection for understanding depositional processes and facies development along the flexural margin of lake edward three depositional sequences were defined in the study area based on top and bottom discordances of seismic reflections climatically induced changes in lake level appear to have forced sequence development sequence i deposits indicate that highstand conditions prevailed earlier in the pleistocene evidence for high frequency lake level change comes from lowstand delta clinoforms in two separate stratigraphic intervals on the flexural margin an older lowstand event occurred during sequence ii time and reduced lake level by at least 37 m a younger lowstand event occurred during the deposition of sequence iii and reduced lake level by 18 m clastic sedimentation on the flexural margin of lake edward is strongly modified by inflow from the kazinga channel and lake george during periods of lowered lake level we suggest that the presence of an incised valley at the margin of the rift controls the stratigraphic development of clinoforms diagnostic of lowstand deltaic sedimentation the morphology and orientation of these clinoforms suggest that discharge from lake george during arid periods utilized the kazinga channel to transport sediment toward the exposed flexural margin of lake edward dipping reflections in sigmoid oblique clinoforms suggest that these flows were capable of transporting sediment loads that included coarse grain sizes deposition by this process occurred several times during the late quaternary history of lake edward and represents an important modern analogue for linked incised valley lacustrine delta sequences in the stratigraphic record

climatic forcing of eastern mediterranean deep water formation and benthic ecosystems during the past 22 000 years abstract lateglacial and holocene faunal and stable isotope records from benthic foraminifers in the eastern mediterranean sea ems suggest a high spatiotemporal variability of deep water oxygenation and biogeochemical processes at the sea floor during that time changes in the oxygenation and food availability of the deep sea ecosystems are closely linked to the hydrology of the ems borderlands they reflect orbital and suborbital climate variations of the high northern latitudes and the african monsoon system during the last glacial maximum cool surface waters and high evaporation resulted in maximum convection and oxic deep waters in all sub basins strong wind induced mixing fostered surface water production with seasonal phytodetritus fluxes during the glacial termination and the holocene oxygenation and food availability of deep sea benthic ecosystems were characterized by a pronounced regional differentiation local deep water formation and trophic conditions were particularly variable in the northern aegean sea as a response to changes in riverine runoff and black sea outflow during the interval of sapropel s1 formation in the early holocene average oxygen levels decreased exponentially with increasing water depth suggesting a basin wide shallowing of vertical convection superimposed by local signals in the northernmost aegean sea deep water ventilation persisted during the early period of s1 formation owing to temperature driven local convection and the absence of low salinity black sea outflow at the same time severe temporary anoxia occurred in the eastern levantine basin at water depths as shallow as 900 m this area was likely influenced by enhanced nutrient input of the nile river that resulted in high organic matter fluxes and related high oxygen consumption rates in the water column in the southern aegean and levantine seas we observe a gradual increase in deep water residence times preceding s1 formation by approximately 1 1 5 kyr once oxygen levels fell below a critical threshold the benthic ecosystems collapsed almost synchronously with the onset of s1 deposition the recovery of benthic ecosystems during the terminal phase of s1 formation is controlled by subsequently deeper convection and re ventilation over a period of approximately 1500 years after the re ventilation of the various sub basins had been completed during the middle and late holocene deep water renewal was more or less similar to recent rates during that time deep sea ecosystem variability was driven by short term changes in food quantity and quality as well as in seasonality all of which are linked to millennial scale changes in riverine runoff and associated nutrient input

climate change in the turkana basin as deduced from a 4000 year long δo18 record abstract a model of the oxygen isotopic composition of lake turkana waters sheds insight into the paleoclimatic interpretation of a 18 o profile of inorganic calcite from a core taken from the lake the model indicates that high frequency fluctuations seen in the isotopic record are probably due to shifts in lake level while the long term increase of 1 4 seen from 4 to 2 kyr bp cannot be due solely to lake level change it must be due to changes in other climatic parameters such as the isotopic composition of the inflow these findings suggests that the abrupt change observed in lake turkana and many other african lakes at 4 ka was the precursor to a 2000 year long adjustment in the climate over the ethiopian plateau the isotopic record from lake turkana agrees with other paleoclimate records from the basin including the diatom stratigraphy and radiocarbon dates on exposed shoreline deposits

organic carbon preservation in southeastern arabian sea sediments since mid holocene implications to south asian summer monsoon variability the earlier studies show a contrasting long term trend of the south asian summer monsoon sasm after attaining the precessional forcing induced mid holocene maximum the increasing total organic carbon toc concentration of marine sediments in the southeastern arabian sea seas has been interpreted to imply strengthening sasm since mid holocene by a few studies however toc concentration is also influenced by redox conditions sedimentation rate and an influx of terrigenous matter depending on the regional settings so it needs to be ascertained whether the toc concentration of the sediments in the seas is a signal of productivity related to the sasm strength or preservation therefore we studied multiple proxies toc total nitrogen atomic c n 13corg caco3 and major and trace elements concentration for determining the productivity redox conditions detrital supply and provenance in a sediment core from the upper continental slope of the seas spanning the past 4700 years at centennial scale resolution the present study shows that the observed toc increase is not a result of enhanced productivity but is because of better preservation due to the increased sedimentation rate along with increasingly reducing conditions since mid holocene we further show that the sasm has been declining since mid holocene after attaining a precession forced maximum which corroborates the earlier model ensemble studies

lake level history of lake tanganyika east africa for the past 2500 years based on ostracode inferred water depth reconstruction assemblages of ostracodes from sediment cores illuminate lake level history at decadal to centennial timescales during the late holocene at lake tanganyika east africa the ostracode based lake level curves for several cores resemble both each other and the only previously published lake level record of comparable resolution for lake tanganyika during this interval successfully reconstructing known highstands improving the chronology of known lowstands and contributing new information on late holocene lake level variability at this important tropical african location in agreement with other late holocene records from east africa the surface level at lake tanganyika reflects predominantly arid conditions throughout this interval interrupted by relatively brief episodes of higher precipitation and lake level the most pronounced lowstand in the record occurs at v200 0 bc with other significant lowstands dating to the intervals v200 500 ad v700 850 ad the medieval warmperiod mwp v1050 1250 ad at lake tanganyika and the latter part of the little ice age lia v1550 1850 ad the most important wet intervals in the lake level record are centered on v500 ad v1500 ad and v1870 ad the highstands and lowstands reported here for lake tanganyika appear to be fairly coherent with other records of rainfall throughout east africa during the mwp and the lia prior to the mwp paleoclimate records are apparently less coherent although this may be a reflection of the resolution and abundance of recent paleoclimatic data available for this climatically complex region

measurement of vertical strain and velocity at siple dome antarctica with optical sensors as part of a larger program to measure and model vertical strain around siple dome on the west antarctic ice sheet we developed a new sensor to accurately and stably record displacements the sensors consist of optical fibers encased in thin wall stainless steel tubes frozen into holes drilled with hot water and stretched from the surface to various depths up to 985 m in the ice sheet an optical system connected annually to the fibers reads out their absolute lengths with a precision of about 2 mm two sets of five sensors were installed in the 1997 98 field season one set is near the siple dome core hole an ice divide and a second set is on the flank 7 km to the north the ice thickness at both sites is approximately 1000 m the optical fiber length observations taken in four field seasons spanning a 3 year interval reveal vertical strain rates ranging from 229 4 ppm a 1 to 7 9 ppm a in addition to confirming a non linear constitutive relationship for deep ice our analysis of the strain rates indicates the ice sheet is thinning at the flank and is in steady state at the divide

an 11 000 year lithostratigraphic and paleohydrologic record from equatorial africa lake edward uganda congo abstract sedimentologic and geochemical analyses of four cores from lake edward uganda congo document a complex record of moisture balance in the edward basin from 11 000 cal yr bp to present highly organic diatomaceous muds provide evidence for shifts in wind intensity and stratification within an early holocene wet phase lake level variations within this period may have shifted due to tectonic lowering of the lake s outlet level the onset of mid holocene aridity as documented by the appearance of authigenic calcite at 5200 cal yr bp initiated a period of falling lake levels that culminated in a late holocene lowstand between 4000 and 2000 cal yr bp this lowstand is documented by coarse sediments whose fabric and mineralogy depend on the core site although different cores yield different ages for this regression it appears that declining lake levels culminated in a maximum lake lowstand of 14 m lake levels then rose rapidly attaining modern positions by 1700 cal yr bp this lake level history suggests that although many paleoclimatic changes in africa are apparently synchronous throughout northern hemispheric africa other events may be spatially heterogeneous these patterns highlight the need for a well dated network of paleoclimate sites within the african continent

large scale ice sheet modelling as a means of dating deep ice cores in greenland the three dimensional ice sheet model sicopolis is used to simulate the dynamic thermodynamic behaviour of the entire greenland ice sheet from 250 000 a bp until today external forcing consists of a surface temperature history constructed from 18 o data of the grip core a snowfall history coupled linearly to that of the surface temperature a piecewise linear sea level scenario and a constant geothermal heat flux the simulated greenland ice sheet is investigated in the vicinity of summit the position where the maximum elevation is taken and where the two drill sites grip and gisp 2 are situated 28 km apart from each other in the region the aggreement between modelled and observed topography and ice temperature tuns out to be very good computed age depth profiles for grip and gisp2 are presented which can be used to complete the dating of these cores in the deeper regions where annual layer counting is not possible however artififical diffusion influences the computed ages in a near basel boundary lar of approximately 15 of theice thickness so that the age at the bottom of the cores cannot be predicted yet

a 23000 year old record of paleoclimatic and environmental changes from the eastern arabian sea abstract climate of the eastern arabian sea is highly influenced by both global climate and indian monsoon we produced multi proxy records of benthic foraminiferal biofacies and stable isotope ratio 13c and 18o planktic foraminifera and total organic carbon wt content of sediments from core sk291 gc11 in the upper continental shelf off mangalore off eastern arabian sea eas to decipher paleoenvironmental and paleoclimatic changes during the past 23 000 years the monsoon was weak during the last glacial maximum 23 000 18 000 calibrated years before present cal yr bp when sea level was lower globigerinoides ruber populations fluctuate from 30 to 60 in the studied interval with a maximum during 6000 5000 cal yr bp and a minimum during 19 000 17 000 cal yr bp the 18o values of benthic foraminifer ammonia gaimardii suggest pronounced cooling phases during 23 000 18 000 cal yr bp and a short term cooling at 6000 cal yr bp several intervals of high organic productivity and low oxygen conditions were identified based on benthic foraminiferal biofacies during 23 247 682 cal yr bp wind induced surface mixing was intense during 3200 530 cal yr bp as evidenced by high globigerina bulloides abundances the core was affected by moderate chemical weathering and solar variability appears to have been an important forcing factor in shaping climate of the region

stochastic perturbation of divide position perturbation of divide position is considered by a linearization about the vialov nye solution and also about related solutions with 0 1 relief relaxation times of one sixteenth the fundamental thickness accumulation rate time scale are found for the vialov nye configuration while substantial basal topography can halve the rate of relaxation steady divide position is most sensitive to anti symmetric accumulation rate distributions near the divide but transient divide motion is most strongly excited by anti symmetric accumulation rate variations halfway between the margin and the divide relaxation times for the antarctic peninsula divide position are estimated to be around 200 years while the larger greenland ice sheet has a divide position relaxation time of around 600 years modelling accumulation rate as a white noise process permits analysis of divide perturbation as a stochastic ornstein uhlenbeck process where the standard deviation of the response is proportional to the standard deviation of the forcing if observed accumulation rate variability in the antarctic peninsula were antisymmetric about the divide it would be sufficient to force the divide position to fluctuate with standard deviation 10 20 times the depth of the ice sheet there appears to be sufficient noise to cause raymond bumps to be spread significantly more data on the statistical variation of accumulation with position are needed random forcing will increase the complexity of any fold structures created in the divide region and in particular the number of such structures intersecting any borehole

orbital forcing of glacial interglacial variations in chemical weathering and silicon cycling within the upper white nile basin east africa stable isotope and biomarker evidence from lakes victoria and edward on quaternary time scales the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae notably diatoms for which si is an essential nutrient large tropical river systems dominate the export of si from the continents to the oceans here we investigate variations in si cycling in the upper white nile basin over the last 15 ka using sediment cores from lakes victoria and edward coupled measurements of stable o and si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and si cycling while the abundances of lipid biomarkers characteristic of terrestrial emergent higher plants submerged floating aquatic macrophytes and freshwater algae document past ecosystem changes during the late glacial to mid holocene 15 5 5 ka bp orbital forcing greatly enhanced monsoon rainfall forest cover and chemical weathering riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower si isotope values since 5 5 ka bp increasingly dry climates and more open vegetation reinforced by the spread of agricultural cropland over the last 3 4 ka have reduced dissolved silica inputs into the lakes centennial to millennial scale dry episodes are also evident in the isotopic records and merit further investigation

ice flow between the greenland ice core project and greenland ice sheet project 2 boreholes in central greenland the ice flow between the greenland ice core project grip and greenland ice sheet project 2 gisp2 boreholes in the summit region of central greenland is modeled with a steady state finite element model the model calculates the free ice sheet surface and the coupled ice flow and temperature fields along the flow line between the boreholes three dimensional effects are expressed in terms of the divergence of the flow and included in the coupled stress and temperature fields in addition to ice core data several geophysical surface programs have provided data that are used to constrain the model the modeled isochrones resemble the bedrock structure along the flow line but rise at the divide the rise at the divide is not seen in internal layers found by surface radio echo sounding an ice particle originally deposited at the surface moves through different stress and temperature regimes a set of trajectory lines are followed from the surface to the boreholes in order to follow the variation of stress deviators temperature and deformation along with the ice movement these variations along a trajectory line constitute a history of the corresponding ice layer the difference between the history of ice layers found in the grip and gisp2 boreholes is mainly due to the shear stress which is 3 4 times higher for gisp2 ice than for grip ice furthermore the grip ice experiences a nonzero longitudinal stress deviator much longer than the gisp2 ice

a 725 yr cycle in the climate of central africa during the late holocene high resolution analyses of the chemical and stable isotope composition of endogenic calcite from lake edward an equatorial rift lake in central africa reveal large oscillations in african climate and moisture balance over the past 5400 yr these oscillations indicate a series of droughts from 4900 to 400 yr b p that define a drought period of 725 yr this period may be correlated to variations in the indian ocean monsoons as documented by similar periods in sea surface salinity records from the northern indian ocean the similarities in periodicity of the lake edward and indian ocean records may indicate that low frequency variability within the indian and pacific oceans dominates millennial scale african moisture balance and the holocene climate of the tropical indian ocean

abrupt changes in the asian southwest monsoon during the holocene and their links to the north atlantic ocean during the last ice age the indian ocean southwest monsoon exhibited abrupt changes that were closely correlated with millennial scale climate events in the north atlantic region1 2 3 suggesting a mechanistic link in the holocene epoch which had a more stable climate the amplitude of abrupt changes in north atlantic climate was much smaller and it has been unclear whether these changes are related to monsoon variability here we present a continuous record of centennial scale monsoon variability throughout the holocene from rapidly accumulating and minimally bioturbated sediments in the anoxic arabian sea our monsoon proxy record reveals several intervals of weak summer monsoon that coincide with cold periods documented in the north atlantic region4 including the most recent climate changes from the medieval warm period to the little ice age and then to the present we therefore suggest that the link between north atlantic climate and the asian monsoon is a persistent aspect of global climate

a 5000 yr record of climate change in varved sediments from the oxygen minimum zone off pakistan northeastern arabian sea the upper holocene marine section from a kasten core taken from the oxygen minimum zone off karachi pakistan at water depth 700 m contains continuously laminated sediments with a sedimentation rate of 1 2 mm yr and a unique record of monsoonal climatic variability covering the past 5000 years our chronostratigraphy is based on varve counts verified by conventional and ams 14 c dating individual hemipelagic varve couplets are about 0 8 1 5 mm thick with light colored terrigenous laminae a deposited mainly during the winter monsoon alternating with dark colored laminae b rich in marine organic matter coccoliths and fish debris that reflect deposition during the high productivity season of the late summer monsoon august october precipitation and river runoff appear to control varve thickness and turbidite frequency we infer that precipitation decreased in the river watershed indicated by thinning varves after 3500 4000 yr b p this is about the time of increasing aridification in the near east and middle east as documented by decreasing nile river runoff data and lake level lowstands between turkey and northwestern india this precipitation pattern continued until today with precipitation minima about 2200 1900 yr b p 1000 yr b p and in the late middle ages 700 400 yr b p and precipitation maxima in the intervening periods as documented by spectral analysis the thickness of varve couplets responds to the average length of a 250 yr cycle a 125 yr cycle the gleissberg cycle of solar activity 95 yr and a 56 yr cycle of unknown origin higher frequency cycles are also present at 45 39 29 31 and 14 yr the sedimentary gray value also shows strong variability in the 55 yr band plus a 31 yr cycle because high frequency cyclicity in the enso band ca 3 5 and 5 yr is only weakly expressed our data do not support a straightforward interaction of the pacific enso with the monsoon driven climate system of the arabian sea

coherent high and low latitude climate variability during the holocene warm period a faunal record of sea surface temperature sst variations off west africa documents a series of abrupt millennial scale cooling events which punctuated the holocene warm period these events evidently resulted from increased southward advection of cooler temperate or subpolar waters to this subtropical location or from enhanced regional upwelling the most recent of these events was the little ice age which occurred between 1300 to 1850 a d when subtropical ssts were reduced by 3 to 4 c these events were synchronous with holocene changes in subpolar north atlantic ssts documenting a strong in phase link between millennial scale variations in high and low latitude climate during the holocene

does the el nino southern oscillation control the interhemispheric radiocarbon offset since the 1970s it has been recognised that southern hemisphere samples have a lower radiocarbon content than contemporaneous material in the northern hemisphere this interhemispheric radiocarbon offset has traditionally been considered to be the result of a greater surface area in the southern ocean and high latitude deepwater formation this is despite the fact that the el nino southern oscillation enso is known to play a significant role in controlling the interannual variability of atmospheric carbon dioxide by changing the flux of old co2 from the tropical pacific here we demonstrate that over the past millennium the southern hemisphere radiocarbon offset is characterised by a pervasive 80 yr cycle with a step shift in mean values coinciding with the transition from the medieval warm period to the little ice age the observed changes suggest an enso like role in influencing the interhemispheric radiocarbon difference most probably modulated by the interdecadal pacific oscillation and supports a tropical role in forcing centennial scale global climate change

complexity of holocene climate as reconstructed from a greenland ice core glaciochemical time series developed from summit greenland indicate that the chemical composition of the atmosphere was dynamic during the holocene epoch concentrations of sea salt and terrestrial dusts increased in summit snow during the periods 0 to 600 2400 to 3100 5000 to 6100 7800 to 8800 and more than 11 300 years ago the most recent increase and also the most abrupt coincides with the little ice age these changes imply that either the north polar vortex expanded or the meridional air flow intensified during these periods and that temperatures in the mid to high northern latitudes were potentially the coldest since the younger dryas event

replicated stalagmite inferred centennial to decadal scale monsoon precipitation variability in southwest china since the mid holocene a lack of agreement between stalagmite records of the centennial to decadal scale asian summer monsoon asm variability since the mid holocene limited our understanding of forcings and the possible impacts on culture evolution here we present replicated high resolution stalagmite 18o profiles from dark cave in guizhou province southwest china established with 512 oxygen isotope data and 28 230th ages providing a continuous history of the asm precipitation from 6100 to 300 years before present yr bp before ad 1950 consistency of stalagmite 18o inferred long term decreasing asm trends since the mid holocene between caves at this site and others in the indian and east asian monsoon realms on orbital timescales supports the effect of primary orbital solar forcings on monsoonal precipitation the dark cave records display significant centennial to decadal scale variations more evident than previous stalagmite records spectral analysis of the dark cave 18o time series yields periodicities of 731

little ice age drought in equatorial africa intertropical convergence zone migrations and el nino southern oscillation variability high resolution analyses of the mg concentration in authigenic calcite in five cores from lake edward provide a water balance history of central equatorial africa spanning the past 1400 yr a high ratio of mg to ca mg indicates strong droughts in central africa during the little ice age a d 1400 1750 in contrast to records from lake naivasha kenya which suggest a wet little ice age this spatial pattern in africa likely arose due to coupled changes in the high latitudes the position of the intertropical convergence zone and the el nino southern oscillation enso system our results further suggest that the patterns and variability of twentieth century rainfall in central africa have been unusually conducive to human welfare in the context of the past 1400 yr

high resolution holocene monsoon record from the eastern arabian sea through stable oxygen and carbon analyses of rapidly accumulating sediment cores from the eastern arabian sea we show that the excess of evaporation over precipitation e p steadily appears to have decreased during the last 10 000 to 2000 years most probably due to an increasing trend in the summer monsoon rainfall contrary to the land based paleoclimatic data from this region which indicate onset of aridity around 4000 years ago our results are consistent with the hypothesis that significant spatial variability in the monsoon rainfall observed today was persistent during most of the holocene alternatively the trend can be seen as an adjustment between two phases one between 10 000 and 6000 years ago of increasing precipitation and another between 3500 and 2000 years ago after the arid episode we also report a significant 700 year periodicity similar to that reported recently from the south china sea indicating that the centennial millennial scale response of the indian and chinese monsoons to high latitude forcing may be alike

palaeolimnological evidence for widespread late 18th century drought across equatorial east africa abstract the sediment records of three shallow climate sensitive lakes in equatorial east africa chibwera and kanyamukali in western uganda and baringo in central kenya contain clear lithological evidence of an episode of complete desiccation in the relatively recent past quantitative stratigraphic analyses of bulk sediment composition texture sand and coarse organic detritus the c n ratio of bulk organic matter and magnetic susceptibility allowed us to define a common succession of sedimentary units in all three lakes this common stratigraphy includes a basal unit of dry and stiff low organic clays capped by a horizon of peaty mud or peat overlain by organic lake sediment we interpret this stratigraphy to represent an episode of nearly complete desiccation of these lake basins this was followed first by an early phase of modest lake refilling during which time extensive littoral and submerged vegetation developed producing peaty lake deposits mostly higher lake levels later on resulted in deposition of fine grained lake muds using 210 pb and 137 cs activity profiles combined with four ams 14 c dates on single terrestrial plant macrofossils or multiple charred grass particles indicate that in all three lakes the episode of most recent complete desiccation occurred in the late 18th and early 19th century these results add to an accumulating body of palaeoenvironmental data revealing a prolonged period of extreme drought in equatorial east africa about 200 years ago also documented in the cultural history of the african great lakes region given the geographical spread of the study lakes across the east african plateau these results indicate that the climatic anomaly of reduced rainfall responsible for this drought was at least sub continental in scale

climate of the last 500 years high resolution ice core records precipitation accumulating on the earth s ice sheets and ice caps records a variety of physical and chemical information about the atmosphere and in some cases provides unique insight to both the history and the mechanics of the earth s environmental system high resolution well dated dust records from both polar ice sheets suggest a linkage between increased atmospheric dust and cooler temperatures over antarctica but a similar relationship is not observed in ice cores from either greenland or china net accumulation histories for the last 490 years reveal no discemible global pattern although interesting regional differences and similarities exist excess non sea salt sulfate profiles provide

flow law hypotheses for ice sheet modeling ice flow modeling requires a flow law relating strain rates to stresses in situ but a flow law cannot be measured directly in ice sheets microscopic processes such as dislocation glide and boundary diffusion control both the flow law for ice and the development of physical properties such as grain size and c axis fabric these microscopic processes can be inferred from observations of the physical properties and the flow law can then be estimated from the microscopic processes a review of available literature shows that this approach can be imperfectly successful interior regions of large ice sheets probably have depth varying flow law constants with the stress exponent n for power law creep less than 3 in upper regions and equal to 3 only in deep ice n probably equals 3 through most of the thickness of ice shelves and ice streams

superplasticity owing to grain growth in polar ices on considere le grossissement des grains de la glace depuis la surface jusqu a plusieurs centaines de metres de profondeur l augmentation de la taille avec leur ge est due a l energie libre des joints de grains cette migration continue des joints de grains semble etre un processus d accommodation beaucoup plus efficace que les processus de diffusion pour le glissement au joints de grains de faibles viscosites devraient resulter de cette deformation superplastique localement heterogene

effects of solar forcing and north atlantic oscillation on the climate of continental scandinavia during the holocene abstract 10 000 year long varved sediment records from lakes nautajarvi and korttajarvi finland provide evidence of climate and environment oscillations at multi decadal to millennial timescales we used two independent methods to extract periodic features from these time series of clastic laminae and assess their statistical reliability analyses revealed that seasonal sediment fluxes correspond to environmental changes with statistically significant periodicities of 1500 1800 1000 600 800 nearly 300 nearly 200 150 170 nearly 90 and 47 years showing variable coherency with different climate forcing factors and other palaeoproxy records in the northern hemisphere results indicate that the holocene winter climate in continental scandinavia was forced by a combination of several factors at least by solar variability and the north atlantic ocean atmosphere circulation patterns with varying influences through time

peregrinations of the greenland ice sheet divide in the last glacial cycle implications for central greenland ice cores abstract the superb quality of the climate chronology archived in the summit greenland ice cores grip gisp2 testifies that the greenland ice sheet divide has been generally stable through the last glacial cycle the ice sheet has experienced a broad range of paleoclimate conditions ice sheet margin configurations and internal dynamical adjustments in glacial interglacial transitions however it is unlikely that the summit region escaped shifts in ice divide position geometry elevation and flow characteristics details of this dynamical history are important to several aspects of ice core studies the magnitudes of pure and simple shearing reconstruction of vertical ice velocity the explicit location of the ice divide and the divide residence time at different locations are all of interest in interpretation of climatic variables and physical properties of ice in the ice cores we apply a three dimensional thermomechanical ice sheet model to examine the evolution of these dynamical variables over the last 160 kyr in central greenland while a high elevation ice dome is present in the summit region throughout the simulation ice divide migrations of up to 150 km are predicted all points in the vicinity of the summit ice cores including the modern divide have been subject to flowline shifts and variable non zero shear deformation during the adjustment from glacial to holocene conditions from ca 10 ka to the present modelled divide peregrinations and strain rate history are consistent with the observed disturbance of deep ice in the grip and gisp2 ice cores which has muddled paleoclimate reconstructions for the last interglacial eemian period in greenland dynamical excursions are also evident north of the modern summit where the ngrip ice core is currently being drilled dahl jensen et al j glaciol 43 1997 300 306 however the prevailing flow direction and deformation regime at the ngrip site are much more stable than those at grip and gisp2 in the simulations combined with the greater depth of ice at this site this lends cautious optimism to the hope that eemian ice at ngrip may contain an intact record of eemian climate

an analytical approach to deformation of anisotropic ice crystal aggregates deformation rates of single hexagonal crystals deforming by glide on the basal plane are described as a function of stress state and crystal orientation these results are used to infer the deformation rate of crystal aggregates assuming that the stress distribution within the crystal aggregate is homogeneous analytical equations for the deformation rate of anisotropic ice aggregates are derived for vertically symmetric girdle fabric this type of fabric is approximated by a uniform distribution of c axis orientations between a cone angle and a smaller girdle angle relative to the symmetry axis for simple shear stress acting on a single maximum fabric there is a slight de enhancement for cone angles of 60 90 in uniaxial compression a maximum enhancement of 1 7 occurs at a cone angle of 57 a pure shear stress state has similar features with the additional complication that it causes a non zero transverse strain rate except for perfect vertical alignment of crystals and isotropic fabric in combined states of stress the contribution of each stress component to the strain rate depends on fabric a single enhancement factor is not adequate to describe the effects of anisotropy for complex stress states

late holocene climate change in the north atlantic and equatorial africa millennial scale itcz migration 1 climate proxy data and numerical models suggest that latitudinal displacements of the intertropical convergence zone itcz occur during millennial scale cold events in the north atlantic region but the potential effects of these itcz movements on the climate of equatorial regions are unclear here we present a 5400 year geochemical record of rainfall and drought from lake edward in equatorial africa we observe a non linear correlation in which drought in equatorial africa occurs during both cold and warm extremes in the north atlantic s 1500 year quasi cycle we propose that this relationship occurs due to northward southward displacement of the itcz from its equatorial mean position during warm cold events our results show that millennial scale high latitude climate events are linked to changes in equatorial terrestrial climate even during the late holocene and suggest important constraints on the mechanisms linking tropical and extratropical climate variability

past precipitation rates derived from ice core measurements methods and data analysis ice cores in which annual layers can be identified or particular horizons dated provide a record of past precipitation rates if measured layer thicknesses can be corrected for thinning by flow since deposition total strain is usually calculated from steady state flow models because of lack of information about the ice cap s history an assumption of uniform vertical strain rate is valid at best in the upper half of the ice column complex models are needed to take account of uneven bedrock and variations in precipitation rate upstream of the borehole drilling at an ice divide avoids these complications new data analyses suggest that precipitation at camp century has fluctuated by only plus or minus 10 for the past 8500 years whereas at devon island on the opposite side of baffin bay precipitation has been uniform only for the past 1300 years with significantly higher values before that estimation of pre holocene precipitation rates is especially difficult because that ice may have an effective viscosity different from holocene ice three published estimates of pre holocene precipitation at camp century are unreliable the authors did not allow for thinning and their time scales differ appreciably data from three antarctic stations show some inconsistencies measuring concentrations of 10be in ice cores may be an alternative method of determining past precipitation rates

variability of the brazil current during the late holocene abstract our understanding of the centennial scale variability of the brazil current bc during the late holocene is elusive because of the lack of appropriate records here we used the mg ca and oxygen isotopic composition of planktonic foraminifera from two marine sediment cores collected at 27 s and 33 s off southeastern south america to assess the late holocene variability in the upper water column of the bc our results show in phase fluctuations of up to 3 c in sea surface temperatures sst and 0 8 in oxygen isotopic composition of surface sea water a proxy for relative sea surface salinity sss time series analyses of our records indicate a cyclicity with a period of ca 730 yr we suggest that the observed cyclicity reflects variability in the strength of the bc associated to changes in the atlantic meridional overturning circulation amoc positive negative sst and sss anomalies are related to a strong weak bc and a weak strong amoc moreover periods of peak strength in the bc occur synchronously to a weak north brazil current negative sst anomalies in the high latitudes of the north atlantic and positive negative precipitation anomalies over southeastern south america equatorial africa further corroborating our hypothesis this study shows a tight coupling between the variability of the bc and the high latitudes of the north atlantic mediated by the amoc even under late holocene boundary conditions

an anisotropic flow law for ice sheet ice and its implications abstract a new flow law or anisotropic polycrystalline ice is presented the strain rate tensor is related by a geometrical actor tensor g to the stress tensor the g factor tensor can be obtained from the c axis fabric data and stress condition this new flow law describes well the direction dependent mechanical properties of anisotropic ice which cannot be demonstrated by glen s flow law for example the new flow law can explain the fact that a strong single maximum fabric ice such as dye 3 vvisconsin ice can deform several times aster than isotropic ice under horizontal shear but can hardly deform under vertical or horizontal normal stress i ve also show that at a deeper part of an ice sheet where a single maximum fabric develops a positi e vertical strain rate can be produced with only a horizontal shear stress 1 introduction ice sheet modelling and studies on ice sheet dynamics carried out so far have been based on glen s flow law