stable carbon and oxygen isotopic evidence for late pleistocene to middle holocene climatic fluctuations in the interior of southern africa stable carbon and oxygen isotope analyses of ungulate grazers from four archaeolog ical sites located in different environs within the caledon river valley have provided a relatively well dated proxy palaeoenvironmental and palaeoclimatic sequence for the period between 16 000 and 6000 calendar cal yr bp within the overall trend towards hot mid holocene temperatures and a summer rainfall pattern stable carbon isotope results show that there were three periods when growth season temperatures were cool enough for c3 grasses to be present 16 000 14 000 10 200 9600 and 8400 8000 cal yr bp similar trends were recorded in stable oxygen isotope values reflecting shifts in either temperature or available moisture although having a similar pattern to that of the lower altitude site sites situated in foothills and montane portions of the valley consistently maintained lower temperatures until the mid holocene altithermal at this time growth season temperatures warmed sufficiently for a 100 c4 grassland to expand in altitude from the warmer low lying localities in relation to present understanding of synoptic and global climatic patterning these findings suggest that the early to middle holocene transition was not a gradual warming trend but rather it was marked by a series of climatic fluctuations of particular note is the possible global rather than regional occurrence of the 8200 cal yr bp event copyright 2002 john wiley sons ltd journal of quaternary science

third millennium b c climate change in syria highlighted by carbon stable isotope analysis of 14c ams dated plant remains from ebla abstract we use ams accelerator mass spectrometry techniques for the simultaneous analysis of carbon stable isotopic values and 14c dating on charred plant remains found in the archaeological site in ebla syria and propose the diachronic variation in 13c plant values as a possible palaeoclimatic tool both anthracological and carpological remains usually employed in palaeoenvironmental palaeoagricultural palaeonutritional and palaeoeconomical reconstructions are used to study stable carbon isotopic variations in a chronological framework defined through radiocarbon ams measurements the results obtained from the samples of the ebla archaeological site show a chronological correspondence with other climatic reconstructions based on various proxy data available for the near east and shed new light on the aridity crisis of the iii millennium b c

warming phases in southern africa during the last 150 000 years an overview abstract firmly based palaeoenvironmental data for southern africa prior to 40 000 yr b p are sparse the only site which is likely to be able to provide useful information for isotope stage 6 is the pretoria saltpan warming at the end of this stage appears to have been rapid and to have been associated with increased precipitation in the interior of the sub continent in contrast dry conditions appear to have prevailed in the southwestern areas during the warmest phases of isotope stage 5 this regional difference first became apparent during the pliocene during the last glacial maximum temperatures based on isotope ratios reached their lowest values of 5 6 c below present in the southern part of the subcontinent and widespread dryness prevailed warming became quite rapid after 16 000 yr b p and was almost everywhere associated with a marked increase in wetness to around or even above present levels this warming phase continued until the beginning of the holocene the terminal pleistocene was however marked by desiccation in the arid western areas of special interest is the holocene altithermal which can be placed between 7000 and 6500 yr b p the extent of the temperature rise cannot be specified with any precision but probably did not exceed the holocene mean by more than 2 c in the southern cape and karoo more summer rain apparently occurred than during isotope stage 2 and in the early holocene and this trend towards year round rainfall appears to have become increasingly pronounced through the later holocene until about 2000 yr b p this argues in a general way for an increase in the proportion of summer rainfall with rising temperature in the present winter rainfall areas of southern africa

the geochemistry of major and selected trace elements in a forested peat bog kalimantan se asia and its implications for past atmospheric dust deposition abstract biogeochemical processes in a forested tropical peat deposit and its record of past atmospheric dust deposition were assessed using the vertical distribution of lithophilic and plant essential elements in a dated core profile from borneo se asia peat formation started 22 120 14c yr before present bp and ca mg mass ratios of the solid peat and very low ash contents indicate a strongly ombrotrophic character throughout the deposit implying that most of the inorganic fraction has been supplied exclusively by atmospheric inputs concentration profiles of mn sr and ca suggest a very minor influence of chemical diagenesis in the underlying sediments silicon ca mg p s and k show a strong and extended zone of enrichment in the top 200 cm of the profile indicating that biological accumulation mechanisms are much more extensive than in temperate peat bogs in the lower core sections where the element distribution is dominated solely by past atmospheric deposition average al ti ratios are similar to the upper continental crust ucc whereas fe is slightly enriched and si is strongly depleted this condition favors highly weathered tropical soil dust as the main inorganic mineral source significant correlation of al fe si s ca and ti with the lithophilic elements y and zr suggests that the distribution of these elements is controlled by sources of atmospheric mineral dust the ca mg ca k and mg k ratios of the collected rainwater samples are similar to the global average of continental rainwater and suggest a continental character for the site this is supported by the similarity of the average concentration of br mg ca and s to that in temperate continental and maritime bogs in switzerland and scotland the concentration profiles of si fe al and ti show distinct peaks within the profile implying enhanced dust deposition reduced rates of peat accumulation or possibly both owing to climatic changes during the holocene enhanced dust deposition between 10 830 and 9060 14c yr bp is tentatively interpreted as a younger dryas like event with dust fluxes of 10 8 mg m2 yr the variations in al ti and fe ti profiles suggest that mineral dust sources have been changing constantly during the holocene with local sources being dominant between 7820 and 9500 14c yr bp and long range transport derived most likely from china being important during the late pleistocene and early holocene and from 7820 14c yr bp to the present

tide induced seawater groundwater circulation in shallow beach aquifers summary in this paper we investigated the tide induced seawater groundwater circulation in shallow beach aquifers using the finite element model marun the numerical solutions were generalized using a dimensionless formulation from a dimensionless tidal period and a dimensionless beach slope of 10 we obtained results that apply to a wide range of beach permeabilities from 10 4 m s to 10 3 m s beach slopes from 3 16 to 31 6 tidal amplitude 0 3 m 2 m and period diurnal or semidiurnal the numerical simulations demonstrated the following the maximum darcy velocity always occurs at the intersection of the watertable and the beach surface the offshore beach groundwater is almost stagnant compared with the onshore groundwater flow which may explain the previous observations that the major portion of the seaward groundwater seepage usually occurs in the shallow part of the submerged beach the outflow from the seepage face accounts for 41 97 average 55 of the outflow from the intertidal zone the amount of seawater infiltrating into the intertidal zone in a tidal cycle increases with the beach permeability and decreases when the inland recharge increases and ranges from 35 5 m 3 yr 1 m 1 to 505 8 m 3 yr 1 m 1 for all the cases considered smaller beach slopes smaller inland freshwater recharges and or greater beach permeability lead to larger saltwater plumes in the intertidal zone of the beach the results are in line with the existing results of field observations and numerical simulations by previous researchers a 2008 elsevier b v all rights reserved

holocene rainfall variability in southern chile a marine record of latitudinal shifts of the southern westerlies geochemical and clay mineral parameters of a high accumulation marine sediment core from the chilean continental slope 41 s provide a 7700 yr record of rainfall variability in southern chile related to the position of the southern westerlies we especially use the iron content measured with a time resolution of ca 10 yr on average of 14 caccelerator mass spectrometry dated marine sediments as a proxy for the relative input of iron poor coastal range and iron rich andean source rocks variations in this input are most likely induced by rainfall changes in the continental hinterland of the core position based on these interpretations we find a pronounced rainfall variability on multicentennial to millennial time scales superimposed on generally more arid conditions during the middle holocene 7700 to 4000 cal yr b p compared to the late holocene 4000 to present this variability and thus changes in the position of the southern westerlies are first compared to regional terrestrial paleoclimate data sets from central and southern chile in order to derive possible wider implications and forcing mechanisms of the holocene latitudinal shifts of the southern westerlies we then compare our data to ice core records from both tropical south america and coastal antarctica these records show similar bands of variability centered at ca 900 and 1500 yr comparisons of band pass filters suggest a close connection of shifts of the southern westerlies to changes within the tropical climate system the correlation to climate conditions in coastal antarctica shows a more complicated picture with a phase shift at the beginning of the late holocene coinciding with the onset of the modern state of el nin o southern oscillation system the presented data provide further evidence that the well known millennial scale climate variability during the last glacial continued throughout the holocene fl 2001 elsevier science b v all rights reserved

numerical modelling of tide induced beach water table fluctuations field studies have shown that the elevation of the beach groundwater table varies with the tide and such variations affect significantly beach erosion or accretion in this paper we present a bem boundary element method model for simulating the tidal fluctuation of the beach groundwater table the model solves the two dimensional flow equation subject to free and moving boundary conditions including the seepage dynamics at the beach face the simulated seepage faces were found to agree with the predictions of a simple model turner 1993 the advantage of the present model is however that it can be used with little modification to simulate more complicated cases e g surface recharge from rainfall and drainage in the aquifer may be included the latter is related to beach dewatering technique the model also simulated well the field data of nielsen 1990 in particular the model replicated three distinct features of local water table fluctuations steep rising phase versus flat falling phase amplitude attenuation and phase lagging

the effects of oscillation period on groundwater wave dispersion in a sandy unconfined aquifer sand flume experiments and modelling summary this paper presents a new laboratory sand flume dataset on the propagation of groundwater waves in an unconfined sandy aquifer with a vertical boundary subject to simple harmonic forcing with a wide range of oscillation period from 10 7 s to 909 s the data is unique in that it covers a much wider range of non dimensional aquifer depths n d k where n is the porosity is the angular frequency d is the aquifer depth and k is the hydraulic conductivity than has been previously investigated both the amplitude decay rate and rate of increase in phase lag of the water table waves are observed to monotonically increase with increasing oscillation frequency increasing n d k this is in contrast to existing theoretical dispersion relations which predict 1 zero phase lag or standing wave behaviour and 2 an asymptotic decay rate as the frequency increases possible influences on the experimental data including sand packing measurement location finite amplitude wave effects unsaturated zone truncation and multiple wave mode effects are unable to explain the discrepancy the data was also compared against numerical solutions of richards equation with and without hysteresis and in both cases the same qualitative behaviour as the analytic solutions described above is found the discrepancy between data and predictions remains unexplained and highlights a knowledge gap that requires further investigation these findings relate directly to practical applications in the field of surface groundwater interactions such as the influence of wave forcing of coastal aquifers on contaminant transport sediment mobility and salt water intrusion all of which are influenced by the dispersion of the groundwater wave

pollen and diatom inferred climatic and hydrological changes in sumxi co basin western tibet since 13 000 yr b p although the tibetan plateau greatly influences the atmospheric circulation of the nortbern hemisphere few continuous paleoclimatic records are available from the plateau a 13 000 yr pollen and diatom record from the sumxi longmu co basin in western tibet gives information on major changes both in regional vegetation and in local hydrology after the basin first filled ca 13 000 yr b p a dry spell occurred about 10 500 yr b p within the interval spanned by the european younger dryas chronozone a major environmental change occurred suddenly at 10 000 yr b p with the establishment of wet conditions and was followed by a long term trend toward maximum aridity which lasted approximately 6000 yr short term oscillations are superimposed on this general climatic change with a major reversal event about 8000 yr b p and a second wet pulse leading to a maximum lake volume ca 7500 6000 yr b p maximum aridity occurred 4300 yr b p the major environmental fluctuations recorded at sumxi longmu co appear in phase with climatic changes recognized in north tropical africa suggesting that the 8000 to 7000 yr b p event was caused by an abrupt disequilibrium in the climatic system as was the younger dryas and possibly the 4300 yr b p event

indian monsoon and north atlantic oscillation signals reflected by cl and na in a shallow ice core from dasuopu glacier xixabangma himalaya

environmental changes and the rise and fall of civilizations in the northern horn of africa an approach combining δd analyses of land plant derived fatty acids with multiple proxies in soil the domains of the ancient polities d mt and aksum in the horn of africa s highlands are a superior natural system for evaluating roles of environmental change on the rise and fall of civilizations to compare environmental changes of the times of the two polities we analyzed stable hydrogen isotopic ratios d of land plant derived fatty acids n c26 30 and other proxies from soil sequences spanning the holocene from the region three results suggest that trends in d values unambiguously reflect changes in rainfall first increases in d coincide with dry periods inferred from studies of eastern african lakes second changes in d values were parallel among sections during overlapping time intervals third consideration of vegetation history did not alter directions of trends in d values over time by unambiguously recording precipitation the d values also enhanced interpretations of proxies that are affected by both climate and land clearing both d mt ca 2750 2350 cal y bp and the aksumite ca 2100 1250 cal y bp rose during wetter intervals of the drier part of the holocene after ca 6000 cal y bp analyses of charred matter indicated that fire had been a common agent of land clearing in all sites the influence of climate on fire varied however prior to the emergence of d mt d values were correlated with c4 c3 plant ratios estimated from 13c values there are no c4 trees and precipitation may have been the main influence on canopy openness after ca 4300 cal y bp there was no significant relationship between d and c4 c3 plant ratios suggesting that factors such as fire influenced canopy openness regardless of climate furthermore the impact of land clearance differed between sites and between d mt and the aksumite s times in one site the interval from 3550 cal y bp to the decline of d mt had several anomalies that suggested dramatic increases in thermal severity of fire and human impact among these were a large contribution of charred matter to a high total organic carbon that low hydrogen and oxygen indices suggest was severely altered by other factors than humification these results support hypotheses about the rise of civilizations being favored by specific climatic conditions but suggest that patterns of land clearing differed during the declines of d mt and the aksumite

geochemistry of the peat bog at etang de la gruere jura mountains switzerland and its record of atmospheric pb and lithogenic trace metals sc ti y zr and ree since 12 370 14 c yr bp a 650 cm core from a swiss bog represents 12 370 14c years of peat accumulation and provides the first complete record of atmospheric pb deposition for the entire holocene excess non atmospheric sr in the peat was calculated by normalizing sr sc to crustal abundance this was used to differentiate between the ombrogenic section of the bog in which inorganic solids are supplied exclusively by atmospheric deposition and the minerogenic zone where mineral water interactions contribute metals to the peat while sediment dissolution contributes significantly to the sr inventory of the minerogenic section of the peat profile there is no measurable effect of this process on the pb burden isotopic analyses 204pb 206pb 207pb 208pb show that effectively all of the pb in the peat profile was supplied exclusively from the atmosphere to separate natural and anthropogenic pb sc was selected over ti y zr hf and ree as a conservative reference element which is supplied by soil dust aerosols derived from rock weathering enrichment factors ef were calculated using the pb sc ratio in the peat samples normalized to the natural background pb sc which is found in peats dating from 8030 to 5320 14c yr bp the results show that anthropogenic source have dominated the supply of atmospheric pb to the peat core continuously since 3000 14c yr bp the aerosols supplied to the bog can be divided into 3 classes a pre anthropogenic older than 3000 14c yr bp with pb ef 1 194 b pre industrial dating from 3000 240 14c yr bp with pb ef 2 but 20 and 206pb 207pb 1 179 elevated soil dust fluxes are observed at 5320 8230 and 10 590 14c yr bp the latter corresponds to the younger dryas aluminum zr hf and ree sc ratios also are elevated at the same depths suggesting differences in particle size wind strength or source regions pre anthropogenic aerosols deposited since 8230 14c yr bp reveal pb sc which is significantly higher and 206pb 207pb which is less radiogenic than during the early holocene while the trend toward increasing pb sc could be due to chemical weathering and soil development this could not explain the shift in pb isotopic composition the changes which took place at 8230 14c bp therefore may be related to a large scale climatic reorganization which at present is poorly understood

climate change during the last deglaciation in antarctica greenland ice core records provide clear evidence of rapid changes in climate in a variety of climate indicators in this work rapid climate change events in the northern and southern hemispheres are compared on the basis of an examination of changes in atmospheric circulation developed from two ice cores high resolution glaciochemical series covering the period 10 000 to 16 000 years ago from a central greenland ice core and a new site in east antarctica display similar variability these findings suggest that rapid climate change events occur more frequently in antarctica than previously demonstrated

influence of variable salinity conditions in a tidal creek on riparian groundwater flow and salinity dynamics while recent studies have revealed that tidal fluctuations in an estuary significantly affect groundwater flows and salt transport in the riparian zone only seawater salinity in the estuary has been considered a numerical study is conducted to investigate the influence of estuarine salinity variations on the groundwater flow and salt dynamics in the adjacent aquifer to extend our understanding of these complex and dynamic systems tidal salinity fluctuations synchronous with estuary stage were found to alter the magnitude and distribution of groundwater discharge to the estuary which subsequently impacted on groundwater salinity patterns and residence times especially in the riparian zone the effects of salinity fluctuations were not fully captured by adopting a constant time averaged estuarine salinity the modelling analysis also included an assessment of the impact of a seasonal freshwater flush in the estuary similar to that expected in tropical climates e g mean estuary level during flood significantly greater than average on adjacent groundwater flow and salinity conditions the three month freshwater flushing event temporarily disrupted the salt distribution and re circulation patterns predicted to occur under conditions of constant salinity and tidal water level fluctuations in the estuary the results indicate that the salinity variations in tidal estuaries impact significantly on estuary aquifer interaction and need to be accounted for to properly assess salinity and flow dynamics and groundwater residence times of riparian zones

observations of the latitude dependence of the location of the martian magnetic pileup boundary 1 we report observations that show the dependence of the altitude of the magnetic pileup boundary mpb at mars on planetary latitude as seen by the mars global surveyor magnetometer electron reflectometer instrument the mpb is further away from mars on average at southern latitudes than at northern latitudes the data are consistent with a mpb distance mapped to the terminator plane that does not vary with latitude in the northern hemisphere but increases with increasing southern latitude in the southern hemisphere we also report increased variability in the mpb distance within the longitude range 90 270 e longitude in the southern hemisphere which is the region that contains the strongest crustal magnetic fields these trends are most obvious in a planet fixed coordinate system indicating a planet fixed driver of the mpb location the proposed mechanism is the local diversion of shocked solar wind flow by crustal magnetic fields

model data comparison for the 8 2 ka bp event confirmation of a forcing mechanism by catastrophic drainage of laurentide lakes abstract to improve our understanding of the mechanism behind the 8 2 ka bp cooling event we compare proxy evidence with climate model simulations in which the thermohaline ocean circulation is perturbed by a freshwater pulse into the labrador sea both the proxy data and model results show a cooling that is mainly concentrated in the north atlantic region ranging from more than 5 c cooling in the nordic seas to about 0 5 1 c over europe and less than 0 5 c over the subtropical north atlantic data and model also indicate a weakening of the summer monsoon and generally a drier circum north atlantic over the south atlantic ocean the model simulates a slight warming mostly less than 0 5 c which falls within the uncertainty of proxy data and thus could not be confirmed to examine in detail the structure of the 8 2 ka bp event we also compare the modeled climatological evolution at two locations with high quality records revealing a generally consistent picture the good model data agreement confirms the hypothesis that the 8 2 ka bp event was forced by a freshwater induced weakening of the thermohaline circulation other forcings are unlikely since they would result in an alternative geographical distribution and expression of the climate response

tidal dynamics of the water table in beaches tidal motions of the water table height inside a sloping beach are investigated via field measurements and theoretical considerations only the movements forced by the tide are considered so a beach with negligible wave activity was chosen for the field measurements the data show that even in the absence of precipitation the time averaged inland water table stands considerably above the mean sea level also the water table at a fixed point inside the beach is far from sinusoidal even though its variation is forced by an essentially sinusoidal tide this latter effect is due to the boundary condition along the sloping beach face which acts as a highly nonlinear filter the observed behavior of the water table is explained in terms of perturbation extensions to the classical deep aquifer solution one extension deals with the nonlinearity in the interior the other with the boundary condition at the sloping beach face

the melting history of the late pleistocene antarctic ice sheet spatial and temporal variations in the sea levels of the past 20 000 years around the globe place constraints on the melting history of the major late pleistocene ice sheets the antarctic ice sheets provided a significant contribution to the sea level rise at a rate that was approximately synchronous with the melting of the laurentide ice sheet except for the interval 9 000 6 000 years ago when it may have lagged behind minor melting of the antarctic ice sheet continued throughout the holocene

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

tidal influence on behaviour of a coastal aquifer adjacent to a low relief estuary the tidal influence on groundwater hydrodynamics salt water intrusion and submarine groundwater discharge from coastal estuarine aquifers is poorly quantified for systems with a mildly sloping beach in contrast to the case where a vertical beach face is assumed we investigated the effect of beach slope for a coastal aquifer adjacent to a low relief estuary where industrial waste was emplaced over the aquifer the waste was suspected to discharge leachate towards the estuary field observations at various locations showed that tidally induced groundwater head fluctuations were skewed temporally frequency analysis suggested that the fluctuation amplitudes decreased exponentially and the phase lags increased linearly for the primary tidal signals as they propagated inland salinisation zones were observed in the bottom part of the estuary and near the beach surface flow and transport processes in a cross section perpendicular to the estuary were simulated using seawat 2000 which is capable of depicting density dependent flow and multi species transport the simulations showed that the modelled water table fluctuations were in good agreement with the monitored data further simulations were conducted to gain insight into the effects of beach slope in particular the limiting case of a vertical beach face was considered the simulations showed that density difference and tidal forcing drive a more complex hydrodynamic pattern for the mildly sloping beach than the vertical beach as well as a profound asymmetry in tidally induced water table fluctuations and enhanced salt water intrusion the simulation results also indicated that contaminant transport from the aquifer to the estuary was affected by the tide where for the mildly sloping beach the tide tended to intensify the vertical mass exchange in the vicinity of the shoreline

late quaternary western mediterranean pollen records and african winds abstract late quaternary vegetation changes of regional and global significance in the mediterranean region are generally interpreted as being controlled by changes in the circulation patterns of north atlantic air masses however the possibility cannot be excluded that they may also be related to winds blowing from africa long pollen records from southwest europe show that cedrus pollen of northwest african provenance is found during the glacial periods and occasionally during abrupt deforestation events in the forest dominated periods of the interglacials a pollen concentration record from central italy shows that during the holocene the presence of cedrus pollen coincides with two abrupt deforestation events around 8 1 and 4 2 cal kyr bp these observations raise the question of a possible influence of eolian activity on arboreal vegetation during phases of climatic stress towards aridity and suggest the use of modern pollen monitoring as a strategy for revealing tendencies towards aridification in southwestern europe

the last deglaciation event in the eastern central arctic ocean oxygen isotope records of cores from the central arctic ocean yield evidence for a major influx of meltwater at the beginning of the last deglaciation 15 7 thousand years ago 16 650 calendar years b c the almost parallel trends of the isotope records from the arctic ocean the fram strait and the east greenland continental margin suggest contemporaneous variations of the eurasian arctic and greenland laurentide ice sheets or increased export of low saline waters from the arctic within the east greenland current during the last deglaciation on the basis of isotope and carbon data the modern surface and deep water characteristics and seasonally open ice conditions with increased surface water productivity were established in the central arctic at the end of termination lb about 7 2 thousand years ago or 6 000 calendar years b c

late weichselian valdaian and holocene vegetation and environmental history of the northern timan ridge european arctic russia abstract lake and peat deposits from the timan ridge arctic russia were pollen analysed reconstructing the vegetation history and paleoenvironment since the last glacial maximum lgm 20 18 000 years ago the sites studied are located inside the margins of a large paleolake of about 20 km 2 by us named lake timan this lake developed in the late weichselian more than 30 000 years after the deglaciation of this region and was formed due to increased precipitation and warmer summers that accelerated the melting of stagnant ice within its catchment the lake was drained during the early holocene when the outlet rivers eroded the spillways a new generation of much smaller lakes formed during the holocene when the last remnants of buried glacier ice melted away causing the exposed floor of lake timan to subside since deglaciation the following regional vegetation development has been recorded 1 during the initial stage of lake timan the dominant vegetation was discontinuous steppe tundra with patches of snow bed vegetation 2 a dwarf shrub tundra established during the late weichselian interstadial allerod probably reflecting warmer and moister conditions 3 the younger dryas cooling is recognised by a reversal to steppe tundra and snowbeds on unstable mineral soils and higher palynological richness 4 soon after the transition into the holocene a birch forest established on the timan ridge 5 a cooling starting around 8200 cal years bp initiated the deforestation of the exposed hills in the most protected sites birch trees persisted until later than 4000 years ago reflecting a gradual development into the present treeless dwarf shrub tundra

dust clouds rain types and climatic variations in tropical north africa abstract dust and processes of raindrop formation in the clouds play a very important role in the climatic evolution of tropical north africa sedimentologic stratigraphic pedologic geomorphologic and palynologic data converge to show that a major environmental change occurred in tropical africa about 7000 yr b p in the sudanian and sudano guinean zones wet tropical zone from 15 000 to 7000 yr b p rivers deposited mostly clay while from 7000 to 4000 yr b p they deposited mostly sand during the first period pedogenesis was vertisolic montmorillonite dominant associated with pollen belonging mostly to vegetation typical of hydromorphic soils while during the second period pedogenesis was of ferruginous type kaolinite dominant with pollen belonging mostly to vegetation typical of well drained soils the great change near 7000 yr b p is linked chiefly to a major hydrological change that appears related to a change in the size of raindrops from fine rains associated with considerable atmospheric dust raindrop diameter essentially less than 2 mm to the second period associated with thunderstorm rains raindrop diameter mostly greater than 2 mm the size of raindrops is related particularly to cloud thickness and dust concentration in the troposphere thunderstorm activity is influenced also by fluctuations of the atmospheric electricity modulated by the sun

a 10 000 year high resolution diatom record from pilkington bay lake victoria east africa a new diatom record from lake victoria s pilkington bay subsampled at 21 to 25 year intervals and supported by 20 ams dates reveals a 10 000 calendar year environmental history that is supported by published diatom and pollen data from two nearby sites with their chronologies adjusted here to account for newly documented ancient carbon effects in the lake these three records provide a coherent finely resolved reconstruction of holocene climate change in equatorial east africa after an insolation induced rainfall maximum ca 8800 8300 cal yr b p precipitation became more seasonal and decreased abruptly ca 8200 and 5700 yr b p in apparent association with northern deglaciation events century scale rainfall increases occurred ca 8500 7000 5800 and 4000 yr b p conditions after 2700 yr b p were generally similar to those of today but major droughts occurred ca 1200 600 yr b p during europe s medieval warm period

reef drowning during the last deglaciation evidence for catastrophic sea level rise and ice sheet collapse elevations and ages of drowned acropora palmata reefs from the caribbean atlantic region document three catastrophic metre scale sea level rise events during the last deglaciation these catastrophic rises were synchronous with 1 collapse of the laurentide and antarctic ice sheets 2 dramatic reorganization of ocean atmosphere circulation and 3 releases of huge volumes of subglacial and proglacial meltwater this correlation suggests that release of stored meltwater periodically destabilized ice sheets causing them to collapse and send huge fleets of icebergs into the atlantic massive inputs of ice not only produced catastrophic sea level rise drowning reefs and destabilizing other ice sheets but also rapidly reduced the elevation of the laurentide ice sheet flipping atmospheric circulation patterns and forcing warm equatorial waters into the frigid north atlantic such dramatic evidence of catastrophic climate and sea level change during deglaciation has potentially disastrous implications for the future especially as the stability of remaining ice sheets such as in west antarctica is in question

drainage of recharge to symmetrically located downstream boundaries with special reference to seepage faces summary the seepage face is an important feature of the drainage process when recharge occurs to a permeable region with lateral outlets examples of the formation of a seepage face above the downstream water level include agricultural land drained by open ditches and the ballast beneath railway tracks the first part of the paper considers vertical section two dimensional analytical and numerical solutions for recharge to permeable regions above horizontal impermeable bases having two symmetrically positioned downstream boundaries with zero or specified external depths of water for vertical downstream boundaries with seepage faces above the downstream water level empirical relationships are developed relating the seepage face height on the downstream boundary to the downstream discharge the same problems are also analysed using the one dimensional dupuit forchheimer approximation improved estimates of the entire water table profile are achieved when the downstream boundary head is specified as a function of the downstream discharge using relationships derived from the two dimensional solutions instead of the customary assumption that the water table is drawn down to the ditch water level finally time variant simulations based on the dupuit forchheimer approximation are introduced for a single layer and for a system of two horizontal layers having different hydraulic conductivities

abrupt early to mid holocene climatic transition registered at the equator and the poles paleoclimatic records from equatorial east africa antarctica and greenland reveal that atmospheric circulation changed abruptly at the early to mid holocene transition to full postglacial conditions a climatic reorganization occurred at all three sites between 8200 and 7800 years ago that lasted 200 years or less and appears to have been related to abrupt transitions in both marine and terrestrial records around the world

tidal effects on sea water intrusion in unconfined aquifers abstract a variable density groundwater model is used to analyse the effects of tidal fluctuations on sea water intrusion in an unconfined aquifer it is shown that the tidal activity forces the sea water to intrude further inland and it also creates a thicker interface than would occur without tidal effects moreover the configuration of the interface is radically changed when the tidal fluctuations are included this is because of the dramatic changes in the flow pattern and velocity of the groundwater near the shoreline for aquifer depths much larger than tidal amplitudes the tidal fluctuation does not have much effect on how far the sea water intrudes into the aquifer nevertheless a significant change in the configuration of concentration contours because of the effect of tidal fluctuations is observed this change is more noticeable at the top of the aquifer near the water table than at the bottom of the aquifer and is caused by the infiltration of salt water into the top of the aquifer at higher tidal levels a flatter beach slope therefore intensifies this phenomenon the interface configurations do not change noticeably over the course of a tidal cycle neglecting tidal fluctuation effects results in an inaccurate evaluation of the water table elevation at the land end of the aquifer although no distinguishable difference is seen between the water tables near the shoreline where the landward boundary condition is a constant head the effects of tidal fluctuations on sea water intrusion are more pronounced than for cases where the landward boundary condition is a specified flux also it is shown that the effects of tidal fluctuations are more significant for a sloping beach than for a vertical shoreline and the salt water intrudes further inland for the sloping case

holocene palaeohydrology of lake titicaca estimated from an ostracod based transfer function lacustrine deposits of holocene age 8000 14c yr bp present were studied in lake titicaca bolivia ostracod assemblages were identified in six cores taken in the three basins which today form lake titicaca factorial correspondence and multiple linear regression analysis of the ostracods have permitted the reconstruction of 1 lake level 2 water salinity and 3 water mg ca ratio variations palaeolake levels indicate a complex evolution of the three basins with some major phases after a flooding at about 8000 yr 14c bp the lake level fluctuated around a mean position until 3900 14c yr bp when a second rise took place it seems however that the present day lake level was not reached until after 680 yr 14c bp some major dry events are also recorded at about 5300 14c yr bp and 2300 14c yr bp the lake titicaca data are consistent with other records from south america and also seem to reflect some world wide events

late pleistocene desiccation of lake victoria and rapid evolution of cichlid fishes lake victoria is the largest lake in africa and harbors more than 300 endemic species of haplochromine cichlid fish seismic reflection profiles and piston cores show that the lake not only was at a low stand but dried up completely during the late pleistocene before 12 400 carbon 14 years before the present these results imply that the rate of speciation of cichlid fish in this tropical lake has been extremely rapid

investigation of the thermal regime and subsurface properties of a tidally affected variably saturated streambed temperature and moisture content in the variably saturated subsurface are two of the most important physical parameters that govern a wide variety of geochemical and ecological processes an understanding of thermal and hydraulic processes and properties of transient vadose zones is therefore fundamental in the evaluation of such processes here an investigation of the thermal regime and subsurface properties of a tidally affected variably saturated streambed is presented field and laboratory measurements as well as a forward numerical model are jointly employed in the investigation temperature soil moisture surface level and water level data were recorded in a transect perpendicular to a tidally driven stream frequency domain analysis of the subsurface temperature measurements revealed the rapid decay of the tidal temperature driver within the top 30 cm of sediment several techniques were used to evaluate subsurface thermal and hydraulic properties including thermal conductivity and the soil water retention curve these properties were used to constrain a forward numerical model that included coupled treatment of relevant variable saturation thermal and hydraulic physics even though the investigated vadose zone is intermittent and relatively shallow 20 cm the results illustrate how error can be introduced into heat transport calculations if unsaturated conditions are not taken into account

tidal effects on variations of fresh saltwater interface and groundwater flow in a multilayered coastal aquifer on a volcanic island jeju island korea summary we conducted various field studies at the seawater intrusion monitoring wells located in the eastern part of jeju island korea to observe the tidal effect on groundwater seawater flow in the coastal aquifer studies included monitoring the fluctuations of groundwater and tide levels electrical and temperature logging and 2 d heat pulse flowmeter tests according to time series analysis tidal effects on groundwater level reached up to 3 km inland from the coastline water level variation was more sensitive to tidal fluctuations near the coast and more related to rainfall toward inland areas temporal and spatial variations in the shape and location of the freshwater saltwater interface were analyzed using data from nine monitoring wells the results indicated that the interface toe is located at a distance of 6 8 km from the coastline and its location was related to geological layers present long term seasonal variations revealed no major changes in the interface minor variations were due to moving boundary conditions induced by tidal fluctuations using the two dimensional heat pulse flowmeter groundwater flow directions and velocities at four tidal stages were measured on three monitoring wells drilled into the multilayered aquifers this direct measurement enabled us to relate the differences of flow velocities and directions with geology and tidal fluctuations combining the results of ec logging and flowmeter tests we found a zone where freshwater and saltwater moved alternately in opposite directions as influenced by the tidal fluctuations integrating various physical logging and flowmeter data with water level fluctuations improved our understanding of the behavior of fresh and seawater flow in the coastal aquifers

a high resolution 11 400 yr diatom record from lake victoria east africa abstract fine interval 30 45 yr sampling of a core from lake victoria s damba channel shows that numerous abrupt changes in the lake s diatom assemblages have occurred in response to climatic fluctuations over the past 11 400 14 c yr four distinct climatic phases bounded by sudden transitions are inferred 1 variably dry 11 400 10 000 yr b p 2 humid 10 000 7200 yr b p 3 more seasonal 7200 2200 yr b p and 4 more arid 2200 0 yr b p with a dry little ice age event 600 200 yr b p the diatom inferred paleoclimatic history for northern lake victoria closely resembles that inferred from a well dated pollen record from pilkington bay spectral analysis of the diatom record reveals strong periodicities including globally distributed 2360 2550 1400 1030 1130 and 500 cal yr cycles repeated rapid shifts between aulacoseira and nitzschia dominated diatom assemblages suggest that post 1960 changes in the lake s phytoplankton communities have had earlier climate driven analogs

periodic seepage face formation and water pressure distribution along a vertical boundary of an aquifer detailed measurements of the piezometric head from sand flume experiments of an idealised coastal aquifer forced by a simple harmonic boundary condition across a vertical boundary are presented the measurements focus on the pore pressures very close to the interface view the mathml source and throw light on the details of the boundary condition particularly with respect to meniscus suction and seepage face formation during the falling tide between the low and the mean water level the response is consistent with meniscus suction free models in terms of both the vertical mean head and oscillation amplitude profiles and is consistent with the observation that this area of the interface was generally within the seepage face above the mean water level the influence of meniscus formation is significant with the mean pressure head being less than that predicted by capillary free theory and oscillation amplitudes decaying faster than predicted by suction free models the reduced hydraulic conductivity in this area due to partial drainage of pores on the falling tide also causes a delay in the response to the rising tide the combined influence of seepage face formation meniscus suction and reduced hydraulic conductivity generate higher harmonics with amplitudes of up to 26 of the local main harmonic to model the influence of seepage face formation and meniscus suction a numerical solution of the richards equation was developed and evaluated against the data the model data comparison shows a good agreement with the behaviour high above the water table sensitive to the choice of moisture retention parameters

rapid climate change during the early holocene in western europe and greenland based on microfacies analyses of seasonally laminated varved sediments from lake holzmaar germany we report evidence of decadal to century scale climate variability during the early holocene the shifts in climate are documented in the thickness variations and changes in the composition of the varves in response to subtle shifts in limnological conditions the close similarity between the holzmaar varve record and the grip oxygen isotope record during 7 4 9 0 calendar cal ka suggests that the high frequency climatic variations in both regions were controlled by the same mechanism our more detailed studies covering the central 409 yr period 7 846 8 255 cal ka encompassing the 8 2 ka event document for the first time on a seasonal scale the changing precipitation regimes in western europe during these climate shifts we show i that winters were drier and summers shorter and cooler in western europe during colder periods in greenland ii in contrast to the present day climate in the holzmaar

postglacial marine diatom record of the palmer deep antarctic peninsula odp leg 178 site 1098 1 total diatom abundance 1 late pleistocene to holocene climate variability in the antarctic peninsula is described from ocean drilling program odp leg 178 site 1098 palmer deep using total diatom abundance valves per gram dry sediment as a paleoproductivity proxy diatom frustules are absent in the diamicton at the base of the core which was deposited prior to age calibrated circa 13 2 ka immediately above the diamicton frustules are in low abundance as rare fragments and become increasingly more abundant and well preserved up core a rapid and major increase in productivity after circa 12 8 ka is evident from distinctly laminated sediments that overlie the diamicton the strongly laminated section was deposited during a deglaciation phase that followed the retreat of grounded ice after the last glacial maximum diatom productivity decreased during a climatic cooling and or change in meltwater supply until circa 9 0 ka between 9 0 and 4 4 ka productivity increased steadily again with diatom abundance maxima reached during a mid holocene climatic optimum the optimum is interrupted by a brief cold event of 1000 years duration 6 0 5 0 ka after 4 4 ka productivity generally decreased and the last 500 years have experienced the lowest diatom productivity of the postglacial period the palmer deep sediment is compared to ice core and lake sediment records from other regions around the antarctic peninsula these diverse data sets confirm that the antarctic peninsula has been a region of rapid and variable climate fluctuation since the last glacial maximum

a giant vampire mammalia chiroptera in the late holocene from the argentinean pampas paleoenvironmental significance abstract the first fossil desmodontine record and the only well documented chiropteran fossil record from argentina is described a complete left upper canine was collected at centinela del mar 38 21 s58 w general alvarado county buenos aires province from fossil bearing sediments referred to the late holocene the tooth size is 25 larger than that of the modern vampire bat desmodus rotundus we assign this tooth to desmodus cf d draculae an extinct species recorded in the pleistocene holocene of south america brazil and venezuela the southernmost distribution of present day desmodus extends to northeast buenos aires province 35 s the presence of desmodus some 600 km south of this present day limit july minimal isotherm of 10 c indicates that around 300 years bp the southeastern buenos aires province was at least 2 c higher than modern july isotherm the desmodus tooth is associated with sigmodontine rodents characteristic of subtropical and temperate warm areas e g pseudoryzomys simplex bibimys cf b torresi and provides additional evidence to support this hypothesis a correlation with a global warming phase is discussed

behavior of dense aqueous phase leachate plumes in homogeneous porous media surface contaminant plumes emanating from waste disposal facilities are often denser than the ambient groundwater under certain conditions these dense plumes may become unstable contaminating larger regions of an aquifer the behavior of contaminant plumes with different densities was examined in three flow containers packed with homogeneous porous media simulating unconfined aquifers glass beads and a medium sand were used as the porous media a horizontal ambient groundwater stream was established in each flow container using deionized water the contaminant plumes consisted of nai or nabr solutions introduced into each flow container from a line source located on top of the porous medium and extending over the total width of the container optical tracers were added to the salt solutions to allow flow visualization results show that for a given porous medium dense plumes were either stable or unstable depending on the magnitude of the horizontal flow velocity the contaminant leakage rate and the density difference between the contaminant solution and the ambient groundwater when a dense plume becomes unstable lobe shaped gravitational instabilities develop which are unsteady and three dimensional our experimental results suggest that the behavior of dense contaminant plumes overlying a less dense groundwater stream in a homogeneous porous medium depends on the magnitude of certain nondimensional parameters it appears that gravitational instabilities begin to develop when the values of these nondimensional parameters exceed certain critical values

state of the antarctic and southern ocean climate system this paper reviews developments in our understanding of the state of the antarctic and southern ocean climate and its relation to the global climate system over the last few millennia climate over this and earlier periods has not been stable as evidenced by the occurrence of abrupt changes in atmospheric circulation and temperature recorded in antarctic ice core proxies for past climate two of the most prominent abrupt climate change events are characterized by intensification of the circumpolar westerlies also known as the southern annular mode between 6000 and 5000 years ago and since 1200 1000 years ago following the last of these is a period of major trans antarctic reorganization of atmospheric circulation and temperature between ad1700 and 1850 the two earlier antarctic abrupt climate change events appear linked to but predate by several centuries even more abrupt climate change in the north atlantic and the end of the more recent event is coincident with reorganization of atmospheric circulation in the north pacific improved understanding of such events and of the associations between abrupt climate change events recorded in both hemispheres is critical to predicting the impact and timing of future abrupt climate change events potentially forced by anthropogenic changes in greenhouse gases and aerosols special attention is given to the climate of the past 200 years which was recorded by a network of recently available shallow firn cores and to that of the past 50 years which was monitored by the continuous instrumental record significant regional climate changes have taken place in the antarctic during the past 50 years atmospheric temperatures have increased markedly over the antarctic peninsula linked to nearby ocean warming and intensification of the circumpolar westerlies glaciers are retreating on the peninsula in patagonia on the sub antarctic islands and in west antarctica adjacent to the peninsula the penetration of marine air masses has become more pronounced over parts of west antarctica above the surface the antarctic troposphere has warmed during winter while the stratosphere has cooled year round the upper kilometer of the circumpolar southern ocean has warmed antarctic bottom water across a wide sector off east antarctica has freshened and the densest bottom water in the weddell sea has warmed in contrast to these regional climate changes over most of antarctica near surface temperature and snowfall have not increased significantly during at least the past 50 years and proxy data suggest that the atmospheric circulation over the interior has remained in a similar state for at least the past 200 years furthermore the total sea ice cover around antarctica has exhibited no significant overall change since reliable satellite monitoring began in the late 1970s despite large but compensating regional changes the inhomogeneity of antarctic climate in space and time implies that recent antarctic climate changes are due on the one hand to a combination of strong multi decadal variability and anthropogenic effects and as demonstrated by the paleoclimate record on the other hand to multi decadal to millennial scale and longer natural variability forced through changes in orbital insolation greenhouse gases solar variability ice dynamics and aerosols model projections suggest that over the 21st century the antarctic interior will warm by 3 4 1oc and sea ice extent will decrease by 30 ice sheet models are not yet adequate enough to answer pressing questions about the effect of projected warming on mass balance and sea level considering the potentially major impacts of a warming climate on antarctica vigorous efforts are needed to better understand all aspects of the highly coupled antarctic climate system as well as its influence on the earth s climate and oceans

holocene variations of sea surface conditions in the southeastern barents sea reconstructed from dinoflagellate cyst assemblages palynomorphs were analysed in two sediment cores from the southeastern barents sea representing the past 8 3 and 4 4 kyr high dinocyst contents and species diversity enabled the application of the best analogue method to quantitatively reconstruct sea surface salinities temperatures and ice cover using 677 modern reference sites from the north atlantic and arctic seas including new data from the barents sea reported here at the southern core site where waters are affected by the atlantic inflow sea surface conditions were relatively warm and stable between ca 8000 and 5000 calendar yr bp in contrast the past 5 kyr had periods with cooler temperatures and extended ice cover fluctuating mostly at 1 1 5 kyr frequencies at both sites most pronounced coolings occurred around 8 1 5 3 5 3 2 and 2 5 ka the northern site additionally shows younger cooling events tentatively dated to 1 4 0 3 and 0 1 ka identified variations in sea surface conditions indicate changes in atlantic water inputs to the barents sea our results generally correlate to palaeoclimatic reconstructions from northwestern eurasia exemplified by palynological records from karelia this correlation suggests that sea surface variations in the barents sea reflect large scale changes in atmospheric and oceanic interactions between the north atlantic and the arctic copyright 2001 john wiley sons ltd

holocene variations of monsoon rainfall in rajasthan two reconstructed histories of the monsoon rainfall in rajasthan show that the monsoon was weak or absent in latest glacial time with the advent of holocene climatic patterns fresh water lakes formed in dune fields and the pollen rain preserved in these reservoirs provides a basis for the reconstruction of the monsoon history the two reconstructions separated by only 150 km have some features in common and some striking differences both show maximum monsoon amounts in the early holocene with a roughly two thirds decrease to the present both show salinization in sub boreal time both show long intervals of near complete desiccation in the last four millennia the shorter term variations however are not closely parallel these shorter term variations may be explained in terms of the behavior of the present day interannual variability

influence of hysteresis on groundwater wave dynamics in an unconfined aquifer with a sloping boundary in this paper the influence of hysteresis on water table dynamics in an unconfined aquifer was examined using a numerical model to solve richards unsaturated flow equation the model was subject to simple harmonic forcing across a sloping boundary with a seepage face boundary condition time series from both hysteretic and non hysteretic models were subject to harmonic analysis to extract the amplitude and phase profiles for comparison with existing sand flume data cartwright et al 2004 the results from both model types show good agreement with the data indicating no influence of hysteresis at the oscillation period examined t 348 s the models were then used to perform a parametric study to examine the relationship between oscillation period and hysteresis effects with periods ranging from 3 min to 180 min at short oscillation periods t 180 s the effects of hysteresis were negligible with both models providing similar results as the oscillation period increased the hysteretic model showed less amplitude damping than the non hysteretic model for periods greater than t 60 min the phase lag in the non hysteretic model is greater than for the hysteretic one for periods less than t 60 min this trend is reversed and the hysteretic model produced a greater phase lag than the non hysteretic model these findings suggest that consideration of hysteresis dynamics in richards equation models has no influence on water table wave dispersion for short period forcing such as waves t 10 s whereas for long period forcing such as tides t 12 25 h or storm surges t days hysteresis dynamics should be taken into account

tidal impacts on riparian salinities near estuaries groundwater dependent riparian biota is known to be sensitive to changes in soil and groundwater salinity in estuarine systems the groundwater flow and salinity behaviour in a phreatic aquifer adjoining a partially penetrating tidal estuary is investigated through two dimensional numerical experiments for a lateral cross section which explore the influence of factors such as aquifer and soil materials tidal amplitudes and regional groundwater hydraulic gradients the density contrast between estuarine water and the fresh groundwater drives saltwater penetration of the aquifer even in the case of a marked groundwater hydraulic gradient towards the estuary we show that tidal fluctuations in estuaries can significantly affect the groundwater salinity distribution in adjacent density stratified phreatic aquifers this has consequences for the expected distribution of salinity sensitive biota in the hyporheic zone as well as vegetation and fauna dependent on water in the riparian soil and aquifer the shape of the dense saltwater wedge propagating into the adjacent groundwater system is also modified by the estuarine tidal signal although this effect appears to have only minor influence on the maximum distance penetrated into the aquifer i e location of the toe of the wedge tide induced changes to riparian groundwater salinity are advection driven as evidenced by the modified time averaged groundwater flow dynamics

climate change in the circum north atlantic region during the last deglaciation a survey of new and published palaeoclimate data indicates that both the high and low latitude north atlantic regions were characterized by at least three synchronous periods of abrupt climate change during the last glacial to interglacial transi tion climate model results suggest that changes in the melting history of the laurentide ice sheet may explain much of this nonlinear response of the climate system to astronomical milankovitch forcing

century scale paleoclimatic reconstruction from moon lake a closed basin lake in the northern great plains estimates of past lake water salinity from fossil diatom assemblages were used to infer past climatic conditions at moon lake a climatically sensitive site in the northern great plains a good correspondence between diatom inferred salinity and historical records of mean annual precipitation minus evapotranspiration p et strongly suggests that the sedimentary record from moon lake can be used to reconstruct past climatic conditions century scale analysis of the holocene diatom record indicates four major hydrological periods an early holocene transition from an open freshwater system to a closed saline system by 7300 b p which corresponds with a transition from spruce forest to deciduous parkland to prairie and indicates a major shift from wet to dry climate a mid holocene period of high salinity from 7300 to 4700 b p indicating low effective moisture p et a transitional period of high salinity from 4700 to 2200 b p characterized by poor diatom preservation and a late holocene period of variable lower salinity during the past 2 200 yr indicating fluctuations in effective moisture

synchronous holocene climatic oscillations recorded on the swiss plateau and at timberline in the alps eight synchronous pre roman cold phases were found at 9600 9200 8600 8150 7550 6900 6600 6200 5350 4900 4600 4400 3500 3200 and 2600 2350 radiocarbon years bp by reconstructing past climate at two sites on the swiss plateau and at timberline in the alps the cooling events during the early and mid holocene represent temperature values similar to today and apparently the onset of cooling events represents a deviation from today s mean annual temperature of about 1 c and is triggered at a 1000 year periodicity at wallisellen langachermoos 440 m a former oligotrophic lake near zurich the correlation between sum mertime lake levels and the seed production of the amphi atlantic aquatic plantnajas flexilis was used to reconstruct lake levels over a 3000 year period during the first part of the holocene at lake seedorf on the western swiss plateau 609 m the sedimentological palynological and macrofossil record revealed fluctuations of lake levels for the complete holocene from lago basso in the

late quaternary vegetation and hydrological change at ljubljansko barje slovenia abstract this paper investigates changes in the vegetation and hydrology at ljubljansko barje slovenia using a multi proxy approach pollen diatoms geochemical and stable isotope analysis during the lateglacial ljubljansko barje was a deep oligotrophic lake surrounded by open woodland comprised predominantly of pinus and betula which in the early holocene when the lake became shallower and mesotrophic fragilaria pinnata 15 n 3 3 13 c 33 7 c n 7 9 was replaced by broadleaved tree taxa tilia ulmus quercus corylus and picea this was followed by the spread of fagus and after 9200 cal yr bp abies and planktonic and oligotrophic diatom taxa aulacoseira italica gomphonema angustum as well as an increased allochthonous influx silt sio 2 al 2 o 3 between ca 6750 and 6000 cal yr bp the diatom data suggest shallower and oligotrophic conditions whereas the stable isotope record increased 13 c and decreased 15 n indicates terrestrial input changes in forest composition increase of quercus alnus corylus and the spread of herb taxa were probably advanced by human impact on the environment the beginning of organic sedimentation dated to 6000 cal yr bp was followed by an increase of fagus and abies and by eutrophication and peat growth at 5200 cal yr bp

two dimensional vertical moisture pressure dynamics above groundwater waves sand flume experiments and modelling this paper presents a new laboratory dataset on the moisture pressure relationship above a dispersive groundwater wave in a two dimensional vertical unconfined sand flume aquifer driven by simple harmonic forcing a total of five experiments were conducted in which all experimental parameters were kept constant except for the oscillation period which ranged from 268 s to 2449 s between tests moisture content and suction head sensor pairings were co located at two locations in the unsaturated zone both approximately 0 2 m above the mean watertable elevation and respectively 0 3 m and 0 75 m from the driving head boundary for all oscillation periods except for the shortest t 268s the formation of a hysteretic moisture pressure scanning loop was observed consistent with the decay of the saturated zone groundwater wave the size of the observed moisture pressure scanning loops decayed with increasing distance landward and the decay rate is larger for the shorter oscillation periods at the shortest period t 268s the observed moisture pressure relationship was observed to be non hysteretic but with a capillary capacity that differs from that of the static equilibrium wetting and drying curves this finding is consistent with observations from existing one dimensional vertical sand column experiments the relative damping of the moisture content with distance landward is higher than that for the suction head consistent with the fact that transmission of pressure through a porous medium occurs more readily than mass transfer this is further supported by the fact that observed phase lags for the unsaturated zone variables i e suction head and moisture content relative to the driving head are greater than the saturated zone variables i e piezometric head harmonic analysis of the data reveals no observable generation of higher harmonics in either moisture or pressure despite the strongly non linear relationship between the two in addition a phase lag of moisture content relative to the suction head was observed indicating that the response time of the moisture content to watertable motion is greater than that of the pore water pressure the observed moisture pressure dynamics are qualitatively reproduced using a hysteretic richards equation model however quantitative differences exist which are likely to be due to previous findings that demonstrated that the richards equation model is unable to accurately reproduce the observed watertable wave dispersion particularly at shorter period oscillations

the holocene climatic record of the salt caves of mount sedom israel mount sedom is a salt diapir on the southwestern shore of the dead sea which has been rising above the local base level throughout the holocene karst development within the salt body has kept pace with the rise forming subhorizontal cave passages with vertical shafts wood fragments found embedded in flood sediments that were deposited in the cave passages yielded 14c ages ranging from c 7100 to 200 bp a palaeoclimatic sequence was constructed based on parameters that include relative abundance of plant types or floral communities the elevations of the corresponding cave passages and the ratio of their width to present passage width the results were correlated to the holocene sedimentary sequence of the dead sea basin and other features associated with shifting lake levels moister climatic stages are indicated by relatively abundant wood remains by wide cave passages and by elevated outlets indicating high dead sea level arid periods are marked by a scarcity of wood remains by narrow cave pa

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

timing cause and consequences of mid holocene climate transition in the arabian sea abstract we reconstruct centennial scale quantitative changes in surface seawater temperature sst evaporation precipitation from mg ca and 18o of surface dwelling planktic foraminifera productivity from relative abundance of globigerina bulloides carbon burial from caco3 and organic carbon corg and dissolved oxygen at sediment water interface covering the entire holocene from a core collected from the eastern arabian sea from the multi proxy record we define the timing consequences and possible causes of the mid holocene climate transition mhct a distinct shift in evaporation precipitation e p is observed at 6 4 ka accompanied by a net cooling of sst the shift in sst and e p is synchronous with a change in surface productivity a concurrent decrease is also noted in both the planktic foraminiferal abundance and coarse sediment fraction a shift in carbon burial as inferred from both the caco3 and corg coincides with a change in surface productivity a simultaneous decrease in dissolved oxygen at the sediment water interface suggests that changes affected both the surface and subsurface water a similar concomitant change is also observed in other cores from the arabian sea as well as terrestrial records suggesting a widespread regional mhct the mhct coincides with decreasing low latitude summer insolation perturbations in total solar intensity and an increase in atmospheric co2

holocene climatic records from agassiz ice cap ellesmere island nwt canada four ice cores from the top of the agassiz ice cap and down a flow line have been variously analysed for 8 18o ecm solid conductivity and ice melt layer stratigraphy stratigraphic correlation

changes in atmospheric circulation and ocean ice cover over the north atlantic during the last 41 000 years high resolution continuous multivariate chemical records from a central greenland ice core provide a sensitive measure of climate change and chemical composition of the atmosphere over the last 41 000 years these chemical series reveal a record of change in the relative size and intensity of the circulation system that transported air masses to greenland defined here as the polar circulation index pci and in the extent of ocean ice cover massive iceberg discharge events previously defined from the marine record are correlated with notable expansions of ocean ice cover and increases in pci during stadials without discharge events ocean ice cover appears to reach some common maximum level the massive aerosol loadings and dramatic variations in ocean ice cover documented in ice cores should be included in climate modeling

mechanisms of holocene palaeoenvironmental change in the antarctic peninsula region the antarctic peninsula is one of the three fastest warming regions on earth here we review holocene proxy records of marine and terrestrial palaeoclimate in the region and discuss possible forcing mechanisms underlying past change with a specific focus on past warm periods our aim is to critically evaluate the mechanisms by which palaeoclimate changes might have occurred in order to provide a longer term context for assessing the drivers of recent warming two warm events are well recorded in the holocene palaeoclimate record namely the early holocene warm period and the mid holocene hypsithermal mhh whereas there are fewer proxy data for the mediaeval warm period mwp and the recent rapid regional rrr warming we show that the early holocene warm period and mhh might be explained by relatively abrupt shifts in position of the southern westerlies superimposed on slower solar insolation changes a key finding of our synthesis is that the marine and terrestrial records in the ap appear to show markedly different behaviour during the mhh this might be partly explained by contrasts in the seasonal insolation forcing between these records circumpolar deep water cdw has been implicated in several of the prominent changes through the holocene but there are still differences in interpretation of the proxy record that make its influence difficult to assess further work is required to investigate contrasts between marine and terrestrial proxy records east west contrasts in palaeoclimate the history of cdw to retrieve a long onshore high resolution record of the holocene and determine the role of sea ice in driving or modulating palaeoclimate change along with further efforts to study the proxy record of the rrr and the mwp

holocene environmental change inferred from a high resolution pollen record lake zhuyeze arid china a high resolution pollen record c 50 yr sample from terminal lake sediments in the shiyang river drainage basin on the present margin of the summer monsoon was used to reconstruct vegetation and climate history during the holocene forest trees from mountainous areas of the drainage including sabina picea and pinus dominated pollen assemblages in the early holocene 11 6 7 1 cal ka in the mid holocene 7 1 3 8 cal ka desert and steppe shrubs and herbs around the lake including nitraria poaceae compositae and artemisia were dominant the late holocene 3 8 0 cal ka was again dominated by alternation of pinus sabina tree pollen and desert steppe pollen the early holocene forest expansion in the mountains and subsequent increase in the river transport of tree pollen corresponds with maximum precipitation during the east asian summer monsoon maximum the timing of these changes in our record from arid china is different from that of east china where the holocene monsoon maximum appeared in

a high resolution stalagmite record of the holocene east asian monsoon from mt shennongjia central china high resolution oxygen isotope 18o profiles of six stalagmites from sanbao cave in hubei province central china established with 1413 oxygen isotope data and 65 230th ages provide a continuous history of east asian summer monsoon easm intensity for the period from 13 0 2 thousand years before present ky bp relative to ad 1950 the 18o record includes four distinct stages in the evolution of the easm 1 an abrupt transition 11 5 ky bp into the holocene 2 a period of gradual increase in monsoon intensity 11 5 9 5 ky bp 3 the maximum humid period 9 5 6 5 ky bp and 4 a period of gradual decline in monsoon intensity 6 5 0 2 ky bp comparison of sanbao with regional records of comparable resolution reveals that the timing of the beginning and end of the holocene optimum as defined by the minimum in 18 o was similar in the indian and east asian monsoon systems this supports the idea that shifts in the monsoon tied to shifts in the mean position of the inter tropical convergen

numerical and experimental study of seepage in unconfined aquifers with a periodic boundary condition the assessment of groundwater conditions within an unconfined aquifer with a periodic boundary condition is of interest in many hydrological and environmental problems a two dimensional numerical model for density dependent variably saturated groundwater flow sutra voss c i 1984 sutra a finite element simulation model for saturated unsaturated fluid density dependent ground water flow with energy transport or chemically reactive single species solute transport us geological survey national center reston va is modified in order to be able to simulate the groundwater flow in unconfined aquifers affected by a periodic boundary condition the basic flow equation is changed from pressure form to mixed form the model is also adjusted to handle a seepage face boundary condition experiments are conducted to provide data for the groundwater response to the periodic boundary condition for aquifers with both vertical and sloping faces the performance of the numerical model is assessed using those data the results of pressure and mixed form approximations are compared and the improvement achieved through the mixed form of the equation is demonstrated the ability of the numerical model to simulate the water table and seepage face is tested by modelling some published experimental data finally the numerical model is successfully verified against present experimental results to confirm its ability to simulate complex boundary conditions like the periodic head and the seepage face boundary condition on the sloping face c 1999 elsevier science b v all rights reserved