a 2100 year trace element and stable isotope record at decadal resolution from rice lake in the northern great plains usa a 2100 year multiple proxy record at decadal resolution from a topographically closed but hydrolog ically open lake basin was used to reconstruct changes in lake salinity and evaporative intensity and to evaluate the reliability of different proxies for inferring climatic change in the glaciated northern great plains ngp at rice lake north dakota the ostracode mg ca ratios show good correlation with other ostracode diatom and aeolian proxy records from other sites in the ngp all of these records show significant century scale periodicities which have been related to solar variability as inferred from atmospheric radiocarbon records this coherent and consistent pattern suggests that these sites record climatic variability at regional scales the interval of the mediaeval climatic anomaly from 900 to 550 cal yr bp contains two pronounced dry periods during the following little ice age a single period of frequent drought that peaked at 300 cal yr bp was bracketed by wet periods before and af

holocene palaeoclimate and sea level fluctuation recorded from the coastal barker swamp rottnest island south western western australia abstract the holocene palaeoclimatic history of south western western australia swwa has received little attention compared to south eastern australia and this has resulted in conflicting views over the impact of climate variability in the region we present here a well dated high resolution record from two overlapping sediment cores obtained from the centre of barker swamp rottnest island offshore perth the records span the last 8 7 ka with the main lacustrine phase occurring after 7 4 ka this site preserves both pollen and several ostracod taxa the pollen record suggests a long term shift from the early mid holocene to the late holocene to drier conditions with less shrubland and more low ground cover and less fire activity a salinity transfer function was developed from ostracod faunal assemblage data and trace metal ratios mg ca sr ca and na ca and stable isotopes 18 o and 13 c analysed on selected ostracod valves these provide a detailed history of evaporation precipitation e p differences that clearly shows that the swwa region was subjected to significant climatic shifts over the last 7 4 ka with a broad shift towards increased aridity after 5 ka the swamp ranged from fresh to saline as recorded in the ostracod valve chemistry and the independently derived salinity transfer function the ostracod record also indicates that a sea level highstand occurred between ca 4 5 and 4 3 ka with probable step wise increases at 6 75 6 2 and 5 6 ka with the last vestiges of salt water intrusion at ca 1 ka after about 2 3 ka the fresh groundwater lens that underlies the western portion of the island intersected the swamp depression influencing the hydrology of the swamp the broad climatic changes recorded in barker swamp are also compared with data from southern south africa and it is suggested that the southern annular mode appears to have been the dominant driver in the climate of these regions and that the indian ocean dipole is of little importance in the southern regions of the south western cape of africa and south western western australia

major hydrological shifts in the black sea lake in response to ice sheet collapses during mis 6 130 184 ka bp abstract the saalian was one of the largest glaciations during the quaternary with an ice sheet extending considerably wider into the eurasian continent than during other glacials orbital variations caused the ice sheet to switch between growing and shrinking the partial retreat of the ice sheet and meltwater discharge resulted in global sea level rise and increased lake levels of inland seas with broader environmental implications during marine isotope stage 6 mis 6 meltwater entered the formerly enclosed black sea at least twice as documented in a 18o record from anatolian speleothems here we present a sedimentary record from the black sea lake covering mis 6 and provide evidence for three meltwater periods coinciding with insolation maxima bswp 6 1 180 167 ka bp bswp 6 2 160 145 ka bp bswp ii 133 130 ka bp while 18oostracods and sr caostracods point to pronounced meltwater supply and decreasing salinity 87sr 86srostracods shed light on meltwater sources and pathways during all three periods meltwater drained most likely via the dnieper and volga into the black and caspian seas and connected both basins relatively low 87sr 86srostracods values during the oldest meltwater period suggest melting solely of the eastern eurasian ice sheet in contrast during the younger meltwater periods exceptional high 87sr 86srostracods values point towards additional meltwater from the western eurasian ice sheet a surplus from melting glaciers in the tian shan and pamir mountains that finally entered the caspian sea via the amu darya and sry darya probably amplified the input of high radiogenic sr isotope water we also show that higher temperatures and productivity suggest dansgaard oeschger like climate variability during the first half of mis 6

2000 year environmental history of a karstic lake in the mediterranean pre pyrenees the estanya lakes spain abstract the sediment record of the estanya lakes pre pyrenees spain indicates that this region was intensely affected by land use during the middle and modern ages which contrasts with the present day poor socioeconomic development of the region the correlation between sediment and biotic data and well known historical events suggests a reliable history of environmental changes in this area during the last 2000 years changes in lake level were mainly controlled by climatic factors whereas properties such as lithology organic carbon ostracods and pollen content are mainly associated with lake management and land use major sedimentological changes resulted from the use of the lake for hemp retting at the start of the xivth century subsequent depopulation of the area in the xxth century led to the progressive recovery of the lacustrine ecosystem

little ice age aridity in the north american great plains a high resolution reconstruction of salinity fluctuations from devils lake north dakota usa the little ice age was an interval between about ad 1500 and 1850 characterized by advancing glaciers in mountainous regions of europe and western north america however it is unclear whether this cool moist period was truly global in extent or how it was manifested in other regions with different climatic controls a high resolution reconstruction of salinity fluctuations in devils lake north dakota based on fossil diatoms ostracode shell geochemistry and bulk carbonate geochemistry indicates that saline conditions prevailed throughout much of the recent past these results suggest an arid climate in the northern great plains throughout the little ice age and that during this interval climatic gradients between the great plains and regions to both the east and west may have been quite steep

a high resolution record of climate variability and landscape response from kettle lake northern great plains north america abstract a decadal scale multiproxy record of minerals pollen and charcoal from kettle lake north dakota provides a high resolution record of climate and vegetation change spanning the entire holocene from the northern great plains ngp in north america the chronology is established by over 50 ams radiocarbon dates this record exhibits millennial scale trends evident in other lower resolution studies but with much more detail on short term climate variability and on the rapidity and timing of major climatic shifts as a proxy for precipitation we utilize the rate of endogenic carbonate sedimentation which depends on groundwater inflow which in turn depends on precipitation independent cluster analyses of mineral and pollen data reveal major holocene mode shifts at 10 73 ka ka cal yr bp 9 25 ka and 4 44 ka the early holocene 11 7 9 25 ka was generally wet with perhaps a trend to higher evaporation associated with warming temperatures a switch from calcite to aragonite deposition associated with a severe but brief drought occurred at 10 73 ka from 10 73 ka to 9 25 ka climate was generally humid but punctuated at 100 300 yr intervals by brief droughts including the most severe drought of the entire holocene at 9 25 ka this event was coeval with the 9 3 9 2 ka event in the greenland ice cores and observed at a number of sites worldwide in contrast the prominent 8 2 ka event in greenland is not remarkable at kettle lake the prominence of the 9 25 event locally in the ngp may be due to a major drawdown and northward retreat of lake agassiz at this time reducing its mesoclimatic effect on the ngp and thrusting the region into an insolation controlled regime the mid holocene 9 25 4 44 ka was characterized by great variability in moisture on a multi decadal scale with severe droughts alternating with more humid periods the high abundance of the weedy but drought intolerant ambrosia generally during the mid holocene and specifically during the multi decadal drought periods is seemingly paradoxical but can be explained by high interannual variability of moisture overlaid on the multi decadal variability the late holocene 4 44 ka present was also characterized by multi decadal variability in moisture but was generally wetter than the mid holocene and the magnitude of variability was less the trends in wet dry mineral pollen and charcoal proxies were similar to the mid holocene but late holocene mineral pollen assemblages are distinct from mid holocene the shift to wetter climate in the late holocene was more gradual than the abrupt shift to arid conditions 9 25 ka which may explain the asymmetric retreat and readvance of forest along the eastern margin of the ngp precipitation variations in the ngp have been linked with pacific and atlantic sea surface temperatures and mid holocene drought in the ngp has been linked with sustained la nina like conditions in the pacific these linkages may explain the decadal and millennial scale variations seen in the ngp but cause of the prominent century scale variations remains elusive

abrupt changes of temperature and water chemistry in the late pleistocene and early holocene black sea new mg ca sr ca and published stable oxygen isotope and 87sr 86sr data obtained on ostracods from gravity cores located on the northwestern black sea slope were used to infer changes in the black sea hydrology and water chemistry for the period between 30 to 8 ka b p calibrated radiocarbon years the period prior to 16 5 ka b p was characterized by stable conditions in all records until a distinct drop in 18o values combined with a sharp increase in 87sr 86sr occurred between 16 5 and 14 8 ka b p this event is attributed to an increased runoff from the northern drainage area of the black sea between heinrich event 1 and the onset of the bolling warm period while the mg ca and sr ca records remained rather unaffected by this inflow they show an abrupt rise with the onset of the bolling allerod warm period this rise was caused by calcite precipitation in the surface water which led to a sudden increase of the sr ca and mg ca ratios of the black sea water the stable oxygen isotopes also start to increase around 15 ka b p although in a more gradual manner due to isotopically enriched meteoric precipitation while sr ca remains constant during the following interval of the younger dryas cold period a decrease in the mg ca ratio implies that the intermediate water masses of the black sea temporarily cooled by 1 2 c during the younger dryas the 87sr 86sr values drop after the cessation of the water inflow at 15 ka b p to a lower level until the younger dryas where they reach values similar to those observed during the last glacial maximum this might point to a potential outflow to the mediterranean sea via the sea of marmara during this period the inflow of mediterranean water started around 9 3 ka b p which is clearly detectable in the abruptly increasing mg ca sr ca and 87sr 86sr values the accompanying increase in the 18o record is less pronounced and would fit to an inflow lasting 100 a

late quaternary paleohydrology in the north american great plains inferred from the geochemistry of endogenic carbonate and fossil ostracodes from devils lake north dakota usa abstract devils lake north dakota is a closed basin lake in which water chemistry is responsive to hydrological changes making it a valuable resource for documenting paleoclimate conditions ostracode trace element and bulk carbonate geochemistry in a core are used to generate a 12 000 year record of salinity fluctuations the relative amounts of sr ca and ba in the bulk carbonate fraction can be linked mechanistically to changes in sediment mineralogy calcite aragonite and or solute chemistry whereas the elemental geochemistry of ostracode valves reflect solute chemistry associated minima in sr ca and maxima in mg ca in ostracode valves from the devils lake core may be due to aragonite mediation of sr chemistry during the precipitation of multiple mineral phases during more saline intervals the devils lake record can be divided into four hydrological periods on the basis of the geochemical proxies a 12 5 9 k y low salinity b 9 5 4 5 k y fluctuating saline conditions with a maximum at about 8 k y c 4 5 3 5 k y low salinity phase but probably not as fresh as at 12 5 9 k y d 3 5 k y present saline phase although generally less so than 9 5 4 5 k y however a large degree of variability characterizes the entire record suggesting the absence of long term climatic stability in this region

an attempt to determine the absolute geomagnetic field intensity in southwestern iceland during the gauss matuyama reversal abstract we have measured the variation in the intensity of the geomagnetic field during the gauss matuyama n4 r3 polarity reversal by application of the thelliers method to specimens of lava flows from hvalfjordur district in western iceland reynivallahals mts eleven lava flows all show very similar directions corresponding to an equatorial vgp plat 2 9 n plong 81 9 e a95 4 2 k 119 twenty nine specimens from nine of the flows were pre selected for palaeointensity determination on the basis that specimens from the same drill cores showed a single component of magnetisation upon thermal or af demagnetisation and possessed low magnetic viscosity and reversible susceptibility curves upon heating at 600 650 c observation that the directional data obtained in the course of the palaeointensity experiments occasionally showed substantial non linearity indicates that a significant chemical remanent magnetization crm can be acquired in the direction of the laboratory field during heating at t for each double heating step we calculated the ratio of crm t to the magnitude of the natural remanent magnetization nrm t in the direction of characteristic remanence obtained independently from another specimen from the same core when this ratio exceeded 15 the paleointensity data was rejected in addition specimens for which the quality factor was less than 5 were rejected twelve reliable palaeointensity values were obtained from specimens representing five lava flows the results confirm that the palaeointensity was substantially reduced during the n4 r3 reversal the range of mean palaeointensity values obtained for the five flows is 8 8 to 20 5 and the overall mean is 14 8 4 6 t this corresponds to an equivalent vdm of 3 81 1 19 1022 a m2 a comparison of all thellier palaeointensity data from the r3 magnetozone in the rayinivallahals mts area reveals a progressive although irregular increase in the palaeointensity between the gauss matuyama and matuyama reunion reversals this trend is opposite to that expected from the saw tooth model of palaeointensity variations

palaeolimnological approaches to climate change with special regard to the biological record abstract lakes systems respond physically chemically and biologically to changes in climate and these responses are registered in many various ways in lake sediment records this paper focuses principally on the biological record and describes recent research that aims to reconstruct i past variability in effective moisture and ii past variability in temperature changes in effective moisture are manifested especially by changes in lake water level in closed lake basins water level change can cause significant changes in water salinity that is registered in sediments by changes in the composition of fossil assemblages especially diatoms in open freshwater lake basins changes in water level are best reconstructed from the aquatic macrofossil record and potentially from the ratio of open water to littoral species of cladocera ostracods and diatoms the most promising biological method for reconstructing past temperature is chironomid analysis and a range of new chironomid temperature transfer functions are being developed however biological responses to changing temperature are usually indirect as a result of temperature effects on water column stratification nutrient cycling and alkalinity generation and in most cases a correct interpretation of the climatic significance of changes in the fossil record requires an understanding of the processes that are operating in the water column a multi proxy whole lake approach is advocated as the best way forward not only as a means of reconstructing past climate but also as a means of assessing the impact of climate change on lake systems

geochemistry of ostracode calcite part 2 the effects of water chemistry and seasonal temperature variation on candona rawsoni abstract reconstruction of lake paleochemistry from the isotope and trace element composition of fossil ostracodes requires an understanding of geochemical variations induced by the seasonality of shell formation monthly field collections of live candona rawsoni from two hyposaline lakes in the eastern dakotas reveal large variations in 18 o values 2 5 o and mg ca and sr ca ratios up to 35 among individual adults within the same assemblage because the measured variations in water chemistry were small the amplitude of ostracode 18 o and mg ca variations must result from seasonal temperature variation ostracode sr ca ratios which should be temperature independent shows strong positive covariance with shell mg ca these results imply that sr uptake in ostracode calcite increases with the mg content of the shell the partitioning coefficient for mg on the other hand appears to decreases at high values of mg ca in the host water the geochemical variability in the field collections indicates that c rawsoni populations are composed of multiple overlapping generations that attain maturity at different times many juveniles apparently undergo several molts in mid summer and persist as penultimate instars a 1 into the following spring geochemical analysis of late instar juvenile c rawsoni from lacustrine sedimentary records is likely to provide information on mid summer conditions large numbers of fossil ostracodes should be analyzed from each stratigraphic interval to eliminate seasonal noise from the paleochemistry records of temperate region lakes

palaeoenvironmental reconstruction of a pleistocene lacustrine sequence from faunal assemblages and ostracode shell geochemistry baza basin se spain abstract we have reconstructed the early pleistocene palaeoenvironmental record for a shallow lacustrine sequence from the baza basin southern spain using faunal assemblages and the shell chemistry of two ostracode species candona sp and cyprideis torosa in the ne basin sector orce area a lower pleistocene lacustrine sequence up to 10 m thick contains alternating phases of two fossil assemblages which differ in their salinity requirements the faunal assemblages record phases of 1 slightly saline ca bicarbonate rich water when freshwater organisms predominated and 2 saline nacl dominant water in which marine like organisms lived the ostracode shells from intervals with a saline fauna have higher 18o values than those from intervals with a freshwater fauna this feature corresponds to a parallel alternation of positive and negative hydrologic balances 18o and 13c values from ostracode calcite display a covariant trend which indicates that the ostracodes lived in a closed lacustrine system the isotopic data show a better agreement with the inferences from faunal assemblages than with trace element trends the trace element data although locally fitting well with the isotope and faunal interpretations suggest overall that trace element contents in ostracodes are not consistent indicators of salinity and or temperature variations but depend on a set of complex factors which may not be easily deciphered the alternation of concentration dilution phases recorded in the studied section can be correlated with climatic cycles described from synchronous ocean basin records from the late matuyama chron

environment change of sihu lake in the past 450 years southern china in recent years climate and environment changes since the little ice age have become a focus in modern research the paper selected the sediment 77 cm of sihu lake which was located in guilin southern china as the research object and analyzed the karst wetland environmental changes within nearly 450 years according to the vertical profile of indicators and geochemical elements in sediment core the regional environmental evolution was divided into four stages the former two stages were mainly influenced by little ice age 1562 1895 the third stage climate has changed slowly the fourth stage was mainly affected by human activities and global warming the factor analysis of geochemical elements indicated that karst wetland regional environmental change was affected by four major factors in the 53 77 cm stage 1562 1703 f1 erosion factor f2 watershed runoff and diagenesis factor and f3 regional background factor were reduced indicating that it was in cold and dry stage of little ice age in the 20 53 cm stage 1703 1895 f1 and f2 increased indicating that it was in cold and wet stage of little ice age in the 4 20 cm stage 1895 1987 f1 f2 and f4 human activity factor increased slowly indicating that the climate was affected by global warming and more precipitation in the 0 4 cm stage 1987 2007 f1 and f2 reduced and f4 increased indicating the reduced precipitation rising temperature and more impact of human activities in addition it was proposed that the little ice age in guilin district in southern china started in the 1560s and lasted until the 1895s or so

oxygen and carbon isotopes and trace element ca ratios in late quaternary ostracods loxoconcha lepida and palmoconcha agilis from the black sea paleoclimatic and paleoceanographic implications abstract oxygen and carbon isotopes and trace element ca ratios are presented for the valves of benthic ostracods palmoconcha agilis and loxoconcha lepida extracted from two composite sediment cores from the black sea shelf the two cores together provide a complete latest pleistocene holocene sedimentary record with 16 radiocarbon dated shells in core m02 45 and 9 in core m05 50 results reveal environmental changes and evolution of water chemistry associated with the last reconnection of the black sea to the global ocean the 18o values in composite cores m02 45 and m05 50 exhibit ranges of 8 0 and 3 0 respectively 13c values show ranges of 2 5 and 2 0 there are three isotopic substages a substage 1c 12 1 9 4 cal ka represents conditions in the neoeuxine lake with relatively high 13c and low 18o b substage 1b 9 4 6 3 cal ka represents the transition from lacustrine to marine conditions with an increase in 18o values as small amounts of mediterranean water flowed into the black sea and c substage 1a 6 3 cal ka to present represents the establishment of marine conditions like today with near constant 18o values and a trend of slowly increasing 13c trace element ca ratios show downcore trends consistent with the three isotopic substages at 6 3 cal ka isotopic data and trace element ca ratios exhibit a major shift with changes in the mg ca ratio suggesting an increase in water temperature from 12 14 c to 17 19 c these mid holocene changes in ostracod valve composition and environmental conditions coincide with the weakening of black sea outflow and the proliferation of marine organisms

groundwater discharge to a gobi desert lake during mid and late holocene dry periods abstract high levels of lake eastern juyanze and a wetter climate in the gobi desert nw china were recorded between 5100 and 4100 cal year bp from ostracod distribution shell geochemistry sedimentology and palynological data after about 4100 cal year bp lake levels decreased and groundwater discharge became dominant source of water during short term regressive events at about 4000 3800 and 3500 cal year bp as suggested by a significant difference in modern river water and groundwater chemistry and the geochemical signal of fossil ostracod shells the overall trend towards lower lake levels culminated in repeated episodes of desiccation of lake eastern juyanze between 3200 and 2900 cal year bp individual regressive events as well as the rapid environmental fluctuations of lake eastern juyanze at about 3000 cal year bp were likely caused by mid to late holocene temperature fluctuations recorded by the dunde ice core yao t thompson l g 1992 trends and features of climatic changes in the past 5000 years recorded by the dunde ice core annals of glaciology 16 21 24 and shifts of the southeastern asian monsoon

late holocene paleoenvironmental change at mid elevation on the caribbean slope of the cordillera central dominican republic a multi site multi proxy analysis abstract high resolution proxy records from the circum caribbean region indicate significant variation in late holocene climate especially precipitation attributed primarily to shifts in the mean annual position of the intertropical convergence zone itcz the paleoenvironmental and cultural impacts of this late holocene climate variability have been analyzed intensively in the western caribbean and to a lesser extent in the southern caribbean however the occurrence and impacts of late holocene climate shifts in the eastern caribbean especially in island interiors has not been well documented here we present sediment records of late holocene paleoenvironmental change from two lakes located on the caribbean slope of the cordillera central in the dominican republic that span the last 3000 years sediment characteristics pollen charcoal biogenic carbonate assemblages and isotopic composition and bulk sedimentary carbon isotope values in laguna castilla and laguna de salvador indicate extreme shifts in hydrology vegetation and disturbance regimes in response to climate change and human activity in the lake watersheds close correspondence between the hydrological histories of the lakes and trace metal concentrations in sediments of the cariaco basin indicate that precipitation variability here responds to the same controls and may similarly reflect shifts in the mean annual position of the itcz human occupation of the watersheds appears to be closely linked to severe dry periods and may indicate larger scale cultural responses to precipitation variability on the island of hispaniola prehistoric human populations strongly affected vegetation and disturbance regimes in the lake watersheds impacts may have lasted several centuries and may have been more severe than impacts of modern populations

drought and dust deposition in the west african sahel a 5500 year record from kajemarum oasis northeastern nigeria a high resolution multiproxy palaeolimnological record from the manga grasslands northeastern nigeria spanning the last 5500 calendar years reveals the episodic deterioration in sahelian climate as significant biogeophysical thresholds were crossed desert dust deposition began to increase 4700 cal bp rainfall during the summer monsoon season declined permanently after 4100 cal bp a further significant change in atmospheric circulation giving rise to multidecadal to centennial scale droughts and enhanced dust deposition occurred 1500 cal bp hence the post 1968 sahel drought is not unique the prolonged arid episode that occurred around 1200 1000 cal bp in ethiopia the sahel and tropical mexico may have been linked to an abrupt cooling event in the north atlantic and to a cluster of intense el nino southern oscillation events in the eastern equatorial pacific

trace element variability in single ostracod valves as a proxy for hydrochemical change in nam co central tibet during the holocene ostracod shells belonging to three taxa leucocytherella sinensis huang 1982 leucocythere dorsotuberosa huang 1982 and l dorsotuberosa f postilirata sensu pang 1985 were collected from two sediment cores from lake nam co on the central tibetan plateau two variants of la icp ms laser ablation inductively coupled plasma mass spectrometry spot and line scan analyses were used to investigate trace element concentrations mg sr ba u and rare earth elements rees in single ostracod shells the results suggest that the line scan method can provide better precision than the spot analysis and is therefore preferred no significant difference in trace element composition between taxa was detected in our study fluctuations of mg ca and sr ca in the shells show good agreement with the reported lake level changes during the holocene indicating that ostracod mg and sr can be related to palaeohydrochemical processes in this area a high correlation between ba ca and sr ca was detected in our study perhaps implying that both metals were controlled by the same mechanisms a possible relationship between ostracod u ca and the past redox conditions on the lake bottom is discussed relatively low and constant la ca was observed which could be due to the ree characteristics in the lake water or ostracod biological processes or even the associated fe mn and or organic contaminants future studies on more specimens from this area especially shells from living ostracods are essential to investigate the potential of ostracod ba u and rees for use as palaeoenvironmental indicators

high geomagnetic intensity during the mid cretaceous from thellier analyses of single plagioclase crystals recent numerical simulations have yielded the most efficient geodynamo having the largest dipole intensity when reversal frequency is low reliable paleointensity data are limited but heretofore have suggested that reversal frequency and paleointensity are decoupled we report data from 56 thellier thellier experiments on plagioclase crystals separated from basalts of the rajmahal traps 113 to 116 million years old of india that formed during the cretaceous normal polarity superchron these data suggest a time averaged paleomagnetic dipole moment of 12 5 1 4 1022 amperes per square meter three times greater than mean cenozoic and early cretaceous late jurassic dipole moments when geomagnetic reversals were frequent this result supports a correlation between intervals of low reversal frequency and high geomagnetic field strength

intensity of the earth s magnetic field evidence for a mesozoic dipole low the variation of the strength of the earth s magnetic field throughout geological time is known only poorly large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations these uncertainties combined with well known experimental difficulties can explain the large scatter in paleointensity data reported in most previous review papers because experimental difficulties are more rigourously dealt with using the thellier method which also offers some opportunities to minimize some fundamental uncertainties we carried out a compilation of all triassic and younger paleointensity determinations made using this method this selected data set indicates the occurrence of large long term changes of the geomagnetic dipole strength with time the average dipole strength seems to have been approximately constant since late cretaceous time but a dipole strength only one third of the cenozoic value prevailed during most of mesozoic time following a period of larger strength ending possibly during early jurassic times the mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle the terrestrial dynamo seems therefore to exhibit different long term stable states which are probably controlled by the thermal structure in the lowermost mantle

mg ca and sr ca of ostracod valves from living species of lake geneva abstract the mg ca and sr ca ratios of living ostracods belonging to 15 different species and sampled monthly over a one year cycle at five sites 2 5 13 33 and 70 m water depths in western lake geneva switzerland are compared to the oxygen and carbon isotope compositions measured on the same samples as well as to the temperature and chemical composition of the water 18 o h2o 13 c dic mg ca h2o and sr ca h2o at the time of ostracod calcification the results indicate that trace element incorporation varied at the species level mainly because of the ecological and biological differences between the different species life cycle micro habitat preference biomineralisation processes and the control thereof on trace element incorporation of the ostracods in littoral zones the mg ca and sr ca of ostracod valves increase as temperature and mg ca and sr ca of water increase during spring and summer hence reflecting mainly seasonal variations however given that for lake geneva the mg ca and sr ca of water also vary with temperature it is not possible to distinguish the effects of temperature from those of changes in chemical composition of water on the trace element content in ostracod valves results support that both water temperature and water mg ca and sr ca ratios control the final trace element content of cyprididae valves in contrast the trace element content of species living in deeper zones of the basin is influenced by variations in the chemical composition of the pore water for the infaunal species trace element content measured for these specimens cannot therefore be used to reconstruct the compositions of the water lake bottom in addition incorporation of mg and sr into the shell differs from one family sub family or even species to the other this suggests that the distinctive mg and sr partition coefficients for the analysed taxa result from different valve calcification strategies that may be phylogenetic

evidence for weak geomagnetic field intensity prior to the cretaceous normal superchron abstract a palaeomagnetic and geochronologic investigation has been carried out on a sequence of 19 lava flows at sihetun in northeastern china age determinations by k ar dating indicate that the sequence spans 124 133 ma that is the time just prior to cretaceous normal superchron cns rock magnetic measurements of thermomagnetic curves and hysteresis loops show the primary carrier of remanence to be pseudo single domain magnetite stepwise thermal demagnetization successfully isolated a well defined direction of characteristic magnetization chrm in all the lavas the mean palaeodirection determined is d i 5 4 58 7 95 2 4 and d i 175 9 60 0 95 6 4 for 15 normally magnetized flows and 3 reversely magnetized flows respectively one lava was found to possess an intermediate direction of magnetization the inferred palaeolatitude of the sampling site is very similar to the present day position supporting the contention that suturing between the mongolia and north china plates was probably completed by the late jurassic and that subsequently little rotation or translation occurred in the region west of the tanlu fault zone thellier thellier palaeointensity experiments with systematic partial thermoremanent magnetization ptrm checks were conducted on 66 samples from 13 lava flows results from the 29 samples 44 which met strict reliability criteria indicate a range of virtual dipole moments vdms from 2 83 to 4 17 1022 a m2 with an average of 3 53 0 02 1022 a m2 this finding helps confirm that a weak geomagnetic field probably occurred just prior to the cns

evidence from western north america for rapid shifts in climate during the last glacial maximum the estancia basin in southwestern united states contains evidence for strong and rapid pulsations in the supply of moisture brought into the region during the last ice age the pulses were recorded during episodes of stream discharge that spread plumes of fresh water laden with quartz sand over the saline lake the largest pulses in stream discharge lasted only a few decades were organized into cycles that were spaced approximately 200 to 250 and 2000 years apart and were of sufficient magnitude to freshen and maintain the lake at its maximum recorded elevation

strontium content of ostracods indicates lacustrine palaeosalinity individual non marine ostracod valves contain sr in proportion to the sr content of their host lake water two commonly coexisting large planktonic species mytilocypris praenuncia and australocypris robusta have a sr distribution coefficient kd sr ca of 0 082 which is independent of temperature in the range 10 18 c if the relationship between salinity and the sr content of a hydrologically simple lake is known palaeosalinities can be calculated from fossil ostracods we present here palaeosalinities for the past 10 000 yr for lake keilambete victoria calculated from ostracod sr ca of fossil material which are in striking agreement with independent interpretations of palaeosalinity the range of salinity spans 14 95 we also show that a reliable quantitative continuous indicator of lacustrine palaeosalinity is important for the elucidation of continental palaeoenvironments for palaeoclimatology and archaeology and ocean continent climatic correlations and interactions

magnesium content of non marine ostracod shells a new palaeosalinometer and palaeothermometer abstract ostracods are microcrustaceans with calcitic carapaces each consisting of two valves or shells that grow by moulting and regeneration of sequentially larger valves up to nine times before reaching maturity the mg content of these valves is species dependent and is a function of their growth stage and the temperature salinity mg content and mg ca ratio of their host water newly formed partly calcified valves of the non marine nektic ostracod mytilocypris henricae have high mg contents 100 000 ppm mg in caco3 at individual shell weights of 2 g this relative mg content decreases as calcification proceeds until shell weights exceed 120 g whereupon the mg content is constant and useful as an indicator of the ostracod s environment mg ca ratios in individual ostracod valves from several species of the genera mytilocypris and australocypris from culture experiments and a series of australian lakes indicate that the valve s mg content increases as a function of increasing temperature and salinity within the range 11 25 c and for waters containing 24 2325 ppm mg the molar mg ca ratio in these ostracods fully calcified valves increases by 0 0015 or 370 ppm mg in caco3 per 1 c and per change of 125 ppm mg in solution for most mainland australian lakes a change in water composition of 125 ppm mg corresponds to a change of 3 5 in salinity for a given mg content of a lake the mg ca ratio of the water may vary within the range of common lacustrine values without significantly affecting mg uptake in the ostracod valves these relationships can provide palaeosalinities and or palaeotemperatures for ostracods recovered from continental sediments for mg partitioning an increase in temperature and salinity produce increased mg content in ostracods whereas for 18o fractionation in biogenic calcite an increase in temperature produces a decrease in 18o of the ostracod and an increase in salinity leads to an increase in 18o of the ostracod thus a combination of 18o and mg measurements on non marine ostracods should enable unique resolution of palaeotemperature and palaeosalinity variations

reduced early holocene moisture availability in the bayan har mountains northeastern tibetan plateau inferred from a multi proxy lake record abstract a late glacial and holocene lake sediment record from the northeastern tibetan plateau was investigated using ostracod and geochemical analysis after lake koucha was established about 16 cal ka bp oligohaline conditions gradually converted to mesohaline conditions about 10 cal ka bp clearly suggesting that the water body was a closed basin lake from about 10 cal ka bp the salinity of the lake rose continuously until hyperhaline levels were reached about 7 5 cal ka bp this increase in salinity was accompanied by a falling lake level extreme rainfall and flooding events in a generally dry climate hit the shallow saline lake around 8 5 cal ka bp causing a unique thin gravel containing layer within the continuous fine grained lake sediments the age control of our record does not allow us to correlate this event exactly to the widely recorded 8 2 cal ka bp event however the recovered sediments from lake koucha provide strong evidence that climate changes around 8 5 cal ka bp had significant environmental implications in northeastern tibet after about 7 3 cal ka bp the available moisture significantly increased causing a continuous salinity decline until the present day freshwater conditions of a through flow lake were established about 6 5 cal ka bp in lake koucha this major shift from a shallow saline to a deeper freshwater lake was probably accompanied by a fundamental change from a macrophyte dominated to a phytoplankton dominated lake a slight reduction in available moisture in comparison to the periods before and afterward was recorded between 4 3 and 2 0 cal ka bp in phase with numerous records from monsoonal asia the reconstructed overall change in moisture availability from dry conditions before about 7 3 cal ka bp to wetter conditions afterwards contradicts most palaeoclimate records from the tibetan plateau and its surroundings which show that wetter conditions in the early holocene were replaced by drier and probably cooler conditions in the mid and late holocene however in the semi arid region of lake koucha with a mean annual precipitation of about 500 mm the insolation driven strengthened monsoon in the early holocene may have resulted in a temperature rise and higher evaporation rates which outweighed the increase in precipitation and therefore led to significantly lower moisture availability our record provides further evidence for a regionally non uniform response of the available moisture evolution to the overall monsoonal signal in the holocene climate history of central asia

paleolimnological records of climatic change in north america lacustrine fossil records provide long time series of data on limnological and climatic conditions these data are useful for establishing natural patterns of climate variability and for generating testable hypotheses about atmospheric circulation and climate ecosystem linkages shoreline features can indicate past lake level fluctuations that may reflect changes in moisture balance but often these records are discontinuous and are evidence of only extreme conditions the organisms geochemistry and sedimentology of lake sediments may provide a more continuous sequence of direct and indirect lake climate interactions in the past the most clearly interpretable paleolirnnological records of climatic change are those that use several lines of evidence to corroborate a climatic hypothesis and are from sites near an ecotone or in regions of extreme climate in all cases hydrologic setting mediates a lake s response to climate and must be considered in interpreting sedimentary sequences discussion of the impact of recent and future climatic changes necessitates a baseline against which the magnitude of observed or predicted changes can be measured commonly we look to recent history as a reference point for comparison however the landscape of recent decades or in some cases even centuries has been altered greatly by human activities thus to determine whether recent or future climatic change is unique or unusual we need long natural preanthropogenic time series against which observed or predicted climatic states and variability can be compared paleolimnological archives are a tool for looking at environmental patterns prior to anthropogenic impact and show a range of climatic states far greater than those documented by written records former strand lines of pluvial lakes bonneville and lahontan in the western u s for example testify to moisture extremes unsurpassed in recent centuries or even millennia in addition one can us the paleolimnological record as an independent climatic proxy against which patterns and rates of terrestrial response to climatic change can be compared here i present a broad overview of paleoclimatic reconstruction in north america from lacustrine records with a focus on the late quaternary the review is limited to climatic inference from paleolirnnological records a term i use in a restrictive sense to refer to records of past lake states physical biological and geochemical as opposed to records of past stream hydrology e g ely et al 1993 knox 1993 or terrestrial conditions i summarize paleolimnological tools available for reconstruction of past climate the capabilities and limitations of these tools

northern hemisphere climate control on the environmental dynamics in the glacial black sea lake abstract the marine isotope stage mis 3 stands out due to its abrupt changes from cold and dry stadials to warm and humid interstadials the so called dansgaard oeschger cycles that also affected temperature and rainfall in the black sea region this study is based on a gravity core from the southeastern se black sea that covers the last glacial lake stage from 64 to 20 ka bp by using the composition of major and trace elements in the sediments terrestrial plant derived n alkane flux and sr ca from benthic ostracods we reconstruct the variability of riverine and aeolian input salinity and productivity in the se black sea region in response to the northern hemisphere climate oscillations during colder and drier stadials the aeolian input increased relative to the riverine discharge potentially due to southward shifted and or stronger westerly winds and due to changes in the vegetation cover an evaporation exceeding freshwater supply by rainfall and rivers possibly caused higher salinity and a lower lake level the environmental status during mis 4 and 2 is very much comparable with the stadial conditions during mis 3 during warmer and more humid interstadials lower salinity and presumably positive lake level changes most likely resulted from increased precipitation and river discharge this likely increased primary productivity through an augmented nutrient supply lowest average salinities are suggested for the middle part of mis 3 in response to enhanced meltwater from the disintegrating fennoscandian ice sheet and or by generally more humid conditions

late cenozoic climate changes in china s western interior a review of research on lake qinghai and comparison with other records abstract we review late cenozoic climate and environment changes in the western interior of china with an emphasis on lacustrine records from lake qinghai widespread deposition of red clay in the marginal basins of the tibetan plateau indicates that the asian monsoon system was initially established by 8 ma when the plateau reached a threshold altitude subsequent strengthening of the winter monsoon along with the establishment of the northern hemisphere ice sheets reflects a long term trend of global cooling the few cores from the tibetan plateau that reach back a million years suggest that they record the mid pleistocene transition from glacial cycles dominated by 41 ka cycles to those dominated by 100 ka cycles during terminations i and ii strengthening of the summer monsoon in china s interior was delayed compared with sea level and insolation records and it did not reach the western tibetan plateau and the tarim basin lacustrine carbonate 18 o records reveal no climatic anomaly during mis3 so that high terraces interpreted as evidence for extremely high lake levels during mis3 remain an enigma following the last glacial maximum lsm several lines of evidence from lake qinghai and elsewhere point to an initial warming of regional climate about 14 500 cal yr bp which was followed by a brief cold reversal possibly corresponding to the younger dryas event in the north atlantic region maximum warming occurred about 10 000 cal yr bp accompanied by increased monsoon precipitation in the eastern tibetan plateau superimposed on this general pattern are small amplitude centennial scale oscillations during the holocene warmer than present climate conditions terminated about 4000 cal yr bp progressive lowering of the water level in lake qinghai during the last half century is mainly a result of negative precipitation evaporation balance within the context of global warming

quantifying closed basin lake temperature and hydrology by inversion of oxygen isotope and trace element paleoclimate records lake systems are important paleoclimate archives that preserve ecosystem and hydrologic responses to critical periods in earth history such as carbon cycle perturbations and glacial interglacial cycles geochemical measurements of biogenic carbonate for example 18o 13c 87sr 86sr li u sr and mg are indicators of hydrologic variability in lake systems throughout the geologic record in this study we present a new closed basin lake modeling approach hybim the hydrologic balance inverse model that employs a system of total differential equations and uses the measured 18o sr ca and mg ca of biogenic carbonate to determine changes in temperature runoff and lake evaporation using equally spaced time steps these equations are simultaneously solved to constrain the hydrologic parameters of the lake as recorded in biogenic carbonate we use a monte carlo approach to account for uncertainty in the input parameters such as 18o temperature relationships partition coefficient uncertainty and watershed solute chemistry for illustrative purposes we apply the model to two ostracod valve datasets covering different timescales 1 the cretaceous songliao basin northeast china and 2 holocene lake miragoane haiti modern water measurements of water isotopes and cation concentrations from each location are required as model inputs we compare our modeling results with author interpretations and geologic observations the modeling approach presented in this study can be applied to other closed basin lake records can be modified for other calcifying species for example gastropods or mollusks or with calibration to inorganic lacustrine carbonate in addition this approach holds promise for extension with additional proxy measurements that is d u ca or li ca and changing source area on tectonic timescales using proxies that reflect changing source lithology that is sr and pb isotopes future incorporation of age model uncertainty in the monte carlo approach will also provide utility by quantifying temporal uncertainty on the hydrologic response recorded by lake sediments

variability in ostracod partition coefficients d sr and d mg implications for lacustrine palaeoenvironmental reconstructions living and sub recent ostracods were collected from four lakes and foul springs located in belgium france and spain characterized by low water mg ca ratios from 0 08 to 0 65 valves of ostracods belonging to cyprideis torosa candona neglecta c marchica and c candida have been extensively analysed for their magnesium and strontium contents with respect to calcium comparisons of these results are made using the same elements from the waters in which the ostracod valves calcified our results offer new and somewhat different interpretations with respect to the uptake of mg and sr in the calcitic ostracod valves these findings include the following 1 the partition coefficient l sr for ostracods belonging to the same genus candona is not a constant the observed variability in d sr values could be due to the rare of biological calcification although more investigations are necessary to assess the influence of this factor it is possible that some taxa grouped under the genus candona belong to a different genus 2 even at the species level cyprideis torosa the ostracod d sr cannot be considered to be constant at low water mg ca ratio 0 65 our d sr value for c torosa is higher than those determined by others for c torosa calcified in evolved marine water 3 the mg ca ratio of the water appears as a major factor influencing d mg for ostracods for low water mg ca values similar to 2 d mg drastically increases this phenomenon is also effective for inorganic calcite implying a non biologic effect that could be linked to mg2 adsorption consequently large variations in me content of fossil ostracod valves from selected lacustrine sequences with expected low water mg ca similar to 2 can be due to small variations in water mg ca c 1998 elsevier science b v all rights reserved

a late quaternary palaeolimnological record from jamaica based on trace element chemistry of ostracod shells abstract the trace element chemistry of ostracod shells provides one of the most promising methods for palaeolinmological reconstruction in this paper molar sr ca and mg ca ratios in shells of the ostracod cypretta brevisaepta furtos 1934 were used to reconstruct the late quaternary palaeolimnology of wallywash great pond a small freshwater coastal lake in jamaica analyses of modern jamaican ostracods were undertaken to establish molar distribution coefficients kd d m s for this species k d sr 0 306 0 069 k d mg 0 0142 0 0088 these values were used to reconstruct past levels of sr ca water and mg ca water for a 9 23 m lake sediment core from wallywash great pond the results were coupled with previously published stable isotope 18 o and 13 c determinations on fine grained calcite to reconstruct the salinity palaeohydrology and palaeoproductivity of the lake over the last 125 ka b p 10 3 calendar years before present three major highstands with slightly elevated salinity occurred during marine isotope stage 5 under elevated sea level and a humid climate the lake dried out 93 5 ka b p as sea level felll and the climate became drier it refilled around 10 7 ka b p three separate highstands during the holocene were sustained by humid conditions and high relative sea level although there were small increases in salinity during the holocene when sea level was close to the deepest part of the lake significant saltwater intrusion appears to have been prevented by freshwater discharge from inland throughout the late quaternary the salinity of the great pond showed low absolute variations from 0 3 to 0 6 o the long term trends in lake level reflect both orbitally induced changes in insolation and eustatic sea level water level fluctuations during the holocene however reflect climatic changes on the millennial timescale rather than orbital forcing or sea level change

wetland evolution in the qinghai lake area china in response to hydrodynamic and eolian processes during the past 1100 years abstract the daotanghe riverine wetland in close proximity to the qinghai lake was investigated to demonstrate the interrelationships between qinghai lake hydrodynamic processes eolian mobility and ecological conditions during the past 1100 years in response to climate change we used ostracod assemblages from various sites east of qinghai lake and from the sediment core qw15 of daotanghe pond and combined them with grain size and geochemical data from the same core the statistical extraction of grain size endmembers em revealed three different transportation processes responsible for pond related fluvio lacustrine pure fluvial and eolian deposits identified seasonal effects eolian mobility phase and timing of ice cover are possible tracers for the competing influence between the asian summer monsoon and the westerlies in the daotanghe wetland and surrounding area our results show that ostracod associations and sediment properties are evidence of a fluvio lacustrine fresh water environment without ingression of qinghai lake into the wetland hydrodynamic variations coupled with phases of eolian input indicate highly variable water budgets in response to climate induced effective moisture supply the medieval warm period mwp until about 1270 ce displays generally moist and warm climate conditions with minor fluctuations likely in response to variations in summer monsoon intensity the three partite period of the little ice age lia shows hydrologically unstable conditions between 1350 and 1530 ce with remarkably colder periods assigned to a prolonged seasonal ice cover pond desiccation and replacement by fluvial deposits occurred between 1530 and 1750 ce superimposed by eolian deposits the phase 1730 1900 ce is recorded by the re occurrence of a pond environment with reduced eolian input principal component analysis pca on ostracod abundances shows similar trends all three phases of the lia developed during a weak summer monsoon influence favoring westerly derived climate conditions until ca 1850 ce in accordance with records from the adjacent regions seasonal freezing periods in excess of the average time of frozen water bodies also occurred in periods of the well known grand solar minima and indicate stronger seasonality possibly independent from variations in summer monsoon strength but with links to global northern hemispheric climate

changes of the geomagnetic field vector obtained from lava sequences on the island of vulcano aeolian islands sicily abstract a combined geochronologic k ar and palaeomagnetic study has been conducted on five volcanic sections 70 flows on the island of vulcano the sections span the interval from 15 2 to 135 4 ka with large hiatuses rock magnetic investigations indicate that the natural remanent magnetisation nrm is carried by titanomagnetites in the four oldest sections with a contribution from high coercivity magnetic minerals in the youngest section palaeomagnetic directions obtained by thermal demagnetisation document a large magnetic excursion at around 110 ka with virtual geomagnetic poles vgps that lie over eastern siberia these directions may represent a volcanic record of the termination of the blake event only about 15 of the samples from 19 flows proved suitable for palaeointensity thellier and thellier determinations these measurements indicate that the geomagnetic field has varied at vulcano between 16 and 53 t in the explored time interval when combined with previous results obtained at mt etna for the period from 140 to 60 ka tric et al 1994 phys earth planet inter 85 113 129 these results document a broad low at around 115 ka the palaeointensity values from vulcano and etna are consistent with global values suggesting the presence of a dominant dipole field contribution these values on the other hand are significantly smaller than those recently obtained from la reunion in the southern hemisphere this suggests that large long lived non dipolar components of the geomagnetic field may be present at la reunion in the interval from 130 to 90 ka these observations are consistent with the fact that the vgp angular dispersion calculated from non excursional data is significantly smaller at vulcano than at la reunion

a continuous high resolution record of late pleistocene climate variability from the estancia basin new mexico lake sediments that accumulated in the estancia basin central new mexico provide a detailed record of submillennial latest pleistocene climatic oscillations sedimentologic biologic and geochemical proxies for changes in salinity and lake level were used to identify episodes of wet and dry climate that occurred between 24 and 12 k y b p as dated by 14c by means of accelerator mass spectrometry from shoreline and basin center deposits these dates determine the timing and duration of the episodes of wet and dry climate the lake expanded and contracted repeatedly during the last glacial maximum and fluctuated near its highest elevation several times during the interval ca 20 to 15 k y b p a pronounced lowstand lasting 1 k y occurred between ca 15 and 14 k y b p followed by two more highstands of the lake between ca 14 and 12 5 k y b p desiccation of the perennial lake after 12 k y b p was followed by a final poorly dated highstand at ca 10 k y b p the record of lake level fluctuations from the estancia basin provides a high resolution record of changes in water budget in a basin the comparatively simple physiography and hydrogeology of which ensured a direct response to the strong climatic fluctuations that occurred during the last glacial maximum and termination

the holocene paleolimnology of lake issyk kul kyrgyzstan trace element and stable isotope composition of ostracodes the stable isotope and trace element geochemistry of ostracode shells from lake issyk kul kyrgyzstan provide an important new paleoclimate record for central asia the n 18 o n 13 c sr ca mg ca and u ca composition of shells from the species candona neglecta found in two piston cores constrains the paleohydrology of the lake from 8700 to 1000 calendar yr before present these data combined with robust chronologies based on 16 ams 14 c dates suggest that the region went from a relatively moist climate in the early holocene to a more arid climate in the late holocene intense freshwater input presumably glacial runoff flowed into the lake from 8700 to 8300 cal yr bp as evident from the n 18 o and n 13 c values from 8300 to 6900 cal yr bp n 18 o and sr ca values indicate that the lake was relatively fresh and an open basin and u ca values suggest that the lake was well mixed with well oxygenated bottom waters there is a transition zone between 6900 and 4900 cal yr bp when n 18 o and sr ca values rapidly increase and mg ca and u ca values rapidly decrease during this period the lake evolved from an open freshwater well mixed lake to a closed more saline less well mixed lake for the remainder of the record n 18 o sr ca mg ca and u ca are relatively constant implying conditions similar to modern conditions although the slight increasing trend in sr ca may suggest an increase in salinity over this period the n 13 c values were approximately in equilibrium with atmospheric co2 in the early part of the record but became relatively negative in the later part of the record this indicates that the dominant mechanism controlling the isotopic composition of dissolved inorganic carbon shifted from exchange with the atmosphere to variations in biological productivity fl 2001 elsevier science b v all rights reserved

ostracode geochemical record of holocene climatic change and implications for vegetational response in the northwestern alaska range abstract trace element analysis of the calcareous shells of ostracodes in a sediment core from farewell lake provides the first limno geochemical record for climatic reconstructions in alaska when compared with pollen data from the same site this record offers new insights into climatic controls over vegetation dynamics during the holocene the low mg ca ratios and high sr ca ratios suggest that a relatively cold dry climate prevailed in this region between 11 000 and 9000 yr b p uncalibrated14c ages are used throughout the paper this result contrasts with previous interpretations of a thermal maximum at this time corresponding to the widespread establishment ofpopuluswoodland forest the trace element record suggests instead that the warmest period of the early holocene at farewell lake was between 8500 and 8000 yr b p during the decline ofpopulus marked decreases in sr ca and mg ca suggest a major increase in effective moisture around 6500 yr b p which coincided with the establishment ofpiceaboreal forests in the farewell lake region this climatic change was probably widespread throughout much of alaska and adjacent canada and might have induced the rapid spread ofalnusand the shift frompicea glaucatop marianadominance across that region our geochemical record also suggests that the late holocene climate history was more complex than previously thought on the basis of palynological studies according to this record growing season temperatures increased 6000 4500 yr b p decreased 4500 1500 yr b p and increased with fluctuations afterward after 6000 yr b p stratigraphic changes in pollen percentages ofpiceaappear to be positively related with those of mg ca this relationship implies that once the threshold of effective moisture was crossed for the establishment ofpiceaforests temperature was the primary control ofpiceapopulation density

sample size implications of the trace element variability of ostracod shells the magnesium and strontium contents of fossil ostracod shell calcite have been used extensively in the reconstruction of water composition and temperature in lakes and to a lesser degree in marginal marine waters and in the deep ocean current instrumentation may allow single shells of these calcite microfossils to be analysed and most studies have taken advantage of this capability however there is now good evidence for large intra sample variability in the trace element content of ostracod shells both in living populations and in fossil assemblages and this has implications for any attempt to distinguish the low frequency similar to decadal or longer trends in water temperature and composition that are often the goals of palaeoenvironmental studies from higher frequency seasonal or interannual noise in this study the mg and sr contents of living and fossil populations of six species of marine marginal marine and shallow marine ostracods from 11 sites are used to investigate sources of variability and to estimate critical sample sizes for stratigraphic studies results confirm that variability amongst individual samples is generally wide and cannot be attributed to instrumental error or sample handling alone in some instances variability can be explained by fluctuations in water composition and or temperature although this has to be evaluated in each case some evidence points to non environmental control on variability in shell composition perhaps connected to genetically controlled differences in physiology that affected calcification early diagenetic alteration or the role of environmental factors other than temperature or water composition however further work is needed to verify this in some stratigraphic studies large numbers 20 of individual shells would need to be analysed in order to detect low frequency environmental changes with any degree of confidence although fewer shells may often suffice the critical sample size depends on the variability at each study site which should ideally be evaluated using pilot studies c 2008 elsevier ltd all rights reserved

late holocene flooding and drought in the northern great plains usa reconstructed from tree rings lake sediments and ancient shorelines during the ad 1990s the waubay lakes complex in eastern south dakota experienced historically unprecedented high water levels property damage from this flooding led to an examination of the occurrence of past pluvial episodes and their relation to climate a 1000 year hydroclimate reconstruction was developed from local bur oak quercus macrocarpa tree ring records and lake sediment cores analysis of lake shoreline and drainage features provides late quaternary geomorphic context for this high resolution record tree ring width and shell geochemistry of the ostracode candona rawsoni show marked coherence indicating synchronous responses to moisture balance in vegetation and lake salinity geomorphic evidence suggests buffering of lake system expansion during pluvial periods by evaporative dynamics pluvial periods display a recurrence frequency of approximately 140 160 years over the past millennium prior to ad 1800 both lake highstands and droughts tended towards greater persistence than during the past two centuries frequency and timing of hydroclimatic oscillations show strong similarities to records from other sites in the northern great plains ngp of north and south dakota and incomplete coherence with records of southern manitoba

paleochemistry of lakes agassiz and manitoba based on ostracodes the ionic composition and salinity of lake manitoba and its late glacial precursor lake agassiz changed significantly over the past 11 000 years the paleochemical record reported here is based on modern analog environments of ostracodes identified in a new 14 5 m core from southern lake manitoba the ionic composition of lake manitoba today is dominated by na cl and hc03 with much less ca2 mg2 and k evaporative concentration of modern lake manitoba water would lead to greater salinity and the near depletion of ca2 due to continued precipitation of calcite during periods of highest salinity in the holocene however lake manitoba supported limnocythere staplini today this species inhabits waters in which ca2 hco3 including springs associated with groundwater in paleozoic bedrock discharging into lake winnipegosis and eventually after much dilution into lake manitoba further complicating the holocene record are intervals containing limnocythere friabilis that suggest periodic

late holocene palaeolimnology of bal lake northern nigeria a multidisciplinary study abstract a multidisciplinary study of lake sediments from bal lake a small saline lake in the sahel zone of northeastern nigeria provides evidence for changing precipitation evaporation ratios over the last 1000 cal years climatic and environmental changes over this period have been reconstructed using evidence from ostracod faunal assemblages and shell chemistry sediment properties and palynology and the sequences dated by a combination of 137 cs 210 pb and ams radiocarbon methods the results testify to major hydrological changes in the sahel over the last millennium the end of a major drought phase that is seen in other records from the manga grasslands is dated to around 950 ad this was followed by a prolonged freshwater phase of the lake from the 12th to the 16th century ad which points to wetter conditions a period of higher salinity and hence drier and more variable conditions from the 1700s to the mid 1900s was followed by a pronounced drying trend into the 20th century

geomagnetic field intensity at hawaii for the last 420 kyr from the hawaii scientific drilling project core big island hawaii four hundred twenty five new paleointensity thellier thellier determinations out of 545 analyzed samples have been obtained from core hsdp which penetrates about 1000 meters 208 flows of the mauna loa and mauna kea volcanic series encompassing the last 420 kyr rock magnetic investigations identify pseudo single domain magnetite as the main magnetic mineral inclinations are shallower than expected from a geocentric dipole field but are consistent with data from other geographical regions at the same latitude the inclination record reveals three episodes of negative inclination whose interpolated age correlates well with that of known geomagnetic events the paleointensity record from the mauna loa sequence is not very detailed and does not allow precise comparison with other data in the 0 50 kyr interval the record from the mauna kea sequence on the contrary is very detailed and documents relatively short lived episodes of low and high field strength from 15 to 60 t the average virtual dipole moment 8 7 3 0 10 22 a m 2 is not significantly different from the value reported by kono and tanaka 1995 for the last 2 5 myr a comparison with other data from hawaii and other geographical regions is described in detail there are no drastic changes in paleointensity with the inclination anomaly in agreement with previous results from hawaii but in contrast with most published results which however consider data from polarity transition spectral analysis of a particularly detailed portion of the record between 420 and 326 kyr documents significant periodicities at 36 8 5 and 4 ka in the inclination record but not in the intensity record suggesting that changes in time of the inclination are to a certain extent independent from those of the intensity

sr isotope constraints on the mediterranean environment at the end of the messinian salinity crisis probably the most profound event that has affected the mediter ranean is the desiccation which occurred at the end of the miocene1 8 during this period there was a very rapid change from deep open marine conditions to environments of shallow water carbonate and massive evaporite deposition1 10 here we show that ostracod valves found in messinian lago mare lake sea sediments have a uniform but generally lower sr isotopic composition than contemporary sea water thus providing evidence that some of the major mediterranean basins for example balearic and levantine basins formed large lacustrine systems that were probably interconnected towards the end of the messinian salinity crisis1 3 after the major evaporites were deposited the most likely source of fresh water is from continental rivers flowing from the eastern european inland paratethys sea or perhaps the nile river in the lower part of the lago mare some ostracods have sr isotopic compositions identical to contemporary sea water indicating a transient influence of marine derived water sr compo sitions greater than that in contemporary sea water are only found in ostracods from onshore sediments in the sorbas basin4 5 of southeastern spain suggesting that this small basin adjacent to the straits of gibraltar became isolated after the deposition of the major evaporite sequences

mid pliocene deep sea bottom water temperatures based on ostracode mg ca ratios abstract we studied magnesium calcium mg ca ratios in shells of the deep sea ostracode genus krithe from a short interval in the middle pliocene between 3 29 and 2 97 ma using deep sea drilling sites in the north and south atlantic in order to estimate bottom water temperatures bwt during a period of climatic warmth results from dsdp and odp sites 552a 610a 607 658a 659a 661a and 704 for the period ma reveal both depth and latitudinal gradients of mean mg ca values shallower sites 552a 610a and 607 have higher mean mg ca ratios 10 3 9 7 10 1 mmol mol than deeper sites 661a 6 3 mmol mol and high latitude north atlantic sites 552a 610a 607 have higher mg ca ratios than low latitude 658a 9 8 mmol mol 659a 7 7 mmol mol 661a 6 3 mmol mol and southern ocean 704 8 0 mmol mol sites converting mg ca ratios into estimated temperatures using the calibration of dwyer et al 1995 dwyer g s cronin t m baker p a raymo m e buzas j s correge t 1995 north atlantic deepwater temperature change during late pliocene and late quaternary climatic cycles science 270 1347 1351 suggests that mean middle pliocene bottom water temperatures at the study sites in the deep atlantic were about the same as modern temperatures however brief pulses of elevated bwt occurred several times between 3 29 and 2 97 ma in both the north and south atlantic ocean suggesting short term changes in deep ocean circulation

ostracod shell chemistry as proxy for paleoenvironmental change abstract the application of ostracod shell chemistry as a paleoenvironmental tool has grown within the past decades most studies have investigated mg and sr in ostracod shells as proxies for temperature and salinity and the use of a wide range of trace elements as prospective paleoenvironmental indicators has yet to be developed for lacustrine systems only a few preliminary studies have used trace metals in paleolimnological studies such as cd ba and zn as paleonutrient indicators or mn fe and u as redox and oxygenation indicators this paper reviews the state of the art of geochemical analyses in microfossils such as ostracods foraminifera and corals and provides insights in new trace element proxies with the goal to promote the use of trace elements in ostracod shells as paleoenvironmental proxies in paleoceanography foraminifera and corals are most prominently used to reconstruct past climate conditions well established proxies such as 18o mg ca and sr ca provide information about changes in sea surface temperatures in addition a great number of new proxies have been developed recently such as radiogenic isotopes and redox sensitive trace elements in paleolimnology ostracod shell chemistry is widely used to assess paleohydrological changes reconstruction of temperature and salinity changes in lake environments is often achieved by oxygen isotopes as well as mg ca and sr ca ratios but depending on the hydrological and geological settings of the lake system local calibrations are needed to assess which proxy is suited to reflect which processes new proxies need to be tested by novel techniques that recently have become available compared to conventional instrumentation used in ostracod shell chemistry methods such as laser ablation icp ms and nanosims allow single shell analysis and provide high resolution data the potential of ostracods in paleolimnology is not yet fully assessed but can be developed by learning from paleoceanographic studies

oxygen isotope evidence of little ice age aridity on the caribbean slope of the cordillera central dominican republic climate change during the so called little ice age lia of the 15th to 19th centuries was once thought to be limited to the high northern latitudes but increasing evidence reflects significant climate change in the tropics one of the hypothesized features of lia climate in the low latitudes is a more southerly mean annual position of the intertropical convergence zone itcz which produced more arid conditions through much of the northern tropics high resolution stable oxygen isotope data and other sedimentary evidence from laguna de felipe located on the caribbean slope of the cordillera central of the dominican republic support the hypothesis that the mean annual position of the itcz was displaced significantly southward during much of the lia placed within the context of regional paleoclimate and paleoceanographic records and reconstructions of global lia climate this shift in mean annual itcz position appears to have been induced by lower solar insolation and internal dynamical responses of the global climate system our results from hispaniola further emphasize the global nature of lia climate change and the sensitivity of circum caribbean climate conditions to what are hypothesized to be relatively small variations in global energy budgets

high resolution saline lake sediments as enhanced tools for relating proxy paleolake records to recent climatic data series abstract lake level in an endorheic saline lake in southern europe has been inferred for the last 105 years 1889 1994 at an annual level of resolution using two independent methods first ordination analyses factor analysis fa correspondence analysis ca and detrended correspondence analysis dca have been used to point out the mineral successions of the sedimentary record these successions are evidenced by the arch disposition of both samples and mineral phases in the plane defined by the first two eigenvectors calculated by these analyses these temporal evolutions are the same as those obtained during the drying and refilling phases of saline lakes relay indices ri were obtained from distances to the first two eigenvectors which accurately reconstructed the lake level evolution during this period second an inferred lake level series was obtained using a multivariate time series model from the average maximum temperatures and total rainfall six main drought periods were found which coincided with known droughts in the area a high level of agreement was found between the two reconstructions which offered the possibility of directly extending the instrumental record back into the past therefore climatic changes could be reconstructed from saline lakes provided that an accurate chronological control of sedimentary processes is available

complex controls on the trace element chemistry of non marine ostracods an example from lake patzcuaro central mexico abstract the calcitic shells of non marine ostracods are sources of geochemical information that have the potential to record changes in the water chemistry of closed basin lakes resulting from variations in the hydrological budget within the catchment in particular the uptake of sr and mg into the shell has been shown to be a function of salinity and salinity and temperature respectively this paper assesses the trace element chemistry records from 4 short 1 5 2 8 m cores from lake patzcuaro central mexico in the light of a detailed limnological study of the lake the following three environmental factors are believed to have influenced the interpretation of the trace element chemistry record i salinity the salinity of lake patzcuaro lies close to the calcite branchpoint below which the linear relationship between mg ca or sr ca and salinity has been lost in the fossil record ii environmental stresses e g low hco3 concentrations produced poorly calcified valves this study found this had no significant effect on the trace element chemistry results and iii solute composition of the host water changes in the solute concentration of inwash waters played a primary role in determining major water chemistry changes within the lake

in search of high fidelity geomagnetic paleointensities a comparison of single plagioclase crystal and whole rock thellier thellier analyses the strength of earth s magnetic field on timescales greater than a million years is known primarily from thellier thellier analyses of igneous rocks in the thellier thellier method the natural remanent magnetization nrm is compared with a thermoremanent magnetization trm imparted during heating in a known laboratory field unfortunately the multiple heating steps required by the method often result in alteration of magnetic minerals in clay bearing groundmass of whole rock samples this alteration may result in the creation of new magnetic minerals that could affect trm acquisition analyses of single plagioclase crystals which contain magnetic inclusions that retain paleofield values may offer a way to retrieve paleointensity data that is less affected by alteration we test this hypothesis through a study of whole rocks and single plagioclase crystals from a mid cretaceous 113 115 ma basalt flow of the rajmahal traps northeastern india magnetic hysteresis and thermal demagnetization data from plagioclase crystals and whole rock samples are similar suggesting a common magnetic mineralogy however hysteresis data from heated whole rock samples indicate that a fine grained magnetic phase has formed this effect was either absent or much less pronounced in the single plagioclase crystals tested paleointensity data from these crystals suggest a paleofield value of 65 1 5 3 t n 15 data from whole rock samples overlap these values but are on average lower we attribute the difference in this study to the preferential growth of new magnetic minerals in the whole rock samples resulting in an enhanced acquisition of trm and shallowing of the slope of nrm lost versus trm gained curves these data suggest that subtle alteration effects may bias thellier thellier results from some whole rock samples toward low paleofield values in these cases the single plagioclase crystal approach may yield more reliable estimates of geomagnetic paleointensity

mass balance and equilibrium line altitudes of glaciers in high mountain environments abstract the mass balance characteristics of glaciers in high mountain environments complicate the relationship between glacier equilibrium line altitudes elas and climatic variables such as precipitation and air temperature therefore methods of ela reconstruction employed in low relief environments are commonly not applicable in high mountains or require some modification we review the concept of the ela with reference to the mass balance of a range of glacier types found in high mountain regions we examine the applicability of several commonly used methods of ela reconstruction for different glacier types and propose some general principles to guide the choice of appropriate methods

paleosalinity from trace metals in fossil ostracodes compared with observational records at devils lake north dakota usa abstract the chemistry of closed basin lakes responds directly to the hydrologic budget through evaporative concentration of dissolved salts and the sediments of these basins contain several excellent fossil and geochemical proxies for past changes in water chemistry and salinity the trace metal content of ostracode shells among the most promising of these paleosalinity methods is further explored in this study of mg and sr partitioning in candona rawsoni a widespread ostracode of the north american great plains the molar distribution coefficients kd mg and kd sr for this species k d mg ca caco 3 0 004 mg 2 ca 2 h 2 o 9 68 10 5 t c k d sr sr ca caco 3 sr 2 ca 2 h 2 oz 0 406 are determined from laboratory experiments in which c rawsoni are cultured under controlled temperatures and salinities our formulation of kd mg contains a correction term for excess mg that is incorporated into the shell during early calcification the accuracy of the distribution coefficients is tested against field collections of live ostracodes from a suite of 12 dakota lakes the kd inferred values for mg2 ca2 h2o and sr2 ca2 h2o are in close accord with the same ratios measured in the lakes a comparison of ostracode reconstructed and historically measured salinities for devils lake north dakota shows good agreement within the salinity range preferred by c rawsoni 1 10 0 although salinity excursions beyond this optimum are poorly represented in the sedimentary record in most lakes salinity is more readily inferred from mg ca h2o than sr ca h2o because sr solubility is a complex function of several carbonate phases as shown for devils lake sr ca h2o may not vary systematically with the salinity of some brines

determination of the dissolved anion composition of ancient lakes from fossil ostracodes the mineralogy of evaporite and other precipitated minerals together with geochemical studies has provided traditional sources of information about the major dissolved ion composition solutes of ancient lakes the paleocompositional resolving power of these methods is generally greatest in high salinity lakes that precipitate numerous solute sensitive evaporite minerals as opposed to dilute saline lakes that precipitate only a few minerals ostracodes live in dilute saline lakes where a species occurrence is determined by the relative proportions of the lake s major dissolved anions so that each species describes specific areas on an anion trilinear diagram moreover the upper salinity tolerance of each species appears to depend upon the types of major anions in solution and is therefore anion specific knowledge about both anion and anion salinity tolerances of an ostracode may ultimately provide a means of estimating absolute anion concentrations in paleolakes because ostracodes are common fossils in lake sediments they provide an important new source of original paleocompositional information suitable for many geologic climatic geochemical and paleontologic studies

controls on ostracod valve geochemistry part 1 variations of environmental parameters in ostracod micro habitats abstract the variations of environmental conditions t ph 13 c dic dic 18 o mg ca and sr ca of ostracod habitats were examined to determine the controls of environmental parameters on the chemical and isotopic composition of ostracod valves results of a one year monitoring of environmental parameters at five sites with depths of between 2 and 70 m in lake geneva indicate that in littoral to sub littoral zones 2 5 and 13 m the chemical composition of bottom water varies seasonally in concert with changes in temperature and photosynthetic activity an increase of temperature and photosynthetic activity leads to an increase in 13 c values of dic and to precipitation of authigenic calcite which results in a concomitant increase of mg ca and sr ca ratios of water in deeper sites 33 and 70 m the composition of bottom water remains constant throughout the year and isotopic values and trace element contents are similar to those of deep water within the lake the chemical composition of interstitial pore water also does not reflect seasonal variations but is controlled by calcite dissolution aerobic respiration anaerobic respiration with reduction of sulphate and or nitrate and methanogenesis that may occur in the sediment pores relative influence of each of these factors on the pore water geochemistry depends on sediment thickness and texture oxygen content in bottom as well as pore water variations of chemical compositions of the ostracod valves of this study vary according to the specific ecology of the ostracod species analysed that is its life cycle and its micro habitat littoral species have compositions that are related to the seasonal variations of temperature 13 c values of dic and of mg ca and sr ca ratios of water in contrast the compositions of profundal species are largely controlled by variations of pore fluids along sediment depth profiles according to the specific depth preference of the species the control on the geochemistry of sub littoral species is a combination of controls for the littoral and profundal species as well as the specific ecology of the species

the chemical composition of lakes in the north central united states1 abstruct lake waters of the north central u s a arc classified into five groups based on increasing specific conductivity and changes in ionic composition from east to west from wisconsin through minnesota to north and south dakota the most dilute group of waters has specific condllctivities 29 pmhos cm at 25 c the most concentrated group has specific conductances that range from 7 000 to 73 000 pmhos as conductivity increases all major ions increase but there is a shift in cation dominance from ca2 to mg2 to na and in anion dominance from hc03 to sod2 this shift partly reflects a westward increase in climatic aridity and partly a westward sequence of glacial drifts from noncalcareous to calcareous and thence to calcareous with abundant sulfllr bearing minerals levels of ph k cl f b and sio also show a distinct westward increase concentrations of no and mn increase from east to west but the trend is less distinct concentrations of fe vary widely without any trend over the range of conductivity color mostly from dissolved organic matter is controlled chiefly by lake depth except for lakes with cxtcnsive peatlands in their drainage basins

climate driven hydrologic transients in lake sediment records calibration of groundwater conditions using 20th century drought abstract the effect of 20th century drought on groundwater and lakes in a climatically sensitive area of the northern great plains grant county western minnesota was investigated by analysis of lake sediments and historical air photos in conjunction with groundwater flow simulations drought caused an observed 4 0 to 5 1 m lake head decline between 1923 and 1938 all but three either dried completely or declined to 210 pb dated sediment core was analyzed for ostracode species distribution and geochemistry of c rawsoni shells 18 o 13 c mg ca sr ca mg ca and 13 c increased during drought at the same time that salinity tolerant ostracodes thrived suggesting increasing salinity however 18 o decreased during drought anti correlative with mg ca a numerical model for transient response of groundwater flow to drought modeled after that of 1923 38 was constrained by drought end recessional strand lines observed on air photos of one lake with strand line elevations inferred from bathymetry simulated lake water level declines in the first 15 drought years were consistent with strand line elevations the rate of decline was exponential at drought onset slowing as water in lakes and wetlands receded to below land surface and lake evaporation declined a near steady state was attained between 40 and 50 simulation years after onset of drought with the water table from 1 5 m below the level of dry lakebeds and only the two deepest lakes still holding water they were however greatly constricted in area and depth simulated evaporation fluxes were reduced by over 50 within 15 years despite increased et rates due to lake area constriction and water table declines both relate to land surface morphology beneath and around lakes model results suggest that lake bed elevations exert control on the rate and ultimate level to which groundwater is depressed by drought the deepest lakes in any region will tend to become the focus of groundwater flow during sustained drought once lakes around it dry out the unexpected finding of decreasing shell 18 o in response to drought is interpreted as some combination of source and or evaporation reduction effects such as seasonality of recharge precipitation or reduced et flux from wetlands that seep back into lakes

aquatic community response in a groundwater fed desert lake to holocene desiccation of the sahara abstract the finely laminated sediment record of a permanent hypersaline desert oasis lake in the ounianga region of northeastern chad presents a unique opportunity to document the hydrological evolution of this groundwater fed aquatic ecosystem during mid and late holocene desiccation of the sahara in this study we reconstruct long term changes in zoobenthos and zooplankton communities of lake yoa as their early holocene freshwater habitat changed into the hypersaline conditions prevailing today chironomid production peaked during the fresh to saline transition period then stabilized at about half that of the earlier freshwater ecosystem quantitative salinity inferences based on fossil chironomid assemblages indicate that the fresh to saline transition occurred fairly abruptly between 4100 and 3400 cal yr bp but that the ecosystem was buffered against shorter term climate fluctuations due to continuous inflow of fossil groundwater the mixture of tropical african and southern palaearctic chironomid faunas in the lake yoa fossil record required us to address several methodological issues concerning chironomid based salinity reconstruction and the applicability of a calibration dataset based on tropical east and west african lakes to this sahara desert locality the most coherent reconstruction was obtained with an inference model that applies a weighted best modern analogue wmat transfer function to the african calibration dataset expanded with six sahara lakes

reflections on cohmap cohmap cooperative holocene mapping project had its start in the 1970s with the intent to compare atmospheric general circulation models with quantitative palaeoclimatic reconstructions based on stratigraphic pollen analysis as the sophistication of the models increased and as additional palaeoclimatic indicators lake levels marine microfossils were incorporated in the reconstructions the programme expanded to global coverage through the participation of numerous collaborators the carefully nurtured dual approach of modelling and data analysis was strengthened by continued testing of one against the other with emphasis on interdisciplinary understanding fostered by workshops with the development of new computer models that have higher spatial resolution and improved boundary conditions and with diversification and enlargement of the geographic and temporal coverage of field and analytical data substantial refinements can be anticipated in the understanding of global climatic changes since the

6000 years of environmental changes recorded in blue lake south australia based on ostracod ecology and valve chemistry abstract a 4 m long core taken from the freshwater blue lake crater near the township of mount gambier in southeastern south australia provided a high resolution palaeoclimatic record for the last six millennia accelerator mass spectrometry ams radiocarbon 14 c dates were obtained from organic plant fibres and biogenic carbonates from the laminated sequence of the core and from a modern water sample large discrepancies between the radiocarbon ages determined from plant fibres and biogenic carbonates indicate the presence of a time variable lacustrine reservoir which is consistent with what is known of the lake s hydrology ostracod assemblages associated with stable isotope 13 c 18 o analyses and in combination with mg ca sr ca and na ca analyses done on ostracod valves infer salinity temperature and water level changes in blue lake over the last 6 millenia the influence of local aquifers through time has also been determined from the na ca of ostracod valves approximately 900 year cycles are evident in the 13 c record from 5 4 ka to 1 8 ka the history of blue lake records an initial period of high hydrological variability around 6 ka becoming increasingly deeper as groundwater flowed into the basin by 4 ka the lake had reached steady state with the lake level fluctuating by as much as 9 m although significant geochemical variations represent temperature fluctuations until european settlement near the lake in 1839

quantification of paleotemperature changes during isotopic stage 2 in the la draga continental sequence ne spain based on the mg ca ratio of freshwater ostracods past summer temperature changes are quantified from the la draga sequence banyoles ne spain during stage 2 24 10 ka bp using trace element content in lacustrine ostracod valves the high resolution results from this new method are compared to summer paleotemperature reconstructions from sequences from the north atlantic area providing a discussion about the magnitudes of some temperature changes in ne spain and offshore portugal the coldest part of stage 2 is not documented at the classical date of similar to 18 c 14 ka bp similar to 21 5 calendar ka bp but rather occurred later date of ca 17 15 ka cal bp in the north atlantic area the onset of the bolling allerod interstadials was relatively synchroneous at similar to 14 6 ka cal bp although the magnitude of the bolling allerod improvement was more important in the southern area than in the northern area the duration of the younger dryas is discussed even if different durations are reported from calendar ages it seems that the summer temperature cooling of the younger dryas was similar around the north atlantic area c 1996 elsevier science ltd

hydrologic and climatic implications of stable isotope and minor element analyses of authigenic calcite silts and gastropod shells from a mid pleistocene pluvial lake western desert egypt abstract authigenic calcite silts at wadi midauwara in kharga oasis egypt indicate the prolonged presence of surface water during the marine isotope stage 5e pluvial phase recognized across north africa exposed over an area of 4 25 km 2 these silts record the ponding of water derived from springs along the libyan plateau escarpment and from surface drainage the 18 o values of these lacustrine carbonates 11 3 to 8 0 pdb are too high to reflect equilibrium precipitation with nubian aquifer water or water of an exclusively atlantic origin mg ca and sr ca of the silts have a modest negative covariance with silt 18 o values suggesting that the water may have experienced the shortest residence time in local aquifers when the water 18 o values were highest furthermore intra shell 18 o sr ca and ba ca analyses of the freshwater gastropod melanoides tuberculata are consistent with a perennially fresh water source suggesting that strong evaporative effects expected in a monsoonal climate did not occur or that dry season spring flow was of sufficient magnitude to mute the effects of evaporation the input of a second isotopically heavier water source to aquifers possibly indian ocean monsoonal rain could explain the observed trends in 18 o and minor element ratios

climate driven hydrologic transients in lake sediment records multiproxy record of mid holocene drought abstract oxygen and carbon isotopes trace element concentrations and ostracode paleoecology are used with ground water flow modeling to assess the magnitude of climate driven hydrologic changes during mid holocene time for elk lake grant county minnesota during mid holocene time two extended droughts were recorded in the sediment record from elk lake at 5385 and 5221 calendar yr bp a drop in lake level to 14 9 m below the 1994 level and 10 0 m below the maximum 1930s decline at about 5385 calendar yr bp is interpreted for elk lake and its water table based on heterocypris fretensis occurrence and higher 18 o 13 c and mg ca values numerical modeling suggests this low water event could have resulted from conditions drier than those of the 1930s drought sustained over a time frame of approximately a century the later drought event at approximately 5221 calendar yr bp is marked by a shift in the dominant source of water to elk lake from surface run off to ground water from the till aquifer the shift in source water is identified by lower 18 o values and the presence of limnocythere staplini estimates of climatic sensitivity for holocene lakes should not be directly based on modern conditions but must take into account the changing shape of the lake basin through time and its effect on the paleohydrology

the role of the earth s mantle in controlling the frequency of geomagnetic reversals a series of computer simulations of the earth s dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magnetic field generation in the fluid core eight different patterns of heat flux from the core to the mantle are imposed over the core mantle boundary spontaneous magnetic dipole reversals and excursions occur in seven of these cases although sometimes the field only reverses in the outer part of the core and then quickly reverses back the results suggest correlations among the frequency of reversals the duration over which the reversals occur the magnetic field intensity and the secular variation the case with uniform heat flux at the core mantle boundary appears most earth like this result suggests that variations in heat flux at the core mantle boundary of the earth are smaller than previously thought possibly because seismic velocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin or because of enhanced heat flux in the mantle s zones of ultra low seismic velocity