holocene glacial activity and climatic variations in the swiss alps reconstructing a continuous record from proglacial lake sediments cores recovered from proglacial lake silvaplana swiss alps were investigated to examine the influence of holocene glaciers on lacustrine sedimentation during times of glacial activity in the catchment the sedimentation in the lake is dominated by the input of silt and clay sized material delivered by glacial meltwaters the deposition of fine grained material and the formation of varves reflect the existence of glaciers in the catchment and can therefore be used for the reconstruction of glacial history sedimentation rates are up to nine times higher in the varved than in the non varved sediments indicating significantly higher erosion rates under glaciated catchment conditions glacial varves were deposited in lake silvaplana during glacial retreat in the latest pleistocene and early holocene until 9400bp non varved deposits having a higher organic content and coarser grain sizes accumulated at lower rates when glacial activity was absent or negligible within the catchment between 9400 and 3300 bp

diatom inferred salinity in palaeolakes an indirect tracer of climate change lakes in arid and semi arid regions respond to climatic change through shifts in lake water volume and ionic concentration because diatom distribution is highly correlated with lake hydrochemistry diatoms can be used to infer changes in salinity and brine composition and thus to infer past climates here we critically examine the use of diatom inferred salinity as a climate proxy with examples taken from both modern waterbodies and sedimentary profiles sediment records may contain assemblages mixed from periods or sites of differing hydrochemistries because of the high degree of spatial and temporal heterogeneity in saline systems dissolution and diagenesis in saline brines may further complicate interpretation of sedimentary assemblages furthermore threshold effects salinity regulation via groundwater seepage antecedent conditions and other aspects of local hydrology may modify the relationship between salinity change and climatic forcing these complexities necessitate critical examination of the ecology and taphonomy of sedimentary assemblages as well as evaluation of potential non linearities in the salinity climate relationship through comparison of diatom inferred salinity with other proxy records of hydrochemical change historical documents where they exist and with other regional lake systems

chironomid based lateglacial climatic reconstruction for southeast scotland this paper presents the first chironomid based climatic reconstruction for the uk lateglacial obtained from a lake basin in southeast scotland whitrig bog comparison of chironomid lithological geochemical and pollen data reveals a marked vegetation lag behind the warming following deglaciation warm water chironomid taxa e g chironomus had replaced cold water chironomid taxa e g paracladius prior to the arrival of shrubs and trees and when local soils were still poorly developed in addition to clear evidence for the younger dryas stadial chironomid data also reveal two short term cold episodes that punctuate the lateglacial interstadial both of which are also reflected in pollen and lithological data as reversals from open birch scrub woodland to open herbaceous tundra coinciding with inwash of minerogenic matter into the basin 1997 john wiley sons ltd

nd sr isotopic composition of granulites and constraints on the evolution of the lower continental crust in a statistical study of granulite facies rocks we have analysed 32 samples with ages ranging from 2 900 to 200 myr for sm nd rb and sr and 143nd 144nd and 87sr 86sr ratios the sm nd ratio is found to be constant compared with the highly variable rb sr ratio using the model age formalism we show that in many cases internal differentiation of continental crust follows external differentiation with a large time interval

geochemistry of granulite facies lower crustal xenoliths implications for the geological history of the lower continental crust below the eifel west germany

reconstructing lake ice cover in subarctic lakes using a diatom based inference model a new quantitative diatom based lake ice cover inference model was developed to reconstruct past ice cover histories and applied to four subarctic lakes the used ice cover model is based on a calculated melting degree day value of 130 and a freezing degree day value of 30 for each lake the reconstructed holocene ice cover duration histories show similar trends to the independently reconstructed regional air temperature history the ice cover duration was around 7 days shorter than the average ice cover duration during the warmer early holocene approximately 10 to 6 5 calibrated kyr b p and around 3 5 days longer during the cool little ice age approximately 500 to 100 calibrated yr b p although the recent climate warming is represented by only 2 3 samples in the sediment series these show a rising trend in the prolonged ice free periods of up to 2 days diatom based ice cover inference models can provide a powerful tool to reconstruct past ice cover histories in remote and sensitive areas where no measured data are available

sm nd age of kambalda and kanowna greenstones and heterogeneity in the archaean mantle a precise sm nd age of 2 790 30 myr and an initial ratio corresponding to nd 3 4 0 6 has been determined for the kambalda sequence from the eastern yilgarn block of western australia the high initial 143nd 144nd implies a long lived light rare earth element depleted high sm nd source for the kambalda volcanics and therefore provides definitive evidence for ancient heterogeneities in the archaean mantle

an expanded calibration model for inferring lakewater and air temperatures from fossil chironomid assemblages in northern fennoscandia chironomid temperature inference models based on an expanded data set of surface sediment and limnological data from 53 subarctic lakes in northern fennoscandia have been developed using eight different numerical techniques each based on slightly different underlying statistical models or ecological assumptions the study sites are mostly small shallow bathymetrically simple oligotrophic lakes with a ph range from 5 0 to 7 8 a total organic carbon range from 2 5 to 12 6 mg l 1 a mean july lakewater temperature ranging from 6 1 to 15 4 c and a mean july air temperature ranging from 8 5 to 14 9 c a series of redundancy analyses rda identified sediment organic content maximum lake depth and lakewater temperature as being the most important explanatory variables variance partitioning by partial rdas further suggested that each of these variables accounted for a significant fraction of variance independent from each other different cali bration models were assessed on the basis of their statisti

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

nd and sr isotopic systematics of central australian granulites chronology of crustal development and constraints on the evolution of lower continental crust nd and sr isotopic data are reported for a granulite terrain in the proterozoic arunta block of central australia sm nd data from a wide range of rock types define a crust formation age of 2 070 125 ma and provide further evidence for voluminous crustal growth in the proterozoic an nd value of 1 5 0 8 indicates a depleted mantle source for this crustal segment and there is no evidence for a large component of significantly older sialic crust field relationships geochemistry and rb sr data for mafic and felsic granulites indicate that intracrustal differentiation and polyphase deformation were followed by granulite facies metamorphism rb depletion at 1 800 ma rb sr data for strongly retrogressed granulites define an age of 1 700 ma which is interpreted as the time of retrograde biotite growth partial melting at the presently exposed crustal level and anatexis at deeper crustal levels were broadly coeval with the retrograde metamorphism sm nd and rb sr isotopic systematics of minerals indicate that the terrain cooled slowly did not experience significant uplift until 1 000 ma and remained at temperatures above 320 c until the late palaeozoic the mineral data are consistent with geological relationships and petrological evidence for a prolonged period of isobaric cooling followed by uplift late in the metamorphic evolution of the terrain the granulite protoliths appear to have formed in a rift which closed within 280 ma of initial separation deformation and granulite facies metamorphism at 1 800 ma are interpreted to be a consequence of collision between the continental blocks which defined the rift regional retrogression and granitoid magmatism at 1 700 ma are attributed to underthrusting of the granulites by lower grade rocks in the final stages of collision subsequent events in the cooling and uplift history appear to have been controlled by the presence of long lived major faults in the crust and a prolonged history of episodic compression in the continental lithosphere the results of this study suggest that granulite terrains in general cannot be equated with lower continental crust but instead represent assemblages of mainly supracrustal rocks which in some instances have been involved in major collision events

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

chironomid response to environmental drivers during the holocene in a shallow treeline lake in northwestern fennoscandia the significance of different environmental drivers for the chironomid compositional changes during the holocene was assessed using partial redundancy analysis these drivers included pollen inferred mean july air temperature vegetation cover as pollen percentages diatom inferred water ph and total organic carbon toc and sediment properties as loss on iginition loi variance partitioning showed that on the holocene timescale chironomid assemblages were significantly responding to changes in betula percentages this is explained by the co occurring response of chironomids to the holocene climatic development which has also been responsible for the development of vegetation in the catchment however during the very early holocene period from 10 500 cal yr bp to 8300 cal yr bp chironomids were responding more directly to the changes in water quality ie ph during the latter part of the holocene no environmental variable appeared to be significant in explaining the variation in chironomid assemblages this may be due to the underestimation of the actual water toc by the diatom based toc model however for the latter part of the holocene from 5000 cal yr bp to the present the importance of limnological changes attributed to the development of marginal mires on the midges is evident our study shows that factors other than climate may have had more direct influence on the chironomid abundances at certain time periods although these factors are probably indirectly connected to climate through catchment processes more understanding of the catchment mediated climate effects on lake ecology and chironomids in high latitudes is needed in addition to careful site selection in order to better understand the environmental reconstructions

mechanisms of formation of deep basins on continental crust in the verkhoyansk fold belt miogeosynclines and cratonic basins abstract the verkhoyansk fold belt is a wide folded area in northeastern asia a large sedimentary basin 10 15 km deep was formed there by a slow subsidence of continental crust during 1000 ma in the late proterozoic middle paleozoic there are no deformations in the sedimentary cover that could indicate significant lithospheric stretching or shortening during this slow subsidence a number of deep basins 5 15 km of the same type exist or existed in other areas they can be called cratonic basins a slow gabbro eclogite transformation in the lowermost crust that occurred in a thick cratonic lithosphere can be suggested as a mechanism for their formation several deep water basins were formed in the verkhoyansk fold belt by rapid subsidence of continental crust during 1 5 ma in the late paleozoic and early mesozoic the subsidence occurred without significant stretching or thrust loading the basins formed were strongly folded in the late mesozoic under the convergent plate motions there were many basins of the same type in the other fold belts among deep basins on continental crust only those produced by rapid subsidence were intensely folded they can be called miogeosynclines rapid gabbro eclogite transformation with the destruction of the basaltic layer may be suggested as the mechanism of their formation this probably occurred under upwelling of hydrous anomalous mantle to the base of the crust

strontium neodymium and lead isotopic compositions of deep crustal xenoliths from the snake river plain evidence for archean basement abstract xenoliths of intermediate to felsic granulites found in evolved lavas from the snake river plain have been analyzed for nd pb and sr isotopic compositions and related trace element contents overall they exhibit wide ranges in present day values of 87 sr 86 sr 0 70235 0 83011 143 nd 144 nd 0 51023 0 51148 and 206 pb 204 pb 13 43 24 65 the rb sr u pb th pb and sm nd decay schemes have been variably affected by granulite facies metamorphism and possibly by transport in the host magmas however pb pb and sm nd isochron systematics seemingly are preserved in many of the xenoliths and indicate essentially concordant metamorphic ages of about 2 8 ga nd model ages are significantly older ca 3 1 3 4 ga for many of the xenoliths precambrian metasediments exposed along the southern margin of the snake river plain have sm nd systematics similar to those of the xenoliths these results suggest that at least two significant thermal events occurred during archean evolution of the crust in this region a early ca 3 1 3 4 ga additions of mantle derived magmas to the crust and b regional metamorphism at 2 8 ga accompanied at least locally by magmatism concordance between the sm nd and pb pb ages suggests that these isotopic systems are little affected by high grade metamorphism in this case the distribution of xenoliths in snake river plain lavas supports the presence of archean crust beneath much of southern idaho although such rocks rarely are exposed at the surface thus nd and pb isotopic studies of crustal xenoliths can provide a useful means of determining the extent of crustal age provinces where surface exposures are lacking

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

new evidence of warm early holocene summers in subarctic finland based on an enhanced regional chironomid based temperature calibration model abstract paleoclimate reconstructions based on biological proxies present methodological challenges especially during non analog conditions such as the early holocene here two chironomid based training sets from finland were amalgamated to create a more accurate transfer function of summer air temperature the aim was to reconstruct holocene paleoclimate in northernmost lapland in an area that has been either too warm or too cold for reliable reconstructions using the original calibration models the results showed that the combined calibration model had improved performance statistics the temperature trends inferred from the downcore chironomid record using the original and combined models were very similar however there were major changes in their absolute values with the combined model showing greatly improved accuracy the chironomid based temperature reconstruction showed significant correlation with the previous pollen based reconstructions from northwestern finnish lapland however differences were observed in the temperature trends of the early holocene when the chironomid inferred temperatures rapidly increased but the pollen based reconstructions lagged behind suggesting that a cool climate continued for much longer however similar to the chironomid record new plant macrofossil evidence from northwestern finland also showed warmer than present early holocene temperatures therefore we conclude that the early holocene was probably warm in northern lapland

isotope and trace element models of crustal evolution some of the isotopic constraints on the development of continental crust from about 3 8 ga ago are reviewed particularly it is noted that archaean granitic sensu lato rocks have initial 143 nd 144 nd ratios close to predicted values for the bulk earth at the time before emplacement whereas those phanerozoic granites investigated so far diverge considerably from the bulk earth and betray the existence of later continental crust in their provenance geochemical evidence for recycling of some continent derived elements into the mantle is examined and the important distinction between selected element recycling and bulk return of continental material is emphasized various transport models that have been proposed to model the development of continental crust are examined and some of their differences and similarities particularly with respect to implications for continental recycling are highlighted

cathodoluminescence and composition of calcite cement in the taum sauk limestone upper cambrian southeast missouri abstract study of calcite cements in the taum sauk limestone upper cambrian southeast missouri using cathodoluminescence petrography and microprobe analysis has shown that 1 distinct luminescent zones correspond to distinct compositional zones 2 the degree of luminescent intensity of a particular calcite zone appears to be controlled by the fe mn ratio of the zone not by the absolute concentrations of either cation 3 manganese may induce luminescence in calcite at concentrations as low as 300 ppm and 4 there may be no critical concentration of ferrous iron in calcite above which there can be no luminescene

isotopic and geochemical studies of nodules in kimberlite have implications for the lower continental crust lower crustal xenoliths from the calcutteroo kimberlitic pipes in south australia show a surprisingly diverse range in nd and sr isotopic compositions and rare earth element ree abundances mafic granulite and garnet clinopyroxenite xenoliths have measured end 0 values between 6 1 and 7 9 both light l ree enrichments and depletions low rb sr ratios 0 005 0 040 and relatively high 87sr 86sr ratios of from 0 70675 to 0 70919 a felsic xenolith has compositions more typical of upper crustal rocks with end 0 21 4 lree enrichments rb sr 1 47 and 87sr 86sr 0 85987 an approximate correlation also exists between sm nd versus 143nd 144nd and rb sr versus 87sr 86sr indicating major intracrustal differentiation in the lower crust at 2 400 myr this produced extremely low rb sr ratios and both relative lree enrichments low sm nd and depletions high sm nd if these results are typical then they indicate that the lower crust has higher 87sr 86sr and more positive end 0 values than previous estimates1 5 this may require revision of total crustal compositions and crust mantle evolutionary models1 5

tripartite climate reversal in central europe 5600 5300 years ago abstract the history of variations in water level of lake constance as reconstructed from sediment and pollen analysis of a sediment sequence from the archaeological site of arbon bleiche 3 shows an abrupt rise in lake level dendrochronologically dated to 5375 yr ago 5320 yr relative to ad 1950 this event paralleled by the destruction of the neolithic village by fire provoked the abandonment of this prehistoric lake shore location established in the former shallow bay of arbon bleiche and was the last of a series of three episodes of successively higher lake level the first occurring at 5600 5500 cal yr b p the dendrochronologically dated rise event was synchronous with an abrupt increase in atmospheric 14 c this supports the hypothesis of an abrupt climate change forced by varying solar activity moreover the three successive episodes of higher lake level between 5600 and 5300 cal yr b p at arbon bleiche 3 coincided with climatic cooling and or changes in moisture conditions in various regions of both hemispheres this period corresponds to the mid holocene climate transition onset of the neoglaciation and suggests inter hemispheric linkages for the climate variations recorded at arbon bleiche 3 this mid holocene climate reversal may have resulted from complex interactions between changes in orbital forcing ocean circulation and solar activity finally despite different seasonal hydrological regimes the similarities between lake level records from lake constance and from jurassian lakes over the mid holocene period point to time scale as a crucial factor in considering the possible impact of climate change on environments

crustal evolution of the hercynian belt of western europe evidence from lower crustal granulitic xenoliths french massif central abstract granulite facies lower crustal xenoliths scavenged by the tertiary basaltic pipes of bournac and roche pointue massif central france have been analysed for major and trace elements and radiogenic sr nd isotope ratios they display wide variations in major elements rare earth elements and isotopes 87 sr 86 sr 0 7041 0 7216 nd 11 9 to 1 5 which tend to correlate with the three different rock types encountered basic and acidic meta igneous and metasediments the basic liquids are of calc alkaline affinity and cumulates are abundant there is a gap in silica content between the basic and acid meta igneous xenoliths the isotopic compositions suggest mixing of old pre existing continental crust represented by the metasediments and mantle derived basic melts which represent juvenile addition to the crust during the hercynian orogeny the basic meta igneous rocks have been contaminated during their emplacement into the crust whereas the surrounding metasediments have suffered partial fusion and contamination by the same liquid producing the hercynian granitoids which have nd and sr initial ratios identical to those of the lower crustal xenoliths the residue from melting may be represented by the acid meta igneous rocks some of which have positive eu anomalies these results indicate that an important basaltic underplating event occurred during the evolution of hercynian orogenic belt although the exact timing of the underplating requires further constraint

the development of continental crust through geological time the south african case nd isotopic compositions and147sm144nd ratios were measured in fifty eight south african shales and greywackes with depositional ages ranging from 0 2 to 3 3 b y elements such as the rare earths which are poorly soluble in water and not fractionated during exogeneous processes preserve the signature of the original crustal source the147sm144nd ratios appear to be approximately constant throughout the time interval sampled we calculated nd model ages of crustal differentiation knowing that the shales represent a true blend of different continental areas we consider these model ages representative of the mean ages of their primitive continental sources then using the inverse technique developed by alle gre and rousseau in 1984 we computed a growth curve for the continental crust in south africa two periods of important crustal genesis archaean and around 1 5 b y can be compared with the observed geology and with other continental crust growth curves obtained in previous studies in southern africa and in australia the observation of large variations in the mgo content and ni cr u and th concentrations between archaean south african shales and post archaean samples compared to the constancy of the147sm144nd ratios leads us to propose that the archaean crust was composed of both granite 70 5 and a mafic component 29 5 which could have been komatiite the small dispersion of 147sm 144nd ratios suggests that erosion and sedimentation processes yielded homogeneous archaean shales the present day continental crust is much more heterogeneous because it has undergone several episodes of recycling thus recent shales are characterized by more variable 147sm 144nd ratios

holocene climatic instability a prominent widespread event 8200 yr ago the most prominent holocene climatic event in greenland ice core proxies with approximately half the amplitude of the younger dryas occurred 8000 to 8400 yr ago this holocene event affected regions well beyond the north atlantic basin as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane widespread proxy records from the tropics to the north polar regions show a short lived cool dry or windy event of similar age the spatial pattern of terrestrial and marine changes is similar to that of the younger dryas event suggesting a role for north atlantic thermohaline circulation possible forcings identified thus far for this holocene event are small consistent with recent model results indicating high sensitivity and strong linkages in the climatic system

water level fluctuations of lake tanganyika in phase with oceanic changes during the last glaciation and deglaciation there has been considerable controversy about the magnitude of fluctuations of the levels of lake tanganyika the earth s second deepest lake 1 470 m following the discovery of submerged valleys extending down to 550 m below present lake levels1 these fluctuations register changes in the precipitation evaporation evapotranspiration ratio in a large equatorial tropical area of catchment south of the equator here we report new palaeohydrological data back to 40 kyr bp from carbon dating of the total organic matter in two diatomaceous cores the results constrain the vertical lake level fluctuations more than has hitherto been possible2 5 and show that the fluctuations from 26 kyr bp are correlated with changes in global sea level and ice volume surpris ingly fluctuations seem to be in phase with those of the african lakes north of the equator which are clearly linked to the milankovitch mechanisms

palaeolimnological and sedimentary responses to holocene forest retreat in the scandes mountains west central sweden a suite of analyses was performed on sediments accumulated duringthe last 10 700 years in lake spaime a small hydrologically open water body in the modern alpine tundra zone of the scandes moun tains west central sweden the study aimed to evaluate 1 the nature of climate changes that forced the late holocene loweringof altitudinal tree limit in the region the timingof which is known from prior stu dies based on radiocarbon dating of subfossil wood and 2 the impact of these vegetational changes on an aquatic ecosystem arboreal pollen and plant macrofossil data confirm the persistence of trees in the lake catchment at least from c 9700 cal bp until c 3700 cal bp although growing season temperature is com monly believed to be the dominant factor drivingboreal forest tree limit variations in the reg ion a chir onomid based reconstruction of mean july air temperature suggests that local deforestation during the late holocene was not accompanied by a significant cooling the tree limit retreat was more likely caused by increasing effective moisture and declining length of the growing season the ecohydrological response of lake spaime to this combination of climate and vegetational changes included a decline in primary pro ductivity as indicated by an abrupt decrease in sediment organic matter content while associated increases in organic d 13 c d 15 n and c n point to diminished fluxes and altered balance of catchment derived nutrients followingdeforestation the decline in aquatic productivity is also marked by a distinct change in the mineral magnetic properties from a high magnetic concentration assemblage dominated by fine grained magnetite of biogenic origin to one dominated by background levels of coarse grained detrital magnetite

lower crustal xenoliths from queensland australia evidence for deep crustal assimilation and fractionation of continental basalts abstract a suite of mafic granulite facies xenoliths from north queensland possesses petrographic and geochemical features of basaltic cumulates crystallized at lower crustal pressures negative correlations between incompatible trace elements and mg and positive correlations between compatible trace elements and mg suggest the xenoliths are genetically related and crystallized from a continuously evolving melt zr hf y hree ti and v do not correlate with mg but show excellent negative correlations with al2o3 content reflecting the proportion of cumulate plagioclase to clinopyroxene these chemical trends also suggest the trace element concentrations have not been affected by subsolidus recrystallization the xenoliths have a large range in sr and nd isotopic compositions 87 sr 86 sr 0 70239 to 0 71467 nd 9 5 to 6 1 which cannot be produced by crystal fractionation alone and excellent correlations between isotope ratios and mg suggest the variable isotope compositions are not due to mantle source heterogeneities these mafic xenoliths are proposed to be cumulate products from a melt undergoing simultaneous assimilation and fractional crystallization afc the data illustrate that only a few percent afc in lower crustal environments can dramatically change the sr and nd isotopic composition of a basaltic melt and suggest the use of caution when inferring mantle source isotopic compositions from continental basalts additionally the nd isotopic data plot on a positive trend on an sm nd isochron diagram with an age of 570 ma however if these xenoliths formed by afc the positive trend reflects mixing between two isotopic end members and has no age significance the correlations between sr and nd isotopic compositions with mg degrade as the isotopic ratios are back calculated to earlier times suggesting the xenoliths are relatively young the xenoliths may be related to the cenozoic igneous activity which occurs throughout eastern australia

midges as quantitative temperature indicator species lessons for palaeoecology calibration data sets give a unique opportunity to establish patterns of biological existence and their statistical associations with environmental variables by use of calibration data sets environmental variables can be inferred quantitatively the resulting long time series may assist in distinguishing natural environmental variability from human induced variability both in terms of climate change and biotic turnover however the validity of the palaeoenvironmental reconstructions depends on their accuracy precision and sensibility before performing palaeoenvironmental inferences key mechanisms controlling contemporary species distribution abundances and dynamics should be identified and understood an inference model is developed to produce reconstructions a major challenge lies in validating and interpreting the reconstructions calibration data sets involving midges diptera chironomidae suggest that climate has a broad scale regional control over midge existence and abundance often ove

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

the sensitivity of a tanzanian crater lake to catastrophic tephra input and four millennia of climate change diatom genera in many large east african lakes change little throughout the holocene period suggesting relatively stable ecological conditions and some resilience to environmental change ecosystem stability is less common in smaller more sensitive lakes such as those within volcanic craters where external impacts can cause abrupt and rapid fluctuations a 4100 year diatom and cyanobacteria pigment record from lake massoko a volcanic crater lake in southern tanzania is used to illustrate important switches in resource ratios following tephra deposition 1190 years ago it is hypothesized that the tephra reduced the rate of p diffusion from the sediments and increased the si p ratio in the lake a period of acute change in planktonic diatom communities resulted from the tephra impact and lasted c 110 years the magnitude of the change shown by the diatoms and their slow recovery from the tephra may be due in part to a coincident fall in lake level caused by a reduction in regional rainfall the statistical significance of the tephra impact relative to that of catchment and climate change has been tested using variance partitioning and rate of change analysis multiproxy indicators show an important period of positive water balance 1700 ago and a relatively dry episode persisting between 1000 and 400 years ago the lake ecosystem is shown to be highly sensitive to both climate change and tephra deposition

pollen analysis of a long upper pleistocene continental sequence in a velay maar massif central france abstract a new 20 m sequence is presented which is based on 279 spectra derived from a coring at le bouchet lake massif central france the vegetational history of this region is described back to the end of the last interglacial ribains interglacial the two preglacial early glacial forested interstadials st geneys 1 and st geneys 2 are evidence for the first time in the massif central in spite of the altitude of the site 1200 m they suggest a quite temperate vegetation with the same mesophilous character as during the holocene the pleniglacial appears to be threefold with two cold episodes lower and upper pleniglacial divided by a long complex period of moderate climatic improvement the vegetation history of this mountain region is compared with that evidenced for the same period on the basis of the sequence of les echets at a lower altitude 267 m botanical and climatic inferences are discussed

exceptionally steep north south gradients in lake temperatures during the last deglaciation during the transition from the last glacial to the present interglacial climate late glacial period high summer insolation1 combined with the presence of the laurentide ice sheet is thought to have promoted the development of strong summer thermal gradients in north america south of the ice margin2 here we use palaeoecological methods to obtain quantitative evidence for the existence of these gradients our palaeoclimate reconstructions are based on water temperatures inferred from fossil assemblages of aquatic midge larvae in five lakes along a 240 km transect extending from central new brunswick canada to southeastern maine usa we show that the temperature gradient shifted during the killarney oscillation ko and younger dryas yd cooling events and that water temperature differences of 9 and 11 c existed over distances of 55 and 240 km respectively during parts of the late glacial period these gradients are much stronger than the current north south trends of lake summer surface water temperatures for these five lakes and for lakes across the northern tree line the rapid motion of such steep temperature gradients may have affected the progressive development of aquatic and terrestrial ecosystems in the region

chemical controls of cathodoluminescence of natural dolomites and calcites new data and review calcites and dolomites from the late precambrian bonahaven formation of scotland exhibit luminescence when mnco3 0 03 wt dolomite or 0 06 wt calcite and feco3 2 2 5 wt dolomite comparison with other published data shows close similarity in the position of fe quenching in dolomite but more variability in the position of mn activation in calcites and dolomites and the onset of fe quenching in calcites these anomalies are thought to relate to chemical controls subsidiary to the dominant factors of fe and mn concentration it is not practical to make a useful comparison between operating conditions of different luminescence instruments because of differences in design and an inability to measure routinely the current actually on the specimen surface

ion microprobe analysis of trace elements in calcite with an application to the cathodoluminescence zonation of limestone cements from the lower carboniferous of south wales u k ion microprobe analysis offers high spatial resolution 10 m and low detection limits for many elements in calcite detection limits are likely to be instrument dependent the values given here are specific to the instrument used a modified aei im 20 and more recent instrument designs should give lower detection limits li na and k signals have been observed and detection limits are estimated to be 10 ppm the detection limit for pb is in the range 10 50 ppm zn and ba suffer from significant molecular interferences which impose higher detection limits mg fe mn and sr were standardised against three chemically analysed calcites detection limits in the range 1 5 ppm were obtained for mg mn and sr but fe is subject to interference by a hydride species so that the detection limit is determined by vacuum conditions calcite cements in sections cut from a lower carboniferous limestone from south wales can be divided texturally into four zones analyses were made for mg fe mn and sr only sr differs among the zones mn is the main activator of cathodoluminescence cl in these cements when the fe content is up to 1600 ppm there is a positive relationship between cl intensity and mn content fe acts as a suppressor of cl intensity for a given mn concentration the cl intensity decreases as the fe content increases for fe contents 1600 ppm the cl intensity may be negatively related to the mn concentration when the mn content is below 125 150 ppm the fe content has little discernible effect on the cl intensity in these cements non luminescence can only be predicted with confidence if the mn concentration is below 25 ppm there is no critical concentration of fe up to 7000 ppm or critical fe mn ratio in the range 0 26 which always causes mn bearing locations to become extinct with respect to cl the present data are compared with those from the literature and a diagram is constructed which represents the region of fe mn composition space where luminescence can occur conflicting evidence regarding the relative importance of fe mn ratio and absolute fe and mn concentrations can be reconciled both concentrations and fe mn ratio are involved in controlling luminescence

holocene environmental changes in bangong co basin western tibet part 3 biogenic remains abstract a 12 4 m core collected from the eastern part of lake bangong provided a continuous holocene record this core was analyzed for diatoms ostracods charophytes content in total organic carbon and identification of organic particles contained in the sediments the record is remarkable for the abundance and diversity of its microorganisms an oligo to mesosaline marsh occupied the core site before 9 5 ka b p bangong was most likely closed a sudden filling of the lake basin by dilute nutrient poor water occurs around 9 4 ka b p maximum lake extent seems to have occurred around 9 4 8 7 ka b p generally freshwater relatively deep conditions were maintained up to 6 3 ka b p although interrupted by short term regressive events between 8 6 and 7 5 ka b p a trend towards palustral conditions begins after 6 3 ka b p and the interval 3 9 3 2 ka b p rich in detrital organic material coincides with an extremely low lake level a last lacustrine episode of minor amplitude is recorded from about 3 to 2 ka b p before the closure of the system leading again to slightly saline conditions the resemblances and synchronism between environmental changes recorded at bangong and in lake sumxi situated 200 km northwards and 800 m higher in elevation indicate that regional climatic changes have been the major driving factor on ecological and hydrobiological changes in lakes of western tibet

oxygen isotopes in fresh water biogenic opal northeastern us allerod younger dryas temperature shift the first oxygen isotope analysis of biogenic opal from lake sediments from the allerod younger dryas transition in a core from linsley pond connecticut gives an average estimate of a 6 c drop in temperature during the younger dryas this shift represents temperatures during the bloom season and may be less than the winter temperature drop the sharp transition itself with a duration of about 200 years suggests that the temperature decrease may have been as large as 12 c previous estimates of the allerod younger dryas temperature shifts are controversial and range from 3 20 c suggesting that further interdisciplinary research on the same samples is warranted

destabilization of a 650 km chemical boundary layer and its bearing on the evolution of the continental crust a mechanism for the production of a chemical change in the mantle from primordial silicate compositions above the 650 km discontinuity to differentiated compositions below is reviewed some consequences of this are the stabilization of two layer convection with a temperature contrast between the anhydrous mantle solidus and the geotherm which at 650 km depth is lower than any other location in the mantle with thermal contributions from the concentration of the heat producing elements u th and k below the 650 km mantle boundary layer and the higher geotherms in the past widespread or catastrophic melting may have taken place at this location an episodic breach of this boundary layer by extensive heat and mass transport may have periodically destroyed any simple two layer convection geometry in the mantle such episodic injections of mass and energy into the upper mantle from below may have been the mechanism responsible for episodes of enhanced surface tectonism and thermal activity which appear to be recorded in apparent polar wandering paths and radiometric ages of continental rocks

a 11 000 yr palaeotemperature reconstruction from the southern boreal zone in finland abstract the ecotone between the boreo nemoral hemiboreal and the southern boreal vegetation zones constitutes the northern distributional limit of a number of thermophilous tree species in northern europe and is to a large extent controlled by climatic conditions we present a quantitative annual mean temperature reconstruction from a high resolution pollen stratigraphy in southern boreal finland using a pollen climate calibration model with a cross validated prediction error of 0 9 c our model reconstructs low but steadily rising annual mean temperature from 10 700 to 9000 cal yr bp at 8000 4500 cal yr bp reconstructed annual mean temperature reaches a period of highest values holocene thermal maximum with particularly high temperatures 2 0 1 5 c higher than at present at 8000 5800 cal yr bp from 4500 cal yr bp to the present day reconstructed annual mean temperature gradually decreases by ca 1 5 c comparison of present results with palaeotemperature records from the greenland ice cores notably with the northgrip 18 o record shows marked similarities suggesting parallel large scale holocene temperature trends between the north atlantic and north european regions the verification of the occurrence timing and nature of the short term temperature fluctuations during the holocene in the southern boreal zone in europe requires replicate high resolution climate reconstructions from the region

holocene humidity changes in northern finnish lapland inferred from lake sediments and submerged scots pines dated by tree rings conclusive evidence for a rise in water levels has been found in connection with lake sediment studies undertaken partly in collaboration with the pine megafossil sampling and dendrochronological work in northern finnish lapland the change in lake level stands is shown by slow sedimentation rate in the early to mid holocene and an increase thereafter these data indicate a regional rise in water levels during the latter part of the holocene following a relatively dry period between 8000 and 4000 bp synchronous changes also indicating rising water levels have been observed in the diatom and cladoceran assemblages of the sediment cores subfossil scots pines pinus sylvestris l have been preserved in large quantities in small lakes in lapland because in many cases the rising water level has inundated the trunks after death the position of the subfossil trunks and stumps often indicates that the pines have been growing on dry land at the sides of the lakes in which they are now submerged traces of

past environmental and climatic changes related to tree line shifts inferred from fossil diatoms from a lake near the lena river delta siberia changes in diatom assemblages in a sediment core taken from a tundra lake near the lena river delta northeastern siberia reflect climatic and environmental changes throughout the holocene significant limnological changes occurred at the same time as trees advanced into the region c 9100 cal yr bp 8500 radiocarbon yr bp and subsequently retreated c 3800 cal yr bp 3500 radiocarbon yr bp prior to this tree line shift diatom assemblages were dominated by small benthic fragilaria species and diatom inferred alka linity values were high suggesting that climatic conditions were cool and relatively dry and that lake pro ductivity was probably low coincident with tree line advance into the region c 9100 cal yr bp inferred alkalinity decreased and remained low throughout the forested period reflecting soil development and increased organic runoff associated with the shift in catchment vegetation the establishment of a larch forest in the catchment corresponded with an initial period of limnolo

late holocene history of waldsea lake saskatchewan canada abstract the post hypsithermal history of waldsea lake a saline meromictic lake located in south central saskatchewan has been deduced from a study of the changes in physical mineralogical and paleobiological parameters in sediment cores from the basin six lithostratigraphic units and three palynological zones are identified in the most recent sediment these units and zones indicate that a shallow hypersaline lake with extensive mudflats existed about 4000 yr b p in response to the subsequent trend toward a cooler and wetter climate deeper water conditions ensued and by about 3000 yr ago a relatively deep stratified lake occupied the waldsea basin a short climatic reversal about 2500 yr b p again caused low water and mudflat conditions but by 2000 yr ago the lake had regained its higher levels the past 2000 yr of waldsea s history have been relatively uneventful except for a minor lowering of the lake about 700 yr b p

the effects of manganese ii and iron ii on the cathodoluminescence signal in synthetic apatite abstract synthetic carbonate fluorapatite cfa minerals produced with a range of manganese ii and iron ii concentrations exhibit varying intensities of luminescence when examined by cathodoluminescence cl petrography manganese ii activates luminescence in cfa with a rapid increase in relative luminescent intensity rli between 40 and 100 ppmw mn a sample with less than 40 ppmw mn is considered non luminescent based on visual criteria above 100 ppmw mn the rli value asymptotically approaches a maximum at about 400 ppmw mn and remains constant to 1000 ppmw mn iron ii quenches luminescence and effectively reduces the rli value of cfa samples with high mn concentrations this quenching behavior is not dependent on absolute fe concentrations but rather on mn fe ratios samples with mn fe weight ratios below 1 have lower rli values than samples with mn fe ratios above 1 regardless of the absolute mn or fe concentrations rli values increase linearly as mn fe ratios increase from 0 to 1 then remain approximately constant at a bright luminescence with mn fe ratios increasing from 1 to 6 for the cfa samples examined here a plot of mn versus fe concentrations with an overlay of observed luminescent intensities reveals fields of bright dull and non luminescence although this is a common procedure in characterizing carbonate luminescence this is the first such plot for cfa both mn ca and fe ca are higher in the solid phase cfa compared with the precipitating solution the best estimate from these experiments of the effective partition coefficient d metal ca in solid metal ca in solution for mn incorporation in cfa is 2 and that for fe is 13 due to changing metal calcium solution concentrations during precipitations these ds were estimated using assumptions about precipitation rates and solution composition changes during the course of experiments and are not representative of equilibrium values

submillennial environmental fluctuations during marine oxygen isotope stage 2 a comparative analysis of diatom and pollen evidence from lago grande di monticchio south italy high temporal resolution pollen and diatom analyses carried out on sediments from lago grande di monticchio span the interval 23 700 21 200 calendar yr bp a brief interstadial during marine oxygen isotope stage 2 both records exhibit marked changes that are interpreted as responses to climatic changes the diatoms and terrestrial vegetation appear to respond at the same time any relative lag in the response of the vegetation was less than the ca 60 yr resolution of the two records the interval coincides at least in part with the campo imperatore stade when the gran sasso ice sheet reached its maximum extent and water level was high in the fucino lake correlation of the marked environmental oscillation with one of the dansgaard oeschger events recorded by stable oxygen isotope records from the greenland ice cap is proposed this follows an interval interpreted as having a cold dry climate and correlated with a heinrich event in the north atlantic together the two records enable a multifactorial interpretation of the palaeoclimate changes that characterise the oscillation providing additional insight to that obtained from studies of ice and ocean sediment cores especially with respect to the seasonality of temperature and precipitation copyright 1999 john wiley sons ltd

allerod younger dryas lake temperatures from midge fossils in atlantic canada remains of freshwater midges are abundant in lake sediments and their species distributions are closely related to the surface water temperature of lakes their distributions thus provide a powerful tool for paleoclimatology the distribution of species in a core from splan pond in atlantic canada indicates that there were abrupt transitions in late glacial temperatures between warm and cold states the transitions are correlative with the well known warm allerod and cold younger dryas events in europe these data thus confirm the inference from palynological data that these events affected regions on both sides of the atlantic

rapid hydrological changes during the holocene revealed by stable isotope records of lacustrine carbonates from lake igelsjon southern sweden a holocene sediment sequence from lake igelsjon south central sweden was studied by stable oxygen and carbon isotope analyses of different carbonate components the deposit which covers the time span from ca 11 500 cal bp to the present was laid down in a small kettle hole lake the hydrological balance of which is presently dominated by groundwater flow isotopic records obtained on bulk carbonates originating mainly from summer produced calcitic algal encrustations exhibit several rapid shifts of more than 2parts per thousand likely reflecting pronounced hydrological variations corresponding isotopic data obtained on calcitic gastropod opercula from parts of the profile show subdued responses to major climatic shifts probably related to an extended calcification season the isotopic records were complemented by studies of modern isotope hydrology and our interpretations are based on a simplistic climate hydrology model in which variations in groundwater generation within the lake catchment produce changes in groundwater level and related adjustments of lake level and surface volume ratio of the basin during the ice free season assumed periods of decreased lake volume in a relatively dry climate low lake level are characterised by enrichment in o 18 and c 13 resulting from increased evaporation inflow ratio and atmospheric equilibration respectively in clear contrast to this situation intervals of more humid climatic conditions give rise to increased lake volume high lake level possibly surface over flow and relatively depleted isotopic ratios relatively humid conditions which may correlate to a wide spread cooling event recorded by various proxies across the north atlantic region are indicated by distinct isotopic shifts at ca 8300 and 8000 cal bp bracketing a period of o 18 depletion the period between ca 8000 and 4000 cal bp was characterised by relatively dry and stable climatic conditions whereas the subsequent part of the holocene experienced a more humid and variable climate following marked and coherent depletions in o 18 and c 13 at ca 4000 cal bp c 2002 elsevier science ltd all rights reserved less

metasomatised lower crustal and upper mantle xenoliths from north queensland chemical and isotopic evidence bearing on the composition and source of the fluid phase a diverse range of ultramafic and mafic granulite xenoliths from cone 32 north queensland display mineralogical and chemical evidence for enhanced fluid activities in the lowermost crust and uppermost mantle metasomatism of spinel lherzolite garnet and spinel websterite and garnet granulite produced variable amounts of amphibole mica locally resulting in development of hornblendite as the end product of reaction between spinel websterite and c o h fluids in contrast associated felsic and 2 pyroxene granulite xenoliths have remained relatively unaffected the garnet granulites and spinel websterites are interpreted as products of underplating of the lower crust by cenozoic magmatic activity whereas nd sr isotope systematics for the 2 pyroxene and felsic granulite xenoliths suggest that they may be of proterozoic age the development of amphibole and mica in the mafic and ultramafic xenoliths was accompanied by increases in tio2 na2o k2o a12o2 sr ba zr nb lree and fe2o3 feo and decreased mgo sio2 cr and ni virtually anhydrous spinel websterite xenoliths have 87sr86sr and 143nd144nd almost identical to amphibole rich variants and hornblendite and very similar values to the host nepheline mugearite and an amphibole megacryst the chemical and isotopic data suggest that the metasomatism is a relatively young feature caused by oxidized fluids exsolved from alkaline magmas similar to the host as they ascended through the lithosphere this style of metasomatism is considered a relatively localized phenomenon which is unlikely to provide an important contribution to the source regions of alkali basaltic magmas

evolution of the lower continental crust granulite facies xenoliths from the eifel west germany the lower continental crust is a key region to understanding processes of crust mantle differentiation and subsequent evolution of the crust the region is metamorphosed into granulite or amphibolite facies1 2 whereas felsic to mafic granulites with an average intermediate major element composition are typically observed in old exposed lower crustal terranes mafic compositions are preponderant among granulite facies xenoliths transported by young alkalic basaltic volcanics and kimberlites1 radiometrie ages for granulite bulk rocks using the rb sr or u pb methods are usually interpreted as metamorphic ages in contrast sm nd ages obtained on such metamorphics are interpreted to reflect pre metamorphic events3 7 although a case of severe disturbance of the sm nd isotope system during granulite facies metamorphism has also been reported8 we present here sm nd and rb sr isotope data for a suite of mafic lower crustal xenoliths from the hercynian belt our data suggest that the igneous precursors of these granulites differentiated from depleted mantle 1 5 gyr ago in most of these granulites this evidence for a mid proterozoic age has been obliterated by a young 172 myr metasomatic overprint by mantle derived fluids or melts

rapid early holocene environmental changes in northern sweden based on studies of two varved lake sediment sequences mineral magnetic organic carbon and pollen studies of two varved holocene lake sediment sequences in the boreal forest of northern sweden indicate that significant environmental changes took place between c 6000 and 5700 bc this interval is characterized by an increase in mineral matter accumulation which is a proxy for winter snow accumulation and a statistically significant decrease in total pollen influx predominantly pinus betula and alnus which may reflect lower spring and summer temperatures and increased frost frequency notable increases in the influx of deciduous tree species including quercus and corylus suggest a rapid change to warmer conditions between 5700 and 5600 bc given dating errors associated with the varve chronologies and the greenland ice core timescales the cold interval can be considered to reflect a regional possibly global climatic cooling which is often referred to as the 8 2 kyr bp cooling event however the younger age of the cold event in northern sweden does not support the hypothesis of forcing by the sudden drainage of laurentide glacial lakes into the north atlantic unless a minimal 300 year delay in ocean atmospheric coupling is accepted the data contribute to a complex picture of early holocene environmental change in response to deglaciation of the northern hemisphere

a 2000 year record of lake ontogeny and climate variability from the north eastern european russian arctic a lake sediment record from the north eastern european russian arctic was examined using palaeolimnological methods including subfossil chironomid and diatom analysis the objective of this study is to disentangle environmental history of the lake and climate variability during the past 2000 years the sediment profile was divided into two main sections following changes in the lithology separating the limno telmatic phase between 2000 and 1200 cal yr bp and the lacustrine phase between 1200 cal yr bp and the present owing to the large proportion of semi terrestrial chironomids and poor modern analogues a reliable chironomid based temperature reconstruction for the limno telmatic phase was not possible however the lacustrine phase showed gradually cooling climate conditions from 1200 cal yr bp until 700 cal yr bp the increase in stream chironomids within this sediment section indicates that this period may also have had increased precipitation that caused the adjacent river to overflow sub

a 200 year mid european air temperature record preserved in lake sediments an extension of the δ18op air temperature relation into the past abstract a detailed history of the water oxygen isotope ratios of lake ammersee southern germany was established for the last two centuries by analyzing oxygen isotope ratios of ostracod shells from a sediment core in a three to five year resolution empirical analysis and modelling of the isotope hydrology of the lake show that the oxygen isotopic composition of the lake water is mainly controlled by the isotopic composition of local precipitation for the last 200 years the lake isotope record is strongly correlated to the air temperature record from hohenpeisenberg situated within the lake s catchment area thus the mid european relation between 18 o in precipitation and air temperature documented for the last 20 years can be extended over a longer time and a larger temperature range the results provide further support for the validity of ostracod 18 o from ammersee sediments as a quantitative proxy for mean annual air temperature probably extending back to the last glaciation

holocene climatic change reconstructed from diatoms chironomids pollen and near infrared spectroscopy at an alpine lake sjuodjijaure in northern sweden the results of a multiproxy study reconstructing the climate history of the last 9300 years in northern sweden are presented it is based on diatom chironomid and pollen analyses as well as near

centennial scale climate cooling with a sudden cold event around 8 200 years ago the extent of climate variability during the current interglacial period the holocene is still debated temperature records derived from central greenland ice cores show one significant temperature anomaly between 8 200 and 8 100 years ago which is often attributed to a meltwater outflow into the north atlantic ocean and a slowdown of north atlantic deep water formation this anomaly provides an opportunity to study such processes with relevance to present day freshening of the north atlantic anomalies in climate proxy records from locations around the globe are often correlated with this sharp event in greenland but the anomalies in many of these records span 400 to 600 years start from about 8 600 years ago and form part of a repeating pattern within the holocene more sudden climate changes around 8 200 years ago appear superimposed on this longer term cooling the compounded nature of the signals implies that far field climate anomalies around 8 200 years ago cannot be used in a straightforward manner to assess the impact of a slowdown of north atlantic deep water formation and the geographical extent of the rapid cooling event 8 200 years ago remains to be determined

diatom based transfer functions for inferring past hydrochemical characteristics of african lakes abstract a new dataset of 282 modern diatom samples and associated environmental information has been created by merging existing regional datasets from north and east africa and niger the relationships between diatom species distributions and hydrochemistry are examined using canonical correspondence analysis cca and partial cca variables reflecting water conductivity ph and cation and anion composition account for significant and independent components of the total variation in the diatom data predictive models transfer functions are developed using the method of weighted averaging for conductivity r 2 0 87 ph r 2 0 77 and ratios between alkali and alkaline earth metals r 2 0 81 and carbonate bicarbonate and sulphate chloride ions r 2 0 82 prediction errors are estimated using the computer intensive method of jackknifing these transfer functions enlarge the potential domain for reconstruction of past hydrochemistry from fossil diatoms preserved in lake sediments

extension of the lac du bouchet palaeomagnetic record over the last 120 000 years a high resolution profile of the palaeomagnetic variations recorded in the upper 20 m of the lac du bouchet sedimentary sequence has been reconstructed from several parallel mackereth and livingstone type cores inter core correlations were established by comparing individual core profiles of magnetic susceptibility individual records of af cleaned directions were then stretched to a common depth scale and merged the mean record was then filtered and smoothed in order to reduce the background noise accelerator radiocarbon determinations and pollen analyses show that the sequence covers the last interglacial glacial cycle back to ca 120 kyr b p and allow to perform a depth to time transformation there is a noted absence of widely divergent directions back to 90 kyr b p it is estimated that the recorded signal a pdrm signal represents the geomagnetic input signal attenuated by about 19 it follows that the excursions reported for the laschamp and olby lavas if they existed at all must have been of rather short duration two durable excursions appear in narrow diameter sections at the base of three independent cores b c and d while a reverse to normal transition is possibly documented in the lowermost meter core d this part of the sequence is dated from the recognition of major climatic events related to the 18 o isotopic stages defined in the oceanic record between 116 and 95 kyr this introduces the possibility of new excursions at ca 104 and 95 kyr i e immediately after the recovery of normal polarity documenting the end of the blake event

continental climate response to orbital forcing from biogenic silica records in lake baikal changes in insolation caused by periodic changes in the earth s orbital parameters provide the primary forcing for global ice ages1 6 but it is not clear to what extent the climates in continental interiors are controlled directly by regional variations in insolation and to what extent they are driven instead by the highly nonlinear response of the oceans and ice sheets here we investigate this question using the record of biogenic silica in lake baikal as a proxy for climate change in this high latitude mid continental region we find a good correlation between this record and that of marine oxygen isotopes4 over the past 250 kyr the baikal record exhibits both a strongly nonlinear component manifested in a 100 kyr periodicity and weaker direct insolation components manifested in the 41 kyr obliquity and 23 and 19 kyr precession orbital cycles these results show that even though extreme continental climates such as this are influenced directly by insolation variations they are dominated by the nonlinear rhythm of the oceans and ice sheets

lake qinghai china closed basin like levels and the oxygen isotope record for ostracoda since the latest pleistocene abstract in 1985 a sino swiss limnogeological expedition to china s largest inland lake recovered seismic profiles and piston cores to help calibrate palaeoclimatic models of monsoon and insolation fluctuations over the northern tibet qinghai plateau since the last glacial maximum lake qinghai a perennial lake with an area of 4278 km2 1985 at 3194 m above sea level receives drainage from 30 000 km2 of catchment near the northwest corner of the plateau the 26 5 m deep terminal lake 1985 with brackish alkaline magnesium sulphate rich waters 14 g 1 1 tds has not overflowed since the last glacial but its levels have responded to climatic and hydrologic changes seismic reflection profiles for the sub bottom sediment structures revealed a complex topography with channels relict terraces tectonic tilting and deformation in deposits buried under a 4 6 m packet of mainly flat bedded sediments from which core samples were recovered the lowermost cored material 14 500 yr b p comprises mainly water lain loess deposited in a smaller flood plain lake of up a few metres depth under cold arid climatic conditions ams radiocarbon dating indicates average sedimentation rates for latest pleistocene and holocene deposits in central basin regions of 0 3 0 5 mm yr 1 the 18o values of fossil ostracod valves provide a record of changes in the isotopic chemistry of the lake waters that can be related to the precipitation evaporation budgets which have controlled the palaeo levels for lake qinghai the precipitation evaporation budget for the lake qinghai region is largely determined by the asian monsoon system the lake qinghai sediment record shows that there was no large influx of glacoer melt waters into the lake basin during the post glacial the regional climate remained arid until at least 12 500 yr b p when an incipient monsoon was possibly initiated river transported silts first reached the basin centre at 11 800 yr b p when the lake depth may have increased to 5 m possibly a monsoonal weakening for several centuries after 10 800 yr b p europe younger dryas caused a further period of aridity at 10 200 yr b p the basin began filling towards modern lake dimensions in response to a permanent strengthening of the asian monsoon substantial lake level rises probably characterised the periods 10 200 9800 9500 8500 8300 7200 yr b p and through perhaps a century at about 6800 yr b p decreasing levels probably characterised the intervening intervals short term lake level fluctuations through the mid holocene climatic optimum were minimal indicating a very stable climatic hydrologic regime an isotopic steady state for the lake waters since 3000 500 yr b p obscures the climatic hydrologic regime dependent 18o gradient but there is little stratigraphic evidence to suggest that the lake level has been significantly below its present level since that time three negative 18o excursions for the lake waters may correspond to known glacier advances on the plateau at 3000 2000 and 1500 yr b p during china s neoglacial

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

comparison of quantitative holocene temperature reconstructions using multiple proxies from a northern boreal lake four biotic proxies plant macrofossils pollen chironomids and diatoms are employed to quantitatively reconstruct variations in mean july air temperatures tjul at lake loitsana northern finland during the holocene the aim is to evaluate the robustness and biases in these temperature reconstructions and to compare the timing of highest tjul in the individual reconstructions the reconstructed tjul values are evaluated in relation to local scale site specific processes associated with the holocene lake development at loitsana as these factors have been shown to significantly influence the fossil assemblages found in the lake loitsana sediments while pollen based temperatures follow the classical trend of gradually increasing early holocene tjul with a mid holocene maximum the aquatic wetland assemblages reconstruct higher than present tjul already during the early holocene that is at the peak of summer insolation the relatively low early holocene july temperatures recorded by the pollen are the

charnockite geobarometers based on coexisting garnet pyroxene plagioclase quartz numerous geothermometers based on compositions of coexisting minerals have been calibrated experimentally or theoretically during the past 10 yr in contrast few continuous reaction geobarometers exist which have had wide application one based on the reaction of cordierite to garnet sillimanite and quartz is limited by a relatively narrow pressure range of application and also by conflicting calibrations1 2 because the mixing relations of mg and fe and the role of h2o in cordierite3 are not understood another successful barometer uses the assemblage garnet plagioclase al2sio5 and quartz4 5 continuous geobarometers based on the reaction of orthofer rosilite to fayalite plus quartz6 and on the aluminium content of enstatite in equilibrium with garnet7 have been used to a lesser extent the former barometer is limited by rarity of the requisite assemblage and the latter by uncertain experimental calibration for crustal rocks we calibrate here two mineralogical geobarometers from measured thermodynamic data the participating minerals pyroxenes garnet plagioclase and quartz are frequently associated in high grade metamorphic rocks calculation of pressures for several terrains show that the geobarometers yield reasonable and consistent results for the entire range of crustal pressures

quantitative multiproxy assessment of long term patterns of holocene environmental change from a small lake near abisko northern sweden quantitative reconstructions are made of holocene changes in climatic and environmental conditions from analyses of pollen chironomids and diatoms in identical stratigraphic levels of a sediment c

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

rb sr and sm nd isotopic variations in dissected crustal xenoliths abstract four crustal xenoliths from scotland have been dissected in order to evaluate the effects of magma xenolith interaction on rb sr and sm nd isotopic systematics and hence the use of the xenoliths for accurate determination of source compositions each xenolith was sampled from the edge to the center the nd isotopic compositions are virtually uniform across each xenolith however there are significant variations in rb sr and ree concentration as well as rb sr and sm nd ratios and sr isotopic composition most of this variation appears to be inherited from the protolith however one sample shows systematic variations in rb and sr concentration and sr isotopic composition with distance from the xenolith edges due to interaction with the host magma this sample displays petrographic evidence for grain boundary melt infiltration these data are consistent with theoretical considerations of transport times and diffusivities that place limits on the amount of modification that is possible via diffusion alone the calculated effects of transport of even large amounts of crustal xenoliths on the compositions of xenoliths and host magmas will be small provided transport times are short overall the isotopic variations within the xenolith suite mirror the source rocks from which they were extracted and are inferred to reflect variations in the deep crust of scotland however small samples such as most crustal xenoliths may be of little use in defining meaningful crustal residence ages because of modal variations in metamorphic minerals and isotopic re equilibration the one xenolith studied has extremely variable sm nd ratios due to the presence of garnet and yields a broad range of geologically meaningless nd model ages but yields an sm nd isochron age of 396 49 ma the loch roag xenolith has a large range of rb sr ratio and the sr isotopic data define an isochron age of 416 28 ma both ages are taken to represent the timing of metamorphism and or cooling from 600 c at the end of the caledonian orogeny in scotland

the significance of chironomid analysis insecta diptera for paleolimnological research abstract consideration of the dipteran family chironomidae is an important aspect of microfossil analysis of lake sediments because chironomids provide information on the ecological characteristics of an earlier period of lake development chironomids are of particular interest because some species are typical of the profundal zone of eutrophic lakes and other of oligotrophic lakes as a result the major part of the literature on chironomid analysis deals with the use of these animals as indicators of trophic states during earlier periods of lake histories indeed some studies have shown that an oligotrophic profundal fauna was completely replaced by eutrophic species during lake development the chironomid fauna is not only influenced by factors connected with eutrophication but also by other events occurring during lake development such as changing climatic conditions which until now have hardly been discussed

a multiproxy record of holocene environmental changes in the central kola peninsula northwest russia a sediment core from chuna lake kola peninsula northwest russia was studied for pollen diatoms and sediment chemistry in order to infer post glacial environmental changes and to investigate responses of the lake ecosystem to these changes the past ph and dissolved organic carbon doc of the lake were inferred using diatom based transfer functions between 9000 and 4200 cal yr bp slow natural acidification and major changes in the diatom flora occurred in chuna lake these correlated with changes in regional pollen the arrival of trees in the catchment changes in erosion sediment organic content and doc during the past 4200 yr diatom based proxies showed no clear response to changes in vegetation and erosion as autochthonous ecological processes became more important than external climate influences during the late holocene the pollen stratigraphy reflects the major climate patterns of the central kola peninsula during the holocene i e a climate optimum between 9000 and 5400 5000 cal yr bp when climate was warm and dry and gradual climate cooling and an increase in moisture during the past 5400 5000 yr this agrees with the occurrence of the north south humidity gradient in fennoscandia during the holocene copyright 2002 john wiley sons ltd

laboratory wave velocity measurements on lower crustal xenoliths from calcutteroo south australia abstract seismological and petrologic studies have suggested the presence of mafic granulite eclogite and amphibolite as the major components of the lower crust in southeastern australia elastic wave velocity measurements conducted to 400 mpa confining pressure of selected nodules from one kimberlite pipe in south australia range after corrections for estimated in situ crustal pressures and temperatures from v p 6 9 7 7 km s 1 and v s 3 9 4 3 km s 1 these values fall between previous measurements for more silicic crustal granulites and mafic eclogites and coincide with the published velocities determined by seismic studies for southeastern australia it is possible that these types of compositions may be widespread components of the lower continental crust and may require revision of currently accepted models of crustal evolution

zabargad and the isotopic evolution of the sub red sea mantle and crust abstract zabargad island in the red sea exposes ultramafic bodies in contact with partially retrograded granulite facies lower crustal rocks the contact could represent the sub red sea moho prior to the opening of the red sea the ultramafic rocks are largely protogranular spinel lherzolites that grade locally to porphyroclastic to mylonitic amphibole peridotites some of the peridotites are infiltrated by high mg gabbros troctolites the peridotites and the lower crustal rocks are cut by diabase dikes the zabargad peridotites are isotopically heterogeneous on a scale of meters or less sr and nd isotopic ratios of clinopyroxenes cpx from spinel lherzolites lie in the depleted portion of the 143 nd 144 nd 87 sr 86 sr mantle array whereas cpx and amphiboles from amphibole peridotites fall off the array towards the low 143 nd 144 nd and sol 87 sr 86 sr field that may characterize portions of the sub continental mantle cpx from troctolites plot in the middle of the 143 nd 144 nd 87 sr 86 sr mantle array and have higher 87 sr 86 sr ratios and more depleted lree patterns than the intruded spinel lherzolites and amphibole peridotites all whole rock lherzolites and peridotites and most diabase dikes from zabargad as well as a variety of rocks from other parts of the red sea area morb glasses a diabase dike from the brothers island basanites and clinopyroxenes from associated nodules from the ataq diatreme in yemen define a linear array on a sm nd isochron diagram with a best fit age of ca 675 ma and an initial nd of 6 7 this array is probably a mixing line between depleted morb type mantle and a low 143 nd 144 nd 87 sr 86 sr metasomatic component the age defined by the linear array may date either an episode of pan african mantle homogenization or in our preferred model the pan african segregation of a previously homogeneous mantle into two or more reservoirs of identical 143 nd 144 nd but different sm nd ratios the subsequent partial remixing of these isotopic reservoirs during metasomatism associated with the opening of the red sea did not perturbed the slope of the array the lree enriched reservoir may have been the pan african lower crust since there is field and textural evidence that the metasomatic fluids were derived from or equilibrated with the adjacent crustal complex alternatively the lree enriched component may have been a mantle reservoir similar to that which generated basalts and nodules from the nearby ataq diatreme the troctolites are isotopically distinct from the host peridotites and retain igneous textures suggesting they intruded the peridotites during the opening of the red sea their isotopic patterns are similar to those of recent volcanics from afar portions of the ataq diatreme and some zabargad dikes indicating derivation from a similar mantle source their very lree depleted patterns suggest they are either re melted fractions of previously very depleted spinel lherzolites or early formed mineral aggregates that became enmeshed in the peridotites as fractionating melt ascended to higher levels our results indicate that at least two major processes metasomatism and intrusion have modified the isotopic pat terns of the spinel lherzolites of zabargad had the resultant chemically and isotopically heterogeneous bodies stayed in the mantle instead of being uplifted to their present positions they would have proven to be enigmatic sources for future basalts and xenoliths

holocene glacier fluctuations of flatebreen and winter precipitation changes in the jostedalsbreen region western norvay based on glaciolacustrine sediment records the history of holocene glacier variations of flatebreen an independent glacier close to the sw part of the jostedalsbreen ice cap has been reconstructed from lacustrine sediments in the proglacial lake jarbuvatnet the sedimentary succession shows evidence of three maini episodes of holocene glacier expansion the first is recorded in the basal part of the core up to 370 cm according to the age depth relationship in the sediment core based on 12 ams radiocarbon dates this glacier expansion episode terminated about 10200 cal yr bp the second major glacier phase lasted from 8400 to 8100 cal yr bp while the third was initiated around 4000 cal yr bp and has continued up to the present at 43 cm in the core the medium silt content increases significantly accompanied by a minor increase in the sand content this textural change is interpreted as the first time that the tenninus of flatebreen extended inlto an upstream lake at 1083 m a s l the age model suggests that this event took place around

granulitic xenoliths from the french massif central petrology sr and nd isotope systematics and model age estimates the protoliths of granulitic xenoliths found in tertiary quaternary alkali volcanics from the french massif central are shown to have been principally immature sediments and calc alkaline intrusives the association may indicate formation of the crust in an island arc or continental margin environment pressure and temperature estimates indicate that they have undergone intermediate to high pressure granulite facies metamorphism 9 15 kbar followed by uplift to lower pressures 4 9 kbar rb sr and sm nd isotopic data and ree abundances are presented for both meta igneous and meta sedimentary suites esr present day values for the granulitic xenoliths range from 2 7 to 325 with end values between 0 2 and 12 0 there are no isochron relationships within the sr and nd data but possible mixing lines in the rb sr system may indicate sedimentation at 700 1000 myr t nd dm model ages indicate a proterozoic 800 2100 myr origin for the protoliths of the meta igneous samples and a similar provenance age for the meta sediments 1986 the geological society